Current Trends and Future Developments on (Bio-) Membranes: Recent Advances on Membrane Reactors 0128236590, 9780128236598

Table of contents :
Cover
Current Trends and Future Developments on (Bio)Membranes
Copyright
List of contributors
Contents
Preface
1 Introduction to membrane and membrane reactors
1.1 Introduction and principles
1.2 Membranes
1.3 Membrane bioreactors
1.4 Combination of membranes and catalytic reactions
1.4.1 Interfacial contactor mode
1.4.2 Flow-through contactor mode
1.5 Conclusions and future trends
Nomenclature
Acronyms
Symbols
References
2 Protonic electrocatalytic membrane reactors
2.1 Introduction
2.2 Ammonia synthesis
2.2.1 The common design of protonic electrocatalytic membrane reactors for the ammonia synthesis
2.2.2 Electrocatalytic nitrogen reduction reaction mechanism
2.2.3 Electrolyte materials
2.2.4 Cathode materials
2.2.5 Anode hydrogen feedstocks
2.3 CO2 reduction
2.3.1 The common design of Protonic electrocatalytic membrane reactors for the CO2 reduction
2.3.2 Mechanisms of the CO2 electrocatalytic reduction
2.3.3 Electrolyte materials
2.3.4 Cathodic materials and catalysts
2.3.5 Anodic materials
2.4 Hydrocarbon dehydrogenation
2.4.1 Methane upgrading
2.4.1.1 Electrocatalytic methane coupling
2.4.1.2 Electrocatalytic methane dehydroaromatization
2.4.1.3 Electrocatalytic methane reforming
2.4.2 Conversion of alkanes to alkenes
2.5 Other reactions
2.6 Conclusion and future trends
Nomenclature
Acronyms
References
3 Packed bed membrane reactors
3.1 Introduction
3.2 Latest developments in packed bed membrane reactors
3.3 Conclusions and future trends
Nomenclature
Acronyms
References
4 Fluidized bed membrane reactors
4.1 Introduction
4.2 Latest developments in fluidized bed membrane reactors
4.3 Conclusions and future trends
Nomenclature
Acronyms
References
5 Microstructured membrane reactors for process intensification
5.1 Introduction
5.2 Design and fabrication
5.3 Examples of microstructured membrane reactors
5.3.1 Polymeric
5.3.2 Metallic membranes
5.3.3 Zeolite membranes
5.3.4 Ceramic oxygen and proton conducting membranes
5.4 Conclusion and future trends
Nomenclature
Acronyms
Symbols
References
6 Pervaporation membrane reactor
6.1 Introduction
6.2 Pervaporation membrane reactors
6.3 Fields of application
6.3.1 Esterification reactions
6.3.2 Etherification reactions
6.3.3 Acetalization reactions
6.3.4 Condensation reactions
6.3.5 Bio-alcohol production (pervaporation bioreactors)
6.4 Conclusions and future trends
Nomenclature
Acronyms
References
7 Polymeric membrane reactors
7.1 Introduction
7.2 Polymeric membranes
7.2.1 Structure of polymeric membranes
7.2.1.1 Dense symmetric membranes
7.2.1.2 Mixed matrix membranes
7.2.1.3 Porous membranes
7.2.1.4 Preparation of porous membranes
7.2.1.4.1 Phase-inversion
7.2.1.4.2 Track-etching
7.2.1.4.3 Electrospinning
7.2.1.5 Ionic liquid membranes
7.2.1.6 Microporous membranes
7.3 Classification of membrane reactors
7.3.1 Extractor-type membrane reactors
7.3.1.1 Pervaporation membrane reactors
7.3.2 Contactor-type membrane reactors
7.3.2.1 Interfacial contactor membrane reactors
7.3.2.2 Forced flow-through membrane reactors
7.3.2.2.1 Non-selective flow-through catalytic membrane reactors
7.3.2.2.2 Selective flow-through catalytic membrane reactors
7.3.3 Distributor-type membrane reactors
7.4 Polymeric membrane microreactors
7.5 Conclusions and future trends
7.6 Acronyms
References
8 Current trends in enzymatic membrane reactor
8.1 Introduction
8.2 Designs of enzymatic membrane reactor
8.3 Membrane characteristics
8.4 Enzyme immobilization in enzymatic membrane reactor
8.5 Enzymatic membrane reactor versus other reactor configurations
8.6 Applications of enzymatic membrane reactor
8.7 Conclusion and outlook
Nomenclature
Acronyms
References
9 Membrane reactors in bioartificial organs
9.1 Introduction
9.2 Bioartificial organs—design issues
9.3 Transport phenomena
9.4 Membrane bioreactor as bioartificial liver
9.4.1 Membrane bioartificial livers in flat configuration
9.4.2 Membrane bioartificial livers in hollow fiber configuration
9.5 Membrane bioreactors for bioartificial kidney
9.5.1 Membranes for BAK
9.5.2 BAK devices in animal studies and clinical trials
9.6 Membrane bioreactor as a biomimetic model for nervous tissue analogue
9.7 Conclusions and future perspectives
Nomenclature
References
10 Photocatalytic membrane reactors
10.1 Introduction
10.2 Basic principles of photocatalysis
10.3 Basic of photocatalytic membrane reactors
10.3.1 Types of photocatalysts
10.3.2 Types of membranes
10.3.3 Membrane modules and system configurations
10.3.3.1 Pressurized membrane photoreactors
10.3.3.2 Depressurized (submerged) membrane photoreactors
10.3.3.3 Coupling of photocatalysis with nonpressure membrane operations
10.4 Applications of photocatalytic membrane reactors
10.4.1 Photocatalytic membrane reactors in photodegradation of pharmaceuticals in water
10.4.2 Photocatalytic membrane reactors in the conversion of CO2 in solar fuels
10.5 Advantages and limitations of photocatalytic membrane reactors
10.6 Conclusion and future trends
List of symbols
List of acronyms
Acknowledgments
References
11 Electrochemical membrane reactors
11.1 Introduction
11.2 Electrochemical reactors
11.2.1 General principles
11.2.1.1 Thermodynamics
11.2.1.2 Kinetics
11.2.1.3 Electrochemical efficiency
11.2.2 Endergonic transformers
11.2.3 Exergonic transformers
11.2.4 Cell separators
11.3 Diaphragms for liquid electrolytes
11.3.1 Asbestos
11.3.2 Thermoplastic diaphragms
11.4 Polymer membrane materials
11.4.1 Proton conducting ionomers
11.4.1.1 Chemistry and microstructure
11.4.1.2 Key physical properties
11.4.1.3 Limitations and perspectives
11.4.2 Hydroxyl-ion conducting ionomers
11.4.2.1 Chemistry and microstructure
11.4.2.2 Limitations and perspectives
11.5 Ceramic membrane materials
11.5.1 Nonorganic proton conductors
11.5.2 Oxide-ion conductors
11.5.2.1 Ionic conductivity
11.5.2.2 Limitations and perspectives
11.6 Selected endergonic applications
11.6.1 Water electrolysis
11.6.2 Main water electrolysis technologies
11.6.3 Brine electrolysis
11.6.3.1 Brief historical perspective
11.6.3.2 Performances and technological developments
11.6.3.3 Perspectives
11.7 Conclusions and future trends
Nomenclature
References
Further reading
12 Modeling of membrane reactors
12.1 Introduction
12.2 Packed bed membrane reactors
12.2.1 1D pseudo-homogeneous model
12.2.1.1 Continuity equation
12.2.1.2 Total momentum balance equation
12.2.1.3 Friction coefficient
12.2.1.4 Component mass balance
12.2.1.5 Energy balance
12.2.1.6 1D heterogeneous model
12.2.1.7 Component mass balance
12.2.1.8 Catalyst phase mass balance
12.2.1.9 Energy balance for gas phase
12.2.1.10 Energy balance for solid phase
12.2.2 2D pseudo-homogeneous model
12.2.2.1 Continuity equation
12.2.2.2 Total momentum balance equation
12.2.2.3 Friction coefficient
12.2.2.4 Component mass balance
12.2.2.5 Energy balance
12.2.3 Modeling of fluidized bed membrane reactors
12.3 Conclusions and future trends
Nomenclature
References
13 Techno-economic analysis of membrane reactors
13.1 Introduction
13.2 Latest developments in techno-economic analysis for membrane reactors
13.3 Conclusions and future trends
Nomenclature
References
Index

Citation preview

Current Trends and Future Developments on (Bio)Membranes

Current Trends and Future Developments on (Bio)Membranes Recent Advances on Membrane Reactors

Edited by

Angelo Basile Hydrogenia, Genova, Italy Unit of Chemical-Physics Fundamentals in Chemical Engineering, Department of Engineering, University Campus Bio-Medical of Rome, Rome, Italy

Fausto Gallucci Sustainable Process Engineering, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, The Netherlands

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2023 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. ISBN: 978-0-12-823659-8 For Information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Susan Dennis Acquisitions Editor: Anita Koch Editorial Project Manager: Moises Carlo P. Catain Production Project Manager: Selvaraj Raviraj Cover Designer: Mark Rogers Typeset by MPS Limited, Chennai, India

List of contributors Luca Ansaloni SINTEF Industry, Oslo, Norway Pietro Argurio

Department of Environmental Engineering, University of Calabria, Rende, Cosenza, Italy

Angelo Basile

Hydrogenia, Genova, Italy; Unit of Chemical-Physics Fundamentals in Chemical Engineering, Department of Engineering, University Campus Bio-Medical of Rome, Rome, Italy

Slamet Budijanto

Department of Food Science and Technology, IPB University, Kampus IPB Darmaga, Bogor, Indonesia

Loredana De Bartolo

CNR-ITM, National Research Council of Italy, Institute on Membrane Technology, Rende, Cosenza, Italy

Kiwinta Diaussie

Department of Food Science and Technology, IPB University, Kampus IPB Darmaga, Bogor, Indonesia

Alberto Figoli

Institute on Membrane Technology (CNR-ITM), Rende,

Cosenza, Italy

Francesco Galiano

Institute on Membrane Technology (CNR-ITM), Rende,

Cosenza, Italy

Fausto Gallucci Sustainable Process Engineering, Chemical Engineering and Chemistry, Eindhoven University of Technology, Eindhoven, The Netherlands Ellen Gapp

Institute for Micro Process Engineering (IMVT) at Karlsruhe Institute of Technology (KIT), Eggenstein-Leopoldshafen, Germany

Hua Huang

Department of Materials Science and Engineering, Clemson University, Clemson, SC, United States

Cristina Lavorato

Department of Environmental Engineering, University of Calabria, Rende, Cosenza, Italy

xiii

xiv

List of contributors

Marco Martino

Department of Industrial Engineering, University of Salerno,

Fisciano, Italy

Eugenio Meloni

Department of Industrial Engineering, University of Salerno,

Fisciano, Italy

Pierre Millet

Paris-Saclay University, ICMMO (UMR 8182), Orsay, France

Raffaele Molinari

Department of Environmental Engineering, University of Calabria, Rende, Cosenza, Italy

Sabrina Morelli

CNR-ITM, National Research Council of Italy, Institute on Membrane Technology, Rende, Cosenza, Italy

Vincenzo Palma

Department of Industrial Engineering, University of Salerno,

Fisciano, Italy

Thijs A. Peters

SINTEF Industry, Oslo, Norway

Peter Pfeifer

Institute for Micro Process Engineering (IMVT) at Karlsruhe Institute of Technology (KIT), Eggenstein-Leopoldshafen, Germany

Antonella Piscioneri

CNR-ITM, National Research Council of Italy, Institute on Membrane Technology, Rende, Cosenza, Italy

Carmella Rosabel

Department of Food Science and Technology, IPB University, Kampus IPB Darmaga, Bogor, Indonesia

Simona Salerno

CNR-ITM, National Research Council of Italy, Institute on Membrane Technology, Rende, Cosenza, Italy

Sergio Santoro

Department of Environmental Engineering, University of Calabria, Rende, Cosenza, Italy

Azis Boing Sitanggang

Department of Food Science and Technology, IPB University, Kampus IPB Darmaga, Bogor, Indonesia

Jianhua Tong

Department of Materials Science and Engineering, Clemson University, Clemson, SC, United States

Hilde J. Venvik

Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), Trondheim, Norway

J. Vital

Department of Chemistry, NOVA School of Science and Technology, LAQV-Requimte, Universidade Nova de Lisboa, Caparica, Portugal

List of contributors

Zeyu Zhao

Department of Materials Science and Engineering, Clemson University, Clemson, SC, United States

Minda Zou

Department of Materials Science and Engineering, Clemson University, Clemson, SC, United States

xv

Contents List of contributors Preface 1.

xiii

xvii

Introduction to membrane and membrane reactors

1

VINCENZO PALMA, MARCO MARTINO, EUGENIO MELONI AND ANGELO BASILE

1.1 Introduction and principles

1

1.2 Membranes

6

1.3 Membrane bioreactors

8

1.4 Combination of membranes and catalytic reactions

2.

10

1.4.1 Interfacial contactor mode

15

1.4.2 Flow-through contactor mode

15

1.5 Conclusions and future trends

15

Nomenclature

16

Acronyms

16

Symbols

16

References

17

Protonic electrocatalytic membrane reactors

21

ZEYU ZHAO, MINDA ZOU, HUA HUANG AND JIANHUA TONG

2.1 Introduction

21

2.2 Ammonia synthesis

22

2.2.1 The common design of protonic electrocatalytic membrane reactors for the ammonia synthesis

23

2.2.2 Electrocatalytic nitrogen reduction reaction mechanism

24

2.2.3 Electrolyte materials

26 v

vi

Contents

2.2.4 Cathode materials

27

2.2.5 Anode hydrogen feedstocks

28

2.3 CO2 reduction

3.

30

2.3.1 The common design of Protonic electrocatalytic membrane reactors for the CO2 reduction

31

2.3.2 Mechanisms of the CO2 electrocatalytic reduction

31

2.3.3 Electrolyte materials

32

2.3.4 Cathodic materials and catalysts

34

2.3.5 Anodic materials

35

2.4 Hydrocarbon dehydrogenation

36

2.4.1 Methane upgrading

36

2.4.2 Conversion of alkanes to alkenes

42

2.5 Other reactions

44

2.6 Conclusion and future trends

45

Nomenclature

46

Acronyms

46

References

47

Packed bed membrane reactors

59

FAUSTO GALLUCCI

4.

3.1 Introduction

59

3.2 Latest developments in packed bed membrane reactors

62

3.3 Conclusions and future trends

73

Nomenclature

73

Acronyms

73

References

74

Fluidized bed membrane reactors

77

FAUSTO GALLUCCI

4.1 Introduction

77

4.2 Latest developments in fluidized bed membrane reactors

79

Contents vii

5.

4.3 Conclusions and future trends

90

Nomenclature

91

Acronyms

91

References

91

Microstructured membrane reactors for process intensification

95

ELLEN GAPP, LUCA ANSALONI, HILDE J. VENVIK, THIJS A. PETERS AND PETER PFEIFER

5.1 Introduction

95

5.2 Design and fabrication

96

5.3 Examples of microstructured membrane reactors

6.

105

5.3.1 Polymeric

105

5.3.2 Metallic membranes

110

5.3.3 Zeolite membranes

113

5.3.4 Ceramic oxygen and proton conducting membranes

115

5.4 Conclusion and future trends

117

Nomenclature

118

Acronyms

118

Symbols

118

References

118

Pervaporation membrane reactor

127

SERGIO SANTORO, ALBERTO FIGOLI AND FRANCESCO GALIANO

6.1 Introduction

127

6.2 Pervaporation membrane reactors

130

6.3 Fields of application

134

6.3.1 Esterification reactions

135

6.3.2 Etherification reactions

139

6.3.3 Acetalization reactions

140

6.3.4 Condensation reactions

141

viii

Contents

6.3.5 Bio-alcohol production (pervaporation bioreactors)

7.

141

6.4 Conclusions and future trends

142

Nomenclature

144

Acronyms

144

References

145

Polymeric membrane reactors

151

J. VITAL

7.1 Introduction

151

7.2 Polymeric membranes

152

7.2.1 Structure of polymeric membranes 7.3 Classification of membrane reactors

8.

152 167

7.3.1 Extractor-type membrane reactors

168

7.3.2 Contactor-type membrane reactors

171

7.3.3 Distributor-type membrane reactors

176

7.4 Polymeric membrane microreactors

177

7.5 Conclusions and future trends

179

7.6 Acronyms

179

References

181

Current trends in enzymatic membrane reactor

195

AZIS BOING SITANGGANG, KIWINTA DIAUSSIE, CARMELLA ROSABEL AND SLAMET BUDIJANTO

8.1 Introduction

195

8.2 Designs of enzymatic membrane reactor

196

8.3 Membrane characteristics

197

8.4 Enzyme immobilization in enzymatic membrane reactor

203

8.5 Enzymatic membrane reactor versus other reactor configurations

209

8.6 Applications of enzymatic membrane reactor

210

8.7 Conclusion and outlook

217

Contents

9.

ix

Nomenclature

217

Acronyms

217

References

218

Membrane reactors in bioartificial organs

227

SABRINA MORELLI, SIMONA SALERNO, ANTONELLA PISCIONERI AND LOREDANA DE BARTOLO

9.1 Introduction

227

9.2 Bioartificial organs—design issues

228

9.3 Transport phenomena

229

9.4 Membrane bioreactor as bioartificial liver

231

9.4.1 Membrane bioartificial livers in flat configuration

232

9.4.2 Membrane bioartificial livers in hollow fiber configuration

233

9.5 Membrane bioreactors for bioartificial kidney

236

9.5.1 Membranes for BAK

236

9.5.2 BAK devices in animal studies and clinical trials

237

9.6 Membrane bioreactor as a biomimetic model for nervous tissue analogue

239

9.7 Conclusions and future perspectives

243

Nomenclature

243

References

244

10. Photocatalytic membrane reactors

251

RAFFAELE MOLINARI, CRISTINA LAVORATO AND PIETRO ARGURIO

10.1 Introduction

251

10.2 Basic principles of photocatalysis

253

10.3 Basic of photocatalytic membrane reactors

257

10.3.1 Types of photocatalysts

257

10.3.2 Types of membranes

260

10.3.3 Membrane modules and system configurations

263

x

Contents

10.4 Applications of photocatalytic membrane reactors

268

10.4.1 Photocatalytic membrane reactors in photodegradation of pharmaceuticals in water

268

10.4.2 Photocatalytic membrane reactors in the conversion of CO2 in solar fuels

273

10.5 Advantages and limitations of photocatalytic membrane reactors

274

10.6 Conclusion and future trends

275

List of symbols

277

List of acronyms

277

Acknowledgments

278

References

278

11. Electrochemical membrane reactors

285

PIERRE MILLET

11.1 Introduction

285

11.2 Electrochemical reactors

286

11.2.1 General principles

286

11.2.2 Endergonic transformers

289

11.2.3 Exergonic transformers

290

11.2.4 Cell separators

290

11.3 Diaphragms for liquid electrolytes

292

11.3.1 Asbestos

293

11.3.2 Thermoplastic diaphragms

293

11.4 Polymer membrane materials

295

11.4.1 Proton conducting ionomers

295

11.4.2 Hydroxyl-ion conducting ionomers

298

11.5 Ceramic membrane materials

300

11.5.1 Nonorganic proton conductors

300

11.5.2 Oxide-ion conductors

302

11.6 Selected endergonic applications

304

Contents

xi

11.6.1 Water electrolysis

304

11.6.2 Main water electrolysis technologies

305

11.6.3 Brine electrolysis

305

11.7 Conclusions and future trends

310

Nomenclature

310

References

311

Further reading

313

12. Modeling of membrane reactors

315

FAUSTO GALLUCCI

12.1 Introduction

315

12.2 Packed bed membrane reactors

316

12.2.1 1D pseudo-homogeneous model

316

12.2.2 2D pseudo-homogeneous model

322

12.2.3 Modeling of fluidized bed membrane reactors

325

12.3 Conclusions and future trends

331

Nomenclature

332

References

334

13. Techno-economic analysis of membrane reactors

337

FAUSTO GALLUCCI

Index

13.1 Introduction

337

13.2 Latest developments in techno-economic analysis for membrane reactors

340

13.3 Conclusions and future trends

351

Nomenclature

353

References

353

355

Preface Membrane reactors (MRs) are multiphase, multifunctional systems in which reactions and separation (through a membrane) are integrated within a single vessel. Membranes can be used to either extract one or more components from the reaction system or to feed a reactant into the reaction system. In any case, the integration of membrane separation and reaction is used to enhance the performance of the reactor in terms of conversion, selectivities, and/or energy efficiency. MRs are especially interesting for reaction systems that are limited by thermodynamic equilibrium or where the reaction system is a combination of reactions in series in which the product can be further converted to by-products. In both cases, by adding a membrane, the system can enhance the yield of the products. Most of the MRs in the literature are used for equilibrium-limited reactions. Shortly, if at least one of the products or the reaction is removed from the reaction system, the equilibrium is shifted toward the products, thus higher conversions can be achieved under the same operating conditions, or similar conversions are achieved under milder conditions. Other MRs are used to feed a reactant in the system, such as in partial oxidation reactions or in bioreactors where air is fed through porous membranes. In this book, at first different types of MRs are shown. Indeed, depending on the catalyst system, there are packed bed MRs (PBMRs), fluidized bed MRs (FBMRs), microstructured MRs, or catalytic MRs. Different types of membranes are also used in MRs: from metallic to ceramic, to polymeric ones. For example, most bioreactors use polymeric (hollow fibers) or ceramic membranes. In these cases, some MRs use also particles to remove or decrease the fouling of the membranes (to be underlined that this is very difficult to completely prevent in membrane bioreactors). For gas-phase reactions in MRs, generally metallic or ceramic membrane reactors are used, as these reactions occur at high to very high temperatures. Going a bit into details, the book starts with a general introduction to both membranes and inorganic MRs, given in Chapter 1 (Palma, Martino, Meloni, and Basile). In this work, hydrogen is considered the most promising energy carrier for sustainable energy systems of the future. This is because the production of high purity hydrogen is essential for several applications, including feeding of proton exchange membrane fuel cell stacks, which is the most promising alternative to the internal combustion engines for several transportation applications. However, the authors also stress that high-grade hydrogen is difficult to store and transport suggesting that these issues may be solved through the generation of hydrogen utilizing MRs systems, which have gained great efforts from the scientific community. In fact, in recent years, a consistent stream of studies addressed investigating the combination of hydrogen production and separation. To summarize, in this chapter, brief descriptions of

xvii

xviii

Preface

both the general principles of membrane separation processes and the possible ways to couple a catalyst and a membrane in catalytic MRs are provided. Moreover, a brief overview of membrane bioreactors is also given. Finally, future trends and current challenges about MRs are discussed. The next chapter, Chapter 2 (Zhao, Zou, Huang, and Tong), concentrates on a particular type of MR called protonic electrocatalytic membrane reactors (PEMFCs). Following the authors, the increasing energy and power generation demand mainly depends on the consumption of fossil fuels, which contributes to climate change. In this context, PECMRs offer promising potentials for sustainable energy conversion and storage with low energy consumption and low emission due to their high process and energy efficiencies. Many practical applications have been demonstrated based on PECMRs, such as ammonia synthesis, the reduction of CO2, the dehydrogenation of hydrocarbons, and other environmental applications. In this interesting scenario, the chapter summarizes the most recent development of PECMRs by sorting application types, including the introduction of theoretical principles, the progress of the material development, and presenting challenges and perspectives. In Chapter 3 (Gallucci), a particular kind of MR, named PBMR, is considered the easiest configuration studied in laboratory settings for the proof-of-concept of MRs. The author reports a few examples of these systems as appearing in the specialized literature in the last 5 years (2018 22). The chapter also illustrated a few of the latest examples of this kind of reactor. The same author continues the discussion in the next Chapter 4 (Gallucci), with the FBMRs, considered an extension or an improvement of PBMRs. In fact, the fluidized bed configuration allows more uniform temperature due to the movement of particles even for very exothermic reactions and decreased concentration polarization. Also, in this case, various examples of MRs appearing in literature in the last 5 years (2018 22) are presented. During the last two decades, a new kind of during the last two decades has attracted wide interest and undergone rapid development. For this reason, Chapter 5 (Gapp, Ansaloni, Venvik, Peters, and Pfeifer) illustrates the importance of microstructured MRs for process intensification. The authors focus on microstructured MRs for process intensification employing membranes with a combined gas-selective and/or catalysis function. Firstly, the design and fabrication strategies of such devices are introduced, then followed by various examples of microstructured MRs employing polymeric, and ceramic. And as well metallic membranes are also given by introducing possible applications of microstructured MRs which involve gas liquid and gas gas reactions. The chapter ends by discussing the potential outlook for the technology. Another kind of MR, the so-called pervaporation MRs (the acronym is PVMRs), is discussed in detail in Chapter 6 (Santoro, Figoli, and Galiano). Shortly, also PVMRs represent an integrated separation system where a chemical (or biochemical) reaction is coupled with a membrane-based separation in, in this case, a pervaporation unit. This hybrid process offers a series of advantages in terms of both investment costs reduction and higher operational performance. To be added that PVMRs are today considered a consolidated technology in esterification reactions, but encompass also condensation reactions, acetylation

Preface

xix

reactions, etherification reactions, and biochemical reactions, where a series of products are generated from a microbial conversion. Chapter 7 (Vital) introduces and deeply illustrates the polymeric MRs. Polymeric membranes, applicable in-low temperature processes (below 150 C), due to their high versatility and variety in types and properties, are very suitable and advantageous for applications in the fine chemistry field, when compared with their inorganic counterparts. In this chapter, an overview of polymeric MRs is presented, and membrane structures, such as dense, porous, symmetric, asymmetric, integral, composite, and mixed matrices, until the membrane role in the reactor is given. Techniques for membrane preparation, such as phase inversion or the methods used to prepare metal nanoparticles loaded mixed matrices, are reviewed in detail. MRs’ classification according to the membrane material's nature, the role of the membrane in the catalytic process, the transport function of the membrane, or the reactor configuration is widely revisited. Moreover, recent progress on new membrane types or MRs, such as ionic liquid membranes, polymeric microporous membranes, or polymeric membrane microreactors is referred to. The current trends in enzymatic membrane reactor (EMR) are deeply discussed in Chapter 8 (Sitanggang, Diaussie, Rosabel, and Budijanto). Following the authors, the term EMR, used for both single or monophasic reactors and multiphasic reactors, is generally accepted for any biochemical reaction catalyzed by a certain enzyme(s) or enzymeproducing cells and coupled with membrane separation. EMR has shown the ability to improve the efficiency of enzyme-catalyzed bioconversion, increase product yield, and is easily scaled up for industrial purposes compared to conventional reactors. Especially in food and pharmaceutical applications, EMR is used mostly for the enzyme-catalyzed hydrolytic reactions to improve the product’s nutritional and functional properties, thus increasing their economic values. In particular, this chapter focuses on the designs, membrane characteristics, and applications of EMR in assorted fields to produce a product in single and multiphase systems. The aspects of MRs in bioartificial organs are deeply discussed in Chapter 9 (Morelli, Salerno, Piscioneri, and De Bartolo), where an overview of the application of membrane bioreactor technology to engineer bioartificial organs that can be used as extracorporeal devices providing temporary support for patients with organ failure waiting for transplantation or as implantable systems is provided. These devices can also offer an in vitro platform for drug toxicity testing and studies. In detail, after a brief introduction on the critical issues in the design of a membrane bioreactor to be used as a bioartificial organ, a summary of the transport phenomena within the bioreactor by using computational modeling is reported, since they have to be analyzed to optimize the overall operational conditions. Then, special attention is given to the membrane bioreactor devices used as bioartificial liver, bioartificial pancreas, and biomimetic model of the nervous system. The current status of their development in in vitro and in vivo studies, as well as in clinical trials performed within the last decades is also discussed. Chapter 10 (Molinari, Lavorato, and Argurio) regards the photocatalytic MRs. Heterogeneous photocatalysis is largely studied in the field of environment recovery by the

xx

Preface

total degradation of organic and inorganic pollutants and for the synthesis of chemicals. In this context, the chapter discusses the basic principles of photocatalysis together with both the advantages and disadvantages related to its coupling with a membrane separation in photocatalytic MRs. The types of membranes used and their criteria of selection are briefly examined. On the base of their configuration and membrane operation, photocatalytic MRs are used in reactions of photodegradation of pollutants and reactions of synthesis, evidencing as the appropriate choice of the configuration is a key step given large scale implementation. Some case studies in water treatment (i.e. pharmaceuticals removal) and the reaction of synthesis (CO2 reduction) are discussed, evidencing potentialities, drawbacks, and future trends. In Chapter 11 (Millet), an overview of some electrochemical MRs of great practical interest for the chemical industry and the energy transition is provided. First, the different types of electrochemical reactors are categorized from general thermochemical considerations. In detail: the general features of electrochemical membrane reactors are summarized, followed by the diaphragms used with liquid electrolytes and then an ion-conducting polymer electrolyte used either directly or soaked in electrolytes is described. Moreover, solid oxides are used as electrolytes and cell separators; and a few selected endergonic electrochemical processes of industrial interest (water electrolysis, brine electrolysis, and electrodialysis) are described. Various processes are described too, including limitations of current materials together with some prospective issues. A very important aspect of MRs is their modeling, which is discussed in Chapter 12 (Gallucci), where a few general models that can be used for membrane reactors are reported and various examples of model results from the literature are also given. Although the list of models is not exhaustive, most of the MRs reported in this book can be modeled with one of the models proposed in this chapter. Of course, closure equations for kinetics and membrane flux are presented for the examples reported in this chapter and should be changed and verified by the users for their specific problems. The books end with Chapter 13 (Gallucci) with a techno-economic analysis of MRs. As said, MRs are often used as the advancement of existing technologies because, by integrating reaction and separation in a single vessel, the efficiency of the systems/processes increases. However, the real indicator that shows if the MR is outperforming a conventional system is the final cost of the product, which can be calculated by using a techno-economic analysis. In this chapter, a few examples of techno-economic studies of membrane reactors appearing in literature in the last 5 years are shown. The editors would like to take this opportunity to express their sincere gratitude to all the contributors to this book, whose excellent support resulted in its successful completion. We are grateful to them for the commitment and sincerity they have shown towards their contributions. Without their enthusiasm and support, the compilation of this book would not have been possible. Last but not least, we would also like to thank the publisher, in particular special thanks to the responsible at Elsevier, Ivy Dawn C. Torre, Santos Veronica, Anita Koch, Narmatha Mohan, and Kostas Marinakis, for their great help. Fausto Gallucci Angelo Basile

1 Introduction to membrane and membrane reactors Vincenzo Palma1, Marco Martino1, Eugenio Meloni1, Angelo Basile2,3 1 2

DEPARTME NT OF INDUSTRIAL E NGINEERING, UNIVERSITY O F S AL ERNO, FISCIANO, ITAL Y

HYDROGENIA, G ENOVA, ITALY 3 UNIT OF CHEMICAL-PHY SICS FUNDAMENTALS IN CHEMICAL ENGINEERING, DEPART ME NT OF ENGINEERING, UNI VERSITY CAMP US BIO-ME DICAL OF ROME , ROME, ITAL Y

1.1 Introduction and principles According to the International Union of Pure and Applied Chemistry recommendations, a membrane is a “structure, having lateral dimensions much greater than its thickness, through which mass transfer may occur under a variety of driving forces” [1]. A membrane allows controlling the mass transfer between two adjacent fluid phases by acting as a sieve to separate different species and controlling the relative rates of transport across the membrane [2]. Through the membrane (Fig. 1
1), a fluid stream (retentate stream), is depleted of some of its original components, to form another fluid stream (permeate stream), which is concentrated in these components. The driving force that regulates the transport process across the membrane typically depends on a gradient of concentration, pressure, temperature, electric potential, etc. [3]. The membrane can be used to continuously remove the products and increase the conversion, thus overcoming the equilibrium limitations (Le Chatelier principle), or to increase the selectivity by distributive feeding a reactant [5]. The membranes can be classified according to their nature (biological or synthetic), geometry, and separation regime, as well as organic, inorganic, or organic/inorganic hybrids [5]. Biological membranes can be easily fabricated; however, they have many limitations, including low temperatures of use, tolerance to a limited pH range, and susceptibility to microbial attack [5]. Synthetic membranes can be organic or inorganic in nature; the organic membranes are polymeric materials [6], such as polyamide or polystyrene and in some cases can be used up to 300 C, while the inorganic membranes can be ceramic, such as zeolites or oxides, or metallic, such as palladium or metal alloys, and show high stability in a wide range of temperatures (up to 1000 C, in some cases) and tolerance to a broad pH range [7]. Inorganic membranes can also be classified based on pore diameter (dp) sizes, microporous (dp , 2 nm), mesoporous (2 nm , dp . 50 nm), and macroporous (dp . 50 nm), and Current Trends and Future Developments on (Bio-)Membranes. DOI: https://doi.org/10.1016/B978-0-12-823659-8.00008-3 © 2023 Elsevier Inc. All rights reserved.

1

2

Current Trends and Future Developments on (Bio-)Membranes

FIGURE 1–1 Conceptual scheme of a membrane reactor system [4].

FIGURE 1–2 The choice of membrane with respect to the size of particles [8].

current membrane processes include microfiltration, ultrafiltration, nanofiltration, gas and vapor separation, and pervaporation (Fig. 1
2). The latter mentioned process in the last two decades is finding an increasing application in liquid hydrocarbons separations (petrochemical application, alcohol/ether separations), removal of volatile organic compounds from water, removal of water from glycerin, and dehydration to intensify esterification reaction [8]. Moreover, the metallic membranes can be classified as supported and unsupported [5]. The supported dense membranes [9], obtained by deposition of metallic layers on a porous support, such as alumina, silica, carbon, or zeolite, are particularly interesting in the field of hydrogen production. The mechanisms that regulate the mass transport through porous and dense membranes are very different; in porous membranes, the mechanism depends on the porosity, while in dense metallic membranes a solution-diffusion mechanism is effective [5]. The gas transport mechanisms through a porous membrane include molecular sieving, Knudsen diffusion, capillary condensation, and laminar flow (e.g., Poiseuille flow), depending on the membrane pore size and diameter of gas molecules [10]. Different mechanisms of gas transport through membranes are shown schematically in Fig. 1
3.

Chapter 1 • Introduction to membrane and membrane reactors

3

FIGURE 1–3 Diffusion mechanisms: (A) bulk flow through pores; (B) Knudsen diffusion through pores; (C) molecular sieving; (D) solution diffusion through dense membranes [10].

The permeability (P) is a characteristic property of the membrane, and in the case of dense membranes, it is proportional to solubility (S) and diffusivity (D), according to the Eq. (1.1) [11]. P5S3D

(1.1)

The solubility is related to the affinity between the gas molecules and the membranes materials, the diffusivity to the free volume, and the size of gases. The perm-selectivity (α) depends on the operative conditions, including temperature and pressure, and has been defined as the ratio between the permeability of two gases [12], according to the Eq. (1.2) αði;jÞ 5

ρi ρj

(1.2)

In the case of dense polymer membranes, the perm-selectivity can be also influenced by the plasticization phenomenon [13], due to the physical dissolution of the penetrant gas in the polymer matrix, which induces an increase in the segmental mobility of the polymer chains [14]. The gas permeance (Pe) depends on the gas permeability and the membrane thickness (δ) according to the Eq. (1.3) [11]. Pe 5 P 3 δ

(1.3)

The efficiency of the separation process is defined as the separation factor (SF), which is related to the molar fractions of the components in the permeate (Xi,p, Xj,p) and feed stream (Xi,f, Xj,f), according to the Eq. (1.4) [11] SF 5

xi;p =xj1 p xi1 f =xj;f

(1.4)

As mentioned above in the case of porous membranes, the mechanisms depend on the size of the pores. When the mean pore diameter is larger than the mean free path of the fluid

4

Current Trends and Future Developments on (Bio-)Membranes

molecules (macroporous materials) the Poiseuille flow (viscous flow) is operating (Fig. 1
4), the collision between the molecules is more probable than the collision between the molecules and the pore walls [15]. In the case of mesoporous materials, the molecules tend to collide more with the pore walls than among themselves [16] (Fig. 1
4). When the mean pore diameter and the mean free path of the fluid molecules are similar, the Knudsen mechanism occurs, and the flow through the membrane is calculated according to Eq. (1.5) [17], where G is the geometrical factor related to the membrane porosity and pore tortuosity. Ji 5

GΔPi 1

lð2Mi RT Þ2

FIGURE 1–4 Mass transport mechanism inside porous materials and their perm-selectivity [4].

(1.5)

Chapter 1 • Introduction to membrane and membrane reactors

5

In the case of Knudsen diffusion, unlike the Poiseuille flow, the flow does not depend on absolute pressure. In this case, the highest separation factor obtainable for a binary mixture, when the vacuum is imposed on the permeate side, is equal to the square root of the ratio between the molecular weights of the two different molecules, thus the smaller molecules are transferred more intensively across the membrane [15]. When the molecules are physisorbed or chemisorb on the pore walls, surface diffusion occurs [17], and selective transport takes place, however, the adsorbed molecules reduce the pore size, hindering the further transferring [15]. Capillary condensation takes place when one of the components condenses within the pores, as a result of capillary forces, the condensate fills the pores and then evaporates at the permeate side where low pressure is kept [15]. Multi-layer diffusion is considered an intermediate flow regime between surface diffusion and capillary condensation [5,18]. In the case of microporous materials, the mechanism is comparable to a molecular sieve, only small molecules can permeate, making it possible to achieve very high selectivity [4]. It has been demonstrated that the permeating flow through the microporous materials increases with the temperature [19], according to Eq. (1.6). 

2 Eact J ~ J0 exp RT

 (1.6)

where Eact is the apparent activation energy, ranging from 2 to 40 kJ/mol, depending on micropore size and gas molecule size. Moreover, De Lange et al. [20] described the gas transport and separation in microporous membrane materials, thus the activated transport may be expressed according to the Eq. (1.7) J52

 q  2 E  ρ12ε st i D0 kO exp l ε RT RT

(1.7)

where Do (m2/s) is the mean intrinsic diffusion coefficient for micropore diffusion, and ko is the intrinsic Henry constant, the membrane porosity, l the membrane thickness, ρ the bulk density, qst the isosteric heat adsorption, Ei the activation energy for gas species, R the universal gas constant, and T the temperature [4]. Pervaporation is a combination of permeation and evaporation, which consist of the separation of liquid mixtures (feed stream) by partial vaporization through a dense membrane, therefore it is based on a liquid-vapor phase change [16], and the permeate stream is recovered as vapor. The process consists of several phases, the liquid feed is heated up to the operating temperature, then sent to the active side of the membrane, where the separation occurs, and finally, the permeate vapor is continuously removed from the other side of the membrane. The continuous removal of the permeate vapor generates a concentration gradient across the membrane which acts as the driving force of the process [21]. The mass transfer of a specie across the membrane can be explained through a solution-desorption model [16], which consists of three main steps: sorption of the permeating species at the feed side, transport across the membrane according to the Fick’s law, and desorption at the permeate side under vacuum [21] or a sweep gas [16] (Fig. 1
5A).

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Current Trends and Future Developments on (Bio-)Membranes

FIGURE 1–5 Schematic representation of pervaporation or vapor permeation process: (A) by vacuum; (B) by a carrier gas.  For pervaporation, the feed is liquid; for vapor permeation, the feed is vapor [16].

In addition to mass transfer, the change in the physical state of the permeate component implies also heat transfer; the decreasing of the temperature brings to the decrease of the partial pressure and consequently to the driving force of the mass transfer, so that sometimes, at industrial scale, upstream heat exchange is used to compensate [16]. Unlike gases, the adsorption from the liquid feed is almost independent of the pressure, thus the driving force for the adsorption depends on the fugacity of the species (Eq. 1.8) [22]. f f^ i 5 xl γ i pi

(1.8)

where xi is the feed mole fraction, ɣi is the activity coefficient and pi is the saturated vapor pressure. The fugacity increases with the temperature, moreover, in mixtures, the adsorption selectivity seems to increase with the increase of the feed fugacity ratio [22]. Contrary to pervaporation, in vapor permeation the feed is a mixture of vapor and gases, therefore there happens no phase change and consequently, a non-heat supply is necessary. In this case, the driving force is the chemical potential gradient due to the difference in the partial pressure through the membranes, thus the separation is regulated by a solution diffusion mechanism [16]. Polymeric and zeolite membranes are used in pervaporation and vapor permeation processes; while the most common applications are water removal from organics and viceversa, separation of organic mixtures, and concentration of aqueous solutions. The main advantage of these processes resides in the possibility to easily separate azeotropic mixtures or mixtures for which a high number of theoretical stages is required [21].

1.2 Membranes As previously mentioned, membranes can be classified according to the materials, and the choice of the type of membrane depends on the type of process in which they are to be used.

Chapter 1 • Introduction to membrane and membrane reactors

7

Polymeric membranes are particularly interesting due to their low cost, however, can be used only at low temperatures, up to 150 C [6]. The separation process depends on several parameters; however, the characteristics of the polymer play a crucial role. The polymers used as membranes can be rubber or glassy, the former is usually high permeable for gases but low selective, on the contrary, the latter is selective but lower permeable. The main physicochemical factors influencing the gas permeability and permselectivity of the polymeric membranes are the free volume of the polymer, the chains mobility, and the solubility of the gas in the polymer [6]. The chain rigidity favors the permselectivity but is disadvantageous for the permeability, as in the case of glassy polymers, however, to improve the permeability an increase in the free volume can be beneficial. On the other hand, the high flexibility of the polymer chain in the rubbery polymers allows the high permeability and high selectivity. Glassy polymer membranes provide high mechanical resistance and good reproducibility; however, they suffer from poor temperature resistance, surface corrosion, and swelling effect due to plasticization phenomena. The most used glassy polymers are polysulfone, polyethersulfone, polyetherimide, and polyimide [11]. Rubbery polymers generally show a high affinity for CO2 gas molecules; the most used polymers are poly(ethylene oxide)-based polymers, poly (amide-6-b-ethylene oxide), poly(dimethylsiloxane), and polyvinyl amine [11]. Really attractive are polymers from natural sources, the so-called bio-polymers, which are mostly synthesized by living organisms, and are biodegradable, compostable, and environmentally sustainable [11]. The most interesting examples are: • Cellulose acetate, a glassy polymer commercialized in the 1980s, is used for CO2 separation. • Thermoplastic starch, obtained from plants, including potatoes, corn, etc., is used for packaging applications. • Cross-linked chitosan, obtained by deacetylation of chitin, is used in water treatment processes. • Polylactic acid, is used in packaging applications and tissue engineering. • Polyhydroxyalkanoates, obtained by microbial fermentation, are characterized by hydrophobicity, optical purity, and high processability. • Polyvinyl alchol, is characterized by high hydrophilicity and good barrier properties. • Polyurethane, is obtainable from plant oil, such as castor or soybean oil. PUs possess good physical and tensile strength, chemical resistance, and mechanical properties. Inorganic membranes consist of metals, oxides carbon, or elementary carbon, they are highly selective and permeable and can operate in severe operative conditions [23]. Although more expensive, inorganic membranes present several advantages compared to polymeric ones: a well-defined stable pore structure, high mechanical stability, and thermal and solvent resistance [5]. The main categories of inorganic membranes are: • Metal membranes are categorized as dense or porous; most are characterized by a gradient composite structure of the metal, metal oxide, or metal alloy, the most used metals are Pd, Ag, their alloys, and steel. The unsupported membranes are made with

8

Current Trends and Future Developments on (Bio-)Membranes

pure metal, while the supported membranes are made by coating with a metal or metal oxide on the primary structure which is a porous metal [24]. Metal membranes are used for gas separation, and food, drug, and beverage applications. • Ceramic membranes are based on alumina, silica, or titanium oxide; they are inert and stable at high temperatures and possess high permeability and moderate selectivity. They are particularly suitable for food, biotechnology, and pharmaceutical applications [5], silica and silica, and functionalized ceramic membranes are used for hydrogen production and separation [25]. Among the negative aspects related to their use, there are membrane sealing problems in high-temperature modules, cracking problems related to temperature gradients, and low chemical stability of perovskite-type materials [5]. • Zeolite membranes are based on microporous crystalline alumina silicate, usually obtained by direct growth on a porous ceramic or metal support. The synthesis is usually carried out by hydrothermal treatments in the presence of an organic structure-directing agent, which allows for controlling the shape and size of the pores [26]. The molecular sieving action, the large surface area, and the controllable interaction host-sorbate are considered the main advantages of using zeolite membranes, moreover, the possibility to combine catalytic activity and separation capability is an attractive feature. Zeolite membranes are used as catalysts and sensors for the separation of gas and/or liquids [27]. The main drawbacks of using zeolite membranes are the low gas flux compared to the other inorganic membranes and the low thermal stability of the zeolite layer, which can expand with the temperature [5]. • Carbon membranes are composed of microporous, amorphous high-carbon materials, can be produced by thermal treatment of a wide variety of polymer precursors, and can be supported (flat or tube) and unsupported (flat, capillary, or hollow fiber). The hollow fiber present a high separation performance, high packing density, and low cost, however, the brittleness makes it difficult to handle, so supported carbon membranes are preferable [28]. The supported membranes are fabricated by the carbonization of a polymeric precursor layer on resistant support. Carbon membranes can be used in gas separation for CO2, N2, and H2 removal, however, the selectivity strongly depends on the precursor used for the fabrication. Although the use of carbon membranes is very promising, it still appears to be immature; the problems of fragility and the optimization of preparation methods constitute a limit to their use.

1.3 Membrane bioreactors A membrane bioreactor (MBR) can be defined as a space in which a biochemical transformation and a membrane separation process occur [29]. In MBR, the membrane can be used for different purposes, such as adding a reactant or for selectively removing one of the reaction products [30]. Moreover, membranes can be utilized to retain the biocatalyst or act as the support for the biocatalyst, or the separation of enzymes by size exclusion [31]. MBR processes are characterized by several advantages, including small footprint, lower sludge

Chapter 1 • Introduction to membrane and membrane reactors

9

production, low maintenance, consistency in effluent water quality independent of sludge properties in the bioreactor, and higher removal of organic matters, nutrients, and persistent organic pollutants in comparison to conventional activated sludge processes [29]. However, membrane fouling is one of the most significant disadvantages of an MBR, since it reduces both water quality and quantity and increases the operating costs by 60%, which is mainly consumed by the air scouring process used for membrane-fouling mitigation. In the last decades, MBRs have been used for different applications, including (i) food and biofuel production, (ii) amino acids, antibiotics, proteins, and fine chemicals manufacturing, (iii) removal of pollutants, (iv) wastewater treatment [30]. In any case, the main application of MBRs in the treatment of industrial, municipal, and domestic wastewater. In fact, only in Europe, more than 400 full industrial-scale MBRs have been built, and with a perspective to have more be built. In literature different configurations of MBRs are present, all having the same main objective of maximizing the selective separation of the biocatalyst (microorganisms, or enzymes) from the substrate or products, so resulting in maximum efficiency of the bioreactor. Depending on the configuration, enzymes can be free to move on the retentate side (reaction media) or immobilized on the surface of a membrane, or fixed within the porous membrane support. In this respect, MBRs can be divided into two main categories: (i) a traditional stirred tank reactor coupled to a membrane separation unit, and (ii) biocatalysts such as enzymes, microorganisms, and antibodies are immobilized on membranes. Moreover, MBRs can be subdivided into two reactors generations: the first one named side-stream MBRs (sMBRs), and the second one named immersed MBRs (iMBR)s. In the former, the membrane module is located outside the reactor (Fig. 1
6A), to reduce cake formation on its membrane and is suitable for applications in biotechnology. In this configuration, the sludge from the MBR is pumped into the membrane module, where the permeate forms via a pressure-driven filtration process. The concentrated sludge stream from the membrane module will then return to the bioreactor.

FIGURE 1–6 Basic MBR configurations: (A) side-stream or sMBR and (B) immersed MBR or iMBR [30].

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Current Trends and Future Developments on (Bio-)Membranes

In the latter (Fig. 1
6B), membranes are submerged inside the reaction media, a circulation pump is required, and aeration will create a crossflow. However, trans-membrane pressure still needs to be created. This configuration was developed in the late 1980s in Japan to significantly lower the energy demand of the MBR process for wastewater treatment. However, in this configuration fouling can happen if no preventive measure (e.g., cleaning the membrane surface by aeration) is considered. One more possibility for dividing MBRs is based on the aeration strategy being utilized in the unit. In this respect, two main categories of MBRs can be found: Aerobic Membrane Bioreactors (AMBRs) and Anaerobic Membrane Bioreactors (AnMBRs). Inside an AMBR, aeration has the two main purposes of (i) supplying O2 to microorganisms, and (ii) keeping the surfaces of membranes clean through a scouring process. When an immersed configuration is used, a diffuser is placed at the bottom of the bioreactor, so allowing the right aeration. While coarser bubbles help with the better scouring of the membrane surface, finer bubbles will create smaller resistance for O2 to be transferred from air bubbles to H2O. One important aspect regarding the scouring of the membranes is that this phenomenon is mainly done in the immersed configuration, but it is also present in the side-stream crossflow configuration. When an anaerobic process is used, and depending on the process, the membrane can be cleaned by other gases, including N2, Ar, or recovered CH4 from the reaction zone.

1.4 Combination of membranes and catalytic reactions A membrane reactor, shown in Fig. 1
1, is a device that coupling a membrane separation or distribution process and a chemical reaction in a single unit can allow a high degree of process intensification [32]. In the last years, several experimental and modeling research papers have evidenced the superior performance of MRs for a wide range of applications and various operating conditions [33]. One of the main advantages of MRs is the possibility to perform reactions under milder operating conditions, obtaining higher product selectivities [34]. Among the high literature production in this field, the main part of research papers studies in recent years has investigated different aspects of MRs for various reaction systems, ranging from the production of hydrogen as well as chemicals and pharmaceuticals to wastewater treatment and CO2 abatement [30
39]. Other typical catalytic processes which have been integrated with membranes include photocatalysis, catalytic ozonation, electrochemical oxidation, and Fenton or Fenton-like processes [40,41]. Particularly important in the last years have been gained by photocatalytic membrane reactors (PMR), which through the use of immobilized photocatalysts play an important role in process intensification strategies; this approach offers a simple solution to the typical catalyst recovery problem of photocatalytic processes and, by simultaneous filtration and photocatalysis of the aqueous streams, facilitates clean water production in a single unit [42]. A typical classification of PMRs distinguishes between the catalyst being suspended in solution and the catalyst being immobilized on the membrane (IPMR), as shown in [42] (Fig. 1
7).

Chapter 1 • Introduction to membrane and membrane reactors

11

FIGURE 1–7 Photocatalytic membrane reactor configurations A and B suspended in solution photocatalytic membrane reactors (SPMR), (A): Membrane module inside the reactor; (B): Membrane module outside the reactor; (C
E) immobilized in a membrane photocatalytic membrane reactors (IPMR) (C): a membrane with support function; (D) (dead-end configuration) and (E) (cross-flow configuration) of a membrane with simultaneous support and filtration function [42].

Among the different types of MRs, Catalytic Membrane Reactors (CMRs) are a good choice to develop efficient and safe catalytic reactions [43,44]. CMRs are defined as devices with catalytic perm-selective membranes or made with an ordinary membrane with a catalyst deposited in or on it, and membranes can be of either polymeric or inorganic nature. The use of inorganic membranes is generally preferred in CMRs since harsh conditions, which include high temperature, high pressure, and the existence of corrosive gases or solutions (including both basic and acidic solutions), may occur. In these hard conditions, inorganic membranes, which are typically ceramic membranes (e.g., metal oxides), show higher (i) chemical and thermal stability, (ii) fouling resistance, (iii) mechanical strength, and (iv) lifetime compared with polymeric membranes [45]. A possible scheme of an inorganic membrane reactor (in this case, a fluidized bed membrane reactor for hydrogen production) is shown in Fig. 1
8. The use of catalytic membranes can also allow a high process intensification devoted to a consistent reduction in equipment/energy cost, as well as to enhancing process efficiency. One example is the possibility to obtain in CMRs product separation degrees higher than that permitted by thermodynamic equilibrium limitations in several reactions (e.g., esterification, acetalization, hydrogenation/dehydrogenation, and water-gas shift reaction). In such a way, higher conversions can be obtained. One more example is the enhancement of reactions in which there is a problem of selectivities (as in the case of several consecutive

12

Current Trends and Future Developments on (Bio-)Membranes

FIGURE 1–8 Schematic of an inorganic membrane reactor. Hydrogen is produced on catalyst particles and removed via Pd-based membranes [30].

FIGURE 1–9 Main application possibilities of (inorganic) membrane reactors [30].

reactions), as shown in Fig. 1
9 [46]. As an alternative, the rejection or degradation of pollutants in membrane processes may be improved through the mitigation of membrane fouling obtained by in situ catalytic degradations on the membrane surface [40]. Several alternatives for coupling a catalyst and a membrane are possible, for example, the former may be either incorporated into the membrane matrix or coated on the membrane surface, also depending on the membrane role. The high number of alternatives makes a global analysis very difficult. Anyway, in the literature, some classifications of CMRs have

Chapter 1 • Introduction to membrane and membrane reactors

13

been proposed, depending on how the catalyst and the membrane are combined [47]. One classification considers if the system is composed of a catalytic membrane (the same material acts as catalyst and membrane) or by the coupling of a conventional catalyst (packed or fluidized bed) and a membrane, which just is responsible for the product separation. In this case, six basic types have been identified, as reported in Table 1
1. A schematic representation of some membrane reactors is reported in Fig. 1
10. A different classification is based on the membrane role [48], as reported in Fig. 1
11. As evident, three different functions can be ascribed to the membrane, according to the Catalytic membrane reactor (CMR) type: (i) The CMR may act as an extractor: the membrane removes a reaction product from the reaction zone. With this type of CMR, certainly the most studied, higher reaction yields can be obtained, either by enhancing the conversion in equilibrium-limited reactions or by the improvement of the selectivity towards a primary product in the case of consecutive reactions. In fact, in the latter case, the membrane assures its selective extraction [50]. (ii) The CMR may act as a distributor: the membrane can properly module the introduction of one of the reactants in the reaction zone. This type of CMR is used to spread a reactant all along the catalytic zone, in which the other reactant is introduced as usual. Therefore, if compared to conventional reactors, the concentration of the distributed reactant is kept at a low level in the entire reaction zone, even if the same (or even larger) amounts of it can be introduced. This low concentration may increase the selectivity of reactions when the distributed reactant is further added. Selective oxidations (hydrogenations) can be improved in distributor-type CMR if a membrane distributes oxygen [51]. Another advantage of such a reactor is related to flammable mixtures [44]. Due to the low O2 concentration in the catalyst bed, the composition of the local reactant can be kept outside the flammability region, though the total amount of reactants introduced corresponds to a ratio that is forbidden in conventional reactors. (iii) The CMR may act as a contactor: a membrane may be used to obtain a more effective contact between reactants and catalysts. In this type of CMR, the membrane pore has a peculiar configuration since, differently from the pores of conventional solids, its access is permitted in two distinct ways, which correspond to the two sides of the membrane. In contactors, the membrane generally also acts as catalyst support (or is intrinsically active). Two modes for using a contactor are possible (Fig. 1
11). Table 1–1

Six basic types of membrane reactors.

Acronym

Description

CMR CNMR PBMR PBCMR FBMR FBCMR

Catalytic membrane reactor Catalytic non-permselective membrane reactor Packed-bed membrane reactor Packed-bed catalytic membrane reactor Fluidized-bed membrane reactor Fluidized-bed catalytic membrane reactor

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Current Trends and Future Developments on (Bio-)Membranes

FIGURE 1–10 Schematic representation of membrane reactors [4].

FIGURE 1–11 Representation of the different CMRs [49].

Chapter 1 • Introduction to membrane and membrane reactors

15

1.4.1 Interfacial contactor mode the reactants are independently introduced from each side of the membrane and meet in the catalyst zone, as in the case of non-miscible reactants (for example, gas
liquid catalytic reactions) [51,52]. Different research papers have evidenced that in this mode the gaseous reactant is no more limiting, differently from what is generally observed in conventional reactors [52]. Some research regarding non-miscible aqueous and organic reactants focused on the use of dense polymeric membranes containing metal catalysts encaged in zeolites [50]. In this case, better performance with respect to conventional reactors may be obtained through an enhanced transfer of the organic reactant to the active phase.

1.4.2 Flow-through contactor mode The mixed reactants are forced to pass through the membrane, i.e. through the catalytic pores. One important advantage is the possibility to easily optimize and/or adapt contact time and permeation regime in the active pore, which is difficult to obtain in conventional reactors. These reactors have been used for gas and gas
liquid reactions and have shown higher activity [53
55] and selectivity [56,57]. As previously mentioned, one strategy to obtain a catalytic membrane reactor is to realize a catalytic membrane, which means to deposit a catalytic coating directly on it [40]. The integration of a catalyst inside the membrane matrix is a complex operation, and if it is not performed in the right way the main properties and functionalities of the membrane may be compromised in terms of physical structures, porosity, mechanical strength, or elasticity and chemical selectivity. Therefore, the preparation of catalytic membranes through the surface coating with a catalyst is the preferred route, since in this way not only the direct catalyst exposure to the reactants is obtained, but also the improvement in mass transfer and reaction rates is allowed via forced convection. So far, different surface coating techniques have been developed for innovative and smart catalytic membranes, and two general sections are used for their classifications: physical surface coating and chemical surface coating techniques [40]. Anyway, a deep understanding of these surface coating techniques is still lacking. Moreover, the sophisticated nature of these techniques, as well as some negative impacts on membrane permeability or catalyst activity, does not still permit catalyst coating on membrane filtration materials at the industrial level.

1.5 Conclusions and future trends This chapter highlighted the general features of the separation process through membranes. Moreover, an overview of the main classifications of the catalytic membrane reactors is given, depending on how the catalyst and the membrane are combined. More recently, different strategies to obtain a catalytic membrane reactor by realizing a catalytic membrane have been proposed. The most preferred route is to realize a surface coating on the membrane by means of either physical surface coating or chemical surface coating techniques.

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Current Trends and Future Developments on (Bio-)Membranes

The sophisticated nature of these techniques, as well as some negative impacts on membrane permeability or catalyst activity, does not still permit the realization of optimal catalytic coatings on membrane filtration materials at the industrial level. Further studies and possibly the development of new coating techniques seem necessary to develop efficient and stable systems. In this context, two critical aspects should be addressed, the flow losses through the membrane and the poisoning of the catalysts. In the first case, the realization of mono-bi-atomic catalytic systems, by means of deposition techniques such as atomic layer deposition or chemical/physical vapor deposition, could enormously increase the catalytic activity without affecting the flow. In the second case, the development of new catalytic formulations resistant to poisoning is crucial. Finally, the design and implementation of new catalytic systems cannot disregard the costs, so the new catalytic systems should be economically competitive with those currently used.

Nomenclature Acronyms AMBRs AnMBRs CMR CNMR FBCMR FBMR (iMBR)s IPMR MBR PBCMR PBMR PMR sMBRs SPMR

aerobic membrane bioreactors anaerobic membrane bioreactors catalytic membrane reactor catalytic non-permselective membrane reactor fluidized-bed catalytic membrane reactor fluidized-bed membrane reactor immersed MBRs immobilized photocatalytic membrane reactor membrane bioreactor packed-bed catalytic membrane reactor packed-bed membrane reactor photocatalytic membrane reactors side-stream MBRs solution photocatalytic membrane reactor

Symbols Latin D Do Eact Ei G J ko l P Pe

diffusivity mean intrinsic diffusion coefficient for micropore diffusion apparent activation energy activation energy for gas species geometrical factor flow through the membrane intrinsic Henry constant membrane thickness permeability gas permeance

Chapter 1 • Introduction to membrane and membrane reactors

pi qst R S SF T X

17

saturated vapor pressure the isosteric heat adsorption universal gas constant solubility separation factor temperature molar fraction

Greek α δ ρ ɣi

perm-selectivity membrane thickness bulk density activity coefficient

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2 Protonic electrocatalytic membrane reactors Zeyu Zhao, Minda Zou, Hua Huang, Jianhua Tong DEPARTME NT OF MATERIALS SCIENCE AND ENGINEERING, CLEMSON UNI VERSITY, CLEMSON, SC, UNIT ED STATE S

2.1 Introduction With the rapid growth in energy consumption and worsening climate crises, the demand for sustainable energy conversion and storage (ECS) devices is increasing to reduce the consumption of finite fossil fuel resources [1]. ECS devices based on oxygen ion-conducting ceramics attracted attention due to their high energy conversion efficiencies and precise control of charge carriers during operation [2]. However, the high operating temperature (800
C1000
C) of oxygen ion conducting ceramic devices hinders their development and commercialization because of significant degradation, sealing problems, and high thermal stress [3]. Proton conducting ceramics show great potential to reduce operating temperatures to the intermediate temperature range (350
C600
C) due to their excellent ionic conductivity at elevated temperatures and low activation energy for H1 transport [46]. The low operating temperature provides several benefits, such as reducing plant costs and wider material choices for system components [7]. For ECS devices, proton conducting ceramics primarily serve as electrolytes in electrochemical cells, and electrocatalyst supports in fuel cells [5,79]. electrolysis cells [1013], and membrane reactors [10,14,15]. Several reviews recently gave a good overview of the applications of protonic ceramics on fuel cells and electrolyzers, including fundamental principles, material development, and presenting the challenges and future directions [1619]. As electrolyzers and fuel cells are contract processes, their primary components are almost the same, and they also face similar challenges. The poor performance and challenges related to material synthesis, device fabrications, and the cost forcefully impeded the rapid development at the early stage. The story changed until sintering aids were introduced into the fabrication [5,8], making the material synthesis and device fabrications much easier. Also, appropriate electrolyte and electrode materials are the impediments [2022]. At the state of arts, the single-phase dense electrolyte BaCe12xZrxY0.1Yb0.1O32δ (BCZYYb) electrolyte material by solid-state reactive sintering method [5,23] and triple-conducting cathode materials are most commonly used for excellent performance [5,24,25]. Also, they enforced that protonic ceramic electrolysis cells can directly compress produced hydrogens as pumps/compressors by applying relatively small potentials Current Trends and Future Developments on (Bio-)Membranes. DOI: https://doi.org/10.1016/B978-0-12-823659-8.00011-3 © 2023 Elsevier Inc. All rights reserved.

21

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Current Trends and Future Developments on (Bio-)Membranes

[11,26]. Such an approach offers the opportunity to scale up for commercial hydrogen storage. Notably, the electrolysis cells mentioned here refer in particular to the water electrolysis process, which is strongly differentiated from other reactions and may generate high-value chemicals that could be treated as electrocatalytic membrane reactors (ECMRs). Catalytic ceramic membrane reactors are devices in which a separation process through the membranes is combined with a chemical reaction that takes place on one or both sides of the membranes [27]. Based on the operation condition, catalytic membrane reactors could be classified into two types. One type is the membrane reactors utilizing mixed ionicelectronic conductors (H1/O22 combined with electronic defects), in which charges compensate inside the membrane with only heat and chemical gradians are required. A detailed review of this type of membrane reactor has been done by Deibert et al. [28], which includes reported processes, products, and materials based on different applications. It is not part of the scope of the present review. Another type is ECMRs which require extra electricity input to the system. By applying an appropriate voltage on both sides of the membrane, the direction of charge carriers transferred crossing the membrane could be well controlled. Based on the species of charge carriers, ECMRs could be further classified into oxygen ionic ones (OECMRs) and protonic ones (PECMRs). PECMRs recently attract more and more attention due to their lower operating temperatures than oxygen ionic ones, as mentioned at the beginning, and benefits in terms of increased process and energy efficiencies and compact design. Many valuable applications could be achieved through PECMRs, such as ammonia synthesis, the reduction of CO2, the dehydrogenation of hydrocarbons, and other environmental applications [29]. This technique potentially results in green and sustainable chemistry with less energy consumption, lower pollution, and enhanced performance in terms of separation, selectivity, and yield [30]. It is very meaningful to have a summary of the development of PECMRs. In past decades, many reviews have been done on this technique. However, some of them are combined with OECMRs [3135], and some only covered a subset of applications mentioned above [2,16,17,29,36]. Furthermore, reviews of specific applications based on PECMRs are also made that include the ammonia synthesis [3745], the CO2 reduction [4652], and methane coupling reactions [5356]. Unfortunately, these reviews are not up to date and cover applications reported recently. Thus, it is necessary to get a brand-new summary that only focuses on PECMRs and involves all possible applications. This chapter aims to summarize the development of PECMRs sorting by application types. After the brief introduction of corresponding backgrounds and theoretical principles, numerous progress on the materials for these applications has been comprehensively reviewed in each section. Moreover, this review also concludes by presenting the challenges and future directions.

2.2 Ammonia synthesis Ammonia is an essential chemical, of which the global production is more than 150 million tons per year. The produced ammonia is extensively used in a variety of industrial sectors,

Chapter 2 • Protonic electrocatalytic membrane reactors

23

including fertilizers (about 80% of the produced ammonia) and other industrial processes (the remaining 20%) such as refrigeration, pharmaceuticals, and cleaning products, explosives, and so on. In addition, ammonia is also known as a promising clean energy carrier and hydrogen storage medium due to its zero CO2 emission nature and high gravimetric hydrogen density of 17.6 wt.% compared with 12.5 wt.% in methanol [44,57,58]. In industry, the Haber-Bosch process is almost the only route for ammonia synthesis. In this process, high temperatures (B500
C) and high pressures (150300 bar) are required to activate Ruor Fe-based catalysts for dinitrogen dissociation and achieve an optimized ammonia formation rate. However, the efficiency of the reaction is still as low as 10%15% due to the thermodynamic limitations [59,60]. Furthermore, since hydrogen is primarily derived from fossil fuels in the industry nowadays, ammonia synthesis not only requires a tremendous energy input (485 kJ/mol NH3) but also brings about substantial greenhouse gases emission, accounting for 2% of the total global energy supply and 1.6% of total CO2 emission worldwide, respectively [61]. Various alternative routes for ammonia synthesis have emerged to overcome these drawbacks, such as biochemical, photocatalytic, and electrocatalytic methods. The electrocatalytic route is one of the most promising alternatives, allowing reduced or near zero CO2 emission and flexible scalability from large-scale plants to small-scale distributed facilities. The electrocatalytic routes can be classified into three categories according to electrolyte materials and the related working temperature, including ammonia synthesis at low temperatures (,100
C) in liquid media, at intermediate temperatures (200
C500
C) in molten salt, and high temperatures ( .400
C) in solid-state ceramics. Compared to its counterparts, the hightemperature one can offer the merits of enhanced faradic efficiency for ammonia production, reduced electricity consumption owing to a lower kinetic overpotential, and improved nitrogen reduction reaction (NRR) catalytic activity.

2.2.1 The common design of protonic electrocatalytic membrane reactors for the ammonia synthesis The application of PECMRs for the ammonia synthesis could date back to 1998 when Marnellos and Stoukides first fabricated the PECMR using a proton conductor SrCe0.95Yb0.05O3 as electrolyte and porous Pd as electrodes [62]. As shown in Fig. 21 and Eq. (2.1) and Eq. (2.2), the N2 and H2 respectively fed to cathode and anode sides, where hydrogen molecules were converted to protons transferred via the electrolyte to the cathode side and then reacted with N2 to form NH3. 3H2 ! 6H1 1 6e2 N2

1

6H1 1 6e2

! 2NH3

(2.1) (2.2)

At 570
C and ambient pressure, around 78% of the transported protons were converted to ammonia with an ammonia formation rate of 3 3 1029 mol/s/cm2 in the electrocatalytic PECMRs. The process enabled ammonia synthesis at low pressure without the thermodynamic

24

Current Trends and Future Developments on (Bio-)Membranes

FIGURE 2–1 Schematic illustration of ammonia synthesis in an electrocatalytic PECMR [37]. Permission from S. Gunduz, D.J. Deka, U.S. Ozkan, A review of the current trends in high-temperature electrocatalytic ammonia production using solid electrolytes, J. Catal. (2020). https://doi.org/10.1016/j.jcat.2020.04.025.

limitations, leading to an enhanced ammonia formation rate that is hundreds of times greater than that of the conventional catalytic reactors. Since then, considerable attention was drawn to the ammonia synthesis via PECMRs, and researchers extensively adopted the sandwich structure design. After this, the advances in exploring the mechanism of ammonia electrosynthesis, the electrolyte materials, cathode materials, and anode hydrogen feedstocks in PECMRs will be respectively presented.

2.2.2 Electrocatalytic nitrogen reduction reaction mechanism During the past decade, there have been many theoretical studies focused on the mechanism of electrocatalytic ammonia synthesis at low temperature and pressure which is also applicable to the case of high temperature in some scenarios. Most of the studies are focused on the mechanism for transition metal-based electrocatalysts following a landmark work reported by Skúlason et al. [63] in 2012. In general, the proposed electrocatalytic nitrogen reduction reaction (ENRR) mechanism on a metal surface can be classified into two possible pathways, the dissociative pathway and the associative pathway, of which the schemes are shown as follows [6365] (Fig. 22). In the dissociate mechanism, the adsorbed nitrogen molecule directly dissociates into individual N-adatoms on the catalyst surface, followed by continuous hydrogenation and electron transfer until ammonia is formed and released. In contrast, in the associate mechanism, the two N atoms in adsorbed N2 keep bonding with each other during the subsequent hydrogenation process together with electron transfer, where NH3 formation and desorption occur only after the break of the last N-N bond [44]. Besides the pure electrochemical reactions in the associative mechanism, there is a nonelectrochemical reaction step (N2(g) 1 2 ! 2Nad) included in the dissociate mechanism,

Chapter 2 • Protonic electrocatalytic membrane reactors

25

FIGURE 2–2 Schematic illustration of possible mechanisms for electrocatalytic ammonia formation via (A) dissociative pathway, (B) dissociative pathway, and (C) Mars-van Krevelen (MvK) pathway [37,63,66]. Adapted was regenerated with permission from S. Gunduz, D.J. Deka, U.S. Ozkan, A review of the current trends in hightemperature electrocatalytic ammonia production using solid electrolytes, J. Catal. (2020). https://doi.org/10.1016/j. jcat.2020.04.025; E. Skúlason, T. Bligaard, S. Gudmundsdóttir, F. Studt, J. Rossmeisl, F. Abild-Pedersen, et al., A theoretical evaluation of possible transition metal electro-catalysts for N2 reduction, Phys. Chem. Chem. Phys. 14 (2012) 12351245. https://doi.org/10.1039/c1cp22271f; Y. Abghoui, A.L. Garden, J.G. Howalt, T. Vegge, E. Skúlason, Electroreduction of N2 to ammonia at ambient conditions on mononitrides of Zr, Nb, Cr, and V: A DFT guide for experiments, ACS Catal. 6 (2016) 635646. https://doi.org/10.1021/acscatal.5b01918.

where the high dissociation energy of N2 (g) (945.41 kJ/mol) makes the associative mechanism a preferable one compared with the dissociative mechanism [65]. In the associative mechanism, the first hydrogenation process after the nitrogen adsorption (N2 (ad) 1 H1 1 e2!N2H (ad)) appears to be the rate-limiting step [63,65,67]. However, a preferable

26

Current Trends and Future Developments on (Bio-)Membranes

competing hydrogen evolution reaction (HER) usually occurs in the most active transition metals (Mo, Rh, Ru, and Fe) for ammonia formation via the associative mechanism, leading to low faradic efficiency. On the other hand, the early transition metals (Sc, Ti, Y, and Zr) were predicted with stronger bonding to N-adatoms than H-adatoms, which is beneficial to suppressing the competing HER [42,63]. In addition to transition metals, transition metal nitrates were also analyzed as electrocatalysts for electrochemical nitrogen reduction at ambient conditions by density functional theory (DFT) calculations [66,68]. Unlike transition metals, metal nitrates could catalyze ammonia formation by a Mars-van Krevelen (MvK) mechanism. In the MvK mechanism, a surface lattice N atom is hydrogenated to NH3, and the desorption of the NH3 from the surface occurs subsequently, where an N vacancy is formed and then replenished with dissociated N atom from a gaseous nitrogen molecule. By comparison to the most active transition metals that need high applied potential for ammonia production and suffer from the competing HER [63,64], the most promising candidates of metal nitrates, VN and ZrN, require low onset potentials of 20.51 and 20.76 V (vs Standard hydrogen electrode) for ammonia formation, respectively, which contributes to suppressing the competing HER [68]. Apart from the theoretical work, several experimental studies have taken advantage of operas Raman spectroscopy and FTIR spectroscopy to probe the ENRR mechanism on the noble metal-based cathode at ambient conditions, where the associative mechanism has been confirmed by the detection of intermediate species N2Hx [6971]. However, there is still a lack of experimental investigations on in-situ detecting the intermediates of ENRR in PECMRs at high temperatures to back up the proposed ENRR mechanisms. Therefore, it is important to explore the high-temperature ENRR mechanism on different electrocatalysts by utilizing advanced in-operando characterization techniques.

2.2.3 Electrolyte materials As one of the main components in the PECMRs, the electrolyte should show very low electronic conductivity and high protonic conductivity in working environments. Besides, there are also requirements for high density to prevent gas cross-over between electrodes and good stability under both oxidation and reduction environments. Three types of proton-conducting electrolytes are investigated for high-temperature ammonia electrosynthesis to meet the requirements mentioned above, including perovskite, pyrochlore, and fluorite oxide. The majority of the studied electrolytes are perovskite oxides, such as SrCe0.95Yb0.05O32δ, BaCe12xYxO32δ, BaCe0.2Zr0.7Y0.1O32δ, BaCe0.85Dy0.15O32δ, SrZr0.95Y0.05O32δ, Ba3(Ca1.18Nb1.82) O92δ, La0.9Sr0.1Ga0.8Mg0.2O32δ, etc. Li et al. [72] investigated different doped perovskite oxides, including Ba3Ca0.9Nd0.28Nb1.82O92δ, Ba3CaZr0.5Nb1.5O92δ, and Ba3(Ca1.18Nb1.82)O92δ, as electrolytes for ammonia synthesis using Ag-Pd alloy as standard electrodes. The ammonia formation rates depended on the electrolyte compositions, and the Ba3Ca0.9Nd0.28Nb1.82O92δ showed the highest formation rate of 2.16 3 1029 mol/s/cm2 at 620
C, indicating that the ionic conductivity of electrolytes plays an important role in the ammonia electrosynthesis. Wang et al. [73] synthesized BaCe0.85Dy0.15O32δ by a microemulsion method, where a high

Chapter 2 • Protonic electrocatalytic membrane reactors

27

protonic conductivity (0.93 3 1022 S/cm) was achieved under a wet hydrogen environment at 600
C. When using Ag-Pd alloys as electrodes and wet H2 and dry N2 as feeding stocks, a maximum ammonia formation rate of 3.5 3 1029 mol/s/cm2 was observed at 530
C under an applied current of 1.2 mA. Fluorite oxides are also demonstrated as promising candidates as electrolyte materials for ammonia synthesis. Liu et al. [74] investigated the proton conductivity of fluorite oxide Ce0.8M0.2O22δ (M 5 La, Y, Gd, Sm) at 400
C800
C, and ammonia production in the Ag-Pd | Ce0.8M0.2O22δ | Ag-Pd based PECMRs. Ce0.8M0.2O22δ exhibited mixed protonic and electronic conduction (MPEC) under the wet hydrogen at high temperatures and ammonia formation rates ranging from 7.2 3 1029 to 8.2 3 1029 mol/s/cm2 for different M elements were achieved at 650
C, which were higher than those of many perovskite oxides and pyrochlore oxides. Pyrochlore oxides, such as La1.9Ca0.1Zr2O72δ and La1.95Ca0.05Ce2O72δ, were studied as the electrolyte of PECMRs ammonia production by Xie and Wang et al. [75,76]. At 520
C, a maximum ammonia formation rate of 2.0 3 1029 mol/s/cm2 and a faradic efficiency of 80% was observed for La1.9Ca0.1Zr2O72δ. In contrast, the La1.95Ca0.05Ce2O72δ based reactor had a lower ammonia formation rate of 1.3 3 1029 mol/s/cm2 regardless of the higher conductivity of the electrolyte under wet hydrogen. It could probably be ascribed to the fact that the La1.95Ca0.05Ce2O72δ had some electronic and oxygen ion conductivity under the operating conditions while the La1.9Ca0.1Zr2O72δ showed nearly pure protonic conductivity.

2.2.4 Cathode materials The cathode, also called the working electrode, is supposed to have great protonic and electronic conductivity, high electrocatalytic activity and selectivity towards ammonia formation, and excellent stability under operating environments. Many works have been performed on cathode materials during the past two decades, seeking to achieve a high ammonia formation rate and faradic efficiency. Initially, the PECMRs were fabricated in an electrolyte-support configuration where bimetallic Ag-Pd was extensively used as a cathode for dissociation and hydrogenation of nitrogen because of its excellent proton permeability and easy preparation [42,72,77,78]. Nevertheless, high cost and reduced surface area under working conditions usually occur on precious metal cathodes [79]. Ru- and Fe-based industrial catalysts for ammonia synthesis were also utilized as cathode components in PECMRs. The former, Ru/MgO, was reported by Skodra et al. [80], however, a very low ammonia formation rate (3.0 3 10213 mol/s/cm2) was obtained in their study. As an Ag thin film was deposited between the electrolyte and Ru/MgO to improve both adhesion and cathode electrical conductivity, which brought about high HER on the Ag surface and a few amounts of protons reaching the Ru surface. The later industrial Fe-based catalyst, reported by Ouzounidou et al. [81], also exhibited a low ammonia formation rate (6.5 3 10212 mol/s/cm2) due to a similar reason. Afterward, Vasileiou et al. [8284] reported the use of cermet, NiBaCe0.2Zr0.7Y0.1O32δ (Ni-BCZY27) cathode, which was cost-effective and showed similar performance as those with noble metal as cathode. Ni itself shows poor catalytic activity towards ammonia synthesis

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Current Trends and Future Developments on (Bio-)Membranes

[85]. The enhanced activity stems from the promoting effects of proton-conducting ceramic BCZY27 on the catalytic activity of ammonia formation [86]. Ferrite-containing perovskite materials such as Ba0.5Sr0.5Co0.8Fe0.2O32δ (BSCF) [59] and La0.6Sr0.4Co0.2Fe0.8O32δ (LSCF) [60] were also utilized as cathodes. These materials have proper electronic conductivity and catalytic activity, showing similar performance as precious metals for electrocatalytic ammonia synthesis. However, stability issues usually occur in these materials due to the volatile nature of cobalt under reducing conditions [87,88]. To achieve good durability, the redox stable perovskite oxides, La0.3Sr0.6TiO32δ (LST) and Ba0.5Sr0.5TiO32δ (BST), were modified for the application as the cathode. Ru-doped La0.3Sr0.6TiO32δ (LSTR) [89,90] and Ba0.5Sr0.5TiO32δ (BSTR) [79] perovskite oxide were developed to improve the catalytic activity of LST and BST, which also exhibited suitable electronic conductivity. Moreover, as BSTR also has appropriate proton conductivity, the triple-phase boundary (TPBs) sites are extended to the whole surface of the cathode rather than just the interface between electrolyte and cathode, which leads to an obvious enhancement in the ammonia formation rate compared with that of the reported perovskite cathode [60,79,8991]. However, it suffers from low faradic efficiency of 5.5% due to the high HER rate under the applied voltage of 1.2 V. Most recently, transition metal nitrate VN mixed with Fe was applied as the cathode, which showed similar performance to previous reports [92]. Although DFT calculations have predicted VN with low HER activity but high activity towards nitrogen electroreduction [66,68,93], VN exhibits low electronic conductivity and poor adhesion with BCZY electrolyte, which requires mixing with metal like Fe to settle these issues down. As a result, poor electrochemical performance was obtained since high HER rates come along with additional Fe, and TPB sites are limited to the small interface area between Fe and VN. Therefore, it is of great significance to develop cathode materials with abundant TPBs, appropriate MPEC property, and high activity and selectivity of ENRR to achieve high electrochemical performance.

2.2.5 Anode hydrogen feedstocks Pure H2 was adopted as the anode feedstock in PECMRs for electrocatalytic ammonia synthesis at the early stage of research. In this process, hydrogen is dissociated to protons (H2 ! 2 H1 1 2e2) in an anode (usually are precious metals or cermet), and the protons are pumped by applied voltage through a protonic ceramic electrolyte to the cathode, where protons react with N2 to form ammonia. This process permits operation at ambient pressure and greatly improved ammonia formation rate by avoiding the thermodynamic restrictions existing in conventional Haber-Bosch catalytic reactors [62]. However, the dominant route for hydrogen production in the industry is neither cost-effective nor environmentally friendly. In industry, hydrogen is usually produced from endothermic methane steam reforming (MSR) at 800
C1000
C, which requires an enormous energy supply [94]. Moreover, expensive and complex purification processes are also required for removing steam, CO, Sulfur, and CO2 from the produced hydrogen [41,95]. Therefore, utilizing pure hydrogen as anode feedstock so far is not a good option to diminish the carbon footprint and the capital costs.

Chapter 2 • Protonic electrocatalytic membrane reactors

29

Later on, the utilization of steam as hydrogen feedstock in PECMRs was reported as an alternative method for ammonia electrosynthesis [60,80,89]. In this process, steam is split into O2 and protons in the anode, and only protons transfer to the cathode side, where the anode reaction can be written as 2H2O ! O2 1 4 H1 1 4e2. Therefore, both greenhouse gas emissions and the cost of hydrogen purification are negligible. However, a high electrical cost is still needed for steam electrolysis, and the stability issue of cell components occurs under a steam-containing environment [92]. It is an unwise option until these issues can be solved in the future. Recently, methane is proposed as a promising hydrogen source for ammonia synthesis by Kyriakou et al. [92]. As shown in Fig. 23, the hydrogen production, hydrogen purification,

FIGURE 2–3 Schematic illustration of electrocatalytic ammonia synthesis using methane as anode hydrogen feedstock reported by Kyriakou et al. [92]. Permission from V. Kyriakou, I. Garagounis, A. Vourros, E. Vasileiou, M. Stoukides, An Electrochemical Haber-Bosch Process, Joule. (2019). https://doi.org/10.1016/j.joule.2019.10.006.

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Current Trends and Future Developments on (Bio-)Membranes

and ammonia synthesis process in conventional Haber-Bosch plants were integrated into a small and simple PECMR, where the methane steam-reforming and the water-gas shifting reactions took place simultaneously at the cermet anode Ni- BaZr0.8Ce0.1Y0.1O32δ to produce hydrogen, shown as CH4 1 2H2O ! CO2 1 8 H1 1 8e2. The theoretical electricity energy demand for ammonia synthesis with MSR in the anode of PECMRs (44.7 kJ/mol at 550
C) is far lower than that of steam electrolysis (344.8 kJ/mol at 550
C). Moreover, the employment of methane as hydrogen feedstock could be environmentally friendly by means of significantly decreasing CO2 emissions from efficient electrocatalytic ammonia conversion. For instance, it was demonstrated that the CO2 emission in PECMRs could be reduced by over 60% if the faradic efficiency of ammonia electrosynthesis could be higher than 35%. Besides, alternative methane sources, instead of natural gas, such as biogas and gasification of solid carbonaceous biofuels, can be employed to diminish the overall carbon footprint [92,96,97]. Nevertheless, the direct utilization of methane as a hydrogen source for ammonia electrosynthesis is rarely investigated by now, for which the only report shows poor performance. Therefore, more efforts could be focused on PECMRs using methane as a hydrogen source to lower capital costs and reduce greenhouse gas emissions for ammonia synthesis.

2.3 CO2 reduction Due to an escalating rate of fossil fuel combustion, the emission of CO2 has reached unprecedented levels and continues to increase, causing significant concern about global climate change [98]. Since the strong dependence on fossil resources in the near future, it is paramount to tackle high atmospheric CO2 levels and mitigate climate change through CO2 capture and conversion techniques [99,100]. In recent decades, electrocatalytic CO2 reduction reaction (CO2RR) has shown remarkable progress in converting CO2 into valuable chemicals or fuels [101103]. Compared with other CO2 reduction methods, such as thermal catalytic reduction, plasma-based catalytic reduction, photochemical reduction, photoelectrochemical reduction, and enzymatic CO2 reduction, the electrocatalytic approach demonstrates several benefits. First, the electrocatalytic route could further reduce embedded carbon emissions during the reduction process by utilizing clean energy sources [104]. Second, this technique is easy to combine with other reduction methods for better performance [105]. Furthermore, another prominent advantage is the convenient realization of the integration of electrocatalytic reduction with the existing fuel consumption techniques [106]. This reduction procedure takes place under ambient conditions and could use various hydrogen sources through usual proton conducting membranes [107]. Room temperature CO2 reduction membrane reactors take advantage of good product selectivity by precisely controlling conditions. However, the performance is restricted by the limited solubility and diffusion of CO2 molecules [108]. Oppositely, protonic ceramic ECMRs, which operate in intermediate temperatures (300
C500
C), show the potential to achieve high-performance CO2RR attributing to the low resistance for CO2 molecule diffusion and the combination of thermal and electrocatalytic processes when use high-energy carrier proton sources (for example, CH4) [13,109].

Chapter 2 • Protonic electrocatalytic membrane reactors

31

2.3.1 The common design of Protonic electrocatalytic membrane reactors for the CO2 reduction While the core component in the membrane reactor is the protonic ceramic as the dense membrane (electrolyte) layer, functional cathode (fuel electrode) and anode (oxygen electrode) layers, and catalysts are also required for better performance. For detailed reactions in the reactor, proton resource molecules (commonly is H2O) at the anode are oxidized to oxygen gas and generate protons. Then, protons pass through the membrane to arrive at the cathode, where they react with the adsorbed-CO2 to produce value-added chemicals (such as CO and CH4), as shown in Fig. 24. In a membrane reactor, the reactions on both sides of the membrane happen at the TPBs, where protons, electrons, and reactant molecules confront [47]. Therefore, with a reasonable structure design, better diffusion of CO2 takes place in PECMRs than that in a low-temperature system, especially in aqueous circumstances. Furthermore, at high temperatures, the activation of CO2 becomes easy, which also results in high current density. In the following, progresses on the components of the reduction of CO2 in PECMRs are separately introduced.

2.3.2 Mechanisms of the CO2 electrocatalytic reduction For the possible mechanism of CO2RR based on PECMRs, theoretical modeling work had been established by different researchers [110,111]. Despite the effect of different features they discussed, the common assumption and conclusion for these two models are that CO2 is not involved in the electrochemical reaction with protons transferred from the electrolyte. Instead, CO2 is reduced into CO by the reverse water-gas shifting reaction with produced H2 at the cathode side. Based on this result, Namwong et al. [110] further, conclude that the

FIGURE 2–4 Schematic illustration of CO2 conversion in a PECMR [49]. Permission from N. Shi, Y. Xie, D. Huan, Y. Yang, S. Xue, Z. Qi, et al., Controllable CO 2 conversion in high performance proton conducting solid oxide electrolysis cells and the possible mechanisms, J. Mater. Chem. A. 7 (2019) 48554864. https://doi.org/10.1039/ c8ta12458b.

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Current Trends and Future Developments on (Bio-)Membranes

optimal steam/CO2 ratio to produce syngas is about 3.9. Moreover, high temperature, lowpressure operation conditions, and the cathode-supported reactor structure are all in favor of better performance. Wang et al. [111] focused on the effect of a porous current collector on the performance of a three-dimensional model. Consequently, the electrolysis efficiency of syngas production can be up to 132.7%, while the H2-production is 119.2% at the same current. And the porous current collector can further improve the syngas production efficiency, especially in high current regions. In contrast, an experimental investigation of the CO2 reduction mechanism, on a BaZr0.8Y0.2O32δ (BZY20, 15 μm) electrolyte, the Ni-BZY20 cathode, and a SrEu2Fe1.8Co0.2O72δ (SEFC) anode had been done by utilizing volt-ampere measurement, electrochemical impedance, Raman and Fourier-transform infrared spectroscopies [49]. For detail, they proposed that CO2 reduction consisted of two main processes. One is the electrocatalytic reduction and the thermal reaction (Fig. 25). In the electrocatalytic process, the CO2 absorption capability of BZY20, which formed CO322, strongly affected the production of CO and promoted the formation of C-H radial over Ni (for CH4 production). Simultaneously, the thermal reaction was taking place on the Ni part, which interacted with H2O and CO2 molecules together to generate CO and protons.

2.3.3 Electrolyte materials In a PECMR, the electrolyte should have high protonic conductivity and negligible electronic conductivity. Also, the electrolyte material should be stable under variable redox conditions and CO2/steam-rich atmospheres. Meanwhile, the electrolyte should be easily fabricated into

FIGURE 2–5 Possible schematic diagram of CO2 reduction reactions in PCECs. H1(red) indicates the protons transferred from the anode; H (green) indicates the hydrogen atoms from hydrogen dissociation in nickel [19]. Permission from D. Medvedev, Trends in research and development of protonic ceramic electrolysis cells, Int. J. Hydrogen Energy 44 (2019) 2671126740. https://doi.org/10.1016/j.ijhydene.2019.08.130.

Chapter 2 • Protonic electrocatalytic membrane reactors

33

a dense, thin, strong film to prevent the combination of the reactants at the anode and cathode sides [32] Solid Electrolytes: Applications in Heterogeneous Catalysis and Chemical Cogeneration. Proton conducting ceramics for electrolytes are commonly perovskite-related structures, such as BaCeO32δ, SrCeO32δ, and BaZrO32δ based materials. For detail, doped BaCeO32δ and SrCeO32δ exhibit the highest proton conductivity among all the hightemperature proton conductors. However, in terms of stability, they easily degrade in H2O and CO2-rich atmospheres. In contrast, doped BaZrO32δ has better chemical resistance against the water vapor and CO2, while the poor sintering activity and high grain-boundary resistance greatly hinder the development in practical applications. Thus, only several materials were successfully demonstrated as the electrolyte in PECMRs. Xie et al. [112] firstly realized the reduction of CO2 utilizing a PECMR with the configuration of 3% H2O/H2, Ni-BCZYZ | BCZYZ (60 μm) | Fe-BCZYZ, CO2 (where BCZYZ 5 BaCe0.5Zr0.3Y0.16Zn0.04O32δ, that was reported as a CO2-resisted protonic conductor [113]). As a result, 65% of CO2 was converted into CO (61%), H2 (B8%), and CH4 (B1%) when applying a current of 1.5 A/cm22 at 614
C, which is two times higher than in conventional Fischer-Tropsch synthesis process. Also, some other research demonstrated the BCZYZ and its derivatives electrolyte for cathode and anode material developments, which will be discussed in other sections [114118]. After that, more doped BaCe12xZrxO32δ materials were demonstrated as stable electrolyte candidates for PECMRs. Series of Ba(Ce12xZrx)0.9Nb0.1O32δ (x 5 0, 0.25, 0.5, 0.75, and 1) were investigated for their conductivity and chemical stability against CO2 dependency of compositions. As a result, conductivity decreases with inherent grain boundary conductivity increase for Zr-rich samples. And chemical stability of these samples in the CO2 atmosphere showed opposite behavior to electrical conductivity: the highest stability was achieved for high-Zr-content materials (when x 5 0.75 and 1) [119]. A reactor achieved a significant achievement of long-term stability with a BaCe0.3Zr0.5Dy0.2O32δ (BCZD) electrolyte [120]. At 700
C, the current density degradation was found to be lower than 0.2 and 0.7% during the cyclic change of pCO2, and 10 h of operation in CO2-enriched atmospheres, respectively, which is a tremendous improvement compared with a current density degradation of about 40% was reported by Wu et al. [117]. Furthermore, the confirmation of performance promotion was also done by introducing CO2 into the cathode side under the water electrolysis process, indicating the benefits of CO2 reduction in a PECMR. For detail, the current density increased from 1.170 A/cm2 (pCO2 5 0 atm) to 1.240 A/cm2 (pCO2 5 0.9 atm) at 700
C with 1.6 V applying voltage. The author claimed that the increased current density upon adding CO2 to the cathode side led to increasing the equilibrium water vapor partial pressure resulting in a corresponding proton transport improvement. A similar phenomenon was also confirmed on the stable electrolyte of BaCe0.2Zr0.7Y0.2O32δ (BCZY70) by diluting 10% H2/Ar cathode gas with 10vol.% CO2 to achieve almost two times current density (from B14 to 26 mA/cm2 at 700
C) [121]. Also, BaCe0.7Zr0.1Y0.1Yb0.1O32δ (BCZYYb) shows promising performance as the electrolyte of protonic ceramic fuel cells, demonstrating excellent CO2 reduction in PECMRs with good stability [13,98]. Ryan et al. [122] developed an electrolyte material BaHf0.3Ce0.5Y0.1Yb0.1O32δ (BHCYYb) for a reversible cell device. Relaying on EIS,

34

Current Trends and Future Developments on (Bio-)Membranes

TGA, XRD, and Raman characterizations, BHCYYb displayed higher CO2 resistivity, higher conductivity, and lower activation energies than BCZYYb, which gave a brand-new direction for electrolyte material development. Besides, more effects on the performance of electrolytes were investigated in different material systems. Sarabut et al. [123] studied the dopant effect on BaCeO32δ by adding Sr, Y, and Zr, and BaCe0.6Zr0.4O32δ (BCZ) showed the best conversion performance from CO2 to CO even though BCZ had relatively lower conductivity compared with other samples of Ba0.6Sr0.4Ce0.9Y0.1O32δ and Ba0.6Sr0.4CeO32δ. The composite effect was confirmed by adding (Li, Na)2CO3 (LNCO) into BaCe0.5Zr0.3Y0.2O32δ (BCZY53) to achieve multi-ionic conduction, which benefited the proton conduction and the methanation of CO2 with a lower bias applied (0.5 V) [124]. Moreover, the influence of sintering aids, which are widely used in the sintering of electrolyte layers to decrease the sintering temperatures, on the performance is carefully investigated by Likhittaphon et al. [125]. The comparison of three sintering additives for BaCe0.9Gd0.1O2.95 (BCG), NiO, and Co2O3 provides maximum relative densities, and ZnO exhibits the highest conductivity. For the CO2 reduction, the reactor with 1wt.% NiO indicates the highest CO2 conversion (62%) and the CO yield (32%) at 700
C.

2.3.4 Cathodic materials and catalysts At the cathode side, the CO2RR takes place by utilizing protons transfer from the anode side through the electrolyte layer, with H2, H2O, CO, and hydrocarbons as products. Thus, the cathode materials also face the stability challenge under H2O/CO2-rich and reducing atmospheres. Therefore, a material that demonstrated good stability as the electrolyte in a PECMR could also serve as the cathode composited with metals that act as electronic conductors and catalysts for the CO2RR Ni-BCZYZ cathode mentioned in the previous section [112]. The effect of the composition of cathode materials on the performance of CO2 reduction with the same electrolyte BCZYZ was studied by Ruiz-Trejo [116]. Among cathodes used Fe-BCZYZ, Cu-BCZYZ, and Ni-BCZYZ, the Fe showed the highest electrochemical activity towards CO2RR. Besides metal-electrolyte composites, some materials were developed to be employed as the cathode in PECMRs. Grünbacher et al. [126] compared the property of La0.6Sr0.4FeO32δ (LSF64) and SrTi0.7Fe0.3O32δ (STF73) and figure that LSF64 showed better stability and bigger capability for CO2 reduction. Furthermore, the flowing H2 and CO treatment lead to Fe segregation on the surface of LSF64 which could further improve the capability to reduce CO2. The electrode material (La0.75Sr0.25)0.97Cr0.5Mn0.5O32δ (LSCM) [127] for solid oxide electrolysis cell based on oxygen-ion conduction electrolyte was borrowed into PECMRs. Gan et al. [114] composited LSCM with BCZYZ, the electrolyte material, which works as both cathode and anode for electrocatalytic reduction of CO2. And that, the Ru was loaded into the LSCMBCZYZ cathode to further improve the performance. After the catalyst loading, the electrode polarization resistance decreased, and the electrocatalytic conversion of CO2 significantly improved with the Faradic efficiency for syngas production being enhanced by 60%100%. It indicates that a proper catalyst is meaningful for achieving better performance. Dual-layer

Chapter 2 • Protonic electrocatalytic membrane reactors

35

cathode structure, consisting with catalyst layer (Ce0.6Mn0.3Fe0.1O2-La0.6Sr0.4Fe0.9Mn0.1O3, CMF-LSFM) and cathode functional layer (La0.6Sr0.4Fe0.8Co0.2O3 (LSCF)-GDC), was developed by Shin et al. [118]. With the presence of the catalyst layer, higher current density values and better coking resistance were achieved, exhibiting promising cycling CO2 utilization. Most recently, Li et al. [98] in Ding’s group identified that the catalytic selectivity could be effectively controlled by tuning the hybridization between Ir and surrounding O atoms. Based on this computational result, a complexing agent tailoring method was developed to synthesize strontium doped CeO2 (SDC)/Ir catalysts. Finally, the validation had been done on a reactor with the configuration of Ni-BCZYYb | BCZYYb | PrBaMn2O51δ 1 BaZr0.7Y0.3O32δ (PBMBZY) with SDC/Ir catalyst. As a result, the ionic-state Ir with Ir-O bonding SDC/Ir prefers to catalyze CO2 towards CO, while the metallic featured Ir presented Ir-Ir bonding promotes C-H, which benefits the CH4 production from CO2 with a remarkable achievement of high selectivity ( .95%). Both ionic Ir and metallic Ir catalyzed the promotion of C1 products. However, economic competitiveness and foster commercialization could be further improved by applying this catalyst design and fabrication strategy depending on abundant metals [128].

2.3.5 Anodic materials In a PECMR, protons are generated at the anode side by the oxidation of chemicals, such as H2O, H2, and C2H6. Specifically for H2O, which is the most used one, it is oxidized to oxygen gas and at the anode side along with the production of protons. Therefore, sufficient protonic and electronic are essential for the reaction. Similar to cathode materials, metal-electrolyte composites were employed at the beginning of the research [112,116,121]. To achieve better performance, new anode materials and catalysts were developed. Wu et al. [117] firstly utilized LSCM as the anode material based on BCZYZ electrolyte and NiBCZYZ cathode. At 600
C, the conversion of CO2 into CO showed 100% selectivity and 90% current efficiency. However, the coking issue at the cathode side strongly affected the performance and led the degradation (B40%). In previously mentioned work reported by Danilov [120], Nd1.95Ba0.05NiO41δ (NBN5) was utilized to demonstrate excellent stability, indicating that the good chemical stability of NBN5 itself and the chemical/mechanical compatibility with BCZD electrolyte. Tarutin et al. [129] investigated the NBN system with different substitution amounts of Ba for Nd (0, 10%, 20%, 30%, and 40%). Among this series of specimens, NBN10 and NBN20 displayed good performance with larger electronic conductivity and the minimum amount of impurity phase(s) formed at the interface with the electrolyte BaCe0.5Zr0.3Dy0.2O32δ, indicating the great potential working as anode electrodes. Additionally, Zhang et al. [130] demonstrated the CO2 reduction utilizing protons produced by the dehydrogenation reaction from ethane to ethylene at the anode side. The cell achieved almost 100% Faradaic efficiency at 700
C, with Nb1.33(Ti0.8Mn0.2)0.67O42δ (NTMO) anode and in-situ exsolved Ni-Cu alloy catalyst based on BCZYYb electrolyte and Ni-BCZYYb cathode. Despite the CO2 reduction into CO, the anodic catalyst enhanced the C-H bond activation, which exhibited 75.2% C2H6 conversion with almost 100% C2H4 selectivity. More promising work could be realized based on this unique anode configuration in further research.

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Current Trends and Future Developments on (Bio-)Membranes

2.4 Hydrocarbon dehydrogenation Hydrocarbons are large-volume chemicals, and their dehydrogenation is typically a hightemperature thermal transformation. PECMRs demonstrated a vast potential for hydrocarbon dehydrogenation, which could decrease operation temperature (400
C600
C). It may revolutionize the energy and chemical industry. PECMRs hydrocarbon dehydrogenation research focuses on several parts: (1). The light hydrocarbons (like methane) upgrades one method to turn methane into valuable fuels or industry feedstocks with higher C numbers compared [131]. (2) The methane reforming, which is the most energy-efficient production pathway for hydrogen [26]. Hydrogen separation can be included in the steam reforming process by PECMRs and shifting the thermodynamic equilibrium simultaneously, further resulting in the intensification of processes. (3) The conversion of alkanes could produce alkenes at reduced temperatures compared with industrial ethane steam cracking [109].

2.4.1 Methane upgrading Methane, the main component in natural gas, gas hydrates, and landfill gas, is an abundant energy resource all over the world and thus a promising alternative for petroleum feedstocks. During the past decades, considerable efforts have been contributed to the conversion of methane into valuable chemicals via PECMRs, including electrocatalytic methane dehydroaromatization (MDA), electrocatalytic methane coupling, and electrocatalytic methane reforming.

2.4.1.1 Electrocatalytic methane coupling The electrocatalytic coupling of methane, also called methane dimerization, can be classified into nonoxidative coupling and oxidative coupling. In 1987, the nonoxidative coupling of methane was firstly reported by Mori et al. [56]. Sr-Ce-Yb perovskite and Pt were utilized as proton-conducting electrolytes and electrodes, respectively, to construct PECMRs. Under an applied current, nonoxidative methane coupling reactions (2CH4 ! C2H6 1 2 H1 1 2e2 and C2H6 ! C2H4 1 H2) occur at the anode, and the produced hydrogen was electrochemically transported from the anode to the cathode, as shown in Fig. 26. The merit of using PECMRs was the separation of two products C2 hydrocarbons at the anode and hydrogen at the cathode. Since then, several research groups have studied the nonoxidative methane coupling in PECMRs [132135], aiming to achieve high C2 hydrocarbon yields, where the PECMRs worked as hydrogen pumps. However, low C2 hydrocarbon yields (,2%) were obtained in all these reports. The poor yield can be mainly ascribed to the inevitable side reaction (methane cracking: CH4 ! C 1 2H2) under an oxygen-free environment [56]. Because elevated operating temperatures are usually adopted to realize the higher conversion of nonoxidative methane coupling, unfortunately, which would bring about a more favorable side reaction. The oxidative coupling of methane with CO2 is another route that can accelerate the removal of the produced hydrogen at the anode side and achieve the recycling of CO2. To

Chapter 2 • Protonic electrocatalytic membrane reactors

37

FIGURE 2–6 Schematic illustration of the nonoxidative coupling of methane [55]. Permission from S. Hamakawa, Electrochemical Methane Coupling Using Protonic Conductors, J. Electrochem. Soc. 140 (1993) 459. https://doi.org/ 10.1149/1.2221068.

perform this reaction, Hibino et al. [136] constructed PECMRs with CaZr0.9In0.1O32δ-based protonic ceramic as electrolyte and porous Ag and Pt as electrodes (Fig. 4.2). The applied current greatly enhanced the C2-hydrocarbon formation rate. However, low faradic efficiency (48.4%) for C2 hydrocarbon formation was obtained at a current of B5 mA/cm2 because the coking issue of methane occurred at the anode side. Hamakawa et al. [55] reported the oxidative coupling of methane with air using an Ag | SrCe0.95Yb0.05O32δ | Ag-based EPCMR. The C2 hydrocarbon formation rate was enhanced by B1.6 times (from 0.52 to 1.35 μ/mol/min/cm2) with an applied current at 900
C when feeding methane at the anode and wet air at the cathode. The enhancement contributed to the mixed protonic and p-type electronic conduction of SrCe0.95Yb0.05O32δ under air, which extended the TPB sites for ionization of hydrogen on the electrolyte surface rather than only at the electrode surface, which further accelerated the methane coupling reaction. However, the insufficient electrocatalytic activity of the silver electrode led to a high operating temperature, which brought about the coking issue and lowered faradic efficiency. Therefore, electrode materials with high activity are required to lower the operating temperature and avoid the coking issue (Fig. 27). Later, Kyriakou et al. [137] investigated the partial oxidative coupling of methane using 5Ce-5Na2WO4/SiO2 (CNWS) composite as the catalyst and Au film electronic conductive phase at the anode as well as SrZr0.95Y0.05O32δ disk as electrolyte. As shown in Fig. 28, the electrochemical reactions happening at the anode could be expressed as 3CH4 1 2H2O ! C2H4 1 CO2 1 12 H1 1 12e2. When formed protons were transported from anode to cathode under an applied voltage, methane conversion and hydrogen production rate were increased by 25% and B10 times, respectively, with nearly no influence on the production rate of C2 hydrocarbons at 850
C. Though the CNWS offered high catalytic activity towards oxidative coupling of methane, its low electrical conductivity resulted in a low fraction of

38

Current Trends and Future Developments on (Bio-)Membranes

FIGURE 2–7 Schematic illustration of oxidative coupling of methane with CO2 [136]. Permission from T. Hibino, S. Hamakawa, T. Suzuki, H. Iwahara, Recycling of carbon dioxide using a proton conductor as a solid electrolyte, J. Appl. Electrochem. 24 (1994) 126130. https://doi.org/10.1007/BF00247783.

FIGURE 2–8 Schematic illustration of oxidative coupling of methane with O2 [137]. Permission from V. Kyriakou, C. Athanasiou, I. Garagounis, A. Skodra, M. Stoukides, Production of H2 and C2 hydrocarbons from methane in a proton conducting solid electrolyte cell using a Au-5Ce-5Na 2WO 4/SiO 2 anode, in: Int. J. Hydrogen Energy, Pergamon (2012) 1663616641. https://doi.org/10.1016/j.ijhydene.2012.02.123.

electrochemically separated hydrogen (45%) due to the limited applied currents from the significant anode resistance. To address these issues, they used a conductive perovskite, La0.6Sr0.4Co0.8Fe0.2O32δ (LSCF), as the anode to investigate the same reaction and also achieved the co-generation of electricity and C2 hydrocarbons under fuel cell operating mode [138]. Under the imposed current, the C2 hydrocarbons formation rates are B10 times higher than that of Au-CNWS based PECMRs. Besides, the fraction of electrochemically separated hydrogen reached a high value of 95%. By feeding air rather than inert gas at the cathode, the PECMR could operate under the fuel cell mode. A 20% enhancement of C2 hydrocarbons formation rate compared to open-circuit condition was achieved along with a low maximum power density close to 1.2 mW/cm2 at 850
C. The low power density could be ascribed to the high ohmic resistance of the thick electrolyte pellet with 2 mm thickness. Therefore, decreasing the thickness of electrolytes and enhancing the anode electrical conductivity are suggested for achieving higher power output and C2 hydrocarbons formation rate.

Chapter 2 • Protonic electrocatalytic membrane reactors

39

2.4.1.2 Electrocatalytic methane dehydroaromatization The catalytic MDA (6CH4 ! C6H6 1 9H2) is a promising route that could directly convert methane into value-added chemicals and exhibits the merits of high efficiency and saving energy as well as low CO2 emission compared with traditional syngas routes for methane conversions. However, the MDA reaction usually suffers from two main issues, including the low methane conversion due to the thermodynamic limitation and the catalyst deactivation by the coke formed with the reaction CH4 ! C 1 2H2. In efforts to solve these issues, Morejudo et al. [139] investigated MDA in the PECMRs of Fig. 29 in 2016. A co-ionic conducting BaCe0.2Zr0.7Y0.1O32δ (BCZY27) membrane was

FIGURE 2–9 The schematic illustration of the catalytic co-ionic membrane reactor [139]. Permission from S.H. Morejudo, R. Zanón, S. Escolástico, I. Yuste-Tirados, H. Malerød-Fjeld, P.K. Vestre, et al., Direct conversion of methane to aromatics in a catalytic co-ionic membrane reactor, Science (80) 353 (2016) 563566. https://doi.org/ 10.1126/science.aag0274.

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Current Trends and Future Developments on (Bio-)Membranes

utilized as the electrolyte, capable of simultaneously transporting protons and oxygen ions in the opposite direction under the operating conditions. On the one hand, the low methane conversion issue from thermodynamic limitation was also addressed by the electrochemical extraction of hydrogen from the anode to the cathode. Simultaneously, the coking issue was alleviated by the electrochemical injection of oxygen into the anode. Therefore, both higher aromatic yields and improved catalyst stability were attained in the PECMR than in the traditional catalytic MDA procedure. Besides, the PECMRs system exhibited up to 80% carbon efficiency, similar to that of large Fischer-Tropsch plants in the industry, which could lay a foundation for the commercialization of MDA PECMRs. However, the electrocatalytic MDA still exhibits a limited methane conversion rate (B12%), which could be ascribed to the inefficient transport of the intermediate species (electrons, protons, and oxygen ions) for the electrocatalytic MDA reaction. First, partial conductivities of H1 and O22 in the electrolyte were fixed, which may not be an optimized value for the trade-off between the methane conversion and catalyst stability. Second, the TPB sites were limited at the interface between anode and electrolyte and resulted in high areaspecific resistance of anode because copper has low protonic conductivity and nearly no oxygen ion conductivity. Third, the reaction intermediates were spatially separated from the catalyst’s surface due to the catalyst bed configuration, leading to a constrained electrocatalytic activity towards MDA. Therefore, more efforts could be devoted to investigating co-ionic electrolyte and triple conducting anode materials with suitable partial conductivities and new reactor configurations integrating the catalyst into the anode to realize highperformance electrocatalytic MDA by PECMRs.

2.4.1.3 Electrocatalytic methane reforming Methane is a refractory molecule that complicates its conversion into upgraded products. MSR is the most efficient industrial technology for upgrading methane to valuable products, such as hydrogen. MSR is a highly endothermic process in which Ni-based catalysts are employed to produce syngas, usually around a 1:3 CO/H2 ratio, at elevated temperatures (800
C1000
C) [140]. The dominant reactions are the highly endothermic reforming reaction (Eq. 2.3) and the slightly exothermic water gas shift (WGS) reaction (Eq. 2.4): CH4 1 H2 O ! CO 1 3H2

(2.3)

CO 1 H2 O ! CO2 1 H2

(2.4)

A proton-conducting solid electrolyte reactor has been selected as a membrane for shifting the MSR process equilibrium because hydrogen could be extracted from the reactor simultaneously by the chemical and electric potential. Harald Malerød-Fjeld presents a 30 μm thick BaZr0.82x2yCexYyO32δ proton-protonic membrane reformer with Ni composite electrode, Ni-BCZY27 cermet catalytic activity towards reforming comparable to the state-of-the-art Ni-based commercial catalysts (Haldor-Topsøe R-677H) [26]. This PECMRS produces high-purity hydrogen from steam

Chapter 2 • Protonic electrocatalytic membrane reactors

41

methane reforming in a single-stage process with near-zero energy loss. Fig. 210 realizes four process steps simultaneously. First, it extracts hydrogen from the reforming side and shifts a thermodynamically limited reaction sequence towards the full conversion of methane. Second, it delivers heat to the strongly endothermic reaction through the electrical operation of the membrane, acting as a separator and a compressor. Third, it compresses hydrogen directly at the sweep side of the membrane. And the last, it produces high-purity compressed hydrogen. It achieves complete methane conversion by removing 99% of the formed hydrogen at 800
C and simultaneously compressing hydrogen electrochemically up to 50 bar. Modeling of a small-scale (10 kg H2/day) hydrogen plant reveals an overall energy efficiency of .87%. In the last few years, to reduce the process’s overall cost, researchers pay attention to MSR at low temperatures (LT-MSR) from 400
C650
C [140]. Kyriakou reports a feasibility MSR reactor, which consisted of a BZCY27 proton-conducting membrane, a tubular NiBZCY72 anodic electrode, and a film of metallic Cu cathode [141]. The WGS result was nearly as extensive as a MSR when the cell reactor was worked under open-circuit conditions. 80% methane conversion and 77% hydrogen yield were reached at 650
C when 2.4 V potential was applied on such PECMRs. A 50% improvement was obtained from the initial value under the open circuit. SrZr0.5Ce0.4Y0.1O32δ (SZCY541) was also used for MSR reactor as it is high durability against CO2 and steam. The Ni-SZCY541 1 Ni-loaded paper-structured catalyst anode and Ni-cathode were applied in this reactor. CH4 was effectively converted to H2, CO, and CO2 at the anode, while the H2 evolution rate from the cathode is 21.5 μmole/min/cm2.

FIGURE 2–10 Energy balance and system micro-integration for operation at 800
C for a feed inlet of one mole CH4. Heat for the endothermic reaction is supplied from separation and compression [26]. Permission from H. MalerødFjeld, D. Clark, I. Yuste-Tirados, R. Zanón, D. Catalán-Martinez, D. Beeaff et al., Thermo-electrochemical production of compressed hydrogen from methane with near-zero energy loss, Nat. Energy 2 (2017) 923931. https://doi.org/ 10.1038/s41560-017-0029-4.

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Current Trends and Future Developments on (Bio-)Membranes

2.4.2 Conversion of alkanes to alkenes Conversion of alkanes to the corresponding alkenes is an industrially important process due to the high demand for feedstocks and high purity alkenes to manufacture polymers [142]. PECMRs are more attractive for hydrocarbon dehydrogenation than oxygen membrane reactors, which can be operated at intermediate temperatures with good electrochemical performance. Protons are the only carriers transferred through the proton-conducting electrolyte with no oxygen available for further reactions of the dehydrogenation product. The principles of hydrocarbon dehydrogenation by PECMRs are schematically shown in Fig. 211 (a) Alkanes are the feed-in anode and cathode side of PECMRs, respectively. The alkanes are dehydrogenated to alkenes and H1. The H1 is transferred to the cathode side to form pure H2 by chemical and electrical potential. (b) O2 is fed in the cathode when alkanes are fed in the anode, the transferred H1 reacts with O2 and generates H2O, accelerating the alkenes product rate. (c) CO2 is also fed in the cathode, which reacts with H1 to generate another byproduct CO. In the last 30 years, alkanes dehydrogenation research focuses on propane and ethane conversion to propylene and ethylene due to those being the key chemicals for polymerization and organic synthesis industries [143]. The primary reactions of the decomposition process as the following: C3 H8 ! C3 H6 1 H2

(2.5)

C2 H6 ! C2 H4 1 H2

(2.6)

FIGURE 2–11 Schematic of alkanes conversion to alkenes by PECMRs.

Chapter 2 • Protonic electrocatalytic membrane reactors

43

protonic solid reactors have been used to either study or influence the rate of heterogeneous catalytic reactions’ selectivity. The difference in the electrochemical potential of protons at the two sides of the reactor extracts the pure hydrogen out at the same reaction process, resulting in a moderate increase in propylene and ethylene yield. The anode is the critical material because the propane dehydrogenation is located on this side. In order to improve propylene selectivity, noble metals Pt, Pd, and alloys are always used as anode in the initial research due to the high catalytic effect [14,142,144]. Porous Pt [14] was used as an anode with high temperature-type proton conductive SrCe0.95Yb0.05O32δ (SCY) electrolyte in Iwahara’s work. The anode exhaust gas contained H2, C2H4, CH4, and C2H6 by gas chromatography. Porous Ni is also used as the anode in this work, which demonstrated a similar performance as the Pt anode, especially when the C2H6 selectivity is 76.1%, slightly higher than on the platinum anode (68.4%) at 800
C. John P reported Pdused for propane dehydrogenation with over 97% propylene selectivity [145]. The Pd membrane showed higher hydrogen permselectivity initially. However, this membrane was deactivated and eventually failed after several hours of exposure to reaction conditions. Karagiannakis compared the catalytic activities for propane dehydrogenation of Pt and Pd catalysts [144]. As a result, Pt was superior to Pd, yielding higher rates of propane decomposition. Furthermore, Pt showed a slightly higher selectivity to propylene because shift reactions (COx formation) were more favored on Pd. Carbon deposition is one of the issues for propane decomposition, which causes gradual blocking of the active sites, leading to a dramatic change in catalytic activity. Karagiannakis added H2O to restrict carbon formation and sustained the catalytic activity [146]. An approximately fourfold increase in the overall rate of propane decomposition was observed when pH2O increased from 0.3 to 2.8 kPa. Simultaneously, the rate of H2 formation was enhanced by a factor of 34, while the enhancement in both C3H6 and COx formation rates were less profound. Additionally, a significant decrease (1018 times) in carbon deposition rates was observed. Ceramic anode materials were applied with a high-performance catalyst to lower the cost. Feng et al. [147,148] reported the Cr2O3/Al2O3 as an anode for propane dehydrogenation. The adsorption of an alkane on a coordinative unsaturated the Cr31 center, then the C-H bond of the alkane was broken, with an O-H bond and a Cr-H bond formed. Subsequently, as-formed alkene was released, and H2 was regenerated at the catalytic surface. As a result, C3H6, CH4, C2H4, C2H6, and H2 were detected in the anode chamber. Furthermore, they found that the current flowing in BaCe0.85Y0.15O32δ (BCY15) PECMRs dramatically inhibited the carbon deposition on the anode catalyst due to the presence of a local electrical field [147]. Cr3C2 was also applied for the anode catalyst [149]. It demonstrated good stability with no noticeable degradation detected during the galvanostatic test for 80 h in ethane fuel at 750
C. Fu et al. [150] used nano-Cr2O3 as an anode for ethane fuel cells. 35% conversion and 88% C2H4 selectivity (31% ethylene yield) were obtained at 750
C. Liu et al. [143] used La0.2Sr0.7TiO32δ (LSTA) as an anode with Cr2O3 1 Cu for ethane dehydrogenation. LSTA shows remarkably high conductivity as 27.78 S/cm at 1150
C in a 10% H2/N2 reducing atmosphere. And the ethylene selectivity of 89.7% at 30.9% ethane conversion at 750
C. Cr catalyst deposit on BaZrO3 (BZ) and BaCeO3 (BC) for ethane dehydrogenation and 1.4 3 1027 mol/m2 ethylene formation rate was obtained on Cr/BZ at 575
C [151]. The

44

Current Trends and Future Developments on (Bio-)Membranes

selectivity was maintained above 94% of overall samples in the temperature range 475
C575
C. Li et al. [152] reported Co-Cr2O3 as dehydrogenation anode catalysts for SOFC reactor conversion of ethane to ethylene and power. The Co-Cr2O3 nanocomposite anode catalyst demonstrated a dehydrogenation performance of 32% ethylene yields (91.6% selectivity) at 700
C, comparable to porous Pt anode. In order to reduce the operating temperature to inhibition side reactions, coke formation, and catalyst deactivation, further reducing the operating temperature of PECMRs has aroused great research interest. Ding et al. [109] reports a BZCYYb base electrochemical cell that reaches almost 100% ethylene selectivity at 400
C500
C. PrBaMn2O51δ (PBM)-BaZr0.4Ce0.4Y0.1Yb0.1O32δ (BZ4CYY)/Pt electrodes was also applied for C2H6 hydrogenation to avoid the carbon deposition of Ni-based electrodes [98], where the hybridization between Pt and O was switched to the metallic Pt using a complexing agent tailoring method in order to facilitate the C2H6 electrodehydrogenation. Zhang et al. [130] repots a new anode with NixCu12x-doped Nb1.33(Ti0.8Mn0.2)0.67O42δ (NTMO). The NixCu12x alloy nanoparticles are exsolved on the NTMO backbone to grow an embedded metal-oxide interface architecture. The strong interfacial interaction at the exsolved metaloxide interface would improve the stability and coking resistance at high temperatures. It shows the enhanced C2H6 conversion of 75.2% with B100% C2H4 selectivity at 700
C. High ethylene and propylene selectivity, high alkane conversation, and low process cost are researchers’ goals. Based on the above research result, PECMRs work at relatively low temperatures conducive to improved alkenes selectivity and inhibited carbon deposit on the catalyst. The high active anode catalyst work on low temperatures is one of the essential development directions for the future. Some researchers have demonstrated excellent work in that. The challenge is the durability of those catalysts for long-term alkane dehydrogenation. High proton conductivity material is another essential development direction that needs to be matched and cooperated with the catalyst.

2.5 Other reactions Besides reactions mentioned in previous sections, other conversion processes correspond to the proton conduction through the dense membrane layer from proton resources. The most representative example is the conversion of nitrogen oxides NOx. Similar to the device design for the ammonia synthesis and the CO2 reduction, the NOx feeds at the cathode side and is reduced by transferred protons. Kobayashi et al. [153] conducted the reduction of NO with the steam as the proton source. The configuration of the devices was H2O, Pt | SrZr0.9Yb0.1O32δ (SZYb) | Pt 1 Sr/Al2O3, NO. Consequently, the potential reduction of NO by protons according to these reactions: 2NOðgÞ 1 4H1 1 4e2 ! N2 ðgÞ 1 2H2 OðgÞ 2NOðgÞ 1 2H1 1 2e2 ! N2 OðgÞ 1 H2 OðgÞ NOðgÞ 1 5H1 1 5e2 ! 2NH3 ðgÞ 1 H2 OðgÞ

Chapter 2 • Protonic electrocatalytic membrane reactors

45

The reduction degree of NO could be tuned by changing applied current densities. N2O and N2 were the main products at low current densities, while NO was converted into N2 and NH3 at high current density conditions. Furthermore, NO reduction can even be achieved with the presence of excess O2, which is close to the actual application conditions [154,155]. For the membrane reactor using oxygen-ion conducting material 8 mol% Y2O3-doped ZrO2 (YSZ), NO cannot be reduced in an atmosphere containing O2. In this case, oxygen reduction took place before the reduction of NO and the steam electrolysis, which could provide hydrogen as an effective reducing agent for NO. It indicated that the PECMRs showed a significant advantage in NO reduction [156]. Kalimeri et al. [157] also confirmed the reduction of NO and N2O with the electrolyte of SrCe0.95Yb0.05O32δ and Pd/Ag electrodes. However, the author claimed that when the O2 and C3H8 presented, NO was not reduced through Pt, or Sr/Al2O3 and Ba/Al2O3 catalysts. The absorbed NOx species partially reduced the hydrocarbon, and N2 and CxHyOz oxygen-contained products were generated. In other words, cathodic overpotentials essentially enhanced NO adsorption producing activated ad-species capable of decomposing organic fragments. Except for the conversions at the cathode side by reduction reactions, potential oxidation reactions could also happen at the anode side, such as the dehydrogenation of H2S. As one of the toxic by-products of petroleum, natural gas, and coal gasification, and the key chemical for the Black Sea anoxic issue, H2S is usually converted into lower toxic compounds S or H2SO4 in the industry. Kraia et al. [158] and Ipsakis et al. [159] demonstrated the dehydrogenation of H2S assisted with the H2O electrolysis process with Co/CeO2 catalyst on BaZr0.7Ce0.2Y0.1O32δ and BaZr0.85Y0.15O32δ electrolytes. Whereafter, Ipsakis et al. [160] further analyzed the feasibility of this technique for the application in the treatment of the Black Sea. As was revealed, the decreased concentration of H2S/H2O mixtures results in a higher generation rate of H2 and H2SO4 at the expense of higher heating/electrical inputs. Based on a parametric sensitivity analysis, an H2S concentration of 1 v/v% and a water intake flow corresponding to a hydrogen production of .40 kg/h can promise favorable financial perspectives, which are also controlled by the minimum product price in the practical market.

2.6 Conclusion and future trends In the past decades, PECMRs show promising potential for electrocatalytic hydrogenation and dehydrogenation reactions. There are several advantageous features of PECMRs compared with conventional catalytic reactors. First, PECMRs can integrate several reactions with valuable products cogeneration and separation simultaneously. Second, the electrical energy applied to the reactor greatly promotes the catalytic reactions with controllable reaction directions and minimized critical condition utilization (e.g., high pressure). Additionally, the transfer of H1 greatly avoids the impurities effects, such as poison working electrodes and catalysts, compared with direct utilization of hydrogen in conventional reactions. Despite all the advantages mentioned above, a few hurdles have to be overcome to improve and commercialize PECMRs. On the one hand, sufficient stability is required by the practical application, which is usually under a wide range of oxygen/hydrogen partial

46

Current Trends and Future Developments on (Bio-)Membranes

pressures and especially in a highly reducing atmosphere. Even some research already demonstrates the relatively stable reactors, more candidate stable materials for PECMRs for diversified reactor configurations in terms of compatibility and cost. On the other hand, there is lacking detailed techno-economic evaluation for PECMRs for most processes. As the particularity for each process, valuable conclusions of economic feasibility can only be drawn with the specific parameters for an investigated system. As well, further efforts should also be made for the scale-up of PECMRs, such as the preparation of membrane reactors with high-quality large ultrathin membrane layer to minimize the energy input, the improvement of the catalytic performance of reactors with optimized catalysts loading and structure design, and the assemble technique development of large-scale reactorsand systems.

Nomenclature Acronyms BC BCG BCY15 BCZ BCZD BCZY27 BCZY53 BCZY70 BCZYYb BCZYZ BHCYYb BSCF BST BSTR BZ BZ4CYY BZY20 CMF-LSFM CNWS CO2RR DFT ECMRs ECS ENRR GDC HER LNCO LSCF LSCF LSCM LSF64 LST

BaCeO3 BaCe0.9Gd0.1O2.95 BaCe0.85Y0.15O32δ BaCe0.6Zr0.4O32δ BaCe0.3Zr0.5Dy0.2O32δ BaCe0.2Zr0.7Y0.1O32δ BaCe0.5Zr0.3Y0.2O32δ BaCe0.2Zr0.7Y0.2O32δ BaCe12xZrxY0.1Yb0.1O32δ BaCe0.5Zr0.3Y0.16Zn0.04O32δ BaHf0.3Ce0.5Y0.1Yb0.1O32δ Ba0.5Sr0.5Co0.8Fe0.2O32δ Ba0.5Sr0.5TiO32δ Ba0.5Sr0.5TiO32δ BaZrO3 BaZr0.4Ce0.4Y0.1Yb0.1O32δ BaZr0.8Y0.2O32δ Ce0.6Mn0.3Fe0.1O2-La0.6Sr0.4Fe0.9Mn0.1O3 5Ce-5Na2WO4/SiO2 CO2 reduction reaction Density functional theory Electrocatalytic membrane reactors Energy conversion and storage Electrocatalytic nitrogen reduction reaction Gadolinium doped CeO2 Hydrogen evolution reaction (Li,Na)2CO3 La0.6Sr0.4Co0.2Fe0.8O32δ La0.6Sr0.4Fe0.8Co0.2O3 (La0.75Sr0.25)0.97Cr0.5Mn0.5O32δ La0.6Sr0.4FeO32δ La0.3Sr0.6TiO32δ

Chapter 2 • Protonic electrocatalytic membrane reactors

LSTA LSTR MDA MPEC MSR MvK NBN5 Ni-BCZY27 NRR NTMO NTMO OECMRs PBM PBMBZY PECMRs SCY SDC SEFC STF73 SZCY541 SZYb TPBs TPBs WGS YSZ

47

La0.2Sr0.7TiO32δ La0.3Sr0.6TiO32δ Methane dehydroaromatization Mixed protonic and electronic conduction Methane steam reforming Mars-van Krevelen Nd1.95Ba0.05NiO41δ NiBaCe0.2Zr0.7Y0.1O32δ Nitrogen reduction reaction Nb1.33(Ti0.8Mn0.2)0.67O42δ Nb1.33(Ti0.8Mn0.2)0.67O42δ Oxygen ionic electrocatalytic membrane reactors PrBaMn2O51δ PrBaMn2O51δ 1 BaZr0.7Y0.3O32δ Protonic electrocatalytic membrane reactors SrCe0.95Yb0.05O32δ Strontium doped CeO2 SrEu2Fe1.8Co0.2O72δ SrTi0.7Fe0.3O32δ SrZr0.5Ce0.4Y0.1O32δ SrZr0.9Yb0.1O32δ Triple-phase boundary triple-phase boundaries Water gas shift Y2O3-doped ZrO2

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3 Packed bed membrane reactors Fausto Gallucci SUSTAINABLE PROCESS E NGINE ERING, CHEMICAL ENGINE ERING AND CH EMISTRY, EINDHOVEN UNIVERSITY OF TECHNOLOG Y , EI N D HO V E N, T HE NE T H E RL AN D S

3.1 Introduction Packed bed membrane reactors are a combination of a catalyst bed in a fixed configuration and a membrane through which either product are removed, or reagents are fed to the catalyst bed. A typical example, and a comparison with other reactors, for the methane reforming, has been reported in Gallucci et al. [1] and shown in the Fig. 3.1. A packed bed membrane reactor is probably the easier configuration for a membrane reactor for the design and operation. Its more classical configuration consists of a catalytic particle bed in contact with one side of a membrane as reported in Fig. 3.2. The particles are general on the high-pressure side of the reactor, and in contact with the selective membrane layer, while the permeate side is on the low-pressure side and the side of the membrane support (if any). Although simple in terms of design, this configuration has several drawbacks. Firstly, packed beds have often affected by excessive pressure drops. This problem is more important in membrane reactors, as the permeation through the membrane is driven by pressure, thus an excessive pressure drop in the reaction zone would result in lower driving force and lower permeation. To avoid this, larger particles need to be used which in turn results in lower catalyst loading per unit of membrane area. The larger the particle size is, the smaller the possibility to use the configuration in Fig. 3.2A. Indeed, for tubular membranes, configuration a in Fig. 3.2 would allow only particles of a few hundred microns. Larger particles can only be used in configuration b (i.e. in the shell side of the reactor). While this is not a huge problem for lab-scale reactors, for industrial-scale reactors this configuration may lead to excessive heat transfer limitations and consequent sub-optimal use of the membranes. In the most extreme cases, the membrane may experience a temperature that is either too low for its correct functioning or too high compared to the maximum membrane working temperature. In this regard, the simulation work of Tiemersma et al. [3] demonstrates that for a highly exothermic reaction such as the autothermal reforming of methane, the low heat transfer limitations may lead to a high-temperature peak at the beginning of the membrane reactor (see Fig. 3.3) that will surely be detrimental to the stability of the membrane.

Current Trends and Future Developments on (Bio-)Membranes. DOI: https://doi.org/10.1016/B978-0-12-823659-8.00004-6 © 2023 Elsevier Inc. All rights reserved.

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FIGURE 3.1 Scheme of the packed bed membrane reactor. Reprinted from F. Gallucci, M. Van Sintannaland, J.A.M. Kuipers, Theoretical comparison of packed bed and fluidized bed membrane reactors for methane reforming, Int. J. Hydrogen Energy 35 (2010). https://doi.org/10.1016/j.ijhydene.2010.02.050.

FIGURE 3.2 Membrane reactor catalyst in a tube (A) and catalyst in shell (B) configurations [2]. Reprinted by F. Gallucci, D.A. Pacheco Tanaka, J.A. Medrano, J.L. Viviente Sole, Membrane reactors using metallic membranes (2020). https://doi.org/10.1016/B978-0-12-818332-8.00010-7.

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FIGURE 3.3 Axial temperature profiles in a packed bed and a packed bed membrane reactor for autothermal methane reforming. Reproduced from T.P. Tiemersma, C.S. Patil, M.V Sint Annaland, J.A.M. Kuipers, Modelling of packed bed membrane reactors for autothermal production of ultrapure hydrogen, Chem. Eng. Sci. 61 (2006) 1602
1616. http://www.scopus.com/inward/record.url?eid 5 2-s2.0-30344455632&partnerID 5 40&md5 5 9fb77a0611e8e6a89745a4b8625bbebf.

Another issue of packed bed membrane reactors is the bed to wall mass transfer limitations affecting their performance, especially for very thin and permeable membranes. These mass transfer limitations are often referred to as concentration polarization, a term very often used in membrane processes like desalination and reverse osmosis. While concentration polarization is frequent in membrane processes where liquid phases and solid phases are involved, for a long time it was not considered for gas-phase reactions. However, with the continuous improvements in membranes fluxes, especially with very thin and selective supported membranes, it has become evident that this phenomenon is very detrimental even for membrane reactors dealing with gas-phase reactions. A typical example of this kind of issue has been reported for hydrogen production in membrane reactors. While this system has been long studied at the lab scale, the early use of thick membranes has masked the presence of concentration polarization issues. However, to make these reactors economically feasible, very thin membranes need to be used. These membranes being developed, have demonstrated that at a certain point, the concentration polarization becomes dominant in the performance of the reactor. An indicative figure is the one reported in Fig. 3.4 where it is evident that the higher the flux through the membrane, the higher is the difference between the hydrogen fraction (and thus partial pressure) in the bulk (r/R 5 0) and the hydrogen fraction at the membrane wall (r/R 5 1). The increased mass transfer limitations result in a much larger membrane area required for the same recovery of the product (in this particular case hydrogen). To circumvent these limitations, other reactor concepts have been identified and studied such as the micro-structured reactor concept and the fluidized bed reactor concept, and these will be discussed in more detail in the coming chapters. In this chapter, we will report

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FIGURE 3.4 Relative H2 weight fraction for the isothermal operation mode at different hydrogen permeabilities. Reproduced from F. Gallucci, M. Van Sintannaland, J.A.M. Kuipers, Theoretical comparison of packed bed and fluidized bed membrane reactors for methane reforming, Int. J. Hydrogen Energy 35 (2010). https://doi.org/ 10.1016/j.ijhydene.2010.02.050.

the latest developments in packed bed membrane reactors. We will consider only the works published in the last 5 years. A Scopus search between 2018 and 2022 on packed bed membrane reactor results in 131 documents available (January 2022). In the coming sections, we will report the most interesting results on packed bed membrane reactors related to these works.

3.2 Latest developments in packed bed membrane reactors This section reports the latest developments on packed bed membrane reactors in the last 5 years. Before diving into the most interesting results, a few analyses can be drawn by using the analytical tool Scopus. The 131 papers were found by using the keywords “packed bed” and “membrane reactor” and by limiting the results between 2018 and 2022. As reported in the Fig. 3.5 the highest amount of papers are reported from Eindhoven University (Netherlands), Ulsan National Institute (South Korea), and University of South California (US). This is also directly correlated with the documents reported per authors as reported Fig. 3.6. However, if we analyze the documents by country we realize that most of the scientific output is done in the US followed by Korea and China, while The Netherlands fall to position 4 (see Fig. 3.7).

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FIGURE 3.5 Documents per affiliation. Scopus (accessed January 20th, 2022).

FIGURE 3.6 Documents by authors. Scopus (accessed January 20th, 2022).

Amongst the documents available in Scopus more than 89% are research papers, around 9% are Review papers and only 1.5% are book chapters. Most of the documents deal with Energy as a subject area, many deals with chemical production, and several with the engineering of the membrane reactor including modeling. Note that the modeling and technoeconomic evaluations will be reported in another chapter of this book, thus in the following, we will focus only on the developments of membrane reactors for chemical or energy applications with a focus on the experimental demonstration.

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FIGURE 3.7 Documents by country/region. Scopus (accessed January 20th, 2022).

Table 3.1 reactors.

Main reactions systems recovered in the latest works on membrane

Reaction system

Product of interest

Ammonia decomposition/cracking Methane reforming, oxidative reforming, coupling Propane Syngas CO2 hydrogenation

H2 H2 or C2 1 , aromatics

Membrane type

Pd-based. Ta-based. Pd/Ta Pd membranes, O2 selective membranes, unselective porous membranes Propylene Pd, Zeolite H2 Pd based Methanol, dimethyl ether Zeolite, carbon, polymeric

The following Table 3.1 reports the main reaction systems investigated in the papers/works considered. From the table, it is clear that most of the research on this kind of reactor goes into the increased efficiency for chemical conversion, in particular for hydrogen production (either as a product itself or as an energy carrier). While a lot of research has been spent on this type of reactor for methane conversion (which is still the case as the table shows) it is interesting to note that more effort is spent nowadays on other conversions, and in particular ammonia. The reason for this is that ammonia can be converted into hydrogen without producing CO2 emissions, while ammonia has a very high energy content compared with hydrogen gas (see Fig. 3.8). Ammonia can thus be produced with green hydrogen (or better with hydrogen produced with renewable energy) and used as transportation media. Once at the location, it can be converted again into hydrogen and used either as industrial gas, gas for hydrogen combustion or in fuel cells (FCs). The conversion of the ammonia to hydrogen is an endothermic

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FIGURE 3.8 Energy densities of various energy storage materials and technologies, illustrating the respective volumetric and gravimetric densities [4]. Reproduced from A. Sartbaeva, V.L. Kuznetsov, S.A. Wells, P.P. Edwards, Hydrogen nexus in a sustainable energy future, Energy Environ. Sci. 1 (2008) 79
85. https://doi.org/10.1039/B810104N.

equilibrium reaction that requires temperatures higher than 550 C for achieving high conversions. At temperatures higher than 600 C the conversion is almost complete. For use in gas turbines, this conversion is more than enough, however, for use of hydrogen in FCs, concentrations of ammonia below 0.1 ppm need to be achieved (thus a separation step is required). Integrating the ammonia conversion and hydrogen recovery in the same reactor would be beneficial for achieving high conversions at lower temperatures and high purities of hydrogen. A conceptual design of the membrane reactor for ammonia decomposition was reported by Kim et al. [5]. They reported a very schematic representation of their packed bed membrane reactor as depicted in the top of Fig. 3.9. The work by Kim reports a technical feasibility study by considering ideal membranes with infinite selectivities. Demonstrating that indeed, from a model point of view, lower temperatures, high recoveries, and high efficiencies can be achieved by using membrane reactors. High conversion rates and high purities have been demonstrated by Jo and co-workers [6] who have carried out the ammonia decomposition using a Ru-based catalyst and a composite membrane made of a thin layer (0.4 microns) of palladium covering a 250 micron Ta layer. The authors reported the conversion of .99.5% at temperatures higher than 475 C using low pressure and very high purity in the permeate (circa 800 ppb by Nessler methods). They have also shown that there is no degradation in the performance of a FC for more than 80 h, confirming very low ammonia content in the permeate. For the relatively short period

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FIGURE 3.9 Schematic diagram for ammonia decomposition in a membrane reactor (Mr) for polymer electrolyte membrane fuel cells (PEMFCs). Reproduced by S. Kim, J. Song, H. Lim, Conceptual feasibility studies of a COX-free hydrogen production from ammonia decomposition in a membrane reactor for PEM fuel cells, Korean J. Chem. Eng. 35 (2018) 1509
1516. https://doi.org/10.1007/s11814-018-0037-5.

of operation, the authors also confirmed that there was no interdiffusion of metals between Pd and Ta, a phenomenon that may influence the ability of the membrane to split and separate hydrogen. Zhang et al. [7] have reported a catalytic membrane reactor, by depositing the catalyst directly in the support of the membrane. This can be considered as a specially packed bed membrane reactor that may be used for decreasing the external mass transfer limitations between the catalyst and the membrane surface. As reported in Fig. 3.10 the authors have used a thin Pd membrane layer on top of a porous support and deposited the layer in the mesoporous zone of the support (around 20 microns thick). The results are very promising as high conversions have been obtained at temperatures of around 500 C with ammonia concentrations in the permeate of about 500
1000 ppm (still too high for direct use in FCs). The configuration used is very interesting, especially for the amount of catalyst used and its very good dispersion. One would however argue about the stability of the membrane layer as it is now used in reverse mode, with high pressure on the inside of the support and low pressure on the outside of the membrane layer, this means that the main reason for using support (mechanical stability) is lost in this configuration.

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FIGURE 3.10 Photographs of the catalytic membrane reactor at various stages of fabrication; cross-section showing and high-resolution TEM obtained from the mesoporous region of the catalytic membrane reactor. Reproduced from Z. Zhang, S. Liguori, T.F. Fuerst, J.D. Way, C.A. Wolden, Efficient ammonia decomposition in a catalytic membrane reactor to enable hydrogen storage and utilization, ACS Sustain. Chem. Eng. 7 (2019) 5975
5985. https://doi.org/10.1021/acssuschemeng.8b06065.

FIGURE 3.11 Stability tests data reported by Cechetto et al. [9].

A similar configuration but with completely different materials have been also reported by Cheng and co-workers [8]. In this case, the authors have reported catalytic dual-layered hollow fiber membranes made of a dense mixed protonic-electronic conducting hydrogenselective layer over a porous Ni activated layer of the same material as the dense one. While the mixed conducting layers need high temperature for activation, still the authors report nice enhancements of the catalytic membrane reactor compared to a conventional reactor operated in the same conditions. This work is very interesting because these kinds of membranes are foreseen to be very interesting in combination with electrically driven processes. For the same reaction system, Cechetto et al. [9] have demonstrated the high conversion and high purity of hydrogen using a supported thin Pd-Ag layer membrane (around 4 microns) and a Ru-based catalyst. Especially interesting is the stability test reported for this system as the data reported are very stable for around 600 h of operation (see Fig. 3.11). Recently, Cerrillo et al. [10] have demonstrated the use of a Co-based catalyst in a packed bed membrane reactor operated at high pressures for ammonia decomposition. The results again prove the high production rates at lower temperatures and the high purity of the hydrogen produced through the membrane ( . 99.7% purity).

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Table 3.1 also reports that methane conversion is a very popular reaction studied in packed bed membrane reactors. Most of the reactions of interest are either reforming methane to produce hydrogen or oxidative coupling of methane (OCM) to produce ethylene and other products. While in the first case the membranes are selective to hydrogen and used to extract the product of the reaction, thus increasing the yields and productivities at the lower temperatures, in the case of coupling the membranes are used as the feeding of the reactant (generally pure oxygen) to improve the selectivity towards the product ethylene and reduce the full conversion of products to CO2. While methane to hydrogen is probably the most studied reaction in membrane reactors, the OCM is relatively new in membrane reactors. The reaction system is a kind of dream for the chemical industry and has been studied extensively in the 80 s. The reason for this is that the OCM reaction promises to convert, in a single step, methane to ethylene, without the need to go through syngas production, Fischer
Tropsch synthesis, and cracking of the products. The reaction goes up and down in the research community as the cost of methane and oil fluctuate in time. The difference between the cost of the two raw materials makes the OCM more or less interesting on an industrial scale. In fact, Cruellas et al. [11] have shown that simply following the historical costs of ethylene (from oil) and natural gas, the actual OCM system would be economically feasible around 2035 as shown in the Fig. 3.12.

FIGURE 3.12 Ethylene price forecast based on historical data (red) and ethylene price forecast using OCM (blue) for the coming period. Reproduced from A. Cruellas, J.J. Bakker, M. van Sint Annaland, J.A. Medrano, F. Gallucci, Techno-economic analysis of oxidative coupling of methane: Current state of the art and future perspectives, Energy Convers. Manage. 198 (2019). https://doi.org/10.1016/j.enconman.2019.111789.

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The OCM reaction is a reaction of methane and oxygen at a high temperature. The primary reactions of OCM are the following: 2CH4 1 O2 ! C2 H4 1 2H2 O2

(3.1)

CH4 1 2O2 ! CO2 1 2H2 O

(3.2)

CH4 1 O2 ! CO 1 H2 O 1 H2

(3.3)

Combustion and other consecutive C2 reactions limit the production of higher hydrocarbons (C21), which thus result in low C21 yields. The research in this area resulted in many reactor configurations. A comparison of the different more interesting reactors configurations has been carried out by Cruellas et al. [12,13]. Several authors have been working on membrane reactors for OCM, especially because it has been already reported that a low concentration of oxygen along the reactor is beneficial both for the C2 yields and for the thermal management of the reactor to avoid temperature runaway. Aseem and Harold [14] have reported an enhancement of the ethylene yield in OCM while using porous membranes to distribute the oxygen along the reactor. The authors report that in this kind of reactor, the transmembrane pressure difference is very important to avoid back permeation and ensure the good distribution of the oxygen in the membrane reactor and ultimately reach an increase in C2 yields compared to conventional packed beds. Same conclusions have been experimentally reported by Cruellas et al. [15] who demonstrated that the back-permeation of gases through the porous membranes is very difficult to control experimentally and results in a low yield of the membrane reactor (or at least not high improvements as models would suggest [16]). Thus it seems that dense membranes that can separate oxygen from the air and feed the oxygen to the reactor are more interesting for this reaction system (although could be more expensive). Integration of catalyst and dense membranes (perovskite-like or anyway oxygen selective ceramic membranes) is very important to tune the oxygen flux, and catalyst activity and avoid chemical interactions between the two that may either decrease the catalyst activity or the membrane flux. Several papers are available on this topic. An interested reader is referred to the works of GarciaFayos et al. [17] and the work of Omoze et al. [18]. If we consider hydrogen production in membrane reactors through methane reforming, the effort in the last years is more related to other types of reactors (microreactors and fluidized beds as reported in the next chapters). For packed bed membrane reactors there is still some research based on modeling (not reported here) or on the use f different membranes than Pd-based. For instance, biogas reforming in a membrane reactor using silica membranes has been reported by Akamatsu et al. [19]. The results are in line with previous literature and do not show any significant improvement in terms of stability of the membrane that would make this system more interesting than the ones already reported for Pd-based membranes. Another interesting reactor system is reported for methane to aromatics reaction as reported by Sakbodin et al. [20]. In reality, the system reported in Fig. 3.13 by the authors is

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FIGURE 3.13 Schematic of the reactor separator system for methane to aromatics. (A) CH4 reactor for DNMC reaction; (B) H2-permeable membrane separator for H2 removal from product effluent; and (C) Reactor-separator loop for enhanced aromatics production from the DNMC process. Reproduced from M. Sakbodin, E. Schulman, Y. Pan, E.D. Wachsman, D. Liu, Methane-to-aromatics in a gas recycle methane reactor/hydrogen membrane separator, Catal. Today (2020). https://doi.org/10.1016/j.cattod.2020.06.028.

not strictly a membrane reactor but a reactor separator recycle system. However, the results are very interesting in terms of enhancements achieved and may pave the way for more research towards an integrated membrane reactor system for this reaction. As reported in Table 3.1, another interesting reaction for packed bed membrane reactors is the propane dehydrogenation that produces propylene. This reaction is very endothermic and also results in carbon deposition that deactivates the catalyst. Thus a membrane reactor that would remove the hydrogen during the reaction could result in higher yields, thus possibly resulting in similar conversion at lower temperatures. This temperature decrease would be beneficial for the efficiency f the system and would also prevent (to some extent) carbon deposition. The system has been studied by Dangwal et al. [21] using a zeolite-based membrane reactor selective to hydrogen, with good results in conversion and selectivities compared to a conventional reactor. This reaction system could be also carried out by using a Pd-based membrane reactor, although Brencio et al. [22] have shown that the carbon deposition may also occur on the Pd membrane inhibiting the membrane flux. In any case, as reported by Dittrich [23] the heat transfer problem in this reaction system makes it very difficult for a packed bed membrane reactor to be applied for propane dehydrogenation. It is foreseen that more research will be carried out in the near future, especially for the increasing request for propylene.

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A very interesting topic for packed bed membrane reactors is the use of water selective membranes in the hydrogenation of CO2 to products like methanol or dimethyl ether (DME). Water is a product of all CO2 hydrogenation reactions (which are generally also equilibrium limited), its selective removal from the reaction system results in a shift in the equilibrium conversion and yields (as reported in Figure by Ateka et al. [24]) and in many cases also results in lower deactivation of the catalyst Fig. 3.14. This application, and in particular for CO2 hydrogenation to methanol, has been experimentally demonstrated long ago using a zeolite membrane to separate preferentially water from the reaction zone. A membrane reactor has shown a much higher conversion to methanol compared to a conventional system without membranes. This is at least true at relatively low temperatures ,260 C, for which water can condense in the pores and be separated by capillary condensation [25]. Nowadays, the hydrogenation of CO2 is becoming very popular, and several papers are reporting the use of packed bed membrane reactors for hydrogenation. Rodiguez-Vega et al. [26] have reported an experimental work involving an Zeolite A zeolite membrane for direct hydrogenation to DME. The authors report very interesting results in terms of enhancement of conversion while using a membrane reactor compared to a conventional packed bed reactor (PBR) operated at the same conditions as reported in the Fig. 3.15. These results are very interesting, including the high perm-selectivities reported by the authors in mixed gas experiments. Similar results were also obtained by Seshimo et al. [27] but for methanol production. At lower temperatures than Rodriguez, the authors report similar enhancement in conversion with the results of the membrane reactor going well above the thermodynamic limit of the conventional reactor as shown in Fig. 3.16.

FIGURE 3.14 Graphical representation of the process intensification in membrane reactors for CO2 hydrogenation. Reproduced from A. Ateka, J. Ereña, J. Bilbao, A.T. Aguayo, Strategies for the intensification of CO2 valorization in the one-step dimethyl ether synthesis process, Ind. Eng. Chem. Res. 59 (2020) 713
722. https://doi.org/10.1021/acs. iecr.9b05749.

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FIGURE 3.15 CO2 conversion at different temperatures. Reproduced from P. Rodriguez-Vega, A. Ateka, I. Kumakiri, H. Vicente, J. Ereña, A.T. Aguayo, J. Bilbao, Experimental implementation of a catalytic membrane reactor for the direct synthesis of DME from H2 1 CO/CO2, Chem. Eng. Sci. 234 (2021) 116396. https://doi.org/10.1016/J. CES.2020.116396.

FIGURE 3.16 Temperature dependence of CO2 conversion to compare with membrane reactor (Mr) and packed-bed reactor (PBR) at 1 MPa. Reproduced from M. Seshimo, B. Liu, H.R. Lee, K. Yogo, Y. Yamaguchi, N. Shigaki, et al., Membrane reactor for methanol synthesis using Si-Rich LTA zeolite membrane, Membrane 11 (2021). https://doi. org/10.3390/membranes11070505.

One of the disadvantages of zeolite membranes is the high cost of production and most importantly the low reproducibility for this kind of application. This is why researchers are searching for alternative membranes to be used in this kind of membrane reactor.

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Juarez et al. [28] have investigated the use of a composite membrane (polymer ceramic composite) for the separation of water in conditions similar to the one needed for CO2 hydrogenation. The membrane was produced by coating a thin layer of silicone or Nafion on ceramic porous filters. The authors reported single-layered and double-layered membranes and showed that high perm-selectivities were achieved with double layers at the expense of the flux. The low selectivities achieved are still interesting for further optimization of these mixed membranes to be used at around 220 C. other membranes used for this reaction are carbon-based membranes. Poto et al. [29] have reported that for these membranes, a water/ H2 selectivity of .5 would be enough for a good enhancement in terms of conversion and yields compared to a conventional system. These are the main reactions reported in the last 5 years, however other interesting studies are reporting the use of membrane reactors. For instance, Jokar et al. [30] reported a study (mostly modeling, however) on the use of a membrane reactor to enhance, by removing the hydrogen, the production of formaldehyde. Both the improved efficiencies as well as the recovery of hydrogen as by-products, improve the economic potential of the process. Other papers report the use of membrane reactors for water gas shift, methanol and ethanol reforming for hydrogen production, decomposition of hydrogen iodide, isobutane dehydrogenation, etc.

3.3 Conclusions and future trends Packed bed membrane reactors have been studied for a long time for several applications. In some cases, as for methane reforming, these reactors have been replaced with other types of more efficient reactors. However, future trends still see the use of PBRs, especially for systems where heat transfer is not a major problem such as milder endo-exothermic reactions such as ammonia decomposition or hydrogenation of CO2. It is foreseen that evolutions of PBRs, such for instance using structured reactors will dominate the research and industrial application of these reaction systems soon. Although not exploited enough in the literature, also electrically heated packed bed membrane reactors are being investigated and will probably result in breakthrough applications in the near future.

Nomenclature Acronyms C2 1 Mr OCM PBR

Hydrocarbons including C2 and higher Membrane reactor Oxidative coupling of methane packed bed reactor

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References [1] F. Gallucci, M. Van Sintannaland, J.A.M. Kuipers, Theoretical comparison of packed bed and fluidized bed membrane reactors for methane reforming, Int. J. Hydrogen Energy 35 (2010). Available from: https://doi.org/10.1016/j.ijhydene.2010.02.050. [2] F. Gallucci, D.A. Pacheco Tanaka, J.A. Medrano, J.L. Viviente Sole, Membrane reactors using metallic membranes, 2020. Available from: https://doi.org/10.1016/B978-0-12-818332-8.00010-7. [3] T.P. Tiemersma, C.S. Patil, M.V. Sint Annaland, J.A.M. Kuipers, Modelling of packed bed membrane reactors for autothermal production of ultrapure hydrogen, Chem. Eng. Sci. 61 (2006) 1602
1616. http://www.scopus.com/ inward/record.url?eid 5 2-s2.0-30344455632&partnerID 5 40&md5 5 9fb77a0611e8e6a89745a4b8625bbebf. [4] A. Sartbaeva, V.L. Kuznetsov, S.A. Wells, P.P. Edwards, Hydrogen nexus in a sustainable energy future, Energy Environ. Sci. 1 (2008) 79
85. Available from: https://doi.org/10.1039/B810104N. [5] S. Kim, J. Song, H. Lim, Conceptual feasibility studies of a COX-free hydrogen production from ammonia decomposition in a membrane reactor for PEM fuel cells, Korean J. Chem. Eng. 35 (2018) 1509
1516. Available from: https://doi.org/10.1007/s11814-018-0037-5. [6] Y.S. Jo, J. Cha, C.H. Lee, H. Jeong, C.W. Yoon, S.W. Nam, et al., A viable membrane reactor option for sustainable hydrogen production from ammonia, J. Power Sources. 400 (2018) 518
526. Available from: https://doi.org/10.1016/J.JPOWSOUR.2018.08.010. [7] Z. Zhang, S. Liguori, T.F. Fuerst, J.D. Way, C.A. Wolden, Efficient ammonia decomposition in a catalytic membrane reactor to enable hydrogen storage and utilization, ACS Sustain. Chem. Eng. 7 (2019) 5975
5985. Available from: https://doi.org/10.1021/acssuschemeng.8b06065. [8] H. Cheng, B. Meng, C. Li, X. Wang, X. Meng, J. Sunarso, et al., Single-step synthesized dual-layer hollow fiber membrane reactor for on-site hydrogen production through ammonia decomposition, Int. J. Hydrogen Energy 45 (2020) 7423
7432. Available from: https://doi.org/10.1016/j.ijhydene.2019.04.101. [9] V. Cechetto, L. Di Felice, J.A. Medrano, C. Makhloufi, J. Zuniga, F. Gallucci, H2 production via ammonia decomposition in a catalytic membrane reactor, Fuel Process. Technol. 216 (2021) 106772. Available from: https://doi.org/10.1016/J.FUPROC.2021.106772. [10] J.L. Cerrillo, N. Morlanés, S.R. Kulkarni, N. Realpe, A. Ramírez, S.P. Katikaneni, et al., High purity, selfsustained, pressurized hydrogen production from ammonia in a catalytic membrane reactor, Chem. Eng. J. 431 (2022) 134310. Available from: https://doi.org/10.1016/J.CEJ.2021.134310. [11] A. Cruellas, J.J. Bakker, M. van Sint Annaland, J.A. Medrano, F. Gallucci, Techno-economic analysis of oxidative coupling of methane: current state of the art and future perspectives, Energy Convers. Manage. 198 (2019). Available from: https://doi.org/10.1016/j.enconman.2019.111789. [12] A. Cruellas, T. Melchiori, F. Gallucci, M. van Sint Annaland, Advanced reactor concepts for oxidative coupling of methane, Catal. Rev.—Sci. Eng. 59 (2017). Available from: https://doi.org/10.1080/ 01614940.2017.1348085. [13] A. Cruellas, T. Melchiori, F. Gallucci, M. van Sint Annaland, Oxidative coupling of methane: a comparison of different reactor configurations, Energy Technol. 8 (2020). Available from: https://doi.org/ 10.1002/ente.201900148. [14] A. Aseem, M.P. Harold, C2 yield enhancement during oxidative coupling of methane in a nonpermselective porous membrane reactor, Chem. Eng. Sci. 175 (2018) 199
207. Available from: https://doi.org/ 10.1016/J.CES.2017.09.035. [15] A. Cruellas, W. Ververs, M.S. Annaland, F. Gallucci, Experimental investigation of the oxidative coupling of methane in a porous membrane reactor: relevance of back-permeation, Membranes (Basel). 10 (2020) 1
24. Available from: https://doi.org/10.3390/membranes10070152. [16] A. Cruellas, J. Heezius, V. Spallina, M. van Sint Annaland, J.A. Medrano, F. Gallucci, Oxidative coupling of methane in membrane reactors; a techno-economic assessment, Processes 8 (2020). Available from: https://doi.org/10.3390/pr8030274.

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[17] J. Garcia-Fayos, M.P. Lobera, M. Balaguer, J.M. Serra, Catalyst screening for oxidative coupling of methane integrated in membrane reactors, Front. Mater. 5 (2018). Available from: https://doi.org/10.3389/ fmats.2018.00031. [18] V.O. Igenegbai, R.J. Meyer, S. Linic, In search of membrane-catalyst materials for oxidative coupling of methane: performance and phase stability studies of gadolinium-doped barium cerate and the impact of Zr doping, Appl. Catal. B Environ. 230 (2018) 29
35. Available from: https://doi.org/10.1016/J.APCATB.2018.02.040. [19] K. Akamatsu, M. Suzuki, X. Wang, S. Nakao, Hydrogen production by steam reforming of methane in biogas using membrane reactors with dimethoxydimethylsilane-derived silica membranes prepared by chemical vapor deposition, J. Chem. Eng. Japan 54 (2021) 387
394. Available from: https://doi.org/ 10.1252/jcej.21we016. [20] M. Sakbodin, E. Schulman, Y. Pan, E.D. Wachsman, D. Liu, Methane-to-aromatics in a gas recycle methane reactor/hydrogen membrane separator, Catal. Today (2020). Available from: https://doi.org/ 10.1016/j.cattod.2020.06.028. [21] S. Dangwal, R. Liu, S. Gaikwad, S. Han, S.-J. Kim, Zeolite membrane reactor for high-temperature isobutane dehydrogenation reaction: Experimental and modeling studies, Chem. Eng. Process.—Process Intensif. 142 (2019). Available from: https://doi.org/10.1016/j.cep.2019.107583. [22] C. Brencio, F.W.A. Fontein, J.A. Medrano, L. Di Felice, A. Arratibel, F. Gallucci, Pd-based membranes performance under hydrocarbon exposure for propane dehydrogenation processes: experimental and modeling, Int. J. Hydrogen Energy (2021). Available from: https://doi.org/10.1016/J. IJHYDENE.2021.09.252. [23] C.J. Dittrich, The role of heat transfer on the feasibility of a packed-bed membrane reactor for propane dehydrogenation, Chem. Eng. J. 381 (2020). Available from: https://doi.org/10.1016/j.cej.2019.122492. [24] A. Ateka, J. Ereña, J. Bilbao, A.T. Aguayo, Strategies for the intensification of CO2 valorization in the one-step dimethyl ether synthesis process, Ind. Eng. Chem. Res. 59 (2020) 713
722. Available from: https://doi.org/10.1021/acs.iecr.9b05749. [25] F. Gallucci, L. Paturzo, A. Basile, An experimental study of CO2 hydrogenation into methanol involving a zeolite membrane reactor, Chem. Eng. Process. Process Intensif. 43 (2004). Available from: https://doi. org/10.1016/j.cep.2003.10.005. [26] P. Rodriguez-Vega, A. Ateka, I. Kumakiri, H. Vicente, J. Ereña, A.T. Aguayo, et al., Experimental implementation of a catalytic membrane reactor for the direct synthesis of DME from H2 1 CO/CO2, Chem. Eng. Sci. 234 (2021) 116396. Available from: https://doi.org/10.1016/J.CES.2020.116396. [27] M. Seshimo, B. Liu, H.R. Lee, K. Yogo, Y. Yamaguchi, N. Shigaki, et al., Membrane reactor for methanol synthesis using Si-Rich LTA zeolite membrane, Membranes. 11 (2021). Available from: https://doi.org/ 10.3390/membranes11070505. [28] E. Juarez, J. Lasobras, J. Soler, J. Herguido, M. Menéndez, Polymer
ceramic composite membranes for water removal in membrane reactors, Membranes 11 (2021). Available from: https://doi.org/10.3390/ membranes11070472. [29] S. Poto, F. Gallucci, M. Fernanda Neira d’Angelo, Direct conversion of CO2 to dimethyl ether in a fixed bed membrane reactor: Influence of membrane properties and process conditions, Fuel 302 (2021) 121080. Available from: https://doi.org/10.1016/J.FUEL.2021.121080. [30] S.M. Jokar, M.R. Keshavarz, M. Zhubin, P. Parvasi, A. Basile, A novel tubular membrane reactor for pure hydrogen production in the synthesis of formaldehyde by the silver catalyst process, Int. J. Hydrogen Energy 46 (2021) 21953
21964. Available from: https://doi.org/10.1016/J.IJHYDENE.2021.04.042.

4 Fluidized bed membrane reactors Fausto Gallucci SUSTAINABLE PROCESS E NGINE ERING, CHEMICAL ENGINE ERING AND CH EMISTRY, EINDHOVEN UNIVERSITY OF TECHNOLOG Y , EI N D HO V E N, T HE NE T H E RL AN D S

4.1 Introduction Fluidized bed membrane reactors are reactor concepts in which a bundle of membranes is immersed in a bed of small catalyst particles that move because of the fluid flow. Indeed, when a fluid flow passes through a bed of particles it creates a frictional force that can be measured by the pressure drop over the bed. As indicated in Fig. 4 1, different regimes exist depending on the fluid flow rate for a given volume of particles. At low flow rates (fluid velocities) the bed remains stationary, in a fixed bed configuration. Here the pressure drop increases as a function of the flow rate. At a certain point, the drag force created by the fluid is equal to the gravitational force. Here the bed expands and the situation is called minimum fluidization (and the corresponding velocity is the minimum fluidization velocity). At higher velocities, the gas over the minimum fluidization will pass through the bed in the form of bubbles that, for certain particles called Geldart B particles, increase in size as they travel inside the bed. If the reactor size is relatively small this increase in bubble size results in a slugging regime. At higher flow, the bed behaves like a turbulent regime and when the flow is so high that the velocity exceeds the terminal velocity of the particles, then we have pneumatic transport. Typical fluidized bed (membrane) reactors are operated in either bubbling or turbulent regimes. As written above, fluidization depends on different factors, including the type of particles. An interested reader is referred to a book on fluidization for more details. The immersion of membranes in a fluidized bed has already been reported previously for several applications. Examples are reported in our previous work [1] and shown in Fig. 4 2 where two membrane fluidized bed concepts have been introduced that use different kinds of membranes immersed in a fluidized bed for both recovering hydrogen (Pd membranes) and supply oxygen (perovskite membranes). The use of a fluidized bed of particles improves the shortcomings of packed bed membrane reactors as discussed in the previous chapter. In particular, the continuous movement of particles improves heat and mass transfer rates. Firstly, even for very exothermic or endothermic reactions, virtually uniform temperature profiles have been observed in fluidized beds. This means that, for reactions like autothermal methane reforming or steam reforming, the Current Trends and Future Developments on (Bio-)Membranes. DOI: https://doi.org/10.1016/B978-0-12-823659-8.00006-X © 2023 Elsevier Inc. All rights reserved.

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Fixed bed

Minimum Fluidizaon

Bubbling bed

Slugging bed

Turbulent bed

Pneumac transport

v v v v

Fluid Flow

Fluid Flow

Fluid Flow

Fluid Flow

Fluid Flow

Fluid Flow

FIGURE 4–1 Fluidization regimes as a function of the fluid flow rate.

FIGURE 4–2 Schematic representation of the two fluidized membrane reactor concepts for autothermal methane reforming with integrated CO2 capture. (A) Methane combustion configuration. (B) Hydrogen combustion configuration. Reproduced from F. Gallucci, M. Annaland, J. Kuipers, Autothermal reforming of methane with integrated CO2 capture in a novel fluidized bed membrane reactor. Part 1: Experimental demonstration, Top. Catal. 51 (2008) 133 145. https://doi.org/10.1007/s11244-008-9126-8.

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temperature profile in the reactor is uniform with clear stability of membranes and catalysts. Secondly, the continuous circulation of particles improves also the mass transfer rate with much decreased external mass transfer limitations and thus less concentration polarization. The main advantages of using fluidized bed membrane reactors can be thus summarized as [2] • Negligible pressure drop compared to packed beds. For this reason, one can use very small particles with no mass transfer limitation inside the particles and thus higher catalyst utilization. • Uniform temperature profile, which allows optimal utilization of the membranes and prevents the formation of cold spots (detrimental to the flux) or hot spots (detrimental to stability of the membranes). • Easy installation of membrane area and the possibility to arrange membranes and heating/cooling bundles inside the bed. • Better fluidization because of the presence of membranes, reduced axial gas back-mixing. Reduced average bubble size due to enhanced bubble breakage, resulting in an improved bubble to emulsion mass transfer. On the downside, the use of fluidization results also in the gas back mixing that may decrease the concentration close to the membranes and thus the permeation flux. Additionally, erosion may pose a problem for thinner membranes. These aspects will be discussed in more detail in the next sections.

4.2 Latest developments in fluidized bed membrane reactors This section reports the latest developments on fluidized bed membrane reactors in the last 5 years. Before diving into the most interesting results, a few analyses can be drawn by using the analytical tool Scopus. The 106 documents were found by using the keywords “fluidized bed” and “membrane reactor” and by limiting the results between 2018 and 2022. As reported in the Fig. 4 3 the highest amount of papers are reported from Eindhoven University (Netherlands), Inha National Institute (South Korea), and Harbin Institute (China). This is also directly correlated with the documents reported per authors as reported Fig. 4 4. When we analyze the documents by country, we realize that most of the scientific output is done in China followed by Netherlands and Korea. Additionally, Europe has several studies in this field (see Fig. 4 5). Amongst the documents available in Scopus more than 75% are research papers, around 13% are Review papers and the rest are book chapters and conference documents. Most of the documents deal with Energy as a subject area, many deals with chemical production, and several with the engineering of the membrane reactor including modeling, there is also a good number of papers dealing with biochemistry. Note that the modeling and technoeconomic evaluations will be reported in another chapter of this book, thus in the following, we will focus only on the developments of membrane reactors for chemical or energy applications with a focus on the experimental demonstration.

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FIGURE 4–3 Documents per affiliation. Scopus (accessed January 25th, 2022).

FIGURE 4–4 Documents by authors. Scopus (accessed January 25th, 2022).

The following Table 4 1 reports the main reaction systems investigated in the papers/works considered. Compared to the case of packed bed reactors, the research on fluidized bed reactors is more distributed for different applications including the treatment of wastewaters. From the energy point of view, the applications are both hydrogen production and CO2 capture. The use of fluidized bed reactors is also well suited for reactions affected by catalyst deactivation because of the easy possibility to interconnect two (o more) fluidized beds and carry out the reaction and regeneration in two vessels makes much easier the system integration.

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FIGURE 4–5 Documents by country/region. Scopus (accessed January 25th, 2022).

Table 4–1 reactors.

Main reactions systems recovered in the latest works on membrane

Reaction system

Product of interest

Membrane type

Methane reforming, oxidative reforming, coupling Propane Syngas CO2 hydrogenation Wastewater treatment

H2 or C21, aromatics

Pd membranes, O2 selective membranes, unselective porous membranes Pd, Zeolite, O2 selective membranes Pd based Zeolite, carbon, polymeric Non-selective membranes, ceramic membranes

Propylene, acrylic acid H2 Methanol, dimethyl ether Clean water

If we start looking at hydrogen production most of the research and development is because hydrogen production from methane (or biogas) is a very endothermic reaction. When combined with partial oxidation, it leads to a very high-temperature peak at the beginning of the reactor (if packed bed) as a consequence of the different reaction rates between oxidation and reforming. Additionally, as discussed in the chapter dealing with packed beds, these reactors suffer from excessive mass transfer limitations towards the membranes (concentration polarization) that make the reactor less efficient. A comparison in terms of membrane area required for reforming methane between fluidized beds and packed beds has shown that because of concentration polarization, a packed reactor would require almost double the area compared with a fluidized bed using similar membranes [3]. This is graphically shown also in the Fig. 4 6, where clearly for high methane conversion the packed bed is twice as long as a fluidized bed. A more literature-based comparison between fluidized beds and packed beds membrane reactors has been reported already in our previous review paper [2] and many researchers

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FIGURE 4–6 Comparison between a staged fluidized bed and a packed bed with a 2D model. 5 bar reaction pressure. Reproduced from F. Gallucci, M. Van Sintannaland, J.A.M. Kuipers, Theoretical comparison of packed bed and fluidized bed membrane reactors for methane reforming, Int. J. Hydrogen Energy 35 (2010) 7142 7150. https://doi.org/10.1016/J.IJHYDENE.2010.02.050.

have done an experimental demonstration of fluidized bed membrane reactors for methane reforming or autothermal reforming (see also our previous paper [4]). However, there were a few criticisms on fluidized bed membrane reactors, which all make sense and in particular: (1) long term stability of membranes in fluidization conditions; (2) Stability of the membranes towards erosion; (3) The efficiency of fluidized bed membrane reactors at high recoveries; (4) Lack of fundamental understanding and model closures for fluidized beds with membrane internals. One of the first long term experiments using ultrathin membranes in fluidized bed reactors has been reported by Helmi et al. [5] who have shown a stable operation of the membrane reactor for up to 900 h in a water gas shift reaction system with stable hydrogen production with CO content below the fuel cell requirements (see Fig. 4 7). The stable operation of the fluidized bed membrane reactor has also been reported by de Nooijer in his Ph.D. thesis for biogas auto thermal reforming. Comparing the results of Helmi and de Nooijer, one would realize that in the case of Helmi et al. [5] the hydrogen purity was much more stable than in the case of de Nooijer. It may be concluded that the stability of the membranes may indeed be an issue in fluidized beds, especially when operating at higher temperatures (as used in biogas reforming). For this reason, de Nooijer et al. [6] have tested thin-film membranes on different supports and support configurations (including finger-like supports) for stability under different conditions. These long-term tests lasting thousands of hours have revealed very interesting features. Firstly it is indeed true that the degradation of the perm-selectivities occurs more remarkably at higher temperatures. At 400 C there is no appreciable degradation while at

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T . 500 C the degradation is faster. Additionally as shown in Fig. 4 8 a lot of degradation is due to the sealings, but also the membrane has its own contribution. The low stability of the membranes in fluidized bed reactors and also the observation made by Fernandez et al. [7] that some catalysts used in fluidized beds may chemically

FIGURE 4–7 Long-term performance of the membrane module during 900 h of continuous operation in the bubbling fluidization regime at high-temperature water gas shift conditions. Reproduced from A. Helmi, E. Fernandez, J. Melendez, D.A.P. Tanaka, F. Gallucci, M. Van Sint Annaland, Fluidized bed membrane reactors for ultra pure H2 production—a step forward towards commercialization, Molecules 21 (2016). https://doi.org/10.3390/ molecules21030376.

FIGURE 4–8 Long-term nitrogen permeance of membranes different membranes in different conditions. Reported by N. de Nooijer, A.A. Plazaola, J.M. Rey, E. Fernandez, D.A.P. Tanaka, M.S. Annaland, et al., Long-term stability of thin-film Pd-based supported membranes, Processes. 7 (2019). https://doi.org/10.3390/pr7020106.

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interact with the membrane surface and deactivate its permeation, has driven research towards new kinds of membranes that may alleviate some of these problems. Arratibel et al. [8] introduced and developed an advanced membrane called doubleskinned membrane that is obtained by depositing a protective porous ceramic layer on the metallic thin layer. Those membranes have been produced on both ceramic and metallic supported membranes. In a later work, Arratibel et al. [9] demonstrated that the use of these membranes in fluidized beds results in a much more stable selectivity compared to the counterpart non-protected membranes, as shown in Fig. 4 9, and reports stability tests in a fluidized bed to over 900 h of fluidization. The new membranes developed by Arratibel solve also the other problems of fluidized bed membrane reactors as reported by Helmi, i.e., the chemical interaction between the catalyst particles and the membranes. In fact, by using the same catalyst used by Helmi, Arratibel et al. [10] have shown (see Fig. 4 10) that the protected membrane does not interact with the catalyst as the additional layer represents a barrier between the catalyst and the membrane, while a conventional membrane would reduce the flux in the same fashion as reported before by Helmi et al.

FIGURE 4–9 Hydrogen and ideal perm-selectivity at 4 bar of a pressure difference for both protected (double skinned) and unprotected membrane in presence of Rh-based catalyst under bubbling fluidization conditions. Reproduced from A. Arratibel, J.A. Medrano, J. Melendez, D.A. Pacheco Tanaka, M. van Sint Annaland, F. Gallucci, Attrition-resistant membranes for fluidized-bed membrane reactors: double-skin membranes, J. Memb. Sci. 563 (2018) 419 426. https://doi.org/10.1016/J.MEMSCI.2018.06.012.

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FIGURE 4–10 Comparison between double skinned and conventional membrane in contact with a reactive catalyst. Reproduced from A. Arratibel, A. Pacheco Tanaka, M. van Sint Annaland, F. Gallucci, On the use of double-skinned membranes to prevent chemical interaction between membranes and catalysts, Int. J. Hydrogen Energy 46 (2021) 20240 20244. https://doi.org/10.1016/J.IJHYDENE.2019.10.203.

Protecting the membrane from attrition is a very interesting solution that may be achieved with an additional protective layer as reported by Arratibel, or by creating a membrane directly inside the pores of support as reported by Alique and co-workers. These membranes called pore-plated membranes have also been tested in fluidized bed reactors with good stability (although the original selectivity was too low to judge the deterioration) [11]. The authors have reported that “the main difference with respect to the classical Electroless Plating is the way to put in contact the two main solutions involved in the plating process” [12]. Basically, by feeding the Pd solution from one side of the membrane support and the reducing agent on the other side, the authors can grow the layer of the membrane inside the pores leading ultimately to pore closure. In this way, the Pd layer is inside the support and protected from attrition with the fluidized bed particles. Theoretical work on the erosion of particles on membrane tubes has also been reported by Yang et al. [13] although the magnitude of erosion is far higher than anything observed experimentally. Finally, the use of membranes in a fluidized bed to reduce the extent of mass transfer limitations has been studied experimentally by several authors. The conclusion of these studies, although not yet definite, is that fluidized bed reactors help in reducing the mass transfer limitations but do not eliminate the limitations, as reported by de Nooijer et al. [14] and by Helmi et al. [15]. The effect of the membrane immersion in fluidized bed hydrodynamics, which may be of interest to understanding the evolution and nature of concentration polarization, has been studied by Helmi et al. using both X-ray analysis [16] and visual analysis through particle image velocimetry [17]. The results are very interesting but not enough to draw solid

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conclusions about the effect of membrane presence and permeation through it on the extent of mass transfer limitations. Surely more research needs to be carried out. Nonetheless, the use of fluidized bed membrane reactors has been extended to many more applications such as hydrogen production via ethanol reforming. An experimental demonstration of this concept has been reported by Spallina et al. [18] who gave an experimental demonstration and a model-based design of such a system for off-grid hydrogen production from bio-ethanol. As already mentioned above, the fluidized bed concept is also very interesting, if not the only choice, when there is a need to combine membrane separation in reactions that need catalyst regeneration (because of cocking or to allow oxidation/reduction) as well as when we want to combine membrane separation with sorbents (as sorbents need regeneration). In all these cases the fluidized bed is the best choice and in some cases the only possible feasible concept. It should be also noted that for carbon deposition deactivation of the catalyst, another concept has also been used by Sanz-Martinez et al. [19] that uses a dual-zone fluidized bed membrane reactor and makes use of the difference in weight of the cocked catalyst to exploit catalyst segregation and oxidation at the bottom of the reactor. An interesting concept that integrates hydrogen production with CO2 capture is the membrane-assisted chemical looping reformer that has been developed and demonstrated in our group [20]. The concept has been experimentally demonstrated by Medrano et al. [21] and it is depicted in Fig. 4 11. In this concept, the energy required for the endothermic steam methane reforming is supplied by the oxidation of the metal to metal oxide that occurs in the air reactor and is very exothermic. The metal oxide is then transferred to the reforming reactor where it is reduced and supplies the energy to the reforming while hydrogen is produced and separated through the Pd membranes. The proof of concept has shown that stable operation can be attained and the model was successfully validated. Medrano et al. [20] have made a comparison of this concept, with several other systems proposed in the literature for hydrogen production including CO2 capture. The Membrane assisted chemical looping reforming (MA-CLR) provides a high degree of process intensification and allows for much higher efficiencies compared to other systems. The MA-CLR has shown very high reforming efficiencies at low temperatures because higher fuel conversion is achieved with relatively low membrane area installed and with lower steam requirements compared to other systems. With this concept, hydrogen can be produced with inherent CO2 capture at the same cost (or even cheaper) than the conventional steam methane reforming without CO2 capture. A variation of this concept has been proposed and demonstrated by Wassie et al. [22] who have demonstrated the concept called membrane-assisted gas switching reforming. Compared to the previous concept by Medrano et al. [21], the gas switching uses a single fluidized bed membrane reactor, and the gas is periodically switched between oxidation and reforming. This reactor concept combines ultra-pure hydrogen production with integrated CO2 capture from steam methane reforming by the use of an oxygen carrier, which acts as catalyst and heat carrier to the endothermic reforming and is periodically fed with either the fuel (methane and steam) or with air for the oxidation step. When air is fed to the reactor, the oxygen carrier is heated by

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FIGURE 4–11 Schematic representation of the MA-CLR concept for pure H2 production via Pd-based selective membranes combined with CO2 capture via chemical looping. Reproduced from J.A. Medrano, I. Potdar, J. Melendez, V. Spallina, D.A. Pacheco-Tanaka, M. van Sint Annaland, et al., The membrane-assisted chemical looping reforming concept for efficient H2 production with inherent CO2 capture: experimental demonstration and model validation, Appl. Energy. 215 (2018) 75 86. https://doi.org/10.1016/J.APENERGY.2018.01.087.

the exothermic solids oxidation reaction to be utilized in the fuel stage, where endothermic reduction and catalytic reactions regenerate the oxygen carrier and produce syngas. While this concept may be easier to operate under pressure (a pre-requisite for efficient use of the membranes) the experiments show that the membranes do not survive under oxidation for the time needed to completely oxidize the metal to metal oxide. This is because Pd-based membranes were used and these oxidize as well on the surface, thus reducing progressively their separation efficiency. Despite this drawback, the concept is interesting and may be exploited with different kinds of membranes. As described above the same concept has been proposed to carry out the sorption enhanced methane reforming with hydrogen removal. An example of the reactor scheme is presented in Fig. 4 12 reproduced from the work of Chen et al. [23]. Here the reforming is driven by both the hydrogen permeation and the CO2 removal, thus a double shift in the equilibrium reaction is obtained. The sorbent needs to be regenerated and thus a second fluidized bed (the calciner) operated at high T is needed. A different application of the membrane-assisted fluidized bed reactor for sorption enhanced reforming has been proposed by Antonini et al. [24]. In this concept (reported in Fig. 4 13) the membranes used are oxygen selective membranes and are inserted in the

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FIGURE 4–12 Schematic of the sorption enhanced reforming ad regeneration proposed by Chen et al. [23].

FIGURE 4–13 (A) Conceptual scheme of the Calcium Looping (CaL) process system for CO2 capture assisted by Oxygen Transport Membranes; (B) Arrangement of a single tubular membrane and related fittings, with input air, retentate, and oxygen flows. Reproduced from T. Antonini, A. Di Carlo, P.U. Foscolo, K. Gallucci, S. Stendardo, Fluidized bed reactor assisted by oxygen transport membranes: numerical simulation and experimental hydrodynamic study, Chem. Eng. J. 377 (2019) 120323. https://doi.org/10.1016/J.CEJ.2018.11.021.

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calciner to supply oxygen to the calcination. In this way, oxy-combustion is carried out in the calciner with integrated air separation through the membrane resulting in higher heat transfer efficiency and higher total efficiency of the process. The stability of the oxygen transport membranes needs to be assessed in fluidized bed reactors and surely more research will be carried out in this field. Fluidized bed membrane reactors have also been extensively proposed and tested in wastewater treatment. In this particular case, the use of the fluidized bed has the effect of decreasing the fouling of the membranes used in the bioreactors. Examples of such techniques can be found in different papers for several applications [25 27]. Wang et al. also studied the effect of sphericity of the fluidized media (particles) on the fouling of the membranes [28]. Another application of the fluidized bed membrane bioreactor (see Fig. 4 14) has been reported in the work of Ding et al. [29] where a combination of membrane bioreactor and internally circulated fluidized bed is applied for pollution degradation and micro-algae enrichment. The combination of both circulated fluidized bed and algae enhanced bioreactor results in very high removal efficiency of pollutants and increase algae enrichment.

FIGURE 4–14 Schematic of the internally circulated fluidized bed membrane bioreactor for wastewater treatment.

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FIGURE 4–15 Schematic of the membrane fluidized bed bioreactor. Reproduced from K. Wei, Z. Wang, C. Ouyang, X. Cao, P. Liang, X. Huang, et al., A hybrid fluidized-bed reactor (HFBR) based on arrayed ceramic membranes (ACMs) coupled with powdered activated carbon (PAC) for efficient catalytic ozonation: a comprehensive study on a pilot scale, Water Res. 173 (2020) 115536. https://doi.org/10.1016/J.WATRES.2020.115536.

Yet another application of fluidized bed membrane bioreactor has been proposed and experimentally studied by Oztemur et al. [30] for biological sulfate reduction. According to the authors, this is the first application of this kind of bioreactors for sulfate reduction and the results show good activity and stable flux for several days of operation. Wei et al. [31] proposed and tested a fluidized bed membrane reactor that combines the enhanced mass transfer rates of a fluidized bed with catalytic and separation functionalities of ceramic membranes for catalytic ozonation. A scheme is reported in Fig. 4 15. The pilot results on this system have shown that using the fluidized bed catalyst (activated carbon in this case), the ozonation reaction performance increase especially because of the enhanced mass transfer that results in better utilization of the dissolved ozone fed through the membranes.

4.3 Conclusions and future trends Fluidized bed membrane (bio)reactors have been introduced because the increased heat and mass transfer rates improve the performance compare with the counterpart packed bed

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systems. Additionally, the moving particles may have a very beneficial effect in decreasing the fouling in the bioreactor (because of the erosion effect) but may be detrimental in gas phase separation membranes when thin layers are used. For these last examples, research is devoted to the production of different (more stable) membranes. Several projects are bringing the fluidized bed membrane reactors for energy application closer to industrial scale, while it is foreseen that several industrial applications for membrane bio-reactors are already at a level close to economic exploitation.

Nomenclature Acronyms C21 Hydrocarbons including C2 and higher CaL Calcium Looping MA-CLR Membrane assisted chemical looping reforming

References [1] F. Gallucci, M. Annaland, J. Kuipers, Autothermal reforming of methane with integrated CO2 capture in a novel fluidized bed membrane reactor. Part 1: Experimental demonstration, Top. Catal. 51 (2008) 133 145. Available from: https://doi.org/10.1007/s11244-008-9126-8. [2] F. Gallucci, E. Fernandez, P. Corengia, M. van Sint Annaland, Recent advances on membranes and membrane reactors for hydrogen production, Chem. Eng. Sci. 92 (2013). Available from: https://doi.org/ 10.1016/j.ces.2013.01.008. [3] F. Gallucci, M. Van Sintannaland, J.A.M. Kuipers, Theoretical comparison of packed bed and fluidized bed membrane reactors for methane reforming, Int. J. Hydrogen Energy 35 (2010) 7142 7150. Available from: https://doi.org/10.1016/J.IJHYDENE.2010.02.050. [4] L. Roses, F. Gallucci, G. Manzolini, M. van Sint Annaland, Experimental study of steam methane reforming in a Pd-based fluidized bed membrane reactor, Chem. Eng. J. 222 (2013). Available from: https://doi. org/10.1016/j.cej.2013.02.069. [5] A. Helmi, E. Fernandez, J. Melendez, D.A.P. Tanaka, F. Gallucci, M. Van Sint Annaland, Fluidized bed membrane reactors for ultra pure H2 production—a step forward towards commercialization, Molecules 21 (2016). Available from: https://doi.org/10.3390/molecules21030376. [6] N. de Nooijer, A.A. Plazaola, J.M. Rey, E. Fernandez, D.A.P. Tanaka, M.S. Annaland, et al., Long-term stability of thin-film Pd-based supported membranes, Processes 7 (2019). Available from: https://doi.org/ 10.3390/pr7020106. [7] E. Fernandez, A. Helmi, K. Coenen, J. Melendez, J.L. Viviente, D.A. Pacheco Tanaka, et al., Development of thin Pd-Ag supported membranes for fluidized bed membrane reactors including WGS related gases, Int. J. Hydrogen Energy 40 (2015). Available from: https://doi.org/10.1016/j.ijhydene.2014.08.074. [8] A. Arratibel, A. Pacheco Tanaka, I. Laso, M. van Sint Annaland, F. Gallucci, Development of Pd-based double-skinned membranes for hydrogen production in fluidized bed membrane reactors, J. Memb. Sci. 550 (2018) 536 544. Available from: https://doi.org/10.1016/j.memsci.2017.10.064. [9] A. Arratibel, J.A. Medrano, J. Melendez, D.A. Pacheco Tanaka, M. van Sint Annaland, F. Gallucci, Attrition-resistant membranes for fluidized-bed membrane reactors: double-skin membranes, J. Memb. Sci. 563 (2018) 419 426. Available from: https://doi.org/10.1016/J.MEMSCI.2018.06.012.

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[10] A. Arratibel, A. Pacheco Tanaka, M. van Sint Annaland, F. Gallucci, On the use of double-skinned membranes to prevent chemical interaction between membranes and catalysts, Int. J. Hydrogen Energy 46 (2021) 20240 20244. Available from: https://doi.org/10.1016/J.IJHYDENE.2019.10.203. [11] E. Tosto, D. Alique, D. Martinez-Diaz, R. Sanz, J.A. Calles, A. Caravella, et al., Stability of pore-plated membranes for hydrogen production in fluidized-bed membrane reactors, Int. J. Hydrogen Energy 45 (2020). Available from: https://doi.org/10.1016/j.ijhydene.2019.04.285. [12] R. Sanz, J.A. Calles, D. Alique, L. Furones, New synthesis method of Pd membranes over tubular PSS supports via “pore-plating” for hydrogen separation processes, Int. J. Hydrogen Energy 37 (2012) 18476 18485. Available from: https://doi.org/10.1016/J.IJHYDENE.2012.09.084. [13] X. Yang, S. Wang, S. Liu, Y. He, Erosion behaviors of membrane tubes in a fluidized bed reactor with hydrogen separation, Powder Technol. 381 (2021) 17 24. Available from: https://doi.org/10.1016/j. powtec.2020.12.017. [14] N. de Nooijer, F. Gallucci, E. Pellizzari, J. Melendez, D.A. Pacheco Tanaka, G. Manzolini, et al., On concentration polarisation in a fluidized bed membrane reactor for biogas steam reforming: modelling and experimental validation, Chem. Eng. J. 348 (2018). Available from: https://doi.org/10.1016/j. cej.2018.04.205. [15] A. Helmi, R.J.W. Voncken, A.J. Raijmakers, I. Roghair, F. Gallucci, M. van Sint Annaland, On concentration polarization in fluidized bed membrane reactors, Chem. Eng. J. 332 (2018) 464 478. Available from: https://doi.org/10.1016/J.CEJ.2017.09.045. [16] A. Helmi, E.C. Wagner, F. Gallucci, M. van Sint Annaland, J.R. van Ommen, R.F. Mudde, On the hydrodynamics of membrane assisted fluidized bed reactors using x-ray analysis, Chem. Eng. Process. Process Intensif. (2016). Available from: https://doi.org/10.1016/j.cep.2017.05.006. [17] A. Helmi, I. Campos Velarde, F. Gallucci, M. van Sint Annaland, Hydrodynamics of dense gas-solid fluidized beds with immersed vertical membranes using an endoscopic-laser PIV/DIA technique, Chem. Eng. Sci. 182 (2018) 146 161. Available from: https://doi.org/10.1016/J.CES.2018.02.038. [18] V. Spallina, G. Matturro, C. Ruocco, E. Meloni, V. Palma, E. Fernandez, et al., Direct route from ethanol to pure hydrogen through autothermal reforming in a membrane reactor: experimental demonstration, reactor modelling and design, Energy 143 (2018) 666 681. Available from: https://doi.org/10.1016/j. energy.2017.11.031. [19] P. Durán, A. Sanz-Martínez, J. Soler, M. Menéndez, J. Herguido, Pure hydrogen from biogas: intensified methane dry reforming in a two-zone fluidized bed reactor using permselective membranes, Chem. Eng. J. 370 (2019) 772 781. Available from: https://doi.org/10.1016/j.cej.2019.03.199. [20] J.A. Medrano, V. Spallina, M. Van Sint Annaland, F. Gallucci, Thermodynamic analysis of a membraneassisted chemical looping reforming reactor concept for combined H2 production and CO2 capture, Int. J. Hydrogen Energy 39 (2014). Available from: https://doi.org/10.1016/j.ijhydene.2013.11.126. [21] J.A. Medrano, I. Potdar, J. Melendez, V. Spallina, D.A. Pacheco-Tanaka, M. van Sint Annaland, et al., The membrane-assisted chemical looping reforming concept for efficient H2 production with inherent CO2 capture: experimental demonstration and model validation, Appl. Energy 215 (2018) 75 86. Available from: https://doi.org/10.1016/J.APENERGY.2018.01.087. [22] S.A. Wassie, J.A. Medrano, A. Zaabout, S. Cloete, J. Melendez, D.A.P. Tanaka, et al., Hydrogen production with integrated CO2 capture in a membrane assisted gas switching reforming reactor: proof-ofconcept, Int. J. Hydrogen Energy (2018). Available from: https://doi.org/10.1016/j.ijhydene.2018.02.040. [23] Y. Chen, A. Mahecha-Botero, C.J. Lim, J.R. Grace, J. Zhang, Y. Zhao, et al., Hydrogen production in a sorption-enhanced fluidized-bed membrane reactor: operating parameter investigation, Ind. Eng. Chem. Res. 53 (2014) 6230 6242. Available from: https://doi.org/10.1021/ie500294k. [24] T. Antonini, A. Di Carlo, P.U. Foscolo, K. Gallucci, S. Stendardo, Fluidized bed reactor assisted by oxygen transport membranes: numerical simulation and experimental hydrodynamic study, Chem. Eng. J. 377 (2019) 120323. Available from: https://doi.org/10.1016/J.CEJ.2018.11.021.

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[25] S. Chang, R. Ahmad, D. Kwon, J. Kim, Hybrid ceramic membrane reactor combined with fluidized adsorbents and scouring agents for hazardous metal-plating wastewater treatment, J. Hazard. Mater. 388 (2020) 121777. Available from: https://doi.org/10.1016/j.jhazmat.2019.121777. [26] N. Horstmeyer, C. Thies, T. Lippert, J.E. Drewes, A hydraulically optimized fluidized bed UF membrane reactor (FB-UF-MR) for direct treatment of raw municipal wastewater to enable water reclamation with integrated energy recovery, Sep. Purif. Technol. 235 (2020) 116165. Available from: https://doi.org/ 10.1016/j.seppur.2019.116165. [27] M. Kim, T.Y.C. Lam, G.-Y.A. Tan, P.-H. Lee, J. Kim, Use of polymeric scouring agent as fluidized media in anaerobic fluidized bed membrane bioreactor for wastewater treatment: system performance and microbial community, J. Memb. Sci. 606 (2020) 118121. Available from: https://doi.org/10.1016/j. memsci.2020.118121. [28] J. Wang, A.G. Fane, J.W. Chew, Characteristics of non-spherical fluidized media in a fluidized bedmembrane reactor: effect of particle sphericity on critical flux, Sep. Purif. Technol. 202 (2018) 185 199. Available from: https://doi.org/10.1016/j.seppur.2018.03.047. [29] Y. Ding, Z. Guo, J. Mei, Z. Liang, Z. Li, X. Hou, Investigation into the novel microalgae membrane bioreactor with internal circulating fluidized bed for marine aquaculture wastewater treatment, Membranes 10 (2020). Available from: https://doi.org/10.3390/membranes10110353. [30] G. Oztemur, S. Teksoy Basaran, Z. Tayran, E. Sahinkaya, Fluidized bed membrane bioreactor achieves high sulfate reduction and filtration performances at moderate temperatures, Chemosphere 252 (2020) 126587. Available from: https://doi.org/10.1016/j.chemosphere.2020.126587. [31] K. Wei, Z. Wang, C. Ouyang, X. Cao, P. Liang, X. Huang, et al., A hybrid fluidized-bed reactor (HFBR) based on arrayed ceramic membranes (ACMs) coupled with powdered activated carbon (PAC) for efficient catalytic ozonation: a comprehensive study on a pilot scale, Water Res. 173 (2020) 115536. Available from: https://doi.org/10.1016/J.WATRES.2020.115536.

5 Microstructured membrane reactors for process intensification Ellen Gapp1, Luca Ansaloni2, Hilde J. Venvik3, Thijs A. Peters2, Peter Pfeifer1 1

INSTITUTE FOR MICR O PROCESS ENGINE ERING (IMVT) AT K ARLSRUH E INSTITUTE OF

T ECHNOL OGY (K IT), E GGE NSTE IN- LEO POLDSHAFEN, GERM ANY 2 SINTEF INDUSTRY, OSLO, NORWAY 3 DEPART ME NT OF CHEMICAL ENGINEERING, NORW EGIAN UNIVERSITY OF SCIENCE AND TECHNOLOGY (NTNU), T RONDHEIM, NORWAY

5.1 Introduction Microstructured reactors have attracted wide interest and have undergone rapid development during the last two decades. A microstructured reactor is a device where chemical reactions take place in a confined environment with typical dimensions below 1000 μm. Compared to traditional reactor systems, microstructured reactors have many benefits, such as enhanced heat management and mass transfer, and a large and well-defined interfacial area. In contrast to conventional packed bed reactors, reduced axial dispersion occurs when catalytic wall coatings are applied, and regardless of the laminar flow typical for microreactors, diffusion in the gas phase ensures the rapid cross-sectional exchange of reactive species allowing operation as an ideal plug-flow reactor. Another attractive feature of microstructured reactors is their modularity, which facilitates the scale-up and the adaptation of the reactor system to changing process needs by multiplication of the confined structure. The membrane microstructured reactor can be divided into extractor type reactors for conversion enhancement (involving reactions such as dehydrogenation, steam reforming (SR), Knoevenagel condensation, and water splitting) and distributor/contactor type reactors for selectivity enhancement (hydrocarbons oxidation, m-xylene oxidation, methane-to-synthesis gas, and partial hydrogenation) [1]. In the former type, a product is removed to overcome equilibrium restriction, while integrating the purification step in the reactor. The latter type operates in kinetically limited conditions, allowing to recover a targeted product that is typically an intermediate or one of the products in a series of parallel reactions. A wide range of reactions, including homogeneous liquid, liquid-liquid, gas-liquid, liquidsolid, and gas-liquid-solid phases, have been demonstrated in microchemical systems [2]. Microfluidic systems have amongst others opened up new concepts and offer several attractive

Current Trends and Future Developments on (Bio-)Membranes. DOI: https://doi.org/10.1016/B978-0-12-823659-8.00010-1 © 2023 Elsevier Inc. All rights reserved.

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possibilities for organic chemistry. Combining membranes with microreactors can magnify their respective advantages to provide more valuable applications. Membranes can be used to selectively remove products from, or supply reactants to the reactor zone in a controlled manner, thereby either surpassing the thermodynamic equilibrium of processes such as steam methane reforming or preventing hot spot formation in exothermic processes such as the oxidative coupling of methane (OCM) by adjusting conveniently the CH4/O2 stoichiometry. Especially for gas-liquid applications, microstructured reactors can be used to enhance the contact between the two phases. In such a system, the membrane separates both phases and establishes an efficient gas-liquid contact through the porous membrane. The microdimensions maximize the interfacial area. As well, it also allows independent control of the flow of the gaseous and liquid reagents. Especially for chemical energy conversions, microstructured membrane reactor technology is receiving considerable attention due to the high heat and mass transport rates that can be achieved [3]. Also heating or cooling channels can be integrated, which further can lead to improved energy efficiency [4], excellent temperature control [5], enabling higher selectivity and catalyst lifetime [6]. Also, membrane microreactors have received attention in various organic syntheses enabling the separation of produced gas from the reaction conditions [7]. This can drastically improve safety and circumvent the need for distillation in cases where the gas is generally synthesized in situ, such as diazomethane and hydrogen cyanide. Many reviews have been published in the last decade providing a more general overview of the fabrication and applications of membranes in microfluidics [7 10], or more specifically on various topics, like compact membrane reactors with hydrogen separation [11]. In this chapter, we will primarily aim to focus on microstructured membrane reactors for process intensification employing membranes with a combined gas-selective and/or catalysis function. Firstly, we will introduce the design and fabrication strategies of such devices. Then examples of microstructured membrane reactors employing polymeric, ceramic, and as well metallic membranes will be given introducing possible applications of microstructured membrane reactors involving gas-liquid and gas-gas reactions. Finally. The chapter will end by discussing the potential outlook for the technology.

5.2 Design and fabrication Microstructured membrane reactors are most commonly based on planar plate-type heat exchangers with channels in the sub-millimeter range for the transport of fluids, i.e., liquids and/or gases. They include thin membranes in between oppositely oriented channels or on porous channels walls that contain solid catalyst as coating or as a fixed bed on at least one side of the membrane to enable a high reaction rate. The catalyst can also be deposited within the porous structure of the wall. Microstructured reactors can be fabricated out of glass, quartz, plastic, silicon, and metals such as stainless steel. Their fabrication is strongly dependent on the type of material, not only regarding the channel fabrication but also their assembly. The pressure tolerance of the total assembly is

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also correlated to the material: glass, quartz, and plastic can only be applied to low-pressure applications, while silicon and metals can also withstand pressures . 10 bar in the correct design. Thus, when a high-pressure difference is needed for large transmembrane flux or sufficient species separation while sweep gas or vacuum on the permeate side of the membrane are not required or permitted from the application, silicon and metal are advantageous construction materials. Options to enhance throughput in microstructured/microchannel designs are to keep the lateral dimensions while multiplying the channel number (numbering-up), possibly also the channel length, and further to form repeated stacks of the base assembly: opposite channels and membrane, see Fig. 5 1. The latter approach is often a synonym for modularity. The numbering-up of the channel number, as well as the increase of the channel length, can be limited by the required fluid velocity (associated pressure drop) as well as the membrane availability with regards to the available surface area. The most critical issue is the leak-tight sealing of the membrane to the outer environment and the associated question of parallelization of such stacked membrane assemblies. Polymerbased membranes can provide the necessary leak tightness upon mechanical compression, i.e., the mass transfer in the perpendicular direction to the desired species transport is then sufficiently low to prevent either gas or liquid permeation towards the environment, see also Fig. 5 1. Similar approaches could also be applied with e.g., palladium (Pd) membranes for H2 separation. The membranes can be sealed between a polished stainless steel feed housing and a polished stainless-steel plate for permeate removal [13]. Hydrogen may leak through very small defects in the membrane or the sealing interface in such configurations and safety measures would be required for upscaling. Thus, sealing by welding metal membranes with the microchannel support may become inevitable to prevent leakage. The sealing procedure depends somewhat on the stack design and the approach chosen when constructing a unit from multiple stacks. Diffusion bonding, a method to build up microchannel systems made from metals or silicon, seems inapplicable for embedding metal or ceramic membranes. Exchange of elements, e.g., diffusion of iron and chromium into the Pd-alloy membrane, would be required for the sealing; however, this can potentially negatively influence the membrane properties. Iron, chromium, and nickel tend to accumulate near the surface of the membrane, reducing the number of accessible palladium active surface sites for hydrogen dissociation, which can lead to a reduced overall transversal hydrogen flux. Hole formation at grain boundaries has also been found at diffusion bonding temperatures of down to 800 C. Laser or electron beam welding produces a local melt of metal-membrane species at the outer boundary of the stack. Thus, this option seems much more suitable to build a system with multiple stacks. One can differentiate between two general ideas: beam direction transversal to the stack or parallel to the membrane surface. The first idea has been applied in a membrane separator module [14] and further reformer constructions [11], see Fig. 5 2. As small melting hills are created on top of the sheets which would prevent further stacking, cavities can be generated in the sheets [15]. These cavities also allow reduced welding power, which in turn leads to less mechanical strain and thus less probability for membrane rupture in cooling down the weld seam.

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FIGURE 5–1 Schematic illustrations of (A) stacked catalytic membrane microreactor, (B) independent perforated microchannel and microchamber, and (C) two patterns of the PDMS membrane with two holes [12]. Reprinted with permission from M. Liu, X. Zhu, Q. Liao, R. Chen, D. Ye, G. Chen, Stacked catalytic membrane microreactor for nitrobenzene hydrogenation, Ind. Eng. Chem. Res. 59 (2020) 9469 9477. Copyright 2020 American Chemical Society.

Welding in parallel to the membrane surface can be applied to generate a larger area of welding and thus the high-pressure resistance of the stack. Nevertheless, the pressure tolerance is limited further by the sheet thickness of the surrounding microchannel plates, thus a second pressure resistant housing may be required. When the metal membrane is supported by a porous metal sheet, the parallel welding further avoids hole formation inside the welded area and thus membrane rupture as observed in transversal welding through the membrane

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FIGURE 5–2 Schematic illustrations of (A) microchannel membrane module and (B) reformer construction base assembly by different sheets with visible weld cavities in several sheets, see also patent WO2020228982. Reprinted from (A) T. Boeltken, M. Belimov, P. Pfeifer, T.A. Peters, R. Bredesen, R. Dittmeyer, Fabrication and testing of a planar microstructured concept module with integrated palladium membranes, Chem. Eng. Process.: Process Intensif. 67 (2013) 136 147, with permission from Elsevier; (B) Reprinted from A. Wunsch, P. Kant, M. Mohr, K. Haas-Santo, P. Pfeifer, R. Dittmeyer, Recent developments in compact membrane reactors with hydrogen separation, Membranes 8 (2018), with permission under Creative Commons license CC BY 4.0.

and porous metal substrate. An indentation in the welded surface appears dependent on the porosity of the supporting metal sheet. Such welding has been applied in high-pressure membrane systems [16]. A sketch of the membrane assembly and a picture of the completed membrane device is shown in Fig. 5 3. With regards to scaling-up by modularity, the potential of membrane failure leads to design ideas such as stacking and sealing multiple base assemblies with gaskets in between the base assemblies to distribute the gas flow over the multiple base assemblies. Approaches in this respect are shown in Fig. 5 4. Individual gaskets (rings and long-hole type) for feed, retentate and permeate are shown in block color to provide internal fluid distribution inside a reformer with multiple base assemblies [17]. Membrane coating is another approach to manufacturing the base assembly. While a foilbased membrane, supported on micro sieves, porous metal support, or free-standing, generally requires welding with another material such as stainless steel, a direct membrane coating on porous metal support evades such a critical step. If one can obtain (1) similar heat expansion coefficients of the membrane, support, and diffusion barrier layer materials, and (2) a defectfree membrane coating overlapping the transition area between the porous support and the dense material, the welding can be done without connection to the membrane. Such an approach was already proposed in 2006 [19] but seems more feasible today due to new efforts in 3-dimensional (3D) printing. Dense and porous metal substrates can be printed with selective laser melting (SLM) from metal powder, 30 40 μm particle size, with different laser power [3].

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FIGURE 5–3 (A) Schematic illustration and (B) completed device of high-pressure microchannel membrane module for separation of hydrogen from gas produced by hydrothermal gasification of biomass. Reprinted from M. Cholewa, R. Dürrschnabel, N. Boukis, P. Pfeifer, High pressure membrane separator for hydrogen purification of gas from hydrothermal treatment of biomass, Int. J. Hydrogen Energy 43 (2018) 13294 13304, with permission from Elsevier.

FIGURE 5–4 Schematic illustrations of (A) a microchannel membrane reformer for separation of hydrogen from methane steam reforming [18] and (B) the identical system but with additional base assembly for retentate combustion in between the reformer base assemblies. Reprinted from R. Bredesen, T.A. Peters, T. Boeltken, R. Dittmeyer, Pd-Based Membranes in hydrogen production for fuel cells, Ch.7, in: Process Intensification for Sustainable Energy Conversion, 2015, with permission from John Wiley & Sons; (B) Reprinted from A. Wunsch, P. Kant, M. Mohr, K. Haas-Santo, P. Pfeifer, R. Dittmeyer, Recent developments in compact membrane reactors with hydrogen separation, Membranes 8 (2018) with permission under Creative Commons license CC BY 4.0.

Thus, the difficulty of making suitable support is reduced since either the welding of a porous substrate into a dense frame or a two-step powder metallurgical process for densification of the surrounding frame can be omitted. Internal channels within porous stainless steel, as illustrated

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FIGURE 5–5 (A) Schematic illustrations of the membrane support sheet of a micro membrane reformer based on 3D printed material, (B) porous microchannel system as part of the membrane reformer sheet, and (C) example of a sheet consisting of a dense frame with interior porous metal from 3D selective laser melting [3]. All examples are fabricated at the Institute for Micro Process Engineering, KIT, Germany. (A) Adapted from P. Pfeifer, T. Boeltken, R. Í Durrschnabel, R. Dittmeyer, E. Hansjosten, T. Gietzelt, et al., Membrane reformer, PCT/EP2020/025200, WO2020228982 2021; (B) Reproduced with permission from D. Xie, Structured Porous Metal-Supported Composite Palladium Membranes via 3D Printing and Suspensions Plasma Spraying, Karlsruhe Institute of Technology, 2021.

in Fig. 5 5A can also be manufactured, see Fig. 5 5B. The porous sheets with dense frames as prepared by SLM can be seen in Fig. 5 5C. Catalyst integration in microreactors can be done by applying a fixed bed of catalyst particles or a coating. Early reviews are mainly devoted to the part of coating [5,6], however, the filling of microchannels provides advantages in terms of catalyst removal. This discussion and additional implications on e.g., mass and heat transport are covered elsewhere [20]. For application with membranes, the particle-membrane interaction needs additional attention with respect to mechanical membrane damage, solid species diffusion, and intra-particular mass transport towards the membrane. Membrane damage must be considered in case of direct contact with the particles. Nevertheless, it can be avoided by the right selection of the particle size and an appropriate microsieve on top of the membrane interface [11,21 23]. This microsieve, however, may reduce the accessible membrane surface area and/or restrict the mass transfer towards the membrane. The latter (so-called diffusion polarization) often limits the hydrogen transport in conventional membrane reactors and cannot be easily judged in a micro-fixed bed compared to empty microchannels due to redirection and mixing in the intra-particle voids. The diffusion time from a catalyst coating to an opposite membrane surface in open microchannels with the laminar flow can be calculated as a function of the diffusion coefficient (Dm) and the dimension of the channel (d) according to: t 5 Dm =d2 Channel

While in micro packed beds this assumption is invalid, a Sherwood correlation needs to be elaborated for the particle surrounding flow, and a film effectiveness factor needs to be determined [22]. A detailed analysis can be found in [24]. The positioning of the membrane with respect to the catalyst zone is also a crucial issue for all applications dealing with reaction products, which are to be removed from the reaction zone through the membrane. This is often the case in hydrogen generators/reforming

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FIGURE 5–6 Schematic illustrations of the pre-reforming zone of a microstructured methane steam reformer. Reprinted from T. Boeltken, A. Wunsch, T. Gietzelt, P. Pfeifer, R. Dittmeyer, Ultra-compact microstructured methane steam reformer with integrated Palladium membrane for on-site production of pure hydrogen: experimental demonstration, Int. J. Hydrogen Energy 39 (2014) 18058 18068 with permission from Elsevier.

reactors. The partial pressure of the product needs to be higher than ambient pressure if the requirement is that the product needs to be removed at ambient conditions without the use of sweep gas. As microchannel reactors are close to ideal plug flow reactors, hydrogen may even back-permeate in sections of the membrane module in certain configurations, reducing the net transport across the membrane. A possible solution by adding a pre-reforming zone is shown in Fig. 5 6 [17]. By adding a pre-reforming zone, a higher H2 partial pressure at the entrance of the membrane reactor is ensured and back-permeation issues are prevented. Although this book chapter is not specially attributed to micro-electrochemical systems (MEMS) technology and is not devoted to small flows and analytical systems, one should note that also silicon wafers typically applied for MEMS can be used to generate microchannel membrane reactors (Fig. 5 7). The methods are quite the same in most cases, irrespectively of the type of membrane. Coated Si wafers are etched from one side to remove all Si until the coating of SiO2 and SiN remains. The overlying SiN is patterned with standard lithography and dry etching on the top side of the wafer to generate small openings towards SiO2. Then the backside is sputtered with e.g., Pd as membrane material. After that, the residual SiO2 is removed by etching to yield a free-standing membrane [25]. This last step could also be replaced by providing macroporous silicon with pore dimensions of 4 8 μm as

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FIGURE 5–7 Schematic illustrations of (A) the production method of a free-standing Pd membrane using a SiN-SiO2Si wafer structure and (B) the corresponding assembly with glass to form channels for feed and permeate [27]. Reprinted with permission from H.D. Tong, F.C. Gielens, J.G.E. Gardeniers, H.V. Jansen, C.J.M. van Rijn, M.C. Elwenspoek, et al., Microfabricated palladium-silver alloy membranes and their application in hydrogen separation, Ind. Eng. Chem. Res. 43 (2004) 4182 4187. Copyright (2014) American Chemical Society.

membrane support by using an anodization process. During this process, a metal membrane could serve as conductive material as a top coating. This electrochemical step has been applied to generate macroporous catalyst-coated flow-through membrane systems for reactions [26]. In this particular study, however, no dense membrane covered the porous silicon. For ceramic membranes, the module design is often based on extruded channel monoliths for compactness and manufacturability. For example, countercurrent two-fluid monoliths with integrated mixed-conducting oxygen-selective membranes (MCM) were developed for the combustion of natural gas in an N2-free environment [28,29]. The module has 24 3 24 channels of B2.2 mm width (B70 mm2), separated by porous walls of B0.5 mm coated with a B50 μm dense membrane. This gives an effective membrane area of 540 m2/m3 monolith volume for high heat and mass transfer in the gas phase. However, there is a need for a complex manifold system, and the low tolerance of pressure differences leads to mechanical stress. The manifolds at the ends of the produced monolith consist of two main parts; a flow distributor plate converts from checkerboard flow to alternating rows, while a plate manifold collects the air and sweeps streams. The module parts are joined using glass-ceramic sealants. An expanded view of the design is shown in Fig. 5 8A, while Fig. 5 8B shows an assembled MCM module with gas flows indicated. In a monolithic system, the reactants and the sweep gas are fed separately into the channel of the monolithic membrane reactor. The two streams can be arranged as parallel or checked as shown in Fig. 5 9. The parallel arrangement (Fig. 5 9A) is the conventional flow configuration because it allows for a much simpler feed flow distributor. On the other hand, the checkerboard pattern (Fig. 5 9B) provides a larger contact area between the two flows but requires a more complicated flow distributor structure. Seemingly obvious, but Computational Fluid Dynamics simulations of a monolithic reactor with integrated Pd-based membranes employed for steam methane reforming have shown that only half of the membrane area was effectively utilized in the parallel arrangement, which resulted in lower hydrogen recovery [30,31].

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FIGURE 5–8 (A) Expanded view of a module showing a 24 3 24 channel monolith, flow converter, and the different parts of the manifold, (B) MCM module indicating gas flows. Reprinted from S.G. Sundkvist, S. Julsrud, B. Vigeland, T. Naas, M. Budd, H. Leistner, et al., Development and testing of AZEP reactor components, Int. J. Greenh. Gas Control 1 (2007) 180 187 with permission from Elsevier.

FIGURE 5–9 Micro reformer configuration. Reprinted from T. Jiwanuruk, S. Putivisutisak, P. Ponpesh, P. Bumroongsakulsawat, T. Tagawa, H. Yamada, et al., Effect of flow arrangement on micro membrane reforming for H2 production from methane, Chem. Eng. J. 293 (2016) 319 326 with permission from Elsevier.

Apart from the extruded channel monoliths, the majority of the reports found on microstructured ceramic membrane reactors employ microstructured hollow fiber membranes to achieve a high surface area to volume ratio and reduced wall thickness to minimize resistance to transport. For example, ceramic hollow fiber membranes with designed structures facilitating catalyst deposition were developed using a viscous fingering induced phase inversion process [32]. The type of bore fluid results in different membrane structures [33], such as conical-shaped microchannels that can act as a structured substrate where catalyst can be deposited for the catalytic reaction to take place. The microchannels inside the hollow fiber membranes could significantly enhance mass transfer and generate a large geometrical surface area that substantially improves the contact between reactants and catalysts. Such hollow fiber membranes were applied in OCM using Bi1.5Y0.3Sm0.2O32δ (BYS) as the catalyst is shown in Fig. 5 10B. In such a membrane reactor for OCM, the oxygen can be distributed along the reactor, ensuring that the local oxygen concentration in contact with the hydrocarbon at any point in the reactor is lower compared to a co-feed mode, favoring the selectivity towards the methane coupling reaction over the combustion [33 37]. Moreover, by distributing the oxygen along the reactor, the formation of hotspots is minimized and results in improved heat management.

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FIGURE 5–10 Schematic diagram of (A) microstructured hollow fiber membrane used as packed bed hollow fiber membrane microreactor and (B) catalytic hollow fiber membrane microreactor where the BYS catalyst can be deposited in finger-like microchannels. Reprinted from N.H. Othman, Z. Wu, K. Li, A micro-structured La0.6Sr0.4Co0.2Fe0.8O3 2 δ hollow fibre membrane reactor for oxidative coupling of methane, J. Memb. Sci. 468 (2014) 31 41 with permission from Elsevier.

5.3 Examples of microstructured membrane reactors In the following, the benefits of close interaction for mass transport of reactive species from the membrane into the reaction zone and/or vice versa in terms of a controlled reactant supply or reaching conversion or selectivity beyond the thermodynamic constraints are demonstrated based on examples of microstructured membrane reactors employing polymeric, ceramic, and as well metallic membranes. Depending on the membrane properties in terms of obtainable selectivity and typical application range, the various membrane types are relevant for different reactions. An overview is given in Table 5 1.

5.3.1 Polymeric In microstructured membrane reactors employing polymeric membranes, the membrane in most cases does not provide a selective function. Also, such polymeric membrane reactors may only be used in processes conducted under mild conditions because of the membrane instability at high temperatures, aggressive solvents/chemicals, and oxidative conditions compared to their inorganic or metallic counterparts. In a typical polymeric membrane reactor, the membrane divides two-reactor compartments and is controlling a gas supply that is already in its pure form (e.g., pure H2, O2) to a chemical reaction, such as hydrogenation or oxidation reactions. In such a system the gas supply side is often supplied in dead-end mode. Alternatively, the membrane can be used to continuously remove one of the products to increase yields through the shift in the chemical equilibrium or to suppress possible sequential side reactions. For systems that involve a liquid reactant feed and one or more gaseous products, this can be achieved with polymeric membranes acting as a stripper/scrubber. The removal of the gaseous component then also increases the liquid residence time. Selective membranes or unselective porous membranes can be applied to achieve this, depending on the pressure difference and the capillary forces keeping the liquid away from polymer membrane pores. In terms of the supply of reactants, Park and Kim investigated the oxidative Heck reaction through a dual-channel membrane microreactor, in which oxygen was supplied through a

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Table 5–1

Membrane types relevant for integration in membrane reactors. Polymeric membranes

Typical material AF-2400, PDMS, Nafion Relevant for which H2, O2, and CO2 gas amongst others. Many gases can be supplied through polymers Transport Solution-diffusion mechanism Temperature ,100 C range Selectivity Low Relevant for which (De)hydrogenation reaction Oxidations Coupling reactions Alkylations

Microporous membranes

Dense metallic membranes

Ceramic mixed conducting membranes

Zeolites, silica H2O, H2, O2

Palladium-alloys H2

Perovskites O2, H2

Molecular sieving

Solution-diffusion

Solution-diffusion

100 C 600 C

300 C 800 C

400 C 900 C

Moderate Condensation reactions CO selective oxidation

High Steam reforming Water-gas shift (De)hydrogenation

High Selective oxidations Oxidative dehydrogenation Oxidative Coupling of Methane

poly(dimethylsiloxane) (PDMS) film from one microchannel to another to increase the yield and selectivity by the increased gas 2 liquid contact [38]. The Heck reaction is receiving much attention in synthetic chemistry for its ability to form C 2 C bonds between various aryl and alkene carbons at relatively mild conditions. Their work shows a significant improvement relative to traditional batch reactors and conventional segmental microreactors in terms of yield, selectivity, and reaction time. Moreover, it also allows independent control of the flow of the gaseous reagent. Lamberti et al. [39] showed that the gas (specifical air) permeability of PDMS can be tuned for specific microfluidic applications by adjusting the ratio between the oligomers and the curing agent. Park et al. [40] reported a dual-channel microstructured reactor for photosensitized oxygenation. A PDMS membrane was used as contacting phase between the liquid products and the gas phase, supplying the required amount of O2 to keep the liquid saturated. The concept allowed significantly shorter reaction time (minutes compared to hours required by batch reactor) and improved productivity. Along the same line, Liu et al. developed a stacked catalytic microstructured membrane reactor for the gas 2 liquid 2 solid multiphase catalytic nitrobenzene hydrogenation employing PDMS or Nylon membranes with coated Pd catalysts [12,41,42]. Their stacked configuration was structurally simple and easily expandable, and conversion remained stable with an average value higher than 90% during the 50 h operation, demonstrating good catalytic activity and durability. Fig. 5 11 shows a schematic of the preparation of an ultrathin freestanding nylon membrane in the microchannel system realized via an interfacial polymerization reaction on the interface between two immiscible reactants (hexamethylenediamine and a solution of adipoyl chloride in xylenes). The two reactants were simultaneously introduced from two inlets into the microchannel and stable and parallel laminar flows were formed.

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FIGURE 5–11 Schematic of the preparation of an ultrathin freestanding nylon membrane in a microreactor. Reprinted with permission from M. Liu, X. Zhu, Q. Liao, R. Chen, D. Ye, G. Chen, et al., Catalytic membrane microreactors with an ultrathin freestanding membrane for nitrobenzene hydrogenation, ACS Appl. Mater. Interfaces 12 (2020) 9806 9813. Copyright (2020) American Chemical Society.

An interfacial polycondensation reaction occurred immediately to yield an ultrathin freestanding nylon membrane along the longitudinal direction in the middle of the microchannel, which divided the main microchannel into two separate and uniform microchannels. In the last step, a palladium catalyst layer was coated on one side of the nylon membrane by the Layer-by-Layer (L-b-L) self-assembly technique and in situ reductions. L-b-L selfassembly has been recently reported as a suitable approach to incorporating catalysts into thin membranes within microstructured reactors [12,41 44]. In membrane reactor systems the membrane material must show good mechanical and chemical stability. The combination of high gas permeability and chemical resistance due to the amorphous fluorinated nature could make Teflon AF-2400 a perfect material for membrane microreactors. Teflon AF has also excellent UV light transmission which allows for UV-initiated chemical reactions. In 2010, Ley et al. [45] published ozonolysis of a series of alkenes, showing the potential of this concept for flow-through chemical reactors involving gas-liquid contact. In this case, a Teflon AF-2400 tube (i.d. 5 600 μm, o.d. 5 800 μm) was used to supply ozone to various bleaching solutions. The first report applying a Teflon AF-2400 tube-in-tube membrane reactor for the production of carboxylic acids was reported by the same group in 2011 [46], supplying CO2 to a liquid reagent (Grignard compound). Furthermore, the same concept used to achieve key C-C, C-N, and C-O bond forming and hydrogenation reactions were published in 2015 [47]. The tube-in-tube approach was used for the fabrication of various APIs (active pharmaceutical ingredients). Depending on the reaction, different gases were supplied through the AF-2400 permeable tube (CO2, O2, H2, CO, C2H2, C2H4, CH2O, F2, O3, Cl2, CH2N2, and NH3) [48].

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Also, in the heterogeneous Pd-catalyzed hydrogenations, homogenous Cu(I)/TEMPO alcohol oxidations [49], and for aerobic oxidation of benzyl alcohol on an Au-Pd/TiO2 catalyst, AF-2400 membranes were applied [50,51]. In this latter work, the AF-2400 allowed for operation with a 20 bara pressure difference between the gas and the liquid phases at 120 C. The high oxygen pressure was shown to have a positive effect on reactor performance. A conversion of benzyl alcohol of 70% with 71% selectivity to benzaldehyde was obtained at 1150 gcat
s/galcohol, 8.4 bara oxygen pressure, and 10 bara liquid pressure [50]. The ease of scale-up was demonstrated by increasing the liquid channel width by approximately ten times, which increased reactor productivity by a factor of eight. Flow maldistribution and non-uniform catalyst packing are indicated as possible reasons for the non-linear behavior observed in the upscaling. Efficient production of 5-hydroxymethylfurfural (HMF), through the dehydrogenation of fructose, was enhanced by liquid-liquid extraction in a membrane dispersion microreactor. In this system, a Hastelloy alloy (type C276) microfiltration membrane (3 3 1 3 0.3 mm) with an average pore size of 5 μm was placed between a mixing chamber and a crossflow channel (10 3 1 3 0.6 mm) [52]. Effectively preventing the sequence side reaction improved extraction performance, and the HMF selectivity and yield were hence improved. The reaction time decreased from 60 min in a traditional stirred reactor to 4 min in the microreactor, enhancing space-time yield by 3 orders of magnitude [52]. Jeong et al. [53] reported the possibility to produce HMF intermediates such as 2,5-diformylfuran (DFF) and 2,5-dimethylfuran (DMF) by integrating a gas-permeable Teflon AF tube (i.d. 5 610 μm, o.d. 5 750 μm, length 5 100 cm) in a catalytic microreactor (Fig. 5 12). By supplying O2 or H2 through the Teflon tube, DFF and DMF were respectively obtained, achieving a high yield (.80%) for both reactions.

FIGURE 5–12 Oxidation and hydrogenolysis of HMF to DFF and DMF under gas liquid solid ternary phase reaction in a tube-in-tube microreactor. Reprinted from G.-Y. Jeong, A.K. Singh, S. Sharma, K.W. Gyak, R.A. Maurya, D.-P. Kim, One-flow syntheses of diverse heterocyclic furan chemicals directly from fructose via tandem transformation platform, NPG Asia Mater. 7 (2015) e173 e173 with permission under Creative Commons license CC BY 4.0.

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To demonstrate the concept of continuous-flow microreactors for sustainable chemical transformations and environmental remediation, Rownaghi et al. reported the use of catalytically active asymmetric hollow fibers membranes for heterogeneous catalyst and continuous-flow reactor assemblies. These microreactors consist of bifunctional groups embedded in the hollow fiber wall, making them bifunctional catalysts for cooperative interactions (i.e., acid-base catalysis). The proof-of-concept was reported for aldol and nitroaldol condensation [54], demonstrating the incorporation of the hybrid organic-inorganic catalyst in the fibers, via aminopropyltriethoxysilanebased grafting. The same approach was also successful for Heck coupling [55], CO2 cycloaddition and hydroxyalkylation of aniline [56], and tandem reactions of glucose and fructose to HMF [57]. The reactor performance was found to be stable up to 150 C but swelling and plasticization phenomena (especially in presence of polar aprotic solvents and aromatics) led to instability of the immobilized catalysts, resulting in leaching of active species (e.g., Pd nanoparticles). In their latest work [58], a new approach is proposed to increase the stability of the metallic catalyst by permanently immobilizing Pd nanoparticles onto a PDMS surface through metal coordination chelation, aiming at the reduction of 4-nitrophenol, see Fig. 5 13. A porous polyamide-imide hollow fiber support was used in this work. Organic compounds are expected to permeate through the PDMS layer via the solution-diffusion mechanism, while the transport of Pd nanoparticles is prevented. This approach can lead to a continuous-flow catalytic system that can be reused over multiple

FIGURE 5–13 Schematic description of the hollow fiber module employed as a continuous-flow microreactor for the reduction of 4-nitrophenol. Adapted from Y. He, N. Cheshomi, S.M. Lawson, A.K. Itta, F. Rezaei, S. Kapila, et al., PDMS/PAI-HF composite membrane containing immobilized palladium nanoparticles for 4-nitrophenol reduction, Chem. Eng. J. 410 (2021) 128326 with permission from Elsevier.

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FIGURE 5–14 (A) CAD sketch of the solvent flow with the two membranes and (B) magnification of a single channel bend with the overlying feed regions for oxygen and hydrogen. Reprinted from M. Selinsek, M. Kraut, R. Dittmeyer, Experimental evaluation of a membrane micro channel reactor for liquid phase direct synthesis of hydrogen peroxide in continuous flow using Nafions membranes for safe utilization of undiluted reactants, Catalysts 8 (2018) with permission under Creative Commons license CC BY 4.0.

cycles without significant loss in the catalytic activity. Complete (100%) 4-nitrophenol conversion has been obtained for a single pass with a short residence time (2 3 s). An exemption with respect to the implementation of the catalytic function is demonstrated by Selinsek et al. [59,60]. A microstructured membrane reactor system for continuous liquid phase H2O2 direct synthesis was designed to reduce safety issues by separate dosing of the gaseous reactants via a 180 μm Nafion membrane into a liquid-flow channel filled with a Pd-based catalyst (carbon or TiO2 support). Fig. 5 14 shows a schematic of the microstructured membrane reactor and a top view of the geometry of the meander-shaped microchannel and the relative position of the two membrane areas supplying repetitively small amounts of dissolved H2 and O2. Productivity is increased by the enhanced mass transfer attainable in microchannels, and by multiple, consecutive saturation of the liquid (water with 0.15 to 4 mM NaBr and 0.15 mM H2SO4) with the reactants over the length of the bent 0.5 mm deep reaction channel. Continuous operation of the reactor at pressures up to 50 bara showed the feasibility of this system to replace the energy-demanding highly complex anthraquinone process. During the experiments, the accurate control of the reactant ratio (pH2/pO2) was found to be crucial in order to maximize product yield. Thereby, yields above 80% were achieved, and it is reasonable to believe that further improvements can be obtained through the optimization of process parameters.

5.3.2 Metallic membranes Dense palladium membranes are mainly used for selective separation of hydrogen from a reaction system, to intensify the process by shifting the boundary conditions of thermodynamic equilibria towards product formation and recovering the highly pure hydrogen as the reaction product. Their high hydrogen permeability and selectivity combined with their good

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thermal stability at the operating temperature of many reactions (373K 973K) [61] make them attractive for processes such as SR of methane, ethanol or methanol, dehydrogenation of cyclohexane, methylcyclohexane and perhydro-dibenzyltoluene and the water-gas shift reaction (WGS). Usually, Pd-alloys (Ag, Cu, Fe, Ni, Pt, and Y) are used as membrane material, as alloying improves the thermal and mechanical stability as well as the chemical resistance towards impurities such as unsaturated hydrocarbons, sulfur, chlorine, carbon monoxide and other components that are poisonous for the membrane [62 69]. An alloy also reduces the overall membrane costs and can as well increase the permeability and avoid embrittlement in the hydrogen atmosphere at low temperatures. Since the hydrogen permeation flux is inversely proportional to the membrane thickness, the thickness also strongly impacts permeability and cost and is therefore minimized to the extent that the long-term chemical and mechanical stability allows [70]. Much work has been reported on either microstructured reactors or membrane reactors for the above-mentioned reactions of SR, dehydrogenation, and WGS [18,70 84]. Already in 2000, the status of microstructured palladium membrane reactors for H2 production was reviewed [28], but since that time many new developments have appeared in terms of design and membrane integration. Most of the presented reactor systems have been examined by feeding pure hydrogen and/or hydrogen mixtures into the module in order to investigate membrane integration, stability, and/or effects of different gas components on the membranes [85 88]. This is because a microstructured configuration is very well-suited for the investigation of surface inhibition effects caused by co-existing species in the gaseous feed under different operating conditions in the absence of any flux-limiting factors such as concentration polarization [14,89]. The microstructured membrane modules that have been operated under reaction conditions are few and almost exclusively systems in which the gaseous reactants react on a solid catalyst. Microstructured membrane reactors for SR of methane and ethanol are mainly designed as on-board reformers to provide highly pure hydrogen for proton-exchange membrane fuel cells (PEMFC). Böltken et al. designed and successfully tested an ultra-compact microstructured methane steam reformer where a 12.5 μm thick dense palladium membrane is integrated into the system by laser welding (Fig. 5 4) [17]. A high H2/N2 permselectivity of 1000 16,000 at 500 C and 6 bar feed pressure could be obtained. Throughout the reforming experiments, the purity of the recovered hydrogen was higher than 99.5%. Reaction experiments at 500 C and 12 bar feed pressure with a W/F ratio of 0.33 gCat h/molCH4 resulted in a maximal methane conversion of 87% and a hydrogen recovery of 92% [17]. In addition, microstructured catalytic hollow fiber Pd-based membrane reactors were evaluated for H2 production through SR of various hydrocarbons, primarily for small-scale, automotive, applications [90]. Due to challenges related to assembling the reactor and the formation of a stable and active catalyst layer, these systems seem less relevant for largerscale applications. For example, Rahman and Garcia-Garcia developed catalytic hollow fiber membrane microreactors (CHFMMR) for pure hydrogen production from ethanol SR [91] and the WGS reaction [92 94]. The outer surface of the employed hollow fibers was coated with a 5 μm dense Pd/Ag membrane via the electroless plating technique. A 10% CuO/CeO2 catalyst gave the highest catalytic activity and was deposited inside the Al2O3 hollow fiber by

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the sol-gel Pechini method for the WGS reaction. Yttria stabilized zirconia hollow fibers were used as a substrate for a 10 wt.% NiO/MgO CeO2 ethanol SR catalyst. The use of microstructures and selective removal of hydrogen from the reaction system resulted in an improved reactor performance compared to the corresponding catalytic hollow fiber micro reactor (not incorporating any membrane; CHFMR) and fixed bed reactor showed (FBR). Regarding the ethanol SR, the hydrogen production of the CHFMMR was two times higher than that of the CHFMR and even three times higher than that of the FBR. For the WGS reaction, the CO conversion of the CHFMMR increased by 20% compared to a CHFMR, whereby the hydrogen flux of the CHFMR triples with respect to an FBR. The microstructure of the hollow fibers improved the surface-to-volume ratio, which lead to an intensification of the interfacial phenomena as well as improved heat and mass transfer. The separation of hydrogen by the palladium membrane shifts the thermodynamic equilibria towards product formation. Considering the performance of the Pd/Ag membranes of the CHFMMRs under reaction conditions, the membranes remained stable and showed high hydrogen permeability and nearly infinite hydrogen selectivity for temperatures between 200 C and 500 C, though for relatively short periods of time (24 h). Microstructured membrane reactors are also used for equilibrium-limited dehydrogenation reactions for enhancing conversions. Alternatively, due to the removal of hydrogen from the reaction side, they have the potential to provide the yield of a conventional process while operating the system under milder conditions [23,95 113]. This could potentially decelerate the catalyst deactivation observed in the conventional dehydrogenation of light alkanes. Additionally, downstream separations are simplified as most of the hydrogen is separated in situ. For the dehydrogenation of cyclohexane to benzene (300 C, ambient pressure), Yamamoto et al. investigated the effect of microchannel size by inserting a stainless-steel rod into a tubular membrane reactor, consisting of a porous α-alumina tube coated with the palladium membrane [114]. By varying the diameter of the stainless-steel rod, they found that the yield with the largest rod was about twice that of the system without rods, which could be attributed to the increase in the membrane surface area per volume of the Pd-membrane reactor. Kreuder et al. designed, fabricated, and successfully tested a planar microstructured membrane reactor with an integrated catalytic fixed-bed for heat storage by methylcyclohexane dehydrogenation as a system component in a Liquid Organic Reaction Cycle using a 1 wt.% Pt/Al2O3 catalyst (dp 5 200 300 μm) [115]. The performance of the microstructured membrane reactor was tested for different membrane module configurations, where the membrane support, as well as the active amount of catalyst, was varied. A significant reduction of mechanical stress on the membrane could be observed by using membrane support. The experimentally obtained conversion of methylcyclohexane exceeded the equilibrium conversion corresponding to the given conditions (T, P, C) up to 17% for all module configurations [116]. As the hydrogen recovery factor was too low for the desired application, Cholewa et al. conducted further studies to optimize the ratio of membrane area to catalyst mass. The resulting module enabled a hydrogen recovery slightly above 80% [22]. Hatim et al. [117] tested a compact multifunctional Pd/alumina hollow fiber membrane reactor for the dehydrogenation of methylcyclohexane. The purity of the separated hydrogen

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exceeded 99.9%. Nevertheless, compared to a FBR or porous membrane reactor that achieves 25% and 50% methylcyclohexane conversion, respectively at 610 C, the performance of the hollow fiber membrane reactor was deemed rather poor; i.e., 26% conversion. The authors assumed that the removal of the generated hydrogen in the membrane reactor leads to increased conversion, but accelerates the formation of carbon that blocks the surface of a catalyst and/or accumulates on the membrane surface as well. In contrast to the abovementioned studies, the development of a coking-resistant catalyst with a great catalytic activity was therefore stated as a prerequisite.

5.3.3 Zeolite membranes Zeolites are ideal membrane materials because they have a uniform, molecular-size pores, and excellent thermal, mechanical, and chemical stability [118]. Zeolite membranes are typically composed of an intergrown polycrystalline thin zeolite layer supported by porous support, normally asymmetric alumina support. Because of their good stability, zeolite membranes are of particular relevance for separations under high temperature and pressure, or chemically challenging conditions. However, preparing large areas of high-quality zeolite membranes is challenging. This is because defects between the intergrown zeolite crystals that are only 2 3 nm are sufficient to severely degrade the membrane separation properties. Innovative applications that include zeolite films or small-scale applications such as micro membranes or reactors and sensors have therefore recently been explored. One of the earliest studies employing a zeolite membrane in a microstructured reactor configuration reported on the Knoevenagel condensation reaction of benzaldehyde and ethyl cyanoacetate to produce ethyl 2-cyano-3-phenylacrylate [119,120]. Hydrophilic ZSM-5 membranes, grown on the backside of a micromachined porous multi-channel plate (Fig. 5 15), were employed for the selective removal of the water byproduct from the reaction. Compared to a conventional FBR, the microstructured membrane reactor displayed both supra-equilibrium conversion and better product purity owing to the membrane separation performance. In separate pervaporation experiments, the 30-μm thick ZSM-5 zeolite membrane (Si/Al ratio of 30) exhibited a separation ratio of water over benzaldehyde of 240,000 in a 1 wt.% water benzaldehyde solution. The same authors reported the application of a similar ZSM-5 membrane reactor for selective oxidation of aniline by hydrogen peroxide to azoxybenzene [121]. A TS-1 catalyst was deposited in the nanochannels. The high surface area to volume ratio achieved, 3000 m2/m3, facilitated greatly the selective water removal and this reduced the TS-1 catalyst deactivation. An improvement in the product yield and selectivity towards azoxybenzene was also observed. The fabrication of such HZSM-5 micro membranes and their performance as proton conducting membranes for a miniature hydrogen PEMFC and direct methanol fuel cell (DMFC) was as well investigated [122 124]. A schematic drawing of the zeolite membrane-electrode assembly (MEA) is shown in Fig. 5 16. Supported and self-supporting ZSM-5 membranes (Si/Al 5 25) were evaluated for their performance. The supported ZSM-5 membrane was prepared by seeding and secondary regrowth method on cellulose paper,

FIGURE 5–15 (A) Image of the multi-channel plate, (B) SEM magnification of the channel cross-section, and (C) SEM micrograph of the ZSM-5 membrane cross-section grown on the backside of the multi-channel plate. Adapted from S.M. Lai, C.P. Ng, R. Martin-Aranda, K.L. Yeung, Knoevenagel condensation reaction in zeolite membrane microreactor, Microporous Mesoporous Mater. 66 (2003) 239 252. Copyright permission 2003 Elsevier.

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FIGURE 5–16 Scanning electron microscope pictures of the self-supporting zeolite membrane showing (A) the array of free-standing micro membranes fabricated on silicon, and (B) the cross-section of the individual micro membranes. (C) Schematic drawing of the zeolite membrane-electrode assembly. After W. Han, S.M. Kwan, K.L. Yeung, Zeolite proton conducting membrane for micro fuel cell applications, Top. Catal. 53 (2010) 1394 1400.

whereas the self-supporting zeolite membrane consisted of an array of micro membranes on a silicon substrate. The self-supporting HZSM-5 membrane had an open-circuit voltage of 0.77 V compared to 0.90 V for the supported HZSM-5 membrane and 0.98 V for Nafion. The differences are attributed to the contact between the electrode, catalyst, and membrane layers in the assembly. The performance of the hydrogen PEMFC displays comparable performance to Nafion MEAs for the similar operating condition, whereas the DMFC performed worse than expected. Even though further membrane improvements are thus required, the results are encouraging because it demonstrates for the first time the use of zeolite micro membranes in fuel cell applications [122]. Zeolite membrane microreactors were as well applied for the alkylation of benzene with propylene to form cumene [125]. In this flow-through reactor system, however, the 1 μm-thick β-zeolite film merely acts as a porous catalyst as the film in itself does not incorporate any (selective) removal or supply of gases. Even though zeolite membrane reactors are frequently applied in other equilibrium-controlled reactions, such as the WGS reaction for the conversion of CO to H2 [126], or the selective CO oxidation reaction for removal of CO from H2-rich gas streams [127], there are surprisingly few recent papers on the use of zeolite membranes employed in microstructured reactors.

5.3.4 Ceramic oxygen and proton conducting membranes Because of the high operating temperature typically required for ceramic membranes, membrane reactors employing ceramic membranes have been mostly researched for reactions such as the SR of hydrocarbons for the production of hydrogen, partial oxidation reactions

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producing syngas, or for methane coupling reactions or dehydro-aromatization reactions producing higher hydrocarbons [128]. Franca et al. have applied microtubular membranes made of the mixed oxygen ion and electron-conducting perovskite, La0.6Sr0.4Co0.2Fe0.8O32δ (LSCF) for oxygen permeation and hydrogen production by membrane-based SR [129]. The dense LSCF microtube membranes were manufactured by the phase-inversion/sintering method and had an outer diameter and inner diameter of 1.5 and 1 mm, respectively. Even though their membrane failed after 400 h of operation, they were able to demonstrate syngas production coupled with water splitting. The system was proven since it was confirmed that the production of hydrogen from water splitting did not originate from transient surface reactions, but from steady oxygen permeation across the membrane. With respect to OCM, CHFMMR, using Bi1.5Y0.3Sm0.2O32δ as the catalyst, were investigated [33,37]. The conical-shaped porosity inside the prepared LSCF hollow fiber membranes significantly reduced mass transfer resistance and generated a high geometrical surface area, which substantially improved the contact between reactants and catalysts. Moreover, the direct delivery of dissociated and ionized oxygen from the membrane towards the reaction sites could limit the non-selective oxidation reaction, which thereby improved the C21 yield. The same authors developed dual-phase BYS-La0.8Sr0.2MnO32δ (LSM) ceramic hollow fiber membranes for the same application [130]. Even though strictly not perceived as micro from the definition of a microreactor, it is worth mentioning the all-ceramic mixed-conducting membrane (MCM) reactor for combustion of natural gas in an N2-free environment facilitating CO2 capture [29]. The assembled MCM module was previously shown in Fig. 5 8, but Fig. 5 17 details the all-ceramic monolith and a magnification of one monolithic channel with 50 μm dense mixed ionic electronic conductor membrane film that can transport oxygen with 100% selectivity. The length of the monolith is about 170 mm.

FIGURE 5–17 (A) All-ceramic MCM monolith, and (B) magnification of one monolithic channel with a 50 μm dense mixed ionic electronic conductor membrane film. Reprinted from S.G. Sundkvist, S. Julsrud, B. Vigeland, T. Naas, M. Budd, H. Leistner, et al., Development and testing of AZEP reactor components, Int. J. Greenh. Gas Control 1 (2007) 180 187 with permission from Elsevier.

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The MCM reactor was tested at up to 10 bar and a temperature of ca. 850 C. Oxygen flux data obtained were in good agreement with modeling results. Using this model an oxygen production rate of the order of 37 mol/(m3 s) of O2 is projected for the membrane in the advanced power plant process design. Owing to the compactness of the monolithic structure, a highly efficient heat transfer was obtained, determined to be 100 W/(m2 K). Similarly as for the zeolite membranes though, few papers on the use of ceramic membranes employed in microstructured reactors have appeared in recent years. Also, the development of the Advanced Zero Emissions Power plant (AZEP) incorporating the MCM monoliths was abandoned despite the high efficiency of the concept. Possible reasons include challenges related to sealing, manufacturing, and material stability at typically high operating temperatures.

5.4 Conclusion and future trends Within this chapter, different strategies for built-up microstructured membrane reactors have been summarized. The challenges of leak-tight integration of membranes, catalysts, and the manufacturing technique of the microstructure itself have been addressed and exemplified. Benefits of close integration for mass transport of reactive species from the membrane into the reaction zone and/or vice versa to reach conversion beyond the thermodynamic constraints have been demonstrated. Very high membrane surface areas can be embedded into microstructured reactors. Thus, membrane surface and catalyst active sites can be effectively utilized in case of fast reactions, high permeation rates, and whenever a high heat transfer is associated with the reaction. Nevertheless, because thermal expansion coefficients need to match between catalyst coatings, membrane, and construction materials, most applications can be found in pure metallic constructions, or in polymer-metal assemblies where mechanical stress is tolerated due to the membrane flexibility. Reviewing the advantages, it can be expected from the membrane microreactor technology that small-scale and decentralized production of chemicals and/or energy applications can be facilitated in the future. Finally, additive manufacturing technology presents a high potential for use in various prototyping and fabrication of many materials. Thanks to the ability to create almost any geometrically complex shape or feature across different scales, additive manufacturing can as well be of large relevance for microstructured membrane reactors. For example, microextrusion 3D printing was revealed to be an effective tool to produce green asymmetric ceramic structures [131] in a material composition relevant to dense membranes for H2 purification. No data on membrane performance of these green samples after sintering, however, is included in the work. Lee et al. reviewed recent developments in enhancing membrane module design with 3D printing technology [132]. Also [133] presents an overview of exciting developments in 3D-printed membrane and module materials, though in the water-related field. Although further developments are required for accurate fabrication on the nano-scale [134], 3D printers are rapidly developing and these methods need further exploration with respect to microstructured membrane reactors.

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Nomenclature Acronyms AZEP BYS CHFMR CHFMMR DFF DMF DFMC FBR HMF LSM LSCF MCM MEA MEMS OCV OCM PDMS PEMFC PMR SLM SR SS TIG WGS YSZ 3D

Advanced zero-emissions power Bi1.5Y0.3Sm0.2O32δ Catalytic hollow fiber microreactor Catalytic hollow fiber membrane microreactor 2,5-diformylfuran 2,5-dimethylfuran Direct methanol fuel cell Fixed bed reactor 5-hydroxymethylfurfural La0.8Sr0.2MnO3-δ La0.6Sr0.4Co0.2Fe0.8O32δ Mixed-conducting membrane Membrane-electrode assembly Microelectromechanical systems Open-circuit voltage Oxidative coupling of methane Poly(dimethylsiloxane) Proton-exchange membrane fuel cells Porous membrane reactor Selective laser melting Steam reforming Stainless steel Tungsten inert gas Water-gas shift Yttria stabilised zirconia 3-dimensional

Symbols d Dimension of a channel Dm Diffusion coefficient t Diffusion time

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δ

hollow fibre membranes for

[33] N.H. Othman, Z. Wu, K. Li, A micro-structured La0.6Sr0.4Co0.2Fe0.8O32δ hollow fibre membrane reactor for oxidative coupling of methane, J. Membr. Sci. 468 (2014) 31 41. [34] A. Cruellas, T. Melchiori, F. Gallucci, M. van Sint Annaland, Advanced reactor concepts for oxidative coupling of methane, Catal. Rev. 59 (2017) 234 294. [35] Y.P. Lu, A.G. Dixon, W.R. Moser, Y.H. Ma, U. Balachandran, Oxygen-permeable dense membrane reactor for the oxidative coupling of methane, J. Membr. Sci. 170 (2000) 27 34. [36] Y.P. Lu, A.G. Dixon, W.R. Moser, Y.H. Ma, U. Balachandran, Oxidative coupling of methane using oxygen-permeable dense membrane reactors, Catal. Today 56 (2000) 297 305. [37] N.H. Othman, Z. Wu, K. Li, An oxygen permeable membrane microreactor with an in-situ deposited Bi1.5Y0.3Sm0.2O32δ catalyst for oxidative coupling of methane, J. Membr. Sci. 488 (2015) 182 193. [38] C.P. Park, D.-P. Kim, Dual-channel microreactor for gas 2 liquid syntheses, J. Am. Chem. Soc. 132 (2010) 10102 10106. [39] A. Lamberti, S.L. Marasso, M. Cocuzza, PDMS membranes with tunable gas permeability for microfluidic applications, RSC Adv. 4 (2014) 61415 61419. [40] C.P. Park, R.A. Maurya, J.H. Lee, D.-P. Kim, Efficient photosensitized oxygenations in phase contact enhanced microreactors, Lab Chip 11 (2011) 1941 1945. [41] M. Liu, X. Zhu, Q. Liao, R. Chen, D. Ye, G. Chen, et al., Catalytic membrane microreactors with an ultrathin freestanding membrane for nitrobenzene hydrogenation, ACS Appl. Mater. Interfaces 12 (2020) 9806 9813. [42] M. Liu, X. Zhu, Q. Liao, R. Chen, D. Ye, G. Chen, et al., Preparation of a catalyst layer by layer-by-layer self-assembly for plate-type catalytic membrane microreactors, Ind. Eng. Chem. Res. 59 (2020) 15865 15874.

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6 Pervaporation membrane reactor Sergio Santoro1, Alberto Figoli2, Francesco Galiano2 1

DEPARTMENT OF ENVIRONMENTAL ENGINEERING, UNIVERSITY OF CALABRIA, RENDE, COSENZA, ITALY 2

INSTITUTE O N M EMBR ANE TECHNOLOGY (CNR-ITM), RE NDE, C OSENZA, ITALY

6.1 Introduction Membrane separation processes are of primary importance in facing some of the most critical issues of modern society including freshwater production, wastewater treatment, and environmental pollution remediation. The efficiency of membranes in separating ions, molecules, and microorganisms, employing an eco-friendly and cost-effective approach, spurred their range of applications in a variety of key sectors such as water treatment, gas separation, desalination, agro-food, and biomedicine. Pervaporation (PV) is a membrane process extensively studied that is becoming more and more a consolidated reality also at an industrial level. PV, in fact, is considered a clean technology able to operate a selective separation of volatile compounds from liquid streams. The observation of the phenomenon of PV dates back to the study of Kober [1] in the early twentieth century who coined the term from the words “permeation” and “evaporation” for defining the transport of a liquid through a collodion bag. The first commercial composite membranes [in polyvinyl alcohol (PVA) deposited on porous polyacrylonitrile (PAN) support] for the pervaporative separation of ethanol aqueous solutions, were developed in the early 1980s by the Gesellschaft für Trenntechnik [2]. In 1983, Brazil established the first dehydration plant with a capacity of 1200 L for the production of high purity ethanol [3]. From that time on, a series of initiatives, oriented toward the integration of PV into traditional distillation columns, saw the light in Asia and Europe. PV aims at separating liquid mixtures by exploiting the different solutions and diffusion of components, constituting the feed solution, through a dense (nonporous) membrane. A binary or a multicomponent liquid mixture is, therefore, put in contact with the active site of the membrane while the vaporized permeate is collected on the opposite side. The uniqueness of PV is the phase change (from liquid to vapor) occurring between the two sides of the membrane. The membranes can be characterized by a symmetric dense structure or consisting of a dense skin layer on a porous support (composite or asymmetric membranes). The driving force in PV is the chemical potential gradient between the two sides of the membrane (feed and permeate sides) that is created by applying a partial pressure difference of the permeating species. The difference in partial pressure can be achieved by either using a vacuum pump or using a sweeping gas that lower the partial pressure at the permeate side. Current Trends and Future Developments on (Bio-)Membranes. DOI: https://doi.org/10.1016/B978-0-12-823659-8.00002-2 © 2023 Elsevier Inc. All rights reserved.

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In Fig. 6 1 a graphic representation of a traditional PV unit is shown. The transport mechanism through PV membranes is generally described by the solutiondiffusion model introduced by Binning et al. in 1961 [4], and consists of three different steps: (1) sorption of the target compound into the selective layer of the membrane, (2) diffusion of the permeating species through the membrane thickness, (3) desorption into the permeate side as a vapor. The permeability (P) of a given compound through the membrane depends on its solubility (S) and diffusivity (D), according to Eq. (6.1): P5D
S

(6.1)

D is a kinetic parameter related to the transport rate of the permeating compounds through the membrane; while S is a thermodynamic parameter related to the amount of adsorbed permeant by the membrane at the equilibrium conditions [5]. The preferential permeation of target compounds depends on their chemical affinity with the material of the membrane selective layer. In this regard, the membranes employed in PV can be categorized into three main groups: hydrophilic, hydrophobic, and organophilic. Hydrophilic membranes are generally made of PVA, chitosan (CS), and PAN and find applications in the dehydration of organic solvents, displaying a greater affinity for polar molecules, such as water. Hydrophobic membranes mostly find application in the removal of organic compounds from water using hydrophobic membrane materials such as polydimethylsiloxane (PDMS), polyoctyl methylsiloxane (POMS), and poly(1-trimethylsilyl-1-propyne) (PTMSP).

FIGURE 6–1 Schematic representation of a PV unit.

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Finally, organophilic membranes are employed for the separation of an organic compound from an organic mixture. Besides membrane chemical properties, a crucial role in determining the separation performance of a membrane is also represented by its morphology (e.g., cavity size, membrane thickness) [6]. The size sieving effect can promote, for instance, the permeation of smaller molecules (such as water) through the membrane over other compounds with a larger molecular size. Polymers are generally the most diffused materials in the preparation of PV membranes. However, inorganic membranes can also be employed, particularly when excellent thermal stability and high mechanical strength are required [7]. Mixed matrix membranes (MMMs) are knowing a growing interest in PV applications. They can combine, in fact, the benefits of polymer materials (e.g., flexibility, low cost) with the advantages of inorganic nanofillers (e.g., high selectivity, high flux) [8]. Finally, a new class of two-dimensional (2D) materials (e.g., graphene oxide) can find also application in the preparation of PV membranes offering the possibility to produce ultra-thin selective layers [9]. Fig. 6 2 summarizes the different materials and the transport channels created in PV membranes. When compared to conventional separation processes, such as distillation, PV is considered a competitive technology in terms of energy requirement and separation efficiency. PV is, in fact, able to separate close-boiling liquids or azeotropic mixtures which can not be separated through common distillation. Moreover, PV can operate at milder temperatures preserving the integrity of thermolabile molecules. PV can count on a series of other benefits such as low environmental impact, flexibility, compact design, and ease of operation and control. PV membrane performance is typically evaluated in terms of permeate flux and separation factor. The total permeate flux (J) is expressed by Eq. (6.2) as: J5

m A
t

(6.2)

where m is the mass of the permeate, A is the membrane area and t is the permeate collection time. The separation factor (β) is defined as:   β5

CA CB PERMATE   CA CB FEED

(6.3)

where C is the concentration of the preferential and secondary components, A and B, in the feed and permeates side. The separation factor values can vary from 1 (nonselective membrane) to infinity. The higher is β, the higher the membrane separation performance which can depend on membrane swelling, operation temperature, vacuum degree, hydrodynamic conditions, or changes in membrane or component solubility [10].

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Inorganic membranes

Polymeric membranes

Feed Liquid

Pervaporation Membrane

Permeate Vapor

2D-material membranes

Mixed-matrix membranes

FIGURE 6–2 PV process and transport channels in various membrane materials. Reprinted from G. Liu, W. Jin, Pervaporation membrane materials: recent trends and perspectives, J. Membr. Sci. 636 (2021) 119557. https://doi. org/10.1016/j.memsci.2021.119557 with the permission of Elsevier.

The “pervaporation separation index” (PSI) is another parameter often employed in evaluating the overall membrane performance being the product of permeation rate and separation factor. It measures the ability of a membrane to separate a binary mixture at specific experimental conditions [11]: PSI 5

J β

6.2 Pervaporation membrane reactors A membrane reactor is a device where a membrane separation unit is combined with a chemical reactor. The device can simultaneously perform a reaction (esterification, steam

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reforming, hydrogenation. . .) and a membrane-based separation. The interest, also at the industrial level, in this type of process lies in the possibility of reducing capital and investment costs, offering also advantages in higher process performance [12]. Membrane reactors are particularly appealing because they are able to operate at mild conditions of temperature and pressure, reduce the formation of undesired by-products, and do not require further additives [13]. Membrane reactors allow the continuous removal of the products produced by the catalytic reaction so that shifting the equilibrium and enhancing the yield. Coupling of PV with reactors, in the so-called pervaporation membrane reactors (PVMRs), represents a great promise explored for both biochemicals and chemicals reactors. Numerous studies have been reported in the literature during the last years testifying to the continuous interest in this type of process (Fig. 6 3). In biochemical reactors, the product is generally obtained from a microbial conversion (e.g., the conversion of sugar for the production of bio-ethanol) or by highly specific enzymes immobilized in the membrane matrix or dispersed within the reactor. The application of PV in chemical reactors is strongly focused on esterification reactions but encompasses also condensation reactions (removal of water) [14], acetylation reactions [15], and etherification reactions [16]. First efforts in using PV in combination with an esterification reaction date back to 1991 [17]. In this type of reaction, where alcohol and a carboxylic acid react, water is obtained as a byproduct. The continuous removal of water from the reaction media using hydrophilic PV allows for obtaining a higher yield of reaction avoiding the use of a large excess of reactants. In PVMRs the membrane and the reactor can be placed in the same unit (Fig. 6 4A) or can be located in two different units (Fig. 6 4B). In the first case, the reaction and separation occur in the same place. This type of configuration is the most popular in biotechnology

FIGURE 6–3 Publications trend on the use of pervaporation in membrane reactors in the last 10 years. Data extrapolated from Scopus, July 2021; keywords: pervaporation, reactor.

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and bioprocesses [14]. In the second configuration, the products of the reaction are sent to an external PV unit. In this case, the stream of the reactor, containing reactants and products, enters the PV unit where the products or by-products are selectively removed while the retentate is recirculated back. The esterification reaction of several organic acids, for instance, has been studied in both configurations where the membrane was placed inside or outside the batch reactor. Bendict et al. [18] studied the esterification of lactic acid employing a closed-loop system of a batch catalytic reactor with a separate PV unit equipped with a GFT-1005 membrane (a composite membrane with a dense PVA active side). Lactic acid reacts with ethanol to generate ethyl lactate (an important chemical intermediate) and water. The results showed that the combination of the esterification reaction with PV (whose function was the stripping of water as a reaction by-product) greatly improved the reaction yield with a near-total conversion of lactic acid. Qing et al. [19] studied the esterification reaction between acetic acid and n-butanol using a catalytically active PVA membrane reactor located inside the reactor. The author studied different variables, including the reaction temperature, the catalysts loading, and the catalysts deposition on the membrane surface (as a dense or porous layer). From the results obtained, they found out that the membrane characterized by the porous catalytic layer performed better in terms of water removal offering a lower mass transfer resistance. A higher amount of catalyst had also a positive effect on the acetic acid conversion. After 45 h at 85 C, the acetic acid conversion was almost complete displaying a 43% higher conversion with respect to the equilibrium conversion. Depending on where the catalyst is located, two could be distinguished: the inert PVMRs (Fig. 6 5A) where the membrane is adjacent to the catalytic zone on the feed side, and the catalytic PVMRs. In this second case, the catalyst can be either packed in a bed membrane reactor (Fig. 6 5B) or embedded into the membrane matrix (Fig. 6 5C). In catalytic PVMR the role of the membrane is twofold since it does not act as a simple separation unit but also actively participates in the catalytic reaction. The membrane can take part in the reaction in different ways: 1. extractor membrane reactor (Fig. 6 6A): the membrane selectively removes one of the reaction products shifting the reaction equilibrium and enhancing the yield (such as in the case of esterification and hydrogenation reactions) [20];

FIGURE 6–4 Representative scheme of the two PVMRs configurations: (A) reactor and membrane in the same unit; (B) reactor and membrane in separate units.

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FIGURE 6–5 Catalytic PVMR configurations: (A) catalyst suspended in the membrane reactor, (B) catalyst packed in a bed membrane reactor, (C) a membrane with catalytic properties. Adapted from N. Diban, A.T. Aguayo, J. Bilbao, A. Urtiaga, I. Ortiz, Membrane reactors for in situ water removal: a review of applications, Ind. Eng. Chem. Res. 52 (2013) 10342 10354. https://doi.org/10.1021/ie3029625. Copyright 2021 American Chemical Society.

2. distributor membrane reactor (Fig. 6 6B): the membrane controls the concentration of a reactant involved in a successive reaction [21]; 3. the membrane controls the addition of one reactant in order to prevent the catalyst deactivation and to avoid, in exothermic reactions, the rise of the temperature (Fig. 6 6C) (such as in the case of partial oxidation of hydrocarbons). Polymeric membranes are the dominant materials employed in PVMRs. Among them, PDMS [22], PVA [23], polyimide [24], polyetherimide [25], and Nafion [26] are the polymers mostly employed for the preparation of organic membranes used in PVMRs. The selection of the nature of the polymer (hydrophilic or hydrophobic) occurs on the basis of the target compounds that need to be separated and recovered. In esterification reactions, for instance, hydrophilic membranes are typically employed due to their ability to remove water, a by-product of the reaction, from the reactor [27]. However, some studies reported also the use of hydrophobic membranes (e.g., PDMS) selective for the removal of the ester [22]. The combination of polymers with inorganic fillers, to obtain MMMs, has been also explored in PVMRs. Zeolite [28] and silica particles [29], carbon nanotubes [30], TiO2 [31], and graphene oxide [32] have been combined with several polymers and biopolymers (e.g., CS and sodium alginate) with the aim of increasing the free volumes in the membrane matrix facilitating the diffusion of permeating molecules and improving membrane selectivity [33]. Inorganic membranes are the less representative class of materials employed in PVMRs. However, some studies showed the use of zeolite-coated (H-USY zeolites)

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FIGURE 6–6 Role of the membrane in catalytic PVMRs to improve yield (A) or selectivity (B C). Reprinted from S.S. Ozdemir, M.G. Buonomenna, E. Drioli, Catalytic polymeric membranes: preparation and application, Appl. Catal. A Gen. (2006). https://doi.org/10.1016/j.apcata.2006.03.058 with the permission of Elsevier.

FIGURE 6–7 Surface (A) and cross-section (B) SEM pictures of an H-USY zeolite-coated membrane. Reprinted with permission from T.A. Peters, N.E. Benes, J.T.F. Keurentjes, Zeolite-coated ceramic pervaporation membranes; pervaporation 2 esterification coupling and reactor evaluation, Ind. Eng. Chem. Res. 44 (2005) 9490 9496. https:// doi.org/10.1021/ie0502279. Copyright 2021 American Chemical Society.

ceramic hollow fiber membranes in the esterification reaction between acetic acid and butanol [34]. In this case, the zeolite coating itself possessed catalytic properties resulting in efficiency in the esterification reaction with comparable performance with respect to the activity of bulk zeolite catalysts. Fig. 6 7A shows the surface of the zeolite-coated membrane obtained from a 20 wt.% zeolite dip coat solution. The zeolite coating was about 10 μm thick as visible by the SEM picture cross-section (Fig. 6 7B). The next paragraphs will review the most important applications of PVMRs providing an outlook on the future potential developments of this technology.

6.3 Fields of application PVMRs represent a very well consolidated technology in assisting esterification reactions. However, new impulses are driving the research in exploring new fields of application such

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as condensation reactions, acetylation reactions, and etherification reactions. Moreover, the increasing interest in products generated by biomass is leading to the interest of PVMRs in the production and purification of bio-alcohols.

6.3.1 Esterification reactions Esters, consisting of organic molecules characterized by alkoxy group (-OR) bonded with a carbonyl group (C 5 O), are widely distributed in nature occurring in fruits and flowers as flavor and aroma. At an industrial level, esters are usually synthesized via Fischer esterification (named also acid-catalyzed esterification) by refluxing a carboxylic acid and an alcohol in the presence of an acid catalyst as follows: H1

R1 COOH 1 R2 OH 2 R1 COOR2 1 H2 O:

Thus, this synthetic route leads to the formation of water as a by-product governing the equilibrium of the reversible reaction. Despite the prominence of Fischer esterification, this reaction is characterized by a moderate yield of conversion (ranging from 58% to 78%) at a slow rate (1 4 h under conventional reflux conditions) [35]. The synthesis of esters is usually performed in the excess reactant (tendentially alcohol) to avoid the backward reaction of hydrolysis. Nevertheless, this strategy compromises the economic viability of the synthesis due to the cost of the recovery of both the unreacted alcohols and the esters from a diluted solution. Alternatively, the continuous removal of water is a valid approach exploited to increase the conversion yield of reaction beyond the equilibrium. The traditional column distillation is feasible in separating solvents with a wide difference of volatility; especially beneficial when the reaction and distillation temperatures fit, otherwise it has been widely recognized as an energy-intensive practice. On the other hand, PVMRs have gained more attention to boost the esterification reactions in mild operative conditions by removing the water (i.e., by-product) or recovering the esters (i.e., product) from the ambient reaction. This concept has been patented in 1960 [36], but a relevant contribution to PVMRs came from Dams and Krug in 1991 purposing different designs for the hybridization of a distillation column with a PV unit in an esterification reactor [37]. This study demonstrated the advantages to treat the distillate with an effective hydrophilic PV membrane obtaining a retentate rich in alcohol recycled to the esterification reactor [37]. Since then, efforts have been focused on the reduction of the capital investment of PVMRs and on the improvement of the performance and the stability of the membranes to secure a practical implementation of the PV-aided esterification. Commercial available PVA-based membranes are considered the benchmark in the hydrophilic PV process because the presence of OH groups confers strong hydrophilicity and affinity toward water [38]. For example, the efficiency in solvents dehydration of PERVAP 2201 membranes by Sulzer Chemtech, made of a cross-linked PVA selective layer on a porous PAN supporting layer cast on a polyphenylene sulfide (PPS) nonwoven fabric

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[39], was exploited for the intensification of ethyl lactate production via esterification of lactic acid [40]. The removal rate of the produced water performed by the PVA-based membrane suggested the opportunity to shift continuously the conversion yield beyond the thermodynamic equilibrium of c. 20% hitting values ranging from 70% to 90% [40]. GFT1005 (Deutsche Carbone AG) and a TexSep 1 (Texaco Research) PVA-based membranes were successfully tested in the PV-aided esterification of lactic acid and succinic acid with ethanol to generate ethyl lactate and diethyl succinate [18]. Noteworthy, GFT-1005 secured also a yield of 99% in the acetylation of benzyl alcohol with acetic acid thanks to efficient dehydration of the ester at a flux of 0.54 kg/m2/h with a selectivity of 96% at 80 C [15]. Similarly, composite membranes are made of a selective PVA coating supported by a microporous polyethersulfone (PES) layer produced by Permionics Membranes Pvt. Ltd facilitated the synthesis of n-butyl lactate via esterification of the lactic acid with butanol [23]. Nevertheless, a serious drawback of PVA is the high risk of esterification of secondary alcohol groups in the backbone chain promoted by the acid, resulting in a decrease in the selectivity of the membrane [41]. Therefore, the modification of PVA-based is pivotal to improving the long-term stability and the performance of the membranes, such as the crosslinking to reduce the swelling and improve the chemical resistance of the polymer. Tendentially, cross-linked PVA membranes are characterized by shelf life and stable performance, but this chemical modification strongly impacts the permeation properties of the PVA reducing the transmembrane flux which is, however, counterbalanced by an improvement of the selectivity [42]. The advantages of PVA hybridization with inorganic nanoparticles have trigged the research on membrane preparation. In fact, a systematic study demonstrated that the equilibrium limit of 79% esterification of acetic acid with ethanol was easily overcome thanks to PVMRs based on PVA-zeolite MMMs [43]. Among the different zeolites (KA, NaA, CaA, and NaX), CaA was the most favorable active filler in facilitating the transport of water molecules [43]. Concisely, batch experiments demonstrated that the embodiment of CaA zeolites into a PVA membrane reduced the 50% time required to obtain a 95% conversion with respect to a bare PVA membrane [43]. Analogously, the conversion of acetic acid with ethanol into ethyl acetate at 70 C raised from 80% in 14 h to 85% in 13 h thanks to the PV-assisted esterification with sodium-alginate membrane reaching a value of 90% in 5 h using sodium-alginate MMMs load with Al-rich zeolite beta [28]. The cages of the nanofillers acted as molecular sieves and selective sites for the sorption of water molecules resulting in an improvement of the flux and the separation factor with respect to the bare sodium alginate membrane [28]. The chemical-physical and the morphological properties of the nanofillers are crucial in tuning the performance: NaAsodium alginate MMM showed a flux of 0.13 kg/m2/h with a βwater/ethanol of 1334 and a βwater/acetic acid of 991 [44], whereas the embodiment of zeolite-beta conferred to the sodium alginate membrane a flux of 0.22 kg/m2/h with a βwater/ethanol of 220 and a βwater/acetic acid of 900 [28]. In the case of sodium alginate MMMs, the effectiveness of the PV separation obeys the permeability/selectivity trade-off governed by the pore size of the cage of the zeolites

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(0.4 and 0.7 nm for zeolites A and beta, respectively) leading to a comparable performance in assisting the synthesis of ethyl acetate esterification [33]. In principle, MMMs offer the opportunity to move beyond the concept of extractor membrane reactor towards catalytic PVMRs by integrating the catalyst into/onto the polymeric network. This approach demands contradicting material properties since: a high permeability of the reactants is highly desirable to facilitate the role of the catalyst; whereas PVMRs facilitate the esterification shifting the equilibrium thanks to the selective removal of the products [45]. Nevertheless, the development of composite membranes accommodating the catalytic and separative purposes in different layers represents an interesting perspective for conflict resolution and optimization of the PVMRs [45]. Focusing on the case of study of the esterification reaction between acetic acid and butanol (Fig. 6 8A); the mechanism of working of a composite catalytic PVMRs is based on 3 steps: (1) facilitated diffusion of reactants (acetic acid and butanol) into the catalytic layer; (2) conversion of the alcohol and the acid into the ester with the production of water as a by-product; (3) rejection of the ester (butyl acetate) towards the feed and removal of the byproduct (water) under the effect of the selective layer [46]. With this regard, a catalytic layer of Amberlyst 15 (an ion exchange resin with a strongly acidic sulfonic group) was deposited via dip-coating onto a composite PVA/ceramic hollow fiber membranes showing promising results in the esterification reaction between acetic acid and butanol [47]. Despite the catalytic layer reducing the permselectivity of the membrane since it offered an extra resistance to the mass-transport; the catalytic activity of the membrane was 1.8
1028
m3/mol/s/gcat similar to the performance of the unsupported powder of catalyst [47]. However, the impact of the catalytic layer on the flux of the PVMR can be easily mitigated by optimizing its morphology inducing a microporous structure. For instance, an aqueous

FIGURE 6–8 (A) Role of the membrane in catalytic PVMR in the esterification reaction between acetic acid and butanol. (B) The conversion rate on the acetic acid conversion of esterification in different reactors. (A) Reprinted from T.A. Peters, J. van der Tuin, C. Houssin, M.A.G. Vorstman, N.E. Benes, Z.A.E.P. Vroon, et al., Preparation of zeolite-coated pervaporation membranes for the integration of reaction and separation, Catal. Today 104 (2005) 288 295. https://doi.org/10.1016/j.cattod.2005.03.065, with the permission of Elsevier; (B) Reprinted from W. Zhang, W. Qing, N. Chen, Z. Ren, J. Chen, W. Sun, Enhancement of esterification conversion using novel composite catalytically active pervaporation membranes, J. Membr. Sci. 451 (2014) 285 292. https://doi.org/10.1016/j. memsci.2013.10.001 with the permission of Elsevier.

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solution of the catalyst and PVA was cast onto a selective PVA layer supported by PES and subsequently immersed in a coagulation bath of ethanol. This protocol, known as nonsolvent-induced phase separation (NIPS), enabled the formation of a porous catalytic layer with minimal resistance to the mass transport and was able to boost the conversion rates of acetic acid to 91.4% at 85 C, much higher than the equilibrium conversion of 71.9% [45] (Fig. 6 8B). Polyetherimide (PEI) is another class of polymers for PV-assisted esterification already employed as a selective coating supported by ceramic membranes. The PEI-based composite membrane presented a flux of 0.5 kg/m2/h and αwater/ethanol of c. 18 useful to raise the conversion of acetic acid and ethanol at 70 C from 73% to 88% [48]. Alternatively, inorganic membranes, mostly based on zeolites, blend remarkably permselectivity with chemical stability at the expense of sophisticated and costly manufacturing methods. Studies revealed the opportunity to reduce the water concentration in the reactor to a value below 0.03 wt.% with NaA inorganic membrane with a flux of 0.063 kg/m2/h and αwater/2-Me-2-BuOH of 135 (T 5 50 C) [49]. This optimized the reaction of fructose and palmitic acid to its mono- and diesters with an efficiency of 66% after 56 h [49]. Besides the removal of the by-products via hydrophilic PV, the continuous recovery of the products (i.e., esters) consists of a valid alternative to alter the thermodynamic equilibrium. PDMS is the most common polymer employed in the organophilic PV characterized by a good selectivity towards esters in an aqueous solution. A PVMR equipped with an esterpermeable membrane made of PDMS was found to be beneficial to overcome the equilibrium conversion of c. 67% at 348K (M 5 1/1 initial molar ratio of reactants) reaching a value of c. 80% thanks to the selective recovery of isobutyl acetate from the reaction media-rich of acetic acid and isobutanol [22]. Again, the hybridization of the polymer (e.g., PDMS) with nanofillers is a common strategy aimed to mitigate the membrane swelling and facilitate the selective absorption of the target product resulting in a superior performance [50]. Besides their employment in food and cosmetics, esters are widely used as volatile solvents for paints and lacquers and in the synthesis of polymers. In the recent year, fatty acid alkyl esters (FAAE) have gained more attention as biodiesel produced via the transesterification of free fatty acids (FFA) with alcohol: H1

FFA 1 ROH 2 FAAE 1 H2 O:

A major issue related to the production of biodiesel from FAAE from vegetable oils is the need to facilitate the reaction with H2SO4 (efficiency 90%), implying the employment of corrosive resistant plants and demanding posttreatment phases for the separation and neutralization of the product and the generation of wastewater [51]. The implementation of PVMRs based on PVA composite membranes secured a yield of 99.9% in the esterification of oleic acid with methanol (1:27 molar feed ratio) using 0.3 wt.% of H2SO4 catalyst at reflux temperature (65 C) [52]. Also, the commercial hydrophilic Pervap 1000 membrane (Sulzer) doubled up the ethyl oleate conversion catalyzed by an ion exchange resin (Amberlyst 15 Wet) thanks to high selectivity to water in the ethanol/oleic acid/water/ethyl oleate system [53].

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6.3.2 Etherification reactions Ethers, such as methyl tert-butyl ether (MTBE) and ethyl tert-butyl ether (ETBE), have gained more attention as green and alternative fuel oxygenators benzene. ETBE is preferred to MTBE because of its lower volatility and water solubility (,26 g/L), superior biodegradability, and higher partition coefficient; thus presenting a minimal impact on the hydrologic cycle [54]. Usually, ETBE is produced from the reaction of ethanol (in large excess) with isobutene catalyzed by an acid ion-exchange resin [55]. The critical issue of ETBE production at the industrial level is the thermodynamic equilibrium of the reaction and its challenging recovery due to the presence of azeotropes (Ethanol-ETBE 20/80 wt.%). The separation of the azeotrope is carried out by expensive and highly energy-intensive multistage distillation processes, whereas PV represents an emerging energy-saving practice [56]. Early studies demonstrated the opportunity to concentrate the ether by selectively removing the ethanol from the reaction media by using cellulose acetate-based membranes evidencing a poor flux and high selectivity [57]. Interestingly, changes in the ester substituent of the chain of the biopolymer governed the tread-off of the permselectivity: the flux decreased from 1.7 kg/m2/h for cellulose acetate butyrate to 0.08 kg/m2/h for cellulose acetate counterbalanced by an improvement of the alcohol concentration from 90 wt. % to 100% for cellulose acetate butyrate and cellulose acetate, respectively [57]. Blending different cellulose esters was an effective approach to enhance the flux reaching values of 0.6 3 kg/m2/h (normalized for a membrane thickness of 5 μm) [58]. Similar results were obtained by grafting cellulose acetate membranes with -g-poly(methyl diethylene glycol methacrylate) [59]. Polylactide (PLA), a well-known bio-based polyester derived from renewable sources such as corn starch, represents an interesting option for cellulose-based membranes. PLA blended with 3 wt.% of (polyvinyl pyrrolidone) (PVP) showed a separation factor of 16 and a flux of 2.7. kg/m2/h in the separation of ethanol/ETBE via PV [60]. Remarkable and longterm (80 h) stable performance were also observed for NaY inorganic membrane showing flux and separation factor of 1.30 kg/m2/h and 1100, respectively [61]. The MTBE/methanol separation presents similar pros and cons since literature studies reported different strategies of preparation and modification of cellulose membranes aimed to improve the transmembrane flux without affecting the selectivity [62] (see Fig. 6 9). For instance, blending cellulose acetate with PVP (15 wt.%) guaranteed a separation factor of 411 and a total flux of 0.43 kg/m2/h for a feed with 20 wt.% methanol at 313K [63]. Cellulose acetate membranes could also play the role of active support for cross-linked PVA obtaining flux above 0.4 kg/m2/h of a permeate containing up to 99.9 wt.% of methanol [64]. Additionally, CS, a compound extracted from the cuticle of crustaceans, is an interesting biomaterial for MTBE/methanol separation suffering from poor mechanical properties and high sorption degrees of polar liquids. Nevertheless, the crosslinking of CS with H2SO4 conferred sufficient stability to the membranes enabling to collect via PV of a permeate containing 98.3 wt.% of methanol with a flux of 0.47 kg/m2/h at 25 C [65]. Outstanding separation factors (from 25,000 to 35,000) from MTBE/methanol were observed by depositing a thin

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FIGURE 6–9 State-of-the-art of MTBE/methanol separation via PV. AA, Acrylic acid; BA, butyl acrylate; EVAc, ethylene-vinyl acetate; HEC, hydroxyethyl cellulose; MMA, methyl methacrylate; PAA, poly (acrylic acid); PAMHEMA, 2-hydroxyethyl methacrylate; PEEKWC, polyether ether ketone; PHB, polyhydroxybutyrate; PPO, poly (2, 6-dimethyl-1, 4-phenylene oxide); PVAc, poly (vinyl acetate). Reprinted from A. Pulyalina, V. Rostovtseva, I. Faykov, A. Toikka, Application of polymer membranes for a purification of fuel oxygenated additive. Methanol/ Methyl Tert-butyl Ether (MTBE) separation via pervaporation: a comprehensive review, Polymers (Basel) 12 (2020). https://doi.org/10.3390/polym12102218. Licensee MDPI, Basel, Switzerland. This article is an open-access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license.

selective layer of polystyrene sulfonate onto a microporous alumina substrate, but the low flux (0.023 0.01 kg/m2/h) hampered their practical application [66].

6.3.3 Acetalization reactions The synthetic route leading to the formation of an acetal by the nucleophilic addition of an alcohol to a ketone or an aldehyde has been widely employed to introduce protective groups into a molecule to obtain chemoselectivity for the preparation of organics, carbohydrates, and drugs [67]: H1

R1 COH 1 R2 OH 2 R1 CHOR2 1 H2 O:

Also, in this case, the reaction is reversible, acid-catalyzed and the production of water as a by-product defines the thermodynamic equilibrium. Thus, the removal of the water from the reaction environment is vital in the logic of the process intensification as confirmed by a PVMR based on HybSi raising the efficiency of the acetylation reaction of ethanol with butyraldehyde to 1,1 diethoxy butane from 40% to 70% at 70 C [68].

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Interesting research revealed the advantages of the acetalization between glycerol and cyclohexanone to high-value glycerol derivatives aided by catalytic PVMRs [69]. Under the action of a multifunctional membrane, based on a porous sponge-like catalytic of Zr (SO4)2
4H2O embedded into PVA deposited on a dense PVA selective layer of 8 μm supported by PES, the conversion rate improved to 92% at 75 C with a gain of c. 52% [69]. Interestingly, the investigation elucidated the dependence of the efficiency of the conversion not solely on the operating conditions of the reaction and the membrane composition (i.e., loading of the catalyst, concentration of reactants, temperature), but also on the morphological and geometrical aspects of the PVMR, such as the membrane area/feed volume ratio to match the rates of water production and removal [69]. One of the benefits of membrane technologies, including PV, is their modularity which allowed the integration with other chemical processes. The hybridization of a simulated moving bed with a PVMR (patented as PermSMBR) showed promising results in facilitating the synthesis of 1,1-diethoxybutane (DEB) and 1,1-diethoxyethane (DEE) usually limited by an equilibrium conversion of 55% and 57% respectively [70,71]. In the absence of a PV membrane, the simulated moving bed reactor, consisting of a column of absorber and catalyst, presented a high affinity towards water impacting the economic feasibility of the process due to regeneration cost with desorbent. Thus, the introduction of hydrophilic membranes packed with the resin (Amberlyst 15-wet acting as both catalyst and adsorbent) improved the performance of the process for the synthesis of DEB at 70 C to c. 70 kg/day saving the 84% on the consumption of the desorbent [70,71].

6.3.4 Condensation reactions The potentialities of PVMRs in other reactions have been poorly explored, but water is a common by-product and its removal with hydrophilic PV intensifies the conversion beyond the thermodynamic equilibrium. Pioneering studies have been focused on the phenolacetone condensation, a step of the synthetic route for bisphenol-A, where the water strongly inhibits the catalyst [24]. Asymmetric polyimide solvent-resistant membranes presented a flux of 0.183 kg/m2/h and selectivity of 310 in the treatment of phenol (93.0 wt.%)-acetone (5.3 wt.%)-water (1.7 wt.%) solution at 80 C reducing the reaction time from 24 to 6 h [24]. Methyl isobutyl ketone (MIBK), a common solvent for paints, is synthesized via a threesteps reaction initiated by base-catalyzed aldol condensation of acetone. This reaction can be facilitated by sulfonated organic ion exchanger catalyst, poisoned by the water which accumulates into the resin hindering the accessibility of the reactants to the catalytic sites [72]. The dehydration of the reaction environment (50 60 wt.% acetone, 30 40 wt.% MIBK, 7 8 wt.% of water) with a PVMR equipped with PVA membranes (especially GFT 1001) showed potentialities to nearly double the acetone conversion [73].

6.3.5 Bio-alcohol production (pervaporation bioreactors) Under the impulse of the green transition, ethanol and butanol have been considered a renewable alternatives to the fossil fuels produced by yeast fermentation from different feedstocks,

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such as lignocellulose. However, the produced alcohol in the bioreactor impedes the fermentation of the sugar to a concentration around 5 8 wt.% [74]. Among energy-intensive practices such as distillation, PV emerged as a sustainable process to recover the ethanol from the fermentation bath, also at the azeotropic point (95.6 wt.% ethanol 4.4 wt.% water) [75]. A critical assessment of a PDMS membrane, the benchmark polymeric membrane for organophilic separation, revealed that silicone plays a passive role in the separation presenting performance similar to the vapor-liquid equilibrium behavior [76], as shown in Fig. 6 10 [77]. PTMSP appeared as the most effective polymer with minimal advantages with respect to the thermodynamic equilibrium, whereas the high free volume responsible for the high permeability undergoes poor long-term stability [78]. Other polymeric membranes, including polyether block amide, has been widely studied but demonstrated a selectivity lower in comparison to the vapor-liquid equilibrium [77]. Definitively, the hydrophobicity and the alcohol absorption of MFI-type zeolites with a high Si/Al ratio (ZSM-5) or pure-silica (silicalite-1) are the first choice for the recovery of ethanol from fermentation baths ensuring a concentrated permeate (70 80 wt.% of ethanol) [79,80]. Nevertheless, the high cost of the synthesis and the relatively low permeation have hindered the large-scale production of inorganic membranes stimulating the development of MMMs to transcend the permeability-selectivity upper-bound for polymeric membranes [6]. Zeolites can be easily tailored because of the presence of Si OH or Al OH groups on the surface of the cage easily bounded with a wide range of silanol groups and siloxane bridges [81]. The achievements in the synthesis of the zeolite enabled the reduction of the particle size to submicrometric values enabling the preparation of composite membranes made of thin MMMs supported by a porous layer. This allowed the development of a PDMS layer of 5 μm loaded with 67 wt.% of modified silicalite-1 of 500 nm with a flux of 5.52 kg/m2/h and a separation factor of 15.5 (T 5 50 C) [82]. Butanol, typically produced from fermentation broth containing acetone butanol ethanol (ABE) with a concentration of the product in the range of 1 2 wt.%, is considered the most suitable biofuel candidate in comparison with ethanol because of its higher heat of combustion [83]. The physicochemical properties of the ABE, including the poor solubility of butanol in water, facilitated the recovery of the biofuel. In fact, PDMS membranes enriched the butanol content with respect to the vapor-liquid equilibrium as shown in Fig. 6 10B. Feasibility studies proved the in situ recovery of butanol from fermentation broths at a rate of 0.486 0.710 kg/m2/h and with a separation factor of 30 using a Silicalite-1 Filled PDMS/PAN composite membrane [84].

6.4 Conclusions and future trends Green strategies for the intensification of chemical synthesis are important to satisfy the growing demand without exacerbating the environmental crisis. As a countermeasure, the integration of membrane technologies in industrial processes is an attractive opportunity to recycle and recover valuable compounds.

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FIGURE 6–10 State-of-the-art of (A) ethanol/water and (B) butanol/water separation via PV in terms of alcohol concentration in the permeate as a function of the ethanol concentration in the feed. The dashed line represents the values of the vapor liquid equilibrium behavior of alcohol/water at 60 C. Reprinted from P. Zheng, C. Li, N. Wang, J. Li, Q. An, The potential of pervaporation for biofuel recovery from fermentation: an energy consumption point of view, Chin. J. Chem. Eng. 27 (2019) 1296 1306. https://doi.org/10.1016/j.cjche.2018.09.025 with the permission of Elsevier.

The synergic combination of the achievements in membrane technologies thanks to the advanced methodologies for membrane preparation and the advent of innovative highperforming materials have opened up the door to the next generation of PV membranes. Besides the separative applications, the selective action of the membranes provided the impulse for the development of PVMR useful to aid biochemical synthesis. PVMR selectively removes a product or a by-product from the environment of reaction shifting the reaction yield beyond the thermodynamic equilibrium according to Le Chatelier’s principle.

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A multitude of membranes and configurations of PVMR have been purposed to assist a wide variety of reactions (e.g., esterification, etherification, acetalization, condensation), often removing the water, a ubiquitous undesirable by-product causing the catalyst poisoning. Despite the benefits in the improvement of the yield and/or in reducing the time of the reaction, the poor selectivity and stability of polymeric membranes (mostly based on PVA and cellulose derivates) limited their practical implementation at an industrial scale. Likewise, PDMS membranes have been widely explored in the recovery of ethanol and butanol from fermentation broths with the aim of mitigating the inhibition of enzymes responsible for the conversion of sugars into alcohols. Literature data demonstrated the lack of PDMS in giving an important contribution to alcohol/water separation with respect to the vapor-liquid equilibrium. On the other hand, inorganic membranes based on zeolite and silicalite-1 have demonstrated outstanding separative performance, but elaborated methodologies of preparation have hampered the prospect of industrialization. With the ever-increasing study of nanofillers, MMMs have opened up unprecedented opportunities to improve the permselective of scalable polymeric membranes by the embodiment of tailored nanoparticles designed for specific separation. Nowadays, newly emerged Metal Organic Frameworks and 2-D materials are under investigation to optimize PV membranes. Nevertheless, significant efforts must be applied to identify optimal nanomaterial/polymer pair for a specific application and individual strategies of dispersion and/or modification of the active filler to guarantee adequate homogeneity to MMMs. Likewise, the addressing of fouling problems arising in real applications and the scaleup of the integrated processes require further investigations to pave the way for the industrialization of PVMR.

Nomenclature Acronyms β ABE CS D DEB DEE ETBE FAAE FFA J MIBK MMMs MTBE P PAN PDMS

Separation factor Acetone butanol ethanol Chitosan Diffusivity 1,1-diethoxybutane 1,1-diethoxyethane Ethyl tert-butyl ether Fatty acid alkyl esters Free fatty acids Total permeate flux Methyl isobutyl ketone Mixed matrix membranes Methyl tert-butyl ether Permeability Polyacrylonitrile Polydimethylsiloxane

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PEI PES PLA POMS PPS PSI PTMSP PV PVA PVMRs PVP S SEM

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Polyetherimide Polyethersulfone Polylactide Polyoctyl methylsiloxane Polyphenylene sulfide Pervaporation separation index Poly(1-trimethylsilyl-1-propyne) Pervaporation Polyvinyl alcohol Pervaporation membrane reactors Poly vinyl pyrrolidone Solubility Scanning electron microscopy

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7 Polymeric membrane reactors J. Vital DEPARTME NT OF CHEMISTRY, NOVA SCH OOL OF SCIENCE AND TECHNOLOGY, LAQVREQUIMTE, U NIVERSIDADE NOVA DE LISBOA, C AP ARICA, PORTUGAL

7.1 Introduction Over the last few decades, the process intensification (PI) strategy has gained increasing importance in the chemical industry. The new challenges are not only the increase in productivity but also safer and cleaner production, meaning that the economic and ecological efficiency of chemical production routes must be significantly increased [1]. Membrane reactors (MRs), as multifunctional reactors combining a chemical reaction with a membrane-based separation, respond efficiently to the requirements of the PI strategy, namely by allowing the decreasing of equipment size, energy consumption, and environmental impact, contributing to the improvement of the process efficiency [2
4]. Due to their ability to keep two process volumes physically separated, MRs offer flexibility in choosing the most suitable reactor configuration for each specific reactive process [5]. In a general way a distinction can be made between MRs operating in the lowtemperature range (,150 C) and in the high-temperature range (.150 C). The ability of inorganic membranes to withstand high pressures and temperatures as well as their resistance to harsh chemical conditions makes them suitable for use in high-temperature MRs. They have been widely studied in a variety of fields such as dehydrogenation, methane steam reforming, water-gas shift, oxidative dehydrogenation of hydrocarbons, methane oxidation to syngas, etc. [6
8]. Pd or Pd-alloy membranes exhibit unique permselectivity to hydrogen and are very effective in hydrogenation or dehydrogenation reactions, methane and methanol steam reforming, and the water-gas shift reaction [4,9
12]. The oxidative coupling of methane and the partial oxidation of methane to syngas has been carried out successfully in MRs equipped with oxygen-selective ceramic membranes [9,13]. In their turn, hydrogen-selective ceramic membranes showed to be effective in the nonoxidative coupling reaction of methane [13]. In the field of fine chemical synthesis much milder conditions are generally applicable and polymeric membranes have been applied successfully with some advantages over the most expensive inorganic membranes [14]. In comparison to the inorganic counterpart, polymeric membranes have brought more possibilities in addition to the general benefits of the reactionseparation coupling [7]. First, polymeric materials are much more abundant in types and Current Trends and Future Developments on (Bio-)Membranes. DOI: https://doi.org/10.1016/B978-0-12-823659-8.00012-5 © 2023 Elsevier Inc. All rights reserved.

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versatile in properties, and a wide range of manufacturing techniques have been developed to allow better control over membrane properties. In addition, the ease of postmodification of polymeric membranes offers more space to further adjust the membrane properties. In this chapter, an overview of polymeric MRs will be given, considering the role played by the membrane, which can be used just for separation or can be catalytically active. However, the different aspects related to biological, photochemical, or electrochemical reactors are omitted, as they are the subject of specific chapters in this book.

7.2 Polymeric membranes We will start by focusing our attention on the central part of the MR: the membrane itself. MRs are generally composed of two chambers separated by the membrane. Between the chambers, material (reactants and/or products) exchange takes place, implying that mass transfer through the membrane must occur. Sorption of reagents and/or products in the polymeric matrix can also occur. The type of mass transfer through the membrane depends on the membrane structure.

7.2.1 Structure of polymeric membranes Based on their structure polymeric membranes can be classified into two main groups: dense membranes and porous membranes (Fig. 7
1). Furthermore, the membrane structure

Porous Symmetric

Dense Symmetric Asymmetric with dense skin and porous layer Sorpon-diffusion transport mechamism

Mass transfer in the pores

Sorpon-diffusion transport mechamism Mass transfer in the pores

Symmetric Mixed-matrix

Asymmetric Mixed matrix catalyc layer

Dense separave layer

FIGURE 7–1 Schematic drawing illustrating the structures of polymeric membranes [16,17].

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may be symmetric, if the structure is identical all over the membrane cross-section, or it may be asymmetric if the structure varies over the membrane cross-section [15]. In addition, asymmetric membranes can exhibit an integral structure consisting of the same material all over the membrane thickness or composites made up of layers of different materials [16]. On the other hand, based on their function, polymeric membranes can be classified as catalytically inert membranes (CIMs) or catalytically active membranes (CAMs). The CIM role is only separation being the catalyst contained in one of the reactor chambers. In its turn, CAMs bear catalytic activity and the chemical reaction takes place in the membrane [7,14].

7.2.1.1 Dense symmetric membranes Homogeneous dense symmetric membranes are often prepared by simple dissolution of the polymer or polymers in an appropriate solvent, followed by casting on a suitable support and evaporation of the solvent until a solid film is formed [7,18]. Dense membranes have no defined pores but fluctuating free volume (the volume not occupied by the polymer chains), depending on de movements of the polymer chain segments, instead [19]. Transport in dense membranes is based on the sorption-diffusion mechanism, Therefore, the interactions between the permeants and the polymer matrix dominate the mass transfer. On the one hand, solubility and chemical affinity determine the permeants’ sorption, while the influence of polymer structure on permeants’ mobility affects mass transfer [20]. The sorption and transport properties of a polymer are determined by its chemical structure, crystallinity degree, crosslinking, average molecular weight, and polydispersity [19]. Namely, the transport properties of the polymer matrix depend on the flexibility of the polymer chains, which are affected by their chemical structure. A polymer chain is all the more flexible the greater the freedom of rotation of the chain segments around single bonds. The introduction of aromatic rings or bulky substituents decreases that freedom of rotation and therefore the chain flexibility [19,20]. The polymer’s average molecular weight and its polydispersity have also a strong influence on chain mobility. If the molecular weight increases, the chain length increases, and therefore, the interactions between chains and chain entanglement increase too, resulting in a decrease in chain mobility. High molecular weight polymers are more stable. However, permeants’ sorption and diffusivity will decrease with increasing molecular weight [20]. When the chain flexibility is very much decreased, the polymer becomes hard and brittle and is said to be in a “glassy” state. In the opposite direction, if the mobility of the polymer chains is increased enough, the polymer becomes flexible and malleable and is said to be in a “rubbery” state. The polymer state is affected by temperature: when temperature increases a glassy polymer becomes rubbery, and the characteristic temperature at which that transformation occurs is called glass transition temperature (Tg). Elastomers exhibit Tg values below room temperature [21]. Polymer crystallinity affects its sorption and transport properties. Highly crystalline polymers are denser. Therefore they exhibit a small free volume, and consequently, the permeant uptake is low [19]. Amorphous polymers can also have crystalline regions—the greater the

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number of these regions, the greater the degree of crystallinity of the polymer. However, the transport of permeants takes place through the amorphous rather than the crystalline regions [17]. The sorption of specific compounds from a liquid solution contacting the polymeric membrane is strongly influenced by the polymer chemical composition, which determines its hydrophobic or hydrophilic properties [22]. The evaluation of the hydrophilichydrophobic properties of a polymeric membrane is usually based on the measurement of the water contact angle between the liquid-gas tangent and the liquid-membrane boundary, in a water droplet deposited on the membrane surface. Hydrophobic membranes show high contact angles while hydrophilic membranes show low contact angles [23]. Examples of polymers with hydrophobic or hydrophilic properties are given in Table 7
1. Nonpolar compounds can be selectively sorbed from an aqueous solution into a hydrophobic membrane. This feature has been used for the immobilization of transition metal complexes in a polydimethylsiloxane (PDMS) membrane, in which supplementary coordination around the transition metal was performed by uptaking a nonpolar ligand from an aqueous solution [24]. On the other hand, hydrophilic polymers have a high affinity to water, and therefore they are suited as materials for dense membranes that should have high permeability and affinity to water, as is the case of hydrophilic pervaporation membranes [20]. The hydrophilic-hydrophobic balance of the polymer matrix can also be tuned by anchoring hydrophobic or hydrophilic groups on the polymer chains [23]. For example, the hydrophilicity of polyamide membranes can be increased by graft polymerization of hydrophilic monomers such as acrylic acid and methacrylic acid [25]. An important example of polymer modification to improve its hydrophilicity is the carboxylation or sulfonation of polysulfone [20,26]. In the opposite direction, the hydrophobic character of the hydrophilic poly(vinyl alcohol) (PVA) matrixes can be increased by esterifying the PVA OH groups with hydrophobic groups [27]. The blending of polymers or copolymers is also used for tuning the properties of a final polymer matrix. A homogeneous polymer blend is indicated by a single Tg value between those of the polymers used in the blending. The existence of multiple Tg values is an indication of heterogeneous blending [20]. Polymer additives such as poly(vinyl pyrrolidone) or poly(ethylene glycol) can be used for enhancing membrane hydrophilicity, and form, at least partially, stable blends with polymers such as polysulfones or poly(ether sulfones) [20]. Polymer blending has also been used for the tuning of other membrane properties such as the ability for the immobilization of enzymes [28], gas separation [29], or bifunctional (catalytic and separative) membrane properties [18]. Dense catalytic membranes can be prepared by either anchoring homogeneous catalysts, such as metal complexes, heteropolyacids, or poly(styrene sulfonic acid), on the polymer chains or entrapping (blending) those catalytically active molecules in the polymer matrix [6,14,18,30,31]. The crosslinking of polymer chains is a way to improve the membrane mechanical stability, control swelling, and decrease membrane solubility [20]. For the same polymer with the same crosslinking degree, the transport phenomena depend on the nature of crosslinks, and usually, for low levels of crosslinking, permeant diffusivity decreases with the increase of the

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Table 7–1

155

Examples of hydrophobic and hydrophilic polymers.

Molecular structure

Name

Abbreviation

Character

Polypropylene

PP

Hydrophobic

Poly(vinylidene fluoride)

PVDF

Hydrophobic

Polydimethylsiloxane

PDMS

Hydrophobic

Poly(vinyl alcohol)

PVA

Hydrophilic

Polyacrylamide

PAM

Hydrophilic

Poly(acrylic acid)

PAA

Hydrophilic

crosslinking degree [19]. However, for highly functionalized polymers with functional groups able to form hydrogen bonding between each other, such as PVA, and in the case of chemical crosslinking, the crosslinker chains can act as spacers. The result is, for low crosslinking levels, the increase of membrane transport properties when crosslinking increases [22,32,33].

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For example, Wang et al. [32] observed the increase of flux with the increase of crosslinking, in pervaporation experiments for the separation of methanol from dimethyl carbonate, with membranes prepared with blends of PVA and poly(acrylic acid) crosslinked with glutaraldehyde. Unlu et al. [34] prepared catalytic membranes consisting of chitosan crosslinked/functionalized with sulfosuccinic acid (SSA) for the pervaporation assisted esterification of levulinic acid with ethanol. Pervaporation experiments of an ethanol/water mixture showed that, although the total flux decreases, the water selectivity increases when crosslinking increases. This increase in water selectivity is explained by the smaller kinetic diameter of water molecules. Crosslinking can also be used to add catalytic activity to the membrane, by introducing the appropriate chemical functionalities, as is the case of sulfonic acid functional groups [7,14,34]. Recent examples are the use of poly(acrylic acid) membranes crosslinked with SSA or 4,40 -diamino-2,20 -biphenyl sulfonic acid in the methanolysis of soybean oil [35], as well as the preparation of membranes consisting of PVA crosslinked with 5-sulfoisophtalic acid for the pervaporation assisted ethyl acetate synthesis [36].

7.2.1.2 Mixed matrix membranes Mixed matrix or hybrid membranes are obtained by embedding an inorganic filler in a polymer matrix [20,22,29,37
39]. This coexistence of an inorganic and an organic phase offers the possibility of exploiting the features of every single phase, allowing the tailoring of the membrane transport properties and the improvement of its mechanical stability [39]. The most important characteristic of dense membranes is their ability to separate different chemical species based on the different permeation rates they can reach across the membrane. The use of inorganic fillers affects drastically the membrane transport properties. For example, the inclusion of impermeable fillers affects negatively the permeants’ diffusion, but bigger molecules are affected to a large extent, increasing selectivity [39]. Metal oxides, zeolites, metal or carbon particles, metal-organic frameworks (MOFs), carbon nanotubes, etc., have been dispersed in several polymeric materials, showing promising results in a variety of fields. The zeolite inclusion in PDMS matrixes, enhancing the membrane hydrophobicity, is an example of the improvement of the membrane performance in the pervaporative removal of VOCs in wastewater treatment [40]. In the opposite direction, the occlusion of the hydrophilic MOF HKUST-1 in polyimide Matrimid 5218 allowed the obtaining of mixed matrix membranes very effective in the separation of ethanol/water mixtures by pervaporation [30,41]. Due to their selective water permeation, these membranes were used in the pervaporation-assisted esterification of acetic acid with ethanol. Another important feature of mixed matrix membranes is the immobilization of active catalysts in the polymer matrix, which gives to the membrane catalytic properties in addition to those of separation [7,18,30,42]. The polymers selected for dense catalytic membranes should allow sufficient loads of the catalyst particles without compromising the membrane plasticity. In addition, polymeric material should be highly permeable to reagents and reaction products [43].

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Classical examples of the preparation of CAMs by entrapping solid catalyst particles in a polymeric phase are the composites consisting of zeolite Y encaged iron phthalocyanine occluded in a PDMS matrix and used in the liquid phase oxidation of cyclohexane and ndodecane [24,44,45]. The membrane, separating the organic substrate and the aqueous peroxide solution, allowed the fine-tuning of the concentrations of the reactants in the close vicinity of the catalyst active sites, therefore eliminating the need for a solvent. In addition, In the case of the n-dodecane oxidation, the alcohol and ketone products were exclusively recovered in the organic phase, demonstrating the possibility of the integrated reaction and separation in one single process unit. The versatility of PDMS-occluded membrane systems was further proven with the Ti-containing catalysts TS-1 and Ti-MCM-41 [46]. These hybrid PDMS matrices were used in the by-phasic solvent-free oxidation of hexane and the epoxidation of cis-cyclooctene [47]. In further developments of catalytically active mixed matrix membranes, the use of pseudo-interpenetrating networks as polymer systems for the catalyst incorporation proved to be advantageous for the preparation of poly(ethyl acrylate)/(Ti- or Ni-zeolite 13X) hybrid membranes. Those systems showed considerably fewer cracks than the membranes obtained by the conventional mixing methods and were active in the dehydrogenation reaction of cyclohexane to benzene at low temperatures (323
360K) [48]. Mixed matrix membranes incorporating solid acid catalysts have been also reported. For example, zirconia sulfate embedded PVA membranes were used in the pervaporationassisted esterification of n-butyl alcohol and acetic acid [18]. On the other hand, H-USY zeolites immobilized in PDMS or PVA matrices were reported to be effective in the hydration of alpha-pinene [46] and the pervaporation assisted acetalization reaction between glycerol and phenylacetaldehyde [7], respectively. More recently, mixed matrix membranes, consisting of the Cd MOF (H3O).[Cd(dppa)] (dppa 5 4-(3,5-dicarboxylphenyl)picolinic acid), which exhibits Lewis acid activity, occluded in a PVDF matrix, were used in the synthesis of benzimidazole and alpha- or beta-amino acid derivatives [49]. Another recent example of mixed matrix membranes applied in acid catalysis is the use of cellulose acetate embedded MOF Cu3(BTC)2 (BTC 5 1,3,5-benzenetricarboxylate) as a catalyst for the acetalization reaction between benzaldehyde and ethanol [50]. An important class of catalytic mixed matrix dense membranes is made up of the palladium-loaded polymeric membranes for hydrogenation reactions of organic compounds [43]. Palladium and its alloys show a unique capability to dissolve huge amounts of hydrogen and, as membrane materials, they also possess a selective permeability toward hydrogen. The early studies, developed in the sixties of the last century, were focused on the use of relatively thick dense metallic membranes. However, thick palladium membranes, in addition to low permeability, are too costly to be used commercially. These problems could be overcome by the development of metal-polymer composites, being the initial proposals based on PDMS matrices loaded with palladium nanoparticles [43]. In further developments on palladium occluded dense polymeric membranes, Fritsch and coworkers reported the use of highly permeable materials based on poly(amide imide) s (PAI) as well as new preparation methods [43]. In the first stage, a membrane is formed

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from a polymer homogeneous solution containing palladium acetate as a precursor of metallic palladium, through a casting procedure. In the second stage Pd(II) is reduced to Pd(0) by dipping the Pd-acetate-containing membrane in a methanol solution of sodium borohydride. The above synthesis procedure has been later adapted for the preparation of CAMs containing palladium nanoclusters occluded in poly(ether-b-amide) (PEBA) and PEBA/poly (vinylpyrrolidone) (PVP) blends. PEBA is an outstanding material to form polymer membranes that show high selectivity and sufficient permeability for aromatics and chlorinated compounds, and PVP is known to stabilize small metal clusters in solution. The membranes were tested in the hydrochlorination reactions of 4-chlorophenol and chlorobenzene, as well as in the hydrogenation of acetophenone, carried out in a pervaporative MR [51
53]. Similar techniques were used for the preparation of Pd/PDMS membranes, which were tested in the hydrogenation of acetophenone [53] and propyne [54]. In more recent works P. Ajayan and coworkers reported the in situ synthesis of particles of several transition metals such as palladium, iron, and nickel in PDMS matrices, by thermal decomposition of their acetylacetonate salts [55]. This synthesis route results in relatively monodisperse nanoparticles with a narrow particle size distribution. Metal acetylacetonates were chosen as the precursor over carbonyls or fatty acids due to their lower decomposition temperature. The metal salt solution was added to the PDMS elastomer which was then cured at room temperature. Further heat treatment resulted in the formation of metal nanoparticles in the polymer matrix. The palladium-PDMS nanocomposite was tested for accessibility and catalytic activity using ethylene hydrogenation as a model reaction. Another technique useful for the preparation of mixed matrix membranes of palladium and other transition metals is the so-called intermatrix synthesis, which is based on the immobilization of the precursors of the metal nanoparticles (metal ions or complexes) in a suitable polymeric matrix followed by reduction. The loading of the polymer matrix is usually performed by ion exchange, so the polymer chains must bear suitable functional groups, such as sulfonic or carboxylic. B. Domènech et al. [56,57] reported the use of sulfonated phenolphthalein poly(ether sulfone) (Cardo-PES or PES-C) as the polymer matrix and tetraamminepalladium(II) as the precursor of the Pd nanoparticles. After membrane loading, the [Pd (NH3)4]21 ions were reduced to Pd0 with a solution of NaBH4, leading to well-dispersed almost spherical palladium particles. The so obtained membranes were tested in the reduction of p-nitrophenol to p-aminophenol. A similar technique was used by S. Prakash et al. [58] for the preparation of Ag and Pb nanoparticles loaded sulfonated poly(ether sulfone) (SPES) membranes. The precursors of the metal nanoparticles were the corresponding nitrates and their reduction to the metallic form was performed by immersing the membranes in a solution of hydrazine hydrate. The SPES-Ag/Pb membranes were tested in the liquid phase oxidation of benzyl alcohol to benzaldehyde, carried out in a MR, being achieved excellent selectivities and conversions. K.-V. Peinemann [59] and coworkers reported a method allowing high loadings of welldistributed nonagglomerated gold nanoparticles inside a polymer matrix. The selected polymer was polythiosemicarbazide (PTSC), which is characterized by containing a chelate site

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in each monomeric unit. With this methodology, the precursor metal ions were loaded into the membrane by complexation in the chelates, being subsequently reduced with NaBH4 to form the metal nanoparticles. The catalytic activity of the gold embedded PTSC membranes was successfully tested in the reduction of 4-nitrophenol to 4-aminophenol as a model reaction. In another article [60], the same group describe the preparation of palladium-loaded PTSC membranes, by using the same methodology, which was catalytically active in the reduction of 4-nitrophenol and in the Suzuki coupling reaction.

7.2.1.3 Porous membranes Porous polymeric membranes consist of a polymer matrix with well-defined pores. According to the IUPAC classification, macroporous membranes have an average pore diameter larger than 50 nm, mesoporous membranes have an average pore diameter between 2 and 50 nm, and microporous membranes have an average pore diameter between 0.1 and 2 nm [17,20]. Unlike what happens with dense membranes, where mass transfer takes place through the fluctuating free volume, in porous membranes mass transfer occurs predominantly in the pores, by viscous flow or Knudsen diffusion according to the pore size [6,19,20]. Separation in porous membranes is based on size exclusion through microfiltration, ultrafiltration, or nanofiltration [7,17]. In catalytically active porous membranes the active catalyst or catalytically active functional groups are preferentially located on the pore walls since is through the pores that reactants flow takes place [14]. The catalyst particles or the functional groups located in the dense membrane phase remain inaccessible to the reactants and, because of that, are ineffective. A porous catalytic membrane or membrane layer not only enhances the accessibility of the catalyst, resulting in higher catalytic activity but also lowers the mass transfer resistance of the membrane [7].

7.2.1.4 Preparation of porous membranes The most commonly used techniques for the preparation of polymeric membranes include phase-inversion, interfacial polymerization, stretching, track-etching, and electrospinning [23]. 7.2.1.4.1 Phase-inversion In the so-called phase-inversion or phase-separation process, the polymer is dissolved in an appropriate solvent and cast on a suitable support (plate, belt, fabric). In a second step, this polymer homogeneous solution is transformed into a two phases system: a polymer-reach solid phase and a polymer-poor liquid phase. There are several ways to accomplish this transformation, namely [16,17,20,23]: Nonsolvent induced phase separation (NIPS)—The polymer solution is immersed in a nonsolvent coagulation bath, which must be miscible with the polymer-solvent. Then, an exchange of the solvent (from the polymer solution) and the nonsolvent (from the coagulation bath) occurs, leading to polymer precipitation.

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Vapor-induced phase separation—The polymer solution is exposed to an atmosphere containing the nonsolvent vapor, which upon absorption causes the polymer to precipitate. Thermally-induced phase separation—The polymer solution is cooled down to a temperature at which the polymer is no longer soluble in the solvent, then occurring demixing/precipitation. After demixing is induced, the solvent is removed by extraction, evaporation, or freeze-drying. Evaporation-induced phase separation—The polymer is dissolved in a volatile solvent or a mixture of a volatile solvent and a less volatile nonsolvent. After casting the solvent is allowed to evaporate leading to demixing/precipitation. This technique is also known as the solution casting method. The NIPS process consists of the following consecutive steps: (i) the polymer is dissolved in an appropriate solvent to form a solution; (ii) the solution is cast into a film, usually in the thickness range of 100
500 µm; (iii) the film is quenched in a nonsolvent, typically water or an aqueous solution. During the quenching process, the polymer solution separates into two phases: a polymer-rich solid phase that forms the membrane structure, and a solvent-rich liquid phase that forms the liquid-filled membrane pores. Generally, the pores at the film surface, where precipitation occurs first and most rapidly, are much smaller than in the interior or at the bottom side of the film. This leads to an asymmetric membrane structure [16]. A variety of membrane structures can be obtained by changing many parameters such as polymer nature, polymer concentration, solvent/nonsolvent system, bath temperature, additives, precipitation time, etc. [23]. Selection of the solvent/nonsolvent system strongly affects the membrane morphology. The low miscibility of the polymer in the solvent leads to nonporous membranes, while porous membranes are obtained when the miscibility is high. The polymer concentration has also a pronounced effect on membrane porosity. Increasing the polymer concentration in the casting solution produces membranes with low porosity and pore size [23]. The preparation of symmetrical porous polyoxadiazole membranes by the phaseinversion process was described by Maab et al. [61]. Polyoxadiazoles are polymers resistant to harsh conditions and show high thermal stability. Polymers were dissolved in concentrated sulfuric acid at two different concentrations, 5 and 7 wt.%, and cast on a nonwoven polyester support. They were then immersed in the nonsolvent bath (water). The average pore diameters for the membranes prepared from the 7 wt.% polymer solutions were below 37 nm, while the membranes prepared from the 5 wt.% polymer solutions exhibited much larger average pore diameters, around 150 nm. Bahadorikhalili et al. [62] reported the preparation of asymmetrical porous poly(ether sulfone) membranes loaded with palladium-supported magnetic nanoparticles by the phaseinversion method. The Pd-supported nanoparticles were suspended in 1-methyl-2-pyrrolidinone 17 wt.% PES solutions. After casting on glass plates, the so obtained mixtures were immersed in water, as the nonsolvent, being obtained finger-like asymmetrical structures. A phase inversion method has been also used for the preparation of porous asymmetric membranes with high loads of metal nanoparticles [59,60,63]. The selected polymers must contain chelate sites in the monomeric units, as was referred to in section 7.2.1.2. The

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method referred to as complexation-induced phase separation (CIPS), uses the complexation of metal ions by the chelates of the monomeric units to form the dense layer of the asymmetric membranes, and the NIPS to form the porous support. CIPS consists of three steps: (i) a thin film is cast with the polymer solution (polymer 1 solvent A); (ii) the polymer solution film is immersed in solvent A, which contains a low concentration of metal ions; and (iii) the skinned film is transferred to a nonsolvent bath. The second step is the key step, where the chelate sites of the polymer chains on the top layer of the viscous polymer solution film form interchain complexes with the metal ions. This complexation reaction leads to the formation of a dense skin floating on the top of the remaining viscous polymer solution film. Finally, in the third step, the remaining polymer solution below the dense layer is precipitated by solvent/nonsolvent exchange to form porous support. Dimethyl sulfoxide (DMSO) was used as a solvent both for polymers like PTSC or polythiourea (PTU), and metal ions, such as Ag1, Pd21 or fourth-period metal ions like Co21, Ni21, and Cu21, which lead to thicker dense layers. Water, isopropanol, or mixtures of both are used as the nonsolvent bath [60,63]. For gold-loaded PTSC membranes, a mixture of 1,4-dioxane and DMSO was used as a solvent, and water was used as nonsolvent [59]. To improve the membrane morphology and other properties, various additives to the casting solution are often used, which can function as pore formers, increase solution viscosity or accelerate the phase-inversion process [23]. Embedding nanomaterials, such as carbon nanotubes, into the polymer matrix is a form of producing porous membranes or increasing porosity [30]. Nanoporous well-ordered polymeric membrane structures have been obtained by embedding arrays of aligned carbon nanotubes in polystyrene matrices [64]. Porous polymeric catalytic membranes consisting of palladium decorated multiwalled carbon nanotubes (Pd-MWCNTs) incorporated in polysulfone matrices have been obtained by the phaseinversion process [65]. The membranes exhibited an asymmetric structure consisting of a dense top layer and a porous finger-like sublayer. By incorporating Pd-MWCNTs in the membranes, the size and length of the finger-like structures were increased, which suggests that the membrane formation rate in the presence of Pd-MWCNTs is faster. Zinadini et al. [66] observed similar effects in the preparation of PES mixed matrix membranes loaded with ZnO nanoparticles and MWCNTs. Alpatova et al. [67] reported the preparation of porous poly(vinylidene fluoride) (PVDF) membranes with the inclusion of Fe2O3 nanoparticles and MWCNT for the Fenton-like catalytic degradation of organic contaminants. The optimum combination of 0.2% MWCNT and 1% Fe2O3 induced pore formation and improved membrane permeability. The membranes were successfully evaluated for the removal efficiency of cyclohexanoic acid and humic acid from aqueous solutions. 7.2.1.4.2 Track-etching The so-called track-etching process allows the preparation of symmetrical porous membranes with very regular cylindrical pores and very narrow pore size distribution [17,20,23,30]. In this technique, a relatively thin nonporous polymer film is irradiated with high-energy heavy particles. The particles pass through the film, breaking polymer chains

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and creating damaged “tracks.” Then, the film is immersed in an etching bath (strong acid or alkaline) so that the film is preferentially etched along the tracks, forming pores. The pore density is determined by the irradiation intensity and exposure time, whereas the etching time determines the pore size [20,23]. Since pore sizes down to 10 nm can be obtained by this process [20], inert membranes can be used in MRs for the separation of macromolecules. CAMs can be prepared by anchoring active catalyst particles or functional groups on the membrane pore walls [30]. Mashentseva et al. [68,69] reported the preparation of gold and silver nanotubes embedded in track-etched membranes, active in the reduction of pnitrophenol. The irradiation of Poly(ethylene terephthalate) films with Kr ions followed by etching in a sodium hydroxide solution led to the formation of cylindrical pores with average diameters of 80
100 nm. The gold and silver nanotube arrays were embedded in the membrane pores by electroless deposition. J. Chen and coworkers [70] described the preparation of track-etched polycarbonate membranes loaded with Ag, Au, and Pd nanoparticles. The track-etched membranes were prepared by irradiation of polycarbonate films with 84Kr ions, followed by etching in a NaOH aqueous solution. Coating the pore walls with polydopamine (PDOP) all over the membrane thickness provides nucleation sites for the formation and stabilization of metallic nanoparticles. The so obtained membranes were catalytically active in the reducing reaction of p-nitrophenol. 7.2.1.4.3 Electrospinning Electrospinning has emerged as a unique and straightforward technique that can produce nanofiber membranes. The produced nanofibers in membrane form have very high specific surface areas, high porosities, controllable pore sizes, membrane thicknesses, and interesting functional properties, and can be made from a wide range of compositions and structures suitable for different applications [7,23,71]. In addition, electrospun nonwoven polymeric materials show porous structures with excellent pore-interconnectivity [72]. Electrospinning from polymer solutions can be combined with phase separation, leading to hierarchical pore structures with porosity in the nanofibers superimposed on the nonwoven macropore structure [73]. The electrospinning setup has three main components (Fig. 7
2): the high voltage power supply, the container (usually a syringe) containing the polymer solution or melt, and a grounded collector that may be in different configuration designs (usually flat plate or drum type). The polymer solution or melt is fed into the syringe and a high voltage is applied to the metallic syringe needle. As the polymer solution is constantly pushed by a syringe pump, the applied strong electric field overcomes the surface tension of the droplet formed at the needle tip, which is stretched to a cone shape. At a critical point, an ultrafine fiber is emitted, flies through space, and is collected on the grounded collector. The emitted fibers are elongated by a whipping process caused by electrostatic repulsion and are thinned to a nanometer scale [71]. The diameter and morphology of the nano/microfibers can be controlled by varying the viscosity of the polymer solution, its flow rate, environmental conditions, and the applied electric potential [23]. Due to their unique properties, electrospun membranes have found application in a wide range of fields, from ultra/nanofiltration to tissue engineering [23,71]. They also have been

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163

FIGURE 7–2 Schematic diagram of an electrospinning setup.

applied in MRs, even as CAMs [7,30]. Polymeric membranes bearing highly accessible sulfonic acid functions, active in acid-catalyzed reactions such as esterification and acetalization, have been prepared by using nanofibers of perfluorosulfonic acid (PFSA) resin. Namely, Xuefei et al. [74] reported the preparation of PVA/PFSA nanofiber mats via the electrospinning process. Homogeneous PVA/PFSA solutions were prepared by mixing an N, N-dimethylacetamide PFSA solution with a PVA aqueous solution at different weight ratios. Changing the weight ratio allowed the modification of the diameter of the resulting PVA/PFSA nanofibers. The catalytic activity of the nanofiber mats was tested in the synthesis of ethyl acetate, having observed that the catalytic efficiency is related to the specific surface area of the PVA/PFSA nanofibers. The use of suspensions of SiO2 nanoparticles in the electrospinning polymer solution has shown to be effective in increasing the specific surface area of the PFSA mats, with high catalytic activity in esterification reactions [75,76]. The coelectrospinning of active catalysts into polymer mats allows the obtention of catalytically active functional groups on the surface or in the interior of the nanofibers. Due to the low diffusion resistance of the fiber mats and the fiber’s small diameters, the active functional groups became easily accessible to the substrate molecules [77]. This concept has been used for the preparation of acid membranes by immobilizing phosphotungstic acid in PVA fibers, active in biodiesel production [78]. The electrospun fiber membranes can also serve as perfect host materials for immobilizing nanoparticles [71]. The introduction of palladium and platinum nanoparticles on the electrospun fiber mats can be made by dip coating or wet impregnation of the nanoparticles precursor solutions, as reported by Ebert et al. [79] for the preparation of Pd loaded PAI membranes, or by Soukup and coworkers [80,81] for the preparation of poly(p-phenylene oxide) (PPO) nanofibrous membranes supporting Pt and Pd nanoparticles. A more effective approach, allowing high loads and good dispersions of the metal nanoparticles, consists of coating the nanofibers with PDOP. The catechol and amine functional groups of PDOP can act as chelation sites to which the nanoparticle precursor ions can be coordinated. Shen et al. [82] used this methodology for the preparation of silver-coated polyacrylonitrile (PAN) nanofiber mats with high silver content. The so obtained Ag/PDOP/PAN

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composite membranes showed high catalytic activity for the complete degradation of methylene blue dyes and the reduction of o-nitroaniline. The PDOP coating of electrospun nanofiber mats was also used by He et al. [83] for the preparation of poly(methyl methacrylate) (PMMA) membranes loaded with platinum-nickel nanoparticles. The PtNi/PDOP/PMMA membranes showed good catalytic activity in the decomposition of formaldehyde at room temperature. The electrospun nanofiber coating can also be achieved by grafting polymer gel brushes on the nanofiber surface. Liu et al. [84] prepared polycaprolactone (PCL) nanofiber mats bearing Pd nanoparticles loaded in grafted gel-brush layers of poly(hydroxyethyl methacrylate) (PHEMA). The electrospinning solutions consisted of mixtures of PCL and Br-PCL-BR, which yield nanofibers bearing Br active sites on their surface. The bromine atoms work as polymerization initiators for the formation of short-chain PHEMA brushes. The treatment of the PCL/PHEMA mats with succinic anhydride provided the PHEMA brush hairs with carboxylic groups, which served as coordination points to Pd21 ions. The reduction of Pd21 to Pd(0) led to the formation of Pd nanoparticles. The so obtained membranes successfully catalyzed the reduction of 4-nitrophenol to 4-aminophenol. The addition of nanoparticle-supported inorganic powders or nanoparticle precursors to the polymer electrospinning solution [85,86] is an alternative methodology to the use of coatings for the immobilization of metal nanoparticles. However, this method has the inconvenience that most of the nanoparticles are embedded inside the nanofibers and, therefore, barely accessible to the reagents. To overcome this drawback, Guiping Ma and coworkers [87] inserted an additional electrode in a PVP/AgNO3 electrospinning solution. The so produced positive electric field induced the migration of the silver ions to an external layer of the emitted fiber, leading to a core-shell structure. The catalytic activity of the PVP/Ag coreshell nanofiber membranes was successfully tested in the reduction of methylene blue. Curious applications of polymeric electrospun fiber mats are the so-called Janus membranes or Janus filters, which are membranes and filters with different or even opposite properties on different faces or zones of the membrane/filter (like the two faces of the Roman god Janus). Miao et al. [88] reported the preparation of catalytically active Ag-nanoparticle-supported PVDF nanofiber mats behaving as Janus filters. Hydrophilic Ag nanoparticles were deposited on the surface of the electrospun PVDF nanofiber mats in a star-shaped pattern, through previous sensitization with SnCl2. This procedure allowed the creation of hydrophilic and hydrophobic different zones on the membrane surface, leading to the Janus filter behavior.

7.2.1.5 Ionic liquid membranes Ionic liquids (ILs) are usually defined as salts with a boiling point below 100 C. Due to several key features of these salts, such as low toxicity, negligible vapor pressure under ambient conditions, and high electrical conductivity, ILs have received considerable attention in a wide and diverse range of applications [89]. Because some ILs are liquid at room temperature and have negligible vapor pressure, they are seen as green alternatives to volatile organic solvents. However, due to their properties, ILs have been explored in many applications, namely in catalysis [89].

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ILs with the capability of dissolving homogeneous inorganic catalysts can be supported or entrapped in polymer matrices leading to catalytic membranes with properties tuned for simultaneous reaction and gas separation. Wolfson et al. [90] reported the preparation of polymeric catalytic membranes containing transition metal complexes dissolved in ILs. Since strong interactions between support and IL were required, the chosen polymer was poly(diallyl dimethylammonium chloride). The membranes showed good activity in the asymmetric hydrogenation of methyl acetoacetate. Nancarrow et al. [91] reported the preparation of catalytic composite membranes consisting of solutions of RuCl3 in ILs based on the cation [C4mim] (1-butyl-3-methylimidazolium) entrapped in polyimide matrices. The so obtained membranes were used in the reactive separation of H2/CO mixtures through the water-gas shift reaction and the simultaneous separation of CO2. The best results were obtained with the IL [C4mim][OTf] (cation: [OTf]—trifluoromethanesulfonate), which allows increased solubilities of H2O and CO within the membrane. Catalytic composite membranes consisting of PVDF-hexafluoropropylene/1,3-dialkylimidazolium-based IL/(Pd/C) were tested in the propene hydrogenation [92]. Polymeric ILs (pILs) can be obtained from ILs with the cation, the anion, or both, bearing polymerizable groups (e.g., vinyl, vinyl benzyl), and have found applications in membrane technologies and heterogeneous catalysis [89]. The incorporation of an imidazolium-based poly(IL) in a porous poly(ether sulfone) membrane through a UV photo-grafting was reported by Gu et al. [93]. The pIL served as a stabilizer for palladium nanoparticles, which were obtained in situ by reduction of K2[PdCl4] previously dissolved in the pIL, and the so obtained catalytic membrane was successfully tested in the Suzuki-Miyaura cross-coupling reaction. A particularly attractive feature of ILs is their high affinity toward CO2 and their ability for its catalytic transformation into valuable products. pILs have been extensively used as heterogeneous catalysts for the CO2 cycloaddition to epoxides (CCE reaction) showing high potentialities for their use as catalytic membranes [89,94]. Recently Yao et al. [95] reported the preparation of IL-modified MOF-polymer membranes exhibiting highly selective adsorption for CO2 over N2 and CH4. In addition, the obtained membranes show high catalytic activity for CO2 transformation by cycloaddition with epoxides under ambient pressure.

7.2.1.6 Microporous membranes In the last two decades, a new class of polymeric materials has emerged exhibiting interconnected nonfluctuating free volumes, high specific surface areas ( .1000 m2/g), and pore sizes ,2 nm, thus falling in the category of the microporous materials, according to the IUPAC definition. These microporous polymers have found application in a variety of areas such as gas adsorption, separation, catalysis, etc. [96,97]. The microporosity within microporous polymers is maintained by a robust network of covalent bonds. Generally, nonnetwork polymers pack space efficiently because the macromolecules can bend and twist to maximize intermolecular interactions. However, if one of the polymer constituent monomers contains a site of contortion, that space packing can be avoided. Consequently, large amounts of interconnected void spaces arise, and the polymer will behave like a conventional microporous material, even without a network structure.

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Among the microporous polymers, the so-called nonnetwork polymers of intrinsic microporosity (PIMs) can be soluble like ordinary polymers, making possible solution-based techniques for membrane preparation [98]. The backbone of the first membrane-forming PIM to be prepared (PIM-1) is composed entirely of fused rings, giving a ladder structure (Fig. 7
3), which hinders rotation from occurring and avoids large-scale conformational changes and space packing [97]. Membrane properties can be tuned by modifying the polymer structure. A variety of polymers with different characteristics have been generated from PIM-1 by chemical modification. Particularly, the nitrile groups in PIM-1 provide sites for chemical modification since they can be easily transformed into other functional groups such as acid carboxylic, amide, or amine [97,99]. PIM membranes have been extensively used for gas separation and adsorption [97,100,101], not only as homogeneous materials but also as PIM/IL blends [102]and mixed matrix membranes [103]. The good separation performances and high fluxes make the PIM membranes also suitable for pervaporative separations [98,104
106]. However, in the last few years, microporous polymers, particularly PIMs, have found application as CAMs. Namely, Halder et al. [107] reported the preparation of PIM-1 electrospun fiber mats supporting Pd nanoparticles. PIM-1 nanofibers were revealed to be excellent support for the Pd nanoparticles. The catalytic activity of the obtained mats was tested in the reduction reaction of p-nitrophenol to p-aminophenol. The Pd/PIM-1 mats showed to be more active than Pd supported fiber mats of PAN, which was attributed to the higher sorption of PIM-1 for p-nitrophenol. A flexible and free-standing conjugated microporous polymer (CMP) membrane loaded with Ag nanoparticles was prepared by Lee et al. [108] using a PVP electrospun membrane as a sacrificial template. The PVP/Ag mat was prepared by electrospinning of an ethanol/ water solution of PVP containing Ag nanoparticles. The nanofibers mat was coated with CMP by immersing in the polymerization reaction solution containing the monomers 1,3,5triethynylbenzene and 1,4-diiodobenzne. The solvent extraction of PVP left a CMP membrane composed of hollow nanofibers exhibiting hierarchical porosity, where macropores inherited from the nonwoven electrospun membrane and the meso- and micropores of the microporous organic polymer coexisted (Fig. 7
4). The so obtained membrane was successfully tested in the reduction of p-nitrophenol.

N

…

C O

… O C N FIGURE 7–3 Ladder structure of PIM-1 [97
99].

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FIGURE 7–4 Synthetic scheme of the hollow fiber CMP membrane. Reproduced from J. Lee, J.G. Kim, J.Y. Chang, Fabrication of a conjugated microporous polymer membrane and its application for membrane catalysis, Sci. Rep. 7 (2017) 1
8. https://doi.org/10.1038/s41598-017-13827-w.

The preparation of PIMs imbedded palladium nanoparticles [109] and chiral bifunctional catalysts consisting of thioureas and squaramides grafted on PIMs [110] have also been recently reported.

7.3 Classification of membrane reactors MRs can be classified according to the nature of the membrane material, the role of the membrane in the catalytic process, the transport function of the membrane, the nature of the catalyst or the catalytic process, or the reactor configuration [3,6,30]:

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• Nature of the membrane material: the membrane can be organic (polymeric) or inorganic (ceramic or metallic) [22]. • Role of the membrane in the catalytic process: inert or membrane-assisted MRs, if the membrane is not catalytically active and the membrane function is just separation, or catalytic MRs if the membrane is itself catalytically active [3,111]. • Transport function of the membrane: extractor-type MRs—a permaselective membrane allows one or more products to leave selectively the reactor; distributor-type MRs—the membrane allows a dose of one of the reactants, thus increasing the selectivity of kinetically controlled reactions; contactor-type MRs—a catalytic membrane contactor is a device in which a membrane containing a catalytically active phase is used to provide the reaction zone [3,22,30,112,113]. • The nature of the catalyst or the catalytic process: the catalyst can be nonbiological or biological and the catalytic process can be chemical, photochemical, or electrochemical [3,22,114,115]. • The reactor configuration: flat-sheet MR—the flat membrane separates two chambers; tubular MR (membrane diameter . 10 mm), capillary MR (10 mm . membrane diameter . 1 mm), hollow-fiber MR (membrane diameter , 1 mm). In all these last three configurations, the membranes are assembled in tube-and-shell modules, with the membranes being packed closely together in multitube arrangements in the cases of capillary or hollow fiber configurations [6].

7.3.1 Extractor-type membrane reactors The most common MRs are extractor-type. In this class of MRs, one or more products generated by the chemical reaction are continuously removed from the reaction mixture by permeation through the membrane. As the membrane divides the reactor into two chambers, the feed chamber is often called the retentate side, and the opposite chamber is called the permeate side. The products permeated to the permeate side are usually removed by a sweeping fluid or by applying a vacuum (Fig. 7
5). For reversible reactions, in which conversion is equilibrium limited, the removal of one of the reaction products from the reaction mixture allows increasing conversion beyond the equilibrium conversion value based on the feed conditions. In addition, undesired products formed in sequential reactions can be strongly depleted by the intermediate continuous removal [6,18,30]. Feed: A, B

Retentate: A, B, C, D

Sweep fluid

Permeate: A, B, C, D

FIGURE 7–5 Scheme of an extractor-type membrane reactor exemplified for the reversible reaction A 1 B # C 1 D, with preferential permeation of the product D and sweep fluid. Retentate rich in product C and permeate rich in product D [3,6,30].

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An example of the application of extractor-type MRs to liquid-phase reactions is the simultaneous liquid phase removal of glycerol with triglyceride transesterification for biodiesel production. This type of reaction has been carried out either in inert or catalytic extractor-type MRs, in a variety of configurations, from flat membranes to tube-and-shell hollow-fiber packs [116
118]. Since triglyceride transesterification takes place through a system of reversible reactions, glycerol removal simultaneously with the reaction allows the equilibrium shift to the product’s side (usually fatty acid methyl or ethyl esters) [116,118,119]. Extractor-type polymeric MRs, using both catalytic and inert membranes, have also been successfully used in biodiesel production through the methanolysis of sunflower oil [120]. Another example of an extractor-type polymeric catalytic MR applied to a liquid-phase system is the synthesis of aliphatic esters reported by Uragami et al. [31]. The reactor consisting of two chambers divided by a PSSA/PVA blend membrane was loaded with an aqueous solution of alcohol and acetic acid in one chamber and chlorobenzene in the other chamber. The extraction of the formed esters to the chlorobenzene chamber was observed. The removal of the reaction product from the retentate side is useful not only for the product to recover itself but also for avoiding further side reactions. This is the case of the liquid phase benzene hydroxylation to phenol with H2O2 in a continuous MR, as reported by Al-Megren et al. [121]. The produced phenol is recovered by using water on the permeate side, which works as a sweep fluid. Furthermore, the continuous removal of phenol from the reaction side (retentate) reduces the possible formation of by-products such as benzoquinone, completely preventing the formation of biphenyl. Extractor-type MRs aiming for the equilibrium shift to the product side have also been applied to gas-phase reactions. Following the scheme alkane$olefin 1 hydrogen, the nonoxidative dehydrogenation of alkanes is a typically equilibrium-controlled class of reactions, usually carried out at high temperatures. There are numerous examples of conversion enhancement of alkane dehydrogenation above the thermodynamic equilibrium value using metallic or ceramic hydrogen-selective membranes [112]. However, Frisch et al. [48] reported the use of a low temperature (323
360K) polymeric MR assembled with Ti- or Ni-13X zeolite loaded polymethyl acrylate membranes for the dehydrogenation of cyclohexane to benzene. The propene hydrogenation in a PCMR with continuous extraction of the formed propane, using a mixed matrix PDMS/Pd nanoclusters membrane, was studied by Brandão et al. [54]. Choi et al. [122,123] reported the decomposition of MTBE in a tube-and-shell type polymeric MR consisting of 12-tungstophosphoric acid (HPW) and poly(2,6-dimethyl-l,4-phenylene oxide) (PPO) blends supported on alumina tubes. The selective removal of methanol through the catalytic membrane shifted the chemical equilibrium toward the favorable direction in the MTBE decomposition. The composite HPW- PPO/PPO showed a better performance than the single-phase blend HPW-PPO or the composite HPW/PPO. The enhanced performance of the HPW-PPO/PPO/Al2O3 CMR was due to the intrinsic permselectivity of HPW-PPO and the additional separation capability of the sublayered PPO membrane. Although the water-gas-shift reaction is usually carried out at a high temperature, its use for the reactive separation of H2 from CO in a PCMR assembled with an IL/polyimide membrane loaded with RuCl3 was reported by Nancarrow et al. [91].

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7.3.1.1 Pervaporation membrane reactors An important class of extractor-type MRs is composed of pervaporation-assisted MRs. Typically, in a pervaporation MR, a liquid feed circulates on the retentate side while mixture components or reaction products permeating the membrane evaporate as they reach the membrane side facing the permeate chamber. The name “pervaporation” is a contraction between permeation and evaporation [124
126]. In pervaporation-assisted catalysis, unlike reactive distillation, the separation between the mixture components does not depend on their relative volatilities but is based on permeability differences. Consequently, lower temperatures are required, and therefore, energy consumption is generally lower compared to distillation [22]. The hydrophilic or hydrophobic nature of the membrane is the main determinant of separation between components. Regarding membrane nature three types of membranes can be distinguished: hydrophilic, hydrophobic, and organophilic. Hydrophilic membranes preferentially transport water, hydrophobic membranes do the opposite, and organophilic have specific affinities to organic solvents [124]. Van der Bruggen [124] distinguishes between R1 and R2 type pervaporation reactors, in which the extracted component is the main product or a by-product, respectively. In reversible reactions, the removal of a side product, which is often water, shifts the equilibrium to the product side allowing high yields of the desired product. Esterification is the most common reversible reaction with the production of water as a side product, which can be conducted in an R2 type MR [124,126]: 0

0

RCOOH 1 R OH$RCOOR 1 H2 O

The permeation of the formed water to the permeate side, in the MR, leads to high yields of ester. There is a great number of published works on pervaporation aided-esterification including some literature reviews [126
128], even in specific areas like biodiesel production [129
131] or the use of mixed matrix membranes [132]. More recently, pervaporationassisted esterification reactions conducted in inert polymeric MRs have been reported for the synthesis of ethyl propionate [133], isobutyl propionate [134,135], n-butyl acrylate [136], methyl palmitate [137,138], ethyl acrylate [139] or ethyl acetate [41,138]. On the other hand, catalytically active polymeric MRs have been used for the pervaporation-assisted synthesis of ethyl propionate [140,141], ethyl lactate [142,143], isobutyl lactate [144], n-butyl acetate [145
147], ethyl acetate [36,138,148
151], ethyl levulinate [34] or methyl oleate [152]. Since water is permeated, hydrophilic dense polymeric membranes, either catalytically active or inert, have been used. A short survey over the last eight years found composite membranes consisting of a catalytic layer supported on a hydrophilic layer such as PVA or PES [36,140,141,145
148,151], biopolymer membranes [34,138], symmetric polymer or polymer blend membranes [133,142,149], mixed matrix membranes containing MOFs [41], PVA composite [150] or mixed matrix membranes containing functionalized ILs [152], homogeneous or composite PVA based membranes [134,136,139,144], polyimide membranes [137].

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Another example of a reversible reaction in which equilibrium can be shifted to the products side by removal of the by-product water is acetalization: aldehyde or ketone 1 alcohol$acetal or ketal 1 water

Recently, Qing et al. [153] reported the acetalization of glycerol and cyclohexanone conducted in a pervaporation MR assembled with a composite membrane consisting of a porous Zr(SO4)20.4H2O loaded PVA catalytic layer coated on a PVA/PES dense separative layer. Under optimized conditions, the glycerol conversion reached 93%, corresponding to a conversion enhancement of approximately 72% compared with the equilibrium conversion under the feed conditions.

7.3.2 Contactor-type membrane reactors Contactor-type MRs can be used in two different configurations: interfacial contactors and forced flow-through MRs [3,6,30] (Fig. 7
6).

7.3.2.1 Interfacial contactor membrane reactors In interfacial contactor polymeric MRs the membrane, usually catalytic, promotes the intimate contact between two different phases, immiscible liquid-liquid or gas-liquid, into the membrane phase and between those phases and the catalyst or the catalytic active sites [6,22,46]. Therefore, the membrane provides the reaction zone, not necessarily having a separation function [112]. When the polymeric membrane separates two immiscible liquid reactants, solvents can be excluded rendering the process environmentally and technically more attractive [46]. In addition, in catalytic membranes, the polymer matrix offers the possibility of fine-tuning the reactant’s concentration in the close vicinity of the catalyst active sites by slight changes in the polymer structure affecting its hydrophilic/hydrophobic balance.

Interfacial contactor

Forced flow-through Feed

A, B

A Feed

B

Products

Products FIGURE 7–6 Scheme of a contactor-type membrane reactor as an interfacial contactor or as a forced flow-through reactor exemplified for an A 1 B!Products reaction [3,6,30].

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In some applications, the membrane function can be simultaneously contactor, by promoting the contact between two immiscible phases, and extractor, by removing a product from the reaction mixture. This is the case, for example, of the methanolysis of vegetable oils to biodiesel, in which the membrane promotes the contact between the oil and methanol phases and simultaneously extracts the by-product glycerol to the methanol phase (permeate side), thereby enhancing conversion [116,154]. Interfacial membrane contactors in a flat configuration have been applied in a variety of situations, usually using dense catalytic membranes. This type of configuration was used by Wu et al. [155] to study the oxyfunctionalization of n-hexane with H2O2 in a liquid-vapor interfacial contactor assembled with a mixed matrix PDMS membrane loaded with zeolite TS-1. In this configuration, the membrane divides the reactor into two compartments: an H2O2 solution introduced in the upper compartment and hexane vapor in the lower one. A three-phase reaction system was also reported by Sing et al. [156,157] for the partial hydrogenation of soybean oil. The flat contactor-type reactor was assembled with an integralasymmetric polyetherimide membrane, bearing an ultrathin dense skin layer decorated with platinum or palladium, through a sputtering deposition process. Hydrogen was fed to the membrane porous side while liquid oil contacted the metal decorated dense skin side. The same flat configuration was used by Vankelecom et al. [44,45] for the roomtemperature liquid phase oxidation of cyclohexane and n-dodecane. In this reactor configuration, the immiscible organic substrate and aqueous oxidant phases are contacted through a PDMS membrane loaded with Y zeolite encaged iron phthalocyanine. Consequently, the need for a solvent was eliminated. The flat configuration contactor-type MR was also applied by Molinari et al. to the liquid phase benzene hydroxylation to phenol, with inert PP [158] or PDMS [159] membranes separating the two immiscible phases: an organic phase consisting of benzene, on one side of the membrane, and an aqueous phase composed by water, H2O2, and the catalyst FeSO4, on the other side. The membrane’s permselective barrier function plays an important role in the continuous extraction of the formed phenol to avoid over oxidation. Similarly, Buonomena and Drioli [160] reported the solvent-free oxidation of benzyl alcohol to benzaldehyde conducted in a flat contactor-type MR. In this case, inert hydrophobic PVDF membranes separate the two immiscible liquid phases, which are fed to both faces of the membrane: the organic phase consisting of pure benzyl alcohol and the aqueous phase containing hydrogen peroxide and the catalyst (ammonium molybdate or sodium tungstate). An identical reactor configuration was used by Prakash et al. [58] for the benzyl alcohol oxidation to benzaldehyde with an Ag/Pb loaded SPES mixed matrix membrane separating the organic and aqueous oxidant phases. Wales et al. [161] used a tubular configuration for the partial hydrogenation of soybean oil. The reactor was assembled as a tube-and-shell module with a composite ceramic/polymer membrane, which consisted of a PVP skin loaded with Pd nanoparticles and coating the inner surface of a porous ceramic tube. In this configuration, hydrogen was fed to the reactor shell side while the liquid oil was circulated in the tube lumen.

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Although flat membranes are easier to prepare, the use of hollow fiber membranes, whenever possible, allows for much higher surface areas per unit equipment volume leading to highly compacted devices [37,162]. Among the first applications of hollow-fiber polymeric catalytic contactors packed in a tube-and-shell configuration, stand up the gas phase hydrogenations of conjugated dienes [163], butadiene [164], or propadiene and propyne [165]. In all situations, one of the reactants was fed to the reactor shell side, while the other was circulated in the hollow-fiber lumen (Fig. 7
7). In addition to the use of hollow-fiber contactor-type polymeric MRs for gas-phase reactions, they have also been applied to gas-liquid or liquid-liquid reactions. Volkov et al. [166] reported the use of hydrophobic polypropylene porous hollow fiber membranes loaded with adlayers of palladium nanoparticles for the removal of dissolved oxygen from water. Based on the hydrogenation reaction H2 1 O2 ! H2 O, the oxygen removal took place in a threephase gas-liquid catalytic interfacial contactor consisting of the hollow fibers packed in a tube-and-shell modulus. Water containing dissolved oxygen was fed to the modulus shell side, flowing over the outer surface of the hollow-fiber membranes, while hydrogen was circulated in the hollow-fiber lumen. The porous polypropylene catalytic membranes served three key functions: (i) a well-defined and easily controlled gas-liquid interface; (ii) accessibility of the catalyst to the reagents (H2 and O2); (iii) high parameters of hydrogen mass transfer. A gas-liquid interfacial contactor was also used by Yao et al. [167] for the degradation (oxidation) of phenol in wastewater under mild conditions. PVDF porous hollow fibers bearing an adlayer of the Keggin-type polyoxometalate H5[PV2Mo10O40] were assembled in a tube-and-shell modulus, in which a phenol aqueous solution was fed to the modulus shell side while the air was circulated through the hollow-fiber lumen. Jia et al. [168] reported the liquid-liquid nucleophilic substitution between octyl bromide and potassium iodide in a contactor-type reactor composed of polyethylene hollow fiber membranes bearing anchored quaternized 4-vinyl pyridine as the active ion-exchange catalyst. A hollow-fiber modulus similar to those described above was used, in which the organic phase was fed to the hollow-fiber lumen while the aqueous phase was circulated through the reactor shell side. More recently, the use of contactors consisting of polymeric hollow-fibers containing palladium nanoparticles in a tube-in-tube microreactor configuration [169] or a tube-and-shell packing configuration [170], has been reported for the hydrogenation of nitroarenes.

B A

FIGURE 7–7 Scheme of a hollow-fiber contactor-type membrane reactor in a tube-and-shell configuration, in which reactant A is fed to the hollow-fiber lumen and reactant B is fed to the reactor shell side.

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7.3.2.2 Forced flow-through membrane reactors In this configuration an unselective porous catalytic membrane is applied in dead-end mode, the reactants being forced to flow through the membrane. Unlike the interfacial contactor-type reactors, in which reactants are fed to the reactor one on each side of the membrane, in forced flow-through catalytic membrane reactors (FTCMR) both reactants are fed from the same side of the membrane [6]. The function of the membrane is to provide a reaction space with a short, controlled residence time and high catalytic activity. As the catalyst is located on the walls of the membrane pores and the reactants flow convectively through those pores, the resulting intensive contact between reactants and catalyst leads to a considerable enhancement of catalytic activity [171]. Westermann and Melin [171] distinguish three different concepts of FTCMRs: (i) complete conversion integral FTCMR, which takes advantage of the high catalytic activity; (ii) selective integral FTCMR, applied to fast reactions, in which a maximum selectivity is reached due to the narrow contact time distribution; (iii) selective differential FTCMR, identical to (ii) but applied to slow reactions, with the reaction mixture being recycled to the feed side. 7.3.2.2.1 Non-selective flow-through catalytic membrane reactors In this mode of operation, the premixed reactants flow through the catalytic membrane in a single pass to achieve a very high conversion, taking advantage of the high catalytic efficiency due to the intensive contact between the reactants and the catalyst [171]. Polymeric porous catalytic membranes obtained by the track-etching technology bearing wide pores with catalysts anchored on their walls have been applied in the concept of complete conversion FTCMR. PCMRs in the flow-through configuration, using polymeric membranes with cylindrical pores bearing on their walls anchored palladium or gold nanoparticles, have shown to be effective in the 4-nitrophenol reduction [70], Congo red or methylene blue degradation [172,173] as well as in the Suzuki-Miyaura C-C cross-coupling reaction [93,174]. Most commercially available track-etched membranes are microfiltration or ultrafiltration membranes. Commercial microfiltration polymer membranes modified by anchoring sulfonated polystyrene on the pore walls, assembled in a flow-through configuration, were effective in the esterification of acetic acid with ethanol [175,176]. The same type of membrane in flat-sheet or hollow-fiber configuration and loaded with Pd nanoparticles have been successfully tested in the liquid phase reduction of 4-nitrophenol with sodium borohydride [177
180]. In addition to the track-etching technique, the thermally induced phase separation method has also been used for the preparation of porous membranes suitable for FTCMR operation. Porous polypropylene membranes prepared by this methodology and loaded with Ag nanoparticles have shown to be effective in reducing methylene blue in a flow-through configuration [181]. Polymeric FTCMRs, based on ultrafiltration membranes loaded with Pd nanoparticles on the pore walls, are also very effective in gas phase reactions, such as the selective hydrogenation of propyne to propene [182,183].

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Also, gas-liquid hydrogenations have been successfully performed using polymeric membranes in a flow-through configuration. Porous PVDF [184] or PES [170,185] membranes loaded with Pd nanoparticles on the pore walls showed to be effective in the hydrogenation of a variety of organic substrates. In this case, the liquid stream fed to the reactor is previously saturated with hydrogen. Polymer mats obtained by electrospinning are very suitable for use in FTCMRs due to their high specific area and porosity. Porous membranes consisting of PCL microfiber mats loaded with Pd nanoparticles have shown to be effective in reducing 4-nitrophenol to 4-aminophenol in a flow-through configuration [84]. PVA/phosphotungstic acid microfiber mats were efficiently used for biodiesel production via esterification in an FTCMR [78]. Zhang et al. [186] used an electrospun polypropylene fibrous membrane functionalized by grafting a sulfonic IL for acetic acid and ethanol esterification in a flow-through configuration. Luo et al. [187] reported the triglyceride methanolysis to biodiesel in an FTCMR using an alkaline composite catalytic membrane supported in a polypropylene nonwoven fabric. In their turn, Shi et al. [188] used a composite catalytic membrane consisting of PES and SPES blends supported in nonwoven PET fabrics, for the continuous esterification of oleic acid with methanol in a flow-through configuration. Microporous polymer membranes such as porous organic polymer (POP) membranes or CMP membranes loaded with metal nanoparticles are a promising solution for highperformance FTCMRs. Pd loaded POP [189] and Ag loaded CMP [108] membranes have been shown to have remarkable effectiveness in reducing 4-nitrophenol with NaBH4 in a flow-through configuration. 7.3.2.2.2 Selective flow-through catalytic membrane reactors In addition to the high catalytic efficiency, this mode of operation takes advantage of a narrow residence time distribution [171]. Selectivity can also be achieved due to membrane molecular weight cutoff properties or to the catalyst itself. Mixed matrix membranes obtained by phase inversion and loaded with fillers supporting active catalysts are suitable for FTCMR operation. The phase inversion methodology provides the membrane with an asymmetric porous structure, while the filler loading provides the necessary catalytic activity and improves membrane permeability. Such membranes allow not only high permeate fluxes but also show some selectivity, namely by size-exclusion or by controlling the contact time, having been used in a flat-sheet [59,60,65,67,190
196] or hollow-fiber [197,198] configuration. The FTCMRs assembled with these membranes were very effective in a variety of applications ranging from alcohol oxidation [65] to the Suzuki reaction [192], Heck and Sonogashira coupling [62], water treatment [67,190,191,199], benzene oxidation to phenol [193
196], trichloroethylene dechlorination [197] or several organic synthetic reactions including asymmetric synthesis [198]. Lee et al. [200] reported the hydrolysis of the nerve agent simulant 4-nitrophenyl phosphate in an FTCMR assembled with a PSf-based mixed matrix membrane filled with Zr-MOFs. Mixed matrix membranes obtained via covalent coupling of functionalized oligomers with nanoscale MOF particles decorated with polymerizable groups have shown to be effective in

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the Knoevenagel condensation of 4-nitrobenzaldehyde with malononitrile, in a flow-through configuration [201]. Hou et al. [50] used cellulose acetate mixed matrix membranes filled with the MOF Cu3(BTC)2 in an FTCMR for aldehyde acetalization. Membrane porosity, catalytic activity, and selectivity are provided by the filler. The hydrogenation of edible oils was successfully performed in the flow-through mode with porous PAI membranes filled with Pd or Pt nanoparticles, with permeate recycling [202
204]. FTCMRs have even been applied in the catalytic resolution of racemic mixtures. Namely, Zhao et al. [205] reported the hydrolytic kinetic resolution of racemic epichlorohydrin performed in an FTCMR assembled with a polyethylene hollow-fiber membrane, which had been modified by UV grafting of a salen complex.

7.3.3 Distributor-type membrane reactors In the distributor-type MR, the role of the membrane (often inert) is to dose the addition of the limiting reactant to increase the selectivity of kinetically controlled reactions and prevent side reactions and hot spots (Fig. 7
8) [3,6,15,112]. Most cases of distributor MRs involve ceramic membranes. However, there are a few examples in which polymeric membranes are used. Buonomena et al. [206] reported the selective oxidation of cyclohexane to adipic acid using porous PVDF membranes in a flat configuration, with the membrane separating the cyclohexene phase and the aqueous phase consisting of 30% H2O2, ammonium molybdate and succinic acid. The symmetric membrane with the highest hydrophobicity (water contact angle . 110 ) on both sides was able to induce a monodirectional transport of cyclohexane to the aqueous phase, across the overall membrane. Under these conditions the reactor is working as a distributor-type MR, quantitative oxidation of cyclohexane takes place and high selectivity to adipic acid (90%) is obtained. Less hydrophobic (water contact angle B 90 ) PVDF membranes favor the partial transport of the aqueous phase. Consequently, incomplete oxidation of cyclohexene occurs, leading to the accumulation of the intermediate 1,2-cyclohexane diol and the decrease of selectivity to adipic acid. Bamperng et al. [207] reported the decomposition of dyes in wastewater by ozonation using porous PTFE or PVDF hollow-fiber membranes. The ozone is supplied through the membrane walls, from the module shell side, while the dye-containing water circulates in the hollow-fiber A Feed

A

B Products

FIGURE 7–8 Scheme of a distributor-type membrane reactor exemplified for a reaction A 1 B!Products, in which the limiting reactant A is distributed to the reaction mixture containing reactant B [3,6,30].

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lumen. One of the reactants, ozone, is distributed through the membrane to a solution of the other reactor. Therefore, the MR is working in a distributer-type configuration. Greene et al. [208] reported the development of a tube-and-shell MR for the aerobic oxidation of alcohols, allowing the continuous delivery of O2 to a liquid phase along the entire tube length. The reactor uses O2 permeable PTFE tubing, which avoids hazardous mixtures of organic vapors and oxygen. The reactor was operated in a distributor-type mode by supplying oxygen through the tube walls, from the reactor shell side to a substrate acetonitrile solution containing a homogeneous catalyst (Cu/TEMPO or Cu/ABNO) circulated in the tubes. A tube-in-tube semipermeable Teflon AF-2400 MR has shown to be effective in the synthesis of 1-phenethylthiourea by reaction between ammonia and 2-phenylethyl isothiocyanate [209]. The best results were obtained when pressurized ammonia was fed to the reactor shell side and the isothiocyanate to the inner tube. The semipermeable Teflon AF-2400 membrane determines the monodirectional controllable mass transfer of NH3 to the isothiocyanate liquid phase, and therefore, the distributor-type mode of operation. The same reactor was also used for the synthesis of carboxylic acids via carboxylation of Grignard reagents [210]. In this case, the semipermeable Teflon membrane allowed the controlled and safe delivery of CO2, fed to the reactor shell, to the Grignard reagent circulating in the inner tube.

7.4 Polymeric membrane microreactors Microreactors have been considered one of the key technologies for PI goals being achieved [211
213]. High surface-to-volume ratios, high heat, and mass transfer rates, short residence times, high operating safety, high energy efficiency, etc., are among the many benefits of microreactors [213]. Membrane microreactors (MMRs), resulting from the integration of microreactors with membrane technologies have great potential to enhance the overall efficiency of microreactors [213]. Although membranes in MMRs are commonly ceramic there are a few examples of MMRs with polymeric membranes. Yamada et al. [214] developed a variety of palladium-nanoparticle membrane-installed microflow devices. Three types of polymers bearing anchored palladium ions were subjected to laminar flow conditions to form polymeric palladium membranes at the laminar flow interface. Reduction with sodium formate or heat allowed the creation of microflow devices incorporating membranes containing Pd nanoparticles. These microflow devices achieved instantaneous hydrodehalogenation of aryl chlorides, bromides, iodides, and triflates in the concentration range of 10 to 1000 ppm and residence times of 2
8 s. The microflow hydrodehalogenation was performed by using aqueous sodium formate instead of hydrogen gas. The same group reported the development of a microflow device with an installed catalytic polymeric membrane containing a dinuclear copper complex [215]. The microflow device was used for the instantaneous Huisgen cycloaddition of a variety of alkynes and organic azides to afford the corresponding triazoles in quantitative yield. Minakawa et al. [216] developed a new acetalization method in the presence of water by using a microflow reactor with an installed polymeric catalytic membrane. The catalytic

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Current Trends and Future Developments on (Bio-)Membranes

membrane was generated by complexation of poly(4-styrene sulfonic acid) with poly(4-vinyl pyridine) at the interface between two parallel laminar flows in a microchannel of the microflow reactor. Condensation of a variety of carbonyl substrates with diols proceeded in the presence of water in the microflow device to give the corresponding acetals in yields of up to 97% for residence times of 19 to 38 s. Selinsek et al. [217,218] reported the use of a MMR for the direct synthesis of hydrogen peroxide. Direct synthesis of hydrogen peroxide from hydrogen and oxygen is a very interesting process since it offers the potential to establish a “green” alternative to the conventional large-scale anthraquinone process. However, its implementation faces major hurdles, namely safety, since H2 and O2 form explosive mixtures in the gas phase over a wide concentration range. Microchannel MRs are especially interesting because the small inner dimensions prevent explosions. Simultaneously, introducing the reactants through a membrane into the liquid solvent containing the catalyst prevents the contact of H2 and O2 in the gas phase [218]. The used reactor was composed of two metal plates separated by a Nafion membrane. Both metal plates had engraved microchannels, one with a meander-like shape on the bottom plate, and two linear on the top plate (Fig. 7
9). Hydrogen and oxygen, fed separately to the microchannels of the top plate at 20 bar, were transferred through the membrane to the aqueous solution containing the catalyst (Pd/C or Pd/TiO2) in the microchannel engraved on the bottom plate, forming a fixed bed. The meander-like shape of the bottom channel allowed the alternate feed of both reactant gases to the liquid phase [218]. In this way, the direct contact of H2 and O2 is avoided, ensuring the process’s safety. Liu et al. [169] reported the hydrogenation of nitrobenzene to aniline in a tube-in-tube hollow-fiber catalytic MMR. The microreactor consisted of an external PTFE capillary tube containing an internal PP hollow-fiber coated with palladium nanoparticles. An ethanolwater solution was fed to the reactor shell side while hydrogen was fed into the hollow fiber.

FIGURE 7–9 Scheme of a membrane microchannel reactor for the direct synthesis of H2O2. Reproduced from M. Selinsek, M. Kraut, R. Dittmeyer, Experimental evaluation of a membrane micro channel reactor for liquid phase direct synthesis of hydrogen peroxide in continuous flow using nafions membranes for safe utilization of undiluted reactants, Catalysts 8 (2018). https://doi.org/10.3390/catal8110556.

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7.5 Conclusions and future trends MRs, offering the possibility of integration of reaction and separation in a single process unit, represent a significant contribution to the strategy of PI. When compared with their inorganic counterparts, polymeric membranes cannot withstand high temperatures or harsh reaction conditions. However, the versatility of organic polymers, the abundance of material types, and the variety of manufacturing and postmodification techniques make polymeric membranes very advantageous, compared to inorganic membranes, in the application of MRs in fine chemistry reactions. This chapter started with a survey on polymeric membrane classification according to membrane structure, namely on dense or porous, symmetric or asymmetric, integral or composite, and mixed matrix membranes. Membrane preparation techniques were referred to, particularly the preparation of metal nanoparticle-loaded mixed matrices, the phase inversion methodologies, track-etching, and electrospinning. Recent advances in IL membranes or microporous membranes such as PIM, COF, or other POP membranes were analyzed. Special attention was given to the classification of PMRs according to the membrane role. Extractor-type MRs make use of the membrane separation capabilities for the removal of the desired product to avoid a further reaction, or the removal of a side product to displace the equilibrium to the product’s side, in reversible reactions. Polymeric catalytically active or inert membranes are especially important in pervaporation-assisted esterification. In distributor-type MRs, a permselective membrane, often inert, allows controlling the slow addition of one of the reactants to the reaction mixture, leading to the improvement of selectivity of kinetically controlled reactions or the prevention of side reactions or hot-spots. In contactor-type MRs, the CAM promotes the contact between immiscible reactants and the catalyst active sites (interfacial contactor configuration), allowing to avoid the use of solvents. The forced flow-through configuration allows the intensive contact of the reactants with the catalytically active sites, leading to high conversions with short residence times. The significant advances achieved in the mixed matrices field, with the efficient incorporation of inorganic nanoparticles, carbon fillers, MOFs, etc., in polymer matrices, open perspectives for the use of PMRs assembled with this type of membranes in a wide variety of reactions. The high permeation fluxes of microporous polymers, namely PIMs, make them very suitable for use in PMRs. Recent developments in IL catalytic membranes make them attractive for CO2 conversion into valuable products. MMRs represent a step forward in PI, greatly enhancing the overall efficiency of microreactors. Promising examples of integration of polymeric membranes with microreactors, as contactors or distributors, have widened the potential of applications of these devices.

7.6 Acronyms ABNO BTC CAI

9-azabicyclo[3.3.1]nonane N-oxyl 1,3,5-benzenetricarboxylate catalytically inert membrane

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CAM CCE CMP CMR DMSO FTCMR HKUST HPW IL MCM MMR MOF Mr MTBE NIPS NMOF PAA PAI PAM PAN PCL PCMR PDMS PDOP PEBA PEI PES PES-C PET PFSA PHEMA PI pIL PIMs PMMA POP PP PPO PSSA PTFE PTSC PTU PVA PVDF PVP SALEN SPES SSA TEMPO

catalytically active membrane CO2 cycloaddition to epoxides conjugated microporous polymer catalytic membrane reactor dimethyl sulfoxide forced flow-through catalytic membrane reactor Hong Kong University of Science and Technology 12-tungstophosphoric acid ionic liquid mobil composition of mater membrane microreactor metal-organic framework membrane reactor methyl tert-butyl ether nonsolvent induced phase separation nanoscale MOF poly(acrylic acid) poly(amide imide) polyacrylamide polyacrylonitrile polycaprolactone polymeric catalytic membrane reactor polydimethylsiloxane polydopamine poly(ether-b-amide) polyetherimide polyethersulfone phenolphthalein polyethersulfone (Cardo-PES) poly(ethylene terephthalate) perfluorosulfonic acid poly(hydroxyethyl methacrylate) process intensification polymeric ionic liquid polymers of intrinsic microporosity poly(methyl methacrylate) porous organic polymer polypropylene poly(p-phenylene oxide) or poly(2,6-dimethyl-l,4-phenylene oxide) poly(styrene sulfonic acid) polytetrafluoroethylene (Teflon) polythiosemicarbazide polythiourea poly(vinyl alcohol) poly(vinylidene fluoride) polyvinylpyrrolidone ligand derived from N,N0 -bis(salicylidene)ethylenediamine sulfonated polyethersulfone sulfosuccinic acid 2,2,6,6-tetramethyl-1-piperidinyl-N-oxyl

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Tg TS VOC

181

glass transition temperature titanium silicalite volatile organic compound

Despite the significant developments achieved in the last two decades, the application of non-bio polymeric membrane reactors to industrial processes is still missing and efforts have to be made to pass from the bench scale to pilot and full scale [7,219].

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8 Current trends in enzymatic membrane reactor Azis Boing Sitanggang, Kiwinta Diaussie, Carmella Rosabel, Slamet Budijanto DE P AR T M E NT O F F O O D S CI E NCE AND TECHNOLOGY , IPB UNIV ERSIT Y, KAM PUS IPB DA RMAGA, BO GO R, INDONESIA

8.1 Introduction Enzymatic membrane reactor (EMR) is a reactor design where a combination of separation process via membrane filtration and biochemical transformation catalyzed by enzymes or cells is facilitated. This design allows selective removal of the product from the reaction side [1]. Nowadays, EMR has received increased interest due to its broad potential applications and beneficial factors, such as reduction of solvent and unconverted substrate losses. The application of EMR may reduce the economic cost of the entire chemical production process [2 4]. The operation of EMR may lead to an increase in conversion rate, especially for product-inhibited enzymatic reactions, due to continuous selective removal of the reaction product [5 9]. As compared to the conventional reactor designs, EMR has shown the ability to improve the efficiency of enzyme-catalyzed bioconversion, increase product yields, and feasibility of scaling up at the industrial scale [10]. The applications of EMR are mostly for typical enzyme-catalyzed hydrolytic reactions where the products have improved nutritional and functional properties [10 14]. EMR is an innovative technology for the hydrolysis of a racemic substrate to produce and separate pure optically active compounds that are useful in the pharmaceutical and food industries [15]. According to Belleville et al. [16], there are three main domains, especially for EMR applications, in the area of food and beverages. These include starch-derived products (e.g., production of sweeteners and oligosaccharides), production of functional protein hydrolysates, and production and modification of fatty acid (FA) esters. Two types of EMR design have been reported based on the physical/chemical properties of the reacting system [15]. The two types of EMR design are single (mono-) phasic reactor and multiphasic reactor. Multiphasic EMR is suitable for different substrate(s) and product solubility like water and organic media for lipid hydrolysis [16 19]. In the multiphasic reactor, the reaction occurs without emulsion formation, and products are obtained in a singlephase [16]. In a single-phase EMR, substrate (aqueous or organic) and enzyme remained in the reaction site (feed solution) and have an advantage for a simple operation [20,21]. Current Trends and Future Developments on (Bio-)Membranes. DOI: https://doi.org/10.1016/B978-0-12-823659-8.00003-4 © 2023 Elsevier Inc. All rights reserved.

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This review focuses on the application of EMR, especially for food technology that includes designs, membrane characteristics, processing or reacting conditions to produce a product in single and multiphase systems as well as EMR future trends. EMR is widely known in the industry sector, especially in food industries due to its efficiency in which separation and catalytic reaction can work simultaneously. EMR can work in single liquid (organic or aqueous) or multiphase liquid (organic/aqueous) for enzymatic hydrolysis reaction to produce products with increased nutritional and functional properties that can be applied in the food industries. In order to get satisfactory results during the production, it is necessary to design a reactor that is suitable for substrate and product characteristics. Therefore, it is important to elucidate the importance of EMR process parameters on the overall reaction performance.

8.2 Designs of enzymatic membrane reactor EMR is a tool that combines the separation process and biochemical transformations [5,9]. It can be done due to the role of membrane filtration and enzyme as biocatalysts in EMR. EMR uses membrane filtration as a selective barrier to separate permeable biocatalyst or substrate from permeable substrate and products based on their molecular weights (MW) with Da (Dalton) as a unit of measurement (1 Da 5 1 g/mol). The separation is done by the action of the driving force (pressure, chemical potential, electric field) present across the membrane that results in a movement (convection, diffusion, electrophoretic migration) of solute and enzyme as a biocatalyst to accelerate the biochemical transformation in the reactor [9,22]. EMR is designed to combine biocatalytic conversion, product separation, purification and concentration, and also catalyst recovery into a single continuous operation [22,23]. For hydrolyzing polymers, such as proteins or carbohydrates, EMR is a continuous flow system that could control MW distribution by adjusting the enzyme concentration and retention time during the reaction [24,25]. The enzyme in EMR could be dissolved freely in feed solution (in suspension) or immobilized on solid support [15,16,26,27]. There are three types of EMR configuration according to the position of the enzyme (biocatalyst) and membrane, namely (1) continuous stirred tank reactor (CSTR) with freely suspended enzyme, combined with membrane module where the enzymatic reaction and separation process in EMR work separately by retaining the enzyme and feed substrate on the retentate side while allowing the newly formed product to permeate through the membrane, (2) STR with enzyme immobilized on or within the matrix of the membrane by physical or chemical attachment where separation process and biocatalytic reaction work simultaneously, and (3) conventional STR with enzyme suspended in a liquid core which entrapped within microcapsules with semipermeable membrane walls that are made from lipid bilayer, hydrogel, and other synthetic polymers (Fig. 8 1) [5,28 31]. In EMR, the main flow types according to the direction of flows are classified into a deadend and cross-flow filtration. In a dead-end filtration, the stream of feed flows through the membrane where the feed stream direction is perpendicular (orthogonal) to the surface of

Chapter 8 • Current trends in enzymatic membrane reactor

Feed

197

Feed

Retentate

Enzyme

Membrane module

(A)

Pump

Enzyme activated membrane (B)

Permeate

Permeate

Feed

Entrapped enzyme

(C)

Permeate

FIGURE 8–1 Configuration of EMR according to the position of enzyme and membrane: (A) continuous STR (Stirred Tank Reactor) combined with membrane module to separate the product and to recycle substrates and free enzymes; (B) STR with enzyme immobilized on or within the matrix of the membrane; (C) conventional STR with entrapped enzyme within microcapsules with semipermeable membrane walls. Adapted from G.T. Vladisavljevi´c. Biocatalytic membrane reactors (BMR), Phys. Sci. Rev. 1 (1) (2016). https://doi.org/10.1515/psr-2015-0015.

the membrane. Rejected articles are retained and accumulated on the membrane surface. Meanwhile, in a cross-flow filtration, the stream of feed flows across the membrane where the feed stream direction is parallel (tangential) to the membrane surface. Rejected particles are transported along with the feed stream out of the system, considered retentate (Fig. 8 2) [32]. Feed solution flows by convective transport due to a difference in the transmembrane pressure (TMP) [2].

8.3 Membrane characteristics The membrane is a thin selective barrier between two fluids in order to separate the substrate from the product due to their molecular size differences [33,34]. The common membrane materials used are polymeric and inorganic membranes. They are classified into

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(A)

Rejected particles

(B)

Pressure

Rejected particles

Pressure

Permeate

Permeate

FIGURE 8–2 Schematic diagram of (A) dead-end filtration and (B) cross-flow filtration mode. A.Y. Kirschner, Y.-H. Cheng, D.R. Paul, R.W. Field, B.D. Freeman, Fouling mechanisms in constant flux crossflow ultrafiltration. J. Memb. Sci., 574 (2019) 65 75. https://doi.org/10.1016/j.memsci.2018.12.001, reproduced with permission.

microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) depending on their pore sizes. For polymeric membranes, the membranes used in UF, NF, and RO are asymmetric membranes that consist of a skin layer (dense layer surface) and a support layer (sponge layer) with large pores. The skin layer has small pores that are formed by a porous structure [22,33,35,36]. The term molecular weight cut-off (MWCO) is defined as a characteristic of UF or NF membrane pore size in which molecules with a larger MW than the MWCO of the membrane are retained for 90%, while smaller molecules will pass through the membrane [37]. The MWCO of a UF membrane is only an estimation since UF membranes have varied pore sizes, in the range of 2 100 nm, depending on membrane material and manufacturing process; also there is no rigid correlation between size (unit in nm) and MW (unit in g/mol or kDa) of a molecule. The molecule size within the reacting medium depends on the molecule type, pH of the solution, and ionic strength between molecules that leads to a change of molecule structure (e.g., primary, secondary, tertiary, quaternary structure of proteins) [33,37 40]. The mechanism of molecule separation using a UF membrane is size exclusion and electrostatic interactions between the feed solution and the membrane that depend on the feed solution and the membrane’s characteristics [38]. UF membranes with MWCOs in the range of 50 100 kDa have the capability to reject macromolecules (e.g., suspended solids, carbohydrates, proteins, unmodified enzymes, and pectins). UF membranes with MWCOs in the range of 4 30 kDa are effective to concentrate high MW compounds (e.g., tannins, proteins, hydrolysates) while MWCOs in the range of 1 3 kDa for low MW compounds (e.g., peptides, low MW sugars) [39]. A UF membrane with 6 kDa MWCO can enrich and separate oligopeptides (MW 0.2 1.2 kDa) [41], PES with an MWCO of 20 kDa can separate oligodextran (MW 5 8 kDa) [24], a membrane with 5 kDa MWCO can be used to concentrate the Angiotensin I-converting enzyme (ACE) inhibitory peptides (bioactive peptides) (MW , 3 kDa) [42 44].

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As indicated above, the material of a synthetic membrane can be classified into inorganic or organic (polymer) membranes. The inorganic membrane is made from ceramic (e.g., gamma-alumina on alpha-alumina, zirconia on stainless steel, and zirconia on porous carbon), metal, carbon, and glass membrane. According to Ebrahimi et al. [45], ceramic hollow fiber membrane (CHFM) has a higher chemical, thermal, and mechanical stability during the filtration process, low risk of fouling, and low adhesion potential for molecular organic compounds. The characteristics of CHFM are confirmed by a study conducted by Ur Rehman et al. [46]. They found that hollow fiber with a TiO2 skin layer and Al2O3 support layer with a membrane pore size of 20 nm was less prone to fouling and the adsorption of organic compounds. At the beginning of filtration, the enzyme only passed through the membrane, but after 30 minutes, the enzyme started to form a cake layer on the membrane surface which leads to enzyme immobilization on the membrane surface. On the other hand, organic membranes are normally made of chains of molecules (polymer) and have the characteristic of either hydrophilic (e.g., cellulose, polyamide, acryl, or polyacrylonitrile) or hydrophobic (e.g., polypropylene, polysulfone, polytetrafluoroethylene, polyvinylchloride, polyvinylidene difluoride, or polyetherimide). Polymeric membranes are relatively cheap as compared to inorganic membranes, and are available in a wide range of pore sizes (Fig. 8 3) [18,33,47,48].

Membrane

Synthetic

Biological

Organic

Hydrophilic

• • • •

Cellulose Polyamide Acryl Polyacrylonitrile

Inorganic

Hydrophobic

• • • •

Lipid bilayer

Ceramic Metal Carbon Glass

• • • • •

Polypropylene Polysulfone Polytetrafluoroethylene Polyvinylchloride Polyvinylidene difluoride • Polyetherimide

FIGURE 8–3 Schematic diagram of membrane classifications. Adapted from R. Field, F. Lipnizki, membrane separation process: an overview, in R. Field, E. Bekassy-Molnar, F. Lipnizki, G. Vatai (Eds.), Engineering Aspects of Membrane Separation and Application in Food Processing. CRC Press, 2017, pp. 3 40.

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Current Trends and Future Developments on (Bio-)Membranes

Permeate Retentate Feed (A)

(B)

FIGURE 8–4 Schematic diagram of (A) flat sheet plate membrane module; and (B) cross-sectional view of flat sheet plate module [2,51]. L. Giorno, R. Mazzei, E. Piacentini, E. Driolli, Food applications of membrane bioreactors, in: R. Field, E. Bekassy-Molnar, F. Lipnizki, G. Vatai (Eds.), Engineering Aspects of Membrane Separation and Application in Food Processing, CRC Press, 2017, pp. 299 360; Z. Su, J. Luo, X. Li, M. Pinelo, Enzyme membrane reactors for production of oligosaccharides: A review on the interdependence between enzyme reaction and membrane separation. Sep. Purif. Technol. 243 (2020) 116840. https://doi.org/10.1016/j.seppur.2020.116840, reproduced with permission.

Membrane modules are membranes packaged into a device or a vessel. There are five main designs of membrane modules: (1) flat sheet plate and frame/cassette module, (2) spiral-wound module, (3) tubular module, (4) capillary module, (5) hollow fiber module [33]. A flat sheet plate and frame/cassette module is like a plate heat exchanger or filter press and is the only module commonly used for electrodialysis. This type of module is an option for highly viscous feed streams [2] (Fig. 8 4). A flat sheet plate module has the advantage of being easy to disassemble for cleaning or replacing the membrane [49]. The spiral-wound module consists of at least two membrane pockets. Each pocket is an envelope consisting of a pair of membrane sheets with an active site facing out and separated by a spacer with a special weave. Three edges of the pair of membrane sheets are glued together with the fourth edge left open. This fourth edge is connected to a perforated central tube for permeate removal. The feed site flows between each of the “envelopes” (Fig. 8 5) [33]. Tubular, capillary, and hollow fiber modules are a group of tubes arranged in porous fabric or plastic support shells. The differences between them are the number of tubes and the dimension of the tubes inside the support shell (Fig. 8 6). Hollow fiber membrane modules have the highest surface-area-to-volume ratio compared to the other four membrane modules (flat sheet plate, spiral-wound, tubular, and capillary membrane module). However, the hollow fiber module is normally operated in a laminar flow condition which can cause clogging by the feed particles. Therefore, pretreatment of the feed solution is often required to filter out particles larger than 40 μm (Fig. 8 7) [33]. Pretreatment like pre-hydrolysis of substrates could reduce substrate’s MW and increase its solubility which leads to the decrease of membrane pore blocking by high MW substrates. Pre-hydrolysis has an important role to maintain the stability of membrane flux and reproducibility of MW distribution across the hydrolysis process [41]. Meanwhile, a tubular module is operated under a turbulent condition which helps to circumvent fouling and also improves the efficiency of cleaning [2]. A study conducted by Nath et al. [50] confirmed the above statement where static turbulence promoter inside of tubular membrane module with high tangential velocity across membrane surface, creates turbulence and vorticity of fluid on the membrane surface. It also provides a centrifugal force on fluid which creates a driving force on the membrane surface. As a result, gel layer or

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201

Retentate

Feed FIGURE 8–5 Schematic diagram of spiral-wound membrane module (SWM) with feed, retentate, and permeate flow direction [52]. Adapted from J. Schwinge, P.R. Neal, D.E. Wiley, D.F. Fletcher, A.G. Fane, Spiral wound modules and spacers, J. Memb. Sci. 242 (1 2) (2004) 129 153. https://doi.org/10.1016/j.memsci.2003.09.031.

Capillary membrane

Hollow fiber Do ~ 5 - 25 mm

Do ~ 0.8 - 7.0 mm

(A)

(B)

Tubular membrane Do ~ 5 - 25 mm (C) FIGURE 8–6 Typical dimensions of (A) Hollow fiber membranes; (B) Capillary membranes; and (C) Tubular membranes (Do 5 outer diameter; Di 5 inner diameter) [53]. Adapted from M. Scholz, M. Wessling, J. Balster, Design of membrane modules for gas separations, in: E. Drioli G. Barbieri (Eds.), Membrane Engineering for the Treatment of Gases Volume 1: Gas-separation Problems with Membranes. RSC Publishing, 2011, p. 125. https://doi. org/10.1039/9781849733472-fp001.

concentration polarization (CP) formation decreases due to the reduction of deposition of solute on the membrane surface and leads to an increase in permeate flux. Membrane filtration is a tool to separate reactants and products during the filtration process in EMR due to molecular size differences. Continuous reaction in the EMR can cause membrane

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Current Trends and Future Developments on (Bio-)Membranes

Retentate

Tube side feed Feed

(A)

(B) Permeate

(C) FIGURE 8–7 Schematic diagram of (A) Hollow fiber membrane module; (B) cross-sectional view of hollow fiber module; and (C) Tubular membrane module. Adapted from L. Giorno, R. Mazzei, E. Piacentini, E. Driolli, Food applications of membrane bioreactors, in: R. Field, E. Bekassy-Molnar, F. Lipnizki, G. Vatai (Eds.), Engineering Aspects of Membrane Separation and Application in Food Processing, CRC Press, 2017, pp. 299 360; Z. Su, J. Luo, X. Li, M. Pinelo, Enzyme membrane reactors for production of oligosaccharides: A review on the interdependence between enzyme reaction and membrane separation. Sep. Purif. Technol. 243 (2020) 116840. https://doi.org/ 10.1016/j.seppur.2020.116840.

fouling due to the accumulation of particles on the membrane surface that are larger than the membrane pore size. In an EMR, the freely dissolved enzyme molecules can be adsorbed on the membrane surface by physical attachment which leads to CP formation and might result in a decreased membrane permeability and a decreased enzyme activity [54]. In order to minimize enzyme adsorption on the membrane, it is better to choose a membrane with a smooth surface (low surface roughness) and a more hydrophilic characteristic. According to Zhong et al. [55], membrane roughness plays an important role in particle adhesion since the adhesion of particles on the membrane surface is the main cause of membrane fouling. Fouling is increased with increasing membrane roughness due to the large surface area and caused stronger adhesive force between foulant and membrane surface [55,56]. A hydrophilic membrane could prevent adsorption of enzyme onto membrane surface due to the formation of water molecules layer on the surface of hydrophilic membrane through hydrogen bonding interaction and decrease hydrophobic-hydrophobic interaction between foulant and membrane since protein, organic, inorganic substances, and many other foulants are hydrophobic in nature [56 59]. The enzymatic hydrolysis of sucrose with free invertase as a biocatalyst at pH 6.5 to produce high fructose syrup (HFS) as a sweetener using EMR with a modified membrane, was conducted by Wang et al. [54]. Membrane surface modification using PDA coating and

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203

L-cysteine (U-PDA-Cys) as an antifouling surface on polyethersulfone with an MWCO of 10 kDa showed improvement in membrane smoothness, negative charge, and hydrophilicity of the membrane (i.e., lower water contact angle). Meanwhile, invertases with an isoelectric point (IEP) of 3.4 4.4 were negatively charged at pH 6.5. As a result, adsorption of invertase on modified membrane led to constant sucrose conversions at 93% 95% after four reaction cycles, and with an HFS purity was higher than 97%. This, in conclusion, indicates modified membrane surface could maintain invertase activity or overall enzyme activity [54].

8.4 Enzyme immobilization in enzymatic membrane reactor Enzymes are proteins composed of amino acids and have a function as biocatalysts (biological catalysts) that accelerate biochemical reactions. They can be found in plants, animals, and also of microbial origin. Nowadays, an enzyme is preferable over a chemical catalyst due to its safety (non-toxic and non-flammable), eco-friendly, and ability to act under mild conditions. Besides that, the enzyme works specifically (only works on one type of substrate or molecule) and has high selectivity. This can entail a lower amount of by-products [9,60 64]. Buffer is a salt solution that helps maintain the pH level of the reacting medium. Normally, any enzyme-catalyzed reaction is performed in a buffered condition. There is a relationship between the solubility of an enzyme and the pH of the buffer solution (Fig. 8 8) [65]. Every

pH < IEP

pH  IEP

Solubility

pH > IEP

IEP

pH

FIGURE 8–8 Relationship between pH solution and solubility of the protein. Adapted from R. Scopes, Protein Purification: Priciples and Practice (C. Cantor, Ed.). Springer-Verlag, 1993.

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Current Trends and Future Developments on (Bio-)Membranes

R

O

R

excess

O

excess

H+ H3N+

CH

C

positive charge (cation)

pH < IEP

OH

R

O

CH

C

OHH3N+

CH

C

neutral charge (zwitterion)

pH

IEP

O-

H2N

O-

negative charge (anion) pH > IEP

FIGURE 8–9 Relationship between pH solution and ionic charge of amino acids. Adapted from Y.J. Yoo, Y. Feng, Y.-H. Kim, C.F.J. Yagonia, Fundamentals of Enzyme Engineering. Springer, 2017.

enzyme has an IEP at certain pH, at which amino acids, in total, have a neutral charge due to the equal amount of positively and negatively charged groups [61]. An enzyme is in the most stable state when the pH of the buffer solution is near or is the same as the IEP of the enzyme. Solvation of an enzyme at IEP can lead to enzyme precipitation due to the electrostatic interaction toward attraction between protein molecules. An enzyme has a positive charge when the pH of the buffer solution is below the IEP of the enzyme due to excessive H1 (hydrogen) in the solution; while a negatively charged enzyme is obtained when the pH of the buffer is above the IEP of the enzyme (Fig. 8 9). The electrostatic repulsion between the protein molecules makes enzymes unstable and more soluble in solution [61,65]. Enzymes are large, fragile molecules and are also expensive, so in order to make enzymes reusable and easily recovered, the enzyme could be immobilized on a solid support. An enzyme has higher activity in an aqueous phase due to the presence of water molecules as lubricants compared to an anhydrous media. Organic or hydrophobic solvents with a low water content can interact directly with the entire enzyme surface. This entails conformational changes of enzymes that lead to a decrease in their activities. A weakened affinity between enzyme and substrate, and often can inactivate enzyme [66,67]. The purpose of enzyme immobilization is to convert water-soluble enzymes into their insoluble form, protect the enzyme from shear stress, extend the enzyme’s lifetime in organic solvents, and also prevent conformational changes of the enzyme or enhance the stability of the enzyme, especially when in contact with anhydrous media. The porous structure of the membrane can act as solid support for enzyme immobilization [38,61,66,68,69]. An immobilized enzyme in comparison to the free enzyme is more robust with a wider variety of reactor designs and more resistant to environmental changes. However, an immobilized enzyme also has drawbacks such as a lower activity or conversion rate due to mass transfer limitations. Herein, a higher Michaelis constant (km) due to relative difficulties in accessing substrate is pronounced [62,68,70]. The enzyme immobilization on or in solid support is carried out through physical attachment (e.g., adsorption and entrapment) and chemical attachment (e.g., covalent bonding and cross-linking) between enzyme and support material (Fig. 8 10) [61,71 73]. The comparison between four methods for enzyme immobilization is shown in Table 8 1 [61,74].

Chapter 8 • Current trends in enzymatic membrane reactor

205

Methods for Enzyme Immobilization

Physical attachment

Physical adsorption

Support

Entrapment

Chemical attachment

Crosslinking

Covalent bond

Support

FIGURE 8–10 The most common methods to immobilize enzyme (small circles: enzyme). Adapted from Y.J. Yoo, Y. Feng, Y.-H. Kim, C.F.J. Yagonia, Fundamentals of Enzyme Engineering. Springer, 2017.

Physical attachment of an enzyme on the surface of the membrane or within the pore of the membrane was conducted without chemical reagents that may damage the enzyme. Adsorption is a physical attachment of the enzyme on the support material surface while entrapment is a physical method to immobilize the enzyme within the support matrix [75]. Forced membrane fouling (fouling technique) can be employed directly as an immobilization strategy for the adsorption and entrapment of enzymes in a membrane by physical attachment [76 78]. The mechanism of the immobilization of the enzyme by fouling formation is done via pressure-driven filtration [79]. Physical adsorption has a weak interaction between the enzyme and the membrane surface, likely due to the van der Waals force and hydrophobic force which lead to easy detachment or leakage of the enzyme [61,74,75]. An enzyme (alpha-CGTase) was immobilized on the cellulose nanofiber (CNF) layer so it could be easily cleaned and removed after the enzymatic process [80]. A study by Shamel et al. [81] proved that immobilization of lipase on a hydrophobic membrane (e.g., polysulfone) yielded a higher amount of immobilized enzyme compared to a hydrophilic membrane (e.g., regenerated cellulose). The study also showed that lipase adsorption increased along with the increased temperature due to higher accessibility of inner membrane pores (i.e., enlargement of inner membrane pores due to high temperature) [81]. The hydrophilic character of regenerated cellulose showed a much higher antifouling performance in comparison to less hydrophilic PES/PS membrane due to hydrophobic adsorption between solute and the PES/PS membrane surface [4,54,82 84]. The pH of the buffer solution can influence the net charge of the enzyme. Therefore, pH may dictate enzyme and membrane surface electrostatic and hydrophobic interaction. These interactions may result in adverse conditions, such as the adsorption of enzyme molecules on the membrane’s surface [24,85].

Table 8–1

Four methods for enzyme immobilization.

Method

Adsorption

Entrapment

Definition

Physical attachment of an enzyme onto the surface of the support material

Chemical attachment of enzyme Physical attachment Chemical attachment which forms onto the surface of the network structures between enzyme, of an enzyme support material support material, and coupling within the support reagent matrix Difficult Difficult Difficult Strong Strong Strong

Process Interaction

Easy and simple Weak (van der Waals, hydrophobic, dispersion force) Cost Low Disadvantages Enzyme leakage due to weak interaction between the enzyme and the support material; Mass transfer limitation (fouling) Advantages High activity; no need for chemical additives; mild preparation condition

Cross-linking

Covalent bonding

Low Mass transfer limitation (fouling)

Moderate Cross-linking reagent may damage the enzyme

High Presence of specific groups on the enzyme (e.g., his, biotin) is needed

Reduces enzyme leakage; stability improvement

Stable

Enzyme is very stable due to strong interaction between the enzyme and the support material

Adapted from Y.J. Yoo, Y. Feng, Y.-H. Kim, C.F.J. Yagonia, Fundamentals of Enzyme Engineering, Springer, 2017; A. Sassolas, L.J. Blum, B.D. Leca-Bouvier, Immobilization strategies to develop enzymatic biosensors. Biotechnol. Adv., 30(3) (2012) 489 511. https://doi.org/10.1016/j.biotechadv.2011.09.003; B. Zhang, Y. Weng, H. Xu, Z. Mao, Enzyme immobilization for biodiesel production. Appl. Microbiol. Biotechnol., 93 (1) (2012) 61 70. https://doi.org/10.1007/s00253-011-3672-x.

Chapter 8 • Current trends in enzymatic membrane reactor

207

The fouling type in the entrapment method occurs by pore blocking where the protein molecules are entrapped within the membrane pore with the drawback of mass transfer limitations [74,75,86,87]. Su et al. [24] showed that “enzyme fouling” by blocking larger membrane pores during filtration leads to an increase of the percentage of 20 nm pore size from 57% to 70% which results in a uniform pore size distribution of fouled membrane. The uniform pore size distribution led to a relatively constant product MW in the permeate and thus, improved the filtration performance [24,88]. Giorno et al. [15] produced FA from olive oil using lipase with an MW 57 60 kDa that was entrapped between the sponge and the dense layer of capillary aromatic polyamide UF membrane with an MWCO of 50 kDa. The organic phase containing the substrate was always streamed along the side of the membrane containing immobilized lipase to keep a close distance between the enzyme and the substrate. Additionally, the diffusion of water-soluble products through the membrane was faster than the oil phase due to the hydrophilic character of the membrane. Immobilized lipase by entrapment in the sponge layer (from the shell side) showed higher activity of 45% and a higher rate of FA production into the aqueous phase compared to lipase immobilization on the surface of the dense layer (from the lumen side). It was due to better enzyme distribution. Additionally, the enzyme molecules were more effective when present in an aqueous solution in membrane pores than present as a gel layer or deposit on membrane surface due to decreased diffusion distance from product to reach the aqueous phase [15]. Chemical attachment of the enzyme on the surface of the membrane was conducted with a chemical reagent which can create a strong bond between the enzyme molecules and the membrane’s surface. A covalent bond is established by a chemical interaction between functional groups of the enzyme and the activated group of the membrane surface [28]. Covalent attachment of an enzyme on a coated ceramic membrane was conducted by Lozano et al. [23] in anhydrous media. There were three main steps to immobilize the enzyme by covalent attachment, namely (1) deposition of two water-soluble polymers (gelatin/PEI) on ceramic (alpha-alumina) membrane to form a dynamic membrane, (2) Activation with a cross-linking agent (glutaraldehyde), and (3) Enzyme attachment on a coated ceramic membrane (Fig. 8 11) [23,89,90]. The benefits of forming a hydrophilic character of ultra-thin gel layer on the ceramic membrane are to obtain good selectivity with high permeability and to provide a hydrophilic condition for immobilized enzyme by a cross-linking agent. This has been reported to maintain the conformations of the enzyme’s active sites in anhydrous media by presenting the required water molecules [23,67,89,91]. Even in an anhydrous media, an enzyme still requires water molecules to keep the conformation of the active sites. On the other side, the formation of a dynamic membrane has been shown to gain a higher processing capacity and lasted longer due to limited membrane fouling [23]. Another study showed immobilized lipase by covalent bond using glutaraldehyde for providing chemical linkage between enzyme and modified membrane PMMA-EGDM (polymethyl methacrylate-ethylene glycol dimethacrylate) clay composite [92] and carbon membrane [48]. The membrane was modified using NOx and then aminated using 50% hydrazine hydrate to reduce the NO2 group into the NH2 group

208

Current Trends and Future Developments on (Bio-)Membranes

first step

Cross-flow

alpha-alumina support

Filtration of water for 60 min

Gelatin and PEI

Deposition of gelatin and/or PEI by cross-flow filtration for 60 min

Rinsing with water for 15 min

second step Activation by treatment with glutaraldehyde at 25 °C

Rinsing with buffer pH 7.8 for 15 min

third step Enzyme attachment at pH 7.8

Enzymatic membrane

FIGURE 8–11 Procedures of hydrophilic ultra-thin gel layer formation on a ceramic membrane for enzyme attachment by a covalent bond. P. Lozano, A.B. Pérez-Marín, T. de Diego, D. Gómez, D. Paolucci-Jeanjean, M.P. Belleville, et al. Active membranes coated with immobilized Candida antarctica lipase B: preparation and application for continuous butyl butyrate synthesis in organic media, J. Memb. Sci., 201 (1 2) (2002) 55 64. https://doi.org/10.1016/S0376-7388(01)00703-7, reproduced with permission.

on the polymer matrix. It showed that modified membranes were more hydrophilic due to the amine groups and more hygroscopic, which could adsorb about 79% moisture. This consecutively led to the increase in enzyme’s activity and was highly stable without unnoticeable change over a period of time [48,92]. The cross-linking method of enzyme immobilization has a strong interaction 3-D network structure between enzyme, support, and coupling reagent [61,74]. The optimum glutaraldehyde concentration (0.5%) as a cross-linking agent forms a solid gel or aggregation of enzymes, that is insoluble in water, on the polysulfone (PSF) membrane [17]. Tan et al. [93] immobilized lipase by a cross-linking agent on a PVA/CS (Polyvinylalcohol/chitosan) composite with the optimum glutaraldehyde concentration of 2.0% (w/v). This immobilization technique showed the highest lipase activity. The excessive amount of the cross-linking agent damages the activity site of lipase due to enzyme denaturation and reduces the accessibility of the catalytic site. On the other hand, a low concentration of cross-linking agents forms an unstable gel that leads to being easily washed off [17,93].

Chapter 8 • Current trends in enzymatic membrane reactor

209

8.5 Enzymatic membrane reactor versus other reactor configurations There are several conventional types of reactors for enzymatic hydrolyzes like stirred batch, fluidized bed, CSTR, and packed bed column [92,94]. The EMR, on the other hand, offers advantages compared to the conventional reactor due to its capability to work simultaneously as a selective barrier and biocatalytic reactor [94]. The batch reactor is a conventional reactor design for enzymatic hydrolysis due to its simplicity and ease of use. The batch reactor has disadvantages compared to the EMR because of its low productivity, high cost for the enzyme due to inactivation of the enzyme is needed to recover the final product, the degree of hydrolysis is not easy to control owing to the substrate, enzyme, and products always remain in homogeneous solution and variations in product characteristics from batch-to-batch production [9,41,95]. This type of reactor operates with no inlet and outlet flow during reaction time (Fig. 8 12A) [28]. CSTR is more cost-effective than the batch (A)

.

(C)

(B) Feed

Vin

Retentate

Enzyme

Membrane module

.

Vout Pump Permeate (D)

(E)

FIGURE 8–12 Schematic diagram of (A) Batch reactor; (B) CSTR; (C) EMR; (D) Fluidized bed reactor; (E) Packed bed reactor. Adapted from J.M. Lema, C. López, G. Eibes, R. Taboada-Puig, M.T. Moreira, G. Feijoo reactor engineering, in: Biocatalysis Based on Heme Peroxidases. Springer Berlin Heidelberg, 2010, pp. 245 290. https://doi.org/10.1007/ 978-3-642-12627-7_10.

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Current Trends and Future Developments on (Bio-)Membranes

reactor with a continuous inlet and outlet flow system and a constant volumetric flow rate (V_ in 5 V_ out 5 constant) [28]. Unfortunately, CSTR has a weakness compared to the EMR where enzymes would be removed together with outlet flow due to the absence of membrane filtration as a selective barrier [28]. The packed bed reactor (or fixed bed reactor) shows a steady flow of substrate through the bed with an immobilized enzyme. In the fluidized bed reactor, the immobilized enzyme is maintained in a suspension and the inlet flow from the bottom causes the immobilized enzyme in the suspension acts like a fluid (Fig. 8 12D) [28,96]. Fluidized bed reactors in comparison to packed bed reactors have advantages such as lower pressure drops at high velocity, reduced coalescence of the emulsion particles, and free of blocking of feed solution [97]. Packed bed reactor using Pickering Emulsions (PE) as beads showed high-pressure losses during the continuous process [98,99]. EMR is a continuous flow system that could control the MW distribution of the reaction products by adjusting enzyme concentration and retention time (Fig. 8 12C) [24,25]. The enzyme in EMR can be used in a free suspension or immobilized on or in the membrane [15,16,27]. As mentioned previously, the advantages of EMR compared to a traditional reactor are higher productivity (yield), reduce enzyme cost due to reusability and its high stability, reduce energy and wasted product by recycling, lower vary in end-product quality (high purity), easy operation and control, and easiness to scale up for industrial purposes [9,29,31,42,100]. On the other side, EMR also has drawbacks due to fouling problems that reduce the performance of the membrane, such as decreased permeate flux, especially when the membrane is used as catalyst support and selective barrier [31,85,101]. Wang et al. [41] mentioned that STR coupled with a hollow fiber membrane module could control the degree of hydrolysis of protein and could reach a higher level of oligopeptides production in comparison to a batch reactor.

8.6 Applications of enzymatic membrane reactor EMR has been successfully used for applications in different areas, such as pharmaceutical, energy and environment, and agro-food sectors. The reactions carried out in EMR can be varied and involve multiple polarities between the substrates and the resulted products [10,11,13,14]. In general, chemical reactions are classified according to the number and type of phases, namely homogeneous and heterogeneous systems [102] (Table 8 2). The homogeneous system is a chemical reaction that is composed of a single phase either gas, liquid or solid, and has no phase boundaries. Meanwhile, a heterogeneous system is a reaction that occurs in two or more different phases (e.g., solid/gas, solid/liquid, or immiscible liquid/liquid) and has boundaries between phases [103] (Table 8 3). The reaction can be carried out with the hydrophobic character of the substrate(s) while having a polar product. Herein, the enzyme is expected to work at the oil-water interface [98]. With this condition, an organic solvent is needed to increase the solubility of hydrophobic substrates while the enzyme needs water to maintain its activity [66]. Hence, there are two strategies that can be adapted to run the reaction, namely (i) using a biphasic membrane reactor where two immiscible liquids are separated by a membrane [15,17 19,27,81,92,93] (Fig. 8 13) and (ii) PE [98]. In a heterogeneous system, the enzymatic transesterification of organic

Chapter 8 • Current trends in enzymatic membrane reactor

Table 8–2

211

EMR applications for biphasic (or multiphase) reactions.

Membrane material and module

MWCO or Pore Filtration size Mode

Enzyme

Reaction

References

Capillary; aromatic polyamide membrane (hydrophilic)

50 kDa

Cross-flow Immobilized lipase from Candida rugosa

[15]

Hollow fiber; tetrafluoroethylene membrane (hydrophobic)

—

Cross-flow

[27]

Hollow fiber; cuprophane membrane (hydrophilic) membrane

5 kDa

Cross-flow

Flat plate; mixed cellulose ester (MCE) membrane (hydrophilic)

0.45 μm Cross-flow

Circular disc; poly (methyl methacrylate-ethylene glycol dimethacrylate) (PMMA-EGDM) clay composite membrane Modified porous carbon membrane

116 nm

Cross-flow

Hydrolysis of olive oil for the production of fatty acid and glycerol as nutrients and emulsifiers Hydrolysis of triolein in Free lipase from isooctane for the Candida production of oleic acid cylindracea and glycerol as nutrients and emulsifiers Immobilized lipase Hydrolysis of palm oil in isooctane as an organic from Candida solvent for the production rugosa of fatty acids and glycerol as nutrients and emulsifiers Immobilized lipase Hydrolysis of babassu oil for the production of fatty from Candida acids and glycerol as cylindracea nutrients and emulsifiers Hydrolysis of olive oil in Immobilized heptane for the production pancreatic lipase of fatty acids and glycerol (covalent linking) as nutrients and emulsifiers

4 nm

[48]

Hollow fiber; polysulfone (PSF) membrane (hydrophobic)

66 kDa

Flat sheet; polyvinylalcohol (PVA)/chitosan (CS) composite membrane (thickness 100 200 μm)

—

Flat sheet; polysulfone (PSF) (hydrophobic) membrane

8 kDa

Flat sheet; Polyethersulfone (PES) (hydrophobic) membrane

10 kDa

Immobilized Hydrolysis of olive oil for the pancreatic lipase production of oleic acids (covalent bond) Cross-flow Immobilized lipase Hydrolysis of olive oil for the from M. miehei production of fatty acids (palmitic acid and oleic acid) Cross-flow Immobilized lipase Hydrolysis palm oil for the synthesis of monoglyceride from Rhizopus (MG) as emulsifier oryzae (crosslinking with glutaraldehyde) Cross-flow Immobilized lipase Hydrolysis olive oil in n-octane as a solvent for the from Candida production of free fatty rugosa (crossacids linking with glutaraldehyde) Dead-end Lipase in Pickering Transesterification of 1-phenyl emulsion (PE) ethanol and vinyl butyrate for the production of 1phenylethyl butyrate as flavoring agent

[18]

[19]

[92]

[81]

[93]

[17]

[98]

Table 8–3

EMR applications for single-phase reactions.

Membrane material and module

MWCO or pore size

Filtration mode

Hollow fiber UF with selective layer TiO2 and support layer Al2O3 Flat sheet UF; PES (hydrophobic) membrane

20 nm

Cross-flow Immobilized enzyme Pectinex Ultra SP-L from Aspergillus aculeatus Dead-end Immobilized dextranase from Penilisilium sp.

Flat sheet UF; PES (hydrophobic) membrane Hollow fiber; Polysulfonic membrane

30 kDa

20 kDa

10 kDa

UF 5 kDa and NF Multistage flat sheet UF; 600 800 Da regenerated cellulose and NF sulfonate polyethersulfone membrane 5 kDa Flat sheet UF; Regenerated cellulose (hydrophilic) membrane Flat sheet plate; PES membrane 5 kDa

Hollow fiber; PES membrane

32 kDa

Hollow fiber; PES membrane

32 kDa

Dead-end

Enzyme

Reaction

References

Hydrolysis of molasses for the production of FOS (Fructo-oligosaccharides as prebiotic and lowcalorie sweeteners Hydrolysis of dextran for the production of oligodextran as precursors of iron-dextran for anemia treatment Catalysis of whey protein cross-linking for the recovery of cheese whey protein Hydrolysis of wheat starch for the production of starch hydrolysates (Maltodextrins, glucose syrups, and dextrose)

[46]

Immobilized transglutaminase (covalent bond) Cross-flow Free enzymes:-Termamyl Supra (heat-stable alpha-amylase) from Bacillus sp. -Dextrozyme (mixture of glucoamylase and pullulanase) from Aspergillus niger and Bacillus acidopullulyticus. -Finizym W (lysophospholipase) from A. niger and Aspergillus oryzae. Hydrolysis of casein glycomacro peptide Dead-end Free A mutated sialidase, Tr6, (CGMP)-sialic acid with lactose for the derived from Trypanosoma production of rangeli 3’-sialyllactose Hydrolysis of casein glycomacro peptide (CGMP)Dead-end Free A mutated sialidase, Tr6, sialic acid with lactose for the production of 3’derived from Trypanosoma sialyllactose rangeli Cross-flow Free alcalase Hydrolysis of wheat germ protein isolate (WGPI) for the production of ACE inhibitory peptides as antihypertension Cross-flow Free CGTase (cyclodextrin Hydrolysis of tapioca starch for the production glycosyltransferase) of cyclodextrin (CD) as emulsifier Cross-flow Free CGTase (cyclodextrin Hydrolysis of tapioca starch for the production glycosyltransferase) of cyclodextrin as antioxidant, emulsifier, and stabilizing agent

[24]

[57] [110]

[83]

[82]

[42]

[31] [111]

Hollow fiber; PES membrane

32 kDa

Cross-flow Free CGTase (cyclodextrin glycosyltransferase) Cross-flow Free protease N “Amano” G from B. subtilis

Flat sheet cassette; PES membrane

10 kDa

Flat sheet cassette; PES membrane

[112]

10 kDa

Cross-flow

[95]

Flat sheet cassette; PES membrane

10 kDa

Cross-flow

Flat sheet; Polysulphone (UFX10) membrane Flat sheet; Polysulphone (UFX10) membrane Flat-sheet; PES membrane

10 kDa

Dead-end

10 kDa

Dead-end

10 kDa

Dead-end

Tubular ceramic (alpha-alumina) 1.4 μm (inorganic) membrane coated with gelatin/ polyeyhyleneimine Flat sheet; PES membrane 8 kDa

Hollow fiber; Polyvinylidene fluoride membrane Flat sheet; PES membrane with U-PDA-Cys coating as antifouling Flat sheet cassette UF membrane

Cross-flow

Dead-end

Hydrolysis of tapioca starch for the production of cyclodextrin Hydrolysis of whey protein isolates for the production of functional and bioactive peptides Free protease N “Amano” G Hydrolysis of whey protein isolates for the from B. subtilis production of whey protein hydrolysate for functional food Free protease N Amano G from Hydrolysis of isolate whey protein for the Bacillus subtilis production of whey protein hydrolysate for functional food Free β-galactosidase from Hydrolysis of D-fructose and lactose for the Aspergillus oryzae production of Lactulose for functional food Free β-galactosidase from Hydrolysis of D-fructose and lactose for the Aspergillus oryzae production of Lactulose for functional food Free enzyme papain Hydrolysis of isolated temperature protein for the production of temperature based bioactive peptides Immobilized lipase from Candida Interesterification between castor oil TAG and Antarctica (cross-linking) methyl oleate (sunflower oil) for the production of Methyl ricinoleate as surfactant and plasticizer Free protease from Bacillus Hydrolysis of WPC (Whey Protein Concentrate) licheniformis Lactalbumin for the production of whey protein hydrolysate for functional food Free alkaline protease Hydrolysis of rice protein for the production of oligopeptide for functional food Free invertase from baker’s yeast Hydrolysis of sucrose for the production of high (S. cerevisiae) fructose syrup as sweetener

6 kDa

Cross-flow

10 kDa

Dead-end

5 kDa

Cross-flow Immobilized neutral protease on Hydrolysis of soluble CII (collagen type II) from chick sternal cartilage for the production of surface resin with immobilized immunomodulatory peptides from CII for arginine treatment of chronic disease

[113]

[114]

[4] [115 117] [84]b

[90]

[12]

[41] [54]

[101]

(Continued)

Table 8 3

(Continued)

Membrane material and module

MWCO or pore size

Filtration mode

Spiral-wound; PES membrane

1 kDa

Cross-flow Free savinase isolated from the Bacillus species

Tubular ceramic membrane

5 nm

Cross-flow Lyophilized trypsin from bovine pancreas

Flat sheet; regenerated cellulose membrane

10 kDa

Hollow fiber; polyethylene (PE) membrane

0.4 μm

Tubular zirconia ceramic UF membrane

10 kDa

Flat sheet; PES membrane

3 kDa

Flat sheet; PVDF membrane with ED (electrodialisis)

50 kDa

Tubular ceramic (alpha-alumina) membrane coated with gelatinpolyethyleneimine

0.2 mm

Immobilized α-CGTase-CNF (Cyclodextrin glucanotransferase on cellulose nanofiber) (covalent bond) Hydrolysis of pectin in olive mill wastewater and Cross-flow Immobilized pectolytic enzyme sulfuric acid for the production of galacturonic from Aspergillus species acid (sugar acid) as acidifying agent and (pectinase) (covalent bond) antioxidant Cross-flow Immobilized lipase from Alcoholysis (trasesterification) of 5,7-diPseudomonas cepacea acetoxyflavone in a polar solvent THF for the production of partially acetylated flavone (5,7dihydroxyflavon) Hydrolysis of BSA for the production of Cross-flow Free thermolysin (TLN) from B. precursors of bioactive peptides thermoproteolyticus rokko (T7902) Cross-flow Free bovine pancreatic trypsin Hydrolysis of milk protein (BiPro containing mainly beta-lactoglobulin protein) for the production of bioactive peptides Esterification of vinyl butyrate and 1-butanol in Cross-flow Immobilize Candida antarctica organic solvent for the production of butyl lipase B (CALB) (covalent butyrate (low molecular weight ester) as bond) flavoring agent Dead-end

Enzyme

Reaction

References

Hydrolysis of potato juice for the production of potato juice protein hydrolysate (PJPH) for functional food Hydrolysis of concentrate milk protein for the production of enzyme-treated liquid milk concentrate for ACE inhibitory peptide Hydrolysis of potato starch for the production of α-Cyclodextrin

[118]

[50]

[80]

[119]

[15]

[120]

[121]

[23]

Chapter 8 • Current trends in enzymatic membrane reactor

Organic phase

Aqueous phase

Enzyme activated membrane

Enzyme activated membrane S

Enzyme

S Organic soluble substrate

215

Aqueous phase P

Organic phase

P Aqueous soluble product

(A)

(B)

FIGURE 8–13 Schematic diagram of (A) cross-sectional view of the enzyme-activated membrane in multiphase membrane reactor process and (B) biphasic (multiphase) membrane reactor with enzyme activated membrane. Adapted from G.T. Vladisavljevi´c, Biocatalytic membrane reactors (BMR), Phys. Sci. Rev. 1(1) (2016) https://doi.org/ 10.1515/psr-2015-0015; S.L. Matson, Multiphase asymmetric membrane reactor systems. Google Patents, 1989.

PRC

Valve

Nitrogen tank

Waterbath

Substrate tank

Substrate Product Silica Enzyme

STR Membrane TIC

Heating plate

Balance

FIGURE 8–14 Schematic diagram of a multiphase EMR with W/O PE. A. Heyse, C. Plikat, M. Grün, S. Delaval, M. Ansorge-Schumacher, A. Drews, Impact of enzyme properties on drop size distribution and filtration of water-in-oil Pickering emulsions for application in continuous biocatalysis, Process Biochem. 72 (2018) 86 95. https://doi.org/ 10.1016/j.procbio.2018.06.018, reproduced with permission.

substrates can occur using PE in dead-end filtration mode [98]. Water-in-oil (W/O) PE is stabilized by nanoparticles, for instance, silica particles. The enzyme is dissolved in an aqueous phase surrounded by nanoparticles while reactant and product are dissolved in the organic phase (Fig. 8 14). Product is filtered through the membrane while PE and substrate are rejected and remain in the reactor side. A high concentration of silica particles leads to

216

Current Trends and Future Developments on (Bio-)Membranes

membrane fouling and a decrease in permeate flux. In order to prevent the fouling problem, the concentration of silica should be kept low as possible [98]. Unfortunately, this system has some drawbacks in terms of maintaining the enzyme’s activity. The preparation of W/O emulsion (where the enzyme is dissolved) has to be done under an extreme shear force homogenization that leads to the inactivation of the enzyme. Additionally, the separation of water- and oil-soluble products are of importance which adds to the system’s complexity [27,104]. Lipase is a biocatalyst that has been widely used in catalytic conversions such as hydrolysis, esterification, and transesterification reaction. This enzyme is often used in biphasic (multiphase) membrane reactors due to its reversible catalytic action and the ability to act in the water/lipid interface owing to its hydrophobic site [15,19,49,81,98,105]. Immobilized lipase on or in the membrane is suitable for the biphasic membrane reactor since it is simple and the product can be recovered from a single-phase [19]. The applications of EMR in biphasic reactions are mainly for oil hydrolysis to produce FAs using hollow fiber [18,27,81], capillary [15], and two different vessels separated by flat sheet membrane [17,93], and flat plate modules [19]. Immobilized lipase on or within a membrane called enzyme activated membrane which has a role as a boundary between two immiscible phases (organic and aqueous phase) (Fig. 8 13) [5,8,16,92,98,106]. The modification of the membrane to have a hydrophilic character on its surface for the immobilized enzyme in anhydrous media leads to the improvement of the enzyme’s activity [23,48,67,89,91,92]. Unfortunately, biphasic membrane reactor also has drawbacks, namely mass transfer limitations due to limited reaction interfacial area and high mass transfer resistance. Therefore, based on the drawbacks mentioned previously, PE is considered a superior alternative for carrying out multiphasic reactions providing a large reaction interface area which leads to an increased conversion rate [107,108]. A monophasic EMR can be used for multiple applications, such as for producing oligosaccharides, protein hydrolysates, and bioactive peptides in the aqueous system, and also for oil modification or FA ester production in the organic system [16]. An EMR operated under a cross-flow mode and based on an enzyme-activated ceramic membrane has been used to produce fatty ester [15,23,90]. Oil as a substrate has high viscosity which can limit its permeability through the membrane. Pomier et al. [90] have developed a new method to reduce oil viscosity by injecting supercritical carbon dioxide (SC CO2). The benefit of using supercritical fluid is that it could work at room temperature and there is no requirement for a solvent. Above critical temperature and critical pressure, CO2 becomes a supercritical fluid. According to Pomier et al. [90], in this particular application, the operating pressure is a key parameter to increasing the process productivity. Membrane fouling is the main problem in a dead-end filtration membrane reactor for enzymatic hydrolysis where a flat sheet membrane was placed at the bottom of the reactor. This typical design is common for lab-scale EMR. The accumulation of rejected particles such as enzymes and substrates on the membrane surface, due to a low ratio of membrane surface area to reactor volume, leads to a cake layer formation. This causes a decrease in permeate flux or an increase in TMP [4,12,109]. In order to avoid the fouling phenomenon, the applied TMP should yield an operation below the critical flux. Critical flux is related to minimal flux and maximal pressure for mass transfer through the membrane [12]. When the scale-up is needed, the configuration of EMR is best when operated under a cross-flow filtration mode.

Chapter 8 • Current trends in enzymatic membrane reactor

217

8.7 Conclusion and outlook EMR is a reactor design where the biocatalytic conversion and separation process work simultaneously. It offers many benefits in comparison to conventional reactors such as stirred batch, fluidized bed, CSTR, and PBR. The advantages of the EMR compared to the traditional reactor designs are having higher productivity (yield), reduced enzyme cost due to reusability and its high stability, reduced energy and wasted product by recycling, and less variation of endproduct quality (high purity). The most critical issue in the application of EMR is fouling. Fouling is an undesirable condition that can lead to a decreased permeate flux or increased TMP. Therefore, for typical hydrolytic reactions in EMR, it is important to have pretreatment and select the best membrane and membrane module characteristics, and optimal process parameters (e.g., pH, pressure, temperature, etc.) for fouling minimization. In the heterogeneous system or anhydrous media, conventional STR with enzyme molecules entrapped as PE combined with membrane module in cross-flow filtration mode is a good alternative reactor design. PE is normally stabilized by nanoparticles (e.g., silica particles). PE has high physical stability that can prevent droplet coalescence by steric interfacial barrier formation. The enzyme molecules in PE can create a large reaction interface area and has a short distance between the substrate and enzyme’s active pockets in comparison to an enzyme-activated membrane system. Meanwhile, in a homogeneous system, continuous STR combined with a membrane module in cross-flow filtration mode is considered an apt strategy. A membrane with a smooth surface (low surface roughness) has the ability to minimize enzyme adsorption on the membrane surface due to weak adhesive force between the enzyme and the membrane surface. Additionally, the utilization of an antifouling membrane might be needed to minimize enzyme and/or substrate and product adsorption on the membrane’s surface.

Nomenclature Acronyms ACE CALB CD CGMP CGTase CHFM CII CNF CP CS CSTR ED EMR FA FOS

Angiotensin I-converting enzyme Candida antarctica lipase B Cyclodextrin Casein glycomacro peptide Cyclodextrin glycosyltransferase Ceramic hollow fiber membrane Collagen type II Cellulose nanofiber Concentration polarization Chitosan Continuous stirred tank reactor Electrodialysis Enzymatic membrane reactor Fatty acid Fructo-oligosaccharide

218

Current Trends and Future Developments on (Bio-)Membranes

HFS IEP MCE MF MG MW MWCO NF PBR PDA PE PE PEI PES PJPH PMMA-EGDM PS PSF PVA PVDF RO SC STR SWM TAG THF TLN TMP UF W/O WGPI WPC

High fructose syrup Isoelectric point Mixed cellulose ester Microfiltration Monoglyceride Molecular weight Molecular weight cut-off Nanofiltration Packed bed reactor Polydopamine Polyethylene Pickering Emulsions Polyethylenimine Polyethersulfone Potato juice protein hydrolysate Polymethyl methacrylate-ethylene glycol dimethacrylate Polysulfone Polysulfone Polyvinylalcohol Polyvinylidene difluoride Reverse osmosis Supercritical Stirred tank reactor Spiral-wound membrane Triacylglyceride Tetrahydrofuran Thermolysin Transmembrane pressure Ultrafiltration Water-in-oil Wheat germ protein isolate Whey protein concentrate

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9 Membrane reactors in bioartificial organs Sabrina Morelli, Simona Salerno, Antonella Piscioneri, Loredana De Bartolo CNR-ITM, NATIONAL RESEARCH COUNCIL OF ITALY, INSTITUTE O N M EMBR ANE TE C HNOL OGY , RE NDE, C OSENZ A, IT AL Y

9.1 Introduction The substitution or restoration of failed organs has been a dream since ancient times. Organ failure is the leading cause of mortality all over the world. There are currently more than 100,000 patients on the US transplant waiting list [1] and despite a growing number of donors, the availability of suitable organs is still insufficient. Owing to this critical organ shortage, many patients died while awaiting an organ transplant. The growing crisis in organ transplantation and the aging population has driven a search for new and alternative therapies based on the combination of biomaterials and devices able to replace the functions of the organ in vivo. Bioreactors can offer innovative solutions for the fabrication of bioartificial organs because they have the capacity to integrate heterogeneous cell types and materials recapitulating the native organ geometry and functions [2]. Indeed, a biomimetic approach that utilizes expandable cells, a cell-affinity biomaterial, and an optimal bioreactor is required for the biofabrication of an organ or tissue. It is very challenging to use highly selective membranes to create a proper microenvironment for cell adhesion and growth that is necessary to build a 3D organ or tissue. Membranes of suitable molecular weight cut-off are able to compartmentalize cells in a well-controlled microenvironment acting as selective barriers to prevent immune system components from getting into contact with the implant while allowing nutrients and metabolites to permeate freely to and from cells. Membranes can serve as an extracellular matrix (ECM) that is able to promote the adhesion of anchorage-dependent cells modulating their physicochemical, mechanical, and structural properties. Several studies have demonstrated the biostability, biocompatibility, and selectivity of membrane bioartificial systems. Membrane bioreactors, through the fluid dynamics modulation, may simulate the in vivo complex physiological environment ensuring an adequate mass transfer of nutrients and metabolites and the molecular and mechanical regulatory signals [3]. These devices allow the hierarchical assembly of cells by emulating the predesigned organ architectures while ensuring the corresponding functionality. Current Trends and Future Developments on (Bio-)Membranes. DOI: https://doi.org/10.1016/B978-0-12-823659-8.00007-1 © 2023 Elsevier Inc. All rights reserved.

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Different types of devices have been proposed using flat sheet or hollow fiber membranes (HFMs), capsules, or coatings for the immobilization of cells [3]. Membrane bioartificial organs are engineered to be used as extracorporeal devices providing temporary support for patients with organ failure waiting for transplantation or as implantable systems. These devices can also offer an in vitro platform for drug toxicity testing and studies. They can be distinguished based on membrane material configuration, molecular weight cutoff or pore size, and cell culture technique. Cells can be compartmentalized in the lumen or shell of HFMs or between flat-sheet membranes, in a network of Hollow fiber (HF) membranes, spirally wound modules, encapsulated, or attached to microcarriers. In particular, HFM bioreactors (HFMBRs) are designed to mimic the native vessels inside a 3D tissue. Indeed, they provide a large surface area for the adhesion of cells and mass exchange in a small volume protecting cells from shear stress. These characteristics allow the culture of cells at high density in suspension or in adhesion, needed for tissue engineering, bioartificial organs, genetic therapy applications, and in vitro systems used to recapitulate the architecture of in vivo organs.

9.2 Bioartificial organs—design issues In the design of a membrane bioreactor to be used as a bioartificial organ, critical issues must be considered: type of cells, membrane properties that are important for mass transfer and cell adhesion, immunoprotection, and biocompatibility. Transport properties of the membranes are determined by pore size and molecular weight cut-off (MWCO), that is molecular weight of species retained by 90% within the membrane. Membranes with MWCOs between 70 and 100 kDa act as an immunoselective barrier, preventing immunoglobulins from passing while allowing the transport of other molecules. Morphological properties (e.g., pore size, pore size distribution, roughness) and physico-chemical properties (e.g., surface charge, wettability, surface free energy) reportedly influence the cell adhesion and metabolic functions [4,5]. Generally, membrane bioartificial organs consist of three compartments: culture medium (in vitro system) or blood/plasma (in clinical studies), membrane, and cellular compartment. Oxygen, nutrients, metabolites, growth factors, hormones, and other proteins, as well as endogenous catabolites, drugs, and toxic compounds (to be metabolized) must be transferred to cells, while catabolites, synthesized proteins, and other cellular products need to be efficiently removed from the cellular microenvironment. A complex exchange of numerous molecules with a wide range of sizes and physico-chemical properties occurs between the compartments. Each component has a different diffusion rate in the liquid solution. Additionally, the three compartments are all coupled together [6]. Therefore, the transport through the porous media (membranes, cells), as well as the consumption or production rates in the cells, affects the overall mass transfer. As previously described, the transport rate through the membrane depends on the MWCO, as well as the morphological and physicochemical properties of both the membranes and the molecules of interest. Moreover, some molecules can adsorb on the membrane and not pass through. In the cellular compartment, the reaction rate for each component is also different. For example, oxygen (small molecule

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present in higher concentrations) uptake rate is relatively high, while protein (large molecules present in very low concentrations) secretion rates are very low. The most critical transport phenomena issues which are often recognized as the limiting factors are oxygen concentration and shear stress. Oxygen is one of the main limiting nutrients due to its low solubility in an aqueous medium and the cell consumption rates [7,8]. In vivo cells are located at a distance of about 100 μm from capillaries which ensures the maintenance of cell viability. In vitro membranes (e.g., hollow fibers) can serve as vessels for the perfusion of cells protecting them from shear stress. One of the most influential yet controversial outcomes of direct perfusion (i.e., convection in the cellular compartment) is the unavoidable shear stress exerted on the cells [9]. Automation and control systems for fluid dynamic stability are required to improve reliability and to extend the operation time of a hollow-fiber membrane bioreactor to be used as the bioartificial organ or cell culture device.

9.3 Transport phenomena Computational modeling has increasingly been employed to analyze the transport phenomena inside the bioreactor, evaluate the cellular microenvironment, and optimize the operational conditions [10]. HFMBRs generally consist of a module packed with straight hollow fibers and the isolated cells are seeded in the extra-capillary space (ECS), either forming a cellular layer on the outer wall of the hollow fiber wall or filling the ECS. Mathematical models for fluid motion between a capillary and the surrounding tissue have long been studied [11]. In the majority of the modeling studies, the membranes and the cellular compartment are considered as porous mediums, and HFMBRs are described by the Krogh cylinder approach, which was originally introduced to describe oxygen supply from blood vessels to tissues. This approach has been employed for decades now to model hollow fibers used for different applications and under different operating modes. Several modeling studies have been reported for HFMBRs for different applications and conditions, including diffusion-controlled [12
14] and convection-enhanced [8,15
17] systems, considering multicomponent interactions [18] or study of hydrodynamics in different operating modes [19]. Despite the common use of HFMBRs and extensive modeling studies performed, some vital properties and parameters seem to be overlooked or underestimated. The effective diffusion coefficient, permeability, and even oxygen uptake rate in the 3D cellular aggregates are quite challenging to measure and are often estimated. Additionally, the porosity of membranes is subject to temporal change, due to adsorption of proteins, cell debris, and other culture components and consequently their transport properties. A convection-diffusion-reaction model has developed to describe momentum and mass transfer in a crossed HFMBR, in which the influential parameters have been parametrized through the implementation of applicable correlations. In particular, a systematic parametric study has been carried out to evaluate the effect of different parameters on oxygen transfer: operational parameters (oxygen tension, perfusion rate), design parameters (hollow fiber spacing, spheroid size), kinetic parameters (Michaelis-Menten kinetics for oxygen uptake) and microstructural properties (porosities of the spheroid and the membrane) [9].

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Mathematical models can be a powerful tool for a deeper understanding of transport phenomena inside a bioreactor. Partial differential equations of continuity and momentum balance customized for each domain (compartment) are coupled between domains through interfacial boundary conditions. The model considers as dependent variables velocity (u), pressure (p), and species concentration (c). Conservation of mass for incompressible fluids reveals: r:u i 5 0

(9.1)

The subscript i refers to the bioreactor compartment as the computational domain: lumen (l), membrane (m), cellular compartment (cc), and ECS. The equation of motion in the free flow regions— i.e., the lumina and parts of the ECS—is described by the NavierStokes equation: ρf

@u i 52ρf ðu i :rÞu i 1 μf @t

r2 u i 2 rpi

(9.2)

HF membranes and cells assembled in a 3D structure are considered uniform, isotropic porous media. The fluid transport in the porous media—i.e., membranes and cellular compartment—is described by the Brinkman equation, ρf

@u i 52ρf ðu i :rÞu i 1μei @t

r2 u i 2 rpi 2

μf ui Kbri

(9.3)

μe, effective viscosity in the porous medium, is generally considered either equal to μf or, defined as a function of μf and porosity. Species material balance for each component in the free fluid regions yields:

@cj;i 5 2 r: cj;i u i 1 r: Dj;i rcj;i @t

(9.4)

The subscript j refers to the molecule of interest. Mass balance in the porous media is described by Eq. (9.5):

@cj;i 5 2 r: cj;i u i 1 r: Dej;i rcj;i 1 Rj;i @t

(9.5)

In the membrane, no reaction term is generally considered, thus Rj;m 5 0. In the cellular compartment, R is the metabolic rate of consumption or production of the molecule, which depends on the molecule of interest and the cell type. Oxygen uptake rate is generally modeled by Michaelis-Menten kinetics [Eq. (9.6)]. Roxygen 5

Vmax co;cc co;cc 1 Km

(9.6)

where Vmax is the maximum consumption rate and km is the concentration at which the rate is half of Vmax. Reaction term is generally considered in the cellular compartment only.

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9.4 Membrane bioreactor as bioartificial liver In the human body, the liver is one of the largest and most complex organs essential for life. Consisting of highly specialized cells—the hepatocytes—, the liver performs metabolism, detoxification of various metabolites and molecules in the blood, protein synthesis, glycogen and vitamin storage, decomposition of red blood cells, production of hormones, bile, and biochemical necessary for digestion and growth [20]. Because of its multiple vital functions, liver failure and diseases can affect the whole body and may lead to death, and liver transplantation is the only option of treatment for patients with liver failure. Due to the shortage of donor organs, liver support systems and technologies are being developed and clinically tested as temporary bridges to transplant, or until the patient regenerates and recovers native liver function. Currently, no artificial organ or device is capable of reproducing all the functions of the liver. Bioartificial livers (BALs), incorporating isolated functional liver cells able to perform liver-specific biochemical detoxification, synthesis and modulation, show more promise as temporary extracorporeal liver-assist devices. A shunt veno-venous or arterio-venous is required to connect the patient to the device. The blood is removed from the body and circulated through the device where it is processed by living cells, and then returned to the patient using a dialysis-type procedure. In some devices, the blood plasma removed from the patient is separated from the other blood constituents by plasma separators before the circulation in the device. Several extracorporeal membrane BALs have undergone extensive animal testing and are currently investigated in the early stages of human clinical trials [21,22]. BALs with homogeneous and stable in vivo-like microenvironments can be further employed as valid tools to investigate the basic biology of hepatic tissue development, study the pathways and mechanism underlying the onset of pathology, develop a therapeutic strategy for drug screening application, and evaluate effects and toxicity of new drugs and therapeutic compounds in preclinical tests. The incorporation of human hepatocytes makes these devices more accurate, rapid, and highly specific tools that can be employed as a valid alternative to in vivo animal experimentation [23]. Critical issues in the design of a BAL are represented by the source and the number of the cells that can maintain for a long time the liver-specific functions, the configuration, the mass transfer resistance and limitation, the oxygenation and gas exchange, the scale-up, the regulation and safety [24]. An ideal device should integrate efficient mass transport, scalability, and the maintenance of liver-specific cellular functions. Membrane bioreactors utilizing a microporous membrane with a large pore diameter (0.2 μm) ensure the hepatocyte compartmentalization allowing the free passage of plasma proteins with the advantage of enhancing the fluid convection and the mass transfer condition. The high and continuous perfusion ensures the simultaneous exchange of metabolites and O2/CO2, allowing the maintenance of liver cells in a homogeneous and in vivo-like microenvironment that is responsible for the long-term maintenance of specific liver cell functionality and viability. Moreover, the membrane surface offers a biomimetic substratum for the adhesion of hepatocytes [25,26]. It has been demonstrated that membrane properties strongly affect and modulate the cell-material interaction guiding the hepatocyte adhesion and functions [4,27,28]. Several papers highlighted

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the role of polymeric membranes as specific cues that mimic the natural microenvironments, able to drive and support a 3D liver tissue formation sustained by ECM protein deposition and capillary development [29,30]. The proper choice of the membranes with specific properties, and the optimization of the device in terms of fluid dynamics, mass transport, and gas exchanges, decisively influence the cellular commitment in terms of differentiation and/or activation of metabolic functions for embryonic or liver progenitor cells, or in the maintenance of the differentiated and metabolically active state for primary hepatocytes [31
33].

9.4.1 Membrane bioartificial livers in flat configuration Membranes in a flat configuration are largely used in small-scale devices because of their operational simplicity. Some concerns in their clinical applications are related to complexity in scaling up the systems with the cell density required, and an optimal ratio between surface area and volume. Flat membranes are mainly utilized either in plate or in spirally wound forms. Several flat membrane bioreactors have been used over time to develop BALs. Perfused microbioreactors containing flat polyester and porous polydimethylsiloxane (PDMS) membranes were assessed with rat hepatocytes as promising tools in drug screening [34]. A membrane bioreactor, with optimized fluid dynamics resembling an ideal continuous stirred tank reactor, was developed by using a polyethersulfone (PES) membrane functionalized with galactose. In this device, the effects of drugs and pro-inflammatory cytokines were investigated on primary human and pig hepatocytes [28,35,36]. Flendring et al. developed The Academic Medical Center Bioartificial Liver (AMC-BAL) in which a spirally wound non-woven polyester matrix was employed for hepatocytes adhesion, among polypropylene (PP) HFMs for oxygen delivery [37]. The AMC-BAL is the only BAL in a flat configuration enrolled in a clinical trial [38,39]. Roy et al. developed a microchannel flat-plate bioreactor with a polycarbonate (PC) membrane on which rat hepatocytes adhere, and a polyurethane (PU) gas-permeable membrane on the top receiving oxygen from an internal membrane oxygenator [40]. A rotating wall-gas permeable membrane system including polytetrafluoroethylene (PTFE) membrane in the bottom, enhanced the viability of hepatocyte spheroids that were formed and maintained in microgravity conditions in controlled oxygenated conditions [7]. A large-scale flat membrane bioreactor was developed by using a multitude of stackable modules, each of them constituted by two flat PTFE membranes containing primary pig hepatocytes cocultured with nonparenchymal cells within a collagen sandwich [41]. With a similar configuration, a small-scale oxygen permeable membrane bioreactor mimicked the sinusoidal environment and maintained up to 32 days of human hepatocytes with high viability and functionality [42]. More recently, a human liver microtissue was created in a gas permeable membrane bioreactor by coculturing human hepatocytes, endothelial cells, and skin-derived mesenchymal stem cells [32] A microporous PC membrane allowed the physical separation of the skin-derived MSCs from the other cells, ensuring the biochemical communication by means of the secreted factors. The designed bioreactor offered cells a highly perfused and homogeneous microenvironment allowing an adequate O2/CO2 mass transfer

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exchange through the flat-sheet gas permeable fluorinated ethylene propylene membranes that delimited the bioreactor walls. Signaling and stimuli from the MSCs allowed the creation of a vascularized liver microtissue displaying specific phenotypical characteristics and enhanced functional performance. On the other side, specific growth factors secreted by the hepatocytes and endothelial cells boosted the differentiation onset of skin-derived MSCs in hepatocyte-like phenotype, opening new opportunities in harvesting autologous cells to differentiate and use as a hepatic cell source.

9.4.2 Membrane bioartificial livers in hollow fiber configuration Among the most common configuration, HFMs membranes are largely employed in the design and development of BAL, taking into account that from the scalable perspective they offer greater surface area in small volume in comparison to flat membranes. In the majority of devices, HF is parallel assembled and hepatocytes are cultured in the extracapillary space of the fibers, while medium or patient’s blood/plasma flows through the lumen. One of the first BALs employed porcine hepatocytes in free suspension in the shell of polyvinylchloride HFs [43]. This BAL underwent the first large clinical trial. In the HepatAssistTM Circe Biomedical, which was tested on a large-scale clinical trial, hepatocytes were maintained in the shell of parallel-assembled polysulfone (PSf) HF membrane (0.15 μm pore size) in adhesion to collagen-coated microcarriers [44]. TECA hybrid artificial liver support system (TECAHALSS), which was tested in phase I clinical trials, loaded hepatocytes spheroids in the extrafiber space of PSf HF 100 kDa MWCO [45,46]. Only a few devices loaded the hepatocytes in the lumen of parallel-assembled HF membranes of PSf 100 kDa MWCO, with the continuous perfusion of the patient’s blood in the shell compartment [47,48]. Cellulose acetate (CA) HF membrane cartridges with 70 kDA MWCO were used in the Extra-corporeal Liver Assist Device (ELAD, Vital Therapies, Inc.) that was enrolled in an extensive phase III clinical trials [49]. Successively the device was upgraded, increasing the membrane cut-off (up to 120 kDa) together with the cell mass [50]. The Excorp Medical Bioartificial Liver Support System, developed at the University of Pittsburg and involved in phase II of clinical trials, was made of cartridges of CA HF membranes 100 kDa MWCO loaded with hepatocytes embedded in a collagen matrix in the extracapillary space [51,52]. A singular configuration was achieved in a membrane bioreactor constituted of modified PES multibore fiber that through a foamy porous and highly permeable support structure clusters in a single fiber seven capillaries in which were compartmentalized primary human hepatocytes [27]. Inversely, in a similar configuration, liver cells were inoculated in the microporous structure of a multicapillary PU foam constituted of capillaries in which flowed the culture media [53]. More complex configurations were developed by assembling different sets of HF membranes. Jasmund et al. developed an oxygenating HF bioreactor by alternating crosswise two mats of porous PP oxygenating HF membranes, and polyethylene (PE) heat exchange HF membranes, among which hepatocytes were maintained in direct contact with the perfused medium under high control of temperature and oxygen [54].

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A network with a counter-current medium perfusion and internal oxygenation was developed by intertwining four independent capillary membranes among which hepatocytes were cultured [55] using polyamide (PA) for medium/plasma inflow, PSf for medium/plasma outflow, hydrophobic PP for oxygen/carbon dioxide exchanges, hydrophilic PP for sinusoidal endothelial coculture. The device concept was used in the Liver Support System (LSS Charitè-Humboldt University, Germany) involved in a phase I clinical trial, and subsequently integrated into a Modular Extracorporeal Liver Support system (MELS) enrolled to phase I clinical trials [56]. The miniaturized membrane bioreactor was used as a small-scale device for testing liver detoxification and specific biotransformation pathway of different drugs [57
59], for the expansion and differentiation of fetal hepatic cells [60
62], and for the hepatic maturation of human-induced pluripotent stem cells [63,64]. A peculiar structure was developed in a multicoaxial HF bioreactor in which concentric tubes with increasing diameter were located one inside the other. The hepatocytes were cultured in a sandwich configuration under radial flow in the space between two polysulfone HFs [65]. A further configuration was adopted by the alternate cross assembling of two bundles of HFs with specific physicochemical, morphological, and transport properties. A set of polyetheretherketone modified with cardo group (PEEK-WC) HFs, 190 kDa MWCO, handled the feeding of the oxygenated medium, and a set of PES HFs, 0.2 μm pore size and with high hydraulic permeance (J 5 15.2 L/m2h  mbar), promoted the removal of the waste medium. In the crossed HFs membrane bioreactor, primary human hepatocytes retained their differentiated morpho-functional behavior up to 19 days of culture, and liver progenitor cells proliferated and differentiated in mature hepatocytes [31,33]. With the same concept, a crossed membrane bioreactor constituted of two bundles of only PES HF membranes, favored the creation and long-term maintenance of functional human liver microtissues. Liver spheroids with uniform size and shape self-assembled giving rise to liver-like organoids without any oxygen limitation in the core of the spheroids, as corroborated by the oxygen uptake rate and by the mathematical modeling (Fig. 9.1) [66]. In the same membrane bioreactor, a sequential seeding and coculturing of human sinusoidal endothelial cells, stellate cells, and hepatocytes mimicked the cellular layering in hepatic lobules [67], and human hepatic mesenchymal stem cells acquired a mature hepatic phenotype profile [68]. More recently, a bottom-up tissue engineering approach allowed the creation of a 3D human vascularized liver tissue that was included in a poly(ε-caprolactone) (PCL) HFs membrane bioreactor [69]. Human endothelial cells were compartmentalized in the lumen of the fibers, and primary human hepatocytes were loaded on the external surface and between the fibers (Fig. 9.2). For this purpose, biodegradable PCL HF membranes were synthesized by the dry-jet wet phase inversion spinning method with microporous interconnected pores along the membrane walls. The morphological and transport properties of PCL HF membranes together with the optimized fluid dynamics of the bioreactor created a physiologically relevant microenvironment that favored a considerable integration and communication among the cells. Vascular channels were formed by the endothelial cells that colonized the lumen of the PCL HFs, and that enhanced the specific functional activity of the hepatocytes through their secreted factors that permeated across the microporous membrane walls.

FIGURE 9.1 Crossed hollow fiber membrane bioreactor for the creation and maintenance of human liver microtissue spheroids. Reprinted from H.M.M. Ahmed, S. Salerno, A. Piscioneri, S. Khakpour, L. Giorno, L. De Bartolo, Human liver microtissue spheroids in hollow fiber membrane bioreactor, Colloids Surf. B, 160 (2017a) 272
280, with permission of Elsevier.

FIGURE 9.2 Poly(ε-Caprolactone) hollow fiber membrane bioreactor for the biofabrication of a human vascularized hepatic tissue. Reprinted from S. Salerno, F. Tasselli, E. Drioli, L. De Bartolo, Poly(ε-caprolactone) hollow fiber membranes for the biofabrication of a vascularized human liver tissue, Membranes, 10 (2020) 112 (license under Creative Common Attribution).

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9.5 Membrane bioreactors for bioartificial kidney Kidney disease affects over 10% of the world’s population and causes more deaths than breast or prostate cancer. Chronic kidney disease (CKD) is a complex and dangerous clinical condition characterized by the alteration of the multiple kidney functions and is caused by several dysfunctions or pathologies that also affect other organs. CKD is often associated with cardiovascular risk factors, such as arterial hypertension, dyslipidemia, and type II diabetes mellitus, which have a very negative impact on the quality of life. Dialysis is the current treatment for patients with kidney disease, but, although this process is used in clinical practice, it does not completely substitute all kidney functions. It can perform only filtration but it is not capable to reproduce renal metabolic and endocrine functions. Kidney transplantation could be a possible solution, but, unfortunately, it is limited due to the lack of donors available. In this context, a good alternative is the realization and employment of artificial organs. Over the last decades, with the aim to realize a complete and more functional device for kidney failure patients, research has been focused on the development of a bioartificial kidney (BAK). The realization of a wearable or implantable BAK for full renal replacement therapy may significantly reduce morbidity and mortality in patients with acute or CKD by improving the quality of life. The first BAKs were extracorporeal devices consisting of a conventional synthetic hemofilter, which removes toxins from the blood, connected in series with a bioreactor unit containing renal tubule cells, with the specific functions of maintaining adequate water volume, blood pressure, electrolyte balance, and metabolic functions, as a renal assist device (RAD), in an extracorporeal blood circuit [70
72].

9.5.1 Membranes for BAK Commercial hemofiltration/hemodialysis cartridges with HF membranes have been used as bioreactor units of BAKs. Researchers employed different kinds of commercial fibers produced by different companies [71
76]. The most employed for the realization of a BAK are HF membranes consisting of polysulfone/polyvinylpyrrolidone (PSf/PVP). The HF synthetic membranes provide immunoprotection and also serve as architectural scaffolds for cells in the long-term implantation in a xenogeneic host [77]. To improve cell adhesion, proliferation, and differentiation, the membranes are coated with an ECM, consisting in most cases of pronectin [75], laminin [71,78,79], and bovine collagen type IV [71,76]. These last two molecules (collagen IV and laminin) were best suited for BAK applications and improved the formation and maintenance of differentiated epithelia by proximal tubule cells [80
82]. Membranes used in BAK have been optimized to be in contact with blood. In particular, hydrophobic membranes have been modified with hydrophilic additives to improve the process. For this purpose, asymmetric membranes were developed by blending PSf with a phospholipid polymer to have one hemocompatible and one cytocompatible surface, with a rough sponge layer for cell growth and the smooth skin layer exposed to the blood, respectively [83]. Asymmetric membranes were also prepared by

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coating the surfaces with antifouling agents such as polyethylene glycol on the blood exposed side and adhesive molecules on the cell-exposed side that have led to good improvements. Recently, PES HFMs were successfully biofunctionalized by culturing human immortalized proximal tubule epithelial cell (ciPTEC) monolayers [84].

9.5.2 BAK devices in animal studies and clinical trials During the past decade, different types of BAKs were employed in animal experiments [71
73,77,78,85
88] and clinical trials [74,89]. In the device developed by Humes and coworkers [74], the blood and the hemofiltrate were directed from the hemofilter into the bioreactor (Fig. 9.3A). The hemofiltrate flew in the lumen of the HFM in the bioreactor, where it was directly exposed to the cells. The blood was directed to the extra HFM space, where it was separated from the renal cells by the semipermeable membrane. This architecture mimicked that of the renal proximal tubule. It was expected that the bioreactor unit would replace functions of the renal proximal tubule. Preclinical studies demonstrated that the RAD possesses multiple transport, metabolic, and endocrine functions representative of

FIGURE 9.3 BAKs configurations in animal studies and clinical trials: (A) RAD; (B) BioKid; (C) implantable bioartificial kidney; (D) WEBAK; (E) lab-on-a-chip bioreactor system. Reprinted from L. De Bartolo, E. Curcio, E. Drioli, Membrane Systems: For Bioartificial Organs and Regenerative Medicine, De Gruyter: Berlin/Boston, 2017, 1
264 with permission from De Gruyter.

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kidney tubular epithelium [73]. Researchers conducted clinical trials in patients with acute kidney injury, and encouraging results were reported [74,89]. Particularly, phase I/II clinical trials clearly indicated that RAD therapy was sufficiently safe, with a substantive clinical impact on survival and renal recovery. The RAD is, up to now, the only BAK device that has been successfully tested in humans. Cytokine levels in serum and plasma were measured in various animal studies [77,78,85,86,88] and in the Phase I/II clinical trial [74]. Some changes in cytokine levels suggested that the beneficial effects of BAK treatment might be a result of immunomodulatory functions. Saito and coworkers realized a BAK characterized by continuous hemofiltration with the culture of renal cells 72,87, which provides a reliable system for the treatment of CKD patients [75,76,90]. The group of the University Medical Center Groningen realized the Biological Support for Kidney Patients, namely BioKid, [91] (Fig. 9.3B). This BAK consisted of HFMs that were optimized by coating the lumen for providing an enhanced microenvironment for the culture of renal cells. BioKid improves the quality of hemodialysis treatment by enabling toxin elimination, which usually persists at the end of hemodialysis, therefore avoiding the toxic effect caused by their accumulation. Ronco et al. developed an implantable BAK to replace conventional dialysis in patients needing chronic renal replacement therapy [92] (Fig. 9.3C). The device presents a filter consisting of highly efficient membranes constructed from semiconductor silicon wafers that enable filtration without requiring pumps or electrical power while protecting the renal cells from rejection by the patient’s immune system. Buffington et al. proposed a new alternative approach to RAD [81]. They developed a Wearable bioartificial kidney (WEBAK) that combines peritoneal dialysis and a bioartificial renal epithelial cell system (BRECS) (Fig. 9.3D). The BRECS is a compact, cryopreservable extracorporeal renal replacement device in which primary renal epithelial cells are grown on porous carbon disks that are designed to come into contact with ultrafiltered blood or peritoneal dialysis. WEBAK utilizes peritoneal fluid to maintain cell viability and functionality and comprises the use of sorbent-based technologies to replace the excretory function of the kidney and the compact BRECS to replace the metabolic function of the kidney. The main advantages of BRECS over RAD include the use of a cryopreservable system to enable distribution, storage, and therapeutic use at the point of care facilities, fast manufacture and low cost, and better and more efficient method to obtain cells and small size and low dead volume. It represents the first all-in-one culture vessel, cryostorage device, and cell therapy delivery system. BRECS was tested in large animals to determine its safety profile and potential efficacy [93]. This device has the potential to be used in both acute and chronic therapeutic applications. Innovative BioTherapies, Inc., the company developing the BRECS, plans to advance the product from preclinical trials to human clinical trials within 3
5 years [94]. Over the last 20 years, bioengineering research has led to the development of several on-chip platforms for modeling human kidney disease in vitro and predicting drug toxicity [95]. Ng and coworkers developed a lab-on-a-chip bioreactor device to engineer a bioartificial renal proximal tubule for improved renal substitution therapy [83]. The system consists of an HF membrane of

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PES-PVP with an outer skin layer that reduces protein adsorption and provides immunoprotection, and an inner surface precoated with fibrin that enhanced cell attachment (Fig. 9.3E). This BAK is capable to mimic the complexity of native tissue and shows great promise for generating advanced experimental models of renal diseases and tools for studying drug toxicity. In this context, recently, 3D bioprinting technology has been employed for the generation of more accurate kidney models that emulate the cellular composition, geometry, and function of the native tissue. By combining bioprinting, 3D cell culture, and organ-on-chip methods, Homan and coworkers fabricated 3D convoluted renal proximal tubules embedded within an ECM on customized perfusion chips, which better recapitulate in vivo microenvironment [96]. In 2019, Lin and coworkers created a vascularized proximal tubule model to evaluate the epithelium endothelium cross-talk in basal and disease conditions [97]. In a next future, 3D bioprinting technology could be applied to the construction of fully functional whole kidneys, resolving the issue of organ availability for kidney transplantation.

9.6 Membrane bioreactor as a biomimetic model for nervous tissue analogue The development of reliable preclinical models is a major goal in neuronal tissue engineering in order to effectively repair nervous tissue injury. Indeed, the reproduction of a physiologically relevant environment is fundamental for a proper investigation of a novel strategy able to mitigate the occurred damages by boosting neuronal regeneration or offering a bridging structure to fill the tissue gap [98]. In this context, the latest advances in membrane technologies in the biomedical field offer valid opportunities opening the path for bioinspired exploitation of membrane systems as a tool for the creation of functional neuronal tissue analogues [99]. The brain structure is extremely complex and this complexity must be reproduced within the in vitro platform in order to obtain consistent and reproducible results. This key prerequisite is fulfilled by using a membrane-based system, where is possible to closely reproduce the native microenvironment. Polymeric membranes act as a biomimetic matrix able to boost neuronal growth and network formation by enhancing cell-cell and cell-ECM interaction. The building up of a realistic biomimetic platform allows using a membrane system for a breakthrough analysis of many aspects related to neurobiology with a wide range of practical applications. Membrane systems are versatile investigational toolset where different conditions can be simulated, thus serving as a platform to model neurodegenerative disorders including oxidative stress, cerebral ischemia, Alzheimer’s disease, and neuroinflammation [100
103]. Neuronal cells are strongly influenced by the geometrical cues offered at the membrane interface, therefore these systems offer also the opportunity to evaluate neuronal behavior in response to a substrate periodic pattern, highlighting the different cellular spatial organization. The need to overcome the drawbacks of traditional culture systems arise the urgency to develop accurate dynamic devices able to control different parameters, offering at the same time a wide range of stimuli to emulate in vivo conditions. For this purpose, membrane

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bioreactors have been used for the implementation of an engineered platform in neuronal tissue. Neuronal cells within the membrane bioreactor are surrounded by a well-controlled biomimetic environment. The maintenance of living and differentiated cells is favored by the precise parameters set in terms of temperature, pH, fluid flow, and supply of gases and nutrients, which in turn enhance the formation of a highly arborized neuronal network. The appropriate membrane permeability and selectivity ensure a constant perfusion flow and a uniform treatment mimicking the dynamics of the natural body environment. Membrane bioreactors have been customized to emulate the natural surroundings of the central nervous system and peripheral nervous system and used as in vitro testbeds. In the next section will be described some representative examples of membrane devices as a neuronal investigational tool used to model neurodegenerative disease, to evaluate the effect of membrane surface topography on neuronal cell elongation according to preferred spatial directions, and as a tool for e realistic blood-brain barrier (BBB) in vitro model. In a work carried out by Morelli et al., an in vitro neuronal platform constituted by an HFMBR was set up for the reconstruction of a functional neuronal network [104]. The overall setting parameters offered to the cells the appropriate stimuli and an adequate milieu that promoted the three-dimensional cellular organization and the formation and retention of intra and intercellular contacts. For the realization of the membrane bioreactor were used polyacrylonitrile (PAN) HF membranes assembled in a parallel disposition. The membrane located within a glass chamber gave rise to two separate regions, the intraluminal and the extraluminal one, which were able to keep in contact with the membrane pores. Neuronal cells were grown on the extraluminal side and have been continuously supplied with an appropriate amount of nutrients and oxygen. Indeed, the fresh medium was constantly provided to the cells through the lumen of the fibers thus reaching the cellular compartment thanks to the high membrane permeability and selectivity. To obtain optimal culture conditions the bioreactor fluid dynamics were characterized through a tracer experiment in order to determine the cumulative residence time distribution, thus reaching a uniform concentration of nutrients throughout the bioreactor area. A further characterization included the identification of the concentration profile of main culture media components such as serum and retinoic acid. The wide spectra of bioreactor characterizations were followed by the realization of the neuronal construct. The optimal parameter setting indeed brought a proper neuronal differentiation and elongation. Cells established a good interaction with the fiber surface covering most of it and forming a complex structural organization (Fig. 9.4). The tightly controlled microenvironment ensured also the morphological and functional maturation as proved by the immunolocalization of specific markers and the maintenance of the basal metabolism over culture time. The creation of nervous tissue architecture with remarkable fidelity allowed the use of this system to model in vitro the β-amyloid (Aβ) induced neurotoxicity associated with Alzheimer’s disease and to evaluate the potential neuroprotection offered by crocin, highlighting its capacity to interfere with the Aβ folding and aggregation. The data collected showed that crocin, a carotenoid found in the fruits of gardenia and stigmas of saffron, was able to neutralize Aβ toxic effect. The simultaneous neuronal cells

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FIGURE 9.4 PAN-HF membrane bioreactor. SEM micrographs and confocal laser images of neuronal cells in the membrane bioreactor. Cells were stained for βIII-tubulin (green), synaptophysin (red), and nuclei (blue). For interpretation of the references to color in this figure caption, the reader is referred to the color version of this paper. Reprinted from S. Morelli, S. Salerno, A. Piscioneri, F. Tasselli, E. Drioli, L. De Bartolo, Neuronal membrane bioreactor as a tool for testing crocin neuroprotective effect in Alzheimer’s disease, Chem. Eng. J. 305 (2016) 69
78 with permission from Elsevier.

treatment with Aβ and crocin maintained the structural and functional integrity of the neurons evidencing the neuroprotection offered by the carotenoid treatment. The overall investigation emphasizes the promising role of membrane bioreactor as a reliable platform to the model pathological event offering the possibility to screen new therapeutic molecules able to restore the tissue functionality or mitigate the pathological onset. The development of bioinspired material able to redirect neuronal processes in terms of axon and neurite elongation remains a big challenge in neuronal tissue engineering; several strategies aim to develop an innovative device that facilitates the bridging of large tissue gaps able to restore the long-distance communication and signal integration within the body after the nerve damage [105
107]. For this purpose, Morelli et al. developed a membrane bioreactor able to promote cell elongation according to a definite path. The topographical stimuli arise from the membrane surface that offers at the cellular interface a precise geometrical pattern. The poly(lactic-L-acid) (PLLA) membrane, used to build up the neuronal device,

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was prepared via an electrospinning process and presents a peculiar structural arrangement; it appears as a continuous array of highly aligned microtube membranes (PLLA-MTA) which offer therefore at the cellular interface, a tightly ordered structure able to promote neuronal polarization and orientation. The simultaneous combination of dynamic perfusion conditions with a high degree of membrane fidelity led to the development of a versatile neuronal membrane device. PLLA-MTA membranes underwent a full characterization to disclose the morphological features as well the physicochemical properties. PLLA-MTA membranes displayed an overall porosity of 46%, with a visible porous structure along the entire membrane wall. Physical-chemical characterization evidenced a moderate wettability with an advancing dynamic contact angle of 87.3 6 3 degrees and a receding one of 50.1 6 2.5 degrees, which is a desirable condition for enhanced cell-membrane interaction. The bioreactor fluid dynamic was characterized and optimized to ensure a homogenous environment within the cell compartment to promote proper neuronal growth and differentiation. Confocal microscopy investigation was used to verify the cellular reorganization over the membrane surface and the occurrence of the appropriate differentiation which in turns ensure the cellular functionality. A deeper investigation was performed to test the effect of the membrane surface topography on neuronal cells analyzing the ability of the cells to follow the same membrane path thus growing along the main membrane axis. Therefore, by using confocal microscopy was performed an isometric analysis of fluorescence intensity for βIII-tubulin, which is a cytoskeletal marker, and for DAPI which stains the nuclei, to better visualize cell positioning and elongation. The investigation showed that neuronal polarization occurred in a parallel manner to the long axis of the membranes. Axon and neurite orientation were also investigated by quantifying the angle formed between the neuronal processes and the aligned PLLA microtubes. This further analysis confirmed that most of neurons followed the predefined path presented at the cell-membrane interface, confirming that the topographical membrane features of the MTA membranes served as guiding structures. Besides guiding neuronal orientation, the PLLA-MTA membrane bioreactor sustained and retained neuronal-specific functions underlying the efficient performance of the system [108]. The synergistic combination of a highly perfused bioreactor together with the stimuli offered by the micro-patterned surface allowed the formation of an efficient neuronal tissue analogue in which a peculiar neuronal self-alignment took place. The PLLA-MTA membrane bioreactor development offers another example of how membrane bioreactors are multifunctional platforms in neuronal tissue engineering with the potential to constantly open innovative investigational insight for a wide variety of applications. Perfused membrane devices are useful platforms also for the realization of rational in vitro BBB models [109,110]. The dynamic system setup includes a bundle of microporous pronectin-coated PP hollow fibers. The system creates a physiological condition that enables coculturing of the brain (astrocytes) and nonbrain vascular endothelial cells: endothelial cells are cultured within the membrane lumen and the astrocytes in the membrane outer layer. The hollow fibers are connected to a gas-permeable tubing set for the appropriate exchange of O2 and CO2, while the constant perfusion is achieved through the media intraluminal flow. The ability to mimic the physiological condition allowed to study the effect of different

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intraluminal pressure on endothelial cells’ behavior. BBB models are a helpful platform that can be used to anticipate animal and human studies allowing to accelerate the screening procedure of new therapeutic molecules able to treat nervous system damages.

9.7 Conclusions and future perspectives Membrane reactors have had a significant impact on biomedical research through the fabrication of 3D tissue/organ models. It is not trivial to foresee how the membrane technologies will further contribute over the next years to the field of bioartificial organs and tissue engineering and how their outputs will be eventually translated to humans. Nevertheless, these systems are nowadays affected by some limitations which should be necessarily addressed. Improvements related to bioreactor design and geometry, mass transport, and delivery of nutrients and oxygen are needed. Vascularization remains a critical issue for complex tissues and organs. Physiological conditions can be achieved and maintained by using properly designed bioreactors with optimized fluid dynamics conditions, and membranes that must be appropriately selected on the basis of their structural, physicochemical, mechanical, and transport properties, which are crucially important to achieve a functionally active organ and tissue. Furthermore, to increase the reproducibility and robustness of a bioartificial device computational tools can be helpful in the prediction and subsequent control of the large number of parameters that allow more efficient multiscale predictions.

Nomenclature Symbols c,i D,i Dei Kbri KM pi Rox RMM Rg T ui Uavg

Concentration of species in domain i (μmol/L) Diffusion coefficient of specie in domain i (m2/s) Effective diffusion coefficient of specie in porous medium i (m2/s) Permeability of the porous medium i (m2) Concentration at which the oxygen uptake rate is half of the maximum rate in Michaelis-Menten kinetics (Pa) Pressure in domain i (Pa) Oxygen consumption rate in the spheroids (mol/(m3.s)) Michaelis-Menten kinetics for oxygen consumption rate (mol/(m3.s)) Universal gas constant (J/(mol.K)) Temperature (K) Fluid velocity (vector field) in domain i (m/s) Mean luminal velocity in a single hollow fiber (m/s)

Greek letters μf μei ρf r

Fluid’s dynamic viscosity (kg/(m.s)) Effective viscosity in the porous medium i (kg/(m.s)) Fluid’s density (kg/m3) Vector differential operator

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Abbreviations AMC-BAL Aβ BAL BAK BBB CA CKD ECM ECS ELAD HF HFMBR LSS MELS MSCs MTA MWCO PA PAN PC PCL PE PEEK-WC PLLA PP PSf PTFE PU RAD WEBAK

Academic Medical Center Bioartificial Liver β-amyloid BioArtificial Liver Bioartificial kidney Blood-brain barrier Cellulose acetate Chronic kidney disease Extracellular matrix Extra-capillary space Extra-corporeal liver assist device Hollow fiber Hollow fiber membrane bioreactor Liver support device Modular Extracorporeal Liver Support system Mesenchymal stem cells Microtube array membranes Molecular weight cut-off Polyamide Polyacrylonitrile Polycarbonate Poly(ε- caprolactone) Polyethylene Polyetheretherketone modified with cardo group Poly(lactic-L-acid) Polypropylene Polysulfone Polytetrafluoroethylene Polyurethane Renal tubule assist device Wearable bioartificial kidney

Subscripts cc ecs l m ox

Cellular compartment (domain) (spheroid) Extra-capillary space (domain) Lumen (domain) Membrane (domain) Oxygen

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328.

10 Photocatalytic membrane reactors Raffaele Molinari, Cristina Lavorato, Pietro Argurio DE PARTMENT OF ENVIRONMENTAL ENGINEERING, UNIVERSITY OF CALABR IA, R ENDE, C OSENZ A, IT AL Y

10.1 Introduction Photocatalytic membrane reactors (PMRs) are devices combining a heterogeneous photocatalytic process and a separation process at the molecular level to achieve chemical transformations. In the last 20 years (2000
20), many efforts have been made to develop these devices for reactions of photodegradation, and, then for the synthesis of chemicals. Many reviews, in the scientific literature, describe the state-of-the-art systems, and some of them are briefly commented on in the following to give a quick overview of the advantages, disadvantages, problems, and applications of such systems. Molinari et al. [1] reported a discussion on the difficulties and advantages of Vis-light versus UV-light irradiation and described the major photocatalysts used under visible light. The authors examined water treatment, hydrogen production, photocatalytic conversion of CO2, acetophenone reduction, hydrogenation of nitro compounds, oxidation of cyclohexane, synthesis of vanillin and phenol, under visible light. In particular, this paper describes various applications of these photocatalysts in PMRs to make an organic synthesis, conversion, and/or degradation of organic pollutants in water treatment. Slurry PMRs and systems equipped with photocatalytic membranes (PMs), methods of PMs production, process parameters affecting the performance of PMRs, configurations, and applications of PMRs for removal of organic contaminants from model solutions, natural water, and municipal or industrial wastewater and applications of PMRs in organic synthesis, have been described by Molinari et al. [2] PMRs are classified mainly on the bases of the photocatalyst location relative to the membranes and distinguished in: (1) PMRs with photocatalyst solubilized or suspended in solution and (2) PMRs with photocatalyst immobilized in/on a membrane (i.e., a PM). The main factors affecting the two types of PMRs are the design and development of efficient PMs by the heterogenization of polyoxometalates in/on polymeric membranes for applications in environmentally friendly advanced oxidation processes (AOP) and fine chemical synthesis, which are deeply discussed by Argurio et al. [3]. Different PMR configurations (pressurized, submerged, with PM, photocatalytic reactors with membrane distillation (MD), with membrane dialysis, with a pervaporation (PV) membrane) have been applied in water treatment for the degradation of different organic pollutants (such as dyes, pharmaceuticals, and other pollutants) and in the synthesis Current Trends and Future Developments on (Bio-)Membranes. DOI: https://doi.org/10.1016/B978-0-12-823659-8.00005-8 © 2023 Elsevier Inc. All rights reserved.

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of organic compounds (such as phenol, vanillin, and phenyl ethanol), they have been described and critically examined by Molinari et al. [4]. A particular focus on the photocatalytic hydrogen production from water splitting, CO2 conversion to solar fuels (CH3OH, C2H5OH, CH4, and HCOOH), and organic syntheses such as oxidation (benzene to phenol, cyclohexane to cyclohexanol, ferulic acid to vanillin) and reduction reactions (acetophenone to phenyl ethanol) in different types of PMRs are described by Lavorato et al. [5]. Hybrid PMRs with ultrafiltration (UF) for the treatment of water and wastewater, the various membrane materials, and the advantages of ceramic membranes over polymeric membranes are reported in detail by Rani et al. [6]. In the same review, membrane fouling as a major issue in slurry PMRs and how it affects permeate flux is discussed with a critical evaluation of potential and commercial photocatalytic UF membrane reactor (PUMR) configurations. Micropollutants (MPs) are emerging contaminants that pose a serious threat to human life even though their concentrations in the aqueous medium are in nanograms. Therefore characteristics of MPs and membranes on the removals of MPs are critically reviewed in that article, as well as economic aspects and process intensification in PUMRs. Photocatalysis is considered a very promising “green” process for the possibility to operate under operative conditions to abate refractory, very toxic, and nonbiodegradable molecules [7,8]. Moreover, photocatalysis: (1) avoids the employment of environmentally and unhealthy hazardous heavy metal catalysts by using safer photocatalysts (mainly TiO2); (2) uses mild oxidants (O2, in some cases from the air); (3) needs very few auxiliary additives; (4) does not generate dangerous chemicals; (5) offers a good alternative to the energyintensive conventional treatment methods; (6) can be combined with other physical and chemical technologies (e.g., membrane separations). The photocatalyst is usually employed as a powder suspended in a liquid medium and, consequentially, the phase of catalyst-recovering from the reaction environment is an important step in large-scale applications. A promising approach to overcome this limitation is represented by the use of PMRs. A PMR improves the potentialities of classical photoreactors (PRs) and those of membrane processes with a synergy of both technologies thus minimizing environmental and economic impacts [9,10]. The membrane allows continuous operation in systems in which the recovery of the photocatalyst (immobilized or in suspension), the reaction, and the products separation simultaneously occur, resulting from competition with other separation technologies in terms of material recovery, energy costs, reduction of the environmental impact, and selective removal of the components. Higher energy efficiency, modularity, and easy scaleup are some other advantages of PMRs over convectional PRs. Water treatment by removing both organic and inorganic pollutants represents the main use of photocatalytic techniques. Water pollution, caused by hazardous organic chemicals employed in industry and agriculture, is today a very serious problem. Environmental laws are very severe and they will become more and more restrictive in the next years. Furthermore, various directives suggest the use of green chemistry concepts and clean technologies inside the manufacturing processes to protect the environment. Traditional

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chemical and biological treatments (e.g., adsorption on active carbon, chemical oxidation, and aerobic biological treatments) to clean up waters often fail to remove the majority of organic pollutants because of the high resistance of these compounds, resulting in high concentrations discharged in treated effluents. On this basis, as a consequence of the highly unselective reactions involved in the oxidative-type photocatalytic processes, a wide range of organic pollutants can be degraded (i.e., mineralized) in very small and harmless species (carbon to carbon dioxide, hydrogen to water, nitrogen to nitrate, etc.) without using chemicals, preventing sludge production and its disposal. In this chapter the basic principles of photocatalysis and PMRs are reported, evidencing the advantages related to their coupling, thanks to the synergistic effects. The correct choice of type of membrane and material is evidenced, discussing the literature and the used criterion of selection. Moreover, a classification of the PMRs based both on their configuration (pressurized and depressurized/submerged) and on the particular membrane separation coupled with photocatalysis (pressure-driven membrane separation, MD, dialysis, and PV) is also presented, evidencing the opportune choice of PMRs configuration and membrane separation is a key step to limit membrane fouling. Finally, some applications of PMRs for water treatment, in particular for pharmaceuticals removal, and in reactions of synthesis (e.g., conversion of CO2 in solar fuels) are reported, evidencing the potentialities, the drawbacks, and the future trends.

10.2 Basic principles of photocatalysis Heterogeneous photocatalysis is an AOP based on the use of a semiconductor (the photocatalyst). Photocatalysis differs from conventional catalysis mainly for the photonic activation mode of the catalyst, which substitutes the thermal activation [11]. The electronic structure of a semiconductor is characterized by a valence band (VB) and a conduction band (CB) which are separated by an energy band gap (Eg). When a semiconductor particle is excited by irradiation with photons of energy (hν) equal to or higher than its band gap energy (Eg), valence electrons (e2) is promoted from VB to CB, thus leaving a positive hole (h1) in the VB. For semiconductor TiO2, this step is expressed as: TiO2 1 hν ! TiO2 e2 1 h1




(10.1)

In the absence of suitable electron and/or hole scavengers, the photogenerated electrons and holes can recombine in bulk or on the surface of the semiconductor within a very short time, releasing energy in the form of heat or photons. Electrons and holes that migrate to the surface of the semiconductor without recombination can, respectively, reduce and oxidize the substrates adsorbed on the semiconductor. A schematization of both the photonic activation of the photocatalyst and the photocatalytic oxidation and reduction reactions that take place onto a photo-activated semiconductor particle is reported in Fig. 10
1.

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FIGURE 10–1 Schematization of the photocatalytic process which occurs on a photo-activated semiconductor particle. ox1, red1, ox2, and red2 represent, respectively, the oxidized and the reduced species of two different redox couples, indicated as (1) and (2).

The process in Fig. 10.1 can be divided into four steps: 1. 2. 3. 4.

absorption of light followed by the separation of the electron
hole couple; adsorption of the reagents; redox (reduction and oxidation) reaction; desorption of the products.

The redox reactions involving the species adsorbed onto the semiconductor surface occur only if the potentials of the redox couple to which the substrates belong (E10, E20) are compatible with both the VB and CB potentials (EVB, ECB). In particular, the redox potential of the species to be oxidized, E20, should be less positive than EVB, so that the photo-produced holes h1 can oxidize the reduced form of this species (red2! ox2, see Fig. 10.1). At the same time, the redox potential of the specie to be reduced, E10, should be more positive than ECB, so that the photo-produced electrons e2 can reduce the oxidized form of this species (ox1!red1, see Fig. 10.1). Only if both E20 , EVB and E10 . ECB conditions are met, are both reduction and oxidation thermodynamically favored, and the overall photocatalytic process takes place. The redox reactions which take place on the semiconductor surface represent, respectively, the basic mechanisms of photocatalytic water/air purification and photocatalytic hydrogen production/substrate hydrogenation. In the VB, photogenerated holes can oxidize surface hydroxyl groups into hydroxyl radicals in the aqueous reacting environment: OH2 1 h1 ! OH

(10.2)

As widely accepted, hydroxyl radicals are the primary oxidants that attack the substrates to be degraded by reducing them to their elemental form.

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In the CB, photo-promoted electrons can reduce the molecular oxygen dissolved in the aqueous phase to superoxide radical, metal ions to their lower oxidation states, or H1 ions to H•: O2 1 e2 ! O2 2

(10.3)

Men1 1 e2 ! Meðn21Þ1

(10.4)

H1 1 e2 ! H

(10.5)

The so generated hydrogen radicals can follow two main fates: combination for H2 production or hydrogenation of adsorbed substrate. Thanks to: (1) the favorable energetics of its band structure, (2) its relatively high quantum yield, (3) its stability under irradiation, and (4) its low cost and availability, TiO2 represents the archetypical photocatalyst, a virtual synonym for photocatalysis. Nevertheless, this material does not present photo-response under visible light illumination because of its wide band gap, taking advantage of only less than 6% of the solar energy. Thus its potential as a green technology cannot be entirely fulfilled. Consequently, in the last years, a great number of new photocatalysts have been synthesized and tested as possible alternatives to TiO2 [12] particularly given a solar application. The most important requirements that such material should possess are a suitable band gap together with chemical and physical stability, nontoxic nature, good availability, and low cost. The most common materials used are oxides or sulfides, which redox potentials for the VBs and the CBs range are between 14.0 and 21.5 V versus normal hydrogen electrode. Their photocatalytic properties depend mainly on the position of the energetic level and the band gap. Table 10
1 summarizes the most common semiconductors used as photocatalysts, reporting their band gap and wavelength of the radiation needed for the activation.

Table 10–1 Band gaps and wavelengths of the most common semiconductors used as photocatalysts. Photocatalyst

Band gap (eV)

Wavelength (nm)

SnO2 TiO2 anatase TiO2 rutile WO3 Fe2O3 ZnO ZnS CdS CdSe GaAs

3.8 3.2 3.0 2.8 2.2 3.2 3.7 2.5 1.7 1.4

318 387 380 443 560 387
390 335
336 496
497 729
730 886
887

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The photoactivity of a semiconductor depends also on the mobility and lifetime of the photo-produced electron/hole couples, the light absorbance coefficient, on the method used to prepare the photocatalytic powder which affects many physicochemical properties of the semiconductor like surface area, distribution of particle size and crystallinity. Despite the photocatalytic process offers interesting advantages with respect to other catalytic processes (e.g., milder operating conditions, use of safer catalyst, etc.), its use in the industry is still limited for three different reasons: [13] (1) recombination of photogenerated electron/hole pairs which dissipate their energy as heat; (2) fast-backward reaction; (3) difficulty to employ visible light that limits the effectiveness of utilization of solar energy. To solve these problems, continuous efforts have been made to enhance the photocatalytic activity and promote the visible light response. A simple approach consists of the addition of electron donors or electron scavengers, able to react irreversibly with the photogenerated VB holes or CB electrons, respectively. Practically electron scavengers or electron donors can be reduced by CB electrons or oxidized by VB holes, respectively; the remaining strong oxidizing VB holes or strong reducing CB electrons can oxidize/reduce the substrate. Operating in this way the electron/hole recombination can be suppressed or reduced, and higher quantum efficiency can be achieved. Another way to suppress or reduce the recombination of photogenerated electron/hole pairs consists in doping the semiconductor with noble metals like Pt, Au, Pd, Rh, Ni, Cu, and Ag. By using this method, photo-promoted electrons can be transferred from the CB of the semiconductor to metal particles deposited on its surface, while photogenerated VB holes remain on the photocatalyst. Thus the possibility of electron
hole recombination is greatly reduced, resulting in an enhancement of photocatalytic efficiency [14
19]. A combined effect of metal ion doping consists in the reduction of the band gap energy of the photocatalyst, thus shifting the radiation absorption toward higher wavelengths, permitting to use of visible light [20,21]. The same effect can be also obtained by doping the catalyst with anions, such as N, F, C, S, and B [22,23]. Other ways to overcome previous limitations consist of dye sensitization, composite semiconductors, metal ion implantation, etc. The choice of the lamp wavelength peak is strictly dependent on the photocatalyst type. For example, in Fig. 10
2, the diffuse reflectance spectrum of P25 Titania (Evonik P25) is reported, which has been compared with the photonic efficiency (calculated, by using monochromatic light, as the ratio of the initial rate (example units: moles/cm2/s) to incident irradiance (example units: photons/cm 2 /s)) and with the formal quantum efficiency (FQE) of the system by using polychromatic irradiation. When the action spectrum and the absorption, or reflectance, spectrum of the semiconductor photocatalyst are similar, this performance indicated that this semiconductor photocatalyst is the light-absorbing species responsible for a photocatalytic reaction. In Fig. 10
2 the reported behavior of TiO2 , irradiated by light, has been obtained with data normalized at 350 nm [24].

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FIGURE 10–2 Normalized photonic efficiency (white square) or modified photonic efficiency (black square) for the photocatalytic oxidative degradation of acetic acid and stearic acid, respectively, and diffuse reflectance spectrum (Kulbelka-Munk function; broken line) of P25 titania film as a function of the peak wavelength of the excitation light [24].

10.3 Basic of photocatalytic membrane reactors The basic principles of photocatalysis and the advantages of their coupling with PMRs are reported in the following subparagraphs. In particular, the appropriate choice of photocatalyst, type of membrane, and PMRs configurations (pressurized and depressurized/submerged, pressure-driven membrane, MD, dialysis, and PV) will be discussed.

10.3.1 Types of photocatalysts The design of a photocatalytic reactor (PR) includes three components such as (1) the substrate, (2) the solid photocatalyst, and (3) the light photons. In general, two main configurations of PMRs are used: (1) slurry PMRs, with the photocatalyst suspended in the reaction environment, and (2) PMR configuration with the photocatalyst immobilized in/on a substrate material acting as a membrane (PM) [25]. An ideal photocatalyst should be inexpensive, stable during photoreaction, nontoxic, very selective toward the substrate, and recoverable [3]. The most used photocatalyst in PMRs under UV-light irradiation is Titanium dioxide for its good photocatalytic activity, high photochemical stability, low cost, and toxicity [26]. Some authors employed photocatalysts immobilized on the membrane to ease their recovery and reuse. For example, Jafri et al. [27] studied the degradation of Bisphenol A (BPA) under UV light irradiation by using hollow titanium dioxide nanofibers (HTNF). The authors reported a photocatalytic degradation of BPA of 97.3% by using HTNF with the following operative conditions: initial BPA concentration of 0.75 g/L, pH 4.1, and 10 ppm BPA. This result was 12.6% higher than those of Degussa P25 TiO2.

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Current Trends and Future Developments on (Bio-)Membranes

Singh et al. [28] studied the chloramphenicol degradation by using Titanium dioxide as a photocatalyst doped with hydroxyapatite (TiO2-HAP) in a low-pressure cross-flow lab-scale PMR. Moreover, to improve the photocatalytic activity and antifouling propensity, the authors incorporated different amounts of TiO2-HAP on polysulfone (PSf) membranes. The results showed the highest degradation of 61.59% for the PSf/4 wt.% TiO2-HAP nanocomposite membrane. Recently, graphene (G) or carbon-based materials have attracted great attention not only for their use as active cocatalysts but also as active photocatalysts for pollutants degradation and organic synthesis. Lin et al. [29] reported the reduction of biofouling by using an S-doped g-C3N4 nanosheet as a photocatalyst for water splitting. They prepared this photocatalyst in combination with Ru/SrTiO3: Rh with the addition of [Co(bpy)3]31/21 as an electron mediator to improve the charge transfer in a Z-scheme system. The authors reported a value of H2 and O2 evolution rates of 24.6 and 14.5/μmol/h, respectively, with a reduction in biofouling on a membrane due to the presence of S-doped g-C3N4 that was previously incubated with a solution of Escherichia coli to verify its antibacterial effect. Different are the advantages and disadvantages of photocatalysts immobilized on a membrane. Diaz-Torres et al. [30] studied hydrogen generation by using ZnAl2O4 (ZAO) powders as photocatalysts. The authors used a combustion method to produce carbon dots (C-dots) on the ZAO surface. This photocatalyst was tested alone or incorporated into a polyacrylate matrix to form a PM (named PAZO) which was subsequently attached to a flexible graphene composite (FGC) to form an FGC/PAZO (GAZO) composite. The results showed a lower hydrogen generation rate under UV irradiation by using GAZO composite (  38% less) than the best ZAO powder (annealed at 700 C). Despite the higher photocatalytic activity of suspended photocatalyst compared to the immobilized one on the membrane (GAZO) it could be attached easily to the inner wall of the photocatalytic reactor which facilitates its removal at the end of the photocatalytic reaction. In the last years the interest in using renewable energy sources, such as solar light, has been expanded [1]. The use of visible light irradiation is preferable for some reasons, for example, UV irradiation can cause the deterioration of the membrane can limit the membrane efficiency, and can influence also by-products formation [31]. Moreover, the use of UV irradiation can cause excessive energy input in the reacting environment with a higher generation of reducing and/or oxidizing agents and consequently a greater by-products formation. On the contrary, the use of visible light as an irradiation source can improve the selectivity values [32]. Furthermore, the solar spectrum is constituted of 45% visible light and 50% of near-infrared, and only a small fraction of UV radiation (about 5%), thus, to have efficient use of solar energy, much effort to develop new photocatalysts or semiconductors combination or their modification to improve the absorption on the visible light range, is needed [33]. For example, Zhang et al. [34] studied the degradation of RhB under visible light irradiation by using a rotating reactor (Fig. 10
3). This system contains a composite membrane consisting of graphene oxide (GO) acting as the separation membrane, activated carbon (AC) as the adsorbent, and Ag@BiOBr as the photocatalyst, respectively. The results showed a rejection rate of RhB up to about 100% by using Ag@BiOBr/AC/GO membrane compared to the slowly reduced rejection rate on AC/GO.

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FIGURE 10–3 Illustration of rotating reactor and preparation of Ag@BiOBr/AC/GO membrane by Zhang et al. [34].

Another dangerous pollutant that causes various undesirable effects in humans, is the BPA. To limit the presence of BPA in several water sources, Kamaludin et al. [35]. studied their removal from water under visible light. For this purpose, they prepared a photocatalytic dual-layer hollow fiber (DLHF) membrane produced via cospinning phase inversion. The degradation efficiency of the photocatalytic N-doped TiO2 DLHF, in comparison to the commercial TiO2-P25 immobilized into DLHF membrane, was evaluated by using a submerged membrane photoreactor. The results showed a BPA removal of up to 90% for both DLHF membranes under UV light irradiation, meanwhile N-doped TiO2 DLHF removed 81.6% of BPA under visible light irradiation. Liu et al. [36] studied the MB degradation by using a photocatalyst obtained by doping the surface of GO and titanate nanotubes (TNTs) with Ag nanoparticles (Ag/GO/TNT). The results showed 90% of MB degradation after 120 min irradiation under visible light with a flux of 34.7 L/m2/h. On the same topic, Hu et al. [37] prepared a photocatalyst obtained by doping g-C3N4 with P (PCN), coated on an Al2O3 substrate with an Al2O3 hollow fiber membrane module as a PMR. The authors reported an MB removal greater than 90% also after repeating tests four times, by using 10 wt.% of PCN. Also, Alyarnezhad et al. [38] studied the degradation of MB under visible light irradiation (λ . 420 nm). They prepared four types of membranes with GO nanosheets. The best photocatalytic performance with an MB removal efficiency of 83.5% was obtained by using the membrane sample indicated by the authors as M8 under simulated solar light irradiation. As previously described, the use of visible light can improve the selectivity of de desired product, thus also the productivity can be enhanced. For example, Molinari et al. [39] studied the photocatalytic hydrogenation of acetophenone (AP) to phenyl ethanol under UV and visible light, comparing the photocatalytic activity of commercial TiO2 and homemade Pd/TiO2.

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The photocatalytic tests were performed by using water as a solvent and formic acid as electron and hydrogen donor, in batch and membrane reactors with various methods for adding the substrate. As previously reported, the doping of titanium dioxide with noble metals, such as palladium, in this case, can improve the visible light activity of the photocatalyst. Indeed, the results showed a productivity value five times higher by using Pd/TiO2 than pure TiO2 (productivity 22.0 mg/g/h vs 4.44 mg/g/h). The same research group [40], studied a further modification of titanium dioxide with faujasite (FAU) to improve its photocatalytic activity on hydrogenation of AP in batch and a membrane reactor under UV light. The best photocatalyst obtained, named TF10P, was then doped with Pd (Pd_TF10P) to obtain a photocatalyst active under visible light. Finally, the results showed that the productivity achieved in the PMR under visible light irradiation was higher by using Pd_TF10P than Pd/TiO2 (productivity 99.6 mg/ gTiO2/h vs 22 mg/gTiO2/h) with an extraction percentage of phenyl ethanol of ca. 25%. Summarizing the results obtained in this paragraph, the use of using visible light as an energy source in reactions of synthesis often shows a lower by-products generation, which is instead induced by high-energy UV-light irradiation. So, the UV source is more interesting in water treatment because it can induce higher mineralization of pollutants for its stronger activity. In the cases of PMR configuration with the photocatalyst immobilized in/on the membrane, the use of visible light is preferable to limit the degradation of the membrane increasing its stability under irradiation.

10.3.2 Types of membranes In general, a membrane can be defined as a barrier that separates two phases controlling the mass transport between them. The morphological properties, transport mechanisms, and type of membrane processes are widely described in a basic membrane textbook [41]. On the bases of the type of photocatalytic reaction involved in a PMR system, the suitable choice of the membrane (material and type) and membrane module configuration is decisive to obtain a good system performance. The membrane can affect several roles, among which the main one consists in separating the catalyst from the reaction environment by choosing a membrane (type and material) with complete catalyst rejection. Indeed, the main objective in coupling a membrane process with a photocatalytic reaction is the need to recover and reuse the photocatalyst. Furthermore, another important role of the membrane consists in the rejection of the substrates and their intermediate products to obtain a good permeate in the case the process is used for the degradation of organic pollutants. Instead, when the photocatalytic process is used as a synthetic pathway, the principal purpose can be the selective separation of the product, for example, minimizing its successive reactions, which can carry out undesirable by-products. In this context, the rejection (R) is a useful parameter that expresses the ability of the membrane to maintain the substrate and its intermediates in the reactive environment:

R 5 Cf
Cp =Cf 5 1
Cp =Cf

where Cf and Cp are the solute concentrations in the feed and permeate, respectively.

(10.6)

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The retention of the substrates and degradation by-products is important also to control the residence time of substrates in the photocatalytic system. The efficacy of the photoreaction is strongly influenced by this parameter. For example, a higher extension of the photocatalytic reaction can be obtained by increasing the contact time between the substrate and the photocatalyst by reducing the permeate flux which causes longer retention times. Nevertheless, since the PMR must be able to offer high water permeate flux, it is important to find a good compromise between the permeate flux and the residence time to achieve a system for application purposes. Since the main problem encountered when the photocatalytic process is coupled to a pressure-driven process as MF and UF (which requires porous membranes) is the decrease of permeate flux caused by concentration polarization and fouling, the choice of membrane material is strongly determined by the need to limit these phenomena and by the chemical and thermal stability of the materials. Polymeric membranes are generally used in photocatalysis, though inorganic ones, which generally present higher chemical and thermal stability, are attractive for PMR applications they have the disadvantage of a significantly higher cost compared to polymeric ones [42,43]. In view of this economic factor, suitable polymeric membranes should be sought for this application. Molinari et al. [44] reported an investigation of the stability, under UV irradiation, of some eligible commercial membranes composed of various materials. Tests of photoresistance under UV light were carried out by irradiating the membranes immersed in distilled water. Samples were periodically withdrawn and analyzed by total organic carbon (TOC), optical microscopy, and scanning electron microscopy (SEM) to verify any damage to the membrane surface. Membrane stability was also determined by measuring the changes in pure water permeation flux (WPF) before and after UV irradiation. Membranes made of polyacrylonitrile (PAN), fluoride 1 PP, and polysulfone 1 PP seemed to be quite stable to UV light over a 24-h period of irradiation. Some years later Chin et al. [42] reported a study focused on the selection of polymeric membranes for PMR applications providing a protocol, similar to that one proposed by Molinari et al. [44] for testing the stability of polymeric membranes before using them with photocatalytic reactions. Membrane stability was characterized by: (1) changes in WPF, before and after UV irradiation; (2) release of TOC before and after the predefined time of UV exposure and (3) SEM to observe the surface morphology of the membranes before and after the exposure to UV light. The overall change in surface hydrophobicity or hydrophilicity of the membranes was determined by contact angle measurements. The initial UV-screening test was carried out on ten types of polymeric membranes. After 30 days of UV illumination, the following membranes resulted quite stable: Polytetrafluoroethylene (PTFE), hydrophobic polyvinylidene fluoride (PVDFphobic), and PAN. However, in a study of the oxidative stability of membranes (under UV, hydrogen peroxide (H2O2), and combined UV/H2O2 conditions), it was found that the stability of PAN membrane declined considerably when it was exposed to 10 days of 200 mM H2O2/UV conditions. Then the author concluded that, based on their UV and oxidative screening tests, PTFE and hydrophobic PVDF are the best choices for photocatalytic applications.

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Despite all the properties which make PVDF one of the most extensively applied UF membrane material (e.g., antioxidation, good thermal and hydrolytic stabilities as well as good mechanical and membrane forming properties), a disadvantage, due to the hydrophobic nature of PVDF, frequently causes severe membrane fouling and permeability decline, owing to the chemical affinity with hydrophobic organic contaminants which cause deposition on this material. Damodar et al. [45] reported a study on the self-cleaning, antibacterial and photocatalytic properties of TiO2 entrapped PVDF membranes. The modified PVDF membranes were prepared by using the phase inversion method, adding different amounts of TiO2 particles (0
4 wt.%) into the casting solution. The TiO2-PVDF membranes were tested for their antibacterial property by using Escherichia Coliform, photoactive properties using Reactive Black 5 dye, and self-cleaning (antifouling) properties using 1% BSA solution. Obtained results evidenced that the hydrophilicity and pore size of composite PVDF/TiO2 membranes was varied by the addition of different amounts of TiO2. This also improves the permeability of the modified PVDF/TiO2 membrane. The PVDF/TiO2 membrane showed better bactericidal ability as compared to the neat PVDF membrane under UV light. On the same topic, Wei et al. [46] proposed a new PVDF-TiO2 nanowire hybrid UF membrane prepared via phase inversion by dispersing TiO2 nanowires in PVDF casting solutions. Obtained results evidenced that microstructure, mechanical property, thermal stability, hydrophilicity, permeation, and antifouling performance of hybrid membranes were improved significantly by the addition of hydrophilic inorganic TiO2 nanowires. In a work by You et al. [47] were evaluated the antifouling and photocatalytic properties of a membrane in which polyacrylic acid was plasma-grafted on a commercial PVDF membrane to introduce functional groups on the membrane surface that can support the TiO2 nanoparticles. Obtained results evidenced that the membrane hydrophilicity was tremendously enhanced by the self-assembly of TiO2, following a direct proportionality to TiO2 loading. However, greater hydrophilicity did not necessarily implicate better antifouling properties, since excessive nanoparticles plugged membrane pores. The membrane with 0.5% TiO2 loading maintained the highest pure water flux and the best antifouling property. Beyond this value, a significant permeate flux decline was observed. Numerous works are reported in the literature on the photocatalytic process coupled with NF membranes which have the advantage to retain molecules with low molecular weight better than low-pressure MF and UF membranes. Molinari et al. [48], tested different NF membranes in the photodegradation of various pharmaceuticals. Obtained results evidenced that membrane retention depended on both the pH of the aqueous solution to be treated and on the chemical characteristics of the particular pharmaceutical compound to be degraded. NF PES 10 at alkaline pHs, NTR 7410 at neutral and alkaline pHs and N 30 F at acidic pHs were the membranes that showed the best rejection percentages for furosemide, at pressures of 4
8 bar. NTR 7410 was the best membrane for ranitidine rejection, over the whole pH range, although its rejection at 8 bar was lower than that found for NF PES 10.

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10.3.3 Membrane modules and system configurations Despite the important advantages of the photocatalytic processes with respect to the traditional ones, their application at the industrial level is limited by different drawbacks related to the involved reactions and reactor configuration. With regard to the configuration of the reactor, and in particular, by considering the photocatalyst, two operative configurations can be identified: the first one, in which the photocatalyst is suspended in the aqueous reacting phase, and the other one with the photocatalyst immobilized on a support. Slurry photocatalytic reactors, in which the photocatalyst is suspended in the aqueous reacting phase, have been widely used. In these reactors, the not-degraded molecules or their by-products are freely transported in the final stream. Photocatalyst recovery and recycling is another problem. The coupling with pressure-driven membrane separations in PMRs can solve these problems.

10.3.3.1 Pressurized membrane photoreactors Different types of PMRs were built to obtain an easy separation of the photocatalyst from the reaction environment and efficient removal of pollutants from aqueous media. The most studied configurations are pressurized systems, in which pressure-driven membrane processes, such as MF, UF, and NF are combined with the photocatalytic process. In these systems, the photocatalyst, used both in the suspended configuration and immobilized on the membrane, is confined in the pressurized side of the permeation cell. In 2002 Molinari et al. [49] reported some experimental results obtained by using two different pressurized PMR configurations for the degradation of 4-NP. In particular, the configurations studied were: (1) a first one, in which the irradiation of the photocatalyst was performed in the permeation cell containing the membrane, with three subcases: (1a) catalyst in suspension; (1b) catalyst deposited on the membrane; (1c) catalyst entrapped in the membrane; (2) a second one, in which the irradiation of the suspended catalyst was performed in the recirculation tank. The results evidenced that the second configuration appeared to be the most interesting for industrial applications in terms of irradiation efficiency and membrane permeability. For example, in reactor optimization, high irradiation efficiency, high membrane permeate flow rate and selectivity can be obtained by sizing separately the “photocatalytic system” and the “membrane system” and taking advantage of all the best research results for each system. Polymeric membranes are widely used in PMRs despite their low resistance to UV light and in some cases to hydroxyl radicals [6]. To prolong the membrane performance in PMRs, it is important to choose a membrane with a high UV resistance, such as PTFE, and PVDF, or it should be kept separately from the photoreactor. An alternative type of membrane is ceramic for its higher physical integrity, chemical resistance, thermal stability, low chemical demand, low cleaning frequency, and longer lifetime [50]. These authors investigated a 100 kDa ceramic membrane made by Al2O3 to assess the performance of a slurry PMR to treat a secondary

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wastewater effluent polluted by anticancer drugs. They observed stable membrane properties during 150 h of photocatalyst filtration at 3 m/s of cross-flow velocity in real and pure water matrices. The results, in terms of removal rates of anticancer drugs added to the pure water in PMR conditions, were 0.080 min21 for 5-fluorouracil, 0.088 min21 for cyclophosphamide, and 0.141 min21 for capecitabine.

10.3.3.2 Depressurized (submerged) membrane photoreactors The most important problem detected by coupling photocatalysis with a pressure-driven membrane process is the decline of permeate flux caused by membrane fouling owing to catalyst deposition on the membrane. The use of submerged membrane systems can limit the problem mentioned previously. In this configuration, the photocatalyst is suspended in an open-air reaction environment, the membrane is immersed in the batch and the permeate is sucked using a vacuum pump. By using a submerged membrane photocatalytic reactor (SMPR) it is possible to decrease the membrane fouling thanks to the aeration that keeps the photocatalyst in suspension and decreases the adhesion of pollutants onto the membrane surface. As an example, Wang et al. [51] studied the performance of a SMPR for the advanced treatment of p-nitrophenol (PNP) wastewater, which is widely applied in the production of medicines, dyes, and pesticides. An efficient advanced treatment of PNP wastewater is important for the protection of the water environment because it is difficult to be completely removed from the aqueous system of its refractory character. To overcome this problem, the authors studied the SMPR combined with Photo-Fenton technology under visible light by using Fe(III)-ZnS/g-C3N4 photo-Fenton synthesized by a microwave hydrothermal method. The schematic diagram of SMPR, in which the photocatalysis and membrane filtration process are combined, is reported in Fig. 10
4. This system consists of an SMPR with a diameter of 20 cm and a volume of 9 L composed of a centered quartz tube, light source, a membrane module, a feedwater tank, a backwash tank, an air compressor, three peristaltic pumps, a cooling tank (which maintains the reaction at room temperature), a pressure gauge, a thermometer, and a flowmeter. The results of photocatalytic tests performed in this SMPR showed a removal rate of 91.6% of PNP in 4 h under simulated solar light irradiation and a 100% rejection rate of the photocatalyst by the MF membrane. A submerged photocatalytic oscillatory membrane reactor was reported by Gupta et al. [52] to remove MPs in different water matrices under UV irradiation. In this system, the effects of oscillation amplitude and frequency, as well as the aeration rate, were investigated by the authors in a continuous mode at two different fluxes (155.7 and 363.3 L/m2/h). This membrane reactor consists of a Plexiglas rectangular cell (26.2 cm 3 14 cm 3 2.3 cm) housing an aluminum membrane module (14.7 cm 3 6 cm 3 1.4 cm), equipped to hold two circular 47 mm flat sheet PVDF membranes, with airflow provided at the bottom of the cell and UV irradiation (365 nm) in front of the quartz window of the reactor (Fig. 10
5). The membrane unit was oscillated using an adjustable eccentric, driven by a variable speed motor.

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FIGURE 10–4 Schematic diagram of the combined photocatalysis-membrane filtration process [51].

FIGURE 10–5 Oscillatory membrane photoreactor experimental setup [52].

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The authors reported that the membrane oscillation with aeration was demonstrated to be efficient in reducing fouling and improving membrane flux compared to only aeration. The maximum removal of the MPs was 90% for sulfamethoxazole, 89% for hydrochlorothiazide, 87% for diclofenac (DCF), and 68% for antipyrine. The application of PMR under visible light was recently tested for the degradation of Reactive Orange 29 as a model organic pollutant by Sheydaei et al. [53]. They used La-ZnO, Ho-ZnO, and Ce-ZnO nanoparticles as photocatalysts in three reactor configurations: simple photocatalysis, sono-photocatalysis, and sono-photocatalysis/membrane separation (SPMS). The best photocatalyst was Ce-ZnO nanoparticles prepared in the following conditions: 8 wt.% of cerium nitrate, pH 10, 1 h of sonication at 60 C, 3 h of calcination at 300 C. This photocatalyst was used in a continuous flow SPMS reactor (Fig. 10
6) under visible light with polypropylene hollow fiber membrane for treatment of dye solution obtaining 97.84% of dye removal in the best operative conditions. Furthermore, Ce-ZnO nanocomposite demonstrated good antibacterial behavior against positive and negative bacteria. Therefore the SMPR with fine-bubble aeration and intermittent membrane filtration can be potentially applied in the photocatalytic oxidation process during drinking water treatment.

10.3.3.3 Coupling of photocatalysis with nonpressure membrane operations Coupling of photocatalysis and MD could avoid fouling problems related to the use of pressure-driven membrane separations. MD is a separation process based on the principle of vapor
liquid equilibrium. The nonvolatile components (e.g., ions, macromolecules, etc.) are retained on the feed side, whereas the volatile components pass through a porous hydrophobic membrane and then condense in a cold distillate (usually distilled water).

FIGURE 10–6 Schematic of SPMS reactor used by Sheydaei et al. [53].

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The possibility to obtain a PMR by coupling photocatalysis and direct contact MD (DCMD) for degradation of azo dyes (Acid Red 18, Acid Yellow 36, and Direct Green 99) in an aqueous solution was investigated by Mozia et al. [54]. The highest efficiency of photodecomposition was obtained in the case of Acid Red 18. The results evidenced that the MD process was very effective in the separation of photocatalyst particles, with a complete rejection of the dye and other nonvolatile compounds (organic molecules and inorganic ions). Contrariwise, some volatile compounds crossed the membrane, in a very low range (0.4
1.0 mg/L). The achieved permeate flux was comparable to those obtained during the process in which ultrapure water was used (0.34 m3/m2/d). Despite MD having a disadvantage of the higher energetic consumption, the authors reported that MD coupled with photocatalysis allowed to prevent the significant fouling. Moreover, the hybrid process, obtained by coupling the photocatalysis and DCMD, seems a very promising method to remove organic compounds from water, such as azo dyes, because the MD membrane is a good barrier for both the photocatalyst particles and the nonvolatile compounds present in the feed solution. On this basis, Azrague et al. [55] studied the combination of dialysis and photocatalysis to mineralize organic compounds (e.g., 2,4-dihydroxybenzoic acid (2,4-DHBA) used as a model pollutant) in artificial turbid waters achieved by using a natural clay named bentonite. In this PMR configuration the dialysis membrane, used as a membrane contactor, acts as a barrier for the photocatalyst particles and permits the separation of the polluted turbid water from the photoreactor compartment and to extract the organic compounds from the turbid water thanks to the different concentration gradient. The absence of transmembrane pressure avoids fouling formation, even in the case of highly turbid water. Moreover, the membrane allows to (1) avoid a final filtration stage maintaining the TiO2 in the photoreactor compartment, and (2) limit the loss of efficient irradiation due to the scattering by bentonite particles by keeping the bentonite away from the photoreactor. The complete removal of a high 2,4DHBA concentration and the advantages above mentioned proved the efficacy of the proposed PMR obtained by combining photocatalysis and dialysis. Moreover, these results could lead to the design of a PMR working in a continuous mode. Another approach was the integration of photocatalysis with PV which resulted in a very promising to improve the efficiency of the detoxification of water streams containing recalcitrant organic pollutants at a low concentration [56] as the integration of the two processes generates a synergistic effect. In PV separation is based on the relative volatility of the components in the mixture and on the relative affinity of the components with the membrane. In fact, the choice of the membrane material is essential to obtain a selective separation of the molecules. The results showed that the rate of disappearance of a model pollutant [4-chlorophenol (4-CP)] is highly improved by using the integrated system as the membrane was efficient in continuously eliminating some intermediate products that could slow down the rate of the photocatalytic reaction and, concurrently, the photocatalyst transforms the weakly permeable 4-CP into organic compounds that PV can remove at a high rate. Process intensification by synergistic effect depends on the optimization of the ratio between the characteristic rates of the photocatalytic and PV processes.

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10.4 Applications of photocatalytic membrane reactors The interest in PMRs is constantly increasing thanks to several advantages obtained by coupling a membrane separation process and a photocatalytic process. Some applications of PMRs for water treatment, in particular for pharmaceuticals removal, and in reactions of synthesis, in particular for conversion of CO2 to solar fuels, will be discussed next.

10.4.1 Photocatalytic membrane reactors in photodegradation of pharmaceuticals in water In the last years the presence of pharmaceutical active compounds (PhACs) and their metabolites in waterways have increased considerably [57,58]. For example, a large amount of PhACs, in terms of thousands of tons, are annually used for therapeutic purposes or in animal farming in each European country and could be excreted both as unmetabolized and as active metabolites, thus reaching the aquatic environment. Furthermore, their presence in an aquatic environment can be increased by the inadequate disposal of industrial waste. The wastewater is generally treated in sewage treatment plants that often fail to remove the majority of PhACs resulting in high concentrations discharged in treated effluents: [59
63] so PhACs are detectable in the aquatic environment with concentration levels up to the μg/L. These amounts are much lower than the maximum concentrations reported for typical industrial contaminants but their toxicological chronic effects on the aquatic environment, due to the continuous exposure to mixtures of pharmaceuticals, are unknown. On this basis, the demand for developing efficient systems, an alternative to the traditional purification methods, to remove PhACs from water has assumed a great research interest. PMRs could represent a useful solution to this problem. Molinari et al. [9] reported the degradation of two pharmaceuticals [Gemfibrozil (GEM) and Tamoxifen (TAM)], selected as a target molecule, using suspended TiO2 as photocatalyst in two configurations of PMRs (pressurized and depressurized) operated in both batch and continuous mode. The latter mode consisted of the continuous withdrawal of the permeate from the reactor and its replacement by an equal volume of polluted water until reaching a steady state (c.190 min) in terms of flux and rejection values of intermediates. The experimental plant consisted of an annular photoreactor with an immersed UV lamp connected to the permeation cell in which a pressurized flat sheet membrane or a submerged membrane module was located. The data obtained for the closed membrane system showed complete photodegradation of GEM and TAM, with an abatement of 99% after 20 min and mineralization higher than 90% after approximately 120 min. Concerning the pressurized system, the membrane used (commercial flat sheet NF membrane NTR 7410, Nitto Denko, Tokyo) allowed to maintain the drug and the photocatalyst in the reaction environment, but it was not able to reject significantly the degradation intermediates which were also found in the permeate. Good operating stability was observed by operating continuously, reaching a steady state in ca. 120 min, with a complete abatement of

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the drug together with values of mineralization (60%) and permeate flux (38.6 L/h/m2) that remained constant until the end of a run. A TOC rejection of about 62% at a steady state confirmed the need to identify a membrane with higher rejection to the intermediate products. Photocatalyst deposition on the membrane surface and fouling caused a flux decline during the photocatalytic process. The depressurized system, in which the submerged membrane module was located separately from the photoreactor and the oxygen was bubbled on the membrane surface, could be of interest both for separating the photocatalytic zone from the separation zone and for reducing membrane fouling thanks to oxygen sparging. The UF membrane (PES commercial capillary membrane, 0.05
0.1 μm average pore size) used in the submerged system showed retention of only the photocatalyst in the reaction environment, while GEM and its oxidation products moved in the permeate. However, the submerged PMR permitted to obtain a steady-state flux of 65.1 L/h/m2, higher than those obtained operating with the pressurized module, showing its interest for application purposes. The results reported by Molinari et al. [9] confirmed that PMRs can be of interest in the removal of organic pollutants from water because they allowed the recovery and reuse of the photocatalyst, permitted to achieve a continuous process, and, if a suitable membrane is found, it is possible to retain the pollutant and its degradation products in the reaction environment. Benotti et al. [64] reported a study on the use of a PMR pilot system, employing UV/TiO2 photocatalysis, in the removal of 32 PhACs from water. The results evidenced that concentrations of all compounds decreased during the treatment. In particular, removal efficiency higher than 70% was obtained for 29 of the targeted compounds, while only for three compounds a removal efficiency lower than 50% was achieved. During the treatment of all the considered PhACs, no estrogenically active transformation products were formed. Then, the PMR developed in this work is a useful technology for water treatment as determined by pharmaceutical and endocrine-disrupting compounds destruction as well as the removal of estrogenic activity. In 2012 Mozia and Morawski [65], based on their previous encouraging results obtained in dyes removal from water, compared the efficiency of photodegradation of ibuprofen sodium salt (IBU) present in tap water in a PMR utilizing DCMD under batch and continuous operation. The results revealed that the influence of the operation mode on the effectiveness of IBU decomposition and mineralization and the product (distillate) quality was not significant. The permeate did not contain IBU, regardless of the process conditions. Despite that, the authors recommended operating the PMR in the continuous mode, due to its higher potential for large-scale application. Membrane scaling during the long-term process was also studied evidencing a decline of permeate flux during the long-term experiments. In particular, after 54 h of the PMR continuous operation, the flux was ca. 7% lower than the pure water flux (272 L/m2/d vs 294 L/m2/d), whereas after 188 h an 86% flux decline was detected. The deposited layer formed on a membrane surface had a composite structure: on a thin porous TiO2 layer the calcite and aragonite crystals were present. Cleaning with an HCl solution allowed for the dissolution of the CaCO3 scale deposit and recovery of the flux.

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However, about 70 h after washing with HCl solution the flux started decreasing at a similar rate as before cleaning. In a successive work, the same authors [66] reported the removal of a nonsteroidal antiinflammatory drug [DCF sodium salt] from water in two-hybrid systems coupling photolysis or photocatalysis with DCMD. A complete DCF decomposition in the feed was obtained in both processes. In the case of the photolysis—DCMD hybrid system the effectiveness of TOC mineralization after 5 h of irradiation ranged from 27.3% to 48.7%, depending on the DCF initial concentration. The addition of TiO2 allowed for improving the efficiency of TOC removal. After 5 h of the hybrid photocatalysis-DCMD process, the mineralization efficiency was in the range of 82.5%
100%, being the highest for the lowest DCF concentration. Application of DCMD in the hybrid systems ensured high efficiency of separation of DCF as well as products of its degradation. Indeed, regardless of the process applied, the distillate was high purity water: DCF was not detected, TOC concentration did not exceed 0.8 mg/dm3 and conductivity was lower than 1.6 μS/cm. The results obtained by Mozia et al. [66] evidenced that, as the MD separated efficiently not only TiO2 particles but also organic contaminants present in the feed solution, the PMR utilizing DCMD represents a promising method for treatment of waters containing pharmaceuticals. Low permeate (distillate) flux assured relatively long residence time of contaminants which resulted in high effectiveness of their photodegradation. Recently, the photocatalytic degradation of DCF has been studied also by Nguyen et al. [67] by using N-doped TiO2 and N-doped TiO2/H2O2 (with H2O2 addition to the reaction environment) as photocatalysts. The photocatalytic experimental tests were conducted in a submerged PMR (SPMR) with a suspended photocatalyst under visible light irradiation. The SPMR shown in Fig. 10
7 was constituted by a cylindrical photoreactor with a tubular ceramic MF membrane immersed and five visible lamps of 50 W (420
720 nm) all

FIGURE 10–7 Submerged photocatalytic membrane reactor (SPMR) setup [67].

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around the reactor, while the oxygen was continuously insufflated under the UF membrane. The results indicated that the addition of H2O2 improved the system performance while the efficiency of the process decreased by using a higher initial concentration. The authors reported that N-doped TiO2 absorbed visible light for DCF degradation, and the addition of H2O2 with the Vis/N-doped TiO2 photocatalytic process enhanced the DCF removal efficiency. The degradation of DCF fitted well with the pseudo-first-order kinetics. The persistent presence of some pharmaceuticals in wastewater is hazardous for aquatic and terrestrial organisms. For this reason, Lumbaque et al. studied the oxidation of four pharmaceuticals: paracetamol (PCT), furosemide (FRS), nimesulide (NMD), and diazepam (DZP), in a continuous-mode operation. The photocatalytic tests were carried out in a tubein-tube membrane reactor, with hydrogen peroxide addition, under UVA or UVC irradiation. Synthetic (SWW) and real (urban wastewater after secondary treatment) (UWW) matrices, were used for the tests, both containing the pharmaceutical mix solution of 200 μg/L. The best results, in terms of removal percentage for each pharmaceutical, PCT (27.4%), FRS (35.0%), nmD (24.2%), and DZP (30.0%), were achieved in SWW, at a steady-state regime of the UVC/H2O2/TiO2 system, by adding H2O2 (20 mg/L). Evaluating the UWW and SWW matrices, the UWW showed a decreased removal of pharmaceuticals (PCT—11.5%, FRS—20.3%, NMD—8.2%, and DZP—12.6%). Recently, the development of new materials, such as magnetic materials in photocatalytic composites and semiconductors coated on optical fibers (OF), has been improved thanks to their adsorptive abilities toward pharmaceuticals, for achieving photocatalytic wastewater treatment [68]. For this purpose, some authors used magnetic composites to remove pharmaceuticals from wastewater, for example, tetracycline by using magnetic FeNi3/SiO2/CuS [69] or amoxicillin by using magnetic fluorinated mesoporous graphitic carbon nitride [70] and a magnetic TiO2-GO-Fe3O4 [71]. Tugaoen et al. [72] used titanium dioxide coated on OF to remove parachlorobenzoic acid (pCBA). The authors performed the photocatalytic degradation tests in a recirculating reactor system with OF/LED showed in Fig. 10
8. This system is composed of a near clear PVC (polyvinyl chloride) cylinder (Harrington Plastics), with an inner diameter of 1.9 cm and a total length of 18 cm with the coated OF/LED couple incorporated into the reactor at the distance of 1 cm. The recirculation of the solution pCBA (0.1 mM) was made by a peristaltic pump at 5 mL/min at pH 4.0. They studied three configurations (OF/LED) units, which varied from one to five in the reactor, connected to a single LED source: (1) an individual fiber, (2) a bundle of three fibers, and (3) a bundle of fifteen fibers. Despite the highest kinetics values being achieved for 1:1 coupling using five OF/5 LEDs, in this configuration, numerous photons emitted by LEDs were not used for the photocatalytic degradation thus highest electrical energy per order (EEO) was required, furthermore, also the lowest quantum yield (Φ) was achieved. Instead, the available photoactive surface area increased by coupling a single LED to bundled OFs. The authors reported that pCBA oxidation increased obtaining the best results in terms of kinetics, Φ, and EEO, with increasing the number of OFs, with fixed photon flux.

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FIGURE 10–8 Flow-through optical fiber OF/LED reactor design [71].

The overall results, reported in this section, validate the use of PMRs in the removal of organic pollutants (e.g., pharmaceuticals) from water. These are interesting for the following reasons: (1) efficient separation of TiO2 particles, thus permitting the recovery and reuse of the photocatalyst; (2) possibility to realize a continuous process; (3) possibility to retain organic pollutants and their degradation products, thus avoiding their release in the permeate and controlling the residence time of contaminants in the reaction environment. Then, PMRs can be considered a useful technology for water treatment as observed by PhACs destruction (mineralization) as well as the removal of estrogenic activity. An important issue to take into consideration in developing photocatalysis-membrane hybrid processes is membrane fouling, mainly due to photocatalyst deposition on the membrane feed side: in this context, the approach proposed by Mozia et al., in which MD is coupled with photocatalysis, seems to be promising, as MD allows to obtain the points (1), (2) and (3) previously reported, and also permits to avoid the significant fouling observed when a pressure-driven membrane process is employed, although the higher energetic consumption of MD could represent a limitation of this hybrid process.

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10.4.2 Photocatalytic membrane reactors in the conversion of CO2 in solar fuels In the last years, many works have been focused on the photocatalytic conversion of carbon dioxide (CO2) due to the increased interest in its conversion to fuels [73]. About this, Cheng et al. [74] studied the use of an optofluidic microreactor with a TiO2/carbon paper composite membrane for CO2 photoreduction to methanol. They obtained a methanol production yield of 111 μmol/gcat. This CO2 photoconversion was studied also by Maina et al. [75] by using TiO2 and Cu-TiO2 as photocatalysts within zeolitic imidazolate framework (ZIF 8) membranes. The results obtained incorporating Cu-TiO2 nanoparticles with ZIF-8 membranes showed a methanol yield of 70% and a CO yield of 233%. Subsequently, the results above mentioned were improved by Baniamer et al. [73] that tried to achieve the simultaneous separation and conversion of CO2 by using two-layer PMs fabricated from a porous polyethersulfone-TiO2 (PES-TiO2) as a photocatalytic layer and a thin nonporous layer of poly-ether-block-amide (PEBAX-1657) as a selective layer. The results showed a high methanol production yield of about 697 μmol/gcat. An interesting semiconductor studied on this topic is graphite carbon nitride (g-C3N4) because the presence of N basic sites facilitates the CO2 adsorption step. In this regard, Brunetti et al. [76] studied photocatalytic CO2 reduction by using an exfoliated C3N4-TiO2 photo-catalyst, embedded in a dense Nafion matrix, in a continuous photocatalytic reactor under UV light. The results showed that MeOH production was 17.9 μmol/gcatalyst/h when only C3N4 was embedded into the Nafion membrane, but improved with the TiO2 content in the catalytic membrane to 45 μmol/gcatalyst/h for 100% TiO2. The use of new materials such as magnetic composites and combining photocatalysts with magnetic material or coating on OFs is another method to recover the photocatalyst from water. In this regard, Nguyen et al. [77] proposed an optical-fiber reactor prepared by coating it with a gel-derived TiO2
SiO2 mixed oxide-based photocatalyst by using UV and sunlight irradiation. The main product obtained, under UVA irradiation by using Cu
Fe/ TiO2, was ethylene with a quantum yield of 0.0235%, while the major product was methane with a quantum yield of 0.05%, by using Cu
Fe/TiO2
SiO2 as photocatalyst. Tests performed under sunlight irradiation showed methane formation as a product by using both TiO2
SiO2 and Cu
Fe/TiO2
SiO2 photocatalysts with production rates of 0.177 and 0.279 mmol/gcat/h, respectively. Also, Chen et al. [78] performed the CO2 photoreduction under visible light. Tests were performed in an optofluidic membrane microreactor, with a mesoporous CdS/TiO2/SBA-15@carbon paper composite membrane. This system is constituted of a syringe pump, a CO2 gas cylinder, an optofluidic membrane microreactor, a simulated sunlight source, and a collection vessel as shown in Fig. 10
9. The results obtained by using CdS/20 wt.% TiO2/SBA-15 at 0.4 M NaOH concentration showed a maximum methanol yield of about 1022 mol/gcat h, which was nearly four times higher than CdS/TiO2.

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FIGURE 10–9 (A) Schematic and (B) photo of the optofluidic membrane microreactor [78].

10.5 Advantages and limitations of photocatalytic membrane reactors Based on the results summarized in the previous sections, the use of PMRs can be proposed as an attractive green process for both reactions of degradation/mineralization, since it is effective to abate harmful and recalcitrant substances present in aqueous effluents, and the reaction of synthesis of various chemicals. The main advantages related to the use of the PMRs are: 1. possibility to be applied to a wide range of compounds in aqueous and gaseous phases; 2. short reaction times and mild experimental conditions, usually ambient temperature and pressure; 3. generally, only oxygen from the air (in the case of degradation) without any additional additive is necessary; 4. effectiveness also with a low concentration of pollutant(s); 5. possibility to destroy a variety of hazardous molecules with the formation of innocuous products, solving the disposal pollutant problem associated with the conventional wastewater treatment methods;

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6. possibility to convert toxic metal ions into their nontoxic forms which can be recovered and reused; 7. synergistic effects by coupling the potentialities of classical PRs and those of membrane processes, that is, photocatalyst recovery and reuse (immobilized or in suspension), rejection of the substrate, and/or separation of products, thus allowing to perform the reaction of interest and the simultaneous separation of the desired product(s) in continuous mode; 8. possibility to control the residence time of substrates in the photocatalytic system, thus controlling the contact time between the substrate molecules and the photocatalyst and then the photocatalytic reaction: this feature is very important when the process is used for synthetic purposes; 9. possibility to use sunlight or new low-energy LED lamps connected to sunlight photovoltaic conversion systems. Despite these important advantages, different drawbacks mainly related to the membrane fouling limited the application of the photocatalytic processes at the industrial level. As reported in the previous sections, the choice of the membrane is a key step for attaining a successful application of PMRs. For example, Molinari et al. [9,48] demonstrated the potentialities of PMR applications in both pressurized and submerged configurations in water detoxification, since they permitted to recovery and reuse of the photocatalyst and allowed to obtain a continuous process, while the membrane rejection toward the pollutants was not very satisfactory. These results show the necessity to identify a membrane, selective to intermediate products, to retain the substrate in the reaction environment while, depending on the type of reaction, retaining or turning away the by-products and removing the main product.

10.6 Conclusion and future trends This chapter describes the advantages arising from a synergic effect of the photocatalytic process coupled with membrane separation. In particular, it has been shown that PMRs can be considered a suitable technology for the removal of organic pollutants (e.g., pharmaceuticals) from water, thanks to PhACs destruction, and for the synthesis of organic compounds (e.g., from CO2 as a carbon source). An important limitation related to photocatalysis-membrane coupling is related to the permeate flux decrease caused by concentration polarization and membrane fouling, mainly due to photocatalyst deposition on the membrane surface. This drawback can be limited by appropriately choosing the membrane material, which must also ensure adequate chemical and thermal stability, and the type and PMRs configuration. The ideal membrane process to be coupled with photocatalysis should possess these features: (1) complete retention of the photocatalyst and controlled fouling; (2) good rejection of the substrates and degradation by-products, to avoid the release of dangerous substances in the treated water and assure a residence time in the photocatalytic system sufficient for

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obtaining complete pollutant degradation and mineralization. Longer residence times are usually obtained by reducing the permeate flux so it is important to find a good compromise between the permeate flux and the residence time to achieve a system for application purposes (reactions of degradation or reactions of synthesis). In fact, the process intensification by synergistic effect depends on the optimization of the characteristic parameters of the photocatalytic and membrane separation processes. To enhance the membrane efficiency various methods have been tested. Among them, a promising approach that produces PMs with antifouling/self-cleaning ability is blending with inorganic materials (e.g., immobilization of TiO2) to improve the hydrophilic character of the membrane surface exposed to the reacting environment of TiO2. This type of membrane is a promising candidate for use in membrane PRs for wastewater treatment and reactions of synthesis. Among different PMR configurations, the use of SMPRs with fine-bubble aeration and intermittent membrane filtration can be potentially employed in the photocatalytic oxidation process in drinking water treatment, because this approach considerably reduces membrane fouling. For the removal of organic pollutants from waters, the hybrid process obtained by coupling photocatalysis with MD appears very interesting because it possesses the ideal features previously reported. Despite this coupling avoiding fouling, it needs the energy to achieve evaporation. Based on this, the combination of dialysis and PV could possess the features of the ideal membrane process to be coupled with photocatalysis. To overcome the limitation related to membrane fouling the use of submerged membrane systems operated with fine-bubble aeration in the intermittent membrane filtration mode and the coupling of photocatalysis with MD, dialysis and PV can be considered future trends. In particular, the coupling with DCMD, seems to be more promising, as MD: (1) separates efficiently TiO2 particles, (2) permits to retain of organic pollutants and their degradation by-products (the MD distillate is practically high purity water), (3) permits to realize a continuous process, and (4) allows to avoid membrane fouling, although the higher energetic consumption of MD could represent a limitation of this hybrid process. On this basis it is foreseen that hybrid systems photocatalysis—membranes will have a significant impact in designing processes for the abatement of organic pollutants contained in waters. On the other side, partial-oxidation and reduction processes in PMRs can be used in the reaction of synthesis. The described cases, concerning photodegradation and reaction of synthesis, show that PMRs represent a promising green technology that could shift on applications of industrial interest using visible light (from the Sun) active photocatalysts. However additional studies are still required. These should be aimed at improvement of process efficiency, mainly by the development of novel membranes resistant to the harsh conditions prevailing in these systems, application of visible light active photocatalysts, and use of low energy-requiring LED lamps able to work with electrical energy from photovoltaic systems.

Chapter 10 • Photocatalytic membrane reactors

List of symbols Cf Cp E10 E20 ECB Eg EVB e2 h h1 ν Φ

solute concentration in the feed solute concentration in the permeate Redox potential of the specie to be reduced Redox potential of the species to be oxidized Redox potential of the Conduction Band energy band gap Redox potential of the Valence Band photogenerated electron Plank’s constant photogenerated hole frequency of light radiation quantum yield

List of acronyms AC AP 2,4-DHBA 4-CP 4-NP AOP BPA CB C-dots DCF DCMD DLHF DZP EEO FAU FGC FRS GAZO GEM G (g-C3N4) GO HAP HTNF IBU MD MF NF NMD OF/LED

Activated carbon Acetophenone 2,4-dihydroxybenzoic acid 4-Chlorophenol 4-nitrophenol Advanced Oxidation Process Bisphenol A Conduction Band carbon dots Diclofenac Direct Contact Membrane Distillation dual-layer hollow fiber Diazepam Highest electrical energy per order Faujasite Flexible graphene composite Furosemide PAZO 1 flexible graphene composite Gemfibrozil Graphene Graphitic carbon nitride Graphene Oxide Hydroxyapatite Titanium Dioxide Nanofibers Ibuprofen Membrane Distillation Microfiltration Nanofiltration Nimesulide optical fiber/LED

277

278

PAA PAN PAZO pCBA PCN PCT PES PhAC PMR PNP PP PR PR PSF PTFE PV PVDF R RO SEM SMPR SPMS SWW TAM TNTs TOC UF UV UWW VB WPF ZAO

Current Trends and Future Developments on (Bio-)Membranes

Polyacrylic Acid Polyacrylonitrile polymer membrane with ZnAl2O4 nanoparticles Parachlorobenzoic acid g-C3N4 doped with P Paracetamol Polyethersulfone Pharmaceutical Active Compound Photocatalytic membrane reactor p-nitrophenol Polypropylene Photoreactor Photocatalytic reactor Polysulfone Polytetrafluoroethylene Pervaporation polyvinylidene fluoride Membrane Rejection Reverse Osmosis Scanning Electron Microscopy Submerged Membrane Photocatalytic Reactor sono-photocatalysis/membrane separation Synthetic wastewater Tamoxifen Titanate nanotubes Total Organic Carbon Ultrafiltration Ultraviolet irradiation Urban wastewater Valence Band pure Water Permeation Flux ZnAl2O4

Acknowledgments One of the Authors (C.L.) thanks POR Calabria FESR FSE 2014
20 for the financial support.

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872. Available from: https://doi.org/10.1016/j. solmat.2008.02.010. [78] R. Chen, X. Cheng, X. Zhu, Q. Liao, L. An, D. Ye, et al., 'High-performance optofluidic membrane microreactor with a mesoporous CdS/TiO2/SBA-15@carbon paper composite membrane for the CO2 photoreduction, Chem. Eng. J. 316 (2017) 911
918. Available from: https://doi.org/10.1016/j.cej.2017.02.044.

11 Electrochemical membrane reactors Pierre Millet PARIS-SACLAY UNIVERSITY, ICMMO ( UM R 8 182), ORSAY, F RANCE

11.1 Introduction Electrochemical reactors are a special type of chemical reactor allowing the interconversion of electrical and chemical energy. As the chemical reaction, the electrochemical reaction is subject to two conservation principles: conservation of matter (Lavoisier’s law) and conservation of energy (thermodynamic principles). The electrochemical engineer responsible for sizing this type of reactor must take these two conservation laws into account at the same time [1]. The energy balance of the electrochemical reaction is dictated by the principles of thermodynamics. The electrochemical transformation can be done in both directions. Either electrical energy is used to carry out a chemical transformation (this is referred to as an exergonic reaction or the charge of the battery). Either the chemical energy of the reagents is transformed into electricity (we speak of exergonic reaction or discharge of the battery). This helps to understand the important role played by electrochemical processes in the chemical industry: whether in the field of synthesis by electrolysis, in the generation of electricity using cells of the first kind, or in the Storage of Electricity Using Second Species Cells, the types of cells used and the applications are extremely numerous and varied. The economic weight of electrochemical processes in the industry is extremely important. Examples of endergonic electrochemical reactors (electrolysis or battery charge) The word “electrolysis” comes from a combination of two Greek words: εlektron (amber) and lysis (to separate). It refers to the operation of transforming a molecule into reaction products using electricity. An endergonic electrochemical reactor is a chemical reactor in which DC power is used to perform nonspontaneous endergonic chemical reactions (electrolysis). The general principles of electrolysis were discovered at the end of the 18th century, during experiments carried out in the Netherlands using a Leyden Jar and then in England using a Volta battery [2,3]. Since then, a lot of processes of great industrial interest have been developed and deployed on a large scale: • • • •

Brine electrolysis (chlorine and caustic soda production). Aluminum electrolysis and molten-salt electrolysis (production of alkali metals). Electro-refining (e.g., copper) and electro-coatings (electroplating). Water electrolysis (hydrogen economy, large-scale storage of renewable electricity).

Current Trends and Future Developments on (Bio-)Membranes. DOI: https://doi.org/10.1016/B978-0-12-823659-8.00009-5 © 2023 Elsevier Inc. All rights reserved.

285

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Examples of exergonic electrochemical reactors (primary cells or battery discharge) An exergonic electrochemical reactor is a chemical reactor in which a spontaneous chemical reaction is used to produce DC power. Primary cells can only be used once, and cannot be recharged. They are mainly used in consumer applications. They are also increasingly used in the frame of the energy transition for clean mobility applications. Examples are: • Bunsen, Daniell, Leclanché cells. • AA-size alkaline cells. • H2/O2 fuel cells (mobility applications). Examples of rechargeable electrochemical reactors (secondary cells or rechargeable batteries) Secondary electrochemical cells can only be recharged and used several times. Examples are: • • • •

Lithium batteries Nickel
cadmium battery Silver
zinc battery Redox batteries In this chapter, the focus is on endergonic electrochemical reactors.

11.2 Electrochemical reactors 11.2.1 General principles 11.2.1.1 Thermodynamics The energetics of chemical reactions are described by the laws of thermodynamics applied to chemistry (thermochemistry). In particular, the Gibbs-Helmholtz relation makes it possible to relate the enthalpic and entropic contributions to each other: ΔH ðT; P Þ 5 ΔGðT; P Þ 1 T:ΔSðT; P Þ

(11.1)

In Eq. (11.1), the three terms are, respectively, the enthalpy change (J/mol), the free enthalpy change (J/mol), and entropy change (J/mol/K) of any chemical reaction of interest. In this equation: ΔH(T, P): the total amount of energy required to perform the reaction of interest. ΔG (T, P): the amount of electricity required for the reaction. ΔS (J/mol/K): entropy change of the reaction of interest. T.ΔSd (J/mol): the amount of heat required to “fuel” the entropy change. The spontaneity criterion (ΔG , 0) is defined both by the endothermic/exothermic type of chemical reaction but also by the sign of the entropy variation of the chemical reaction considered. A spontaneous transformation is called an exergonic transformation and a nonspontaneous one is an endergonic transformation. The situation is summarized in Table 11
1. The first principle of thermodynamics (which reflects the conservation of energy) makes it possible to establish the relationship between the variation of free enthalpy (ΔG) of a

Chapter 11 • Electrochemical membrane reactors

Table 11–1

287

Enthalpy and entropy contributions to the spontaneity of a chemical reaction.

ΔH(T, P)

ΔS(T, P)

T. ΔS(T, P)

ΔG(T, P)

Spontaneity

,0 ,0

.0 ,0

.0 ,0

.0

.0

.0

.0

,0

,0

,0 , 0 if |ΔH(T,P)| . |T.ΔS(T,P)| . 0 if |ΔH(T,P)| , |T.ΔS(T,P)| , 0 if |ΔH(T,P)| , |T.ΔS(T,P)| . 0 if |ΔH(T,P)| . |T.ΔS(T,P)| .0

Spontaneous at (T,P) Depends on the weight of enthalpic and entropic contributions Depends on the weight of enthalpic and entropic contributions Nonspontaneous at (T,P)

chemical reaction, the electrical voltage (E in volts) to be applied (endergonic reaction) or the electromotive force to be measured (exergonic reaction) at the endplates of the electrochemical cell. For endergonic transformations: ΔGðT; P Þ 2 nFEcell ðT; P Þ 5 0

(11.2)

ΔGðT; P Þ 1 nFEcell ðT; P Þ 5 0

(11.3)

where ΔG . 0 For exergonic transformations:

where ΔG , 0 In Eqs. (11.2) and (11.3), n is the number of electrons exchanged during the electrochemical transformation; F (  96 485 C/mol) is the Faraday constant; ΔG is free energy change in J/mol associated with the electrochemical transformation of interest; E is the thermodynamic electrolysis voltage in Volt. The thermodynamic electrolysis voltage is, therefore: Ecell ðT; P Þ 5

ΔGðT; P Þ n F

(11.4)

Since most electrochemical reactors are operated in the exothermic mode (heat is produced by internal dissipations), is it sometimes more interesting to define and use the thermos-neutral voltage V(T, P) in volt defined as: Vcell ðT; P Þ 5

ΔH ðT; P Þ n F

(11.5)

11.2.1.2 Kinetics Ecell (T, P) is the minimum voltage to be applied to the endplates of the electrochemical cell to transform one mole of a reactant at (T, P) operating conditions of temperature and pressure. Below that value, no electricity can flow across the cell. Vcell(T, P) is the minimum voltage required to transform one mole of reactant in thermoneutral conditions (the heat

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required for the entropy increase is provided by internal dissipations). In practical conditions, the cell operator sets large operating current densities to minimize the capital expenditures of the process [4]. In modern electrolyzers, operating current densities up to several A/ cm2 are commonly achieved. The internal cell resistance is overcome by applying a cell voltage larger than the thermoneutral voltage. Because the charge transfer kinetics at each interface of the cell exhibit a logarithmic behavior (Butler
Volmer relationship), the relationships between cell voltage and current density of endergonic electrochemical transformation are quite similar to those shown in Fig. 11
1 which therefore has a general character.

11.2.1.3 Electrochemical efficiency The definition of the cell efficiency ε(T, P) used in the academia usually relates two quantities: on one hand, the theoretical amount of energy Wt (in J/mol) required to react one mole of reactant; on the other hand, the real amount of energy Wr (in J/mol) required by the process. The cell efficiency is defined as the ratio of both quantities: εcell ðT; P Þ 5

Wt ðT; P Þ ,1 Wr ðT; P Þ

(11.6)

In practical operating conditions, Wr . Wt because of cell irreversibilities and therefore ε(T, P) , 1. Wt can be explicated by considering either the thermodynamic or the thermoneutral cell voltage. Two different expressions of the efficiency of any electrolysis cell can therefore be used. Let Ucell(T, P, j) be the cell voltage when the cell is operated at T, P, j conditions and j be the operating current density (in A/cm2): εΔG ðT; P; j Þ 5

E ðT; P Þ V ðT; P Þ εΔH ðT; P; j Þ 5 Ucell ðT; P; j Þ Ucell ðT; P; j Þ

FIGURE 11–1 Typical voltage—current density curve measured during electrolysis.

(11.7)

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289

At very low current densities ( j , 10 mA/cm2), the cell efficiency is close to unity since the cell voltage is similar to the thermodynamic one. Since Ucell (T, P, j) increases with j, the efficiency decreases with j. For example, in water electrolyzers, and efficiency εΔH of approximately 70% is obtained at a current density of 1 A/cm2, T 5 90 C and P 5 1 bar. The best operating current density which minimizes the production cost of reaction products is determined by analyzing its impact on both capital (capex) and operational (opex) expenditures [4] for example in water electrolysis.

11.2.2 Endergonic transformers Looking closely at the degree of oxidation of the active reactants and reaction products of any chemical reaction, we see that they change during transformation. Indeed, the breaking and formation of chemical bonds are equivalent to electronic transfers between atoms. This means that any chemical reaction can be split into the sum of two half-cell reactions, explicitly showing the exchanged electrons and the two associated redox couples. Whereas in a chemical reactor the exchanges of electrons take place from molecule to molecule over very small distances, in an electrochemical reactor, the processes of oxidation and reduction are separated from each other. This is made possible by the use of metallic electrodes whose particular electronic structure (cf. the Drude-Sommerfield theory on the metallic bond) makes it possible, by monitoring the electric potential, to give or receive electrons. An electrode that allows electrons to be injected into an electrolytic solution is called a cathode and one which receives electrons is called an anode. The diagram is shown in Fig. 11
2 shows a sectional view of an electrochemical cell operating in endergonic mode (electrolysis) [5]. The electrochemical cell type consists of two metal electrodes placed face to face at a short distance (from a few tens of micrometers to a few cm) depending on the technology. A rectifier (DC voltage/DC current generator) is used to polarize the cell and as a result, the Fermi level of the two electrodes is adjusted so that they come to bracket those of the two redox couples present in solution in the interpolar space. In doing so, an energy level cascade is obtained and the electric current (electrons in the metallic phases and ions in the electrolyte solution) is set in motion. Electroneutrality is preserved: the electronic flow (cathodic current) injected from the cathode into the electrolytic solution is equal to the electronic flow (anode current) ejected into the anode from the electrolyte. It should be noted that the production of chemical species at the two electrodeelectrolyte interfaces leads to the formation of concentration gradients that transport these species to the center of the cell. If nothing is done to prevent them from mixing, then they react spontaneously with each other and restitute chemical energy as heat: the electrochemical cell behaves like a radiator which is of course useless. To avoid this, separators are inserted in the middle of the interpolar space. They must both avoid the mixing of the reaction products (whether in solution or in the form of gaseous compounds, as is the case for example in the electrolysis of water) and not prevent the circulation of the ionic charge carriers from one interface to the other, under penalty of stopping the operation of the cell.

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Current Trends and Future Developments on (Bio-)Membranes

FIGURE 11–2 Cross-sectional diagram of an endergonic electrochemical cell showing the cascade of energy levels under polarization.

11.2.3 Exergonic transformers The diagram is shown in Fig. 11
3 shows a sectional view of an electrochemical cell operating in exergonic mode (cell discharge). The DC rectifier of Fig. 11
2 is replaced by a variable resistor which is used to adjust the current (which, according to Faraday’s law of mass conservation, is a measure of the kinetics of the chemical reaction). During operation, the Fermi levels of the two electrodes (the terms anode and cathode are replaced by positive and negative electrodes in secondary batteries because the redox role of each electrode change when the cell is switched from charge to discharge modes) adjust themselves to satisfy the overall electroneutrality of the cell (Poisson’s law). In doing so, an energy level cascade is obtained and the electric current (the electrons in the metallic phases and the ions in the interpolar electrolyte solution) is set in motion. The overall kinetics is driven by the impedance of the resistor placed in the external circuit. Again, electroneutrality is preserved: the electronic flow (cathodic current) injected into the electrolytic solution from the cathode (negative electrode) is equal to the electronic flow (anode current) ejected from the electrolyte into the anode (positive electrode).

11.2.4 Cell separators A quick look at the diagrams of Figs. 11
2 and 11
3 tell us that species of the two redox couples can move freely in the region between the two electrodes (migration and convection are preferred modes of transport). Hence, the risk that reaction products meet and react spontaneously has to be mitigated. This is usually obtained by inserting a so-called cell

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291

FIGURE 11–3 Cross-sectional diagram of an exergonic electrochemical cell showing the cascade of energy levels under polarization (discharge).

separator inside the cell, between the two electrodes. Such cell separators are sometimes used as diaphragms (thin and porous layers of chemically inert materials) or membranes (usually ion-selective materials). Separators must have a set of sometimes contradictory mechanical, physicochemical and economic properties. From a mechanical point of view, they must be thin (to minimize the interpolar distance and the ohmic drop) but thick enough to have a mechanical resistance suitable for handling, especially for large surfaces. From an electrical point of view, they must be good ionic conductors but poor electronic conductors. They must also effectively separate the reaction products formed at each interface. In the electrochemical industry, the main types of separators are: (1) inert porous materials impregnated with liquid electrolytes: their ionic conductivity is the same as that of the electrolyte; (2) selective ionic conductive polymers (either in cationic form or in anionic form); (3) ion-conductive ceramics (generally at high operating temperatures, above 600 C). The choice of a particular material will depend essentially on the target operating temperature and the pH of the electrolyte when this is an aqueous medium. The different types of cell arrangements are schematically described in the next figure. The first configuration (Fig. 11
4A) is called the “gap cell.” In this type of cell, there is a space (gap) between the (electro-active) surface of the electrodes and the surface of the separator placed in the middle of the interpolar field. This space is occupied (in the case of a stagnant electrolyte) or swept (in the case of a circulating liquid electrolyte) by the ionically conductive electrolyte. This type of configuration is very penalized when the redox half-reactions which occur at the interfaces lead to the formation of gaseous compounds (case of the electrolysis of water) because the gas bubbles tend to coalesce and form an insulating gaseous film which severely limits the value of the operating current density.

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Current Trends and Future Developments on (Bio-)Membranes

FIGURE 11–4 Schematics of different electrochemical cells (A) gap-cell; (B) zero-gap cell; (C) confined electrolyte cell.

The separator is generally a porous solid material whose thickness and pore size are optimized to maximize mechanical strength and ionic conductivity. The thickness of the gap is optimized to reduce both the electrical resistance (low gap) and the hydraulic resistance (large gap) of the cell. The second configuration (Fig. 11
4B) is called a zero-gap cell. Porous electrodes (usual grids) are firmly pressed onto the cell separator which is also a porous medium, impregnated with a conductive electrolyte. The cell resistance (and associated ohmic losses) are thus minimized. The reaction products are released through the grids, behind the electrodes, and therefore outside the electric field, without introducing a parasitic ohmic drop. The cell is more compact and when gases are produced, the limiting current density is significantly increased compared to the “gap cell.” This is used for example in advanced alkaline water electrolysis cells to increase efficiency. The third configuration (Fig. 11
4C) used to be called an SPE-cell where SPE stands for Solid Polymer Electrolyte. The electrolyte is strictly confined inside a selective ionconducting membrane (cationic or anionic conduction) and the cell is fed by the reactant in the gas phase. The membrane can be a polymer with either protonic (PEM 5 Proton Exchange Membrane or Polymer Electrolyte Membrane) or hydroxyl-ions (HEM 5 Hydroxylions Exchange Membrane) conduction. It can also be a ceramic (oxygen ions conduction). This is the configuration used in PEM/HEM water electrolysis cells and PEM/HEM fuel cells but also in different “sister” technologies used H2 purification/compression [6], CO2 reduction in stand-alone or coupled reactors [7], etc.

11.3 Diaphragms for liquid electrolytes The diaphragm is a term used in electrochemistry, in particular in alkaline water electrolysis, to designate a thin (δ 5 0.05
0.5 mm) porous membrane of a chemically inert and nonconductive to electrons material that is placed between the two working electrodes; its role is to allow ionic transport and to prevent the mixing of reaction products (H2 and O2 in this case) which would otherwise spontaneously recombine. Thin diaphragms are required to

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293

minimize the distance between the working electrodes and to minimize ohmic losses during electrolysis. Chemical inertness is required to allow operation for tens of thousands of hours of operation. Nonelectronic conductivity is mandatory to avoid short-circuiting the cells. The diaphragm has to be porous and is impregnated with liquid electrolyte. The pore structure needs to be adjusted and a compromise has to be found between large pores which improve ionic conductivity and small pores which prevent gas transport and mixing. The porosity of the diaphragm also dictates the maximum pressure difference between the two compartments under which the cell can be operated. Operation under differential pressure is particularly advantageous in water electrolysis to produce pressurized hydrogen (typically up to 80 bars) while releasing the oxygen at a pressure close to atmospheric pressure for safety reasons. The greater the porosity of the diaphragm, the smaller the pressure difference that can be reached, and the more difficult it is to change the operating setpoint.

11.3.1 Asbestos Asbestos is a group of six naturally occurring silicate minerals, made of soft and flexible jagged or curly fibers (Crocidolite, Tremolite, Amosite, Actinolite, Anthophyllite, Chrysolite). It can be found in large natural deposits or mixed with other minerals (e.g., vermiculite or talc). It has long been exploited commercially for its physical properties (mainly its resistance to heat in the residential sector or industry, but also its sound absorption properties, its mechanical properties resulting from its average tensile strength, and those of electrical and chemical insulation). It has been largely used in Chlor-alkali and alkaline water electrolysis as a cell diaphragm all along the 20th century because of some key properties such as good water wettability and chemical resistance to caustic electrolytes at elevated temperatures. First commercial separators were made of micro-fibrous chrysolite asbestos (Mg3(Si2O5) (OH)4). However, exposure to asbestos and inhalation of asbestos fibers is linked to several diseases, including lumb cancers. For this reason, all activities (extraction, manufacturing, and processing) related to the manufacture of asbestos-based products have been banned by the European Union [8] and many other countries. Asbestos is not banned in the United States, but its use is highly regulated. A derogative measure to the replacement of existing diaphragms containing chrysotile in electrolysis installations (hydrogen production by alkaline water electrolysis and Chlor-alkali installations) was adopted and will remain until December 31st, 2025 [9]. The toxic mineral is still used in Russia, China, India, and Mexico. Regarding electrochemical rectors, less hazardous materials have been developed and are now used as cell separators.

11.3.2 Thermoplastic diaphragms Asbestos used as a diaphragm in electrochemical reactors has been replaced by different types of chemically inert synthetic polymers. These are fabrics or porous membranes that, when immersed in the electrolyte, become impregnated with it to ensure complete electrical contact from one compartment of the cell to another. Both cationic and anionic species contribute to the transport process, in proportion to their mobility. Water is the most commonly

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Current Trends and Future Developments on (Bio-)Membranes

used solvent in electrochemical processes operated in near-ambient conditions of temperature and pressure (NATP). Acidic liquid electrolytes are not used because of associated corrosion issues. Sodium chloride (brine electrolysis for the production of chlorine) and potassium hydroxide (water electrolysis) are two examples of ionic species used as charge carriers in electrochemical processes of industrial interest. The ionic conductivity of an electrolyte depends markedly on the concentration of ionic species dissolved in the solvent and concentrations as high as 26 wt.% (brine electrolysis) and 40 g/L (alkaline water electrolysis) are used. Hydrophilic materials are generally used to improve ionic conductivity in the separator and reduce the cross-flow of gases from one compartment to another. Controlling the porosity, size, and tortuosity of the pores ensures the passage of electric current while minimizing ohmic resistance and preventing the transport of reaction products from one compartment to another. After the ban on asbestos, PPS (Polyphenylene Sulfide) fabrics were used as diaphragms in alkaline water electrolysis cells. It is an organic polymer made up of aromatic rings linked by sulfides. The diaphragms obtained are chemically and thermally resistant under the usual operating conditions of these cells. Another popular diaphragm commonly used in alkaline water electrolysis cells is a composite material consisting of a polysulfone substrate (Fig. 11
5A) impregnated with hydrophobic zirconium oxide. This material was developed by Vito Research in Belgium and is marketed by Agfa-Gevaert NV under the trade name Zirfon Perl. Zirfon contains about 85% by powder weight of ZrO2 (a hydrophilic compound furthermore having a high specific surface area of B22 m2/g) which serves as a hydrophobic agent, and 15% by weight of polysulfone, a thermoplastic material which provides the necessary mechanical resistance [10]. The separator contains a polymer fabric with open mesh for further mechanical reinforcement. Zirfon separators (typically 0.5 mm thick) are characterized by excellent chemical stability and a low ionic resistance. As a result, reduced ohmic drops form during water electrolysis operation. Zirfon Perl diaphragms are commercially available. Two main types of polymeric mesh are used. They have an open porosity between 50% and 70%:

FIGURE 11–5 (A) Chemical structure of polysulfone. (B) Top cross-sectional view of a 500 μm thick ZIRFON Perl separator (500 UTP).

Chapter 11 • Electrochemical membrane reactors

295

• Zirfon LT (LT 5 Low Temperature): contains a PP (polypropylene) based mesh, which can operate up to a maximum recommended temperature of 80 C (it is this material that is used in alkaline water electrolysis cells); • Zirfon HT (HT 5 High Temperature): contains an ETFE (ethylene tetrafluoroethylene) based mesh allowing operation at higher temperatures up to approximately 120 C. Other materials have been used successfully as diaphragms in alkaline water electrolyzers. For example, porous membranes are made from sulfonated poly-ether-ether-ketone (s-PEEK) [11]. The performances measured with this material under stationary conditions (i.e., 400 mA/cm2, 80 C, 10 bars of pressure, in a KOH electrolyte at 30% by weight) and under transient conditions (including several on/off cycles) were compared with those obtained with commercial Zirfon HTP 500. The performance and stability results showed that cells fitted with s-PEEK separators have a similar voltage (i.e., approximately 2.5 V) but higher gas purity. More recently [12], a polysulfone diaphragm containing TiO2 particles (used to reduce hydrogen cross-permeation) has been successfully used to perform alkaline electrolysis of water at high pressure up to 100 bar and under a current density of 1 A/cm2. Although they did not necessarily lead to commercial applications, these alternative solutions show that the asbestos used in the past could be successfully replaced and that the electrolysis of alkaline water has finally succeeded in overcoming the difficulties that resulted from the ban on this once highly prized material.

11.4 Polymer membrane materials At the dawn of the US space program in the 1950s, many research efforts were made to develop electrochemical fuel cell separators that could operate in the absence of gravity. The efforts made in the field of grafting ionic functions onto polymer matrices have resulted in different materials and the appearance of ionomers, that is, polymers combining electrically neutral repeating units combined with ionized units (with either cationic or anionic mobile species), covalently bonded to the polymer backbone for better chemical stability. Regarding the ionic transport mechanism, the situation prevailing in polymer electrolytes is quite different from the one in electrolyte-impregnated diaphragms. In ionomers, there is only one charge carrier. This can be a cation in cation-exchange materials (e.g., protons in PEM water electrolysis cells or sodium ions in brine electrolysis cells) or anion in anion-exchange materials (usually hydroxyl ions). In such materials, the permselectivity (which is a measure of the preferential permeation of certain ionic species across ionomers) is equal to unity.

11.4.1 Proton conducting ionomers Cationically (proton) conductive perfluorosulfonated ionomers are widely used in modern fuel cell (for mobility purposes) and PEM-type water electrolysis (for energy storage purposes) technologies. They are therefore called upon to play a significant role in the frame of the energy transition.

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Current Trends and Future Developments on (Bio-)Membranes

11.4.1.1 Chemistry and microstructure The most commonly used proton-conducting polymer materials used in modern electrochemical cells are made of perfluorosulfonic acid (PFSA) ionomers. PFSAs form a group of compounds that are a member of a much wider family of fluorinated chemicals known as (PFAs). More specifically, PFSAs are sulfonated tetrafluoroethylene-based fluoropolymercopolymer membranes. Currently, they provide the best association of conductivity, chemical stability, and mechanical strength required for practical application in electrochemical cells. They were originally developed by DuPont Co. and marketed under the Nafion brand. Different types of ionomers (Nafion, Aquivion developed by Dow Chemicals and manufactured by Solvay Solexis, 3M Corporation ionomers, etc.) are used (Fig. 11
6). Each ionomer contains a hydrophobic PTFE backbone. Hydrophilic side chains, each terminating in a sulfonic acid group (
SO3H), are arranged at regular intervals along the main chain. The hydrophobic PTFE backbone provides the mechanical stability that allows membrane shaping, while the pendant sulfonic acid groups come together and form percolated and interconnected nano-domains (called clusters). By impregnation in water, the material swells and the liquid water absorbed provides the continuum responsible for the transport of protons. Proton mobility appears in response to the formation of concentration gradients or electric fields and occurs according to the famous hopping (Grotthus) mechanism. These ionomers differ by the length of their hydrophilic side chain and their concentration of charge (the equivalent weight EW, that is, the reciprocal of the ion exchange capacity, that is, the amount in grams of dry PFSA per mole of sulfonic acid groups; EW can be determined experimentally by simple acid-base titration). The side-chain length of the Aquivion is the shortest and that of the Nafion is the longest [13]. The high proton conductivity of PFSA-membranes is directly related to their EW and their morphology [14].

11.4.1.2 Key physical properties 1. In electrochemical cells, the membranes are used first of all to separate the anode and cathode compartments and to avoid the direct mixing of the reactants (case of exergonic transformations) or the recombination of products resulting from the main reaction (case of endergonic transformations). The transport properties (permeability) of the different

FIGURE 11–6 Three example types of ionomers used in electrochemical membrane reactors. (A) Nafion (du Pont); (B) 3 M ionomer, and (C) Aquivion (Solvay Solexis).

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297

chemical species involved in electrochemical reactions must be carefully measured before a particular material can be qualified for a particular application. 2. For the cell to function properly, the membranes must also be electronic insulators while being permeable to the ions which circulate (under the effect of the electric field) between the electrodes. Ionic conductivity is therefore a key property of membrane materials. For example, the proton-conductivity of conventional PFSA materials used in the 60 C
80 C temperature range is in the 0.1
0.5 S/cm range. In PEM water electrolysis, membranes have a typical thickness δ of between 80 and 200 μm (depending on the type of PFSA used and the supplier). The surface resistance r of such membrane is rB0.1 Ω.cm2. Typical operating current densities range between several hundred and several thousands of mA/cm2. The resulting ohmic drop [15] is up to 100 mV at 1 A/cm2. When pressurized gases are produced during electrolysis (e.g., hydrogen pressures of up to 80 bar are obtained in PEM water electrolysis cells), the transport of gases through the membrane should be monitored closely and at all times. This is done by measuring the volumetric fraction of hydrogen in oxygen but also the volumetric fraction of oxygen in hydrogen, both expressed in vol.%. Taking into account that 4% vol. of hydrogen in oxygen is the lower limit of flammability of the gas mixture, it is of course obligatory to monitor this content in industrial machines. For a given membrane material, this content depends essentially on the operating conditions (T, P, j) of the cell. The situation is more critical at the anode because molecular hydrogen is more diffusive than oxygen. Taking into account the dilution effect, the percentage of H2 in O2 is an inverse function of the current density [4]. To give a few orders of magnitude, hydrogen and oxygen diffusion coefficients (Di) and permeability (Pm) values measured at different operating temperatures on Nafion 117 are compiled in Table 11
2 [16,17].

11.4.1.3 Limitations and perspectives Existing PFSA materials found in PEM technology (mainly water electrolysis and fuel cells) have adequate physical properties for such applications. Their main drawback is an elevated cost (partly due to manpower costs resulting from the fluorination step) which may be prohibitive for the large-scale dissemination of PFSA-based electrochemical technologies. Another problem arises from the strength of carbon-carbon bonds in perfluorinated materials. Of course, this stability is necessary to prevent corrosion phenomena during operation in Table 11–2 H2 and O2 permeability and diffusion coefficient values measured in hydrated Nafion 117. Temperature/ C

10

20

40

60

85

PmO2/cm2/Pa/s DO2/cm2/s PmH2/cm2/Pa/s DH2/cm2/s DH2/DO2 (adim)

2.1 3 10212 2.1 3 1027 3.8 3 10212 3.9 3 1027 1.9

2.3 3 10212 2.5 3 1027 4.6 3 10212 4.9 3 1027 2.0

3.7 3 10212 4.2 3 1027 7.6 3 10212 8.7 3 1027 2.1

5.3 3 10212 6.5 3 1027 1.2 3 10211 1.5 3 1026 2.3

8.4 3 10211 1.1 3 1026 2.0 3 10211 2.6 3 1026 2.4

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electrochemical reactors. But on the other hand, these materials are not very biodegradable and when they are found in the environment, they remain intact and can gradually contaminate the food chain. It should be noted that the problem does not only concern PFSA (whose volumes produced are moreover still very low) but the whole family of perfluoroalkyl compounds, some of which are suspected of being endocrine disruptors. Evaluations are underway at the European level within the framework of the REACH legislation.

11.4.2 Hydroxyl-ion conducting ionomers Polymers conductive by hydroxyl ions are increasingly studied. The targeted applications are anion-exchange membrane (AEM) fuel cell and water electrolysis (AEMWE) technologies. The idea is to combine the advantages of conventional alkaline systems (reduced cost thanks to the use of transition metal electrocatalysts) and those of conventional PEM systems (compactness, great flexibility of operation, and reactivity). The main requirements of membranes with anionic conductivity are: • electronic insulating properties; • high anionic conductivity; • good chemical, mechanical and thermal stability in contact with water at operating T, P,j conditions; • very low gas permeability to avoid gas crossover between the anodic and cathodic compartments, especially for pressurized AEMWE operation; • low cost.

11.4.2.1 Chemistry and microstructure AEM materials usually contain a polymer backbone with periodically anchored cationic groups that confer anion conductivity and selectivity to the material (Fig. 11
7). Various chemical backbones have already been described in the literature, mainly for fuel cell applications: perfluorinated-type [19], polysulfone-type [20], polyphenylene-type [21], polybenzimidazole-type [22], polyethylene-type [23], polystyrene-type [24], poly(ether ketone)-type [25], poly(ether imide)-type [26]. The cationic groups used, which play a role similar to the sulfonic groups used in PEM technology, are less numerous. These are usually quaternary amines substituted by different groups with different electronic properties [27]. The two main synthetic approaches used to incorporate the cation functional groups into AEMs are (1) the direct polymerization of cationic monomers and (2) the postpolymerization functionalization of cationic functional groups onto preformed polymeric backbones. Direct polymerization makes it easier to control the distribution of functional groups along the polymer backbone, and postpolymerization functionalization facilitates synthesis [28]. An example is provided in (Fig. 11
8). This is a quaternary ammonium polysulfone. Its microstructure is similar to that of Nafion, that is, a continuous network of hydrophobic polysulfone, containing nanosized and interlinked hydrophilic domains where ionic conductivity takes place. The ionic conductivity of such materials is good ( . 1022 S/cm at room

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FIGURE 11–7 Diagram describing the transport of hydroxide ions through an AEM [18].

FIGURE 11–8 Chemical formula of the elemental unit of quaternary ammonium polysulfone.

temperature). Its mechanical strength (Young’s modulus . 1000 MPa), is large enough and compatible with alkaline fuel-cell and electrolysis applications.

11.4.2.2 Limitations and perspectives The polymers used in the AEM alkaline fuel cell technology can potentially be used as SPEs for water electrolysis applications if chemical stability is sufficient. AEMs form the core of these technologies and until they reach a sufficient level of development, it will be difficult if not impossible to develop applications with competitive levels of performance with PEM technologies. The principle expected benefits of AEMWE compared to PEM are: (1) reduced polymer cost: (2) significant cost reduction resulting from the use of nonnoble metal catalysts; (3) lower sensitivity of catalysts to trace amounts of cations and the possibility to electrolyze less-purified water. Impressive progress has been made over the past decade. In particular, the chemical stability at the anode at a high potential value under oxygen evolution has been solved (in fact, this doesn’t need to use perfluorinated polymer materials because, as indicated above, the alkaline pH allows the potential level of the anode to be lowered and this has a direct impact on chemical stability). Despite this, these materials still face two main limitations at the current stage of development: (1) insufficient thermal stability

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incompatible with industrial needs (particularly in water electrolysis where competing technologies have already demonstrated lifespans close to 100 khours of operation); (2) the difficulty to operate without KOH solution and only with deionized water.

11.5 Ceramic membrane materials 11.5.1 Nonorganic proton conductors Solid-state (nonpolymer) proton conductors [29] form a class of compounds that can be used as a membrane and cell separator in electrochemical reactors such as fuel cells or water electrolysis cells. They can be used in temperature ranges between those of polymeric materials and those of oxides ion conduction oxides, that is approximately between 200 C and 600 C. Many solid materials have a more or less marked proton conductivity [30], although it is generally much lower than those measured in conventional materials operated in an aqueous medium. These materials can be arbitrarily classified into low and high-temperature conductors (Fig. 11
9). Among the good, long-known, nonorganic solid proton conductors, many are too unstable at temperatures above 300 C and tend to decompose before, releasing water (Fig. 11
9A). Among the most stable ones (Fig. 11
9B), several perovskite-type oxides have been identified as interesting solid oxide

FIGURE 11–9 (A) Examples of low/temperature proton conductors. (B) Examples of high-temperature proton conductors.

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proton conductors for application in electrochemical technologies when exposed to water vapor. Proton conductivity is observed sometimes at elevated (400 C
1000 C) temperatures. In particular, ABO3 perovskite structure (e.g., barium cerates and zirconates), show excellent proton-conducting properties. The incorporation of protonic defects in the host matrix on the extrinsic oxygen vacancies after water absorption (acceptor doping) induces Proton conductivity. This class of materials has recently expanded after the identification of new proton-conducting structures such as those of semiconducting oxides, high entropy oxides, and lithium-intercalation materials [31]. All these materials differ in their water sorption and their proton transport mechanisms. In recent years, porous metal-organic structures such as metal-organic frameworks (MOF) or porous coordination polymers have also emerged as proton conductors and have been widely studied. They form a new type of proton conducting materials due to their specific crystallinity, design versatility, and high porosity. Such a combination of properties is appropriate and favors the adsorption of guest molecules capable of bringing the desired ionic conductivity. Major advances have been obtained over the last years in the development of proton-conductive MOFs with high-performance conductivity levels (σ . 10
2 S/cm), similar to conventional materials [32] (Fig. 11
10).

FIGURE 11–10 Summary of the proton-conducting properties of H2SO4@MIL-101 and H3PO4@MIL-101 together with those of some well-known proton-conducting reference materials [33].

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Current Trends and Future Developments on (Bio-)Membranes

11.5.2 Oxide-ion conductors Oxygen-transporting membranes play a central role in oxygen separators, membrane reactors, and HT (700 C
900 C) electrochemical cells such as H2/O2 fuel cells and water electrolysis cells. In electrochemical devices, such solid electrolytes must be chemically stable in the presence of oxygen, good ionic conductors, and poor electronic conductors. Oxygen transport is a thermally-activated process, hence the need to reach sufficiently high operating temperatures. At such temperatures, conductivity values as large as σ 5 1 S/cm can be reached: this is of the same order of magnitude as for liquid electrolytes. Perovskite oxides are commonly used for that purpose. The interest in using perovskite oxides (usually ABO3 mix-oxides) comes from the fact that different A-site and B-site metal elements can be incorporated into their ABO3-δ lattice structure to form (mainly) A12xA0 xB12yB0 yO32δ compounds with tailored oxygen nonstoichiometry and ionic conductivity [34]. A-site cations are usually either alkaline earth, alkaline, or lanthanide ions whereas B-site cations are usually smaller transition metal ions. Yttria-stabilized zirconia (YSZ) is probably the most popular material of this type. This is a zirconium oxide-based ceramic (ZrO2 or zirconia) made stable by the addition of yttrium oxide (Y2O3 or yttria). As a result of yttria addition to pure zirconia, some of the Zr41 ionic sites in the zirconia lattice are replaced with trivalent Y31 ions. The resulting oxygen vacancies are the origin of ionic conductivity by oxygen ions (O22), provided the mobility of vacancy sites is sufficient, a property that increases steadily with temperature. YSZ is one of the most useful so-called electro-ceramics, well suited for applications in solid oxide fuel cells (SOFC) and solid oxide water electrolysis (SOWE) cells. The ohmic resistance of the membrane is minimized by using thin (,100 μm) electrode (cathode or anode)-supported structures.

11.5.2.1 Ionic conductivity A large variety of oxygen-ion conducting materials have been identified and characterized. Examples are provided in (Fig. 11
11) [39], where the specific ionic conductivity of selected solid oxide materials is plotted as a function of the reciprocal temperature. By taking a cell surface resistance of 0.15 Ω.cm2 as the maximum value, it is possible to deduce the maximum thickness of the membrane as a function of the conductivity of the material. Depending on the membrane thickness, it is possible to classify materials into two types: self-supported electrolytes and supported (either by the anode or the cathode) electrolytes. The limit is approximately 15 μm because thin oxide films can be produced routinely using conventional ceramic fabrication routes down to thicknesses equal to 15 μm. According to Fig. 11
11, the specific electrolyte conductivity must therefore exceed 1022 S/cm. This is satisfied by Ce0.9Gd0.1O1.95 at temperatures exceeding 500 C and by (ZrO2)0.9(Y2O3)0.1 at temperatures above 700 C. A cell design that implements a self-supported 150 μm thick YSZ membrane would have to operate at temperatures greater than 900 C to meet the surface resistance criterion of 0.15 Ω.cm2. Indeed, YSZ is one of the most studied and used oxide-ion conducting solid electrolytes.

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FIGURE 11–11 Ionic conductivity of some oxygen-ion conductors of practical interest for applications in solid oxide electrochemical devices. (1) Bi2V0.9Cu0.1O5.35 [35]. (2) Ce0.9Gd0.1O1.95 [36]. (3) La0.9Sr0.1Ga0.8Mg0.2O2.85 [37]. (4) (ZrO2)0.9(Y2O3)0.1 [38]. Top: conductivity of SOFC bipolar plate materials is given for comparison.

11.5.2.2 Limitations and perspectives The ionic conductivity of the membrane material used as an oxide-ion conductor for SOWE applications is a critical parameter. A specific resistivity of approximately 0.1
0.2 Ω.cm2 is required for practical applications in electrochemical cells. Using a self-supported membrane (thickness  150 μm) of YSZ, the ionic conductivity is such that an operating temperature above 900 C is required to reach the target resistivity value. The situation can be significantly improved by reducing the membrane thickness. In conventional situations, a thin (30
40 μm thick) layer of YSZ is coated onto a porous tubular cathode (usually Lanthanum Strontium Manganite or LSM) [40]. The classical synthetic routes of ceramics (such as the deposition of YSR powders on a substrate and the subsequent sintering of the powder, with or without application of isostatic pressure), are constraint by the necessity to restrict temperatures to less than 1250 C, to avoid parasite reactions between LSM and YSZ. An electrochemical vapor deposition process can be used alternatively but cost considerations require cheaper fabrication routes. In terms of perspectives, the need to use ceramics containing rare earth elements is a limiting factor that negatively impacts the prospects for large-scale deployment, but alternatives are scarce. The development of new oxide-ion conducting ceramics, having suitable properties for application in solid oxide electrochemical devices, remains a challenging task. Results obtained with Sr- and Mg- doped LaGaO3 (lanthanum gallate) perovskite as an alternative electrolyte to YSZ have been reported in the literature [41]. Together, the (partial) substitution of lanthanum for strontium and then the incorporation of divalent magnesium cations in the gallium sub-lattice is an interesting approach since it can increase the concentration of oxygen vacancies. The electrical conductivity is improved compared to YSZ and doped ceria as a result of transition metal doping. The introduction of a small amount of Ni on the B-site of LSGM was also found useful to improve performance, without introducing undesired electronic conductivity.

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Current Trends and Future Developments on (Bio-)Membranes

11.6 Selected endergonic applications 11.6.1 Water electrolysis Electricity is used in water electrolysis cells to decompose water molecules into gaseous hydrogen and oxygen. The principal thermochemical features of the reaction are shown in Fig. 11
12 an over an extended range of operating temperature at atmospheric pressure. The enthalpy change ΔH(T) which is a measure of the total energy required for splitting one mole of water under specific (T, P) conditions remains almost constant up to 1000 C. Over the same temperature range, the entropy change ΔS(T) is also approximately constant and the thermal energy T.ΔS(T) required to maintain the system at a constant temperature during the transformation is a linear function of temperature. As a result, the Gibbs free energy change ΔG(T) tends to decrease with temperature. At 1000 C (approximately the maximum temperature reachable in chemical reactors), one-third of the total energy required for splitting one mole of water vapor demand is required as heat. This is not to say that hightemperature water splitting is cheaper because the electricity consumption is reduced compared to the amount required at lower temperatures. The kWh of heat is cheaper than the kWh of electricity as long as its exergy content is low (i.e., provided at temperatures close to ambient temperature). In the high-temperature range, the Carnot factor is close to unity and the exergy content of heat is close to the exergy content of electricity.

FIGURE 11–12 (A) plots of ΔG(T,1), ΔH(T,1) and T.ΔS(T,1) for the water dissociation reaction. (B) Comparison of iV curves (electrolysis on the left and fuel cell model on the right) obtained for (a) alkaline, (b) PEM, and (c) solid oxide water electrolysis. (C) Role of electrolyte pH on half-cell redox potentials.

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The advantage of choosing a higher operating temperature is an increase in the energy efficiency of the cell through improved kinetics (Fig. 11
12B). The sigmoidal shape of the iV curve measured under NATP conditions is due to the slow kinetics of the oxygen-water redox couple (during both the water oxidation and oxygen reduction reactions). At HTs, the halfcell reactions become completely reversible, in both directions, which makes it possible to obtain an appreciable efficiency bonus (admittedly at the cost of a more complex and therefore more expensive reactor architecture). Another interesting feature of the water electrolysis cell is the pH dependence of half-cell potentials in cells operating under NATP conditions with liquid water. The situation is summarized in Fig. 11
12C. In alkaline electrolytes, the Fermi level of the two electrodes is shifted down along the potential axis, to regions where most conventional metals (transition metals) are chemically stable because a layer surface layer is protective (nonetheless electronically conductive) is formed there. It is a passivation. The situation is reversed in an acidic medium: the energy levels of the two electrodes are found in the trans passivation regions and only the noble metals can be used as catalysts, a situation of course penalizing in terms of cost.

11.6.2 Main water electrolysis technologies Fig. 11
13 describes in more detail the operating principles of the main water electrolysis technologies currently on the market [42]. Half-cell reactions are specified for each case. Main operating characteristics are summarized and main strengths/weaknesses are specified.

11.6.3 Brine electrolysis In brine electrolysis (also known as the chloralkali process) DC electricity is used to decompose aqueous solutions of sodium chloride (brine) and to produce gaseous chlorine while sodium hydroxide is obtained at the cathode. Both chemicals are among the top ten ones produced in the world. They find application as synthetic intermediates in the manufacture of quite different products, most of them being used in daily life (e.g., plastics, herbicides and pesticides, deodorants, detergents, disinfectants, and pharmaceuticals). Hydrogen, a byproduct, is also produced at the cathode (Fig. 11
14). Although it is usually burned at the exhaust of the factory, it could be recovered and used as a fuel when hydrogen mobility will be sufficiently developed. Half-cell reactions are as follows: anode: 2 Cl2 ! Cl2(g) 1 2 e2 (a) cathode: 2 H2O(l) 1 2 e2 ! H2(g) 1 2 OH2 (b) full reaction: 2 NaCl(sol) 1 2 H2O(liq) —. Cl2(g) 1 H2(g) 1 2 NaOH(sol) (c)

11.6.3.1 Brief historical perspective The production of chlorine by electrolysis of brine was invented by Cruikshank in the early 1800s [43]. It took almost a century before industrial developments led to commercial activity. Since that time, three types of electrochemical cells have been developed and used for

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Current Trends and Future Developments on (Bio-)Membranes

FIGURE 11–13 Comparison of the main types of water electrolysis cells.

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FIGURE 11–14 Drawing of a conventional brine electrolysis cell (gap-cell type).

the simultaneous production of chlorine and aqueous solutions of sodium hydroxide. They differ in the type of separator used to prevent the mixing of the evolving chlorine gas with the solution of sodium hydroxide and the resulting formation of sodium hypochlorite. The most-recent type, the membrane process, is using a “gap” or “zero-gap” type of cell (Fig. 11
14). In most cases, the separator placed in the electrolyte is a bi-layer membrane combing a thin layer of perfluorosulfonic (cationic ion-exchanger) membrane pressed against a perfluorocarboxylic (anionic ion-exchanger) membrane; this type of hybrid separator is used to prevent the transport of caustic soda from the cathodic to the anodic compartment of the unit cell where the parasite and undesired reaction can take place spontaneously. The transport of charge carriers across this hybrid membrane is selectively ensured by hydrated sodium ions. Diaphragm process—In electrochemical reactors, diaphragms are porous barriers placed in the interpolar space between the two working electrodes, that is between the anode and cathode compartments. In the case of chlorine-soda electrolysis, the pores are impregnated with electrolyte which ensures the transport of sodium ions from one compartment (anode side) to the other (cathode side) to ensure the electroneutrality of electric charges during operation; the transport of these ions through the diaphragm makes it possible to ensure the circulation of electric charges in the circuit, to compensate for the charge of the hydroxide ion formed at the cathode and also to increase the sodium hydroxide concentration in the cathode compartment since the cationic membrane opposes the transport of hydroxyl ions. The company Griesheim in Germany (1888) is generally credited with manufacturing the first commercial cell used for the industrial production of chlorine. It used a nonpercolating porous cement diaphragm, invented by Brauer in 1886, and designed for the electrolytic production of Cl2 in the early 1900s. This type of cell works in sequential mode (batch). It uses

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Current Trends and Future Developments on (Bio-)Membranes

saturated aqueous solutions of potassium chloride. The electrolysis reaction takes place at an operating temperature of 80 C
90 C and under a fairly low current density (10
20 mA/ cm2). The operation lasts several days until a KOH solution having a concentration of approximately 7% is obtained. The voltage of the cell close to 4 V is quite high but the low current density allows to obtain a faradic efficiency of about 70%
80%. Also according to the literature, the first diaphragm chlorine-soda electrolysis cell developed and installed in Great Britain was the so-called Hargreaves-Bird cell. It was operated commercially from 1890 by the United Alkali Company. Each cell was made up of a large rectangular iron box covered with cement. The space inside the box contained two separate diaphragms of an asbestos sheet and was divided into three distinct parts. Six carbon electrodes placed in the anode compartment served as the anode. The cathode was formed by a copper gauze connected directly to the diaphragms. This ancient technique of anchoring the diaphragm to the cathode is still used today in modern diaphragm cells. During electrolysis, gaseous molecular chlorine (a toxic green compound) was released at the anode, in the anode compartment filled with brine saturated with salt. At the same time, sodium ions, and water passed through the diaphragm to the cathode compartment. Retro-migration of hydroxyl ions, a deleterious phenomenon as indicated above, was suppressed by injecting CO2 and water vapor into the cathode compartment to form sodium carbonate. The main feature of this cell was the vertical position of the diaphragm, parallel to the electrodes, particularly well suited to the collection of gas production at the anode. It is still used today in modern cells. In a typical production unit, twelve cells connected in series were traversed by a current density of about 20 mA/cm2, at a voltage of 4.0 to 4.5 V, leading to the conversion of 60% of the salt into sodium carbonate. Membrane process—Historically, the US company Diamond and Hooker Chemicals first became interested in ion-exchange membranes, starting in the mid-1940s. S. Osborne et al. from HCC (Hooker Chemical Corporation), filed the first patent for a membrane for chloralkali electrolysis cells as early as 1952 [44]. Intense and inventive R&D was carried out from then, until the end of the 1950s. The introduction to the chemical industry of the Nafion membrane (Dupont de Nemours) in the early 1970s increased interest in membrane cells tenfold. Gradually, the problems of these cells that limited their development have been resolved and the technology has matured. The arrival at the same time of dimensionally stable metal anodes or DSA developed by the Italian company De Nora (until then, only graphite anodes subjected to slow erosion during electrolysis were used) has tipped the balance in favor of this membrane technology. In 1975, Asahi Chemical built a pilot plant in Japan incorporating Nafion membrane cells for the first time. Subsequently, other membrane cell technologies were developed to achieve low energy consumption and long service life, notably in Japan. This allowed Japan to convert its industrial park of Chlor-alkali electrolysis to this process without major environmental impact and to become the first country to do so.

11.6.3.2 Performances and technological developments Fig. 11
15 show the photograph of a chlor-alkali plant equipped with membrane cells. The main characteristics of brine electrolysis in membrane cells are: (1) an operating current

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FIGURE 11–15 Photograph of a chlor-alkali electrolysis room equipped with BL-2.7 membrane cells (Uhde GmbH).

density between 300 and 500 mA/cm2; (2) a cell voltage of between 3.0 and 3.6 V; (3) the production of an aqueous sodium hydroxide solution having a concentration of 33%
35% by weight; (4) an energy consumption of 2650 kWh/MTCl2 when operating at 500 mA/cm2; (5) an electrical yield of about 50%; (6) a steam consumption of 180 kWh/MTCl2 to obtain 50% NaOH solutions.

11.6.3.3 Perspectives Membrane technology is the one that has gradually established itself in the chlorine market. It still has significant room for improvement. Indeed, the anode and cathode circuits are separated in the process. It is therefore possible to replace the saturated brine directly with purified solid salt (sodium chloride). This is a significant advantage since, to avoid fouling of the membrane (e.g., by insoluble species of calcium and magnesium), complex by-processes are required: the NaCl solution must be purified beforehand by adding a mixture of NaOH 1 Na2CO3, to reduce the concentrations of Ca and Mg to 1
5 ppm; it is filtered on ion exchange resins (which makes it possible to reduce the Ca
Mg concentration down to levels of the order of 0.02 ppm); it is also necessary to reduce the concentration of other impurities such as Fe, Al, SiO2, iodine, sulfates, chlorates, barium, strontium. Under these operating conditions, the service life of the membranes is estimated at 3 years after which they must be changed. Compared to the two competing technologies (Hg and diaphragm cells), the main advantages of membrane processes are established: (1) the chlorine produced is purer; (2) consumption of electrical energy is reduced; (3) the manufacturing process is much more flexible. The membrane process also has drawbacks: (1) the brine must be highly purified since the membranes are very sensitive to fouling resulting from the presence of traces of otherwise toxic chemicals; (2) membranes are rather expensive materials, which changes the cost equation.

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11.7 Conclusions and future trends In this chapter, the principles of electrochemical membrane reactors used for the interconversion of electrical energy into chemical energy are described. The energy balance of the electrochemical reaction is dictated by the principles of thermodynamics. The electrochemical transformation can be done in both directions. Either electrical energy is used to carry out a chemical transformation (this is referred to as an exergonic reaction or the charge of the battery). Either the chemical energy of the reagents is transformed into electricity (we speak of exergonic reaction or discharge of the battery). Different types of cell separators are inserted between the electrodes into the interpolar space to avoid recombination and direct chemical reaction of reaction products (endergonic or nonspontaneous chemical transformations) or reactants (exergonic or spontaneous chemical transformations). The different types of ion-conducting cell separators are presented and their key properties are described: porous solid diaphragms impregnated with liquid electrolytes, ion-conducting (cationic and anionic) polymer electrolytes, proton and oxygen-ion conducting solid oxides. A few selected endergonic electrochemical processes of industrial interest (water electrolysis and brine electrolysis) are described. Processes are described, and limitations of current materials are discussed together with some prospective issues.

Nomenclature Acronyms AEM AEMFC AEMWE EW KOH MOF PEM PEMFC SOFC

Anion-Exchange Membrane Anion-Exchange Membrane Fuel Cell Anion-Exchange Membrane Water Electrolysis Equivalent weight of membrane (eq/g) Potassium Hydroxide Metal-Organic Framework Proton Exchange membrane Polymer Electrolyte Membrane Fuel Cell Solid Oxide Fuel Cell

Symbols Ecell Di F G H I j n P Pm r

thermodynamic electrolysis voltage (V) diffusion coefficient of species i in cm2/s Faraday constant (96 485 C/mol) Gibbs free energy (J/mol) enthalpy (J/mol) current (A) current density (A/cm2) number of electrons exchanged during unitary half-cell reaction pressure (Pa) permselectivity (m2/Pa/s1) specific resistance (Ω.m2)

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S T Ucell Vcell

311

entropy (J/mol/K) absolute temperature (K) cell voltage (V) thermoneutral electrolysis voltage (V)

Greek symbols δ Δ εcell ρ σ

membrane thickness (m) difference cell efficiency (%) resistivity (Ω.m) conductivity (S/m)

Subscripts or superscripts cell i r t

electrolysis cell species i real theoretical

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226. [30] H. Iwahara, in: P. Colomban (Ed.), Proton Conductors, Solids, Membranes and Gels, Materials and Devices, Cambridge University Press, 1992, pp. 122
137. chapter 8.

Chapter 11 • Electrochemical membrane reactors

313

[31] S. Fop, Solid oxide proton conductors beyond perovskites, J. Mater. Chem. A. 9 (2021) 18836
18856. [32] D.-W. Lim, H. Kitagawa, Proton transport in metal-organic frameworks, Chem. Rev. 120 (16) (2020) 8416
8467. [33] V. Ponomareva, K. Kovalenko, A. Chupakhin, D. Dybtsev, E. Shutova, V. Fedin, Imparting high proton conductivity to a metal-organic framework material by controlled acid impregnation, J. Am. Chem. Soc. 134 (2012) 15640
15643. [34] J. Sunarso, S.S. Hashim, N. Zhu, W. Zhou, Perovskite oxides applications in high temperature oxygen separation, solid oxide fuel cell and membrane reactor: a review, Prog. Energy Combust. Sci. 61 (2017) 57
77. [35] R.K. Nimat, R.S. Joshi, S.H. Pawar, Temperature dependent conductivity and dielectric properties of Bi2V0.9Cu0.1O5.35 solid electrolyte thin films, Mater. Sci. Eng. B 137 (2007) 93
98. [36] J.G. Cheng, S.W. Zha, H. Jia, X.Q. Liu, G.Y. Meng, Sintering behavior and electrical conductivity of Ce0.9Gd0.1O1.95 powder prepared by the gel-casting process, Mater. Chem. Phys. 78 (2003) 791
795. [37] K. Yamaji, T. Horita, M. Ishikawa, N. Sakai, H. Yokokawa, Compatibility of La0.9Sr0.1Ga0.8Mg0.2O2.85 as the electrolyte for SOFCs, Solid State Ion. 108 (1998) 436
442. [38] M.T. Colomer, J.R. Jurado, Structure, microstructure, and mixed conduction of [(ZrO2)0.92(Y2O3)0.08]0.9(TiO2)0.1, J. Solid State Chem. 165 (2002) 79
88. [39] B. Steele, A. Heinzel, Materials for fuel cell technology, Nature 414 (2001) 345. [40] N.Q. Minh, T. Takahashi, Science and Technology of Ceramic Fuel Cells, Elsevier, Amsterdam, 1995. [41] K. Huang, J. Wan, J.B. Goodenough, Oxide-ion conducting ceramics for solid oxide fuel cells, J. Mater. Sci. 36 (2001) 1093
1098. [42] C. Lamy, P. Millet, Efficiency of near-ambient temperature water electrolysis, J. Power Sources 447 (2020) 227350
227364. [43] P. Millet, Chlor-Alkali Technology: Fundamentals, Processes and Materials for Diaphragms and Membranes, Handbook of Membrane Reactors, Elsevier, 2013. [44] K.J. O’Leary, Diaphragm cells for chlorine production, in: Proceedings of a Symposium Held at the City University, London, June 1976, The Society of Chemical Industry, London, 1977, p. 103.

Further reading D. Bessarabov, P. Millet, PEM water electrolysis, in: B.G. Pollet (Ed.), Hydrogen and Fuel Cells Primers, Elsevier, 2018. J.O.’M. Bockris, A.A.K. Reddy, Comprehensive Treatise of Electrochemistry, Plenum Press, 1982. M. Faraday, Experimental researches in electricity, London Edinb. Philos. Mag. J. Sci. 5 (1834) 27. C.H. Hamann, A. Hamnett, W. Vielstich, Electrochemistry, Wiley-VCH, 1998.

12 Modeling of membrane reactors Fausto Gallucci SUSTAINABLE PROCESS E NGINE ERING, CHEMICAL ENGINE ERING AND CH EMISTRY, EINDHOVEN UNIVERSITY OF TECHNOLOG Y , EI N D HO V E N, T HE NE T H E RL AN D S

12.1 Introduction In the previous chapters, several membrane reactors have been introduced and discussed. If we distill the essence of all other chapters we can surely define a general membrane reactor as a multiphase system when a chemical reaction and a separation through a membrane occur at the same time. The catalytic reaction can be in a homogeneous phase or the heterogeneous phase. The membrane separation occurs because of the selective interaction of one or more components of the reactive mixture with the membrane. The model should then surely take into account the reaction rates and the permeation rates. The difference in the modeling of a membrane reactor with the counterpart conventional reactor is simply the addition of the permeation term in the mass balance equation and, in the case of nonisothermal models, in the energy balance equation. For the mass balance, the membrane permeation term will be a sink term for membranes that extract components, or a source term for membranes used to feed a component. In most cases, we should also add the energy and mass balance equations to describe the permeate side of the membranes. In this chapter, we will report the equations used for packed beds, fluidized beds, and other types of membrane reactors and report some examples of the model results. We will not describe the way to calculate the equilibrium conversion (or limit conversion) of membrane reactors. An interested reader can find a proposal to calculate this conversion in our previous work [1] or can use software like Aspen plus to calculate this conversion by coupling a stochiometric reactor, a separator, and a Gibbs reactor unit. We will first start with packed bed membrane reactors models. The models used for packed bed membrane reactors foresee the addition of the membrane flux as a source/sink term in one-dimensional (1D) models and two-dimensional (2D) models. In this last case, if the membranes are seen as the wall of the 2D model (as often done in such reactors) then the membrane flux will be incorporated into the boundary conditions of the models. These models can be easily extended with other packed beds such as structured catalysts or micro-structured reactors. We will give a few examples. In the second part, we will discuss the modeling of fluidized bed membrane reactors (FBMRs), where both solid movements and gas flow are taken into account. We will show some examples of Current Trends and Future Developments on (Bio-)Membranes. DOI: https://doi.org/10.1016/B978-0-12-823659-8.00001-0 © 2023 Elsevier Inc. All rights reserved.

315

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Current Trends and Future Developments on (Bio-)Membranes

simplified models as well as the use of closure equations to account for solid exchange, segregation, and the effect of temperature on the fluid dynamics. Finally, we will report the more advanced models [Computational Fluid dynamics (CFD)-based] and some examples.

12.2 Packed bed membrane reactors As already described in one of the first chapters, a packed bed membrane reactor is a reactor in which one or more membranes are in contact with catalyst particles or structures that are kept in a fixed position. These catalysts can be uniform particles, 3D printed structures, or a combination of the two, or can be coated on the membrane or the walls of the channels (in microreactors). The membrane (generally tubular) is either immersed inside the bed, or the bed is inside the membrane tube (so that the membrane is the wall of the reactor). When we look closely at the catalyst bed, the prevailing phenomena can be summarized as: • • • • • • • •

A mass transfer from the bulk of the fluid to the catalyst surface, A mass transfer within the catalyst particle inside the pores Adsorption on the surface of the pores Chemical reaction Desorption of products and unreacted species Mass transfer towards the bulk of the fluid Mass transfer towards the membrane surface Permeation through the membrane

The last one can also be a combination of different steps, and in the model of the membrane reactor, it can be described as a single permeation equation or as a submodel combining the different steps. For instance, Nordio et al. [2] have reported how to ling the permeation submodel for thin supported membranes to a reactor model. Depending on the limiting steps among the one enumerated above, the description of packed bed membrane reactors is done by using simplified models that capture the most important prevailing phenomena. In general, the model should be as easy as possible but elaborated enough to describe accurately the limiting step(s). Simulations of packed bed (membrane) reactors are generally carried out with continuum models that can be classified in either pseudo-homogeneous or heterogeneous models [3]. These can be also one-dimensional or multidimensional models. In pseudo-homogeneous models, it is assumed that the catalyst surface is exposed to the bulk fluid conditions, that is, that there are no fluid-to-particle heat and mass transfer resistances. On the other side, heterogeneous models take conservation equations for both phases into account separately. Below we will report the main equations used for these models.

12.2.1 1D pseudo-homogeneous model The 1D pseudo-homogeneous model is the most used model to describe packed bed membrane reactors. The model considers the gradient of mass and energy occurring in the axial

Chapter 12 • Modeling of membrane reactors

317

direction of the reactor. The model can be the unsteady state and can consider axial dispersion. the model can always be simplified by considering steady-state and plug flow conditions. We report here the full model, simplification to steady state is possible especially if gradients are not too steep and thus the steady-state solution can be found without numerical instabilities. In case of catalyst deactivation, or case of sorption enhanced membrane reactors, or in case of gas-solid reactions the unsteady state model needs to be used. The main equations would be as follows:

12.2.1.1 Continuity equation  @
 @
ερg 1 ερg u 5 0 @t @z

(12.1)

12.2.1.2 Total momentum balance equation   @
@
@p @  ερg u 1 ερg u2 5 2 ε 2 βερg u 2 Eτ g 1 ερg g @t @z @z @z

(12.2)

12.2.1.3 Friction coefficient β 5 150

ð12εÞ2 μg ð1 2 εÞ εu 1 1:75 ε3 dp ε3 ρg dp2

(12.3)

where τ g is calculated depending on the nature of the fluid. It should be noted that membrane separation is very often a pressure-driven process, thus it is very much advisable to always include the momentum balance equation in the model and refrain to use isobaric reductions as in this last case the membrane flux may be greatly overestimated.

12.2.1.4 Component mass balance     @
@
@ @ωi ερg ωi 5 2 ερg uωi 1 ρg Dax;i 1 Sr;i 2 Ji @t @z @z @z

(12.4)

where the source term Sr, i and the trans-membrane flux term Ji depend on the reaction system considered and membrane used, respectively. In particular, the source term has the following general formula: Sr;i 5 ð1 2 εÞρs Mi

nr X

γ ij rj

(12.5)

j51

The flux term depends on the membrane used (porous or dense) and the reaction system used as well.

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For instance for hydrogen production using dense Pd based membranes the permeation flux is given by the Richardson equation that relates the flux to the difference of the square root of the partial pressure of hydrogen from the two sides of the membrane: 

Ea Pe exp 2 RT



0

JH2 5



pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pH2;upstream 2 pH2;downstream (12.6)

δm

This formula assumes that the diffusion of hydrogen atoms through the bulk of Pd is the limiting step. Also for this permeation, one can have different other limiting steps. If other mass transfer rates are to be included in the permeation flux (such as the resistance of the support or a porous layer) the formula can be augmented with the submodel as described for instance by Brencio et al. [4] who added membrane inhibition because of adsorption and mass transfer resistances through porous supports and the protective porous layer. Thus the flux can be either a simple closure equation or a complete submodel to be coupled with the main reactor model.

12.2.1.5 Energy balance


ερg Cp;g 1 ð1 2 εÞρs Cp;s

   @
@ @T 5 2 Cp;g ερg uT 1 λax 1 Sh @t @z @z @z

 @T

(12.7)

where the source term Sh is calculated as Sh 5 ð1 2 εÞρs

nr X

rj ΔHj

(12.8)

j51

Danckwerts’ boundary conditions at the reactor inlet and outlet can be assumed for the gas phase balance [5].

12.2.1.6 1D heterogeneous model The pseudo-homogeneous model considers that all catalyst is exposed to the surface conditions, such as the transport of species, and energy inside the pores of the catalyst is not limiting. For several applications, this is a valid assumption and the pseudo-homogeneous model is enough to describe accurately the membrane reactor. However, with a very fast reaction and/or very exo-endothermic reaction, this assumption may be not true and other mass/heat transfer limitations need to be taken into account in the reactor model. This is done by adding the submodel of the particle within the reactor model described above. For a porous spherical particle of diameter dp the equations will be written as: In the gas (fluid) phase:

Chapter 12 • Modeling of membrane reactors

319

12.2.1.7 Component mass balance

    @
@
@ @ωi ερg ωi 5 2 ερg uωi 1 ρg Dax;i 2 ans;i 2 Ji @t @z @z @z

(12.9)

12.2.1.8 Catalyst phase mass balance

  @ 1 @ @ωi r 2 Deff;i ρ ðρωi Þ 5 2 1 Sr;i @t r @r @r

(12.10)

where Sr;i 5 ρs Mi

nr X

γ ij rj

(12.11)

j51

The boundary conditions for the catalyst phase are Deff;i ρ

@ωi 5 ns;i @r r5R

@ωi 50 @r r50

(12.12)

(12.13)

The interphase mass transfer is written as   ns;i 5 ρks;i ωi 2 ωs;i

(12.14)

with ks,i 5 interphase mass transfer coefficient

12.2.1.9 Energy balance for gas phase

   @T b @
@ @T b b ερg uT 1 λax 5 2 Cp;g 1 aq r5R ερg Cp;g @z @z @t @z

(12.15)

12.2.1.10 Energy balance for solid phase


ερg Cp;g 1 ð1 2 εÞρs Cp;s

 @T @t

5

1 @ 2  r q 1 Sh r 2 @r

(12.16)

where q 5 λeff;i Sh 5 ð1 2 εÞρs

@T @r

nr X j51

(12.17)

rj ΔHj

(12.18)

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Current Trends and Future Developments on (Bio-)Membranes

The boundary conditions for the solid phase are @T 5 0 and Tjr5R 5 T b @r r50

(12.19)

The remaining the constitutive equations can be written as [6]: Effective dispersion of mass

Effective dispersion of energy

Radial

λr λbed;0 Pe λ Pe  x  5 bed;0 1 x (12.21) 5 1 λg λg λg 8 KN f Dt =dp

 pffiffiffiffiffiffiffiffiffiffiffi Dr;i 5 1 2 1 2 ε Dm i 1

udp   PeN f Dt =dp


pffiffiffiffiffiffiffiffiffiffiffi udp 5 1 2 1 2 ε Dm i 1 8

(12.20) λz λbed;0 Pe λ Pe  x  5 bed;0 1 x (12.23) 5 1 λg λg λg 2 KN f Dt =dp

Axial
pffiffiffiffiffiffiffiffiffiffiffi udp Dz;i 5 1 2 1 2 ε Dm i 1 2 0 1 pffiffiffiffiffiffiffiffiffiffiffi λbed;0
λrad A @ 5 12 12ε 11 λg λg

(12.22)

8 9 2 3 > > λg > > > > B > > 6 12 7 > > λ = 6 7 cat pffiffiffiffiffiffiffiffiffiffiffi< 2 λcat B11 B11 7 1 6 2 1 12ε ln 2 1 6 7 ! 2 λ λ λ λ > > 6 7 2 λg B g g g g > > > > B4 λ B5 1 12 12 > 12 λcatg B > > λcat λcat λrad λcat > : ;

with λrad 5

0:23 2 21

εrad B5C

12ε ε

!2 T 100

Pex 5

dp (12.25)

!10=9

usup ρg Cp;g XF λg

(12.24)

(12.26)

with XF 5 1:15 for spherical particles

C 5 1:4

Additional closure equations may be needed to account for different fluids, porosity gradients, and different dispersion coefficients. The interested reader is invited to check the applicability of the given equations and to add the missing closure equations for their application.

Chapter 12 • Modeling of membrane reactors

321

FIGURE 12–1 Comparison of the membrane reactor performance in adiabatic conditions (red lines) and with the heat management strategy. (A) Temperature profiles and (B) yields. Reproduced from S. Poto, F. Gallucci, M. Fernanda Neira d’Angelo, Direct conversion of CO2 to dimethyl ether in a fixed bed membrane reactor: influence of membrane properties and process conditions, Fuel 302 (2021) 121080, https://doi.org/10.1016/J. FUEL.2021.121080.

In any case, these models are quite easy to use and very powerful in terms of reactor design and optimization. Using the full nonisothermal model for instance can be used to optimize the cooling system of the reactor and identify the optimal conditions for improving the yield of the reactor. Recently, using a 1D nonisothermal model Poto et al. [7] have reported the design and a membrane reactor for CO2 hydrogenation to DME. After optimizing the membrane flux, the authors have also reported how to improve the reactor performance by using the right cooling strategy compared with an adiabatic reactor (see Fig. 121). Similarly, the simplified nonisothermal model has been used by Cruellas et al. [8] to identify the limitations of packed bed reactors and simulate the advances in the performance of a packed bed membrane reactor for oxidative coupling of methane. Lee et al. [9] have used 1D models to make a comparison of the performance of packed beds, packed bed membrane reactors, and sorption enhanced membrane reactors. They found out which reactor and in which flow condition can produce more hydrogen using reforming of methane as a reaction of interest (see Fig. 122). There are several applications of the 1D reactor model for a variety of membrane reactor applications, ranging from hydrogen production, (de)hydrogenation reactions, FTS, biochemical conversion, etc. This shows the versatility and simplicity of such a reactor model for assessing reactor performance and carrying out reactor design. However, as per definition, the 1D model neglects the radial mass transfer phenomena that in some cases are extremely important in the performance of the membrane reactors. Especially bed-to-wall mass transfer limitations, often referred to as concentration polarization (CP), play an important role in the performance of reactors when the membranes are very permeable and selective. This CP can be accounted for in the 1D model

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Current Trends and Future Developments on (Bio-)Membranes

FIGURE 12–2 H2 production rate of 5 different reactor systems at 773K. Reproduced from H. Lee, A. Kim, B. Lee, H. Lim, Comparative numerical analysis for an efficient hydrogen production via a steam methane reforming with a packed-bed reactor, a membrane reactor, and a sorption-enhanced membrane reactor, Energy Convers. Manag. 213 (2020) 112839, https://doi.org/10.1016/J.ENCONMAN.2020.112839.

either by using a CP factor that decreases the flux through the membrane or even better by using Sherwood correlations to calculate a mass transfer coefficient and include this in the definition of the flux [1012]. These corrections are not very accurate in calculating the exact extent of CP, especially because the Sherwood correlations used are often derived for completely different reactive systems and not permeation systems. Additionally, these do not cure the effect of heat transfer limitations in the radial direction. For a more accurate determination of these limitations, it is much better to use 2D models that account also for the radical changes in momentum heat, and mass.

12.2.2 2D pseudo-homogeneous model A pseudo-homogeneous, two-dimensional reactor model for membrane reactors is based on the standard dispersion model that describes the gas phase mass and energy transport as convective flow with radial and axial dispersion. The model equations can be written as:

12.2.2.1 Continuity equation
 @
 ερg 1 r  ερg u 5 0 @t

(12.27)

Chapter 12 • Modeling of membrane reactors

323

12.2.2.2 Total momentum balance equation 

@
τ g Þ 1 ερg g ερg u 1 r  ερg uu 5 2 εrp 2 βερg u 2 r  E 5 @t

(12.28)

12.2.2.3 Friction coefficient β 5 150

ð12εÞ2 μg ð1 2 εÞ εjuj 1 1:75 ε3 dp ε3 ρg dp2

(12.29)

where 5 τ g is calculated as (Newtonian fluid):

 

 2 5 τ g 5 2 λg 2 μg ðr  u Þ 5 I 2 μg ðru Þ 2 ðru ÞT 3

(12.30)

where ju j 5

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u2r 1 u2z

(12.31)

and ρg 5

Mg p ideal gas RTg

(12.32)

Boundary conditions for a 2D model for Packed Bed Membrane Reactor Center (r 5 0)

@uz @r

Wall (r 5 R)

uz 5 0

Inlet (z 5 0)

uz 5

Outlet (z 5 L)

50

ur 5 0 ur 5

Φ}m ερg

p5p0

JH2 ερH2

@ur @z

50

@ur @z

50

12.2.2.4 Component mass balance

  
 @
@ωi 1 Sr;i ερg ωi 5 2 r  ερg uωi 1 r  ρg Di @z @t

(12.33)

where Di 5

Dr;i 0

0 Dz;i

(12.34)

where the source term Sr,i equals: Sr;i 5 ð1 2 εÞρs Mw;i

nr X j51

γ ij rj for i 5 1; 2::nc

(12.35)

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Current Trends and Future Developments on (Bio-)Membranes

12.2.2.5 Energy balance


ερg Cp;g 1 ð1 2 εÞρs Cp;s

 
 @T 5 2 Cp;g r  ερg uT 1 r  λ 1 Sh @t @z

 @T

(12.36)

where Di 5

λr 0

0 λz

where the source term Sh reads Sh 5 ð1 2 εÞρs

nr X

rj ΔHj for j 5 1; 2:nr

(12.37)

j51

Boundary conditions position Center (r 5 0) Wall (r 5 R)

Inlet (z 5 0)

Outlet (z 5 L)

Mass balance

Energy balance

@ωi 50 @r JH2 5 ur ρH2 ε @ωi 5 0 i 6¼ H2 @r

@T 50 @r @T 5 0 adiabatic @r T 5 Twall heated wall

2ρg Dz;i

2λ

@ωi 50 @z

Φ}m;i @ωi 1 ερg uz ωi 5 (12.38) @z Areactor

00

Cp;g T0 Φm @T 1 Cp;g ερg uz T 5 (12.39) @z Areactor

@T 50 @z

The application of such 2D models is very important for the more accurate determination of scaled-up reactors. An example of this modeling approach has been given by Walter and co-workers [13] who made a modeling analysis of packed bed membrane reactors for propane dehydrogenation reactions. The authors used COMSOL to solve the numerical problem. The 2D model was essential in their study as the distribution of oxygen through a membrane diffuses radially in the membrane reactor and its radial concentration profiles would influence the extent of heat production inside the reactor. The same oxygen distribution through membranes is very important for oxidative coupling of methane and Cruellas et al. [8] already indicated the importance of a detailed 2D model of such a reactor system. Such an effort has been recently presented by Onoja et al. [14] who did report the 2D models of a membrane reactor for oxidative coupling of methane. The results reported show as diffusion in the radial direction is one of the main causes of loss in the selectivity of the target products and thus a major drawback to be solved by a proper design of the membrane reactor.

Chapter 12 • Modeling of membrane reactors

325

A 2D model has also been used by Murmura et al. [15] to account for mass dispersion in membrane reactors packed with foam catalysts. While the model is very similar, except for the closure equations used for the foam packing, the results are interesting because structured catalysts are increasingly being proposed to improve mass and heat transfer in packed bed membrane reactors. 2D models have been also used in our previous work on packed bed membrane reactors for hydrogen production. The results have shown that with more permeable membranes (thinner supported membranes) higher mass transfer limitations in packed bed membrane reactors occur and consequently lower flux is observed compared to the case without mass transfer limitations. The effect of mass transfer limitations [16].

12.2.3 Modeling of fluidized bed membrane reactors As described for the packed bed membrane reactors, also for the FBMRs the model consists in adding the flux through the membrane as a sink term (or a source term) in the mass balance equations. This is because the effect of the presence of membranes in fluidized beds on their hydrodynamics is often not considered (or better neglected). This means that often the models presented for fluidized beds are augmented with the permeation law through the membrane. Generally, FBMRs are operated in a bubbling fluidization regime, especially because there is a limit to the flux through the membrane that should match the residence time of the gas in the reactor as well as to avoid excessive erosion of the membranes. These fluidized bed reactors are often modeled with phenomenological models like the one introduced by Kato and Wen [17]. A typical one-dimensional two-phase model for a membrane-assisted fluidized bed reactor can be used for the simulation of the FBMR for production via membranes. A schematic representation of the gas flows between the compartments of the bubble and emulsion phases are depicted in Fig. 123. The model's main assumptions are: • Membranes are immersed in the reactor and do not influence the hydrodynamics. • The reactor consists of two phases, the bubble and emulsion phase. Extension of this model when accounting also the wake phase is available. • The gas flowing through the emulsion phase is considered to be completely mixed in each section and at incipient fluidization conditions. • The bubble phase gas is assumed to be in plug flow, where the bubble size and the bubble rise velocity change for each section according to closure equations. • The heterogeneous reactions take place only in the emulsion phase, assuming that the bubble phase is free of catalyst particles. Extension of this can be found in literature when accounting for a small fraction of catalyst in the bubbles • Gas removed from the fluidized bed via membranes is assumed to be extracted from both the emulsion phase and bubble phase The gas extracted from the emulsion phase is subsequently instantaneously replenished via exchange from the bubble phase (as described by Deshmukh et al. [18]).

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Current Trends and Future Developments on (Bio-)Membranes

FIGURE 12–3 A schematic representation of the two-phase fluidized bed reactor model (FBMR) (E 5 emulsion phase, B 5 bubble phase).

Chapter 12 • Modeling of membrane reactors

327

• The bed is considered at a uniform temperature, only the overall heat balance is considered. • The mass and heat balance equations are as follows: Total mass balance usb;n21 AT ρb;n21 2 usb;n AT ρb;n 1 use;n21 AT ρe;n21 2 use;n AT ρe;n nc n X  o 00 00 membrane membrane 1 ϕi;mol Mw;i Amembrane eb;n 1 ϕi;mol Mw;i Amembrane 1 2 eb;n 5 0

(12.40)

i51

Bubble phase component mass balances usb;n21 AT ρb;n21 2 usb;n AT ρb;n 2

nc X   Kbe;i;n Vb;n ρb;n ωb;i;n 2 ωe;i;n i51

nc X

 00 membrane ϕi;mol Mw;i Amembrane eb;n 1 ωe;i;n SF ðQÞ 2 ωb;i;n SF ð 2QÞ 5 0 1

(12.41)

i51

Emulsion phase component mass balances nc X   Kbe;i;n Vb;n ρb;n ωb;i;n 2 ωe;i;n i51 ! nc nrxn X X   00 membrane ϕi;mol Mw;i Amembrane 1 2 eb;n 2 ν j;i rj Ve;n ρp;n ð1 2 ee Þ 2

use;n21 AT ρe;n21 2 use;n AT ρe;n 2

i51

(12.42)

j51

 2 ωe;i;n SF ðQÞ 2 ωb;i;n SF ð 2QÞ 5 0

Transfer term Q 5 use;n21 AT ρe;n21 2 use;n AT ρe;n 6

nc X   ϕ}membrane Amembrane 1 2 eb;n i;mol i51

nc X   Kbe;i;n Vb;n ρb;n ωb;i;n 2 ωe;i;n 1 i51

where   use;n AT 5 ue;n AT 1 2 eb;n usb;0 AT 5 utot AT eb;0   use;0 AT 5 utot AT 1 2 eb;0  SFðxÞ 5

x if x . 0 0 if x # 0

(12.43)

328

Current Trends and Future Developments on (Bio-)Membranes

Energy balance nc nc
 X
 X T Hi feed usb;n50 AT ρb;i;n50 1 use;n50 AT ρe;i;n50 2 HiTout usb;n5N AT ρb;i;n5N 1 use;n5N AT ρe;i;n5N i51 ( i51 ) nc
00 X   00 Tout membrane membrane 1E50 Hi ϕi;mole Mw;i AT eb;n 1 ϕi;mole Mw;i AT 1 2 eb;n 6

(12.44)

i51

The closure equations often used for fluidized bed reactors are: Parameter

Equation

Archimedes Number

Ar 5

Minimum fluidization velocity

Bed voidage at minimumfluidization velocity

umf


 dp3 ρg ρp 2 ρg g

(12.45)

μ2g

μg 5 ρg dp

!qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  ð27:2Þ2 1 0:0408Ar 2 27:2

εmf 5 0:586Ar

20:029

ρg ρp

(12.46)

!0:021 (12.47)

Projected tube area for a square bed Rise velocity of a single bubble

AT 5 D2T

(12.48)

ubr 5 0:711ðgdb Þ1=2

(12.49)

Velocity of the rise of a swarm of bubbles Initial bubble diameter(Porous plate distributor) Maximum bubble diameter

ub 5 u0 2 umf 1 0:711ðgdb Þ1=2

(12.50)

 2 db0 5 0:376 u0 2umf

(12.51)

Superficial bubble gas velocity

db;max 5 DT usb;max 2 usb 5 exp usb;max 2 usb;0

(12.52) 

0:55z hmf DT

 (12.53)

Maximum superficial bubble gas velocity Initial superficial bubble gas velocity

usb;max 5 u0 2 umf

(12.54)

usb;0 5 ubr;0 δb0   where δb0 5 1 2 hmf =hf

(12.55)

Superficial emulsion gas velocity

use 5 u0 2 usb

(12.56)

Bubble phase fraction

δb 5

usb ub

(12.57)

(Continued)

Chapter 12 • Modeling of membrane reactors

329

(Continued) Parameter

Equation

Emulsion phase fraction

δen 5 1 2 δbn

(12.58)

The volume of the emulsion phase in the nth compartment

Ve;n 5 AT

hf Nb

(12.59)

The volume of the bubble in the nth compartment

V b;n 5 AT

hf δb;n Nb

(12.60)

Bubble diameter

  20:3z db 5 db;max 2 db;max 2 db;0 e DT

Height of bed expansion

hf 5 hmf

(12.61)

C1 C1 2 C2

where;

0 1 ub;0 0:275 A exp @ 2 C1 5 1 2 DT ub;avg 2 0 13 usb 4 0:275 A5 C2 5 1 2 exp @ 2 DT ub;avg

Average bubble rise velocity Gas exchange coefficient

 1=2 ub;avg 5 u0 2 umf 1 0:711 gdb;avg 0

0

1

(12.62)

(12.63)

1

1=2 1=4 umf A BDg g C 1 5:85@ 5=4 A Kbc 5 4:5@ dp db

!1=2

Kce 5 6:77

Dg εmf ub db3

(12.64)

1 1 1 5 1 Kbe Kbc Kce

The bed voidage at minimum fluidization can also be a function of the temperature and another correlation to account for this dependency has been reported by Campos Velarde et al. [19]. A very similar model has been used by Spallina et al. [20] who carried out experiments on membrane-assisted fluidized bed reactor for ethanol autothermal reforming to produce pure hydrogen. The authors have reported a comparison between the experimental results and their modeling approach and the results are ported in Fig. 124. The results are in general in good agreement with the model, whereas the deviations have been attributed to either catalyst activity or mass transfer limitations.

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FIGURE 12–4 Comparison between experiments and models from Spallina et al. [20].

FIGURE 12–5 Permeated hydrogen flux at different total pressures from de Nooijer et al. [21].

The same model has been used for FBMR for biogas reforming by de Nooijer et al. [21], who showed that still, mass transfer limitations exist in the FBMRs that are not properly accounted for by the model. De Nooijer attributed this to CP and accounted for it in the model assuming a boundary layer around the membranes of thickness delta where their polarizations are concentrated. The results are shown in Fig. 125 have good correspondence with the experimental results when using a variable delta thickness. However, the model is not predictive in this fashion. An attempt to make the model more predictive has been done by Helmi et al. [22] who has coupled the phenomenological model, with a two-fluid model and experiments, and came out with a correction of the 1D model by using a mass transfer coefficient in the radial direction to account for CP. These results show how important is to properly account for all

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331

FIGURE 12–6 Three-phases model for fluidized bed reactors from Zambrano et al. [24].

relevant phenomena in FBMRs. A more detailed analysis of CP in FBMRs has been proposed by Yang et al. [23] who used a CFD approach to study the effect of CP and gave very interesting guidelines for the design of such reactors but did not yet provide equations that can be used in simplified models to account for CP. Nonetheless, the simple model is still very useful for the design of membrane reactors and the comparison of different reactor configurations. Using an extension of the model that also accounts for the wake and the solid circulation (see Fig. 126) Zambrano et al. [24] have simulated and compared different fluidized bed reactors for dry reforming of methane and identified the ones with lower deactivation of the catalyst. Finally, we have also reported in a previous chapter, as fluidized bed reactors are used in the bioreactor to exploit the erosion effect of particles to decrease the fouling of the membranes. Of course also, in this case, several papers on modeling have been presented and generally, different CFD approaches are used for this kind of study for example the work of Ramirez et al. [25] who have shown with experiments and CFD simulations as indeed swirling fluidization helps is greatly reduce the fouling of the membranes.

12.3 Conclusions and future trends In this chapter, we reported the typical models for packed beds and FBMRs. With a few examples, we demonstrated as the models are very easily implemented for membrane reactors, whereas the membrane permeation has to be implemented in a standard model. The

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models can be used for the design and optimization of the reactors. However, when we insert membranes in a chemical reactor, often phenomena like CP are difficult to simply implement and often require more detailed models like 2D models for packed beds or full CFD models for FBMRs. With the development of powerful CFD tools (open access or not), it becomes easier to implement these phenomena and be able to properly design membrane reactors. It is thus foreseen that even more CFD simulations will be available in the future for membrane reactors. It is however advisable to always validate the models towards relevant experimental data, although in several papers this common-sense rule is often not followed.

Nomenclature List of acronyms CMR CFD CSTR CP FBMR FC Mr PBR PEM SEMR SER

Catalytic membrane reactor Computational Fluid dynamics Continuously stirred tank reactor Concentration polarization Fluidized bed membrane reactor Fuel Cell Membrane reactor Packed bed reactor Polymer electrolyte Membrane Sorption enhanced membrane reforming Sorption enhanced reforming

List of symbols Ar AT Amembrane,n dp Cp D Dg eb ee Ea g Hj Hi,xT J ki Kbc,i,n Kbe,i,n Kce,i,n KCO Keq,i ks,i Mw

Archimedes number Area of bed cross-section (m2) Membrane surface area per cell, n (m2) Particle diameter (m) heat capacity (J/(kg K)) Dispersion coefficient (m2/s) Gas diffusivity (m2/s) Bubble phase fraction Emulsion phase fraction Activation energy for hydrogen permeation (J/mol) Gravitational acceleration (59.81) (m/s2) Enthalpy of specie j, (J/mol) Enthalpy of component i at temperature T at position x (J/mol) Permeation flux through a membrane (mol/(m2  s)) Reaction rate constant for ith reaction Bubble-to-cloud phase mass transfer coefficient for component I in cell n (s21) Bubble-to-emulsion phase mass transfer coefficient for component I in cell n (s21) Cloud-to-emulsion phase mass transfer coefficient for component I in cell n (s21) Adsorption constant for CO (Pa21) Equilibrium constant for jth reaction (depending on the reaction) Interphase mass transfer coefficient (s21) Molar mass for component i (kg/mol)

Chapter 12 • Modeling of membrane reactors

hM i pi Pe0 r R rj R Sh Sr;i t T u us V vj,i Y z

Average molar mass (kg/mol) Partial pressure for component i (Pa) Pre-exponential factor for permeation of Pd membrane (mol/(s  m2  Pa0.5)) Radial coordinate (m) Gas constant (58.3145) (J/(mol  K)) Reaction rate for jth reaction (mol/(kgcat  s)) Inner tube radius (m) source/sink term for heat balance (J/(m3 s)) source/sink term for mass balance (kg/(m3 s)) Time (s) Temperature (K) Mixture velocity (m/s) Superficial gas velocity m/s Volume (m3) Stoichiometric coefficient for jth reaction and ith component mole fraction axial coordinate (m)

Greek α β δ ρ ε εe λ μg τg ϑðT Þ ϕ}membrane i;mol ω

heat transfer coefficient (J/(m2 K s)) friction factor membrane thickness (m) Density (kg/m3) Porosity Emulsion phase porosity Thermal conductivity, (J/(m K s)) Viscosity of gas (Pa  s) Stress tensor (kg/(m s)) Correction factor for SievertsLangmuir’s model Molar flux component i through the membrane per cell (mol/(m2  s)) weight fraction

Subscripts 0 ax b cat e eff g i j n r s z

Reactor inlet axial Bubble phase Catalyst Emulsion phase effective gas phase Component i Number of reaction Number of CSTRs for emulsion or bubble phase radial coordinate solid phase axial coordinate

Superscripts b bulk condition

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References [1] F. Gallucci, M. De Falco, A. Basile, A simplified method for limit conversion calculation in membrane reactors, Asia-Pac. J. Chem. Eng. 5 (2010) 226234. Available from: http://www.scopus.com/inward/ record.url?eid 5 2-s2.0-77649169160&partnerID 5 40&md5 5 7ba588485286c2fff415f5083f30b521. [2] M. Nordio, S. Soresi, G. Manzolini, J. Melendez, M. Van Sint Annaland, D.A. Pacheco Tanaka, et al., Effect of sweep gas on hydrogen permeation of supported Pd membranes: experimental and modeling, Int. J. Hydrogen Energy 44 (2019). Available from: https://doi.org/10.1016/j.ijhydene.2018.12.137. [3] F. Froment, K.B. Bischoff, J. De Wilde, Chemical Reactor Analysis and Design, third ed., John Wiley and Sons, Inc, 2011. [4] C. Brencio, F.W.A. Fontein, J.A. Medrano, L. Di Felice, A. Arratibel, F. Gallucci, Pd-based membranes performance under hydrocarbon exposure for propane dehydrogenation processes: experimental and modeling, Int. J. Hydrog. Energy (2021). Available from: https://doi.org/10.1016/J.IJHYDENE.2021.09.252. [5] P.V. Danckwerts, Continuous flow systems: distribution of residence times, Chem. Eng. Sci. 2 (1953) 113. Available from: https://doi.org/10.1016/0009-2509(53)80001-1. [6] U. Kürten, M. Van Sint Annaland, J.A.M. Kuipers, Oxygen distribution in packed-bed membrane reactors for partial oxidations: effect of the radial porosity profiles on the product selectivity, Ind. Eng. Chem. Res. 43 (2004) 47534760. Available from: http://www.scopus.com/inward/record.url?eid 5 2-s2.03543040007&partnerID 5 40&md5 5 17969ebe4c7f2e7d0abed2c3e6ee963c. [7] S. Poto, F. Gallucci, M. Fernanda Neira d’Angelo, Direct conversion of CO2 to dimethyl ether in a fixed bed membrane reactor: influence of membrane properties and process conditions, Fuel 302 (2021) 121080. Available from: https://doi.org/10.1016/J.FUEL.2021.121080. [8] A. Cruellas, T. Melchiori, F. Gallucci, M. van Sint Annaland, Oxidative coupling of methane: a comparison of different reactor configurations, Energy Technol. 0 (2019) 1900148. Available from: https://doi. org/10.1002/ente.201900148. [9] H. Lee, A. Kim, B. Lee, H. Lim, Comparative numerical analysis for an efficient hydrogen production via a steam methane reforming with a packed-bed reactor, a membrane reactor, and a sorption-enhanced membrane reactor, Energy Convers. Manag. 213 (2020) 112839. Available from: https://doi.org/10.1016/ J.ENCONMAN.2020.112839. [10] M. Nordio, M.E. Barain, L. Raymakers, M.V.S. Annaland, M. Mulder, F. Gallucci, Effect of CO2 on the performance of an electrochemical hydrogen compressor, Chem. Eng. J. (2019) 123647. Available from: https://doi.org/10.1016/j.cej.2019.123647. [11] A. Caravella, L. Melone, Y. Sun, A. Brunetti, E. Drioli, G. Barbieri, Concentration polarization distribution along Pd-based membrane reactors: a modelling approach applied to Water-Gas Shift, Int. J. Hydrog. Energy 41 (2016) 26602670. Available from: https://doi.org/10.1016/J.IJHYDENE.2015.12.141. [12] S. Bellini, G. Azzato, F. Gallucci, A. Caravella, 3 - Mass transport in hydrogen permeation through Pd-based membranes, in: A. Basile, F. Gallucci (Eds.), Current Trends and Future Developments on (Bio-) Membranes, Elsevier, 2020, pp. 6390. Available from: https://doi.org/10.1016/B978-0-12-818332-8.00003-X. [13] J.P. Walter, A. Brune, A. Seidel-Morgenstern, C. Hamel, Model-based analysis of fixed-bed and membrane reactors of various scale, Chem. Ing. Tech. 93 (2021) 819824. Available from: https://doi.org/ 10.1002/cite.202000227. [14] O.P. Onoja, X. Wang, P.N. Kechagiopoulos, Influencing selectivity in the oxidative coupling of methane by modulating oxygen permeation in a variable thickness membrane reactor, Chem. Eng. Process.— Process Intensif. 135 (2019) 156167. Available from: https://doi.org/10.1016/J.CEP.2018.11.016. [15] M.A. Murmura, S. Cerbelli, A.-S. Kyriakides, S. Voutetakis, P. Seferlis, S. Papadopoulou, et al., Preliminary analysis of mass dispersion in solid foams: separation of nitrogen/hydrogen mixtures in a packed membrane module as a case study, Chem. Eng. Trans. 74 (2019) 961966. Available from: https://doi.org/10.3303/CET1974161.

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[16] F. Gallucci, M. Van Sintannaland, J.A.M. Kuipers, Theoretical comparison of packed bed and fluidized bed membrane reactors for methane reforming, Int. J. Hydrogen Energy 35 (2010). Available from: https://doi.org/10.1016/j.ijhydene.2010.02.050. [17] K. Kato, C.Y. Wen, Bubble assemblage model for fluidized bed catalytic reactors, Chem. Eng. Sci. 24 (1969) 13511369. Available from: https://doi.org/10.1016/0009-2509(69)85055-4. [18] S.A.R.K. Deshmukh, J.A. Laverman, A.H.G. Cents, M. Van Sint Annaland, J.A.M. Kuipers, Development of a membrane-assisted fluidized bed reactor. 1. Gas phase back-mixing and bubble-to-emulsion phase mass transfer using tracer injection and ultrasound experiments, Ind. Eng. Chem. Res. 44 (2005) 59555965. Available from: http://www.scopus.com/inward/record.url?eid 5 2-s2.023844495521&partnerID 5 40&md5 5 efd53538ad1f3c307fcf5f7059af62e6. [19] M. van, S.A.I. Campos Velarde, F. Gallucci, Development of an endoscopic-laser PIV/DIA Technique for high temperature gas-solid fluidized beds, Chem. Eng. Sci. (2016). IN PRESS. [20] V. Spallina, G. Matturro, C. Ruocco, E. Meloni, V. Palma, E. Fernandez, et al., Direct route from ethanol to pure hydrogen through autothermal reforming in a membrane reactor: experimental demonstration, reactor modelling and design, Energy. 143 (2018) 666681. Available from: https://doi.org/10.1016/J. ENERGY.2017.11.031. [21] N. de Nooijer, F. Gallucci, E. Pellizzari, J. Melendez, D.A. Pacheco Tanaka, G. Manzolini, et al., On concentration polarisation in a fluidized bed membrane reactor for biogas steam reforming: modelling and experimental validation, Chem. Eng. J. 348 (2018) 232243. Available from: https://doi.org/10.1016/J. CEJ.2018.04.205. [22] A. Helmi, R.J.W. Voncken, A.J. Raijmakers, I. Roghair, F. Gallucci, M. van Sint Annaland, On concentration polarization in fluidized bed membrane reactors, Chem. Eng. J. (2017). Available from: https://doi. org/10.1016/j.cej.2017.09.045. [23] X. Yang, S. Wang, B. Hu, K. Zhang, Y. He, Estimation of concentration polarization in a fluidized bed reactor with Pd-based membranes via CFD approach, J. Memb. Sci. 581 (2019) 262269. Available from: https://doi.org/10.1016/j.memsci.2019.03.068. [24] D. Zambrano, J. Soler, J. Herguido, M. Menéndez, Conventional and improved fluidized bed reactors for dry reforming of methane: mathematical models, Chem. Eng. J. 393 (2020) 124775. Available from: https://doi.org/10.1016/j.cej.2020.124775. [25] J.E. Ramírez, S. Esquivel-González, B. López-Rebollar, H. Salinas, J.R. Rangel-Mendez, G. Buitrón, et al., Swirling fluidization in an anoxic membrane bioreactor as an antifouling technique, J. Memb. Sci. 600 (2020) 117856. Available from: https://doi.org/10.1016/j.memsci.2020.117856.

13 Techno-economic analysis of membrane reactors Fausto Gallucci SUSTAINABLE PROCESS E NGINE ERING, CHEMICAL ENGINE ERING AND CH EMISTRY, EINDHOVEN UNIVERSITY OF TECHNOLOG Y , EI N D HO V E N, T HE NE T H E RL AN D S

13.1 Introduction Membrane reactors are reactor concepts in which a reaction step and a separation step occur in the same system. The combination of reaction and separation in the same reactor promises a high degree of process intensification and thus an increase in efficiency and a reduction of capital expenditures and operational costs. Despite the integration of functions seems a clever idea, the promised advantages in terms of costs and increase efficiency is not obvious for all reaction systems. One should consider that the integration of two functions brings also several limitations. Amongst these, it is worth reminding that it becomes a challenge to perfectly tune the permeation rates with reaction and heat transfer rates in a single system. Most importantly, when integrating the two functions in one vessel we lose the possibility of optimizing reaction conditions and separation conditions. For instance, if we consider the hydrogenation of CO2 to methanol, water separation would increase the selectivity and rate toward methanol. However, the water separation (from gases) in porous membranes would better be achieved at low temperatures where capillary condensation and selective separation are easier. On the other hand, the activity of the catalyst would require around 250 C to appreciate the conversion. When integrating the two, we are forced to work at intermediate temperatures (below 220 C) where the catalytic activity is low, but also the membrane selectivity is low. This is to say, that integration can give benefits but one should have a clear reason to integrate the two. As already reported in one of our previous chapters [1] the integration of membrane separation (or any separation) in a reactor is beneficial either if thermodynamics constraints are limiting the conversion, or if there is a possible consecutive reaction that would limit the selectivity towards the product of interest (see Fig. 13 1). Once it has been decided to use an integrated reactor, in this case, a membrane reactor, for a given reaction system it is essential to compare the performance of the reactor with conventional systems. This comparison should be done sequentially and logically. The suggestion here is to first compare the performance of the reactor, followed by its rough design, Current Trends and Future Developments on (Bio-)Membranes. DOI: https://doi.org/10.1016/B978-0-12-823659-8.00013-7 © 2023 Elsevier Inc. All rights reserved.

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FIGURE 13–1 Systems in which integration of (membrane) separation and reaction is beneficial. Reproduced from F. Gallucci, D.A. Pacheco Tanaka, J.A. Medrano, J.L. Viviente Sole, Membrane reactors using metallic membranes, in: A. Basile, F. Gallucci (Eds.), 10 - Current Trends and Future Developments on (Bio-) Membranes, Elsevier, 2020, pp. 235 260, https://doi.org/10.1016/B978-0-12-818332-8.00010-7.

and then proceed with a complete cost analysis. This sequence is intended only to decrease the number of calculations because in any case, the full techno-economic analysis (TEA) is the only one that will prove if the integration makes sense. In these comparisons, whatever level of details we decide to use, it is essential to pay attention to the boundary conditions for the comparison; in particular, the state of the reagents and of the products must be the same in the different systems under evaluation. This is because, the membrane process is driven by a difference in pressure, thus the product (if the membrane is used to remove a product) will leave the membrane at low pressure. It is thus not right to compare this system with a conventional system where the product leaves the system at high pressure. To be fair in the comparison, the membrane system should also have included in the calculation the costs for the compression steps. This has been well represented by Medrano et al. [2] where the thermodynamic comparison of different reaction systems for hydrogen production is carried out, including membrane reactors. As well represented by the Fig. 13 2, the comparison is done also considering the compression steps needed such that all products and reagents (thus the battery limits for the comparison) are in the same conditions. A technical comparison like the one reported by Medrano et al. [2] does already give a detailed picture of the limitations and strong points of the different technology and helps identify what are the conditions at which the membrane reactor could outperform the conventional system (see Fig. 13 3) and to which extent this increased performance justify further research and development into the concept. This kind of comparison is interesting but

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FIGURE 13–2 Scheme used for a fair comparison between hydrogen production technologies. Reproduced from J. A. Medrano, V. Spallina, M. Van Sint Annaland, F. Gallucci, Thermodynamic analysis of a membrane-assisted chemical looping reforming reactor concept for combined H2 production and CO2 capture, Int. J. Hydrogen Energy 39 (2014), https://doi.org/10.1016/j.ijhydene.2013.11.126.

FIGURE 13–3 Reforming efficiency profiles of the systems studied as a function of the reactor temperature. Reproduced from J.A. Medrano, V. Spallina, M. Van Sint Annaland, F. Gallucci, Thermodynamic analysis of a membrane-assisted chemical looping reforming reactor concept for combined H2 production and CO2 capture, Int. J. Hydrogen Energy 39 (2014), https://doi.org/10.1016/j.ijhydene.2013.11.126.

does not yet give a complete picture of the real benefit of the membrane reactors compared with conventional systems. More detailed analysis needs to be carried out, now including also kinetics, permeation rates, and heat transfer. This second level of detailed analysis has been reported by several authors for different systems. Remaining in the field of hydrogen production, Brunetti et al. [3] reported that the

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FIGURE 13–4 Volume index decrease as a function of feed pressure for WGS. Reproduced from A. Brunetti, A. Caravella, G. Barbieri, E. Drioli, Simulation study of water gas shift reaction in a membrane reactor, J. Memb. Sci. 306 (2007) 329 340.

membrane reactor would result in a large decrease in reactor volume compared to conventional systems for water gas shift (WGS) reaction (see Fig. 13 4). The reader is referred to the original paper for more details about the calculation, however, we can mention here that this decrease in volume when using membranes in WGS systems is a side effect of the membrane permeation. This is because when removing hydrogen from the reaction system the equilibrium constraints of conventional systems are circumvented so that for WGS it is possible to operate the membrane reactor at higher temperatures and thus achieve higher reaction kinetics and thus a reduction of volume. Again, this info on the reactor design is interesting and offers even more indications compared to the thermodynamics analysis only. However, it is still not enough for judging the real benefit of the membrane reactor. For this particular case, questions may still arise such as: is the decrease of volume in the reactor paying off for the additional cost of the membranes? Or is the increase of temperature in the reactor compensated by a lower production cost of the hydrogen? Many other questions can be asked. To be able to answer these questions the next level of analysis needs to be performed, a full TEA. The following sections will report a guideline for TEA. To be consistent with the discussion till now we will report first the TEA of membrane reactors for hydrogen production and then report other examples of TEA applied to other systems.

13.2 Latest developments in techno-economic analysis for membrane reactors This section reports the latest developments in the TEA of membrane reactors in the last 5 years. Again, before diving into the most interesting results, a few analyses can be drawn by using the

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FIGURE 13–5 Documents per affiliation. Scopus (accessed 25.01.22).

analytical tool Scopus. The 116 documents were found by using the keywords “economic analysis” and “membrane reactor” and by limiting the results between 2018 and 2022. As reported in the Fig. 13 5 the highest amount of papers are reported from Ulsan National Institute (South Korea), Politecnico di Milano (Italy), and Eindhoven University (the Netherlands). This is also directly correlated with the documents reported per authors as reported Fig. 13 6. When we analyze the documents by the country we realize that most of the scientific output is done in the United States followed by China and South Korea. Additionally, Europe has several studies in this field (see Fig. 13 7). As an example of TEA, we will first continue with hydrogen production. For example, we take the paper reported by Wassie et al. [4] who compared different systems for hydrogen production with CO2 capture. One of the typical shortcuts people take is to make a technical, economic, or TEA of a chemical plant and compare the results with literature data on other systems. This is very dangerous as the results of a TEA strongly depend on the level of details used in the calculations, on the equations and methods used for the economic calculations, and strongly depend on the costs assumed for utilities and fuels. It is thus very advisable to carry out the simulations of the reference plant with the same level of detail as the new plant and be able to check the influence of the assumptions on the costs of both plants. For instance, in the paper Wassie reports several plants and here we report the impression of the conventional plant with CO2 capture and the new system introduced in the paper (the gas switching membrane reforming). Figs. 13 8 and 13 9, report the two plants considered in the analysis by Wassie and are detailed enough to understand the different units used in the plant. Additionally, as already discussed above, the plant in Fig. 13 9 needs an additional compression station for the pure hydrogen as the products of the two plants (H2 and CO2) should be delivered at the same

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FIGURE 13–6 Documents by authors. Scopus (accessed 25.01.22).

FIGURE 13–7 Documents by country/region. Scopus (accessed 25.01.22).

purity and the same conditions of pressure (and temperature). Another take-home message is that the utilities need to be considered in the TEA, and often these make the real difference in terms of costs between the two plants compared. The first thing to do when performing the TEA is to perform a technical comparison between the different concepts in the possible most ideal conditions for the different parts of the plants. Wassie has reported the following table summarizing the different terms of comparison between the plants. The table gives us the second take-home message, that is, when

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FIGURE 13–8 Schematic description of a reference fired tubular plant for H2 production with CO2 capture. Reproduced from S.A. Wassie, S. Cloete, V. Spallina, F. Gallucci, S. Amini, M. van Sint Annaland, Techno-economic assessment of membrane-assisted gas switching reforming for pure H2 production with CO2 capture, Int. J. Greenh. Gas Control. 72 (2018) 163 174, https://doi.org/10.1016/J.IJGGC.2018.03.021.

FIGURE 13–9 Schematic description of the membrane-assisted gas switching reforming plant for H2 production with CO2 capture. Reproduced from S.A. Wassie, S. Cloete, V. Spallina, F. Gallucci, S. Amini, M. van Sint Annaland, Techno-economic assessment of membrane-assisted gas switching reforming for pure H2 production with CO2 capture, Int. J. Greenh. Gas Control. 72 (2018) 163 174, https://doi.org/10.1016/J.IJGGC.2018.03.021.

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comparing two systems the scale of comparison should be similar. It is not fair to compare systems at different scales because costs scale in different ways. The scale to be compared could be on the product amount or the reactant amount (of course generally it is not possible to have both at the same scale and otherwise from a technical point of view the only difference will be made by the other utilities). As it can be seen in the table, the benchmark technologies are at the same product scale, while the new technology has the same input as the first benchmark. Another take-home message is that the performance should be compared using also the utilities. For instance, in Table 13 1 the emissions and energy efficiencies are calculated also considering the emissions related to the electricity used as a utility. It is not important now to provide also the equations used for calculating these efficiencies, the interested reader is referred to the work of Wassie. However, the same equations should be used for all technologies, and that is why it is better to calculate the benchmark technology also in the your own study and not assuming from literature. Table 13–1 capture.

Performance of the three concepts for hydrogen production with CO2

Process

Benchmark

Benchmark

MA-GSR

CO2 capture technologies Tref [ C]/Pref [bar]/SC/Pperm [bar] NG flow rate [kg/s] NG thermal input [MWLHV,NG] Steam-to-carbon ratio H2 mass flow rate [kg/s] Electricity production/consumption Air compressor-Air/exh Fan [MWel] Gas turbine [MWel] H2 compressors [MWel] CO2 compressors [MWel] Steam turbine [MWel] Pumps [MWel] Other auxiliaries [MWel] Net electric power [MWel] Steam export (160 C, 6 bar) [kg/s] H2 production efficiency1lH2 (H2,LHV/NGLHV) Equivalent NG input mNG,eq [kg/s] H2 yield [molH2/moleq,CH4] Eq. H2 production efficiency 1lH2,eq (H2,LHV/NGeq,LHV) Heat rate (HR) [Gcal/kNm3H2] CO2 specific emission Eco2 [kgco2/Nm3H2] Equivalent CO2 specific emission Eco2,eq [kgCO2/NM3H2] Eq. CO2 avoided (CCReq) [%] SPECCAeq [MJ/kgCO2]

N/A 890/32/2.7/2.62 121.94 2.7 0.75

CA-MDEA 890/32/4/2.81 130.79 4 0.75

H2O cond 662/50/2.1/1 2.62 121.94 2.1 0.91

20.68 22.27 3.27 20.21 20.05 0.07 4.02 0.74 2.41 2.49 0.81 3.24 0.82 0.76 -

20.91 22.28 22.23 3.79 20.29 20.15 22.07 0.27 0.69 2.88 2.48 0.67 3.79 0.14 0.16 79 4.57

25.73 6.84 29.00 20.28 20.03 20.09 28.29 0.65 0.89 2.89 2.97 0.81 3.05 0.14 0.20 72 0.02

Source: Reproduced from S.A. Wassie, S. Cloete, V. Spallina, F. Gallucci, S. Amini, M. van Sint Annaland, Techno-economic assessment of membrane-assisted gas switching reforming for pure H2 production with CO2 capture, Int. J. Greenh. Gas Control. 72 (2018) 163 174, https://doi.org/10.1016/J.IJGGC.2018.03.021.

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Once the technical comparison has been done we have to go to the economic comparison. Firstly the heat and material balances need to be calculated. This can best be done with flowsheet software like Aspen or similar. The reason is that for a complete plant several equations are used, and software like aspen allows recalculating all material and energy balances when changing some parameters. This is especially required when trying to optimize the system and decrease the overall energy use. Of course, any software is ok for this comparison, but again it is suggested that for the different systems the same software is used and especially the same closure equations like the thermodynamic model, etc. often differences are seen in calculations simply because different thermodynamic models are used. Once the heat and material balances are calculated it is better to report these in the work (or in the appendix) so that it is easier to compare systems for future works. The next step in the calculation will be the sizing of the equipment. Some of these can be done directly in the flowsheet model. For instance separations and compression units, pumps, etc. can easily be calculated by Aspen. Of course, it is highly recommended that the data used for the calculation (for instance efficiencies assumed for compressors) are well reported in the work/paper for a better comparison. Other equipment like for instance reactors are often calculated outside the flowsheet. Again, there is no specific software to be used for such dimensioning. However, the same approach should be best used for the different plants to be considered. Once material and energy balances as well as dimensioning of the equipment have been done, the next step is to make the economic analysis. First and most importantly, the different costs for the different units may come from different sources, but they need to be actualized to the same base. Once actualized costs for the different units are obtained, it is time to put together all costs in a cost analysis. It should be noted that several flowsheet software has their cos analyzer. This can be used as long as is used for all systems to be compared. Otherwise, several papers report the basic equations for the economic analysis. For hydrogen production, we often use the cost estimation methodology for NETL assessments of power plant performance. This is because most of the components are very similar as well as sized are often similar. The interested reader is urgent to check if the methodology is applicable also for their plant. Here is the methodology used for hydrogen production plants as reported by Spallina et al. [5] starts with the calculation of the Total Overnight Cost (TOC). This is calculated according to Table 13 2. As it can be seen the TOC is calculated starting from the actualized costs of all components in the plant. These are summed on the bare erected cost. To these costs, we should add the costs for installation, engineering, contingency, etc. Once these costs are calculated, we can now calculate the operational costs that will include the utilities, feed chemicals, consumables as well as other costs directly calculated from the TOC. The methodology is reported in Table 13 3. This table should report the costs of all consumables, including the replacement costs in case of a lifetime shorter than the lifetime of the plant. Once these costs are calculated then the cost of production can be calculated based on the capital cost, the capital charge rate, the operational costs, the productivity of the plant, and the hours of operation per year.

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Current Trends and Future Developments on (Bio-)Membranes

Table 13–2

Methodology for the calculation of the TOC from NETL.

Plant component

Cost (Mh)

Component W Component X Component Y Component Z Bare erected cost [BEC] Direct costs as a percentage of BEC Includes piping/valves, civil works, instrumentation, steel structure, erections, etc. Total installation cost [TIC] Total direct plant cost [TDPC] Indirect cost [IC] Engineering procurement and construction [EPC] Contingencies and owner’s cost (C&OC) Contingency Owner’s cost Total contingencies & OC [C&OC] Total overnight cost [TOC]

A C C D A1B1C1D

80% BEC BEC 1 TIC 14% TDPC TDCP 1 IC 10% EPC 5% EPC 15% EPC EPC 1 C&OC

Source: Reproduced from V. Spallina, D. Pandolfo, A. Battistella, M.C. Romano, M. Van Sint Annaland, F. Gallucci, Techno-economic assessment of membrane assisted fluidized bed reactors for pure H2 production with CO2 capture, Energy Convers. Manag. 120 (2016) 257 273, https://doi.org/10.1016/J.ENCONMAN.2016.04.073.

Table 13–3

Assumptions of the calculation of the O&M costs.

O&M Labor cost costs Maintenance cost Insurance Catalyst and sorbent replacement Oxygen carrier cost Reforming catalyst cost Water gar shift catalyst cost Desulphurization catalyst cost Lifetime Consumables Cooling water mate-up cost Process water cost Natural gas cost Miscellaneous Steam cost Electricity cost

Mh % TOC % TOC

1.5 2.5 2.0

$/kg Kh/m3 Kh/m3 $/ft3 Years

15 50 14 355 5

h/m3 h/m3 h/GJLHV

0.35 2 9.15

h/tonsteam (h/MWh)

0.13 76.36

Source: Reproduced from V. Spallina, D. Pandolfo, A. Battistella, M.C. Romano, M. Van Sint Annaland, F. Gallucci, Techno-economic assessment of membrane assisted fluidized bed reactors for pure H2 production with CO2 capture, Energy Convers. Manag. 120 (2016) 257 273, https://doi.org/10.1016/J.ENCONMAN.2016.04.073.

Chapter 13 • Techno-economic analysis of membrane reactors

347

If all calculations have been done then a full comparison can be carried out and we can finally assess if the membrane reactor is more beneficial than a conventional system. To continue with the work of Wassie the authors have reported the results in Table 13 4. The example has been selected exactly for the table below. The comparison shows that the use of the gas switching membrane reactor for hydrogen production with CO2 capture gives only marginal benefits compared to the conventional benchmark system with an additional CO2 separation step. These very limited improvements do not justify the implementation of the gas switching reforming for this application, despite the system studied as a standalone Table 13–4

Economic comparison between gas switching and other technologies.

Process

Benchmark

Benchmark

MA-GSR

CO2 capture technologies Tref [ C]/Pref [bar]/SC/Pperm [bar] MA-GSR reactor geometry Reactor diameter [m] Reactor length [m] Number of reactors Membrane length [m] Number of Membranes Economic evaluation Bare erected cost [Mh] (% of tot BEC) Reactors Convective cooling HEX Turbomachines H2 compressors Syngas coolers & heat rejection PSA unit MDEA unit CO2 compressors H2 membranes Total overnight cost 3 CCF [Mh/y] O&M fixed (others) [Mh/y] O&M variable (90% plant availability) [Mh/y] Water (process 1 cooling) Natural Gas Steam export Electricity H2 mass flow rate [kg/s] Cost of hydrogen (COH) [h/NM3H2] COHvariablecost [h/Nm3H2] COHfixedcost [h/Nm3H2] Cost of CO2 avoided [h/tCO2] Cost of CO2 avoided equiv. em. [h/tCO2.eq]

N/A 890/32/2.7/-

CA-MDEA 890/32/4/-

H2O cond 662/50/2.1/1 3.5 10 5 6 0.05 5984

10.56 (27.3%) 10.67 (27.6%) 3.42 (8.8%) 1.46 (3.8%) 4.17 (10.18%) 8.45 (21.85%)

11.11 (18.7) 13.27 (22.3%) 3.7 (6.3%) 1.38 (2.3%) 6.58 (11.1%) 5.9 (10.0%) 14.29 (24.1%) 3.12 (5.2%)

25.13 (27.4%) 5.73 (6.2%) 2.13 (2.3%) 5.71 (6.2%) 7.70 (8.4%)

14.15 6.60

21.70 9.75

0.69 (0.8%) 44.62 (48.7%) 33.18 10.10

0.63 31.67 21.37 20.02 0.75 0.216 0.129 0.087

0.82 33.97 20.09 1.14 0.75 0.282 0.150 0.132 97.06 110.00

0.36 31.67 21.01 4.99 0.91 0.274 0.124 0.150 85.14 105.31

Source: Reproduced from S.A. Wassie, S. Cloete, V. Spallina, F. Gallucci, S. Amini, M. van Sint Annaland, Techno-economic assessment of membrane-assisted gas switching reforming for pure H2 production with CO2 capture, Int. J. Greenh. Gas Control. 72 (2018) 163 174, https://doi.org/10.1016/J.IJGGC.2018.03.021.

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Current Trends and Future Developments on (Bio-)Membranes

concept seemed to give a lot of benefits in terms of process integration. The detailed technoeconomic comparison has shown that this is not true at a system level. A similar analysis but for another concept (the membrane-assisted chemical looping reforming) has been reported by Medrano in his PhD thesis [6] and has shown that for this other concept the hydrogen can be produced with much lower costs and higher CO2 capture efficiencies. A warning has to be given when using the same approach, these costs calculations are tailormade for relatively large plants. When calculating the costs of portable or somehow small plants care has to be taken to the calculations as the procedure described above may lead to a large overestimation of the costs of the plant. A TEA is required to show that a membrane reactor is more interesting than a conventional system. But this is not the only way to use techno-economics. This methodology can also be used to assess the best-operating conditions of a membrane reactor or to compare different membrane reactor configurations even just novel reactor configurations like Lee and coworkers did [7]. Remaining to the hydrogen production case, Dimarcoberardino et al. [8] have used the TEA to select between different membrane reactor concepts for two different biogas conversions systems at a small scale and identified a cost-based “best reactor concept” showing that the best reactor concept in terms of costs is not necessarily the most efficient reactor concept in terms of hydrogen recovery and biogas conversion. The techno-economics have been used also by Lim et al. [9] for the conversion of landfill gas for hydrogen production. As shown in the Fig. 13 10, the authors have shown how to use techno-economic sensitivity analysis to identify and optimize the parameters that make the membrane reactor more economic in terms of normalized hydrogen cost compared to conventional packed bed reactors. Using a techno-economic approach Giaconia et al. [10] have reported a comparison of solar-driven methane reforming with a conventional system and optimized the solar drive system. The results reported in Fig. 13 11, shows that for a relatively small plant the solardriven system is much more expensive (in terms of hydrogen costs) compared to conventional reforming. On the other hand, a solar/methane mix can be used to reduce the costs, although the paper also reports that with the steady increase of the cost of natural gas, the solar-driven plant could be already more economic soon, while surely results already in a big decrease of emissions compared to the conventional system. Techno-economic analyses are also applied to a variety of other membrane reactor systems, from chemical production to water treatment. Szima and coworkers [11] have used a technoeconomic analysis to study the coproduction of hydrogen and power starting from a coal power plant. They used a membrane-assisted WGS system in their analysis and compared it with different systems. It turns out that, despite the membrane reactor being one step hydrogen production, the final cost of energy and cost of capture is not better than other options. Heo et al. [12] have studied the membrane reactor for ethane reforming for hydrogen production showing a much lower cost of hydrogen for a membrane reactor compared to a conventional reactor. Bekkering et al. [13] have studied a small-scale (farm-scale) power to methane system and compared it with different scenarios including one with membranes. In this case, however, the scenario including membrane was not the most economic one.

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349

FIGURE 13–10 Results of sensitivity analysis for a membrane reactor in terms of operating temperature (600 C 650 C) and membrane properties. Reproduced from D. Lim, M. Byun, B. Lee, A. Lee, A. Kim, B. Brigljevi´c, et al., H2 production from catalytic dry reforming of landfill gas utilizing membrane reactor with combined heat and power system: 3E (energy, economic and environmental) feasibility analysis, Energy Convers. Manag. 247 (2021) 114704, https://doi.org/10.1016/j.enconman.2021.114704.

Cruellas et al. [14] have compared different membrane reactor configurations for oxidative coupling of methane (OCM) and compared with conventional reactors and with naphtha steam cracking (NSC). The analysis shows that some membrane reactors configurations can produce ethylene at a much lower price compared to the conventional NSC (see Fig. 13 12). Additionally, the results reported in Fig. 13 13 also shows that carrying out the TEA helps identify the configurations that are best from a cost point of view but are not necessarily best from a reactor performance point of view. Gao et al. [15] have carried out an extensive techno-economic comparison between the conventional activated sludge and membrane bioreactor for a wastewater treatment plant. The results, reported in Fig. 13 14 and valid for the type of wastewater considered, show that the membrane bioreactor gives improvements in terms of net profit compared to the conventional system, although energy requirements are higher, these are counterbalanced by the higher pollutant removal rates. And although the net profit is increased only slightly (per cubic meter) the amounts are so high that the total benefit of upgrading conventional systems with a membrane bioreactor can be huge.

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Current Trends and Future Developments on (Bio-)Membranes

FIGURE 13–11 Estimated Hydrogen Production Cost for the solar reforming plant reproduced. From A. Giaconia, G. Iaquaniello, B. Morico, A. Salladini, E. Palo, Techno-economic assessment of solar steam reforming of methane in a membrane reactor using molten salts as heat transfer fluid, Int. J. Hydrog. Energy 46 (2021) 35172 35188.

FIGURE 13–12 Cost distribution of the OCM process when using four different reactor configurations. Reproduced from A. Cruellas, J.J. Bakker, M. van Sint Annaland, J.A. Medrano, F. Gallucci, Techno-economic analysis of oxidative coupling of methane: current state of the art and future perspectives, Energy Convers. Manag. 198 (2019) 111789, https://doi.org/10.1016/J.ENCONMAN.2019.111789.

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FIGURE 13–13 OCM reactor yield at different reactor temperatures and operating CH4/O2 ratios. Reproduced from A. Cruellas, J.J. Bakker, M. van Sint Annaland, J.A. Medrano, F. Gallucci, Techno-economic analysis of oxidative coupling of methane: current state of the art and future perspectives, Energy Convers. Manag. 198 (2019) 111789, https://doi.org/10.1016/J.ENCONMAN.2019.111789.

Ramesh et al. [16] have also used a TEA of membrane-based wastewater treatment from the textile industry. The paper is interesting as it contains equations used for actualizing the costs of components and to calculate the TOC that is different from the one used for hydrogen production. These calculations seem more applicable for wastewater treatments and the interested reader is referred to the original paper. The paper also links the TEA with the environmental analysis. Several other interesting papers appeared in the last few years on the TEA of different membrane-assisted systems and are not reported as a review, because they use very similar approaches described in this chapter.

13.3 Conclusions and future trends Membrane-assisted (bio)reactors have been regarded as systems that introduce a high degree of process intensification. While this is true in general, the process intensification does not directly translate into cost benefits for the final product. One should keep in mind that, when using a membrane reactor instead of a conventional reactor, the complete system around the reactor will change. Some equipment used for conventional systems is obsolete, while other equipment will be needed (like compressors for instance). It is essential that new technology comes also with a cost analysis and this can be done with a TEA as described in the paper. Another thing that should also be taken into account is that the approach used for

352

Current Trends and Future Developments on (Bio-)Membranes

FIGURE 13–14 Distribution of net profit (NP), energy efficiency (EE), cost efficiency (CE), and other-cost efficiency (OE, other-cost 5 operating costs minus energy consumption) before and after the retrofitting from CAS to MBR under different scenarios. Reproduced from T. Gao, K. Xiao, J. Zhang, W. Xue, C. Wei, X. Zhang, et al., Techno-economic characteristics of wastewater treatment plants retrofitted from the conventional activated sludge process to the membrane bioreactor process, Front. Environ. Sci. Eng. 16 (2021) 1, https://doi.org/10.1007/s11783-021-1483-6.

bigger plants does not necessarily hold for a smaller plant, and that membrane systems are very modular compared to conventional systems. How to implement this is TEA deserves surely some work. The chapter does not discuss how to take into account the different technology readiness levels of membrane-assisted systems compared to conventional systems in techno-economic analysis. There are some ways to do this but there is still no consensus on how to properly account for the difference in maturity and different works surely ignore this topic.

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353

Nomenclature List of acronyms CAS CCF CE CLR CMR CO&M CPO DME DS EE FBR FC LTA MA-CLR MA-GSR MDEA MEA Mr NP NSC OCM OE OTM PBR TEA TOC WGS

Conventional activated sludge capital charge factor Cost efficiency Chemical looping reforming Catalytic membrane reactor operating and maintenance costs Catalytic partial oxidation Dimethyl Ether Double skinned Energy efficiency fluidized bed reactor Fuel Cell Zeolite A Membrane-assisted CLR Membrane-assisted gas switching reforming methyl diethanolamine monoethanolamine Membrane reactor Net profit Naphtha steam cracking Oxidative Coupling of Methane Other costs Oxygen Transport membranes packed bed reactor Techno-economic analysis total overnight cost Water gas shift

References [1] F. Gallucci, D.A. Pacheco Tanaka, J.A. Medrano, J.L. Viviente Sole, 10 - Membrane reactors using metallic membranes, in: A. Basile, F. Gallucci (Eds.), Current Trends and Future Developments on (Bio-) Membranes, Elsevier, 2020, pp. 235 260. Available from: https://doi.org/10.1016/B978-0-12-8183328.00010-7. [2] J.A. Medrano, V. Spallina, M. Van Sint Annaland, F. Gallucci, Thermodynamic analysis of a membraneassisted chemical looping reforming reactor concept for combined H2 production and CO2 capture, Int. J. Hydrog. Energy 39 (2014). Available from: https://doi.org/10.1016/j.ijhydene.2013.11.126. [3] A. Brunetti, A. Caravella, G. Barbieri, E. Drioli, Simulation study of water gas shift reaction in a membrane reactor, J. Memb. Sci. 306 (2007) 329 340. Available from: http://www.scopus.com/inward/record.url? eid 5 2-s2.0-35948940853&partnerID 5 40&md5 5 6aa04457541acd1c290ea8878ff3f599. [4] S.A. Wassie, S. Cloete, V. Spallina, F. Gallucci, S. Amini, M. van Sint Annaland, Techno-economic assessment of membrane-assisted gas switching reforming for pure H2 production with CO2 capture, Int. J. Greenh. Gas Control. 72 (2018) 163 174. Available from: https://doi.org/10.1016/J.IJGGC.2018.03.021.

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[5] V. Spallina, D. Pandolfo, A. Battistella, M.C. Romano, M. Van Sint Annaland, F. Gallucci, Technoeconomic assessment of membrane assisted fluidized bed reactors for pure H2 production with CO2 capture, Energy Convers. Manag. 120 (2016) 257 273. Available from: https://doi.org/10.1016/J. ENCONMAN.2016.04.073. [6] J.A. Medrano, Membrane-assisted chemical looping reforming, 2017. https://pure.tue.nl/ws/portalfiles/ portal/74677925/20170911_Medrano_Jimenez.PDF. [7] H. Lee, B. Lee, M. Byun, H. Lim, Comparative techno-economic analysis for steam methane reforming in a sorption-enhanced membrane reactor: simultaneous H2 production and CO2 capture, Chem. Eng. Res. Des. 171 (2021) 383 394. Available from: https://doi.org/10.1016/j.cherd.2021.05.013. [8] G. Di Marcoberardino, S. Foresti, M. Binotti, G. Manzolini, Potentiality of a biogas membrane reformer for decentralized hydrogen production, Chem. Eng. Process. - Process Intensif. 129 (2018) 131 141. Available from: https://doi.org/10.1016/j.cep.2018.04.023. [9] D. Lim, M. Byun, B. Lee, A. Lee, A. Kim, B. Brigljevi´c, et al., H2 production from catalytic dry reforming of landfill gas utilizing membrane reactor with combined heat and power system: 3E (energy, economic and environmental) feasibility analysis, Energy Convers. Manag. 247 (2021) 114704. Available from: https://doi.org/10.1016/j.enconman.2021.114704. [10] A. Giaconia, G. Iaquaniello, B. Morico, A. Salladini, E. Palo, Techno-economic assessment of solar steam reforming of methane in a membrane reactor using molten salts as heat transfer fluid, Int. J. Hydrog. Energy 46 (2021) 35172 35188. Available from: https://doi.org/10.1016/j.ijhydene.2021.08.096. [11] S. Szima, C. Arnaiz del Pozo, S. Cloete, P. Chiesa, Á. Jiménez Alvaro, A.-M. Cormos, et al., Finding synergy between renewables and coal: flexible power and hydrogen production from advanced IGCC plants with integrated CO2 capture, Energy Convers. Manag. 231 (2021) 113866. Available from: https://doi. org/10.1016/j.enconman.2021.113866. [12] J. Heo, B. Lee, H. Lee, H. Lim, Integrative technical, economic, and environmental feasibility analysis for ethane steam reforming in a membrane reactor for H2 production, ACS Sustain. Chem. Eng. 8 (2020) 7011 7019. Available from: https://doi.org/10.1021/acssuschemeng.0c00328. [13] J. Bekkering, K. Zwart, G. Martinus, J. Langerak, J. Tideman, T. van der Meij, et al., Farm-scale biopower-to-methane: comparative analyses of economic and environmental feasibility, Int. J. Energy Res. 44 (2020) 2264 2277. Available from: https://doi.org/10.1002/er.5093. [14] A. Cruellas, J.J. Bakker, M. van Sint Annaland, J.A. Medrano, F. Gallucci, Techno-economic analysis of oxidative coupling of methane: current state of the art and future perspectives, Energy Convers. Manag. 198 (2019) 111789. Available from: https://doi.org/10.1016/J.ENCONMAN.2019.111789. [15] T. Gao, K. Xiao, J. Zhang, W. Xue, C. Wei, X. Zhang, et al., Techno-economic characteristics of wastewater treatment plants retrofitted from the conventional activated sludge process to the membrane bioreactor process, Front. Environ. Sci. Eng. 16 (2021) 1. Available from: https://doi.org/10.1007/s11783-021-1483-6. [16] K. Ramesh, B.M. Gnanamangai, R. Mohanraj, Investigating techno-economic feasibility of biologically pretreated textile wastewater treatment by electrochemical oxidation process towards zero sludge concept, J. Environ. Chem. Eng. 9 (2021) 106289. Available from: https://doi.org/10.1016/j.jece.2021.106289.

Index Note: Page numbers followed by “f” and “t” refer to figures and tables, respectively. A Ammonia synthesis anode hydrogen feedstocks, 28 30 cathode materials, 27 28 electrocatalytic formation, 25f, 29f in electrocatalytic PECMR, 24f electrolyte materials, 26 27 nitrogen reduction reaction mechanism, 24 26 protonic electrocatalytic membrane reactors for, 23 24 Anode, 289 Asbestos, 293 B Bioartificial organs, 228 229 Bio-polymers, 7 Brine electrolysis, 305 309 C Cathode, 289 Cell separators, 290 292 Clusters, 296 CO2 reduction anodic materials, 35 cathodic materials and catalysts, 34 35 conversion in PECMR, 31f electrocatalytic reduction, 31 32 electrolyte materials, 32 34 experimental investigation of, 32 protonic electrocatalytic membrane reactors for, 31 reactions in PCECs, 32f D Design and fabrication catalyst integration, 101 ceramic membranes, 103 channel monolith and flow converter, 104f

electron beam welding, 97 free-standing Pd membrane, 103f high-pressure microchannel membrane, 100f hollow fiber membrane, 105f independent perforated microchannel and microchamber, 98f leak-tight sealing of membrane, 97 membrane coating, 99 101 membrane support sheet, 101f methane steam reformer, 102f microchannel membrane module, 99f micro-electrochemical systems (MEMS) technology, 102 103 micro reformer configuration, 104f monolithic system, 103 PDMS membrane, 98f scaling-up by modularity, 99 sealing by welding metal membranes, 97 separation of hydrogen, 100f stacked catalytic membrane microreactor, 98f weld cavities in several sheets, 99f Designs of enzymatic membrane reactor configuration of, 197f enzyme in, 196 Diaphragms for liquid electrolytes asbestos, 293 chemical structure of polysulfone, 294f thermoplastic diaphragms, 293 295 Diffusion polarization, 101 Double skinned membrane, 84 E Electro-ceramics, 302 Electrochemical efficiency, 288 289 Electrochemical membrane reactors ceramic, 300 303 developed and deployed, 285 diaphragms for liquid electrolytes, 292 295

355

356

Index

Electrochemical membrane reactors (Continued) polymer membrane materials, 295 300 reactors, 286 292 selected endergonic applications, 304 309 Electrochemical reactors cell separators, 290 292 electrochemical cell, endergonic, 290f electrochemical cell, exergonic, 291f endergonic transformations, 287, 289 exergonic transformations, 287 exergonic transformers, 290 gap cell, 291 292, 292f general principles, 286 289 electrochemical efficiency, 288 289 kinetics, 287 288 thermodynamics, 286 287 voltage—current density curve, 288f spontaneity of a chemical reaction, 287t Endergonic transformers, 289 Enzymatic membrane reactor applications, 210 216 for biphasic reactions, 211t catalytic conversions, 216 dead-end filtration membrane reactor, 216 EMR with W/O PE, 215f enzyme-activated in multiphase, 215f hydrophobic character of the substrate, 210 216 monophasic EMR, 216 for single-phase reactions, 212t characteristics, 197 203 designs of, 196 197 enzyme immobilization in, 203 208 cross-linking method of, 208 four methods for, 206t fragile molecules, 204 functional groups of enzyme, 207 208 hydrophilic ultra-thin gel layer formation, 208f immobilization of, 205 methods to immobilize enzyme, 205f pH solution and ionic charge of amino acids, 204f solubility of the protein, 203f

surface electrostatic and hydrophobic interaction, 205 versus other reactor configurations, 209 210 batch reactor, 209f CSTR and EMR, 209f packed bed reactor, 209f Enzyme activated membrane, 216 Exergonic transformers, 290 F Fields of application acetalization reactions, 140 141 acetic acid and butanol, 137f bio-alcohol production, 141 142 by-products via hydrophilic PV, 138 condensation reactions, 141 esterification reactions, 135 138 etherification reactions, 139 140 MTBE/methanol separation via PV, 140f state-of-the-art, 143f Fluidized bed membrane reactors autothermal methane reforming with integrated CO2 capture, 78f developments application of membrane-assisted fluidized bed reactor, 87 89 bed and a packed bed with a 2D model, 82f Calcium Looping (CaL) process system, 88f documents by authors, 80f documents by country/region, 81f documents per affiliation, 80f double skinned and conventional membrane, 85f hydrogen and ideal perm-selectivity, 84f long-term nitrogen permeance of, 83f long-term performance of, 83f low stability of membranes in, 83 84 MA-CLR concept for pure H2 production, 87f membrane immersion in, 85 86 and packed beds membrane reactors, 81 82 reactions systems, 81t sorption enhanced reforming ad regeneration, 88f tested in wastewater treatment, 89, 89f

Index

testing, 90 ultrathin membranes, 82 function of the fluid flow rate, 78f immersion of, 77 use of, 77 79 G Gap cell, 291 292 Geldart B particles, 77 Glass transition temperature (Tg), 153 H Hargreaves-Bird cell, 307 308 Hydrocarbon dehydrogenation conversion of alkanes to alkenes, 42 44 ceramic anode materials, 43 44 high ethylene and propylene selectivity, 44 by PECMRs, 42f propane dehydrogenation, 43 methane upgrading, 36 41 catalytic co-ionic membrane reactor, 39f coupling, electrocatalytic, 36 38 dehydroaromatization, electrocatalytic, 39 40 energy balance and system microintegration, 41f nonoxidative coupling of methane, 37f oxidative coupling of methane, 38f partial oxidative coupling of, 37 38 proton-conducting solid electrolyte reactor, 40 reforming, electrocatalytic, 40 41 Hydroxyl-ion conducting ionomers, 298 300 I Industry sector, 196 Intermatrix synthesis, 158 Ionic conductivity, 302 J Janus membranes, 164 K Kinetics, 287 288

357

M Main water electrolysis technology, 305 Membrane-assisted gas switching reforming, 86 87 Membrane bioreactors animal studies and clinical trials, 237 239, 237f for BAK, 236 237 as bioartificial liver, 231 235 as a biomimetic model for nervous tissue analogue, 239 243 optimal culture, 240 PAN-HF, 241f perfused devices, 242 243 crossed hollow fiber, 235f definition, 8 9 in hollow fiber configuration, 233 235 immersed MBR or iMBR, 9f livers in flat configuration, 232 233 poly(ε-caprolactone) hollow fiber, 235f reactors generations, 9 side-stream or sMBR, 9f Membrane characteristics capillary membranes, 201f cross-flow filtration mode, 198f dead-end filtration, 198f flat sheet plate membrane module, 200f flat sheet plate module, 200f hollow fiber membrane module, 202f hollow fiber membranes, 201f hollow fiber module, 202f schematic diagram of, 199f spiral-wound membrane module (SWM), 201f tubular membrane module, 202f Membrane principles diffusion mechanisms, 3f Knudsen diffusion, 5 permeation and evaporation, 5 pervaporation or vapor permeation process, 6f porous materials and their perm-selectivity, 4f reactor system, 2f with respect to size of particles, 2f Membrane reactors in bioartificial organs for bioartificial kidney, 236 239 as bioartificial liver, 231 235

358

Index

Membrane reactors (Continued) biomimetic model for nervous tissue analogue, 239 243 design issues, 228 229 transport phenomena, 229 230 bioreactors, 8 10 and catalytic reactions, 10 15 classification contactor-type, 171 176 distributor-type, 176 177, 176f exemplified for reversible reaction, 168f extractor-type, 168 171 flow-through catalytic membrane, 175 176 forced flow-through, 174 176 gas-liquid interfacial contactor, 173 hollow-fiber membrane reactor, 173f hollow-fiber polymeric MRs, 173 hydrophilic dense polymeric membranes, 170 interfacial contactor, 171 173, 171f non-selective flow-through catalytic, 174 175 tubular configuration, 172 water-gas-shift reaction, 169 membranes, 6 8 principles, 1 6 Membranes and catalytic reactions basic types of, 13t Catalytic Membrane Reactors (CMRs), 11 catalytic use, 11 12 flow-through contactor mode, 15 inorganic membrane reactor, 12f interfacial contactor mode, 15 (inorganic) membrane reactors, 12f photocatalytic, 11f representation of, 14f Methane dimerization, 36 Microstructured membrane reactors All-ceramic MCM monolith, 116f CAD sketch of the solvent flow, 110f ceramic oxygen and proton conducting, 115 117 continuous-flow microreactor, 109f good mechanical and chemical stability, 107 HMF to DFF and DMF under gas, 108f

5-hydroxymethylfurfural (HMF), 108 integration in, 106t metallic membranes, 110 113 multi-channel plate, 114f one monolithic channel, 116f polymeric, 105 110 self-supporting zeolite membrane, 115f SEM magnification of the channel crosssection, 114f ultrathin freestanding nylon membrane, 107f zeolite membranes, 113 115 ZSM-5 membrane cross-section, 114f Microstructured membrane reactors for process intensification chemical energy conversions, 96 design and fabrication, 96 104 examples of, 105 117 microstructured, 95 range of reactions, 95 96 Modeling of membrane reactors packed bed, 315 331 N Nonorganic proton conductors, 300 301 Nonsolvent-induced phase separation (NIPS), 137 138 O Oxide-ion conductors, 302 303 P Packed bed membrane reactors axial temperature profiles in, 61f bubble phase component mass balances, 327 closure equations, 328 329 developments ammonia decomposition, 66f Co-based catalyst, 67 CO2 conversion at different temperatures, 72f documents by authors, 63f documents by country/region, 64f documents per affiliation, 63f energy density, 65f ethylene price forecast, 68f high conversion rates and high purity, 65 66

Index

hydrogen production in membrane reactors, 69 OCM reaction, 69 oxidative coupling of methane (OCM), 68 packed bed membrane reactors, 70 Pd membrane layer, 66 process intensification in membrane reactors, 71f reactions systems, 64t reactor system, 69 70 stability tests data, 67f temperature dependence of CO2 conversion, 72f 1D pseudo-homogeneous model, 316 322 catalyst phase mass balance, 319 component mass balance, 317 319 constitutive equations, 315 continuity equation, 317 1D heterogeneous model, 318 energy balance, 318 friction coefficient, 317 for gas phase, 319 H2 production rate of 5 different reactor systems, 322f membrane reactor performance in adiabatic conditions, 321f nonisothermal model, 321 reactor, 321 322 for solid phase, 319 322 total momentum balance equation, 317 2D pseudo-homogeneous model, 322 325 component mass balance, 323 continuity equation, 322 energy balance, 324 325 friction coefficient, 323 total momentum balance equation, 323 emulsion phase component mass balances, 327 energy balance, 328 experiments and models, 330f hydrogen flux at different total pressures, 330f isothermal operation mode, 62f latest developments in, 62 73 membrane reactor catalyst, 60f modeling of fluidized bed, 325 331 scheme of, 60f

359

three-phases model for, 331f total mass balance, 327 transfer term, 327 328 two-phase fluidized bed reactor model, 326f Permeate side, 168 Pervaporation membrane reactors (PVMRs), 130 134 catalytic PVMR configurations, 133f catalytic PVMRs to improve yield, 134f driving force in PV, 127 esterification reaction, 131 132 fields of application, 134 142 H-USY zeolite-coated membrane, 134f organophilic, 129 permeate flux and separation factor, 129 publications trend on, 131f PV process and transport channels, 130f PV unit, 128f two-dimensional (2D) materials, 129 Pervaporation membrane reactors (PVMRs), 131 Phase-inversion, 159 Photocatalytic membrane reactors advantages and disadvantages of, 258 advantages and limitations of, 274 275 applications in the conversion of CO2 in solar fuels, 273 flow-through optical fiber OF/LED reactor design, 272f magnetic materials in, 271 photocatalytic degradation of, 270 in photodegradation of pharmaceuticals in water, 268 272 submerged setup, 270f applications of, 268 273 basic of, 257 267 basic principles of, 253 256 MB degradation, 259 modules and system configurations, 263 267 coupling of photocatalysis with nonpressure operations, 266 267 depressurized (submerged) photoreactors, 264 266 oscillatory membrane photoreactor experimental setup, 265f

360

Index

Photocatalytic membrane reactors (Continued) photocatalysis-membrane filtration process, 265f pressurized photoreactors, 263 264 SPMS reactor, 266f photocatalysis, 252 preparation of Ag@BiOBr/AC/GO membrane, 259f renewable energy sources, 258 types of, 257 262 use of visible light, 259 260 Polymeric membranes microreactors development of a microflow device, 177 the direct synthesis of H2O2, 178f hydrogenation of nitrobenzene to aniline, 178 reactors classification of, 167 177 microreactors, 177 178 polymeric membranes, 152 167 structure of, 152 167, 152f dense symmetric, 153 156 electrospinning, 162 164, 163f hollow fiber CMP, 167f hydrophobic and hydrophilic polymers, 155t ionic liquid, 164 165 ladder structure of PIM-1, 166f microporous, 165 167 mixed matrix, 156 159 phase-inversion, 159 161 porous, 159 preparation of porous, 159 164 track-etching, 161 162 Polymer membrane materials, 295 300 Polymer membranes Aquivion, 296f chemistry and microstructure, 296 H2 and O2 permeability and diffusion, 297 298 key physical property, 296 297 limitations and perspectives, 297 298 3 M ionomer, 296f Nafion, 296f proton conducting ionomers, 295 298 Pore-plated membranes, 85

Principles of photocatalysis band gaps and wavelengths, 255t electron donors, 256 heterogeneous, 253 hydroxyl radicals, 254 lamp wavelength, 256 normalized photonic efficiency, 257f photocatalytic process, 254f photogenerated electrons, 253 photo-promoted electrons, 255 redox reactions, 254 Proton conducting ionomers, 295 298 Protonic electrocatalytic membrane reactors ammonia synthesis, 22 30 CO2 reduction, 30 35 hydrocarbon dehydrogenation, 36 44 other reactions, 44 45 S Solution casting method, 160 SPE-cell, 292 T Techno-economic analysis for membrane reactors documents by authors, 342f documents by country/region, 342f flowsheet software, 345 gas switching and other technology, 347t H2 production with CO2 capture, 343f hydrogen production with CO2 capture, 344t cost, 350f membrane-assisted gas switching reforming plant, 343f OCM process, 350f reactor, 351f O&M costing calculation, 346t operating temperature and membrane property, 349f operational costing, 345 per affiliation, documents, 341f profit (NP), energy efficiency (EE), cost efficiency (CE), 352f techno-economics, 348

Index

TOC from NETL calculation, 346t Techno-economic analysis of membrane reactors feed pressure for WGS, 340f hydrogen production technology, 339f reforming efficiency profiles, 339f separation and reaction is beneficial, 338f technical comparison, 338 339 thermodynamic comparison, 338 Thermodynamics, 286 287

Thermoplastic diaphragms, 293 295 Track-etching process, 161 162 W Water electrolysis, 304 305 Working electrode, 27 Z Zero-gap cell, 292

361