Materials and processes for CO2 capture, conversion, and sequestration [First edition] 9781119231035, 1119231035, 9781119231066, 111923106X, 9781119231080, 1119231086

Table of contents :
Content: Preface xiList of Contributors xiii1 CARBON CAPTURE IN METAL-ORGANIC FRAMEWORKS 1Mehrdad Asgari and Wendy L. Queen1.1 Introduction 11.1.1 The Importance of Carbon Dioxide Capture 11.1.2 Conventional Industrial Process of Carbon Capture and Limitations: Liquid Amines 31.1.3 Metal-Organic Frameworks and Their Synthesis 41.1.4 CCS Technologies and MOF Requirements 61.1.5 Molecule Specific 101.2 Understanding the Adsorption Properties of MOFs 111.2.1 Single-Component Isotherms 111.2.2 Multicomponent Adsorption 141.2.3 Experimental Breakthrough 151.2.4 In Situ Characterization 161.3 MOFs for Post-combustion Capture 301.3.1 Necessary Framework Properties for CO2 Capture 301.3.2 Assessing MOFs for CO2/N2 Separations 321.3.3 MOFs with Open Metal Coordination Sites (OMCs) 341.3.4 MOFs Containing Lewis Basic Sites 371.3.5 Stability and Competitive Binding in the Presence of H2O 451.4 MOFs for Pre-combustion Capture 481.4.1 Advantages of Pre-combustion Capture 481.4.2 Necessary Framework Properties for CO2 Capture 491.4.3 Potential MOF Candidates for CO2/H2 Separations 501.5 MOFs for Oxy-Fuel Combustion Capture 541.5.1 Necessary Framework Properties for O2/N2 Separations 541.5.2 Biological Inspiration for O2/N2 Separations in MOFs 551.5.3 Potential MOF Candidates for O2/N2 Separations 561.6 Future Perspectives and Outlook 61Acknowledgments 63References 632 METAL-ORGANIC FRAMEWORKS MATERIALS FOR POST-COMBUSTION CO2 CAPTURE 79Anne M. Marti2.1 Introduction: The Importance of Carbon Capture and Storage Technologies 792.1.1 Post-combustion CO2 Capture Technologies 802.1.2 Metal-Organic Frameworks: Potential for Post-combustion CCS 822.2 Metal-Organic Frameworks as Sorbents 842.2.1 Criteria for Choosing the Best CO2 Sorbent 842.2.2 Discussion of Defined Sorbent Criteria 872.3 Metal-Organic Framework Membranes for CCS 992.3.1 Membrane Performance Defined 992.3.2 MOF Membrane Fabrication 1022.4 Summary 104References 1043 NEW PROGRESS OF MICROPOROUS METAL-ORGANIC FRAMEWORKS IN CO2 CAPTURE AND SEPARATION 112Zhangjing Zhang, Jin Tao, Shengchang Xiang, Banglin Chen, and Wei Zhou3.1 Introduction 1123.2 Survey of Typical MOF Adsorbents 1163.2.1 CO2 Capture and Separation at Low Pressure 1163.2.2 CO2 Capture and Separation at High Pressure 1393.2.3 Capture CO2 Directly from Air 1403.2.4 CO2/CH4 Separation 1453.2.5 CO2/C2H2 Separation 1483.2.6 Photocatalytic and Electrochemical Reduction of CO2 1493.2.7 Humidity Effect 1523.3 Zeolite Adsorbents in Comparison with MOFs 1583.4 MOFs Membrane for CCS 1633.5 Summary and Outlook 165Acknowledgments 166References 1674 IN SITU DIFFRACTION STUDIES OF SELECTED METAL-ORGANIC FRAMEWORK MATERIALS FOR GUEST CAPTURE/EXCHANGE APPLICATIONS 180Winnie Wong-Ng4.1 Introduction 1804.1.1 Background 1804.1.2 In Situ Diffraction Characterization 1814.2 Apparatus for In Situ Diffraction Studies 1824.2.1 Single-Crystal Diffraction Applications 1824.2.2 Powder Diffraction Applications 1854.3 In Situ Single-Crystal Diffraction Studies of MOFs 1864.3.1 Thermally Induced Reversible Single Crystal-to-Single Crystal Transformation 1874.3.2 Structure Transformation Induced by Presence of Guests 1884.3.3 Dynamic CO2 Adsorption Behavior 1904.3.4 Unstable Intermediate Stage During Guest Exchange 1904.3.5 Mechanism of CO2 Adsorption 1924.4 Powder Diffraction Studies of MOFs 1934.4.1 Synchrotron/Neutron Diffraction Studies 1934.4.2 Laboratory X-ray Diffraction Studies 2044.5 Conclusion 207References 2075 ELECTROCHEMICAL CO2 CAPTURE AND CONVERSION 213Peng Zhang, Jingjing Tong, and Kevin Huang5.1 Introduction 2135.2 Current Electrochemical Methods for Carbon Capture and Conversion 2145.2.1 Ambient-Temperature Approach 2155.2.2 High-Temperature Approach 2185.3 Development of High-Temperature Permeation Membranes for Electrochemical CO2 Capture and Conversion 2245.3.1 Development of MECC Membranes 2245.3.2 Development of MOCC Membranes 2355.4 Summary and Outlook 255Acknowledgments 258References 2586 ELECTROCHEMICAL VALORIZATION OF CARBON DIOXIDE IN MOLTEN SALTS 267Huayi Yin and Dihua Wang6.1 Introduction 2676.2 Thermodynamic Analysis of Molten Salt Electrolytes 2696.2.1 Thermodynamic Analysis of Alkali Metal Carbonates 2696.2.2 Thermodynamic Analysis of Alkaline-Earth Metal Carbonates 2756.2.3 Thermodynamic Viewpoint of Variables Affecting Electrolytic Products 2776.2.4 Thermodynamic Analysis of Mixed Melts 2786.3 Electrochemistry of Cathode and Anode 2826.3.1 Electrochemical Reactions at the Cathode 2826.3.2 Electrochemical Reaction Pathway of CO2 and CO3 (C or CO?) 2856.3.3 Electrochemical Reaction at the Anode 2876.4 Applications of Electrolytic Products 2896.5 Conclusion and Prospects 289Acknowledgments 292References 2927 MICROSTRUCTURAL AND STRUCTURAL CHARACTERIZATION OF MATERIALS FOR CO2 STORAGE USING MULTI-SCALE X-RAY SCATTERING METHODS 296Greeshma Gadikota and Andrew Allen7.1 Introduction 2967.2 Experimental Investigations of Subsurface CO2 Trapping Mechanisms 2987.3 Comparison of Material Measurements Techniques for Microstructure Characterization 3007.4 Usaxs/Saxs Instrumentation 3027.5 Analyses of Ultrasmall- and Small-Angle Scattering Data 3047.5.1 Determination of the Volume Fractions, Mean Volumes, and Radius of Gyration Using Guinier Approximation and Scattering Invariant 3047.5.2 Determination of the Surface Area from the Porod Scattering Regime 3057.5.3 Shapes and Size Distributions 3057.5.4 Fractal Morphologies 3067.6 USAXS/SAXS/WAXS Characterization of CO2 Interactions with Na-Montmorillonite 3077.6.1 Experimental Methods 3077.6.2 Results and Discussion 3107.7 Summary 312Acknowledgments 313References 3138 CONTRIBUTION OF DENSITY FUNCTIONAL THEORY TO MICROPOROUS MATERIALS FOR CARBON CAPTURE 319Eric Cockayne8.1 Microporous Solids 3208.2 Overview of DFT 3238.2.1 Local Density Approximation 3248.2.2 General Gradient Approximation 3258.2.3 Meta-GGAs 3258.2.4 Hybrid Methods 3258.2.5 DFT+U 3268.2.6 Van der Waals (Dispersion) Forces 3278.2.7 Accuracy of DFT 3278.3 DFT: Applications 3288.3.1 CO2 Location and Binding Energetics 3298.3.2 Bandgap 3328.3.3 Elastic Properties 3328.3.4 Phonons 3338.3.5 Thermodynamics 3358.3.6 NMR 3368.3.7 Ab Initio Molecular Dynamics 3368.3.8 CO2 Diffusion 3378.4 Conclusions and Recommendations 337References 3389 COMPUTATIONAL MODELING STUDY OF MNO2 OCTAHEDRAL MOLECULAR SIEVES FOR CARBON DIOXIDE-CAPTURE APPLICATIONS 344I. Williamson, M. Lawson, E. B. Nelson, and L. Li9.1 Introduction 3449.2 Atomic Structure Versus Magnetic Ordering 3459.3 Pore Size and Dimensionality 3469.4 CO2 Sorption Behavior 3479.4.1 Experimental Observations 3479.4.2 DFT Studies 3489.5 Comparison of Cation Dopant Types 3489.5.1 Cation Effects on CO2 Sorption in OMS-2 3499.6 OMS-5 3519.7 Summary 353References 354Index 357

Citation preview

Materials and Processes for CO2 Capture, Conversion, and Sequestration

Materials and Processes for CO2 Capture, Conversion, and Sequestration Edited by

Lan Li Boise State University

Winnie Wong-Ng National Institute of Standards and Technology

Kevin Huang University of South Carolina

Lawrence P. Cook The Catholic University of America

This edition first published 2018 © 2018 The American Ceramic Society All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions. The rights of Lan Li, Winnie Wong-Ng, Kevin Huang, and Lawrence P. Cook, to be identified as the authors of the editorial material in this work have been asserted in accordance with law. Registered Office John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA Editorial Office 111 River Street, Hoboken, NJ 07030, USA For details of our global editorial offices, customer services, and more information about Wiley products visit us at www.wiley.com. Wiley also publishes its books in a variety of electronic formats and by print-on-demand. Some content that appears in standard print versions of this book may not be available in other formats. Limit of Liability/Disclaimer of Warranty In view of ongoing research, equipment modifications, changes in governmental regulations, and the constant flow of information relating to the use of experimental reagents, equipment, and devices, the reader is urged to review and evaluate the information provided in the package insert or instructions for each chemical, piece of equipment, reagent, or device for, among other things, any changes in the instructions or indication of usage and for added warnings and precautions. While the publisher and authors have used their best efforts in preparing this work, they make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives, written sales materials or promotional statements for this work. The fact that an organization, website, or product is referred to in this work as a citation and/or potential source of further information does not mean that the publisher and authors endorse the information or services the organization, website, or product may provide or recommendations it may make. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for your situation. You should consult with a specialist where appropriate. Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read. Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. Library of Congress Cataloging-in-Publication Data Names: Li, Lan (Materials scientist), editor. | Wong-Ng, W. (Winnie), editor. | Huang, Kevin, editor. | Cook, L. P., editor. Title: Materials and processes for CO2 capture, conversion, and sequestration / edited by Lan (Samantha) Li, Winnie Wong-Ng, Kevin Huang, Lawrence P. Cook. Description: Hoboken, NJ : John Wiley & Sons, 2018. | Includes bibliographical references and index. | Identifiers: LCCN 2018003941 (print) | LCCN 2018006054 (ebook) | ISBN 9781119231066 (pdf) | ISBN 9781119231080 (epub) | ISBN 9781119231035 (cloth) Subjects: LCSH: Carbon sequestration. | Carbon dioxide mitigation. Classification: LCC TD885.5.C3 (ebook) | LCC TD885.5.C3 M38 2018 (print) | DDC 628.5/32—dc23 LC record available at https://lccn.loc.gov/2018003941 Cover image: Courtesy of Lan Li, Winnie Wong-Ng, Kevin Huang, and Lawrence P. Cook Cover design by Wiley Set in 10/12pt Times LT Std by Aptara Inc., New Delhi, India Printed in the United States of America 10 9 8 7 6 5 4 3 2 1

Contents

Prefacexi List of Contributors

1

xiii

CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS 1 Mehrdad Asgari and Wendy L. Queen 1.1 Introduction 1 1.1.1  The Importance of Carbon Dioxide Capture 1 1.1.2 Conventional Industrial Process of Carbon Capture and Limitations: Liquid Amines 3 1.1.3  Metal–Organic Frameworks and Their Synthesis 4 1.1.4  CCS Technologies and MOF Requirements 6 1.1.5  Molecule Specific 10 1.2  Understanding the Adsorption Properties of MOFs 11 1.2.1  Single-Component Isotherms 11 1.2.2  Multicomponent Adsorption 14 1.2.3  Experimental Breakthrough 15 1.2.4  In Situ Characterization 16 1.3  MOFs for Post-combustion Capture 30 1.3.1  Necessary Framework Properties for CO2 Capture 30 1.3.2  Assessing MOFs for CO2/N2 Separations 32 1.3.3  MOFs with Open Metal Coordination Sites (OMCs)  34 1.3.4  MOFs Containing Lewis Basic Sites 37 1.3.5  Stability and Competitive Binding in the Presence of H2O45 1.4  MOFs for Pre-combustion Capture 48 1.4.1  Advantages of Pre-combustion Capture 48 1.4.2  Necessary Framework Properties for CO2 Capture 49 1.4.3  Potential MOF Candidates for CO2/H2 Separations 50

v

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Contents

1.5  MOFs for Oxy-Fuel Combustion Capture 54 1.5.1  Necessary Framework Properties for O2/N2 Separations54 1.5.2  Biological Inspiration for O2/N2 Separations in MOFs 55 1.5.3  Potential MOF Candidates for O2/N2 Separations 56 1.6  Future Perspectives and Outlook 61 Acknowledgments63 References63

2

3

METAL–ORGANIC FRAMEWORKS MATERIALS FOR POST-COMBUSTION CO2 CAPTURE 79 Anne M. Marti 2.1 Introduction: The Importance of Carbon Capture and Storage Technologies79 2.1.1  Post-combustion CO2 Capture Technologies 80 2.1.2 Metal–Organic Frameworks: Potential for Post-combustion CCS 82 2.2  Metal–Organic Frameworks as Sorbents 84 2.2.1  Criteria for Choosing the Best CO2 Sorbent 84 2.2.2  Discussion of Defined Sorbent Criteria 87 2.3   Metal–Organic Framework Membranes for CCS 99 2.3.1  Membrane Performance Defined 99 2.3.2   MOF Membrane Fabrication 102 2.4 Summary 104 References104 NEW PROGRESS OF MICROPOROUS METAL–ORGANIC FRAMEWORKS IN CO2 CAPTURE AND SEPARATION 112 Zhangjing Zhang, Jin Tao, Shengchang Xiang, Banglin Chen, and Wei Zhou 3.1 Introduction 112 3.2  Survey of Typical MOF Adsorbents 116 3.2.1 CO2 Capture and Separation at Low Pressure 116 3.2.2 CO2 Capture and Separation at High Pressure 139 3.2.3  Capture CO2 Directly from Air 140 3.2.4 CO2/CH4 Separation 145 3.2.5 CO2/C2H2 Separation 148 3.2.6  Photocatalytic and Electrochemical Reduction of CO2149 3.2.7  Humidity Effect 152

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3.3  Zeolite Adsorbents in Comparison with MOFs 158 3.4  MOFs Membrane for CCS 163 3.5  Summary and Outlook 165 Acknowledgments166 References167

4

5

IN SITU DIFFRACTION STUDIES OF SELECTED METAL–ORGANIC FRAMEWORK MATERIALS FOR GUEST CAPTURE/EXCHANGE APPLICATIONS 180 Winnie Wong-Ng 4.1 Introduction 180 4.1.1  Background180 4.1.2  In Situ Diffraction Characterization 181 4.2  Apparatus for In Situ Diffraction Studies 182 4.2.1  Single-Crystal Diffraction Applications 182 4.2.2  Powder Diffraction Applications 185 4.3  In Situ Single-Crystal Diffraction Studies of MOFs 186 4.3.1 Thermally Induced Reversible Single Crystal-to-Single Crystal Transformation 187 4.3.2  Structure Transformation Induced by Presence of Guests 188 4.3.3  Dynamic CO2 Adsorption Behavior 190 4.3.4  Unstable Intermediate Stage During Guest Exchange 190 4.3.5  Mechanism of CO2 Adsorption 192 4.4  Powder Diffraction Studies of MOFs 193 4.4.1  Synchrotron/Neutron Diffraction Studies 193 4.4.2  Laboratory X-ray Diffraction Studies 204 4.5 Conclusion 207 References207 Electrochemical CO2 Capture and Conversion Peng Zhang, Jingjing Tong, and Kevin Huang 5.1 Introduction 5.2 Current Electrochemical Methods for Carbon Capture and Conversion 5.2.1  Ambient-Temperature Approach 5.2.2  High-Temperature Approach

213 213 214 215 218

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Contents

5.3  Development of High-Temperature Permeation Membranes for Electrochemical CO2 Capture and Conversion 224 5.3.1  Development of MECC Membranes 224 5.3.2  Development of MOCC Membranes 235 5.4   Summary and Outlook 255 Acknowledgments258 References258

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7

ELECTROCHEMICAL VALORIZATION OF CARBON DIOXIDE IN MOLTEN SALTS 267 Huayi Yin and Dihua Wang 6.1  Introduction267 6.2  Thermodynamic Analysis of Molten Salt Electrolytes 269 6.2.1  Thermodynamic Analysis of Alkali Metal Carbonates 269 6.2.2 Thermodynamic Analysis of Alkaline-Earth Metal Carbonates275 6.2.3 Thermodynamic Viewpoint of Variables Affecting Electrolytic Products 277 6.2.4  Thermodynamic Analysis of Mixed Melts 278 6.3  Electrochemistry of Cathode and Anode 282 6.3.1  Electrochemical Reactions at the Cathode 282 6.3.2 Electrochemical Reaction Pathway of CO2 and CO32− (C or CO?) 285 6.3.3  Electrochemical Reaction at the Anode 287 6.4  Applications of Electrolytic Products 289 6.5  Conclusion and Prospects 289 Acknowledgments292 References292 MICROSTRUCTURAL AND STRUCTURAL CHARACTERIZATION OF MATERIALS FOR CO2 STORAGE USING MULTI-SCALE X-RAY SCATTERING METHODS 296 Greeshma Gadikota and Andrew Allen 7.1 Introduction 296 7.2 Experimental Investigations of Subsurface CO2 Trapping Mechanisms298 7.3 Comparison of Material Measurements Techniques for Microstructure Characterization 300

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ix

7.4  Usaxs/Saxs Instrumentation 302 7.5  Analyses of Ultrasmall- and Small-Angle Scattering Data 304 7.5.1 Determination of the Volume Fractions, Mean Volumes, and Radius of Gyration Using Guinier Approximation and Scattering Invariant 304 7.5.2  Determination of the Surface Area from the Porod Scattering Regime 305 7.5.3  Shapes and Size Distributions 305 7.5.4  Fractal Morphologies 306 7.6 USAXS/SAXS/WAXS Characterization of CO2 Interactions with Na-Montmorillonite 307 7.6.1  Experimental Methods 307 7.6.2  Results and Discussion 310 7.7 Summary 312 Acknowledgments313 References313

8

Contribution of Density Functional Theory to Microporous Materials for Carbon Capture Eric Cockayne 8.1  Microporous Solids 8.1.1  Oxide Molecular Sieves 8.1.2  Rigid MOFs 8.1.3  Flexible MOFs 8.2  Overview of DFT 8.2.1  Local Density Approximation 8.2.2  General Gradient Approximation 8.2.3 Meta-GGAs 8.2.4  Hybrid Methods 8.2.5 DFT+U 8.2.6  Van der Waals (Dispersion) Forces 8.2.7  Accuracy of DFT 8.3  DFT: Applications 8.3.1 CO2 Location and Binding Energetics 8.3.2 Bandgap 8.3.3  Elastic Properties 8.3.4 Phonons 8.3.5 Thermodynamics

319 320 320 321 322 323 324 325 325 325 326 327 327 328 329 332 332 333 335

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8.3.6 NMR 336 8.3.7  Ab Initio Molecular Dynamics 336 8.3.8 CO2 Diffusion 337 8.4  Conclusions and Recommendations 337 References338

9

COMPUTATIONAL MODELING STUDY OF MnO2 OCTAHEDRAL MOLECULAR SIEVES FOR CARBON DIOXIDE–CAPTURE APPLICATIONS344 I. Williamson, M. Lawson, E. B. Nelson, and L. Li 9.1 Introduction 344 9.2  Atomic Structure Versus Magnetic Ordering 345 9.3  Pore Size and Dimensionality 346 9.4 CO2 Sorption Behavior 347 9.4.1  Experimental Observations 347 9.4.2  DFT Studies 348 9.5  Comparison of Cation Dopant Types 348 9.5.1  Cation Effects on CO2 Sorption in OMS-2 349 9.6 OMS-5 351 9.7 Summary 353 References354

Index357

Preface

Our present dependence on fossil fuels increases, so do emissions of greenhouse gases, notably CO2. To avoid the obvious consequences of climate change, the concentration of such greenhouse gases in the atmosphere must be mitigated. However, as populations grow and economies develop, future demands almost ensure that energy will be one of the defining issues of this century. This unique set of challenges also means that science and engineering have a unique opportunity—and a burgeoning challenge—to apply their understanding to provide sustainable energy solutions. Integrated carbon capture, and subsequent sequestration, is generally advanced as the most promising option to tackle greenhouse gases in the short to medium term, and efficient conversion of CO2 into sustainable, synthetic hydrocarbon or carbonaceous fuels is regarded as a mid- to long-term strategy. Since 2014, a symposium entitled “Materials and Processes for CO2 Capture, Conversion, and Sequestration (CCS)” has been held at the annual MS&T (Materials Science and Technology) Meeting and Exposition. The symposium has brought together experts from the different areas of CCS research to address the scientific and engineering issues involved in the CCS processes. Topics included (1) selective CO2 capture based on the principles of physical and chemical absorption/adsorption using liquid solvents, solid sorbents, and membranes; (2) new materials and structure/property relationships; (3) electrochemical capture of CO2; (4) chemical conversion of CO2 into hydrocarbons; (5) electrochemical conversion of CO2 into hydrocarbons; (6) CO2 sequestration; (7) computational modeling and modelingexperiment connection. Partly based on the symposium content, we invited the symposium speakers and other experts in the field to contribute nine chapters. The resulting book encompasses up-to-date research topics of CCS and complements existing CCS technical publications with the newest research work as well as with reviews that present new evaluation and analyses of published work. The book also addresses the key challenges involved in CCS materials design, processing, and modeling. The topics include state-of-the-art synthesis, characterization, and measurement techniques applied to CCS materials, such as metal organic framework materials, electrochemical and physical sorptions, different membranes, sorbents, and solvents. This book can serve as a source material for researchers and managers working in the field.

xi

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Preface

The success of the symposium and the publication of the book could not have been possible without the effort and support of John Wiley & Sons and other organizers of the program. Special thanks are due to the symposium speakers, authors, and John Wiley & Sons book coordinators for their contributions. Editors: Lan Li Boise State University Winnie Wong-Ng National Institute of Standards and Technology Kevin Huang University of South Carolina Lawrence P. Cook The Catholic University of America

List of Contributors

Andrew Allen  National Institute of Standards and Technology, Gaithersburg, MD Mehrdad Asgari  Laboratory of Functional Inorganic Materials, Ecole Polytechnique Federale de Lausanne, Sion, Switzerland Banglin Chen  Department of Chemistry, University of Texas at San Antonio, San Antonio, TX Eric Cockayne  Materials Measurement Science Division, Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD Lawrence P. Cook  The Catholic University of America, Washington, DC Greeshma Gadikota  Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ Kevin Huang  Department of Mechanical Engineering, University of South Carolina, Columbia, SC M. Lawson  Micron School of Materials Science and Engineering, Boise State University, Boise, ID Lan Li  Micron School of Materials Science and Engineering, Boise State University, Boise, ID, and Center for Advanced Energy Studies, Idaho Falls, ID Anne M. Marti  Oak Ridge Institute for Science and Education, Pittsburgh, PA E. B. Nelson  Micron School of Materials Science and Engineering, Boise State University, Boise, ID Wendy L. Queen  Laboratory of Functional Inorganic Materials, Ecole Polytechnique Federale de Lausanne, Sion, Switzerland Jin Tao  Fujian Provincial Key Laboratory of Polymer Materials, College of Materials Science and Engineering, Fujian Normal University, Fuzhou, Fujian, China Jingjing Tong  Department of Mechanical Engineering, University of South Carolina, Columbia, SC Dihua Wang  School of Resource and Environmental Sciences, Wuhan University, Wuhan, China I. Williamson  Micron School of Materials Science and Engineering, Boise State University, Boise, ID xiii

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List of Contributors

Winnie Wong-Ng  Materials Measurement Division, National Institute of Standards and Technology, Gaithersburg, MD Shengchang Xiang  Fujian Provincial Key Laboratory of Polymer Materials, College of Materials Science and Engineering, Fujian Normal University, Fuzhou, Fujian, China Huayi Yin  School of Resource and Environmental Sciences, Wuhan University, Wuhan, China Peng Zhang  Department of Mechanical Engineering, University of South Carolina, Columbia, SC Zhangjing Zhang  Fujian Provincial Key Laboratory of Polymer Materials, College of Materials Science and Engineering, Fujian Normal University, Fuzhou, Fujian, China Wei Zhou  NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD

1 CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS Mehrdad Asgari and Wendy L. Queen Laboratory of Functional Inorganic Materials, Ecole Polytechnique Federale de Lausanne, Sion, Switzerland

1.1  Introduction 1.1.1  The Importance of Carbon Dioxide Capture Carbon dioxide, an important chemical gas found in the atmosphere, is critical for the continuation of life on earth. This molecule is required for photosynthesis that fuels plants, which serve as the main source of food for all humans and animals and further produce oxygen that is essential for human respiration [1]. Studies have shown that a small accumulation of CO2 in the atmosphere is necessary to warm earth to a level where glaciation is inhibited, producing an environment where plant and animal life can thrive [2]. However, there is recent evidence that human activity related to energy production is generating an abundance of CO2 in the atmosphere that can no longer be balanced by earth’s natural cycles, an act that is expected to confront mankind with serious environmental problems in the future. Since CO2 is the most abundantly produced greenhouse gas (Figure 1.1) [3], it is directly

Materials and Processes for CO2 Capture, Conversion, and Sequestration, First Edition. Edited by Lan Li, Winnie Wong-Ng, Kevin Huang, and Lawrence P. Cook. © 2018 The American Ceramic Society. Published 2018 by John Wiley & Sons, Inc.

1

2

CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS

Carbon dioxide (fossil fuel and industrial processes) Carbon dioxide (forestry and other land issue) Flurinated gases Methane Nitrous oxide

11%

2% 16%

6%

65%

Figure 1.1  The contribution of different constituent in the greenhouse gas emission. Source: Victor et al. 2014 [3].

implemented in global warming. It is predicted that if the negligent release of CO2 persists, it could have detrimental effects on our environment that include melting ice caps, rising sea levels, strong changes in weather patterns, ocean acidification, ozone layer depletion, poor air quality, and desertification; all of these things could lead to the potential demise of the human, plant, and animal life, making CO2 mitigation an urgent need [4, 5]. Eighty percent of the world’s energy is currently supplied by the combustion of carbon-based fossil fuels [6], an anthropogenic activity that has led to steady increase in atmospheric CO2 levels. Since the beginning of the industrial revolution in the 1750s, atmospheric CO2 concentration has increased from 280 ppm [7] to above 400 ppm in March 2015 [8, 9]. While the best remediation method is to transition from traditional carbon-based fuels to clean energy sources, like wind and solar, energy transitions are historically slow [9]. As such, it is projected that the use of fossil fuels will continue for years to come, requiring the development of materials that can remediate the effects of CO2 through direct carbon capture and sequestration (CCS) and/or conversion of this greenhouse gas into value-added chemicals and fuels. While CO2 capture directly from air is considered to be an unfeasible task, carbon capture from large point sources, such as coal- or gas-fired power plants, could be realized. Currently, 42% of the world’s CO2 emissions come from production of electricity and heat [10] and it is anticipated that approximately 80–90% of these emissions could be eliminated with the implementation of adequate CCS technology [11]. CCS is a multi-step process that includes the capture of CO2 and its transport to sites where it is subsequently stored. While the processes of storage and transport

3

Introduction

are well-developed technologies, the actual implementation of capture process on a global scale is still constrained by the development of an adequate gas separation technology. Thus, the discovery of new materials with high separation ability is a pertinent obstacle that must be overcome.

1.1.2  Conventional Industrial Process of Carbon Capture and Limitations: Liquid Amines The most mature capture technology, which has been around since the 1930s, includes aqueous alkanolamine-based scrubbers [12]. These chemical absorbents feature an amine functionality that undergoes a nucleophilic attack on the carbon of the CO2 molecule (Figure 1.2) to form either a carbamate (in the case of primary or secondary amines) or a bicarbonate species (in the case of tertiary amines) [13]. While amine scrubbers are highly selective in the capture of CO2 relative to other components in a gas stream, operate well at low partial pressures, and can be readily included into existing infrastructure at power plants, they have several limitations that inhibit their implementation on scales large enough for post-combustion carbon capture [14]. The materials are quite corrosive to sources of containment requiring their dilution with water to concentrations ranging from 20 to 40 wt% of the amine [15]. The high heat capacities of the aqueous amine solutions combined with high

(a)

−

O C O

Low T O C +

R'

+

+ H2 N

N R

R

carbamate

Δ H 2O

NH R

O

R'

R'

−

O C

O

+

HO

(b)

+

H N

R' R + HN R

H

Hydrogen carbonate R'

O C + O

R'

N R''

−

O + R

H2O

C

O

HO

R' + H +NH

R

H

Hydrogen carbonate

Figure 1.2  Reaction scheme for carbon dioxide with a (a) primary, secondary, or (b) tertiary amine.

4

CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS

adsorption enthalpies of CO2, approaching −100 kJ mol−1, creates a large parasitic energy cost for the subsequent release of CO2. While the strength of CO2 binding can be tuned to some degree with amine substitution (1° > 2° > 3°, i.e., monoethanolamine, diethanolamine, or triethanolamine) [13], the regeneration process typically requires temperatures that range from 120°C to 150°C [16–18]. The instability of the materials at these temperatures leads to a slow decomposition and hence a decrease in the materials’ performance with subsequent absorption cycles. Given all of these problems, this technology, which has already been employed in hundreds of plants worldwide for CO2 removal from natural gas, hydrogen, and other gases, requires that approximately 30% of the energy produced from a power plant be put back into the carbon-capture process [12]. It is projected that solid adsorbent materials with lower heat capacities might cut the energy consumption assumed from the current carbon-capture technology considerably [19]. For this to be realized, much further work is required to design porous solid adsorbents that show (i) high stability in the presence of various components in the gas stream, particularly water, (ii) high selectivity and adsorption capacity, (iii) low cost, (iv) reversibility, and (v) scale ability [20]. To date, there are several classes of porous adsorbents studied for applications related to carbon capture including zeolites, activated carbons, and covalent organic frameworks; however, all of these materials suffer quite significantly from a minimal adsorption capacity and/or low selectivity [19, 21–25].

1.1.3  Metal–Organic Frameworks and Their Synthesis One materials solution to the aforementioned carbon-capture problem is a relatively new class of porous adsorbents known as metal–organic frameworks (MOFs), which are constructed by metal ions or metal-ion clusters linked together by organic ligands (Figure 1.3) [26, 27]. Since the discovery in the late 1990s that these materials can exhibit permanent porosity [28], they have rapidly moved to the forefront of materials research. Looking at publications related to carbon dioxide adsorption in MOFs, one can see a significant increase in the number since 2005, with over 500 publications in 2015 alone [29]. This is in part due to their unprecedented internal surface areas, up to 7000 m2 g−1 [30], which allows the adsorption of significant amount of guest species. Further, the molecular nature of the predefined organic linkers offers a modular approach to their design (Figure 1.3). Through judicious selection of the building blocks, MOF structures can be chemically tuned for a variety of environmentally relevant applications such as gas storage and separation, sensing, and catalysis [31–39]. MOFs have become particularly attractive due to recent reports of materials with high capacities and selectivities for the adsorption of various guest molecules [40, 41]. Currently, MOFs hold several world records related to small molecule adsorption that include (i) surface area [30], (ii and iii) room-temperature hydrogen [42] and methane storage [43], and (iv) carbon dioxide storage capacity [44]. The facile chemical tunability of MOFs is their primary advantage relative to other more traditional porous adsorbents such as activated carbons and zeolites.

5

Introduction

Prop

erties

Function

Functional group

Applicatio

n

Organic linker

Metal center

Void fraction

Number of Publications

500 400 300

CO2 H2O

N2

O2

200 100 0

2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 Year

Figure 1.3  (Left) Ball-and-stick model of an MOF, MOF-5 or Zn4O(1,4-benezendicarboxylate)3 [27], showing the modular nature of the frameworks, which can be used to tune MOF properties for the selective binding of gas molecules, making the materials of particular interest in applications related to carbon capture. Source: Li et al. 1999 [27]. Reproduced with permission of Nature Publishing Group. (Right) The number of publications related to “CO2 adsorption” and “metal–organic frameworks” increased significantly from 2005 to 2015. Source: Gallagher et al. 2016 [29]. Reproduced with permission of Royal Society of Chemistry.

Further, their highly crystalline nature combined with a non-homogenous van der Waals potential energy landscape on the internal MOF surface dictates that incoming guest molecules bind in well-defined positions and orientations; this allows diffraction techniques to be used to readily unveil their site-specific binding properties. Understanding the structure function relationship allows one to tune the properties of existing materials or rationally design new materials with specified function. MOFs are typically synthesized using a combination of metal salts and ligands via standard hydrothermal or solvothermal methods; reactions are usually carried out inside of sealed vessels or using Schlenk line techniques with reaction times that range from hours to days. The aforementioned methodologies are typically limited to small-scale reactions, from milligram to gram size yields, making them only suitable for standard laboratory-based characterization. To reduce the energy requirements associated with these traditional procedures, recent efforts have been made to search for reaction conditions necessary to produce MOFs at room temperature; however, many of these methods involve non-aqueous solvents such as DEF, DMF, and ETOH [45]. Given this, more recent efforts have been made to develop MOF syntheses in water, an effort that makes industrial production of these materials more feasible [46, 47]. Other research has abandoned the more traditional forms of laboratory-based techniques and moved toward more innovative methods to assist

6

CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS

in materials scale-up; some examples of non-traditional techniques include microwave [48], mechanochemical methods [49, 50] (such as solvent-free neat grinding or extrusion), continuous flow reactions [51, 52], and spray drying [53]. Of these techniques, the highest space time yields (STY, kg per m3 per day), a process parameter that is used to determine industrial profitability, are reported for the mechanochemical extrusion methods developed by James et al.; these methods have remarkable STY values that range between 1 and 3 orders of magnitude greater than those for other methods, a result of the absence or near absence of solvent and high reaction rates. Further, it should be noted that the reported surface areas and pore volumes for the as-prepared materials are similar to those produced on small scales [50, 51]. The latter is an important note because many reports show that surface areas and CO2 adsorption properties suffer quite significantly in the scale-up procedure [54]. While industrial scale synthesis of MOFs is currently limited to a handful of iconic frameworks, it is expected to become a developing trend as companies like BASF have shown proof of concept for the production of MOFs on large scales [55–57] using green synthetic methods (in aqueous media). These materials, targeted for applications related to on-board storage in natural gas and hydrogen powered vehicles, are currently available under the trade name Basolite® and include a few eminent frameworks such as HKUST-1 (Basolite C300 or [Cu3(1, 3,5-benzenetricarboxylate)2]), MIL-53 (Basolite A100 or [Al(OH)(1,4-benzenedicarboxylate)]), ZIF-8 (Basolite Z1200 or [Zn(2-methylimidazole)2]), and Fe-BTC (Basolite F300 or [Fe(1,3,5-benzenetricarboxylate)]), and MOF-177 (Basolite 7377 or Zn4O(1,3,5-benzenetribenzoate)2]) [58–60]. The most critical parameters that must be considered for the industrial scale-up of MOFs have been recently identified as the following: (i) the cost of raw material per kg of obtained MOF, (ii) the amount of MOF produced per m3 of reaction mixture per day, (iii) conditions required for reaction agitation during synthesis, (iv) length of time required and amount of solvent required for sample filtration, and (v) washing conditions necessary for drying (activating) prepared solids [61].

1.1.4  CCS Technologies and MOF Requirements Growing energy demands related to continued population growth and the industrialization of developing countries, like China, imposes the need for the continued combustion of fossil fuels, including coal, natural gas, and oil [62, 63]. Considering that carbon capture from air is not a feasible task, capture at large point sources is certainly one of the best-case scenarios to significantly reduce global CO2 emissions despite the tremendous effort that is required. It is projected that global reserves of coal, which has the highest carbon content and is responsible for 43% of CO2 emissions from fuel combustion [64], will last over 110 years at the current production rate [65]. For comparison, oil reserves are projected to exist for the next 40–55 years [65–67]. Currently, there are three existing chemical processes used for the combustion of fossil fuels at large point sources such as coal and gas-fired power plants. These

7

Introduction

three processes, which include (i) post-combustion capture, (ii) pre-combustion capture, and (iii) oxy-fuel combustion capture, result in the need for a collection of separation materials capable of operating at different temperatures and pressures and offer selective adsorption for several different gas mixtures (Table 1.1). The three processes are briefly described below.

(i) Post-combustion capture at a coal-fired power plant (Figure 1.4a) involves the separation of CO2 from flue gas (1 bar) that consists primarily of CO2 (13–15% by volume), N2 (73–77%), H2O (5–7%), O2 (3– 4%), and other minor contaminates like SOx and NOx. Flue gas is generated after the combustion of fuel in air [78]. The high N2 content in air lends to flue gas mixtures with low partial pressures of CO2; as such, the selectivity for CO2/N2 is one of the most critical factors considered in the selection of a separation material. As in the case of the liquid amine-based scrubbers, finding a balance between CO2 selectivity and binding affinity in MOFs

Ta b l e   1 . 1  Typical composition of gas for three carbon capture technologies Molecules and conditions

Post-combustiona [23, 62, 68–73] by volume

Pre-combustiona [23, 74, 75] by volume

Oxyfuela [76, 77] by volume

Natural gas

Coal

Nautral gas Coal

Air purificationb

CO2 N2 H2O H2 CH4 O2 H2S SO2 SO3 HCl Hg CO NOx Ne Kr Xe Ar Temperature Pressure

3–9% 70–76% 7–18% — — 2–15% — — — — — 200–300 ppm 10–300 ppm — — — — 40–75°C 1 bar

13–15% 73–77% 5–7% — — 3–4% — 800 ppm 10 ppm 100 ppm 1 ppb 20–50 ppm 500 ppm — — — — 40–75°C 1 bar

15–25% Trace — 70–80% 3–6% — Trace — — — — 1–3% — — — — — 40°C 5–40 bar

26–34% 0.3–2.2% 18–38% 35–45% — — 0.1–0.2% — — — — 0.5–0.6% — — — — 0.04% 40°C 5–40 bar

400 ppm 78% — 0.5 ppm — 21% — — — — — — 0.3 ppm 18 ppm 1 ppm 0.087 ppm 0.9% 25°C 1 atm

The values are from some references reporting typical values for these streams. Although the values for other power plants may slightly differ from each other, they will be in the same range. b This value is for the dry air. a

8

CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS

is necessary. Very high CO2 binding energies on the sorbents affords high regeneration energies significantly reducing power plant efficiency [79]. (ii) Pre-combustion capture (Figure 1.4b) involves the separation of CO2 from H2 prior to the combustion process and hence zero carbon emission afterward. In this process, coal undergoes gasification to produce a syngas that typically consists of CO, CO2, H2, and H2O. Afterward, the syngas is reacted with steam in a process called the water-gas shift reaction to form CO2 (26 –34%), H2 (35–45%) with small amounts CO, H2S, and N2. It should be noted that there is also a significant amount of water present in the flue gas stream after the water-gas shift reaction. However, much of the water could be removed using existing technologies. From this point, the separation is carried out to remove CO2 producing a nearly pure H2 fuel that is then combusted to form water. This separation is significantly easier, relative to post-combustion capture, due to the higher partial pressures and concentrations of CO2, approximately 5–40 bar and up to 34% CO2, respectively, making the consideration for the separation medium a bit more versatile to include solid adsorbents, liquid absorbents, and membranes [80, 81]. (iii) As the final alternative, rather than using air for the combustion of fossil fuels, oxy-fuel combustion (Figure 1.4c) involves a separation of O2 from air before the combustion process. Post-separation this technology involves a nearly pure feed of O2 (purity usually >95%) that is then used in the combustion step, eliminating the need for the separation of CO2 and N2 later. The problems with this separation is that it is currently limited to energetically unfavorable distillation as most adsorbents designed to date, such as lithium-containing zeolites, only show limited selectivity of N2 over O2 giving rise to gas mixtures with inadequate purity levels [82]. After combustion, the final gas mixture has CO2 (72–85%) with some amount of water (6–7%) that can easily be condensed giving rise to CO2 capture rates higher than 95%, a feat not yet achieved by pre-combustion and postcombustion capture separations. Compared with aforementioned processes utilizing N2-rich air for combustion, the formation of NOx is largely inhibited due to the initial removal of N2; this will allow for a significantly smaller NOx removal than in typical power plants. For the aforementioned carbon-capture cases, there are three potential processes for regeneration after adsorbent bed saturation including (i) temperature swing adsorption (TSA), pressure swing adsorption (PSA), and vacuum swing adsorption (VSA). TSA is a process where the temperature of the bed will be increased (likely using heat from the power plant) post saturation allowing desorption of the small molecules from the surface of the adsorbent. The resulting pressure increase drives the adsorbate out of the bed, and once no further desorption is observed at the target temperature, a purge gas can additionally be run through the bed to push out

9

Introduction

(a) Post-combustion capture system Electricity

Flue Gas to atmosphere

Steam Turbine CO2 free stream Low Pressure Steam

Natural Gas/Coal Air

High Pressure Steam

CO2 Capture System

The type of fuel can be varied

Boiler

CO2 rich stream

CO2 To Storage

(b) Pre-combustion capture system

Fuel Cell

OR

CO2+H2 mixture Toward CO2 capture

CO2 Capture System

Toward CO removal

Water Gas Shift Reactor

Syn gas mixture: CO+H2

As a feed to petrochemical units

Pure H2 stream

CO2 rich stream

CO2 To Storage

Air Separation Unit

Air

O2

Gasification Unit

If natural gas is used, there will be no need for the air separation unit. Steam reforming will be used instead of coal gasification.

Coal Water

Electricity

N2

Air and/or O2 can be used for coal gasification depending on the process design.

(c) Oxy-fuel combustion capture system Flue Gas to atmosphere

Electricity Steam Turbine

Air

Air Separation Unit

Low Pressure Steam

O2

Fuel O2

High Pressure Steam

Boiler

CO2 free stream

CO2 Capture System

N2

CO2 rich stream

CO2 To Storage

Natural Gas/Coal

Figure 1.4  Schemes for the three different carbon capture technologies including (a) post-combustion capture, (b) pre-combustion capture, and (c) oxy-fuel combustion capture.

10

CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS

additional adsorbate. Subsequently, the bed can be cooled and additional adsorption cycles can be run. On the other hand, PSA and VSA processes entail lowering the pressure from which adsorption takes place to permit removal of the surface-bound guests. For PSA, the inlet valve, where high-pressure gas is allowed to flow into the bed, is simply closed allowing the pressure inside the bed to approach atmospheric pressure. Albeit similar, VSA entails lowering the bed pressure below atmospheric. The low partial pressure of CO2 in post-combustion capture makes TSA the most plausible method for bed regeneration, as it would be energetically unfeasible to expand the bed or pull vacuum on such a large volume of gas. Considering pre-­ combustion capture involves high-pressure flue gas, it is much more feasible to employ PSA for the regeneration method. Considering the parameters for these capture technologies and their subsequent regeneration methods are not easily modified, it is necessary to design adsorbents with all of these parameters in mind. As MOFs are the most chemically tunable adsorbent available, they offer unmatched opportunity to find the necessary balance between various parameters such as binding energies and densities of adsorption sites, capacities, and selectivities, which influence the ability to achieve high working capacities and low regeneration energies. The final decision related to which adsorbent should be applied to the carbon-capture process should be taken after a detailed evaluation of the technical performance and assessment of economic feasibility. Such an evaluation is imperative for implementation of carbon-capture processes on a global scale.

1.1.5  Molecule Specific The elevated temperature at which carbon capture is carried out, combined with low boiling points (Table 1.2) of many of the small molecules in flue gas and air, makes cryogenic distillations, carried out on scales large enough for CCS, energetically unfeasible; hence, large energy savings could be realized with the use of solid adsorbents that function at much higher temperatures. When working to design adsorbent materials capable of separating gases, one must first consider the differences in the physical properties of the molecules of interest. The similarities in the kinetic diameters for most of the molecules in flue gas or air make separations dependent on size exclusion difficult; this makes thermodynamic-based separations that are dictated by the nature of the adsorptive interactions between the guest molecules and internal framework surface more feasible. For physisorptive-type interactions, the separation process relies on guest molecules having small disparities in their physical properties that include polarizability, quadrupole moment, and dipole moment. For most of the components in flue gas and air, the values for these aforementioned physical properties are listed in Table 1.2. While some important differences exist for instance between CO2 and N2, regarding the nature of their intermolecular interactions and their chemical reactivity, these differences are minimal and necessitate the careful design of carbon-capture materials that exhibit strong, molecule-specific chemical interactions on their internal surface.

11

Understanding the Adsorption Properties of MOFs

Ta b l e   1 . 2  Chemical properties of small molecules involved in carbon capture Molecules CO2 N2 H2O H2 CH4 O2 H2S SO2 HCl CO NO NO2 Ne Kr Xe Ar

Normal boiling point (K) 216.55 77.35 373.15 20.27 111.66 90.17 212.84 263.13 188.15 81.66 121.38 302.22 27.07 119.74 165.01 87.27

Kinetic diametera [62]

Quadrapole momenta [62]

Dipole momentb

Polarizabilityc [40]

3.3 3.64 2.64 2.89 3.76 3.46 3.62 4.11 3.34 3.69 3.49 — 2.82 3.66 4.05 3.54

43.0 15.2 — 6.62 0 3.9 — — 38.0 25.0 — — 0 0 0 0

0 0 18.5 0 0 0 9.78 16.3 11.1 1.1 1.59 3.16 0 0 0 0

29.1 17.4 14.5 8.04 25.9 15.8 37.8 37.2 26.3 19.5 17.0 30.2 3.96 24.8 40.4 16.4

The numbers are expressed with the following unit: 10−27 esu−1 cm−1. The numbers are expressed with the following unit: 10−19 esu−1 cm−1. c The numbers are expressed with the following unit: 10−25 cm3. a b

1.2  Understanding the Adsorption Properties of MOFs There are a variety of techniques used to assess MOFs for CO2 capture applications. These include single-component adsorption isotherms, breakthrough analysis, multicomponent adsorption and a host of in situ techniques. Several studies have shown that pairing many characterization methods, particularly adsorption, with in situ characterization can provide molecular-level insight into the adsorption process giving direct evidence of the structural components that give rise to enhanced or diminished properties [83]. There is hope that in-depth experimental efforts like these can provide the insight necessary for the eventual deliberate design of new MOF for energetically favorable carbon-capture technologies.

1.2.1  Single-Component Isotherms Nitrogen adsorption isotherms, collected at 77 K and up to 1 bar, are typically used to first assess the pore volume, pore size distribution and surface area of as-­prepared MOF materials. Subsequently, adsorption isotherms can also be used to further assess a materials performance related to carbon-capture processes. For this, the isotherms are collected using carbon dioxide (or other small molecules) as probes. These experiments are typically carried out using commercially available equipment

12

CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS

at temperatures ranging from 25°C to 40°C and from low pressures up to 50 bar. It should be noted that these measurements provide insight into a materials (i) adsorption capacity, (ii) selectivity, and (iii) enthalpy of adsorption [84]. These three metrics will be briefly discussed below. 1.2.1.1 Adsorption Capacity Adsorption capacity is expressed gravimetrically or volumetrically as the amount of adsorbed CO2 per unit volume or mass of adsorbent, respectively. While reports of gravimetric capacity are more predominate throughout the literature, it is equally important to look at the volumetric properties of materials as it dictates the required volume of the adsorbent bed and both parameters also influence the efficiency with which the materials can be regenerated. It was recently shown that MOF-177, a high surface area adsorbent (BET surface area >4500 m2 per gram of adsorbent), exhibits a volumetric capacity at room temperature and 35 bar of 320 cm3 (STP) per cm3, a value that is over nine times larger than the quantity of CO2 that can be stored in the same empty container without the MOF [85]. More often than not, high surface areas lend to high capacities in the high-pressure regime, while low-pressure adsorption measurements 10 bar) MIL53–H2O V = 1012.8Å3

s = d–1 / Å–1

Figure 1.8  (a) The illustration of structural change of hydrated MIL-53(Cr) unveiled using in situ X-ray powder diffraction carried out as a function of CO2 pressure. (b) Schematic diagram of corresponding breathing behavior of MIL-53(Cr) as a function of pressure. Source: Llewellyn et al. 2006 [140]. Reproduced with permission of John Wiley & Sons.

large pore phase (Figure 1.8b). In this structural transition, the unit cell increases by over 30% from 1012.8 Å3 for the hydrated analog to 1522.5 Å3 after CO2 loading. While the narrow pore material adsorbs minimal CO2, the structural phase transition opens up a significant amount of accessible surface area depicted as a step in the CO2 adsorption isotherm [140]. Most crystallographic reports in the MOF literature include either ­neutron or X-ray powder diffraction [141, 142], while only a few studies have used single-crystal methods [63]. Although the data obtained by single-crystal diffraction are more complete and hence can give more structural detail, the technical aspects of the experiment is more difficult. MOF single crystals often experience a decrease in the quality with sample activation. There are also inherent limitations in MOF chemistry, due to weak coordination-type bonding that renders metal–ligand interactions quite labile, which can inhibit the growth of sizeable single crystals for structural analysis. Hence, there is a bottleneck in the field related to isolating the reactions conditions necessary for single crystal growth. Further, due to the extremely small sample size, very minor leaks in gas dosing equipment can cause contamination and hence problems in the final data refinement. This is particularly a problem for MOFs with OMCs, which typically bind water relatively strong compared to CO2 and other small molecules of interest for postcombustion carbon capture. As such, in situ neutron or synchrotron X-ray powder diffraction, which requires significantly larger samples sizes (gram or milligram scale for neutrons an X-rays, respectively), is more straightforward. For powder neutron or X-ray experiments, activated samples are loaded into either a vanadium sample can or glass/ kapton capillary (Figure 1.6). After data collection, Fourier difference analysis, followed by subsequent Rietveld refinement is used to elucidate CO2 locations and

20

CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS

orientations (Figure 1.9a) [63]. Figure 1.9b shows three adsorption sites identified in Cu2(dobdc) (dobdc4− = 2,5-dioxido-1,4-benzenedicarboxylate) with site I bound at the OMC as determined by neutron powder diffraction. The plot of the diffraction data (Figure 1.9c) shows an excellent match with the final structural model [63]. (a)

(b)

I

I III

(c) 32,000

III

II

II

1,600

28,000

1,400 1,200

24,000 Intensity

1,000

20,000

800 600

16,000

400

12,000

200 0

8,000

70

80

90

100

110

120

130

140

150

4,000 0 5

15

25

35 2θ

45

55

65

Figure 1.9  (a) Fourier difference map revealing excess scattering density in Cu2(dobdc) that results from CO2 adsorption in the framework channel. (b) A ball-and-stick model of the finalized structure of Cu2(dobdc) showing three CO2 adsorption sites (determined from Rietveld analysis). (c) NPD data from Cu2(dobdc) (10 K) after dosing with 0.5 CO2 per Cu2+. The green line, crosses, and red line represent the background, experimental, and calculated diffraction patterns, respectively. The blue line represents the difference between experimental and calculated patterns. Source: Queen et al. 2014 [63]. Reproduced with permission of Royal Society of Chemistry.

Understanding the Adsorption Properties of MOFs

21

While crystallographic tools have been used in the characterization of a number of MOF systems, there are also a few limitations to be considered. The time and position averaged diffraction data can suffer from static or dynamic disorder that inhibits the elucidation of fine structural detail associated with small molecule bond distances and angles [63]. Further crystallographic problems can arise due to poor crystalline quality and improper sample handling or activation. 1.2.4.2  Spectroscopic Techniques IR, Raman, and INS  While PXRD can be used to probe the static structure of materials, IR, Raman, and INS are often used to look at local vibrations and rotations of guest species throughout MOF materials. These spectroscopic tools are highly sensitive to molecular interactions in the frameworks. As such, they can also be used to indirectly probe binding configurations, binding enthalpies, and loading levels. However, this information is usually extracted from peak shifts, broadening, or intensities and so interpretation should proceed with caution. There are many false assumptions throughout the literature that peak shifts, which are very sensitive to the coordination environment around the adsorption site, are directly correlated with the adsorption enthalpy. Last, the integrated spectral intensities are often assumed to correlate with loading level. While this can hold true in some instances, sometimes intermolecular interactions can strongly influence the peak intensities as highlighted in a recent study H2 adsorption in Mg2(dobdc), which was published by Nijem et al. [143]. They found that high H2 loadings resulted in a counterintuitive decrease in IR intensity due to a decrease in the effective charge of H2 at the OMC. There are three vibrational modes associated with CO2, the asymmetric stretching mode, v3 = 2349 cm−1 and bending v2 = 667 cm−1, mode are IR-active, while the symmetric stretching mode, v1 = 1388 cm−1, is IR-inactive [144, 145]. If there is significant polarization of the surface-bound molecules, as in the case of OMCs, the IR-active modes offer a way to probe the nature of the CO2 framework interaction. Dietzel et al. carried out one of the earliest in situ IR studies on Ni2(dobdc) framework to unveil the nature of the CO2 interaction with the Ni2+-OMC [113]. They observed several bands associated with the CO2 adsorption that were assigned to the CO2 in an end-on orientation at the OMC, an observation that agreed well with in situ diffraction experiments. The v3 mode was slightly red shifted by 8 cm−1 with respect to the gas phase, a result of the charge transfer between the lone-pair of electrons on the CO2 to the OMC. They additionally observed a combination mode of ν3 + νM–O, where νM–O is the stretching mode of the Ni2+–(O)CO2 adduct. The position of νM–O was determined to be 67 cm−1, a value that is comparable to the expected value of approximately 70 cm−1. Further, there was observation of the aforementioned bending mode. While it is expected to be doubly degenerate, a doublet was instead observed at 659 and 651 cm−1. This observation was used to support bending of the CO2 molecule at the OMCs observed in the diffraction data. It should also be noted that the CO2 angle found in the diffraction studies carried out in this work was

22

CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS

162(3)° [113], a huge deviation from the expected linear geometry. Later studies showed that this unexpected bending was due to a misinterpretation of the time and position averaged diffraction data and that the CO2 angle should not deviate greatly from 180° [63]. Since this initial work, IR and Raman have both become highly active tools for the characterization of many MOFs. Valenzano et al. used variable temperature IR data to obtain the enthalpy and entropy of adsorption for CO2 adsorbed at the OMC in Mg2(dobdc). The calculated enthalpy of adsorption was estimated to be −47 kJ mol−1, which agreed very well with previously reported values obtained from adsorption isotherms [146]. An additional in situ IR study of CO2 adsorption was carried out in the rht-type MOF known as Cu-TDPAT [147] (Cu-TDPAT = [Cu3(TDPAT)(H2O)3]·10H2O·5DMA and TDPAT = 2,4,6-tris(3,5-dicarboxylphenylamino)-1,3,5-triazine), a framework that consists of Cu paddlewheel clusters and triazine ligands (Figure 1.10a). CO2 adsorption studies show not only high carbon dioxide uptake (0.072 grams of CO2 per gram of MOF at 0.15 bar and 298 K) [148], but also high CO2 to N2 selectivity (34.2 determined via IAST). An in situ IR study carried out on the Cu-TDPAT framework, which exhibits both Lewis acidic OMCs and Lewis basic amine functionality on the triazine linker, gives evidence of two strong adsorption sites [149]. A follow up study of Lockard et al. monitored certain Raman-active vibrational frequencies associated with the metal-containing building unit, the ligand, and surface-bound CO2 (Figure 1.10b) [150]. They were able to monitor a Cu–Cu stretching mode in the paddlewheel, revealing that the Cu–Cu interaction became stronger with dehydration and with CO2 loading, implying a shorter Cu–Cu distance. Additional Raman active modes were used to show a rearrangement of the linker configuration to become more planar and hence less strained upon activation, which was verified using DFT simulations of the Raman spectra. Further, the Raman-active CO2 vibrational modes gave additional insight into the presence of two strong adsorption sites, one associated with the metal and one associated with the triazine linker. After poisoning the OMC with the addition of water, and effectively blocking that adsorption site, the linker reverts back to a non-planar configuration, enhancing the Lewis basicity of the linker. The latter, in turn, enhances the CO2 interaction with the Lewis base functionality, an observation that is consistent with theoretical work that predicts that the selectivity for CO2 over N2 will be enhanced in wet gas streams [147]. In situ DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) studies were utilized to study CO2 adsorption in mmen-CuBTTri (Cu-BTTri = H3[(Cu4Cl)3(BTTri)8(mmen)12] [151] and mmen: N,N0-dimethylethylenediamine) [152, 153]. The idea was to mimic the chemical reactivity observed in liquid amine scrubbers through the appendage of alkyl amines to the OMC on the internal MOF surface. After dosing the mmen-CuBTTri framework with CO2, there is disappearance of a peak at 3283 cm−1, which is related to the N–H stretching mode. There is also

23

Understanding the Adsorption Properties of MOFs

(a)

OMC

(b)

Raman Intensity

1 1CO2/H2O 1activated 1CO2

200

400

600

800

1000

Wavenumber

1200

1400

1600

(cm−1)

Figure 1.10  (a) Ball-and-stick model of Cu-TDPAT that shows interconnected cuboctahedral building units, consisting of Cu-paddlewheels and isophthalate ligands. The pink balls represent void space. Cu, C, O are represented by blue, grey, and red spheres, respectively. (b) Full Raman spectra of as-synthesized Cu-TDPAT (black), Cu-TDPAT treated with H2O-saturated CO2 (pink), activated Cu-TDPAT (blue), and activated Cu-TDPAT treated with pure CO2 gas (red). Source: Chen et al. 2015 [150]. Reproduced with permission of Royal Society of Chemistry.

24

CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS

CO2 asymmetric stretch NH2 stretch

298 K 308 K 323 K 338 K 353 K 373 K 393 K

Absorbance

an emergence of several other peaks that were unobserved before the introduction of CO2. These significant changes in the spectrum have been attributed to the formation of zwitterionic carbamates or carbamic acid, which are the expected species via chemisorption of CO2 to the amine functional groups. Last, recent work of Wright et al. showed that synchrotron-based IR microspectroscopy with polarized IR light could be used to determine the orientation of adsorbed CO2 molecules, the CO2 loading level, and the enthalpy of adsorption in single crystals of a small pore amine-containing MOF, Sc2(BDC-NH2)3 (BDC-NH2 = 2-amino-1,4-benzenedicarboxylate) [154]. The measurements, collected during CO2 uptake at partial pressures 0.025–0.2 bar at 298–393 K (Figure 1.11), have 100-fold higher photon flux density relative to lab sources. Further, this technique yields a large improvement in the signal-to-noise ratio so that high-quality direction-dependent polarized IR spectra can be measured for anisotropic crystals. As such, in this study, measurements could be done at extremely low CO2 coverage of ∼0.1 mmol g−1 which is equivalent to a site occupancy of 1.5%. This value is within the error of what can be measured by diffraction experiments and certainly outside the regime of what can be seen with most other in situ techniques. While it was not shown in this study, due to the high rate of CO2 diffusion at the temperatures probed, it is suggested that the technique could be used to elucidate CO2 diffusivities. This can be done by following the temporal and spatial variations over a single crystal [154].

393 K 373 K 353 K 338 K 323 K 308 K 298 K 3600 3400 3200 3000 2800 2600 2400 2200 2000

Wavenumber (cm–1)

Figure 1.11  Series of spectra taken from an isobar from a single site of a single Sc2(BDC-NH2)3 crystal. As temperature increases, the magnitude of the adsorbed CO2 asymmetric stretch (red arrow) at 2335 cm−1 wavenumbers decrease relative to the NH2 stretching modes, one of which is shown with a blue arrow. Source: Greenaway et al. 2014 [154]. Reproduced with permission of John Wiley & Sons.

Understanding the Adsorption Properties of MOFs

25

It should be noted that while Raman and IR have been used extensively due to their sensitivity to CO2 and several other small molecules of interest, INS, a technique that does not suffer from selection rules, rarely has been used to study materials for carbon-capture applications. The reason is that the incoherent scattering cross sections for C, O, and N are weak relative to those of H [155]. As such, these studies require long measurement times. Further, for INS measurements, data are first collected for the activated framework. This is followed by data collection for the framework loaded with gas, and the spectrum of the bare framework is then subtracted from the spectrum obtained for the CO2 loaded sample. Given the data subtraction, when possible, frameworks should be deuterated in order to lower the contribution of the framework H atoms in the background spectrum. Further, in cases where CO2 binds very strongly and causes significant changes in the unit cell parameters, this can shift the framework phonon frequencies. These two problems were highlighted in an INS study of CO2 adsorption in Mg2(dobdc). Despite framework deuteration, Queen et al. could still see no convincing indication of vibrational modes associated with framework adsorbed CO2. The difference spectrum instead revealed negative and positive intensities, indicating significant shifts in the framework modes upon CO2 adsorption. This was further verified by neutron powder diffraction, which showed a significant reduction in unit cell volume from the bare framework to the CO2 dosed one. Both softening and hardening of the vibrational modes are observed, likely due to the inverse effect CO2 adsorption has on the lengths of the a/b and c-axes, as supported by the diffraction data [63]. NMR  In situ NMR, a powerful technique that can be used to complement diffraction studies, locally probes the nuclear spins present in the sample, and hence the local environment around each atom. This technique, which does not require any long-range order, can provide binding configurations, information on adsorption-­ induced framework flexibility, as well as the dynamic properties of imbibed guest molecules throughout the frameworks (in the millisecond regime). While the number of studies is growing, in situ NMR experiments reported to date for the elucidation of host–guest interactions are few in number. To the best of our knowledge, in situ studies have been carried out with the following adsorbates including Xe [156], CO2 [118, 157], H2 [158–160], H2O [158, 161], NH3 [162], and hydrocarbons [163, 164]. It is thought that the lack in utilization of in situ NMR is due to the lack of accessible instruments, difficulty in data interpretation, and probe restrictions. It has been shown that CO2 adsorption in frameworks can result in anisotropic or restricted motions of CO2. As such, the 13C NMR signals exhibit characteristic line shapes that in turn allow determination of information pertaining to the motion of the adsorbed molecules [165], such as rotational axes of surface-bound CO2. These characteristic line shapes were observed in in situ 13C NMR spectra of CO2 adsorbed in Ni2(2,6-ndc)2(dabco) or DUT-8 (2,6-ndc = 2,6-naphthalenedicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane) [166] (Figure 1.12), an should be a MOF that exhibits a gate opening effect at a characteristic pressure of 5 bar. Below this pressure, the only signal observed in the NMR is from unbound CO2 at 335 ppm; however,

26

CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS

Free CO2

Adsorbed CO2

200

150

100

50 [ppm]

Figure 1.12  C NMR spectrum of DUT-8(Ni) pressurized with 9.5 bar carbon dioxide meas13

ured at 237 K. Note that the initially closed structure opens during the adsorption experiment at the temperature-dependent gate-opening pressure (∼5 bar at 237 K) whereas it remains open during desorption down to 1 bar (hysteresis). (Blue: experimental spectrum, green: simulated gas phase signal, magenta: simulated signal of adsorbed CO2; red: Sum of the simulated signals.). Source: Hoffmann et al. 2012 [167]. http://www.mdpi.com/19961944/5/12/2537/htm. Licensed under CC BY 4.0.

at higher pressures, the framework pops open and a broad signal is observed with a characteristic shape due to chemical shift anisotropy, Δav of 52 ppm. From these data, the tilt angle, θ, defined as the angle between the symmetry axis and the rotational axis, can be calculated using the following equation:

 3 cos2 θ − 1  ∆av = ∆   (1.5) 2  

The tilt angle determined for DUT-8(Ni) was calculated to be 49(2)° [167]. Following this study, a similar effort was made by Kong et al. to elucidate the CO2 adsorption behavior in Mg2(dobdc) [118]. This study is briefly described in the next section; however, an image showing the determined θ value for CO2 in Mg2(dobdc) is shown in Figure 1.13. QENS  Quasielastic neturon scattering (QENS), often combined with molecular dynamics simulations, has on several occasions been used to understand diffusive properties of small molecules in MOFs. One thing to note for QENS is that the incoherent scattering cross section is minimal for many elements other than H and so in the case of CO2, collective motions of molecules are measured rather than

Understanding the Adsorption Properties of MOFs

27

CO2 Hopping

θ

Figure 1.13  Ball-and-stick model of Mg2(dobdc) with CO2 adsorbed at the open metal site, as determined by NPD. The arrows represent the CO2-hopping mechanism proposed by molecular simulations used to interpret NMR data. The higher-magnification view represents the fixed rotation angle, θ. The green, red, and grey spheres represent Mg, O, and C, respectively. Source: Lee et al. 2015 [83]. Reproduced with permission of John Wiley & Sons.

diffusivities of individual molecules that can be measured for some small molecules like H2 and CH4, for instance [125, 168, 169]. Most QENS studies are used to extract three parameters, the self-diffusivity (Ds), which describes the diffusion of individual molecules,  the transport diffusivity (Dt), which describes the mass transport that is induced in the presence of a gradient, and the corrected diffusivity (Do) which is related to the transport diffusivity via a thermodynamic correction factor that is calculated from the experimental adsorption isotherm. When looking at many industrial applications regarding separations, particularly membrane separations, where there might be mass transport in non-equilibrium environments, the transport diffusivity becomes very important and so it is necessary to assess these terms separately. Because the scattering from CO2 is coherent in nature, the transport diffusivity is probed. Compared to NMR, QENS measures fast diffusive motions on time scales ranging from 1010 to 1013 sec−1. Further, the distances over which the diffusive properties are observed are well defined and variable: typically, for QENS, the distances range from 2 to 100 A, while in NMR the time scales and distances are longer compared to QENS. Time scale for NMR is normally in the millisecond regime and the distances are normally in the microns range. One of the first QENS studies of CO2 adsorption in an MOF was carried out by Maurin et al. in MIL-47(V) or VO(1,4-BDC) (1,4-BDC = 1,4-benzenedicarboxylate) [170] to extract the aforementioned three diffusivity parameters over a wide range of loadings. They found that while Ds and Do decrease as functions of loading, Dt shows a non-monotonous response that is rationalized to be the result of intermolecular CO2 interactions that deviate from the behavior expected for an ideal gas.

28

CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS

They show that through coupling QENS with molecular dynamics simulations (QENS-MD), transport diffusivity in MOFs can be directly probed, and hence can be applied to understand the dynamics associated with industrially relevant gas mixtures [170]. Since this time, the diffusivity of H2 and CO2 in the small pore Zr-based MOF MIL-140A(Zr) has been evaluated using the QENS-MD technique to determine the self-diffusivities of H2, and the corrected and transport diffusivities of CO2 as single components and also in a binary mixture. Of these two guests, H2, with its smaller kinetic diameter, was shown to diffuse through the narrow triangular channel of MIL140A(Zr) at a significantly faster rate than CO2 at the same temperature. In the binary mixture, H2 is still faster, but it shows a slightly slower self-diffusivity, while CO2 gets a bit faster. Despite these slight changes, there is still a significant difference in the observed diffusivities in the binary mixture, suggesting that this material might be a good candidate for kinetic-based separations of CO2 and H2 [171, 172]. Computational Aid for Data Interpretation  While spectroscopic tools are generally accepted as a means to assess small molecule interactions in MOFs, because of the aforementioned reasons, data interpretation should proceed with caution. When relevant, it is helpful to utilize theoretical tools capable of incorporating vdW interactions to reduce error in data interpretation. There are several examples throughout the literature where computational tools have proven essential to interpret spectra. This was highlighted by the work of Lin et al. [119] who used molecular simulations to reproduce chemical shift anisotropy (CSA) powder patterns of 13C NMR, work that was proven to be essential for the interpretation of diffusive motions of CO2 in Mg2(dobdc). In this study, the NMR measurements of CO2-adsorbed Mg2(dobdc) [118] revealed a distinct CSA powder pattern, which at the time was interpreted to be the result of a uniaxial rotation with a fixed rotation angle θ that ranged from 56° to 69° (at temperatures from 200 to 400 K). However, a more recent study used molecular simulations to probe the free-energy landscape of CO2 in Mg2(dobdc) under conditions similar to those used in the NMR study. The Monte Carlo simulations indicated that NMR signature was instead the result of a molecular-hopping motion between metals within the crystallographic ab plane (Figure 1.14). This study implies that the dynamics of CO2 within Mg2(dobdc) were more complex than originally expected [119]. X-ray absorption spectroscopy (XAS), an element-specific technique used to probe changes in the electronic environments with the adsorption or desorption of guest species, is highly sensitive yet difficult to interpret without the help of theory. Near edge X-ray absorption fine structure (NEXAFS) was used to probe the Mg K-edge first in the activated Mg2(dobdc), and then again with the DMF and CO2 bound to the open Mg2+ site. Spectra were simulated using DFT and then compared with the experimental spectra. This theoretical analysis proved necessary to understand the variations in the local electronic environment around the OMC from the activated MOF to the one with surface-bound molecules [120]. Using in situ XAS (X-ray absorption spectroscopy), a similar study was carried out to study CO2 adsorption

Understanding the Adsorption Properties of MOFs

29

CO2 H-bonding

SO2

H2O ∆E1

∆E2

13 20 Exchange barrier (kJ mol−1)

Figure 1.14  Ball-and-stick model of the OMC in M2(dobdc) with CO2 being displaced by either H2O or SO2 and their energy barriers to activation. M2+, O, C, H, and S are represented by orange, red, great, white, and yellow spheres, respectively. Source: Tan et al. 2015 [173]. Reproduced with permission of American Chemical Society.

around the OMC in Cu-TDPAT [150]. The constant edge position at 8990 eV has confirmed the oxidation state of the Cu2+ throughout the adsorption and desorption process. In addition, the activated framework was exposed to both water and carbon dioxide, and due to the fact that the edge feature intensity for the CO2 dosed framework was between that of H2O loaded and activated one, it has been concluded that the interaction of the OMC with the CO2 molecule is not as strong as that of water, giving a strong indication that in a competitive environment, water would preferentially bind over CO2. More recently, Chabal et al. used in situ IR combined with ab initio simulations to investigate competitive binding of small molecules in M2(dobdc) where M = Mg, Co, and Ni [173]. They investigated the displacement of CO2 with several molecules including H2O, NH3, SO2, NO, NO2, N2, O2, and CH4. They found, despite the higher binding energy of SO2, NO2, and NO (∼70–90 kJ mol−1), that H2O and NH3 (∼60–80 kJ mol−1) are the only molecules able to sufficiently displace the CO2 (38–48 kJ mol−1 for the three metals). They used DFT simulations to evaluate the energy barrier associated with CO2 displacement by H2O and SO2. They found the energy barrier to be approximately 13 and 20 kJ mol−1, for H2O and SO2, respectively. The calculations revealed that, instead of differences in binding energies, the kinetic barrier for this exchange is dictated by the interaction of the second guest molecule (in this case H2O or SO2) with the MOF ligands. Hydrogen bonding between the H2O and oxygen on the organic linker facilitate the positioning of the oxygen atom of the water molecule toward the metal center (Figure 1.14). These interactions reduce the exchange barrier for the CO2 displacement by H2O. In contrast, the SO2 instead interacts with the benzene ring that is more distant from the metal center, an occurrence that hinders the exchange process. To the best of our knowledge, this is the first in situ work that provides insight into competitive co-adsorption [173].

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CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS

Due to the aforementioned problems with negligible incoherent scattering cross sections of many of the elements of interest for CO2 capture, many INS studies actually pair DFT-based calculations to aid in understanding of the host– guest interactions. For instance, Al2(OH)2(bptc) (where bptc = biphenyl-3, 3′, 5,5′-­tetracarboxylate alternatively known as NOTT-300), shows relatively high CO2 uptake with a value of 2.64 mmol per gram of MOF at 0.15 bar and 298 K and minimal uptake of CH4, N2, Ar, O2, or H2 [174]. INS, paired with DFT-derived simulations of the INS spectrum, was used as the primary method for characterization of the surface-bound CO2, a process that was then validated via powder X-ray diffraction. The sample was loaded with 1.0 CO2 per formula unit, and the INS data revealed an increase in the intensity of two peaks located at 30 meV and 125 meV. Further, peaks above 100 meV were shifted to higher energy indicative of a hardening of the framework modes with CO2 adsorption (Figure 1.15a). The DFTderived INS spectrum showed very good agreement with the experimental one and indicated that the preferential CO2 adsorption site was located in an end-on orientation approximately 2.33 Å from the framework hydroxyl group. This implies that moderate H-bonding interactions were responsible for the observed CO2 adsorption properties (Figures 1.15b and 1.15c). The low energy peak in the INS spectrum was assigned to a wagging mode of the –OH that was induced by the presence of CO2, while the higher energy peak was assigned to the wagging mode of four aromatic C–H bonds that are found adjacent to the surface-bound CO2. This work shows that pairing DFT and INS made it possible to visualize the binding mechanism of CO2 and gain necessary insight into the high CO2 absorption capacities of the framework at low pressures. Additionally, the low H2 uptake for this material was rationalized using the same methodology [174].

1.3  MOFs for Post-combustion Capture 1.3.1  Necessary Framework Properties for CO2 Capture Maximizing the framework adsorption capacity (both volumetric and gravimetric) and selectivity of CO2 at low pressures and in the presence of other components in the flue gas stream is of principal importance to post-combustion carbon capture. As shown in Table 1.1, this process is carried out at a pressure of ≈1 bar with CO2 volumes that range from 3% to 15% depending on the fuel source and at temperatures between 40°C and 75°C. As such, absorbents used for post-combustion capture must show high CO2 adsorption capacity at partial pressures less than 0.15 bar, a property that can be positively influenced by designing frameworks with high densities of strong adsorption sites. Other properties required for post-combustion capture include the ease of regeneration (likely to occur between 100°C and 200°C in a TSA

31

MOFs for Post-combustion Capture

Neutron energy loss (cm–1) 0

200

400

600

800

1,000 1,200 1,400 1,600

S(Q,ω) (a.u.)

Bare NOTT-300 NOTT-300·CO2 2.298 Å

S(Q,ω) (a.u.)

Simulated bare NOTT-300 Simulated NOTT-300·CO2

0

25

50

75 100 125 150 Neutron energy loss (meV)

3.920 Å

175

200

2.335 Å

3.029 Å 3.190 Å

II I

Figure 1.15  (a) Experimental and simulated INS spectra for the CO2-loaded Al(OH)2(bptc). The yellow bars highlight the position of peak I and II. (b) Wire view of the Al(OH)2(bptc) obtained using PXRD analysis. The two identified CO2 adsorption sites in the pore channel are represented by a ball-and-stick model. Site I (with the grey carbon) was determined by INS/DFT and verified via powder x-ray diffraction, and site II (with a blue carbon) was determined by powder x-ray diffraction. The dipole interaction between neighboring CO2(I,II) molecules is highlighted in orange. (c) Detailed view of the interactions between MOF –OH and –CH groups with CO2 molecules in a pocket-like cavity (determined by DFT simulation). The modest hydrogen bond between O(δ−) of CO2 and H(δ+) from the Al–OH moiety is highlighted in cyan. The weak cooperative hydrogen-bond interactions between O(δ−) of CO2 and H(δ+) from –CH are highlighted in purple. Each O(δ−) center therefore interacts with five different H(δ+) centers. Framework aluminium, carbon, oxygen, and hydrogen are represented by green, grey, red, and white, respectively. Source: Yang et al. 2012 [174]. Reproduced with permission of Nature Publishing Group.

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CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS

process), rapid diffusion of gases through the adsorbent, and long-term stability of materials under application-relevant conditions. While it is considered difficult to design materials to meet all of these criteria, the facile structural tunability of MOFs offer an unprecedented opportunity to target new materials with tunable interactions for the energy-efficient capture of CO2. Due to the similarities in the kinetic diameter of the two most abundant molecules in the flue gas stream, CO2 and N2 (3.3 and 3.6 Å, respectively) size exclusion is difficult. As such, most of the focus is on carrying out this separation via physisoprtion or chemisorption methods. Knowledge of the fundamental differences in the physical properties and reactivity of CO2 and N2 has aided the design of multiple frameworks that exhibit strong CO2 adsorption in the low-pressure regime that is of interest for post-combustion carbon capture [175]. For example, Table 1.2 shows that CO2 has higher polarizability (CO2 = 29.1 × 10−25 cm3; N2 = 17.4 × 10−25 cm3) and quadrupole moment (CO2 = 43.0 × 10−27 esu−1 cm−1; N2 = 15.2 × 10−27 esu−1 cm−1) compared to N2. As such, the introduction of structural components that exhibit high charge densities on the framework surface can be used to polarize incoming CO2 molecules and hence manipulate both the selectivity and the adsorption capacity at low pressures [176]. As an alternative, CO2 is also known to be susceptible to attack by nucleophiles as demonstrated with the aforementioned liquid amine-based scrubbers. Chemisorptive interactions, achieved via Lewis base functionality appended on the internal MOF surface, are also becoming a common trend in MOF chemistry [177–180].

1.3.2  Assessing MOFs for CO2/N2 Separations The most common method to assess applicability of MOFs in post-combustion flue gas capture is through the assessment of CO2 and N2 single-component isotherms collected between 293 and 313 K at pressures up to 1 bar [23]. From extraction of isosteric heats obtained from variable temperature adsorption data and analysis of the CO2 uptake in the low-pressure regime, one can gain immediate insight into the materials affinity for CO2, its potential regarding adsorption capacity, and the potential ease of regeneration. It should be noted that, compared to many reports in the literature, flue gas will be introduced to the adsorbent at temperatures ranging from 50°C to 75°C and then released at temperatures that range from 100°C to 200°C. As such, there is a clear need in the literature for materials assessment at significantly higher temperatures. This fact was highlighted in a recent report by Mason et al. which show assessment of CO2 and N2 adsorption in two MOF frameworks, Mg2(dobdc) and MOF-177, at temperatures ranging from 20°C to 200°C [62]. With this work, they report a methodology to assess the performance of materials in a likely scenario using temperature swing adsorption. Given the partial pressures of CO2 and N2 in the flue gas of a coal-fired power plant, MOF selectivities reported in the literature for CO2 over N2 are usually calculated using the molar ratio of the CO2 uptake at pressures of 0.15 bar over the

33

MOFs for Post-combustion Capture

Adsorbed CO2 (g/g) at 0.15 bar

N2 uptake at 0.75 bar. It should be considered that the significantly larger quadrupole moment of CO2 over N2 will lead to an overestimated N2 adsorption in MOFs that contain highly polarizing adsorption sites because CO2 will preferentially bind to strong adsorption sites and hence reduce the actual amount of adsorbed N2 in a binary mixture. While this molar ratio method is the simplest way to calculate selectivity, it has been shown in a recent report by Mason et al. that the obtained values cannot be taken too literally. In assessment of CO2 adsorption isotherms of Mg2(dobdc) between 40°C and 60°C, they show an unrealistic increase in selectivity for CO2 over N2 demonstrating the importance of using IAST [181] to calculate the selectivities for MOFs containing strong adsorption sites [111]. One very important metric for assessing a material for post-combustion carbon-capture applications is the isosteric heat of adsorption, −Qst. The −Qst at low coverage is indicative of the strength of binding of the strongest adsorption site and strongly influences the low-pressure adsorption capacity and selectivity of MOFs and further influences their ability to undergo regeneration [182, 183]. Figure 1.16 shows the CO2 adsorption capacity and isosteric heats for a number of frameworks in the low-pressure regime of interest for post-combustion capture [63, 96, 111, 114, 152, 184–191]. It should be noted that because the density of adsorption sites is not taken into account in this image, the strength of

0.3 43.5

OMCs

0.25

Hydroxyl

44

0.2 0.15 0.1 0.05

Alkyl amine

120

No functional group

34.1 71

Heterocycle and/or aromatic amine 96 98 26.8 113 45 21

35

44

26.8 27

17

19

14

M g2 M (do g2 b (d dc ) o N bpd PE i2 c I-C (do ) r-M bdc m IL ) m en Fe2 -10 -M (d 1 g o m 2(d bdc m ob ) e C r-M n-C pdc u IL - ) -1 BT 01 Tr i Zn -DE 2( TA d C obd D -M c) Bi O oF M -2 O FC u- 11 B H TT KU ri C r-M ST1 C IL-1 u2 0 (d 1 ob dc ) ZI IR F-8 M O IR F-1 M O M FO 3 F17 7

0

38.6

Figure 1.16  CO2 adsorption at 0.15 bar, the pressure relevant for post-combustion capture. It should be noted that all for the data presented were obtained from isotherms collected at 298 K with the exception of HKUST-1, Cr-MIL-101, and Cr-MIL-101-DETA, which were collected at 293 and 296 K, respectively. The values in red represent the low-coverage experimental isosteric heats of adsorption, −Qst [63, 96, 111, 114, 152, 184–191].

34

CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS

the adsorption site does not correlate very well with the low-pressure capacity. However, for the high surface area materials with no polarizing surface functionality, such as MOF-177, the adsorption capacity is only 6 mg of CO2 per gram of adsorbent and the isosteric heat is 14 kJ mol−1, while Mg2(dobdc), a material with much stronger adsorption sites due to the presence of exposed metal cations (43.5 kJ mol−1) and a modest BET surface area of only 1200 m2 g−1 of adsorbent, takes up 290 mg CO2 per gram of adsorbent at 0.15 bar and 298 K. This shows that surface area has minimal impact on the low-pressure adsorption properties. Further, it can be seen in Figure 1.16 that the strongest binding sites for CO2 (hence the highest −Qst values) are related to frameworks with alkyl amine-based functionality or highly polarizing open-metal coordination sites (OMCs). These two structural components have been used quite extensively throughout MOF chemistry to enhance the overall low-pressure CO2 adsorption properties.

1.3.3  MOFs with Open Metal Coordination Sites (OMCs) In most reported MOFs, weak van der Waals (vdW) forces are the dominant interactions between the framework and incoming guests; however, recent work has shown that an effective strategy to increase the surface packing density of adsorbates is through the generation of MOFs that contain high concentrations of open metal coordination sites (OMCs) [192]. The incorporation of highly reactive, electron deficient OMCs into frameworks can also enhance both binding energy and selectivity and permit charge transfer between the framework and surface-bound guest species [82, 128], a property of much interest for the conversion of small molecules into other useable products. While many of the frameworks with OMCs have been discovered serendipitously [26], there are emerging methodologies for their controlled introduction including the incorporation of metalloligands [193] and using synthetic protocols where metal clusters containing OMCs are used ab initio [98]. In all of these cases, the frameworks form with solvent remaining in the channels and in the metal coordination sphere. Post-synthetic treatment of the materials with a combination of heat and vacuum, a process called activation, can liberate the solvent molecules from the framework, and if the MOF porosity is maintained, the newly generated OMCs are available for guest inclusion. Figure 1.16 shows the low-pressure adsorption capacity for several MOFs at 0.15 bar and 298 K. Those highlighted in blue are the adsorption capacities for MOFs with OMCs while those highlighted in purple are frameworks dominated by weak vdW-type interactions [98]. It can easily be seen from this plot that the electron deficient OMCs, which strongly polarize incoming CO2, lend to significantly higher adsorption at low pressures of interest for post-combustion carbon capture. It should also be noted that while OMCs provide strong interactions allowing CO2 adsorption

MOFs for Post-combustion Capture

35

at higher temperatures and lower pressures than typically used for energy-consuming cryogenic distillation processes, the adsorbate–adsorbent interactions are often weak relative to the formation of chemical bonds, providing facile release of the molecules during the regeneration step of a separation process [98]. One of the most well-studied OMC-containing MOFs to date is M2(dobdc), alternatively known as M-MOF-74, CPO-27-M, or M2(dhtp) where M = Mg, Mn, Fe, Co, Ni, Cu, or Zn and dobdc4− = 2,5-dioxido-1,4-benzenedicarboxylate (Figure 1.17a) [99, 105, 112, 113, 135, 194–196]. The framework consists of one-dimensional honeycomb like channels that are constructed by metal oxide chains interlinked by dobdc4− ligands. The significance of this framework is related to the interesting adsorption properties that derive from the existence of unique structural features. For instance, upon solvent removal, M2(dobdc) offers one of the highest densities of OMCs of any framework discovered to date. It also undergoes chemical substitution with a wide range of first-row transition metals. The extent of the metal substitution is only rivaled by a few MOF families such as M-BTT (BTT3− = 1,3,5-benzenetristetrazolate), where M = Cr, Mn, Fe, Co, Ni, or Cu [133, 197–200], and M3(btc)2(btc 3− = 1,3,5-benzenetricarboxylate), where M = Cr, Fe, Ni, Cu, Zn, Mo, or Ru [201–206]. The high concentration of electron deficient OMCs in Mg2(dobdc) leads to an unprecedented adsorption at low pressures of ≈0.29 g CO2 per gram MOF (0.15 bar and 298 K) and an isosteric heat of −43.5 kJ mol−1 [63]. The low-pressure adsorption is still the highest reported to date. Multiple in situ techniques including NMR, IR, EXAFs, and in situ diffraction have shown that the high-initial isosteric heat is directly related to the presence of OMCs [63, 118, 120, 143, 146]. Rietveld analysis of neutron powder diffraction data followed by subsequent Fourier difference analysis has revealed CO2 bound to Mg2+ in an end-on orientation that is angled with respect to the framework surface, a direct result of secondary vdW interactions between the CO2 and ligand atoms (Figure 1.13). The high isosteric heat is reflected in the ability of the CO2 molecule to get close to the framework with short Mg-CO2 distances approaching ≈2.3 Å [207]. Britt et al. also showed a facile release of CO2 in Mg2(dobdc) at a moderate temperature of 80°C [98]. Later, Mason et al. calculated the working capacity for the material to be approximately 17.6 wt% with a temperature swing process from 40°C (at 0.15 bar CO2) to 200°C [98]. This value is much higher than MOF-177, a material without strong adsorption sites, which has a negative working capacity at all evaluated temperatures. It has recently been demonstrated that chemical substitution of the framework metals offers tunability with regard to the CO2 adsorption properties, Figure 1.17b. The low-coverage isosteric heats show the following trend: (Cu < Zn < Mn < Fe < Co < Ni < Mg) for the M2(dobdc) series, one that does not correlate with the expected ionic radii or Irving Williams series [63]. A first principles study of Yu et al. show that the trend is instead dictated by the effective nuclear charge

OMC

0.0

0.2

0.4

0.6

0.8

1.0

(b) 1.2

0.0

0.2

0.4

0.6 P (bar)

0.8

1.0

Mg Ni Fe Co Zn Mn Cu

Figure 1.17  (a) Ball-and-stick model of M2(dobdc) showing open metal coordination sites (OMCs). Orange, red, and grey spheres represent metals, oxygen, and carbon, respectively. (b) Excess CO2 adsorption isotherms collected for the M2(dobdc) series at 298 K showing variability in the low-pressure adsorption properties with metal substitution. The red line, located at 0.15 bar, represents a pressure relevant to post-combustion flue gas capture in a coal-fired power plant. Source: Queen et al. 2014 [63]. Reproduced with permission of Royal Society of Chemistry.

(a)

CO2 adsorbed (mol/mol M2+)

MOFs for Post-combustion Capture

37

seen by the CO2 as it approaches the OMC [208]. Much additional theoretical work has been focused on predicting the structures and properties of this family of compounds. The wide range of chemical substitution and vast amount of experimental data provide a platform to test the efficacy and accuracy of developing computational methods in slightly varying chemical environments [83]. On the experimental side, in addition to metal substitution, recent efforts have been made to elongate the framework ligands to tune the pore size. Deng et al. have constructed an isoreticular series of MOFs that contain expanded versions of the linker with as many as 11 additional benzene rings giving rise to pore diameters as large as 98 Å [44]. Another iconic framework that has been the subject of a large number of CO2 adsorption studies throughout the literature is Cu3(BTC)2 (alternatively known as HKUST-1 or Cu-BTC), which has a cubic structure that consists of copper-containing paddlewheels, Cu2(COO)4, interlinked by BTC3− ligands. This material is shown to have an isosteric heat of −35.0 kJ mol−1 with a modest CO2 uptake of 4.56 wt% at 0.15 bar and 25°C [209]. This value is significantly lower than Mg2(dobdc) due to a lower density of OMCs and the smaller charge density of the Cu2+ cation. This work was followed by Wade et al. performed CO2 adsorption studies on the M3(BTC)2 analogs that are capable of maintaining their permanent porosity post-activation [206]. They found the following trend for isosteric heats of adsorption: Ni > Ru > Cu > Mo ≈ Cr, which range from −36.8 to −26.7 kJ mol−1. However, it should be noted that the OMC on the Ni analog is incapable of undergoing complete activation and instead is blocked by coordinated guest molecules including H2O and MeNH2. As such, it is thought that Coulombic interactions occurring between the surface-bound CO2 and guest molecules could be the culprit for the high isosteric heat [206]. This is further supported by a recent study of Snurr et al. that shows that slightly hydrated variations of Cu3(BTC)2 show improvements in the low pressure CO2 adsorption capacity and zero-coverage isosteric heats of adsorption when compared to the completely activated framework [123]. Framework families like the aforementioned ones that undergo a broad range of chemical substitution provide a mechanism for tuning the adsorption properties (Figure 1.17b) while retaining the same structural motif. These types of in-depth studies offer an unprecedented opportunity to gain insight into the structure-derived function of MOFs, knowledge that is necessary to understand how to design new candidates with optimized properties for post-combustion capture applications.

1.3.4  MOFs Containing Lewis Basic Sites Much work has been focused on functionalizing the surface of MOFs with a variety of functional groups that, unlike the Lewis acidic OMCs, can additionally function as electron-donating Lewis bases. This work has primarily been focused on (i) the generation of framework ligands with strongly polarizing functional groups and/or

38

CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS

heterocycles and the (ii) appendage or infusion of MOFs with Lewis base containing substituents that lend to chemisorptive interactions. The nature of the framework –CO2 interaction, whether it be physi- or chemisorption, respectively, is dictated by the type of the functional group. (i) Strongly Polarizing Ligands for Physisorption. The quadrupole moment of CO2 can become polarized by the existence of dipole moments from a variety of framework functionality. This dipole moment can be generated for instance by the introduction of heterocycles in the organic ligand or through direct functionalization of the ligand with a variety of chemical groups including amine, hydroxyl, thio, cyano, and halides. The idea is that the stronger polarizing the functionality, the stronger the interaction with CO2. This effort is highlighted in the zeolitic imidazolate frameworks (ZIFs), a class of MOFs that have zeolite-based topologies; ZIFs are generally described by a formula, M(Im)2, that is similar to their zeolite counterparts Al(SiO2) where the Si nodes and oxygen bridges are replaced by metal cations and imidazolate ligands, respectively. Due to their high porosity and chemical and thermal stability, they have been investigated for carbon-capture applications. A variety of ZIF topologies with varying functionalities on the imidazole ligands have been prepared. Figure 1.18a for instance shows the introduction of various functional groups into the RHO ZIF framework family reported by Yaghi and coworkers [210]. It is reported that the improved CO2 adsorption of the amine functionalized ZIF-96 relative to the others (Figure 1.18b) is likely due to a combination of two effects: a large contribution arising from electrostatic interactions, due to the asymmetric functionalization, and strong vdW interactions arising from the polarizability of the functional groups. While this framework family shows some tunability in the CO2 adsorption properties, the low-pressure adsorption is moderate, leaving little room for potential applications in post-combustion carbon capture. As a significant improvement, An et al. introduced Bio-MOF-11 (also known as Co2(ad)2(CO2CH3)2·2DMF·0.5H2O where ad = adeninate) whose structure consists of Co2+ paddlewheels capped with two acetate ligands and interlinked by two heterocyclic adeninate ligands, which have amine-based functionality [211]. This MOF has a high initial isosteric heat of adsorption of −45 kJ mol−1 and an impressive selectivity of 75:1 for CO2:N2 calculated from the molar ratios of CO2 and N2 uptake at 298 K. While these values are very impressive, it should be noted that the isosteric heat drops off rapidly with higher loading and stabilizes around −35 kJ mol−1, likely due to the rapid saturation of strong adsorption sites. As a result, the low pressure CO2 adsorption has a modest value of ≈5.8 wt% at 0.15 bar. While this value is significantly improved from the aforementioned ZIFs and any reported framework containing aromatic amines, it is well-below many of the OMC-containing MOFs (Figure 1.16) and frameworks made with alkylamine functionality [211]. However, a follow-up study of Chen et al. report molecular simulation studies on Bio-MOF-11. They show that, relative to other nanoporous adsorbents including

ZIF-25

ZIF-71

HN N dmelm ZIF-25

Cl

Cl

ZIF-93

HN N dclm ZIF-71

ZIF-96

HN N almelm ZIF-93

O NH2 HN N hymelm ZIF-97

ZIF-97

HN N cyamlm ZIF-96

N

OH

(b)

0

0.5

1

1.5

2

0

ZIF-25 ZIF-71 ZIF-93 ZIF-96 ZIF-97

200

400 Pressure (torr)

600

800

Figure 1.18  (a) Isotructural series of RHO Zeolitic Imidazolate frameworks with varying functionality and (b) experimental (circles) and theoretical (triangles) excess CO2 adsoprtion isotherms collected at 298 K for the RHO ZIF series. Source: Morris et al. 2010 [210]. Reproduced with permission of American Chemical Society.

(a)

Uptake (mmol CO2/g)

40

CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS

MOFs and zeolites, this framework has one of the highest CO2/N2 selectivities, and it is even slightly enhanced in the presence of water, a factor that makes the material of high interest for post-combustion capture applications. They further attribute the preferential adsorption of CO2 to the presence of pyrimidine and amino Lewis basic adsorption sites [212]. More recently Woo and coworkers used in situ single-crystal diffraction to provide the first observation of CO2 binding to an amine in Zn2(C2O4) (C2N4H3)2⋅(H2O)0.5 (alternatively known as Zn2(atz)(ox) where atz = aminotriazole and ox = oxalate, Figure 1.19) [213], which features an isosteric heat of −40.8 kJ mol−1 and low-pressure capacity of 13.6 at 0.15 bar and 22°C [214]. This MOF has two crystallographically distinct CO2 adsorption sites. The first is located near the free amine group, while the second adsorption site is near the oxalate, both with distances above 3 Å. The intermolecular CO2 distances imply strong intermolecular interactions. Using this experimental work combined with molecular simulation studies, they concluded that the observed CO2 adsorption properties are a combination of appropriate pore size, strong interaction between CO2 and functional groups on the pore surface, and intermolecular interactions between neighboring CO2 molecules [213]. (ii) Lewis Bases for Chemisorption. Amines do not only polarize CO2, in many instances they can strongly and selectively bind CO2 via chemisorptive interactions. Considering the lower heat capacity of solid adsorbents compared to the liquid amine-based scrubbers, which are already implemented industrially for various CO2 separations, it seems feasible that decorating the surface of MOFs with pendant alkylamines could lower regeneration energies, and hence give rise to an overall energy penalty that is closer to the projected thermodynamic minimum (projected to be about 11% energy penalty) [215]. Some recent work in the MOF field have been focused on either appending alkylamines to the internal surface of MOFs via OMCs or through impregnation of MOF frameworks with polymers, such as polyethyleneimine. The first report of post-synthetic appendage of small molecules to OMCs was carried out by Hwang et al. who appended ethylenediamine to Cr3+ sites in the MIL-101 framework (alternatively known as Cr3O(1,4-BDC)3(H2O)2X, where X = F− or NO3−) [216]. While the targeted application was Knoevenagel condensation catalysis, several works reported since are focused solely on post-combustion carbon capture. The first report of an amine-appended MOF for flue gas separation was by McDonald et al. in 2011. At 25°C mmen-CuBTTri (mmen = N,N′-dimethylethylenediamine and H3BTTri = 1,3,5-tri(1H-1,2,3-­triazol-4-yl) benzene) adsorbs 0.105 g CO2 per gram MOF with an IAST selectivity of 327,

(b)

(c)

3.023Å

3.152Å

2.287Å

CO2 (I)

2.122Å

Figure 1.19  (a) Partial structure showing Zn-atz layer that is made up of Zn2 dimers rotated by 90° with respect to each other. (b) Ball-and-stick model of the Zn2(atz)(ox) structure formed from Zn-atz layers that are pillared by the oxalate units. (c) Single crystal x-ray structure of CO2 binding in Zn2(atz)(ox) at 173 K. (c) (Top) The amine group of Atz ligand is shown bound to CO2(I). (Bottom) The H atoms of the amine group form H-bonds to the oxalate O atoms, directing the N lone pair toward the C atom of the CO2 molecule. (Bottom) The CO2 molecules are found in a cavity with short intermolecular distances (CO2-I and CO2-II). There are secondary interactions between the CO2-I and Ox (drawn in orange). Zn, N, O, C, and H are represented by cyan, blue, red, grey and purple spheres, respectively. Source: Vaidhyanathan et al. 2010 [213]. Reproduced with permission of American Association for the Advancement of Science.

(a)

42

CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS

a direct result of the high isosteric heat of CO2 adsorption, which was calculated to be −96 kJ mol−1 at zero coverage (−Qst for Cu-BTTri = 21 kJ mol−1). In addition to this, in situ IR spectra gave evidence of the formation of a zwitterionic carbamate species indicative of a chemisorptive interaction. Despite the large initial isoteric heat of adsorption, the CO2 uptake was also fully reversible and the framework could be easily regenerated at 50°C, enabling a cycling time of just 27 min and no loss of capacity over the course of 72 adsorption/ desorption cycles tested [152]. Later, Hu et al. appended several amines to the surface of Cr-MIL-101 and showed a threefold enhancement of the low pressure CO2 adsorption properties compared to the bare framework (from 0.022 to 0.085 g CO2 per gram MOF at 0.15 bar and 23°C). The isosteric heats of CO2 adsorption increased from −44 [114] to −98 kJ mol−1 and the selectivity of the aminebased analog again showed nearly no N2 uptake at 296 K leading to a very high selectivity factor for CO2/N2 [186]. More recently, Lin et al. reported polyethyleneimine (PEI) infused Cr-MIL-101 [184]. Although the surface area and pore volume of MIL-101 decreased significantly with a 100 wt% PEI loading (BET surface areas range from 3125 to 608 m2 g−1), there is a dramatic enhancement in the CO2 adsorption capacity at 0.15 bar that ranges from 0.0145 to 0.185 g CO2 per gram MOF at 25°C, respectively (Figure 1.16). Further, at 50°C, a temperature more relevant to post-combustion capture, the adsorption capacity is still 0.150 g CO2 per gram of 100 wt% PEI-Cr-MIL-101 [184]. In 2012, McDonald et al. reported mmen-appendage to Mg2(dobpdc) (dobpdc4– = 4,4′-dioxido-3,3′-biphenyldicarboxylate) (Figures 1.20a and 1.20b), which showed 0.088 g CO2 per gram MOF at 0.39 mbar (298 K) and 0.138 g/g at 0.15 bar (at 40°C), conditions relevant to CO2 capture from air and flue gas, respectively [185]. The material also shows excellent performance in the presence of water [217]. Adsorption/desorption cycling experiments, carried out via TGA, demonstrate that mmen-Mg2(dobpdc) can be repeatedly regenerated (at 150°C under N2 flow) after many 15 min exposures to simulated flue gas. The working capacity, calculated to be 0.11 g CO2 per gram of MOF, corresponds to a total CO2 removal of 98% [185]. With DSC (differential scanning calorimetry), the authors further estimated that regeneration would require approximately 2.34 MJ of energy to release 1 kg of CO2 from mmen-Mg2(dobpdc) compared to the 3.6 to 4.5 MJ energy requirement for the state of the art MEA scrubbers [218–220]. Later, in 2015, this study was extended to append mmen onto the internal surface of multiple M2(dobpdc) analogs (where M = Mg, Mn, Fe, Ni, Co, and Zn). They show significant tunability in the steep steps observed in the adsorption isotherm. Using a combination of IR, synchrotron X-ray diffraction, and computational studies they were able to reveal the origin of the sharp adsorption step in the isotherm (Figure 1.20c) is the result of a cooperative insertion process in which CO2 molecules insert into metal-amine bonds, inducing a reorganization of the

Mg2(dobpdc) (2)

2.100 °C in vacuo 6h 3. CO2

1. mmen C6H14 24 h

(b)

mmen-Mg2(dobpdc) + CO2

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

(c) 4.0

0

200

600

P (mbar)

400

3.0 390 ppm 2.5 2.0 1.5 1.0 0.5 0.0 0.01 0.1

800

1

1000

10

25 °C 50 °C 75 °C

(dobpdc) at various temperatures showing a sharp step at 0.39 mbar (the partial pressure of CO2 in air) that is shifted with temperature. Source: McDonald et al. 2012 [185]. Reproduced with permission of American Chemical Society.

Figure 1.20  Ball-and-stick model of (a) Mg2(dobpdc), (b) mmen-Mg2(dobpdc), and (c) excess CO2 adsorption isotherms for mmen-Mg2

(a) CO2 Adsorbed (mmol g−1)

44

CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS

(a)

(b)

(c)

(d)

2.29(6) Å 2.10(2) Å

(e)

Figure 1.21  Space-filling models of the solid-state structures of (a) mmen-Mn2 (dobpdc) and (b) CO2-mmen-Mn2(dobpdc) at 100 K. Portions of the crystal structures for mmen-Mn2(dobpdc) (c) before and (d) after CO2 adsorption, as determined from synchrotron powder x-ray diffraction data. The latter shows CO2 insertion between the amine and Mn metal. (e) A portion of the crystal structure shows the formation of an ammonium carbamate chain along the MOF pore. Green, grey, red, blue and white spheres represent Mn, C, O, N, and H atoms, respectively; some H atoms are omitted for clarity. Source: McDonald et al. 2015 [221]. Reproduced with permission of Nature Publishing Group.

amines into well-ordered chains of ammonium carbamate (Figures 1.21a–1.21e). As a consequence, large CO2 separation capacities can be achieved with small temperature swings, and regeneration energies appreciably lower than achievable with state-of-the-art aqueous amine solutions become feasible [221]. Beyond alkyl amine-containing frameworks, there are few examples of other Lewis base functionality in MOFs that lend to chemisorptive-type interactions with CO2. Recently, Gassensmith and coworkers reported the synthesis of an MOF constructed by γ-cyclodextrin linked together by alkali metals such as Rb (CD-MOF-2) [157]. The ligand in this MOF, which is decorated with free hydroxyl groups, is a

MOFs for Post-combustion Capture

45

natural product produced from starch. Further, the MOF can be prepared readily in green solvents such as water using slow evaporation methods. While this material was not assessed for room temperature N2 adsorption, it shows extremely high uptake of CO2 at very low partial pressures indicative of very strong binding. As such, solid-state cross-polarization magic-angle-spinning (CP/MAS) 13C NMR spectroscopy was used to monitor the material before and after exposure to CO2. Upon introduction to CO2, CD-MOF-2 shows a new peak at 158 ppm that was interpreted as the formation of carbonic acid functionality, due to a direct interaction between the CO2 and surface hydroxyls. This additional resonance was also accompanied by chemical shifts of other signature peaks in the MOF supporting the hypothesis that a chemical reaction was occurring. Later, calorimetry revealed a zero coverage differential enthalpy of CO2 adsorption equal to approximately −113.5 kJ mol−1 at 25°C.. This value quickly dropped to around −65.4 kJ mol−1 binding event, which was attributed to less reactive hydroxyls and then another plateau at −40.1 kJ mol−1 that was attributed to physisorptive-type interactions. It should be noted that the strongest binding sites appear to be irreversible; however, this only reduces the capacity slightly with cycling [222].

1.3.5  Stability and Competitive Binding in the Presence of H2O One of the major drawbacks for the utilization of MOFs in many applications is the widespread belief that they are unstable in the presence of water, a result of many reports that show frameworks that break down due to hydrolysis. While these materials do exhibit coordination-type bonding, that is considered to be weaker than that of their covalent counterparts, there have been many MOFs synthesized to date that exhibit water stability [223]. This effort has been driven by the need for materials that maintain high performance in wet environments, such as post-combustion flue gas separations. While the amount of water present in a flue gas stream could be reduced, it is energetically costly and complete removal is likely unfeasible [224]. As such, many synthetic strategies have been taken to provide water stable materials. These methodologies include the use of high oxidation state metals, multidentate ligands, metal nodes with large coordination numbers, MOF modification with hydrophobic ligands, guests or polymers, and the use of ligands with limited acidity, like pyrazoles and imidazoles [223, 225–232]. While many of the azole-based ligands can bind metals with similar geometries as carboxylates, the higher Lewis basicity creates stronger metal-ligand bonds improving both thermal and chemical stability. It is expected that MOF stability will increase with increasing pKa to give the following stability trend: pyrazole > imidazole > triazole > tetrazole. A recent report of Long et al. reveals the synthesis of a new pyrazolate framework, Ni3(BTP)2 (H3BTP = 1,3,5-tris-1H-pyrazol-4-yl)benzene), which exhibits stability in boiling water for 14 days with varying pH levels that range from 2 to 14 [228]. Later, in situ IR studies revealed weak interactions of Ni3(BTP)2 with CO, CO2, and H2. It is proposed that the pyrazolate ligand forces the Ni2+ into a diamagnetic low-spin state with a large energy barrier to transition from low spin to high spin of 75 kJ mol−1, a value derived from ab initio molecular

46

CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS

modeling [233]. This low-spin state might also inhibit the interaction of the metal with water and hence add to the overall framework stability. While this study has focused on improving the strength of the metal-ligand bond other studies have added hydrophobic functionality to framework ligands such as –CH3, –CF3, and –F [234, 235]. A later study of Navarro et al. reports construction of an isoreticular series of MOFs denoted as [Ni8(OH)4(H2O)2(L)6]n where L = a series of azolate ligands with increasing levels of hydrophobicity shown in Figure 1.22. Moreover, the length and functionalization of the linkers impact on the pore size as well as on the fine tuning of the surface polarity and the incorporation of trifluoroalkyl groups gives rise to a significant enhancement in the hydrophobicity and an overall improvement in hydrolytic stability. Although this study was mostly focused on overcoming problems related to the adsorption of harmful volatile organics in wet environments, these same principles can be applied to MOFs for various gas separation applications [236]. While the aforementioned studies worked to improve hydrolytic stability and hydrophobicity in the initial framework design, others have utilized post-synthetic modifications to manipulate these parameters. A recent report demonstrated that the encapsulation of HKUST-1 into polystyrene microspheres not only improved the hydrolytic stability of the framework, but additionally allowed retention of most of the CO2 uptake capacity after exposure of the composite to 80% relative humidity at 27°C for 1 month [235]. This is a significant achievement considering that HKUST-1 readily decomposes in the presence of water, which coordinates to the Cu-OMC on the paddlewheel cluster, promoting hydrolysis of the metal–ligand bond and hence framework degradation [237]. For MOFs that exhibit highly charged functionality on their internal surface, such as OMCs, water will compete for adsorption sites with CO2 and in most cases water will win this battle due to an existing dipole moment. As such, even if MOFs are deemed water stable, their separation performance must still be assessed in wet multicomponent streams or at minimum after exposure to water vapor. While Mg2(dobdc) is currently the best-performing MOF at ambient pressure and dry conditions, it cannot be used for the capture of CO2 under flue gas conditions due to diminished adsorption properties in the presence of water. In a study of Matzger et al., the breakthrough performance of a series of M2(dobdc) (where M = Zn, Ni, Co, and Mg) MOFs were evaluated in dry streams of CO2:N2 (0.16 bar and 0.84 bar, respectively) after exposure to relative humidity levels that range from 0% to 70%. For Mg2(dobdc), after exposure to 70% RH and subsequent thermal regeneration, only about 16% of the initial CO2 capacity was recovered. While the other metal-analogs experienced less of a performance decline, it is clear that in the event that water does not cause framework decomposition, it will be in competition for the OMCs and with further cycling the materials will become saturated [106]. Subsequent removal of water adsorbed at the OMCs can often be energy intensive, requiring high heat and vacuum. This work was additionally supported by a recent study of Mason et al., which carried out multicomponent adsorption studies of several MOFs in CO2/N2/H2O mixtures. It was found that for every OMC containing MOF

47

MOFs for Post-combustion Capture

N–NH N–NH

N–NH

N–NH

N–NH

HO

CH3 H3C

O

H2L1

HO

O

H2L2

HN–N H2L3

1.6 nm

1.1 nm

1.9 nm

CF3

F3 C

HN–N H2L4

N–NH H2L5

0.9 nm

N–NH

N–NH

N–NH

N–NH

H2L5-CH3 H2L5-CF3

1.4 nm

2.4 nm

Figure 1.22  (a) The crystal structure of [Ni8(OH)4(H2O)2(L5-CF3)6]n viewed as a combination of n octahedral (yellow polyhedron) and 2n tetrahedral (gray polyhedron) cavities. (b) Pyrazolate-based ligands used in the synthesis of the [Ni8(OH)4(H2O)2(L)6]n MOFs. H2L1 = 1H-pyrazole-4-carboxylic acid; H2L2 = 4-(1H-pyrazole-4-yl)benzoic acid; H2L3 = 4,4′-benzene-1,4-diylbis(1H-pyrazole); H2L4 = 4,4′-buta-1,3-diyne-1,4-diylbis(1Hpyrazole); H2L5 = 4,4′-(benzene-1,4-diyldiethyne-2,1-diyl)bis(1H-pyrazole); and H2L5-R (R = methyl, trifluoromethyl). (c) View of the tetrahedral (top) and octahedral (bottom) cages found in the crystal structures of [Ni8(OH)4(H2O)2(L3)6]n (left), [Ni8(OH)4(H2O)2(L4)6]n (middle), and [Ni8(OH)4(H2O)2(L5)6]n (right), and the corresponding metric descriptors. Ni, N, C, O, and F and H are depicted as green, blue, grey, red, and white spheres, respectively. Source: Padial et al. 2013 [236]. Reproduced with permission of John Wiley & Sons.

studied, that the low-pressure CO2 adsorption capacity decreased significantly compared to the capacities in pure streams of CO2 (Figure 1.5) [87]. To alleviate this problem, appending amines to the OMCs, which are known to preferentially bind CO2, even in the presence of water, can be a useful solution. Given that the amine

48

CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS

appended frameworks are likely hydrophilic due to the high concentration of functional groups that readily form hydrogen bonding interactions, it also promotes water adsorption inside of the MOF that could inadvertently induce structural changes or simply decrease the overall capacity. As such, there is still a need to assess the separation ability of many amine-appended frameworks in wet streams [238]. However, it should be noted that the aforementioned study of Mason et al., also probed several alkyl amine-containing frameworks using multicomponent adsorption. For every amine-containing MOF studied, the decreases in the CO2 adsorption capacities are minimal in the presence of water (Figure 1.5). Even in some cases, a slight increase can be observed upon the introduction of water. Recently, an alternative strategy for achieving water stability was reported by Cohen et al., who introduced polyMOFs, which are constructed by ligands with long hydrophobic polymeric chains. Some of these polyMOF materials are shown to exhibit relatively high CO2 sorption with minimal N2 sorption, making them promising materials for CO2/N2 separations. Although the parent MOFs are generally unstable to water, the polyMOFs demonstrated excellent water stability due to the hydrophobic polymer, as well as the cross-linking of the polymer chains within the MOF. Further, the polyMOFs exhibit minimal change in their CO2 adsorption properties before and after water exposure [239]. While many of the studies focused on enhancing water stability and assessing the performance of MOFs in wet environments are still in their infancy, this work has already led to significant improvements in materials performance providing optimism toward the eventual implementation of MOFs in various energy-relevant gas separations.

1.4  MOFs for Pre-combustion Capture 1.4.1  Advantages of Pre-combustion Capture Pre-combustion capture, which primarily involves the separation of CO2 from H2 and a few other impurities, has several advantages over other carbon-capture technologies. First, the separation is carried out at high pressure ranging from 5 to 40 bar [81] with much higher CO2 concentrations. As a result, it will be less energy intensive to regenerate the material. The high pressure allows implementation of a PSA-type regeneration process where the pressure is simply dropped to atmospheric, eliminating the need to heat the material through a temperature swing process, as in post-combustion capture. Further, the actual separation of CO2 from H2 is significantly easier due to much larger disparities in their chemical properties such as polarizability and quadrupole moment for CO2/H2 compared to CO2/N2 and O2/N2 in post-combustion and oxy-fuel processes (Table 1.2). These differences provide much higher selectivity for CO2 over H2 in solid adsorbents allowing the separation to be done using a purely physisorptive process. All of these added benefits could allow a more rapid development of separation materials and their subsequent implementation into industrial separations.

MOFs for Pre-combustion Capture

49

Currently, the separation of CO2 and H2 is already carried out on extremely large scales worldwide, 50 million tons per year, for the purification of H2, which is used primarily in the production of ammonia and various hydrocarbons [240]. This process is typically carried out via PSA with solid adsorbents such as activated carbons or zeolites [84]. While the process is still too energetically inefficient to make pre-combustion capture economically viable, significant improvements in the efficiency of solid adsorbents for the separation of CO2/H2 could render the technology workable on a large scale or provide further energy savings in existing hydrogen purification infrastructure worldwide. It is projected that a 10% energy savings in already implemented hydrogen purification industries would be the equivalent of closing 18 coal-fired power plants [88]. The already existing infrastructure for hydrogen purification and the scale with which this separation is already carried out, implies that the implementation of pre-combustion capture could be expedited relative to post-combustion and oxy-fuel technologies.

1.4.2  Necessary Framework Properties for CO2 Capture Most of the energy expended in the separation of CO2 and H2 is related to mass transport of the gas and PSA regeneration process. As such, much energy savings could be realized through an improvement in the selectivity and working capacity of the solid adsorbent. Working capacity is defined as the difference between the amount of gas adsorbed at the flue gas stream pressure and the amount of gas adsorbed at the regeneration pressure. High gravimetric and volumetric working capacity lowers the amount of material required and/or size of the fixed bed for the separation and hence also lowers the overall energy input for the PSA regeneration process. Other factors to be considered when selecting materials for pre-combustion capture should include their long-term stability and that their separation properties are maintained in the presence of other minor impurities in a flue gas stream such as CO, H2O, and H2S. While there are many studies assessing the hydrolytic stability of MOFs previously mentioned, little is known about their behavior in H2S. Only a few studies are included in the literature; in one of these Eddaoudi et al. show that SIFSIX-3-Ni (SIFSIX = hexafluorosilicate) MOF has high selectivity for CO2 and is stable in the presence of H2S [241]. Further, De Weireld et al. have studied H2S adsorption in a series of MILframeworks. They show that two MOFs, including MIL-53(Al, Cr) and MIL-47(V), maintain their methane adsorption properties after H2S treatment, whereas MIL-100 and MIL-101 show significant decreases in their CH4 adsorption capacities [242]. Considering our limited ability to tune the pore size, pore shape, and surface functionality of activated carbons and zeolites, it is expected that only minor improvements can be made regarding their efficiency of the separation could be realized. Indeed, MOFs already offer record-breaking capacity for CO2 adsorption in the pressure regime of interest for pre-combustion capture [85, 105, 243–245]. To date, the highest high pressure carbon dioxide adsorption belongs to NU-11 with the absolute uptake of 856 cm3 per gram of MOF at 30 bar and 25°C. Further, the

50

CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS

facile structural tunability of MOFs can allow significant improvements in binding strength of CO2 and hence the selectivity of CO2 over H2. Last, their unprecedented internal surface areas, a factor of strong importance for high-pressure separations, offer significant promise with regard to this separation [88, 246–250].

1.4.3  Potential MOF Candidates for CO2/H2 Separations To date, while there are a number of studies looking at high-pressure CO2 adsorption (up to 50 bar) in MOFs, there are very few studies focused specifically on assessing their properties for hydrogen purification or pre-combustion capture. Currently, more work is needed to assess the properties of existing frameworks and in turn gain more insight into the structural features that give rise to enhanced separation ability of CO2/H2 in MOFs. For the most part, current CO2/H2 studies are limited to a few frameworks and are based on the use of single-component CO2 and H2 adsorption isotherms to estimate the separation ability of the MOFs in question. The first experimental study of MOFs for CO2:H2 separations by PSA was carried out by Herm et al. This work features single-component adsorption isotherms for a series of 5 MOFs, which are further compared to state of the art separation materials including zeolite 13X and activated carbon JX101 [88]. The MOF series were comprised of two frameworks with OMCs and modest surface areas, i.e. Mg2(dobdc) (1800 m2 g−1) [62] and Cu-BTTri (1750 m2 g−1) [151], two frameworks with high surface areas and no polarizing functionality, i.e. MOF-177 (4690 m2 g−1) [245] and Be-BTB (4400 m2 g−1, BTB = benzene-1,3,5 tribenzoate) [42], and one flexible framework, Co-BDP (2030 m2 g−1, BDP = 1,4benzenedipyrozolate) [251]. Single-component adsorption isotherms were collected for H2 and CO2 at pressures up to 40 bar and 40°C and then IAST was used to estimate the materials behaviors in a binary mixtures of 80:20 or 60:40 H2:CO2. It was revealed that the materials with highly polarizing functionality on their internal surface such as the OMC-containing MOFs and zeolite 13X yielded much higher selectivities, between 75 and 859. This was also true for the activated carbon and was rationalized based on the small pores and overlapping van der Waals potential. Although the MOFs with high surface areas and no polarizing functionality show appreciable CO2 uptake at high pressures compared to the other materials assessed, they have shown inadequate selectivities of significantly less than 10 (Figure 1.23a), limiting their performance in an actual separation process. In addition to selectivities, the gravimetric and volumetric working capacities were also estimated from IAST (Figures 1.23b and 1.23c). Further, if we consider the MOF with the highest selectivity among those tested in this work (Figure 1.23a) Mg2(dobdc), the gravimetric and volumetric working capacities can even climb up to 6.4 and 5.9 mol per kg or mol per Liter, respectively. It is suggested that if the separations are carried out in the high pressure regime with Mg2(dobdc) replacing zeolite 13X, then the mass and volume of the required adsorbent would be decreased by a factor of 2 and 2.7, respectively. This study shows while materials with high surface areas can have high adsorption capacities, adsorbents with polarizing functional groups lend to higher

51

MOFs for Pre-combustion Capture

(a) 800 Mg2(dobdc)

Selectivity

600

Zeolite 13X

400

Carbon JX101 Cu-BTTri

200

MOF-177

10

Co(BDP) Be-BTB

5 0

10

(b) CO2 Working Capacity (mol kg−1)

30

20

40

Pressure (bar) 9 8

Cu-BTTri

7

Mg2(dobdc)

6

Be-BTB

5

Co(BDP)

4

MOF-177

3

Carbon JX101

2

Zeolite 13X

1 0

0

10

20

30

40

Pressure (bar) CO2 Working Capacity (mol L−1)

(c)

7

Cu-BTTri

6

Mg2(dobdc)

5

Co(BDP)

4

MOF-177

3

Be-BTB

2

Carbon JX101

1

Zeolite 13X

0

0

10

20

30

40

Pressure (bar)

Figure 1.23  IAST-calculated (a) selectivity, (b) gravimetric, and (c) volumetric CO2 working capacities for an 80:20 H2/CO2 mixture at 40°C for the metal-organic frameworks MOF-177, Be-BTB, Co(BDP), Cu-BTTri, and Mg2(dobdc), the activated Carbon JX101 and zeolite 13X. Source: Herm et al. 2011 [88]. Reproduced with permission of American Chemical Society.

52

CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS

CO2:H2 selectivities and enhanced performance with regard to working capacity. This study is further the proof of the concept that MOFs can indeed potentially outperform the state of the art materials [88]. In light of the previous work, Vaidhzanathan et al. began working with small framework ligands for the synthesis of ultramicroporous MOFs [250]. They were inspired by the knowledge that OMCs are susceptible to poisoning by even trace amounts of water. As such, they began to look at a small pore material without OMCs, Ni-(4-pyridylcarboxzlate)2 (Figure 1.24a) that might lend to overlapping van der Waals potential and hence high selectivities as in the case of the aforementioned carbon. This Ni-MOF exhibits a cubic framework with ultramicropores ranging (b) Working capacity (mmol g−1)

(a)

Ni O N C

Ni-4PyC Carbon JX101 Zeolite 13X MgMOF-74 Cu-BTTri

8 6 4 2

80H2:20CO2

0

5

10

b c

(c)

15

20 25 30 Pressure (bar)

0

S(CO2/H2)

500

I

III II

IV

40

Ni-4PyC Carbon JX101 Zeolite 13X MgMOF-74 Cu-BTTri

600

a

35

400 300 200

80H2:20CO2

100 0

5

10

15 20 25 30 Total pressure (bar)

35

40

Figure 1.24  (a) Structure of Ni-(4-pyridylcarboxzlate)2 obtained from single-crystal x-ray structure green, Ni dimers reduced to one node. The green cones trace the six-connected distorted cubic arrangement formed by collapsing the Ni dimers to nodes and the PyC linkers as lines. The yellow ball represents the cages in the structure. Below the structure is the Connolly surface diagram. The channels labeled I and III are interconnected and run along the a and c axes, respectively, whereas the channel labeled II propogate along the c axis. IV represents the cages, which are lined with terminal water molecules in addition to the ligand groups. (b) IAST working capacities and (c) selectivity characteristics from a 20:80 mixture of CO2:H2 at 40°C for a 1–10 bar PSA process. Comparison of the H2/CO2 selectivity of Ni-(4-pyridylcarboxzlate)2 is compared to other known MOFs and industrial sorbents determined under the same conditions. Source: Herm et al. 2011 [88] and Nandi et al. 2015 [250]. Reproduced with permission of American Chemical Society.

MOFs for Pre-combustion Capture

53

from 3.5 to 4.8 Å, labeled I–IV in Figure 1.24a. High pressure adsorption isotherms, collected up to 10 bar at 40°C, were used to calculate the IAST selectivties and capacities for a 1–10 bar PSA process. The results show that the selectivities of the material is 285 and 230 for 20:80 and 40:60 CO2:H2 mixtures, respectively, and despite a modest surface area (945 m2 g−1), the gravimetric capacity is relatively high, 3.95 mmol per gram of MOF (Figure 1.24b). While the selectivities are lower than Mg2(dobdc), they are similar to those observed for zeolite 13X. Further, the gravimetric working capacity bests all materials analyzed by Herm et al. (at 10 bar) and more importantly Ni-(4-pyridylcarboxzlate)2 is shown to retain its CO2 adsorption properties after exposure to H2O. Last, the CO2 self-diffusivities (3 × 10−9 m2 s−1) were determined to be as much as two times higher than zeolite 13X and comparable to the top performing CO2 adsorbing MOFs [250]. Other experimental studies have included one by Chen et al. that form a NbO-type MOF referred to as UTSA-40 (1630 m2 g−1, also known as [Cu2(L) (H2O)2]  ⋅  6DMF  ⋅  2H2O where L = 6,6-dichloro-2,2-diethoxy-1,1-binaphthyl-4,4di(5-isophthalic acid)) that consists of a tetracarboxylate ligand and dicopper paddlewheel cluster. The authors found that the material outperforms several traditional zeolites, and while the performance is lower than two OMC-containing MOFs, including Mg2(dobdc) [62] and Cu-TDPAT [148], this framework has a significantly lower energy cost for regeneration [238]. One weakness in many of the aforementioned studies is the lack of assessment of the materials performance in the presence of other impurities in the flue gas stream such as H2O, H2S, CO, and CH4 (the latter in the case of methane reforming). The adsorption behavior of each impurity can vary widely in MOFs as it is dictated by the pore size, shape, and surface functionalization. It is expected, for instance, that H2O, H2S, and/or CO could poison the OMCs and block the adsorption of CO2. While most of the water could be removed by condensation or other adsorbents placed in route to the fixed bed intended for CO2/H2 separation, it is likely that all of these impurities cannot be removed from the flue gas stream. As such, a more thorough assessment of materials performance in multicomponent streams is needed. Ideally a study would include both the adsorption properties in the multicomponent gas stream containing all potential impurities. The best way to test these properties is via experimental or simulated breakthrough curves. Because experimental breakthrough analysis containing very minor impurities or mixtures of more than two or three components can be extremely time-consuming or in some cases experimentally intractable, Krishna et al. have used computational methods to simulate breakthrough curves for a number of MOFs, zeolites, and carbons in the tertiary mixture, CO2/CH4/H2, and their binary combinations [252]. Their study revealed the utility of breakthrough simulations for MOFs and further implied that Mg2(dbodc) was the top performer of the materials tested under dry conditions. As such, this work was followed by an experimental one of Herm et al. for validation of the computational methods [253]. For this, breakthrough curves were generated for Mg2(dobdc) in several multicomponent streams including CO2/CH4, CH4/H2, and CO2/CH4/H2

54

CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS

[253]. The experiment not only validated the aforementioned simulations, but also showed that this MOF additionally outperformed zeolite 13X for all three-gas mixtures. Later, Wu et al. simulated several MOFs and zeolites in quaternary mixtures including CO2/CO/CH4/H2 and identified a new pre-combustion capture candidate, Cu-TDPAT [254] and then Banu et al. carried out simulations for a quinary mixture including CO2/CO/CH4/H2/N2 for a series of four Zr-containing MOFs [255]. The results of these studies suggest that MOFs with small pores and open metal-sites, or other sources of charged functionality could be used to achieve both high selectivity and working capacity necessary to improve the efficiency of the CO2:H2 separation. However, considering the few number of data points, more work is needed to screen a large number of existing MOFs with varying structural features to gain more clarity related to their structure-derived function. Studies of this kind will provide insight into how to find the intricate balance between strength of CO2 adsorption and high working capacity. Additionally, while it is clear that MOFs have a high potential to outperform the state of the art zeolites or activated carbons, more understanding of MOF properties in flue gas mixtures containing minor impurities must be obtained.

1.5  MOFs for Oxy-Fuel Combustion Capture 1.5.1  Necessary Framework Properties for O2/N2 Separations The implementation of oxy-fuel combustion in the power industry is limited by an adequate, low-cost gas separation technology for the separation of O2 from air. Currently, air purification, which predominately involves the separation of O2 from N2, is most widely carried out via cryogenic distillation; while it provides high purity O2 (99%), due to very low boiling points of O2 and N2 (−196°C and −183°C for N2 and O2, respectively), it also poses a large energy and economic cost for execution on the scale that is necessary for CCS. As such, other technologies have been studied; these included various adsorbents like zeolites and activated carbons and membrane-based separations. All of these aforementioned technologies can be implemented; they are limited to processes that can utilize O2 at a purity level that is less than 94%, a direct result of limited selectivity for N2 over O2 [84]. The development of adsorbent materials that exhibit higher selectivities (lending to O2 purity levels >95%) and are also operational at ambient temperature and pressure could afford significant energy savings. Compared to post-combustion and pre-combustion technologies, the capture step for an oxy-fuel process is relatively easy, using existing condensation protocol to isolate CO2 (55–65 wt%) from water (25–35 wt%) after the combustion process [256]. As such, existing power plants could be easily retrofitted to accommodate this process, which has shown capture rates of CO2 on the order of 95%, a value significantly higher than pre- or post-combustion capture technologies [257]. In addition to easy implementation,

MOFs for Oxy-Fuel Combustion Capture

55

the O2 stream used for combustion in an oxy-fuel process is first diluted with CO2 to a partial pressure of approximately 0.21 bar to control the flame temperature, an act that limits the formation of NOx impurities [258]. The most energy consuming part of the oxy-fuel process is generating large quantities of nearly pure O2 [259] from air whose main component is N2. As such, separation materials must show high selectivities and capacities for O2 in the pressure and temperature regime of interest, which is approximately 0.2 bar and 25°C. The modularity of MOFs makes them ideal candidates for this separation; however, compared to CO2 and N2 discussed in the previous section, O2 and N2 have even smaller disparities in their physical properties including kinetic diameter, quadrupole moment, polarizability, and boiling point, creating a challenge for the design of adsorbents. Looking at Table 1.2, the polarizability and quadrupole moment of N2 is slightly higher than O2, making most framework materials like MOFs and zeolites with highly polarizing adsorption sites only slightly more selective for N2 over O2. This will cause the selectivities and hence resulting O2 purity to be quite low. As such, recent efforts in MOF chemistry are instead focused on the differences in the chemical properties of O2 and N2. Of these two small molecules, O2 exhibits a significantly higher electron affinity than N2 making redox active MOFs that might give rise to a reversible electron transfer to O2 of great interest for oxy-fuel combustion.

1.5.2  Biological Inspiration for O2/N2 Separations in MOFs Knowledge of the reactivity of O2 with transition metal complexes is not new as this property is exploited in heme containing metaloproteins that are responsible for transport and storage of O2 in mammalian systems. Heme species, also known as porphyrins, are a group of heterocycles composed of four modified pyrrole subunits with Fe2+ bound in the center (Figure 1.25a). Much research has been dedicated to producing synthetic molecular complexes that can mimic the reversible O2 binding observed in nature; [84] however, these molecular species have a strong propensity to react and combine upon formation of the metal-O2 complex making them highly difficult to isolate [260]. The instability of the mononuclear species has prompted the development of porphyrin-based supports that provide isolation of the reactive species to inhibit their decomposition and allow experimental observation of the porphyrin-O2 adduct [29]. While observation of this species has been limited to spectroscopic evidence at low temperature, recent work of Harris et al. used a heme-based Zr-MOF, PCN-224 [261], to give the first crystallographic evidence of a five-coordinate heme-O2 adduct (Figure 1.25b) [262]. Relative to their molecular counterparts, MOFs contain immobilized, separated active sites and offer facile structural tunability, allowing control over important metrics such as binding enthalpy and adsorption capacity for O2 and regeneration conditions. Further, given their highly crystalline nature, crystallography techniques can be used to unveil their structure-derived function. As such, MOFs provide an ideal platform to study O2 adsorption and separation.

56

CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS

(a)

CH2

CH3

CH2

H3C N

N Fe

N

N CH3

H3C

O

O

OH

OH

(b)

O2

ΔH = −34(4) kJ/mol

5-coord heme-O2

Figure 1.25  (a) Heme-based metaloprotein and (b) PCN-224-Fe (left), which forms PCN224-Fe-O2 at −78°C (right). The structure consists of Zr6 clusters interlinked by TCPP ligands (where TCPP = tetrakis(4-carboxyphenyl)porphyrin. Green octahedra represent Zr atoms; Fe, N, O, and C atoms are represented by orange, blue, red, and gray spheres, respectively. The distances and angles for PCN-224-Fe-O2 are Fe−O 1.79(1), O−O 1.15(4), Fe···N4 plane 0.526(2), Fe−O−O 118(4), N−Fe−O 104(1). Source: Anderson et al. 2014 [262]. Reproduced with permission of American Chemical Society.

1.5.3  Potential MOF Candidates for O2/N2 Separations While studies related to O2 adsorption are still limited, over the last few years, there have been several reports that show O2 adsorption in redox active OMC-containing MOFs. These materials are thus far limited to a few SBUs constructed by metals such as Cr2+, Fe2+, and Ti3+, all of which have a propensity to undergo redox activity in the presence of oxygen. One of the first studies of O2/N2 adsorption in a redox active MOF was carried out on Cr3(BTC)2 (BTC3− = 1,3,5 benzenetri-carboxylate) (Figures 1.26a and 1.26b) [203]. The material, which is isostructural with the

(b)

− O2

+ O2

(c)

Gas adsorbed (wt %) 0

2

4

6

8

10

12

0.0

0

4

8

12

0

0.2

2

4

10

0.4 0.6 Pressure (bar)

6 8 Cycle

12

0.8

14

adsorption desorption

N2

0.0

0.2

0.4

0.6

0.8

O2 (cycle 2)

O2 (cycle 1)

1.0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Figure 1.26  (a) Ball-and-stick model of O2-adsorbed Cr3(BTC)2 and (b) close-up view of the activated and O2 adsorbed Cr-paddlewheel where the large red spheres represent bound O2 molecules. The structures were obtained from Rietveld refinement of powder neutron diffraction data. (c) Uptake of O2 and N2 by Cr3(BTC)2 at 298 K. The compound saturates with O2 at ∼2 mbar but shows little affinity for N2. Upon evacuation, the O2 isotherms reveal reduced capacity. The inset shows O2 adsorption in Cr3(BTC)2 over 15 consecutive cycles at 25°C. Desorption was carried out by heating at 50°C under vacuum for 48 h. Source: Murray et al. 2010 [203]. Reproduced with permission of American Chemical Society.

(a)

Gas adsorbed (O2 per Cr)

58

CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS

aforementioned HKUST-1 and has a BET surface area of 1810 m2 g−1, displayed both a high O2 loading capacity and high selectivity, ≈22, for binding O2 (0.73 mmol per gram O2 at 0.21 bar) over N2 (0.033 mmol per gram N2 at 0.78 bar) at 25°C; however, the material showed a significant loss in capacity with cycling (Figure 1.26c) [203]. More recently, this same group studied O2 and N2 adsorption in a redox active Fe2(dobdc) [82] (Figure 1.27a), which has a BET surface area of 1360 m2 g−1 and features a hexagonal array of one-dimensional channels lined with coordinatively unsaturated Fe2+ OMCs. Single-component gas adsorption isotherms collected at 298 K indicate that this framework binds O2 preferentially over N2 (Figure 1.27c); however there is an irreversible capacity of 9.3 wt%, corresponding to the adsorption of 0.5 O2 molecule per Fe2+–OMC. Upon cooling the material to 211 K, O2 uptake becomes fully reversible and the capacity increases to 18.2 wt%, a value that corresponds to the adsorption of one O2 molecule per Fe2+-OMC. Several techniques, including Mössbauer spectroscopy, infrared spectroscopy, and neutron powder diffraction were used to investigate this crossover from the physisorption (211 K) to the chemisorption regime where O2 adsorption becomes irreversible. All characterization pointed to a partial charge transfer from Fe2+ to O2 at low temperature and a complete charge transfer to form Fe3+ and O22− at room temperature. Rietveld analysis of powder neutron diffraction data confirms this interpretation, revealing O2 bound to Fe in a symmetric side-on configuration with an O–O distance of 1.25(1) Å at low tempearture, labeled as site I in Figure 1.27b. This value is only slightly elongated compared to the distance found in a free O2 species (1.2071(1) Å) [263]. Neutron diffraction carried out after the exposure of Fe2(dobdc) to O2 at room temperature reveals a different structure with the O2 species in a slipped side-on mode with O–O distance of 1.6(1) Å (Figure 1.27b), a value that is consistent with a two electron reduction of O2 to a peroxide species. These measurements also unveiled two secondary adsorption sites at low temperatures labeled as II and III in Figure 1.27a. Simulated breakthrough curves, which were calculated via single-component gas adsorption isotherms and IAST, indicate that the material should be capable of the high-capacity separation of O2 from air at temperatures as high as 226 K, well above the current temperatures employed in cryogenic distillation [82]. Cr3(BTC)2 and Fe2(dobdc), are representative members of large isostructural framework families that have been synthesized with a wide number of transition metal cations (M3(btc)2 where M = Cr, Fe, Ni, Cu, Zn, Mo, or Ru [201–206] and M2(dobdc) where M = Mg, Mn, Fe, Co, Ni, Cu, or Zn [99, 105, 112, 113, 135, 194– 196]). Despite this, no other material in the framework families has shown utility in the separation of O2 from N2. As such, recent work of Bloch et al. have explored a new framework family known as M-BTT with the sodalite-type structure, M3[(M4 Cl)3(BTT)8]2. While the aforementioned MOF family has also been synthesized with a variety of M2+ OMCs (M-BTT where M = Mn, Fe, Co, Cu, Cd); only the most recently synthesized Cr2+ analog (with a BET surface area of 2300 m2 g−1) shows

III

II

I

1.25(1) Å

Fe2(O2)2(dobdc)

2.09(2) Å

Fe2(dobdc)

2.10(1) Å

(b)

2.30(1) Å

1.6(1) Å

Fe2(O2)(dobdc)

2.19(3) Å

Fe2(N2)2(dobdc)

2.60(9) Å

(c) 1.13(2) Å

20 18 16 14 12 10 8 6 4 2 0

O2/N2 adsorbed (wt %) 0.0

0.2

0.4

0.6 P (bar)

0.8

1.0

N2 (298 K)

O2 (298 K)

O2 (226 K)

O2 (211 K)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Figure 1.27  Ball-and-stick model showing the partial structure of O2 adsorbed in Fe2(dobdc) with three O2 adsorption sites, labeled as I–III. Site I is bound in a side-on fashion to the Fe-OMC. (b) A close-up view of the Fe-OMC after activation and then after dosing with N2 and O2. The structures are for samples under vacuum (upper left), dosed with N2 at 100 K (upper right), dosed with O2 at 100 K (lower left), and dosed with O2 at 298 K (lower right). All structures were determined via Rietveld analysis of neutron powder diffraction data. Orange, blue, and red spheres represent Fe, N, and O atoms, respectively. All diffraction data were collected below 10 K. (c) Adsorption isotherms collected for Fe2(dobdc) at 211 (orange), 226 (purple), 298 K (red), and N2 adsorption at 298 K (blue). Filled and open circles represent adsorption and desorption, respectively. Source: Bloch et al. 2011 [82]. Reproduced with permission of American Chemical Society.

(a)

O2 adsorbed (per Fe)

60

CARBON CAPTURE IN METAL–ORGANIC FRAMEWORKS

utility in O2/N2 separations (Figure 1.28a) [133]. The single-component adsorption isotherms indicated a high selectivity for O2 over N2 as the material has a reasonably high O2 uptake of 7.01 wt% at 0.2 bar and 298 K and a low adsorption capacity for N2, which is less than 0.6 wt% at 0.8 bar and 298 K (Figure 1.28b). Consistent (a)

1.11(1) Å

1.26(2) Å 1.84(2) Å

180° 129(2)°

2.35(3) Å

Cr-BTT-O2

8

8

O2

7 6 5 4 3 2

N2

1 0

O2 adsorbed (wt %)

O2 or N2 adsorbed (wt %)

(b)

Cr-BTT-N2

6

4

2

0 0.0

0.2

0.4

0.6

P (bar)

0.8

1.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Cycle

Figure 1.28  (a) Ball-and-stick model of Cr-BTT and the first coordination spheres for the Cr centers within O2- and N2-dosed Cr-BTT as determined from Rietveld analysis of powder neutron diffraction data. Atom colors: Cr dark green, Cl purple, O red, N blue. Values in parenthesis give the estimated standard deviation in the final digit of the number. (b) Excess O2 and N2 adsorption isotherms collected for Cr-BTT at 298 K; the solid lines represent Langmuir–Freundlich fits to the data. Bottom: Uptake of O2 at 200 mbar in Cr-BTT over 15 cycles at 298 K. Adsorption experiments were performed over 30 min and desorption was carried out by placing the sample under a dynamic vacuum at 423 K for 30 min. Source: Bloch et al. 2016 [133]. Reproduced with permission of John Wiley & Sons.

Future Perspectives and Outlook

61

with the significantly higher O2 capacity compared to N2, the material exhibits an isosteric heat of −65 kJ mol−1 and −15.3 kJ mol−1 for O2 and N2, respectively. IAST calculations reveal that the material has a selectivity factor greater than 2500 under conditions necessary for O2 separation from air. As shown in Figure 1.28b, Cr-BTT displays only a moderate capacity loss after the first adsorption/desorption cycle, with a decreased uptake from 7.01 wt% to 5.6 wt%. After the second cycle, the capacity drops a bit further to 4.6 wt%, but then appears to plateau for the next 13 consecutive cycles. Despite this slight loss in capacity, the material shows rapid adsorption/desorption and a reversible capacity of 4.6 wt%. While the capacity loss is not yet fully understood, several in situ techniques were employed to understand the O2 binding mechanism. Infrared spectroscopy and neutron diffraction were again used to show experimental evidence of the electron transfer to form a Cr3+ superoxo species. The diffraction data are consistent with what was observed in the IR spectrum, showing an end-on coordination of the guest species and a Cr–O2 distance of 1.84(2) Å, a value that is in excellent agreement with other previously reported Cr3+-superoxo species (1.876(4) Å) [264]. As observed in the case of Fe2(dobdc), the O–O distance is slightly elongated to 1.26(2) compared to free O2, consistent with the formation of a superoxo. Further, Cr–N and Cr–Cl distances decrease from 2.064(3) Å and 2.57(2) Å to 2.026(4) Å and 2.52(2) Å, upon oxidation from Cr2+ to Cr3+, respectively [133]. All of the aforementioned work demonstrates the importance of having redox active OMCs in MOFs for selective separations of O2 over N2. While this work is dependent on existing framework families that can undergo metal substitution, there is a lot of room for further MOF development for air purification through the use of (i) metaloligands, which may offer a way to post-synthetically decorate MOF surfaces with under coordinated metals or (ii) through the infusion or appendage of other metal-containing small moleucles on the internal surface of MOFs. A proof of concept for redox active MOF-composites was highlighted by Zhange et al., which infused a well-known Cr-MIL-101 with the redox active Fe2+ containing ferrocene molecule [265]. Heating the material above 350°C led to a transformation of the ferrocene into maghemite nanoparticles rendering a composite MOF materials that exhibited a high selectivity for O2 over N2. Experimental breakthrough obtained using a custom-built apparatus equipped with a residual gas analyzer (in a gas mixture of 0.21% O2 and 0.79% N2) showed N2 breakthrough within 1 min, while O2 required 40 min. At any rate, MOFs offer an unprecedented opportunity to tailor-make materials with controlled capacities, selectivities, and regeneration energies [265].

1.6  Future Perspectives and Outlook The previous sections have highlighted some of the recent progress made in the advancement of MOFs toward carbon-capture applications. While we have confirmed that there are a number of frameworks currently available for the efficient

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separation of CO2/N2, O2/N2, and CO2/H2, there are still a number of factors that need to be addressed for their eventual implementation. More understanding of how MOF materials function in wet environments and in the presence of other minor impurities present in gas streams is a necessity. To achieve this, it will require experimentalists to characterize the materials in more application-relevant environments as in multicomponent adsorption and breakthrough analysis. Further, ongoing work should pay more attention to the temperatures and pressures necessary for the varying applications as these two factors vary a great deal throughout the literature, making it difficult on some occasions to gain an accurate comparison between two differing materials. Also, there are a number of reports that show disparities in measured values for various properties ranging from surface areas, adsorption capacities, selectivities, and isosteric heats. As such, more emphasis should be placed on sample quality and efforts should be made to understand how varying reaction conditions affect the crystallization and hence subsequent framework properties. Once the more fundamental problems are addressed, it is expected that MOF chemistry will naturally gravitate away from the synthesis of new materials and look toward addressing more engineering-related issues. Some of these areas include: (i) material scale-up and cost analysis, (ii) nanostructuring materials for pelletization, (iii) the impact of pelletization on MOF performance, (iv) assessing their performance over many adsorption/desorption cycles, and (v) accurate determination of the energy penalty associated with MOF regeneration. While we know that the parasitic energy cost of liquid amine scrubbers is approximately 30%, to the best of our knowledge, there are no thorough studies addressing the energy penalty and economic cost related to MOF adsorbents and so it is a necessity for the future. It is assumed throughout the literature that these materials will have better performance due to lower heats capacities, but a number of factors must be considered. For instance, there is not a lot of information pertaining to MOF thermal conductivity [266], a parameter that dictates the efficiency of the adsorbent bed and the duration of the regeneration cycle of a TSA capture process. In the same regard, there are few studies addressing the desorption of CO2 via PSA in any great detail either. While it is not discussed to a large extent throughout this chapter, it should be noted that there are a number of developing computational tools that might allow accurate structure and property prediction in MOFs [267]. Advancements in this area could provide experimentalists with target frameworks that will perform well in predefined gas mixtures and hence deliver these materials more rapidly to industry [268]. Though progress has been made through cooperative work between theoreticians and experimentalists to understanding MOF–small molecule interactions, the rate at which theoretical tools are being actively used to provide optimized MOF targets remains slow. One challenge, for instance, is the difficulty of developing synthetic pathways toward a specific structure containing the desired building blocks or the inability to predict structural changes that occur with adsorption. While these challenges are large, the partnership between experimentalists and theoreticians is

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becoming more prevalent throughout the literature and hence significant progress has been made in the area [83]. As such, we are confident that computational tools, including high throughput screening methods, could push MOF chemistry past the era of largely serendipitous discoveries and allow for engineering porous media for solving specific problems. Given the huge scientific progress made in the last 10 years within MOF research, we have an optimistic outlook on MOF chemistry for their eventual implementation in a wide number of energetically relevant gas separations. Unprecedented internal surface areas, facile structural tunability, and the ease with which MOFs readily undergo post-synthetic modification clearly distinguish this class of materials from other porous counterparts. It is demonstrated throughout the literature that MOFs offer a unique opportunity for controlled design, allowing one to find the intricate balance necessary between a variety of properties such as selectivity, gravimetric and volumetric working capacity, regeneration energy, and framework stability, making them likely candidates for future carbon-capture technologies.

ACKNOWLEDGMENTS This work was supported by the Swiss National Science Foundation under Grant PYAPP2_160581 and the Swiss Commission for Technology and Innovation (CTI).

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2 METAL–ORGANIC FRAMEWORKS MATERIALS FOR POST-COMBUSTION CO2 CAPTURE Anne M. Marti National Energy and Technology Laboratory (NETL), Pittsburgh, PA

2.1  INTRODUCTION: THE IMPORTANCE OF CARBON CAPTURE AND STORAGE TECHNOLOGIES The emission of carbon dioxide (CO2) from anthropogenic sources is one of the most growing worldwide environmental concerns today, as it is directly linked to global climate change [1]. The origins of such environmental concern trace back to the start of the industrial revolution; since then, the consumption of fossil fuels for electricity has increased in parallel to population rise attributing to the climbing CO2 levels, as depicted in Figure 2.1 [2, 3]. Specifically, the U.S. Department of Energy (DOE) estimates that in 2014, the concentration of CO2 in the atmosphere was ~400 ppm, a 40% increase since the start of the industrial revolution with a concentration of ~280 ppm CO2 [4–6]. The U.S. DOE estimates that the high CO2 concentration within the atmosphere is largely due to the combustion of fossil fuels (coal, natural gas, and oil), and levels will continue to climb as fossil fuels are expected to produce a large amount of energy in 2040 [7]. Carbon capture and storage (CCS) programs

Materials and Processes for CO2 Capture, Conversion, and Sequestration, First Edition. Edited by Lan Li, Winnie Wong-Ng, Kevin Huang, and Lawrence P. Cook. © 2018 The American Ceramic Society. Published 2018 by John Wiley & Sons, Inc.

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120

US total primary energy consumption Quadrillion BTU

60

0 1950

1975

2000

2015

Figure 2.1  The following graphical representation describes a trend of the total U.S. energy consumption from approximately 1950 to recent. BTU is British thermal unit, a traditional unit of work used to express power consumption. Source: Courtesy of U.S. Department of Energy 2015 [3].

are thus being pursued by the U.S. DOE and many organizations in the United States to reduce CO2 emissions from large point sources to combat climate change [8]. Challenges associated with CCS include the identification of materials that demonstrate a high affinity for CO2 with fast adsorption/desorption kinetics. Additionally, materials must be chemically and thermally stable, selective, scalable, and require minimal energy for regeneration [9]. The U.S. DOE considers implementing CCS technologies for three types of fossil fuel power plant systems, which include post-combustion, pre-combustion, and oxy-fuel combustion [10]. Post-combustion involves the capture of CO2 from flue gas streams, while pre-combustion methods capture CO2 from fuel before combustion, and oxy-fuel combustion burns the fuel in oxygen and produces a highly concentrated CO2 stream. Of these three methods, post-combustion is currently the most utilized method for power production. Therefore, CCS technologies that can be retrofitted to existing post-combustion power plants will be the focus of this chapter. The implication of CCS program technologies targeted for post-combustion should achieve 90% capture at an increase in cost of less than 20 percent. This translates to a cost of CO2 separation and compression of $30–$50 per ton of CO2 [2, 10].

2.1.1  Post-combustion CO2 Capture Technologies Post-combustion carbon capture involves the removal of CO2 from flue gas streams generated from the combustion of coal. Flue gas consists of ~10–15% CO2,

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~70–75% N2, with the remaining components water (5–7%), O2 (3–4%), CO (~200  ppm), SOx (40 kJ mol−1 reflecting its strong interaction between the MOF and CO2. The OMS are induced binding sites for incoming guests that are generated within the MOFs’ pores at the metal node upon solvent removal through vacuum and heat. The OMS once generated can bind to incoming CO2 molecules through dipole/quadrupole interactions thus reflecting the relatively high Qst value. Overall, OMS often lead to a high adsorption capacity for CO2, fast adsorption kinetics, and good selectivity; however, a few drawbacks are also commonly observed such as competitive adsorption with water [75], and a large energy penalty upon regeneration. The classic example of this is demonstrated with MOF-74, also referred to as M/ DOBDC (M = Mg, Ni, Co, Zn, Mn; DOBDC = dioxybenzenedicarboxylate) [76, 77]. MOF-74 consists of hexagonal, large one-dimensional channels, is 11 Å in size, and is highly studied for CCS [78, 79] as its OMS can interact with CO2 [80]. MOF-74 is also an interesting MOF that shows promise for other applications as well; one example is H2 storage [81]. Specifically, Mg MOF-74 exhibits the highest CO2 uptake at low pressure as compared to other MII MOF-74 analogs due to the high Mg–O ionic bond character (Figure 2.5) [76]. Mg MOF-74 has a Qst for CO2 = 42 kJ mol−1 at 25°C and 0.15 bar pressure. This will be referred to as general capture conditions throughout the chapter. The Qst for CO2 of Mg MOF-74 exceeds its other isostructural structures, MII = Ni, CO, Zn, and Mn (Figure 2.5) making it the more promising analog for CCS. Although the CO2 adsorption capacity and selectivity of Mg MOF74 are impressive, it suffers from competitive adsorption with water, and also experiences reduced capture performance not recoverable in the presence of humidified flue gas [75, 83]. As demonstrated by Canepa et al. [84], water is anticipated to bind more strongly with the MOFs’ OMS than CO2. However, Canepa hypothesized that MOF-74 containing select MII noble metals such as Rh, Pd, Os, Ir, and Pt does not demonstrate such competitive adsorption and is therefore a promising candidate to be considered for future CCS studies.

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~ CO2 Adsorption M x+ MOF-74 Qst CO2 (kJ/mol) capacity (mmol/g) 4.95 Mg 42 3.9 Ni 37 3 Co 33 1.25 Zn 24 2.67 Mn 28 Figure 2.5  MgMOF-74 graphic (left) [82]; MIIMOF-74 characteristics at 20°C and 0.15 bar pressure (right) [56]. Source: Andirova et al. 2016. Reproduced with permission of Elsevier.

Other MOFs such as HKUST-1 and MIL MOFs also contain OMS and are widely studied for CCS [85] and other applications [86–89]. HKUST-1, also referred to as Cu-BTC, consists of copper metal ions connected by benzene-1,3,5-tricarboxylate linkages forming a cubic lattice with 9 × 9 Å square-shaped pores. HKUST has a good adsorption capacity for CO2, however, not at post-combustion capture conditions (0.55 mmol g−1 at 25°C, 0.15 bar). Although HKUST-1 has poor CO2 capture performance, it is a highly studied MOF for CCS as it can be synthesized using a variety of methods [90] and is scalable [91] and thermally stable [92]. One major drawback is HKUST-1’s hydrophilicity; it is susceptible to competitive adsorption with water, similar to MOF-74 [93]. Al-Janabi et al. [94] studied HKUST-1 in the presence of water vapor for CO2 adsorption from flue gas. They found that the OMS of HKUST-1 bind strongly to water leading to long desorption times (i.e., slow desorption kinetics). The high binding strength of the OMS of HKUST-1 to water reflects an expected energy intensive regeneration, as it requires high temperature and vacuum to reactivate (evacuation of the pores) the MOF for repeatable adsorption. Adsorbed moisture within HKUST-1 also leads to decomposition of the framework [93, 94], an occurrence similar to the one found to happen in traditional MOF-5 [95]. MOFs from the MIL class of materials (MIL = Matérial Institut Lavoisier) are flexible frameworks that often respond to external stimuli. For example, MOF MIL-53, consisting of MO4(OH)2 octahedra (M = Al3+, Fe3+, Cr3+), connected by 1,4-benzenedicarboxylic acid, demonstrates a two-stage step isotherm for CO2 adsorption dependent on the pressure [96]. Such flexibility is not favored for post-combustion, however, may be useful for high-pressure systems. MIL-101 Cr, consisting of CrIII metal nodes connected by 1,4-benzenedicarboxylic acid linkers, forms a porous super-tetrahedron structure with 5.5 and 8.6 Ǻ pore windows and a cage size of 1.2 and 1.6 nm. MIL-101 Cr can be synthesized using hydrothermal synthesis routes and is chemically and thermally stable. MIL-101 Cr does not have the same flexibility as exhibited in MIL-53, and has permanent porosity instead. MIL-101 Cr has a CO2 adsorption capacity of 1.86 mmol g−1 at 46°C and 1 bar pressure (~0.2 mmol g−1 capacity at 0.15 bar, 46°C), and high CO2 adsorption capacity at high pressures (40 mmol g−1 at 10 bar [97, 98]. MIL-101 Cr also maintains a high

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89

level of performance in the presence of trace contaminants such as NOx, SOx, and water, even after multiple adsorption cycles with mild regeneration [99]. Therefore, its high stability and adsorptive nature make it a promising candidate for pre-combustion capture. Functionalization employed to OMS MOFs often improves their CO2 adsorption capacity and selectivity. For example, the water stable MOF, CuBTTri [100], consisting of Cu2+ ions connected between 1,3,5 tris(tetrazol-5-yl) benzene linkers was functionalized with the ethylenediamine (em). In this study, the MOFs’ Cu2+ OMS were modified post-synthesis with em in order to enhance its affinity for CO2 [101]. Demessence et al. [101] studied the adsorption differences between the as synthesized CuBTTri and en-CuTTri MOF, and observed CO2 adsorption enhancement at pressures 3 mmol g−1). MOF, mmen-CuTTri also suffers from framework instability, and pore blocking from the excess amines [102]. Following this, McDonald et al. [82] functionalized a modified MOF-74 structure (M2dobdc, M = Mg, Mn, Fe, Co, Ni, Zn) that has an increased channel size of 18.4 Å with mmen. In this study, mmen-Mg2dobdc was found to have exceptional CO2 adsorption capacity of 3.13 mmol g−1 at post-combustion capture conditions and 3.86 mmol g−1 at 1 bar, 25°C. The Qst for CO2 is ~70 kJ mol−1 at low pressure (ppm range) as a result of the amine binding the intended CO2. However, as these sites saturate near capture conditions, the Qst value drops to ~30 kJ mol−1. The IAST selectivity for CO2/N2 was found to be ~200, which is less than the value found for mmen-CuTTri (327), but with a higher adsorption capacity [82]. MOF mmen-M2dobdc also exhibited fast adsorption/ desorption kinetics in the presence of dry flue gas; however, its structure still has a strong affinity toward moisture. To deviate from this problem, elevated temperatures can be used so moisture will not have a chance to bind to the structure; as such at 100°C, mmen-M2dobdc sorbent adsorbed ~90% less water as compared to capture conditions at 40°C. However, operating at such elevated temperatures reduces the MOF’s CO2 adsorption capacity [103]. Lee et al. [104] then investigated primary/tertiary amine functional group N,N-dimethylethylenediamine (dmen) within the Mg2dobdc structure. They found that changing the amine chemistry increased the CO2 adsorption capacity further to 3.77 mmol g−1 at capture conditions with a

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selectivity of 554. Similar amine functionalization schemes have been employed for the moisture stable MIL-101 Cr MOF. Lin et al. [105] incorporated polyethyleneimine (PEI, 300 g mol−1 molecular weight) into the MIL-101 Cr structure which resulted in a high CO2 adsorption capacity with fast kinetics with good selectivity at capture conditions. The OMS of MIL-101 Cr were used to anchor the PEI at various weight percent loadings to the framework, as shown in Figure 2.6. At 100 weight percent loading PEI in MIL-101 Cr, the CO2 adsorption capacity increased from ~0.33 mmol g−1 to 4.20 mmol g−1, as described in Figure 2.6. Lin et al. [106] then studied the effects of smaller particle sizes of MIL-101 Cr in order to enhance the surface area and maximize the PEI loading. They determined that the smaller particle size enhances the adsorption capacity with low molecular weight PEI. However, large

OH

H2O

H 2O

open site NH2 H2N

CO2 adsorbed (mmol g−1) 50°C

25°C PEI loading SBET (m2 g−1) VTotal (cm3 g−1) Occupancy (wf%) *(%) *(%) *(%) 0.15 bar 1 bar MIL-101 PEI-MIL-101-50 PEI-MIL-101-75 PEI-MIL-101-100 PEI-MIL-101-125

0 50 75 100 125

3125.4 1802.7 1112.6 608.4 182.9

1.629 0.901 0.526 0.292 0.095

0 31 46 61 77

0.33 2.40 3.67 4.20 3.85

1.60 4.00 4.64 5.00 4.35

0.15 bar

1 bar

0.20 1.86 3.17 3.40 3.95

1.00 3.07 4.02 4.14 4.51

Figure 2.6  Top: A representation of the interaction between PEI and the OMS of MIL-101 Cr. Bottom: A measure of the properties of MIL-101 Cr with different PEI loadings. Source: Lin et al. 2013 [105]. Reproduced with permission of Nature Publishing Group.

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molecular weight PEI cannot readily diffuse into the pores of MIL-101 Cr; therefore, a larger pore MOF will be needed for large molecular weight incorporation. Overall, Lin [105–107] deduced that the optimization of the amine molecular weight is the determining factor for the effectiveness of this capture sorbent system. Although their functionalization strategy demonstrates promising attributes, the decomposition of the PEI within the framework is inevitable, as at elevated temperatures the kinetics slow and the amine begins to degrade from the harsh regeneration steps. 2.2.2.2  MOFs with Saturated Metal Centers  Several examples of MOF sorbents studied for CCS consisted of OMS and amine functionality, leading to high CO2 adsorption capacity, fast kinetics, and good selectivity in the presence of dry flue gas. Drawbacks of such MOF sorbents include performance poisoning from moisture in the gas stream and/or lack of scalability and potential high cost from material synthesis and the regeneration energy penalty. To combat these drawbacks, scientists also considered microporous MOFs for CCS that contained saturated metal centers (SMC), relatively narrow pores, and functional linkers. SMC MOFs are often thermally and chemically stable even to water, and have high selectivity for CO2 over N2 and fast adsorption kinetics to saturation. Drawbacks include a relatively low CO2 adsorption capacity as a result to the MOF’s small pores, which can be a cost concern as more material may be needed to capture the intended CO2. Overall, if a microporous MOF is chosen for CCS, proper engineering of the capture system will have to be implemented in order to meet post-combustion capture targets. One example of a well-characterized SMC MOF is the zirconium (IV)-based MOF called UiO66 Zr (UiO for University of Oslo) [108], which is a highly studied microporous MOF for CCS [109, 110] because of its high water and thermal stability up to 500°C. The UiO66 Zr structure consists of Zr6 octahedron connected with twelve 1,4-benzenedicarboxylic acid linkers forming the three-dimensional structure with a pore between 6 and 7 Å. UiO66 Zr is a highly characterized material that can be synthesized in a variety of particle sizes (nm to µm sizes [111], see Figure 2.7), is scalable [112], and can be functionalized easily [113–115]. UiO66 Zr, without any additional functionality within its structure, has a CO2 adsorption capacity of ~0.54 mmol g−1 (2.37 weight percent) at post-combustion capture conditions and a selectivity ~32 (IAST; 15:85, CO2:N2) [116]. Although its initial CO2 uptake is low, it can be easily functionalized, thus changing its properties. For instance, Cmarik et al. [116] studied the adsorption properties of functionalized UiO66; functional groups NH2, NO2, 1,4-napthyl, and 2,5-dimethoxy-terephthalic acid (O(CH3)2). They found that the amine functional group had the largest impact on the UiO66 structure. The CO2 uptake was enhanced to ~0.88 mmol g−1 (3.9 weight percent) and the selectivity for CO2/N2 increased to ~50 at capture conditions. The increased adsorption capacity and selectivity are reflected in the CO2 heat of adsorption; the Qst for UiO66 was ~22 kJ mol−1 and was increased to ~27 kJ mol−1 upon functionalization. The addition

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UiO66

1 μm

View: 1 0 0

Figure 2.7  Structure representation of UiO66 Zr in the 1 0 0 direction (left) and SEM image of ~200 nm nano-crystals (right).

of the amine functional group also improved the materials selectivity for CO2 over water [117]. Although UiO66 is stable in the presence of moisture, it is still relatively hydrophilic; however, the stronger interaction between the MOFs framework and CO2 reduces its affinity for moisture improving its performance. The functionalization of UiO66 Zr is one example of a strategy to improve the capture performance of SMC MOFs. Other strategies such as framework ionization [118], ligand exchange [119], and mixed ligand MOF synthesis [120] also demonstrate adsorption enhancements, increased Qst for CO2 at low pressures, and improved selectivity upon treatment. Zeolitic imidazolate frameworks (ZIFs) are a sub-class of MOFs that are structurally similar to inorganic zeolites, they are also microporous, have SMCs, and are very stable. ZIFs consist of metal ions or clusters connected by imidazole-based linkers to form one-, two-, and three-dimensional materials that crystallize in zeolite topologies. More specifically, a ZIF’s metal node connects to the imidazole at a 145° angle, which is the same as a traditional zeolite; see Figure 2.8 [121, 122]. ZIFs are typically chemically and thermally stable [123], are scalable [124], and can be synthesized using a large variety of methods [74, 125]. Overall, many ZIFs have been considered for CCS due to their ease of functionalization [125–127]. Banerjee et al. [128] studied ZIF-78 and other functionalized ZIFs crystalizing in the GME zeolite topology for its CO2 capture performance. ZIF-78 consists of Zn2+ metal centers connected by 5-nitroimidazole and has a pore size of 7.1 Å and polar NO2 functionality to induce dipole–quadrupole interactions with CO2. ZIF78 was estimated to have a high selectivity of ~50 at low pressure for CO2/N2 exceeding other functionalized ZIFs and an adsorption capacity of ~0.68 mmol g−1 at capture conditions. Huang et al. [129] studied the mixed ligand material, JUC-160, consisting of Zn2+ metal centers connected by ligands benzimidazole and 2-methylbenzimidazole. JUC-160 crystalizes in the GIS topology, is thermally and chemically stable, and

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ZIF

Zeolite O

N

Si

N

M

M

Si 145°

145°

ZIF-7

ZIF-8

Figure 2.8  Structural representation of a zeolitic imidazolate framework as compared to a zeolite (top); two examples of ZIFs: ZIF-7, 8 with corresponding linkers shown (bottom). Source: Park et al. 2006 [121]. Reproduced with permission of PNAS.

has potential for CCS, exhibiting a higher uptake as compared to other CCS ZIFs (~1 mmol g−1 CO2 uptake). ZIF-78 and JUC-160 are prime examples of the trade-off between an MOF sorbent containing SMC and the choice of an MOF with OMS; the ZIF with SMC is highly stable, and selective, whereas an MOF with OMS has high gas uptake using less material and is selective, but typically not stable and expensive. On another note, ZIF materials, although they have not reached capture targets yet, demonstrate creative chemical and structural design expected to transform CCS and other applications combined. For instance, Pan et al. [130] produced N-decorated porous carbons upon carbonization of JUC-160 ZIF, they observed a large CO2 adsorption capacity enhancement from ~1 mmol g−1 to 5.50 mmol g−1 at 1 bar, 0°C (3.50 mmol g−1 at 25°C). Liu et al. [131] also reported a creative method to employ a ZIF for CCS through the combination of ZIF-8 with a liquid glycol adsorbent. The hybrid, slurry, system up-took 1.25 mmol g−1 CO2 (as shown in Figure 2.9) with a selectivity of ~394 at 1 bar, and low energy requirement (enthalpy of 29 kJ mol−1) as compared to other liquid glycol solvents, demonstrating that ZIFs can be used in creative ways and setting new limits for porous materials [131]. Rational synthetic design of SMC MOFs has also extended to interpenetrating square grid nets. Gable et al. [132] developed the first recognized porous material with interpenetrating square grids known as Zn(bipy)2SiF6·2H2O (bipy = 4,4′bipyridine). The framework consists of independent sheets forming a square grid; one infinite sheet contains four bipy linkers (propeller arrangement) connected to each octahedral zinc metal center, which is also connected to two water molecules.

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METAL–ORGANIC FRAMEWORKS MATERIALS FOR POST-COMBUSTION CO2 CAPTURE

Ab(d)sorption amount (mol l−1)

3.0 Gas phase

ZIF-8

Slurry phase CO2

Other gas molecule

CO2 CH4 N2 H2

2.5 2.0 1.5 1.0 0.5 0.0

0

10

20

30 40 50 Pressure (bar)

60

70

Figure 2.9  Representation of the ZIF-8 glycol slurry and gas isotherms for CO2, CH4, N2, and H2. Source: Liu et al. 2014 [131]. Reproduced with permission of Nature Publishing Group.

Another infinite sheet is perpendicular to the first forming a square grid, and SiF6 anions are found hydrogen-bonded to the zinc corners, see Figure 2.10. The interpenetrating square grid design grabbed the attention of Zaworotko’s [133] and Kitagawa’s [134] research groups. They built on this chemistry and developed square grid MOFs consisting of metal centers connected by charged inorganic SiF6 pillars and neutral organic ligands forming an overall uncharged framework that has strong interactions with CO2. Metal centers such as Zn2+ and Cu2+ formed 3D-SMC MOFs with high surface areas and promising gas storage capabilities, and they are called SIFSIX MOFs [135–138]. The applicability of SIFSIX MOFs was realized for carbon capture by Zaworotko’s group when Burd et al. [137] b

c

C (1) C (2) C(3)

Zn N C(5)

C(4)

a

Figure 2.10  Unit cell of Zn(bipy)2SiF6·2H2O is shown on the left demonstrating the Zn SMC connected to four bipy linkers, SiF6 omitted for clarity. The framework interpenetrating sheets are shown on the right, zinc centers only shown for clarity. Source: Gable et al. 1990 [132]. Reproduced with permission of Royal Society of Chemistry.

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METAL–ORGANIC FRAMEWORKS AS SORBENTS

produced SIFSIX-2-Cu bipy (4,4-bipyridine) and SIFSIX-2-Cu bpy (1,2-(bispyridyl) ethylene)). Within this study, they found that the SMC combined with moderate pore size and surface area, and the electrostatic contributions from the inorganic pillars induced a high CO2 affinity with exceptional selectivity. MOFs SIFSIX-2-Cu bipy and bpy were thus found to adsorb CO2 strongly at capture conditions and had excellent selectivity for CO2 over methane [137]. Nugent et al. [139] then studied similar MOFs: SIFSIX-2-Cu, SIFSIX-2-Cui, and SIFSIX-3-Zn. MOF, SIFSIX-2-Cu, is a 3D-primitive cubic net material consisting of CuSiF6 metal nodes connected by linear linker, dipyidylacetylene (dpa), [Cu(dpa)2(SiF6)]n. It has a pore size of 13.05 Ǻ and a high surface area of ~3000 m2 g−1. SIFSIX-2-Cu has a CO2 adsorption capacity of 0.45 mmol g−1 at capture conditions, thus reflecting on its mild Qst value (22 kJ mol−1), which may be attributed to its relatively large pore size. The properties of SIFSIX-2Cu, however, can be enhanced for CCS. Nugent et al. [139] fabricated SIFSIX-2-Cui (i = interpenetrated) by introducing a second interpenetrating net into SIFSIX-2-Cu; here, the pore size is reduced and the density of SiF6 anions is enhanced. Its structure is shown in Figure 2.11. SIFSIX-2-Cui has a pore size of 5.1 Ǻ, and half the surface area of SIFSIX-2-Cu; however, its CO2 adsorption capacity has increased by more than six times (2.8 mmol g−1 at capture conditions). The enhancement in CO2 uptake is also reflected in its Qst for CO2,

Cu SiF6

Zn/Cu SiF6

N

N SIFSIX-2-Cui

N

N

SIFSIX-Zn/Cu-3

Figure 2.11  Representation of SIFSIX-2Cui (left), and SIFSIX- Zn or Cu-3 (right). Source: Shekhah et al. 2014 [140]. Reproduced with permission of Nature Publishing Group.

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32 kJ mol−1, as compared to 22 for the non-interpenetrated analog. In addition, Nugent [139] demonstrated the versatility of the inorganic pillar by synthesizing SIFSIX3-Zn, [Zn(pyr)2(SiF6)]n. It is a 3D-primitive cubic material that has a reduced pore size of 3.8 Å, the narrow pores are a result of the combination of pyrazine linkages instead of dipyridylacetylene, connected to ZnSiF6. No interpenetration is observed with SIFSIX-3-Zn as shown in the structure representation in Figure 2.11. SIFSIX3-Zn exhibits stronger interactions between its framework and CO2 in comparison to SIFSIX-2Cui aspiring from the SiF6 inorganic pillars crowded within a smaller pore space. SIFSIX-3-Zn has a higher CO2 adsorption capacity at low pressure reflected in both its very steep adsorption slope and its Qst for CO2 (45 kJ mol−1). SIFSIX3-Zn demonstrates remarkable CO2/N2 selectivity of 1818, considerably higher than SIFSIX 2Cui (140) and other MOFs such as OMS Mg MOF-74 (182) [62]. Following Nugent’s work, researchers Shekhah et al. [140] and Elsaidi et al. [141] developed iso-structural SIFSIX-3 analogs with metal centers (Cu, Ni, and Co). Herein, they observed that through the reduction in pore size, SIFSIX-3 Zn < Ni < Cu, the affinity for CO2 increases as observed in the Qst for CO2. The higher the Qst value, the more selective the SIFSIX MOF is. For example, SIFSIX-3-Cu, pore size of 3.5 Ǻ, has a Qst of 54 kJ mol−1, and an overall CO2/N2 (10:90) selectivity estimated using IAST calculations to be ~2000. SIFSIX-3-Zn (Qst = 45 kJ mol−1) has a CO2/N2 selectivity of 1818; as such, the 0.3 Å pore reduction between Cu and Zn structures induces a stronger interaction with CO2. The varying properties of all SMC SIFSIX MOFs are described in Table 2.1 and compared to other OMS MOFs as well. The tolerance to moisture and ultimate stability of the SIFSIX series has been under investigation, as the SIFSIX MOFs are of interest for not only post-combustion carbon capture but for direct air capture [142] and acetylene capture from ethylene [143]. The high adsorption capacity for CO2 and selectivity with mild regeneration conditions make these materials promising as CO2 sorbents. In addition, Madden et al. [117] found that the SIFSIX MOFs all adsorb 60% and above CO2 in humidified gas streams and thus outperforming top performing OMS MOFs HKUST-1, Mg MOF 74. 2.2.2.3  MOF Sorbent Summary  Within sections 2.2.1 and 2.2.2, several top performing MOFs that are either highly characterized or of interest today have been discussed. Two distinct MOF types have been introduced for CCS sorbents, MOFs with OMS and SMS, both having positive and negative aspects. For example, MOFs with OMS such as Mg MOF-74 are typically mesoporous and are selective for CO2 over N2, as they have binding sites available to interact with incoming CO2. The OMS MOFs may also be post-functionalized with amines to enhance its already high adsorption affinity for CO2, as described in Section 2.2.1. The strength of the interaction between CO2 and the MOF is represented by the heat of adsorption (Qst) value for CO2, which is typically greater than 40 kJ mol−1. To summarize, OMS MOFs often demonstrate a high CO2 adsorption capacity, approaching DOE targets with only a small amount of material, and high selectivity for CO2/N2. The drawbacks of OMS include difficulty in regeneration; the MOF may bind too strongly to the

870 3000

11

—

Mg MOF74 [56, 62]

mmen-CuTTri [102]

3.5

SIFSIX3 Cu [62, 140, 141]

300

223

368

1468

FG, functional group. Dash (—) refers to data that were not available. a CO2:N2 composition of 15:85. b Determined using breakthrough measurements.

3.5

SIFSIX 3 Co [141]

250

3.8

3.7

SIFSIX 3 Zn [139]

SIFSIX 3 Ni [141, 144]

735

8

5.1

SIFSIX 2 Cu bipy [137]

SIFSIX 2 Cui [139]

740

13

10.6

SIFSIX 2 Cu [139]

SIFSIX 2 Cu bpy [137]

1495

3270

18.4

mmen-Mg2 dobdpdc [82]

Surface area (m2 g−1)

Pore size (Å)

MOF and references

54

47

51

45

32

27

21

22

48

47

70

Qst CO2 (kJ mol−1)

2.45/2.60

2.50/2.8

2.65/2.93

2.38/2.54

1.72/5.41

0.9/8.5

0.70/2.8

0.45/1.84

2.38/4.2

4.95/—

3.13/3.86 No FG 4.82/6.42

CO2 (mmol g−1 at 0.15/1 bar)

MIL-101 > LTA-5A. Overall, ZJU-35a can be ranked as one of the few best porous MOFs for high pressure hydrogen purification when regeneration cost, gravimetric and volumetric production capacities need to be balanced.

3.2.3  Capture CO2 Directly from Air In addition to removing CO2 from flue-gas, it is also crucial to develop an economic approach to reduce CO2 content in the atmosphere through CO2 removal from air. But it is particularly challenging because the suitable adsorbents have to combine optimum uptake, kinetics, energetic, physical/chemical stability and CO2 selectivity over competing gases and vapors at atmospheric CO2 concentrations [208]. To date, direct air capture systems have typically employed solid organo-aminebased chemisorbents, requesting elevated temperatures (>100°C) for regeneration. Recently, some research work has demonstrated that MOFs provide a possible way to realize practical CO2 capture directly from air.

−O

H

O

−O

−O

O

O

O

H

O

O

X

O−

H

O−

H

O−

O

O

O

H

O

H

O

X

H

+ Cu2(CO2)4

O−

+ Cu2(CO2)4

O−

+ Cu2(CO2)4

O−

−O

O

H

O

O− −O H

O H

X

(III) H

H

O O−

0

20

40

60

80

100

10

20

30

40

50

0

0

4

6

8

10

CO2 CH4 H2

MgMOF-74 NaX Cu-TDPAT CuBTC LTA-5A ZJU-35a ZJU-36a MIL-101

15

20

25

30

Breakthrough calculations: CO2/CH4/H2 mixture: ρ1 = 1.5 MPa; ρ2 = 1 MPa; ρ3 = 2.5 MPa; ZJU-35a 298K;

10

Dimensionless time, τ = t u l ε L

5

(c)

Absolute loading, g / mol kg−1

2

pure CO2

(a)

(b)

0

2

4

6

8

10

12

0

(d)

1

2

4 Total gas pressure, ρt / MPa

LTA-5A

MgMOF-74 NaX MIL-101

Cu-TDPAT ZJU-36a

3

5

1 6 Total gas pressure, ρ1 / MPa

MgMOF-74 NaX Cu-TDPAT MIL-101 UT SA-40a LTA-5A ZJU-35a ZJU-36a CuBTC

ZJU-35a CuBTC

0 0.1

5

10

15

20

25

IAST calculations: CO2/CH4/H2 mixture: 298 K: ρCO2/ρCH4/ρH2 = 30/20/50

6

Figure 3.7  Left: (a) m-Benzenedicarboxylate organic building unit (I) and two new expanded ones ((II) and (III)). Self-assembly of (b) H3btc and (c–d) two new organic linkers with paddle-wheel Cu2(CO2)4 unit leads to the construction of isoreticular porous MOFs (b) Cu-btc, (c) ZJU-35, and (d) ZJU-36 whose pores (spheres in purple and orange) are systematically enlarged (Cu, green polyhedra C, gray O, red; H atoms are omitted for clarity). Right: (a) Comparison of isosteric heats of adsorption, Qst, of CO2 in ZJU-35a, ZJU-36a, Cu-btc, Mg-MOF-74, Cu-TDPAT, MIL-101, NaX, and LTA-5A. The calculations of Qst are based on the use of the Clausius-Clapeyron equation. (b) Calculations using ideal adsorbed solution theory (IAST) of Myers and Prausnitz (CO2 + CH4) uptake capacity, expressed as moles per kg of adsorbent, in equilibrium with a ternary CO2/CH4/H2 30/20/50 gas mixture maintained at isothermal conditions at 298 K. (c) Breakthrough characteristics of an adsorber packed with ZJU-35a maintained at isothermal conditions at 298 K and 5 MPa. (d) Influence of operating pressure on the number of moles of 99.95% + pure H2 produced per kilogram of adsorbent material during the time interval 0–τbreak. The breakthrough times, τbreak, correspond to those when the outlet gas contains 500 ppm (CO2 + CH4). Source: Reproduced with permission of John Wiley & Sons.

(d)

(c)

(b)

O

O

(II) Isosteric heat of adsorption, - Qst/kJ mol−1 Mol % in outlet gas

(I)

99.95% pure H2 produced during 0-𝜏Dec-at / mol kg−1

(a) (CO2 + CH4) uptake capacity / mol kg−1

165

1.89

0.930

2.22

1.183

1.06

MOF-177

Cu-tdpat

DUT-25

ZJU-25a

PCN-16

0.630

0.63

401

446

1.599

2.16

ZJU-36a

MOF-205

UTSA-20

282

423

1.074

2.15

ZJU-5a

MIL-101c

Mg-MOF-74

250

1.36

1.01

PCN-61

PCN-46

289

227

299

467

193

450

154

300

312

1.156

1.29

ZJU-35a

280

NU-125

1.080

NOTT-101

121

193

0.91

0.47

PCN-11

Ni-MOF-74

NU-111 0.748

0.724

0.622

0.415

0.783

0.43

0.909

0.909

0.38

0.495

0.44

0.678

0.619

0.56

0.578

0.657

0.683

1.206

382

447

660

360

654

321

323

750

578

641

426

476

523

518

458

445

260

416

777

493

2.09

MOFs 0.409

Excess uptake Vp (cm3 g−1) BET (m2 g−1) Dc (g cm−3) (cm3 g−1)

Ta b l e 3.3  CO2 uptakes of some reported MOFs at RT and 30 bar

276

278

274

282

281

291

294

285

287

282

289

295

293

299

301

304

313

311

318

416

486

732

390

716

342

343

821

629

710

461

507

568

560

495

481

275

445

856

301

302

304

306

308

310

312

312

312

313

313

314

318

324

326

328

332

333

350

Excess uptake Absolute Absolute uptake (cm3 per cm3) uptake (cm3 g−1) (cm3 per cm3)

[198]

[204]

[190]

[49]

[42]

[196]

[189]

[42]

[207]

[188]

[187]

[186]

[185]

[184]

[207]

[183]

[196]

[198]

[199]

Reference

510

624

198

0.603

3.60

0.650

3.59

MOF-505

MOF-210

ZIF-8

MOF-200

163

453

154

178

0.650

0.701

UTSA-40

Cu-btc

385

614

2.82

1.47

NU-100

PCN-80

403

1.701

Be-BTBb

190

230

1.10

1.08

MIL-100

209

175

SNU-50'

2.13

0.830

PCN-68

IRMOF-11

380

0.713

1.55

Cu-BTTri

MOF-5

400

216

1.63

0.853

PCN-66

547

367

2.91

1.52

DUT-49

SNU-77H

IRMOF-3

251

0.993

IRMOF-6

0.22

0.924

0.25

0.926

0.879

0.827

0.574

0.279

0.423

0.650

0.70

0.760

0.38

0.593

0.789

0.634

0.45

0.586

0.308

0.650

766

194

766

226

240

259

376

793

547

361

339

325

646

422

331

415

573

459

851

426

169

179

192

209

211

214

216

221

231

235

237

247

245

249

261

263

258

269

263

277

883

215

884

246

263

285

424

896

599

397

375

352

721

473

353

443

631

490

947

458

194

199

221

228

231

236

243

250

253

258

262

268

274

279

279

281

284

287

292

298

[42]

[198]

[42]

[55]

[55]

[207]

[206]

[192]

[188]

[83]

[188]

[55]

[185]

[185]

[202]

[55]

[185]

[191]

[195]

[55]

144

NEW PROGRESS OF MOFs IN CO2 CAPTURE

Amine grafting chemistry has been adopted for the first time in Mg-MOF-74 as a support for CO2 adsorption from ultradilute gas streams such as ambient air [209]. Long and co-workers investigated the effect of N,N-dimethylethylenediamine grafting into Mg2(dobpdc). At 25°C and 0.39 mbar, near the current partial pressure of CO2 in Earth’s atmosphere, mmen-Mg2(dobpdc) adsorbed 2.0 mmol g−1 (8.1 wt%), which is 15 times the capacity of Mg2(dobpdc). Besides high capacity, its especially high selectivity and fast kinetics for adsorbing CO2 from dry gas mixtures with N2 and O2 makes it an attractive new adsorbent for applications in which zeolites and inorganic bases are currently used, including the removal of CO2 from air [100]. However, another MOF with NH2 group reported by Xiang’s group showed up that the amine amino groups have negligible interaction with the CO2 guests in air condition. They synthesized four isoreticular MOFs [Zn(Trz)(R-BDC)1/2] (FJU-40-R, R = H, NH2, Br, or OH; Trz = 1,2,4-triazole; H-BDC = terephthalic acid) and demonstrated that FJU40-NH2 is the first MOF example for which direct structural evidence of CO2 capture from air has been obtained. The structures of CO2-loaded FJU-40-NH2 suggest that the mechanism for CO2 loading into the cages depends on the CO2 partial pressure. In contrast to the direct structural evidence observed at high pressure, the C–H groups from triazolate act as the functional sites playing the predominant roles in CO2 capture under low CO2 partial pressures. The amine groups only capture the competitive water molecules rather than CO2 upon exposure of the sample to air (Figure 3.8). Also, to tune the pore size of physical adsorption-based MOFs is also available to target trace and low-concentration CO2 removal. Shekhah et al. built up an isostructural MOF (SIFSIX-3-Cu) based on pyrazine/copper(II) two-dimensional periodic 44 square grids pillared by silicon hexafluoride anions allowing further contraction of the pore system to 3.5 versus 3.84 Å for the parent zinc(II) derivative. The resultant SIFSIX-3-Cu exhibits very high CO2 sorption energetics but fully reversible adsorption–desorption operations at very mild conditions. Further, Eddaoudi’s group explored NbOFFIVE-1-Ni, with the appropriate pore size, shape, and fluorine moieties, ideal for the effective and energy-efficient removal of trace carbon dioxide [211]. The precise localization of the adsorbed CO2 at the vicinity of the periodically aligned fluorine centers, promoting the selective adsorption of CO2, is evidenced by the single-crystal X-ray diffraction study on NbOFFIVE-1-Ni hosting CO2 molecules. Markedly, the CO2-selective NbOFFIVE-1-Ni exhibits the highest CO2 gravimetric and volumetric uptake (~1.3 mmol g−1 and 51.4 cm3 (STP) cm−3) for a physical adsorbent at 400 ppm of CO2 and 298 K. Practically, NbOFFIVE-1-Ni offers the complete CO2 desorption at 328 K under vacuum with an associated moderate energy input of 54 kJ mol−1, typical for the full CO2 desorption in conventional physical adsorbents but considerably lower than chemical sorbents. Cyclic CO2/N2 mixed-gas column breakthrough experiments under dry and humid conditions corroborate the excellent CO2 selectivity under practical carbon capture conditions. Pertinently, the notable hydrolytic stability positions NbOFFIVE-1-Ni as the new benchmark adsorbent for direct air capture and CO2 removal from confined spaces.

145

SURVEY OF TYPICAL MOF ADSORBENTS

(a)

FJU-40-NH2-a dry ice > 1 atm

(b)

(c)

NH2-PMOF-55:1.5 CO2

(d)

FJU-40-NH2∙(CO2)0.401

(f)

O2a

O1a

(i)

C8 O2c

O1w O2

O8A C5b

C6b

FJU-40-NH2∙(CO2)0.145∙(H2O)0.161.161

(h) O2b

C7 O7A

(e)

FJU-40-NH2∙(CO2)0.203

(g) O1

air 1 atm

15:85 CO2/N2 1 atm

pure CO2 1 atm

C5

O2b O2a

3.050λ O2c

N4b 3.216Å

N4

O2w O2

C6c

C6 C6a

Figure 3.8  View of cage-I (blue ball), cage-II (green ball), and cage-III (purple ball) along the b axis for the activated FJU-40-NH2-a (a) and various samples after subsequent exposure to dry ice (b) in Reference [210], and to pure CO2 (c), 15:85 CO2/N2 (d), or air (e) in this study. View of the multiple-point interactions between CO2 and the framework: (f) in cage-I of FJU-40-NH2-a after exposure to pure CO2, 15:85 CO2/N2, and air atmosphere; (g) in cage-II after exposure to pure CO2 and 15:85 CO2/N2. View of the interactions between water and the framework in cage-II (h) and cage-III (i) of FJU-40-NH2-a after exposure to air. Source: Reproduced with permission of John Wiley & Sons.

Kumar et al. investigated five materials, including one chemisorbent [TEPASBA-15 (amine-modified mesoporous silica)] and four physisorbents (Zeolite 13X, HKUST-1, Mg-MOF-74/Mg-dobdc, and SIFSIX-3-Ni), for their ability to adsorb CO2 directly from air and other gas mixtures. The four physisorbents were found to be capable of carbon capture from CO2-rich gas mixtures, but competition and reaction with atmospheric moisture significantly reduced their DAC performance.

3.2.4  CO2/CH4 Separation Separation of CO2 and CH4 is an important issue in the processing of low-quality natural gas such as biogas, coal-seam and land fill gases. Typical pipeline specifications for natural gas require a CO2 concentration below 2–3%, while the concentrations CO2 in low-quality natural gas wells are as high as 70% [212, 213]. Moreover, the coexistence of CO2 with CH4 reduces the energy content of natural gas and also

146

NEW PROGRESS OF MOFs IN CO2 CAPTURE

causes pipeline corrosion. Both the thermodynamic and kinetic aspects of separation will affect the CO2/CH4 separation. The strategies for CO2 capture and separation in post-combustion process are also observed in nature gas separation. Among them, most studies are largely limited to equilibrium uptake measurements, which are realized in many MOFs with polar functional sites. The highly polar groups will have great positive effect on the CO2/ CH4 separation because of the difference in their electronic properties (quadrupolar moment for CO2 being 13.4 × 10−40 C m2 and CH4 being nonpolar). For example, it has been predicted that the selectivity of CO2 from CO2/CH4 mixtures in Li-modified MOFs would be greatly improved within IRMOF-5 series based on the simulation studies, due to the enhancement of electrostatic potential in the materials through the addition of extra metal ions [214]. Such prediction has been confirmed by Hupp and Snurr’s works on Li-(Zn2(NDC)2(diPyNI) and Li-(Zn2(TCPB)(DPG) [215, 216]. Recently, Zaworotko and co-workers utilized functionalized porphyrins as nodes or linkers to get porph@MOM-11, which has cation and anion binding sites to facilitate cooperative addition of inorganic salts (such as M+Cl−) in a stoichiometric fashion [217]. After PSM, All the porph(Cl−)@MOM-11-(M) (PSM; M = Na+, Ba2+, Mn2+ and Cd2+) at 298 K were found to exhibit higher Qst of CO2 than porph@MOM-11 at both low and high loading of CO2. Qst increases of up to approximately 36% (10.9 kJ mol−1) were observed in porph(Cl−)@MOM-11-(Cd2+) at low loading. The increase of Qst for CO2 may be attributed to smaller pore size as well as the influence of Cl− and cations. IAST calculations revealed that the PSM variants exhibit higher selectivity for CO2 versus CH4 than the parent MOF. Porph(Cl−)@MOM-11(Mn2+) exhibits the largest increase in selectivity (up to ~42% (~3.0) in the low-pressure region. Functionalization of MOFs with LBS is another highly efficient strategy to enhance the CO2/CH4 selectivity and capacity of the materials [58]. Yan et al. incorporated PEI into the as-prepared amine-MIL-101(Cr) pores, leading to a remarkable CO2/CH4 selectivity up to 931 at a partial pressure of 0.5 bar and 25°C and CO2 adsorption capacity up to 3.6 mmol g−1 at 1 bar and 25°C [218]. Balbuena and Zhou et al. reported a zeolite-like microporous tetrazole-based [Zn(btz)] with 24 nuclear zinc cages, in which high CO2 adsorption capacity up to 35.6 wt% (8.09 mmol g−1) and excellent CO2/CH4 selectivity (21.1) were observed at 273 K/1 bar due to the multipoint interactions between CO2 molecules and frameworks. Other polar groups have also been confirmed to have great positive effect on the CO2/CH4 separation in some cases. The highly polar –CF3 groups contribute to a high equilibrium selectivity of 66 in [Cd2L(H2O)]2 (L = 4,4′-(hexafluoroisopropylidene)diphthalate), estimated by employing the Langmuir constants at 293 K and 1 atm [219]. In addition, the kinetic diameters of the gases are 3.30 and 3.80 Å for CO2 and CH4, respectively, leading to high CO2/CH4 selectivity through size-exclusive of micropores. For instance, (Me2NH2)In(NH2BDC)2 shows essentially no uptake for CH4 (0.01 cm3 g−1) up to 3.0 MPa, and therefore has a highly selective CO2 separation [220]. The very low CH4 uptake may be largely related to the kinetic effect of the micropore as well as the blocking counter ions in the pores. Li et al. reported a flexible microporous

SURVEY OF TYPICAL MOF ADSORBENTS

147

MOF structure [Zn2(bpdc)2bpe], which exhibits high selectivity in adsorbing CO2 over CH4. The value is 53:1 (v/v) at 298 K and 0.16 atm (a pressure that is well within a typical CO2 partial pressure range in flue gases), considerably higher than 49:1 for Mg-MOF-74 under the same conditions [221]. Notably, the method of composite has been proposed recently to improve the CO2/CH4 selectivity. Xiang’s groups proposed a method of graphene oxide composites to improve thermal stability and selectivity for biogas decarburization [222]. A series of MOF (UTSA-16)–graphene oxide composites was synthesized. These composites are the first reported examples of core–shell type MOF composites armored with graphene oxide film. It showed a greatly improved thermal stability compared with their parent materials. More interestingly, the UTSA-16–GO19 composite has a CO2/CH4 selectivity of 114.4, which is three times greater than that of UTSA-16 alone; of the previously reported MOFs, only the polyamine-incorporated amineMIL-101(Cr) has a higher CO2/CH4 selectivity. More recently, Long et al. demonstrated that the incorporation of relatively small amounts (~20 wt%) of Ni2(dobdc) nanocrystals into various polyimides, including four upper-bound 6FDA-based polyimides as well as the commercial polymer Matrimids, can improve the performance of membranes for separating CO2 from CH4 under mixed-gas conditions [223]. For the adsorption of CO2/CH4 binary mixtures in MOFs, most reported works are based on molecular simulations [224–229] CO2/CH4 kinetic separation experiments are more reliable for real natural gas separation, which has been operated in some typical MOFs. For example, Hamon et al. [230] reported experimental breakthrough results for CO2–CH4 and CO2–CH4–CO mixtures in a bed packed with Cu-btc powder. The experimental coadsorption results are compared to predictions from coadsorption models that rely on pure component isotherms. By using the independently established equilibrium isotherm parameters and micropore diffusivities as fitting parameters, they were, however, unable to fit the binary breakthrough results. Farooq et al. [231] further carried out adsorption equilibrium and dynamic column breakthrough measurements of CO2, CH4, and N2 on the chosen Cu-btc sample over a wide pressure range at different temperatures. Meanwhile, they combined the calculation to understanding the transport mechanism and quantifying the transport parameters, and confirmed that the transport of all three gases was controlled by a combination of molecular diffusion in the adsorbent macropores and diffusion across the external fluid film around the particles. Mg-MOF-74 is another typical case. By calculation, Herm et al. expected that the selectivity of Mg-MOF-74 is up to 150 at 298 K and 1 atm during the separation of CO2/CH4 mixtures with equimolar composition [232]. Yaghi performed breakthrough separation experiments on Mg-MOF-74 to demonstrate that Mg-MOF-74 provides complete separation of CO2 from the CH4 stream in a flow of a 20% mixture of CO2 in CH4. Moreover, Mg-MOF-74 offers an excellent balance between dynamic capacity and regeneration, which can be fully regenerated after exposure to water without any effect on its CO2 adsorption performance [43]. The last example is based on UiO-66(Zr). Llewellyn et al. used a joint experimental and modeling strategy to understand the

148

NEW PROGRESS OF MOFs IN CO2 CAPTURE

thermodynamic and kinetic behavior of the CO2/CH4 gas mixture within UiO-66(Zr) solid [233]. It shows UiO-66(Zr) is very promising for CO2/CH4 separation with a good selectivity, very high working capacity and low regeneration cost together with its stability under various conditions. More importantly, both coadsorption and codiffusion mechanisms in this MOF have revealed a very unusual dynamic behavior with the slower molecule (CO2) enhancing the mobility of the fast one (CH4), which is quite different from what has been previously reported for CO2/CH4 mixture in zeolites such as LTA and DDR.

3.2.5  CO2/C2H2 Separation Such a separation is essential to get a high purity of acetylene for its commercial usage. But it is still as challenging because these two gas molecules have very similar shapes, dimensions (332 × 334 × 570 pm3 vs. 318.9 × 333.9 × 536.1 pm3), and boiling points (−84°C vs. −78.5°C). A few MOFs with relevant differences in capacity for C2H2/CO2 have been developed, mainly through tuning the cross-section size of the pore [147, 234, 235] For example, Zhang and Chen [147] illustrated a C2H2/CO2 sorption behaviors with C2H2/CO2 uptake ratio (3.7) at 298 K and 1 atm in a metal azolate framework, [Cu(etz)]n (MAF-2, Hetz = 3,5-diethyl-1,2,4-triazole). As demonstrated by single-crystal X-ray crystallography, C2H2/CO2 hexamers are confined inside the nanocages of MAF-2 in different configurations. The subtle difference between C2H2 and CO2 is magnified by consequent framework dynamics. Kitagawa groups reported an MOF [Mn(bdc)(dpe)] (dpe = 1,2-di(4-pyridyl)ethylene) with zero-dimensional pores, which shows an adsorbate discriminatory gate effect [236]. As the gate opening pressure increases with temperature, the selectivity of separation will increase with increasing temperature. The compound shows gate opening–type abrupt adsorption for C2H2 but not for CO2, leading to an appreciable selective adsorption of CO2 over C2H2 at 273 K. Recently, several MOFs have been found with actual C2H2/CO2 breakthrough experiments. Xiang’s group achieved {[Cu(L)]·(DMA)(H2O)1.5}n (FJU-22, L = 5-triazole isophthalic acid; DMA=N,N′-dimethylacetamide) with good robustness controlled by adjusting the helical chain SBUs [237]. Activated FJU-22a with BET 28.19 m2 g−1 can adsorb 111.3 and 114.8 cm3 g−1 of CO2 and C2H2, respectively, at 296 K and 1 bar. Its actual CO2/C2H2 column breakthrough curve under a total flow of 5 cm3 min−1 showed the separation selectivity of 1.9 and the separation capacity for C2H2 of 44.13 cm3 g−1. Based on first-principles calculations, the extraordinary separation performance of C2H2 for FJU-22a was attributed to hydrogen-bonding interactions between the C2H2 molecules with the open O donors on the wall. Chen’s group synthesized Zn2(H2O)(dobdc)·0.5(H2O) (UTSA-74, H4dobdc = 2,5dioxido-1,4-benzenedicarboxylic acid), Zn-MOF-74/CPO-27-Zn isomer [238]. It has a novel four coordinated fgl topology with one-dimensional channels of about 8.0 Å. Unlike metal sites in the well-established MOF-74 with a rod-packing structure in which each of them is in a five coordinate square pyramidal coordination

149

SURVEY OF TYPICAL MOF ADSORBENTS

(a)

(c) 1.0

C/C0

0.8

(b)

0.6 0.4 C2H2

0.2

CO2

0.0 0

10

20 30 40 Time (min)

50

60

Figure 3.9  DFT-D optimized structure of (a) UTSA-74 ⊃ C2H2 and (b) X-ray single crystal structure of UTSA-74 ⊃ CO2 in which the local coordination environments are shown on the right. (c) Experimental column breakthrough curve for an equimolar C2H2/CO2 mixture (298 K, 1 bar) in an adsorber bed packed with UTSA-74a. Source: Reproduced with permission of American Chemical Society.

geometry, there are two different Zn2+ sites within the binuclear secondary building units in UTSA-74 in which one of them (Zn1) is in a tetrahedral while another (Zn2) in an octahedral coordination geometry. After activation, two accessible gas binding sites per Zn2 ion can be achieved, leading to a moderately high and comparable amount of acetylene (145 cm3 per cm3) to Zn-MOF-74. Interestingly, the accessible Zn2+ sites in UTSA-74a are bridged by carbon dioxide molecules instead of being terminally bound in Zn-MOF-74, so UTSA-74a adsorbs a much smaller amount of carbon dioxide (90 cm3 per cm3) than Zn-MOF-74 (146 cm3 per cm3) at room temperature and 1 bar, leading to a superior MOF material for highly selective C2H2/CO2 separation. Moreover, it showed a complete separation of C2H2 from the equimolar C2H2/CO2 mixture with their separation factors of 20.1 by a column packed with activated UTSA-74a solid with a total flow of 2 cm3 min−1 at 298 K (Figure 3.9c).

3.2.6  Photocatalytic and Electrochemical Reduction of CO2 The conversion of CO2 into valuable organic products or fuels is one of the best solutions to solve the problems of global warming and energy shortage. MOFs could be functionalized with a variety of metal centers and organic linkers, responsible for the many catalytic reactions. A large number of MOFs have been examined as heterogeneous catalyst [239], particularly MOFs with carboxylate linkers have high surface area and wide range of pore size with much attention to catalytic reactions. Photo-driven CO2 reduction is one of the most promising scenarios for solar energy conversion. It is a convenient and efficient way to realize MOF catalysts

150

NEW PROGRESS OF MOFs IN CO2 CAPTURE

through the introduction of photocatalytic activity moieties into MOFs. Lin’s group successfully doped [Re(CO)3(5,5′-dcbpy)Cl] into a UiO-67 framework to obtain an MOF photocatalyst Zr6(μ3-O)4(μ3-OH)4(bpdc)5.83(L8)0.17 (bpdc = 5,5′-biphenyldicarboxylate; H2L8 = Re(CO)3(5,5′-dcbpy)Cl), which catalyzed highly selective photocatalytic CO2 reduction toward CO in acetonitrile solution with triethylamine as a sacrificial reducing agent [240]. A turnover number (TON) of 10.9 was achieved in 12 h, which is almost three times higher than that of the homogeneous complex. The higher activity was believed to result from site isolation of catalytic centers, which blocks bimolecular catalyst decomposition pathways. However, the recovered solid was inactive for CO generation. Spectroscopic studies showed the loss of CO stretching vibrations and the MLCT absorption band, indicating that the rhenium– carbonyl moieties detached from the MOF backbone during the catalytic cycle. In some MOFs, the organic linkers can act as antennas to absorb light upon irradiation and activate the metal clusters, which make MOFs promising photocatalysts. The light absorption ability of MOFs also can be more easily tuned by modifications on the metal ions and the organic linkers to achieve an efficient utilization of solar energy. For example, Li’s group successfully obtained NH2-MIL-125(Ti) and NH2-Uio-66(Zr) by facile ligand substitutions, which are active in photocatalytic CO2 reduction under visible light irradiation [241, 242]. They also reported a series of earth-abundant Fe-containing MOFs (MIL-101(Fe), MIL-53(Fe) and MIL-88B(Fe)) with photocatalytic activity for CO2 reduction to give formate under visible light irradiation, which can be attributed to the direct excitation of the Fe−O clusters [243]. Among the three investigated Fe-based MOFs, MIL-101(Fe) showed the best activity due to the existence of the coordination unsaturated Fe sites in its structure. All three amine-functionalized Fe-containing MOFs showed enhanced photocatalytic activity in comparison to the mother MOFs, due to the existence of dual excitation pathways: that is, excitation of an NH2 functionality followed by an electron transfer to the Fe center in addition to the direct excitation of Fe−O clusters. Jiang demonstrated that a porphyrin involved MOF, PCN-222, can selectively capture and further photoreduce CO2 with high efficiency under visible-light irradiation in the presence of triethanolamine (TEOA) as a sacrificial agent. The H2TCPP ligand (H2TCPP = tetrakis(4carboxyphenyl)-porphyrin)) in PCN-222 behaves as a visible-light-harvesting unit, and the high CO2 uptake might facilitate the enrichment of CO2 molecules around the catalytic Zr6 centers, thereby enhancing the photocatalytic efficiency. The presence of a deep electron trap state in PCN-222 effectively inhibits the detrimental, radiative electron–hole recombination. As a direct result, PCN-222 significantly enhances photocatalytic conversion of CO2 into formate anion compared to the corresponding porphyrin ligand itself [244]. These studies demonstrate the high potential of using MOFs as photocatalysts. However, studies on MOF-based photocatalysis are still in the infancy stage, considering the large number of MOF materials that have already been reported. Another attractive approach toward providing carbon-neutral energy is the ­electrochemical conversion of atmospheric carbon dioxide (CO2) into energy-dense

151

SURVEY OF TYPICAL MOF ADSORBENTS

carbon compounds to be used as fuels and chemical feedstock [245]. Recently, some MOF films have been utilized as efficient electrocatalyst for selective reduction of CO2 to CO or CH4. Hupp’s group first demonstrated electrophoretic deposition of crystallites of appropriately chosen MOFs is an effective means of heterogenizing and surface-concentrating catalysts for the electrochemical reduction of CO2 [246]. For electrocatalytic CO2 reduction, they used Fe-porphyrin-based MOF-525 that contains functionalized Fe-porphyrins as catalytically competent, redox-conductive linkers (Figure 3.10). The approach yields a high effective surface coverage of electrochemically addressable catalytic sites (~1015 sites cm−2). The chemical products of the reduction, obtained with ~100% Faradaic efficiency,

(a)

e− Fe

Fe

CO2

Fuel

FTO

Fe

Potential (V vs. CO2/CO)

Current density (mA/cm2)

(b)

−0.5

0.0

Potential (V vs. CO2/CO)

0.5

0.0

−0.2

Fe_MOF-525 N2

−0.4

(c) Current density (mA/cm2)

−1.0

1

−1.0

−0.5

0.0

0.5

Fe_MOF-525 N2 Fe_MOF-525 CO2 Fe_MOF-525 CO2 + 1M TFE

0

−1

−2

−3 −1.5

−1.0

−0.5

Potential (V vs NHE)

0.0

−1.5

−1.0

−0.5

0.0

Potential (V vs NHE)

Figure 3.10  (a) Illustration of heterogeneous electrochemical CO2-to-fuel conversion in Fe_ MOF-525. Cyclic voltammograms of Fe_MOF-525 films in a 1 M TBAPF6 acetonitrile solution: (b) under an N2 atmosphere, demonstrating the redox hopping ability of the Fe-MOF-525 film; (c) comparing behavior in N2- versus CO2-saturated solutions, with and without addition of a 1 M trifluoroethanol (TFE) proton source, showing electrocatalytic CO2 reduction behavior.

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NEW PROGRESS OF MOFs IN CO2 CAPTURE

are mixtures of CO and H2. Yaghi and Yang also utilized Co−porphyrin MOF, Al2(OH)2TCPP-Co (TCPP-H2 = 4,4′,4″,4″′-(porphyrin-5,10,15,20-tetrayl) tetrabenzoate) as atomically defined and nanoscopic materials that function as catalysts for the efficient and selective reduction of carbon dioxide to carbon monoxide in aqueous electrolytes [247]. It revealed a selectivity for CO production in excess of 76% and stability over 7 h with a per-site TON of 1400. In situ spectroelectrochemical measurements provided insights into the cobalt oxidation state during the course of reaction and showed that the majority of catalytic centers in this MOF are redox-accessible where Co(II) is reduced to Co(I) during catalysis. An interesting electrochemical reduction of CO2 to CH4 was observed in MOF Zn–BTC (BTC = 1,3,5-benzenetricarboxylic acid) [248]. Zn-BTC deposited on carbon paper was used as cathode in electrochemical reduction of CO2 with ionic liquids as the electrolytes, which was the first work on combination of an MOF electrode and a pure ionic liquid electrolyte in the electrochemical reduction of CO2. The efficiency of the reaction strongly depended on the morphology of the Zn-MOFs. Compared with the commonly used metal electrodes, the electrochemical reaction showed much higher selectivity to CH4 and current density, and the overpotential for CH4 is much lower.

3.2.7  Humidity Effect Sensitivity to water liquid and vapor is widely considered to be a major weakness of MOFs that could negate potential advantages of the MOF materials from an application’s perspective. To a certain degree, the stability of MOFs to water vapor is more important than to water liquid in industrial CCS process. Water has been reported to attack the metal connectors within MOFs, displacing ligands and causing phase changes, loss in crystallinity, and/or decomposition to reduce or destroy the porosity of the materials. Besides, change of CO2 adsorption capacity in the presence of moisture is also observed. Understanding the behavior of MOFs under humid conditions as well as the parameters that contribute to this sensitivity is critical for elevating MOFs to the applied level, which have been widely studied by experimental and theoretical methods. The first step is to consider the structural stability of MOFs under the humid environment because a large majority of MOFs are not water-stable. The reaction of water with the metal oxide clusters (hydration) in MOFs could involve ligand displacement and/or hydrolysis, which may result in hydrolysis reactions. The metal–ligand bonds are broken and hydroxide anions dissociated from water bond to metals. A typical example, IRMOF series including MOF-5, is constructed with zinc atoms as metal centers and terephthalic acid (BDC) molecules as ligands, which is particularly sensitive to water and lose their porosity at room temperature in air because of carboxylic groups substituted by water molecules to coordinate with zinc centers [179, 249–252]. Cu-btc, is formed by the copper paddlewheel and 1,3,5-benzene tricarboxylic acid (btc), which shows structural stability to water to

SURVEY OF TYPICAL MOF ADSORBENTS

153

a certain degree. There still exists discrepancy in the literature on the water/humid stability of Cu-btc although quite a lot of work has been carried out on this prototypical MOF. Based on our own experience to handle this MOF, we concur to Long et al.’s judgment that the Cu-btc exhibits intermediate stability [97]. Wang et al. reported the first water isotherm for Cu-btc, but no structure or surface area analyses were performed [253]. Low et al. reported that Cu-btc is stable up to 200°C when exposed to 50 mol % steam [257], and Cychosz and Matzger found that Cu-btc exhibits good structure retention in a 7:1 mixture of H2O:DMF even after 21 mo of exposure [234]. In contrast, Kaskel et al. [254] determined from powder X-ray diffraction that Cu-btc breaks down after immersion in pure water at 323 K for 24 h. The investigation on M-MOF-74 series is also operated by several groups. The Dietzel [255] group has investigated the stability of the M-MOF-74/CPO-27 materials (Co, Mg, Ni) throughout dehydration/rehydration cycling. The M-MOF-74 analogs were found to be stable during cyclic adsorption testing while using inert gases (Ar/N2). The bulk of the water consisting of the non-coordinating water molecules can be removed easily and reversibly as long as the framework structure is intact. However, the coordinating water is more tightly bound in Mg-MOF-74 than in Ni-MOF-74, so it is more difficult to remove water molecules in Mg-MOF-74. LeVan group [256] found that Mg-MOF-74 is less stable than Ni-MOF-74 toward steam conditioning and long-term storage. They found that the redox properties of metal sites within M-MOFs-74 might play important roles for their different stability toward water/humidity. It has been approved that strength of the bond between the metal oxide cluster and the bridging linker as well as the stability of metal cluster nodes are important in determining the hydrothermal stability. And the flexibility of the framework plays a role, but it is not as important as the metal–linker bond strength [257, 258]. Some new strategies have been employed to improve hydrolytic stability of MOFs. First, M3+ or M4+-containing MOFs, such as MIL materials and UiO-66, were approved to be more resistant than M2+-containing SBUs with respect to reaction with water. Several MIL materials are known to maintain good structural integrity after water exposure due to high coordination numbers [259, 260]. It has been recently shown that Zr-based MOFs (e.g. UiO-66) are particularly stable to water vapor, partly due to the reversible arrangement of the Zr6O4(OH)4(CO2)12 cluster upon hydroxylation and dehydroxylation [261]. Zr-based MOFs like like UiO-66, 2MIL-140, and PIZOF are air-stable up to 813 K. Second, a variety of MOFs, such as azolate-type frameworks based on imidazolate-, triazolate- and pyrazolate, have been reported in recent years that do not lose structural integrity in the presence of water due to the stronger M–N bond interactions compared with M–O [104, 105, 141, 262–264]. For example, after soaking the solid for 3 d in boiling water or 1 d in a solution of HCl (0.001 M pH = 3), the powder X-ray diffraction pattern of Cu-BTTri remained unchanged [106]. In fact, the most common for the waterunstable MOFs is to introduce hydrophobic groups at ligands of MOFs, such as −CH3, −C2H5, −CF3, and trifluoromethoxy, which could shield the metal ions from

154

NEW PROGRESS OF MOFs IN CO2 CAPTURE

attack by water molecules, and thus enhance the water resistance of the MOF structure significantly, as verified in [Zn(bdc)(dabco)0.5] and MOF-5 [265–267]. Shimizu obtained barium tetraethyl-1,3,6,8-pyrenetetraphosphonate (CALF-25) containing a new phosphonate monoester ligand. The presence of the ethyl ester groups makes the pores hydrophobic in nature, also protects CALF-25 from decomposition by water vapor, with crystallinity and porosity being retained after exposure to harsh humid conditions (90% relative humidity at 353 K) [268]. Hou et al. showed that the hydrophobic –CF3 groups can enhance structural resistance of [Cd2L(H2O)] to moisture in flue gas [73]. Zhou et al. [269] used reactions of ZrCl4 and single or mixed linear ligands bearing methyl or azide groups to obtain highly stable isoreticular Zr-MOFs with tunable loadings of azide groups inside the pores. These MOFs can not only survive water treatment but also remain intact in dilute acid (pH = 2 solution with HCl) and base (pH = 11 solution with NaOH) for some time. Choi et al. [209] modified Mg-MOF-74 by functionalization of its open metal coordination sites with ethylene diamine (ED) to introduce pendent amines into the MOF micropores. The new ED-Mg-MOF-74 material synthesized in this work is a more stable adsorbent compared to the parent Mg-MOF-74 because it can completely regain its adsorption capacity under mild regeneration conditions. In addition, the strategy of using metal–cation exchange into prototypical metal nodes has been employed. Ni-doped MOF-5 not only exhibited higher specific surface areas and larger pores, but also enhanced hydrostability toward ambient moisture compared to the undoped MOF-5 [270]. Moreover, it is worth noting that the combination of MOFs with other gas storage materials is also a feasible way to enhance the hydrostability of MOFs. Park and co-workers successfully synthesized a novel hybrid composite CNT@ MOF-5 to increase the hydrostability [271]. Interestingly, polymer−MOFs (polyMOFs), a new class of hybrid porous materials that combine advantages of both organic polymers and crystalline MOFs, has been explored as water tolerant materials for selective CO2 separations [272]. The enhanced water stability is attributed to the incorporation of the hydrophobic polymer ligands, as well as the cross-linking of the MOF lattice by the polymer chains. In addition to the stability of a material in the presence of water vapor, its effect on the CO2 adsorption process will also be significant but without systematical research. Evidence suggests that the presence of water may have different influence on CO2 uptake in terms of capture capacity. Ideally, materials developed for CCS applications will either react synergistically or be unreactive toward water vapor. For example, Llewellyn group showed a remarkable fivefold increase in CO2 uptake of MIL-100(Fe) at 40% RH, in parallel with a large decrease in enthalpy measured [273], while CO2 uptake of UiO-66 remained similar whatever the relative humidity. Nugent showed CO2 uptake and selectivity for SIFSIX-2-Cu-i and SIFSIX-3-Zn were only slightly reduced in the presence of CO2/H2 mixture (30:70) [166]. And a high CO2/N2 sorption selectivity has been observed in a Ca-sdb (sdb = 4,4′sulfonyldibenzoate) even under high relative humidity (RH) because of the linker

SURVEY OF TYPICAL MOF ADSORBENTS

155

geometry [274, 275]. The V-shaped SDB linker provides a “pi-pocket” formed by two phenyl rings and CO2 is positioned at equal distance to both rings, resulting in a high heat of adsorption ~31 kJ mol−1. In some flexible MOFs, promising CO2 adsorption properties in the presence of water have been observed. Flexible MOF MIL-53(Cr) is found in the narrow pore form when pre-equilibrated at 50% RH. In this state, negligible CO2 uptake occurs up to 20 bar, at which point the narrow pore → large pore transition occurs and CO2 is adsorbed with displacement of the pre-adsorbed water [276]. Significant decrease in CO2 capacity will occur in many MOFs because water molecules act in competition with CO2 for adsorption sites, in spite of the good stability in humidity [212]. For example, Barbarao et al. found that even with 0.1% water in the CO2/CH4 mixture, the CO2/CH4 selectivity in rho-ZMOF decreases by 1 order of magnitude [277]. For MOFs with OMS, OMS plays an important role in their performance for CO2 capture from a practical stream containing water and other impurities. Through GCMC and density functional theory simulations, Balbuena et al. found that the effect of water on MOFs with OMS mainly depends on the difference of binding energies between CO2···OMS and CO2···coordinated water. By comparing these binding energies, it may be possible to predict effects of water on CO2 adsorption in MOFs with OMS [278]. Being best CO2 capacity in dry conditions, M-MOF-74 is the most promising absorbent for industrial CCS if the co-adsorption of CO2 with water could be diminished. Matzger et al. [279] investigated the effect of humidity on the performance of this series by collecting N2/CO2/H2O breakthrough curves at relative humidities in the feed of 9, 36, and 70%. After exposure at 70% RH and subsequent thermal regeneration, only about 16% of the initial CO2 capacity of Mg-MOF-74 was recovered. However, in the case of Ni- and Co-MOF-74, approximately 60% and 85%, respectively, of the initial capacities were recovered after the same treatment. It turns out that Ni- and Co-MOF-74 exhibit better CO2 capture performance under moderate humid flue gas despite the highest CO2 capacity of Mg-MOF-74 at dry condition. A fixed-bed CO2/N2 breakthrough study on Ni-MOF-74 exhibited trace amounts of water can affect CO2 adsorption capacity as well as CO2/N2 selectivity for Ni-MOF-74 [51]. Ni-MOF-74 can retain a significant CO2 capacity of 2.2 mol kg−1 and a CO2/N2 selectivity of 22 at 0.15 bar CO2 with 3% RH water. Unfortunately, Ni-MOF-74 could not adsorb significant CO2 when a large amount of water is pre-adsorbed; further, the Ni-MOF-74 may lose its high CO2 capacity after full saturation by water and repeated regeneration processes [256, 280]. Recently, some interesting research work has focused on maintaining, even improving, carbon dioxide capture in the presence of water. In some cases, evidence suggests that the presence of immobilized amines may be beneficial to CO2 uptake under humid conditions. Montoro carried out the functionalization of the coordinatively unsaturated Cu(II) in Cu-btc with different bifunctional amines, including ethylenediamine, 3-picolylamine and 4-picolylamine [281]. Interestingly,

156

NEW PROGRESS OF MOFs IN CO2 CAPTURE

there is a strong effect of amine stereochemistry on CO2 capture with 3picolylamine in Cu-btc@3pico giving rise to the best performance. Moreover, pulse gas chromatographic experiments are indicative of the enhancement of the interaction of CO2 in Cu-btc@3pico compared to Cu-btc as a consequence of the incorporation of the amine basic sites while the interaction with H2O diminishes as a consequence of the blockage of open metal sites. Yaghi’s group show that the interior of IRMOF-74-III can be covalently functionalized with primary amine (IRMOF-74-IIICH2NH2) and used for the selective capture of CO2 in 65% relative humidity [282]. Carbon dioxide isotherms and breakthrough experiments show that IRMOF-74-III-CH2NH2 is especially efficient at taking up CO2 (3.2 mmol of CO2 per gram at 800 Torr) and, more significantly, removing CO2 from wet nitrogen gas streams with breakthrough time of 610 ± 10 s g−1 and full preservation of the IRMOF structure. Monodentate hydroxide has been employed by Zhang’s group as a super strong yet reversible active site for CO2 capture from high humidity flue gas [283] [CoIICoIII(OH)Cl2(bbta)] (MAF-X27ox, H2bbta = 1H,5H-benzo(1,2-d:4,5-d′)bistriazole) exhibited ultrahigh CO2 adsorption affinity (124 kJ mol−1), adsorption capacity (9.1 mmol cm−3 at 298 K and 1 bar), and CO2/N2 selectivity (262 at 298 K) by the reversible formation/decomposition of bicarbonate in the adsorption/desorption processes. More importantly, it can capture up to 4.1 mmol cm−3 or 13.4 wt% of CO2 from simulated flue gases (CO2 pressure 0.10–0.15 bar at 313 K) even at high relative humidity (82%) and quickly release it under mild regeneration conditions (N2 purge at 358 K). Also, Ding et al. developed a new strategy by partitioning the channels of MOFs into confined, hydrophobic compartments by in situ polymerization of aromatic acetylenes [284]. Compared with pristine MOF-5, the resultant material (PN@MOF-5) exhibits a doubled CO2 capacity (78 vs. 38 cm3 g−1 at 273 K and 1 bar), 23 times higher CO2/N2 selectivity (212 vs. 9), and significantly improved moisture stability. The dynamic CO2 adsorption capacity can be largely maintained (>90%) under humid conditions during cycles as shown in Figure 3.11. In some cases, CO2 capacity is closely related to the degree of humidity during CO2 capture in MOFs. A typical example is the influence of different water loadings on CO2 uptake of microporous Cu-btc [280–285]. Yazaydin et al. showed small amounts of water (4 wt%) may increase CO2 uptake in Cu-btc by 45% at 1 bar [285], due to the enhanced binding energy between CO2 and Cu-btc framework. Liu et al. further studied CO2 uptake on Cu-btc that was pre-equilibrated with different water loadings [280]. At low water loadings, the CO2 capacity effectively increased by 10%, but with further increases in water loading, the uptake of CO2 gradually fell. Cycling measurements also showed a decrease in CO2 uptake, attributed to the partial degradation of the network. Using 1H and 13C solid-state NMR, GulE-Noor et al. highlighted decomposition of the Cu-btc framework at high relative humidity [286]. The structure was stable when only a small amount of water (0.5 mol equiv. with respect to copper) was absorbed, but decomposition occurred at higher water contents.

MOF-5

DEB

Bergman cyclization Δ

CO2

Δ

PN@MOF-5

Radical polymerization

H2O

PN

n

(c)

0

80 20

0 100

80 20

(e) 100

0

0

50

100

150

200

250

300

CO2

N2

CO2

N2

500

Switch On

Switch On

1000

0.2 0.4 0.6 0.8 1.0 Pressure (bar)

PN@MOF-5 MOF-5

0

10

20

30

40

Time (s)

1500

MOF-5

Wet-2nd

Wet-1st

Dry

3000

MOF-5, dry MOF-5, humid

3500

PN@MOF-5, dry PN@MOF-5, humid

2500

PN@MOF-5

2000

Dry N2 purge

Dry N2 purge

(d)

Calculated IAST selectivity for CO2 over N2 at 273 K for a 14:86 CO2/N2 gas mixture. (d) Capacities and (e) dynamic sorption curves of PN@MOF-5 and MOF-5 under dry conditions (blue) and in the presence of water (red). Source: Reproduced with permission of American Chemical Society.

Figure 3.11  Illustration of Competitive Adsorption of CO2 against H2O at the surface and edge of PN; (b) Polymerization of DEB in MOFs. (c)

(b)

(a)

IAST Selectivity Effluent / influent (%)

Dynamic adsorption capacity (cm3/g)

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NEW PROGRESS OF MOFs IN CO2 CAPTURE

3.3  ZEOLITE ADSORBENTS IN COMPARISON WITH MOFs Zeolites [287], a class of porous crystalline aluminosilicates built of a periodic array of TO4 tetrahedra (T = Si or Al), having uniform pore sizes in the interval 0.5–1.2 nm. The substitution of aluminum atoms in these silicate-based molecular sieve materials lead to negative charges of zeolites, which are compensated with exchangeable cations in the pore space. The main advantages of zeolites over MOFs are homogeneity, low cost, and high temperature and pressure stability, as well as easy cation exchange to provide good adsorption sites for the quadrupolar CO2 molecule, which facilitate the tuning of gas–solid interaction energy. As a result, zeolites are also promising for CO2 capture from post-combustion flue gas and from natural gas, which have shown high CO2 uptake capacity and selectivity [288]. We list some zeolites with the best performance in Table 3.4. On the other hand, zeolites face the challenge of regeneration difficulty attributed to strong interactions between cation ions and CO2 molecules, which are also observed in MOF materials. Because water removal is essential for CCS, it is certainly necessary to take into account of the cost for the water removal. In an actual implementation of post-combustion CCS, the flue gas will be saturated with water after desulfurization. Since the presence of water is incompatible with zeolites, the feed requires dehydration prior to the PSA–VSA process. Flouds et al. have previously studied several alternatives for feed dehydration from power plant flue gas [289, 290] and found that TEG (triethylene glycol) absorption is the most economical option. Through calculation, they estimated that the feed dehydration uniformly adds an additional $10.22 per ton of CO2 captured for the 1 kmol s−1 feed considered [291]. For example, thetotal cost of an adsorption process starting from a wet feed is $33.85 per ton of CO2 captured for AHT and $42.05 for zeolite 13X. It is important to consider this cost when comparing alternative technologies; however, it does not affect the cost-based ranking of zeolite sorbents. Generally, CO2 capture capacity of zeolites is greatly determined by Si/Al ratios, exchangeable cations as well as polarities and size of pores. Synthetic zeolites (type A, 13X, and NaY) with low SiO2/Al2O3 ratios have been widely used in commercial adsorption processes. But they are limited by high regeneration penalty and sensitivity to water vapor and sulfur compounds. Among the more than 180 known zeolites, Na-X (Si/Al = 1–1.7) is considered to be one of the bestperforming zeolites for CO2 capture, especially at low feed and adsorbent regeneration pressures, and is widely used as a benchmark for evaluating the performance of new solid adsorbents. Structured adsorbents containing zeolite Na-X havebeen reported with a CO2 working capacity of 1.5–3.5 mmol g−1 and a separation factor of 50 [292]. Notably, the reported value for commercial Na-X is 34 cm3(STP) per cm3 [293]. In comparison with MOF mateials, H2O does not inhibit CO2 adsorption for Cu-btc and Ni-MOF-74 as much as it does for Na-X zeolites [261, 280]. In an experimental study, Wang and LeVan determined both the pure component

159

ZEOLITE ADSORBENTS IN COMPARISON WITH MOFs

Ta b l e 3.4  CO2 and N2 uptake in selected zeolites at pressures relevant to postcombustion CO2 capture at RT Material chemical formula

CO2 uptake at 0.15 bar mmol g−1 (wt%)

N2 uptake at 0.75 bar mmol Enthalpy g−1 (wt%) Selectivity (kJ mol−1)

Reference

NaKA(K+17.4%) NaKA(K+8.4%) NaKA(K+ 0%)

2.35 (10.36) 2.92 (12.86) 3.19 (14.05)

0.06 (0.16) 0.22 (0.63) 0.28 (0.77)

205.2 65.0 57.8

[310] [310] [310]

Zeolite 5A Zeolite 13X Na-A Ca-A Na-KFI Li-KFI Na-CHA Li-CHA Ca-CHA Na-LEV K-KFI Li-ZK-5 Na-ZK-5 K-ZK-5 Na-X Na-X K-CHA Na-X NaK(17.3)A Na-X ZIF-8 BPL AC HiSiv-3000 DT-100 DT-200 DT-300 Cu-SSZ-13 H-SSZ-13 390 HUA 890 HOA 980 HOA

3.97 (17.48) 3.47 (15.27) 1.78 (7.84) 4.19 (18.45) 3.45 (15.20) 3.18 (14.00) 4.06 (17.84) 4.88 (21.47) 3.05 (13.4) 1.63 (7.16) 2.30 (10.14) 3.78 (16.64) 3.33 (14.66) 2.95 (12.97) 4.06 (17.86) 2.51 (11.03) 1.85 (8.15) 3.04 (13.39) 2.48 (11.91) 0.08 (0.37) 0.12 (0.54) 0.59 (2.59) 0.42 (1.85) 0.54 (2.36) 0.56 (2.47) 0.49 (2.15) 1.80 (7.90) 1.79 (7.88) 0.65 (2.86) 1.16 (5.09) 0.37 (1.62)

0.44 (1.24) 0.57 (1.60) 0.065 (0.18) 0.42 (1.18) 0.33 (0.93) 0.34 (0.97) 0.59 (1.67) 0.63 (1.77) 0.73 (2.05) 0.09 (0.26) 0.25 (0.70)

44.7 30.4 148.3 49.9 51.9 46.2 34.16 38.57 20.88 86.7 46.7

[180] [180] [311] [311]

0.27/0.76

75.19

0.20 (0.56) 0.22 (0.61) 0.014 (0.04) 0.14 (0.38) 0.08 (0.21) 0.22 (0.62) 0.13 (0.36) 0.16 (0.45) 0.16 (0.44) 0.16 (0.44) 0.25 (0.71) 0.21 (0.60) 0.066 (0.18) 0.15 (0.43) 0.11 (0.30)

46.6 69.4 843.6 3.0 8.0 13.4 16.3 16.6 17.7 15.6 35.4 42.0 49.3 37.5 17.0

−30.2 −58

42.1 48.4 40.2 49.2

[316] [316] [316] [316] [311] [293] [53]

32.3 33.6

[317] [317] [303] [303] [303] [303] [313] [313] [303] [303] [303]

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NEW PROGRESS OF MOFs IN CO2 CAPTURE

and mixture isotherms of H2O and CO2 on commercial 13X and 5A samples and noted that small amounts of water reduce the CO2 capacity slightly. When water is adsorbed near its saturation loading, the CO2 capacity is an order of magnitude lower than on the dry materials [294, 295]. The investigations on CO2 post-combustion capture of Na-X by PSA [296–298], VSA [299], and TSA [300] have been reported. Lu group used a dual-column temperature/vacuum swing adsorption (TVSA) with Na-X to study cyclic CO2 capture from a gas stream, which showed less moisture sensitivity below 30°C and a stable adsorption performance for CO2 under humid conditions [301]. Ojuva et al. showed freeze-cast laminated Na-X monoliths with an improved CO2 uptake capacity of 4–5 mmol g−1 and reduce the cycle time [302]. Comparatively, High silica zeolites get less attention due to their low capacity. However, they have a good compromise between high CO2 selectivity and easy regeneration, which usually exhibit relatively low CO2 capacity for the real industrial trial [303]. As high silicazeolites have high SiO2/Al2O3 ratios, only dispersion and polarizationinte ractions are involved in the adsorption, and a high desorption rate can be easily achieved compared to hydrophilic adsorbents. More importantly, high silica zeolites can be used in the presence ofwater vapor and other polar contaminants [303, 304]. Amine-functionalized porous adsorbents can be apromising alternative in post-combustion CO2 capture efforts, which have been made recently to improve CO2/N2 selectivityby introducing amine species to zeolites in a fixed bedor in membranes [128]. As observed in the MOF field, the energy penalty issue as well as the balance between increasing polarity and decreasing surface area need to be considered to improve capacity and selectivity of CO2 in zeolites. The mesoporous materials are reported to have high CO2 adsorption capacities of 184 mg g−1 (4.18 mmol g−1) at 25°C for reformulated immobilized amine sorbent and 151 mg g−1 at 100°C for tetraethylenepentamine (tepa)-loaded SBA-15 [305, 306]. In contrast, amine-functionalized materials such as NH2-MCM-22, NH2-ITQ-2 [307] and NH2-beta zeolite [308], and NH2-SAPO-34 [309] showed lower CO2 capacities due to their lower surface areas. But their CO2 adsorption selectivities over N2 or CH4 were found enhanced than their original unsupported materials. In comparison with other porous adsorbent, cations exchange is a unique, convenient and powerful method to modify the adsorption properties of the zeolite, which has been considered in two important ways: (1) it creates stronger adsorption sites as a result of the additional interactions between the CO2 molecules and the cations, and (2) it decreases the saturation uptake of CO2 because of the reduction in the free volume [310]. In the adsorption isotherm, the modification is reflected as an increase in the CO2 uptake at low pressures and a decrease in the uptake at high pressures. For the purpose of postcombustion CO2/N2 separation, these property changes can present an attractive trade-off. For example, Long group [311] evaluated potential application of a series of zeolites in

ZEOLITE ADSORBENTS IN COMPARISON WITH MOFs

161

post-combustion CO2 capture, including PS- MFI (SiO2),Na-A (NaAlSiO4),Mg-A (Na0.48Mg0.26AlSiO4), Ca-A(Na0.28Ca0.36AlSiO4), Na-X (NaAlSi1.18O4.36),Mg-X (Na0.38Mg0.31AlSi1.18O4.36), and Ca-X (Na0.06Al0.47Si1.18O4.36). Among them, Ca-A is best-performing, which exhibits higher volumetric CO2 uptake (5.63 mmol cm−3) and selectivity than Mg-MOF-74 (5.07 mmol cm−3) at 0.15 bar and 40°C. But the low-coverage isosteric heat of adsorption for CO2 in Ca-A (−58 kJ mol−1) is also higher than that of Mg-MOF-74 (−42 kJ mol−1). In fact, pore sizes of zeolites also significantly affect CO2 capture capacity, especially selectivity. The cage-like structures in particular with eight-membered windows, have been very promising in CO2 capture over CH4, and N2, because the orifice diameters would provide size-exclusion in the adsorption of gas molecules, and the larger cavity in the hole would be ideal for gas uptake. Via configurational Monte Carlo simulations and molecular dynamics, Krishna and van Baten demonstrated that zeolitic frameworks with narrow 0.35–0.45 nm sized windows (e.g., CHA, ERI, DDR, and ITQ-29) yield the highest selectivities for the CO2/CH4 separation [312]. Hudson et al. [313] studied both acidic and copper-exchanged forms of SSZ-13, a zeolite containing an eight-ring window. Both exhibit the ideal adsorbed solution theory selectivity large than 70 under ideal conditions for industrial separations of CO2 from N2. The isosteric heat of adsorption for CO2 was found to be 33.1 and 34.0 kJ mol−1 for Cu- and H-SSZ-13, respectively. From in situ neutron powder diffraction measurements, they ascribed the CO2 over N2 selectivity to differences in binding sites for the two gases, where the primary CO2binding site is located in the center of the eight-membered-ringpore window; at the same time, the Cu2+ site is responsible for very little of the overall CO2 uptake ability, making Cu-SSZ-13 an ideal candidate for practical CO2/N2 flue-stack separations, where humidity is likely. Therefore, they speculated that Cu-SSZ-13 will not suffer to a great extent from humidity effects that other MOFs and zeolites are subject to since any water should preferentially adsorb at the Cu2+ site. In some cases, the cooperation of cations exchanges and size-exclusive effect has been observed. For example, Li et al. [314] reported the ion (Li+, Na+, and Ca2+) exchange based on three basic gas diameter grade structures of zeolites: KFI (0.39 × 0.39 nm), CHA (0.38 × 0.38 nm), and LEV (0.36 × 0.48 nm). CHA was synthesized with a lower Si/Al ratio, and the surfaces and microporous volumes were changed greatly by ion exchange. KFI had a higher Si/Al ratio, and the scope of the surface could be kept smaller. The samples were exchanged by Li+ and Na+ with bigger surfaces and greater adsorption volumes. Na-LEV had an excellent sieving effect because N2 and CH4 could not diffuse into its structure like CO2. From the viewpoint of the separate adsorption equilibria, Na-zeolites had the highest data adsorption equilibrium selectivity for CO2 and N2 or CO2 and CH4, followed by Li-zeolites, which only had a strong adsorption potential of CO2; K-zeolites with high SCO2/N2 and SCH4/N2, based on the strong adsorption of CO2 and CH4. They concluded that

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the adsorption potential order was K-zeolites > Na-zeolites > Li-zeolites, so the bigger ions had a stronger affinity. Another case with eight-member-ring windows is KFI zeolite, consisting of larger α-cages (11.6 Å in diameter) and smaller γ-cages (6.6 × 10.8 Å). More recently, Denayer et al. [315] obtained a series of alkali (Li, Na, K) and alkaline-earth (Mg, Ca, Sr) exchanged samples of the new LS KFI. LS Li-KFI showed the largest pore volume; whereas LS Na-KFI and LS K-KFI were inaccessible for Argon at 87 K. Adsorption of CO2 at 303 K demonstrated the dominant quadrupolar interaction on alkali-exchanged LS KFI samples. LS Li-KFI showed the largest capacities upon high pressure isotherm measurements of CO2 (4.8 mmol g−1), CH4 (2.6 mmol g−1), and N2 (2.2 mmol g−1) up to 40 bar at 303 K. The performance of the new LS KFI was compared to a KFI sample (ZK-5) with a higher Si/Al ratio. Isotherm measurements and dynamic breakthrough experiments demonstrated that ZK-5 samples show larger working capacities for CO2/CH4 separations at low pressure. Li-ZK-5 and Na-ZK-5 show the highest capacities and high selectivities (similar to benchmark 13X). Hybrid method has also been observed in a zeolite-active carbon composites [318, 319]. Zeolites have higher adsorption capacities for CO2 as well as a higher equilibrium selectivity for CO2 over N2 than activated carbons, but the heat of adsorption of CO2 on activated carbons is lower than on zeolites, so that the use of activated carbon in the PSA process may result in less severe heat effects on the PSA performance. Li et al. obtained a series of zeolite X/activated carbon composites with different ratios of zeolite and activated carbon and then modified them with diluted NH4Cl solution [320]. As shown, there is a higher capacity and steeper nature of the CO2 isotherm on the unmodified composites and a lower capacity and lower slope of the isotherm on the modified composites. In general, the CO2 adsorption capacity and corresponding adsorption heat on both unmodified and modified composites decreased with increasing activated carbon in composites, indicating a weaker interaction between CO2 and activated carbon than with zeolite. After modification, higher adsorption selectivity of CO2/N2 and lower adsorption heat, especially in the low-loading regions, were obtained compared to unmodified composites which results in their favorable utilization in a PSA process. In all, MOFs are superior to zeolite adsorbents for pre-combustion CO2/H2 separation because of the higher porosities of MOFs and the resulting larger CO2 uptakes at moderately high pressures as demonstrated in Figure 3.8. For post-combustion CO2 capture, both of them are expected to have very high adsorption capacity comparable with the value of amine scrubbers but require substantial research efforts to be suitable under flue gas conditions. Zeolite-based sorbents without any nitrogen functionality have advantages such as high temperature and pressure stability and low cost with fast adsorption kinetics and low regeneration energy. From the laboratory-based experimental results, the strategy of ion changes can greatly increase CO2 capacity, but has barely positive influence on separation [54], as shown in

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900 800 700

MOFs

Selectivity

600

Zeolites

500 400 300 200 100 0 2.0

2.5

3.0

4.5 3.5 4.0 Capacity (mmol/g)

5.0

5.5

6.0

Figure 3.12  The capacities and selectivities of some selected MOF materials calculated from the experimental single-component gas adsorption isotherms in comparison with those of some zeolite materials at 0.15 bar and 298°C.

Figure 3.12. The zeolites are limited by the low separation for CO2/N2. Comparatively, MOF materials have advantages of both high capacity and selectivity because of the easy modification of surface affinity and window size as we mentioned above. Notably, the Lewis-base functional groups can benefit the improve CO2/N2 selectivity in both solid materials, which also lead to higher regeneration cost. CO2 adsorption of zeolite sorbents is strongly influenced by the temperature, pressure, and the presence of water vapor and other contaminants in flue gas, leading to a rapid decrease of CO2 capacity. Such a CO2 adsorption decreasing has been also observed in porous MOFs; however, in some cases, such a decreasing trend has been relieved in MOF materials.

3.4  MOFs MEMBRANE FOR CCS Membrane separation is a special separation methodology which can be also utilized for CCS. The membrane separation technology has some advantages of high energy efficiency, low cost, easy of operating, and recycling, and thus needs to be fully explored as well. The separation mechanism using membranes is usually based on the molecular-sieving effect of the membranes toward gas molecules. Because the pore sizes and shapes within MOFs are easily tuned to direct size selective sieving effects of the resulting membranes, MOF membranes are very promising for gas separation and carbon dioxide capture and separation. The performances of MOF membranes for CCS can be also readily improved through the incorporation of

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special functional sites to direct their different interactions with gas molecules [321] and/or through the post-synthetic functionalization to further tune the pores and to enhance the quality of MOF membranes [331]. The research on MOF membranes for CCS lags signicantly behind the bulky MOF materials. The first example of MOF membranes was realized in 2009 by Lai and co-workers [322]. Zhu and Qiu [323] obtained the HKUST-1 membrane on oxidized copper nets. It shows preferential selectivities for H2 over other gases (CO2, N2, and CH4), because H2 molecules with the smallest size can go through the membrane more easily compared to other gases (CO2, N2, and CH4). The results were also confirmed independently by another group [324]. Ben et al. prepared a stainless steel net/PMMA–PMAA-supported HKUST-1 membrane, which also shows high selectivity for H2/CO2 separations[325]. The zeolitic imidazolate frameworks (ZIFs), a subclass of MOFs, have emerged as candidates for membrane materials in gas and hydrocarbon separation, owing to their attractive crystallographic pore sizes (0.3-0.5 nm), and good thermal and chemical stability [326]. So far, a series of supported ZIF membranes, including ZIF-7, ZIF-8, ZIF-22, IF-69, and ZIF-90 have been reported for permeation of single gases or gas separation of mixed gases [97]. ZIF-8 is the first-reported ZIF membrane by Caro and coauthors [327], which presents the Knudsen selectivity for H2/CO2 separation because of the pore size (3.4 Å) being a little larger than the kinetic diameter of CO2 (3.3 Å) and very flexible. After that, Caro group did deeply research work in ZIF-7 membrane and its selectivity. They first obtained a new rigid ZIF-7 membrane with suitable pore sizes of ZIF-7 (~3.0 Å, between the kinetic diameters 2.9 Å for H2 and 3.3 Å for CO2) [328]. A clear cut-off between two gases is observed, when an H2/CO2 50/50 gas mixture permeates through the activated ZIF-7 thin membrane [329]. This molecular sieving effect for these two gases leads to an imposing high H2/CO2 ideal selectivity (6.7) and separation factor (6.5), which are higher than the Knudsen separation factors (4.7). Further, they enthanced the H2/CO2 separation factor to 8.4 for an equimolar H2/CO2 mixture at 200°C by altering the fabricative method to obtain c-out-of-plane oriented ZIF-7 membranes. In addition, they also revealed that the H2/CO2 separation on ZIF-7 membranes can be varied with the temperature change. [330]. The experimental results found that the H2 permeance increased quickly with a temperature increase, while the permeance of CO2 was almost constant, leading to a significant enhancement in the H2/CO2 separation factor from 5.4 at 50°C to 13.6 at 220°C. Interestingly, an LTA topological framework ZIF-22 membranes with the same pore size as ZIF-7 (~3.0 Å) exhibited high molecular-sieving performance on separation of H2 from CO2 and other larger gas molecules [331]. Another imspiring example is ZIF-90 membrane, which has been functionalized by different methods to improve selectivities. Compared to the as-prepared ZIF-90 membrane, the post-synthetic functionalization of ZIF-90 membrane [332] by ethanolamine can remarkably improve the separation capability of H2 from CO2 and other gases, with a separation factor for H2/CO2 ~15.7. Further, they obtained APTES-functionalized ZIF-90 molecular sieve membranes by a facile

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post-functionalization road, in which both narrowing of the pore mouth and sealing of intercrystalline defects of the polycrystalline ZIF-90 layer are achieved, and thus the selectivity for H2/CO2 (20) was enhanced for binary mixtures at 225°C and 1 bar. And the APTES-functionalized ZIF-90 molecular sieve membranes display a high thermal and hydrothermal stability [333]. Nair et al. [334] obtained ZIF-90/Torlon membranes, in which ZIF-90 membranes can be fabricated by a technologically scalable low temperature process on polymeric hollow fiber supports, with complete surface coverage and a lack of mesoscopic or macroscopic defects. Its gas separation factors (3.5 for CO2/N2 and 1.5 for CO2/CH4) are modest and consistent with previous literature on ceramic-supported ZIF membranes. Incorporating MOFs into polymers to obtain mixed-matrix membranes (MMMs) has been explored to be an alternative strategy to introduce MOFs into membrane-based applications. Musselman et al. [335] reported the first example of ZIF-based polymer MMM in which ZIF-8 is the filler phase and Matrimid® is the polymer phase. The permeability values for H2, CO2, O2, N2, CH4, C3H8 were progressively increasing as the ZIF-8 loading increased to 40% (w/w). In particular, the high ZIF-8 loadings of 50% or 60% (w/w) into Matrimid can enhance the selectivities for many gas separations. For example, pure Matrimid exhibited an ideal selectivity of 2.58 in a 50/50 H2/CO2 gas mixture, which jumped to 7.01 at 60% ZIF-8 loading. The similar increase was also observed in a 10/90 CO2/CH4 gas mixture. Pure Matrimid gave selectivity of 42, which increased to 89 at 50% ZIF-8 loading. The MMM composed by poly(vinyl acetate) (PVAc) and the framework Cu-TPA also displays increased selectivity for CO2/N2 and CO2/CH4 separations [336]. Furthermore, Car and co-workers [337] reported that the HKUST-1/PSf and HKUST-1/PDMS membranes with 10 wt% HKUST-1 loading result in high selectivity for CO2/N2 and CO2/CH4.

3.5  SUMMARY AND OUTLOOK Metal-organic frameworks, with tremendous choices of metal centers and organic linkers, show great potential applications in CO2 capture and separation because of their large surface area, high adsorption affinity, diverse structures and pore topologies, and accessible functionalization of tunnels. Capacity, selectivity, and regeneration play most important roles in the CO2 capture and separation process. We reviewed some typical research work about CO2 capture and separation, including post-combustion and pre-combustion CO2 capture, the direct CO2 capture from air, CH4/CO2 and C2H2/CO2 separation, photocatalytic and electrochemical reduction of CO2, and the humid effect on CO2 capture. There are several issues that need to be solved in the future. First, it is challenging to keep the deliberate balance among capacity, selectivity, and regeneration during CO2 capture and separation process. For the studies on post-combustion CO2 capture, some reported MOFs with openmetal-sites or amine-functionalized frameworks can reach the desired working capture capacity large than 3 mmol g−1 under working conditions due to the strong interaction between frameworks and CO2 molecules; however, they suffer from the

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high regeneration costs because of their large adsorption enthalpies. Those MOFs with functionalized groups of weak polarity can lead to lower adsorption enthalpies but are limited by their low CO2 capture capacity. The joint cooperation of size-exclusive effect and functional groups with relatively weak polarity can balance the enthalpy, capacity and selectivity of the MOF materials; so, such an approach is very promising to target new MOFs for efficient CCS. During the investigations of size-exclusive effect, porous cage structures with small window and large pore size as well as flexible frameworks with tunable micropores are particularly attractive for high selectivity; more research endeavors will be carried out in this direction. The unprecedented discoveries by Chen on UTSA-16 [39] and Zaworotko [166] on a prototypic MOF for the efficient post-combustion CCS are really encouraging. It indicated that some promising MOFs might have been already reported in the literature. Reevaluation of some reported MOFs through in-depth studies on the crystal structures deposited in CCDC guided by the outlined strategies mentioned above, together with some comprehensive screen, will certainly facilitate the realization of a few porous MOFs with even better performance for the post-combustion CCS. Second, the negative effect of the presence of water vapor and other ubiquitous traces of flue gas contaminants (sulfur oxides [SOx], nitrogen oxide [NOx]) also need to be considered. In particular, the computational results demonstrated that SOx or NOx with ppm levels had significant implications on both the equilibrium and separation behavior of the MOFs. Experimental data regarding these contaminants are extremely limited both for MOFs and zeolites, which cannot be ignored during industrial process. Third, it is certainly necessary to eventually implement some attractive MOF materials for their environmental and industrial large-scale CCS. The dynamic adsorption simulation of a fix-bed study can provide us some guidance. MOF chemists have primarily focused on the discoveries of porous MOFs for CCS through adsorption isotherms of pure gases. Because both the capacities and selectivities of MOF materials for such an application might be heavily changed when exposed to mixtures of gases under dynamic conditions, as is the case in power plant flue gas and methane mining applications; while the evaluation of the regeneration energy from the initial heat of adsorption of MOFs is over simplified, mixed gas breakthrough measurements in a laboratory settings are highly valuable tools. Although some breakthrough measurements have been processed on MOFs, studies on this direction are still in the infancy stage, considering the large number of MOF materials that have already been reported. Last, MOF chemists will be still actively working on searching for new porous MOF materials for CCS, while more and more chemical engineers will be involved in such a very important research topic in order to target some practically promising MOF materials for CCS in the future.

ACKNOWLEDGMENTS This work was financially supported by the Award AX-1730 (B.C.) from the Welch Foundation, the National Natural Science Foundation of China (21273033, 21673039,

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and 21573042), and Fujian Science and Technology Department (2014J06003 and 2014H6007). S. X. gratefully acknowledges the support of theRecruitment Program of Global Young Experts, Program for New Century Excellent Talents in University (NCET10-0108), and the Award “MinJiang Scholar Program” in Fujian Province, China.

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4 IN SITU DIFFRACTION STUDIES OF SELECTED METAL–ORGANIC FRAMEWORK MATERIALS FOR GUEST CAPTURE/ EXCHANGE APPLICATIONS Winnie Wong-Ng Materials Measurement Division, National Institute of Standards and Technology, Gaithersburg, MD

4.1  INTRODUCTION 4.1.1  Background CO2 is known to be the main anthropogenic contributor to the global climate change, and carbon mitigation approaches are critical for maintaining a sustainable future [1, 2]. At the current rates of energy consumption, known world coal reserves will last for more than 300 years, while known world natural gas reserves will last for about 60 years and oil reserves will last for about 40 years. Carbon capture/sequestration strategies are being developed for power plants worldwide which generally can be summarized in four steps: (1) CO2 capture, (2) separation at point sources such as fossil-fueled power plants, followed by (3) transportation, and finally (4) long-term storage, primarily via deep underground injection [3]. Materials and Processes for CO2 Capture, Conversion, and Sequestration, First Edition. Edited by Lan Li, Winnie Wong-Ng, Kevin Huang, and Lawrence P. Cook. © 2018 The American Ceramic Society. Published 2018 by John Wiley & Sons, Inc.

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Capture is the key step for the capture/sequestration process, and is the most research-intensive. Gas retention in porous solids is technically and economically feasible; therefore, development of novel solid sorbent materials could provide a cost-effective way to capture CO2. Porous materials offer a wide range of compositions and structures that are suitable for adsorption and capture of CO2 [4] and other guest molecules. These materials include zeolites [5, 6], activated carbon [7], smectites [8], oxide materials such as calcium oxide [9], lithium zirconates [10], and hydrotalcites [11]. There are also a vast number of reported metal–organic frameworks (MOFs) [12–40] that show diverse capture properties and applications at different pressures and temperatures. The field of MOF research has been rapidly expanding since the past decade. According to Zhou and Kitagawa [12], the surge of MOF research in recent years has been due to five factors: (1) advances in cluster chemistry, (2) maturation of organic synthesis (ligand design and post-synthetic modification of linkers), (3) improvements in evaluation of sorption and structural properties, (4) increase in interdisciplinary MOF investigations, and (5) an expanding potential for applications. Various techniques have been applied for characterization of MOFs and the associated guest molecules, including structural techniques, adsorption techniques, spectroscopic techniques, and modeling techniques. Up to the present, considerable knowledge of MOF structure and properties has been accumulated; however, there is still a lack of detail concerning guest adsorption mechanisms and guest–host interactions, which are necessary for designing more efficient sorptive materials.

4.1.2  In Situ Diffraction Characterization Instrumental designs [41–46] for in situ diffraction experiments concerning adsorption of guest molecules in the cavities of MOFs and the synthesis and monitoring MOF phase formation have attracted increasing attention in recent years [32, 47–76]. These experiments continue to facilitate our understanding of adsorption/desorption properties and detailed adsorption mechanisms of various guest species. In particular, neutron and X-ray synchrotron-based diffraction have been used successfully to identify CO2 adsorption sites in MOFs. Time-resolved, in situ single-crystal X-ray diffraction and powder diffraction techniques provide the opportunity to study crystalline nanoporous materials under realistic ambient and non-ambient conditions. These measurements will provide experimental data to compliment data that are obtained theoretically or by spectroscopic methods. In situ single-crystal and powder diffraction techniques are being utilized in national laboratory facilities worldwide. As these special facilities have limited availability and large expenses associated with their usage, a summary of the applicability of laboratory powder X-ray diffraction to study the location of CO2 in solid sorbents is also included here.

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Example of applications of in situ diffraction experiments include detection of unstable intermediate stage during guest exchange [41]; materials screening for post-combustion CO2 capture [45]; phase transformation (reversible phase transformation [47]; guest-induced structural transition [48, 49], and multiple-phase transition upon selective adsorption [57, 66]); dynamic CO2 sorption behavior [50]; identification of adsorption/binding site of CO2 and other gases [51, 67]; breathing modes of flexible MOF [52–56]; monitor of mechanochemical reaction-metastable intermediate [58]; gas adsorption mechanism [59]; capture of metastable intermediates in during the MOF formation [60]; internal pore location of CO2 [62]; in situ synthesis and phase formation of MOFs [61, 63, 65, 70–72]; dynamic gas adsorption sites [64]; guest–host intermolecular interactions [68]; topological change/ framework distortion related to solvent exchange [69]; kinetics of liquid-enhanced gas uptake [73]; high speed of transient molecular adsorption [74]; and crystal engineering of large discrete cavities [75]. The two goals of this chapter are (1) to give a brief summary of in situ studies of MOFs with regard to sorption of guests (i.e., H2O, CO2), for single crystals as well as powder samples, and (2) to illustrate examples emphasizing the importance of in situ techniques, particularly for unraveling the processes related to the complex behavior of MOF materials.

4.2   APPARATUS FOR IN SITU DIFFRACTION STUDIES Since 2005, there have been a number of papers discussing in situ studies of porous materials for capture of gases and liquids [41–46]. We will briefly describe several representative cell- and chamber-based designs.

4.2.1   Single-Crystal Diffraction Applications 4.2.1.1  Environmental Control Cell  For in situ single-crystal diffraction studies, it is important to have appropriate goniometers that house the single crystals and allow the presence of gas in either a closed system or an open system where the gas is allowed to flow through. Recently, an environmental control cell (ECC) has been incorporated as part of a single-crystal goniometer head [41]. This ECC can be used with any commercially available single-crystal X-ray instrument that is equipped with a flexible tubing source capable of delivering static or dynamic vacuum, liquids, or gases. Figure 4.1 gives the description of the device and the image of the assembled ECC mounted on a standard goniometer head [41]. The base of the device is modeled after a standard magnetic base and is held by a setscrew. The components of this simple setup include a bent needle, the ECC body, a capillary retaining screw, an o-ring, a capillary, a setscrew, and a thin glass fiber for sample mounting. The cell is easy to use and is completely reusable. The device is nearly identical in size to standard single-crystal mounts, so a full unrestricted range of motion is expected for most commercial goniometers.

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APPARATUS FOR IN SITU DIFFRACTION STUDIES

g

e f

d c

b f a

Figure 4.1  Image of the assembled ECC mounted on a standard goniometer head (left), with exploded view of ECC components at right: a, bent 18 gauge tube; b, ECC body; c, capillary retaining screw; d, o-ring; e, capillary (shortened and sealed with glue); f, setscrew; g, sample mounted on thin glass fiber. Source: Cox et al. 2015 [41]. Reproduced with permission of the International Union of Crystallography, http://journals.iucr.org/

4.2.1.2  Environmental Gas Cell  Another environmental gas cell (EGC) which also allows a single crystal to be placed either under vacuum or exposed to a pure gas or a gas mixture was designed by Warren et al. [42] (Figure 4.2). The cell utilizes the Hampton Research short XYZ goniometer head1 with no modification to allow space for the gas/vacuum pipe connection. The cell body is made of borosilicate or quartz capillary tubing. The capillary tube is glued into a small recess of the EGC head. The EGC base and head were fabricated from stainless steel. A simple Swagelok stainless steel two-way valve was connected to the EGC Nalgene tube allowing vacuum or gas to be fed to the EGC head. Details are given in References [42] and [43]. 4.2.1.3  Quartz Pressure Cell  A simple and inexpensive pressure cell for single-crystal study at pressures up to 1 kbar and a special attachment for mounting the cell onto a goniometer were designed by Yufit and Howard [44]. Based on a number of different previous versions, their improved design is much more convenient to use. It also takes advantage of modern diffractometer capabilities, including the use of area detectors. Certain trade names and company products are mentioned in the text or identified in illustrations in order to specify adequately the experimental procedure and equipment used. In no case does such identification imply recommendation or endorsement by National Institute of Standards and Technology, nor does it imply that the products are necessarily the best available for the purpose.

1

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Figure 4.2  Prototype environmental gas cell (EGC) (SRS on Station 9.8). Source: Adapted from Warren et al. 2009 [42] and Cernik et al. 1997 [43].

A schematic of the quartz pressure cell (QPC) is shown in Figure 4.3a, where the basic components are illustrated. Figure 4.3b illustrates a QPC on the geometer of the Bruker Smart CCD 6000 diffractometer. Monitoring the pressure was performed using a standard pressure gauge attached to the pressure line. (a)

(b)

A B E

C D

F

G

Figure 4.3  (a) Schematic of the QPC. A, crystal; B, quartz capillary; C, brass part of quartzmetal seal; D, valve 30-12HF2; E, removable handle; F, standard 1/8″ connector to a pressure line; G, stud for attachment of the QPC to the goniometer head. (b) QPC on the goniometer of the Bruker Smart CCD 6000. Source: Yufit and Howard 2005 [44]. Reproduced with permission of the International Union of Crystallography, http://journals.iucr.org/

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4.2.2   Powder Diffraction Applications 4.2.2.1  Environmental Chambers  Different options for environmental chambers are provided by manufacturers such as Panalytical, Bruker, and Rikagu Inc. There are also various custom-built chambers such as those being used at national beamlines. Figure 4.4 illustrates several non-ambient chambers (Anton Paar Instruments, Inc.) used by diffractometer manufacturers. The XRK900 chamber can be used with gases such as CO2 up to 10 bars (300–1175 K). The TTK450 chamber operates from 80 to 723 K, whereas the HTK16 chamber operates from 300 to 1875 K. 4.2.2.2  Simultaneous PXRD and DSC Techniques  The XRD-DSC system that was developed by Woerner et al. [45] is composed of a Rigaku Ultima IV diffractometer (CuKα radiation with a D/TeX high-speed linear position sensitive detector), a Rigaku XRD-DSC stage, Rigaku HUM-1 humidity generator, ULVAC vacuum pump (6.66 × 10−5 kPa), a vacuum manifold, and a custom-built humid atmosphere swing chamber (Figure 4.5). The humid atmosphere swing chamber (HASC) was necessary to buffer between the humidity generator and a XRD-DSC stage so that both humid atmosphere and vacuum swings could be performed. The design of the XRD-DSC system allows for three types of sample treatments, namely, in situ activation, vacuum swing, and atmosphere swing. 4.2.2.3 Oxford-Diamond In Situ Cell  Oxford-Diamond In Situ Cell (ODISC) [46] is a versatile, infrared-heated, chemical reaction cell developed by Moorhouse for in situ study of a range of chemical syntheses using time-resolved, energy-dispersive X-ray diffraction (EDXRD) on beamline I12 at the Diamond Light Source. A specialized reactor configuration has been constructed to enable in situ EDXRD investigation of samples under non-ambient conditions. One can use various sample vessels such as alumina crucibles, steel hydrothermal autoclaves, and

XRK900

TTK 450

HTK16

Figure 4.4  Non-ambient chambers for in situ X-ray diffraction studies. Source: Courtesy of Anton Paar Instruments, Inc.

186 IN SITU DIFFRACTION STUDIES OF SELECTED METAL–ORGANIC FRAMEWORK MATERIALS

(a)

(b)

(c)

(d)

Gas manifold

Flowmeter Humidity generator

Humid atm swing chamber

Needle valve XRD DSC Exhaust Vacuum manifold

760 Digital vacuum strain gauge

Figure 4.5  (a) Photo of a complete XRD-DSC system, (b) XRD-DSC stage with sealing cap, (c) XRD-DSC attachment without cap showing the Al2O3 standard and sample powder on aluminum pans, and (d) schematic of the entire XRD-DSC system. Source: Woerner et al. 2015 [45]. Reproduced with permission of American Chemical Society.

glassy carbon tubes, at temperature up to 1200°C. Figure 4.6a gives the schematic of the ODISC furnace and Figure 4.6b shows the cross section of the lower section of the cell.

4.3  IN SITU SINGLE-CRYSTAL DIFFRACTION STUDIES OF MOFs In situ diffraction studies as applied to porous MOF materials are illustrated with the following 12 examples, which include neutron, synchrotron, and conventional laboratory X-ray diffraction under ambient and non-ambient conditions. ­Crystallographic

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IN SITU SINGLE-CRYSTAL DIFFRACTION STUDIES OF MOFs

Centring tube Furnace body

16 mm

15'

Stirrer insert (a)

143 mm

Polished aluminium reflector

95 mm

IR lamp LOW OFF

167 mm

Reaction vessel

HIGH

180 mm

(b)

Figure 4.6  (a) Schematic of the ODISC furnace. (b) Cross-sectional diagram of the lower section of the ODISC cell. Source: Moorhouse et al. 2012 [46]. Reproduced with permission of American Institute of Physics.

observation of adsorbed gas molecules at high temperatures is a highly challenging task due to their rapid motion. In the selected examples, the following features are emphasized: crystal-to-crystal phase transformation, structure transformation due to the presence of guests, phase transformation induced by changes of guests (CO2/ N2), intermediate stage of guest exchange, mechanism of CO2 adsorption, breathing mode of flexible MOFs, multiphase-transition upon selective CO2 adsorption, in situ metastable intermediate transformation, location of CO2 in the pores of MOFs, reversible gas sorption, in situ study of framework formation, and fast screening of sorbents for CO2 capture using combined in situ XRD and DSC.

4.3.1   Thermally Induced Reversible Single Crystal-to-Single Crystal Transformation Allan et al. [47] demonstrated the detailed reversible phase transformation of single-­ crystal Cu2(OH)(C8H3O7S)(H2O)·2H2O (Cu-SIP-3) due to the presence/absence of water using in situ X-ray diffraction as a function of temperature. X-ray diffraction data were collected over a range of 150–500 K at the Advanced Light Source, Lawrence Berkeley National Laboratory. In situ loading experiments were performed using the EGC goniometer head designed by Warren et al. [42]. Cu-SIP-3 undergoes a phase transformation on dehydration. As a result of the loss of coordinated water, there is a change in metal coordination which involves the breaking of several bonds in the low-temperature structure. At temperatures below a dehydration-induced phase transition (T < 370 K) the structure was confirmed as being hydrated (left drawings of Figures 4.7a and 4.7b). In the

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temperature range where the transition takes place (370 K < T < 405 K), no discrete, sharp Bragg peaks were found in the X-ray diffraction patterns, indicating a significant loss of long-range order. At T > 405 K, the Bragg peaks return and the structure corresponds to a dehydrated phase, Cu-SIP-3 (right drawings of Figures 4.7a and 4.7b). At a temperature up to 150 K, the structure of hydrated Cu2O11C8S is monoclinic P21/n (a = 7.2949(4), b = 18.2726(6), c = 10.1245(6), β = 139.172(4)°). At 500 K, the sample becomes dehydrated Cu4O16C16S2 (Cu-SIP-3). While the dehydrated form is also monoclinic P21/n, the cell parameters are different: a = 13.792(2) Å, b = 19.430(4) Å, c = 12.057(2) Å, β = 139.172(4)°. After the crystal was exposed to moisture at 293 K, the structure of the rehydrated sample (Cu2O11C8S) returns to monoclinic P21/n (a = 7.3333(14) Å, b = 18.153(3) Å, c = 10.1729(19) Å, β = 94.379(4)°). Therefore, one can conclude unambiguously using in situ X-ray experiment that the loss of coordinated guest water molecules necessitates coordination changes, leading to lowering of symmetry and almost doubling the size of the asymmetric unit.

4.3.2   Structure Transformation Induced by Presence of Guests Takamizawa et al. [48] used combined single-crystal and in situ powder X-ray to study crystal transformation from an empty host Rh2(bza)4(pzy) (where bza = benzoate and pzy = pyrazine) to the material after CO2 molecules were adsorbed. The structure of Rh2(bza)4(pzy) consists of parallel one-dimensional chains and isolated cavities of 9 × 4 × 3 Å with narrow gaps of approximately 1 Å at the four corners of the cavities (Figure 4.8). The crystal structure of the host crystal without CO2 and with saturated (sealed-in) CO2 is rather different, namely, triclinic P-1 versus monoclinic (C2/c at 90 K and C2/m at 298 K), respectively [35]. The structure transformation was a result of the changing of interchain distances, slipping of neighboring chains, and tilting of the π–π stacked phenyl rings of the (bza) ligand

−H2O

−H2O

O

+H2O

Cu

Cu

C O (a)

+H2O

C S (b)

Figure 4.7  (a) Chains of copper tetramers in the low-temperature (left) and high-­ temperature (right) structures of Cu-SIP-3. (b) Copper tetramers in the low-temperature (left) and high-temperature (right) structures (Color scheme: blue balls-Cu, red balls-O, yellow balls-S, and gray balls-C). Source: Allan et al. 2010 [47]. Reproduced with permission of American Chemical Society.

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IN SITU SINGLE-CRYSTAL DIFFRACTION STUDIES OF MOFs

(a)

(b)

Figure 4.8  The single-crystal structures of (a) [Rh2(bza)4(pyz)] and (b) CO2-included [Rh2(bza)4(pyz)] measured at 90 K. Thermal ellipsoids are drawn at 30% probability. Source: (a) Zhang et al. 2014 [35]. Reproduced with permission of Royal Society of Chemistry. (b) Takamizawa et al. 2010 [48]. Reproduced with permission of American Chemical Society.

(i.e., tilting of the benzene ring about 9° away from the chain). Figure 4.8b shows that the adsorbed CO2 molecules were confined in the channel and interact with the phenyl rings via phenyl and CO2-quadrupole interaction. In situ high-temperature XRD showed the amount of uptake of adsorbed CO2 decreases as the temperatures increase. Evidence for the diffusion of gas into the crystal through the narrow gaps was obtained from sorption measurements. Interestingly, variable-temperature 2H NMR spectroscopy in a constant CO2 pressure (0.1 MPa) showed that the “rotating door” motions of the flipping benzene rings of the host skeleton were responsible for CO2 diffusion between cavities (Figure 4.9) [49].

Figure 4.9  Schematic diagram of the “rotating door” motion of the flipping benzene rings of the host skeleton consisting of inner channel walls in [Rh2(bza)4(pyz)]. The light gray arrow indicates the channel direction where CO2 diffuses. Source: Takamizawa et al. 2010 [49]. Reproduced with permission of American Chemical Society.

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4.3.3   Dynamic CO2 Adsorption Behavior Using a bis-triazolate ligand and tetrahedral Zn(II) ion, Liao et al. [35, 50] synthesized a flexible porous coordination polymer [Zn2(btm)2].4H2O, H2btm = (bis(5-methyl-1H-1,2,4-triazol-3-y1) methane) ((MAF-23·4H2O). This porous polymer is functionalized with pairs of uncoordinated triazolate N-donors that can be used as guest-chelating sites to give CO2/N2 selectivity. The dynamic CO2 sorption behavior was monitored by single-crystal X-ray diffraction. According to Zhang et al. [35], MAF-23 showed high saturation CO2 uptake (3CO2 per formula unit). Single-crystal structures of MAF-23·xCO2 (x = 0.00, 0.07, 1.5, and 2.8) were measured at 195 K. In MAF-23·0.07CO2, a CO2 molecule was chelated by one of the two crystallographic independent chelating claws, which possessed a methyl group at the ortho-position of the N-donor, demonstrating the weak electron donating effect of the methyl group (Figure 4.10). When the CO2 uptake increases, the unit cell volume of the MAF-23 continuously increases due to the distortion of coordination geometries of the Zn(II) ions and ligand shapes around the methylene groups. As CO2 increases, there is a strong binding affinity due to the action of the guest-chelating claws. The cage continues to deform and there is a balance between attraction of N-donor with the CO2 molecule and the repulsion between the CO2 molecules. With x = 1–4, MAF-23·xCO2 crystallizes in monoclinic P21/n. At 195 K, MAF-23·xCO2 showed a Type I CO2 sorption isotherm without hysteresis (Figure 4.11), giving an apparent Langmuir surface area of 622(5) m2 g−1 and a pore volume of 0.21 cm3 g−1.

4.3.4   Unstable Intermediate Stage During Guest Exchange A successful trapping of a metastable intermediate hydrate phase was illustrated by Cox et al. using the ECC with a previously reported MOF [Co(5-NH2-bdc) (a)

(b)

X=0

X = 0.07

C N (c)

X = 1.5

(d) X = 2.8

Figure 4.10  Single-crystal structures of MAF-23•xCO2 measured at 195 K (a−d). x = 0.00, 0.07, 1.5, and 2.8, respectively. Hydrogen atoms are omitted for clarity. Short intermolecular contacts are shown as dashed bonds. Source: Liao et al. 2012 [50]. Reproduced with permission of American Chemical Society.

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IN SITU SINGLE-CRYSTAL DIFFRACTION STUDIES OF MOFs

80 70

V (cm3 g−1)

60 50 40 30

CO2 273 K

20

CO2 298 K N2 273 K

10

N2 298 K

0 0

20

40

60

80

100

P (kPa)

Figure 4.11  CO2 and N2 adsorption (solid) and desorption (open) isotherms of MAF23⚫xCO2. Source: Liao et al. 2012 [50]. Reproduced with permission of American Chemical Society.

(bpy)0.5(H2O)]·2H2O (5-NH2-bdc = 5-aminoisophthalate, bpy = 4,4′-bipyridine) [41]. In situ single-crystal X-ray diffraction experiments performed under dynamic gas flow conditions revealed this highly flexible framework is capable of exchanging a wide variety of guest molecules in a single-crystal-to-single crystal manner. It was reported that the anhydrous framework [Co(5-NH2-bdc)(bpy)0.5] was prepared by heating the trihydrate phase under a vacuum at 393 K for 2 h. As the crystal structures contain multiple crystallographically unique solvent molecules, Cox et al. were interested to see if guest exchange, in particular the dehydration reaction of the crystal, occurred via the formation of discrete structures possessing an intermediate number of solvent molecules. The initial trihydrate structure (Figure 4.12b) has one water molecule coordinated to the Co metal center and two “free” water molecules held within the pore by hydrogen bonds. The second structure, obtained after 15 min of nitrogen flow, is that of a previously unreported dihydrate phase [Co(5-NH2-bdc)(bpy)0.5(H2O)]·2H2O. When the flow of dry nitrogen gas is halted, the dehydrate phase (Figure 4.12a) is converted back to the starting trihydrate phase within 5 min, presumably by reabsorbing moisture from the air. Sealing the capillary effectively halts the rehydration back reaction and thereby enables the collection of diffraction data on the metastable dihydrate phase. In summary, the ECC was capable of stabilizing a novel metastable intermediate in the dehydration reaction.

192 IN SITU DIFFRACTION STUDIES OF SELECTED METAL–ORGANIC FRAMEWORK MATERIALS

(b) a

b

c

(a)

a

H2O b

c

Figure 4.12  The reversible conversion of a trihydrate (a), single-crystal structure to dehydrate (b), is conducted by flowing dry N2 through the crystal, and then by halting the flow. The appearance and disappearance of the water molecule can be seen. Source: Cox et al. 2015 [41]. Reproduced with permission of the International Union of Crystallography, http://journals.iucr.org/

4.3.5   Mechanism of CO2 Adsorption Miller et al. [51] studied the adsorption mechanism of the microporous scandium terephthalate, Sc2(O2CC6H4CO2)3 (ScBDC), an attractive small-pore model sorbent for small molecules relevant to CO2 separation technologies. The mechanism of adsorption of CO2 has been determined by single-crystal synchrotron X-ray diffraction at ≈230 K at the European Synchrotron Radiation Facility, Grenoble, France. A custom-made gas delivery system was used for the in situ gas pressure studied from 1 to 9 bar of CO2. The details of the adsorption mechanisms were obtained with in situ studies from pCO2 = (0–1) bar. At pCO2 = 1 bar, the symmetry of the crystal has changed from orthorhombic Fddd to monoclinic C2/c through tilts in the terephthalate linkers with the presence of CO2. CO2 molecules take up different sites in two symmetrically different channels that result from the symmetry change. Figure 4.13a depicts the ScO6 octahedra, and three different terephthalate groups, namely, group 1, group 2a, and group 2b. Group 1 in channel A was found having the orthorhombic structure. Groups 2a and 2b were a result of the rotations of the terephthalate groups. In channel A, the occupancy of CO2 is close to one and the CO2 molecule axis is aligned so that the O atom is pointed toward the hydrogen atoms of the framework phenyl group. In channel B, there are two symmetry-related sites for CO2 but they

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POWDER DIFFRACTION STUDIES OF MOFs

(a)

(b)

H8

H8 C100 H8

O100

O100

H8

1 2a

A (c)

2b

B

H11 H11

O201

H11

H11

C200 O200 H3

H3

Figure 4.13  (a) Slices of CO2 molecules adsorbed in ScBDC at 1 bar and 235 K, viewed down the channel axes. In rows of channel types A and B, the channels are no longer identical due to different rotation of the terephthalate groups labeled as types 2a and 2b. The environments of CO2 in the triangular pores in channels of types A and B are illustrated further in (b) and (c), respectively. Source: Miller et al. 2009 [51]. Reproduced with permission of American Chemical Society.

cannot be simultaneously occupied. The environments of CO2 in the triangular pores in channel types A and B are shown more clearly in Figures 4.13b and 4.13c.

4.4   POWDER DIFFRACTION STUDIES OF MOFs 4.4.1   Synchrotron/Neutron Diffraction Studies The breathing behavior of MOFs for gas adsorption has been reviewed by Alhamami et al. [52]. The unique breathing behavior upon adsorption of gases or solvents underlies their potential application as host materials in gas storage for renewable energy. This unique behavior has attracted widespread attention to designing, understanding, and utilizing properties of these materials. The tools of investigation, in addition to the use of in situ diffraction, also include calorimetry, nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy, and theoretical modeling.

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4.4.1.1  Breathing Modes of Flexible MOFs  A unique feature, which distinguishes some MOFs from conventional porous materials, is the ability to “breathe,” that is, to expand or contract in response to external stimuli such as variation in temperature. Such flexible networks are sometimes referred to as “breathing” MOFs. The best-known materials exhibiting breathing mode characteristics comprise the MIL-53 series [52–56]. Structurally, these materials are formed by connections of corner-sharing MO4(OH)2 octahedra linked by 1,4-benzenecidarboxylic (BDC) acids. As the MIL53 (Al, Cr) materials are synthesized hydrothermally (MIL-53as), the channels are filled with disordered BDC and H2O molecules (narrow pore (np) structures) [53]. The narrow pore structure is formed by hydrogen bonding between the water molecule and the carboxylic and hydroxyl groups of the host molecules. Upon dehydration at high temperature, the MIL-53 gives rise to a large pore (lp) structure due to the absence of interactions. Therefore, using heat as an external stimulus, a reversible conversion between the hydrated MIL-53 (np) and the dehydrated MIL-53 form (lp) is observed (Figure 4.14a).

(a)

16.83 Å 19.69 Å

−H2O 13.04 Å

7.85 Å +H2O

na (mmol g−1)

(b)

(c) 9 8 7 6 5 4 3 2 1 0

P (bar) 0 2

MIL-53LP

4 6 8 10

MIL-53HP

8 6 4 2

0

5

10 P (bar)

15

20

3

5 7 2-Theta (°)

9

0

MIL-53LP

Figure 4.14  (a) Schematic representation of the reversible hydration–dehydration of MIL-53np and MIL-53lp. Source: Seere et al. 2002 [53]. Reproduced with permission of American Chemical Society. (b) Adsorption isotherms of MIL-53 (Cr) showing hysteresis loop. (c) Powder XRD patterns of MIL-53 (Cr) under various pressures of CO2 at 293 K. Source: Seere et al. 2007 [54]. Reproduced with permission of John Wiley & Sons.

POWDER DIFFRACTION STUDIES OF MOFs

195

An interesting adsorption behavior that has been observed in MIL-53 (Cr) is the adsorption of a large amount of CO2 molecules at room temperature [54] (Figure 4.14b). After a fast uptake at low pressure (approximately 2–3 mmol g−1), the isotherm reaches its first plateau between 1 and 4 bar followed by an adsorption of more than a double amount of CO2 at higher pressures. The desorption branch returns to coincide with the adsorption branch at approximately 2 bar. This unusual result could be interpreted as follows: while the degassed solid exhibits an expected lp structure, the first small portion of CO2 would force the pore to be np due to host– guest interaction. A further absorption of CO2 at higher pressure reopens the pore to accept more CO2. This interpretation agrees with in situ diffraction experiments performed at synchrotron radiation facilities under 1–10 bar CO2 (Figure 4.14c). 4.4.1.2  Multiple-Phase Transitions upon Selective CO2 Adsorption  Another interesting system (provided by Henke et al. [57]) that exhibits multiple-phase transition and remarkable breathing behavior with respect to the presence/absence of polar molecules in the pores is the flexible alkyl ether-functionalized MOF [Zn2(BMEbdc)2(dabco)]n, (BME-bdc = 2,5-bis(2-methoxyethoxy)-1,4-benzenedicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane) [57]. Apparently, the dried sample exhibits a narrow pore form with almost no accessible porosity. Upon adsorption of polar guest molecules (DMF, EtOH), the framework transforms to an open pore form, with accompanying increase of unit cell volume. This flexibility (contraction of network) is because of the strong interaction between the 2-methoxyethoxy substituents, when guest molecules are absent. This situation is opposite to the situation of MIL-53 discussed previously. When guest molecules are present in this case (CO2), the unit cell volume increases. Due to the slow adsorption kinetics, an unexpected metastable intermediate form could be identified. Figure 4.15 gives a sorption isotherm of [Zn2(BME-bdc)2(dabco)]n at 195 K. The feature of this isotherm includes a stepwise adsorption and a large hysteresis, typical for flexible MOFs that undergo a structural transition upon adsorption of guest molecules. The quadrupole moment of CO2 enables the gas to penetrate into the pore because of weak intermolecular interactions. It stays at the narrow pore region from pCO2 = 0 to 200 mbar. The steep uptake at 200 mbar signifies a structural transformation from the narrow pore to the open pore form. On the desorption branch a large hysteresis was observed which was due to the “breathing” framework. The in situ XRD patterns corroborate the transformation explanation. Figure 4.16 shows a close-up view of the low angle region of the X-ray patterns recorded as a function of partial pressure of CO2. The lowest angle peak reflection (2.73° – (001)) is in roughly the same position for all three phases. Structural models of the three different states of [Zn2(BME-bdc)2(dabco)]n upon CO2 adsorption at 195 K are shown in Figure 4.17. The cell parameters are all rather similar to each other in the three forms. Up to 300 mbar, only “narrow pore form” is present. The unit cell is slightly expanded at 200 mbar and shrinks at 300 mbar. From 400 to 900 mbar, an intermediate pore form appears which has a mixture of the narrow, intermediate, and open pore form. At 1000 mbar, the “open pore form” almost exclusively exists.

196 IN SITU DIFFRACTION STUDIES OF SELECTED METAL–ORGANIC FRAMEWORK MATERIALS

100

Vads (cm3 g−1)

80

CO2

60 40 20 N2 0 0.0

0.2

0.4

0.6

0.8

1.0

p (bar)

Intensity (a.u.)

Figure 4.15  Sorption isotherms at 195 K. Adsorption and desorption branches are shown with closed and open symbols. Source: Henke et al. 2011 [57]. Reproduced with permission of Royal Society of Chemistry.

0.0 on Adsorpti on Desorpti (bar) p(CO 2)

Intermediate pore

0.5

Open pore

1.0 Intermediate pore

0.5

Narrow pore

0.0 2.5

3.5

3.0

4.0

2θ (°)

Figure 4.16  Lower angle region of the PXRD patterns recorded at variable CO2 pressure (λ = 0.4592 Å). Patterns representing the narrow pore (blue), open pore (red) and intermediate pore form (green). Source: Henke et al. 2011 [57]. Reproduced with permission of Royal Society of Chemistry.

197

POWDER DIFFRACTION STUDIES OF MOFs

Narrow pore form

Intermediate pore form

Open pore form

1943 Å3

2092 Å3

2275 Å3

Figure 4.17  Structural models of the three different states of [Zn2(BME-bdc)2(dabco)]n upon CO2 adsorption at 195 K as a function of increasing pCO2 (L → R from 0 to 1000 mbar). Disordered 2-methoxyethoxy substituents and hydrogen atoms are not included in the model structures. Source: Henke et al. 2011 [57]. Reproduced with permission of Royal Society of Chemistry.

4.4.1.3  Metastable Intermediate Transformation  Katsenis et al. [58] used in situ, real-time synchrotron powder X-ray diffraction to capture and to monitor a metastable, novel-topology intermediate of a mechanochemical transformation. While amorphization on milling is a well-known phenomenon, spontaneous recrystallization of the amorphous phase by continued milling has not been described prior to the cited work. During the synthesis of MOF Bis (2-methylimidazolyl)-zinc (also known as ZIF-8) by applying the grinding technique, they observed the unexpected amorphization. On further milling, recrystallization into a non-porous material via a metastable intermediate new phase, katsenite (kat), took place. The sequence of solid-state transformations in the reaction of ZnO and 2-methylimidazole (HMeLm) is shown in Figure 4.18. The new topology provides evidence that milling transformation can involve short-lived, structurally

ZnO + N

Amorphous Zn(Melm)2

NH CH3

ZIF-8 SOD-Zn(Melm)2 T/V = 2.4 nm−3

kat-Zn(Melm)2 T/V = 3.8 nm−3

dia-Zn(Melm)2 T/V = 4.2 nm−3

Figure 4.18  Sequence of solid-state transformations in the reaction of ZnO and sodalite (SOD) framework, ZIF-8. Source: Katsenis et al. 2015 [58]. Reproduced with permission of Nature Publishing Group.

198 IN SITU DIFFRACTION STUDIES OF SELECTED METAL–ORGANIC FRAMEWORK MATERIALS

(a)

(b)

Figure 4.19  Structure of kat-Zn(MeIm)2 viewed along the crystallographic c direction: (a) ball-and-stick representation and (b) the kat framework with different coloring for each type of vertex, represented by its vertex figure. Source: Katsenis et al. 2015 [58]. Reproduced with permission of Nature Publishing Group.

unusual phases. Structure of kat-Zn(Melm)2 is shown in Figures 4.19a and 4.19b. kat-Zn(Melm)2 contains pores consisting of tight channels and pockets. It consists of four crystallographically independent zinc ions, each in a tetrahedral environment defined by four nitrogen atoms of four different ligands. This work was also corroborated by in situ NMR studies. The solid-state 13C NMR spectrum of kat-Zn(Melm)2 indicates four symmetrically non-equivalent Melm-ligands. Samples of dia-Zn(MeIm)2 are non-porous, exhibiting an almost completely flat nitrogen sorption curve. 4.4.1.4  Reversible Gas Sorption Driven by Temperature  Baek et al. (2015) [59] used combined in situ single-crystal and synchrotron powder X-ray diffraction to determine reversible CO2 sorption processes in a self-assembled and apparently non-porous organic crystal, 1,3,5-tris-(4-carboxyphenyl) benzene (H3BTB) (Figure 4.20a). The host material is formed by a hydrogen bond network between H3BTB and N,N-dimethylformamide (DMF) and by the π–π stacking between the H3BTB moieties (Figure 4.20b). The material can be viewed as wellordered array of cages which are tight-packed so that the cages are inaccessible from outside. The hydrogen bonds and π–π stacking are strong enough to give relatively high thermal stability up to ≈373 K at 1 bar. The DMF-forming cages could be flexible, providing transient pathways for guest diffusion. The encapsulated DMF molecules are disordered around a site of a threefold rotary inversion symmetry. These molecules can be completely removed in vacuum at 323 K, resulting in very stable crystal (Figure 4.20c). The unit cell volume decreased by 3.23% after removal of DMF molecules. Each cage has about 83 Å which is large enough for capturing small gas molecules such as CO2.

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POWDER DIFFRACTION STUDIES OF MOFs

(a) (b)

B (8.13 Å)

(c) A (3.69 Å)

B

(d) A

Figure 4.20  (a) Structure of (H3BTB +DMF) showing the hydrogen bonding network. (b) Packing of (H3BTB +DMF) moieties. Red color represents “O.” (c) Structure after DMF molecules have been evacuated, showing the cages. (d) Views of the cage with a CO2 molecule (H3BTB·CO2). The CO2 molecule is disordered around a site of a threefold rotary inversion symmetry and is depicted by a space-filling model. Source: Baek et al. 2015 [59]. Reproduced with permission of PNAS.

In situ single-crystal X-ray diffraction experiments were done under varying pressures at 323 K. It was found that even though there are no permanent channels connecting the isolated cages, all cages are filled at 25 bar of CO2 pressure. One cage contains one CO2 molecule and the cage is composed of two H3BTB and six DMF molecules, with stoichiometry (H3BTB)2(DMF)6(CO2). Therefore, the empty cages are permeable to CO2 at high temperatures due to thermally activated transient pathways (molecular gating) between the cages. Figure 4.20d shows the cage

200 IN SITU DIFFRACTION STUDIES OF SELECTED METAL–ORGANIC FRAMEWORK MATERIALS

(2 −1 0)

(2 0 2) (4 −1 1) Change of PCO2 (bar)

(3 0 0) (3 −2 −1)

0 1 5 10 Intensity (a.u.)

15 20 25 20 15 10 5 1 0 as synthesized 5

6

7

8

9

10

11

12

13

14

2θ (°)

Figure 4.21  In situ synchrotron PXRD patterns under varying pressures of CO2 at 323 K. The red dotted square represents the changes in diffraction peaks at the (2 −1 0) plane accompanying CO2 adsorption and desorption. The blue dotted square represent the intensity changes in diffraction peaks accompanying framework expansions and contractions upon reversible CO2 sorption. Source: Baek et al. 2015 [59]. Reproduced with permission of PNAS.

of (H3BTB) with a CO2 molecule. The disordered CO2 molecule is depicted by a space-filling model. Further variable-temperature in situ synchrotron powder X-ray diffraction studies also confirmed that the CO2 sorption is reversible and driven by temperature increase (Figure 4.21). With combined solid-state NMR, they were also able to study the dynamical motion of CO2. 4.4.1.5  Framework Formation in Action  In order to understand the details of the complex crystallization process of lithium tartrate MOF, Yeung et al. [60] employed high-energy in situ synchrotron X-ray powder diffraction at the Diamond Light Source Beamline I12 to study the crystallizations and the metastable intermediates in the phase formation of a lithium tartrate MOF, Li2(meso-C6H6O6). Li2(meso-C6H6O6) has a monodentate binding site. They observed the successive crystallization and dissolution of three competing MOF phases in one reaction. As shown in Figure 4.22, which gives the three possible products, namely, the low-density phases 1 (Li2(meso-C6H4O6)(H2O)0.5, a metastable intermediate

201

POWDER DIFFRACTION STUDIES OF MOFs

1

O (meso-C6H6O6)·H2O + 2 Li(OAc)2H2O

H

Li 2a

H O H

CO2H CO2H

CO O C H HO

H

HO

Li

O

OH

O

Li

H

2b O

H

C

C

OH O HO

Li

O

Figure 4.22  Three possible products in the formation of lithium meso-tartrates, showing ligand conformations and major binding modes. Source: Yueng et al. 2016 [60]. Reproduced with permission of John Wiley & Sons.

2a (Li2(meso-C6H4O6)(H2O) (with gauche ligand conformation which is the preferred conformation of the mesotartaric acid), and 2b, a denser Li2(meso-C6H4O6) phase, which has a monodentate binding feature. In situ X-ray diffraction confirmed compound 2b to be the final thermodynamically stable product (Figure 4.23). They further determined the extent of crystallization as a function of reaction time, and quantified the reaction energy by determining the reaction rate constants and activation energies. The structure relationships between reactants and products were used to interpret different reaction rates (i.e., larger changes in conformation gave rise to higher activation energy). 4.4.1.6  Resin-Assisted Solvothermal MOF Synthesis  The [Co(NDC) (DMF)] (NDC = 2,6-naphthalenedicarboxylate) system is a system which is known to produce at least three distinct frameworks during the synthesis process. In order to determine the structural diversity observed in coordination polymers formed using Co(II) NDC and DMF, the system was probed by Moorhouse et al. [61] to determine, in particular, the factors influencing the preferential formation of one structure over another. The process involved the use of cation-impregnated polymer resin as both template and metal source in a relatively new technique in MOF synthesis.

202 IN SITU DIFFRACTION STUDIES OF SELECTED METAL–ORGANIC FRAMEWORK MATERIALS

(a)

40.1(4)°C

0.0 0

1000

104.4(3)°C 1.0

2a

2a

2b

α

1

α

1.0

(b)

2000

0.0 3000

3000

4000

t (s)

5000

t (s)

(c)

(d)

0 2θ (°)

0

2

2θ (°)

4

2

4 1000

2000 t (s)

3000

3000

4000 t (s)

5000

Figure 4.23  Solvothermal conversions between lithium tartrate MOFs, showing (a, b) the extent of crystallization as a function of reaction time (t). (c, d) In situ XRD data, where 2θ is the diffraction angle. (a, c) Hydrated phase 1 (gray points) is converted into metastable phase 2a (green points), followed by (b, d) conversion of 2a into the thermodynamic product 2b (orange points). λ = 0.2326 Å. Source: Yueng et al. 2016 [60]. Reproduced with permission of John Wiley & Sons.

Reactions were conducted using angular-disperse XRD at Beamline I12, Diamond Light Source (with the Oxford-Diamond In Situ Cell (ODISC)). Further in situ energy-disperse XRD experiments were performed by using HASYLAB Beamline F3 of the DORIS synchrotron at the Hamburg campus. It was found that the resin-­assisted synthesis resulted in the preferential formation of a topology previously impossible to synthesis in bulk form. The [Co(NDC)(DMF)] framework viewed along the z-axis (showing the diamond shape channels and coordinated DMF) is shown in Figure 4.24. The stack plot in Figure 4.25 shows in situ energy-dispersive XRD data obtained during the solvothermal formation of [Co(NDC)(DMF)] at 200°C. Reflections consistent with undissolved H2NDC are observed initially at 2θ = 2.48° and 3.64°, then diminished and finally disappeared before the autoclave reaches 200°C. All reflections consistent with the [Co(NDC)(DMF)] phase appear concurrently. The framework MOF was formed as a pure phase which is monoclinic with a space group C2/c (a = 23.436(3) Å, b = 8.7733(6) Å, c = 7.219(1) Å, β = 97.80(2)°).

203

POWDER DIFFRACTION STUDIES OF MOFs

CoO6

Figure 4.24  Structure of [Co(NDC)(DMF)] viewed along the c-axis showing coordinated DMF. Octahedral coordination spheres are shown as polyhedral. Source: Moorhouse et al. 2015 [61]. Reproduced with permission of Elsevier.

(110) (200)

(400) (111)

(510) (220)

H2NDC

70 60 50

t (mi

40

n) 30 4.0

3.5

2.5

2.0

1.5

10

3.0

20

°)

2θ (

1.0

Figure 4.25  Three-dimensional-stack plot showing in situ angular-dispersive XRD data at 200°C. Source: Moorhouse et al. 2015 [61]. Reproduced with permission of Elsevier.

204 IN SITU DIFFRACTION STUDIES OF SELECTED METAL–ORGANIC FRAMEWORK MATERIALS

4.4.2   Laboratory X-ray Diffraction Studies 4.4.2.1  Location of Adsorbed CO2 Molecules  The highly porous compound HKUST-1 [62, 33] is composed of dimeric cupric tetracarboxylate units. HKUST-1 has a three-dimensional channel structure connecting a system of cages. The square-shaped pores are about 10 Ǻ diameter, and the largest ones are hexagonal pores of about 18 Ǻ in diameter. Using neutron diffraction, Wu et al. found that the unsaturated open Cu site plays an important role in enhancing binding of CO2 molecules [32]. In Figure 4.26a, in situ experiments with CO2 delivered at different partial pressures were conducted by Wong-Ng et al. [62] with a Panalytical X-ray diffractometer using the Anton Paar environmental chambers XRK 900 (gas flow conditions) under a series of CO2 partial pressures (0.2, 0.4, 0.8, 1, and 6 bar). Several features of the X-ray patterns that are sensitive to occupancy of the cages by guest species were found. The X-ray patterns containing 311 and 222 reflections were shown: (1) refers to the as-received sample, (2) is the one that received treatment for 24 h at 150°C under flowing He, and exposure to mixture of CO2/He with increasing fractions. The 311 intensity increases from near zero for hydrated sample 1 to dehydrated 2, then decreases back to small value as pCO2 decreases, suggesting the empty pores become occupied by CO2. Figure 4.26b gives an asymmetric unit structure. One can identify two principal adsorption sites: at “open space” A-site near Cu, and around the B-site, the “octahedral site.” Highly disordered CO2 molecules were found. Using first principles calculations, Zhou et al. [76] predicted CO2 to be linear with the Cu–O1–C angle of 110° (Figures 4.26c and 4.26d). The large open space in the vicinity enables the rotation of the CO2 molecules. The CO2 molecules in the octahedral cage are also disordered. It gives a single quadruple split site at the center of the cage (1/4, 1/4, 1/4) and four overlapped oxygen sites (unresolved double split) along the directions. A plot of the occupancy of the “open space” site and the “octahedral cages” shows progressive increase as pCO2 increases (Figure 4.26e). The open-space site is occupied preferentially. In summary, the adsorption is mainly on two sites, one is on top of the open Cu atom and another one is in the window opening of an octahedral cage. The metal–CO2 binding is due to enhanced electrostatic interaction, and the framework CO2 interaction is of the van der Waals type. The highly disordered CO2 molecules result from the flowing versus static gas situation. 4.4.2.2  Materials Screening for Post-Combustion CO2 Capture (  In Situ XRD/DSC)  Combined use of in situ X-ray diffraction and simultaneous differential scanning calorimetry (DSC) is a promising technique for rapid evaluation of the suitability of microporous materials for post-combustion CO2 capture in both dry and wet conditions (particularly in the presence of water vapor which is a major component of post-combustion CO2 capture). Using calorimetry, the enthalpy of adsor­ ption for a gas can be directly measured at experimental conditions. This is especially advantageous in the situations of phase transitions, or gate-opening in the presence

205

POWDER DIFFRACTION STUDIES OF MOFs

30,000

222

20,000 15,000 10,000

(2) (3) (4) (5) (6) (7) (1)

Cu

0 10.5

11

11.5 2θ (°)

C1

O1

Cu

C2

Cu

311

5,000

O1 C3

O2

O2 C O1

C3

Cu-O1-C

H1 Cu

O3 C4 O4 C5 O5

12

(a)

(b)

(c)

O O

O

0.6 0.5 Green - Cu Blue - O in CO2

O

A

Grey - C in CO2 Black - C in HKUST-1 Yellow - O in HKUST-1

0.4 0.3 0.2

B C Cu

CO2/Cu

Intensity (counts)

25,000

O

C

0.1

0.2

0.4

0.6

0.8

1

Partial pressure of CO2

O

(d)

(e)

Figure 4.26  (a) In situ X-ray diffraction patterns under gas flow conditions under a series of partial pressures, from 0.2, 0.4, 0.8, 1 bar and higher, (1) as-received sample and (2) evacuated at 150°C under He for 24 h; (b) Atomic labeling scheme, including the disordered CO2 molecules; (c) The structure of CO2 molecule adsorption on the open Cu sites of HKUST-1; (d) A projection of the unit cell along ⟨110⟩ direction. Rotational disorder proposed for the CO2 molecules located next to the Cu ions is illustrated by showing multiple molecule positions and further emphasized using arc arrows; (e) The occupancy of the “open space” site (round dots) and the “octahedral cages” (squares) increase progressively as pCO2 increases [numbers in (a); colors in (e)]. Source: Wong-Ng et al. 2016. Reproduced with permission of Elsevier.

of guests. Woerner et al. [45] used this combined technique to study the performance and structural effect of CO2 adsorption under static or dynamically varying humidity, temperature, and gas on a class of potential materials for CO2 capture. One example illustrating the applications of the simultaneous XRD-DSC technique is the open-framework compound ZIF-7, Zn(bIm)2 (where bIm = 2-benzimidazolate). ZIF-7 is formed by connecting Zn metal clusters through the benzimidazole (BIM) linkers. It has a sodalite topology with a crystallographic six-membered ring pore opening (Figures 4.27a and 4.27b). The position of the benzene rings in the

206 IN SITU DIFFRACTION STUDIES OF SELECTED METAL–ORGANIC FRAMEWORK MATERIALS

(a)

a b

CO2

Enthalpy (J g −1 ZIF-7)

(c) 60

(b)

50 40 30 20 10 0 0.0

Dry 75% RH 0.2

0.4

0.6

0.8

1.0

CO2 Pressure (atm)

Figure 4.27  (a) Pore morphologies and accessible volumes (indicated by yellow surfaces) of ZIF-7; (b) structure of ZIF-7 (gate open) at 1 atm of CO2. Zinc tetrahedra are shown in gray tone nitrogen in blue, carbon in black, and CO2 in red. The CO2 molecules are coordinated to the benzimidazolate rings; (c) Cumulative measured enthalpy isotherm of ZIF-7 during CO2 adsorption–desorption with the XRD-DSC under dry (red) and 75% RH (blue) conditions. Closed symbols are from enthalpy measurements during adsorption, and open symbols are from desorption enthalpy measurements. Source: Woerner et al. 2015 [45]. Reproduced with permission of American Chemical Society.

optimized structure in vacuum narrows the pore entrance to ≈0.3 nm. However, the linkers have some freedom to rotate over a certain angle, allowing molecules larger than 0.3 nm to enter the main cavities. In Figure 4.27c, cumulative enthalpy isotherms during CO2 adsorption−­ desorption with XRD-DSC under dry (red) and 75% RH (blue) conditions are shown. The presence of water vapor thus reduces the measured enthalpy and therefore the capacity of ZIF-7 (85.2% of dry). Figure 4.27c shows the results of vacuum-humid atmosphere swings. However, relative to other studied sorbents [45], ZIF-7 still shows strong CO2 affinity even in the presence of 75% RH. Successive XRD patterns under vacuum and under 75% RH CO2 vacuum-humid 1 atm CO2 swings are shown in Figure 4.28. As a summary, ZIF-7 appears to be ideally suited for post-combustion flue gas CO2 capture.

207

REFERENCES

Intensity (a.u.)

15 °)

2θ (

20

25 0

6 15 2θ ( 20 °)

4

2

25

r

m be

8 10

nu

5

ng

10

12 10

Sw i

Intensity (a.u.)

5

12 10 8 er b 6 m u 4 gn 2 win S

30

Figure 4.28  (a) Successive XRD patterns of ZIF-7 under vacuum and (b) under 75% RH CO2 during vacuum. Source: Woerner et al. 2015 [45]. Reproduced with permission of American Chemical Society.

4.5  CONCLUSION In situ diffraction techniques have been shown to be effective means for study of interactions of MOF with adsorbed guest molecules. In this chapter, we illustrated these techniques with seven examples, representing the capability of in situ diffraction for study of a gas adsorption mechanism, breathing of flexible MOFs, guest exchange studies, a guest-induced structural transition, a reversible transformation, probing of adsorption sites and binding sites of CO2 and other gases, and in situ synthesis and phase formation of MOFs. In addition, the in situ diffraction techniques can also uniquely be used for monitoring a mechanochemical reaction-metastable intermediate, to capture metastable intermediates in successive crystallizations during the formation of an MOF, and to perform a snapshot analysis of transient molecular adsorption. Some of the studies used in situ diffraction combined with other techniques such as in situ NMR, and in situ DSC. As new in situ technology continues to evolve, we will be able to further our understanding of adsorption mechanisms, to facilitate the improvement of adsorption capability and the design of new applications.

REFERENCES 1. Etheridge, D.M.; Steele, L.P.; Langenfelds, R.L.; Francey, R.J.; Barnola, J.M.; Morgan, V.I. Natural and anthropogenic changes in atmospheric CO2 over the last 1000 years from air in Antarctic ice and firn. J. Geophys. Res. Atmos. 1996, 101, 4115–4128.

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5 Electrochemical CO2 Capture and Conversion Peng Zhang, Jingjing Tong, and Kevin Huang Department of Mechanical Engineering, University of South Carolina, Columbia, SC

5.1  INTRODUCTION Burning fossil fuels for power, heat, and transportation is a primary source of CO2 emissions that leads to global warming and climate change. Mitigating human-activity induced CO2 emissions is a key step to battle this grand challenge [1, 2]. There are three strategies that have been formulated to control CO2 pollutions: (1) adopting environmentally benign energy sources (e.g., solar, wind, nuclear); (2) improving the efficiency of current energy conversion processes (e.g., fuel cells); (3) capturing CO2 from the emission point sources followed by either geological sequestration or conversion into useful products [3, 4]. In these methods, increasing renewable energy production is attractive and growing rapidly in recent years, but the slow development of energy storage technology has hindered a widespread adoption of the intermittent renewable power generation. Improving energy conversion efficiency can effectively reduce CO2 emissions per unit power generated, but it takes longer time and costs more [5]. Therefore, CO2 capture and storage (CCS) or conversion (CCC)

Materials and Processes for CO2 Capture, Conversion, and Sequestration, First Edition. Edited by Lan Li, Winnie Wong-Ng, Kevin Huang, and Lawrence P. Cook. © 2018 The American Ceramic Society. Published 2018 by John Wiley & Sons, Inc.

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is currently deemed the most realistic near-term solution to stabilize the atmospheric CO2 concentration and thus mitigate the climate change that has been increasingly observed to intensify in recent history. Capturing CO2 from centralized fossil-fueled (coal, natural gas, or biogas) power plants is a vital first step in CCS implementation as they represent highemission sites responsible for ∼78% of global stationary carbon emission [6]. The current industrial methods to capture CO2 at emission point sources are mainly based on reversible chemical/physical sorption processes using liquid solvents and solid sorbents as a CO2 scrubber [7–10]. Typical examples of sorption materials under development include solvent-based amine [11] and ionic liquids [12], sorbent-based activated carbon [9], molecular sieves [13], and metal-organic framework (MOF) [14, 15]. However, the cost and energy penalty to implement these scrubbing technologies into existing power plants are so high that the overall plant efficiency and cost of electricity could be significantly impacted. A system analysis by National Energy Technology Laboratory (NETL) projects that implementing an amine-based scrubbing system to a new carbon-intensive coal-fired power plant would increase the cost of electricity by 80% and lower the plant efficiency by onethird [16], not to mention that the thermal degradation and corrosive nature of amine could incur additional costs. Therefore, there is a great need to develop alternative technologies for the next generation of advanced carbon capture. Electrochemical methods have emerged in recent years as new approaches to capture CO2. In these methods, an electrical field is typically applied across an electrochemical cell consisting of an electrolyte and two electrodes (cathode and anode) to drive the CO2 capture process. One immediate example is the use of reverse mode of molten carbonate fuel cell to concentrate CO2 [17, 18]. Compared to conventional solvents/sorbents sorption methods, electrochemical methods are more efficient, thus requiring less energy. In addition, the electrochemical process is also compatible with CO2 conversion by electrochemistry, making capture and conversion in single step technically feasible. Because of the efficiency advantage, research in electrochemical CO2 capture and conversion (e-CCC) has intensified in recent years. The objective of this article is to review the recent developments in the field of e-CCC, particularly high-temperature e-CCC processes, because a large body of low-temperature e-CCC has been previously reviewed [3, 19–23].

5.2  CURRENT ELECTROCHEMICAL METHODS FOR CARBON CAPTURE AND CONVERSION There are two types of electrochemical cells studied for e-CCC. The first one is the electrolytic cell consisting of one aqueous electrolyte and two solid electrodes. Electrons provided through external circuit are the driving force for the e-CCC process. The second one is the permeation cell consisting of only one mixed-conducting membrane allowing CO2/O2 to pass under a pure electrochemical gradient of active

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species without an external electric field. The aqueous-based electrolytic cell approach is designed for ambient-temperature e-CCC, whereas the permeation cell is more suited for high-temperature e-CCC, where ionic/electronic diffusion in solid phases can be thermally activated. In the following sections, we review primarily the principles and materials involved in these two methods.

5.2.1  Ambient-Temperature Approach 5.2.1.1 CO2 Capture Using Electrolytic Cells  Ambient-temperature CO2 capture from air through electrochemical cells was first demonstrated by Wynveen and Quattrone in 1971 for aircraft [24] and was also later proved applicable for spacecraft by Wynveen et al. [25] and Winick et al. [26]. The electrochemical cell is virtually an alkaline fuel cell with aqueous Cs2CO3 solution as the electrolyte, and the cathode and the anode were comprised of fine mesh screens upon which a Teflon and platinum mixture was applied. The cathode and anode gases were ambient-temperature CO2-containing moistened air and moistened H2, respectively. The electrode reactions are: At the cathode side:

O2 + 2H 2 O + 4e − → 4OH − (5.1)



CO2 + OH − → HCO3− (5.2)



HCO3− + OH −  CO32 − + H 2 O (5.3)

At the anode side:

H 2 + 2H 2 O → 2H 3 O + + 2e − (5.4)

The lower pH value on the anode side is needed to achieve a higher bicarbonateion concentration via reaction (NaN). The release of CO2 can be carried out in two steps: direct dissociation of bicarbonate-ions into CO2 (reaction 5.5) and indirect dissociation of carbonic acid into CO2 (reaction 5.6) [26, 27].

HCO3− → CO2 + OH − (5.5)



HCO3− + H 2 O − OH −  H 2 CO3 → CO2 + H 2 O (5.6)

One major challenge of this aqueous electrochemical cell is the low CO2 transport rate, that is, 0.0304 ml min−1 cm−2. Eisaman et al. [28] further improved the CO2 capture efficiency of the process with two new membrane-supported electrolytes: cellulose filter paper saturated with Cs2CO3 solution and anion exchange membrane (AEM, fumasep® FTAM) soaked in a 0.1 M aqueous K2CO3 solution. The results showed a 5% faradaic efficiency with an energy consumption of 2170 kJ mol−1 CO2 for the cellulose filter paper saturated with the Cs2CO3 method, and a 23% faradaic

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efficiency with an energy consumption of 350 kJ mol−1 CO2 for the AEM membrane. However, the current density of the alkaline fuel-cell CO2 concentrator was quite low, only 0.5 mA cm−2, so that to capture the CO2 emitted from combusting a gallon of gasoline would require 2141 m2 membrane area of a fuel-cell concentrator. In addition, the humidity of the gas must be very high to prevent formation of precipitates from the electrolyte solutions. Recently, Eisaman et al. [29, 30] further studied the electrochemical CO2 capture from the atmosphere using a K2CO3/KHCO3/KOH electrolyte solution through a bipolar membrane electrodialysis (BPMED) unit. By measuring the CO2 generation rate, faradaic efficiency, voltage and energy consumption at different K2CO3/KHCO3/KOH concentrations, it was found that the minimum energy consumption of CO2 separation from the atmosphere was about 300 kJ mol−1 CO2, 200 kJ mol−1 CO2 of which was directly used for the capture of CO2 [31] and the rest was used for CO2 regeneration from the BPMED unit. They also studied the pressure effect (1.5–10 atm) on the performance of the BPMED system [30], and found that at large current densities (139 mA cm−2), a high-pressure operation at >6 atm could reduce the energy consumption by 29% compared to operating at 1.5 atm. Hatton et al. [32] recently reported an electrochemical CO2-capture cell utilizing a quinone redox-active carrier and ionic liquid (1-ethyl-3-methylimidazolium tricyanomethanide) electrolyte. The CO2 source containing 15% CO2 and 85% air was fed to the cathode. The electrode reactions are (Q is the quinone):

At the cathode side: Q + 2CO2 + 2e − = Q (CO2)2 2 − (5.7)



At the anode side: Q(CO2)2 2 − = Q + 2CO2 + 2e − (5.8)

The electrical energy consumption for CO2 capture from the cathode to the anode was ∼43 kJ mol−1. Watkins et al. [33] have also investigated the CO2 permeation through a cell consisting of a polypropylene liquid membrane carbon paper anode and a cathode mediated by quinone redox couple through H+-coupled electron transfer mechanism in NaHCO3 solution. It was found that the amount of CO2 transported depended upon the applied potential and the metal catalysts used. The best efficiency was achieved at 1.0 V with Pt as a catalyst. As usual for all the low temperature fuel cells, noble metal electrodes are needed to catalyze the electrode reactions. 5.2.1.2 CO2 Conversion Using Electrolytic Cells  Among many existing CO2 conversion methods, the electrochemical reduction of CO2 attracts much attention due to its potential in utilizing renewable resources as energy input, reaction at atmospheric pressure, and the product diversity offered by the types of electrocatalysts and electrolytes. Liu et al. [34] reported N-doped nanodiamond/ Si rod arrays as an efficient non-metallic electrocatalyst with a 0.5 M NaHCO3 electrolyte for converting CO2 to acetate and formate with an onset potential of −0.36 V (vs. RHE). The production rate of acetate is 1.65–1.90 times higher than

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217

that of formate at −0.55 to −1.30 V. The maximum total faradic efficiency of 91.8% was achieved for CO2 reduction at −1.0 V. Ponnurangam et al. [35] optimized a Cu electrode coated with a poly(4-vinyl pyridine) film to synthesize formate in a 0.1 M KHCO3 electrolyte with a faradaic efficiency of 40% at −1.3 V. Verma et al. [36] further studied the influence of electrolytes (KOH, KCl, and KHCO3) on the electro-reduction of CO2 to CO on Ag-based electrocatalysts. They found that the CO production rate increased with the electrolyte concentration from 0.5 to 3.0 M. The faradaic efficiency and CO production rate were observed to vary in the order of OH− > HCO3− > Cl−. A faradaic efficiency was achieved at 99.5%, 87.0%, and 79.6% for CO2 electro-reduction to CO at −2.75 V with 1 M KOH, KHCO3, and KCl electrolytes, respectively. There are a number of reviews available in the literature on the low-temperature electrochemical conversion of CO2 to fuels [19, 23, 37–39]. In general, four groups can be divided according to the conversion products: (1) formic acid, which can be yielded by In, Sn, Hg, Cu, and Pb electrodes in aqueous-supported electrolytes [3, 40, 41]; (2) carbon monoxide, which can be produced by Zn, Au, Ag electrodes in aqueous electrolytes and Ni, Pd, Pt, Cu, Ag, Au, In, Zn, Sn electrodes in non-aqueous-supported electrolytes [42–44]; (3) hydrocarbons, aldehydes, and alcohols can be produced using Cu-based electrodes in aqueous-supported electrolytes [45–50]; (4) oxalic acid can be obtained on Pb, Tl, and Hg electrodes in non-aqueous-supported electrolytes [51]. The effects of different electrocatalysts and electrolyte solutions for electrochemical CO2 conversion have also been reviewed by Kuhl et al. [52], Jhong et al. [37], and Qiao et al. [22]. The high product yield by the low-temperature electrochemical cells for CO2 conversion has been demonstrated as two-, four-, six-, or eight-electron reduction reactions depending upon the specific experimental conditions [53]. The most common halfelectrochemical-reactions of the cathode for electrochemical CO2 reduction into formate, CO, methanol, methane, and ethylene are summarized as follows: [37, 53]

CO2 + H + + 2e − → HCOO − (5.9)



CO2 + 2H + + 2e − → CO + H 2 O (5.10)



CO2 + 6H + + 6e − → CH 3 OH + H 2 O (5.11)



CO2 + 8H + + 8e − → CH 4 + 2H 2 O (5.12)



2CO2 + 12H + + 12e − → C2 H 4 + 4H 2 O (5.13)

However, the low product selectivity is a major challenge for the low temperature electrochemical CO2 conversion, as the electro-reduction does not always yield a single species, rather a mixture of different products. In addition, low energy efficiency and high electrode overpotentials are also issues for the low-temperature approach.

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5.2.2  High-Temperature Approach 5.2.2.1 CO2 Capture through Electrodeless Permeation Cells  Flue gas from coal-fired power plants represents a major stationary CO2 emission source on the earth, which contributes significantly to global warming and climate change [54]. Therefore, effectively capturing CO2 from flue gas is crucial to mitigate the carbon pollutions. The flue gas emitted from a coal-fired power plant is hot with a temperature ranging from a few hundreds to one thousand degrees Celsiusdepending on the specific locations in the emission line. For amine washing CO2 capture process, the hot flue gas must be cooled down to 100–150°C prior to entering the amine columns [1, 55, 56]. In many cases, it is highly desirable and economically advantageous to capture CO2 directly from a hot flue gas at a high temperature (e.g., ∼350°C after the economizer) without cooling [57]. At higher temperatures, the hot concentrated CO2 as a feedstock can be converted directly into value-added chemicals such as methanol in the presence of catalysts [58–60]. Two well-studied high-temperature electrochemical cells for CO2 capture are reverse molten carbonate fuel cells (MCFCs) [18, 61–66] and electrodeless permeation cells [67–103]. Winnick et al. [17] and Weaver et al. [18] have previously demonstrated MCFCs as a CO2 concentrator with many distinct advantages over ambient-temperature aqueous-based counterparts. For example, MCFCs CO2 concentrator can operate at/with: (1) any humidity, from 0% to 100%; (2) noble metal– free electrodes; (3) higher current densities [18]. The working principle of MCFC CO2 concentrator is schematically shown in Figure 5.1 [61], where Figure 5.1a depicts separation of CO2 and O2 at the anode side with external electricity supplied to the cell and feeding gas CO2 and O2 (representing a flue gas) at the cathode side. Since the anode reaction is CO32− = CO2 + 1/2O2 + 2e−, producing the anode gas with CO2:O2 = 2:1, the operation is termed decomposition mode [104]. Another mode of MCFC CO2 concentrator is the power generation shown in Figure 5.1b [18]. The CO2 produced at the anode through oxidation reaction of hydrocarbon fuels can be recycled back to the cathode to sustain CO32− transport cross the molten carbonate electrolyte. In this case, the CO2 concentration is determined by fuel gas (H2 and CO) utilization. The higher the fuel utilization, the higher the CO2 concentration in the exhaust gases recycled back to the cathode chamber. The major disadvantage of MCFC CO2 concentrators is the low voltages allowable in order to avoid decomposition of the molten carbonate electrolyte, which results in lower currents, thus lower flux of CO32− or CO2 across the electrolyte. Hence, a large number of stacks are needed to achieve a significant CO2 capture capacity, which in turn considerably increases the cost. In addition, the degradation of electrode at high temperatures and by sulfur poisoning from flue gas are the critical challenges to the development of MCFC CO2 concentrators [105]. A new promising electrochemical cell for CO2 capture and conversion emerged in recent years consists of a mixed-conducting membrane without the use of electrodes as in MCFC CO2 concentrators. In the original concept first proposed by Jerry Lin [67],

CURRENT ELECTROCHEMICAL METHODS FOR CARBON CAPTURE AND CONVERSION

(a)

1/2O2+CO2

Anode

CO32− 2− CO32− CO3

Electrolyte plate

CO32− 2−

33.3O2/66.7CO2

2e−

DC power source

CO32−

2− Cathode CO3

219

2e−

CO3 Cathode exhaust

Oxidant gas

1/2O2+CO2

Cathode reaction: 1/2O2 + CO2 + 2e− = CO32− Anode reaction: CO32− = 1/2O2 + CO2 + 2e−

(b)

Fuel gas

H2O+CO2

H2

CO32−

Anode

CO32−

2−

CO3

Electrolyte plate Cathode

Anode exhaust

2e−

Load

CO32− CO32−

CO32− 2−

2e−

CO3 Cathode exhaust

1/2O2+CO2

Oxidant gas

Cathode reaction: 1/2O2 + CO2 + 2e− = CO32− Anode reaction: H2 + CO32− = H2O + CO2 + 2e−

Figure 5.1  Schematic illustrations of CO2 removal via MCFCs: (a) decomposition mode and (b) power generation mode. Source: Sugiura et al. 2003 [61]. Reproduced with permission of Elsevier.

two types of mixed conductors were envisioned: (1) mixed electron and carbonate-ion conductor (MECC) membrane consisting of a dual-phase metal (e-conductor)carbonate composite; (2) mixed-oxide-ion and carbonate-ion conductor (MOCC) membrane consisting of a dual-phase ceramic (O2−-conductor)-carbonate composite. These CO2-selective separation membranes have been demonstrated in the laboratory with high flux and selectivity. They are particularly more attractive than electrically driven MCFC CO2 concentrators since no external electronics are needed [18, 61].

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Electrochemical CO2 Capture and Conversion

Figure 5.2 shows the working principles of these two types of membranes along with the enabling surface reactions for CO2 separation. The CO2 transport through an MOCC membrane is taken in the form of CO32− that is charge-compensated by the concomitant opposite flow of O2−, as shown in Figure 5.2a. The ionization of CO2 (a)

Pure CO2 or CO2, H2O

MOCC (carbonate+oxide) H2

CO2 , H

2

Vacuum or H2O

2−

CO 3 O2

CO2+O2− = CO32−

(b)

CO2, H2O

CO32−=CO2+O2−

MECC (carbonate+metal) N2

CO , O 2 2, N

2

2−

CO, H2

CO 3 e−

CO2+1/2O2+2e−=CO32−

CO32−+H2=CO2+H2O+2e−+Δ

CO32−+CO=2CO2+2e−

Figure 5.2  Working principles of selective electrochemical CO2 separation membranes: (a) MOCC and (b) MECC. Source: Zhang et al. 2012 [94]. Reproduced with permission of Royal Society of Chemistry.

CURRENT ELECTROCHEMICAL METHODS FOR CARBON CAPTURE AND CONVERSION

221

takes place on the surface of feed-gas side with O2− delivered from a flux driven by electrochemical potential gradient of oxygen. The permeated CO2 can be collected by either vacuum or flushed with steam for easy separation. Similarly, the CO2 transport across MECC membrane is also in the form of CO32−, but charge-compensated by electrons, see Figure 5.2b. The ionization of CO2 is accomplished in oxidizing atmospheres by O2 and e−. Based on these fundamental reactions, it is evident that MOCC is more suitable for CO2 separation from a reducing stream such as water-gas shift gas where the chemical gradients of CO2 and O2 exist in opposite direction across the membrane. On the other hand, MECC is more adequate for CO2 separation from an oxidizing stream such as flue gas (CO2, O2, and N2) where the chemical gradient of CO2 and O2 exists in the same direction across the membrane. In this case, both CO2 and O2 will permeate through the membrane, the mixture of which can be further separated by a downstream oxygen transport membrane (OTM) or recycled back to the combustion chamber for oxy-combustion [94]. In MOCC and MECC membranes, the driving force for the CO2 (and O2) transport is the electrochemical potential gradient(s) of CO2 (and O2) existing across the membrane, making the design of a reactor simple and system cost low because of the elimination of external electronics. In addition, the high working temperature of these membranes, which can be practically sustained by the heat in combustion exhausts from which CO2 is separated, also enables a faster surface reaction kinetics in the absence of noble metal catalysts that are typically required for those ambienttemperature cells [17, 18]. 5.2.2.2 CO2 Conversion Using High-Temperature Electrochemical Cells  The most studied high-temperature electrochemical cells for CO2 reduction is solid oxide electrolysis cells (SOECs), by which external electricity is supplied to the cell to reduce CO2 to CO. From both thermodynamic and kinetic perspectives, high-temperature SOECs are energy efficient, kinetically fast and noble-metal-free. The first high-temperature electrolysis of CO2 was demonstrated at NASA as a means of producing oxygen for spacecraft using a SOEC with platinum and nickel as the cathode and the anode, respectively [106, 107]. The most attractive application of SOECs is the co-electrolysis CO2 and H2O into syngas, first demonstrated on large stacks by Idaho National Laboratory (INL) and Ceramatec [108]. The working principle of the co-electrolysis H2O and CO2 is schematically shown in Figure 5.3. At the anode, O2− is oxidized to oxygen:

O2 − → 1 / 2O2 + 2e − (5.14) At the cathode, CO2 and/or H2O are reduced to CO and/or H2, respectively:



CO2 + 2e − → CO + O2 − (5.15)



H 2 O + 2e − → H 2 + O2 − (5.16)

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Electrochemical CO2 Capture and Conversion

2e−

(a) CO2

CO Gas diffusion in porous electrodes

Porous cathode

CO2 + 2e− → CO + O2−

Electrolyte

O2− O2− → 1 O2 + 2e− 2 Gas permeation in porous electrodes

Porous anode O2 2e− e− (b)

x=0

H2O, CO2

H2, CO

Cathode

CO2 + H2

x = dc Electrolyte x = da Anode

x=0

Ni YSZ

H2O + CO2 +

O2−

O2

2e− → 2e− →

CO + H2O H2 + O2− CO + O2−

2O2− → O2 + 4e−

e−

LSM

Figure 5.3  Working mechanisms of (a) CO2 electrolysis by an SOEC [110], (b) co-electrolysis of H2O and CO2. Source: Ni 2012 [111]. Reproduced with permission of Elsevier.

An important reaction accompanying the co-electrolysis is the reverse watergas shift (RWGS) reaction that effectively converts CO2 into CO via

CO2 + H 2 → CO + H 2 O (5.17)

Since reaction (5.16) is much easier than (5.15) with much lower overpotential, the pathway (5.17) is a very likely route for CO production [109]. There is a large body of publications for electrolysis of CO2 [110–117] and co-electrolysis of CO2/H2O [118–125] for syngas production in recent years. The most popular cathode material for SOECs is Ni-based cermet, which performs

CURRENT ELECTROCHEMICAL METHODS FOR CARBON CAPTURE AND CONVERSION

223

worse with pure CO2 electrolysis due to coking on Ni. The cell performance can be enhanced by introducing H2O, producing syngas for further synthesis of methanol and other hydrocarbons. Thus, the co-electrolysis of H2O/CO2 is a very promising approach for high-temperature CO2 conversion. More details on SOECs for coelectrolysis of CO2/H2O can be found in the review by Uhm et al. [126]. High-temperature CO2 electrochemical conversion also can be achieved by both MECC and MOCC membrane reactors [127]. An example is to feed CH4 into the membrane reactors in which the permeated CO2/O2 can reform CH4 into syngas through the dry methane reforming (DMR) reaction. The working principles of MOCC and MECC membrane reactors for DMR are schematically shown in ­Figure  5.4. ­Compared to SOEC-based CO2 conversion, these kinds of membrane reactors show unique advantages: (1) DMR uses two greenhouse gases, CO2 and CH4, to create a valuable chemical feedstock; (2) no electricity is needed for the DMR reactors; and (3) simple membrane reactor design enabled by electrodeless feature. More importantly, the co-permeated O2 by the MECC reactor can decrease coking and heat requirement for the reforming, a major problem for the conventional DMR process.

Flue gas (CO2, O2, N2…)

Cleaned flue gas

CO2 + 1/2O2 + 2e − = CO32−

CH4

(a)

CO32− e − CO3 CO2 + 1/2O2 + 2e − CO2 + CH4 = 2CO + 2H2 1/2O2 + CH4 = CO + 2H2 CO32− + 2CH4 = 3CO + 4H2 + 2e − 2− =

MECC membrane

Flue gas (CO2, O2, N2…)

3CO + 4H2 Syngas out e − CO32−

Cleaned flue gas

CO2 + O2− = CO32−

CH4

CO32− O2− CO32− = CO2 + O2− CO2 + CH4 = 2CO + 2H2 CO32− + 2CH4 = 2CO + 2H2 + O2−

(b)

MOCC membrane

2CO + 2H2 Syngas out 2− O2− CO3

Figure 5.4  Schematic of “all-in-one” membrane reactor for DMR: (a) MECC-based, (b) MOCC-based.

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Electrochemical CO2 Capture and Conversion

5.3   DEVELOPMENT OF HIGH-TEMPERATURE PERMEATION MEMBRANES FOR ELECTROCHEMICAL CO2 CAPTURE AND CONVERSION The aforementioned dual-phase permeation membranes MOCC and MECC operated at elevated temperatures are well-suited for capturing CO2 directly and continuously from a hot CO2-containing combustion stream. In MECC membranes, a solid porous metal is used to immobilize the molten carbonate phase as well as to conduct electrons, whereas in MOCC membranes, a solid porous oxide-conducting ceramic is used to immobilize the molten carbonate phase as well as to conduct O2−. The representative materials for the metals and O2−-conducting ceramics are silver and Gd2O3-doped CeO2, respectively. In the following, we review the recent development of these two classes of membranes for high-temperature CO2 capture from a variety of CO2 sources.

5.3.1  Development of MECC Membranes 5.3.1.1  Stainless-Steel-Molten Carbonate Dual-Phase Membrane  The first proof-of-concept of MECC membrane was demonstrated by Jerry Lin’s group at Arizona State University in 2005 [67], in which porous stainless-steel (SS) was used as the electronic phase immobilizing a eutectic mixture of Li2CO3Na2CO3-K2CO3 (Tm∼397°C, Li:Na:K = 42.5:32.5:25.0 (mol%)) as CO32− conducting phase. Figure 5.5 shows a schematic of experimental setup preparing the dualphase membrane via dipping the porous SS matrix into the molten carbonate. The CO2 permeation rate of such SS-MC composite membrane in Figure 5.6 shows an increase with temperature from 450°C to 650°C, reaching a maximum value of Dip a membrane on the surface of molten carbonate

Molten carbonate

Membrane

Crucible Furnace

Figure 5.5  Schematic illustration of dip-coating infiltration method to synthesize the dual-phase SS-MC membrane. Source: Chung et al. 2005 [67]. Reproduced with permission of American Chemical Society.

225

DEVELOPMENT OF HIGH-TEMPERATURE PERMEATION MEMBRANES

Permeance (mol s−1 m−2 Pa−1) x 10−10

300 250

CO2 N2 CO2 + O2

200 150 100 50 0 400

450

500

550

600

650

700

750

800

Temperature (°C)

Figure 5.6  Plotted results of various gas permeation tests at different temperatures. Source: Chung et al. 2005 [67]. Reproduced with permission of American Chemical Society.

1.67 × 10−8 mol s−1 m−2 Pa−1 at 650°C, followed by a decrease in permeation rate with temperature. It was speculated that the reaction between SS and MC in oxidizing atmosphere promotes the formation of LiFeO2 at a temperature >650°C. The low conductivity of LiFeO2 scale formed on the surface of SS considerably lowered the effective conductivity of SS, thus resulting in a decrease in CO2 flux. Another important observation in Lin’s work [67] was the confirmation of the active surface reaction CO2 + 1/2O2 + 2e− = CO32− taking place during the CO2/O2 permeation by varying the ratio of CO2/O2 in the feed gas as shown in Figure 5.6. It is clearly shown that without O2 in the feedstock, the CO2 permeation was virtually shut down. 5.3.1.2  Silver-Molten Carbonate Dual-Phase Membrane  To solve the reaction problem between SS and MC, Kevin Huang’s group at University of South Carolina demonstrated that silver is a better metal than SS for the dual-phase metalcarbonate MECC membrane because silver is unreactive with MC and possesses a much higher electronic conductivity [92]. More importantly, the operatability of silver in the temperature range of 550–650°C matches well with the operating temperature of MCs. All of these compatibilities between silver and MC promises the Ag-MC MECC to be a better membrane than SS-MC counterpart. Indeed, Xu et al. [92] first demonstrated that Ag-MC membrane exhibited a CO2 flux density as high as 0.82 ml cm−2 min−1 at 650°C with 41.67% CO2, 41.67% O2, and 16.66% N2 as the feed gas and pure He as the sweep gas, which is ∼6× higher than that of SS-MC dual-phase membrane. The membrane was fabricated by a solid-state

226

Electrochemical CO2 Capture and Conversion

reaction method with 50 vol% Ag and 50 vol% MC (Li2CO3:K2CO3 = 62:38 (mol%)) at 600°C for 2 h. The enabling surface reaction CO2 + 1/2O2 + 2e− = CO32− was also confirmed by the observed flux ratio of CO2:O2 = 2:1 and a close activation energy for the CO2 and O2 flux density; the latter signals that the transport of CO2 and O2 is coupled, probably by the enabling surface reaction CO2 + 1/2O2 + 2e− = CO32−. A similar decrease in CO2 and O2 permeation flux above 650°C was also observed in the Ag-MC dual-phase membrane, but with a different mechanism from that of the SS-MC membrane. For the Ag-MC membrane, Xu et al. [92] speculated that the decrease of flux density at >650°C was related to the loss of MC as a result of easy Ag sintering, according to SEM examinations. The sintering of the porous silver matrix at 650°C essentially “squeezed” out MC from the Ag matrix. The long-term stability of CO2 and O2 flux densities in the Ag-MC membrane shown in Figure 5.7 seems to support this assertion. Xu et al. [92] observed that the fluxes increased with time during the first 20 h and then slightly decreased in the following 60 h, which was explained by the excessive grain growth of Ag. First, grain growth reduces the effective thickness of the membrane, which account for the initial increase of CO2 and O2 flux densities. Second, the repelled MC from the Ag matrix gradually accumulated on the surfaces, preferably on the bottom sweep surface, blocking kinetics of the enabling surface reaction, thus degrading flux. Nevertheless, throughout the dynamic change of fluxes, the ratio between CO2 and O2 remained roughly 2:1, suggesting that while the rate of CO2/O2 transport decreases with time, the nature of the enabling surface remained unchanged. To improve the flux stability of Ag-MC membranes, alleviating Ag sintering and enhancing the wettability between silver and MC so as to increase the 1.2 CO2 O2

J (ml·cm−2·min−1)

1.0 0.8 0.6 0.4 0.2 0.0 0

10

20

30

40 t (h)

50

60

70

80

Figure 5.7  Long-term stability of CO2 and O2 flux densities of Ag-MC membrane measured at 650°C. Source: Xu et al. 2012 [92]. Reproduced with permission of Elsevier.

DEVELOPMENT OF HIGH-TEMPERATURE PERMEATION MEMBRANES

227

Flux densities (ml min−1cm−2)

ability to retain MC within porous silver seem to be the two rational approaches. Following this strategy, Zhang et al. [97] tested the use of Al2O3 coating over the surfaces of porous Ag as a means of mitigating Ag-sintering and improving the wettability with MC. Al2O3 is known to form LiAlO2 when in contact with MC; the latter has a zero contact angle with MC. Zhang et al. [97] demonstrated the fabrication of the Al2O3-coated Ag-MC membranes in two steps: first, coating porous Ag with an Al2O3 colloidal solution through infiltration method, followed by high-temperature infiltration of molten carbonate (Li2CO3:K2CO3 = 62:38 (molar concentration)). The results showed that 5 wt% Al2O3 concentration represents the upper limit of Al2O3 colloidal solution [7], above which CO32− transport could be hindered by the insulating Al2O3 and/or LiAlO2. The most noticeable improvement, however, was observed on the long-term stability shown in Figure 5.8a, where 90% of the original flux can still be retained for the coated membrane after 130 h, whereas only ∼15% of the original flux can be maintained for uncoated sample after 60 h. The post-test microstructural examinations shown in Figure 5.8b indicate the presence of large pores in the uncoated sample, implying that loss of MC has occurred. A further EDS analysis shown in Figure 5.9 also supported that coated sample exhibited less silver sintering than uncoated sample. One issue with the colloidal deposition of Al2O3 thin film on porous silver is the poor controllability over coverage and thickness, which could affect the consistency and repeatability of improvement from batch to batch. To obtain a uniform Al2O3 coating and therefore better stability and repeatability, Tong et al. [99] used chemical

0.5

(a) CO2-O2-N2MECCHe T = 1.23 mm At 650°C

sample A-CO2

(b)

sample A-O2 sample B-CO2 sample B-O2

0.4 0.3

10 μm (c)

0.2 0.1 0.0

0

20

40

60 80 Time (h)

100

120

140 10 μm

Figure 5.8  (a) Long-term stability of CO2 and O2 flux densities of samples A and B measured at 650°C. Microstructures of (b) sample B and (c) sample A after long-term stability test. (Samples A and B are Ag-MC membranes without and with 5 wt% Al2O3 coatings, respectively.)

228

Electrochemical CO2 Capture and Conversion

(a)

(e)

50 μm (b)

50 μm (f)

Ag L (c)

Ag L (g)

Al K

Al K (d)

(h)

KK

KK

Figure 5.9  Microstructure and elemental distributions of sample A after a long-term stability test. (a) SEM image, (b) Ag mapping, (c) Al mapping, (d) K mapping. Microstructure and elemental distributions Sample B after a long-term stability test. (e) SEM image, (f) Ag mapping, (g) Al mapping, and (h) K mapping [97]. (Samples A and B are Ag-MC membranes without and with 5 wt% Al2O3 coatings, respectively.)

DEVELOPMENT OF HIGH-TEMPERATURE PERMEATION MEMBRANES

0.30 0.25 J (ml min−1 cm−2)

(a)

CO2-CVD MECC CO2-MECC

0.20

(b)

229

Sweep side

≈2X O2-CVD MECC

0.15

Feed side

(c)

0.10

100 μm Sweep side

O2-MECC

0.05 0.00

Feed side

0

20

40

60

80

100 μm

100

Time (h)

Figure 5.10  (a) CO2 flux as a function of time measured under a simulated flue gas composition at 650°C. Cross section of an MECC membrane after a 100 h test: (b) coated, (c) uncoated. Source: Tong et al. 2015 [99]. Reproduced with permission of Royal Society of Chemistry.

vapor deposition (CVD) method to deposit a conformal Al2O3 over layer on the surface of a porous silver skeleton. The long-term stability of such CVD-Al2O3-coated MECC membrane shown in Figure 5.10 indicates a much better stability than the uncoated baseline sample. Over the 100-h testing, the CVD-Al2O3-coated MECC showed virtually no sign of degradation, whereas the uncoated baseline sample lost nearly 50% of its original flux in just the first 20 h. From the SEM images of posttested samples shown in Figures 10b and 10c, it is evident that the coated MECC membrane still exhibited an intact and dense microstructure of sliver and MC, while the uncoated membrane showed a large amount of porosity accompanied by apparent sintering of silver grains, implying that a loss of MC and Ag-sintering has likely happened during the operation. Tong et al. [103] have also studied the flux behavior of an Ag-MC membrane overcoated by atomic layer deposition (ALD)-derived nanoscale Al2O3 thin films. The results are interesting: not only is the CO2 flux enhanced (0.71 vs. 0.20∼0.25 ml cm−2 min−1 at 650°C), but also an irregular flux-density ratio of JCO2:JO2 ≈ 1:1.5 was observed. This is contradictory to 2:1 as predicted from the enabling surface reaction CO2 + 1/2O2 + 2e− = CO32−. A subsequent in situ Raman investigation revealed a new active species CO42− only on the surface of ALD-Al2O3 overcoated MECC, but not on the uncoated baseline sample. Based on this new finding, Tong et al. proposed the following enabling surface reaction to interpret the observed reversed JCO2:JO2 ratio.

3 CO2( g ) + O2( g ) + CO32 −( MC ) + 2e −( Ag ) = 2CO 4 2 −( MC ) (5.18) 2

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Electrochemical CO2 Capture and Conversion

1.4

J (ml min−1 cm−2)

Feed gas: 15% CO2, 10% O2, Bal N2 Sweep gas: He Membrane thickness: 0.82 mm 650°C

JO2

1.2 1.0

JCO2

0.8

JO2:JCO2 ≈ 1.5:1

0.6 0.4

JO2

0.0

JO2:JCO2 ≈ 1:2

JCO2

0.2

0

20

ALD-AI2O3-coated Ag-MC

Uncoated Ag-MC

60

40

80

100

t (h)

Figure 5.11  Time dependence of JO2 and JCO2 of the ALD-coated and uncoated Ag-MC

membrane. The red and blue colors are O2 and CO2 flux densities, respectively. Source: Tong et al. 2016 [103]. Reproduced with permission of Royal Society of Chemistry.

The migration of CO42− in MC has also been proposed via “cogwheel” mechanism. The fluxes of CO2 and O2 degraded with time for the ALD-Al2O3-coated MECC membrane as shown in Figure 5.11. However, the unusual JCO2:JO2 ≈ 1:1.5 remained unchanged, suggesting degradation mechanism is independent of the new surface active species. The pore size of the porous silver matrix was also studied as an important variable to affect flux densities and long-term stability by Huang’s group [96, 97, 102]. It is well known that capillary forces depend on the size of pores. Too large a pore would not generate sufficient capillary forces to withhold molten carbonate during high-temperature operation, thus leading to a gradual loss of carbonate and degradation of flux. Furthermore, large pores of low-surface-area possess a low density of triple phase boundaries (TPBs), which is a critical factor determining the rate of permeation, thus resulting in a lower CO2/O2 flux density. Therefore, decreasing the pore size of porous metal-phase matrix (e.g., SS and Ag) is a promising method to improve both permeation flux and long-term stability of MECC membranes. Zhang et al. [96, 97] compared the use of microcrystalline methylcellulose versus carbon black as a pore-former in making porous silver matrix for Ag-MC MECC membranes. The pore size of the silver matrix shown in Figure 5.12 was decreased from 15–20 to 10 μm by replacing microcrystalline methylcellulose with carbon black as the pore-former. The results showed a roughly 1.5× enhancement of the CO2 flux densities at 500–650°C. Fang et al. [102, 128] further demonstrated that using chemical/electrochemical dealloying (CD), a well-established method to produce nanoporous metal structures for various applications [129], can produce microporous silver matrix for high performance MECC membranes. For example,

231

DEVELOPMENT OF HIGH-TEMPERATURE PERMEATION MEMBRANES

(a)

(b)

Ag

10 μm

10 μm

Figure 5.12  SEM-BSE images of a porous Ag network created by (a) microcrystalline methylcellulose [97] and (b) carbon black pore-former. Source: Zhang et al. 2014 [96]. Reproduced with permission of Elsevier.

the microstructure of a microporous silver matrix fabricated by the chemical dealloying method is shown in Figure 5.13. Compared to Figure 5.12, the pore size and distribution in porous silver matrix derived from chemical dealloying are undoubtedly smaller and more uniform. (a)

(b) 3D channel

2

1

Intergranular region

4

4

2

1

3 10 μm

(c)

3

10 μm

(d)

1 μm

1 μm

Figure 5.13  Microstructure of (a) Ag50Al50 (leached in 3 M HCl at 90°C for 3 min to reveal grains and grain-boundaries), (b) 48 h-Ag50Al50 (overall), (c) α-Al-derived porous Ag matrix, (d) γ-Ag2Al-derived porous Ag matrix. 1, α-Al; 2, γ-Ag2Al; 3, porous Ag derived from α-Al phase; 4, porous Ag derived from γ-Ag2Al phase. Source: Fang et al. 2016 [102]. Reproduced with permission of Elsevier.

232

Electrochemical CO2 Capture and Conversion

The fine microstructure exhibited by the CD-derived porous silver matrix increases TPB density significantly, thus resulting in enhanced CO2 flux density. At 650°C, the CO2 flux density is as high as 1.02 ml min−1 cm−2, ∼3× higher than that using carbon black pore former [97, 99]. More interestingly, in the same study, Fang et al. also showed that using a reducing 9.41% H2-Ar as the sweep gas can further double the CO2 flux density. The instantaneous reaction between H2 and O2 permeated increases the driving force for the CO2/O2 transport. The overall reaction at the sweep side becomes: H 2 + CO32 − = CO2 + H 2 O + 2e − + ∆(heat ) (5.19)



The generated products, CO2 + H2O + Δ, can be either condensed to obtain a pure stream of CO2 or fed into high-temperature solid oxide electrolyte cells for co-electrolysis to convert CO2 and H2O into syngas. The reuse of captured CO2 to yield syngas is a promising approach to realizing a carbon-neutral energy future. The long-term stability test of a CD-derived Ag-MC MECC membrane was conducted at 600°C for 900 h with a simulated flue gas composition (75% N2, 10% O2, and 15% CO2) and 9.41% H2-Ar sweep gas. The results are shown in Figure 5.14, where CO2 and O2 flux densities did not show significant degradation over the entire 900-h testing period, despite some fluctuations at the beginning. As a type of mixed-conducting membranes similar to OTMs and proton transport membranes (PTMs) [130, 131], the thickness of an MECC membrane should

1.4

J (ml/min·cm2)

1.2

CO2

1.0 0.8

O2

0.6 72h-Ag50Al50 T = 600°C t = 1.02 mm Feed gas: 10%O2, 15%CO2, 75%N2 Sweep gas: 10%H2-Ar

0.4 0.2 0.0 0

200

400

600

800

1000

Time (h)

Figure 5.14  CO2 and O2 flux densities as a function of time operated at 600°C under a simulated flue gas 10% O2, 15% CO2, and 75% N2 as the feed gas and 9.41% H2-Ar as the sweep gas. Source: Fang et al. 2016 [102]. Reproduced with permission of Elsevier.

DEVELOPMENT OF HIGH-TEMPERATURE PERMEATION MEMBRANES

233

have significant effect on flux. Zhang et al. [96] systematically studied the effect of thickness on the flux of an Ag-MC MECC membrane. The critical thickness, below which surface exchange kinetics becomes rate limiting, of the MECC membrane was found to be 0.84 mm; this value is higher than those of OTMs and PTMs since the size, molecular complexity, and slower surface-exchange rate of CO32− in comparison to simpler O2− and H+ ionic species can easily make the surface CO2/O2 exchange kinetics a dominant factor for MECC membranes. In addition, Zhang et al. [96] also studied the dependence of CO2 and O2 flux of the MECC membrane on CO2 partial pressure. It was found that CO2 flux density can be better expressed by the following equation assuming the conductivity of CO32− is dependent on partial pressure of O2 and CO2:

 ε  3 RT ′′ PO′′ 1/2 − PCO ′ PO′ 1/2 ) (5.20) J CO2 = −   2 ϕσ0 (PCO 2 2 2 2  τ  8F L

where JCO2 is the CO2 flux density, ɛ and τ are porosity and tortuosity, respectively; R, T, and F have their usual meanings; L is thickness of the membrane; ϕ is volume fraction of the MC phase,; σ° is the pre-exponential factor in Arrhenius equation of ′′ , PCO ′ , PO′′ , and PO′′ are partial pressures of CO2 and O2 at CO32− conductivity, PCO 2 2 2 2 the feed side and permeate side, respectively. In summary, the Ag-based MECC membranes show a better CO2 flux density than the SS-based MECC membranes. Table 5.1 summaries the performance of all MECC membranes studied so far. However, the conventional Ag matrix is prone to sinter at the temperature higher than 650°C, which will lead to a fast degradation of the CO2 flux density. Two kinds of methods were studied to retain the stability of the Ag-based MECC membranes: (1) surface modification of the Ag matrix by different Al2O3 coating methods such as colloidal [97], CVD [99], and ALD [103], which increases the sintering resistance of the Ag matrix at ≥650°C; and (2) microstructural optimization of the Ag matrix to enhance the CO2 permeation flux, thus to decrease the operation temperature of the Ag-based MECC membranes to ≤600°C [102, 128]. 5.3.1.3  Silver-Molten Carbonate Dual-Phase Membrane Reactor for Dry-oxy Methane Reforming  Converting the captured CO2 into useful products is a better alternative to carbon storage since it offers a solution to realize carbon neutral ecosystem. As shown in Figure 5.4a, the MECC membrane reactor can simultaneously capture and convert CO2 into useful product in single reactor. Among many CO2 conversion methods, DMR (dry methane reforming) stands out as an appealing catalytic process to upgrade methane into syngas for liquid fuels and methanol synthesis [132–134]. A distinctive feature of DMR is that it reutilizes the captured CO2 to combine with today’s widely accessible and cost-competitive CH4 to make value-added products with a profound environmental implication. Since the DMR is a highly endothermic reaction favorable at a temperature ≥ 700°C

0.13 0.82 0.39 0.25 0.71 0.61 0.61 0.32 0.28 0.23 1.02

∼10/1670

15–20/1230

∼10/800

∼10/820

∼10/630

∼10/840

∼10/1140

∼10/1210

∼10/1450

∼1/960

Silver/ Li2CO3:K2CO3 = 62:38

Silver-Al2O3 colloidal/ Li2CO3:K2CO3 = 62:38

Silver-Al2O3-CVD/ Li2CO3: Na2CO3 = 52:48

Silver-Al2O3-ALD/ Li2CO3: Na2CO3 = 52:48

Silver-Al2O3 colloidal/ Li2CO3:K2CO3 = 62:38

Silver-Al2O3 colloidal/ Li2CO3:K2CO3 = 62:38

Silver-Al2O3 colloidal/ Li2CO3:K2CO3 = 62:38

Silver-Al2O3 colloidal/ Li2CO3:K2CO3 = 62:38

Silver-Al2O3 colloidal/ Li2CO3:K2CO3 = 62:38

Silver/ Li2CO3:Na2CO3 = 52:48

650

600

600

600

600

600

650

650

650

650

650

CO2:O2:N2 = 3:2:15/ Ar:H2=90.6:9.4

CO2:O2:N2 = 5:5:2/He

CO2:O2:N2 = 5:5:2/He

CO2:O2:N2 = 5:5:2/He

CO2:O2:N2 = 5:5:2/He

CO2:O2:N2 = 5:5:2/He

CO2:O2:N2 = 5:5:2/He

CO2:O2:N2 = 3:2:15/He

CO2:O2:N2 = 5:5:2/He

CO2:O2:N2 = 5:5:2/He

CO2:O2 = 2:1/vacuum

Matrix pore size/ Flux density thickness (µm) (ml min−1 cm−2) Temp. (°C) Feed gas/sweep gas

Stainless-steel/ Li2CO3:Na2CO3: 5–10/1580 K2CO3 = 42.5:32.5:25.0

e−/CO32− dual-phase composition

Ta b l e   5 . 1   Comparative analysis of data on CO2 flux density with the MECC membrane

44.6

/

/

/

/

/

68

35

81.0

65.6

/

[102]

[96]

[96]

[96]

[96]

[96]

[103]

[99]

[97]

[92]

[67]

Ea (kJ mol−1) Reference

DEVELOPMENT OF HIGH-TEMPERATURE PERMEATION MEMBRANES

235

[135, 136], neither Al2O3 surface coating and microstructural optimization of the Ag matrix can enable the Ag-based MECC membrane to operate at a temperature higher than 700°C. Thus, Zhang et al. [137] developed an Ag matrix overcoated by the atomic layer deposition (ALD) with a nanoscaled ZrO2. They demonstrated an excellent CO2-O2 (CO2:O2 = 2:1) co-capture performance at 850°C with the highest CO2 flux density reaching 0.9 ml min−1 cm−2. Furthermore, Zhang et al. [138] further demonstrated a combined the CO2-O2 co-capture and dry-oxy methane reforming (DOMR) within reactor-based ALD-ZrO2-coated Ag membranes. The catalyst used for the catalytic DOMR was a commercial standard Ni supported on MgO impregnated with 1 wt% Pt (NMP). The feed gas to the MECC membrane reactor was a simulated flue gas containing 75% N2, 15% CO2, and 10% O2, while a mixture of CH4 and Ar was used as the sweep gas. They found that the conversion rates of CH4 and CO2 at 800°C with a 4.4% CH4 inlet concentration were 86% and 70%, respectively. Simultaneously, the rates of H2 and CO production were as high as 4.0 and 4.2 ml min−1 cm−2, respectively, as shown in Figure 5.15. It was also demonstrated that the CH4 preferentially reacted with O2 over CO2 during the DOMR process. The combined CO2/O2 capture and DOMR reactor was operated for 115 h, exhibiting a reasonably stable performance and the technical feasibility of such a new capture and conversion “all-in-one” CO2 reactor.

5.3.2  Development of MOCC Membranes One of the unique features of MECC membranes is that both CO2 and O2 can transport through the membrane in the form of CO32− charge-compensated by electrons. As a result, CO2 and O2 are needed to be present in the feed gas and the permeated CO2 and O2 need a further separation. The enabling reaction CO2 + 1/2O2 + 2e− = CO32− for MECC membranes, however, suggests that MECC membranes are better suited for CO2 separation from flue gas, a major stationary CO2 emission source on the earth. The permeated product CO2 + O2 can be either recycled back to oxy-combustion chamber [139] or reacted with H2 (CO)-containing sweeping gas to convert O2 into H2O (CO2) for easy downstream CO2 separation or conversion into CO + H2 through high-temperature co-electrolysis [65, 109, 119, 122–124]. Different from MECC membranes, O2−-conductor-MC-based MOCC membranes transport solely CO2 because CO2 can be directly reduced by O2− in the membrane without electrons via the enabling surface reaction CO2 + O2− = CO32−. The transport of CO32− is charge compensated by O2− in the bulk of membrane. Therefore, MOCC membranes are more suited for separating CO2 from O2-free fuel gas such as water-gas-shift-reaction (WGSR) product H2 + CO2. 5.3.2.1  MOCC Membranes Based on MIEC Ceramic Phase  A mixed ionic-electronic conducting (MIEC) ceramic phase was first employed by Lin et al. [71] in the development of MOCC membranes. The MIEC porous skeleton is a well-known perovskite La0.6Sr0.4Co0.8Fe0.2O3-σ (LSCF), within which a eutectic

236

Electrochemical CO2 Capture and Conversion

(a) 100

3.0 2.5

80 70

2.0

60 1.5

50 40

1.0

30 20

CH4 conversion

10

CO2 conversion

CO production rate H2 production rate CO2 permeation rate

0 740

750

760

770

780

790

Rate (ml min−1 cm−2)

CH4/CO2 conversion (%)

90

0.5 0.0

800

Temperature (ºC) (b) 100

6

90 5

CH4 conversion

70

CO2 conversion

60

4

CO production rate

50

3

40 2

30 20

H2 production rate

10

Rate (ml min−1 cm−2)

CH4/CO2 conversion (%)

80

1

CO2 permeation rate

0 1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 4.5

CH4 concentration (%)

Figure 5.15  (a) Effect of temperature on DOMR performance of ALD-ZrO2-Ag-MC membrane reactor with NMP catalyst. Feed gas: 75 ml min−1 N2, 15 ml min−1 CO2, and 10 ml min−1 O2; sweep gas: 0.94 ml min−1 CH4 and 50 ml min−1 Ar. (b) Effect of CH4 concentration on the DOMR performance of ALD-ZrO2-Ag-MC membrane reactor with NMP catalyst at 800°C. GHSV = 5800–6500 h−1. Source: Zhang et al. 2017 [138]. Reproduced with permission of American Chemical Society.

ternary mixture of Li2CO3:Na2CO3:K2CO3 = 42.5:32.5:25 (molar concentration) molten carbonate phase is withheld. The porous LSCF skeleton had an average pore size of 182 nm, and the CO2 flux density measured at 900°C was 2.01, 3.73, 4.63, and 4.77 × 10−8 mol m−2 s−1 Pa−1 for an MOCC membrane with thicknesses

DEVELOPMENT OF HIGH-TEMPERATURE PERMEATION MEMBRANES

237

3.0, 1.5, 0.75, and 0.375 mm, respectively. The selectivity for CO2 was ≥225. The permeation flux densities at the membrane thicknesses of 3.0 and 1.5 mm agreed well with theoretical prediction by a model hypothesizing that O2−-conduction in the LSCF phase is the rate-limiting step [70]. For thinner membranes at 0.75 and 0.375 mm, however, the predicted CO2 flux density deviated from the experimental data, suggesting involvement of surface reaction kinetics in the permeation process [71]. The determined critical thickness for the LSCF-based MOCC membrane was between 0.75 and 1.5 mm, which agrees with that of the silver-based MECC membrane previously determined by Zhang et al. [96]. Ortiz-Landeros et al. [77] further investigated the influence of the pore structure of the LSCF phase as well as carbonate volumetric fraction on CO2 flux of an LSCF-MC membrane. The different porosities of porous LSCF skeletons were created by sintering them at various temperatures. CO2 permeance of the resultant membranes exhibited first an increase, followed by a decrease with the sintering temperature of the porous LSCF skeleton as a result of decreased porosity. A peak CO2 permeance of ∼6 × 10−8 mol s−1 m−2 Pa−1 was obtained at 900°C with the LSCF porous matrix sintered at 1000°C. The study showed that the CO2 permeance of MOCC membranes was controlled by not only the intrinsic conductivities of molten carbonate and ceramic phase, but also the ratio between carbonate or solid fraction and tortuosity. Norton et al. [83] extended the study to long-term stability of the LSCF-MC dual-phase membrane at high temperatures under various experimental conditions. As shown in Figure 5.16, the CO2 flux decreases sharply in the temperature range of 800–900°C after an initial exposure to O2-free atmosphere containing equimolar CO2/N2 as the feed gas and pure Ar as the sweep gas. The instability and large drop in CO2 flux of the LSCF-MC membrane were attributed to the reaction between LSCF and MC at high temperatures in CO2-concentrated environments. The alkaline earth metals, such as Sr in LSCF, are prone to react with CO2 at high temperatures, resulting in the formation of a SrCO3 layer on the membrane surface [140, 141]. This surface layer could block the enabling surface reaction between CO2 and O2− in LSCF phase. In contrast, when oxygen is present in the feed gas, SrCO3 can no longer be formed at temperatures higher than 800°C [142], which ensures a stable perovskite structure of LSCF. Figure 5.17 shows much improved stability of LSCF-MC membrane at different temperatures with feed gas containing both CO2 and O2. The CO2 and O2 permeated followed a 2:1 ratio, which was explained by a higher electronic conductivity than ionic conductivity in LSCF phase. Furthermore, the CO2 flux was ∼3.0 ml cm−2 min−1 at 900°C, 10× higher than that with O2-free feed gas as shown in the Figure 5.16. This is because the rate-limiting step of CO32− transport changed from O2− conductivity (∼0.1 S cm−1 at 900°C) in LSCF to CO32− conductivity (∼3.5 S cm−1) in MC due to high electronic conductivity of LSCF (∼1000 S cm−1 at 900°C) after introducing O2 to the feed gas. However, the parallel O2− permeation to the CO32− counterpart through the LSCF phase seemed to have been overlooked.

238

Electrochemical CO2 Capture and Conversion

0.25

Membrane sealing time period

JCO2 (mL·min−1·cm−2)

0.20

0.15

0.10

0.05

Membrane 1 - 900°C Membrane 2 - 850°C Membrane 3 - 800°C

0.00 0

20

40

60 Hours

80

100

Figure 5.16  Time dependence of CO2 permeation flux of three LSCF-carbonate membranes at different permeation temperatures (thickness = 1.0 mm, feed gas is equimolar CO2/N2, sweep gas is pure Ar, feed and sweep flow rate = 100 ml min−1, both feed and sweep gases at 1 atm for membranes 1–3; membrane 1 tested at 900°C, membrane 2 at 850°C, and membrane 3 at 800°C). Source: Norton et al. 2014 [83]. Reproduced with permission of American Chemical Society. 5

900

950 925

875

850

900

JCO2 (mL·cm−2·min−1)

4

3

2

1

0

0

100

200

300

400

500

600

Hours

Figure 5.17  Time dependence of CO2 permeation flux of LSCF-carbonate membranes (thickness = 1.0 mm, feed and sweep flow rate = 100 ml min−1, feed CO2 = 0.5 atm, feed O2 = 0.25 atm, T = 850–950°C). Source: Norton et al. 2014 [83]. Reproduced with permission of American Chemical Society.

239

DEVELOPMENT OF HIGH-TEMPERATURE PERMEATION MEMBRANES

Lan et al. [79] reported that by adding approximately 10 wt% LiAlO2 into La0.5Sr0.5Fe0.8Cu0.2O3-σ (LSFCu) can improve the CO2 flux for LSFCu-MC MOCC membrane at a thickness of 1.5 mm as a result of improved wettability between molten carbonate and oxide. The highest CO2 flux of 1.55 ml min−1 cm−2 was achieved at 750°C with a 20% CO2/80% O2 mixture as the feed gas. Different from the LSCF-MC MOCC membrane [83], O2 flux as shown in Figure 5.18 was only 0.15 ml min−1cm−2 for LSFCu-(Li,Na)2CO3-LiAlO2 membrane, even though the LSFCu phase possesses a relatively higher electrical conductivity (∼75 S cm−1 at 700°C). The results were explained based on the hypothesis that CO2 permeation was primarily dominated by a coherent interaction between O2− and CO32− of the following reactions:

At the feeding side: CO2 + O2 − → CO32 − (5.21)



At the sweeping side: CO32 − → CO2 + O2 − (5.22)

Since the alkaline earth metals are prone to react with CO2 to form metal carbonates [141, 143–145], A-site deficient perovskites have been considered to mitigate the CO2 attack [146]. Norton et al. [81] studied the use of an A-site deficient perovskite oxides such as La0.85Ce0.1Ga0.3Fe0.65Al0.05O3-σ (LCGFA), which was previously reported as a CO2-tolerant oxygen permeation membrane at high temperatures [146], as the ceramic phase for ceramic-carbonate (Li2CO3/Na2CO3/K2CO3 = 42.5/32.5/25 1.8 CO2

Flux (ml cm−2 min−1)

1.5

O2

1.2 0.9 0.6 0.3 0.0 10

20

30

40

50

60

70

CO2 concentration (%)

Figure 5.18  The CO2 and O2 flux of an LSFCu-(Li,Na)2CO3-LiAlO2 membrane as a function of CO2 concentration in the CO2/O2 mixture at 750°C. Source: Lan et al. 2014 [79]. Reproduced with permission of Elsevier.

240

Electrochemical CO2 Capture and Conversion

(molar concentration)) MOCC membrane. A maximum CO2 flux of 0.044 and 0.024 ml cm−2 min−1 was obtained for 0.75 and 1.5 mm thick membranes, respectively, at 900°C with 50% CO2-He as the feed gas, which was significantly lower than that of LSCF-carbonate MOCC membrane, as shown in Table 5.2, due to the much lower oxide-ion conductivity of LCGFA (0.03 S cm−1 at 900°C) compared to that of LSCF (0.1 S cm−1 at 900°C), further confirming that the oxide-ion transport in MOCC membrane was the rate-limiting step for CO2 permeation. The stability of the membrane with a thickness of 1.5 mm was also investigated. Figure 5.19 shows a stable flux between 0.021 and 0.025 ml cm−2 min−1 for 275 h at 900°C. However, CeO2 peaks were detected after testing, which was attributed to the partial decomposition of LCGFA phase at the surface exposed to a low CO2 partial pressure. Comparing to planar membranes, tubular membranes have the potential to provide a higher surface/volume ratio, easier sealing and a smaller thickness for less bulk transport resistance. Zhuang et al. [85] fabricated an LSCF-carbonate MOCC membrane based on hollow fibers and found that the CO2 flux density can be as high as 1.0 ml min−1 cm−2 at 900°C versus 0.36 ml min−1 cm−2 for a 0.375 mm thick diskshaped LSCF-MC dual phase membrane under the same measurement conditions. Jiang et al. [87] fabricated a hollow fiber MOCC membrane with SrFe0.8Nb0.2O3-σ (SFN) as the support matrix and Li2CO3/Na2CO3/K2CO3 = 42.5/32.5/25 (mol%) as the carbonate phase. The configuration and the transport mechanism of the hollow

0.04

JCO2 (mL·cm−1·min−1)

0.03

0.02

0.01

0.00 0

50

100

150 Hours

200

250

Figure 5.19  Time dependence of CO2 permeation flux of LCGFA-carbonate membrane (thickness = 1.5 mm, feed and sweep flow rate = 100 ml min−1, feed CO2 = 0.5 atm, T = 900°C). Norton and Lin 2014 [81]. Reproduced with permission of Elsevier.

La0.6Sr0.4Co0.8Fe0.2O3-σ/Li2CO3:Na2CO3:K2CO3 = 42.5:32.5:25.0 La0.6Sr0.4Co0.8Fe0.2O3-σ/Li2CO3:Na2CO3:K2CO3 = 42.5:32.5:25.0 La0.6Sr0.4Co0.8Fe0.2O3-σ/Li2CO3:Na2CO3:K2CO3 = 42.5:32.5:25.0 La0.6Sr0.4Co0.8Fe0.2O3-σ/Li2CO3:Na2CO3:K2CO3 = 42.5:32.5:25.0 La0.6Sr0.4Co0.8Fe0.2O3-σ/Li2CO3:Na2CO3:K2CO3 = 42.5:32.5:25.0 La0.6Sr0.4Co0.8Fe0.2O3-σ/Li2CO3:Na2CO3:K2CO3 = 42.5:32.5:25.0 La0.6Sr0.4Co0.8Fe0.2O3-σ/Li2CO3:Na2CO3:K2CO3 = 42.5:32.5:25.0 La0.5Sr0.5Fe0.8Cu0.2O3-σ/Li2CO3:Na2CO3 = 53:47 La0.5Sr0.5Fe0.8Cu0.2O3-σ-LiAlO2/ Li2CO3:Na2CO3 = 53:47 La0.5Sr0.5Fe0.8Cu0.2O3-σ-LiAlO2/Li2CO3:Na2CO3 = 53:47 La0.5Sr0.5Fe0.8Cu0.2O3-σ-LiAlO2/ Li2CO3:Na2CO3 = 53:47 La0.5Sr0.5Fe0.8Cu0.2O3-σ/Li2CO3:Na2CO3 = 53:47 La0.85Ce0.1Ga0.3Fe0.65Al0.05O3-σ/Li2CO3:Na2CO3: K2CO3 = 42.5:32.5:25.0

O2−/CO32− dual-phase composition

0.63 0.65 0.816 0.05 2.0 0.35 0.55 0.65 1.55 0.29 0.044

0.18/750 0.18/0.375 ∼0.8/1200 /1000 /1000 /1500 /1500 /1500 /1500 /1500 /750

750 900

750

750

750 750

850

900

900

900

900

900

144

147

89.9

89.6

/ 96

/

CO2:O2 = 1:4/He CO2:air = 1:9/He CO2:He = 1:1/Ar

57.9

46.3 74.3 CO2:N2 = 2:1He

CO2:N2 = 1:1/He CO2:N2 = 1:1/He

CO2:O2:N2 = 2:1:1/Ar 108

CO2:N2 = 1:1/Ar

CO2:N2 = 1:1/He

CO2:Ar = 1:1/He

CO2:Ar = 1:1/He

CO2:Ar = 1:1/He 87.7

0.51

0.18/1500

(Continued)

[79] [81]

[79]

[79]

[79] [79]

[83]

[83]

[77]

[71]

[71]

[71]

[71]

86.4

CO2:Ar = 1:1/He

0.27

0.18/3000

900

Ea (kJ mol−1) Reference

Matrix pore size/ Flux density Temp. Feed gas/ thickness (µm) (ml min−1 cm−2) (°C) sweep gas

Ta b l e   5 . 2   Comparative analysis of data on CO2 flux density with the MOCC membrane

650 650 650 650 650 900 700

0.2–3/200–400 0.05 0.2–2/200–400 0.03 0.07 0.08 0.52 1.56 0.88 1.84

// 0.29/50 /∼10 1–5/150 /∼150 0.5–0.6/1200 1–2/1200 1–2/1200 0.4/1500

Ce0.8Sm0.2O1.9/Li2CO3:Na2CO3 = 52:48

Ce0.8Sm0.2O1.9/Li2CO3:Na2CO3 = 52:48

Ce0.8Sm0.2O1.9/Li2CO3:Na2CO3 = 52:48

0.8

0.13

0.11

850

0.64

1–5/∼220

900

650

700

700

900

1.0

/∼300 44.84

CO2:N2 = 1:1/He

H2:CO2:N2 = 1:10:10/ He CH4:CO2:N2 = 14:3:2/He O2:CO2:N2 = 2:3:15/ He CO2:N2 = 1:1/Ar

CO2:N2 = 1:1/He

CO2:N2 = 1:1/He

CO2:N2 = 1:3/He

CO2:Ar = 1:1/He

63

34.7

48.9

74.3

/

60.3

106

113

CO2:He = 1:1/Ar:CO2 77±6 = 99:1 CO2:He = 1:1/Ar:CO2 84±14 = 99:1 CO2:Ar = 1:1/He 113.4

58.6

[82]

[101]

[100]

[94]

[80]

[75]

[76]

[74]

[69]

[73]

[73]

[87]

[85]

Ea (kJ mol−1) Reference

CO2:N2 = 1:1/He

Matrix pore size/ Flux density Temp. Feed gas/ thickness (µm) (ml min−1 cm−2) (°C) sweep gas

La0.6Sr0.4Co0.8Fe0.2O3-σ/Li2CO3:Na2CO3:K2CO3 = 42.5:32.5:25.0 SrFe0.8Nb0.2O3-σ/ Li2CO3:Na2CO3:K2CO3 = 42.5:32.5:25.0 Y0.16Zr0.84O2-σ/Li2CO3:Na2CO3:K2CO3 = 43:31:25.0 Ce0.9Gd0.1O1.95/ Li2CO3:Na2CO3:K2CO3 = 43:31:25.0 Bi1.5Y0.3Sm0.2O3/Li2CO3:Na2CO3:K2CO3 = 42.5:32.5:25.0 Bi1.5Y0.3Sm0.2O3/Li2CO3:Na2CO3:K2CO3 = 42.5:32.5:25.0 Y0.16Zr0.84O2-σ/Li2CO3:Na2CO3:K2CO3 = 42.5:32.5:25.0 Ce0.8Sm0.2O1.9/Li2CO3:Na2CO3:K2CO3 = 42.5:32.5:25.0 Ce0.8Sm0.2O1.9/Li2CO3:Na2CO3:K2CO3 = 42.5:32.5:25.0 Ce0.8Sm0.2O1.9/Li2CO3:Na2CO3 = 52:48

O2−/CO32− dual-phase composition

Ta b l e   5 . 2   (Continued )

243

DEVELOPMENT OF HIGH-TEMPERATURE PERMEATION MEMBRANES

CO2+N2

lean CO2 gas

He+CO2 He

te MCMHF

SFN-Carbona

Molten carbonate Outer surface O2−

CO32−

O2−

CO32−

O2−

Inner surface MIEC support Cross-section

Figure 5.20  Schematic configuration of an asymmetric SFN-carbonate MOCC membrane and CO2 transport mechanisms through the membrane. Source: Jiang et al. 2015 [87]. Reproduced with permission of American Chemical Society.

fiber MOCC membrane are shown in Figure 5.20. The maximum of CO2 flux was 0.64 ml min−1 cm−2 with a 220 µm thickness membrane at 850°C and a 50% CO2-N2 feed gas. The long-term stability was also tested for 200 h at 700°C. As shown in Figure 5.21, no obvious flux degradation was observed with the CO2 flux density. 5.3.2.2  Pure Oxide-Ion Conductor as the Porous Matrix for MOCC Membrane  Different from MIECs, pure ionic conductors such as yttria stabilized zirconia (YSZ) [78], samarium doped ceria (SDC) [94], and gadolinium stabilized ceria (CGO) [84] have also been tried in MOCC membranes for CO2 separation. Wade et al. [73] showed an in situ method to fabricate YSZ-, CGO- and α-Al2O3-MC MOCC membranes for CO2 separation, in which three kinds of MCs (pure Li2CO3, Li/ Na/K eutectics, and Na/K2CO3 eutectics) were particularly compared. The Al2O3 was used as a control sample against O2− conducting YSZ and CGO-based membranes, to understand the relevance of oxide-ion conductivity in the transport mechanism. During their 4000-min test, the CO2 flux density of Al2O3-MC MOCC membrane never exceeded 8 × 10−13 mol m−1 s−1 Pa−1 at 750°C, demonstrating that the non-ionic conducting Al2O3 membrane did not support a meaningful flux of CO2. With the YSZLi2CO3 membrane, the flux decreased with time and failed after 36 h because of the formation of an insulating zirconate phase. The MOCC membranes containing (Li/ Na/K)2CO3 and/or (Na/K)2CO3 eutectic mixtures and YSZ and/or CGO all showed

244

Electrochemical CO2 Capture and Conversion

JCO2 (mL·min−1·cm−2)

0.4

0.3

0.2

0.1 0

50

100 Time (h)

150

200

Figure 5.21  Time dependence of the CO2 permeation flux of the SFC-carbonate membrane (feed side, CO2 flow rate of 50 ml min−1 and N2 flow rate of 50 ml min−1; sweep side, helium flow rate of 100 ml min−1, T = 973 K). Source: Jiang et al. 2015 [87]. Reproduced with permission of American Chemical Society.

a similar permeability trend; that is, the CO2 permeability started relatively low, ∼1 × 10−12 mol m−1 s−1 Pa−1, but gradually increased to 5–6 × 10−12 mol m−1 s−1 Pa−1 at 750°C over a 2500-min period. Figure 5.22 shows the overall results of the longterm testing.

CGO + M2CO3 (M = Li, Na, K) YSZ + M2CO3 (M = Li, Na, K) YSZ + M2CO3 (M = Na, K) YSZ + M2CO3 (M = Li) Al2O3 + M2CO3 (M = Li, Na, K)

8x10−12 (mol m−1 s−1 Pa−1)

Leak corrected CO2 permeability

10x10−12

6x10−12 4x10−12 2x10−12 0

0

1000

2000

3000

4000

Minutes

Figure 5.22  CO2 permeability at 750°C with dual-phase MOCC membranes consisting of CGO+(Li/Na/K)2CO3, YSZ with Li2CO3, (Li/Na/K)2CO3, and (Na/K)2CO3 and Al2O3 (Li/Na/K)2CO3 mixtures. Membrane thickness was varied from 200 to 400 µm.

DEVELOPMENT OF HIGH-TEMPERATURE PERMEATION MEMBRANES

245

Since CO2 permeation through the dual-phase MOCC membrane is primarily dominated by oxide-ion transport [68, 70, 73], improving oxide-ion conductivity of the ceramic phase becomes an effective way to enhance the overall CO2 flux density for the MOCC membranes. Li et al. [69] and Rui et al. [74] employed the fluorite structured Bi1.5Y0.3Sm0.2O3 (BYS) known to be the best oxide-ion conductor in the dual-phase MOCC membrane. The MC phase was infiltrated into the porous BYS support by the capillary force. A schematic illustration of such MOCC membrane is shown in Figure 5.23, in which a thin γ-Al2O3 film was also introduced to enhance the wettability between the BYS surface and MC. As expected, a higher CO2 flux density than that of YSZ-MC MOCC membrane was obtained [73]. The CO2 flux density was increased from 6.5 × 10−3 to 6.6 × 10−2 ml min−1 cm−2 as the temperature was increased from 500°C to 650°C with an apparent activation energy of 113.4 kJ mol−1 for the CO2 permeation. The CO2 permeation flux density was also shown to increase with sweep gas flow rate and achieved 8.3 × 10−2 ml min−1 cm−2 at 650°C with a He sweep gas flow rate of 125 ml min−1 (STP). Decreasing the thickness of the membrane is another effective way to achieve a high CO2 flux density. Lu and Lin [76] demonstrated a 5–50 µm thick MOCC membrane for CO2 separation. The thin YSZ/LSCF-carbonate (Li/Na/K2CO3 = 42.5/32.5/25 (mol%)) dual-phase membrane was synthesized on an asymmetric support of a two-layer structure shown in Figure 5.24. The support consisted of a thick (1–2 mm), large-pore (0.5–5 µm) base layer and a thin (5–50 µm), small-pore

(1) CO2 + O2 → CO32

(2)

O2−

(2)

CO32−

(2)

O2−

(3) CO32− → CO2 + O2

(4)

CO2

Bi1.5Y0.3Sm0.2O3 Molten carbonate Modification film

Figure 5.23  Schematic illustration of CO2 transport through solid-state oxide-molten carbonate dual-phase membrane with a pore surface modification film (γ-Al2O3). Source: Rui et al. 2012 [74]. Reproduced with permission of Elsevier.

246

Electrochemical CO2 Capture and Conversion

Small pore top layer Large pore base support

Porous ceramic asymmetric support

Infiltrated with molten carbonate Molten carbonate Thin dual-phase membrane No carbonate and porous

Figure 5.24  Schematic configuration of asymmetric thin dual-phase membrane. Source: Lu and Lin 2013 [76]. Reproduced with permission of Elsevier.

(< 100 nm) O2− conducting or MIEC top-layer. A key point to this concept is to ensure that the top layer remains gas tight while the base layer remains porous for gas transport after infiltration with carbonate. This was achieved by utilizing different capillary forces created by different pore sizes and wettability. BYS, YSZ, and α-Al2O3 were, respectively, selected as the base layer supports, while YSZ and LSCF were selected as the ionic conducting thin top layer. Based on the different wettability properties between the above ceramics and MC, the best combination of thin top-layer/thick porous base-layer was found to be YSZ/BYS. The CO2 flux for such asymmetrical dual-phase MOCC membrane shown in Figure 5.25 was at least 10× higher than those reported for the thick LSCF-MC membrane at 700°C [71] and YSZ-MC and CGO-MC membranes at 650°C [73] and over 5× higher than that of BYS-MC [74] and SDC-MC membrane (70 vol% SDC) at 650°C [94]. Figure 5.26 shows an approximately 20 h stability testing on the asymmetrical dualphase MOCC membrane, and no degradation was observed. It was also found that the measured CO2 flux for the 10 µm membrane only exhibit 10× improvement rather than 30× over 200–400 µm thick membranes, implying that the surface reaction becomes a limiting factor as the membrane thickness decreases. Similar to MECC membranes, the microstructure of the ceramic phase and the ratio of ceramic phase to carbonate phase in MOCC membranes also have a significant effect on the CO2 flux density. Dong et al. [75] developed an asymmetric tubular SDC-carbonate MOCC membrane, which consists of a thin and dense membrane layer on a porous support. SDC was used as the thin porous inner layer withholding the MC phase, while a SDC-BYS mixture was used as the thick porous outer support layer. Since BYS is a MC non-wettable material, MC will be immobilized primarily by the porous SDC thin layer, not by the thick SDC-BYS porous substrate, thus creating a thin and dense MOCC membrane supported on a porous SDC + BYS

247

DEVELOPMENT OF HIGH-TEMPERATURE PERMEATION MEMBRANES

CO2 flux (mol m−2.s−1)

0.01

1E-3 thin YSZ-MC YSZ-MC [73] CGO-MC [73]

1E-4

BYS-MC [74] LSCF-MC [71] SDC50-MC [94] SDC30-MC [94] 1E-5 600

500

700

800

900

Temperature (°C)

Figure 5.25  Comparison of CO2 flux for thin YSZ-carbonate dual-phase membrane developed in this study with those for thick dual-phase membranes made of LSCF, YSZ, CGO, BYS, and SDC available in the literature. Source: Lu and Lin 2013 [76]. Reproduced with permission of Elsevier.

CO2 flux (mol m−2·s−1)

0.005

0.004

0.003

0.002

0.001

0

2

4

6

8

10 12 Time (h)

14

16

18

20

Figure 5.26  Time dependence of CO2 permeation flux through the thin YSZ-carbonate dual-phase membrane at 650°C. Source: Lu and Lin (2013, 2014) [76, 80]. Reproduced with permission of Elsevier.

248

Electrochemical CO2 Capture and Conversion

(b)

(a)

150 μm

100 μm

5 μm

(c)

(d) SDC 150 μm

100 μm

5 μm

Carbonate

Figure 5.27  SEM images of (a) cross-section of the porous SDC/SDC-BYS substrate and (b) the porous SDC layer without infiltration. (c) Cross-section of the SDC/SDC-BYS asymmetric membrane and (d) the SDC-carbonate membrane layer after infiltration. Source: Dong et al. 2013 [75]. Reproduced with permission of Royal Society of Chemistry.

substrate [74, 76]. The tubular membranes had a typical outer diameter of 7 mm with a total thickness of 1.5 mm and a thin dual-phase functional layer of 150 µm on the inner surface as shown in Figure 5.27. At 900°C, the CO2 flux and permeance of the asymmetric membrane were 1.56 ml cm−2 min−1 and 2.33 × 10−7 mol m−2 s−1 Pa−1, respectively, which were 3× higher than that of the symmetric SDC-MC membrane at a thickness of 1.5 mm. The effect of CO2 concentration on the feed side on CO2 flux was also studied. Figure 5.28 shows that the CO2 flux increased from 0.17 to 0.50 ml min−1 cm−2, while the permeance decreased from 1.27 to 0.41 × 10−7 mol m−2 s−1 Pa−1 at 700°C by increasing the feed CO2 concentration from 10% to 90%. Lu and Lin [80] also fabricated an asymmetric but disk-shaped thin SDC-carbonate MOCC membrane in a thickness of 150 µm supported by a macroporous 60 vol% SDC and 40 vol% BYS mixture. The thin SDC-carbonate membrane exhibited a CO2 flux of 0.18–0.88 ml min−1 cm−2 at 550–700°C with a steady-state operation for more than 160 h. Zhang et al. [94] used a combined “co-precipitation” and “sacrificial-template” synthesis method to fabricate the porous SDC layer for the SDC-MC MOCC membrane and observed a very high and efficiently interconnected three-dimensional

249

DEVELOPMENT OF HIGH-TEMPERATURE PERMEATION MEMBRANES

CO2 flux (ml·cm−2·min −1)

CO2 Flux 1.2

CO2 Permeance

0.5

1.0 0.4 0.8 0.3 0.6 0.2

0.4

CO2 Permeance (10−7 mol·m−2·s−1·Pa−1)

1.4

0.6

0.2

0.1 0

20

40

60

80

100

Feed CO2 concentration (%)

Figure 5.28  CO2 flux and permeance of asymmetric tubular dual-phase membranes as a function of feed CO2 concentration at 900°C. Feed side: CO2 flow rate 5–45 ml min−1, CO2 and N2 total flow rate 50 ml min−1; sweep side: He flow rate 50 ml min−1.

ionic channels [90, 93, 94]. Figure 5.29 shows that the average pore size of the prepared SDC porous matric was 550–600 nm. The CO2 flux density of a 1.2 mm-thick MOCC membrane at 700°C was increased from 0.26 to 1.84 ml min−1 cm−2 by increasing the MC loading from 30 to 50 vol% with 4.8% H2, 47.6% CO2, and 47.6% N2 as the feed gas and He as the sweep gas. Figure 5.30 shows that the permeability of such SDC-MC MOCC membrane was at least two orders of magnitude higher than that reported in YSZ-MC and CGO-MC at 650°C [73] and at least one order of magnitude higher than the LSCF-MC membrane at 700°C [71]. The significant improvement in CO2 flux was mainly attributed to the highly efficient microstructure, which provides a large amount of intra- and interconnected ionic channels for high-rate ionic transport in the dual-phase MOCC membrane. The activation energy values of the tested CO2 flux densities were 0.80, 0.83, 0.78, and 0.77 eV, respectively, for the four MC loadings, which was close to 0.78 eV for oxide-ion conduction in SDC electrolytes [147]. Hence, the ionic transport in SDC-MC MOCC membranes is the rate-limiting step. The early CO2 capture using MOCC membranes were primarily focused on flue gas and fuel gas. In a recent study, MOCC membranes have also been explored to separate CO2 directly from natural gas (NG) [100]. CO2 as an acidic gas in NG must be removed prior to pipeline transportation. The CO2 concentration in an NG can vary from 0% to 70%. During the removal process, avoiding accidental CH4 release to the atmosphere is important since CH4 is a heat-trapping gas 86× more

250

Electrochemical CO2 Capture and Conversion

(a)

(b)

Normalized Size Distribution, μm–1

4.0 3.0 μm 2.0 1.0 0 1.0 2.0 μm

0.4

0.2

0 0 0.8

0.5

1

1.5

2

2.5 Pore SDC

RVE2

0.6 0.4 0.2

3.0

0 0

4.0 (c)

Pore SDC

RVE1

0.6

0.5

1

1.5

Diameter, μm

2

2.5

(d) Log Differential Intrusion(mL/g)

0.25 0.2 0.15 0.1 0.05 0

1 μm

10

1E+2

1E+3

1E+4

Pore size (nm)

Figure 5.29  Microstructural features of a SDC50 (50 vol% SDC). (a) Reconstructed three-dimensional microstructure and (b) pore and phase size distributions of two RVEs (representative volume elements) obtained by X-ray nanotomography. (c) SEM twodimensional microstructure and (d) pore size distribution obtained by mercury porosimetry. Source: Zhang et al. 2012 [94]. Reproduced with permission of Royal Society of Chemistry.

powerful than CO2. Therefore, a safe, efficient, and low-cost separation process for CO2 removal from raw natural gas is technologically and environmentally important. Tong et al. [100] tested a SDC-MC MOCC membrane for CO2 separation from a simulated NG containing 75% CH4, 15% CO2, and 10% N2—N2 was purposefully added as a tracer gas to indicate and quantify any leakage. With a 1.18 mm-thick membrane, the CO2 flux density reached 0.11 ml min−1 cm−2 at 700°C. Figure 5.31 shows that

251

CO2 Permeability (mol m−1 s−1 Pa−1)

DEVELOPMENT OF HIGH-TEMPERATURE PERMEATION MEMBRANES

D C B A

1E-10

1E-11 LSCF-MC [71] 1E-12

YSZ-MC [73]

1E-13

CGO-MC [73]

1E-14

500

600 700 Temperature (°C)

800

900

Figure 5.30  Comparison of CO2 permeability of the MOCC developed by Zhang et al. with LSCF-MC, YSZ-MC, and CGO-MC membranes available in the literature. MOCC: A, 70 vol% SDC and 30 vol% MC; B, 65 vol% SDC and 35 vol% MC; C, 60 vol% SDC and 40 vol% MC; and D, 50 vol% SDC and 50 vol% MC. Source: Zhang et al. 2012 [94]. Reproduced with permission of Royal Society of Chemistry.

JCO2 (ml·min−1·cm−2)

0.20 0.16 0.12 0.08 Feed gas: 70% CH4, 15% CO2, 10% N2 Thickness: 1.15 mm Temperature: 650°C

0.04 0.00 0

20

40

60

80

100

Time (h)

Figure 5.31  CO2 flux stability measured with a simulated natural gas containing 15% CO2 at 650°C. Source: Tong et al. 2015 [100]. Reproduced with permission of the Electrochemical Society.

252

Electrochemical CO2 Capture and Conversion

In J (ml min−1 cm−2)

−1.0

1.00

Feeding Gas: 15% CO2, 10% O2, 75% N2 Sweeping Gas: Helium Thickness: 1.21 mm

−2.0

Ea = 34.72 kJ mol−1

CO2

r2 = 0.9939 0.10

−3.0

−4.0

−5.0 1.05

Ea = 37.93 kJ mol−1

O2

J (ml min−1 cm−2)

0.0

r2 = 0.9997

1.10

1.15

1.20

0.01 1.25

1000/T, K−1

Figure 5.32  Arrhenius plots of CO2 and O2 flux densities measured with a simulated flue gas. Source: Tong et al. 2015 [101]. Reproduced with permission of Elsevier.

the membrane tested at 650°C was relatively stable over a 100-h period, including an initial increase during the first 20 h, followed by a very slow degradation. Tong et al. [101] also demonstrated that SDC-MC MOCC membranes can separate low-concentration CO2 from a N2-rich flue gas. Figure 5.32 shows an Arrhenius plot of CO2 and O2 flux densities measured with a simulated flue gas containing 15% CO2, 10% O2, and 75% N2 as the feed gas. A high CO2 flux density of ∼0.13 ml min−1 cm−2 was achieved at 650°C. It was also found that a small fraction of O2 permeated through the MOCC membrane, which is contradictory to the MOCC enabling surface reaction mechanism. Early concurrent CO2 and O2 permeation was only observed in the MECC membranes with a flux ratio of CO2:O2 is 2:1. However, the flux ratio in Figure 5.32 was found to be 7.5:1, implying a large portion of CO2 still permeated from the mechanism CO2 + O2− = CO32− as previously perceived. Based on the results of the effect of oxygen partial pressure on the CO2 flux, a parallel pathway for CO2/O2 transport was proposed by Tong et al. [90] following the surface reaction CO2 + 1/2O2 + 2e− = CO32−, with O2 from the flue gas and electrons from the sealant silver. Figure 5.33 illustrates the parallel mechanism proposed. From a practical application perspective, especially for pre-combustion processes, long-term stability under a large system pressure is necessary. Norton et al. [82] investigated the CO2 flux and stability of SDC-MC MOCC membrane under CO2/N2 and simulated syngas conditions at high temperatures and high pressures. Figure 5.34 shows that the membrane exhibited a steady-state CO2 permeation of ∼0.43 ml cm−2 min−1 for more than 80 h at 700°C with a 2.5 atm CO2 partial pressure in a CO2/N2 feed gas. The CO2 flux varied from 0.68 to 0.74 ml min−1 cm−2 for more

253

DEVELOPMENT OF HIGH-TEMPERATURE PERMEATION MEMBRANES

Feed side: CO2, O2, N2

Path way 2: CO2 + 1/2O2+ 2e− CO32−

e−

CO32− CO32−

Path way 1: CO2 + O2− = CO32−

O2−

CO32−

e−

MOCC Membrane Silver sealant

CO32− = CO2 + 1/2O2+ 2e− CO32− = CO2 + O2− Sweep side: He

Figure 5.33  Schematic illustration of parallel transport mechanisms in MOCC membranes with flue gas as the feedstock.

JCO2 (ml·cm−2·min−1)

0.78

0.65

(a) Temperature: 900°C Feed CO2: 0.5 atm

0.52

(b) Temperature: 700°C Feed CO2: 2.5 atm

0.39

0

2

4

6

8

10

12

14

Days

Figure 5.34  Time dependence of CO2 permeation flux of SDC-MC membrane: (a) at 900°C (atmospheric pressure CO2:N2 feed) and (b) at 700°C (high pressure CO2:N2 feed). Source: Norton et al. 2014 [82]. Reproduced with permission of Elsevier.

254

Electrochemical CO2 Capture and Conversion

0.5

(a)

JCO2 (ml·cm−2·min−1)

0.4

(b) 0.3

0.2

0.1

0.0 0

5

10

20 15 Days

25

30

35

Figure 5.35  Time dependence of CO2 flux of SDC-MC membrane with a simulated syngas feed at 700°C: (a) 5 atm feed gas, (b) atmospheric pressure. Source: Norton et al. 2014 [82]. Reproduced with permission of Elsevier.

than 330 h at 900°C with 0.5 atm CO2 partial pressure in the feed gas. Furthermore, with a simulated fuel gas composition containing 50% CO, 35% CO2, 10% H2, and 5% N2, Figure 5.35 shows that the CO2 flux measured at 700°C stabilized at ∼0.31 ml min−1 cm−2 under atmospheric pressure for more than 30 days, and at 0.4 ml min−1 cm−2 for 5 days when exposed to a total feed pressure of 3 atm. From the XRD patterns of the surface of SDC-carbonate dual-phase membrane after testing, the sweep side of membranes maintained a pure fluorite structure, whereas some Sm2O3 peaks were observed on the feed side, which was speculated to be caused by the decomposition of SDC under H2-containing environments. 5.3.2.3  MOCC Membrane Reactor for Dry Methane Reforming  Since the operating temperature of an MOCC membrane matches with the preferred reaction temperature of DMR reaction, CO2 capture and instant reaction with CH4 within an MOCC membrane reactor is possible. Anderson et al. [127] first reported the concept with an LSCF-MC dual-phase membrane. The work clearly proved the concept, but the interaction between LSCF and MC led to a poor performance: with a 10% Ni/γ-Al2O3 as the catalyst, the CH4 conversion rate and syngas production rate were only 8.12% and 0.3 ml min−1 cm−2 at 850°C, respectively. Zhang et al. [148] improved the MOCC membrane reactor comprising GDC-MC dual-phase membrane loaded with a robust Ni-MgO-1 wt% Pt (NMP) catalyst or a ceramicbased LaNi0.6Fe0.4O3-σ (LNF) catalyst. The MOCC membrane reactor with NMP catalyst generally outperformed the LNF counterpart in CH4 conversion rate and

SUMMARY AND OUTLOOK

255

syngas production yield. For example, at 850°C and over the NMP catalyst, the membrane reactor yielded a CO2 permeation flux of 2.25 ml min−1 cm−2, H2 and CO production rates of 3.75 and 3.24 ml min−1 cm−2, respectively, and a CH4 conversion of 93.9%. At the same temperature 850°C, but over the LNF catalyst, a CO2 permeation flux of 2.1 ml min−1 cm−2, H2 and CO production rates of 2.7 and 3.4, respectively, and a CH4 conversion of 73% were achieved after ∼100-hour activation. However, the LNF catalyst showed a better stability and coking resistance, which showed no sign of degradation within 200 h of operation. One of the shortcomings of the combined CO2 capture and DMR in Zhang et al.’s [148] study is the lower H2/CO ratio (7 bar), CO2 is able to bypass the K+ and diffuse farther down the tunnels. This causes CO2 to become trapped when the pressure is reduced back to ambient conditions. The type of cation dopant was found to affect CO2 diffusion. A comparison of different dopant types (K+, Na+, and Ba2+) revealed three different scenarios for CO2–dopant interaction. Scenario I occurred in all three cases and for Ba2+, CO2 remained at the equilibrium position with no further diffusion. For the K+ dopant, CO2 bypasses it to further diffuse along the OMS-2 tunnel (Scenario II). For Na+, CO2 continued diffusing by pushing the dopant along the tunnel (Scenario III). These mechanisms suggested a sorption hysteresis. The phenomenon of sorption hysteresis commonly occurs in gas molecule adsorption and desorption isotherms and indicates that the path to adsorption of gas molecules by a porous host differs from that of desorption. Even though it has the lowest CO2 uptake capacity, the Ba2+-doped OMS-2 is expected to have the smallest hysteresis. Upon desorption (i.e., a pressure decrease) CO2 can easily exit the OMS-2 tunnel. In contrast, CO2 can diffuse farther in the OMS-2 tunnel by passing through a K+ dopant and pushing a Na+ dopant. These mechanisms require a longer time to vacuum CO2 in the OMS-2 tunnel, yielding the phenomenon of sorption hysteresis. The Na+-doped OMS-2 should have a slightly smaller hysteresis than the K+-doped OMS-2. In Scenario II, upon desorption, CO2 has to pass through the K+ dopant in reverse. This action costs an energy penalty and requires more time than the Na+ cation case, where CO2 can directly exit the tunnel with no need to pass any Na+. These results suggested that the charge, size, and mobility of the dopant, accommodated in a porous material, control the CO2 uptake capacity and sorption hysteresis. OMS-5 has a larger pore dimensionality than OMS-2 and has potential for similar carbon-capture applications. OMS-5 was determined to be more stable when a

354

COMPUTATIONAL MODELING STUDY OF MnO2 OCTAHEDRAL MOLECULAR SIEVES

Na+ dopant is added to the tunnel. This could be due to the larger frame and volume of OMS-5, compared to that of the OMS-2. Without Na+, the OMS-5 channel compressed and its cell volume decreased by nearly 5%. A larger pore dimensionality also allows more degrees of freedom for CO2 molecules which need to be further understood.

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Index

Note: Page numbers followed by “f” indicate figures.

A ab initio molecular dynamics (AIMD), 336–337 adsorption capacity, 12 advanced photon source (APS), 310 alkanolamine-based scrubbers, 3 aminated graphite oxide (AGO), 137 amine scrubber, 3, 115 anion exchange membrane (AEM), 215 Argonne National Laboratory (ANL), 310 Arizona State University, 224 arylamines, 126 atomic force microscopy (AFM), 301 atomic layer deposition (ALD), 229, 235

B Barrer units, 99 Basolite®, 6 1,4-benzenecidarboxylic (BDC), 91, 194 1,3,5-benzenetricarboxylic acid (BTC), 152 bipolar membrane electrodialysis (BPMED), 216 Bonse–Hart crystal, 302 Bragg peaks, 188 Bruker Smart CCD 6000 diffractometer, 184

C CALF-25, 154 carbamate, 3 carbon-capture process/technology, 4 chemical properties of molecules in, 11

composition of gas for, 7 oxy-fuel combustion, 8–9 post-combustion capture, 7–9 pre-combustion capture, 8–9 carbon capture and storage (CCS) technologies, 79–80 MOF, 82–83 post-combustion, 80–81 carbon capture, utilization and storage (CCUS), 268 carbon dioxide detrimental effects of, 2 importance of, 1 reaction scheme for, 3 remediation method, 2 Cartesian direction, 333 cathode and anode, electrochemistry of CO2 and CO32−, electrochemical reactions pathway, 285–287 electrochemical reaction at the anode, 287–289 electrochemical reaction at the cathode, 282–284 Ceramatec, 221 chemical accuracy, 327 chemical shift anisotropy (CSA), 28 chemical vapor deposition (CVD), 227, 229 Clausius–Clapeyron equation, 13, 86 CO2 capture and conversion (CCC), 213 CO2 capture and storage (CCS), 213

Materials and Processes for CO2 Capture, Conversion, and Sequestration, First Edition. Edited by Lan Li, Winnie Wong-Ng, Kevin Huang, and Lawrence P. Cook. © 2018 The American Ceramic Society. Published 2018 by John Wiley & Sons, Inc.

357

358

CO2 capture, utilization, and storage (CCUS), 296 computational modeling study, of MnO2 octahedral molecular sieves, 344–345 atomic structure versus magnetic ordering, 345–346 cation dopant types, 348–349 CO2 sorption, 349–351 CO2 sorption behavior DFT studies, 348–348 experimental observations, 347 OMS-5, 351–353 pore size and dimensionality, 346–347 computational models, 102 cross-polarization magic-angle-spinning (CP/MAS), 45 crystallographic techniques, 18–21 cyclic voltammograms (CV), 283

D Decco/Essentiv™, 83 density functional theory (DFT), for carbon capture, 319–320, 323–324 accuracy of, 327–328 applications of, 328 ab initio molecular dynamics, 336–337 bandgap, 332 CO2 diffusion, 337–337 CO2 location and binding energetics, 329–332 elastic properties, 332 NMR, 336 phonons, 333–335 thermodynamics, 335–336 DFT+U, 326–327 general gradient approximation, 325 hybrid methods, 325–326 local density approximation, 324–325 meta-GGA, 325 microporous solids flexible MOF, 322–323 oxide molecular sieves, 320–321 rigid MOF, 321–322 vdW forces, 327 diamond light source beamline I12, 200 differential scanning calorimetry (DSC), 42

Index

diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), 22 2,5-dimethoxy-terephthalic acid, 91 dioxybenzenedicarboxylate (DOBDC), 87 dipyidylacetylene (dpa), 95 dry methane reforming (DMR), 223, 233 MOCC membrane reactor for, 254–255

E EGC goniometer, 187 elastic single-molecule trap, 132 electrochemical CO2 capture and conversion, 213–214 current methods, 214–215 CO2 capture through electrodeless permeation cells, 218–221 CO2 capture using electrolytic cells, 215–216 CO2 conversion using electrolytic cells, 216–217 CO2 conversion using high-temperature electrochemical cells, 221–223 high-temperature permeation membranes for MECC membranes, 224–235 MOCC membranes, 235–255 electrochemical valorization, of carbon dioxide, 267–269 electrolytic products, applications of, 289 energy-dispersive X-ray diffraction (EDXRD), 185 environmental control cell (ECC), 182 environmental gas cell (EGC), 183 Environmental Protection Agency (EPA), 83 ethylene diamine (ED), 154 exchange correlation (xc), 324 exponential wall, 323

F Faraday constant, 269, 278 Fe-BTC, 6 flue gas, 7, 80–81, 218 Fourier transform infrared spectroscopy, 193

G gadolinium stabilized ceria (CGO), 243 Gamma function, 307 gas diffusivity, 99

359

index

gas permeation units (GPU), 99 general gradient approximation (GGA), 325 Gibbs free energy, 279–280 grand canonical Monte Carlo (GCMC), 13, 128 graphene oxide (GO), 137 greenhouse gas emission, constituent in, 2 Guinier approximation, 304–305

H halide-oxide melts, 281 Hall Heroult cell, 269 Hartree–Fock exchange, 325 Helmholtz free energy, 335 Henry’s constant, 122 high-resolution TEM (HRTEM), 301 HKUST-1, 6 properties of, 88 HSEO6 hybrid calculations, 326 Hubbard parameter, 326 humid atmosphere swing chamber (HASC), 185 hybrid materials and MOF, 137–138 hybrid methods, 325–326

I Idaho National Laboratory (INL), 221 ideal adsorbed solution theory (IAST), 12, 85, 115 inelastic neutron scattering (INS), 16 infrared spectroscopy (IR), 16 in situ diffraction studies, of MOF apparatus for powder diffraction applications, 185–186 single-crystal diffraction applications, 182–184 background, 180–181 characterization, 181–182 integrated gasification and combined cycle (IGCC), 139 ionic liquids (IL), 81 Irena, 310 Irving Williams series, 35 isosteric heat of adsorption (−Qst), 13–14

J Jacob’s ladder, 324

K Kelvin, 278 Knudsen selectivity, 164 Kohn–Sham formalism, 332

L La0.85Ce0.1Ga0.3Fe0.65Al0.05O3-σ (LCGFA), 239 Langmuir model, 13–14, 133, 146 La0.5Sr0.5Fe0.8Cu0.2O3-σ (LSFCu), 239 Lawrence Berkeley National Laboratory, 187 Lewis bases, for chemisorption, 40 Lewis basic sites, 122–127. See also metal organic frameworks (MOF) local density approximation (LDA), 324–325 local density of states (LDoS), 350

M Material Institut Lavoisier (MIL), 88 Membrane Technology and Research (MTR) Center’s Polaris™, 100 meta-GGA methods, 325 metal organic frameworks (MOF), 90 adsorption properties of experimental breakthrough, 15–16 in situ characterization, 16–30 multicomponent adsorption, 14–15 single-component isotherms, 11–14 ball-and-stick model of, 5 capture CO2 directly from air, 140–145 carbon dioxide capture importance, 1–3 CCS technologies and, 6–10 CO2 capture and separation at high pressure, 139–140 CO2 capture and separation at low pressure, 116–121 conjunctive effects, 129–131 endeavors, 134–137 flexible frameworks, 131–134 functional sites, 127 hybrid materials, 137–138 Lewis basic sites, 122–127 open metal sites, 121–122 size-exclusive effect, 127–129 uptake/release kinetics, 138 CO2/C2H2 separation, 148–149

360

metal organic frameworks (MOF)  (cont’d) CO2/CH4 separation, 145–148 flexible, 322–323 future perspectives of, 61–63 graphical representation of, 82 humidity effect, 152–156 industrial process and limitations of, 3–4 industrial scale synthesis of, 6 in situ single-crystal diffraction studies of, 186–187 dynamic CO2 adsorption behavior, 190 mechanism of CO2 adsorption, 192–193 structure transformation induced, 188–189 thermally induced reversible single crystal-to-single crystal transformation, 187–188 unstable intermediate stage during guest exchange, 190–191 membrane for CCS, 99, 163–165 fabrication, 102–103 membrane performance defined, 99–101 molecule specific, 10 with open metal sites, 87–91 oxy-fuel combustion capture biological inspiration for O2/N2 separations, 55–56 O2/N2 separations, 54–61 photocatalytic and electrochemical reduction, 149–152 post-combustion capture CO2 capture, 30–32 CO2/N2 separations, 32–34 Lewis basic sites, 37–45 open metal coordination sites, 34–37 stability and competitive binding, 45–48 powder diffraction studies of laboratory X-ray diffraction studies, 204–206 synchrotron/neutron diffraction studies, 193–203 pre-combustion capture advantages of, 48–49 CO2/H2 separations, 50–54 properties for CO2 capture, 49–50 properties of, 97

Index

rigid, 321–322 with saturated metal centers, 91–96 sorbents, 84 CO2 sorbent, 84–86 criteria, 87–99 synthesis of, 4–6 microporous metal–organic frameworks, 112–116 microstructure characterization, material measurements techniques for, 300–302 MIL-53, 6 mixed electron and carbonate-ion conductor (MECC), 219 development of dry-oxy methane reforming, 233–235 silver-molten carbonate dual-phase membrane, 225–233 stainless-steel-molten carbonate dualphase membrane, 224–225 mixed ionic-electronic conducting (MIEC), 235 mixed matrix membrane (MMM), 102, 165 mixed-oxide-ion and carbonate-ion conductor (MOCC), 219 development of dry methane reforming, 254–255 MIEC ceramic phase, 235–243 pure oxide-ion conductor, 243–254 MOF-177, 6 molten carbonate fuel cells (MCFC), 218 molten salt carbon capture and electrochemical transformation (MSCC-ET), 268 molten salt electrolytes, thermodynamic analysis of alkali metal carbonates, 269–275 alkaline-earth metal carbonates, 275–277 electrolytic products, 277–278 mixed melts, 278–281 monoethanolamine (MEA), 81 Monte Carlo simulations, 28 Mössbauer spectroscopy, 58

N 2,6-naphthalenedicarboxylate (NDC), 201 National Energy Technology Laboratory (NETL), 214

361

index

near edge X-ray absorption fine structure (NEXAFS), 28 Nernst equation, 278 neutron diffraction, 58 neutron powder diffraction, 58 Ni10Cu11Fe alloy, 288 Nika, 310 NIST Center for Neutron Research (NCNR), 312 nitrogen adsorption isotherms, 11 N,N-dimethylethylenediamine (dmen), 144 N,N-dimethylformamide (DMF), 198 nodes, 113 nuclear magnetic resonance (NMR), 16, 193, 336 NuMat, 83

O open metal coordination sites (OMC), 18, 34 MOF with, 34–37 properties of, 97 open metal sites (OMS), 87–91, 114, 121–122. See also metal organic frameworks (MOF) optical microscopy (OM), 301 Oxford-Diamond In Situ Cell (ODISC), 185–186 oxide molecular sieves (OMS), 320–321 oxygen transport membrane (OTM), 221

P permeability (P), calculation of, 99 permeance, 99 perturbation theory, 333 phonons, 333–335 3-picolylamine, 156 Polaris membrane, 100 polyethyleneimine (PEI), 42, 90, 125 poly(vinyl acetate) (PVAc), 165 pore space partition (PSP), 127 porous coordination polymers (PCP), 112 porphyrins, 55 post-synthetic exchange (PSE), 128 powder diffraction applications, 185–186 environmental chambers, 185 Oxford-Diamond In Situ Cell, 185–186

simultaneous PXRD and DSC techniques, 185 powder diffraction studies, of MOF laboratory X-ray diffraction studies, 204–206 synchrotron/neutron diffraction studies, 193–203 breathing modes, 194–195 framework formation in action, 200–201 metastable intermediate transformation, 197–198 multiple-phase transitions, 195–196 resin-assisted solvothermal MOF synthesis, 201–203 reversible gas sorption driven, 198–200 pressure swing adsorption (PSA), 8, 134 proton transport membranes (PTM), 232

Q quartz pressure cell (QPC), 183–184 quasi-elastic neutron scattering (QENS), 16, 26, 312

R Raman frequencies, 335 Raman spectroscopy, 16, 102 reticular synthesis, 113 reverse water-gas shift (RWGS) reaction, 222 Rietveld analysis, 58 of neutron powder, 35 Rigaku Ultima IV diffractometer, 185 Robeson plot, 101

S samarium doped ceria (SDC), 243 saturated metal centers (SMC), 91 scanning electron microscopy (SEM), 301 Schlenk line techniques, 5 Schrödinger’s equation, 323 selectivity factor (S), 12 selectivity/separation factor, 99–100 SIFSIX-2-Cu bipy (4,4-bipyridine), 95 SIFSIX-2-Cu bpy (1,2-(bispyridyl) ethylene)), 95 SIFSIX MOFs, 94

362

single-crystal diffraction applications, 182–184 environmental control cell, 182 environmental gas cell, 183 quartz pressure cell, 183–184 single-molecule traps (SMT), 129 small-angle neutron scattering (SANS), 298 small-angle X-ray scattering (SAXS), 298 sodium montmorillonite (SWY-3), 307 solid oxide electrolysis cells (SOEC), 221 solution-diffusion model, 99 space time yields (STY), 6 spectroscopic techniques, 21–30 subsurface CO2 trapping mechanisms, 298–300 syngas production, 222

T temperature swing adsorption (TSA), 8, 84, 134 temperature/vacuum swing adsorption (TVSA), 160 thermal gravimetric analysis (TGA), 85, 347 thermodynamic analysis, of mixed melts, 278–281 tilt angle, 26 Torlon®, 103 total carbon (TC), 299 total inorganic carbon (TIC), 299 transmission electron microscopy (TEM), 301 triethanolamine (TEOA), 150 triethylene glycol (TEG), 158 triple phase boundaries (TPB), 230 TruPick, 83 turnover number (TON), 150

U UiO66 Zr, 91 structure representation of, 92 ultrasmall-angle neutron scattering (USANS), 298 ultrasmall- and small-angle scattering data, analyses of fractal morphologies, 306–307 Porod scattering regime, 305

Index

shapes and size distributions, 305–306 volume fractions, mean volumes, and radius of gyration, 304–305 University of South Carolina, 225 “upper bound,” 101 USAXS/SAXS instrumentation, 302–303 USAXS/SAXS/WAXS characterization experimental methods, 307–310 outcomes, 310–312 U.S. Department of Energy (DOE), 79–80 UTSA-16–GO19 composite, 147

V vacuum swing adsorption (VSA), 8, 84, 134 van der Waals (vdW) forces, 327 very small angle neutron scattering (vSANS), 313

W water-gas-shift reaction (WGSR), 235 wide-angle neutron scattering (WANS), 302 wide-angle X-ray scattering (WAXS), 302 working capacity, defined, 49. See also metal organic frameworks (MOF)

X X-ray absorption spectroscopy (XAS), 16, 28 XRD-DSC system, 185

Y yttria stabilized zirconia (YSZ), 243

Z zeolite, 83 adsorbents, in comparison with MOF, 158–163 zeolitic imidazolate frameworks (ZIF), 38, 92, 164 structural representation of, 93f ZIF-8, 6 Zn(bipy)2SiF6∙2H2O, 93 unit cell of, 94f zwitterionic carbamates, 24