Solid Acid Catalysis: From Fundamentals to Applications 9789814463294

Table of contents :
Cover
Contents
Preface
Chapter 1: Introduction
Chapter 2: Solid Acid Catalysis
Chapter 3: Characterization of Solid Acid Catalysts
Chapter 4: Catalytic Properties of Solid Acid Catalysts
Chapter 5: Hydrocarbon Transformations: Mechanism and Industrial Processes
Chapter 6: Synthesis of Organic Chemicals through Solid Acid Catalysis
Back Cover

Citation preview

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Solid Acid Catalysis From Fundamentals to Applications

editors

Preben Maegaard Anna Krenz Hideshi Hattori Wolfgang Palz

| Yoshio Ono

The Rise of Modern Wind Energy

Wind Power

for the World

CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2015 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Version Date: 20150311 International Standard Book Number-13: 978-981-4463-29-4 (eBook - PDF) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www. copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com

Contents Preface

1. Introduction









1.1 Types of Solid Acid Catalysts 1.2 Advantages of Solid Acid Catalysis 1.3 Historical Overviews of Solid Acid Catalysis 1.3.1 Dawn of Solid Acid Catalysis 1.3.2 Establishment of the Concept “Solid Acid Catalysis” 1.3.3 Progress of Solid Acid Catalysts and Their Industrial Applications 1.4 Future Outlook

2. Solid Acid Catalysis

2.1 Definition of Acid and Base 2.1.1 Brønsted Acid and Lewis Acid 2.2 Acid Sites on Surfaces 2.2.1 Origin of Brønsted Acid Sites 2.2.2 Origin of Lewis Acid Sites 2.2.3 Identification of Brønsted and Lewis Acid Sites 2.3 Acid Strength 2.3.1 Definition of H0 Acidity Function in Homogeneous Phase 2.3.2 H0 Scale of Acidic Sites on Solid Surfaces 2.3.3 Determination of Acid Strength of Surface Acid Sites 2.4 Role of Acid Sites in Catalysis

xv

1

1 2 4 4

8

10 16

23

23 23 24 25 28

29 30 30 31 33 36

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2.4.1 Reaction of Hydrocarbons with Brønsted Acid Sites: Formation of Carbocations and Their Reactions 2.4.1.1 Carbocation, carbonium ion, and carbenium ion 2.4.1.2 Formation of carbenium ions 2.4.1.3 Reactions of carbenium ions 2.4.1.4 Protonated cyclopropane intermediates 2.4.1.5 Carbenium ions as transition state 2.4.2 Reactions of Alcohols and Carbonyl Compounds over Brønsted Acid Sites 2.4.3 Catalytic Action of Lewis Acid Sites 2.5 Bifunctional Catalysis 2.5.1 Bifunctional Catalysis by Acidic and Basic Sites 2.5.2 Bifunctional Catalysis by Acid Sites and Metal/Metal Cations 2.5.2.1 Skeletal isomerization of light alkanes 2.5.2.2 Aromatization of lower alkanes 2.5.2.3 One-step synthesis of methyl isobutyl ketone 2.6 Pore Size Effect on Catalysis: Shape Selectivity 2.6.1 Reactant Selectivity 2.6.2 Product Selectivity 2.6.3 Transition State Shape Selectivity 2.6.4 Different Origins of Pore Size Effect

3. Characterization of Solid Acid Catalysts

3.1 Indicator Method 3.2 Temperature-Programmed Desorption of Ammonia 3.3 Calorimetry of Adsorption of Basic Molecules

36

36 37 38 41 42

45 48 53 54

60 60 64

67 68 70 71 72 73

81

82 86 91

Contents





3.3.1 Ammonia 3.3.2 Other Basic Molecules 3.4 Infrared Spectroscopy 3.4.1 IR of Adsorbed Pyridine 3.4.2 IR of Adsorbed Ammonia 3.4.3 IR of Adsorption of CO and N2 3.5 NMR Spectroscopy 3.5.1 1H MAS NMR 3.5.1.1 Chemical shift 3.5.1.2 Mobility of acidic protons 3.5.2 31P MAS NMR 3.6 Test Reactions 3.6.1 Acidic and Basic Properties 3.6.1.1 Butene isomerization 3.6.1.2 Alcohol dehydration and dehydrogenation 3.6.1.3 Cyclization of acetonylacetone 3.6.1.4 Reactions of 2-methyl-3-butyn-2-ol 3.6.2 Brønsted and Lewis Acids 3.6.2.1 Rearrangement of cyclic acetals of a-bromoalkyl phenyl ketone 3.6.3 Strength of Brønsted Acid Sites 3.6.4 Number and Strength of Acid Sites by Poisoning 3.6.5 Strength Estimation through Cracking Activity

4. Catalytic Properties of Solid Acid Catalysts

4.1 Zeolites

4.1.1 Characteristics of Zeolites as Solid Acid Catalysts 4.1.2 Structure of Zeolites

91 92 93 93 98 99 103 103 103 106 111 116 116 116 121 123

124 126

126 129 131

134

141

141 141 143

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4.1.3 Nomenclature of Structures of Zeolites and Zeolite-Like Materials 4.1.4 Synthesis of Zeolites 4.1.5 Acidic Sites in Zeolites 4.1.5.1 Acidic OH groups in proton form of zeolites 4.1.5.2 Acidic OH groups in zeolites with multivalent cations 4.1.5.3 Formation of acidic OH groups by reduction of exchangeable cations 4.1.6 Factors Affecting Acid Strength of Acidic OH Groups 4.1.6.1 Si/Al ratio 4.1.6.2 Isomorphous substitution 4.1.6.3 Structure of zeolites 4.1.6.4 Dealumination of zeolites 4.1.7 Catalysis by Metal Cations in Zeolite Framework 4.1.8 Structure and Use of Representative Zeolites 4.1.8.1 Zeolites of industrial importance 4.1.8.2 Zeolites of interest for future applications 4.2 Aluminophosphate Molecular Sieves 4.2.1 Structures of Aluminophosphate Molecular Sieve 4.2.2 MeAPO Materials 4.2.3 SAPO Materials 4.3 Ordered Mesoporous Materials 4.3.1 Synthesis of Ordered Mesoporous Silica 4.3.2 Development of Acidity in Ordered Mesoporous Materials

145 147 147 147 149 150 151 151 152 153 154 157 157 157

167 168 168 170 170 175

175 177

Contents



4.3.2.1 Catalysis by mesoporous silica



4.3.2.3 Generation of Lewis acid sites on mesoporous silica













4.3.2.2 Incorporation of heteroatoms into the structure of mesoporous silica

4.3.2.4 Mesoporous silica functionalized with sulfo groups

4.4 Heteropolyacids

4.4.1 Isopolyanions and Heteropolyanions 4.4.2 Behaviors of Protons in Solid HPAs 4.4.3 Pseudoliquid Phase in Solid HPAs 4.4.4 Supported Heteropolyacids 4.4.5 Cs Salts of Heteropolyacids 4.4.6 Catalysis by Metal Salts of Heteropolyacids

177 178

182

183 187 187 189 190

192 197

200

4.4.7 Acid Sites Formation by Reduction of Metal Cations

201

4.4.9 Catalytic Reactions by Heteropolyacids

206

4.4.8 Bifunctional Catalysis by Metal–HPA Composite Catalysts

4.5 Clays

4.6 Alumina and Modified Alumina 4.6.1 Al2O3

4.6.1.1 Structure and preparation of alumina

4.6.1.2 Surface properties of alumina 4.6.1.3 Catalytic properties

4.6.2 Al2O3–Cl and Al2O3–F

4.7 Zirconium Oxide and Related Catalysts 4.7.1 ZrO2

4.7.1.1 Preparation and phase change 4.7.1.2 Characterization

4.7.1.3 Catalytic properties

204 210

214

214 215

217 224

228

229

229 230

237 239

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Contents





4.7.1.4 Morphology dependence of catalysis 4.7.2 S​O​2–   ​ – ​ ZrO 2 4 4.7.2.1 Preparation 4.7.2.2 Characterization 4.7.2.3 Catalytic activity 4.7.3 Pt/S​O2– ​ ​  ​–ZrO2 4 4.7.3.1 Preparation 4.7.3.2 Characterization 4.7.3.3 Catalytic activity 4.8 SiO2–Al2O3 4.8.1 Preparation 4.8.2 Catalytic Activity 4.8.3 Characterization 4.9 Tungsten Oxide and Related Catalysts 4.9.1 WO3 4.9.2 WO3–Al2O3 4.9.3 WO3–SnO2 4.9.4 WO3–ZrO2 4.9.4.1 Preparation 4.9.4.2 Characterization 4.9.4.3 Catalytic activity 4.9.4.4 Pt/WO3–ZrO2 4.10 Niobium Oxide and Related Catalysts 4.10.1 Nb2O5 4.10.1.1 Preparation 4.10.1.2 Acidic properties 4.10.1.3 Catalytic activity 4.10.2 Nb2O5–Al2O3 4.10.3 Nb2O5–WO3 and Nb2O5–MoO3 4.10.4 Layered Compounds Containing Nb 4.11 Supported Acids 4.11.1 Solid Phosphoric Acid 4.11.2 Other Supported Acids

244 245 246 247 248 250 250 251 253 256 256 257 257 263 263 264 265 271 272 272 275 279 282 282 282 282 283 285 285 286 288 288 291

Contents





4.11.2.1 Supported Brønsted acids 4.11.2.2 Supported Lewis acids 4.12 Ion Exchange Resins

291 295 297

5.1 Introduction 5.2 Catalytic Cracking (FCC) 5.2.1 Reaction Mechanisms 5.2.2 Fluid Catalytic Cracking Process 5.2.2.1 Reactor 5.2.2.2 Catalyst design 5.3 Synthesis of Ethylbenzene and Cumene 5.3.1 Synthesis of Ethylbenzene 5.3.2 Synthesis of Cumene 5.4 Isomerization and Hydroisomerization of Alkanes 5.4.1 Isomerization of Alkanes 5.4.2 Mechanism of Alkane Isomerization on Acidic Catalysts 5.4.2.1 Main reaction pathway 5.4.2.2 Side reactions 5.4.2.3 Isomerization by bimolecular mechanism 5.4.3 Bifunctional Catalysts 5.4.4 Industrial Processes for Isomerization of Lower Alkanes 5.4.5 Isomerization of Long-Chain Alkanes 5.5 Production of p-Xylene 5.5.1 Isomerization of Xylenes 5.5.1.1 Introduction 5.5.1.2 Mechanism of xylene isomerization 5.5.1.3 Industrial processes for xylene isomerization 5.5.2 Disproportionation of Toluene

321 323 323 325 325 326 329 330 332

5. Hydrocarbon Transformations: Mechanism and Industrial Processes

321

333 333 334 334 335

335 336

338 341 346 346 346 347 349 351

xi

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5.5.2.1 Catalysts for toluene disproportionation 5.5.2.2 Mechanism of toluene disproportionation 5.5.2.3 Shape selective formation of p-xylene 5.5.2.4 Industrial processes 5.5.3 Alkylation of Toluene with Methanol 5.6 Conversion of Methanol to Hydrocarbons 5.6.1 MTH, MTG, and MTO Reactions 5.6.2 Methanol to Gasoline Reaction over ZSM-5 5.6.3 Methanol to Alkenes 5.6.3.1 Methanol to alkenes over SAPO-34 5.6.3.2 Methanol to alkenes over ZSM-5 5.6.4 Mechanism of MTH Reaction

6. Synthesis of Organic Chemicals through Solid Acid Catalysis

6.1 Alkylation 6.1.1 Alkylation of Aromatics with Alcohols/Alkenes 6.1.1.1 Synthesis of cymene by isopropylation of toluene 6.1.1.2 Alkylation of toluene with t-butyl alcohol or isobutene 6.1.1.3 Alkylation of toluene with benzyl alcohol 6.1.1.4 Alkylation of phenols with t-butyl alcohol 6.1.1.5 Alkylation of phenols with benzyl alcohol 6.1.1.6 Alkylation of anisole with benzyl alcohol

351 353

354 356 358 359 359 360 361 361 364 365

375

375 375

375 378 379 380 383 384

Contents







6.1.2 N-Alkylation of Heterocycles with Alcohols 6.1.2.1 N-Alkylation of imidazoles and pyrazoles 6.1.2.2 N-Alkylation of pyridine-2-one with methanol 6.2 Acylation 6.2.1 Overview 6.2.2 Acylation of Aromatic Hydrocarbons 6.2.3 Acylation of Anisole 6.2.4 Acylation of Veratrole 6.2.5 Acylation of 2-Methoxynaphthalene 6.2.6 Acylation of Heterocycles 6.2.7 Acylation of Ferrocene 6.2.8 Acylation of Phenols, Alcohols, and Amines 6.2.9 Acylation of Alkenes 6.2.10 Geminal Diacylation of Aldehydes 6.3 Esterification and Transesterification 6.3.1 Esterification with Solid Acids 6.3.2 Esterification of Lower Carboxylic Acids with Alcohols 6.3.3 Esterification of Acids with Alkenes 6.3.4 Transesterification 6.3.5 Biodiesel Synthesis 6.3.5.1 Esterification of long-chain fatty acids 6.3.5.2 Biodiesel synthesis by transesterification of oils with methanol 6.4 Reactions of Epoxides 6.4.1 Ring Opening of Epoxides with Amines and Alcohols 6.4.2 Hydration of Ethylene Oxide 6.4.3 Isomerization of Styrene Oxide

384 384 387 387 387 389 391 394 396 399 399

401 402 403 404 404

405 413 414 417 418 423 425

425 432 432

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6.4.4 Rearrangement of b-Pinene Oxide into Myrtanal 6.4.5 Rearrangement of a-Pinene Oxide to Camphenolic Aldehyde 6.5 Dehydration 6.5.1 Dehydration of Glycerole to Acrolein 6.5.2 Dehydration of 1,2-Propanediol 6.5.3 Dehydration of Fructose and Glucose to 5-Hydroxymethylfurfural 6.5.4 Dehydration of Xylose to Furfural 6.5.5 Dehydration of Ethanolamine to Ethyleneimine 6.6 Conversion of Trioses into Lactates 6.7 Hydration of Alkenes 6.8 Isomerization/Rearrangement 6.8.1 Isomerization of a-Pinene to Camphene 6.8.2 Bamberger Rearrangement 6.8.3 Beckmann Rearrangement: Production of e-Caprolactam 6.9 Acetalization 6.10 Prins Reaction: Nopol Synthesis 6.11 Synthesis of Xanthenes 6.12 Pechmann Condensation 6.13 Friedländer Reaction 6.14 Synthesis of Amides 6.14.1 Ritter Reaction 6.14.2 Amidation of Amines with Carboxylic Acids 6.15 Biginelli Reaction 6.16 Strecker Reaction

Subject Index

Chemical Formula Index

433

434 435 435 438 439 443 447 450 452 455

455 457

458 461 466 467 470 471 472 472 474 476 477

495 511

Preface The application of solid acid catalysts started in the late 1930s with Houdry cracking process, in which an acid-washed clay was used as a catalyst. Since then, various solid acids and their applications as catalysts have been extensively studied. As a result, a number of new solid acid catalysts have been developed for industrial processes. Solid acid catalysts have advantages over liquid acid catalysts in the construction of efficient and environmentally benign processes. Replacement of liquid acid catalysts with solid acid catalysts is being actively undertaken in many processes. The first book (Solid Acids and Bases) on solid acids and bases and their catalysis was written by Prof. Kozo Tanabe in 1970 and the second one (New Solid Acids and Bases) by Tanabe and three co-authors, including both of us, was published in 1989. Since 1989, a great progress has been achieved in the acid–base chemistry of solid materials and their application to the industrial processes. This book, Solid Acid Catalysis, complements the book Solid Base Catalysis, written by us in 2011, and offers a comprehensive survey of solid acid catalysts and their applications to chemical reactions. The book is composed of six chapters. The first three chapters describe the fundamental aspects of solid acid catalysis and the remaining three chapters focus on more advanced aspects. Chapter 1 introduces various types of solid acid catalysts and historical overviews of their development. Chapter 2 explains the fundamentals of solid acid catalysis. Chapter 3 covers the characterization methods of solid acids. Chapter 4 describes the preparation of important solid acids and their catalytic properties. Chapters 5 and 6 explain a variety of reactions, from hydrocarbon transformations to fine chemical synthesis, catalyzed by solid acids in detail. H. H. expresses his deep thanks to the late Prof. Tadao Shiba, who led H. H. as a supervisor to the research on solid acid catalysis in the 1960s. Y. O. expresses his heartfelt thanks to the late Prof. Tominaga Keii for his long-time encouragements.

xvi

Preface

In writing the book, we received cooperation of many people and organizations. We are grateful, in particular, to Dr. Y. Nishimura (ex-member of Catalysts & Chemicals Ind. Co., Ltd) for sending the articles on the industrial use of zeolites, Prof. N. Katada (Tottori University) for useful discussion on acidity measurement, and Prof. T. Baba (Tokyo Institute of Technology) for collecting useful information. H. H. acknowledges the support of King Fahd University of Petroleum & Minerals (KFUPM) and Catalysis Research Center, Hokkaido University (CRC), by providing him with the positions and environments suitable for book writing.

Hideshi Hattori Yoshio Ono February 2015

Chapter 1

Introduction

1.1  Types of Solid Acid Catalysts Acid-catalyzed reactions are the reactions that are initiated by acid–base interactions in which the catalysts act as acids and the reactants act as bases. The acids are regenerated on the completion of the reaction to be ready to act as acids for further reactions. Acid-catalyzed reactions are either homogeneous or heterogeneous. In heterogeneous acid-catalyzed reactions, the surfaces of solids act as acids toward reactant molecules. The solids are called solid acid catalysts. Many homogeneous acid-catalyzed reactions had been studied for a long time before heterogeneous acid-catalyzed reactions were recognized in the 1940s. Sugar conversion, ester hydrolysis and its reverse reaction (esterification), etc., had been known to be catalyzed in aqueous solutions of an acid. Some general rules had been established until the time when heterogeneous acid-catalyzed reactions started to be investigated: the Brønsted relationship (a linear free energy relationship) and the relation of the rate of acidcatalyzed reactions with Hammett acidity function H0. Extensive studies of cracking catalyst, SiO2–Al2O3, in the 1940s and 1950s revealed that the active sites of the catalyst are acidic sites present on the SiO2–Al2O3 surface. The reactions taking place on the surface are initiated by acid–base interaction between the acidic sites and the reactants. Solid Acid Catalysis: From Fundamentals to Applications Hideshi Hattori and Yoshio Ono Copyright © 2015 Pan Stanford Publishing Pte. Ltd. ISBN  978-981-4463-28-7 (Hardcover), 978-981-4463-29-4 (eBook) www.panstanford.com



Introduction

Following the recognition of SiO2–Al2O3 as a solid acid catalyst, a number of different types of materials were found to have acidic properties and applied to a wide variety of acid-catalyzed reactions. The materials exhibiting acidic properties include zeolites of various crystalline structures, mixed metal oxides, mounted acids, single metal component metal oxides, metal salts, heteropolyacids, cation exchange resins, and mesoporous materials. Examples of these classifications of solid acids are summarized in Table 1.1.

Table 1.1 Class

Solid acid catalysts Example

Zeolite

X-, Y-zeolites (faujasite), chabasite, ferrierite, beta-zeolite, mordenite, erionite, ZSM-5, MCM-22

Clay

montmorillonite, saponite

Zeolite-like

Metal oxide

Mixed metal oxide Supported acid Sulfated oxide

Layered transition metal oxide Metal salt

Heteropoly compound

Cation exchange resin

metalloaluminophosphate (e.g., silicoaluminophosphate (SAPO)), gallosilicate, beryllosilicate, titanosilicate (TS-1), stanosilicate Al2O3, TiO2, SiO2, Nb2O5, WO3

SiO2–Al2O3, SiO2–ZrO2, SiO2–MgO, TiO2–SiO2 WO3–ZrO2, WO3–Al2O3, WO3–SnO2, Nb2O5–Al2O3, B2O3–Al2O3

H3PO4/SiO2 (SPA), heteropolyacids/SiO2, HClO4/ SiO2, SO3H/SiO2, SO3H/C, AlCl3/SiO2, BF3/SiO2, SbF5/ SiO2–Al2O3, SbF5/TiO2, CF3SO3H/SiO2 SO4/ZrO2, SO4/TiO2, SO4/SnO2 HNbMoO6, HTaWO6, HNbWO6

AlPO4, Nb3(PO4)5, FePO4, NiSO4

H3PW12O40, H4SiW12O40, H3PMo12O40, H4SiMo12O40 and their salts (e.g., H0.5Cs2.5PW12O40, etc.) Amberlyst-15, Nafion, Nafion–silica composite

1.2  Advantages of Solid Acid Catalysis The advantages of solid acid catalysts in comparison with homogeneous ones in industrial uses are as follows:

Advantages of Solid Acid Catalysis

(1) Continuous production systems can be employed for solid acid catalysts, while batch-type reaction systems are commonly used for homogeneous acid catalysts. This advantage is great for industrial processes in particular. (2) The reaction can be carried out at a high temperature for solid acid catalysts, whereas the maximum temperature is limited to the boiling point of the solvent for homogeneous acid catalysts. A high reaction rate can be attained by hightemperature operation. In addition, we do not have to worry about the selection of solvent. For a homogeneous acid catalyst system, the solvent should dissolve both the reactant and the catalysts. In a solid acid catalyst system, on the other hand, the solvent needs to dissolve only the reactant, or the reaction can be carried out without any solvent. The reaction at a high temperature is beneficial for the endothermic reactions where the equilibrium favors formation of the products at a high temperature. (3) The separation of the catalyst from the reaction mixture is easy for solid acid catalyst systems; the catalysts are automatically separated for vapor phase reactions, or can be separated by simple filtration for liquid phase reactions. In most cases, the separated catalysts are easily regenerated by burning the residues deposited on the surface of the catalyst. Thus, recycle use of the catalysts is easy. For homogeneous systems, on the other hand, the separation of the catalyst from the reaction mixture needs tedious procedures, and the catalysts are difficult to be regenerated for recycle use. (4) Solid acid catalysts are not corrosive and can be used in the reactor systems made of usual materials, whereas homogeneous acids are corrosive and the reactor systems should be made of anti-corrosive materials to acids. (5) By use of solid acid catalysts, the production of by-products can be minimized. Homogeneous acid catalysts are normally neutralized on the completion of the reactions by an alkaline solution to form metal salts (e.g., Na2SO4), which are byproducts and need further treatment to be disposed of. Solid acid catalysts need not be neutralized on the completion of the reaction, and by-product formation can be minimized. (6) Multifunctional catalysts can be designed based on the solid acid catalysts. Catalytically active components other than





Introduction

acid sites can be located on the solid acid catalyst surfaces to promote reactions other than acid-catalyzed ones. The reactions requiring more than one step can proceed over a single catalyst that has both acidic function and another catalytic function. For example, platinum supported on zeolite can catalyze hydrocracking of alkanes, in which Pt acts as a dehydrogenation/hydrogenation catalyst and zeolite acts as a cracking catalyst.

All six items listed here contribute to the construction of environmentally benign catalytic processes.

1.3  Historical Overviews of Solid Acid Catalysis 1.3.1  Dawn of Solid Acid Catalysis

Acidic properties of the solid surface were first found out in 1902 by Kobayashi for a clay named Kambara earth produced at Niigata Prefecture, Japan. He observed a color change of blue litmus paper into red when he put the clay on it. His observation was later reported in 1912 together with some applications of the clay [1]. He named the Kambara earth “acid earth.” He utilized it in chemical processes such as the production of hydrocarbons from fish oil, soybean oil and turpentines, and decolorizing mineral oils but gave up to apply it in the petroleum-refining process. The use of a clay (Fuller’s earth) in the cracking of crude oils had been already disclosed by Thiele in the literature [2]. Later in 1933, Houdry accomplished the cracking process in petroleum refining using a different type of clay as a catalyst. Houdry was a French mechanical engineer. He was interested in the motor car as many young men were. He was particularly interested in the automobile as a machine. He learned of the limitations on performance of an engine resulting from fuel quality; the most efficient way to run the racing cars fast is to improve the fuel quality. He decided to produce an improved fuel and organized a team constituting of researchers and engineers of different fields such as chemistry, chemical engineering, and mechanical engineering in 1922. At that time, gasoline was produced by the distillation and thermal cracking of the heavier fraction of petroleum oils. Through his experience in lignite (brown coal) upgrading, he was convinced

Historical Overviews of Solid Acid Catalysis

that the catalytic treatment of the heavier fraction would produce high-quality gasoline. He tried various clays and soon achieved very promising results. A sample of the clay produced at San Diego that had been acid treated showed high performance as a catalyst for producing high-quality gasoline [3]. Gasoline demand was increasing at that time when motorization became popular. He completed a semi-commercial unit of a size 2000 bbl/day (bbl. Barrel = 159 L) in 1933, and the first large-scale commercial unit of a size 15,000 bbl/day of heavy gas oil started operation in 1937. The reactor was a fix-bed type operating in short cycles. The commercialization of the Houdry cracking process was announced at the annual meeting of the American Petroleum Institute in 1938. In the same year, a massive oil field was discovered in Saudi Arabia. This discovery caused a dramatic change in the structure of chemical industries. Coal-based chemical industries changed into petroleumbased ones, which have continued up to present. Many petroleum companies in the United States and Europe pursued extensive research to develop catalysts and processes in petroleum refining. The catalyst used in the first commercial cracking process was the clay of bentonite, which contained ca. 90% montmorillonite. The main components of montmorillonite are SiO2 and Al2O3. A typical commercial catalyst contained SiO2 and Al2O3 in a weight ratio of 3.0:4.5, and was low in sodium (0.5 wt% as Na2O), iron (2.0% as Fe2O3), calcium (1.8% as CaO) and magnesium (3.8% as MgO). Because the composition of natural clays varied depending on the mining site, the catalytic performance of the clays was not stable. The variability in the catalytic properties was not suitable for the natural clays to be used as a catalyst in the industrial cracking process. In 1938, synthetic cracking catalyst composed of SiO2 and Al2O3 was prepared by UOP researchers represented by Thomas. They called the catalyst “silica–alumina.” The Houdry process brought on stream the first synthetic SiO2–Al2O3 cracking catalyst in 1940. The synthetic catalyst eliminated the variability in the catalytic properties that the clay catalyst encountered. Some catalysts other than SiO2–Al2O3 such as SiO2–MgO and SiO2–ZrO2 were also examined. These were, however, not as good as SiO2–Al2O3 in the production of gasoline fraction. The cracking reactor was a fixed-bed type in the first Houdry process. Since catalyst deactivation was severe, a frequent





Introduction

regeneration (every 10–20 min) of the catalyst was required. For the regeneration of the catalyst, a fluidized-bed type reactor is more convenient because a part of the catalyst can be continuously withdrawn from the reactor and fresh catalyst can be supplied continuously. In 1942, the first commercialized fluidized-bed reactor started up at Standard Oil Development, Baton Rouge in the United States. Fluidized-bed reactors have been used until now with some modifications. The process is called fluid catalytic cracking (FCC). The fluidized catalytic reactor employed first in the cracking process has been widely used thereafter for many catalytic processes that evolve a large exothermic heat like oxidation and that cause rapid activity decay.

Figure 1.1

Framework of X and Y zeolites (faujasite type).

Distinct improvement of cracking catalysts took place in 1957 at Union Carbide [4]. Rabo found that X and Y zeolites showed far higher activity than amorphous silica–alumina for hydrocarbon reactions. Plank and Rosinski, Mobil researchers, disclosed the performance of Ca-X zeolite to exhibit an activity in the cracking far higher than amorphous silica–alumina catalysts in 1961 [5]. The findings laid the foundation for the modern catalytic cracking process. The activity of the zeolite was so high that the reaction completed during transferring a mixture of the reactant and catalyst in the pipe (riser) to the fluidized bed. The reactor is called “riser reactor.” The fluidized bed acted as a cyclone to separate the catalyst from the products. Essentially the same faujasite-type zeolites are still used in the current cracking processes after modification to some extent and with some additives (see Section 5.2).

Figure 1.2

Petroleum refining and petrochemical industry.

Historical Overviews of Solid Acid Catalysis 



Introduction

Figure 1.2 illustrates the relationship between the petroleum refining industry and the petrochemical industry. The cracking process is a key process in refining and the largest chemical process among all chemical industrial processes. Refining of petroleum produces fuels and raw materials for use in petrochemical industries.

Crude oils are fractionated by atmospheric distillation into several distillates and residues. Residues are fractionated by vacuum distillation into vacuum gas oils and vacuum residues. The vacuum gas oils consist of large molecules, which should be cracked into smaller molecules. Because the percentage of the residues in crude oils exceeds 50%, vacuum gas oils are produced in large amounts, which should be cracked in the cracking process. Accordingly, the catalytic cracking process is the largest process in the petroleum and petrochemical industries. Out of crude oils, about 85% goes to liquid fuels and the rest about 15% to petrochemical raw materials. The liquid fuels produced from crude oils provide 33% of world total primary energies: the largest primary energy source among coal 30%, natural gas 24%, nuclear 4.5%, etc., in 2012 [6].

1.3.2  Establishment of the Concept “Solid Acid Catalysis”

The improvement of catalytic activity and selectivity is always pursued for all catalytic processes. This is true of the cracking process. The understanding of the nature of the catalysts and chemistry involved in the process is the essential requisite for the improvement. The surface properties relevant to cracking were not clarified until about 1950. To clarify the surface properties relevant to the cracking activity, cracking catalysts and the cracking mechanisms were studied extensively from different viewpoints in the late 1940s and 1950s. It turned out that the acidic properties of the catalyst have direct relevance to cracking, and the concept of solid acid catalysis was established. Through extensive studies of the properties of cracking catalysts and their catalysis in the related reactions performed in 1945–1960, the following points were clarified. These are the

Historical Overviews of Solid Acid Catalysis

reasons for which the acidic properties of the catalyst are believed to be relevant to the reactions catalyzed by silica–alumina and similar types of catalysts:

(1) The existence of acid sites on the surfaces: The characterization of the surfaces by various methods such as titration of acid sites with amine using acid–base indicators, the adsorption of basic molecules, and several spectroscopic measurements indicate that acid sites exist on the surface. The generation of acidic sites on SiO2–Al2O3 was reasonably interpreted. (2) Parallel relation between catalytic activity and the amount and/or strength of acid sites: The catalytic activities correlate well with the amounts of acid sites with certain strengths measured by various methods. Also, the active sites are poisoned by basic molecules such as amines and eliminated by doping the catalysts with alkali ions. (3) The similarity of the catalytic actions of solid catalysts to those of homogeneous acids: There are a number of reactions known as acid-catalyzed reactions in homogeneous systems. Certain solid materials also catalyze these reactions to yield the same products. The carbenium ions, which were proposed by Whitmore [7] as intermediates for low-temperature acidcatalyzed reactions such as alkylation and polymerization in liquid acids, are believed to be involved in the catalytic cracking on solid catalysts. The similarities in catalytic actions between liquid acids and certain solid materials include high reactivity of alkenes compared with alkanes, dealkylation of side chains from alkylaromatics, high reactivity of compounds containing tertiary carbon atoms in hydrocarbon conversions, etc. Carbenium ion chemistry can explain the product distribution in catalytic cracking both in homogeneous and heterogeneous phases. (4) The indication of carbenium ions as the reaction intermediates: Mechanistic studies of the reactions by, for example, product distribution analysis, isotopic tracer study, and spectroscopic observation of the adsorbed species indicate that carbenium ions are involved in the reactions taking place over the solid catalysts. The carbenium ions are formed by the interaction of the reactants with surface acid sites.



10

Introduction

1.3.3  Progress of Solid Acid Catalysts and Their Industrial Applications ~1950 In parallel with the development of cracking process, the other processes of hydrocarbon conversions such as the oligomerization of propene, alkylation of benzene with propene to cumene, and reforming of naphtha were explored in this period. These processes aimed at producing high-octane gasoline components and used solid acid catalysts such as solid phosphoric acid and halogen treated Al2O3. Solid phosphoric acid was developed by Ipatieff for the conversion of hydrocarbons [8]. Ipatieff was a Russian chemist who moved to the United States in 1929 and worked at UOP and Northwestern University. Solid phosphoric acid was prepared by the heat treatment of a mixture containing phosphoric acid and kieselguhr. Kieselguhr is a natural occurring material and composed mostly of SiO2. The solid phosphoric acid catalyst was used for the oligomerization of propene and the copolymerization of isobutene and propene. The products were hydrogenated with Ni catalyst to produce high-octane components for gasoline. This process was the first commercial catalytic process using solid acid introduced to petrochemistry, a few years before the Houdry process started operation. During World War II (1943–1945), solid phosphoric acid was used in the manufacture of cumene by the alkylation of benzene with propene. Cumene is a good component of the aviation fuel. The process using solid phosphoric acid is still a major process for cumene production at present, using essentially the same catalysts. Ipatieff is considered one of the founding fathers of modern petrochemistry in the United States [9]. In 1947, Shell Oil started the production of ethanol by ethylene hydration using solid phosphoric acid. The process has been operated until the present; a large fraction of synthetic ethanol production depends on this process at present, though the synthetic ethanol production is only 5% of total ethanol production, the rest 95% being fermentation ethanol.

Historical Overviews of Solid Acid Catalysis

Reforming is one of the refining processes to enhance the octane number of the naphtha fraction and includes several types of reactions such as the dehydrogenation of alkylcyclohexanes, ring contraction and enlargement of naphthenes, isomerization of n-alkanes to branched alkanes, and hydrocracking of alkanes and alkyl benzenes. Two different catalytic sites, metallic sites and acidic sites, are required for the isomerization of n-alkanes to branched alkanes and the hydrocracking of alkanes and alkyl benzenes. Toward the development of the reforming process, Ciapetta (Atlantic Refining) and Haensel (UOP) contributed much independently. Ciapetta used supported Ni or Pt on silica–alumina for “Catforming process,” while Haensel used supported Pt on chlorided or fluorided alumina for the “Platforming process.” Both processes realized commercial operation in the late 1940s. These two catalysts are the first two examples of the industrial use of bifunctional catalysts, a combination of the metallic function and the acidic function. 1950–1970

Zeolites were applied to the cracking process in this period and completely replaced silica–alumina catalysts. The concept of “solid acid catalysis” was established through extensive studies of solid acid catalysts from various points of view. Syntheses of zeolites possessing the same structures as those of naturally occurring zeolites were performed and their catalytic properties were examined. The synthesis of zeolites relies greatly on Barrer, who systematically studied the synthesis from the 1940s to the 1980s. Barrer is called the founding father of zeolite chemistry. One of his most important contributions is the use of alkylammonium compounds in zeolite synthesis. This influenced the synthesis of new zeolites undertaken in the 1970s and 1980s for which structure-directing agents (SDAs) were used. Following Barrer’s achievements, the synthesis of zeolites was investigated by Milton and Breck in 1949–1954 at Union Carbide. They succeeded in synthesizing A-, X-, and Y-type zeolites in a commercial scale. X- and Y-type zeolites have the same topology as natural zeolite faujasite. They differ in the Si/Al ratio; the ratio is 1–1.5 for X and more than 1.5 for Y. Rabo examined the catalytic

11

12

Introduction

activities of the X- and Y-type zeolites and found in 1957 that X and Y zeolites showed far higher activity than amorphous silica– alumina for cracking, hydrocracking, and isomerization. Y-type zeolite was used after ion exchange with Mg and Pd addition (Pd/ Mg–Y) for the first zeolite-based hydrocracking process jointly developed by Unocal and Union Carbide in 1959 [4]. Plank and Rosinski, Mobil Oil, used faujasite-type zeolites for cracking process and confirmed the high activity as described earlier [5]. The first cracking process using faujasite was commercialized in 1962. Zeolites showed far higher activity than amorphous SiO2–Al2O3 not only for cracking but also for other hydrocarbon reactions. The modification of Y zeolites by steaming improved the catalytic activity and thermal stability of the zeolites. The zeolites thus obtained are rich in SiO2 content and tolerant to high-temperature treatment required for the regeneration process of cracking catalysts. The steam-treated zeolite Y is named ultra stable Y zeolite (USY). It was soon revealed that a distinct feature of catalysis by zeolites is shape selectivity, which is caused by the relative sizes of the pores of zeolites to those of the molecules involved in the reactions [10]. The feature of shape selectivity was utilized in industrial applications. The first application in the industrial process was reforming of naphtha by erionite-type zeolite in 1968 (Selectforming, Mobil). Toluene disproportionation to selectively form p-xylene and benzene was commercialized in 1969 by use of mordenite-type zeolite and successively by faujasite-type zeolite in the same year. In 1970, Tanabe wrote the book Solid Acids and Bases [11]. This book is the first comprehensive book on solid acid catalysis. The book emphasizes that the combination of various metal oxides offers acidic properties and that preparative conditions of the catalysts, in particular calcination temperature, affect much the acidity generation in the resulting materials. Afterward, he wrote a successive book, New Solid Acids and Bases, with three other authors in 1985 [12]. The publication of the books has contributed much to the development of solid acid catalysis. 1970–1990

The synthesis of zeolites was extended in this period, and the famous artificial zeolite ZSM-5 was synthesized by Mobil Oil Co.

Historical Overviews of Solid Acid Catalysis

in 1972 (ZSM-5 stands for Zeolite Socony Mobil No. 5.) [13]. ZSM5 became famous for its catalytic activity for the conversion of methanol to gasoline in one step (MTG reaction) [14]. The synthesis of ZSM-5 has exerted a strong impact on the zeolite synthesis and catalyses by zeolites thereafter in the following points: (1) The zeolite possessing 10-membered O ring pore mouth was synthesized for the first time. (2) Because of the 10-membered O ring, shape selectivity was distinctly observed for many reactions. In addition, coke formation was very small compared with other types of zeolites having 12-membered O ring such as X, Y, and mordenite. (3) In the synthesis, SDA was used. ZSM-5 was applied to many processes, including dewaxing, xylene isomerization, and the conversion of methanol into gasoline. Among them, the alkylation of benzene with ethylene to ethylbenzene is important in the sense that the homogeneous Friedel–Crafts catalyst (AlCl3) was replaced by solid acid catalysts. The gas phase reaction of the alkylation of benzene with ethylene using a ZSM-5 catalyst known as the second generation of the Mobil-Badger process started operation in 1980. In the 1980s, new compositions and structures of zeolites and zeolite-like materials appeared explosively. One of the new compositions was a material containing Al and P. Microporous crystalline aluminophosphate (ALPO) was disclosed by Flanigen et al. at Union Carbide in 1982 [15]. Aluminophosphate-based zeolitelike materials were subsequently disclosed by 1986, such as SAPO, MePO, MeSAPO, ElAPO, and ElSAPO, where S, Me, and El stand for Si, metal, and element, respectively. These elements are included in the frameworks. The process for the conversion of methanol to olefins (MTO process) using SAPO-34 was developed in 1988. Isodewaxing process using SAPO-11 was commercialized in 1997 by Chevron. Applications of ion exchange resins to industrial processes were realized in this period, which were the results of development of macroporous ion exchange resins undertaken in the early 1960s. The applications include esterification, methyl t-butyl ether (MTBE) production from methanol and isobutene, the synthesis of bisphenol-A, and the hydration of isobutene to t-butanol. Ion exchange resins are water tolerant and, accordingly, have been used in the processes in which water is included in the reactants or products.

13

14

Introduction

Meanwhile, Olah, Nobel laureate in 1994, proved the existence of non-classical carbocations in very strongly acidic media such as SbF5, FSO3H, and CF3SO3H (triflic acid) as well as mixed acids solutions such as HF–SbF5, FSO3H-SbF5 (magic acid), CF3SO3H– SbF5, and CF3SO3H–B(O3SCF3)3, in 1972. The non-classical carbocations are penta-coordinate carbocations (carbonium ions) as illustrated in Fig. 1.3 together with classical tri-coordinate carbocations (carbenium ions). This finding contributed much to the advancement of the carbocation chemistry. Carbocations are formed from a variety of carbon compounds in high concentrations in the strongly acidic media. The structures of carbenium ions were exactly determined by spectroscopies, and the reactivities were also examined. The strongly acidic media are called “superacids,” which are defined as protic acids stronger than 100% sulfuric acid (H0 = 12).

H H

C+

H

Carbenium ion

Figure 1.3

H

H C

H

H

+

H

Carbonium ion

Carbenium ion vs. carbonium ion.

Triggered by liquid superacid, some trials were undertaken to synthesize solid superacid. Some of the catalysts that catalyze alkane isomerization at room temperature were synthesized, such as SbF5 mounted on SiO2–Al2O3, TiO2. They were, however, not used for practical processes probably because they contain halogen and are difficult to be prepared and handled. The only catalyst prepared for the purpose aiming at solid superacid and actually utilized for practical processes is sulfated zirconia (S​O​2– 4​  ​/ZrO2). S​O​2– ​  / ​ ZrO was first reported by Arata in 1978 to possess 2 4 superacidity and catalyze alkane isomerization at room temperature [16]. Although there is controversy whether the catalyst is stronger than 100% sulfuric acid or not, S​O​2– 4​  ​/ZrO2 showed an excellent catalytic behavior for the isomerization of alkanes when modified with Pt. The modified catalyst, Pt/S​O​2– 4​  ​/ZrO2, was used

Future Outlook

as an industrial catalyst for the hydroisomerization of light naphtha for the production of high-octane gasoline components in 1996 (UOP-Cosmo Oil, Par-Isom process). 1990–2010

The synthesis and applications of new types of zeolites continued in this period. Among these zeolites, MCM-22 was used in industrial processes for the alkylation of benzene with ethylene and propene to produce ethylbenzene and cumene, respectively. The ethylbenzene production process had used AlCl3–HCl, and the cumene production had used solid phosphoric acid. MCM-22 was used in a liquid phase reaction for both ethylbenzene production (1995, Mobil-Raytheon EBMax process) and cumene production (1996, Mobil-Badger process). These are examples of a replacement of problematic acid catalysts such as AlCl3–HCl and solid phosphoric acid by easyhandling solid acid catalysts. The cumene production using beta zeolite was also industrialized (ENICHEM). In addition to MCM-22 and beta zeolite described above, several acid-treated solid acid catalysts successfully replaced homogeneous catalysts such as AlCl3, HF, and H2SO4 in alkylation processes in this period. These included fluorinated SiO2–Al2O3, BF3-treated Al2O3 and CF3SO3H-treated SiO2 for different alkylation processes; fluorided SiO2–Al2O3 for the alkylation of benzene with linear olefins for surfactant detergent intermediate (UOP and CEPSA, Detal process), and BF3-treated Al2O3 and CF3SO3H-treated SiO2 for the alkylation of isobutane with alkenes for high-octane gasoline components. One successful achievement in replacing the use of sulfuric acid by use of solid acid catalyst is the production of e-caprolactam, which is a raw material of nylon-6 production. The Beckmann rearrangement of cyclohexanone oxime to e-caprolactam uses fuming sulfuric acid (oleum) and ammonia as stoichiometric reaction reagents in the current processes, which produces a large amount of (NH4)2SO4 as a by-product. Sumitomo Chemical Co. established a vapor phase Beckmann rearrangement of cyclohexanone oxime to e-caprolactam using a high-silica ZSM-5 as a catalyst, and industrialized in 2003. This process can produce e-caprolactam without the use of fuming sulfuric acid and the

15

16

Introduction

production of a large amount of the by-product. The process is environmentally benign (see Section 6.8).

1.4  Future Outlook

Solid acid catalysts that began with a cracking catalyst have rapidly expanded their applications to a wide variety of the chemical industrial processes accompanied by understanding the properties of cracking catalysts, characterization of the acid sites, and development of preparation methods. Different types of solid acid catalysts have been developed, and each type of solid acid catalysts exhibits its unique catalytic activity. Solid acid catalysts are not merely replacements of homogeneous acid catalysts but catalyze reactions for which homogeneous acid catalysts cannot pursue. Shape selectivity derived from pore structure and multifunctional catalysis arising from the coexistence of acid sites with other catalytically active sites on the surfaces are specific to solid acid catalysts. The important issues with solid acid catalysts in future are as follows:

(1) Crystalline mesoporous materials: There are many ordered mesoporous materials with a variety of compositions. The walls of mesoporous materials, however, are all amorphous at present. As the evolution of the catalysts from amorphous silica–alumina to crystalline microporous zeolites drastically improved the activity and selectivity for the acid-catalyzed reactions, the crystallization of the walls will bring about great improvements in the catalytic performance of mesoporous materials. The preparation of such mesoporous materials will open a new area of solid acid catalysis, in particular, in shapeselective liquid phase reactions. (2) Water- and temperature-tolerant solid acid catalysts: For many reactions in aqueous solutions or involving water as a reactant or product, ion exchange resins have been used. The ion exchange resins can be used only at low reaction temperatures. Although there are some water-tolerant nonresin type catalysts, they are not compatible to ion exchange resins in many reactions. Water-tolerant solid acid catalysts,

Future Outlook

which can be used at a high temperature, will be replacement for ion-exchange resins. (3) Multifunctional catalysts with acidic properties: Many chemical processes contain more than one type of reaction; successive reactions are required to occur to produce a target product. A classical example is alkane isomerization, in which the hydrogenation/dehydrogenation function and the acidic function operate successively. Another example is aldol condensation, which results from successive reactions of aldol addition and dehydration. Two functions are required to complete the reaction. There should be many cases in which two or more than two functions are required, such as acid–base, acid–redox, and acid–metal. Since the location of different active sites on the surface is possible for solid acid catalysts, the design of such multifunctional catalysts is promising. (4) Exploration of the reactions over solid Lewis acid catalysts and preparation of solid Lewis acid catalysts: In most of the reactions studied with solid acid catalysts so far, active sites are Brønsted acid sites. Lewis acid–catalyzed reactions have not been studied extensively. The existing solid Lewis acid catalysts are mostly of types of mounted-Lewis acids, such as AlCl3, BF3, FeCl3, SbF5, and SnCl4. These suffer from the leaching of the acidic components during the reaction. Recently, metal cations such as Sn, Zr, and Ti incorporated in the structures of mesoporous silica or beta zeolites have shown to serve as Lewis acid centers. Catalysis by solid Lewis acid is an area of research that remains largely unexplored and that offers many opportunities to discover novel chemistry. (5) Further application of solid acid catalysts to fine chemicals synthesis: The application of solid acid catalysts to fine chemicals synthesis is still limited and should be explored more extensively in future. Chemical transformations of biomass-derived compounds and reactions pertinent to the synthesis of pharmaceuticals and fragrances are the area of increasing importance. For this purpose, solvent-free reactions, reactions in water (or water-tolerant catalysts), and one-pot reactions should be explored. Selected industrial processes and important developments of solid acid catalysts are summarized in Table 1.2.

17

18

Introduction

Table 1.2

Year

Selected industrial processes and important developments of solid acid catalysis in chronological order

Process

Catalyst

Company

1935 Isooctane production

Ni on solid UOP phosphoric acid

1937 Cracking

Acid-treated Bentonite clay

1941 Fluid catalytic cracking FCC

SiO2–Al2O3

1942 Catalytic cracking with moving bed reactor

SiO2–Al2O3

Houdry process, SoconyVacuum Oil, Sun Oil

Remarks

The first commercial process using heterogeneous catalyst in the petrochemistry

Standard Oil SoconyVacuum Oil

1945 Alkylation of Solid UOP benzene with phosphoric acid propene to cumene 1947 Hydration of ethylene

1949 Reforming of naphtha

1949 Synthesis of A and X zeolites 1954 Synthesis of Y zeolite

1958 Isomerization of alkane C5/C6 (light naphtha)

Solid Shell Oil phosphoric acid Pt/Al2O3–F

Pt/Al2O3–Cl

Platforming UOP

Union Carbide, by Milton Union Carbide, by Breck Penex, UOP

1959 Hydrocracking

Pd/Mg–Y

Unocal and First process Union Carbide using zeolite

1968 Reforming of naphtha (hydrocracking of light paraffins)

Erionite

Selectforming, First process Mobil utilizing shape selectivity of zeolite

1962 Cracking using zeolite

1969 Toluene disproportionation

X, Y

Mordenite

Mobil

Tatoray, Totay/ UOP

Future Outlook

Year Process 1970 Isomerization of alkanes (light naphtha)

Catalyst Pt/mordenite

1973 Etherification of isobutene with methanol to MTBE

Amberlyst15

1972 Synthesis of ZSM-5

Company Hysomer, Shell Mobil

1972 Reforming of naphtha

ZSM-5

1974 Xylene isomerization

ZSM-5

1975 Toluene disproportionation

ZSM-5

MTDP, ExxonMobil

1980 Aromatization of C3~C4 olefins

Ga–ZSM-5

Cyclar process, BP/UOP

1981 Hydration of isobutene

Ion-exchange resin

Mitsui Petro. Chem.

1985 Amination of isobutene with ammonia to t-butylamine

ZSM-5

BASF

1986 Selective cracking of linear alkanes to propene and butenes

ZSM-5

Mobil

1988 Methanol to olefins (MTO) 1988 Toluene disproportionation

SAPO-34

1989 p-Diethylbenzene from ethylbenzene

ZSM-5

UOP and Norsk Hydro

CVD-ZSM-5

Taiwan Styrene Monomer

1974 Dewaxing (cracking ZSM-5 of linear alkanes) 1977 MTG (methanol to gasoline) 1980 Alkylation for ethylbenzene production in gas phase

ZSM-5

ZSM-5

Mordenite 1985 Amination of methanol to monoand di-methylamine

Remarks

M-Forming, Mobil

Snamprogetti WCTR Mobil Mobil Mobil

Mobil-Badger

Nitto Chem.

MSTDP, Mobil

(Continued)

19

20

Introduction

Table 1.2   (Continued) Year

Process

1990 Liquid phase alkylation for ethylbenzene production

1990 Hydration of cyclohexene to cyclohexanol

Catalyst

Company

Y zeolite

Lummus, Unocol, UOP

ZSM-5

Asahi Kasei

F-SiO2–Al2O3 1992 Alkylation of benzene with linear alkenes

UOP

Modified ZSM-5 a-process 1993 Production of Asahi Kasei aromatics from light naphtha 1994 Alkylation of isobutane with alkenes (Refinery alkylation)

CF3SO3H/SiO2

Haldor TopsoeKellog

BF3/Al2O3

1995 Alkylation of benzene with ethylene to ethylbenzene in liquid phase

MCM-22

Neste, Oy, Conoco, Catalytica

1994 Alkylation of isobutane with alkenes (Refinery alkylation)

MCM-22 1995 Alkylation of benzene with propene to cumene in liquid phase

β-zeolite 1996 Alkylation of benzene with propene to cumene in liquid phase 1996 Isomerization of alkanes (light naphtha)

1996 Acylation of anisole with acetic anhydride

Pt/ZrO2–SO4 Y zeolite

Mobil-Badger EBMax, Mobil Mobil-Badger ENICHEM Par-Isom Process, UOP Rhodia

Remarks

References

Year

Process

Catalyst

Company

1996 Toluene disproportionation

ZSM-5 modified PxMax, ExxonMobil with SiO2 and Pt

1997 Alkylation of ethylbenzene to p-diethylbenzene

CVD-ZSM-5

1999 Ethyl acetate from ethylene and acetic acid

H4SiW12O40/ SiO2

1997 Acetic acid by ethylene oxidation in vapor phase

Pd/ H4SiW12O40/ SiO2

1998 p-Xylene production ZSM-5 from toluene and C9 aromatics

2000 Ring opening polymerization of THF

ZrO2/SiO2

2002 Bisphenol A from Ion-exchange phenol and acetone resin

Showa Denko Paschim/IPCL

Remarks

The first heterogeneous Wacker process

TransPlus, ExxonMobil

Showa Denko Mitsubishi Chemical Mitsubishi Chemical

Rare earth– Nippon 2003 Diethanolamine from ethylene oxide modified ZSM-5 Shokubai and ammonia 2003 Beckmann rearrangement of cyclohexanone oxime to e-caprolactam

High silica ZSM-5

Sumitomo Chemical

References

1. K. Kobayashi, J. Ind. Eng. Chem., 4, 891 (1912). 2. C. F. Thiele, Petroleum Age, 7, 45 (1920).

3. A. G. Oblad, in Heterogeneous Catalysis, Selected American Histories (B. H. Davis, W. P. Hettinger, Jr., ed.), ACS Symposium Series 222, American Chemical Society, p. 61, 1983.

4. J. A. Rabo, M. W. Schoonover, Appl. Catal. A, 222, 261 (2001).

5. C. J. Plank, E. J. Rosinski, W. F. Hawthorne, Ind. Eng. Chem. Prod. Res. Dev., 3, 165 (1964).

21

22

Introduction

6. BP Statistical Review of World Energy 2013, http://www.bp.com/en/ global/corporate/about-bp/statistical-review-of-world-energy-2013. html.

7. F. C. Whitmore, J. Am. Chem. Soc., 54, 3274 (1932); Chem. Eng. News, 26, 668 (1948). 8. V. N. Ipatieff, V. I. Komarewsky, Ind. Eng. Chem., 29, 958 (1937).

9. H. Pines, in Heterogeneous Catalysis, Selected American Histories (B. H. Davis, W. P. Hettinger, Jr., ed.), ACS Symposium Series 222, American Chemical Society, p. 23, 1983.

10. P. B. Weiz, V. J. Frilette, J. Phys. Chem., 64, 382 (1962).

11. K. Tanabe, Solid Acids and Bases, Kodansha-Academic Press, 1970.

12. K. Tanabe, M. Misono, Y. Ono, H. Hattori, New Solid Acids and Bases, Kodansha-Elsevier, 1989. 13. R. J. Argauer, G. R. Landolt, US Patent, 3702886, 1972.

14. D. Chang, A. L. Silvestri, J. Catal., 47, 249 (1977).

15. S. T. Wilson, B. M. Lok, C. A. Messina, T. R. Cannan, E. M. Flanigen, J. Am. Chem. Soc., 104, 1146 (1982). 16. M. Hino, S. Kobayashi, K. Arata, J. Am. Chem. Soc., 101, 6439 (1979).

Chapter 2

Solid Acid Catalysis

2.1  Definition of Acid and Base 2.1.1  Brønsted Acid and Lewis Acid There are many kinds of definitions for acids and bases in homogeneous phase [1]. Among them, the definitions by Brønsted– Lowley and Lewis are the most important in relation to the acid– base chemistry of solid surfaces. In the definition by Brønsted and Lowley, an acid (AH) donates a proton and a base (B–) accepts a proton.



AH + B– → A – + BH

(2.1)

In the reverse reaction, BH is an acid and A– is a base. The species AH and A– are referred to as conjugate acid–base pairs. Similarly, BH and B– are also conjugate pairs. The acids and bases in accordance with this definition are called a Brønsted acid and a Brønsted base, respectively. In the definition by Lewis, a base :B donates a lone pair and an acid (A) accepts a lone pair.

Solid Acid Catalysis: From Fundamentals to Applications Hideshi Hattori and Yoshio Ono Copyright © 2015 Pan Stanford Publishing Pte. Ltd. ISBN  978-981-4463-28-7 (Hardcover), 978-981-4463-29-4 (eBook) www.panstanford.com

24

Solid Acid Catalysis



A+ 　 :B  A:B A + :B  A : B

(2.2)



BF3 + :NH3  F3B:NH3

(2.3)

The acids and bases in accordance with this definition are called a Lewis acid and a Lewis base, respectively. In a following reaction, NH3 is a Lewis base and BF3 is a Lewis acid:

A base molecule such as ammonia can be a Brønsted base (formation of N​H+4​ ​)​  or a Lewis base (formation of F3B:NH3), depending on the nature of the combining acid molecule. In 1963, Pearson classified Lewis acids into three categories: hard, soft, and borderline based on the various chemical facts such as reaction rates and chemical equilibrium [2, 3]. Similarly, Lewis bases are also classified into three categories, hard, soft, and borderline. Hard acids are small in size, of high positive charge and do not contain unshared pairs of electrons in their valence bond. They have high electronegativity and low polarizability. Soft acids generally have acceptor atoms large in size, of low positive charge, and containing unshared pairs of electrons. Hard acids include H+, Na+, Mg2+, Ti4+, and AlCl3, whereas soft acids include Ag+, Pd2+, and GaCl3. Examples of borderline acids are Fe3+, Cu2+, and Zn2+. Pearson formulated the rule saying that hard acids prefer to bind to hard bases and soft acids prefer to bind to soft bases (HSAB principle). Although the HSAB principle is qualitative, there have been proposals to find underlying theoretical reasons [3].

2.2  Acid Sites on Surfaces

The definition of Brønsted acid and Lewis acid can be applied to solid acids. When a surface site has a property of proton donation, the site is called Brønsted acid site. Similarly, when a surface site has a property of electron pair acceptor, the site is called Lewis acid site. The solids having the sites that serve as a Brønsted acid or a Lewis acid are called solid acids. Accordingly, the solids that have the sites that serve as a Brønsted base or a Lewis base are called solid bases. Obviously, a very same site can work as a proton acceptor (Brønsted base) and also as an electron pair donor (Lewis base).

Acid Sites on Surfaces

Here, the origin of acidic sites on solid surfaces is briefly discussed. The acidic and catalytic properties of representative solid acid materials are described in more detail in Chapter 4.

2.2.1  Origin of Brønsted Acid Sites

The simplest type of Brønsted acid is the acid in the solid form. A typical one is heteropolyacids such as H3PW12O40. Heteropolyacids can be used also in the supported form (see Section 4.4). Sulfuric acid and benzenesulfonic acids are also used in the supported form. Solid phosphoric acid entrapped in silica is an important industrial catalyst (see Section 4.11). The acid may be present in its condensed form such as pyrophosphoric acid in the working conditions. HClO4 supported on silica is often used for various organic syntheses. Typical cation exchange resin is the styrene-divinylbenzene copolymer in which phenyl groups are sulfonated. Thus, acidic sites are in the form of benzenesulfonic acid. The Nafion resin is a copolymer of tetrafluoroethylene and perfluoro-2-(fluorosulfonylethoxy)propyl vinyl ether. After the hydrolysis of the fluorosulfonyl group, it yields the strongly acidic terminal –CF2CF2SO3H group. The resin has much higher acid strength compared with ordinary cation exchange resins. The drawback of the Nafion resin is the very low surface area (0.02 m2 g–1). In order to overcome this drawback of the material, a Nafion resin–silica nanocomposite with high surface area (200 m2 g–1) has been developed. In the nanocomposite, nanometersized Nafion resin particles are entrapped within a highly porous silica network, which significantly improves the accessibility to the acid groups. More details on cation exchange resins, perfluorinated resin, and Nafion-silica composite materials are given in Section 4.10. Sulfonic acid–type Brønsted acids can be introduced on the surfaces of solids such as silica gel and mesoporous silica. The methods of tethering sulfo groups to the surfaces of mesoporous silica are described in Section 4.3.2.

O O O

Si

SO3H

25

Solid Acid Catalysis

Amorphous carbon material bearing sulfo groups is very active solid acids [4]. The material can be prepared by the partial carbonization of natural organic compounds such as sugar, cellulose, and starch, followed by the sulfonation of the resulting amorphous carbon. It can incorporate large amounts of hydrophilic molecules, including water, into the carbon bulk and are active for many reactions, including the hydrolysis of cellulose. Acidic hydroxyl groups exist on the surface of certain metal oxides such as Nb2O5, WO3, and Al2O3. The hydroxyl groups on metal oxide surfaces exist as the residual hydroxyl group after the dehydration of metal hydroxide or they are formed by the reaction of metal oxide with water. Thus, the number of acidic hydroxyl groups depends largely on the calcination temperature of the oxides. Metal oxides can often be used in supported form. The supports most often used are silica gel, alumina, titania, and zirconia. By loading on the support, the surface area can be increased and at the same time the acidic properties can be greatly modified. The acidic properties are also dependent on the calcination temperature, supported amount, etc. Tungsten oxide supported on zirconia is known to have very high acid strength (see Section 4.9.4). Mixed oxides also have acidic OH groups. In the case of SiO2– Al2O3, Thomas proposed a model shown in Fig. 2.1 [5]. Here, in the SiO2-rich mixed oxides, Si4+ cations in SiO2 are isomorphously replaced by Al3+ cations. To compensate charge imbalance by the incorporation of Al3+, the proton remains on the surface. Actually, the protons are attached to oxygen anions and exist in the form of hydroxyl groups. Si

O

Si

O

Si

O

O

Figure 2.1

O

Si

O

——

26

O

Al

O

Si

H+

Si

O

O

Si

O

Si

O

A model for the formation of a Brønsted acid site on SiO2–Al2O3 surface. Reprinted with permission from C. L. Thomas, Ind. Eng., Chem., 41, 2564 (1949). Copyright (1949) American Chemical Society.

Acid Sites on Surfaces

Zeolites are an inorganic cation exchanger and a kind of aluminosilicates and the origin of Brønsted acidity is also explained by the model similar to the model by Thomas for SiO2–Al2O3 (see Section 4.1). This type of Brønsted acid sites formation can be applied to aluminophosphate-type molecular sieves (see Section 4.2) and natural clays such as montmorillonite (see Section 4.3). In zeolites or in the metal salts of heteropolyacids, water can be dissociated by metal cations.

( n – 1)+

Mn+ + H2O  M(OH)

+ H+

(2.4)



Here, Mn+ is multivalent metal cations such as Mg2+, Al3+, and La3+. The protons thus formed are trapped by oxygen anions on the surface to generate acidic hydroxyl groups. The reduction of metal cations on surfaces with hydrogen gives protons. Mn+ +

n H  M + nH+ 2 2



(2.5) Here, Mn+ is metal cations such as Cu2+ and Ag+. This type of acidsite generation is observed in Cu2+ and Ag+-ion exchanged zeolites and Ag salt of heteropolyacids [6–9]. In the case of transition metals (Pt, Pd) supported on heteropolyacids or sulfated zirconia, hydrogen is dissociated and the spilt-over hydrogen is turned into protons. For example, in the case of Pd supported on heteropolyacids, the reaction is expressed as follows [7–9]:



Pd  H2  2H



3–

4–

 PW12O40    H+ +  PW12O40   H +

(2.6)

(2.7)

Thus, number of acid sites is dependent on the hydrogen partial pressure and the hydrogen effect is often reversible. Transition metals supported on sulfated zirconia and tungsten oxides supported on zirconia are highly active catalysts for alkane isomerization, which is operated under hydrogen pressure (see Sections 4.7.2, 4.9.4, 5.4.3, and 5.4.4). Brønsted acid sites are formed from spilt-over hydrogen. Some of metal organic frameworks have OH groups, which work as Brønsted acid sites [10].

27

28

Solid Acid Catalysis

2.2.2  Origin of Lewis Acid Sites Surface Lewis acid sites are usually with coordinatively unsaturated metal cations. In the case of metal oxides, when surface hydroxyl groups or adsorbed carbon dioxide are eliminated by hightemperature calcination or by heating under a vacuum, metal ions are exposed and serve as Lewis acid sites. Metal cations (Al, Sn, Ti) serve as Lewis acid sites. These metal cations can be incorporated in the structure of zeolites or mesoporous silica materials. For example, Sn-containing beta zeolite is active for the isomerization of citronellal to isopulegol (see Section 2.4.3) [11]. Pentavalent ions, Nb5+ and Ta5+, also work as Lewis acid sites [12]. XANES and EXAFS studies suggest the structure of Lewis acid centers (Nb5+) in beta zeolite as follows. The Nb5+ ions can coordinate a water molecule, inducing an expanding the coordination shell, from tetravalent to pentacoordination.



 

Lewis acid sites can be introduced by supporting metal compounds on the surface. For example, the reaction of metal (Al, Zr) alkoxides or chlorides (Al, Sn) with OH groups of mesoporous silica results in the formation of exposed metal cations on the surface [13–15]. These metal cations work as efficient catalysts for the Meerwein–Ponndorf–Verley reduction. The acylation of naphthalene with p-toluoyl chloride over metal triflates immobilized in SBA-15 has also been reported [16]. Metal centers in metal-organic frameworks play the role of Lewis acid centers in catalytic reactions. For example, CuII ions in [Cu3(BTC)2] (BTC = benzene-1,3,5-tricarbonate) is active for various Lewis acid–catalyzed reactions such as the cyclization of citronellal to isopulegol and the cyanosilylation of benzaldehyde and aldol synthesis in liquid phase [17–19].

Acid Sites on Surfaces

2.2.3  Identification of Brønsted and Lewis Acid Sites The presence of acid sites can be evidenced by the adsorption of basic molecules such as ammonia and amines. The amount of chemisorbed basic molecules is a measure of the number of acid sites. The amount of adsorption, however, does not tell whether the acid sites are Brønsted-type or Lewis-type in nature. In order to know the reaction mechanism, it is essential to clarify whether the active sites are Brønsted or Lewis acid sites. Spectroscopic measurements of adsorbed base molecules offer the convenient way for distinguishing Brønsted-type and Lewis acid sites [20, 21]. Infrared spectrum of adsorbed pyridine is most often used. Pyridine reacts with acidic OH groups to form pyridinium ion, which shows the band at 1540–1550 cm–1 and at the same time, the band due to acidic OH groups disappears. Pyridine also reacts with Lewis acid sites to show the bands at 1440–1445 cm–1. Thus, the appearance of these bands gives a firm evidence for the acid sites of the respective types. 2,6-Dimethylpyridine also reacts with Brønsted acid sites to give the corresponding pyridinium ion but does not react with Lewis acid sites. Infrared spectra of ammonia also identify Brønsted and Lewis acid sites. The adsorption of carbon monoxide on metal cations gives rise to the appearance of the band at 2150–2240 cm–1. The band position can be a measure of the acid strength as a Lewis acid. The adsorption of CD3CN is also used for discriminating Brønsted acid and Lewis acid sites. Brønsted and Lewis acid sites can be also discriminated by 31P MAS NMR and CP/MAS NMR using trialkylphosphines and trialkylphosphine oxides as probe molecules. Trimethylphosphine interacts with Brønsted acid sites in Y-zeolite to form [(CH3)3PH]+ adducts that give 31P chemical shifts at ca. 3 ppm and a JP–H coupling constant of ~550 Hz. On the other hand, 31P atom of the trimethylphosphine-Lewis acid site adduct appears at considerably higher fields, –32 to –58 ppm. Direct quantification of Lewis acid sites can be carried out by monitoring 31P MAS NMR of the adsorption of the more stable trimethylphosphine oxide (see Section 3.5). Various test reactions can be used for distinguishing Brønsted and Lewis acid sites (see Section 3.6). For example, the reaction of

29

30

Solid Acid Catalysis

ethylene acetal of 2-bromopropiophenone provides information on the relative population of Brønsted and Lewis acid sites and on the hardness and softness of the Lewis acid sites.

2.3  Acid Strength

2.3.1  Definition of H0 Acidity Function in Homogeneous Phase Discussion on the solid acids in many cases is based on the concept developed for acid–base chemistry in solutions. It is appropriate to briefly introduce the definition of H0 acidity function, one of the most important concepts for the acidity of solutions. In 1932, Hammett and Deyrup suggested that a convenient measure of acidity would be provided by determination of the extent of protonation of neutral basic indicators in acidic solutions [22].

B + H+  BH+



(2.8)

Hammett and Deyrup defined an acidity function H0 by Eq. (2.9) and proposed that H0 could be taken as a quantitative measure of the acidity of the solution.

C +  H0 = pK BH+ – log10 BH   CB 

(2.9)

Here, pKBH+ is the acid dissociation constant of BH+, which is the conjugate acid of B. To determine H0 value of a solution, the concentrations of B and BH+ have to be measured accurately. When a half of a solute B is protonated in the solution, i.e., [B] = [BH+], the H0 value of the solution is equal to the pKa value of BH+. The acid strength of solutions is determined by using indicator molecules, B, with known pKBH+ values. Usually, the concentration ratio, [BH+]/[B] is determined spectrophotometrically. In this way, the H0 values of various acidic solutions have been determined [23]. It should be noted that decreasing values of H0 indicates increasing acid strength. The H0 values of typical acidic solutions are listed in Table 2.1.

Acid Strength

Table 2.1

Hammett acidity function (H0) of acidic solutions

0.15M H3PW12O40 in water

0.25M H3PW12O40 in water

0.20M H3PW12O40 in CH3COOH

30% H2SO4 60% H2SO4 96% H2SO4 100% H2SO4 HClO4 CF3SO3H HSO3F SbF5(10 mol%)-HSO3F SbF5(25 mol%)-HSO3F HSO3F-SbO5 (1:1) HF-SbF5(1:1)

–0.15 –0.50

–1.41

–1.82 –4.51 –9.88

–11.94 –13 –14.1 –15.07 –18.94 –21.0 –23 –28

It is most important to note that the acidity function H0 represents the properties of “solutions.” The acidity functions are not a property of individual molecules, such as H2SO4 and CH3COOH. Obviously, the H0 values are solvent dependent.

2.3.2  H0 Scale of Acidic Sites on Solid Surfaces

The concept of the H0 acidity function is applied to estimate the acid strength of acidic sites of solid surfaces [24–26]. Suppose that molecules of an indicator, B, interact with acidic sites AH+ on the surface.

B + AH+ → BH+ + A The H0 value of the acidic sites, AH+, is defined as  [BH+ ]s   H0 = pK BH+ –log   [B]s 

(2.10) (2.11)

Here, [B]s and [BH+]s are the surface concentration of B and BH+, respectively. The value of H0 is independent of any particular indicator. Upon adsorption of an indicator B from the solution, the solid surface exhibits the color of BH+ when [BH+]s/[B]s >> 1, whereas it shows the color of B when [BH+]s/[B–]s Secondary (–18) > Primary (0)

Thus, tertiary carbenium ions are most easily formed and the reactions involving formation of primary carbenium ions are slow. The primary carbenium ions tend to transform into secondary or tertiary carbenium ions.

2.4.1.2  Formation of carbenium ions

Carbenium ions, key intermediates in hydrocarbon chemistry, can be generated by several different routes by the interaction of hydrocarbons with Brønsted acid sites of solid acids. (i)  Addition of proton to alkene +



H2C=CHCH3 + H+  CH3–CHCH3



H2C=CHCH3 + H+  H2 C–CH2CH3

(2.13)

Because of the stability difference between primary and secondary carbenium ions, the formation of primary propyl cation hardly occurs. +

(2.14)

(ii)  Addition of proton to aromatics



(2.15)

The addition of protons to alkylaromatics is the elementary step of the isomerization of alkylaromatics such as xylenes.

37

38

Solid Acid Catalysis

(iii)  Reaction of alkanes with protons

Carbenium ions are generated through carbonium ions, which are formed by the reaction of protons with C–C or C–H bonds of alkanes. For example, butane reacts with proton to form three kinds of carbenium ions. In each case, a two-electron, three-center bond (carbonium ions) is considered to be an intermediate.

n—C4H10 + H+



[  [  ​​

​​

CH3CH2CH2 CH3CH2

[ 

H  

CH4 + C3​H+7​ ​​ 

​​ ​

CH2CH3 + H

  ​​ ​

]

CH3CHCH2CH3 +

  ​​ ​

​​ H  H 



] ]

CH3 +

C2H6 + C2​H+5​ ​​  H2 + C4​H+9​ ​​ 

(2.16)

(iv)  Abstraction of hydride ions from alkanes by Lewis acid sites, L.

(CH3 )3 CH + L → (CH3 )3 C+ + H–L

(2.17)

2.4.1.3  Reactions of carbenium ions

Carbenium ions are involved in the elementary steps of the hydrocarbon transformation. Following are some of typical reactions of carbenium ions: (i)  Intramolecular hydrogen shift

+



+  CH3CH2CH2 CHCH2CH2CH3 (2.18)

(ii)  Intramolecular methyl shift

+

CH3 CHCH2CH2CH2CH2CH3  CH3 CH2 CHCH2CH2CH2CH3

CH3

CH3CHCHCH2CH3 +

CH3

CH3CHCHCH2CH3 +

(2.19)

Role of Acid Sites in Catalysis



+ CH3CH2CHCH2CH3

+ CH3CHCHCH3 CH3

(2.20)

The intramolecular methyl shift on aromatic ring occurs as shown in reaction 2.15. The intramolecular methyl shift of carbenium ions is a key step in the skeletal isomerization of alkenes and xylenes. In addition to the methyl shift, the shift of ethyl group is also known. (iii)  Ring transformation



(iv)  Addition to alkenes

+ CH3CHCH3 + H2C=CHCH3



(2.21)

+ CH3CHCH2CHCH3 (2.22) CH3

The addition of carbenium ions to alkenes is the key step in the oligomerization of alkenes (v)  Addition to aromatics





(2.23)

The addition of carbenium ions to aromatic hydrocarbons is the basis for alkylation of aromatics with alkenes or alcohols, since carbenium ions are easily formed from alkenes and alcohols.

39

40

Solid Acid Catalysis

(vi)  Hydride transfer reaction CH3

CH3CCH2CH3  +  CH3CH2CH2CH2CH3 + CH3

+ CH3CHCH2CH3  +  CH3CH2CHCH2CH3

(2.24) The hydride transfer offers a facile route to convert a neutral molecule to a carbenium ion, and thus important step for alkane cracking. The successive hydride transfer of alkene to carbenium ion results in the formation of aromatic compounds. Thus, propene can be transformed into benzene in the following scheme.

(vii)  Cracking (b-scission)

CH3 +   RCH2CHCH2CHCH2R¢

(2.25)

+ RCH2CH=CH2 + CH3CHCH2CR¢ (2.26)

This reaction is the reverse of reaction 2.22 and the key reaction in the cracking of hydrocarbons. The C–C bond scission occurs at the bond located b to the carbon atom bearing the positive charge.

Role of Acid Sites in Catalysis

Since primary carbenium ions are not easily formed, methane, ethane and ethylene are not formed by this mechanism.

2.4.1.4  Protonated cyclopropane intermediates

As described above, skeletal isomerization of hydrocarbons can be explained by the conventional carbenium ion mechanism. In this mechanism, the isomerization of 2-methylpentane to 3methylpentane involves the methyl shift of carbenium ion

H3C

H C–C–C–C–C + H

H3C

CH3 CH3~ H H C–C–C–C–C C–C–C–C–C + H H +

CH3 H C–C–C–C–C H +

(2.27)

This mechanism involves the transformation of tertiary carbenium ions to secondary carbenium ions. Actually, the activation energy of this reaction is not much different from the value expected from the stability difference of the secondary and tertiary carbenium ions. On the other hand, if a similar mechanism is operative in the isomerization of 2,3-dimethylbutane into 2-methylpentane, the reaction could be expressed as follows: C C

H3C–C–C–C + H

C C

H2C–C–C–C + HH

C HH C–C–C–C–C H   + H

C HH C–C–C–C–C H   H +

(2.28)

Here, tertiary carbenium ions have to be converted into the primary carbenium ions in the pathway. The activation energy observed is smaller than the value expected from the energy difference between primary and tertiary carbenium ions. Therefore, the real intermediate has to be energetically more stable than the primary carbenium ions. Thus, Brouwer and coworkers introduced protonated cyclopropane intermediates instead of primary carbenium ions [41].

(2.29)

41

42

Solid Acid Catalysis

Thus, the merit of this intermediate is to explain the methylshift without assuming the presence of primary carbenium ions [41, 42]. The protonated cyclopropane mechanism can be applied to explain the slow rate of the isomerization of butane to isobutane. As shown below, the reaction has to proceed through the primary carbenium ion as the intermediate even if the protonated cyclopropane is the intermediate.

(2.30)

On the other hand, butane-1-13C rapidly isomerizes to butane-2-13C.

CH3CH2CH213CH3 → CH3CH213CH2CH3

(2.31)

As shown in the above scheme, this isomerization can proceed through protonated cyclopropane intermediate but does not require the intermediacy of the primary carbenium ion. This mechanism explains also the facile isomerization of pentane to isopentane. In the case of C5 cations, protonated cyclopropane-type cation is much more stable (ca. 14 kcal mol–1) than the primary carbenium ion and slightly less stable (ca. 3 kcal mol–1) than the secondary carbenium ion. Protonated cyclopropane intermediates are also postulated in the acid-catalyzed cracking of alkanes [43].

2.4.1.5  Carbenium ions as transition state

Most of hydrocarbon conversions over solid acid catalysts are accounted for by the carbenium ion reactions described above. However, carbenium ions on surfaces have not been detected by spectroscopic methods except very stable ions such as triphenylcarbenium ion. Reaction products differ depending on

Role of Acid Sites in Catalysis

solid acids used. For example, 1-butene gives 2-butene in one case, but isobutene in another case. Furthermore, when the reaction of 1-butene was performed over ZSM-5 zeolite ([Al]-ZSM-5) and [B]-ZSM-5, which contains boron instead of aluminum in the framework, the reaction products over the two catalysts at 773 K differ markedly, as shown in Table 2.4. In the case of [B]-ZSM-5, main products are lower alkenes, which are expected from oligomerization-cracking mechanism. On the other hand, the reaction products mainly consist of lower alkanes and aromatics (mainly, benzene, toluene and xylene) over [Al]-ZSM-5. This indicates that the extensive hydrogen transfer reactions occur (reaction 2.25). Since the first carbenium ion formed from 1-butene is 2-butyl cation in both cases, exact behavior of the carbenium ions greatly depends on the kind of solid acid, and carbenium ion intermediates may not be free from the catalyst surface. Table 2.4

Product distribution in the conversion of 1-butene over H-[Al]-ZSM-5 and H-[B]-ZSM-5 H-[Al]-ZSM-5

H-[B]-ZSM-5

Conversion/%

92.6

68.4

C2H4 + C3H6

16.5

47.1

Products (carbon base)/% CH4 + C2H6 + C3H8 C4H8

C4H10

}

C5+ aliphatics

Aromatics

31.5 10.4 3.0

38.6

1.6

31.8 3.1

14.3 2.3

Note: Reaction conditions: For H-[Al]-ZSM-5, 773 K. 1-butene –23 kPa; for H-[B]-ZSM5, 823 K, 16.5 kPa.

Kazansky and coworkers made quantum chemical calculations on the adsorbed state of ethylene on the zeolite surface [44, 45]. Figure 2.2 shows the interaction of ethylene with bridging OH groups in high silica zeolite. Thus, Figs. 2.2(a)-(c) show the p-complex of ethylene, the transition state of adsorption, and the structure of the final state, respectively. The energy diagram of the adsorption process is shown in Fig. 2.3. At first, ethylene is adsorbed as a p-complex (Fig. 2.2(c)). No considerable change in the C2H4 molecular geometry is found after adsorption. The most stable

43

44

Solid Acid Catalysis

structure is a covalent ethoxy group (Fig. 2.2(c)). The positive charge of the ethyl fragment is only +0.384e typical for covalent organic compounds. Figure 2.3b corresponds to the transition state of adsorption. The geometry and the electronic structure of the C2H5 fragment is very similar to those of the classical form of the ethyl cation; the positive charging of the C2H5 group increases to 0.565e and the C–C bond length is in between those of a double bond and a single bond. It follows that “carbenium ions” exist only as a transition state and that many hydrocarbon reactions go through the carbenium ion-like transition states, but not through stable carbenium ions.

Figure 2.2

The structures of the quantum chemically calculated intermediates in ethoxylation of the zeolite surface: (a) pcomplex of ethylene; (b) the transition state of the ethoxylation reaction; (c) the final ethoxide structure. transition state

8.51 cluster + ethylene

15.41

6.90 10.97

19.48

surface -complex surface alkoxide

Figure 2.3

The energy diagram (kcal mol–1) of surface ethoxy group formation. Reprinted with permission from V. Kazanski, Acc. Chem. Res., 24, 379 (1991).

Role of Acid Sites in Catalysis

As described in reaction 2.16, the cracking of alkanes such as butane occurs via carbonium ion intermediates. The transition state of butane cracking over acidic zeolite is predicted by quantum chemical calculation [46]. Here, butane is protonated to result in the elongation of the C–C bond and undergoes cracking to form ethane and s-bonded carbenium ions. It should be noted that the surface oxygen anion is involved in the reaction besides acidic OH groups.

2.4.2  Reactions of Alcohols and Carbonyl Compounds over Brønsted Acid Sites Alcohol reacts with the strong Brønsted acid site to form oxonium ions, which in turn afford carbenium ions by dehydration.

ROH + H+

+ ROH2

R+ + H2O



(2.32)

1-Butanol reacts over strong Brønsted acid sites to form 2-butene (E1 mechanism).

C–C–C–C OH

+ H+

–H2O C–C–C–C C–C–C–C + OH2 +

C–C–C–C +

–H+

C–C=C–C

(2.33)

Carbenium ions formed from alcohols are the intermediates for various reactions such as alkylation and Ritter reactions. t-Butyl alcohol reacts with phenol to give 2-t-butylphenol and 2, 6-di-tbutylphenol, and with nitrile to give amide.

45

46

Solid Acid Catalysis

(Alkylation) (CH3)3COH

+H+   

(Ritter reaction) (CH3)3C+



+CH3CN

+ –H2O (CH3)3COH2

(CH3)3C+

2,6-di-t-BuPhOH

  CH –C 3

+

N–C(CH3)3 

+H2O –H+

+PhOH 2-t-butylphenol –H+

(2.34)

CH3CO–NH–C(CH3)3

(2.35)

Aldehydes (or ketone) give their enol form by their reaction with proton. R4

R4

R3—C—C—R5

R3—C—C—R5 +O

O H H

H

R3—C

H

OH

– A—B—A

A—B—A

R4

C—R5

H

A—B—A

The addition of another molecule of aldehyde (or ketone) to the enol leads to aldol condensation. R3—C

:OH

H

R4

C—R5

A—B—A

R1

R2 C +d O–d H

A—B—A

R4

R1

R3—C—C—C—R2 OH O R5 H

A—B—A

H

A—B—A

Protons attack the carbonyl oxygen of esters. This step is the first step in esterification and transesterification reactions. The proposed reaction scheme for the esterification of isoamylalcohol with acetic acid over H4SiW12O40/ZrO2 immobilized on SBA-15 is shown in Fig. 2.4 [47].

Role of Acid Sites in Catalysis

+ H

O C

H3C

O 

O +

 H

H3C

C

 H  H +  R 

O 

H3C

 OH

+ H 

 

+H3C 

 C 

 O 

 R

Where, R:CH2CH2CH(CH)2 and + H : acid site on catalyst

–H3O+

+H3O+

H3C  R

+O   C

O

 H

H 

 O

H

 O

C 

R 

O

 H

O 

H3C

O +

H

O 

 H

O

H

C 

R

 H

 O +

 H

Figure 2.4

Proposed reaction scheme for the esterification of isoamylalcohol by acetic acid over solid acids. Reprinted with permission from D. P. Sawant, A. Vinu, S. P. Mirajkat, R. Lefebvre, K. Ariga, S. Anandan, T. Mori, C. Nishimura, S. B. Halligudi, J. Mol. Catal., A, 271, 46 (2007).

Figure 2.5

Proposed reaction scheme for the transesterification of methyl salicylate and phenol over solid acids. Reprinted with permission from S. Z. M. Shamshuddin, N. Nagaraju, J. Mol. Catal., A., 273, 55 (2007).

47

48

Solid Acid Catalysis

CH3COOH + (CH3 )2CHCH2CH2OH CH3COOCH2CH2CH(CH3 )2 + H2O (2.36)

For the transesterification of methyl salicylate and phenol over modified ZrO2, the reaction scheme shown in Fig. 2.5 is proposed [48].

C6H4(OH)COOCH3 + C6H5OH    C6H4(OH)COOC6H5

2.4.3  Catalytic Action of Lewis Acid Sites

(2.37)

The coordination of carbonyl group to the Lewis acid site induces the polarization of the C=O double bond. The Meerwein–Ponndorf– Verley (MPV) reaction, the reduction of aldehydes and ketones with alcohols, is an important reaction in organic chemistry. Aldehydes or ketones are generally converted to the corresponding alcohols without reduction of any double or triple bonds present in the molecules. Following is the reaction of cinnamaldehyde (3-phenyl3-propenal) with 2-propanol to afford cinnamyl alcohol (3-phenyl2-propen-1-ol):

(2.38)

The MPV reaction (transfer hydrogenation reaction) is known to be catalyzed by metal alkoxides such as aluminum isopropoxide. The reaction of the homogeneous reactions proceeds via cyclic sixmembered transition state in which both the reductant and the oxidant are coordinated to the metal center of a metal alkoxide catalyst (Fig. 2.6). The alcohol reactant is coordinated as an alkoxide. The activation of the carbonyl by coordination to metal centers initiates the hydride transfer reaction from the alcoholate to the carbonyl.

Role of Acid Sites in Catalysis

Figure 2.6

Transition state for MPV reaction over metal alkoxide.

The MPV reactions are also catalyzed by Lewis acid centers on solid surfaces [49–52]. The metal centers can be introduced to the surface by various ways. Metal ions (Al3+, Ti4+, Zr4+, Sn4+, Nb5+, Ta 5+) in the framework of zeolite beta serve as Lewis acid centers. Metal alkoxides can be grafted to silica materials such as silica gel and MCM-41.

Figure 2.7

Proposed MPV reaction scheme starting with an aluminum alkoxide species in zeolite Beta. Reprinted with permission from P. J. Kunkeler, B. J. Zunnrdeeg, J. C. van der Waal, J. A. van Bokhoven, D. C. Koningsberger, H. van Bekkum, J. Catal., 180, 234 (1998).

49



50

Solid Acid Catalysis

Metal-cation containing beta zeolites are highly active and regioselective catalysts for the reduction of 4-t-butylcyclohexanone to the thermodynamically less stable cis-t-butylcyclohexanol [51, 53, 54]. The high selectivity toward the cis-alcohol has been explained by a restricted transition state around a Lewis acidic metal atom in the straight channels of the zeolite beta pore system. In the case of [Al]-beta, the active species for MPV reactions are partially hydrolyzed Al-sites still attached to the zeolite framework, formed by the high temperature treatment. On the other hand, in the case of [Ti]-beta, the active Ti sites are in the zeolite framework. The reaction mechanism on [Al]-beta zeolite are considered to be similar to the one proposed for that in homogeneous phase as shown in Fig. 2.7 [51, 54]. It should be noted that MPV reactions are also catalyzed by solid bases [55]. Zeolite beta containing Sn in the framework catalyzes the Baeyer–Villiger oxidations. For example, cyclohexanone and adamantanone react with hydrogen peroxide to afford the corresponding lactones at room temperature [56].





H2O2

O

H2O2

(2.39)

O

O



(2.40)

The proposed mechanism of this Baeyer–Villiger oxidation is shown in Fig. 2.8. Here, the ketone is coordinated to the Lewis acid center (Sn), and thereby, the carbonyl group is activated. Hydrogen peroxide is activated by an adjacent weakly basic site (oxygen associated to a Sn-OH bond) by a hydrogen bond and attacks the more electrophilic carbonyl carbon atom. After the rearrangement step, the lactone product is replaced by a new substrate molecule. The activation of the carbonyl group with the catalyst is evidenced by infrared spectroscopy. Thus, the band of C=O group shifts toward

Role of Acid Sites in Catalysis

lower wavenumbers by 48 cm–1. The 18O tracer experiment shows that oxygen in the carbonyl group in the reactant ketone remains in the carbonyl group in the product.

Figure 2.8

Baeyer–Villiger oxidation of methylcyclohexanone by Sncontaining beta zeolite. *O represents isotopic 18O.

Scandium ions in montmorillonite serve as active centers in a variety of Michael reactions [57]. For example, the reaction of ethyl 2-oxocyclopentanecarboxylate with 3-buten-2-one affords the addition product in 99% yield in water in the presence of montmorillonite exchanged with Sc3+ ions. The Michael reaction proceeds via the formation of a scandium complex II, in which both the 1,3-dicarbonyl compound and the enone coordinate to the Sc center (Fig. 2.9). Subsequently, the successive C–C bond formation produces an intermediate Sc-alcoholate III, followed by protolysis to afford the Michael adduct together with the regeneration of the original Sc species I. In fact, the treatment of the catalyst with acetylacetone gives an acetylacetonato Sc species as evidenced by infrared spectroscopy.

51

52

Solid Acid Catalysis

Figure 2.9

Michael reaction in Sc3+-exchanged montmorillonite. Reprinted with permission from T. Kawabata, T. Mitsugaki, K. Ebitani, K. Kaneda, J. Am. Chem. Soc., 125, 10486 (2003).

Prins condensation of b-pinene with paraformaldehyde gives nopal in the presence of Fe–Zn double cyanide in high selectivity [58].

OH

+

Paraformaldehyde

b-pinene

Nopal

(2.41)

It is proposed that Zn2+ is the active center for the condensation; the reaction proceeds by the reaction of formaldehyde coordinated to Zn2+ cation with b-pinene. Intramolecular carbonyl-ene reaction of citronellal to isopulegol proceeds over ZnBr2 loaded on mesoporous silica [59]. Beta zeolites with Sn, Zr, Nb, and Ta ions in the framework are also active for this reaction [52, 60, 61]. Isopulegol is a precursor for menthol.



Lewis acid

CHO

rac.-citronellal

catalytic hydrogenation OH

rac.-isopulegol

OH (2.42)

rac.-menthol.

Sn-containing beta zeolite is an effective catalyst for the isomerization of glucose to fructose [62, 63]. The reaction of a

Bifunctional Catalysis

45% glucose solution in the presence of a catalytic amount of [Sn]beta for 6 h at 383 K gave the product distribution of 46% glucose, 29% fructose, and 9% mannose. The reaction mechanism where Sn in the zeolite framework serves as a Lewis acid site is shown in Fig. 2.10, based on the 13C NMR study of the reaction using glucose deuterated at the C-2 position [63]. This reaction also proves that [Sn]-beta can be used in aqueous solutions as a solid Lewis acid.

Figure 2.10 Glucose isomerization mechanism over [Sn]-beta zeolite. Reprinted with permission from Román-Leshkov, M. Davis, ACS Catal., 1, 1566 (2011).

2.5  Bifunctional Catalysis

Some catalytic reactions proceed effectively only when two different types of active centers are present. The catalysts with two different functions are called bifunctional catalysts. In the case of heterogeneous catalysis, bifunctionality is often provided by the combination of an acid site and a basic site or by the combination of an acid site (basic site) and metals supported on the solid acid (or solid base). Supported metals offer hydrogenation and/or dehydrogenation function. From a mechanistic viewpoint, bifunctional catalysis can be divided into two groups. In one group, two functional sites on the catalyst interact with two different positions of the reactant at the same step of the reaction in a concerted manner. In the other groups, two functional sites interact with the reactant(s) or reaction intermediate(s) at the separate steps of the reaction. The following section describes bifunctional catalysis with acidic and base sites and with metals supported on solid acids.

53

54

Solid Acid Catalysis

2.5.1  Bifunctional Catalysis by Acidic and Basic Sites The synergy of acidic and basic sites is one of the characteristics of solid acid–base catalysis. In dehydration of alcohols over solid catalysts having both moderately acidic and moderately basic sites, the reaction proceeds via the E2 mechanism, where the elimination of a proton and a hydroxyl group from alcohols is concerted without the formation of ionic intermediates (see Section 3.6.1). Both acidic and basic centers are required in this mechanism. Lack of but-2-ene or exclusive formation of 1-butene from 1-butanol is an indication of the E2 mechanism. Alumina is a typical E2 oxide. Alumina is the most typical dehydration catalyst, and the extensive studies on dehydration over alumina have been reviewed by Pines and Manassen [64]. In contrast, over strongly acidic catalysts, 1-butanol gives mainly 2-butene via intermediacy of the carbenium ion. From 2butanol, preferential formation of 2-butene (Saytzev orientation) is observed (see Section 3.6.1). Concerted action of an acidic site and a basic site is also seen in the case of SiO2–Al2O3 grafted with amino- or diethylaminogroups (SA–NR2) [65–67]. Here, acidic OH groups on SiO2–Al2O3 are acidic sites, whereas amino groups grafted serve as basic sites. The catalysts are very active for cyano-O-ethoxycarbonylation, the Michael addition, and nitroaldol reaction.

R

O





+ NC

O

+ NC

Cinnamaldehyde

O

SA–NEt2

OEt

O

OEt

O + CH3NO2 Cinnamaldehyde

SA–NEt2

SA–NH2

Cinnamyl alcohol

R

O

O

CN

OEt

(2.43)

O CN

Cinnamyl alcohol

CO2Et NO2



(2.44)

(2.45)

Bifunctional Catalysis

For these reactions, the activity of SA-NR2 is much higher than SiO2 grafted with the same groups. The high activity of SA-NR2 is attributed to the high acidity of the OH groups on SiO2–Al2O3 compared with those on SiO2. A proposed mechanism involves the basic (–NEt2) and acidic (–OH) groups as active center working in a concerted manner. For example, Fig. 2.11 shows the scheme of the cyano-O-ethoxycarbonylation of benzaldehyde with ethyl cyanoacetate.

Figure 2.11 Reaction scheme of cyano-O-ethoxycarbonylation of benzaldehyde with ethyl cyanoacetic acid. Reprinted with permission from K. Motokura, N. Viswanadham, G. M. Dhar, Y. Iwasawa, Catal. Today, 141, 19 (2009).

Both acidic and basic groups can be grafted to silica materials. The effects of dual functionalities of mesoporous silica (SBA-15) on the aldol condensation of aldehydes and acetone are shown in Table 2.5 [68, 69]. Here, the acidic functional group is the sulfo group and the basic function is offered by the aminopropyl group. As shown in Table 2.5, materials with only aminopropyl groups gave a conversion of 33%. The materials with only an acidic functionality were totally inactive. When the materials have both acidic and basic functionalities, high activities for the reaction were attained. The results indicate the importance of synergetic effects of acidic and basic sites. The proposed role of acidic and basic pair centers is shown in Fig. 2.12, which demonstrates the potential importance of acid/base group in the generation of the enolate from acetone.

55

Figure 2.12 Role of acid–base pairs in aldol condensation over bifunctional mesoporous silica. Reprinted with permission from R. K. Zeiden, M. E. Davis, J. Catal., 247, 379 (2007).



56 Solid Acid Catalysis

Bifunctional Catalysis

Table 2.5

O

Catalytic activities for aldol condensation over acid-base centers on SBA-15 H O +

O2N

50°C, 20 h cat. O2N

Entry Catalyst (10 mol%) 1

2

3

4

5 6

B [%]

B

Conv. [%][b]

8

8

16

30

14

NH2

0

0

0

NH2

3

5

8

SO3H

SO3H

SBA-15

NH4

SBA-15-A/SBA-15-B[b]

8

A [%]

+

O2N

62

BBA-15

7

A

O

17

NH2

SBA-15

+

O

45

SBA-15

SBA-15

OH

25 0

SO3H

3

SO3H

8

33

0

0

44

1

4

[a] Total conversion. Yields determined through 1H NMR spectroscopic analysis with THF as the internal standard. [b] 1:1 mixture of sulfonic acid functionalized SBA-15 (SBA-15-A) and amine-functionalized SBA-15 (SBA-15-B). Source: Reprinted with permission from R. K. Zeiden, M. E. Davis, J. Catal., 247, 379. (2007).

HO

The reaction of glycerol with urea gives glycerol carbonate. OH

OH + NH2CONH2

-NH3

OH

-NH3 O O O O C NH2 OH O

Glycerol urethane

OH

Glycerol carbonate

(2.46)

57

Solid Acid Catalysis

ZnO and Al2O3–ZnO show high activity and selectivity for this reaction [70]. It has been shown that these catalysts have both Lewis-acid and Lewis-base sites of appropriate strength. The reaction scheme shown in Fig. 2.13 is proposed for the activation of the reactants. OH

HO H2N

O

C

NH2

— —

58

O

M

H O

O M

Figure 2.13 Activation of the reactants in the carbonylation of glycerol with urea. Reprinted with permission from M. Climent, A. Corma, P. de Frutos, S. Iborra, M. Nozy, J. Catal., 269, 140 (2010).

Figure 2.14 Acid–base catalyzed mechanism of aldol condensation of benzaldehyde with heptanal. Reprinted with permission from M. J. Climent, A. Corma, V. Fornés, R. Gull-Lopez, S. Iborra, J. Catal., 197, 385 (2001).

In some cases, two reactants are activated separately by acid sites and basic sites. In the cross aldol condensation of

Bifunctional Catalysis

benzaldehyde and heptanal, an amorphous aluminophosphate catalyst containing weak acid and base centers shows much higher rates and selectivities than conventional solid acid catalysts (mesoporous aluminosilicate, H-Beta, H-Y) or solid base catalysts (MgO, hydrotalcite, KF/Al2O3) [71]. The reaction mechanism proposed involves the activation of benzaldehyde by protonation of the carbonyl group on the acid sites and the abstraction of a proton from heptanal on the basic sites (Fig. 2.14). Acidic sites and basic sites may act at different stages of the reaction scheme. MgO, a solid base, is a good catalyst for transfer hydrogenation between ketones and alcohols. For example, the reaction of 7-tridacanone with 2-propanol affords 7-trindecanol in a 41% yield at 623 K (Eq. 2.47) [72]. A small amount of undecene (7%) is also formed by the dehydration of the alcohol (Eq. 2.48).

(C6 H13 )2 C = O + (CH3 )2 CHOH → (C6 H13 )2 CH – OH + (CH3 )2 C = O (2.47) (C6 H13 )2 CH – OH → C13H26 + H2O

(2.48)

When MgO was loaded with H3PO4, the acidic property of the solid developed. This enhances the dehydration of the alcohol. Thus, H3PO4/MgO affords exclusively undecene in a 90% yield at the same temperature [73]. MCM-41, which has two functionalities, sulfo group and amino group, is effective for the reaction of benzaldehyde diacetal 1 and ethyl cyanoacetate 2 to benzylidene ethyl cyanoacetate 4 [74].

(2.49)

The reaction is considered to proceed by the tandem reactions. The deacetalization of 1 is catalyzed by the sulfo groups, and the amino groups catalyze the Knoevenagel condensation of 2 with benzaldehyde 3 formed by the first step. The catalyst with only sulfo groups catalyzes the first step to give 3 as the sole product.

59

60

Solid Acid Catalysis

The catalyst with only amino groups has no activity. When both functionalities are present, the catalyst gives 100% conversion of 1 and 95.2% yield of 4. Direct conversion of ethanol to isobutene proceeds over ZrO2– ZnO mixed oxides [75].

3CH3CH2OH+H2O → iso–C4H8 + 2CO2 + 6H2



(2.50)

With ZrO2–ZnO (Zn/Zr = 1/10), 100% conversion was attained with isobutene selectivity of 83% under the reaction conditions of W/F = 0.23 s g mL–1, ethanol = 1.8 mol%, steam/carbon ratio = 5, reaction temperature = 723 K. Here, ethanol is converted into acetone over basic sites (reactions 2.51 and 2.52) and then acetone is converted into isobutene over Brønsted acid sites (reaction 2.53).





CH3CH2OH → CH3CHO + H2



2CH3CHO + H2O CH3COCH3 + CO2 + 2H2

3CH3COCH3 → 2iso–C4H8 + CO2 + H2O

(2.51)

(2.52)

(2.53)

2.5.2  Bifunctional Catalysis by Acid Sites and Metal/ Metal Cations 2.5.2.1  Skeletal isomerization of light alkanes

Skeletal isomerization of light alkanes (C4H10, C5H12, C6H14) is an important industrial application of catalysis (see Section 5.4.2). The primary use of the branched light alkanes is the gasoline-blending component for raising the octane number. The research octane numbers (RONs) of pentane and hexane are 62 and 29, respectively, whereas their isomers, 2-methylbutane and 2,3-dimethylbutane, have RONs of 93 and 104, respectively. In commercial isomerization processes, the reaction takes place in a fixed-bed reactor in the presence of hydrogen with the use of noble metals (Pt) supported on acidic oxides such as zeolites, chlorinated alumina, and sulfated zirconia. Alkanes can be isomerized via carbocation intermediates over strongly acidic catalysts [76]:

Bifunctional Catalysis



+ n -C5H12 + H+ → n -C5H11 + H2

(2.54)

+ + n-C5H11 → iso-C5H11

+ iso-C5H11 + H2  iso-C5H12 + H+

(2.55)

(2.56)

Alkenes are formed from carbenium ions during isomerization:

+ n-C5H11  n-C5H10 + H+

+ iso –C5H11  iso–C5H10 + H+

(2.57) (2.58)

Alkenes thus formed react with carbenium ions to form C10species or higher oligomers, which causes the deactivation of catalysts. Oligomers are also a main source of by-products. To avoid deactivation, the metal is supported on the acidic catalyst and the reaction is performed under high hydrogen pressure.

Figure 2.15 Isomerization of pentane over mordenite and Pt/mordenite. Numbers in parentheses show the selectivity for branched isomers. Reaction temperature, 523 K; Pressure, 30 kg cm–2; H2/n-C5 = 2.5. Data from H. W. Kouwenhoeven, W. C. van Zijll Langhout, Chem. Eng. Progr., 67, 65 (1971).

61

62

Solid Acid Catalysis

Figure 2.15 shows the catalyst performance of mordenite in the presence and absence of the Pt component [77]. Although mordenite shows a high initial activity even without Pt, higher selectivity and stability of catalysts are obtained in the presence of Pt. This is caused by high hydrogenation–dehydrogenation activity of platinum. Alkenes are promptly hydrogenated into alkanes over the metal, and thus the formation of oligomers and coke precursors is greatly suppressed. A similar role of Pt is also found in the isomerization over chlorinated alumina, sulfated zirconia, and tungsten oxide–zirconia. The metal contributes also to the dehydrogenation of alkanes. The catalysts have two main functions: acid sites, which serve the isomerization function, and the metal, which has the hydrogenation– dehydrogenation function. Thus, the catalysts are often called bifunctional catalysts. The reaction of the mechanism over the bifunctional catalysts can be expressed as follows:



n-C5H12  n-C5H10 + H2 + n-C5H10 + H+  n-C5H11 + + n-C5H11 → iso–C5H11

+ iso-C5H11  iso-C5H10 + H+

iso-C5H10 + H2  iso-C5H12

on Pt 

on solid acid

(2.59)



on solid acid



on solid acid on Pt 



(2.60) (2.61)

(2.62)

(2.63)

When a loaded amount of Pt exceeds a certain level, the dehydrogenation–hydrogenation steps (reactions 2.59 and 2.63) reach the equilibrium under hydrogen. The rate-determining step of the isomerization is the rearrangement of the carbenium ions (reaction 2.61). Since hydrogen shifts the equilibrium (reaction 2.59) to the lefthand side, the concentration of the carbenium ion decreases with H2 pressure. This results in the rate retardation of the isomerization of the rate-determining step (reaction 2.61). The rate of pentane isomerization, r, is expressed as r = k(PC5H12/PH2)1/2. Pt supported on sulfated zirconia is also used for the isomerization of light naphtha under hydrogen. The unique role

Bifunctional Catalysis

of hydrogen is operative in this catalytic system (Section 4.4). Hydrogen dissociates on Pt. The spilt-over hydrogen atoms react with the surface Lewis acids to convert them into protonic acid sites, which are active for isomerization. The rate of isomerization and the number of protonic sites reversibly change with the change in the partial pressure of hydrogen. It is also proposed that the spilt-over hydrogen serves as hydrogen transfer agent in isomerization. Thus, the rate of hexane isomerization is enhanced, whereas the cracking is retarded in the presence of hydrogen. The role of the spilt-over hydrogen is also seen in hexane isomerization and conversion of methanol to hydrocarbons over Pd–heteropoly acid [7, 8]. Catalytic reforming is a refinery process in which a feedstock composed of primarily alkanes and cycloalkanes with a 340–470 K boiling range is converted into a a high-octane gasoline [78, 79]. In naphtha reforming, normal alkanes and cycloalkanes are converted to branched alkanes and aromatics. The reforming reactions are very complex and involve dehydrogenation, dehydrocyclization, isomerization, and hydrocracking. The catalyst is usually platinum, palladium, or platinum–rhenium on alumina and operates in a high-pressure hydrogen atmosphere. Alkanes are isomerized by the bifunctional mechanism, as described above. The dehydro- cyclization of alkanes and the dehydroisomerization of alkylcyclopentanes also proceed by a bifunctional mechanism, as shown in Fig. 2.16 [80].

Figure 2.16 Dehydrocyclization of hexane to benzene in catalytic reforming. Adapted with permission from G. A. Mills, H. Heinemann, T. M. Milton, A. G. Oblad, Ind. Eng. Chem., 45, 134 (1953).

63

64

Solid Acid Catalysis

2.5.2.2  Aromatization of lower alkanes The direct conversion of lower alkanes into aromatic hydrocarbons is an interesting route to produce both high-ocatane gasoline components and benzene/toluene/xylenes (BTX) as feedstocks of petrochemicals. BP and UOP have jointly developed the CYCLAR process to convert C3–C5 alkanes and alkenes to aromatic-rich products. This process uses a gallium-loaded ZSM-5-type catalyst. Aromatic hydrocarbons (benzene, toluene, and xylenes) are fundamental raw materials in petroleum chemistry. ZSM-5 zeolites in their H-form are active for the production of aromatics from lower alkanes such as propane and butanes. The formation of aromatics, however, is accompanied by the production of alkanes, since aromatization takes place simultaneously with cracking and hydrogen transfer reactions. These reactions limit the maximum yield of aromatic hydrocarbons. The selectivity for aromatics is considerably improved by introducing Zn, Ga or Ag into ZSM-5 [81]. Molecular hydrogen is formed as a valuable by-product. Table 2.6 shows the result of the conversion of butane over H-ZSM-5, Zn-ZSM-5, and Ga-ZSM-5 [82]. By introducing Zn or Ga, the conversion of butane and the selectivity for aromatics are greatly enhanced. The main aromatic products are benzene, toluene, and xylene. The production of hydrogen is also enhanced. Zn or Ga can be introduced by various ways: ion exchange, impregnation, or isomorphous substitution of Si in the framework. The active state of these cations is discussed extensively [81]. The proposed species under working conditions includes isolated metal cations, the reduced form of metal cations such as Ga+, metal hydrides, and intracrystalline metal oxides [81, 83]. Industrially, Ga species are used since Zn component is too volatile under the high process temperatures employed. As described earlier (Section 2.4.1), over solid acids such as H-ZSM-5, alkanes are activated by protonic sites. For example, butanes are activated by reaction 2.29. The reactions can be expressed by the following three equations:



C4H10 C4H8 + H2 C4H10 → C3H6 + CH4

(2.64) (2.65)

Bifunctional Catalysis

C4H10 → C2H4 + C2H6



Table 2.6

(2.66)

Conversion of butane over H-ZSM-5, Zn-ZSM-5, and Ga-ZSM-5a Catalyst

Catalyst

H-ZSM-5

Zn-ZSM-5

Ga-ZSM-5

Conversion (%)

44.7

38.3

58

CH4

4.3

5.8

5.0

C2H4

9.3

4.7

6.2

Yield of aromatics (%)

Product distribution (%) C2H6 C3H8

C3H6

iso-C4H10 C4H8

C5+

3.2 9.9

44.5 9.5 4.9 7.4 3.0

17.6 7.2

16.4 7.1

8.0

32.6

7.5

6.2

9.5 10.2 1.2

7.9

4.6

2.1

Aromatics

7.2

45.9

28.3

Benzene

14.9

28.1

19.2

Aromatics distribution (%) Toluene

Xylenes + ethylbenzene

C9+

Hydrogen selectivityb H2

aReaction

41.2

41.4

47.7

39.4

27.8

28.6

10.7

124.9

66.9

4.5

2.7

4.5

conditions: 773 K, 34.2 kPa C4H9 and W/F = 4.2 g h mol–1. in moles produced per 100 moles butane converted. Source:  Reprinted with permission from Y. Ono, K. Kanae, J. Chem. Soc., Faraday Trans. 87, 669 (1991).

bSelectivity

Studies of the reaction at low pressure over Zn-ZSM-5, GaZSM-5, and Ag-ZSM-5 revealed that the primary reactions at low conversion levels are also expressed by the three reactions. The ratio

65

66

Solid Acid Catalysis

of the three reactions in the presence of these cations is, however, quite different from that over H-ZSM-5, indicating the difference in the reaction mechanism. Over Zn-ZSM-5, the rate of dehydrogenation (reaction 2.64) is about 27 times higher than over H-ZSM-5. The rate of reaction 2.65 is also higher over Zn-ZSM-5 than over H-ZSM-5, whereas the rate of reaction 2.66 is lower over ZnZSM-5 than over H-ZSM-5. Considerably higher rate of dehydrogenation compared with H-ZSM-5 is also observed over Ga-ZSM-5 and Ag-ZSM-5. The rate of dehydrogenation (reaction 2.64) is compared for butane and pentane isomers. The order of dehydrogenation rates is as follows [84]: C C C C–C–C  >  C–C–C–C  >  C–C–C–C–C  >  C–C–C–C  >  C–C–C C 36 19 8.9 5.9 0 (over Zn-ZSM-5) 27 20 7.8 4.4 0 (over Ag-ZSM-5) (numbers show the rate in 10–2 mol h–1 g-catalyst–1)

The order indicates that the rate of dehydrogenation can be correlated with the ease of carbenium ion formation from the reactants. Isobutane and isopentane give tertiary carbenium ions by the abstraction of a proton. n-Pentane and n-butane give secondary carbenium ions, whereas neopentane gives primary carbenium ion. This means that the activation of alkanes occurs by the heterolytic cleavage of C–H bond to form carbenium ion R+ (or surface alkoxide) and hydride ion H– by the action of surface oxide ion and metal cations.

RH + Mn+ + OZ– → H–Mn(n–1) + + R + –OZ–

(2.67)

The carbenium ions thus formed are decomposed into alkenes and protons. The protons combine with hydride ions to complete dehydrogenation. The formation of surface methoxide is observed in the 13C MAS NMR spectrum upon the adsorption of methane (13CH4) over Zn-ZSM-5 [85, 86]. The methoxy group reacted with benzene to give C6H513CH3 [85, 86]. Alkenes are oligomerized to form higher alkenes, which finally converted into aromatics as a mechanism shown in reaction 2.24. These steps are catalyzed by Brønsted acid sites of the zeolite. It is

Bifunctional Catalysis

also shown that alkenes are also dehydrogenated in the presence of the metal cations and effectively converted into aromatic hydrocarbons [87, 88]. For the activation step of alkanes, several groups also proposed heterolytic dissociation of alkanes over Zn- or Ga-loaded ZSM-5, but, instead of carbenium ion formation, they propose the formation of a metal-alkyl bond and a hydroxyl group mainly based on MAS NMR studies [89, 90]. In summary, a high activity of metal-containing zeolites for the production of molecular hydrogen from alkanes and alkenes leads to the high selectivity for aromatics and lower selectivity for alkanes.

2.5.2.3  One-step synthesis of methyl isobutyl ketone

Methyl isobutyl ketone (MIBK) is mainly used as a solvent for inks and resins, and an important reagent in dewaxing mineral oils. MIBK is produced commercially in three steps from acetone: basecatalyzed liquid-phase aldol condensation of acetone to diacetone alcohol (DAA) (reaction 2.68), acid catalyzed dehydration to DAA to mesityl oxide (MSO) (reaction 2.69), and selective hydrogenation of MSO to MIBK (reaction 2.70).





(2.68)







(2.69)

(2.70)

A problem associated with the three-step method is the unfavorable thermodynamic equilibrium of the first condensation step. To avoid the thermodynamic limitation, the one-step process

67

68

Solid Acid Catalysis

has been explored extensively [91]. Since the first two steps are catalyzed either acids or bases and the third step by metals, metals supported on solid acids or bases are used for the one-step process. Acid supports include metal oxides, cation exchange resins, zeolites, and zirconium phosphate. Pd supported on zirconium phosphate gives 30–50% conversion with selectivity of 95–96% at 393 K under pressure of 120 bar with acetone/H2 molar ratio of 0.3–0.4 [92]. Pd supported on MCM-41 (Si/Al = 25) gives acetone conversion of 34.0% and a selectivity of 86.9% under the conditions of 433 K, 40 bar, and H2/acetone molar ratio of 0.2 [93]. The balance of the acid function and the hydrogenation function is very important to obtain high selectivity for MIBK. Excessive acid function enhances further condensation of MIBK with acetone to C9 compounds. Excessive function of hydrogenation enhances direct hydrogenation of acetone to 2-propanol.

2.6  Pore Size Effect on Catalysis: Shape Selectivity

Size and structure of pores show profound effects on the catalytic reactions, especially selectivities, when the size of pores is close to the molecular dimensions of the reactants and/or products. Only molecules whose dimensions are less than a critical size can enter the pores, access the internal catalytic sites, and react there. Furthermore, only molecules that can diffuse through pores appear in the products. For example, the channels of the MFI structure allow the diffusion of benzene and monosubstituted benzenes as well as p-xylene. The diffusion of ortho- and meta-disubstituted benzenes is far more difficult. This allows shape selectivity in favor of mono- or para-disubstituted benzenes in alkylation and disproportionation of alkylaromatics. The concept of “shape-selective catalysis” was first introduced in catalysis over zeolites by Weisz and Frilette in 1960 [94, 95]. Since then, many shape-selective reactions have been found and utilized in industrial processes [96, 97]. Most of those reactions are zeolite catalyzed, since the pore sizes of zeolites are close to the molecular dimensions of reactants and/or products and are uniform and more discrete. Degnan Jr. listed 18 industrial processes using zeolites as shape-selective catalysts in petroleum and petrochemical areas [96].

69

Pore Size Effect on Catalysis Bifunctional Catalysis

Generally, shape selectivity is classified into three types (Fig. 2.17) [98]: (a) Reactant selectivity: Some of the molecules in a reactant mixture are too big to diffuse through the catalyst pores, whereas other less bulky molecules are able to enter. (b) Product selectivity: Some of the products formed inside of the pores are too bulky to diffuse out as products. Only the less bulky molecules are found as observed products. (c) Transition state selectivity: Certain reactions are prevented because the corresponding reaction intermediates (or transition states) would require more space than available in the pores or cavities. Reactions requiring smaller transition states proceed unhindered. Transition state selectivity is governed by intrinsic properties of the crystal structure and therefore it does not depend on crystal size. On the other hand, reactant and product selectivity are mass transfer– related phenomena and, therefore, depend on the particle size.

Figure 2.17 Different types of shape selectivity. Adapted with permission from S. M. Csiscery, Zeolites, 4, 202 (1984). 

 

70

Solid Acid Catalysis

2.6.1  Reactant Selectivity Cracking of n-alkanes is one of the first examples of shape selectivity. Table 2.7 compares the cracking of 3-methylpentane and hexane over amorphous silica–alumina and Ca-exchanged A-type zeolite whose pore diameter is ca. 0.5 nm [94, 99]. Over amorphous silica– alumina, 3-methylpentane cracks much faster than hexane at 773 K. This is expected since the formation of carbenium ion is much easier for the former than for the latter. Over CaA, however, hexane does react, but 3-methylpentane hardly reacts at the same temperature. This is because the branched isomer cannot enter the pores of zeolite A. Erionite, a small-pore zeolite, is used in Selectforming, where, among alkanes, aromatics, and naphthenes, only n-alkanes are cracked mostly to propane. A hydrogenation component (Ni) prevents catalyst deactivation by inhibiting coking. Table 2.7

Reactant and product selectivities: C6 paraffin cracking 3-Methylpentane

Catalyst

Cracking at 773 K (%)

Silica–alumina 28 Linde Ca-A

ferrierite > faujasite. Suzuki et al. measured the heat of adsorption of ammonia DH by TPD and examined the correlation between the acid strength and the O–H stretching frequency of OH groups in IR. Figure 3.5 shows DH plotted against O–H frequency for different zeolites [12]. A clear tendency is observed between DH and the frequency for OH groups located on 12-, 10-, and 8-membered rings. The DH values are larger as the OH frequencies are lower, provided that the OH groups are not perturbed. The acid strength of the zeolites with 12-, 10-, and 8-membered rings was in the order, H-MOR > H-ZSM-5 ~

89



90

Characterization of Solid Acid Catalysts

H-b > H-Y. Deviations from the relation observed for OH groups in 6-membered rings are caused by perturbation of the OH groups with zeolite walls (H-bonded to O atoms in the wall).



Figure 3.4

Determination of DH applied to mordenite (HMn), ferrierite (HFn), and ZSM-5 (nH), where numbers show the silica to alumina molar ratio of the zeolites. Reprinted with permission from M. Niwa, N. Katada, M. Sawa, Y. Murakami, J. Phys. Chem., 99, 8815 (1995).

Figure 3.5

Plots of acid strength DH with band position of OH located in 12-, 10-, and 8-member rings (), and 6-member ring (). Lines are drawn in order to show the extent of experimental errors. Reprinted with permission from K. Suzuki, T. Noda, N. Katada, M. Miwa, J. Catal., 250, 151 (2007).

Calorimetry of Adsorption of Basic Molecules

Although TPD of ammonia is widely used, some difficulties have been pointed out. Sites that adsorb ammonia are not always acidic sites. Solid bases such as Al2O3, MgO, and CaO adsorb ammonia in such a way that basic sites abstract an H+ from ammonia to form N​H–2​  ​​  [13, 14]. Adsorbed states of ammonia should be examined when ammonia is used as a probe molecule of TPD analysis.

3.3  Calorimetry of Adsorption of Basic Molecules

Calorimetry of adsorption of basic molecules like amines gives information of the strength of acid sites and acid amount. The strength is expressed in the scale of heat of adsorption of the basic molecules. Ammonia is most frequently used as a probe molecule. The heat evolved during adsorption of the probe molecule is directly measured by microcalorimeter connected to a volumetric system with a sensitive pressure gauge for measuring adsorbed amount. For measurement of differential heats of adsorption, small amounts of the probe molecule (1–10 µmol/g-cat) are sequentially admitted to the adsorbent. A variety of microcalorimeters developed are classified into three categories; adiabatic, isothermal, and heatflow calorimeters. For acidity measurement, heat-flow calorimeters are normally used. More detailed description of the calorimetry is found in some reviews [15–17].

3.3.1  Ammonia

Figure 3.6 shows the calorimetry of ammonia adsorption on mordenite-type zeolites with different Si/Al ratios. H-forms of mordenite show large heats of adsorption about 170 kJ mol–1 at the initial adsorption of ammonia [18]. The heats of adsorption gradually decrease and become constant with further increase in the amount of ammonia adsorbed until distinct steps are observed at which heats of adsorption abruptly decrease. The step points correspond to the amounts of acid sites that coincide with the number of Al atoms in the mordenites. The heat values decrease to ca. 80 kJ mol–1, which are the same as those observed for the Naform of mordenite (M-10).

91

92

Characterization of Solid Acid Catalysts

Figure 3.6

Calorimetrically determined molar differential heats of adsorption at 473 K of various mordenites evacuated at 773 K. Filled symbols represent heats of re-adsorption on samples that were evacuated at 473 K after the first run of the adsorption measurement. Reprinted with permission from K. Tsutumi, K. Nishiyama, Thermochim. Acta, 143, 299 (1989).

The adsorption temperature affects the results of calorimetry. Ammonia molecules are supposed to be adsorbed first and selectively on the stronger acid sites. Adsorption equilibrium should be established in a short time. However, at a low temperature, adsorption equilibrium may not be established. Ammonia molecules adsorbed first on the weaker acid sites or on the acid sites outside of the cavities may not move to the stronger acid sites or acid sites in the cavities. In this case, the distinct step of heat of adsorption with adsorbed amount does not appear. For calorimetric measurement of ammonia on H-mordenite, the measurement should be done above 423 K. The minimum temperature for adsorption varies with the types of solid acid. For H-ZSM-5 and faujasite, the minimum adsorption temperature is 373 K.

3.3.2  Other Basic Molecules

Basic molecules other than ammonia can also be used for calorimetry such as butylamine, trimethylamine, pyridine, and piperidine. Ammonia is weaker base than the other basic molecules. The use of basic molecules other than ammonia enables us to



Infrared Spectroscopy

measure weaker acid sites that may interact weakly with ammonia and emit a small heat of adsorption. The heats of adsorption of these molecules on solid acids are in the following order piperidine > trimethylamine > pyridine ≈ butylamine > ammonia [19]. This order is in accord with proton affinity (PA) in the gas phase, defined as the negative of the change in enthalpy for the reaction

Base (g) + H+(g)    Base-H+(g)



Base(sol) + H+(sol)    Base-H+(sol)

The values of pKa, as shown in Table 3.4 (Ka is a dissociation constant of conjugate acid in aqueous solution), are not in the same order of heats of adsorption on the acidic sites. In the case of solution phase acids, following is the reaction: Table 3.4

Proton affinities (PA) and pKa of basic molecules

Probe molecule

PA (kJ mol–1)

pKa

Piperidine

943.1

11.1

Trimethylamine

Pyridine

Butylamine

Ammonia

938.5

922.2

916.3

857.7

9.8

5.2

10.6

9.3

If the solvent is water, H+(sol) is the hydronium ion and –log K (K is an equilibrium constant for dissociation of the conjugate acid (Base-H+)) is defined as pKa. It is realized that the energies associated with placing both Base and Base-H+ are comparable to the energy of the protonation energy itself, so that solvation effects can be very large. The pKa values include the solvent effects that are not included in the adsorption of basic molecules on the solid acids. Accordingly, the proton affinity rather than pKa value was recommended as a measure of the basic strength of the probe molecule [20].

3.4  Infrared Spectroscopy 3.4.1  IR of Adsorbed Pyridine

The IR of adsorbed pyridine clearly distinguishes between Brønsted acid sites and Lewis acid sites, and can be used to evaluate the

93

94

Characterization of Solid Acid Catalysts

strengths and amounts of these acid sites. Brønsted acid sites are distinguishable from Lewis acid sites by IR band positions of pyridinium ion formed on Brønsted acid sites (~1540 cm–1) and pyridine coordinated to Lewis acid sites (~1450 cm–1). The strengths are expressed in term of the temperature at which pyridine is desorbed from the sample by outgassing or flushing with an inert gas. The amounts of both acid sites are evaluated from the band intensities for pyridinium ion (~1540 cm–1) and pyridine coordinated to Lewis acid sites (~1450 cm–1). This method is based on the assignments of modes associated with pyridinium ion and the coordination complexes by Parry [21]. He measured IR spectra of pyridine in chloroform, pyridine:BH3 in chloroform and pyridine:H+ Cl– in chloroform, and summarized IR bands as shown in Table 3.5. Table 3.5

Infrared bands of pyridine on solid acids in the 1400–1700 cm–1 region

Hydrogen bonded pyridine

Coordinatedly bonded pyridine

1440b,c

1447 b,c,d

~ 1447 (v.s.)

1485~1490 (w)

1580 b,c ~1600 (s)

~1460 (v.s.)

1488~1503 (v) ~1580 (v)

1600 b,c ~1633 (s)

Pyridinium ion

1485~1500 (v.s.) 1540 e (s)

~1620 (s)

~1640 (s)

Source: Reprinted with permission from E. P. Parry, J. Catal., 2, 371 (1963). aBand

intensities: v.s., very strong; s, strong; m, medium; w, weak; v, variable.

bThe split between hydrogen-bonded and coordinately bonded pyridine (Lewis base)

is not well defined by band frequency alone. Other conditions need be used where possible, such as ease pumping off.

cFrequency dThis

appears to increase with increasing bond strength.

band is not present in the pyridinium ion spectrum. This, with auxiliary bands listed, can be used to determine Lewis acidity.

eThis

band cannot be present in coordinately bonded pyridine (Lewis base) since the N+–H bending motion is involved in the vibration. Thus, a proton has to actually transfer to the nitrogen in order for this band to appear. This band together with auxiliary bands listed, is used to characterize proton sites.

Infrared Spectroscopy

The spectra in Fig. 3.7 are obtained for pyridine adsorbed on mordenite, H-Y, H-ZSM-5 and amorphous SiO2–Al2O3 [22]. The band at 1540 cm–1 is assigned to pyridinium ions formed by adsorption of pyridine on Brønsted acid sites, and the band at ~1450 cm–1 to pyridine coordinatively bonded to Lewis acid sites. Both pyridinium ion and coordinated pyridine show a band at 1490 cm–1. Pyridine hydrogen-bonded to non-acidic sites gives bands at 1449 and 1599 cm–1, which are diminished by evacuation at 523 K.

Figure 3.7

Difference spectra obtained during addition of pyridine, before saturation occurred. MOR 1~3, three types of mordenite; Y, Y  zeolite; ASA 1 and 2, two types of amorphous silica–alumina. Reprinted with permission from C. A. Emeis, J. Catal., 141, 347 (1993).

In most cases, addition of H2O brings about a decrease in the band intensity at about 1450 cm–1 and an increase in the band intensity at 1540 cm–1, indicating that Lewis acid sites convert into Brønsted acid sites on addition of water.

95

96

Characterization of Solid Acid Catalysts

The quantitative analysis of each type of acid sites is possible on the basis of extinction coefficients of the bands at 1450 and 1540 cm–1. Under the conditions where the amount of adsorbed pyridine is constant and no hydrogen-bonded pyridine exists, introduction of water converts Lewis acid sites to Brønsted acid sites. Increase in the integrated absorbance for the band at 1540 cm–1 and decrease in the integrated absorbance for the band at 1450 cm–1 are observed. The changes in the integrated intensity relate with the absorptivity (extinction coefficient) for the two bands as expressed by the following equation:

DA1540/–DΑ1450 = a1540/a1450,

(3.3)

where DΑ1540 and –DΑ1450 represent the increase in the integrated intensity for the band at 1540 cm–1 and the decrease in the integral intensity at 1450 cm–1, respectively, and a1540 and a1450 represent the absorptivities of the bands at 1540 cm–1 and 1450 cm–1, respectively. The ratios a1540/a1450 thus determined were reported to be 0.9 and 0.64 for fluorinated alumina [23] and mordenite [24], respectively. The other values, 0.75 [22] and 1.08 [25], were reported for a1540/a1450. These values were obtained by different methods and for different sample wafers, mordenite, HY, ZSM-5 and SiO2– Al2O3 for 0.75, and HY and Al2O3 for 1.08. The difference in a1540/ a1450 values reported is partly due to the difference in the optical properties among the samples. It is desirable to calculate using the same wafer of the sample under study, because scattering of light during transmission of the sample, for example, is not the same for different sample wafers even the sample is the same. Knowing the ratio a1540/a1450, the amounts of Brønsted acid sites and Lewis acid sites can be estimated as described in the following box. Absorptivity,

a, is a Beer–Lambert absorption coefficient. The decadic absorbance (A10) divided by the product of sample path-length, l, and mass concentration, r, of the absorbing material: a = A10/rl.

The term “absorptivity” is used in this text because the use of “extinction coefficient” has been discouraged since the 1960s, when the international agreement with nonchemical societies reserved the word “extinction” for the diffusion of radiation. (J. E. Bertie in Handbook of Vibrational Spectroscopy, Vol. 5, Wiley and Sons Ltd. 2002.)

Infrared Spectroscopy

Calculation of Brønsted acid sites and Lewis acid sites



­



First, we need to measure IR spectrum of the sample that adsorbs a known amount of pyridine X. X should be small so that all pyridine molecules should be adsorbed in the forms of pyridinium ion and pyridine coordinated to Lewis acid sites, no physically adsorbed pyridine existing. Under these conditions, X can be expressed by X = A1540/f a1540 + A1450/f a1450,

(1)

where A1540 and A1450 are the integrated intensities (peak area of the spectrum plotting absorbance vs. wavenumber) of the bands at 1540 cm–1 and 1450 cm–1, respectively, and the factor f is a function of the optical properties such as light scattering and transparency. The optical properties vary with the particle size of the sample powder, thickness or weight of sample, the pressure under which the powder sample is formed into a wafer, roughness of the wafer surface, etc. Therefore, f is unique to a specific wafer under study and may be different for different wafers even the sample is the same type. Let us write

Ra = f a1450/f a1540 = a1450/a1540,

which can be obtained by the way described in the text (Eq. 3.3). Insertion of (2) into (1) gives

(2)



X = (Ra A1540 + A1450)/Ra f a1540.

(3)



f a1540 = (Ra A1540 + A1450)/Ra X

(4)







f a1540 and f a1450 can be obtained as f a1450 = (Ra A1540 + A1450)/X.

(5)

The amounts of pyridine adsorbed on Brønsted acid sites (B) and Lewis acid sites (L) after a certain condition, for example, after evacuation of the pyridine adsorbing sample at 423 K, can be counted as follows: B = A1540/f a1540

L = A1450/f a1450,

(7)

(8)

where A1540 and A1450 are the integrated intensities of the bands at 1540 cm–1 and 1450 cm–1, respectively. The insertion of the values fa1540 and f a1450 obtained from (4) and (5) into (7) and (8) gives the amounts of pyridine adsorbed after a certain condition on Brønsted acid sites (B) and Lewis acid sites (L). For more accurate values of fa1540 and fa1450, different amounts of pyridine (X) are adsorbed, and (Ra Α1540 + Α1450)/Re is plotted against X. The slope gives the value of f a1540, and then fα1450 is obtained by (2). These values are more reliable than those obtained by the measurement of single X.

97

98

Characterization of Solid Acid Catalysts

The use of a large probe molecule such as 2,6-di-t-butylpyridine in place of pyridine enables us to measure the acid sites located at the external surface of microporous materials. Protonated 2,6-di-t-butylpyridine gives bands at 3370, 1616 and 1530 cm–1. These bands are assigned to the N–H stretching (3370 cm–1) and the ring signals (1616 and 1530 cm–1) of the protonated 2,6-dit-butylpyridine. By measuring IR spectra of adsorbed 2,6-di-t-butylpyridine on different zeolites, zeolites could be classified into three types in terms of the accessibility to 2,6-di-t-butylpyridine [26]:

(1) Total accessibility to 2,6-di-t-butylpyridine: Beta and Y zeolites possessing a tridirectional 12-membered ring pore system are included in this type. (2) Partial accessibility (30–40%) to 2,6-di-t-butylpyridine: Mordenite and SSZ-24 possessing unidirectional 12-membered ring pore system, and SSZ-26 possessing tridirectional 12/10-membered ring pore system are included in this type. (3) Nonaccessible zeolites to 2,6-di-t-butylpyridine ( O2 (422 kJ mol–1) > Ar (371 kJ mol–1). N2 is suitable for probing acidic strength of O–H considering small size of the molecule and large proton affinity. Two types of OH groups in CoAPO-18 (Co/(Co + Al + P) = 0.02) were distinguished by the shift in OH vibration bands caused by adsorption of N2 [39]. CoAPO-18 has two types of OH groups; one is isolated P-OH (stretching at 3681 cm–1, bending at 956 cm–1) and the other is bridged Co-(OH)-P (stretching at 3573 cm–1, bending at 905 cm–1) (Figs. 3.11 and 3.12). On admission of N2 at 77 K, the stretching band for isolated P-OH shifted to a lower frequency by 70 cm–1 and that for bridged Co-(OH)-P shifted by 110 cm–1, indicating that the bridged OH groups are stronger than the isolated ones. The shift of bending band caused by N2 adsorption was also large for the stronger OH groups, though the direction of the shifts was to a higher frequency. The isolated P-OH bending band shifted to a higher frequency by 14 cm–1 and that of the bridged OH shifted by 24 cm–1.

Figure 3.11 Band shift of the OH stretching and bending mode regions of the bridged Co-OH-P of CoAPO-18 (Co/(Co + Al + P) = 0.02) caused by adsorption of N2 at 77 K.

IR spectroscopy of OH groups associated with CO and N2 adsorption gives valuable information about strength of acid sites, in particular Brønsted acid sites as described above. However, low temperature as low as 77–100 K should be used for adsorption of these weakly interacting molecules on the surfaces. A cryogenic

NMR Spectroscopy

IR cell is required, which is not commonly available. This point is a drawback of this method.

Figure 3.12 Band shift of the OH stretching and bending mode regions of the isolated P-OH of CoAPO-18 (Co/(Co + Al + P) = 0.02) caused by adsorption of N2 at 77 K.

3.5  NMR Spectroscopy 3.5.1 

1H

MAS NMR

3.5.1.1  Chemical shift Nuclear magnetic resonance (NMR) spectroscopy is a powerful analytical tool; providing quantitative measurements of populations of nuclei discriminated by their chemical shift and other parameters related to the local structural environment. One of the most important techniques for the study of solid acids such as zeolites is 1H MAS NMR. Generally, the chemical shifts of hydroxyl groups increases with acid strength [40, 41]. As shown in Table 3.6, several distinct types of protons in zeolites can be identified by their chemical shifts; terminal silanol (1.3–2.2 ppm), isolated bridging hydroxyl groups SiOHAl (3.6–5.6 ppm), and AlOH at non-framework aluminum (2.6–3.6 ppm). In HY zeolites, the resonance of bridging hydroxyl groups (Brønsted sites) is split into two components for OH-groups pointing into the supercage (at about 3.9 ppm) and into the sodalite cages (at 4.9 ppm) as shown in Fig. 3.13 [41]. These two resonances correspond to HF (high frequency) band at 3650 cm–1 and LF (low frequency) band at 3540 cm–1 of the stretching vibrations of the hydroxyl groups in the IR spectra, respectively. Further resonances may appear from residual ammonium ions

103

104

Characterization of Solid Acid Catalysts

(6.5–7.5 ppm) and OH-groups connected with metal cations (–0.5 to 0.5 ppm), e.g., [CaOH]+ or [MgOH]+ in CaY or MgY zeolites. Table 3.6 1H

1H NMR chemical shifts and assignments of hydroxyl groups in dehydrated zeolites

NMR shift d1H (ppm)

Abbreviation Type of hydroxyl groups

–0.5…0.5

MeOH

1.2…2.2

SiOH

2.8…6.2

CaOH, AlOH, LaOH

2.8…3.6

3.6…4.3

4.6…5.2 5.2…7.0

AlOH

Metal or cation OH groups in large cavities or at the outer surface of particles

Silanol group at the external surface or at lattice defects

OH groups bonded to extra-framework aluminum species located in cavities or channels involved in hydrogen bonds Cation OH groups located in sodalite cages of zeolite Y and in channels of ZSM-5 involved in hydrogen bonds

SiOHAl, SiO1HAla

Bridging OH groups in large cavities or channels of zeolites

SiOHAl

Disturbed bridging OH groups in zeolite HZSM-5 and HBeta

SiOHAl, SiO2HAla

Bridging OH groups in small channels and cages of zeolites

Source: Reprinted with permission from M. Hunger, Solid State NMR, 6, 1 (1996). aAssignment of bridging OH groups in faujasite-type zeolites.

The 1H MAS NMR spectrum of H-ZSM-5 has two peaks: a peak at 4.3 ppm attributed to the acid sites and a peak at 2.0 ppm due to silanol groups terminating crystal surfaces or defects. The existence of the third peak at 6.9 ppm has also been reported, but the assignment is controversial. From the results of 1H{27Al} double resonance experiments, the resonance was shown to a second type of Brønsted site also located in close proximity to an aluminum atom [42]. Alternatively, the peak at 6.2–6.5 ppm (at lower temperature) is assigned to the residual water associated with a Brønsted acid site, either water molecules Hbonded to Brønsted acid sites or H3O+ ions [43].

NMR Spectroscopy

Figure 3.13 The 1H MAS NMR spectra of calcined zeolites HNaY (nsi/nAl = 2.6) after exchange of 36, 52, 71, and 88% of sodium cations by ammonium ions. The spectra were recorded at 400.13 MHz with a spinning rate of 12 kHz. The individual peaks used for the simulation are shown below the spectra. Reprinted with permission from M. Hunger, Solid State NMR Reson., 6, 1 (1996).

The acid strength of OH groups that are not involved in hydrogen-bonding or other chemical interaction correlates with the 1H chemical shift; the acid strength of hydroxyl groups increases with decreasing electronic charge of the hydrogen atom, which corresponds to a larger value of the 1H chemical shift. However, it may be difficult to determine whether the observed 1H chemical shift of the hydroxyl protons originates from intrinsic acid strength or from the formation of hydrogen bonds [40]. The chemical shift, d1H is correlated with the wavenumber, nOH, of the IR stretching vibration [41].

d1H (ppm) = 57.1 – 0.0147nOH (cm–1)

(3.4)

The chemical shift of the protons in zeolites changes with temperature. This is related to the delocalization of the protons [44]. The chemical shift values are 4.1 and 4.7 ppm at 298 K and 674 K, respectively. The 1H MAS NMR spectra of the SAPO-5 consist of signals at 1.5 ppm due to SiOH groups and at 3.7 ppm and 4.8 ppm caused by bridging OH groups in 12-ring pores and 6-membered oxygen rings, respectively [41].

105

106

Characterization of Solid Acid Catalysts 1H

MAS NMR spectra of ZrO2 and SnO2 gave the signals at 5.2 ppm and 5.1 ppm, respectively. These signals were assigned to weakly acidic ZrOH and SnOH, respectively. When these oxides were sulfated, signals appeared at 6.4 ppm for ZrO2 and 8.1 and 7.5 ppm for SnO2, indicating the development of strongly acidic Brønsted acid sites [45].

3.5.1.2  Mobility of acidic protons

The dynamic properties of protons have been rarely discussed compared with static properties, but they can be vital in the catalysis of solid acids. If the protons are mobile and not fixed at the specific “sites,” they would undergo the collective influence of the environment. It has been established that the mobility of protons can be estimated by the detailed analysis of NMR line shape. Brunner described the theoretical model for the line shape of 1H MAS NMR to be a function of correlation time when proton mobility is a decisive factor for the residual linewidth [46]. Figure 3.14 shows the temperature dependence of 1H MSAS NMR spectra of H-ZSM-5 with SiO2/Al2O3 ratio of 106 [47]. The temperature of the measurements was raised stepwise from 298 to 473 K. Two peaks were observed at 4.3 and 2.0 ppm. As described above, the former is attributed to acidic protons (acidic OH groups) and the latter non-acidic ones (silanol OH groups). The line shape due to acidic protons strongly depended on temperature. Upon raising temperature, the linewidth increased and through a maximum around 350 K, it decreased. The change in the linewidth was reversible. The change of the linewidth with temperature for acidic protons of H-ZSM-5 with different SiO2/Al2O3 ratios is shown in Fig. 3.15. As for non-acidic protons, neither the chemical shift value nor the linewidth changed with raising temperature for all the samples. Fenzke et al. theoretically treated the influence of isotropic thermal motion upon MAS NMR for spin I = 1/2 [48]. When the broadening results from a heterogeneous magnetic dipole interaction, which is the dominating line-broadening mechanism in the case of bridging OH groups in zeolites, the line shape is expressed by Lorentzian line with the linewidth given by the following equation:

NMR Spectroscopy



DnMAS = (M2/3p){2t/[1 + (wt)2] + t/[1+(2wt)2]},

(3.5)

where w (rad s–1) is the spinning frequency of the sample, M2 (s–1) the second moment for the central line; t(s) is the correlation time. The linewidth of 1H MAS NMR spectrum expressed by Eq. 3.5 exhibits a maximum as a function of correlation time at (wt) ≈ 0.8, and a simple decrease of the value of the linewidth both at smaller (line broadening by thermal motion) and at larger (line narrowing caused by MAS) values of the correlation time. Thus, the linewidth increases and through a maximum, it decreases upon decreasing the correlation time. Since the correlation time decreases with temperature, the linewidth increases and through a maximum, it decreases upon raising the temperature. The theoretical prediction is in conformity with the feature of the spectra as shown in Fig. 3.15.

Figure 3.14

1H MAS NMR spectra of H-ZSM-5 (SiO/Al

2O3 = 106): (a) 298, (b) 333, (c) 353, (d) 373, (e) 393, (f) 423, (g) 453, (h) 473 K, and (*) denotes a spinning sideband. Reprinted with permission from T. Baba, Y. Ono, Appl. Catal., A 181, 227(1999).

107

108

Characterization of Solid Acid Catalysts

Figure 3.15 Effect of temperature on the line width of the peak due to acidic protons in H-ZSM-5 zeolites: () SiO2/Al2O3 = 24; () SiO2/Al2O3 = 39; (D) SiO2/Al2O3 = 72; () SiO2/Al2O3 = 106. Reprinted with permission from T. Baba, Y. Ono, Appl. Catal., A 181, 227 (1999).

Equation 3.5 also shows that the linewidth depends on the spinning frequency by the isotropic thermal motion of protons. Figure 3.16 shows the dependence of the 1H MAS NMR spectra of H-ZSM-5 (SiO2/Al2O3 = 106) on the spinning frequency at 373 K. The linewidth of the peak due to acidic protons was sharply influenced by the spinning frequency. This is further evidence for the contribution of thermal motion to the linewidth. The protons may move from one bridging oxygen ion to another around Al3+ ions. On the other hand, the linewidth did not change with the spinning frequency at 298 K, indicating that the acidic protons are fixed at this temperature. From the dependence of the linewidth on the spinning frequency, the activation energy for hopping was estimated to be 17–20 kJ mol–1, depending on the SiO2/Al2O3 ratio. Sarv et al. estimated that the activation energy of proton mobility is 45 kJ mol–1 from the line shape analysis of the spinning side bands [49].

NMR Spectroscopy

Figure 3.16 Effect of the spinning frequency of the sample on the line shape of 1H MAS NMR spectrum: the 1H MAS NMR spectra of H-ZSM-5 (SiO2/Al2O3 = 106) recorded at 373 K; spinning frequency of the sample: (a) 2.5; (b) 3.0; (c) 3.5; (d) 4.0 kHz. Reprinted with permission from T. Baba, Y. Ono, Appl. Catal., A 181, 227 (1999).

The linewidth of the peak due to silanol protons did not change with the spinning frequency. The bridging hydroxyl group in borosilicate [B]-ZSM-5 did not change with temperature, indicating the low mobility of the proton in Si–OH–B bonds [47]. Silver salt of dodecatungstosilicic acid (Ag3PW12O40, AgTP) is a unique solid acid. While it has no catalytic activity for acidcatalyzed reactions, very high catalytic activities for acid-catalyzed reactions such as hexane isomerization and methanol conversion to hydrocarbons arise when the salt is reduced under hydrogen [50, 51]. The activities of the reduced AgTP are much higher than the parent acid, H3PW12O40. The activities appear only when hydrogen coexists in the system and depend on the pressure of hydrogen. The effect of hydrogen is reversible; when hydrogen is

109

110

Characterization of Solid Acid Catalysts

removed from the system, the activity disappears. It follows that the Ag+ ions in AgTP is reversibly reduced with hydrogen.



Ag + +

1 H2  Ag o + H+ 2

(3.6)

The 1H MAS NMR spectrum of AgTP showed a peak at 9.3 ppm. When hydrogen existed in the system, another peak appeared at 6.4 ppm. The intensity of 6.4 ppm peak depended on the partial pressure of hydrogen and the effect of hydrogen was reversible, as seen in the catalytic activities [52, 53]. Thus, the 6.4 ppm peak is responsible for the high catalytic activities of the reduced AgTP. The linewidth of the peak at 6.4 ppm depended on the spinning frequency even at 298 K, whereas the peak at 9.3 ppm did not [53]. Thus, the protons of 6.4 ppm peak are mobile even at 298 K. Further evidence for high mobility of 6.4 ppm protons was evidenced by the temperature dependence of the peaks due to the two species in the presence of hydrogen. Figure 3.10a shows the spectrum at 298 K under hydrogen pressure of 40 kPa. As described above, two kinds of protons were observed at 6.4 ppm and 9.4 ppm. When temperature was raised, both two peaks shifted, and at the same time, both peaks broadened significantly as shown in Figs. 3.17a–d. The extent of broadening of 6.4 ppm peak was more pronounced than that of 9.3 ppm peak, indicating the higher mobility of 6.4 ppm protons compared with 9.3 ppm protons in accordance with the conclusion from the effect of spinning frequency at 298 K. At 373 K, 6.4 ppm peak was further broadened and merged into 9.3 ppm. The original shape was completely reproduced by cooling the sample down to 298 K, indicating the phenomenon is reversible. The temperature dependence of the spectrum clearly shows that the exchange reaction proceeds between the two types of protons [53]. Exchange of protons between two types of protons in H-Y zeolite was reported based on the variable temperature experiments of 1H MAS NMR [54]. At 298 K, protons in the supercages and sodalite cages gave peaks at 4.0 and at 4.7 ppm, respectively. By raising the temperature, two peaks merged and a single peak was observed at 4.4 ppm at 673 K. the two peaks reappeared when temperature was decreased to 298 K.

NMR Spectroscopy

(e) (d) (c) (b) (a) 20

15

10

ppm

5

0

–5

Figure 3.17 Change in the 1H MAS NMR spectrum of R-AgTP with temperature. Each spectrum was recorded by heating the sample stepwise from 298 to 373 K under hydrogen (40 kPa). Temperature: (a) 298; (b) 333; (c) 353; (d) 373 K. The spectrum (e) recorded at 298 K, after the spectrum was measured at 373 K. Reprinted with permission from T. Baba, Y. Ono, Appl. Catal., A 181, 227(1999).

3.5.2 

31P

MAS NMR

Although 1H MAS NMR spectroscopy is capable of providing structural information about various hydroxyl groups on solid acid catalysts directly (including bridging OH groups, i.e., Brønsted acid site), it is limited by spectral resolution because of the narrow chemical shift range (ca. 20 ppm) available at the 1H nucleus. Therefore, acid characterization by solid-state MAS NMR spectroscopy normally requires the adsorption of suitable probe molecules; valuable acid features (such as type, strength, and distribution of acid sites) can be obtained through the interactions between the probe molecules and the Brønsted or Lewis acid sites. A variety of solid-state 13C and 15N MAS NMR have been applied to the characterization of solid acids. For example, 13C MAS NMR

111

112

Characterization of Solid Acid Catalysts

spectroscopy of adsorbed 2–13C-acetone was used to estimate the acid strength [55]. The chemical shifts of 15N in pyridine may be used to distinguish between Brønsted and Lewis sites [56]. However, these studies are hampered by low sensitivity and/ or limited chemical shift ranges of the observed nuclei. 13C and 15N are the NMR-active isotopes used in these studies, but these species have natural abundances of 1.1% and 0.4%, respectively. Therefore, isotopic labeling is generally necessary. Both 13C and 15N also have moderate to low gyromagnetic ratios, and this further contributes to their low sensitivity. The use of phosphorus-containing bases in conjunction with solid-state 31P NMR overcomes the most difficult experimental limitations associated with 13C and 15N studies [57]. The 31P isotope is 100% abundant in nature and it also has a larger gyromagnetic ratio than 15N and 13C. The advantage of using the phosphorous molecules over other NMR probes such as pyridine, are a higher sensitivity and a wider chemical shift range (>300 ppm) possessed by the 31P nucleus compared with 13C or 15N. A solid-state 31P NMR technique revealed the propensity for simultaneous determination of the types and strengths of acid sites in zeolites and related catalysts using trialkylphosphines or trialkylphosphine oxides as probes. Lunsford and coworkers showed that trimethylphosphine (TMP) reacts with Brønsted acid sites in various zeolites to form protonated TMPH+ ions to give a 31P resonance peak in the range from –2 to –5 ppm, whereas the resonance responsible for TMP’s interaction with Lewis acid sites is located at a higher field, typically within the range from ca. –20 to –60 ppm [57–60]. Because of the large chemical shift range, TMP is sensitive for probing acid sites with different Lewis acid centers. In the 31P MAS NMR spectra of TMP taken without high power proton decoupling, the peak due to trimethylphosphonium ion (TMPH+) on zeolites was split into a doublet that results from 1H–31P J coupling [58–61]. Figure 3.18 shows the spectra with and without proton-decoupling of TMP in H-mordenite after removal of weakly bound molecules [61]. The resonance at –2.2 ppm for H-mordenite is due to the protonated adduct, TMPH+ with proton decoupling, and, in the spectrum taken without proton decoupling, the resonance due to TMPH+ adduct is split into a well-resolved doublet that results from the 1H–31P J coupling. The observed value

NMR Spectroscopy

for the JP–H coupling of TMPH+ on zeolites agrees with the reported coupling of ~500 Hz for TMPH+ in an aqueous solution. The value of JP–H coupling in the 31P NMR spectrum of zeolites provides definitive evidence that there is essentially complete transfer of the Brønsted proton to TMP.

Figure 3.18 Room temperature 31P MAS NMR spectra of TMP/H-MOR acquired (a) with proton decoupling, and (b) without proton decoupling (spinning speed = 10 kHz). Reprinted with permission from H. M. Kao, C. Yu, M.-C. Yeh, Micropor. Mesopor. Mater., 53, 1 (2002).

TMP adsorbed on ZrO2 exhibited a resonance at –43 ppm, which was attributed to a Lewis bound adduct [62, 63]. Adsorption of TMP on sulfated ZrO2 resulted in the formation of the protonated adduct [62, 63]. Adsorption of TMP on sulfated zirconia also gave rise to TMP molecules located on different Lewis acid centers [62]. The 31P MAS NMR spectra of TMP adsorbed on various solid acids, including silica–alumina, alumina, and SnO2–Al2O3 were also reported [60]. Certain drawbacks exist while using TMP as the probe molecule. First, the narrow 31P chemical shift range (ca. 3 ppm) of the TMPH+ complexes makes it difficult to embody various Brønsted acid sites. Second, the flammable and air-sensitive nature of TMP in bulk demands diligent and cumbersome sample preparation procedures. In these contexts, trialkylphosphine oxides, such as trimethylphosphine oxide, TMPO are more preferable as probes. In contrast to TMP, the phosphorous oxide molecules, which possess partially negatively charged oxygen atom, tend to interact

113

114

Characterization of Solid Acid Catalysts

with bridging hydroxyl groups (which act as proton donors) in zeolitic adsorbents to form O–H hydrogen bonds. Consequently, the density of the electron cloud surrounding the 31P nucleus neighboring the oxygen atom on the phosphorus oxides decreases with increasing acid strength of the Brønsted acid sites, which in turn causes the 31P resonance to shift downfield [57]. Adsorption of TMPO on amorphous silica alumina exhibited several peaks [64]. The peak at 40 ppm was assigned to physisorbed TMPO and the shoulder at 53 ppm to Lewis bound TMPO. The peak at 65 ppm was assigned to TMPOH+ bound to surface Brønsted sites. Proton decoupled 31P MAS NMR spectra of TMPO adsorbed on H-mordenite and H-beta displayed multiple resonances in the range of 45–90 ppm as shown in Fig. 3.19 [61]. The spectra could be deconvoluted by six and five components for H-mordenite and H-beta, respectively. Five peaks at 86.4(I’), 80.3(I), 72.9(II), 69.6(III), and 62.1(IV) ppm for mordenite, and four peaks at 77.6(I), 71.3(II), 67.3(III), and 57.9(IV) ppm for beta were assigned to TMPO on Brønsted acid sites. The resonance of site IV was suggested to be related with the dealumination of the zeolites. The resonance V was assigned to physisorbed TMPO [61]. On H-ZSM-5, five peaks due to TMPO adsorbed on Brønsted acid sites were observed [65].

Figure 3.19 Proton decoupled 31P MAS NMR spectra of (a) TMP/H-MOR and (b) TMP/H-β, acquired at room temperature and at a spinning speed = 10 kHz, together with the deconvoluted spectra (---). Reprinted with permission from H. M. Kao, C.-Yu, M.-C. Yeh, Micropor. Mesopor. Mater., 53, 1 (2002).

Adsorption of TMPO on g-alumina gave rise to the peaks at 37 ppm, which was attributed to its chemisorption on Lewis

NMR Spectroscopy

sites [66]. The adsorption of TMPO on H-Y zeolites gave the peaks at 50–55 ppm and 63 ppm. These peaks were ascribed to the molecule adsorbed on Brønsted acid sites [66]. In the case of ultrastable Y zeolite, five peaks were observed [67]. The peaks at 63 and 53 ppm were assigned to TEMPO adsorbed on Brønsted acid sites, the former corresponded to the stronger sites. The peak at 37 ppm was assigned to the molecule adsorbed on Lewis acid sites. The peaks at 43 and 39 ppm were due to physisorbed and crystalline TEMPO molecules. The numbers of Brønsted acid and Lewis acid sites were estimated to be 676 and 165 µmol g–1. A total of four resonance peaks were identified to TMPO adsorbed on H-MCM-41 (SiO2/Al2O3 = 25) [68]. The resonance peaks at 68.5 and 56.7 ppm were assigned due to TMPO adsorbed on Brønsted acid sites with different acid strengths, whereas the peaks at 45.7 and 40.5 ppm were attributed to TMPO adsorbed on the silanol groups and physisorbed TMPO, respectively. The latter peak has a chemical shift resembling that of crystalline TMPO (39 ppm). No Lewis acidity was found in the H-MCM-41. The 31P MAS NMR of TMPO on heteropolyacid (H3PW12O40) gave broad peaks in the range of 80–90 ppm [69]. These peaks were deconvoluted into 89.9, 86.7, 83.7, and 81.7 ppm and assigned to TMPOH+ adsorbed on the acid. Increasing loading of TMPO gave additional peaks within 56–72 ppm, which were assigned to (TMPO)2H+ species. Sulfated zirconia (S​O​2– 4​  ​–ZrO2) showed the signals of protonated TMPO at 69.2 and 62.1 ppm [63]. They correspond to relatively strong Brønsted acid sites and weak ones (ZrOH), respectively. Besides, the peak due to Lewis acid sites appeared at 52.3 ppm. Sulfated tin oxide (S​O​2– 4​  ​–SnO2) showed signals at 72.8 and 66.2 ppm, indicating the presence of strong Brønsted acid sites. These results 2– indicate that S​O​2– 4​  ​–SnO2 is stronger Brønsted acid than S​O​4​  ​–ZrO2 [63]. Different chemical shifts and the assignments were also given for S​O2– ​ ​  ​–ZrO2; the peaks at 87, 68 ppm for Brønsted acid sites and 4 the peaks at 90 and 63 ppm for Lewis acid sites [70]. Layered metal oxide, HNbMoO6, is highly active catalysts for acid-catalyzed reactions such as alkylation and esterification [71, 72]. Adsorption of TMPO on the layered oxide led to the appearance of the peaks at 86.4 and 81 ppm. Mesoporous Nb–W oxides are highly active solid acids [73]. They are much more active than Amberlyst-15 or Nafion-H for

115

116

Characterization of Solid Acid Catalysts

the alkylation of anisole with benzyl alcohol and hydrolysis of sucrose. Adsorption of TMPO on the oxides developed a main peak at 75 ppm together with peaks at 63 ppm and at 86 ppm, indicating that the acid strength of the Brønsted acid sites of the oxides is comparable to those of zeolites.

3.6  Test Reactions

Catalytic activity and selectivity reflect the surface properties of the catalyst. The reaction for which the catalytic behaviors reflect the acidic properties can be a proper test reaction. The items that can be characterized by test reactions are the distinction between acidic and basic properties of the catalyst, determination of quality of the acid sites (Brønsted or Lewis acids), estimation of softness and hardness of the Lewis acid sites, evaluation of strength of Brønsted and Lewis acid sites, and counting the number of active sites.

3.6.1  Acidic and Basic Properties 3.6.1.1  Butene isomerization

Because the reaction mechanisms are well established for both acid-catalyzed and base-catalyzed reactions, butene isomerization is a good test reaction for estimating the nature of active sites as to whether the active sites are acidic or basic. Isomerization among 1-butene, cis-2-butene and trans-2-butene is catalyzed by both solid acids and solid bases. The nature of active sites is reflected on the selectivity of the two isomers produced in the reaction starting from one isomer. The product selectivities are different from those expected from the equilibrium values listed in Table 3.7 for both acid-catalyzed and base-catalyzed isomerization. In addition, a deuterium tracer experiment can give a definite answer to the question whether intermolecular H transfer, which is expected for acid-catalyzed reaction, or intramolecular H transfer, which is expected for base-catalyzed reaction, is involved in the isomerization. The deuterium tracer experiment to distinguish between intermolecular H transfer and intramolecular H transfer is the

Test Reactions

coisomerization of non-deuterio butene (d0) and perdeuterio butene (d8) in which the starting isomer is a C4H8:C4D8 = 1:1 isotopic mixture. If the reaction involves intermolecular H transfer, the products consist of d0, d1, d7, and d8 isotopic mixture. On the other hand, if the reaction involves intramolecular H transfer, the products consist only of d0 and d8 isotopic mixture. This method was developed by Hightower and Hall, and applied to butene isomerization over SiO2–Al2O3 and Al2O3 [75]. The diagnostic method of the coisomerization can clarify whether butene isomerization is catalyzed by acidic sites or basic sites. Table 3.7

Equilibrium percentages of butene isomers

Temperature (K) 1-Butene (%) trans-2-Butene cis-2-Butene (%) 273 323 373

1.9

78.9

25.7

6.9

65.4

28.0

4.2

423

10.0

523

16.8

473

13.3

Note: Calculated from Eqs. (1) and (3) in Ref. 74.

70.6 60.2 56.0 52.1

25.2 29.8 30.6 31.1

The mechanism for acid-catalyzed isomerization is illustrated in Fig. 3.20.

Figure 3.20 Mechanism of acid-catalyzed isomerization of butenes.

117

118

Characterization of Solid Acid Catalysts

Addition of surface H+ to any of butene isomers results in the formation of 2-butyl cation as a common reaction intermediate. In the 2-butyl cation, C1, C2–H, and C3 atoms all lie in a plane, parallel to the surface. The C4 methyl group extends away from the surface, leaving the two hydrogens (labeled Ha and Hb) on C3 directed toward the surface. The two hydrogen atoms on C3 are geometrically different, for loss of Ha will result in the formation of cis-2-butene, whereas the trans isomer will result from loss of Hb. Probability of losing either the Ha or the Hb is equal, since the two C–H bonds are energetically quite similar. Accordingly, the cis/trans ratio in the 1-butene isomerization is very close to unity for solid acid catalyst regardless of the reaction temperature. The 1/trans ratio in the cis-2-butene isomerization over acid sites varies with the reaction temperature and perhaps with strength of acid sites. The probability of losing any of three hydrogen atoms on C1 leading to 1-butene is three times higher than that of losing Hb leading to trans-2-butene. However, the primary C1–H bonds are stronger and more difficult to be cleaved than the secondary C3–H bonds, and this is reflected in the higher activation energy for the formation of 1-butene than trans from the cis isomer. Effect of activation energy difference normally exceeds the effect of the probability difference. In many cases, 1/trans ratios are less than unity in the cis-2-butene isomerization over solid acid catalysts. In all interconversions between each of three isomers, the H+ added to reactant isomer to form the 2-butyl cation is retained in the isomerized products. The H+ added to the reactant should originate from the other butene molecules except when the original surface protons interact with the reactant molecules for the first time. The original surface protons are quickly replaced by the protons originating from the reacting molecules. Accordingly, the intermolecular H transfer is involved in the acid-catalyzed isomerization. The isotopic distribution in the coisomerization over typical acid catalyst SiO2–Al2O3 is shown in Table 3.8. The products (1-butene and trans-2-butene) consist essentially of d0, d1, d7, and d8, demonstrating occurrence of intermolecular H transfer over SiO2–Al2O3.

Test Reactions

Table 3.8

Product cis1-

trans-

Isotopic distribution of butene isomers in coisomerization of cis-2-butene d0/d8 over SiO2–Al2O3 at 292 K

% each product 91.4

1.1

7.5

Isotopic composition of products/%

d0

d1

d2

d3 d4 d5

d6

d7

d8

43.4

1.9

0.2

0

0.4

4.7

49.4

33.7

32.6

24.4

18.3

0

2.7 0.1 0

1.4 0.1 0

0

0.2 3.8 21.3 13.8

0.3 2.7 21.4 23.2

The mechanism of base-catalyzed isomerization of butenes is shown in Fig. 3.21.

Figure 3.21 Mechanism of base-catalyzed isomerization of butenes.

The intermediates are cis and trans forms of allylic carbanion. The characteristic selectivity of the product isomers in basecatalyzed isomerization is caused by the relative stability of two allylic carbanions as well as a slow direct interconversion between the two allylic carbanions. Allylic carbanion is more stable in the cis form than in the trans form as proposed by Bank et al. [76, 77]. Direct interconversion between cis and trans form of allylic carbanion has a high energy barrier to cross over since C2–C3 bond has a double bond character. Isomerization is initiated by abstraction of an allylic H from each isomer by basic site. Abstraction of an H+ from 1-butene results in the formation of both cis and trans allylic carbanions. Since the cis form of allylic carbanion is more stable, the concentration

119

120

Characterization of Solid Acid Catalysts

of allylic carbanions is higher in the cis form than in the trans form on the surface of solid base catalysts. The geometrical structure is retained during addition of an H+ to allylic carbanion; addition of an H+ to the cis form and the trans form of allylic carbanions results in the formation of cis-2-butene and trans-2-butene, respectively. Accordingly, cis-2-butene is predominantly formed over trans-2-butene in the initial stage of 1-butene isomerization. Abstraction of an H+ from cis-2-butene results in the formation of cis form of allylic carbanion. Interconversion of cis form of allylic carbanion to trans form is slower than the addition of an H+ to form 1-butene. Accordingly, 1-butene is predominantly formed over trans-2-butene in the initial stage of cis-2-butene isomerization. Intramolecular H transfer is involved in the base-catalyzed butene isomerization. The H atom that is abstracted from a molecule returns to the same molecule to form isomerized product. The occurrence of intramolecular H transfer can be evidenced by coisomerization of butene-d0/d8. The isomerized butene isomers consist essentially of d0 and d8 isomers. Table 3.9 summarizes the characteristic features of acidcatalyzed isomerization of n-butenes in contrast to those of basecatalyzed one. Table 3.9

Reaction features in acid-catalyzed and base-catalyzed isomerization of butene Acid-catalyzed

Base-catalyzed

Intermediate

2-Butyl cation

Allylic carbanion

1/trans from cis-2-butene

Close to the equilibrium value

Higher than the equilibrium value

Poisoning effect by NH3

Strong

cis / trans ratio from 1-butene

H transfer involved (Isotopic distribution in the product of coisomerization of d0/d8) Poisoning effect by CO2

Close to 1

Intermolecular (d0, d1, d7, and d8)

None

Higher than 1

Intramolecular (d0 and d8)

Slight

Strong

Test Reactions

3.6.1.2  Alcohol dehydration and dehydrogenation Alcohols undergo dehydration to alkenes and dehydrogenation to aldehydes or ketones over acidic and basic catalysts. The product distribution varies with the acid and base properties of the catalysts, the acid and base properties can be estimated by the product distribution. It is often assumed that acidic sites are responsible for dehydration of alcohols and basic sites are responsible for dehydrogenation of alcohols. Therefore, the reactions of alcohols have frequently been used for characterizing acid–base sites of solid catalysts. For this purpose, 2-propanol has been used most commonly. Butanols and cyclohexanol have also been used. However, a simple assumption that acidic sites are responsible for dehydration and basic sites for dehydrogenation is not always valid from a mechanistic viewpoint. Base-catalyzed dehydration occurs over some solid bases. The mechanism of dehydration and dehydrogenation of alcohols over acid–base catalysts can be classified as follows (Fig. 3.22): E1 mechanism: The first step of dehydration is the formation of a carbenium ion by abstraction of an OH group. This mechanism occurs with strongly acidic catalysts such as the H form of zeolites. The acidic center A may be either Brønsted or Lewis type. In the former case, the carbenium ions may be produced with intermediacy of oxonium ions. The isomerization occurs at the carbenium ion stage. Thus, the formation of 2-butene from 1-butanol is indicative of E1 mechanism. E2 mechanism: The elimination of a proton and a hydroxyl group from alcohols are concerted without formation of ionic intermediates. Both acidic and basic centers are required in this mechanism. Lack of but-2-ene or exclusive formation of 1-butene from 1-butanol is an indication of E2 mechanism. From 2-butanol, preferential formation of 2-butene (Saytzev orientation) is observed. Alumina is a typical E2 oxide. E1cB mechanism: The first step of dehydration is the formation of a carbanion; an H+ is abstracted by basic site in the first step. This mechanism occurs with strongly basic catalysts such as alkaline earth oxides. High selectivity for 1-

121

122

Characterization of Solid Acid Catalysts

butene (Hofmann orientation) from 2-butanol is indicative of E1cB. Whenever E1cB mechanism is found, dehydrogenation is also found in addition to dehydration. The H– is abstracted from the anion for dehydrogenation, whereas OH– is abstracted for dehydration. Usually stronger bases show higher selectivity for dehydrogenation.

Figure 3.22 Mechanisms of 2-propanol dehydration and dehydrogenation.

Table 3.10 summarizes the predominant products in decomposition of alcohols over the catalysts with different acid–base properties. If dehydrogenation occurs, we can safely state that the catalyst is basic. If dehydration occurs, we have to use 2-propanol, 1-butanol, and 2-butanol before stating the acid base properties of the catalyst. Table 3.10 Reaction

Main products in decomposition of alcohols Dehydration

Dehydrogenation

Catalyst property

Acidic

Acid-Base Basic bifunctional

Mechanism

E1

E2

Example

2-Propanol 1-Butanol

2-Butanol

H-zeolites Al2O3

ZrO2

E1cB

Dehydration product C3H6

2-C4H8 2-C4H8

C3H6

1-C4H8 2-C4H8

Basic MgO

Dehydrogenation product

C3H6

CH3COCH3

1-C4H8

CH3COC2H5

1-C4H8

C3H7CHO

Test Reactions

3.6.1.3  Cyclization of acetonylacetone Acetonylacetone undergoes both acid- and base-catalyzed intramolecular cyclizations. Acid catalysis produces 2,5-dimethylfurane (DIMF), whereas base catalysis leads to 3-methyl-2cyclopenten-1-one (MCPO). The mechanistic schemes for the two reactions are shown in Fig. 3.23 [78]. The selectivity of reaction is used for distinguishing acidic and basic surfaces. +

C

OH–

C

C

C

C

Figure 3.23 Reactions from acetonylacetone.

Table 3.11

O

Selectivity in the reaction of acetonylacetone Selectivity/%

Catalyst

Conversion/%

DIMF

MCPO

H-ZSM-5

96.8

98.7

1.2

[78]

SiO2–Al2O3

20

99.0

1.0

[79]

Nb2O5

15

54

42

[80]

Na-ZSM-5 MgO

Al2O3

99.9

20

71

4.4

0

13

89.1

100 77

Ref. no.

[78]

[79] [80]

Selected data are listed in Table 3.11. Over H-ZSM-5, DIMF was produced at greater than 97% selectivity, whereas over Na-ZSM-5

123

124

Characterization of Solid Acid Catalysts

MCPO was obtained with selectivity of >90%. SiO2–Al2O3 shows a 99% selectivity for DIMF, whereas MgO shows a 100% selectivity for MCPO. Hydrated and calcined Nb2O5 shows amphoteric nature. Al2O3 shows basic character rather than acid, though Al2O3 show very high selectivity for dehydration in 2-propanol dehydration.

3.6.1.4  Reactions of 2-methyl-3-butyn-2-ol

Product distribution of the decomposition of 2-methyl-3-butyn2-ol (MBOH) is an effective measure for distinguishing acidic, basic and amphoteric catalysts [81, 82]. MBOH undergoes dehydration over solid acids to 3-methyl-3-buten-1-yne (Mbyne) and decomposition to acetylene and acetone over solid bases. Over amphoteric catalysts, 3-hydroxy-3-methyl-2-butanone (HMB) is mainly formed (Fig. 3.24). Acidic reactivity

HO

MBOH

H

Amphoteric reactivity

Basic reactivity

H

H and

O Prenal

Mbyne HO

O

HMB O

O

and +

MIPK H

H

Figure 3.24 The reaction products from MBOH over acidic, amphoteric and basic catalysts. Reprinted with permission from H. LauronPernot, F. Luck, J. M. Popa, Appl. Catal. A, 78, 213 (1991).

The product distribution in the decomposition of MBOH on various catalysts reported by Lauron-Pernot et al. is listed in Table 3.12. SiO2–Al2O3, a typical acidic catalyst, gave 90% selectivity to Mbyne, 3-methyl-2-butenal (prenal) also being formed. Over MgO, a typical basic catalyst, the conversion is very high and acetylene and

Test Reactions

acetone are formed exclusively. Though the conversion is low, ZnO also gave a basic character. Over ZrO2, a typical amphoteric oxide, HMB, a hydration product, is mainly formed. Other products are Mbyne, acetone and acetylene. Water is necessary for the hydration of MBOH to HMB. The hydration of MBOH to HMB is presumed to involve the surface hydroxyl groups or traces of water either contained in MBOH or formed by side reactions such as acetone condensation. Over Al2O3 with a low Na2O content, MIPK(3methylbutene-2-one) is the predominant product, but acetylene, acetone and Mbyne are also formed, indicating that Al2O3 has both acidic and basic characteristics. By doping Na2O onto Al2O3, the products shift to acetylene and acetone, indicating that the material is shifted to a solid base. Table 3.12

Product distribution in the reaction of MBOH over various catalysts Selectivity/%

Catalyst

Conv./% Mbyne Prenal Acetylene Acetone HMB MIPK

SiO2–Al2O3 25

90

9

0.5

0.5

0

0

ZnO

0

0

50

50

0

0

MgO

70

ZrO2

8

Al2O3b

9.5

Al2O3a Al2O3c

20

7

100

0

18 16

4.5 0

0 0 0

0

0

52 2

22

41

50

48 18 23

43.5 50

0 76 0

4.5 0

0 0

39

6.5 0

Source: Adapted with permission from H. Lauron-Pernot, F. Luck, J. M. Popa, Appl. Catal., 78, 213 (1991). aNa O content, 0.025%. 2 bNa O content, 0.27%. 2 cNa O content, 4.1%. 2

The mechanisms for the dehydration to Mbyne and isomerization to prenal over acid sites are proposed as shown in Fig. 3.25. The mechanism of the decomposition of MBOH to acetylene and acetone over basic sites is proposed as shown in Fig. 3.26.

125

126

Characterization of Solid Acid Catalysts

Figure 3.25 Mechanism of MBOH reaction over acidic catalysts. Reprinted  with permission from H. Lauron-Pernot, F. Luck, L. M. Popa, Appl. Catal. A, 78, 213 (1991).

Figure 3.26 Mechanism of MBOH reaction over basic surface. Reprinted with permission from H. Lauron-Pernot, F. Luck, L. M. Popa, Appl. Catal. A, 78, 213 (1991).

3.6.2  Brønsted and Lewis Acids

3.6.2.1  Rearrangement of cyclic acetals of a-bromo phenyl ketone The products are different for Brønsted acid-, hard Lewis acidand soft Lewis acid-catalyzed reactions of cyclic acetals of

  

Test Reactions

2-bromopropiophenone [83] Fig. 3.27. Brønsted acid sites promote hydrolysis in the presence of moisture to 2-bromopropiophenone. On the other hand, Lewis acid sites yield either 2-bromoethyl 2phenylpropanoate (hard Lewis acid sites) or 5-methyl-6-phenyl-2,3dihydro-1,4-dioxin (soft Lewis acid sites) arising from the 1,2-aryl or 1,2-alkoxy migration, respectively. Br in the product 2-bromoethyl 2-phenylpropanoate can be replaced by Cl and OH if chlorinated catalyst or solvent is used, and water is present in the reaction mixture, respectively. O

Br 2‐bromopropiophenone B acid

O

O

O Hard L acid

Br 22‐bromopropiophenone b i h ethylene acetal

Br

O 2‐bromoethyl 2‐phenylpropanoate

Soft L acid

O O 5‐methyl‐6‐phenyl‐2,3‐dihydro 1,4‐dioxin

Figure 3.27 Reaction route of 2-bromopropiophenone ethylene acetal over different types of acid catalysts.

Difference in the product between hard Lewis acid-catalyzed reaction and soft Lewis acid-catalyzed one arises from the different coordination of the reactant to soft and hard Lewis acids. The interaction of the reactant with the soft Lewis acid occurs at the Br atom and p-system of the benzene ring, then the leaving group (Br) and the oxy group are antiperiplanar, and alkoxy migration occurs. On the other hand, the hard Lewis acid sites interact with the bromine and oxygen atom of the cyclic acetal, then the benzene ring is the group that occupies the anti disposition with respect to the bromine, and 1,2-phenyl migration occurs Fig. 3.28.

127

128

Characterization of Solid Acid Catalysts

Figure 3.28 Coordination of 2-bromopropiophenone ethylene acetal to soft and hard Lewis acids.

Table 3.13 show the results of the reaction of ethylene acetal of a-bromoacetophenone in the presence of solid acid catalysts [84, 85]. HY gave high selectivity for the ketone, reflecting Brønsted acid nature of the catalyst. ZnNaY and AgY were more selective for the rearrangement products. The ratio of dihydrodioxin and 2-phenylpropanoate indicates that AgNaY is softer Lewis acid than ZnNaY. Table 3.13

Catalyst

Results of the reaction of the cyclic ethylene acetal of abromoacetophenone in chlorobenzene at 403 K for 20 h in the presence of solid acid catalysts

Selectivity (%) Conversion (%) Ketone a-Phenylpropionate Dehydrodioxane

HY

100

91

0

0

ZnNaY

86

23

38

19

3

11

75

ZnHY ZnCl2

AgNaY

HgNaY Hg2Cl2

98

89 97

94 6

66 15 73

8

56 3

71

2

14 9

28

Note:  Ketone, a-phenylpropionate and dehydrodioxane correspond to acetal hydrolysis by Brønsted acid, 1,2-phenyl rearrangement by hard Lewis acid and alkoxy rearrangement by soft Lewis acid, respectively. Source: Reprinted with permission from F. Algara, A. Corma, V. Fomés, H. Garcia, A. Martinez, J. Primo, Stud. Surf. Sci. Catal., 78, 653 (1993).

Test Reactions

The reaction of 2-bromopropiophenone in the presence of tin-tungsten mixed oxide gave >99% selectivity for the ketone, indicating the acidic sites on the mixed oxide are Brønsted-type [86]. A strong cationic resin Dowex 50 W × 4–100 also shows Brønsted acid nature [83]. The microporous metal-organic framework [Cu3(BTC)2] (BTC = benzene-1,3,5-tricarboxylate) is a rigid zeolite-like structure and with free coordination sites on the CuII ion. The compound was a highly selective catalyst for the rearrangement of a-pinene oxide to camphene aldehyde and the cyclization of citronellat to isopulegol. The test reaction could indicate that the active sites in [Cu3(BTC)2] are hard Lewis acid [83].

3.6.3  Strength of Brønsted Acid Sites

Strength of Brønsted acid sites can be estimated by r values in Hammett rs equation (log kX/kH = rs) for H/D exchange between surface protons and substituted benzene derivatives. Hammett used the ionization constant of substituted benzoic acids in water to assess the s parameter for each substituent. The ionization constant for the substituted benzoic acid is affected by the electronic factors of the substituent. Electron-withdrawing substituents such as nitro (–NO2) and cyano (–CN) have positive s values (increasing the ionization constant relative to benzoic acid), whereas electronreleasing substitutes such as methyl and methoxy have negative s parameters. The s parameter developed by Hammett does not include resonance effect on the substituent. Brown introduced a new parameter s+ by which the resonance of positive charge in the ring during the reaction is taken account. The magnitude of the r values in the Hammett–Brown equation (log kX/kH = rs+) for the following H/D exchange is a measure of acid strength [87] Fig. 3.29. The basic hypothesis is that stronger acid sites lead to a more polar transition state with an increased degree of proton transfer to the aromatics. The degree of proton transfer is affected by the electronic effects of the substituent. The degree of proton transfer is reflected in the magnitude of the r. The more the positively charged is the intermediate, the more the negative value of the r is. (The absolute value of the r is large because of more polar intermediate.)

129

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Characterization of Solid Acid Catalysts

D +  D-Catalyst

d+

H

+ +

+  H-Catalyst

X

X

D

X

Figure 3.29 H/D exchange of benzene, showing the transition state with charge development in the ring. Reprinted with permission from V. L. C. Goncalves, R. C. Rodrigues, R. Lorencato, C. J. A. Mota, J. Catal., 248, 158 (2007).

Goncalves et al. measured the rate of H/D exchange of substituted benzenes with deuterated solid acids as well as with aqueous solutions of deuterated sulfuric acid of different concentrations [87]. The r values obtained from the plots of log kx/kH against s+ are summarized in Table 3.14. Among the solid acids examined, Amberlyst-15 shows the highest magnitude of the r. The strength of Brønsted acid sites is in the order; Amberlyst-15 > HUSY > Nb2O5 > K-10 montmorillonite > HUSY partly poisoned by butylamine. As compared with sulfuric acid solution, Amberlyst-15 and HUSY were in between 80% and 98% D2SO4, and Nb2O5 and K-10 montmorillonite were in between 60% and 80% D2SO4. Table 3.14

r values for H/D exchange of substituted benzenes with solid acids at 373 K

Acid system

r valuea

Amberlyst-15

–1.3

zeoliteb

–0.3

HUSY zeolite HUSY

Nb2O5

K-10 montmorillonite

–1.1

–0.7 –0.5

D2SO4 (98%)

–1.5

D2SO4 (60%)

–0.38

D2SO4 (80%)

–0.75

Source: Adapted with permission from Data taken from V. L. C. Goncalves, R. C. Rodrigues, R. Lorencato, C. J. A. Mota, J. Catal., 248, 158 (2007) aCalculated using Hammet-Brown s+ value. bPartly poisoned by n-butylamine.

Test Reactions

3.6.4  Number and Strength of Acid Sites by Poisoning

(ke + kp)/1017 × molecules g–1s–1

Pre-adsorption of strongly basic molecules on solid acid catalysts retards the rates of the acid-catalyzed reactions. The number of the pre-adsorbed basic molecule required to stop the reaction corresponds to the number of acid sites relevant to the reaction. This is the underlying concept by which the number of active (acidic) sites is counted by poisoning method. The molecules that retard the reaction are called “poisons.”

Pyridine adsorbed/molecules per unit cell Figure 3.30 Progressive pyridine poisoning at 370 K. ke and kp stand for the rate constants for ether formation and propene formation, respectively. Catalysts used are HY-2.5, -5.5, -8.4, -11.0, -14.0, -17.0 and -20.0, where the numbers represent the concentrations of structural OH group per unit cell. Reprinted with permission from R. Rudham, A. I. Spiers, A. W. Winstanley, Zeolites, 11, 850 (1991).

Figure 3.30 shows the activity decays with the amount of preadsorbed pyridine for 2-propanol dehydration over H-Y zeolites with different ion-exchanged degrees [88]. The “lethal doses,”

131

132

Characterization of Solid Acid Catalysts

defined as the amount of poison required to completely stop the reaction, were close to the numbers of acidic OH in the H-Y zeolites. The numbers of acidic OH were calculated from ion-exchange degree and confirmed by IR of adsorbed pyridine. All ion-exchanged H+ acted as active sites for 2-propanol dehydration to propene and diisopropyl ether. Although the lethal dose of the poison coincided with the number of the H+ in the case of 2-propanol dehydration, there reported several cases in which the lethal doses are significantly smaller than the number of Brønsted acid sites. For some reactions over zeolites, on the other hand, the number of acid sites as determined by poisoning methods is far smaller than that of acidic OH groups (Al atoms in the framework). Poisoning of no more than 10% of the sites (counted as lattice Al ions) with ammonia was sufficient to eliminate the activities for neopentane decomposition over H-mordenite, HY and HZSM-5 [89]. Ion exchange of dealuminated HY with Na+ in the amount of one fifth of the Brønsted acid sites eliminated the catalytic activity for hexane cracking. The IR band at 3602 cm–1, assigned to a framework OH group that functioned as a Brønsted acid, was found to decrease in a manner parallel to the catalytic activity [90]. Similar poisoning effects were observed for cracking of hexane and isomerization of cyclopropane over ZSM-5 [91]. Ion exchange of HZSM-5 with alkali cations with a minute degree drastically decreased the catalytic activity of the zeolite. The extent of the activity decrease differed from one reaction to another. The complete activity loss of HZSM-5 was observed by 50% and 25% exchange with Na+ and K+ cations, respectively, in the cracking of hexane at 623 K. For the isomerization of cyclopropane to propene at 373 K, only 1% exchange of Na+ with H+ led to 80% loss of the original activity of H-ZSM-5. In addition, the temperature dependence of the linewidth dependence of the 1H MAS NMR signal of the acidic OH groups was greatly affected by substituting only 1% of Na+ and N​H+4​ ​​  ions for protons [91]. The reasons for the smaller number of the lethal doses as compared with Brønsted acid sites are not well understood. Poisoning method can be applied to estimate the strength of acid sites relevant to acid-catalyzed reactions. It is assumed that easy reactions proceed over weakly acidic sites, whereas difficult reactions require strongly acidic sites.

Test Reactions

133

One point that we should take care of is the way of adsorption of poisons. As the molecules used as poisons are normally strongly basic such as ammonia, amines and pyridine, these strong bases are irreversibly adsorbed even on weak acid sites at a low temperature. Stepwise addition of the poisons to the catalytic system may result in irreversible adsorption on weaker sites and stronger sites may remain uncovered by the poisons. To avoid the problem, stepwise desorption of the preadsorbed poison at increasingly high temperature is recommended. Figure 3.31 shows the influence of the desorption temperature of pre-adsorbed pyridine (TD) on the recovery of the activities (AR) for the transformation of several hydrocarbons [92]. The temperature at which the activity appears on desorption of pyridine is the lowest (~493 K) for the easiest reaction of 3,3-dimethyl-butene and the highest (~793 K) for the most difficult reaction of n-hexane cracking. The weak acid sites that are able to retain pyridine at 493 K are active for 3,3-dimethyl-1-butene isomerization, whereas only strong acid sites that are able to retain pyridine at 793 K are active for n-hexane transformation.

Figure 3.31 Influence of the temperature of pyridine desorption (TD) on the activity of HY for the transformation of hydrocarbons (AR). nC = n-hexane; 2mC5 = 2-methylpentane; 2,4dmC5 = 2,4-dimethylpentane; 2,2,4-tmC5 = 2,2,4-trimethlpentane; ox = o-xylene; 1,2,4tmb = 1,2,4-trimethylbenzene; 3,3 dmb1 = 3,3-dimethyl-1-butene. Reprinted with permission from G. Bourdillon, C. Gueguen, M. Guisnet, Appl. Catal., 61, 123 (1990).



134

Characterization of Solid Acid Catalysts

3.6.5  Strength Estimation through Cracking Activity Cracking is a typical acid-catalyzed reaction. The activity for cracking is one of the most important catalytic properties of solid acid catalysts. The relationship between cracking activity and strength of acid sites is also useful in estimation of acid strength. These points were studied for cracking of hexane and isobutane over different solid acid catalysts [93].



Figure 3.32 Rate constants for n-hexane cracking measured for various crystalline aluminosilicate compositions. Fau represents faujasite. (Ce, La, H-Fau)s is steam-treated (Ce, La, H-Fau). Adapted with permission from J. N. Miale, N. Y. Chen, P. B. Weisz, J. Catal., 6, 278 (1966).

The activities of different types of zeolites and amorphous SiO2–Al2O3 for hexane cracking were measured in a reaction temperature range 443–813 K. Arrhenius plots of the observed rate constants with temperature for different types of zeolites and amorphous SiO2–Al2O3 are shown in Fig. 3.32. Apparent activation energies are similar to all catalysts (about 125 kJ/mol). By extrapolation of each line in Arrhenius plot, the activity relative to that of amorphous SiO2–Al2O3, defined by “a,” is obtained for each catalyst.

a = Rate constant for solid acid catalyst/rate constant for amorphous SiO2–Al2O3

Test Reactions

The a values for NH4-exchanged mordenite and (rare earth + NH4)exchanged faujasite exceeded 10,000, those for NH4-exchanged faujasite and steam treated (rare earth + NH4)-exchanged faujasite were 6,400 and 20, respectively. Later, the catalytic activities of mordenite and ZSM-5 zeolites for hexane cracking were evaluated by the a-test. The relative activity increases in the order; SiO2-Al2O3 < TCC-DB-1(commercial cracking catalyst) < NaY < NaZSM-5 < MgX < LaY < HY < H-ZSM-5 (120) < H-mordenite < HZSM-5 (60) < H-ZSM-5 (30), where the number in parentheses represents the SiO2/Al2O3 ratio of ZSM5 [94]. The a value for Ca-exchanged A was 0.6. The a value of H-ZSM-5 was proportional to the content of aluminum in the framework. This indicates that all the acid sites in H-ZSM-5 have the same activity for the cracking of hexane [95]. The relation between cracking activity and acid site strength was studied by Umansky et al. [96]. The activities of mordenite, beta zeolite, HY zeolite and amorphous SiO2–Al2O3 for isobutane cracking were measured in a temperature range 494–795 K. The temperature required for 0.5% conversion was taken as an activity scale. The H0 values of the catalysts were determined by the absorption peak position in UV-Vis spectrum from 4-nitrotoluene and 4-nitrofluorobenzene adsorbed on the sample. The peak positions of these indicators vary with the concentration of H2SO4 in the aqueous solutions. By the peak positions of adsorbed indicators, the acid strength of the sample is estimated in the scale of H2SO4 concentration. From the relation of H2SO4 concentration with H0 value, the acid strength of the sample is expressed in H0 scale. The correlation of the cracking activity with H0 is shown in Fig. 3.33. Stronger the acid strength, higher the isobutane cracking activity. For a certain solid acid catalyst, the cracking activity could be estimated from the H0 value of the catalyst, the difference in number of active sites of different catalysts being not taken into consideration. Fraenkel et al. extended the method to estimate the H0 value of sulfated zirconia (S​O​2– 4​  ​–ZrO2) from the comparison of the catalytic activities of S​O​2– ​  – ​ ZrO 2, mordenite and HZSM-5, and listed the acid 4 strength order; S​O​2– ​  – ​ ZrO 2 (H0 ~ –18) > mordenite (–14) > HZSM-5 4 (–9) [97].

135

136

Characterization of Solid Acid Catalysts

Figure 3.33 Correlation of the temperature required for 0.5% conversion of i-C4H10 with –H0. Reprinted with permission from B. Umansky, J. Engelhardt, W. K. Hall, J. Catal., 127, 128 (1991).



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Chapter 4

Catalytic Properties of Solid Acid Catalysts

4.1  Zeolites 4.1.1  Characteristics of Zeolites as Solid Acid Catalysts Zeol ites are crystalline aluminosilicates with a three-dimensional framework structure that forms uniformly sized pores. The term zeolite was originally coined in 1756 by Swedish mineralogist Axel  F. Cronstedt, who observed that upon rapidly heating a natural  zeolite mineral, it produced a large amount of steam from water that had been adsorbed by the material. Based on this, he called the material zeolite, from two Greek words, “zeo,” meaning boil and “lithos,” meaning stone. Although Cronstedt’s original sample was widely reported as stilbite, it was concluded recently that  Cronstedt’s zeolite was most likely predominantly stellerite [1]. There are over 40 naturally occurring zeolites. Besides, there  are more than 200 synthetic structures (including the related materials), which do not appear in nature.

Solid Acid Catalysis: From Fundamentals to Applications Hideshi Hattori and Yoshio Ono Copyright © 2015 Pan Stanford Publishing Pte. Ltd. ISBN  978-981-4463-28-7 (Hardcover), 978-981-4463-29-4 (eBook) www.panstanford.com

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Catalytic Properties of Solid Acid Catalysts

Synthetic zeolites are one of the most important solid acids as catalyst. They are used for a variety of industrial processes, especially in the field of petroleum refining and the syntheses of petrochemicals. The first academic paper on catalysis by synthetic zeolites appeared in 1960 [2, 3]. In the following years, Y-type  zeolites completely replaced the position of amorphous silica– alumina in fluid catalytic cracking (FCC). This is due to the very high activity of the zeolite compared with amorphous silica–alumina though the composition and the local structure of acid sites are seemingly very similar. As shown in Table 4.1, the catalytic activity  of hydrogen form of Y-zeolites in hexane cracking is about 30,000 times higher than that of amorphous silica–alumina [4]. The significant properties of zeolites and the related materials as  solid acids can be summarized as follows: (1) The catalytic activities of zeolites are very high compared with that of amorphous silica–alumina, as mentioned above. The amount of acid sites is directly related to the framework aluminum content, and, thus, can be adjusted through the framework Al content. (2) Isomorphous replacement of especially Al (e.g., by Ga, Fe, B, Ti) in zeolites is well established. The intrinsic activity of acidic sites can be modified by the isomorphous substitution. The activity decreases in the order Al > Ga > Fe > B. (3) Zeolites, in particular, the silica-rich materials, often exhibit a high thermal and hydrothermal stability. This allows their use as catalysts at high temperatures, and their frequent  oxidative regeneration even in the presence of some steam. This is a sharp contrast with catalysts with organic groups, such as ion exchange resins. The thermal stability is much higher than that of heterpolyacids. (4) Zeolites work as shape-selective catalysts because of welldefined pore sizes and cages inside the structures. (5) Zeolites allows a relatively high concentration of reactant molecules inside the zeolite cages; thus, in the presence of a zeolite, the reaction will proceed as if it were carried out at higher partial pressure. (6) Zeolites are non-corrosive and non-toxic.

Zeolites

Table 4.1 Catalysts

Relative activities of the faujasite family of zeolites for hexane cracking Relative activity

Amorphous silica–alumina

1

Rare earth hydrogen Y

460

Sodium zeolite X

Rare earth hydrogen X Hydrogen Y

Steamed-USY

Steamed, chemically dealuminated-USY

Source: B. Szostak, Stud. Surf. Sci. Catal., 137, (2001), p. 269.

1.2

7,800

30,000 23,000

870,000

4.1.2  Structure of Zeolites

The basic structural unit of all zeolite frameworks consists of silicon or aluminum atoms tetrahedrally coordinated to four oxygen atoms. Because of the presence of nanometer-sized channels and cages, zeolites have high porosity and a large surface area. They consist of robust, crystalline silica frameworks. Si4+ can be replaced by Al3+ at some places in the framework, and a negative charge is created in the framework. All such framework charges are neutralized by extra-framework cations that reside inside the  zeolite pores, where they are rather loosely held. The structural formulas of zeolites are represented by the following empirical formula:

Mx/n[(AlO2)x(SiO2)y]· wH2O,

where n is the cation valence and w represents the water contained in the voids of zeolites. The water may be reversibly removed by heat. The cations are readily exchanged with other cations such as Na+, K+, N​H+4​ ​,​  Ca2+, Mg2+, and Cu2+. Because of these properties of zeolites, they are highly effective as ion-exchangers and adsorbents. When protons neutralize framework charges, they constitute acid sites. Thus, zeolites are used for a variety of industrially important reactions as solid acid. Analogs of zeolites such as (silico) aluminophosphates have been synthesized (see Section 4.2).

143

144

Catalytic Properties of Solid Acid Catalysts

Zeolites are often called “molecular sieves.” The term molecular sieve refers to a particular property of these materials, i.e., the  ability to selectively sort molecules based primarily on a size  exclusion process. This is due to a very regular pore structure of the molecular dimensions. The maximum size of the molecular or ionic species that can enter the pores of a zeolite is controlled by the dimensions of the channels. These are conventionally defined by  the ring size of the aperture. Typical zeolite pore sizes using  oxygen-packing models are shown in Fig. 4.1 [5], where, for example, the term “8-ring” refers to a closed loop that is built from eight tetrahedrally coordinated silicon (or aluminum) atoms and eight oxygen atoms. They include small pore zeolites with eightring pores with the diameters of 0.30–0.45 nm, e.g., zeolite A; medium pore zeolites with a 10-ring, 0.45–0.60 nm in free diameter, e.g., ZSM-5; large pore zeolites with 12-ring pores, 0.6–0.8 nm,  e.g., zeolite X and Y; and extra-large pore zeolites with 14- or  18-ring pores, e.g., UTG-1. The fact that the pore openings are of similar dimensions to hydrocarbon molecules, i.e., from about 0.3  to 1.2 nm, is the characteristic features that forms the basis of the  catalytic shape-selective behavior of zeolites (see Section 2.6).  For example, n-octane readily accesses the internal void through  the pore of zeolite A, whereas isooctane (2,2,4-trimethypentane) is larger than the pore and is totally excluded.

Figure 4.1

Typical zeolite pore sizes illustrated with oxygen packing models.

Zeolites

The pore system can be one, two- or three-dimensional. It can contain pores of different sizes, and consists of channels and/or cages, etc. In the two- and three-dimensional systems, two types of channels are interconnected and at the intersections the cage-like voids are formed. In a one-dimensional system, rather uniform and straight channels exist.

4.1.3  Nomenclature of Structures of Zeolites and Zeolite-Like Materials

There is no systematic nomenclature developed for zeolites and related materials. The structure commission of International Zeolite Association assigns the three-letter code for framework topologies of the zeolites and related materials [6]. For example, FAU is the code for molecular sieves with a faujasite topology, e.g., zeolite X and Y; MFI is the code for the ZSM-5 and silicalite topologies, CHA for chabasite. It is important to note that the three letter codes are not the name of the particular zeolite, but assigned for the  structure of the materials. Thus, besides X- and Y-zeolites, the structure code of a silicoaluminophosphate, SAPO-37, is also FAU. SAPO-34 and chabasite have the same structure code of CHA. The structural features of some important zeolites are listed  in Table 4.2. Table 4.2

Zeolite A

Structural features of zeolites Pore Structure aperture code Ring sizea

LTA

8

0.41 × 0.41*

3

0.38 × 0.38*

1

Erionite

ERI

8

0.51 × 0.36***

EU-1

EUO

10

0.41 × 0.54*

ZSM-23

MTT

10

Chabasite ZSM-5 Ferrierite

CHA MFI

FER

8

10

10

10 8

Connectivity Dimensionlity between of pore different pore system systems

3 1

0.51 × 0.55*

3

0.45 × 0.52*

1

0.53 × 0.56*

0.42 × 0.54* 0.35 × 0.48*

2

Interconnected Interconnected

(Continued)

145

146

Catalytic Properties of Solid Acid Catalysts

Table 4.2

(Continued)

Zeolite

Pore Structure aperture code Ring sizea

ZSM-57

MFS

10

0.51 × 0.54*

MCM-68

MSE

12

0. 64 × 0.68*

8

10 10

Connectivity Dimensionlity between of pore different pore system systems 2

Interconnected

0.52 × 0.56*

3

Interconnected

2

Not interconnected

0.33 × 0.48*

0.52 × 0.52*

MCM-22 (ERB- MWW 1)

10

0.55 × 0.40**

SSZ-35

STF

10

0.54 × 0.57*

Mordenite

MOR

12

0.70 × 0.65*

L

LTL

12

0.71 × 0.71*

1

ITQ-7

ISV

12

0.65 × 0.61**

3

Interconnected

3

Interconnected

7.0 × 5.9*

3

Interconnected

6.2 × 7.2*

1

Theta-1 (ZSM22)

Faujasite Beta

TON

FAU

*BEA

CIT-1 (SSZ-33) CON ITQ-4 (MCM58, SSZ-42) SSZ-53

UTD-1

CIT-5

ECR-34 (gallosilicate)

aThe

IFR

SFI

DON

CFI

ETR

10 8

12 12

0.51 × 0.41**

2

0.46 × 0.57*

1

0.57 × 0.26***

0.74 × 0.74*** 0.66 × 0.59*

12

6.6 × 6.7**

12

6.4 × 7.0*

10

5.1 × 4.5*

12 12 12

14

14

14

18 8

5.6 × 5.6*

1 1 3

0.64 × 0.87*

1

0.72 × 0.75*

1

0.81 × 0.82* 1.01 × 1.01* 0.25 × 0.60**

1 3

Interconnected

number of asterisk indicates whether the channel is one-, two-, or threedimensional.

Zeolites

4.1.4  Synthesis of Zeolites Zeolites are mainly prepared by reactive crystallization of a silica– alumina gel from an aqueous mixture of reagents at 6 < pH < 14 and temperatures between 350 and 500 K. One of the important processes used to carry out zeolite synthesis is sol-gel processing. The product properties depend on reaction mixture composition, pH of the system, synthesis temperature, nature of the cations, pre-reaction “seeding” time, reaction time, and the structuredirecting agents used. In the sol-gel process, other elements (metals, metal oxides) can be easily incorporated. An amine or quaternary ammonium salt may be used as a structure-directing agent. The  dry gel conversion technique is also useful especially for the synthesis of zeolite films. In this technique, a hydrogel is dried and the resultant dry gel is converted into zeolite crystals in steam or  in a mixed vapor of steam and organic structure-directing agents  [7, 8]. The chemistry of zeolite synthesis has been reviewed  [9–11]. The synthetic procedures of various zeolites are given by  the Synthesis Commission of International Zeolite Association [12].

4.1.5  Acidic Sites in Zeolites

4.1.5.1  Acidic OH groups in proton form of zeolites As described above, the structures of zeolites are composed of extralattice cations and negative framework. The cations may be alkali metal cations or alkylammonium cations in as-synthesized zeolites. To obtain acidic zeolites, these cations have to be replaced by protons. For example, in the case of Y-type zeolites, as-synthesized Na-form of the zeolite (NaY) is first converted into ammonium  form (NH4Y) by ion-exchange with ammonium nitrate. NH4Y is  then converted into proton (or hydrogen) form (HY) by decomposing ​ NH​+4​​  ions at 650–750 K. The transformations can be expressed by  the following scheme. O

Na+ Na+ O O O O O O Si Al Si Si Al Si H Ion-exchange

147

148

Catalytic Properties of Solid Acid Catalysts

O

O

​ +4​ ​​  NH

N​H4+​ ​​ 

O O O O O O (NH4Y) Si Al Si Si Al Si H

650–750 K

H –NH3

H

O O O O O O Si Al Si Si Al Si

(HY)

The infrared spectra of the H-form of zeolites show that  protons exist as surface OH groups [13]. In the case of Y-type  zeolites, two kinds of OH groups are observed at 3562 and  3626 cm–1. The two bands (3626, 3562 cm–1) have been assigned to bridging OH groups (Si–OH–Al) located in the supercage and those located near the middle of the six-membered rings connecting the sodalite cages. The acidic nature of these OH groups is evidenced by spectroscopic studies of adsorbed probe molecules. Adsorption of pyridine results in the formation of pyridinium ions and the disappearance of the OH groups. This indicates that both OH groups act as Brønsted acid sites. On HY, another band is observed at  3740 cm–1, which does not interact with pyridine, and is assigned to the nonacidic surface silanol groups. HY is dehydrated at higher temperatures (>630 K). After dehydration, Lewis acid sites are developed as evidenced by the IR spectrum of adsorbed pyridine. The source of the Lewis acid sites  is the Al species such as AlO+, which is dislodged from the zeolite framework (dealumination). The effect of the temperature of calcination temperature of NH4Y on the amount of Brønsted acid sites and Lewis acid sites is shown in Fig. 4.2 [13]. The amount of acid sites was determined by the intensity of IR spectra of adsorbed pyridine. The changes of acid site concentration are in conformity with the structural change of the zeolites. At about 250–300°C (523–573 K), ammonia evolves to  leave acidic OH groups, which serve as Brønsted acid sites. Above 500°C (773 K), dehydroxylation of HY occurs resulting in the decrease of Brønsted acid sites and Lewis acid sites develop due to dislodged aluminum species. In the case of zeolites synthesized using cationic organic templates such as ZSM-5, the protonic form of zeolites may be obtained by controlled calcination of as-synthesized zeolites.

Zeolites

Figure 4.2

Intensity of absorption bands of chemisorbed pyridine on Brønsted and acid sites. Reprinted with permission from J. W. Ward, J. Catal., 9, 223 (1967).

4.1.5.2  Acidic OH groups in zeolites with multivalent cations

Acidic OH groups can be produced also by ion-exchange with multivalent cations such as Ca2+, Mg2+, and La3+. The development of acidity can be expressed as

[Ca(OH2)]2+  [Ca(OH)] + + H+

(4.1)

Thus, water molecules coordinated to multivalent cations are dissociated by heat-treatment to give the following structure: O

H [Ca(OH)]+ O O O O O O Si Al Si Si Al Si

The absorption band due to OH groups in [Ca(OH)]+ is observed in the infrared spectrum besides the bands of acidic OH groups [14, 15]. The amount of the Brønsted acid sites increases with decreasing cation size:

MgY > CaY > SrY > BaY.

149

150

Catalytic Properties of Solid Acid Catalysts

In the case of rare earth Y, formation of Brønsted acid sites can be expressed as follows [15, 16]:

La(H2O)3+ → La(OH)2+ + H+ + + La(H2O)3+ 2 → La (OH)2  + 2H

(4.2)



(4.3)

Stabilization of hydrogen Y could be achieved by ion exchange with multivalent cations such as La3+. Though the activity of the rare earth-exchanged Y-zeolite (RE-Y) is lower than hydrogen-Y (HY) (Table 4.1), RE-Y has been a preferable catalyst for FCC because of  its higher thermal and hydrothermal stability.

4.1.5.3  Formation of acidic OH groups by reduction of exchangeable cations

Acidic OH groups are formed by the reduction of transition metal ions.

Cu2+  + H2 → Cu0 + 2H+ 1 Ag + +     H2 → Ag 0 +  H+ 2

(4.4)



(4.5)

Protons thus formed are trapped by the framework oxygen anions  to form acidic OH groups [17]. In the case of AgY zeolite, the catalytic activity is greatly enhanced by the presence of gaseous hydrogen. The activity of AgY in the presence of hydrogen is much higher than that of HY whose  activity is not affected by hydrogen. Figure 4.3 shows the  dependence of the catalytic activity for ethylbenzene disproportionation on the partial pressure of hydrogen [18]. The effect of hydrogen is reversible, i.e., elimination of hydrogen reduces the activity, which can be regained by reintroduction of hydrogen to the system. These facts show that there is a mechanism of  interconversion of molecular hydrogen and proton. It is assumed that silver cluster ions are involved in the hydrogen-proton interconversion.

1 Ag +n +   H2  Ag nH  +  H+ 2



(4.6)

Zeolites

In fact, the reversible formation of acidic OH groups and AgnH is confirmed by 1H MAS NMR. In the case of AgA, [Ag3H]+ species has been observed by 1H MAS NMR spectroscopy [19].

Ethylbenzene conversion/%

5 4 3 2 1

0

Figure 4.3

50 Hydrogen pressure/kPa

100

Dependence of the catalytic of () AgY and () HY on the pressure of hydrogen. Reaction condition: 473 K,  W/F = 7.62 g h mol–1, ethylbenzene = 10.1 kPa. Reprinted with permission from T. Baba, Y. Ono, Zeolites, 7, 292 (1987).

4.1.6  Factors Affecting Acid Strength of Acidic OH Groups

As described above, the number of acidic OH groups is basically  equal to the number of aluminum in the framework. The acid strength of the OH groups depends on various factors such as  Si/Al ratio and the crystal structure of zeolites.

4.1.6.1  Si/Al ratio

The acid strength of the acidic OH groups generally increases with the Si/Al ratio of the framework if the structure of the zeolite is the same. Y-type zeolite is much higher activity than X-type zeolite.

151

152

Catalytic Properties of Solid Acid Catalysts

The activities of Y-type zeolite is further enhanced by removing  aluminum (dealumination) from the framework. The phenomenon is explained by Al site distribution as the primary determinant of acid strength. In zeolites, a given Al atom has four Si atoms in the first surrounding layer (nearest neighbors) and nine Al or Si atoms in the second layer (next nearest neighbors, NNN). In the case of X-type zeolite, almost all of these nine atoms will  be Al, while increasing the Si/Al ratio, some Al will be replaced by Si. At a sufficiently large ratio, all nine will be replaced by Si and  the original Al site is isolated. The strength of the original Al site  will depend on the number of Al NNN, reaching a maximum at  0 NNN. Essentially all Al sites are isolated at Si/Al ratio of ~7. In  fact, the catalytic activity of Pt/mordenite for the isomerization  of n-pentane shows the maximum at Si/Al ratio of 8.5 [20]. In the case of hexane cracking over H-ZSM-5 with varying Si/Al  ratio, the activity per aluminum is independent of the ratio [21].  This is due to the fact that Si/Al ratio of H-ZSM-5 is far larger than  7. In contrast, for the faujasite structure, the frequency of the hydroxyl stretching band decreases as Si/Al ratio increases and  a limiting value is obtained when Si/Al ≥ 6.

4.1.6.2  Isomorphous substitution

The synthesis of zeolites containing various elements such as B, Ge, and Fe has been carried out for a long time. The replacement of aluminum with other elements (isomorphous substitution) greatly modifies the acidic and redox properties of the zeolites. The elements introduced include Be, B, Ti, Cr, Fe, Zn, Ga, Ge, and V.  These elements were usually introduced by adding metal salts as one of the starting materials for the synthesis of the zeolites. Isomorphous substitution can also be made by post-synthesis. For example, boron can be introduced by reacting ZSM-5 with boron trichloride. Figure 4.4 shows the TPD profile of ammonia adsorbed on various metallosilicate with MFI structure [22]. The acid strength  of metallosilicate changes in decreasing order as follows:

[Al]-ZSM-5 > [Ga]-ZSM-5 > [Fe]-ZSM-5 > [B]-ZSM-5

Here, [M]-ZSM-5 denotes the metallosilicate with metal M in  the framework.

Zeolites

Ratio of ammonia desorption/ 10–6 mol K–1

7.5

[Fe]-ZSM-5 [A]-ZSM-5

5.0 2.5

[Ga]-ZSM-5

0

300 Figure 4.4

[B]-ZSM-5

500

700 900 Temperature/K

1100

Temperature-programmed desorption of ammonia from metallosilicates. Reprinted with permission from C. T. W. Chu, C. D. Chang, J. Phys. Chem., 89, 1569 (1985).

The band position of OH groups in IR changes in conformity with TPD profile. Thus, the OH band appears at 3610, 3620, 3630,  and 3725 cm–1 for [Al]-, [Ga]-, [Fe]-, and [B]-ZSM-5, respectively.

4.1.6.3  Structure of zeolites

Extensive theoretical calculations show that the structural factors greatly affect the acidity of bridging hydroxyl groups. When proton is removed from bridging hydroxyl groups, the local structure of  the framework is relaxed; the Al–O–Si bond angle and Al–O and Si–O bond length change by deprotonation. This indicates the deprotonation energy depends on the local structure of the zeolite. Thus, the acidic strength of the OH groups differs from zeolite to zeolite. Among the structural factors, the Al–O–Si angle is often considered the most influential factor for determining acid  strength. The bond angles range of ZSM-5, mordenite, and Y-type  are 137–177°, 143–180°, and 138–147°, respectively. Carson et al. have calculated the energy for protonated and unprotonated forms of zeolites and noted that the unprotonated forms show little change with the Al–O–Si angle in the range of 130–175°, whereas the protonated forms become markedly less stable for larger angles [23]. This suggests that the deprotonation energy decreases with increasing Si–O–Al angle and thus the  proton becomes more acidic.

153

154

Catalytic Properties of Solid Acid Catalysts

Katada et al. have reported that Al–O distance a is a major structural factor for determining the acid strength of bridging OH groups [24]. The ammonia adsorption energy Eads calculated with density functional theory was taken as a measure of the acid strength. The relationship between Eads and Al–O distance a is  given by the following equation:

Eads = 515 – 2090a

(4.7)

where Eads and a are given in kJ mol–1 and nm, respectively. No clear relationship was found between Eads and the Si–O–Al bond angle. It  is suggested that the intrinsic origin of the Brønsted acidic property is an electron withdrawing of the Lewis acidic Al from the bridging OH. A shorter Al–O distance a yields a stronger interaction between Al and the OH, resulting in a smaller charge and higher acid strength of the OH group. This is in conformity with the acid  strength order MFI > Beta > FAU.

4.1.6.4  Dealumination of zeolites

The catalytic properties of zeolites such as activities, shape-  selectivity and thermal stability can be greatly improved by various treatments of as-synthesized zeolites. The method used for the modification is called the post-synthesis of zeolites. One of the most important methods of the post-synthesis is the removal of aluminum from the framework. Most typical ways of dealumination are hydrothermal treatment and the chemical treatment with an acid, EDTA or SiCl4. The combinations of hydrothermal treatment and the reaction with chemicals are also employed. The chemistry  of dealumination by acid treatment may be expressed as follows: Si O H  Si   O   Al   O   Si    +  3H2O O Si



Si O H  Si   O   H  H   O   Si    +  Al(OH)3 H O Si

(4.8)

Zeolites

Removal of aluminum leaves the “hydroxyl nest” in the structure. The vacated aluminum sites are partly filled by silica from other parts of the lattice, and mesopores are generated in the dealuminated zeolites. This alleviates diffusion limitation of reactants during catalytic reactions.

Si O H  Si   O   H  H   O   Si    +  SiO2 H O Si

Si O  Si   O   Si   O   Si    +  2H2O O Si

(4.9)

The aluminum species extracted from the framework remain in the zeolites. Different types of extra-framework aluminum species  (EFAL) may stay in the zeolite channels and/or supercages. Thus, aluminum cations such as AlO+, Al(OH​)​+2​,​  and AlOH2+, and some neutral species can be formed depending on the conditions of the hydrothermal treatment and the characteristics of the starting zeolites, though the detailed structure are not certain. Extraframework aluminum species are responsible for the appearance of Lewis acidity and also for the presence of new IR hydroxyl bands generally considered non-acidic. Dealumination of zeolites gives the favorable effects on the catalytic properties of zeolites as follows: (i) As described above, the defects sites generated by dealumination are filled in partially by silicon, which leads to a high Si/Al ratio and more stable framework. It is well known that the higher the SiO2/Al2O3 ratio the more stable the zeolite structure. The SiO2/Al2O3 ratio of Y-zeolite is limited to around six by primary synthesis. A mild hydrothermal treatment of faujasite Y zeolite framework gives the materials with higher Si/Al ratio. This improves the thermal stability of the zeolites. The Y-type zeolite stabilized by dealumination is called ultrastable Y zeolite (USY). The reduced ion exchange capacity and a smaller unit cell are the indications of the removal of

155

156

Catalytic Properties of Solid Acid Catalysts

aluminum from the framework. In the catalytic cracking, the catalyst must be able to withstand temperature in the range of 773–1123 K and a steam environment. Because of high thermal and hydrothermal stability, USY is the most favorable catalyst for the catalytic cracking. The part of EFAL can be removed by acid treatment. (ii) Besides its higher thermal stability, USY exhibited a much higher activity than the non-activated samples in spite of the decrease in the number of acid sites. The hydrocarbon cracking activity increases sharply during the initial phase of dealumination, then decreases as more Al is pulled out of the framework. A maximum in acidity is often reached at a Si/Al ratio of 9–12.

Many explanations have been put forward for the high  catalytic activity of USY. First, the increase in the Si/Al ratio results in the higher activity. This may be explained by the next nearest neighbor (NNN) effect, as described above. Second, extra-framework aluminum has been suggested to modify the catalytic activity because it stabilizes the lattice,  or has a synergistic effect on nearby Brønsted acid sites. Acidity enhancement is observed even for high-silica  zeolites such as ZSM-5. This cannot be explained by the NNN concept.

A mechanism of Brønsted/Lewis acid synergy is proposed  from NMR experiments and DFT calculations. The coordination of the Lewis acid site Al(OH)3 or Al(OH​)+2​ ​​  to the oxygen atom nearest to the Brønsted acid site is capable of causing an enhanced acid strength though there is no direct interaction between them [25].

H

Al(OH)3 or AlOH2+

O     O     O Si     Al     Si     Si

An infrared band of Y-zeolites showed that several new bands developed after dealumination in the hydroxyl region [26, 27].

Zeolites

The new bands at 3675 and ca. 3600 cm–1 are attributed to protons associated with EFAL. The band at ca. 3600 cm–1 was found to be responsible for the catalytic activity for hexane cracking and was assigned to a framework hydroxyl groups that function as a strong Brønsted acid due to the interaction with dislodged aluminum species [27].

(iii) Dealumination often changes the porosity of zeolites, altering the total pore volume and the pore size distribution. This may bring about the creation of mesopores, which facilitates the diffusion of the reactants and/or products, and modifies shape selective properties of the zeolites. In the case of mordenite, it is suggested that the channel system changes from one-dimensional to two- or three-dimensional. The dealuminated mordenite (SiO2/Al2O3 = 2600) shows very high activity and selectivity in the production  of 4,4¢-diisopropylbiphenyl in the alkylation of biphenyl with propene [28].

4.1.7  Catalysis by Metal Cations in Zeolite Framework

As described above, aluminum species dislodged from the zeolite framework work as Lewis acid and modify the strength of acidic  OH groups. On the other hand, metal cations incorporated in the zeolite framework may directly participate in catalytic reactions as Lewis acid sites as described in Section 2.4.3. The Meerwein– Ponndorf–Verley reaction is catalyzed by beta zeolites containing  Al3+, Ti4+, Sn4+, and Zr4+ ions in the framework [29]. The  isomerization of citronerall to isoplugenol is also catalyzed by beta zeolite with Sn4+, Al3+, and Ti4+ (Reaction (2.42)) [30]. Sn-beta zeolite is also active for the Prins condensation of a-methylstyrene with paraformaldehyde [31].

4.1.8  Structure and Use of Representative Zeolites 4.1.8.1  Zeolites of industrial importance

Faujasite (Fig. 4.5a): The faujasite structure is formed by wide supercages (1.3 nm diameter) accessed through 12-membered silicate ring with 0.74 nm diameter, much smaller sodalite cages

157

158

Catalytic Properties of Solid Acid Catalysts

accessed through six-membered silicate rings, and hexagonal prisms connecting the sodalite cages. All of the catalytic chemistry of faujasite is supposed to occur in the supercages. The aluminum content in faujasite is generally very high. Faujasite-type zeolites  with Si/Al ratio near 1 are usually denoted by X-zeolites, whereas those with Si/Al ratio higher than 2 are usually denoted by Y-zeolites. Y-zeolites are more stable than X-zeolites because of high silicon content. (a) Faujasite (FAU)

(c) Mordenite (MOR)

(e) MCM-22 (MWW)

Figure 4.5

(b) Beta *BEA

(d) ZSM-5 (MFI)

(f) Ferrierite (FER)

Pore structures of zeolites. From the database of Structure Commission of International Zeolite Association. http://www. iza-structure.org/databases/.

Zeolites

The two well-defined bands arising from bridging OH groups  are observed in the IR spectrum of Y-zeolites [13]. The highfrequency HF band (3626 cm–1) has been assigned to bridging OH groups located in the supercage and accessible to most molecules, whereas the low-frequency LF band (3562 cm–1) has been assigned to OH groups located near the middle of the 6-membered rings  connecting the sodalite cages, being possibly weakly H-bonded with molecules located in the supercage through the cavity. Sarria et al. observed a third band at 3501 cm–1 [32]. This band was not perturbed at all by adsorption of trimethylamine and  assigned to OH groups in the hexagonal prisms. Suzuki et al.,  identified four kinds of OH groups by the study of infrared mass spectrometry/temperature desorption (IRMS-TPD) of ammonia [33]. The 1H MAS NMR spectrum shows a peak at 3.9 ppm that is hydroxyl groups in the supercages. Another peak at 4.8 ppm is  not accessible to the probe molecules and is consequently assigned to hydroxyl groups in the sodalite cage. Faujasite zeolite is important as catalyst for the industrial reactions in the petroleum refining and petrochemistry such as cracking, hydrocracking, and isomerization. Its widespread applications are mainly attributed to the acidic property. The large pore openings and high surface area make it the catalyst of choice in fluid catalytic cracking, one of the most important reactions in petroleum refining. As described above, H-Y zeolites for their practical application at high-temperature reactions must be stabilized by steam dealumination, generally performed at ca. 773 K on the NH4-Y precursor. The resulting materials are hydrothermally more stable (the so-called ultrastable Y zeolite, USY). Their structure and acidic properties are greatly influenced by the dealumination process, which generates extra-framework aluminum possessing Lewis acidity and inducing enhanced Brønsted acidity within the material, as described above. The main component of fluid catalytic cracking (FCC) catalysts today is rare earth (RE)–containing USY zeolites. USY is also a typical component or support of hydrocracking catalysts, to provide acidity. The catalyst contains a sulfur-resistant hydrogenation phase, such as Ni-W sulfide. The reaction is performed at 570–670 K under 50–200 atm of hydrogen. A heavy low-value feed is transformed into linear fractions. Hydrodesul- 

159

160

Catalytic Properties of Solid Acid Catalysts

furization, hydrodenitrogenation, hydrodearomatization, and hydroalkylation occur. The wide dimension of the channels of faujasite allow quite heavy molecules to be cracked. Beta zeolite (Fig. 4.5b): The framework of *BEA zeolite gives rise to three-dimensional systems composed of two different channel types, both formed by 12-membered rings but with different diameters, one (0.56 × 0.56 nm) in the medium pore range, the  other (0.66 × 0.76 nm) in the large pore range. The Si/Al ratio is typically in the 10–30 range, although particular preparations  allow this ratio range to be expanded down to 5 or up to infinity. Typical beta zeolites can be regarded as highly disordered intergrowths between two end member structure types. Because of the distorted connection of the two polymorphs, a high concentration of internal structural defects is generated and a large amount of extra-framework Al species is formed by calcinations or steaming around 673 K. This causes the appearance of strong acid sites due to the interaction between Brønsted acid site and Lewis acid sites (dislodged aluminum) in H-beta. The complex nature of the OH groups in H-beta zeolites is evidenced by IR and 1H MAS NMR spectroscopy. For example, Gabrienko et al. revealed various bands in the IR spectrum and  their acidic strength by the shift of the bands upon CO adsorption  [34]. They also correlated the band position of IR with the chemical  shift in 1H NMR. Their results can be summarized as follows:









• 3785 cm–1 (0.6 ppm): low-acidity OH groups bonded to extralattice aluminum (AlOH, AlOOH) or aluminum species still connected to the framework; weakly acidic • 3745 cm–1 (2.1 ppm): isolated SiOH groups at external  surface; weakly acidic • 3740 cm–1 (1.8 ppm): Isolated SiOH groups at internal surface formed at defects; strongly acidic • 3660 cm–1 (2.2–2.6 ppm): acidic OH groups connected  to Al partially attached to the framework and located at external surface; strongly acidic • 3610 cm–1 (composed of two bands, 4.0 and 5.1 ppm):  bridged hydroxyl groups; strongly acidic • 3500–3300 cm–1 (broad): bridged hydroxyls that are  perturbed by H-bond interactions with the zeolitic framework

Zeolites

The order of the acidic strength of the three strongly acidic OH bands as determined by 1H NMR chemical shift and CO adsorption is  as follows:

3610 cm–1 > 3740 cm–1 > 3660 cm–1

The size of the largest channels of H-beta allows quite easily the diffusion of aromatics. The size of the cavity may perhaps be reduced by dealumination, producing extra-framework species. H-Beta is an active catalyst in a variety of reactions, like hydrocarbon cracking, alkylation of aromatics, and hydroisomerization of alkanes. H-Beta finds industrial application in the liquid-phase synthesis of cumene by alkylation of benzene with propene (Section 5.3.2). H-Beta-based catalysts selectively catalyze in multi-fixed-bed catalytic reactors both the alkylation reaction with a large excess of benzene and transalkylation reaction by which benzene reacts with polyisopropylbenzene to produce additional cumene in a separate fixed-bed reactor. H-Beta gives rise to better selectivity to cumene than other zeolites such as mordenite with lower coproduction of propene oligomers and n-propylbenzene. As already described, metal ions (Sn or Zr)-incorporated beta zeolites act as Lewis acid (Section 2.4.3). They are active for Meerwein–Ponndorf–Verley reactions and Baeyer–Villiger oxidations [35]. The catalytic activity of beta zeolites for Meerwein– Ponndorf–Verley reactions is correlated with the IR band of the  OH groups at ca. 3785 cm–1. Zr-incorporated beta zeolite is also  active and selective for the transformation of citronellal to  isoplugenol (Reaction (2.42)) [36]. A high diastereoselectivity for  (±)-isopulegol of ~93% was obtained together with the >98% overall selectivity to isopulegol isomers. The reaction mechanism involving Lewis acid sites and weak Brønsted acid sites was  proposed (Fig. 4.6). H

Si

: : O

Figure 4.6

Zr

O

H+ O

Si

H

Si

: : O

Zr

O

H+ O

Si

Si

H+ O

OH : : O Si Zr

Proposed mechanism for cyclization of citronellal over (Zr) beta zeolite Z. Reprinted with permission from Z. Yuntong, N. Yuntong, S. Jaenicke, G.-K. Chuah, J. Catal., 229, 404 (2003).

161

162

Catalytic Properties of Solid Acid Catalysts

Mordenite (Fig. 4.5c): The mordenite structure denoted by MOR exhibits a two-dimensional network of channels consisting of  nearly straight main channels running along the [110] crystallographic directions, which are accessible through 12-membered rings (0.65 × 0.7 nm). Additionally, eight-ring “side pockets” exist in the [010] direction, the opening of which is 0.34 × 0.48 nm. The  main channels are connected by the side pockets with an elliptical small openings, 0.57 × 0.26 nm wide. Because of small openings,  only very limited access from one main channel to the next is  possible. Consequently, the channel system is effectively onedimensional. IR spectra show, for typical H-mordenite with a Si/Al ratio of 10, a strong slightly asymmetric band centered at 3605–3610 cm–1. Several authors reported that the OH groups in the so-called side pockets and smaller channels are associated with a band located  at distinctly lower frequencies (near 3580 cm–1) with respect to those located in the main channels. Brønsted acid sites in  H-mordenite have also been identified through NMR techniques. The adsorption of different probes shows that monosubstituted benzenes diffuse easily in the main channels but are not allowed to enter the side pockets. Even the access of pentane and hexane in the side pockets is hindered. The entrance of ortho-disubstituted benzenes may be hindered even in the main channels. In agreement with this, H-mordenite catalyzes selective conversion of aromatics. Dealuminated H-mordenite is the catalyst of the Dow–Kellog  cumene synthesis process. Noble-metal containing H-mordenite is applied for the disproportionation of toluene to benzene and an equilibrium mixture of xylenes. Dealuminated mordenite is the basic structure of commercial catalysts for C4–C5 alkane skeletal isomerization based on aluminabound Pt–H–mordenite with SiO2/Al2O3 of 15–17. Dealumination  to a framework/extra-framework Al ratio of ~3 improves the catalytic activity. The catalyst works near 520 K, at a definitely  higher temperature than those based on chlorinated alumina. Therefore, the thermodynamics is less favorable, but the catalyst performance over the mordenite is more stable and more environmentally friendly. Mordenite is a catalyst for the industrial process for the production of di- and monomethylamines from methanol and

Zeolites

ammonia by gas phase reaction [37]. The mordenite was modified by carefully controlling the acidity and pore-mouth-opening with alkali cation-exchange and steaming. Mordenite is a selective catalyst for carbonylation of dimethyl ether to methyl acetate at low temperatures (400–460 K) without homologation reaction or catalyst deactivation [38, 39]. The unique point of this reaction is the fact that the reaction rates are  proportional to the number of OH groups within eight-membered rings, indicating the remarkably higher reactivity of C​H+3​ ​  ​groups located within 8-MR relative to those within 12-membered  channels. This suggests that the specificity of 8-MR ring channels  for C​H+3​ ​–CO ​  reactions arises from selective stabilization of carbocationic transition states via interaction with framework oxygen anions. ZSM-5 (Fig. 4.5d): The structure of MFI zeolite contains two types of intersecting channels, both formed by 10-membered silicate rings, characterizing this material as a medium-pore zeolite. One channel type is straight and has a nearly circular opening  (0.53 × 0.56 nm) along [010], whereas the other one is sinusoidal and an elliptical opening (0.51 × 0.55 nm) along [100]. The Si/Al ratio may vary from infinity to near 10. The bridging hydroxyl  groups show a single IR band that shifts from 3595 to 3620 cm–1  by varying the Si/Al ratio and measurement temperature. 1H MAS NMR studies allowed peaks usually in the range of 4.1 to 4.3 ppm  to be assigned to the acidic protons. The channels of the MFI structure allow the diffusion of  benzene and monosubstituted benzenes as well as p-xylene. The diffusion of ortho- and meta-disubstituted benzenes is far more difficult. This allows shape selectivity in favor of mono- or paradisubstituted benzenes. A typical example of product selectivity is found in the toluene alkylation with methanol on H-ZSM-5.  m-, p-, and o-Xylene are formed inside the ZSM-5 channels, but the product is enriched in p-xylene since this isomer has the smallest kinetic diameter and diffuses out rapidly. An industrial example of this behavior is the selective toluene disproportionation process, allowing the highly selective production of p-xylene and benzene from toluene. H-ZSM-5 catalysts find a number of other applications in the  field of gas-phase aromatics chemistry. They are utilized for the

163

164

Catalytic Properties of Solid Acid Catalysts

Mobil-Badger process of benzene alkylation of benzene with ethylene for the ethylbenzene synthesis, performed in the vapor phase at 660–720 K. A product shape selectivity effect is also the basis of the development of H-ZSM-5 catalysts for the Mobil methanol to gasoline (MTG). The main products of the process are hydrocarbon mixtures useful as gasoline components. A reactant shape selectivity effect allows the use of H-ZSM-5 (usually containing also a hydrogenation metal) for the selective cracking of linear alkanes in the catalytic dewaxing of lube oils such as the Mobil selective dewaxing process (MSDW). Linear alkanes enter and diffuse easily in the MFI cavities, whereas the entrance of branched isomers is hindered. Thus, conversion of linear  compounds is favored with respect to those of branched isomers. ZSM-5 zeolites are rather stable to deactivation by coke. This is due to its channel dimensions, which do not allow the formation of aromatic compounds with more than 11 carbon atoms, nor the formation of polyaromatics, which can lead to coke deposits. Moreover, three-dimensional channel structure of ZSM-5 makes less critical the diffusional limitations created by coke deposits as compared with other one-dimensional zeolites such as mordenite. An important recent application of H-ZSM-5 is as a component of FCC catalysts based on RE-USY zeolites. H-ZSM-5 cracks selectively linear C5+ alkanes, resulting in a better gasoline quality and  increased gaseous alkene production albeit at the cost of a lower gasoline yield. H-ZSM-5 zeolites can also be applied as a heterogeneous acid catalyst in the water phase. A process for cyclohexene hydration to cyclohexyl alcohol in water using highly siliceous H-ZSM-5 zeolite from Asahi Kasei operates successfully at ~390 K (see Section 6.7). ZSM-5 zeolite with reduced acidity (or silicalite) is a selective catalyst for the Beckmann rearrangement of cyclohexanone oxime  to e-caprolactam (Section 6.8.3). MCM-22 (Fig. 4.5e): MCM-22, isotypic with the so-called ERB-1 zeolite (IZA structure code MWW), possesses a unique layered structure containing two independent noninteracting pore  systems. The first consists of two-dimensional channels with 10membered ring openings (diameter 0.41 × 0.51 nm), whereas the other consists of larger supercages of 12-membered ring with dimensions of 0.71 × 0.71 × 1.82 nm. The supercages stack one

Zeolites

above the other through double prismatic six-membered ring connecting channels and accessed by slightly disordered elliptical 10-membered connecting channels (0.40 × 0.55 nm). These cavities are opened to the exterior at the termination of crystallites forming half-supercage or cup structures, with 12-ring openings and an approximated depth of 0.7 nm (Fig. 4.7). In general, the synthesized MCM-22 zeolites crystallize as very thin plates with large external surface area on which 12-membered cups are exposed.

Figure 4.7

MCM-22 structure, large cavities opened to the exterior at the termination of crystal. Reprinted with permission from A. Corma, V. Martínez-Soria, E. Schnoeveld, J. Catal., 192, 163 (2000).

Four hydroxyl bands were detected in the IR spectrum of pure MCM-22 samples [40]. The large band centered at 3626 cm–1 was resolved into two single components of 3628 and 3618 cm–1, which were assigned to Si(OH)Al groups located at supercages and in sinusoidal channels, respectively. The shoulder at 3585 cm–1 was attributed to Si(OH)Al groups positioned in a hexagonal prism between the two supercages. The fourth component (3670 cm–1) was ascribed to the AlOH groups linked to extra-framework Al species. The existence of Lewis acid sites and their location mostly at the external surface of the zeolite have been suggested from the IR study of adsorption of nitriles.

165

166

Catalytic Properties of Solid Acid Catalysts

MCM-22 is the catalyst for an industrial ethylbenzene synthesis process EBMax by the alkylation of benzene with ethylene  performed in liquid phase. MCM-22 is also the component of the catalyst of the liquid-phase Mobil Raytheon process for cumene synthesis. MCM-22 based catalysts also show excellent behavior for the liquid-phase alkylation of phenol with alcohols such as t-butyl alcohol. MCM-22 zeolite catalyst is more monoalkylate-selective than most large-pore zeolites such as beta and is very stable. The excellent selectivity to monoalkylated products is attributed to the presence of half-supercage (or cups) at the external surface, which ensures the easy desorption of alkylbenzenes from these cavities [41]. This hypothesis is further supported by observation that the catalytic activity of MCM-22 is significantly depressed by  deactivating with 2,6-di-t-butylpyridine, a large base that should not penetrate into the channels. However, spectroscopic data suggest that mono-substituted aromatics as well as para-disubstituted  benzenes (such as p-xylene) and isopropyl group containing molecules (such as isobutyronitrile), enter easily not only the halfsupercages but also both internal channel systems of MCM-22.  From these data, Busca and coworkers suggest that alkylation of benzene could occur both at the external half-supercages (with no diffusion limits) and in the internal cavities (without strong diffusion limit), especially above 473 K [40]. Ferrierite (Fig. 4.5f): This zeolite presents two kinds of one-dimensional channels, 10-membered rings with diameters  0.42 × 0.54 nm and eight-membered ring with diameters of  0.35 × 0.48 nm. These two kinds of channels are perpendicularly intersected. It is consequently classified as a medium pore zeolite.  It frequently has quite high Al content (Si/Al ratio = 8), but may be also prepared in a very highly siliceous form. It presents an OH stretching band near 3595 cm–1, with slight shifts depending on the Al content and recording temperature. Ferrierite is an active and selective catalyst for the isomerization of n-butene to isobutene. An industrial process for the isomerization has been developed  by Shell Research. Ferrierite is also useful to hydrocrack diesel and to improve the fluidity at low temperatures of the resultant diesel. Omega: Zeolite omega, a large-pore zeolite with a silica– alumina ratio in the range of 4–10, is the synthetic isotype of the mineral mazzite (topological code MAZ). The framework of mazzite features a unidimensional channel system accessible through 

Zeolites

quasi-circular 12-membered ring windows, approximately 0.74 nm in diameter. The acidic properties of omega have been characterized by various techniques. Strong acid sites exist in dealuminated  omega [42]. The dealuminated omega zeolite shows high activity and selectivity for the isomerization of C5–C6 alkanes [42].

4.1.8.2  Zeolites of interest for future applications

TNU-9 is classified as TUN in the IZA structural code. Its framework contains two distinct straight 10-ring channels (0.52 × 0.60 and 0.51 × 0.55 nm). TNU-9 exhibits acidic properties similar to  ZSM-5 [43]. TNU-9 shows higher catalytic activities than ZSM-5  in the disproportionation of toluene and alkylation of toluene  with methanol [44]. SSZ-35 has a system of one-dimensional 10-ring channels that periodically open into wide and shallow cavities with 18-membered ring. The acidic properties of SSZ-35 have been studied [45]. The unique properties of this zeolite in aromatic transformations have been described [46]. Dealuminated SSZ-35 shows very high selectivity for 2,5-dimethylcumene in the alkylation of p-xylene with 2-propanol at 423 K [47]. MCM-58, MCM-68: MCM-58 is isostructural with SSZ-42 and ITQ-4, and classified by IZA under the code name of IFR. The zeolite possesses one-dimensional structure compiled by sinusoidal 12-ring channels. Its most characteristic feature is large void volume fraction 0.21 cm3 g–1. The peculiar sinusoidal shape of the channel is forcing the formation of a shallow cavity in between two  adjacent 12-rings. The maximum size of the cavity is 1.12 × 0.73 × 0.5 nm. MCM-68 of the MSE topology is a three-dimensional zeolite with 12–10–10-ring channels where 12-ring straight intersects  with two twisted 10-ring channels. MCM-68 contains also 18-ring cavities, accessible only through the 10-ring apertures. The acidic features of MCM-58 and MCM-68 have been reported [48]. CIT-1 (SSZ-33) has a channel system of intersecting 12- and 10-membered ring pores. The acidic properties of CIT-5 have been reported [15]. In the cracking of heptane, CIT-1 gives a behavior typical of that of large pore zeolites. It produces a remarkably high selectivity to isobutene [49]. ITQ-7 has a three-dimensional channel system composed of intersecting 12-membered ring pores and classified under the name

167

168

Catalytic Properties of Solid Acid Catalysts

of ISV. Two bands due to acidic OH groups (3610 and 3630 cm–1)  are observed by IR spectroscopy [50]. Both bands shifted to about 3340 cm–1 upon adsorption of CO. The extent of band shifts (270–290 cm–1) indicates that the acidic strength of these OH groups are lower than those of H-beta, H-mordenite, H-ZSM-5 and  H-MCM-22 (310–320 cm–1), but higher than that of SAPO34 (270 cm–1). ITQ-7 is an active catalyst for isomerization,  disproportionation, and alkylation of alkylaromatics [51]. SSZ-53, CIT-5 and UTD-1 are high-silica molecular sieves containing extra-large pores. These materials contain onedimensional pores with 14 rings and have high thermal and hydrothermal stabilities required for industrial use. The catalytic properties of these zeolites have been reported [52–55]. ECR-34 is a gallosilicate. It contains one-dimensional pores of 1.01 nm and is the first silicate molecular sieve containing 18-ring pore openings. It can adsorb large molecules like perfluorotri-nbutylamine [56]. ITQ-33 exhibits straight large pore channels with circular openings of 18-rings along the c-axis interconnected by a bidimensional system of 10-membered channels, yielding a structure with very large micropore volume. The material is highly active for cumene synthesis by the alkylation of benzene with propene, while giving an extremely low yield of the undesirable n-propylcumene [57].

4.2  Aluminophosphate Molecular Sieves

4.2.1  Structures of Aluminophosphate Molecular Sieves Ordered microporous aluminophosphates were first synthesized by Wilson et al. [58]. Many types of aluminophosphate with different micropore sizes and topologies have been synthesized by changing the template and/or crystallization conditions. The aluminophosphate molecular sieves known as AlPO-n (n refers to a distinct structure type) are built from strictly alternating AlO4 and PO4 tetrahedra. Their general formula can be expressed as [(AlO2)x(PO2)x] . yH2O. Some of AlPOs have the same structure as zeolites. The aluminophosphate framework is neutral in contrast to the regularly charged aluminosilicate one. While the aluminum atoms in the aluminosilicate frameworks are always tetrahedrally

Aluminophosphate Molecular Sieves

coordinated, the four-, five-, or six-coordinated aluminum atoms  are present in the aluminophosphate framework. The main  drawback of these molecular sieves as catalytic materials is low acidity because of neutral tetrahedral framework; they only provide low   catalytic activity for acid catalysis.   The pore structure of some of AlPO materials and the  AlPOͲ5 AlPOͲ5 AlPOͲ11 AlPOͲ11 code is described below (Fig. 4.8). structural     AlPOͲ5 AlPOͲ5   AlPO-5

AlPOͲ11 AlPOͲ11

AlPO-11

           

 

  AlPOͲ36 AlPOͲ36 AIPO-36 AlPOͲ36 AlPOͲ36

 

AlPOͲ41 AlPOͲ41

AIPO-41

AlPOͲ41 AlPOͲ41

                 

Figure 4.8  

 

 

Pore structure of AlPO-n materials. From the databases of     Structure Commission of International Zeolite Association, http://www. iza-structure.org/databases/.

 Figure4.8PorestructureofAlPOͲnmaterials.FromtheIZAStructureCommission.  Figure4.8PorestructureofAlPOͲnmaterials.FromtheIZAStructureCommission.

Figure4.8PorestructureofAlPOͲnmaterials.FromtheIZAStructureCommission.  Figure4.8PorestructureofAlPOͲnmaterials.FromtheIZAStructureCommission.  AlPO-5 (AFI). The channel system consists of 4-,

6-, and 12rings parallel to the 001 direction. The 12-membered channel is nearly circular having a diameter of 0.73 nm. AlPO-11 (AEL) is characterized by a one-dimensional pore system formed of 10-rings with an elliptical shape of 0.63 × 0.39 nm.   membered

169

170

Catalytic Properties of Solid Acid Catalysts

AlPO-31 (ATO) has circular monodimensional channels  0.54 × 0.54 nm in diameter, which is defined by a circular 12-ring window and significantly smaller than the circular 12-ring pore present in AlPO-5. AlPO-36 (ATS) possesses a 12-ring channel system with annular side pockets. The channels are elliptical and 1.01 × 0.92 nm across in projection and have a pore opening of 0.74 × 0.66 nm.  The side pockets consist of two pairs of four-rings and one boatshaped hexagon. AlPO-41 (AFO) has a one-dimensional elliptical 10-ring pore 0.43 × 0.70 nm in diameter. The elliptical 10-ring pore opening  is distorted and slightly larger in size than the 10-ring pore in  AlPO-11.

4.2.2  MeAPO Materials

The porous aluminophosphate frameworks can be modified by introducing other elements [59]. The incorporation of divalent metal cation (Me) in the AlPO famework gives metalloaluminophosphate (MeAPO-n). Here, the metal cations substitute rather exclusively for aluminum. The metal cations that can be incorporated into the framework include Zn2+ (ZnAPO-n), Mg2+ (MAPO-n), and Co2+ (CoAPO-n). The substitution of divalent metal ions for Al3+ generates the Brønsted acid sites (acidic bridging OH groups) as well as the Lewis acid sites (anionic vacanceies deriving from missing lattice oxygens). Incorporporation of a transition-metal cation (Co, V, Cr, Ti), which can easily change the oxidation number, creates also a redox active site.

4.2.3  SAPO Materials

The incorporation of silicon results in the silicoaluminophosphate molecular sieves, SAPO-n [60]. When both silicon and a metal are incorporated in the framework, the materials are called MeAPSOn. In the SAPO materials, silicon substitutes for phosphorus or for an aluminum–phosphorous pair (Fig. 4.9). No evidence for  the presence of Si–O–P is found, indicating that these bonds are not likely. Thus, Si-rich islands tend to be formed. The bridging  hydroxyl groups (Si–OH–Al) thus formed are considered to be the origin of Brønsted acidity. As shown in Fig. 4.9, a variety of Si centers with different local environments can be present in SAPO

Aluminophosphate Molecular Sieves

171

materials, depending on the silica content, the synthetic conditions, and procedures [61].

Figure 4.9  

Scheme for Si incorporation in a hypothetical bidimensional network Al–O–P. Reprinted with permission from M. Briend,  138, 90 M. J. Peltne, A. Lamy, P. P. Man, D. Barthomeuf, J. Catal., (1992).

The nature and quality of hydroxyl groups in SAPOs have Figure4.9SchemeforSiincorporationinahypotheticalbidimensionalnetworkAl–O–P.FromM.Briend, Catal.,138 (1992),p.93. etal., beenJ.studied by IR [62]. The spectra of the SAPO materials in the OH-stretching domains are shown in Fig. 4.10. In the spectra of all SAPO materials, three bands at ca. 3743, 3677, and 3625 cm–1 are  observed. The bands at 3743 and 3677 cm–1 are assigned to the  Si–OH and P–OH groups on the external surface. The band at ca. 3625 cm–1 (high-frequency band, HF) is attributed to Brønsted acid  site of the Si–OH–Al groups. In the case of SAPO-5, an additional –1  band at 3570 cm (low frequency band LF) is observed. This band can be attributed to also Si–OH–Al groups where the OH groups  form hydrogen bonds with the neighboring oxygen atoms of the framework. In many SAPO materials, two types of the bridged OH groups (HF and LF) are observed. In general, SAPO-n’s are generally characterized by milder acidity. Figure 4.11 shows the ammonia TPD spectra of mordenite, ZSM-5, SAPO-11, and AlPO-11 [63]. On all materials, weakly acidic sites are present, which result in the maximum at lower temperatures (~373 K). On mordenite, ZSM-5, and SAPO-11, strongly acidic sites are additionally present. The difference in the second desorption maximum indicates that mordenite and ZSM-5 are more strongly acidic than SAPO-11. As expected, AlPO-11 has no strong acid sites.



Catalytic Properties of Solid Acid Catalysts

3744

3677

ABSORBANCE UNITS

3743

3742

3628

3570

SAPO-5 3676 3628

SAPO-11

3677 3622

3744

3677

3627

SAPO-31

SAPO-41 3800



3700 3600 3500 WAVENUMBER CM-1

3400



Figure 4.10 IR spectra of hydroxyl groups in SAPO-5, SAPO-11, SAPO-31   andFigure4.10IRspectraofhydroxylgroupsinSAPOͲ5,SAPOͲ11,SAPOͲ31andSAPOͲ41.FromP. SAPO-41. Reprinted with permission from P. Mériadeau, V. J. Catal.,169 Mériaudeau,etal., A. Tuan, V. T. Nghiem, S.(1997),p.60. Y. Lai, L. N. Hung, C. Naccache, J. Catal.,  169,  55 (1997). 

Desorption rate [a.u.]

172

a b

0

100

200 300 400 Temperature [°C]

500

c d 600

Figure 4.11 Comparison of the NH4-TPD on (a) mordenite, (b) ZSM-5, (c)   SAPO-11 and (d) ALPO-11. Reprinted with permission from A. Figure4.11ComparisonoftheNH Jentys, J. A.4ͲTPDon(a)mordenite,(b)ZSMͲ5,(c)SAPOͲ11and(d)ALPOͲ11.From Lercher, Stud. Surf. Sci. Catal., 137, 345 (2001). A.Jentys,J.A.Lercher,Stud. Surf. Sci. Catal.,137(2001),p.379. 



Aluminophosphate Molecular Sieves

SAPO-11 (AEL) has a monodimensional medium-pore system with a medium diameter. As shown in Fig. 4.10, SAPO-11 has acid sites with mild acid strength. Because of its mild acidity and pore dimension, SAPO-11 is used as selective catalyst for several processes. Pt-loaded SAPO-11 is an efficient catalyst for the hydroconversion of long-chain hydrocarbons (Section 5.4.3). In the hydroisomerzation of n-tetradecane, high yields of branched isomers  (81%) could be achieved over Pt(0.4%)/SAPO-11 with high selectivity (89%) and high stability under the conditions as shown in Fig. 4.12 [64]. The high selectivity for isomerization (low extent of cracking) and high stability are suggested to be due to the mild acidity and the steric constraints of the micropores. The  isodewaxing process by Chevron might use the SAPO-11, where long-chain linear alkanes are eliminated by hydroisomerization [65]. SAPO-11 is also used for skeletal isomerization of butenes  to isobutene.

Figure 4.12 Hydroisomerization of n-C14 as a function of time-on-steam over Pt/SAPO-11. Conditions: temperature = 653 K; pressure = 3.0 MPa; WHSV = 2.0 h–1; H2/C14 = 8.7 mol mol–1 (note: sampling period 3 h; data for changing reaction parameters in between not shown). Reprinted with permission from C.H. Geng, F. Zhang, Z.-X. Gao, L.-F. Zhao, J. L. Zhou, Catal. Today, 93–95, 485 (2004).

SAPO-34 is isomorphous to chabazite (CHA). The chabazite topology might be described as layer of double six-rings that

173

174

Catalytic Properties of Solid Acid Catalysts

are interconnected by units of four-membered rings. The  double six-membered ring layers stack in an ABC sequence. This leads to a framework with a regular array of barrel-shaped cage with 0.94 nm diameter, interconnected by eight-membered-ring  windows (0.38 × 0.38 nm). The chabazite structure contains only one unique tetrahedral site but four different oxygen atoms in the asymmetric unit, giving four possible acid site configurations, depending on to which of the oxygen atoms the proton is attached. SAPO-34 gives five IR peaks in the range of 3000–4000 cm–1 representing five types of hydroxyl groups. The peaks at 3675, 3743, and 3748 cm–1, with very low intensity, are assigned to P–OH,  Si–OH, and Al–OH groups, respectively, which are generated by the defect sites of SAPO-34 surface. Two peaks at 3625–3628 cm–1 and 3598–3605 cm–1 are assigned to bridged hydroxyl groups. The hydroxyl groups at ~3600 cm–1 (LF band, OHC) are assumed to be localized in the hexagonal prism, forming an H-bond with adjacent oxygen atoms of the framework. The isolated bridging OH groups pointing toward the center of the elliptical cage give rise to a vibrational frequency at ~3625 cm–1 (HF band, OHA). The lower frequency OH groups are considered to be a little less acidic than  the high-frequency ones. The existence of the third acidic OH groups at 3617 cm–1 (OHB) is proposed by deconvolution besides the LF (3600 cm–1)-HF bands (3631 cm–1) [66]. Adsorption of CO led to the downward shift of the stretching frequency of the OH groups, the values of the shifts being 276, 331 and 150 cm–1 for OHA, OHB and OHC, respectively. This indicates that the acid strengths of the order of the OH  groups are OHB > OHA > OHC. The value of the shift of OHB upon CO adsorption (DνOH =  –331 cm–1) is too large to be explained in terms of isolated Si sites in a SAPO framework. In fact, the acidic strength of OHB sites is comparable to that of aluminosilicate zeolites with homologous structure (e.g., H-SSZ-13, DνOH = –316 cm–1), where Si–OH–Al Brønsted sites are formed. The strong acidity of the OH groups is explained in terms of protons either at the borders of silica patches/islands or inside aluminosilicate domains: When Si islands are formed in SAPO materials, these regions resemble the zeolite framework. Brønsted

Ordered Mesoporous Materials

sites at the borders of silica islands (or inside aluminosilicate regions) experience a chemical environment similar to that of analogous sites in aluminosilicate. As a consequence, the presence of Si-rich regions in SAPO-34 could induce the formation of  Brønsted sites with stronger acid character than those produced by isolated silicon sites. SAPO-34 is an excellent catalyst for the conversion of methanol to ethylene and propene in the so-called methanol-to-olefin (MTO) process. The small pore size of SAPO-34 restricts the diffusion of heavy and branched hydrocarbons, and this leads to high  selectivity to the desired small linear olefins (see Section 5.6).

4.3  Ordered Mesoporous Materials

4.3.1  Synthesis of Ordered Mesoporous Silica In 1990, Yanagisawa et al. reported the synthesis of the ordered mesoporous material from kanemaite, a layered silicate. The synthesis pathway is thought to proceed via surfactant intercalation into the silicate sheets, wrapping of the sheet, and transformation to the hexagonally packed material [67, 68]. The obtained materials are designated as FSM-n (Folded Sheets Mesoporous Material);  here n is the number of carbon atoms in the surfactant alkyl chain used to synthesize the material. Soon after, in 1991, scientists in Mobil Oil described in patents the discovery of the mesoporous  silica materials. In subsequent papers, the Mobil researchers  showed that MCM-41 was a molecular sieve with a highly ordered regular one-dimensional pore system [69, 70]. The walls, however, closely resemble amorphous silica. Other related phases such as  MCM-48 and MCM-50, which have a cubic and lamellar  microstructure, respectively, were also reported. The synthesis of the materials is controlled by the micelle formation of the surfactant molecules or their silicate salts (a liquidcrystal templating mechanism) [69, 70]. MCM-41, the hexagonally ordered one-dimensional molecular sieve, is formed under conditions where cylindrical micelles are favored in the surfactant– silicate system (Fig. 4.13). On the other hand, MCM-48 is formed under conditions where spherical micelles are more stable.

175

176

Catalytic Properties of Solid Acid Catalysts

Surfactant Micelle

Hexagonal Array Micelle Rod Calcination

Silicate

 MCM-41 Silicate

 Figure 4.13 Possible mechanistic pathways for the formation of MCM-41: (1) liquid crystal phase initiated and (2) silicate anion initiated. Reprinted with permission from J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicicz, C. T. Kresge, K. D. Schmidt, C. T. W. Chu, D. H. Olson, E. W. Sheppard, S. B. McCullen, J. H. Higgins, J. L. Schlenker, J. Am. Chem. Soc., 114, 10834 (1992).

Pore sizes of MCM-41 materials obtained using various surfactants and synthesis procedures fall into the range between 1.5 and 4.5 nm. Adding expander molecules, such as 1,3,5trimethylbenzene, which prefer to be in the hearts of the micelles leads to wider pores up to 8.5–12 nm. The mesoporous materials have the surface areas in the range of 400–1000 m2 g–1 and high pore volumes. A post-synthesis procedure to enlarge the pores of as-synthesized MCM-41 silica has also been developed [71]. Here, a conventional MCM-41 is first synthesized typically in the presence of cetyltrimethylammonium bromide, followed by post-synthesis hydrothermal treatment in the presence of dimethyldecylamine.  By this method, pore-expanded mesoporous silica exhibits large pores (up to 20 nm), large pore volume (up to 3.5 cm3g–1) and  large surface area often exceeding 1000 m2g–1. Since the discovery of ordered mesoporous solids such as  MCM-41, there has been impressive progress in the development of many new mesoporous solids based on templating mechanism. Different materials, denoted with abbreviations HMS, SBA-1,  SBA-15, SBA-16, MSU, KIT-1, MSA, ERS-8, and UVS-7 with different mesoporous pore structures, may be obtained by different preparation procedures [72]. For example, a 2D-hexagonal mesoporous material, SBA-15 is synthesized under acidic conditions by using the triblock co-polymers consisting of poly(ethylene oxide)xpoly(propylene oxide)y-poly(ethylene oxide)z as the surfactant. SBA-15 exhibits a thick wall of 3–7 nm thickness and large pore sizes adjustable between 6 and around 15 nm. The thick wall of



Ordered Mesoporous Materials

this material significantly improves the thermal and hydrothermal stability compared with MCM-41. For improvement of the surface acidity and thermal or hydrothermal stability, partial or local zeolitization of the wall of mesoporous silica is performed. Generally, following methods seem feasible for this process: (i) conversion of the ordered mesoporous silica into zeolite units [73], (ii) synthesis of the wall from solutions that already contain zeolitic structural elements or seeds  [74–78]. (iii) dual templating with both small amines and long chain  surfactant molecules [79], and (iv) coating of the walls of  mesoporous silica with zeolite fragments [80]. Partial recrystallization of MCM-41 or HMS showed enhanced catalytic activities compared with parent MCM-41 or HMS, respectively, for the cracking of cumene [73]. Mesoporous materials synthesized by self-assembly of preformed aluminosilicate nanoclusters with templating micelle show high activities for the catalytic cracking of triisopropylbenzene and high hydrothermal stability [75]. While the original materials were synthesized as pure silica or aluminosillicate, further investigations resulted in successful synthesis of mesoporous materials such as alumina and zirconia [81–84]. Mesoporous organosilicas (PMOs) that contain bridging organic groups such as ethane, ethylene, benzene, and biphenyl  have been synthesized [85].

4.3.2  Development of Acidity in Ordered Mesoporous Materials 4.3.2.1  Catalysis by mesoporous silica

Since the surface OH groups are very weak in acid strength, ordered mesoporous silica is not very often used as a solid acid catalyst except for a few cases. A unique pore size effect is observed in the acetalization of cyclohexanone on pure-silica MCM-41 [86]. For this reaction, silica gel has no activity. The highest catalytic activity  was obtained by MCM-41 with a pore diameter of 1.9 nm and the reaction rate constant decreased when either narrower or wider pores were used (Fig. 2.19). The high activity of the 1.9 nm pore size catalyst was attributed to the fact that the surface silanol groups  are all oriented towards the center of pores, and they are thus

177

178

Catalytic Properties of Solid Acid Catalysts

assumed to work as an ensemble. It is proposed that such an  assembly might create highly active sites for acid catalysis. Pure silica-MCM-41 is very active for the synthesis of amide  from fatty acids and long-chain amines as shown in Table 4.3 [87]. The reactions were carried out with an equimolar mixture of fatty acid and amine in toluene under a Dean-Stark apparatus at  azeotropic reflux temperature for 6 h. Silica gel also shows the catalytic activity, but the activity is lower than MCM-41. Nafion  and Y zeolites show even lower activities. Table 4.3 Acids

Amidation of fatty acids with long-chain amines over MCM-41 catalysta

Palmitic acid

Amines

Hexylamine

Yield/%b 94

Palmitic acid

Octylamine

>99

Palmitic acid

Tetradecylamine

>99

Palmitic acid Palmitic acid Palmitic acid

Octanoic acid

Docylaminwe

Dodecylamine

Hexadecylamine Hexylamine

Decanoic acid

Hexylamine

Stearic acid

Hexylamine

Dodecanoic acid Myristic acid Oleic acid

Hexylamine Hexylamine

Hexadecylamine

>99 >99 >99 90

89 85 88

>99

>99

Source: From Komura et al., Green Chem., 13, 828 (2011). aReaction was carried out by using acid (3 mmol) and amine (3 mmol) in the presence of 20 wt% of MCM-41 in toluene (15 mL) at reflux for 6 h. bYield

was determined by GC.

4.3.2.2  Incorporation of heteroatoms into the structure of mesoporous silica As described above, ordered mesoporous silicas are not often used as catalyst as such. Much more frequently, additional catalytic functions are introduced by incorporation of active sites in the  silica walls or by deposition of active species on the inner surface of the material. Metal ions replacing silicon atoms in the framework

Ordered Mesoporous Materials

can act as acid or redox active sites and may be used for different classes of catalytic reactions. Incorporation of aluminum is particularly important for acid catalysis. Typically, incorporation of aluminum is achieved by a direct method in which an aluminum source is included in the solutions or gels for the hydrothermal synthesis [88–92]. Indirect method is also used. In this case, aluminum source is introduced to preformed siliceous mesoporous materials by an impregnation method [92]. Synthesized aluminum-containing mesoporous silicas have both tetrahedrally and octahedrally coordinated aluminum. The catalytic activities of aluminum-containing MCM-41 ([Al]MCM-41), USY and amorphous silica–alumina(ASA) were compared for cracking of heptane and gas oil [89]. The order of activity for heptane cracking was USY (12.37) > ASA (3.20) > [Al]-MCM-41 (2.04). The numbers in parentheses indicate the first order rate constants. When the activities are compared by per-Al site basis,  the activity order is USY (1237) > ASA (35.6) > [Al]-MCM-41 (8.87). Thus, the intrinsic activity for heptane cracking is 139 times  larger on USY than on [Al]-MCM-41, indicating the much weaker acid strength of [Al]-MCM-41. The local environment around  the acid sites seems similar to ASA and rather than to zeolites. The first-order rate constants of the three catalysts for gasoil cracking showed different tendency, USY (3.32) > [Al]-MCM-41 (2.04) > ASA (1.71). The difference in the activity order for the two  reactions is due to the effect of the pore size. In the gas-oil cracking,  MCM-41 may encounter less diffusional limitation for the larger molecules. It turned out, however, the thermal, especially hydrothermal stability of MCM-41 is not good enough for the severe reaction conditions in the regenerator. In the isomerization of m-xylene isomerization, silica–alumina is more active than [Al]-MCM-41 (Si/Al = 10), H-Y zeolite being  500 times more active than [Al]-MCM-41 [93]. Mesoporous silica–alumina (MSA) was found to be a very stable acidic component for preparing metal-supported bi-functional catalysts. Thus, Pt/MSA is more selective towards isomerization than USY zeolites and conventional silica–alumina during hydroconversion  of n-decane [94]. Typical reaction conditions are total pressure = 30 bar, H2/decane = 4 and reaction temperature 523 K. Three factors of MSA favorably work for this reaction; a mild Brønsted acidity,

179

180

Catalytic Properties of Solid Acid Catalysts .

a very high surface area and a narrow distribution of pores in the mesoporous region. The lower Brønsted acidity decreases cracking of the primary decane isomers. As in zeolites, different heteroelements provide different strength of the resulting acid sites. The Brønsted acid strength of  Al-, Ga-, and Fe-substituted MCM-48 investigated by NH3-TPD  were in the order of Al > Ga > Fe, whereas the Lewis acid sites  showed an order Ga ≈ Al >> Fe [95]. Although initially substantial efforts are focused on the conversion of large hydrocarbons in refinery applications, it turned out that low hydrothermal stability and low acid strength limit the applications in these fields. The majority of research efforts focus  on applications that are closer to fine chemicals by taking advantages of high surface area, larger pore diameters, and low acidity. In the alkylation of naphthalene with propene, 2,6diisopropylnaphthalene (2,6-DIPN), which can be used as a raw material for the production of advanced polymers, is selectively formed over mordenite zeolite. The selective production of the slim isomer, 2,6-DIPN, is due to the shape selective effect of the zeolite. On the other hand, DIPN isomeric mixture is used as a high quality solvent for copying materials. The content of DIPN isomers with high solidification point, such as 2,6-DIPN should be limited to ensure liquid state of the solvent at low temperatures. SBA-1 containing Al or Fe is very active for the alkylation of  naphthalene with propene [96]. Because of large free space of the cages in SBA-1, the product distribution is free from the shapeselectivity constraint; the products contain only small amount of 2,6-DIPN. Furthermore, low acid strength of these mesoporous materials favorably work for lower yield of 2,6-DIPN, since the isomerization of the primary DIPN products (a,a- and a,b-isomers) to the b,b-isomers (2,6- and 2,7-DIPN) is slow. It was reported that alumiosilicate MCM-41 was a suitable catalyst for the alkylation of the bulky 2,4-dibutylphenol with cinnamyl alcohol [97]. While this alkylation does not occur in the restricted environment of an H-Y zeolite (pore size about  0.75 nm), the primary alkylation product 6,8-di-t-butyl-2-phenyl2,3-dicyclo[4H]benzopyrane is formed using the large pore  diameter mesoporous molecular sieves.

Ordered Mesoporous Materials

OH

CH2OH

{  ​

OH

}

 ​

o

Ph

(4.10)

Acetalization is an important reaction for the protection of carbonyl functional groups. This reaction does not require strong  acid sites and therefore mesoporous silicas with weak to  intermediate acid sites strength are suitable catalysts in this reaction. Preparation of dimethylacetals by the reactions of aldehydes (heptanal, 2-phenylpropanal, diphenylacetaldehyde) with trimethyl orthoformate was studied in the presence of [Al]-MCM-41 and zeolite beta [98]. The initial rate of reaction was very similar regardless of the size of the reactants for [Al]MCM-41. When beta zeolite was used as catalyst, the rate of the acetalization of heptanal was slightly higher than that of [Al]MCM-41. For the acetalization of 2-phenylpropanal, [Al]-MCM-41 was more active than beta-zeolite. The rate of the acetalization of  diphenylacetaldehyde was very low over beta zeolite because of diffusion limitation. It was suggested that bridging hydroxyl groups (Si–OH–Al) are not the only active groups for the acetalization reaction, but other weaker acid sites such as silanols or at least silanol groups in the neighborhood of AlIV can be strong enough  for the acetalization reaction. The hydrophobic/hydrophilic properties of the catalyst surface also have a strong influence on the catalytic activity. In the acetalization of d-glucose with butanol on [Al]-MCM-41, the initial rate for the disappearance of d-glucose increased with an increase of the Si/Al ratio [99]. Since the number of acid sites decreases  with an increase of the Si/Al ratio, it is considered that the  hydrophobic/hydrophilic properties are involved in the catalytic process. When the Si/Al ratio of [Al]-MCM-41 is increased, the hydrophobicity of the catalyst will increase as well. Due to the difference of polarity of d-glucose (hydrophilic) and butanol (hydrophobic), the more polar glucose could preferentially and strongly adsorbed on hydrophilic surface. This leads to inhibition of adsorption of the alcohol, and desorption and diffusion of the products.

181

182

Catalytic Properties of Solid Acid Catalysts

[Al]-SBA-15 is also used for a variety of acid-catalyzed reactions such as esterification of acetic acid with alcohols and alkylation of anisole with benzyl alcohol [100]. [Al]-SBA-16, which possesses three-dimensional mesostructural channels, is more active than [Al]-SBA-15 for t-butylation of phenol [101]. More examples of the catalytic applications of ordered mesoporous materials can be found in review articles [102, 103].

4.3.2.3  Generation of Lewis acid sites on mesoporous silica

Lewis acid sites can be developed on the surface of mesoporous  silica materials by various methods. Sn and Zr can be incorporated into the mesoporous silica by the direct hydrothermal method [104–111]. The materials obtained are active for the Baeyer–Villiger oxidation, Meerwein–Ponndorf–Verley reactions, Prins reactions and pinacol rearrangement. Sn can be loaded on mesoporous silica by grafting Sn compounds followed by calcination (Eq. (4.11)) [105, 112]. These Sn-loaded materials are active for the Baeyer–Villiger oxidation and Meerwein– Ponndorf–Verley reactions. Si

OH

Si

OH

Si

OH

Si Si

OH

OH

RnSnX4–n Et3N

Si

OH

Si

O

Si

OH

Si Si

O

Sn

OH

R

Calcination 580°C, air R H2O

Si

OH

Si

O

Si

OH

Si Si

O

Sn

OH

OH

OH

(4.11)

Zirconim 1-propoxide grafted on mesoporous silica is highly active for the Meerwein–Ponndorf–Verley reaction without calcinations [109, 113]. Thus, zirconium 1-propoxide grafted SBA15 has higher activities in the Meerwein–Ponndorf–Verley reduction of 4-t-butylcyclohexanone than SBA-15 grafted with aluminum  2-propoxide. Here, the Zr species is supposed to work as Lewis  acid centers. It has been reported that strong Lewis acid catalysts are obtained by treating Sn-containing mesoporous silica (MCM-41 and UVM-7) with triflic acid [114, 115]. The model of the active centers is considered to be as follows:

Ordered Mesoporous Materials

O Si

O

CF3 S

O

Sn

O O

Si

The materials are active for the acylation of aromatic sulfonamide (4.12), synthesis of (dl)-[a]-tocophenol by the condensation of 2,3,6-trimethylhydroquinone (TMHQ) with isophytol (IP) (4.13), etherification of 1-octanol, and etherification of ethylene glycol  with 1-octanol.  R

+  CH3

SO2NH2

OH

HO

OH

TMHQ O

+



C

R

OH

O

SO2NH–CO–CH3

(4.12)

OH

–H2O

IP 

(4.13)

  4.3.2.4  Mesoporous silica functionalized with sulfo groups

In order to obtain catalysts with higher acidic strength, sulfo  groups are incorporated in the structure of mesoporous silica. In general, mercaptoalkyl groups are introduced directly in the  synthesis step (co-condensation  method) or by grafting mercaptoalkyl groups to the as-synthesized silica (grafting or  post-synthesis method). The mercapto groups are then transformed into sulfo-groups. In this way, highly acidic catalysts are formed  with advantages of large and uniform pore sizes [102, 116, 117].

183

184

Catalytic Properties of Solid Acid Catalysts

O O O

Si

O O O

SH

Si

O

O

S

OH

(4.14)

The post-synthesis method without using the expensive reagent, mercaptoalkyl trialkoxy silane has been reported [118]. In this method, SO3H-MCM-41 is prepared through condensation of  MCM-41 with benzyl alcohol followed by sulfonation with chlorosulfonic acid. The material has the acid amount as high as  8.2 mmol g–1.

(4.15)

Some examples of the reactions catalyzed by mesoporous materials with sulfo groups are shown below.

(i) Synthesis of 2,2-bis(5-methylfuryl)propane from 2-methylfurane and acetone [119]



Me

o

+

Me

Me

c = o

Me

o

Me

C

Me

o

Me (4.16)



Catalyst: MCM-41-SO3H (post synthesis) Reaction conditions: 323 K, 24 h, furane: acetone ratio = 1:2.5 Conversion: 85%, Selectivity: 96%

(ii) Synthesis of p,p¢-bisphenol A from phenol and acetone [120] OH



+

O

H+ –H2O

HO

HO

(4.17)

Ordered Mesoporous Materials

Catalyst: MCM-41-SO3H (post synthesis) Reaction conditions: 373 K, 24 h, phenol : acetone ratio = 5:1 Phenol conversion: 35.3%, Selectivity: 88.6%

(iii) Decomposition of cumene hydroperoxide to phenol and acetone [121] OH +



O

OOH

(4.18)



Catalyst: SBA-15-SO3H (Direct method) Reaction conditions, room temperature, 24 h, hexane as solvent Phenol yield: 95%, Selectivity: 100% (iv) Dehydration of d-xylose to furfural [122]

Catalyst: MCM-41-SO3H (post synthesis) Reaction conditions: 413 K, 24 h Conversion: 91%, Selectivity: 82% HO

OH

O

OH –3H2O

O

CHO

(v) Claisen–Schmidt condensation acetophenone [123]. O

R

O

H

+ H3C

R, R¢ = H, OCH3, Cl

R¢

R



(4.19) of

benzaldehyde

and

O



HO

+  H2O R¢

(4.20)

185

Catalytic Properties of Solid Acid Catalysts

Catalyst: Organo(ethane)silica-SO3H, (post synthesis) Reaction conditions: 423 K, 6 h, benzaldehyde: acetophenone  = 1:1 Conversion: 72%, Selectivity: 95% (for R = R¢ = H) (vi) Prins condensation of styrene with formaldehyde [124]

o

o

o

O



H+

H





+

–H+

o

(4.21)

Catalyst: SBA-15-SO3H (post synthesis) Reaction conditions : 413 K, 4 h, styrene : formaldehyde = 1 : 2 in dichloromethane Conversion: 100%, Selectivity : 100%

(vii) Synthesis of substituted aryl-14H-dibenzoxanthenes (a) and bis(indoyl)methanes (b) [125] R

2

CHO

OH

2

+

N H

+

R

CHO R

O

186

HN

(4.22)

R

N H

(4.23)

Catalyst: SBA-15-SO3H (post synthesis) Reaction conditions: (a) 358 K, 24 h, 2-naphthol : benzaldehyde = 2 : 1 in dichloromethane,

Heteropolyacids

(b) 333 K, 6 h, indole : benzaldehyde = 2 : 1 in CCl4. Yield: (a) 95% and (b) 52% for R = H. (viii) Fisher indole synthesis [118] NHNH3CI



CH2CH3

+

O O

OH

SO3H-MCM-41 Et

N

H

(4.24)

Catalyst: MCM-41-SO3H, (post synthesis) Reaction conditions: 363 K, 4 h, 2-ethylphenyl hydrazine hydrochloride:dihydrofuran = 1:1. Solvent, EtOH-water. Yield: 50%

4.4  Heteropolyacids

4.4.1  Isopolyanions and Heteropolyanions Molybdate ions condense in an aqueous solution of low pH to form heptamolybdate ions.

6– + 7Mo​O​2– 4​  ​ + 8H   Mo7​O​20 ​​  + 4H2O

(4.25)



+ 3–   P​O3– ​ ​  ​ + 12W​O​2– 4 4​  ​ + 24H   PW12​O​40 ​​  + 12H2O

(4.26)

Polyoxometallate ions like Mo7​O​6– 20 ​​ , which are formed by a condensation of a single kind of monomeric oxometallate ions, are called isopolyanions. Likewise, polyoxometallate ions, which are supposed to be formed by a condensation of two or more kinds of monomeric oxometallate ions, are called heteropolyanions.

  Si​O​4– ​  ​+ 12W​O​2– ​  ​+ 24H+  SiW12​O​4– 40 ​​  + 12H2O 4 4

(4.27)

There are a variety of structures of heteropolyanions. Some of them are called Keggin (XM12O40), Dawson (X2M18O62), Anderson (XM6O24), and Preyssler (NaX5M30O110) structures. Here, the

187

188

Catalytic Properties of Solid Acid Catalysts

central atom (heteroatom), X, can be P, As, Si, Ge, or B and the polyatoms, M, is usually Mo or W. Free acids of heteropolyanions  are called heteropolyacids (HPAs). For catalysis, heteropolyanions with Keggin structure are most important, since the anions with  this structure are easily prepared and thermally most stable. The Keggin structure is shown in Fig. 4.14 [126]. In the case of SiW12​O​4– 40 ​​ , at each vertex of a central SiO4 tetrahedron, there are three edge-shared WO6 octahedra. These W3O12 groups are linked  to one another on the surface of cluster. Charge in the Keggin anions is delocalized throughout the structure. There are four kinds of oxygen atoms. Four oxygen atoms are located inside the anion and connect Si and W atoms. Twelve oxygen atoms connect W3O12  groups by corner sharing and the other 12 connect WO6 octahedra  by edge sharing. The last 12 oxygen atoms are terminal ones  bonding to only one W atom.

Figure 4.14 Keggin structure of a-XM12O40 anions.

Heteropolyacids are soluble in water and in polar solvents such as acetone and methanol. Heteropolyacids such as dodecatungstophosphoric acid, H3PW12O40 (HPW), dissociate completely in the aqueous solution. They are strong general acids in organic solvents. They can be used also as solid acid catalysts. Heteropolyacids containing Mo and V are easily reduced and the reduced forms are rather easily reoxidized. Because of the redox property, these HPAs and their salts are efficient catalysts for oxidation reactions in both homogeneous and heterogeneous systems. One of the drawbacks of HPAs is their low thermal stability.  Here, the thermal stability of Keggin HPAs, defined as the 

Heteropolyacids

temperature at which all acidic protons are lost, decreases in the order [127]:

HPW (738 K) > HSiW (818 K) > HPMo (628 K) > HSiMo (623K)

Here, HPW, HSiW, HPMo and HSiMo stand for H3PW12O40, H4SiW12  O40, H3PMo12O40 and H4SiMo12O40, respectively. The decomposition of the Keggin structure begins at these temperatures. In the case  of HPW, the decomposition is complete at about 880 K to form  P2O5 and WO3 [128]. The low thermal stability limits reaction temperatures and the method of regeneration, i.e., coke burning. Various methods  to overcome these drawbacks have been attempted [128].

4.4.2  Behaviors of Protons in Solid HPAs

Since HPAs such as HPW are strong acids, they exhibit high catalytic activities for heterogeneous acid-catalyzed reactions [127, 129– 132]. The catalytic activities of Keggin HPAs as solid acid generally follow the order

HPW > HSiW ≈ HPMo > HSiMo.

This order of the catalytic activity as solid acid is identical with that of acid strength in solutions. The activity of HPAs is generally  much higher than those of typical solid acids such as zeolites. The nature of protons in HPAs has been studied by  various techniques [127, 129, 130]. 1H MAS NMR investigation of H3PMo12O40 . 29H2O has shown that the protons exist as H3O+. The electrical conductivities of H3PW12O40 . 29H2O and H3SiW12O40 . 29H2O are remarkably high and the activation energies for proton transportation are low. It is suggested that the protons in the solid can be transported through the hydrogen-bonded  network as in an aqueous solution, The X-ray and neutron  diffraction studies show that the structure of the hexahydrate can  3– be expressed as (H5O2​)+3​ ​ [PW ​  12O40] . The removal of water of hydration under vacuum results in the formation of acids in which some of protons are directly  attached to the heteropolyanions. The linewidth of 1H MAS NMR spectra of dehydrated HPAs changes very little up to 371 K, 

189

190

Catalytic Properties of Solid Acid Catalysts

indicating that the protons are rigidly fixed in the structure. At temperature higher than 423 K, the linewidths of HPW and HSiMo reduce to a small value. This shows that the protons in dehydrated HPA move rather freely in the bulk above this temperature. It is  beyond doubt that the high catalytic activity of HPA at high temperatures is related to the dynamic nature of the protons in  the bulk. 1H MAS NMR also shows that the small quantity of water (0.5 mol/mol-HPA) stimulates the transfer of protons. Six molecules of pyridine per anion are sorbed by solid HPW  at room temperature. After evacuation at 373–423 K, three  pyridine molecules per anion remain in the solid. The IR spectrum shows that the pyridine molecules are present exclusively in the  form of pyridinium ion. This indicates that pyridine can interact  with all the protons of the solid HPW: Pyridines molecules are  trapped not only on the surface of HPW, but also in the bulk of the solid. The unique feature of HPAs as solid acid may be summarized  as (1) high acid strength, (2) high proton mobility, and (3) sorption of polar molecules in the bulk of the acids.

4.4.3  Pseudoliquid Phase in Solid HPAs

As described above, about six molecules of pyridine are absorbed by a solid HPW. In general, a large amount of polar molecules such as alcohols and ammonia are absorbed rapidly into the bulk of the solid and 2–6 molecules of the adsorbates per anion are  retained even after evacuation at room temperature [129]. This  causes the expansion of the interstitial space, and the volume of the solid also expands. The infrared spectra show that the Keggin structure of the HPW is retained by sorption of polar molecules. Misono et al. conclude that the “primary structure” (the Keggin unit) is stable, but that the secondary structure” (three-dimensional arrangement of the Keggin ions, counter cations, and sorbed molecules) is largely modified. Because of this flexible nature,  Misono proposed the concept of “pseudo-liquid” phase. The interstitial space of HPAs behaves like a solution and sorbed  molecules undergo the catalytic reactions in the interstitial space  of the HPAs [129–133]. On the other hand, molecules that are not absorbed in the solid bulk undergo the catalytic reactions only on

Heteropolyacids

the surface of the solid. Schematic models for the surface type and  pseudo-liquid phase type (or bulk type) reactions are shown  in Fig. 4.15.

Figure 4.15 Schematic models for surface-type and bulk-type (pseudoliquid phase) reactions.

The pseudo-liquid phase behavior of HPAs in catalysis has been demonstrated in various reactions, including dehydration of alcohols. The number of the effective active sites of HPW for dehydration of 1-butanol was estimated by a pyridine poisoning method. The dehydration was started by feeding 1-butanol at  433 K. After the dehydration reaches a stead state, pyridine was added to the feed. Most of the pyridine introduced was retained  by the catalyst. The catalytic activity after pyridine sorption  decreased linearly with the amount of the sorbed pyridine, a  complete loss of the activity being attained when the molar ratio of the sorbed pyridine to HPW used as the catalyst was three. The amount of acid sites thus estimated indicates that all the  protons in the solid HPA are accessible to pyridine or 1-butanol [133]. This strongly indicates that the dehydration of the alcohol  proceeds in the bulk of the solid HPW. A similar phenomenon  was observed in the transformation of methanol into hydrocarbons over solid HPW [134]. Pseudo-liquid phase behavior is also seen in the catalysis by Dawson-type heteropolyacids [135].

191

192

Catalytic Properties of Solid Acid Catalysts

4.4.4  Supported Heteropolyacids Since the surface area of solid HPAs is small (2–5 m2g–1), HPAs are often used as catalyst in a supported form. The nature of the supports, loading amount and pretreatment temperature show a profound effect on the activity and the stability of the catalysts.  HSiW supported on SiO2 is an industrial catalyst for the production  of ethyl acetate from ethylene and acetic acid [136]. The effect of the support on the catalytic activity of HPMo has been studied for the vapor-phase synthesis of methyl t-butyl ether (MTBE) from isobutene and methanol [137]. The activity depends on the support. The activity decreased in the following order: SiO2 > SiO2–Al2O3(13% Al2O3) > SiO2–Al2O3 (26% Al2O3) > Al2O3 > MgO

Most of the supported acids are decomposed in the case of  Al2O3 and MgO, probably because of their basic nature. The catalytic activities of bulk HPW and supported HPW at submonolayer coverage (15 wt%) for dehydration of 2-propanol were studied [138]. The order of the 2-propanol conversion at  393 K was as follows: HPW/SiO2-300 (99%) > HPW-300 (81%) > HPW/TiO2-300 (67%) > HPW/TiO2-500 (19%) > HPW/Nb2O5-300 (9%) > HPW/Nb2O5-500 (4%) > HPW/ZrO2-300 (5%) > HPW/ ZrO2-500 (2%)

The numbers after the name of the catalysts show the  pretreatment temperature (°C). This shows that silica is a good support for HPW. The activity decreases in the order, SiO2 > TiO2 > Nb2O5 > ZrO2. This order suggests the order of the interaction between HPW and the support. The stronger interaction leads to the weakening of the acid strength of supported HPW or the decomposition of HPW. The catalysts pretreated at 573 K is always higher than those pretreated at 773 K, indicating the partial decomposition of the Keggin structure at 773 K. For liquid-phase reactions, leaching out of HPA to the liquid  phase must be avoided. Activated carbon is able to entrap a certain amount (7.2–11.9%) of HPA and that the HPA entrapped are hardly

Heteropolyacids

removed even by extraction with hot water or hot methanol.  The catalyst was active for the liquid-phase synthesis of butyl t-butyl ether by the dehydration of 1-butanol and t-butyl alcohol at 379 K. In contrast to activated carbon, silica gel was incapable of immobilizing HPAs, the acid being completely removed by solvent extraction [139]. In the acylation of benzene with benzoyl chloride over HPMo on silica at 311 K, the acid was completely decomposed during the reaction, and the active species, which might be formed from the acid and benzoyl chloride, was soluble in the liquid phase. On the other hand, HPW and HSiW supported on silica were hardly  soluble, though the catalysts were gradually deteriorated in the course of the reaction [140]. The number of acid sites of HSiW supported on silica was estimated by adsorption amount of ammonia and benzonitrile determined by temperature programmed desorption [141, 142].  The adsorption amount of ammonia was proportional to the supported amount of HSiW and corresponded to the number of protons of supported HSiW, indicating that ammonia was sorbed  in the bulk of the supported HSiW. Benzonitrile is supposed to be adsorbed only on the external surface of the supported HPW. Temperature-programmed desorption of benzonitrile showed two desorption peaks (600 and 730 K) due to chemisorbed benzonitrile. The peaks at 600  and 730 K were assigned to desorption from medium and strong acid sites, respectively. Dependence of the amounts of the two types of acid sites on the loaded amount of HSiW on silica is shown  Fig. 4.16 [142]. The amounts of acid sites with medium and high strength show their maxima at 30% and 50% loading, respectively. The catalytic activities for 1-butene isomerization and n-butane skeletal isomerization are correlated with the amount of acid  sites with medium and high strength, respectively [142]. 1H and 31P MAS NMR studies indicate a chemical interaction of HPW with the supports [127, 143, 144]. Impregnation of SiO2 with an aqueous solution of HPW gives catalysts with two HPA species, which are characterized by 31P MAS NMR: one at –15 ppm with intact Keggin structure (A) and the other at –14 ppm with a  different structure (B) (Fig. 4.17) [127, 143]. The relative amount of species A and B depends on HPW loading, with A dominating.

193

194

Catalytic Properties of Solid Acid Catalysts

At 30–50%, the species A is practically the only one present on the SiO2 surface. Below 30%, both species exist, the amount of B increasing as the loading decreases. The species A is due  to be bulk crystalline HPW, whereas B is attributed to the  “interacting” species (SiO​H+2​ ​)​  (H2PW12​O​–40  ​) ​  [127, 143] or products  of a partial decomposition or transformation of the Keggin structure, e.g., H6P2W18O62 and H6P2W21O71 [127]. Similar spectrum change was also observed in HPW supported on mesoporous  silica [127] and Nb2O5 [144].

Amount of surface acid sites/μmol g–1

150

100

50

0 0

20 40 60 80 100 H4SiW12O40 loading/wt%

Figure 4.16 Dependence of the amounts of the surface acid sites estimated from BN-TPD for H4SiO2. () Total acid sites, () medium  strength acid sites (peak at 600 K), and (D) strong acid sites (peak at 730 K). Reprinted with permission from J. Zhang, M. Kanno, Y. Wang, H. Nishi, Y. Miura, Y. Kamiya, J. Phys. Chem. C, 115, 14762 (2011).

In contrast, catalysts prepared by impregnation with a methanol solution Keggin of structure over the whole range of Figure 4.16 DependenceofofHPW theretain amounts the surface acid sites estimated from HPW loading [127]. Heteropolyacids can be immobilized in the silica matrix by BN-TPD for H4SiOmeans 2.Ƶ Total acid sites, (ʊ) medium strength acid sites (peak at 600 K), of sol-gel technique involving hydrolysis of tetraethoxysilane. The silica-included catalyst thus obtained catalyzed the water and (') strong acidparticipating sites (peakreactions at 730 K). J. Zhang et and al., J. Phys. Chem. C, 115, suchFrom as esterification isobutene  hydration [145].

14762 (2011).

Heteropolyacids

195

31P MAS NMR spectra of HPW/SiO at various loadings. Figure 4.17 4.17 31P MAS NMR spectra of HPW/SiO 2 Figure 2 at various loadings. From I. Reprinted with permission from I. Kozhevnikov, Chem. Rev., 98, Chem. Rev., 96, 171 (1998). Kozhevnikov, 171 (1998).

HPW supported by MCM-41 has been characterized by various techniques [146]. The HPW/MCM-4145samples with loadings from 10–50 wt% have ~3 nm uniformly sized mesopores. HPW retains the Keggin structure on the MCM-41 surface at an HPW

196

Catalytic Properties of Solid Acid Catalysts

loading above 20 wt%; at lower loadings a partial decomposition  of HPW was observed. HPA is finely dispersed on the MCM-41  surface and mainly located inside the MCM-41. HPW/MCM-41 exhibits a higher catalytic activity than bulk HPW and shape  selectivity. In the alkylation of 4-t-butylphenol with isobutene  in liquid phase, HPW/MCM-41 was very active. Furthermore, very high selectivity for 2,4-di-t-butylphenol (91% at 95% conversion) was obtained. This is the pore-size effect of MCM-41. Over neat  HPW and Amberlist-15, 2,4,6-tri-t-butylphenol was the main  product [146]. OH



OH

OH



(4.28)

MCM-41-supported heteropolyacids (HPW, H4SiW12O40) are  also active catalysts for the liquid-phase esterification of hexanoic acid with 1-propanol and for the gas-phase estrification of acetic acid with 1-butanol. However, the initial high dispersion of HPA  units is lost during reaction, large clusters (~10 nm) of HPA being formed on the surface of the MCM-41 support. Water, formed in  esterification, is expected to play a major role in the transport  of the HPA from the MCM-41 pores to the outer surface [147]. The mesoporous silica-supported HPW can be prepared also  by co-condensation method. HPW/SBA-15 was prepared by a direct sol-gel method combined with hydrothermal treatment in the presence of a non-ionic surfactant. The catalyst is active for the solventless condensation of phenol and levulinic acid. No  leaching of HPW was detected, but the catalyst was deactivated by the strong adsorption of the products. The catalytic activity  was restored by calcinations of the used catalyst at 693 K [148]. HO



O O

+

OH

H3PW12O40/SBA-15

HO

HO

O

DPA(p, p¢) isomer

+

OH

HO

HO

O

OH

DPA(o, p¢) isomer

(4.29)

Heteropolyacids

HPW/supported on SBA-15 is active for the vapor-phase alkylation of phenol with t-butyl alcohol [149]. The loading of  30 wt% HPW gave the best results. Zirconia is a good support for HPAs [150–154]. HPW supported on zirconia shows high catalytic activity and stability  for acylation [151], esterification [152], and dehydration of glycerol [153]. The interaction of HPAs with zirconia is stronger than that with SiO2 [151]. In the acylation of veratrole with benzoic anhydride, leaching of HPW into the liquid phase was observed for HPW on SiO2, whereas HPW on ZrO­2 acted as efficient and stable catalysts. The deactivated catalyst could be regenerated by calcination without appreciable loss in activity [151]. HPW  supported on hydrous zirconia is a good catalyst for liquid-phase alkylation of phenol with t-butyl alcohol [154]. HPW supported on TiO2 is effective for alkylation of p-cresol with t-butanol [155]. The catalyst calcined at 973 K gave the best performance [155]. The composite of HPW and Ta2O5 prepared by sol-gel method is an efficient catalyst for the esterification of  lauric acid with ethanol [156]. High catalytic activity develops when HPAs are supported on a strongly acidic ion exchange resin, Amberlyst-15 [157, 158]. The activities of the supported HPAs are much higher than those of Amberlyst-15 and the acids supported on activated carbon. The reactions studied involve the synthesis of MTBE, the esterification of acetic acid with 1-pentanol and hydration of isobutene in vapor phase. The synergism was explained by the interaction of the heteropolyanions and protons of the ion exchangers.

4.4.5  Cs Salts of Heteropolyacids

Cs salts of HPAs have unique properties as solid acid [129, 159]. Figure 4.18 shows the variation of the catalytic activities for acid catalyzed-reactions (hydrolysis of cyclohexyl acetate and alkylation of 1,3,5-trimethylbenzene with cyclohexene) as a function of x of the catalysts, H3–xCsxPW12O40 [159]. The catalytic activity decreases to almost zero as x increases from 0 to 2, whereas high catalytic activity appears around x = 2.5. The salts were prepared by adding predetermined amount of an aqueous solution of Cs2CO3 to an aqueous solution of HPW, and the precipitates were subsequently

197

198

Catalytic Properties of Solid Acid Catalysts

evaporated to dryness together with solution. Thus, x stands for  the overall composition of the precipitate plus the species  dissolved in the solution. The compound in the original precipitates was found to be HCs2PW12O40 and Cs3PW12O40 for x = 0–2, and  for x = 2–3, respectively. The remaining HPW in solution is  deposited on these precipitates during drying the solution. The  31P MAS NMR of the Cs salt with x =2.5 after heat treatment at  473 K indicates that protons are nearly homogeneously  distributed throughout the solid bulk, indicating the migration of  Cs ions and protons during heat treatment. The surface area of  the Cs salts changes greatly with the value of x; the surface areas  of the salt with x = 0, 1, 2, 2.5, and 3.0 are 6.0, 1.0, 1.2, 135, and 156, respectively. In the high-surface area Cs salts, the structure is  rather rigid and even polar molecules are not absorbed.

Figure 4.18 Catalytic activities for () decomposition of cyclohexenyl acetate and () alkylation of 1,3,5-trimethylbenzene with cyclohexene as a function of Cs content. Reprinted with permission from T. Okuhara, T. Nishimura, H. Watanabe, K. Na, M. Misono, Stud. Surf. Sci. Catal., 90, 419 (1994).

The Catalytic change inactivities the catalytic with theof value of x as  Figure 4.18 for (ʊ)activity decomposition cyclohexylacetate and (ŏ  shown in Fig. 4.18 can be explained as follows: In the range of  x = 0–2, catalytic activity decreases mainly because number  alkylation of the 1,3,5 with cyclohexene as a the function of Cs content. -trimethylbenzene of protons decreases with increase in the value of x. Over x > 2,  surface area of Stud. the CsSurf. saltsSci. greatly Thus, the number Catal.increases. , 90, 419 (1994). Fromthe T. Okuhara et al.,

Heteropolyacids

of protons on the surface increases in spite of the decrease in  the number of protons in the bulk. The catalytic activity of the  salt with x ~ 2.5 becomes even higher than that of H3PW12O40.  The activity is completely lost at x = 3. Similar dependence of the catalytic activities on the composition (x) of the Cs salts of HPW has also been found in the benzylation of benzene with benzyl chloride and benzoylation of p-xylene  with benzoyl chloride [160]. For the H4–xCsxSiW12O40, the highest activity was observed at x = 2 [160]. The activity of H2Cs2SiW12O40 was higher than that of H0.5Cs2.5PW12O40 for the benzylation of benzene with benzyl chloride. The Cs salts of HPW have micropores and the pore size of Cs salts depends on the Cs content [129, 161]. The pore sizes of the Cs salts estimated from the adsorption of molecules with different sizes are in the range 0.62–0.72 nm for x = 2.2, smaller than 0.85 nm for  x = 2.5, and larger than 0.59 nm for x = 2.1. Thus, the Cs salts exhibit shape-selective catalysis. The Cs salt with x = 2.2 effectively catalyzes the dehydration of 2-hexanol (molecular size of 0.50 nm) and the decomposition of 2-propyl acetate (0.50 nm), but is much less active for the reactions involving larger molecules such as cyclohexyl acetate (0.60 nm) and 1,3,5-trimethylbenzene (0.75 nm). This is contrast to the salt of x = 2.5, for which reactions proceed smoothly for all of these reactions. The Cs salt with x = 2.5 is moderately hydrophobic similar to low-silica H-ZSM-5 [131]. Thus, the salt is very active as a watertolerant solid acid catalyst. It is usually much more active than other inorganic solid acids for the hydrolysis of esters and hydration of 2,3-dimethyl-2-butene, as shown in Table 4.4 [162]. A drawback of the Cs salts is its small crystal size (about 10 nm). Thus, the salts easily disperse in water forming colloidal solutions, so become inseparable by filtration. By encapsulation of the salts in silica, formation of the colloid can be avoided and the catalysts become separable from aqueous media [161]. The Cs salts of HPW with x = 1.7–2.0 can be obtained by eliminating the silica matrix by HF from the composites of the Cs salt and SBA-15. The Cs salts thus obtained have high surface area (41–93 m2g–1) and show 2–3 times higher activity than that of  the Cs salt with x = 2.5 in the synthesis of methyl t-butyl ether and the dehydration of 2-propanol [163].

199

200

Catalytic Properties of Solid Acid Catalysts

Table 4.4

HO

CH2OH O OH CH3

Catalytic activities of solid acids for acid-catalyzed reactions in excess water

OH

O

CH3

CH3 CH3

CH2OH O OH +  H2O OH OH

+  H2O

Catalyst Cs2.5H0.5PW12O40

H-ZSM-5 (Si/Al = 40) S​O​2– 4​  ​–ZrO2

H-mordenite

HO

CH3 CH3

2 HO

CH3

Reaction (1)

Reaction (2)

51.0

10.5

0

0

17 0

SiO2–TiO2

6.3

g-Al2O3

Amberlyst 15 Nafion-H

(2)

Reaction rate /µmol g–1 h–1

0.4

SiO2–Al2O3

OH

CH3

H-Y

Nb2O5

(1)

CH2OH O OH OH

5.7 0 0

3.0

0.1

4.6

0

0

95.0 52.0

0

0

14.0 1.8

Reaction temperatures are 373 and 343 K for reactions 1 and 2, respectively. Source: From T. Okuhara, Chem. Rev., 102 (2002), p. 3641.

The Cs salt with x = 2.5 promoted by Pt or Pd shows a very high activity together with high selectivity under hydrogen for the isomerization of butane to isobutane [129, 164]. The catalyst is  more active and selective than Pt-sulfated zirconia and Pt-ZSM-5.  The conventional bifunctional mechanism is considered to be operative for the isomerization.

4.4.6  Catalysis by Metal Salts of Heteropolyacids

The “neutral salts” of HPAs show catalytic activities for various  acid-catalyzed reactions. Therefore, there must be some mechanisms  by which protons are generated in apparently “neutral” salts.

Heteropolyacids

The activity of metal salts of HPMo for the isomerization of cis-2-butene was correlated with the electronegativity of the metal cations. The activity of the salts with divalent ions increased with the electronegativity of metal cations, whereas the activity of the salts with trivalent cations showed a volcano-type dependence on the electronegativity [165]. It has been proposed that the  formation of protons in the metal salts is due to the dissociation of water [165–167]. For example, in the case of Al salt, the generation of protons can be expressed as

  Al(H2O​)​3+ n​  ​

 



  Al(H2O​)​3+ n​  ​



Al(OH​)(3 – n)+ ​ ​  ​ + nH+  n

Al(OH​)​+2​​ + 2H+ + (n – 2) H2O

  Al(OH)2+ + H+ + (n – 1) H2O

(4.30)

(4.31)

These processes of proton generation are supported by 1H and  31P MAS NMR study of the Al salt of HPW [166]. At higher temperature, protons are lost by recombination to water.   Al3+ + nH2O

(4.32)

This process is reversible. Infrared study of adsorbed pyridine demonstrated that the number of Brønsted acid sites in aluminum salt of HPW increased when it was exposed to water vapor at 573 K  [167]. The catalytic activity of the salt for o-xylene isomerization increased upon its exposure to water vapor at 573 K as well [167].

4.4.7  Acid Sites Formation by Reduction of Metal Cations

The silver salts of HPAs as prepared have no activity for o-xylene isomerization and pyridinium ion was not formed upon adsorption of pyridine. After the salt is reduced with hydrogen at 573 K, the activity for o-xylene develops and pyridinium ions are formed upon adsorption of pyridine. A unique phenomenon was observed in several acid-catalyzed reactions in the catalysis by the Ag salt of HPAs [130, 168–174]. The catalytic activities varied reversibly with hydrogen pressure. Figure 4.19 shows the effect of hydrogen partial pressure on the

201

Catalytic Properties of Solid Acid Catalysts

rate of the isomerization of 1-butene over partially reduced silver salt of HPW (AgPW) [168]. The rate increased almost linearly  with hydrogen partial pressure PH2 and the effect was reversible. Thus, the rate of the isomerization (R) is expressed as



R = Ro (1 + aPH2)

(4.33)

Here, Ro, PH2, and a are the rate of isomerization in the absence of hydrogen, hydrogen pressure, and a constant, respectively. The rate of the isomerization was 1.2–1.5 times larger when AgPW was reduced with H2 than when it was reduced with D2, confirming that protons are active centers for the isomerization. The activity of  the Ag salt is much higher than HPW. As expected, no hydrogen  effect is observed for HPW.

14

Isomerization rate/10–4 mol g–1 min–1

202

12 10 8 6 4 2 0 0

5 10 15 20 Hygrogen pressure/kPa

Figure 4.19 Dependence of the catalytic activity of AgPW/SiO2 for  1-butene isomerization on hydrogen pressure. T = 304 K,  1-butene = 16.5 KPa; pretreatment with H2 at 488 K for 1 h.

For the isomerization of hexane, the catalytic activity of AgPW Figure 4.19 Dependence of the catalytic activity of AgPW/SiO2 for 1-butene supported on silica also reversibly depends on hydrogen pressure.  on hydrogen pressure. T = 304, 1-butene = 16.5 KPa; pretreatment with In thisisomerization case, AgPW/SiO 2 shows no activity in the absence of hydrogen. H2 at 573 K for 1 h. From T. Baba, Y. Ono, Appl. Catal., 55, 301 (1989).

47

Heteropolyacids

The Ag salt of HPW (AgPW) is a very active catalyst for the conversion of methanol into hydrocarbons. Furthermore, the catalytic activity depends on the partial pressure of hydrogen in  the gas phase. The catalytic activity is much higher in the presence  of hydrogen than in its absence, and the effect of hydrogen is  reversible. Similar enhancing effect of hydrogen has been  observed also in the alkylation of toluene with methanol and disproportionation of ethylbenzene. The results shown above indicate that two types of acid sites exist in the working catalyst; one is formed reversibly by the  presence of hydrogen and the other is present in the as-synthesized AgPW, the amount of the latter being independent of hydrogen pressure. Existence of the two types of protons was confirmed by 1H MAS NMR [169]. Two kinds of protons, 6.4 and 9.3 ppm, appeared in the spectrum of the partially reduced AgPW. The peak at 6.4 ppm appeared only when hydrogen was present in the gas phase. The dependence of the line shapes of the two peaks on the spinning rate indicates that the protons at 6.4 ppm are mobile even at room temperature, whereas the protons observed at 9.3 ppm are not.  Only protons at 6.4 ppm are active for hexane isomerization, whereas both kinds of protons are active for butene isomerization. The temperature dependence of the linewidth was not observed  for HPW. When AgPW is treated with hydrogen, silver cations are  reduced to metallic silver. However, it is well known that metallic silver does not chemisorb hydrogen molecules. Therefore, the cationic silver, which remains unreduced may be the chemisorption centers for hydrogen molecules. It is plausible that Ag cluster  cations, A​g​n+​ ​, are formed from silver atoms and a silver cation and serve as the adsorption centers for hydrogen. Thus, hydrogen molecules dissociate on the cluster ions to afford protons.



2A​gn​+​ ​ + H2  A​g​n+​ ​ + H2 

  2Agn + 2H+

  AgH + H+

(4.34)

(4.35)

The reactions may be reversible and protons thus formed are mobile and more active than those in as-synthesized AgPW or the parent acid (HPW). The dissociative nature of hydrogen  adsorption has been confirmed by H2–D2 equilibriation reaction.

203

204

Catalytic Properties of Solid Acid Catalysts

Similar chemistry of Ag+ ions can be found in silver-exchanged zeolites (see Section 4.1.5).

4.4.8  Bifunctional Catalysis by Metal–HPA Composite Catalysts

The results shown in AgPW indicate that protons can be formed by the dissociative adsorption of hydrogen. For example, in the case  of palladium salts of HPW (PdPW), the protons can be formed by  the following reactions:



H2 

Pdo

  2H

H + [PW12O40]3– 

  H+ + [PW12O40]4–

(4.36)

(4.37)

Palladium ions are easily reduced to metallic Pd, which dissociates hydrogen, which in turn, reduce the heteropolyanions and form protons. 1H MAS NMR study shows that the exposure of hydrogen to  the Pd-HPW system shows that protons are generated reversibly and that protons thus formed have much higher mobility than those in HPW [175]. Figure 4.20 shows the time course of methanol conversion over PdPW supported on silica [130, 174]. The catalyst was treated with hydrogen prior to the reaction. The feed gas was composed of 51 kPa of methanol and 51 kPa of hydrogen (or nitrogen). The yield of hydrocarbons with two or more carbon atoms was much higher under hydrogen than under nitrogen. The distribution of hydrocarbons was quite different; alkenes are almost missing because of high hydrogenation activity of Pd metal. The yield of hydrocarbons changed reversibly by changing the accompanying gas from hydrogen to nitrogen or nitrogen to hydrogen. The yield of a by-product, carbon monoxide, did not change with hydrogen pressure, showing that hydrogen promotes acid-catalyzed reactions, but not the decomposition of methanol over the Pd metal. The same enhancing effect of hydrogen was also found in methanol  conversion by Pt-HPW supported on SiO2 (Pt/HPW = 0.2).

Heteropolyacids

205

​ ​¢2 ​​  Hydrocarbon yield/% C

100 80 (a)

60

(d)

40 20 (c)

0 0

(b)

1 2 3 4 Time on stream/h

5

Figure 4.20 Effect of hydrogen on the hydrocarbon yield in the methanol conversion over PdPW/SiO2 (T = 573 K, CH3OH = 51 KPa,  W/F 50 g h mol–1) (a) H2, (b) N2 = 50 KPa, (c, d) accompanying gas was changed from H2 to N2, and N2 to H2, respectively, after reaction of 2 h. Reprinted with permission from Y. Ono, M. Taguchi, Gerile, S. Suzuki, T. Baba, Stud. Surf. Sci. Catal., 20, 167(1985).

Involvement ofofspilt-over wasyield confirmed by separate Figure 4.20 Effect hydrogen on hydrogen the hydrocarbon in the methanol conversion over experiments. A physical mixture of Pd/SiO2 and –1HPW/SiO2 showed PdPW/SiO2 (T = 573 K, CH3OH = 51 KPa, W/F 50 g h mol ) (a) H2, (b) N2 =50 KPa, (c, d) a high activity for the methanol conversion in the presence of accompanying gas was changed from H2, to N2, and N2 to H2, respectively, after reaction hydrogen, whereas HPW/SiO 2 alone showed a low activity. The effect in metal-HPW catalysts has Surf.of Sci.hydrogen Catal., 20 (1985), p. 167 of 2 h.enhancing Ono et al., Stud. been observed also in the isomerization of alkanes. Table 4.5 shows ȱ the conversion of hexane at three different temperatures over  PdPW/SiO2. The catalyst showed a 53.0% conversion of hexane  with 89.8% selectivity for the isomers at 483 K. The isomerization  rate greatly depended on hydrogen pressure. The activity of HPW/  SiO2 was small under the same reaction conditions. The bifunctional nature of the 48catalysis was confirmed by performing the isomerization with HPA supported on Pd(5%)/ carbon from a commercial source. As shown in Table 4.5, the  HPA on Pd/carbon catalyst showed higher activity and selectivity than PdPW/SiO2 for hexane isomerization. The selectivity to the isomers was 96.4% at hexane conversion of 77.9% at 523 K.

206

Catalytic Properties of Solid Acid Catalysts

Table 4.5

Catalyst

Product distribution in hexane isomerization over PdPW supported on silica (A) and HPW supported on Pd/carbon (B) A

A

A

B

Reaction temperature (K)

443

483

523

523

Selectivity for isomers (%)

94.2

89.8

78.5

96.4

CH4

0.0

0.0

0.1

0.1

C3H8

0.3

1.7

6.1

0.7

Hexane conversion (%)

Product distribution (%) C2H6 C4H10 C5H12

2,2-Dimethylbutane 2,3-Dimethylbutane 3-Methylpentane

Methylcyclopentane Cyclohexane C7H16

27.1

0.0 1.1 0.6 2.7

63.7 27.8 1.6 2.2 0.0

53.0

0.1 4.2 3.4 4.3

59.1 26.4 0.5 0.2 0.1

Note: C6H14 = 30 kPa, H2 = 71 kPa, WF = 100 g h mol–1.

53.8

0.3 9.6 5.1 3.1

51.5 23.9 0.2 0.1 0.1

77.9

Trace 0.9 0.5 5.2

49.0 20.9 0.3 0.1 0.1

Source: From Y. Ono, Perspectives in Catalysis (J. M. Thomas, K. I. Zamaraev, eds.),  p. 431, Blackwell, London, (1992).

HPW/SiO2 is also effective for the isomerization of heptanes and pentane. For pentane, the conversion of 49% was obtained with the selectivity for isopentane of 97% at 453 K. In the isomerization  of heptanes, the selectivity for the isomers was 70% at the  conversion of 20% at 453 K.

4.4.9  Catalytic Reactions by Heteropolyacids

Because of strong acidity, heteropolyacids and their salts are extensively studied for a variety of reactions as solid acid catalyst. Table 4.6 lists recent reports on the acid reactions catalyzed  by HPAs. More examples can be found in review articles [127, 131, 132, 136].

HPW–Ta2O5 composite

Regiospecific  reusable

Yield: 80–96% reusable Room temp. in CH2Cl2 2–10 min

Room temp. in CH2Cl2, Yield: >90% 0.5–2 h

H14NaP5W30O110

HPMo/Al2O3

Thioacetalization of carbonyl compounds (4.38)

Ring opening of oxiranes with amines

(Continued)

[183]

[182]

[180]

[179]

[178]

[177]

[176]

Ref.

No leaching, regenerable by [181] calcination

Conv. 80.3%

Gas phase

HPW–SiO2 composite 373 K, 8 h

Conv. 100% Select. = 75%

No leaching, regenerable

No leaching

Gas phase

Remarks

Synthesis of diphenolic acid from levulinic acid and phenol

548 K

Conv. 99,9%

Conv.: 59% Select: >96%

81–96%

Conversion

HPW/SiO2–Al2O3

351 K, 3 h

373 K, 3 h

343 K, 1 h

303–403 K

Conditions

Dehydration of glycerol to acrolein

HSiW/ZrO2/SiO2

Esterification of lauric acid with ethanol

Esterification of  acetic acid with benzyl alcohol

H6P2W18O62 . 24H2O H6P2W18O62/SiO2

H6P2W18O62

Ethyl t-butyl ether from isobutene and ethanol

Diphenylmethyl ethers from diphenylmethanol and alcohols or phenols

Catalyst

Reactions catalyzed by heteropolyacids

Reaction

Table 4.6

Heteropolyacids 207

373 K, 0.5–1.5 h

Yield 80–96%

Esterification of cinnamic acid with phenols and imidoalcohols (4.44)

H14[NaP5W29MoO110] Refluxed in toluene 7 h for phenols 24 h for imidazoles

87% 82%

Solvent-free

Solvent-free

Remarks

Yield: 50–99% Solvent-free

Conv., 100% Selectivity 60%

Yield 92%

Yield 100%

Conversion

Synthesis of 6-aryl-1H-pyrazolo[3,4- H14[NaP5W29MoO110] Refluxed in acetic acid Yield: 83–92% 1h d]pyrimidine-4[5H]-ones (4.43)

HPW, HPW/SiO2 H6P2W18O62, H6P2W18O62/SiO2

363 K, 0.5–2 h

H14[NaP5W30O110]

Synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes (4.42)

333 K 2h

HPW/SiO2

Synthesis of highly substituted pyrroles (4.40)

Methoxylation of a-pinene (4.41)

373 K in toluene 180 min

Conditions

353 K 1.5 h

H6P2W18O62 HPMo/SiO2

Catalyst

Reaction

Deprotection of aldehyde from  1,1-diacetals (4.39)

Table 4.6  (Continued)

[190]

[189]

[188]

[187]

[186]

[185]

[184]

Ref.

208 Catalytic Properties of Solid Acid Catalysts

Heteropolyacids



R

O

+ HS R¢ OAc



NH2

CH3CH2CHO +

R¢

R CHO

OAc

S

S

SH



(4.39) NO2

Me

+

N

Me

Me

O2N



O2N

O-CH3



(4.41) Ar

R N

Ph

NH2

+  R1CHO

NH2

(4.42)

O

R N

O

O

(4.40)



OH H14[NaP5W30O110] (0.03g) + ArCHO Heat

2



(4.38)

N

Ph

NH N

R1



(4.43)

209

210

Catalytic Properties of Solid Acid Catalysts



4.5 Clays (Montmorillonite and Saponite)

(4.44)

Clay minerals, once used for the Houdry cracking process as described earlier, are one type of mixed oxides, but have regular layered structure. Among clays, montmorillonite and saponite are most commonly used as catalysts. Both of them belong to a smectite clay mineral group. Smectite clays have sheet structure as shown in Fig. 4.21. The basic unit structure of this group of clays consists of two tetrahedral Si sheets separated by one octahedral Al or Mg  sheet. Two basic minerals are pyrophyllite in which ions in  octahedral sheet are all Al3+, and talc in which ions in octahedral sheet are all Mg2+. In these minerals, the clay layers are neutral; no negative charges on the layers.

Figure 4.21 Schematic illustration of smectite clay. Each layer consists of  a sheet of octahedrally coordinated cations (mostly Al3+),  which is sandwiched between two tetrahedrally coordinated silicate sheets, charge-compensating cations being located between the layers if the layers are negatively charged.

In montmorillonite, Al3+ ions in the octahedral sheet of pyrophyllite are partly replaced by Mg2+ ions, which results in localized negative charges on the clay layers. The negative charges

Clays (Montmorillonite and Saponite)

are balanced by hydrated cations (normally Na+, K+, or Ca2+) in the interlayer space. In saponite, Si4+ ions in the tetrahedral sheets of talc are partly replaced by Al3+ ions, and charge balancing cations exist in interlayer space. Smectites are swelling and ion-exchangeable clays. Water can enter into the interlayer spaces to expand the clay layers. Acid treatment of the clays causes the ion-exchange of the interlayered cations with H+ ions and results in the generation of acid sites. Swelling and ion-exchangeable properties enable to prepare  pillared clay materials by ion-exchange with larger hydrolyzed metal cations or organic/inorganic complexes. Acid-treated montmorillonite and smectite are used in a limited number of chemical industrial processes as solid acid catalysts and adsorbents for purification and decolorization of oils. Pillaring in between the sheets results in a two-dimensional mesoporous material (pillared clay). For pillaring materials, Al2O3 is most commonly used. The other materials include oxides of Ti, Zr, Cr, and Fe and mixed oxides of Fe–Al, Ga–Al, Si–Al, Zr–Al. The common procedures for the preparation of pillared clays are (1) swelling of smectite in water, (2) exchanging the interlayer cations by partially hydrated polymeric or oligomeric metal cation complexes in the interlameller region of the clay, and (3) drying and calcining of the wet cake of expanded clay to transfer the  metal polyoxocations into metal oxide pillars. Preparation of pillared clays is reviewed [191]. Table 4.7 lists the catalytic application of pillared days in acidcatalyzed reactions [192]. In addition to the reactions listed in Table 4.7, montmorillonite acts as effective catalysts in fine chemical synthesis. Some examples are described below. Addition of 1,3-dicarbonyl compounds to alkenes are efficiently catalyzed by acid-treated montmorillonite (H-mont) at 373–423 K in heptane (Fig. 4.22). A proposed reaction path involves the dual activation of the alkene and the 1,3-dicarbonyl compound: (1) protonation of an alkene at an acid site and coordination of a 1,3-dicarbonyl compound to a neighboring H+ site, (2) formation of an activated enol intermediate; nucleophilic attack of the enol  Fig. 4.23 [212].

211

212

Catalytic Properties of Solid Acid Catalysts

Table 4.7

Catalytic application of pillared clays in acid-catalyzed reactions

Reaction

Reactant/product

Catalyst (Pillared mater./Clay)

Ref.

Dehydration

2-Propanol/propene

Cr/Mont

[193]

Methanol/hydrocarbons

Alkylation

Isomerization

Al/Mont

[195]

Ti/Mont

Methanol/hydrocarbons glucose/5-hydroxymethylfurfural

Fe, Cr/Mont

[194]

[196]

1-phenylethanol/

Al/Mont

[197, 198]

Cumene

Fe, Cr/Mont

[196]

Acetic acid,   2-methoxyethanol/ 2-methoxyethanol acetate

Al/bentonite

[201]

Biphenyl, propene/ alkylated biphenyls

Al/Mont, others

[202]

trimethyl benzene

Al, Ga/Al/Mont

3-oxa-2,4- diphenylpentane Ti/Mont

Disproportionation Toluene Esterification

[193]

1-butanol/butenes

1-pentanol/pentenes

Cracking

Cr/Mont

Toluene, methanol/xylene,

Cr or Al or Zr/Mont

Benzene, propene/cumene Al or La/Al 1-Butene/isobutene

Heptane/isoheptanes

Al

Pt/Al (or Zr)/Sapo Pt/ Al/Mont, Sapo

[199]

[200]

[207]

[203– 205] [208] [206] [207]

[208]

[209]

Pt/Zr, Zr–Al/Mont, Sapo [210]

Mont, montmorillonite; Sapo, saponite. Source: Adapted from Table 3 in ref. [192].

Pt/Al/Mont, Sapo

[211]

Clays (Montmorillonite and Saponite) O

R1

O 2

R3 R H O

O

R1

R2

O

R1

O

R3

n

H-mont

R3 R2

n R1

R4

R4

O O R3

R2

Figure 4.22 Addition of 1,3-dicarbonyl compounds to alkenes over Hmontmorillonite.

Figure 4.23 A proposed dual-activation reaction pathway for the addition of 1,3-dicarbonyl compounds to alkenes or alcohols in the presence of H-montmorillonite catalyst.

213

214

Catalytic Properties of Solid Acid Catalysts R3

R1

R3-NH2

R4

O

O R6

R5

SiMe3

H- or M+-mont R1

OH

R2

R4

O

R1

R1

NH

R2

O

R6

R2

R5

R2

Figure 4.24 Montmorillonite-catalyzed substitution of nitrogen compounds, 1,3-dicarbonyls and allylsilane. Reprinted with permission from K. Motokura, N. Nakagiri, T. Mizugaki,  K. Ebitani, K. Kaneda, J. Org. Chem., 72, 6006 (2007).

Nucleophilic substitution of nitrogen compounds, 1,3dicarbonyls, and allylsilane with alcohol are also effectively catalyzed by an acid-treated montmorillonite at 323–373 K in heptane  (Fig. 4.24) [213]. It is suggested that the efficient catalytic performance of montmorillonite for the above reactions is due to weak acid  strength and delocalized counter anions of two-dimensional  silicate sheets of montmorillonite.

4.6  Alumina and Modified Alumina 4.6.1  Al2O3

Aluminas (Al2O3) is used both as a catalyst for various kinds of reaction and as a support for metals and metal oxides. Al2O3 is very  frequently used as a support of industrial catalysts for its  mechanical strength as well as its strong interaction with metals and metal oxides that enables high dispersion of the supported compounds. As for the surface properties, alumina is generally regarded as acidic rather than basic, but basic sites coexist. Participation of both acidic and basic sites in the catalytic behavior of alumina is observed in many reactions.

Alumina and Modified Alumina

4.6.1.1  Structure and preparation of alumina Alumina is prepared from hydroxide (Al(OH)3) and oxyhydroxide (AlO(OH)) by dehydration at elevated temperatures. The hydroxide, oxyhydroxide, and oxide of Al exist in a and g forms. It is common to name them by mineral names of these Al compounds because they are often referred to by name and also because the American nomenclature is sometimes different from that used in England [214]. Chemical formula

Form

Al(OH)3

a

AlO(OH)

a

Al2O3

a

Al(OH)3

g

AlO(OH)

g

Al2O3

g

Mineral name Bayerite,

Gibbsite, Hydrargillite, Nordstrandite Diaspore

Boehmite

Corundum —

Al2O3 exists in different crystal phases depending on the precursors and the conditions of heat treatment. The most stable crystal form of alumina is a-alumina, and heating the precursors above 1470 K results in the formation of a-alumina. At lower temperatures a variety of phases are formed, which are collectively known as g-alumina or transition alumina, and in various studies, the Greek alphabets have been used in naming them, such as c, d, e,  g, h, k, q, and r. These phases represent various degrees of ordering  of Al atoms in an essentially cubic closest packing of O atoms, described as a defect spinel structure, since there are only 21 and 1/3 metal atoms arranged at random in the 16 octahedral and  8 tetrahedral positions of that structure. Among these transition aluminas, g- and h-aluminas are important as catalysts primarily because of high surface area. The proposed pathways for the formation of different transition aluminas have some discrepancies between researchers, but the pathways shown in Fig. 4.25 are typical of the schemes  postulated as the result of X-ray and DTA studies [215] (Fig. 4.25).

215

216

Catalytic Properties of Solid Acid Catalysts

Figure 4.25 Pathways for formation of different transition alumina.



4.6.1.1.1  Conventional preparation of h-alumina

One preparation method for h-alumina through bayerite reported by Kul’ko et al. is described below [216]. Bayerite was prepared by the precipitation from an aqueous solution of Al(NO)3 with a concentrated aqueous ammonia at constant pH 10.0 ± 0.2 and at  295 ± 2 K, followed by aging the suspension at room temperature  for 10 days. After aging, the precipitate was filtered off and washed with distilled water until no N​O​–3 ​​  ions were detected in the filtrate. The filtered cake was dried at 383 K for one day. The obtained material was bayerite, which contained 2.99 mol H2O/(mol Al2O3), which is close to the theoretical value of 3.0 mol H2O/(mol Al2O3). h-Alumina was obtained by calcination of the bayerite at 873 K or 1073 K in air for 4 h. The h-alumina thus obtained changed gradually into q-Al2O3 on heat treatment in the range 1173–1273 K, and then a-Al2O3 above ca. 1573K. Surface areas of the h-alumina obtained  by calcination at 873 K and q-Al2O3 obtained by calcination at  1173 K were 280 and 110 m2/g, respectively. The other preparation methods for h-alumina as well as  g-alumina were reported by MacIver et al. [217].

4.6.1.1.2  Sol-gel preparation of alumina

A sol-gel preparation of Al2O3 reported by Wang et al. is as follows [218]. Aluminum s-butoxide (25.73 mL) was dissolved into a given amount of butanol. Oxalic acid (1 g) was added to the solution as

Alumina and Modified Alumina

a hydrolysis catalyst. The pH of the solution was kept at 5. Then  15 mL of water was slowly added into the solution followed by refluxing for 3 h at 343 K with continuous stirring until a gel was formed. The gel was dried at 343 K and calcined at 673, 873 or  1073 K. The Al2O3 calcined at 673, 873, and 1073 K had surface  areas of 504.7, 346.8, and 172.9 m2/g, respectively. The Al­2O3’s calcined at 873 K and 1073 K were composed of g- and q-Al2O3,  the relative composition of q-alumina was higher for 873 K  calcined sample compared with 1073 K calcined sample.

4.6.1.1.3  Mesoporous alumina

Vaudry et al. prepared mesoporous alumina as follows [219]. An aluminum hydroxide suspension was obtained by hydrolysis of  43.8 g of aluminum s-butoxide with 10.3 g of deionized water in  275 g of 1-propanol. After 60 min of stirring, 10.8 g of lauric acid was added. The mixture was aged for 24 h at room temperature and heated under static condition at 383 K for 2 days. The solid was filtered, washed with ethanol, and dried at room temperature. A mesoporous alumina having pores of 2.1nm and surface area  of 710 m2/g was obtained after calcining at 703 K for 2 h in air. Zhang et al. also prepared mesoporous alumina [220]. An aqueous mixture of [Al13O4(OH)24(H2O)12]Cl17 (27.78 g of a 12.5 wt% Al solution, 100 mmol Al) and the triblock polymer, poly(ethyleneglycol)-poly(propylene glycol)-poly(ethylene glycol) ((EO)19(PO)39(EO)19 surfactant Pluronic P84) (4.2 g, 1.0 mmol)  was aged at 313–353 K for 6–10 h, and then hydrolyzed with concentrated NH4OH (3.02 g 50 mmol). The precipitate was aged  at 353 K for 6 h and then 373 K for 24 h, and air-dried. The resulting mixture of the mesostructured surfactant/boehmite precursor and NH4Cl was calcined at 598 K for 3 h and then 823 K for 4 h.  A mesostructured g-alumina was obtained with a surface area of  306 m2/g and pore size of 6.4 nm.

4.6.1.2  Surface properties of alumina

Surface properties of alumina have been studies primarily by IR  of the surface OH groups and the adsorbed probe molecules. On the surface, there exist O atoms with different coordination numbers, Al atoms with different coordination numbers and OH groups 

217

218

Catalytic Properties of Solid Acid Catalysts

of different forms. Distribution of these sites varies primarily  with pretreatment condition.

4.6.1.2.1  IR study of OH groups

The surface hydroxyl groups on transition aluminas were studied  by IR, and the exact assignments of the O–H bands have been a subject of discussion [221]. Peri observed five distinct O–H stretching bands, three major OH bands (3800, 3744, and 3700 cm–1), and two minor bands (3780 and 3733 cm–1) [222, 223]. These bands became distinct by pretreatment of Al2O3 in a vacuum above 773 K. Relative intensities of these bands varied with the temperature of evacuation. Peri proposed the surface model of Al2O3 by assuming that the (100) plane of a cubic, close-packed oxide lattice and that aluminum ions are located in all interstices between oxide ions. Based on the  model, he assigned the five bands to isolated hydroxyl groups in different environments, each with a different number of oxide ions as nearest neighbors on the surface. The bands at 3800, 3780, 3744, 3733, and 3700 cm–1 were assigned to the OH groups with the number of the nearest neighbor oxide ions of 4, 3, 2, 1, and  0, respectively. The limit of Peri’s model is the assumption of the (100) crystal face as the only possible termination for the  crystallites of alumina, in that only octahedrally coordinated Al atoms (AlVI) would be present in the uppermost layer. Tsyganenko and Filimonov examined the νOH vibrations of a very large number of metal oxides, including alumina and classified them according to the crystal structure [224, 225]. Considering the most probable terminations of the crystallites and the geometry of the OH groups in these terminations, they concluded that the number of nearest neighbors has a negligible effect on the frequency of the OH species, whereas the determining factor is the number of  lattice metal atoms that OH groups are attached to. There exist  three types of OH groups of type I, II, and III, differing in the coordination number of the OH groups, 1, 2, and 3, respectively  (Fig. 4.26). They also suggested that the further splitting of the  each band could be caused by the difference in the coordination  number of the aluminum atoms. Knözinger and Ratnasamy proposed a very detailed model for the surface of transition alumina [226]. The basic assumptions

Alumina and Modified Alumina

of the model are the following: (1) The termination of alumina crystallites occurs along crystal planes, the (111), (110), and (100) planes, and (2) the frequency of OH species is imposed by the net electrical charge at the OH group. The net charge is determined by the coordination number of both OH groups and Al cations. Based  on these considerations, they singled out, within the three crystal plains and nine possible OH configurations, five configurations as shown in Fig. 4.26 by neglecting possible differences in relative orientation of the OH group with respect to Al. Knözinger and Ratnasamy(13)

H O 3785–3800

Tsyganenko and Filimonov(12,12) H O 3800

(111), (110), (100)

H O 3790

AlVI

Al

AlIV

AlIV

H 3740 O

AlIV

H O 3760–3780 (111), (110)

AlVI AlIV AlVI

Busca et al.(14,15)

H 3740–3745 O (111), (110)

AlVI

H 3730–3735 O

AlVI

H 3700–3701 O

AlVI

AlVI

Al

Al

H 3700 O

Al Al Al

H O 3770

O

H O 3740

AlVI Al

H 3680 O

Al

H 3580 O

Al Al Al

Digne et al.(14,15) H 3785–3800 O (110)

AlIV H O

AlVI

3760–3780 (100)

H 3745–3740 O (111)

AlVI AlVI Al

VI

H O 3730–3735 (110)

AlV

H 3690–3710 O (110)

AlVI AlVI

H 3590–3650 O (110)

Al Al Al

Figure 4.26 Types of OH groups and IR absorption frequencies (cm–1) proposed by researchers. Roman numbers of superscript to Al are coordination numbers of Al. Reprinted with permission  from Y. Ono, H. Hattori, Solid Base Catalysis, Springer-Tokyo  Institute of Technology Press (2011).

Busca et al. modified the model of Knözinger and Ratnasamy, who considered only regular surface terminations by taking  account of cation vacancies, and reassigned the various OH species as shown in Fig. 4.26 [227, 228]. Digne et al. made the vibrational analysis of the OH groups on (110), (100), and (111) surfaces of the bulk model of g-Al2O3

219

220

Catalytic Properties of Solid Acid Catalysts

based on DFT (density functional theory) calculation [229, 230]. The model they used was also constructed by DFT calculation for the dehydration of boehmite to g-Al2O3. The most stable structure turned out to be nonspinel with 25% of tetrahedral Al atoms,  which is not traditional defective spinel-like structure [231].  They calculated OH vibration frequencies for 12 different OH  groups. Selected OH groups and their frequencies are included in Fig. 4.26.

Figure 4.27 Linear relation between experimental OH stretching  frequency and formed charge on the OH. Reprinted with permission from J. J. Fripiat, L. J. Alvarez, J. S. Sánchez, E. M. Morales, J. M. Saniger, N. A. Sánchez, Appl. Catal., A, 215, 91 (2001).

Although the assignment of the O–H vibration peaks is not always the same for all investigators, they agree the point that the OH groups giving the bands at lower frequencies are stronger in acidic properties than the other OH groups with some exceptions. Knözinger and Ratnasamy suggested that on adsorption of CO, the three bands at lower frequency, 3725, 3715 and 3695 cm–1, were completely eroded, whereas the highest two peaks at 3785 and  3775 cm–1 remained almost entirely unaffected by CO. CO interacts with acidic OH groups. The OH groups showing bands at 3725, 3715, and 3695 cm–1 are stronger Brønsted acid than the OH groups showing bands at 3785 and 3775 cm–1. In general, the lower the frequency of the O–H stretching band, the higher the acid strength  of the OH groups. A linear relationship between the frequency and 



Alumina and Modified Alumina

the formal charge of the OH group was presented by Fripiat et al.  [232] (Fig. 4.27).

4.6.1.2.2  IR study of adsorbed CO

CO is adsorbed not only on the OH groups but also on Al atoms  with different coordination numbers. IR spectrum of adsorbed CO can monitor the acidic nature of the Al atoms. CO is a weak base and can monitor the acidic sites with different strength. This is different from strong bases such as ammonia and pyridine. Strong bases are adsorbed on both strong acidic sites and weak acidic sites, and adsorption bands are not distinctly different. Because of a weak adsorption, the measurements are normally undertaken at a low temperature ca. 173 K. The CO stretching frequencies are sensitive to the chemical environment. On adsorption on Al2O3, the s-donation from 5s COorbital to the empty 3p orbitals of the unsaturated Al atoms is the driving force for the adsorption. Since the 5s CO-orbital is slightly antibonding, the CO-bond becomes stronger upon adsorption and the CO frequency shifts to higher frequency compared with gasphase CO stretching. The more acidic the Al site, the higher the upward shift of the CO frequency. Four bands are observed: (A) 2240–2220 cm–1, (B) 2200–  2190 cm–1, (C) 2165–2155 cm–1, and (D) 2150–2140 cm–1. Band (D), which is hardly shifted from the gas-phase CO-band, is commonly assigned to physically adsorbed or H-bonded molecules. CO  adsorbed on the OH groups show a band in this region. There is no consensus as to the assignment of the other three bands. According  to Zecchina et al. [233], (A) is assigned to CO adsorbed on Al atom  at the surface defects, (B) to tetrahedral Al atoms, and (C) to  octahedral Al atoms. Digne et al. assigned, on the basis of DFT calculation, bands (A), (B), (C) and (D) to AlIII(110), AlIV(110) and AlV(110), AlV(100), and AlV (on highly hydrated surface) or Brønsted sites, respectively [230]. The

formal electrostatic charges on the O and OH are calculated as a function of the neighboring Al cations. An AlVI–O bond intervenes for +0.5e charge, whereas an AlIV–O is counted as +0.75e charge according to Pauling’s definition of electrostatic bond strength. The charges on the bridging O atoms in AlVI–O–AlVI and AlV–O–AlIV, for example, can be calculated to be –1e and –0.65e, respectively. Complementarily, the formal charges on the bridging OH’s are 0e and +0.35e, respectively.

221

222

Catalytic Properties of Solid Acid Catalysts

4.6.1.2.3  IR study of adsorbed CO2 Adsorption of CO2 on Al2O3 results in the formation of several  surface species such as linear CO2, bridged carbonate, bidentate carbonate, monodentate carbonate, and hydrogencarbonate [234– 237]. The spectra of adsorbed CO2 reported by various authors  differ considerably, which is due primarily to the degree of dehydration of Al2O3. The phase of alumina does not significantly influence the spectra [238]. Below ca. 673 K of heat treatment, the Al2O3 surface is full of  OH groups, and adsorption of CO2 results in the predominant formation of hydrogencarbonate species and the formation of  linear CO2 to a small extent. Above ca. 673 K, oxide ions and exposed Al ions exist together with OH groups. Adsorption of CO2 on alumina heat-treated at high temperatures results in the formation of bidentate carbonate, monodentate carbonate, bridged carbonate, linear CO2, and hydrogencarbonate. For the formation  of bidentate carbonate and bridged carbonate, both exposed  Al ion and coordinatively unsaturated O ion (OCUS) are required  to exist on the surface. For the linear CO2, coordinatively unsaturated Al atom of tetrahedral coordination (AlIVCUS) or that of octahedral coordination (AlVICUS) is responsible. For the monodentate  carbonate, only oxide ions are required to exist. Accordingly, except for the formation of the monodentate carbonate, the surface  Al atoms in different coordination states are involved in the formation of the carbonates species. In all cases, these Al atoms undergo electrophilic attack to O in CO2, and, therefore, act as an  acid toward CO2. The linear CO2 shows three bands at 2347, 2370, and 2407 cm–1.  Morterra et al. measured the dependency of the peak intensity  on the CO2 pressure, co-presence of CO, and the sample heattreatment temperature [235]. They assigned the band at 2347 cm–1  to CO2 interacting with a coordinative vacancy on an AlVI, and the two bands at 2370 and 2407 cm–1 to that on an AlIV. Their assignment coincides with Peri’s assignment that the band at  2370 cm–1 is ascribed to CO2 held by a strained Al–O–Al linkage  (a site) by ion–quadrupole interaction if Peri’s a site is AlIV. In  addition, they observed a connection between sites responsible  for the strongly held linear CO2 and those for bridged carbonate as  suggested by Peri that the 2370 cm–1 and 1870 cm–1 bands are

Alumina and Modified Alumina

related through an equilibrium between the linear CO2 and bridged carbonate.

4.6.1.2.4  IR of adsorbed pyridine

On adsorption of pyridine on Al2O3, the bands due to pyridine coordinated to Lewis acid sites and H-bonded pyridine are  observed, no bands ascribed to pyridinium ion are observed. No OH groups on Al2O3 act as sufficiently strong Brønsted acid to protonate pyridine to form pyridinium ions. The strength of Lewis acid sites reflects on the frequency of the ring vibrational modes 8a, 8b, 19a and 19b, according to the assignment of Kline and Turkevic [239]. The stronger the Lewis acid site, the larger the frequency shift of  the 8a mode. The absorption band of 8a mode appears at 1574 cm–1 for gasphase pyridine. Four bands are observed on adsorption. The band at ca. 1590 cm–1 is assigned to pyridine H-bonded to surface OH groups, the band at 1598 cm–1 to AlV site, the band at 1610–1620 cm–1 to  AlIV site, and the band at 1625 cm–1 to the AlIII site [221]. It is to be noted that although the OH groups on Al2O3 cannot protonate pyridine, they can protonate more strong bases such as butylamine and piperidine. Therefore, it would be misleading to state that Al2O3 possesses only Lewis acid sites on the basis of IR study  of adsorbed pyridine. The OH groups on Al2O3 may act as Brønsted acid toward strongly basic molecules or at a high temperature.

4.6.1.2.5  IR of adsorbed ammonia

Peri studied IR of NH3 adsorbed on Al2O3 in detail [240]. Several bands appeared on adsorption of NH3 on Al2O3 pretreated at 1073 K  in the N–H stretching region (3600~3100 cm–1) and in the –NH2 or NH3 deformation region (1700~1500 cm–1). No band was observed near 1400 cm–1, as should have resulted from formation of N​H​+4​.​  In addition, a new band appeared near 3725 cm–1, which is ascribed to O–H stretching. Peri suggested based on the observed spectra and their changes with desorption of adsorbed NH3 by increasing the desorption temperature that most NH3 held at 323 K is undissociated and some forms N​H–2​  ​ ​  and OH– by abstraction of H+ from NH3 by basic site O2–; little forms N​H+4​ ​ ​  by protonation of NH3. The undissociated NH3 is coordinated with Al atom (Lewis acid site) and the formation of N​H–2​  ​​  with simultaneous 

223

224

Catalytic Properties of Solid Acid Catalysts

formation of OH groups occurs on AlCUS3+–OCUS2– ion-pair or “acid– base” sites. AlCUS stands for coordinatively unsaturated site of Al.

4.6.1.3  Catalytic properties

Reactions catalyzed by Al2O3 include dehydration of alcohols, isomerization of alkenes, H–D exchange between D2 and CH4 or alkenes, H–D exchange between CH4–CD4, H-D exchange among alkenes, and H2/D2 equilibration for which both acidic sites (AlCUS atoms and OH groups) and basic sites (O atoms and OH groups) are involved in different ways. For the diagnostic reaction of 2-methyl-  3-butyn-2-ol (MBOH), alumina gives a dehydrated product 3-methyl3-butene-2-one (MIPK) indicating amphoteric properties.

4.6.1.3.1  Dehydration of alcohols

Alumina catalyzes intramolecular dehydration of alcohols to form alkenes and intermolecular dehydration to form ethers. The selectivity of alkene formation versus ether formation is primarily determined by the reaction temperature as well as the structure of alcohols. Alkene formation favors at a high temperature, whereas ether formation favors at a low temperature. The alcohols without b-H atoms (such as benzylalcohol) yield only ethers. The tendency toward ether formation is reduced as the chain length and chain branching of alcohols increase; tertiary alcohols (such as t-butyl alcohol) form exclusively alkenes. Reaction mechanisms  and, therefore, surface sites involved in the reaction are  different for alkene formation and ether formation. Acidic sites (OH groups) and basic sites (O2– ions) participate in both types  of dehydration. Intramolecular dehydration of alcohols on Al2O3 proceeds via E2 mechanism in which elimination of the OH group and b-H of  the alcohol is concerted without formation of ionic intermediates. Both surface OH groups and basic sites (O2– ions) are involved.  Lewis acid sites (coordinatively unsaturated Al3+ ions) are not participated in the reaction. This is based on the poisoning  experiments with varying amounts of preadsorbed pyridine in dehydration of t-butyl alcohol and isobutyl alcohol. The dehydration  is not retarded by preadsorption of pyridine, which is adsorbed  on Lewis acid site. Preadsorption of TCNE, which is adsorbed  on the basic sites, on the other hand, strongly retards the  dehydration [241].

Alumina and Modified Alumina

Figure 4.28 Dehydration of 2-butanol over Al2O3 by E2 mechanism.

Knözinger proposed the mechanism of the activation of alcohol as follows (Fig. 4.28) [242]. The activation is assumed to be initiated by interaction of alcoholic OH group with surface OH group, which may result in polarization of the alcohol molecule so that b-H  becomes more acidic. The b-H may approach a basic O2– ion and abstracted by the basic site (O2–). The rate-determining step is the abstraction of b-H, which is estimated from the primary  isotope effect observed when the b-H attached is substituted  for D [243]. The mechanism for intramolecular dehydration of  alcohols is shown in Fig. 4.28 [244]. For intermolecular dehydration of alcohols to ethers, three types of centers are involved: Al–O pair sites and OH groups.  Unlike intramolecular dehydration, intermolecular dehydration is strongly poisoned by pyridine, indicating the participation of  Lewis acid sites (Al3+). TCNE also retards the ether formation, indicating the participation of basic sites. It was shown that as the  chain length of alcohol is reduced, the tendency to form alkoxides on Al2O3 increases, and the tendency to form ethers also increases. These suggest that alkoxides formed on Al–O pair sites are  reaction intermediates. The second molecules to react with the alkoxides are assumed to be the alcohol molecules, which are Hbonded to the surface OH groups.

4.6.1.3.2  Isomerization of alkenes

Butenes undergo double bond and cis-trans isomerization over  Al2O3 above room temperatures. Hightower et al. studied coisomerization of 1-butene d0/d8 and cis-2-butene d0/d8 on alumina  at 300 K, and showed that the cis-trans isomerization involves primarily intramolecular H (or D) transfer, but no decision could be reached concerning double bond migration. Reactivity of d0  butene isomers was higher than d8 isomers; definite isotope effects

225

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Catalytic Properties of Solid Acid Catalysts

were observed for all the isomerization reactions. The cis to trans ratio in 2-butene produced in 1-butene isomerization was 6.25  at 300 K. The ratio of 1-butene to trans-2-butene was 0.22 in  cis-2-butene isomerization; double bond migration was slower  than cis-trans isomerization, which is not typical for base-catalyzed  cis-2-butene isomerization. As they described, the reaction may proceed by different pathways [245]. Whatever the active sites and mechanisms are, one point has been clearly established: All the isomerization reactions over alumina involve C–H bond cleavage in the rate-determining step. For the main pathway for double bond migration, Gerberich and Hall proposed a cyclic intermediate that is draped over a surface oxide ion in the form of cis configuration to explain the high cis/trans  ratio in 1-butene isomerization [246]. Although they did not state that the reaction intermediates are carbanion, it is plausible to assume that the double bond migration of 1-butene proceeds via abstraction of an allylic H+ by basic site (O2–) to form an allylic carbanion intermediate, which then accepts the abstracted H+ at  the terminal C to form primarily cis-2-butene. Peri observed that 1-butene isomerization was poisoned by NH3 [240]. He also measured the adsorbed ammonia on alumina by IR spectroscopy. Adsorption of ammonia occurs in several  ways. Certain sites that adsorb ammonia as N​H–2​  ​​  and hydroxyl ions appear essential for butene isomerization. The sites are suggested  to be “acid–base” or “ion-pair” sites. Corad et al. postulated two types of sites active for double bond isomerization of 2,3-dimethyl-1-butene at 353 K [247]. The catalyst deactivates during the reaction. The mode of hydrogen transfer (intra- or inter-molecular) during the isomerization changes from predominantly intra-molecular to intermolecular during the deactivation. The sites of the first type are predominating on the fresh catalyst, but they are blocked by self-poisoning during the reaction. The sites of the second type are responsible for a stable activity. They specified the active site of the second type to be made up by a basic oxygen ion, incompletely coordinated Al ion (Lewis  acid site) and a hydroxyl group. A cyclic allylic carbanion-like species was proposed as an intermediate for the intermolecular isomerization (Fig. 3.29). Although basic sites appear to be responsible for alkene isomerization over Al2O3, CO2 does not poison the isomerization,

Alumina and Modified Alumina

whereas CO2 strongly poisons the H–D exchange reactions such as C4H8–D2 and C6H6–C6D6 [248, 249]. Different poisoning effects by CO2 on the isomerization and exchange reactions indicate that the isomerization and the exchange reactions occur independently, though exact structures of these sites are not clear.

4.6.1.3.3  H–D exchange

Alumina exhibits particularly high activity for the H–D exchange  of CH4 with D2, CD4, and surface OD groups. Larson and Hall reported that the mixing of isotopes between CH4 and CD4 took place at a readily measurable rate at room temperature with an activation energy of 23.8 kJ/mol [250]. The H–D exchange of CD4–H2 occurred at about the same rate as CD4–CH4 equilibration. The H2–D2 equilibration was much faster (virtually instantaneous at 195 K). The mixing of CH4 with D2 proceeds about 1.8 times faster than the rate of mixing of CD4 with H2. It was inferred that the rate-determining step in the mixing  of CD4 with CH4 is the breaking of C–D bonds. Alumina is also active for exchange of D2 with alkenes and cyclic alkenes. At temperature below 373 K, only those H atoms that were initially vinyl, or which could become vinyl by isomerization of alkene underwent exchange. As methylenecyclopentane isomerizes to 1-methylcyclopentene, only 6 out of 10 H atoms undergo exchange with D2. 3-Methylcyclopentene does not isomerize, and only 2 H atoms undergo exchange [251]. Hydrogen atoms in benzene also exchange with D2 at room temperature over alumina [252].  Utilizing the high ability of alumina for exchange with D2 without considerable hydrogenation, Larson et al. prepared perdeuterio alkenes and cyclopropane in the temperature range 300–483 K [253]. All the H–D exchange reactions are poisoned by CO2. This is contrary to the alkene isomerization, for which CO2 does not poison, as described in the previous section. Rosynek et al. reported CO2 poisoning effects on 1-butene isomerization and C4H8–D2 exchange. They postulated that the exposed Al3+ ions are responsible for  the exchange [254]. CO2 is adsorbed on an exposed Al3+ ion to give an IR band at 1780 cm–1, which was assigned by Parkyns to the linear CO2 adsorbed on Al3+. Later, they preferred a bicarbonate on an exposed Al3+ ion showing a band at 1480 cm–1 as the CO2 species that blocked the exchange sites [248].

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Catalytic Properties of Solid Acid Catalysts

4.6.1.3.4  Reaction of 2-methyl-3-butyn-2-ol (MBOH) Lauron-Pernot et al. carried out the reaction of MBOH over  alumina at 453 K [255]. The products were sensitive to the amount of Na2O contained as an impurity. Among aluminas examined,  the alumina containing the smallest amount of Na2O at 250 ppm yielded 3-methylbut-3-ene-2-one (MIPK) as a main product, indicating amphoteric properties of alumina. The alumina containing larger amount of Na2O at 2700 ppm, on the other hand, yielded acetylene and acetone indicating that basic properties  are predominating.

4.6.2  Al2O3–Cl and Al2O3–F

Alumina enhances its acidity on modification with halogens, in particular, with Cl and F. Aluminas modified with Cl and F are called chlorinated alumina (Al2O3–Cl) and fluorinated alumina (Al2O3–F),  respectively. Al2O3–Cl’s are prepared by treatment of Al2O3 with chlorine compounds such as HCl, CHCl3, CCl4, and AlCl3, and  Al2O3–F were prepared mostly with NH4F. Al2O3–Cl and Al2O3–F originated from reforming catalysts Pt/ Al2O3–Cl and Pt/Al2O3–F. UOP researchers lead by Haensel who was the first student of Ipatieff, were searching for the catalyst  for reforming in 40’s [256]. They found the Pt/Al2O3 prepared  from AlCl3 showed much higher activity than one prepared from Al(NO3)3. They confirmed the acidity generation due to Cl by testing the activity of fluorided Al2O3 (Al2O3–F) in reforming,  which yielded the highest octane product. In 1949, UOP announced a new reforming process, platforming, in which Pt/Al2O3–F  catalyst was used. At present, Al2O3–Cl is more frequently used than Al2O3–F for supporting metallic components in acid-metal bifunctional catalysts. Al2O3–F is now used frequently in the reactions involving fluorocarbons and fluorochlorocarbons. Berteau et al. examined the acid–base properties of Al2O3’s 3– – – modified with S​O​2– 4​  ​, P​O​4​  ​, Cl , and F by IR of adsorbed pyridine,  TPD of CO2, and the activity measurement for dehydration of  4-methyl-2-pentanol [257]. The strength of acid increased in the 2– – – order of P​O​3– 4​  ​ < Cl < S​O​4​  ​ < F . Generation of acidic sites on fluorination was studied with reference to the influence of the amount of F on the type of acid

Zirconium Oxide and Related Catalysts

sites, their strength and their number. Fluorinated Al2O3 presented both Brønsted and Lewis acid sites [258]. The total number of  Lewis acid sites decreased when F content increased, whereas the number of strongly acidic sites exhibited a maximum for the  samples with 2–4% F content, though only a small fraction of the sites created by fluorination exhibited high acid strength. Kytokivi and Lindblad studied the reaction of HCl with Al2O3 by IR and 1H MAS NMR. The two primary reactions of HCl were the exchange reaction with OH groups and dissociation reaction of HCl as represented below [259].

The exchange reaction occurred preferentially with more  basic OH groups (isolated OH groups) than with less basic ones (associated ones) [260]. The new OH groups formed from the dissociation of HCl are acidic in nature due to OH…Cl interaction. The water molecules released in the exchange reaction also attach to (Al–O) sites and form acidic OH groups. Increased Brønsted acidic strength of chlorinated Al2O3 was confirmed by IR measurement of adsorbed CO at liquid nitrogen temperature. The band ascribed to C–O stretching in Al–OH…CO adducts appeared at 2154 cm–1 for non-chlorinated Al2O3, whereas the band shifted to 2159 cm–1 for chlorinated Al2O3, indicating that acid strength of the OH groups increased [261].

4.7  Zirconium Oxide and Related Catalysts 4.7.1  ZrO2

Zirconium oxide (ZrO2) possesses both acid sites and base sites. The reactions that are catalyzed by ZrO2 are mostly base-

229

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Catalytic Properties of Solid Acid Catalysts

catalyzed reactions in the sense that the reactions are initiated by  abstraction of an H+ by base sites to form anionic intermediates. The acid sites also participate in the anionic intermediate formation. The catalytic behavior of ZrO2 is frequently interpreted by acid–base bifunctional catalysis. The acid sites on ZrO2 are not  relevant to acid-catalyzed reactions which initiate with the formation of cationic intermediates. Nevertheless, ZrO2 converts ​ ​  ​ or combined  to solid acid catalysts by supporting WO3 and S​O2– 4 with SiO2. Zirconium oxide is used in many industrial processes such as dehydration of 1-cyclohexylethanol to cyclohexylethylene [262, 263], dehydration of propylamine-2-ol [264], dimerization of isobutyraldehyde to isobutyl isobutyrate (Tishchenko reaction) [265], and hydrogenation of aromatic carboxylic acids to aromatic aldehyde [266]. ZrO2 modified with Pt and S​O​2+ 4​  ​, and combined with SiO2 are used for industrial processes of isomerization of  light naphtha [267] and polymerization of THF [268], respectively.

4.7.1.1  Preparation and phase change

Zirconia has three crystalline structures: monoclinic phase, tetragonal phase, and cubic phase. The monoclinic phase is stable up to 1473 K, the tetragonal phase is stable up to 2173 K, and the cubic phase is stable above 2173 K. When hydrous zirconia is calcined  at progressively higher temperature to obtain ZrO2, the phases of the resulting ZrO2 do not follow the thermodynamic stabilities of  the phases. The resulting phases depend on the preparation conditions as described below. The phases of ZrO2 used for catalyst are amorphous, metastable tetragonal, and monoclinic. The surface area of ZrO2 is small in the form of stable tetragonal and cubic phases. Tetragonal phase and monoclinic phase can be determined clearly by XRD. Monoclinic phase shows XRD peaks at ~28° and __ 31° in 2q for (11​1​ ) and (111) reflections, respectively, whereas tetragonal phase shows at 30° for (111) reflection. However, it is difficult to distinguish tetragonal phase from cubic phase by XRD. The XRD patterns of the cubic and the tetragonal zirconia are  nearly identical though small differences are observable; the tetragonal phase gives a limited number of additional high order and low intensity reflections due to its lower degree of symmetry.

231

Zirconium Oxide and Related Catalysts

For the sample whose crystallization degree is low, it is not  possible to determine clearly from broad XRD patterns whether  the phase is tetragonal or cubic. The tetragonal, monoclinic and cubic phases may be readily identified and distinguished from one another by means of Raman spectroscopy. Tetragonal zirconia is expected to give a spectrum consisting of six Raman bands with frequency at about 148, 263, 325, 472, 608, and 640 cm–1, whereas cubic zirconia is expected to give a single Raman band around 490 cm–1 [269]. Figure 4.29 shows Raman spectra of ZrO2 samples prepared by calcination of  hydrous zirconia at different temperatures together with a pure monoclinic sample [269]. ZrO2 is tetragonal when hydrous zirconia is calcined at 723 K. The tetragonal phase completely converts to monoclinic phase by calcination at 1123 K.



Figure 4.29 Typical Raman spectra of different ZrO2 samples: (a) a predominantly “metastable” tetragonal sample obtained by calcining hydrous zirconia at 723 K (triangles indicate strongest bands resulting from the “metastable” tetragonal phase); (b) a predominantly monoclinic sample obtained by calcining hydrous zirconia at 1123 K; and (c) a 100% monoclinic sample (CERAC Chemicals, spectro grade monoclinic ZrO2). Reprinted with permission from P. D. L. Mercera, J. G. Van Ommen, E. B. M.  Doesburg, A. J. Burggraaf, J. R. H. Ross, Appl. Catal., 57, 127 (1990).

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Catalytic Properties of Solid Acid Catalysts

Appearance of the phases and transformation of the phases are strongly dependent on the preparation conditions and procedures for ZrO2. The main factors determining the crystalline phase  are pH of the mother liquid during hydrothermal treatment (aging, digestion), period of hydrothermal treatment, temperature and environment (type of gas(es) in contact with the oxide) during  heat treatment, and presence of contaminants or additives. The effects of pH of mother liquid are complicated. Denkewicz et al. reported that at a high pH (>13) of the mother liquid during hydrothermal treatment of the precipitate at about 383 K, tetragonal phase of hydrous zirconia formed predominantly, whereas at  a low pH (70 %) has been observed with Lewis acids, and a lower selectivity (50–70 %) was observed with Brønsted acids (3.4.2). The selectivity for 2a over WO3–SnO2 was 58 %, indicating Brønsted acid catalyzed reaction.

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Catalytic Properties of Solid Acid Catalysts

1

O

OH

2a

+

2b

OH

+

2c

OH

+

2d

OH

(Reprinted with permission from Ogasawara, S. Uchida, K. Yamaguchi, N. Mizuno, Chem. Eur. J. 15, 4343 (2009).)

Diels–Alder reaction is another C–C bond forming reaction catalyzed by WO3–SnO2. The reaction of dienes with a,b−unsaturated carbonyl compounds (dienophile) is known to be catalyzed by acids [354]. One example of Diels–Alder reactions examined in the presence of WO3–SnO2 is

+

O

O

O

O

The yield of the Diels–Alder adduct exceeded 95 % in the reaction with 3 mmol diene, 3 mmol dienophile, 100 mg catalyst in 6 mL dichloromethane at 293 K for 1 h. The activity of WO3–SnO2 was higher than those of Fe-montmorillonite, H3PW12O40/SiO2, Zn-[Al]MCM-41, Ru-hydroxyapatite, [Al]-MCM-41, Cu-montmorillonite, and Ca-V-apatite. For many other Diels–Alder reactions of different dienes with a, b−unsaturated carbonyl compounds, WO3–SnO2 showed high activities. One more example of C–C bond forming reaction catalyzed by WO3–SnO2 is cyanosilylation of carbonyl compounds with trimethylsilyl cyanide [354]. An example is

The yield of the product was 99 % under the reaction conditions,  1 mmol acetaldehyde, 4 mmol trimethylsilyl cyanide, 50 mg  WO3–SnO2 with Sn/W mol ratio = 2 calcined at 1073 K in 0.5 mL  dichloroethane at 295–296 K for 0.5 h. Various ketones and 

Tungsten Oxide and Related Catalysts

aldehydes reacted with trimethylsilyl cyanide to afford the corresponding products in high yields under the same reaction conditions. WO3–SnO2 is an efficient catalyst for the reactions involving water such as hydration of alkynes, dehydration of aldoximes to form nitriles and dehydration of sugars to furfurals. WO3–SnO2 catalyzes hydration of various kinds of alkynes to form corresponding ketones in high yields at 373 K [355]. R1

R2 + H2O

R1

O

R2

The hydrations of para- and meta-substituted ethynylbenzene were carried out to examine the electronic nature of the intermediates. The relationship between log(kX /kH) and the BrownOkamoto σ* constant for each compound. A linear relationship  was observed and the slope (Hammett’s ρ+ value) was –2.31. This large negative value indicates an electrophilic nature of the  hydration, in which the reaction likely proceeds via positively charged transition state, with the positive charge on the a-carbon atom adjacent to the phenyl ring, that is, a vinyl cation. The hydration of alkyne begins with protonation of alkyne to form a vinyl cation. Then, nucleophilic attack of water on the vinyl cation gives a vinyl alcohol, followed by tautomerization to form  the corresponding ketone as the final product. For the above hydration of alkynes, the catalyst with Sn/W ratio of 2 (43 wt%  WO3, 34 wt% W) and calcined at 1073 K showed the highest activity. Dehydration of various aromatic and alkyl aldoximes proceeds quantitatively over the WO3–SnO2 to form nitriles at 438 K in oxylene [353].

R

NOH

R–CN + H2O

It is to be noted that WO3–SnO2 can catalyze dehydration of unsaturated aldoximes and the aldoximes containing heteroatoms such as N and S as shown below even though the active sites are  acid sites.

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Catalytic Properties of Solid Acid Catalysts

NOH

,

S

NOH

,

N

NOH

The optimum calcination temperature of WO3–SnO2 is 1073 K  for the dehydration of aldoximes. The dehydration of aldoximes proceeds over the uncalcined WO3–SnO2 too, that is Sn–W  hydroxide obtained by condensation of SnCl4 in an aqueous solution containing [H2W12O40]6– ions. The activity is about the same as  that of WO3–SnO2 calcined at 1073 K. Sn–W hydroxide acts as a catalyst for the dehydrative condensation of aldehydes with hydroxylamine to form aldoximes as well as dehydration of aldoximes. Accordingly, it is possible for Sn–W hydroxide to synthesize nitriles through the dehydrative condensation of aldehydes with hydroxylamine followed by dehydration in one-pot at 406 K. R-CHO + NH2OH    R-CH=NOH + H2O

R-CH=NOH    R-CN + H2O

R-CHO + NH2OH    R-CN + 2H2O

For this reaction, Brønsted acid sites are active. WO3–SnO2 is an effective catalyst for conversion of saccharides  to furan derivatives. The WO3–SnO2 prepared via condensation of SnCl4 in an aqueous solution containing [H2W12O40]6– ions catalyzed both the isomerization of d-glucose to d-fructose and dehydration  of d-fructose to 5-hydroxymethylfurfural (HMF) at 343–353 K [356]. The WO3–SnO2 (Sn/W ratio 2) calcined at 1073 K was more active than the other solid acid catalysts such as Amberlyst-15,  H-mordenite, H-Y for both the isomerization and dehydration. The production of HMF from glucose proceeds by two-step reaction. Lewis acid sites are relevant to the first step isomerization,  whereas Brønsted acid sites to the second step dehydration. OH

HO HO

O

OH

D-glucose

HO

OH

Lewis acid

O

HO

HO

D–fructose

OH

–3H2O OH B-acid

HO

O HMF

O

Tungsten Oxide and Related Catalysts

Natural lignocellulose is mainly composed of various  aldohexoses (mainly glucose) and aldopentoses. Utilizing the ability of WO3–SnO3 for conversion of saccharides to furane derivatives, two step production of HMF and furfural from wood (Japanede cedar sawdust) became possible as shown in Fig. 4.48. The  WO3–SnO2 was used in step B at 393 K, whereas a solution of heteropolyacid (H5BW12O40) was used in step A at 333 K. O

HO

O

36% yield Japanese ceder

A

Saccharide mixture

B O

62% yield 32% overall yield based on holocellulose in Japanese ceder

Figure 4.48 Two-step production of HMF and furfural from Japanese cedar sawdust. Reaction conditions A: Japanese cedar sawdust  (100 mg), H5BW12O40 solution (0.7 M aqueous solution, 2 mL),  60°C. By the reaction, a mixture of glucose (53%), galactose (4%), mannose (11%), xylose (8%), arabinose (1%), and  cellobiose (3%) was obtained. Reaction condition B: Saccharides obtained, Sn–W oxide (18 mg), THF/water  (5 mL/1 mL), 120°C, 18 h.

4.9.4  WO3–ZrO2

Arata and Hino presented the first paper describing the synthesis and catalytic activities of WO3–ZrO2 for acylation and alkane isomerization at 9th International Congress on Catalysis in 1988 [357]. They reported preparation conditions for WO3–ZrO2 possessing high activities for acylation of toluene with benzoic anhydride, skeletal isomerization of pentane and hexane, and cumene cracking. The optimum calcination temperature range was 1073–1123 K and optimum loading of WO3 was ca. 13 W wt%. They also reported the optimum drying temperature of Zr(OH)4 before impregnation with ammonium metatungstate was 373–573 K.  They ascribed the high activities for the acid-catalyzed reactions  to strong acid sites generated on WO3–ZrO2.

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Catalytic Properties of Solid Acid Catalysts

4.9.4.1  Preparation The most common preparation is by impregnation. Hydrated  zirconia or zirconium oxyhydroxide (ZrO(OH)2) is impregnated with aqueous ammonium metatungustate followed by drying and calcining in the temperature range 873–1023 K. The content of W should be 10–25 WO3 wt% on ZrO2 corresponding approximately  to monolayer to double layer of WO3 on the surface of ZrO2. A coprecipitation method is also often used for WO3–ZrO2 preparation. One example is as follows [358]. A solution of zirconium oxychloride octahydrate (ZrOCl2 . 8H2O) was combined with stirring to a solution containing ammonium metatungstate  and ammonium hydroxide. The pH of the resulting slurry is about 9.0. This slurry is placed in a steam box at 373 K for 16 h. The  product is recovered by filtration, washed with excess water,  dried overnight at 368 K, and calcined in the temperature range 873–1023 K.

4.9.4.2  Characterization

The structure of WO3–ZrO2 has been characterized by XRD, surface area measurement, Raman and IR spectroscopy, and UV-vis  diffuse reflectance spectroscopy. Characterization of acid sites has been investigated by an indicator method, IR of adsorbed pyridine,  IR of adsorbed CO, and TPD of NH3. All results indicate that WO3 loading and calcination temperature exert strong effect on generation of acidic sites; near monolayer coverage of polytungstate composed of peripheral WO6 octahedra on tetragonal ZrO2 phase  is the preferable state for acting as solid acid catalyst. Transformation of tetragonal ZrO2 to monoclinic ZrO2 occurs much easier for the WO3–ZrO2 with low loading of WO3 than for the one with high WO3 loading. As shown in Fig. 4.49, a half of  pure ZrO2 transforms from tetragonal to monoclinic phase below 800 K, whereas WO3–ZrO2 containing 19 wt% WO3 transforms a half of tetragonal phase to monoclinic phase at about 1173 K  [359]. Loading WO3 on ZrO2 stabilizes ZrO2 in tetragonal  phase. As a result, surface areas keep high against calcination temperature.

Tungsten Oxide and Related Catalysts

Figure 4.49 Variations in specific surface are and % monoclinic ZrO2 as a function of calcination temperature for ZrO2 and WO3–ZrO2  containing 19 wt% WO3. Based on M. Scheithauer, R. K. Grasselli, H. Knözinger, Langmuir, 14, 3019 (1998).

The UV-vis absorption edge energy depends on the domain  size of WOx cluster. As the content of WO3 increased, the absorption edges shifted to lower energies. Absorption edge energies obtained from UV-vis spectra were plotted against WOx surface density  (Fig. 4.50) [360]. The absorption edge energies are grouped into three regions, 0–4 W nm–2, 4–8 W nm–2 and > 8 W nm–2. In the first region, the absorption edge energy is 3.49 eV corresponding to the structure of WOx species of octahedral WOx species isolated on the surface of ZrO2. In the second region where the  absorption edge shifts linearly from 3.49 to 3.16 eV, the size of polytungstate domain increases to reach polytungstate monolayer at 8 W nm–2. The formation of WO3 crystallites begins to occur  when the surface density of W exceeds 8 W nm–2. The formation  of different W species with increasing WOx density is illustrated in Fig. 4.51. As shown by a broken line curve in Fig. 4.50, the activity for  acid-catalyzed reaction of o-xylene isomerization has maximum  at about 10 W nm–2. The 1b state of WOx in Fig. 4.51 is active  phase for acid-catalyzed reactions [360].

273



274

Catalytic Properties of Solid Acid Catalysts

Figure 4.50 Indirect absorption edge energies of WOx–ZrO2 samples at several oxidation temperatures and tungsten loading. Two crystalline tungsten oxide materials (monoclinic WO3 and ammonium metatungstate) are shown for reference. Dashed  curve is a summary of o-xylene isomerization rates per W atom [523 K, 0.66 kPa o-xylene, 100 kPa H2]. Reprinted  with permission from D. G. Barton, M. Shtein, R. D. Wilson, S. L. Soled, E. Iglesia, J. Phys. Chem. B, 103, 630 (1999).



Figure 4.51 Evolution of octahedral WOx species on ZrO2 surface with increasing WOx surface density. Reprinted with permission from D. G. Barton, M. Shtein, R. D. Wilson, S. L. Soled, E. Iglesia, J. Phys. Chem. B, 103, 630 (1999).

The IR spectra of WO3–ZrO2 show a broad O–H stretching band at about 3620–3630 cm–1. This band shifted 160 cm–1 on adsorption of CO. The shift that correlates with the Brønsted acid strength is smaller compared with the shift observed for H-ZSM-5 (320 cm–1), indicating that Brønsted acid sites are weaker on WO3–ZrO2 than  on H-ZSM-5.

Tungsten Oxide and Related Catalysts

275

The IR study of adsorbed pyridine indicated that both Brønsted acid sites and Lewis acid sites exist on WO3–ZrO2. Exposure of the WO3–ZrO2 to hydrogen above 373 K results in the formation of Brønsted acid sites even without loading of Pt [361]. Heat of adsorption of NH3 calculated from TPD plot showed that the strength of acid sites is similar for WO3–ZrO2 and H-ZSM-5; a heat of adsorption of 130 kJ mol–1 was measured for both catalysts [362].

4.9.4.3  Catalytic activity

The studies of WO3–ZrO2 became accelerated since 11th International Congress on Catalysis in 1996 where Iglesia et al. and Larsen et al. presented papers describing high activities and selectivities  of Pt/WO3–ZrO2 for alkane isomerization [363, 364]. Since then,  the catalytic activities of the support, WO3–ZrO2, have been extensively studied together with those of Pt/WO3–ZrO2.

Figure 4.52 Influence of oxidation temperature on o-xylene isomerization turnover rates on WOx–ZrO2 samples at three WOx concentrations (5, 12, and 21 wt% W) (reaction conditions: 0.67 kPa o-xylene, 106 kPa H2, 532 K). Reprinted with permission from D. G. Barton, S. L. Soled, G. D. Meitzner, G. A. Fuentes, E. Iglesia, J. Catal., 181, 57 (1999).



276

Catalytic Properties of Solid Acid Catalysts

o-Xylene undergoes isomerization to m- and p-xylenes over WO3–ZrO3 at 523 K in a closed recirculating reactor. The activity of WO3–ZrO3 depends on the WO3 loading, calcination temperature,  and hydrogen pressure in the reaction mixture. A maximum conversion rate was observed at a W loading of 12 wt% [365]. The optimum loading of W varied with the calcination temperature of the catalysts as shown in Fig. 4.52. The optimum loading of W decreased with the calcination temperature. As the surface areas changed with the calcination temperature, the conversion rates  for all samples from Fig. 4.52 are replotted as a function of WOx surface density (Fig. 4.53). They merge into a single curve with a maximum rate at about 10 W nm–1. The value 10 W nm–1 is slightly larger than the monolayer of tungsten species calculated on the assumption of corner sharing WOx octahedra with bond distances derived from monoclinic WO3.

Figure 4.53 o-Xylene turnover rates replotted from Fig 4.52 as a function  of nominal WOx surface density on three WOx–ZrO2 (5, 12  and 1 wt% W) samples oxidized at various temperatures (800–1300 K). The theoretical monolayer capacity for ZrO2 (7 W nm–2) is marked. Reprinted with permission from  D. G. Barton, S. L. Soled, G. D. Meitzner, G. A. Fuentes, E. Iglesia,  J. Catal., 181, 57 (1999).

Hydrogen has significant effects on the activity as well as on deactivation. The activity for o-xylene isomerization increases

Tungsten Oxide and Related Catalysts

2.0

0.08

1.5

0.06

1.0

0.04

0.5

0.02

0.0 0

Para/meta-Xylene Isomer Ratio

Rate per W-atom (103 s–1)

with the hydrogen pressure as shown in Fig. 4.54. In the absence of hydrogen, deactivation occurs rapidly. The presence of hydrogen inhibits coke formation, which is initiated by dehydrogenation of xylenes on Lewis acid sites. Promoting effects of hydrogen on activity and suppression of deactivation seem to be well interpreted by “Molecular hydrogenoriginated protonic acid site” as illustrated in Fig. 4.39 [365]. By reduction of WO3–ZrO2 in the pretreatment, hydrogen activating centers are formed on WO3–ZrO2 even without Pt. In the presence of hydrogen, Brønsted acid sites as well as hydride ions form, which is evidenced by IR study of pyridine [361]. The presence of  hydride ions suppresses coke formation by blocking Lewis acid  sites, which otherwise act as dehydrogenation sites. Isomerization of alkanes is the main reaction for which the catalytic activity of WO3–ZrO2 was studied. The other reactions catalyzed by WO3–ZrO2 include acylation of toluene with benzoic anhydride, acylation of veratrole, toluene alkylation, and Beckmann rearrangement of cyclohexanone oxime to e-caprolactam.

0.00 25 50 75 100 125 150 H2 Pressure (kPa)

Figure 4.54 Reaction rate and product xylene distribution dependence on H2 pressure. Para/meta-Xylene isomer ratios measured  at 4.0% (±0.5%) conversion (523 K, 0–124 kPa H2, 0.66 kPa o-xylene, 15 wt% WOx–ZrO2; 8.4 WOx/nm2 surface density). Reprinted with permission from R. D. Wilson, D. G. Barton, C. D. Baertsch, E. Iglesia, J. Catal., 194, 175 (2000).

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Catalytic Properties of Solid Acid Catalysts

Following are the characteristic features of WO3–ZrO2 for  alkane isomerization:

(1) Induction period is observed. (2) reaction rate increases in the presence of hydrogen. (3) pretreatment with hydrogen enhances the activity.

The first feature is associated with the alkane activation forming carbenium ions at the initial stage of the reaction. The second and third indicate that the active sites are generated by interaction with hydrogen. In the steady state, the reaction proceeds via carbenium ion mechanism. The process for alkane activation at the initial stage of the reaction is controversial. Two explanations for the induction period exist. One explanation is protonation of alkanes to form carbonium ion followed by converting into carbenium ions with H2 evolution.

RH + H+    [RH2]+    R+ + H2



RH    R. + H.

The other explanation is redox mechanism forming carbenium ions. In the redox mechanism, the following reactions are suggested to occur at the initial stage of the reaction.



R. + W6+    R+ + W5+ H. + O2– + W6+    OH– + W5+

It is suggested that the slight reduction in the tungsten phase is accomplished by oxidation of the alkane, and that activation of alkane proceeds via a redox initiation step that involves the abstraction of two electrons and one proton that are transferred to the tungsten phase, leading to the formation of W5+ centers and OH groups. This idea is based on IR and EPR measurements of  the samples contacting with the alkanes. OH stretching band appeared at 3620 cm–1, and W5+ centers and organic radicals were observed to form simultaneously. One-electron transfer from an organic radical is postulated to lead to carbenium ions that act as chain carriers in the isomerization catalysis [359, 366]. Support of the protonation mechanism in alkane activation was reported for pentane isomerization. It is noted that pentane isomerization is poisoned by 0.004 and 0.002 meq-mol/g-cat of 2,6-dimethylpyridine for co-precipitated and impregnated  WO3–ZrO2’s, respectively [258]. These numbers are about one to

Tungsten Oxide and Related Catalysts

two orders of magnitude smaller than those of acid sites measured  by NH3 TPD or XPS of adsorbed 2,6-dimethylpyridine. Only very strong Brønsted acid sites are relevant to pentane isomerization activity. It is suggested that the small number of very strong Brønsted acid sites act as the protonation sites producing carbonium ion followed by the formation of carbenium ions with H2 evolution.

4.9.4.4  Pt/WO3–ZrO2

4.9.4.4.1  Preparation Pt/WO3–ZrO2 is prepared by impregnation of WO3–ZrO2 with an aqueous solution of Pt-containing compound followed by drying and calcination at about 723 K. As a Pt-containing compound, tetrahydrogen hexachloroplatinate (H4PtCl6), tetraammine platinum chloride ((NH3)4PtCl2), and tetraammine platinum nitrate ((NH3)4Pt(NO3)2) are used. Use of H4PtCl6 as a Pt source gave a catalyst with higher activity and selectivity for heptane isomerization as compared with (NH3)4PtCl2 as shown in Fig. 4.55. Concerning Pt loading, the activity increased with Pt content up to 0.5 wt%, and gave a constant activity for further Pt loading [367, 368]. Pt/WO3– ZrO2 is normally reduced with H2 before use.



Figure 4.55 Isomerization selectivity vs. conversion for heptane isomerization over Pt/WO3–ZrO2 catalysts prepared by various methods. Cat. No. 2 was prepared with (NH3)4PtCl2, and the other catalysts were prepared with H4PtCl6. Cat. No. 8 was prepared by calcination at 873 K, and the other catalysts were prepared by calcination in the range 923–1123 K. Reprinted with permission from B. R. Jermy, M. Khurshid, M. A. Al-Daous, H. Hattori, S. S. Al-Khattaf, Catal. Today, 164, 148 (2011).

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Catalytic Properties of Solid Acid Catalysts

4.9.4.4.2  Characterization When Pt/WO3–ZrO2 is exposed to H2, reduction of WO3 occurs to produce W5+ and OH at room temperature. Pt/WO3–ZrO2 is much easily reduced compared with unpromoted WO3–ZrO2, which requires a higher temperature (>473 K) [369]. Similar to the case of Pt/S​O​2– 4​  ​–ZrO2 and WO3–ZrO2,  the conversion of Lewis acid sites into Brønsted acid sites in the presence of hydrogen occurs for Pt/WO3–ZrO2. In the case of Pt/WO3–ZrO2, the conversion occurs even at room temperature [361]. Kinetic study of H2 adsorption on Pt/WO3–ZrO2 showed that  spillover of H atoms has an activation energy of 35.5 kJ mol–1,  and surface diffusion of H atoms has an activation energy of  25.9 kJ mol–1. The surface diffusion of H atoms is much faster on WO3–ZrO2 than on S​O​2– 4​  ​–ZrO2 over which the surface diffusion  has an activation energy of 84 kJ mol–1 [370].

4.9.4.4.3  Catalytic activity

As Pt is loaded on WO3–ZrO2, the activity, selectivity and catalyst  life for isomerization of alkanes are substantially improved, but  higher performance is observed only in the presence of H2. These features are common to what is observed for Pt/S​O​2– 4​  ​–ZrO2 as described in Section 4.7.2. The isomerization activity is higher for Pt/S​O​2– 4​  ​–ZrO2 than for Pt/WO3–ZrO2, but the selectivity for isomerization against cracking is higher for Pt/WO3–ZrO2. The activity of Pt/WO3–ZrO2 for alkane isomerization is dependent on the H2 pressure. In the absence of H2, the activity for heptane isomerization is low as shown in Fig. 4.56, and deactivation occurs quickly. The activity sharply increases with the H2 pressure to reach a maximum at about 0.2–0.3 MPa, and gradually decreases with further increase in the pressure. There are two different interpretations for promoting effect  of Pt in the isomerization of alkanes in the presence of H2. (1) The promoting effect of Pt is attributed to its dehydrogenation and hydrogenation activity. The alkanes are dehydrogenated by Pt to form alkenes, which undergo isomerization on Brønsted acid sites by carbenium ion mechanism. The isomerized alkenes are hydrogenated by Pt to form the products. This is a classical bifunctional catalysis involving metal and acid sites.

Tungsten Oxide and Related Catalysts

Isomerization yield/%

(2) The role of Pt is essentially only the activation of H2. H2 is dissociated on Pt to form H atoms, which undergo spillover onto polytungstate overlayer and lead to the formation of W5+ centers and OH groups. The hydrogen on the polytungstate overlayer converts the carbenium ions to alkanes. Two electrons and one proton are transferred in this desorption step, and the adsorbed carbenium ion is replaced by a proton. The high selectivity is explained by the fast hydride transfer rates, which lead to fast desorption of the carbenium ions before b-scission leading cracking.

H2 pressure/MPa Figure 4.56 Variation in isomerization yield as a function of H2 pressure. Catalyst: Pt/WO3–ZrO2 (0.75 wt% Pt, WO3–ZrO2 calcined at 1053 K). Pressure of heptane, 0.047 MPa. [WSHV]553: 7.89 L h–1g-cat–1. Reprinted with permission from M. Khurshid, M. A. Al-Daous, H. Hattori, S. S. Al-Khattaf, Appl. Catal., A, 362, 75 (2009).

The second explanation is essentially the same as the  explanation by “molecular hydrogen-originated protonic acid site” as illustrated in Fig. 4.39. The role of Pt is to dissociate H2 into H  atoms and supply Lewis acid sites on WO3–ZrO2 with the H atoms through spillover and surface diffusion. Two H atoms convert into  an H+ and an H– as described earlier. Since the rate of surface diffusion of H atoms is fast on WO3–ZrO2, the concentration of hydride (H–) becomes high. Carbenium ions quickly react with the H– to form alkanes to be desorbed, resulting in a low concentration of carbenium ions, which lowers the rate of b-scission leading 

281

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Catalytic Properties of Solid Acid Catalysts

to cracking. The high selectivity for isomerization against  cracking is due to the fast surface diffusion of H atoms over WO3– ZrO2. Slow cracking results in a reduced alkene formation, which minimizes coke formation.

4.10  Niobium Oxide and Related Catalysts 4.10.1  Nb2O5 4.10.1.1  Preparation

Nb2O5 used as a catalyst is normally prepared by calcination of  niobic acid, hydrated form of Nb2O5. Commercially available niobic acid is used as a starting material. TG-DTA curves of hydrated  Nb2O5 are shown in Fig. 4.57 [371]. Dehydration is completed by heating up to 300°C (573 K) and the gravity change of the sample was not observed in the range 300–750°C (573–1023 K). The exothermic peak in DTA at about 843 K indicates the occurrence  of crystallization.

Figure 4.57 TG and DTA curves of hydrated Nb2O5. Reprinted with permission from T. Iizuka, K. Ogasawara, K. Tanabe, Bull. Chem. Soc. Jpn., 56, 2927 (1983).

4.10.1.2  Acidic properties

The acidic properties of hydrated niobia evacuated at 373, 573, and 773 K were examined by IR of adsorbed pyridine (Fig. 4.58). The amount of Brønsted acid sites shows a maximum on evacuation at 373 K, and decreases monotonously with the temperature. 

Niobium Oxide and Related Catalysts

On evacuation at 773 K, no more Brønsted acid sites are present.  The maximum amount of Lewis acid sites is observed when evacuating at 573 K. On evacuation at 773 K, Lewis acid sites  almost disappear.

Figure 4.58 Acidity change of Nb2O5 . nH2O with pretreatment temperature : Evacuated at room temperature after adsorption of pyridine, : at 100°C, : at 200°C, : at 300°C. Variations in Lewis acid sites and Brønsted acid site as a function of pretreatment temperature. Reprinted with permission from  T. Iizuka, K. Ogasawara, K. Tanabe, Bull. Chem. Soc. Jpn., 56, 2927 (1983).

4.10.1.3  Catalytic activity

Various reactions catalyzed by Nb2O5 are reviewed by Tanabe  and Okazaki [372] and Tanabe [373]. For 1-butene isomerization, Nb2O5 shows a maximum activity when evacuated at 373 K. Evacuation at 573 K results in a considerable reduction of the activity. Rehydration of the catalyst evacuated at 573 K followed

283

284

Catalytic Properties of Solid Acid Catalysts

by evacuation at 373 K restores the activity to the level for the catalyst evacuated at 373 K. These indicate that presence of water or OH groups on the surface is required for Nb2O5 to act as solid acid catalyst. This might relate the catalytic feature of Nb2O5 that  an efficient catalytic behavior is observed in the reactions  that involve water as a reactant or product such as hydration, dehydration, and esterification. Hydration of ethylene to ethanol takes place over Nb2O5 at 493–513 K. The maximum activity was observed when hydrated Nb2O5 was calcined at 573 K. Calcination at 673 K eliminated  the activity. Nb2O5 catalyzes dehydration of alcohols such as 2propanol, 2-butanol, cyclopentanol, cyclohexanol and 1,4-butanediol. Nb2O5 can be a catalyst for esterification of ethanol with  acetic acid to ethyl acetate, and methanol with acrylic acid to form methyl methacrylate. Friedel–Crafts alkylation (benzylation of anisole with benzyl alcohol) takes place over Nb2O5 at 423 K in liquid phase. Hydrolysis and condensation are also catalyzed by Nb2O5 [374]. Phenyloxirane (styrene oxide) undergoes hydrolysis to form phenyl-1,2-ethenediol and isomerization to form phenylacetaldehyde (Fig. 4.26) [375]. The reaction was carried out in a solvent of water/ dioxane mixture (10/90). The selectivity for hydrolysis was much higher over Nb2O5 than that over other solid acid catalysts such as SiO2–Al2O3, H-ZSM-5, and H-Nafion. Nb2O5 showed a high activity for hydrolysis in 100% water solvent at 373 K, indicating Nb2O5  functions in aqueous solutions. O

CH2CHO

OH

+ H2O

Isomerization

(II)

OH

Hydrolysis

(I)

(III)

Figure 4.59 Reaction of phenyloxirane: (I) phenyloxirane, (II) phenylacetoaldehyde, and (III) phenyl-1,2-ethenediol.

Nb2O5 catalyzes the Prins reaction of isobutyraldehyde with isobutene (condensation) [376]. Among various products, 2,5dimethyl-2,4-hexadiene is the desired product, which is formed

Niobium Oxide and Related Catalysts

by the Prins reaction followed by dehydration. The reaction was carried out at 525 K in a flow reactor. The yield of 2,5-dimethyl2,4-hexadiene was the highest for Nb2O5 compared with other solid acid catalysts such as S​O2– ​ ​  ​/ZrO2, SiO2–Al2O3, WO3–TiO2, and Al2O3 4 The process using hydrated Nb2O5 has been commercialized by Sumitomo Chemical Co. [377].

4.10.2  Nb2O5–Al2O3

Although Nb2O5 alone loses its acidic properties on calcination above 673 K, Brønsted acid sites are generated when N2O5 is supported on Al2O3 and calcined at higher temperature [378, 379]. The amount of Brønsted acid sites increased with calcination temperature up to 1173 K and decreased with further increase in the temperature.  The amount of Brønsted acid sites varied with the loading of  Nb2O5, 16 wt% Nb2O5 gave the maximum amount of the acid sites. By calcining 16 wt% Nb2O5/Al2O3 at 1173 K, two-dimensional  Nb–O–Nb network of stabilized niobic acid–like compound is  formed on the Al2O3. The compound is a precursor of AlNbO4,  which would be formed by calcination above 1173 K. The catalytic activities of Nb2O5/Al2O3 catalysts with different loading of Nb2O5 for Friedel–Crafts alkylation of anisole with  benzyl alcohol correlated well with the amount of Brønsted acid sites.

4.10.3  Nb2O5–WO3 and Nb2O5–MoO3

Mixed oxides, Nb2O5–WO3 and Nb2O5–MoO3, show acidic properties, which exceed those present in Nb2O5. These mixed oxides show acidity stronger than Nb2O5 [380]. These mixed oxides can be prepared with mesoporous porosity and exhibit efficient catalytic behaviors in a liquid-phase acid-catalyzed reactions as described below. Mesoporous Nb2O5–WO3 was prepared from NbCl5 and WCl6 in the presence of a poly block copolymer surfactant Pluronic P-123 as a structure directing agent [381]. Mesopores were formed in NbxW(10 – x) oxide with x values from 2 to 10. XRD peaks attributed to (110) and (200) of the two-dimensional hexagonal structure were observed from an x = 10 sample (mesoporous Nb

285

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Catalytic Properties of Solid Acid Catalysts

oxide). With the samples with x values from 0 to 2, presence of crystallized WO3 is observed. The crystallization of WO3 resulted in the destruction of mesoporous structure by calcination at  673 K to remove the template. The number of Brønsted acid sites increased with an increase in the W content for the samples holding mesoporous structure, the largest number of Brønsted acid sites being observed for Nb3W7 oxide by IR of adsorbed pyridine.  The strength of Brønsted acid sites was examined by 31P MAS NMR study of adsorbed trimethylphosphine oxide to be comparable to those of H-ZSM-5 and H-mordenite. Mesoporous Nb2O5–WO3’s showed high activities for Friedel–Crafts alkylation of anisole  with benzyl alcohol, and hydrolysis of sucrose, the maximum  activity being obtained with Nb3W7 oxide. Mesoporous Nb2O5–MoO3 can be prepared from NbCl5 and MoCl5 in the presence of block copolymer surfactant P-123. Mesoporous materials could be prepared in the range x = 9–10 in NbxMo10–x oxide. In the range x = 3–8, large pores were formed. The highest activity for Friedel–Crafts alkylation of anisole with  benzyl alcohol was obtained with Nb3Mo7 oxide. Strong Brønsted acid sites were formed by the isomorphous substitution of Nb5+  ions by higher valence Mo6+ ions [382].

4.10.4  Layered Compounds Containing Nb

KTiNbO5 and KSr2Nb3O10 form layered structure. K+ ions in these compounds can be ion exchanged with H+ to form HTiNbO5 and HSr2Nb3O10 layered compounds as shown in Fig. 4.60 [383]. The layered structures should be exfoliated for utilization of the H+ present between the negatively charged sheets containing Nb, Ti, and Sr oxides. Such exfoliation can be done using  soft-solution such as tetrabutylammonium hydroxide solution.  The resulting exfoliated materials retain two-dimensional crystal structures and are referred to as “nanosheets.” Strong acid sites appeared on HTiNbO5 nanosheets, but only weak acid sites appeared on HSr2Nb3O10 nanosheets. The HTiNbO5 and HSr2Nb3O10 nanosheets are active for the esterification  of acetic acid with ethanol to ethyl acetate, cumene cracking, and dehydration of 2-propanol. The HTiNbO5 is more active than the HSr2Nb3O10 for all these reactions. The layered HTiNbO5 and

Niobium Oxide and Related Catalysts

HSr2Nb3O10 do not catalyze these reactions. The NH3-TPD results indicate that the HTiNbO5 nanosheets have stronger acid sites  than those of H-ZSM-5, whereas the HSr2Nb3O10 nanosheets have  no strong acid sites. The strong acid sites in HTiNbO5 nanosheets  act as active sites for these reactions.

TiO6, NbO6

NbO6

H+ Sr2+

H+

c a

HTiNbO5

c a HSr2Nb3O10

Figure 4.60 Schematic structures of layered HTiNbO5 and HSr2Nb3O10. Reprinted with permission from A. Takagaki, M. Sugisawa,  D. Lu, J. N. Kondo, M. Hara, K. Domen, S. Hayashi, J. Am. Chem. Soc., 125, 5479 (2003).

In the layered HTiNbO5, H+ is located between an oxygen atom  at the vertex of a TiO6 or NbO6 octahedron and an oxygen atom  shared by Ti4+ and Nb5+. However, OH in Ti–(OH)–Nb is oriented to a space between layers, and not accessible to reactants. In [TiNbO5]– sheets obtained by the exfoliation of layered HTiNbO5, TiO6, and  NbO6 octahedra arrange in such a way that OH in Ti–(OH)–Nb is oriented to external surface of the sheets, and accessible to the reactant. Layered HNbMoO6 . nH2O is easily obtained from layered compound LiNbMoO6 . nH2O by H+ exchange. The HNbMoO6 . nH2O consists of randomly sited MO6 (M = Nb and Mo) octahedral with H2O in the interlayer. The layered HNbMoO6 . nH2O exhibits remarkable activity for certain types of acid-catalyzed reactions without exfoliation [384].

287

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Catalytic Properties of Solid Acid Catalysts

The acid sites measured by TPD of NH3 are stronger than those on H-ZSM-5. The reactions that the HNbMoO6 . nH2O exhibited  high activity include Friedel–Crafts alkylation of anisole, toluene,  and benzene with benzyl alcohol, esterification of lactic acid  with ethanol, acetalization of cyclohexanone with methanol, and hydration of 2,3-dimethyl-2-butene. In particular for Friedel– Crafts alkylation of alkyl aromatics with benzyl alcohol, the  HNbMoO6 . nH2O showed much higher activity than Nb2O5 . nH2O, Nafion NR50, Amberlyst-15, H-ZSM5, and H-b zeolite. For  esterification of acetic acid with ethanol, however, HNbMO3  exhibited a low activity because acetic acid is not intercalated.  The catalytic performance of HNbMoO6 is attributed to strong Brønsted acid sites and intercalation of reactants into the interlayers.

4.11  Supported Acids

4.11.1  Solid Phosphoric Acid Solid phosphoric acid-mounted catalysts were developed first by Ipatieff. The first solid phosphoric acid was prepared by heat treatment of a mixture containing phosphoric acid and kieselguhr. Kieselguhr is a deposit of the shells of diatomite and composed mostly of SiO2. Today, the favored support for solid phosphoric acid is synthetic SiO2. Solid phosphoric acid is typically prepared by heating viscous pastes consisting of SiO2 and 85% phosphoric acid above 573 K. The heat treatment causes partial polymerization of orthophosphoric acid H3PO4 to diphosphoric acid H4P2O7, and higher polymers  such as H5P3O10 as well as the formation of crystalline silicon orthophosphate Si3(PO4)4 and silicon pyrophosphate SiP2O7, silicon hydrogen phosphate monohydrate Si(HPO4)2 . H2O, and silicon hydrogen triphosphate SiHP3O10. The solid phosphoric acid is composed of different forms of phosphoric acids spread over the support, which is composed of different forms of silicon phosphates. Phosphoric acids exist in different forms (ortho-, di-, tri-, and polyphosphoric acids, with increasing degrees of condensation). The relative amounts of phosphoric acids vary with the contents

Supported Acids

of P2O5 and water [385]. Figure 4.61 shows the relative amounts  of the different species as a function of the water content in P2O5/ H2O mixtures.

Figure 4.61 Distribution of phosphoric acids as a function of water content in P2O5/H2O mixtures. Reproduced with permission from  F. Carvani, G. Girotti, G. Terzoni, Appl. Catal. A, 97, 177 (1993).

Figure 4.61 indicates that at relatively low water content the average degree of condensation of acids is higher. It is also known that the acid strength of various phosphoric acids varies considerably. Accordingly, the acidic nature of solid phosphoric acids containing the same amount of P2O5 depends strongly on the P2O5/H2O ratio on the catalyst surface. Orthophosphoric acid and pyrophosphoric acid existing on the surface are water soluble. They are designated “free-P2O5” and active phase of solid phosphoric acid. H0 is –4.7 for 70% P2O5 and –6.96 for 85% P2O5. From these values, the H0 value for solid phosphoric acid is estimated in the range of –5 to –7. NMR studies (1H13C31P double cross polarization experiments) indicate that among several phosphorous species, phosphoric acid and oligomers like pyrophosphoric acid form complexes with probe molecules such as acetone and propene,

289

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Catalytic Properties of Solid Acid Catalysts

and that these “free-P2O5” are the origins of Brønsted acid sites on  solid phosphoric acid catalysts. No Lewis acid sites are present  on solid phosphoric acid [386]. IR spectrum of the solid phosphoric acid shows a broad OH  band around 3500 cm–1 indicating strong hydrogen bonds between POH groups and H2O molecules. On adsorption of pivalonitrile, a negative band appeared at 3668 cm–1 in the subtracted spectrum. These free POH groups are the most available to pivalonitrile.  The POH groups appear to act as Brønsted acid sites of solid phosphoric acid [387]. The strength of acid sites of solid phosphoric acid was examined by the chemical shift of 13C NMR for adsorbed acetone on solid phosphoric acid. The chemical shifts were greater than 228 ppm, which is larger than that observed when acetone is adsorbed on HZSM-5 (223 ppm), indicating solid phosphoric acid is stronger than HZSM-5 [386]. Solid phosphoric acid is being used in some industrial  processes such as ethylene hydration to ethanol, alkylation of benzene with propene to cumene and oligomerization of propene and isobutene to produce gasoline components. The activities of solid phosphoric acid depend on the water content in the reaction mixtures. For the process of cumene production by alkylation of benzene with propene, 100–300 ppm of water is introduced at a reaction temperature range 453–633 K, and for propene oligomerization process, 250–300 ppm of water is introduced at a reaction temperature range 423–473 K. The water content in feed has double effect: (1) It affects the activity and selectivity and  (2) it causes a hydrolysis of the main component of the catalyst,  the silicon phosphates, with a destruction of the the catalyst itself [388]. The effect (1) results from the composition of phosphoric acids in the active phase. Introduction of an optimum content of water keeps the concentrations of phosphoric acid and pyrophosphoric acid high, and maintains the Brønsted acidity of the catalyst. The optimum content of water varies with the reaction condition. The effect (2) is the most stringent drawback of the solid phosphoric acid in industrial use. Hydrolysis of the silicon phosphates results in the formation of phosphoric acid and SiO2, which result in the leaching of phosphoric acid and weakening the

Supported Acids

mechanical strength. The spent catalyst cannot be regenerated and must be disposed of. Although the solid phosphoric acid has the disadvantages of leaching and structural deterioration, it is still used in a variety of chemical processes. This is because its activity and selectivity are so high. To solve disadvantages, two kings of trials were performed;  (1) replacement of solid phosphoric acid by other non-leaching  sold acid catalysts, and (2) use of new supports to retain phosphoric acids firmly on the surfaces. The replacement of solid phosphoric  acid by other solid acid catalysts was successful for cumene production process, but not yet successful for the other processes. MCM-22 and b-zeolite are being used in some cumene production processes, and the replacement is being extended. Use of supports other than SiO2 was examined for ethylene hydration, but no supports better than SiO2 were found. On TiO2, ZrO2 and Nb2O5, phosphoric acids are more strongly held than on SiO2. The activities, however, were lower than that of SiO2 supported phosphoric acid. Strong interaction of TiO2, ZrO2, and Nb2O5 with phosphoric acids diminishes the active components found in SiO2supported phosphoric acid [389].

4.11.2  Other Supported Acids

Besides phosphoric acid, many Brønsted acids and Lewis acids are supported on solids such as SiO2, graphite, clays, polymers and Al2O3. Except for those supported on Al2O3, the supported acids  act as solid acids possessing catalytic properties similar to the  original acids. The supported acids have the advantages that heterogeneous catalysts have over homogeneous catalysts.

4.11.2.1  Supported Brønsted acids

Heteropolyacid (H4SiW12O40/SiO2) supported on SiO2 has been  used in the industrial process producing ethyl acetate from  ethylene and acetic acid since 1999. CH2 CH2  +  CH3—C

O

OH

CH3—C

O

O-CH2

CH3

291

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Catalytic Properties of Solid Acid Catalysts

The activity of H4SiW12O40 for the reaction is enhanced by supporting on SiO2, the activity being much higher than solid phosphoric acid, ion exchange resin, p-toluenesulfonic acid [390]. Activity enhancement by supporting on SiO2 is due to a high dispersion of the heteropolyacid on SiO2 surface to increase the surface area of the heteropolyacid. For preparation of industrial catalyst, a small amount of alkali metal is included to minimize dimerization of ethylene, which caused activity decay with time on stream. The reaction is carried out at 423~443 K in a flow-type reactor. H3PW12O40/SiO2 also shows an activity similar to H4SiW12O40/ SiO2 for the formation of ethyl acetate, but H3PMo12O40/SiO2 and H4SiMo12O40/SiO2 do not show an activity as high as H3PW12O40/ SiO2 and H4SiW12O40/SiO2 as shown in Fig. 4.62.

Figure 4.62 Comparison of heteropolyacid catalysts for production of ethyl acetate from ethylene and acetic acid. Reprinted with permission from T. Nakajo, Shokubai (Catalyst), 48, 505 (2006).

Heteropolyacid supported on solids other than siliceous materials also show activity for acid-catalyzed reactions. H3PW12O40 and H4SiW12O40 supported on mesoporous siliceous materials such as MCM-41, FSM-16 and SBA-15 were tested as catalysts in benzylation of alkyl substituted benzenes with benzyl alcohols 

Supported Acids

[391, 392]. H3PW12O40 supported on ZrO2 was reported to be  active for liquid phase alkylation of benzene with 1-octene and 1-decene, which is an important reaction in production of linear alkylbenzene sulfonates [393]. TiO2 supported H3PW12O40 was studied as a catalyst for benzylation and butylation of phenol, the activity being higher than those of WO3–ZrO2, SO4–ZrO2, K10 (montmorillonite) and zeolites [394, 395]. HClO4 supported on SiO2 is used as a solid acid catalyst for  a wide variety of organic synthesis such as acylation of alcohols  and aldehydes, Ferrier rearrangement, cleavage of benzylidene acetals, and Hantzsch condensation in liquid-phase reaction. An example is electrophilic substitution of indoles with aldehydes and ketones (Fig. 4.63). The reaction proceeds at room temperature [396]. O R

C

+

R1

aldehyde, ketone

N indoles

H

X

HClO4–SiO2 MeOH, r.t.

R

N

H

R1

X  X

N

H

bisindoylmethane

Figure 4.63 Electrophilic substitution of indoles with aldehydes and ketones.

Sulfo groups (–SO3H) are introduced to the surface of  mesoporous silicas such as FSM-16, MCM-41, SBA-15 and HMS.  They are effective catalysts for a wide variety of typical Brønsted  acid-catalyzed reactions such as dehydration of carbohydrates, esterification and transesterification of fatty acids, condensation of acetone and phenol to bisphenol-A, rearrangements and isomerization, and cyclizations to form cyclic ethers, etc. Silica materials with sulfo groups can be prepared in a onestep approach of co-condensing inorganic–organic reagents in the presence of different surfactant templates with in situ oxidation  of the thiol groups to the sulfonic acid groups [397]. Esterification  of glycerol with long-chained carboxylic acids proceeds smoothly over the sulfonic acid supported on different mesoporous silicas [398].

293

294

Catalytic Properties of Solid Acid Catalysts

Supported sulfonic acid can also be prepared by post-  modification of silica by the reaction with 3-mercaptopropyltrimethoxysilane followed by oxidation with H2O2. FSM-16 supported sulfonic acid by this method showed a high activity for the acetalization of carbonyl compounds with ethylene glycol. MCM-41 supported sulfonic acid prepared by this method is a  good catalyst for dehydration of d-xylose (pentose) to furfural.  d-Xylose can be sourced from agricultural waste rich in pentosan polymer [399]. More examples of ordered mesoporous silica  functionalized with sulfo groups are found in Section 4.3.2.

Figure 4.64 Simplified reaction mechanism of acidic degradation of pentosan to furfural. Reprinted with permission from A. S. Dias, M. Pillinger, A. Valente, J. Catal., 229, 414 (2005). 

Based on a silica–sulfonic acid, a bifunctional catalyst was prepared by addition of –NH2 groups (Eq. 4.45) [400]. (MeO)3Si (MeO)3Si

+

NH2

O

S Cl

O

O O Si O O O Si O

SBA-15-A/B

NH2

SO3H

(4.45)

The catalyst was an SBA-15 material that incorporates both  –SO3H and NH2 and tested for aldol condensation of 4nitrobenzaldehyde with acetone to give the resulting aldol addition (A) and the dehydration (B) products (Eq. 4.46). The total conversion was the highest for the bifunctional catalyst modified with both –SO3H and –NH2. Synergy effect of acidic site and basic site was suggested. Trifluoromethanesulfulic acid (CF3SO3H) supported on SiO2 is strongly acidic catalyst and used for alkylation of isobutane with

Supported Acids

propene and butene to produce gasoline fraction of high quality. The conversion of butane was >98%, selectivity to octanes 97% and trimethylpentane selectivity >60% were obtained at 298 K and  22 bar [401]. It was estimated that CF3SO3H reacts with silanol groups to form Brønsted acid in the form either 1a or 1b in Fig. 4.65, which act as active sites. O

O2N

H

+

O

323 K 20 h

OH O

O2N

O

+

O2N

A

B (4.46)

CF3SO3-H3O+ OH OH Si O Si

+

1a

CF3SO3H F3C

O-H3O+

S O O O Si O Si

Figure 4.65 Trifluoromethanesulfonic acid supported on SiO2.

4.11.2.2  Supported Lewis acids

Soluble strong Lewis acids are heterogenized by supporting on highsurface-area solids such as graphite, SiO2, Al2O3, clays, zeolites, and mesoporous materials to be used in environmentally benign way. The supported Lewis acids act similarly to the original Lewis acids or produce Brønsted acid sites on interaction with OH groups on the supports and in the presence of moisture. Lewis acids used for this purpose include metal chlorides, BF3, SbF5, and metal triflates. Among metal chlorides, AlCl3 is most

295

296

Catalytic Properties of Solid Acid Catalysts

widely used. The other metal chlorides are SnCl4, SnCl2, FeCl3, GaCl3, InCl3, and InCl2. They are used for a wide variety of reactions, including alkylation of aromatics, acylation, transesterification, Prins condensation, and Diels–Alder reactions. Some examples are as follows [402]. AlCl3 supported on MCM-41 is active for alkylation of benzene with normal 1-alkenes. The selectivity to linear mono-alkylation is higher for MCM-41 supported AlCl3 than for pure AlCl3 [403]. The same catalyst shows high activity for alkylation of naphthalene with 2-propanol to diisopropylnaphthalenes, of which 61% is most important 2,6-isomer [404]. AlCl3 intercalated in graphite catalyzes Friedel–Crafts reactions such as alkylation of benzene with ethyl bromide, ethylene, and propylene. Compared with pure AlCl3, the intercalated AlCl3 is a milder catalyst for the alkylation reactions and gives less polysubstituted reaction products [405]. K10 (montmorillonite) supported FeCl3, InCl3 and GaCl3 are active for benzylation of benzene with benzyl chloride [406]. BF3 supported on SiO2 shows activity for alkylation of phenol with 1-octene or cyclohexene. The product distribution (phenylcyclohexyl ether (O-alkylation) or cyclohexylphenol (Calkylation)) was dependent on the presence of Brønsted acid sites produced in the presence of moisture. In the absence of moisture, the ether was primarily formed, whereas in the presence of moisture, both products were formed [407]. BF3 supported on Al2O3 has activity for alkylation of isoalkanes with alkenes, in particular isobutane with C3–C5 alkenes, an important reaction for production of gasoline fraction of high quality. SbF5 supported on SiO2–Al2O3 can isomerize n-butane to isobutane at room temperature [408]. SbF5 intercalated in graphite promote cracking and isomerization of methylpentanes [409]. Metal triflates (–OSO2CF3) supported on solids catalyze Friedel– Crafts reactions. Rare earth triflates supported on MCM-41 catalyze  the t-butylation of phenol to produce 2,4-di-t- and 2,4,6-tri-tbutylphenol. When supported on Y-zeolite, 2,4,6-tri-t-butylphenol  was not obtained. Among La, Ce, Yb, and Sc, Sc gave the best result in producing 2,4,6-tri-t-butylphenol. The reaction proceeded well in supercritical CO2, which is believed to keep the catalyst clean by removing coke precursors from the catalyst surface [410]. Cu(OSO2CF3)2 supported on SiO2 catalyzes Friedel–Crafts acylation

Ion Exchange Resins

of methyl salicylate by acetyl chloride/acetic anhydride (Fig. 4.66) [411]. OH

O

O

+

O

R

Acid Catalyst

R=Cl or OCOMe

O

O

O

OH

O

+

O

O

O

Figure 4.66 Friedel–Crafts acylation of methyl salicylate by acetyl chloride/ acetic anhydride.

As opposed to pure AlCl3 where stoichiometric amount of AlCl3 is required, the reaction proceeded catalytically over Cu(OSO2CF3)2 supported on SiO2. The reaction proceeded efficiently under microwave irradiation; the turnover number defined by number of moles of product per mole of catalyst was as high as 23 in 7.8 min.

4.12  Ion Exchange Resins

Ion exchange resins are polymer with functional groups capable of cation or anion exchange ability. Cation exchange resins show acidic properties and work as Brønsted and/or Lewis acids when in the forms of H+ and/or M+. Ion exchange is supposed to be carried out in an aqueous solution, and, therefore, ion exchange resins are able to function in an aqueous solution. Cation exchange resins catalyze acid-catalyzed reactions in the presence of water. This point is  one of the favorable features of cation exchange resins as compared to many of the other solid acid catalysts which lose their activity in the presence of large amount of water. One drawback of the cation exchange resins is a poor thermal stability. The polymers degrade and the functional groups decompose at a high temperature, though resins with relatively high thermal stability are synthesized. For common resins, the maximum reaction temperature is 373 K. This limits the reaction types in which cation exchange resins can be used as a catalyst. Regeneration of a spent catalyst by burning the cokes cannot be adopted for the resin catalysts. Prior to around 1960, ion exchange resins were essentially gel-type resins whose swelling characteristics depended upon the

297

298

Catalytic Properties of Solid Acid Catalysts

solvent or reactants. In non-swelling media, the active sites were largely inaccessible to reactants. This problem was solved by the development of macroporous ion exchange resins in the 1960s [412]. Copolymerization of styrene and divinylbenzene forms crossliked polymer in which divinylbenzene is a cross-linking agent  (Fig. 4.67). The cross-linked polymers are macroreticular and  more rigidly structured than non-linked polymers. n

[  ] ​

CH CH2

styrene

 ​

+  m

[  ] CH CH2

​

CH CH2

 ​

divinylbenzene

… —CH—CH2—CH—CH2—CH—CH2—CH—CH2— …

… —CH—CH2—CH—CH2— .

poly(styrene-divinylbenzene)

Figure 4.67 Formation of poly (styrene-divinylbenzene).

The most frequently used ion exchange resins as solid acid catalysts are macroreticular sulfonated polystyrene-divinylbenzene such as Amberlyst-15 and Amberlyst-35 from Rohm and Haas. The acidity of these materials arises from arylsulfonic acid groups  Ar-SO3H (Fig. 4.68, left). Their thermal stability is poor. They can be used below ca. 373 K except for some resins. The surface area  of Amberlyst-15 is about 0.35 m2 g–1.

Figure 4.68 Functional groups of ion exchange resins. Reprinted with permission from M. A. Harmer, Q. Sun, Appl. Catal. A, 221, 45 (2001).

Nafion produced by DuPont is a copolymer of tetrafluoroethylene and perfluoro-2-(fluorosulfonylethoxy)propyl vinyl ether. It shows





Ion Exchange Resins

strong acidity arising from terminal –CF2CF2SO3H groups (Fig. 4.68, right). The acid strength of Nafion is –11 to –13 in H0 scale.  It may be called superacid according to the definition. Nafion is hydrophobic, and can be used for the reactions in aqueous phase. In the presence of water, the –CF2CF2SO3H groups generate  solvated protons. Nafion is thermally stable up to about 553 K.  The surface area is approximately 0.02 m2 g–1. In order to increase the acid site accessibility of Nafion, Nafion silica nanocomposites were developed in 1996, where  nanometer sized Nafion particles are entrapped within a highly porous silica network [413]. Schematic illustration of Nafion silica nanocomposite is shown in Fig. 4.69 [414]. The microstructure may be regarded as a porous silica network, which contains a large number of pocket of very strong acid sites (the Nafion polymer) in domain of about 10–20 nm. The surface area of the nano-  composites is typically 150–500 m2 g–1, which is approximately 10,000~20,000 times greater than that of the starting polymer [415]. The thermal stability of the nanocomposites is high; they  can be used up to 453 K.

Figure 4.69 Schematic of the Nafion silica nanocomposite. Reprinted with permission from D. E. Lopez, J. G. Goodwin, Jr., D. A. Bruce, J. Catal., 245, 381 (2007).

The catalytic activity of the Nafion silica nanocomposite is much higher than the other pure polymer resins. An example is shown

299

300

Catalytic Properties of Solid Acid Catalysts

in Table 4.9 for alkylation of benzene with propene in liquid phase  at 333 K [416]. The rate per unit weight catalyst was 3–6 times  higher for Nafion silica nanocomposite than for pure Nafion or Amberlyst-15. Upon unit acid capacity, the differences were much larger; about two orders of magnitude higher for Nafion silica nanocomposite than for pure Nafion or Amberlyst-15. Table 4.9

Comparison of activity of three resins for alkylation of benzene with propene in liquid phase at 333 K Rate (m mol/ meq H+h)

Acid capacity (meq H+/g)

Rate (m mol/g cat h)

Amberlyst-15

4.4

0.61

0.14

Nafion 13 wt%/SiO2

0.12

1.98

16.50

Catalyst

Nafion-NR50

0.89

0.30

0.34

Source: Data from M. A. Harmer, W. E. Farneth, Q. Sun, J. Am. Chem. Soc., 118, 7708 (1996).

Because of water-tolerant properties, the resins have been often used as catalysts for the reactions in which water is produced or involved. Hydration, dehydration, etherification, and esterification are examples. H3C

O

+

HO

CH3

+

OH

H+

H2O

H3C CH3

HO

Figure 4.70 Bisphenol-A synthesis from phenol and acetone. O

O

O

+

2CH3CH2OH

H+

O

O

OCH2CH3

OCH2CH3

OH

+  H2O

Figure 4.71 Esterification of maleic anhydride with ethanol to diethyl maleate.

References

Following are the processes using cation exchange resins: dehydration of t-butanol to isobutene and reverse reaction (hydration) (UOP 1981), dehydration of butanediol to THF (DavyMcKee 1985), condensation of two phenol molecules with acetone to bisphenol-A (Bayer, DOW/Kellog, 1994) (Fig. 4.70), dehydration of diacetonealcohol produced from acetone to methyl isobutyl ketone, esterification of maleic anhydride with ethanol to diethyl maleate (BASF and Kvaerner 1995) (Fig. 4.71), and etherification of isopentene with methanol to t-amyl alcohol (TAME) (IFP/ELF,  1984), esterification of isobutene with ethanol to ethyl t-butyl ether (ETBE) (SNAM/Ecofuel, 1993), etherification of isobutene with methanol to methyl t-butyl ether (MTBE) (SNAM, 1996)), etc. All the above reactions involve water molecules as the product or the reactant.

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319

Chapter 5

Hydrocarbon Transformations: Mechanism and Industrial Processes

5.1  Introduction Started with the clay catalyst for cracking process, a number of solid acid catalysts have been applied in industrial processes. A statistical survey of the industrial processes and the types of catalysts up to 1999 was reported by Tanabe and Hoelderich [1]. Their results are shown in Tables 5.1 and 5.2, which include the process using solid acid and base catalysts. Solid acid catalysts are applied in various fields, from petroleum refining to fine chemicals synthesis. Among solid acid catalysts, zeolites are used in the largest number of processes. More than 40% of all collected processes are catalyzed by zeolites of different types. Among zeolites, MFI type zeolites (ZSM-5) are most frequently used in the industrial processes. However, faujasite type zeolites (Y-type) are used most abundantly; Y-type zeolites for FCC process share more than 90% mass-wise because FCC process is the largest process in chemical processes. Price-wise, Pt/Y zeolites share most because expensive Pt is used abundantly. In this chapter, selected hydrocarbon transformations important in catalytic chemistry and in industry are described. Solid Acid Catalysis: From Fundamentals to Applications Hideshi Hattori and Yoshio Ono Copyright © 2015 Pan Stanford Publishing Pte. Ltd. ISBN  978-981-4463-28-7 (Hardcover), 978-981-4463-29-4 (eBook) www.panstanford.com

322

Hydrocarbon Transformations

Table 5.1

Industrial processes using solid acid–base catalysts

Dehydration and condensation

18

Alkylation

13

Isomerization

15

Etherification

10

Amination

9

Cracking

8

Aromatization

7

Hydration

7

Oligomerization and polymerization

6

MTG/MTO processes

5

Hydrocracking

4

Hydrogenation

4

Esterification

3

Disproportionation

2

MTBE

1

Others

15

Total

127

Source: Reprinted with permission from K. Tanabe, W. F. Hoelderich, Appl. Catal. A. 181, 399 (1999).

Table 5.2 Zeolites

Type of catalysts used in industrial processes

Oxides, complex oxides Ion-exchange resins Phosphates

Solid acids (not specified) Clays

Immobilized enzymes Sulfate, carbonate

Sulfonated polysiloxanes Total

74 54 16 16 7 4 3 3 3

180

Source: Reprinted with permission from K. Tanabe, W. F. Hoelderich, Appl. Catal. A. 181, 399 (1999).

Catalytic Cracking

5.2  Catalytic Cracking (FCC) Cracking is a cleavage of C–C bonds. The incentive for the catalytic cracking process in petroleum refining is to produce gasoline fraction, middle distillate, and liquid petroleum gas containing much of alkenes. Feeds are heavy oil fractions such as atmospheric gas oil, heavy and light vacuum gas oils. These fractions are cracked into lighter fractions by solid acid catalyst. As described in Chapter 1, the first solid catalyst used for cracking process was a clay, which was followed by amorphous SiO2–Al2O3 and faujasitetype zeolites. The faujasite zeolites have been improved since 1960s until present to enhance their catalytic behaviors and adjust to the change in the feeds. The faujasite zeolite (ultrastable Y type zeolite modified with rare earth elements) is the main catalyst at present in FCC (fluid catalytic cracking). The percentage of the heavy fractions in crude oil normally exceeds 50%. These fractions should be cracked into lighter fractions mainly in the cracking process. This is the reason why the cracking process is the largest chemical process in chemical industries.

5.2.1  Reaction Mechanisms

Cracking proceeds via b-scission of carbenium ion intermediates. As described in Section 2.4.1, carbenium ions are formed by various routes such as (1) addition of an H+ to alkenes and aromatics, (2) addition of an H+ to alkanes to form carbonium ions followed by elimination of H2, CH4, or C2H6, and (3) H– transfer from alkanes to carbenium ions. Carbenium ions also undergo various reactions such as (1) intramolecular H shift, (2) intramolecular CH3 shift (isomerization), (3) ring contraction (cyclohexyl to methylcyclopentyl compounds), (4) addition to alkene (oligomerization), (5) addition to aromatics (alkylation), (6) H– transfer from alkanes to form other new carbenium ions, [7] H+ transfer from the carbenium ions to alkenes to form new carbenium ions, and (8) b-scission to form alkenes and the carbenium ions of cracked fragments. The industrial cracking process is carried out typically in the temperature range 773–823 K, which is higher than the temperatures required for other hydrocarbon transformations such as isomerization, alkylation, cyclization, ring contraction, hydrogen transfer, etc. Therefore, these reactions are occurring in addition to the b-scission in the cracking reactor.

323

324

Hydrocarbon Transformations

Because of the carbenium ion mechanism, several features are observed in the reactions in FCC. The fractions of methane, ethane, and ethylene are small in the products. For the formation of these products, the reactions should go through primary cabenium ions, which are energetically unstable. Ethylene is produced by thermal cracking in the current industry. Hydrocarbons containing less than six carbon atoms hardly undergo cracking, which would form primary carbenium ions. Linear-chained alkanes do not crack directly but isomerize first to branched hydrocarbons and crack, which avoids going through primary carbenium ion intermediates. Among the reactions occurring simultaneously with the b-scission, hydrogen transfer is an important reaction that exerts a strong influence on the product distribution and coke formation. Examples of hydrogen transfer are as follows:



4 CnH2n    3 CnH2n + 2 + CnH2n – 6

5 butene    2 xylene + 2 butane

benzene + 3 butene    6 C (coke) + 3 butane

These hydrogen transfer reactions are composed of repeated occurrences of hydride transfer and proton transfer in alternation. Proton transfers from carbenium ion either to alkenes to form carbenium ion or to catalyst surface to restore acidic site. For butene to p-xylene and butane, hydride transfer and proton transfer occur three times after dimerization to isooctane, cyclization and ring expansion being also included as shown below. As a result of hydrogen transfer in general, alkenes decreases and alkanes, aromatics and coke increase.

Catalytic Cracking

To suppress the hydrogen transfer, the reaction conditions and the catalyst are properly selected. To keep the contact time short is one of the ways to suppress the hydrogen transfer. Concerning the catalyst, selection of the catalyst with a high Si/Al ratio (a low density of acid sites) is one of the ways. The adsorption of hydrocarbons becomes low for the zeolites with a high Si/Al ratio. Hence, a bimolecular reaction of the hydrogen transfer becomes less favorable than the monomolecular reactions such as cracking. Control of these reactions by adjusting reaction conditions and catalyst design according to the feeds is important to produce desired products. Coke formation results in the deactivation of the catalyst. However, coke formation has some meanings in the cracking process. Cracking is endothermic process. Heat should be supplied, which is obtained by burning the deposited coke on the catalyst during regeneration of the spent catalyst.

5.2.2  Fluid Catalytic Cracking Process 5.2.2.1  Reactor

The reactors employed in the cracking process have been changed with the change in the efficiency of the catalyst. At first, the fixed-bed reactor was used in the first Houdry process. Since the catalyst deactivation was severe, frequent regeneration of the catalyst was required. Moving-bed reactor was then employed once but quickly changed to fluidized-bed reactor. Fluidized-bed reactor is convenient for the catalytic systems that need frequent regeneration, since continuous catalyst regeneration is easy. Since highly active zeolite catalysts appeared, the reaction did not need a large fluidized-bed; the reactions completed during transport of the mixture of the feed and the catalyst to the fluidizedbed in a short contact time. The transporting pipe is called “riser.” As actual reactions occur in the transporting pipe, the pipe is called “riser reactor.” The fluidized-bed functions as a stripper separating the spent catalyst from the products. However, all the catalytic cracking processes at present are customary called “fluid catalytic cracking” (FCC). An example of FCC reactor is schematically shown in Fig. 5.1. Oil and the regenerated catalyst are fed into the bottom of the riser reactor. The reactions take place during transportation of the feed

325

326

Hydrocarbon Transformations

and the catalyst in the riser reactor to the spent catalyst stripper. The reaction temperature is in the range 750–850 K, and the contact time is in the range 2–10 s. The spent catalyst is collected in the stripper and transported by an airlift to the bottom of the regenerator where the combustion air is also introduced. In the regenerator, coke deposited on the spent catalyst is burned in the temperature range 900–1000 K to restore the catalyst activity. Heat evolved in the regenerator is utilized for heating the riser reactor. Heat balance is completed within the reactor system. No heat supply from outside of the reactor is required. Products Flue Gas

Catalyst Reganerator 900–1000 K

Riser Reactor 750–850 K

Dry Gas C3/C4 Light Naphtha Middle Distillate Heavy Cycle Gasoil

Spent catalyst stripper (Cyclones inside)

Combustion Air Lift Air Oil Feed

Figure 5.1

FCC reactor diagram.



5.2.2.2  Catalyst design

The catalysts used in FCC are based on the ultrastable zeolite Y (USY). USY is prepared from Y zeolite by steaming at a high temperature, which causes dealumination of Y. USY is thermally stable in the regenerator up to 1000 K. Rare earth metal ions may be introduced to USY, which also increase the thermal stability. These catalysts are used in the matrix composed of SiO2–Al2O3 or clay; use of pure zeolites causes severe deactivation. Catalysts used for FCC are designed to enhance the fractions of desired products and to be effective in cracking different types of feedstock. Recently, increase in the production of light alkene fractions, in particular propene, and light naphtha with high octane number are demanded. Also, because the crude oils have

Catalytic Cracking

become heavier and heavier, the FCC catalysts need to crack heavier fractions. These requirements are fulfilled by addition of various components to the zeolite catalysts. For enhancing the propene production and increasing the octane number of the light naphtha fraction (gasoline fraction), ZSM-5 is added to the FCC catalyst. An example of product distribution is given in Table 5.3 when VGO (vacuum gas oil) was allowed to react over USY at 773 K to reach 70% conversion. In the third column is shown the product distribution when ZSM-5 was added to USY by 3 wt%. Table 5.3

Product distribution of FCC of VGO and effects of ZSM-5 additive in a pilot plant Product wt% without Product wt% with additive ZSM-5 (3 wt%)

Compound Dry gas (C1 and C2)

2.0

2.0

Isobutane

2.9

3.7

Propane Propene

n-Butane Butenes

Gasoline (C5–477

K)a

LCO (477–616 K)a Slurry (616 K

Coke

Gasoline RON

aBoiling

+)a

0.8 4.3 0.6 4.5

1.1 6.4 0.7 5.6

49.9

45.6

5.0

4.9

20.5 9.5

90.9

20.5 9.5

92.6

point range. Source: Data taken from T. Masuda, p. 35, in Technology and Applications of Zeolite Catalysts (T. Tatsumi, Y. Nishimura, eds.), CMC Publication.

Multi-branched alkanes that have high octane numbers are difficult to enter the pores of ZSM-5, but straight-chained and monobranched alkanes that have low octane numbers can easily enter the pores where the alkanes crack to produce small alkenes, including propene. Thereby, addition of ZSM-5 results in the increased yields of propene and multi-branched alkanes at the expense of the yields of gasoline fraction. Although the yields of gasoline fraction decrease, the research octane number (RON) of the fraction increases.

327

328

Hydrocarbon Transformations

Extreme case of enhancing production of light alkenes is the deep catalytic cracking of vacuum gas oil in which ZSM-5 alone is used as a catalyst. The products consist of up to 20% propene and other light alkenes with less gasoline. Processing of heavier feedstocks can be improved by addition of the catalysts possessing cracking ability and large pores. SiO2– Al2O3 is an example of the additives, and added either to the matrix or as a separate solid phase. SiO2–Al2O3 has large pores and accessible to heavier fractions, which undergo cracking to a medium extent. The cracked products are further cracked on the zeolite catalyst in the matrix. The feedstocks of FCC contain various impurities other than hydrocarbons. Main impurities are the compounds containing V, Ni, S, and N. These are contained in crude oil, and large fractions of these compounds are removed in the hydrotreating process, which precedes the FCC unit. Still these compounds are included in the feeds of FCC to some extent. Vanadium destroys the crystalline structure of the zeolites and should be trapped before it reaches the zeolites. For the capture of V compounds, rare earth elements are either included in the matrix together with the zeolite catalyst or added to the FCC catalyst as a separate solid phase. The rare earth elements react with the V compounds to form stable compounds. Nickel accelerates the cracking by radical mechanisms and the dehydrogenation resulting in the coke formation. To diminish the activity of Ni, oil-soluble compounds containing Sb and Bi are fed into the feed. These compounds react with Ni compounds to form inactive compounds. Thereby, methane formation and dehydrogenation are suppressed, and coke formation is reduced. If the feed contains sulfur compounds, SO2 and SO3 are formed during regeneration by oxidation of sulfur compounds present in the coke. Removal of SO2 and SO3 can be done by addition of metal oxides such as MgO, CeO2, and Al2O3 to the FCC catalyst. These metal oxides capture SO2 and SO3 in the regenerator and release H2S in the reactor and stripper. The reactions occurring in the regenerator, reactor, and stripper are as follows in which MgO is exemplified as the metal oxide. In the regenerator, oxidation of SO2 to SO3 and subsequent reaction with metal oxides.

Synthesis of Ethylbenzene and Cumene



2SO2 + O2  2SO3

SO3 + MgO  MgSO4

In the riser reactor, reduction of metal sulfate and release of H2S.

MgSO4 + H2  MgSO3 + H2O



MgSO3 + H2  MgS + H2O



MgSO3 + H2  MgO + H2S

In the stripper, regeneration of metal oxide with release of H2S.

MgS + H2O  MgO + H2S

The released H2S can be removed by absorption and subsequent conversion to S in a Claus unit together with the effluent gas from the hydrodesulfurization units in the same refinery. Promotion of oxidation in the regenerator is required for complete regeneration of the catalyst. For this purpose, the particles of Pt supported on Al2O3 are added to the FCC catalyst. The Pt/Al2O3 facilitates the oxidation of CO to CO2 to increase the temperature and to decrease the unburned coke on the catalyst in the regenerator to restore the activity and selectivity of the catalyst.

5.3  Synthesis of Ethylbenzene and Cumene

Alkylation of aromatics with alkenes is important in the chemical industry. In particular, the processes of alkylation of benzene with ethylene and propene to produce ethylbenzene and cumene (isopropylbenzene), respectively, are two important processes of benzene conversion. Out of all benzene consumption in the chemical industry, 50~60% and 15~20% are converted to ethylbenzene and cumene, respectively, by acid-catalyzed alkylation. Most of ethylbenzene produced are further converted to styrene, which is a raw material for polymers with various functions. Cumene is primarily used as an intermediate for the production of phenol and acetone. Phenol is a raw material for polymer, bisphenol A, etc. Both alkylation of benzene with ethylene and propene are acid-catalyzed reactions in which homogeneous acid catalysts or problematic heterogeneous catalysts were used and have been gradually replaced by solid acid catalysts. This topic is reviewed by Perego and Ingallina [2] and Degnan Jr. et al. [3].

329

330

Hydrocarbon Transformations

5.3.1  Synthesis of Ethylbenzene Alkylation of benzene with ethylene was performed in the presence of the Friedel–Crafts catalyst such as AlCl3–HCl in liquid phase, which was developed in the 1930s. Because of the drawbacks of usage of liquid acid, acid-mounted catalysts were proposed. BF3/ Al2O3 and solid phosphoric acid (SPA) (Kieselguhr supported phosphoric acid) were used in 1960s. However, these catalysts have problems with release of acids during the reaction and could not be regenerated after use for the reaction. SiO2–Al2O3 and faujasite zeolites such as HY and HX were examined, but these catalysts were suffering from severe deactivation. Accordingly, AlCl3–HCl had been used industrially until 1980s. In 1980s and 1990s, several industrial alkylation processes were realized by use of zeolites. The first commercial process using zeolite occurred in 1980, in which a medium pore zeolite, ZSM-5, was used. The reaction was performed in a gas-phase fixed-bed reactor (second-generation Mobil-Badger process). In a liquidphase, a large pore zeolite, Y type zeolite, could be used without severe deactivation. A process using Y zeolite was commercialized in 1989 (Lummus/Unocal/UOP). In 1995, a new process using a new zeolite, MCM-22, appeared (Mobil-Raytheon EBMax). Ethylbenzene is formed by acid-catalyzed ethylation of benzene (Eq. 5.1). The produced ethylbenzene undergoes further alkylation to di- and other polyethylbenzenes by acids (Eq. 5.2). The polyethylbenzenes are separated from ethylbenzene to be allowed to react with benzene to produce ethylbenzene by transalkylation to increase the yield of ethylbenzene (Eq. 5.3). Transalkylation is also an acid-catalyzed reaction, but the catalysts are different from those used in the alkylation step. Accordingly, alkylation processes are normally composed of two steps: alkylation step and transalkylation step. C2H5



C2H5

+  CH2=CH2

+  CH2=CH2

C2H5

(5.1)



C2H5

(5.2)

Synthesis of Ethylbenzene and Cumene

C2H5



C2H5 +

C2H5

2



(5.3)

The catalysts currently used in the alkylation processes are HY, ZSM-5, MCM-22, and b-zeolite. Because the size of polyethylbenzene is larger than ethylbenzene, the pores of the catalysts used in transalkylation step are slightly larger than those used in the alkylation step. An appropriate size of the pores depends on the reaction phase, gas-phase reaction or liquid phase reaction. Selected processes for ethylbenzene production are listed in Table 5.4 and an example of the process flow for alkylation is shown in Fig. 5.2. Table 5.4

Selected processes for ethylbenzene production

Name of process Monsanto-Lummus

Second-generation MobilBadger

Year

Phase Catalyst

Phase

Alkylation step

Trans alkylation step

~1975 Liquid AlCl3

1980

Vapor ZSM-5

Catalyst

No step used

No step used

Lummus/Unical/ UOP

1989

Liquid Y

Lummus/UOP EBOne

1996

Liquid EBZ-500* Liquid EBZ-100*

Third generation Mobil-Badger 1990 Mobil-Raytheon EBMax

1995

Vapor ZSM-5

Liquid MCM-22

Liquid Y Vapor

Vapor

ZSM-5

ZSM-5

*Estimated to be modified b-zeolite. Source: Data taken from C. Perego, P. Ingallina, Catal. Today, 73, 3 (2002).

Figure 5.2

Process flow of alkylation of benzene with ethylene to  ethylbenzene (Lummus/UOP EBOneTM Process).

331

332

Hydrocarbon Transformations

5.3.2  Synthesis of Cumene Cumene production process is similar to ethylbenzene production process in both process development and process composition. For over 50 years, the chemical industry has used SPA or AlCl3 catalysts to synthesize cumene from benzene and propene. The process using SPA is still a major process for cumene production. Replacement of SPA and AlCl3 by zeolites started to occur later for cumene production than for ethylbenzene production. This is due to differences in the reactivity of alkenes and the size of the product between ethylbenzene and cumene production. Propene is much more reactive than ethylene toward acid catalysts, and cumene is slightly larger than ethylbenzene. The reaction scheme for alkylation of benzene with propene is shown in Fig. 5.3. CH2=CH–CH3 

H+

CH3 CH3

 CH3–CH+–CH3 C3H6

Higher alkylbenzenes

–H+

Higher alkylbenzenes

–H+

Figure 5.3

C6​H+​1 3​ 

–H+

C3H6

C9​H+​1 9​ 

–H+

CH

–H+ CH2–CH2–CH3

C3​H+7​ ​​ 

CH3 CH3 CH

C6 alkenes cracking C9 alkenes

CH3 CH CH3

C2, C4, C5, C7  alkenes

Other alkylates

Acid-catalyzed alkylation of benzene with propylene and accompanying reactions. Reprinted with permission from C. Perego, P. Ingallina, Catal. Today, 73, 3 (2002).

Propene is protonated by the acid catalyst to form secondarypropyl cation, which reacts with benzene followed by release of a proton back to the catalyst. Secondary-propyl cation also undergoes oligomerization to form hexyl cation (C6​H+13 ​   ​)​  and nonyl cation ​   ​)​  . These carbocations release a proton to form alkenes or (C9​H+19

Isomerization and Hydroisomerization of Alkanes

react with benzene to higher alkylbenzenes. The cumene undergoes further reaction to form n-propylbenzene and diisopropylbenzene. The produced polyisopropylbenzenes undergo transalkylation with benzene to increase the yield of cumene. The alkylation reaction is carried out at 393–443 K, 20–40 MPa, and benzene to propene ratio of 3–5. ZSM-5, which is successfully used for ethylbenzene production, is inadequate for cumene production because of the narrow pore opening of a 10-membered ring zeolite. The industrial processes were successful by use of zeolites with 12-membered ring pore openings such as Y, mordenite, ZSM-12, and b in liquid phase. MCM-22 also behaves efficiently in industrial processes. Although MCM-22 is classified into a medium pore zeolite, it behaves like a large pore zeolite because of the existence of 12-membered cups, which are open to the exterior at the termination of crystal (see Fig. 4.7). The cups could allow the cumene to be formed and diffuse out without great difficulty.

5.4  Isomerization and Hydroisomerization of Alkanes 5.4.1  Isomerization of Alkanes

The isomerization of n-alkanes plays an important role in the petroleum industry. The isomerization of C4–C6 hydrocarbons has found applications in the production of gasoline with high octane numbers, whereas that of long-chain alkanes (C7–C18) has been used in the dewaxing processes for production of high quality diesel fuel and lube base oil with improved flow properties. The isomerization of alkanes proceeds in the presence of acids via carbenium ion intermediates. In industrial practice, the isomerization process usually takes place over noble metals supported on acidic support in the presence of hydrogen, and in this case it is referred as hydroisomerization. The isomerization of lower alkanes needs strongly acidic solid acids such as chlorinated alumina, zeolites, sulfated zirconia, and heteropolyacids. For hydroisomerization of longer alkanes, solid acids with lower acid strength such as WO3–ZrO2 and SAPO-11 are used to avoid excessive cracking of the alkanes.

333

334

Hydrocarbon Transformations

5.4.2  Mechanism of Alkane Isomerization on Acidic Catalysts 5.4.2.1  Main reaction pathway Isomerization of alkanes in the presence of Brønsted acids proceeds through following steps:

n

n

RH + H+  n R + + H2 R+ 

iso

iso

R+



RH+ nR +

R + + nRH 

iso

R + + H2 →

RH + H+

(5.4) (5.5) (5.6)

The formation of carbenium ions (reaction (5.4)) is induced by the reaction of protons with alkanes (see Section 2.4.1.1). The carbenium ions thus formed undergoes skeletal rearrangement (reaction (5.5)). The newly formed carbenium ions react with reactant alkenes by a hydride transfer reaction to form the isomerized alkane (reaction 5.6). The chain reactions composed of reactions (5.5) and (5.6) make the isomerization catalytic. The detailed mechanism of the skeletal isomerization of carbenium ions, reaction (5.5) is described in Section 2.4.1. Reaction (5.6) usually converts a tertiary carbenium ion into a secondary carbenium one. Therefore, this step is not easy in the energetic respect. Under high pressure of hydrogen, the reaction of carbenium ions with dihydrogen becomes more important than reaction (5.6).

iso

iso

(5.7)

The occurrence of elementary reaction (5.7) was confirmed in the homogeneous systems [4]. Minachev et al. found that the rate of isomerization of pentane over mordenite under hydrogen is expressed as r = kPC5/PH2, and proposed a mechanism composed of the reactions (5.4), (5.5), and (5.7), reaction (5.5) being the rate-determining step. Hydrogen in this case is considered to be a chain transfer agent [5]. The formation of carbenium ions occurs very easily if the feed contains alkenes as impurities. Addition of a small amount of alkenes to the reaction system often enhances the rate of isomerization.

Isomerization and Hydroisomerization of Alkanes

5.4.2.2  Side reactions The side reactions in alkane isomerization lower the selectivity for the desired products and lower the catalyst life. They always involve carbenium ions or alkenes as reactive intermediates. Alkenes are formed by deprotonation of carbenium ions.

CnH2+n+1  CnH2n + H+

(5.8)



C4H8 C4H8 + C+4  → C+8  → C12

(5.9)



C+8 → C3= + C5+

Alkenes react easily with carbenium ions to form new carbenium ions with longer chains.

Desorption of higher alkenes from the surface does not occur easily at the reaction temperature and accumulate as coke or coke precursors. When the number of carbon atoms is 6 or more, carbenium ions undergoes b-scission to form alkenes (​C=n​ ​ ​) and carbenium ions, whose carbon numbers are different from the starting alkane.

C+8 → C5= + C3+

(5.10) (5.11)

The repetition of oligomerization (reaction (5.9)) and cracking (reaction (5.10) and (5.11)) lowers the selectivity of alkane isomerization significantly. Therefore, to obtain high selectivity, it is essential to keep the concentration of alkenes very low.

5.4.2.3  Isomerization by bimolecular mechanism

In the isomerization of alkanes, elementary reaction (5.5) proceeds via protonated cyclopropane (CPC) mechanism (see Section 2.4.1). This monomolecular mechanism is applicable to the isomerization of higher alkanes. However, as described in Section 2.4.1, the isomerization of butane with this mechanism is difficult since the reaction involves the conversion of a secondary to primary carbenium ion. It is often observed that 1,4-13C-butane over a solid acid catalyst undergoes intermolecular scrambling of 13C, isobutane with the number of 13C atoms other than two being found. This cannot be explained by a monomolecular mechanism and indicates that the

335

336

Hydrocarbon Transformations

existence of bimolecular mechanism (oligomerization-cracking). The intermolecular scrambling occurs through the skeletal rearrangement of ​C​+8​​  carbenium ions, whose decomposition give isobutene with the number of 13C atoms other than 2 [6, 7]. n







C+4 + C4=  → C8+ (oligomerization)

 → C¢8+ (rearrangements) C+8 ←  C¢8+ +  →

iso = 4

C + isoC+4 (cracking)





(5.12)

(5.13) (5.14)

The characteristic of butane isomerization via bimolecular mechanism is that the reaction products always contain C3 and C5 hydrocarbons as a result of reactions (5.10) and (5.11). The catalysts on which extensive 13C scrambling during butane isomerization occurs include mordenite, sulfated zirconia, Cs2.5H0.5PW12O40, and chlorinated alumina. On the other hand, 13C scrambling is not significant in butane isomerization in the presence of Pt/Cs2.5H0.5PW12O40 under hydrogen [8]. This indicates that the reaction of carbenium ions and alkene molecules hardly occur due to low alkene concentration under hydroisomerization conditions. The extent of the contribution of mono- and bimolecular mechanisms depends on the reaction conditions such as temperature and hydrogen pressure.

5.4.3  Bifunctional Catalysts

As described above, isomerization of alkanes is performed by solid acids loaded with noble metals (Pt, Pd) under hydrogen. A stable activity and higher activity is obtained when both a metallic component and hydrogen are present. This was observed in the case of hexane isomerization over Pt/Al2O3 [9]. Without Pt, Al2O3 has no activity even in the presence of hydrogen. Pt/Al2O3 is not active if hydrogen is replaced by nitrogen. The mechanism of alkane isomerization over Pt loaded on mordenite under hydrogen is described in Section 2.5.2. This mechanism is called a conventional (or classical) bifunctional mechanism, because the catalysts offer two functionalities,

Isomerization and Hydroisomerization of Alkanes

hydrogenation-dehydrogenation function by noble metals and isomerization function by solid acids. The role of noble metals with hydrogen in the conventional bifunctional mechanism is summarized as follows.

(1) Noble metals promote dehydrogenation of the starting nalkanes to form n-alkenes, which are easily protonated by solid acids to form carbenium ions. When the loaded amount of noble metal exceeds a certain level, the dehydrogenationhydrogenation of alkanes reaches the equilibrium under hydrogen. In this case, the rate-determining step is the step of carbenium ion rearrangement reaction (5.5). This function may not always be essential, since some solid acids show higher catalytic activity without noble metal-hydrogen than with (Fig. 2.16). (2) Noble metals hydrogenate alkenes or carbenium ions formed during isomerization. This shortens the life time of alkenes on the surface and eliminates the probability of oligomerization of the alkenes. This inhibits coke formation and leads to the stable activity, as shown in Fig. 2.16, and enhances the selectivity by reducing by-products caused by reactions (5.8–5.11). The conventional bifunctional mechanism occurs for the hydroisomerization over Pt/Al2O3–Cl and Pt (or Pd) supported on zeolites. The bifunctional characteristics (stable activity, high selectivity) by noble metals and hydrogen are also observed in the isomerization over Pt (or Pd) supported on sulfated zirconia, WO3–ZrO2, and heteropolyacids. The mechanism of the synergism on these systems is different from the conventional bifunctional mechanism. The roles of noble metal-hydrogen on sulfated zirconia and WO3–ZrO2 are described in Sections 4.7.2 and 4.8.2, respectively. In short, noble metals dissociate dihydrogen to form hydrogen atoms, which spillover onto the support and convert into a proton and a hydride ion (or an electron). Protons thus formed are trapped on the surface oxygen anions and serve as strong Brønsted acid sites [10, 11]. Hydride ions are trapped by surface Lewis acid sites. Or electrons may reduce the surface metal oxides (W6+ to W5+) [12, 13]. Hydrogen also facilitates to combine the carbenium ions and hydride ions to form alkane products. This shortens the life time of carbenium ions and results in the reduction of oligomers and coke [14].

337

338

Hydrocarbon Transformations

5.4.4  Industrial Processes for Isomerization of Lower Alkanes The purpose of skeletal isomerization of lower alkanes is to increase the octane rating of the light naphtha stream by converting low octane straight chain alkanes (C5–C6) into their high octane branched isomers (Table 5.5). This alkane skeletal isomerization conversion is an equilibrium-limited reaction, as shown in Fig. 5.4 (C5, C6 distribution at equilibrium), with a low reaction temperature favoring isomerization and thus high octane number products. The characteristics of the typical technologies in industrial use are summarized in Table 5.6 [15]. They are all based on bifunctional catalysts consisting of a noble metal (Pt, 0.3–0.5%) dispersed on an acidic support and operate in the presence of hydrogen. Process conditions for C5/C6 isomerization depend on the catalyst and the composition of the feed. Feeds rich in C6 hydrocarbons require higher H2 pressure, i.e., 35–100 bar to maintain low coke make. Pentane-rich streams can be processed at temperatures between 390 and 520 K and H2 pressure of less than 27 bar with an LHSV of about 2 [16]. The acidic components used in industrial processes are chlorinated alumina, sulfated zirconia, and zeolites [15–18].

Figure 5.4

Equilibrium of hexanes and temperature range of typical isomerization catalysts.

Isomerization and Hydroisomerization of Alkanes

Table 5.5 Compound pentane Isopentane

Research octane number (RON) of pentanes and hexanes RON 61.7 92.3

Hexane 3-Methylpentane 2-Methylpentane 2,2-Dimethylbutane 2,3-Dimethylbutane

Table 5.6

24.8 73.4 74.5 91.8 102.0

Process features of isomerization of lower alkanes

Catalyst–process

Chlorinated Metal oxide Common Modern alumina (zirconia) zeolite zeolite

Feedstock conditions   Feedstock type

C5/C6

  Aromatics/benzene (%)

Y > mordenite. The activity of beta increased with dealumination. This was attributed to the generation of

393

394

Synthesis of Organic Chemicals through Solid Acid Catalysis

mesopores during dealumination and the corresponding ease of diffusion of the reactants and products. Acylation of anisole with benzoyl chloride was studied over zeolite beta supported on silicon carbide in a trickle-bed mode at 393 K [69]. The catalyst exhibited a high catalytic activity. The selectivity toward p-methoxybenzophenone was higher than 90%. In addition, the catalytic activity was very stable. The conversion of benzoyl chloride slightly decreased from 73% to about 60% after 3 h on stream but remained unchanged up to more than 12 h. On the other hand, bulk zeolite exhibited a strong deactivation with time mainly due to the formation of carbonaceous residues inside the channel network. The stability of the supported zeolite was attributed to the high dispersion of the zeolite layer and the small primary particle size, which make it possible to maintain high diffusion rates of the reactants and the products. The thin layer of zeolite allows the increase of the product escaping rate and thus, significantly reduces the secondary acidic condensation reaction, which could lead to the carbonaceous species formation. Acylation of anisole with Ac2O proceeded in the presence of H3PW12O40/SiO2 to give 65% yield of 4-MOAP at 363 K. The catalyst was strongly deactivated by the product [40].

6.2.4  Acylation of Veratrole

Acetoveratrone (3,4-dimethoxyacetophenone) is used in the synthesis of papaverine, an opium-alkaloid antispasmodic. It can be synthesized by acylation of veratrole (1,2-dimethoxybenzene) with acetic anhydride (Ac2O). OMe

OMe

OMe + Me

veratrole

O

O O

Me

Catalyst

OMe

+ H2C

H3C O acetoveratrone

O

OH

(6.15)

Acylation of veratrole with Ac2O to afford acetoveratrone has been studied using various solid acids. The catalysts used include aluminum-incorporated mesoporous silica, zeolites, and supported heteropolyacids, as listed in Table 6.6. All the catalysts show almost 100% selectivity for acetoveratrone.

Acylation

Table 6.6

The catalytic activities of solid acids for acetylation of veratrole with acetic anhydride

Catalyst

Surface Acid sites Veratole area (mmol Temp. (Ac2O Conv. (m2g–1) g–1) (K) ratio) (%) Ref.

[Al]-SBA-15 (Si/Al = 45)

930

0.44

333

[Al]-KIT-5 (Si/Al = 10)

989

0.50

333

—

—

5

353

0.5 2

2

[73]

2

12

[70]

540

0.94

HY

530

2.25

333

2

H-ZSM-5

364

0.82

333

2

15 wt% HPW/22.4% ZrO2/ SBA-15

33.6 wt% HPW/SBA-15 22.4 wt% ZrO2/SBA-15

15% HPW/22.4% ZrO2/MCM-41 15% HPW/22.45 ZrO2/MCM-48

Zn-montmorillonite

372 —

341

426

516

540 —

0.72 0.42 —

0.29

0.24 0.33

0.10

0.67

[70]

95a

H-beta

490

53

97.5 [70]

333

H-mordenite

[72]

6

0.94 —

64

353

526

620

[71]

100 [66]

H-MCM-22

—

30

12.5

333

—

[70]

333

0.36 —

95

100 [71]

1210

1324

[70]

3

[Al]-MCM-41(Si/Al = 22) [Al]-SBA-1 (Si/Al = 39)

92

333

383

333 353

353

353

353

353

333

353

Reaction conditions: veratrole/Ac2O = 5; Catalyst = 0.15 g. aCatalyst = 0.25 g. bCatalyst = 0.09 g.

2

2

2

63 9

73b

1

83b

1

37b

2

1

1

1

[70] [70] [74]

[74]

30b

[74]

44b

[75]

27b 20b

[74]

[75]

[75]

Ordered mesoporous materials incorporated with Al, such as [Al]-SBA-15 [70], [Al]-KIT-5 [63] and [Al]-SBA-1 [71] show high activities for the acylation. The activity depends on the Si/Al ratio of the materials. Catalytic activities are higher than zeolites due to higher accessibility of the reactants to the active sites in the case of mesoporous materials [70].

395

396

Synthesis of Organic Chemicals through Solid Acid Catalysis

Among the zeolites, MCM-22, beta, and faujasite are much more active than mordenite and ZSM-5 [63, 72]. The high activity of MWW is attributed to the presence of three-dimensional cage type structure with two kinds of independent pore systems [72]. The deactivation of Y zeolite during the reaction is ascribed to a partial adsorption of acetoveratrone and/or acetic acid on the active sites of the catalyst [76]. Heteropolyacid supported on mesoporous materials are also used for the acylation of veratrole with Ac2O [77]. Dodecatungstophosphoric acid (HPW) on HMS (hexagonal mesoporous silica) gave ca. 70% yield of the desired product. HPW over zirconia dispersed uniformly in mesoporous silica (MCM-41, MCM-48) channels were synthesized and tested for the catalytic activity in veratrole acylation with acetic anhydride [74, 75]. Among the catalysts, 15 wt% HPW/22.4 wt% ZrO2/MCM-41 calcined at 1123 K was found to have the highest acidity and at least four times more active than 15 wt% HPW/MCM-41 for veratrole acylation. Acylation of 1,4-dimethoxybenzene with Ac2O was studied over various solid acids. Cation exchange resins gave the best results. The product was 2,5-dimethoxyacetophenone [78].

6.2.5  Acylation of 2-Methoxynaphthalene

The synthesis of S-naproxen, an important anti-inflammatory drug, involves a Friedel–Crafts acetylation of 2-methoxynaphthalene (2-MN) to 2-acetyl-6-methoxynaphthalene with acetyl chloride followed by various reactions, the last one being an asymmetric hydrogenation over a Ru catalyst [45]. OCH3 CH3COCI –HCI

OCH3

OCH3

COCH3

CH CH3 COOH

(6.16)

The acylation of 2-MN with acetic anhydride (Ac2O) has been studied by various solid acids such as zeolites. Products are 1-acetyl-2methoxynaphthalene 5, 2-acetyl-6-methoxynaphthalene 6, 1-acetyl7-methoxynaphthalene 7 and 2-acetyl-3-methoxynaphthalene 8.

Acylation

COCH2 OMe 5

OMe 6 OMe

+ (CH3CO)2O CH3CO –CH3COOH

COCH3 OMe 7 OMe 8 COCH3



(6.17)

Among zeolites, zeolite beta is most active. Figure 6.3 shows the time course of the acylation of 2-MN with Ac2O in nitrobenzene in the presence of zeolite beta (Si/Al = 15) under the condition of 2-MN/Ac2O = 5 at 393 K [79, 80].

Figure 6.3

Acetylation of 2-methoxynaphthalene with zeolite Beta. Total yield (×) and yields in isomers 5 (), 6 () and 7 (D) versus reaction time. 393 K, 2MN/Ac2O= 5, nitrobenzene solvent. Reprinted with permission from E. Fromentin, J. –M. Coustard, M. Guisnet, J. Mol. Catal., A, 159, 377 (2000).

In the initial stage, parallel reactions gave three acylation products: 5, 6, and 7. The product 8 was formed only in a trace amount. Acylation occurred preferentially at the kinetically controlled 1-position with formation of 5. At 45 min of the reaction, the yield based on Ac2O reached 100%, the yields of

397

398

Synthesis of Organic Chemicals through Solid Acid Catalysis

5 and (6 + 7) being 63.6% and 36.4%, respectively. However, acylation is followed by the isomerization of 5 into 6 and 7, thermodynamically more stable products. The decrease of total yield indicates the deacylation of 5. The rates of isomerization and deacetylation increase with reaction temperature. At 443 K, 5 was completely converted after 4 h with a final yield of 6 and 7 of 66% and 10%, respectively, the rest being lost by deacetylation. The maximum yield of (6 + 7) was 83% after a 30 min reaction. Isomerization of 5 into 6 proceeds faster in the presence than in the absence of 2-MN. This suggests an intermolecular mechanism for this reaction (transacylation). COCH3 OCH3 +

5

OCH3

OCH3 2–MN

OCH3

+ 2–MN

CH3CO

6

(6.18)

This transacylation mechanism was confirmed by the reaction of a mixture containing 5 with a deuteriated methoxy group (OCD3) and light 2MN as a reactant [81]. The ratio of the formation rates of 5 and 6 depends very much on the nature of beta zeolites such as crystal size, extent of dealumination and passivation of the outer surface [82, 83]. Formation of bulkier product 5 is suppressed by shape selectivity of the zeolite. Thus, the poisoning of the outer surface dramatically decreases the formation of 5 and increases the selectivity for the desired product 6. Nafion silica composite is an effective catalyst for acylation of 2-MN with Ac2O [84]. Nonpolar solvents such as toluene favor the formation of 5, whereas polar solvents such as nitrobenzene and sulforane favor the formation of the desired product 6. The ion exchange of Nafion silica composite with Ag+ and Cu2+ enhanced the selectivity for 6. The reaction of 2-MN with Ac2O over Ag+ -exchanged Nafion silica (80% Nafion) at 413 K for 24 h gave 96% conversion of Ac2O with product distribution of 5:6:7 = 4:69:26. A high selectivity for 5 has been observed in the acylation of 2-MN with MCM-22 [72], mesoporous materials containing aluminum [66, 70], supported heteropolyacid [74], Al salt of heteropolyacid [85] and cation-exchanged montmorillonite [86].

Acylation

6.2.6  Acylation of Heterocycles The acylation of heterocycles such as thiophene, furan and pyrrole proceeds in the gas phase on zeolites with high selectivity [87]. + (CH3CO)2O

X x = O, S, NH

X

C

CH3

+ CH3COOH

O



(6.19)

The reaction of thiophene with Ac2O at 523 K on a [B]-ZSM5 zeolite led to 2-acetylthiophene with 99% selectivity at 24% conversion. On a Ce-doped [B]-ZSM-5, 2-acetylfuran was obtained with 90% selectivity at 23% conversion at 473 K. Liquid-phase acylation of thiophene, pyrrole, furan, benzofuran, and benzothiophene with beta zeolite was reported [73]. 2-Acetyl heterocycles were obtained in high selectivity.

6.2.7  Acylation of Ferrocene

Acylferrocene derivatives are used as intermediates for the production of functional materials such as functional polymers, chiral catalysts, combustion catalysts for propellants, pharmaceutical treatment, etc. Acylation of ferrocene with acetic anhydride was studied at 413 K with different zeolites: beta, USY, mordenite, SSZ-33, ZSM-5, and SSZ-13 [88].

O

C CH3 Fe   +   (CH3CO2)O 

zeolite

  Fe      +   CH3COOH (6.20)

The highest conversion was observed for large-pore zeolites, beta and USY. Small or medium pore zeolites, SSZ-35 and ZSM-5 were least active. The total amount of ferrocene was converted over beta (Si/Al = 12.5). The product was exclusively monoacylferrocene

399

400

Synthesis of Organic Chemicals through Solid Acid Catalysis

due to shape selective property of the zeolite. Zeolite with more acid sites (lower Si/Al ratio) showed higher activity. Acylation of ferrocene with long-chain acid anhydrides and acyl chlorides also proceeds over beta zeolite (Si/Al = 12.5) [52]. Acetic, propionic, and butyric anhydrides gave 100% ferrocene conversion in 300 min at 413 K. The conversion dropped to 74.6% and 45.7% for hexanoic and benzoic anhydrides, respectively. As for chlorides, the conversions of ferrocene were almost 100% for acetyl, propionyl and butyryl chloride. The ferrocene conversion decreased with further increase in the number of carbon atoms in the chain; the conversion of ferrocene was 62.2% and 60.4% for octanoyl and decanoyl chloride, respectively. The conversion was 41.6% for benzoyl chloride. The selectivity for 1-acylferrocene was in all cases 100%. In the acylation of ferrocene with adamantoyl chloride 9 and cinnamoyl chloride 10 was studied over different structural types of zeolites [89]. Fe

+

CI

O

Zeolite

Fe



9

O

O CI

Fe +

10

(6.21)

Zeolite

Fe

O



(6.22)

The acylation with both chlorides led exclusively to monoacylated products. Three-dimensional large-pore zeolites are highly active catalysts for these reactions. The ferrocene conversion decreased in the order: beta > USY > ZSM-5 > mordenite > ferrierite for both acylating agents. Under the same reaction conditions, a higher conversion of ferrocene was observed with 10 in comparison with 9. The ferrocene conversions over zeolite beta (Si/Al ratio of

Acylation

12.5) with 9 and 10 were 45.9% and 63.5%, respectively, under the ferrocene to acylating agent molar ratio of 1:6 at 373 K.

6.2.8  Acylation of Phenols, Alcohols, and Amines

The acylation of phenol affords phenyl acetate (PA) by O-acylation and 2-hydroxyacetophenone (2-HAP) and 4-hydroxyacetophenone (4-HAP) by C-alkylation. (6.23) Furthermore, phenyl acetate may undergo Fries rearrangement during the reaction. The acylation of phenols are reviewed by Saetor and Maggi [44]. Vapor-phase acylation of phenol with Ac2O over ZSM-5 gave mainly PA and 2-HAP [90]. The C- and O-acylation ratio was 1.5 at 70% conversion at 523 K, the ratio of 2-HAP and 4-HAP being 98.5:1.4. Vapor-phase acylation of phenol with acetic acid in gas phase was studied in the presence of H-ZSM-5 [91]. PA and 2-HAP are the primary products, O-acylation being much faster than C-acylation. At high conversion, part of the 2-HAP resulted from the acylation of phenol with PA. 4-HAP did not form through direct acylation of phenol, but formed by the hydrolysis of 4acetoxyacetophenone, which was formed through the autoacylation of PA. Vapor-phase acylation of phenol with acetic acid proceeds over aluminum-containing mesoporous silica ([Al]-KIT-6) to give PA with high selectivity [92]. A steady conversion (80%) and selectivity (90%) were obtained at 573 K. Vapor-phase acylation of phenol with ethyl acetate over H3PO4 supported on TiO2–ZrO2 mixed oxide (Ti/Zr = 1) was studied [93]. The best result was obtained when 15 wt% of H3PO4 was supported on the mixed oxide prepared by sol-gel method using surfactant. 2-HAP was the major product and PA and alkylated products were minor products at 623 K. 4-HAP was not detected

401

402

Synthesis of Organic Chemicals through Solid Acid Catalysis

at a short time on stream (TOS). Conversion decreased with TOS. The conversions were 100% and 35.7% at the time on stream of 1 h and 11 h, respectively. The selectivity for 2-HAP also decreased from 96.5% at TOS = 1 h to 35.3% at TOS = 11 h. The formation of PA increased rapidly with TOS. Sulfated zirconia effectively catalyzes the acylation of a variety of phenols, alcohols and amines with Ac2O under solvent-free conditions [94]. Very high yields of the acylated products were obtained within 15 min at room temperature. In this case, phenol gave PA selectively. The catalyst can be recycled several times. Sulfated ceria–yttria (Ce:Yb:S = 80:15:5) catalyzes the acylation of alcohols and amines at 403 K under solvent-free conditions [95]. High yields of the acylated products were obtained in 4–10 h. The acylating agents used were acetic, propionic, and pivalic anhydrides. Alcohols acylated include hexanol, cyclohexanol, menthol, geraniol, and prenyl alcohol. Catechol, glycerol, and d-mannitol gave di-, tri-, and hexa-acylated product, respectively, with Ac2O. The reaction was chemoselective for 2-aminoethanol giving the N-acetate only. Based on FT-IR spectrum of adsorbed pyridine, the authors proposed that strong Lewis acid sites are active centers for the acylation reactions.

6.2.9  Acylation of Alkenes

Acylation of alkenes with carboxylic acid derivatives leads to the formation of unsaturated ketones, which are versatile intermediates in the preparation of more elaborated compounds. Reaction of cyclohexene and methylcyclohexene with acetic anhydride (Ac2O) proceeds over zeolites [96]. Possible reaction products are shown below.

1

R

R

R

Ac2O zeolite



Ac

+ 2

R

Ac

OAc Ac

+

+ 3

R

OAc

4

5

R

Ac

+ 6

Ac

(6.24)

The reaction of cyclohexene (1. R = H) with Ac2O over mordenite at 323 K gave mainly1-acetylcyclohexene 2 and acetoxycyclohexane

Acylation

4. On the other hand, in the reaction of methylcyclohexene (1, R = CH3) over dealuminated Y zeolite (Si/Al = 15) at 373 K, the main product was 6-acetyl-1-methylcyclohexene 3 (selectivity = 83%), 1-acetyl-2-methylcyclohexene 2 (selectivity = 4%) and 1-acetoxy-1-methylcyclohexane 4 (selectivity = 4%). Acylation of cyclohexene with anhydrides of carboxylic acids was investigated over zeolite beta, USY, mordenite, [Al]-MCM-41, and [Al]-SBA-15 at 353 K [97]. The desired product from cyclohexene is a,b-unsaturated ketone. For the acylation with propionic anhydride, the cyclohexene conversion over beta zeolite with Si/Al = 37.5 was higher than that over USY. Selectivity to cyclohexenyl ethyl ketone by acylation with propionic anhydride over beta was 50.3% at cyclohexene conversion of 78.7%. The byproducts include cyclohexyl ester of propionic acid and diacylated products. Cyclohexene conversion decreased with increasing chain length of the anhydrides: acetic (82%) > propionic (78.7%) > butyric (69.3%) > hexanoic (57.8%). Mordenite, [Al]-MCM-41 and [Al]-SBA15 did not show any activity. In contrast to acylation of cyclohexene, acylation of 1methylcyclohexene with propionic anhydride proceeded in a very high selectivity [97]. The major product was ethyl 2-methylcyclohex2-enyl ketone. It indicates that methyl substituent in position 1 prevents subsequent reactions; the reaction of methylcyclohexene with carboxylic acid did not occur. The acylation of methylcyclohexene with acyl chlorides gave higher conversions in comparison with the acylation with anhydrides. Selectivity to ethyl 2-methylhex2-enyl ketone was, however, lower for acyl chlorides, probably because of the reaction of HCl with the acylated product.

6.2.10  Geminal Diacylation of Aldehydes

Perchloric acid immobilized on silica gel is an effective and expedient for germinal diacylation of aldehydes with Ac2O [98]. A wide variety of aromatic and aliphatic aldehydes can be easily transformed into the corresponding acetals in the presence of 0.5 wt% HClO4/SiO2 within 2–20 min under solvent-free conditions at room temperature. For example, 4-chlorobenzaldehyde was converted to give 98% yield of the corresponding acetal in 2 min.

403

404

Synthesis of Organic Chemicals through Solid Acid Catalysis



CI

CHO

Ac2O

OAc

CI

OAc



(6.25)

Similarly, 1-heptanal was converted to 86% yield of the diacylated product in 5 min. Only stoichiometric amount of Ac2O was required and the catalyst was recyclable. Beta zeolite modified with chlorosulfonic acid is very active for the synthesis of diacetates from a variety of aromatic or aliphatic aldehydes and Ac2O under solvent-free conditions [99]. Benzaldehyde and 1-hexanal (aldehyde/Ac2O molar ratio = 1) were converted into the corresponding diacetals in 1 min at room temperature. Benzaldehydes substituted with electron-donating and electron–withdrawing groups were diacylated with Ac2O in excellent yields in 2–4 h at 333 K under solvent-free conditions in the presence of amorphous carbon-silica composites bearing sulfo-groups [100]. Cinnamaldehyde and 2-furfuryl aldehyde were also effectively converted to the diacylated products. Methacrolein (MEL) can be converted into the diacetate by the reaction with Ac2O in the presence of a mesoporous strong cation exchange resin [101].

CH3

CH2   C — CHO + (CH3CO)2O 

K1 K2

CH3

CH2   C— CH(OOCCH3)2 (6.26)

Methacrolein conversion was 92.1% under the following reaction conditions: Ac2O/MEL molar ratio of 1.2, reaction time of 4 h and 258 K.

6.3  Esterification and Transesterification 6.3.1  Esterification with Solid Acids

Esterification of carboxylic acids with alcohols is well known to be catalyzed by protonic acid, sulfuric acid being usually used. The homogeneous esterification reactions using sulfuric acid in industrial processes involve many problems to be solved. For

Esterification and Transesterification

instance, corrosion of the reaction vessel, and difficulty in treatment of the acid after the reaction. Thus, it is desired to use the heterogeneous process using solid acid catalysts in place of sulfuric acid. A variety of solid acids such as ion exchange resins, zeolites, heteropolyacids, and WO3–ZrO2 have been explored for esterification reactions. The esterification and transalkylation reactions are generally catalyzed by Brønsted acid sites. Examples of the proposed mechanisms for esterification and transesterification over solid acids are shown in Figs. 2.5 and 2.6, respectively. For both esterification and transesterification, the first step is the protonation of carboxylic acid (or ester) [102–104]. Thus, the reaction kinetics is often described by the Eley–Rideal mechanism with retardation by the product water [104]. The catalysts for esterifications require hydrophobicity to avoid strong adsorption of water. Because the reactants involve highly polar reactants (or products) such as acids, alcohols, and water, leaching of the active components to the liquid phase is an important factor for selecting the proper catalysts. Furthermore, proper acid strength of the Brønsted sites is required to avoid side reactions such as the etherification of alcohols.

6.3.2  Esterification of Lower Carboxylic Acids with Alcohols

Ion exchange resins are versatile catalysts for esterification reactions. Application of strong ion exchange resins, Nafion and Nafion/silica composite to solid acid catalysis, including esterification reactions, has been reviewed [105]. The esterification of maleic anhydride with ethanol to produce diethyl maleate has been commercialized by Rohm and Haas and is successfully used in BASF in large-scale plants [105].

O

O

O + 2CH3CH2OH O

H+ O

OCH2CH3 +  H O (6.27) 2 OCH2CH3

This esterification process is a two-stage process involving an autocatalytic exothermic first stage to produce a mono ester, followed by a catalytic second stage to produce the diester [105].

405

406

Synthesis of Organic Chemicals through Solid Acid Catalysis

The effect of carbon chain length on the liquid-phase esterification of carboxylic acid (acetic, propionic, butyric, hexanoic, and capric acid) with methanol over Nafion–silica composite (SAC-13) was studied [106]. The reaction rate decreased with the number of carbons in the linear alkyl chain. This was attributed to the restricted conformational freedom of molecules adsorbed on the catalyst surface. Swelling mesoporous polyvinylbenzenes tethered with sulfogroups were synthesized by sulfonation of polyvinylbenzenes with chlorosulfonic acid in CH2Cl2 [107]. Esterifications of acetic acid with cyclohexanol, hexanoic acid with ethanol, and lauric acid with ethanol showed that the materials were more active than strongly acidic ion exchange resin (Amberlyst-15) or zeolites (beta, Y). The superior performance of the materials was attributed to their unique features, including large surface area (380 m2g–1), abundance of mesoporosity, and high content of sulfo-groups (3.9–4.1 mmol g–1). The esterification proceeds efficiently with hydrous zirconia for a great variety of combinations of carboxylic acids and alcohols to give the corresponding esters in vapor phase as well as in liquid phase [108]. The dehydration of alcohols does not occur in spite of a high reaction temperature. In the vapor-phase reaction, the reactions were carried out by feeding carboxylic acid (0.1 mol dm–3) in alcohol at 474–523 K. The reaction of butanoic acid, 2-methylpentanoic acid and benzene carboxylic acid with ethanol gave 100% conversion with 100% selectivity. In the case of reaction of secondary alcohols (2-propanol, cyclohexanol), the conversion of carboxylic acids increased in the order, tertiary < secondary < primary. The esterification with tertiary alcohols gave lower ester yields. In the liquid-phase esterification (alcohol excess) under reflux conditions, very high yields of esters were obtained. Thus, the reaction of ethanol with hexanoic acid, cyclohexane carboxylic acid, and 2,2-dimethylpropanoic acid gave the corresponding esters in 100, 87, and 50% yield, respectively, in 5 h. Thus, the reactivity of the carboxylic acids increases in the order, tertiary < secondary < primary, in accordance with the esterfication in vapor phase. The reaction of salicylic acid with methanol (methanol excess) at 583 K in autoclave gave methyl salicylate with selectivity of 91% at 100% conversion in 1.5 h.

Esterification and Transesterification

The reaction of acetic acid with ethylene glycol and glycerol (acid excess) at 453–473 K, gave selectively the diacetate and triacetin, respectively, in high yields. Sulfated titania gives a high yield of the ester in the reaction of sebatic acid with 2-ethylhexanol (acid/alcohol ratio = 3) at 368 K and the catalyst could be effectively reused [109].

(6.28) Esters of lactic acid find wide applications in food, pharmaceutical and cosmetic industry. Esterification of lactic acid with 1-butanol in the presence of TiO2–ZrO2 catalysts has been reported [110].



CH3CH(OH)COOH + C4H9OH  CH3CH(OH)COOC4H9 + H2O (6.29)

The best performance was obtained at the composition of Ti:Zr = 3:1. The maximum butyl lactate yield was 94.3% at the conversion of 98.5% at 443 K for 8 h. The catalytic activities of four solid acid materials were compared for esterification of acetic acid with methanol [111]. The catalysts studied were a zeolite (H-beta), sulfated zirconia, tungstated zirconia (WO3–ZrO2) and Nafion/silica (SAC-13). Activities on a per weight basis decreased in the order H-beta ~ SAC-13 >> sulfated zirconia > WO3–ZrO2. Sulfated zirconia is highly active for the esterification of acetic acid with methanol, ethanol, 1-propanol, 1-butanol, and isobutyl alcohol at 303–333 K [112]. The sulfated zirconia was calcined at 848 K. The reaction of acetic acid (1.04 M) in ethanol gave ethyl acetate yield of 93% at 333 K. A fibrous sulfated zirconia, which is prepared using collagen fiber as the template, exhibited

407

408

Synthesis of Organic Chemicals through Solid Acid Catalysis

high catalytic activity for the esterification of acetic acid with 1-butanol [113]. The catalyst could be reused 6 times without significant loss of catalytic activity. The catalytic activity of tungstated zirconia (WO3–ZrO2) for esterification reactions was studied [114]. Figure 6.4 shows the dependence of surface W density on the activities and acid site concentration of WO3–ZrO2 for (a) gas-phase reaction of acetic acid with methanol (393 K), (b) liquid-phase reaction of acetic acid with methanol and (c) liquid-phase reaction of triacetin with methanol. In all cases, the maximum activities (or optimum W density) were obtained at the calcination temperature of 1073 K. The selective poisoning of the acid sites with 2,6-di-t-butylpyridine revealed that Brønsted acid sites play a major role in the esterification reactions.

Figure 6.4

Relative catalytic activity (with respect to the activity of the catalyst calcines at 1073 K) for the esterification of acetic acid in the gas phase (), the liquid phase (), and transesterification of triacetin in the liquid phase (). Surface concentration of acid sites determined by exchange-titration method () as  a function of the surface concentration of tungsten atoms. Reprinted with permission from D. E. López, K. Suwannakarn, D. A. Bruce, J. G. Goodwin, J. Catal., 247, 45 (2007).

Zeolites are effective catalyst for the synthesis of acetates, benzoates and phthalates by esterification [115]. Hydrophobicity of zeolites and segregation of water from the reaction systems are important factors for obtaining higher yields of esters. The esterification of benzyl alcohol with acetic acid proceeds over zeolites, H-beta, H-Y and H-ZSM-5 [116]. Benzyl alcohol

Esterification and Transesterification

conversion was in the order of H-beta > H-Y > H-ZSM-5. The conversion of benzyl alcohol was high over H-beta, the selectivity for benzyl acetate being low because of the formation of dibenzyl ether. On the other hand, the selectivity for the ester was 100% over H-ZSM-5, formation of dibenzyl ether being not observed. This is attributed to shape selectivity due to the smaller pore size of H-ZSM-5 [Al]-MCM-41 shows the higher catalytic activity than zeolites (mordenite, Y zeolites) for liquid-phase esterification of acetic acid with 1- and 2-propanol. Thus, about 85% yields of the esters were obtained for both reactions at alcohol/acid molar ration of 1/2 at 423 K in 2 h [117]. The catalytic activity of [Al]-MCM-41 materials with varying Si/Al ratios (29, 52, 74 and 110) were tested for esterification of acetic acid with 1-hexanol, 2-ethyl-1-hexanol, and 3-methyl-1-butanol, and propionic acid with 3-methyl-1-butanol [118]. A hydrophobic [Al]-MCM-41 (Si/Al = 110) showed higher alcohol conversion with acetic acid than the other materials at 498 K. The conversion with respect to the alcohols followed the order 1-hexanol > 3-methyl-1-butanol > 2-ethyl-1-hexanol, the selectivity being 100% in every case. [Al]-MCM-41 with different Si/Al ratio was studied for the esterification of acetic acid with alcohols [119, 120]. In the esterification of acetic acid with isobutyl, t-butyl, and n-pentyl alcohols, higher conversion was obtained over more hydrophobic MCM-41 (higher Si/Al ratio). Because of hydrophobicity, the water produced could be easily expelled from the pores, thus avoiding its poisoning effect. In the esterification with alcohol/acid ratio of 2 at 523 K, the conversion of 1-pentanol was nearly 100%. [Al]-MCM-41 is also effective for the vapor-phase esterification of butyric acid with 1-pentanol at 523 K, the activity being higher than that of beta zeolite [121]. Al-SBA-15 is active for esterification of acetic acid with C4–C9 alcohols [122]. Heteropolyacids supported on solid surfaces are often explored. H4SiW12O40 (HSiW) supported on hydrous zirconia is active for esterification reactions [123]. The optimum of loading is 15 wt% HSiW on hydrous zirconia. This catalyst gave 91% conversion of acetic acid in its esterification with 1-butanol (acid/ alcohol molar ratio of 1/16) at 371 K. The percentage of conversion of formic, acetic, propionic, and butyric acid with various alcohols follows the order 1-butanol > isobutyl alcohol > s-butyl alcohol.

409

410

Synthesis of Organic Chemicals through Solid Acid Catalysis

This difference in conversion is supposed to be due to the degree of positive charge of the carbenium ion. The esterification of different acids with the above-mentioned alcohols follows the order formic > acetic > propanoic > n-butyric. This order is the same as that of the strength of the aliphatic acids. In all cases the selectivity toward the formation of ester is 100%. The catalyst can be regenerated easily and reused at least five times. HSiW supported on neutral zirconia is active for the synthesis of butyl formate, butyl acetate, butyl propionate, isobutyl acetate, 2-butyl acetate and cyclohexyl acetate by esterification reactions at 353 K [124]. HSiW/Al2O3 is more effective than HSiW/ZrO2. H3PW12O40 (HPW) and HSiW supported on MCM-41 are active for the esterification of hexanoic acid with 1-propanol [125]. They are also effective for the esterification of acetic acid with 1-butanol in vapor phase. Both HPW/MCM-41 and HSiW/MCM-41 gave 95% conversion of 1-butanol. The selectivity of the ester, based on 1butanol conversion, were 85% and 80% for the HPW/MCM-41 and HSiW/MCM-41, respectively. These catalysts, when subject to the esterification conditions, have a tendency to form large heteropolyacid clusters on the external surface of MCM-41. HSiW supported on zirconia-immobilized on mesoporous silica, SBA-15, shows catalytic activity for esterification of acetic acid with 1-pentanol and benzyl alcohol [102, 126]. Under the optimized conditions, the 15%HWSi/22%ZrO2/SBA-15 calcined at 1123 K was found to have the highest acidity and gave 59% benzyl alcohol conversion with selectivity for benzyl acetate as high as 96% at a reaction time of 2 h. The activity is higher than that of HSiW/ZrO2 and comparable to that of zeolite beta. HSiW supported on MCM-41 is active for the esterification of succinic acid and malonic acid with alcohols [127]. In the presence of HSiW (30 wt%)/MCM-41, the diester was obtained by the esterification of malonic acid with butanol in 80% yield after 4 h at 353 K. The diester of succinic acid was obtained in 90% yield from succinic acid and butanol after 14 h at 353 K. Preyssler heteropolyacids, H14[NaP5W29MoO110] (PWMo), H14 [NaP5W30O110] (PW), and the two compounds supported on silica (PWMo/SiO2, PW/SiO2), are effective catalysts for the esterification of cinnamic acid with imidoalcohols and phenols [128].

Esterification and Transesterification



(6.30)

Among the four catalysts, PWMo/SiO2 is most active. Table 6.7 shows the synthesis of different 2-phthalimidoethyl cinnnamates using PWMo/SiO2 at 383 K. The reaction of cinnamic acid (1 mmol) and phenol (1 mmol) gave 87% yield of phenyl cinnamates at 383 K in 7 h. Table 6.7

Synthesis of different 2-phthalimidoethyl cinnamates using PWMo/SiO2 as catalyst

Entry Cinnamic acid

Yields (%)

Product O

COOH

1

82

N O O

COOH 2

O

O

O

N

H3C

H3C

O

O

84

O COOH

CI

CI

O

3

N O

H3CO

OCH3

H3CO

27

O

COOH H CO 3

H3CO 4

O

O

O N

OCH3

O

62

Reaction conditions: Cinnamic acids (1 mmol) and 2-(N-phthalimido)ethanol (1 mmol); PWMoSiO2 (1 mol%) at 110°C, toluene, 24 h stirring. Source: Reprinted with permission from D. M. Ruiz, G. P. Romanelli, P. G. Vázques, and J. C. Autio, Appl. Catal., A, 374, 110 (2010).

411

412

Synthesis of Organic Chemicals through Solid Acid Catalysis

Silica-supported sulfonate, which is prepared by the reaction of silica with chlorosulfonic acid, showed high activity for esterification reactions [129]. For example, the reaction of ethanol with acetic acid and propionic acid (ethanol excess) gave 99% conversion of the acids with 99% selectivity to the corresponding esters at 373 K in 4 h. A carbon material treated with H2SO4 carries a large amount of sulfo groups (4.30 mmol g–1) and highly active as solid acids. The activity for the esterification of acetic acid with ethanol was much higher than Nafion-H [130]. A mesoporous carbonaceous material sulfonated by H2SO4 is very active for esterification of diacids. Four different diacids (succinic, fumaric, levulinic, and itaconic) were reacted in aqueous ethanol [131]. The diesters were obtained in high selectivity at conversion levels of ca. 90% at 353 K. The catalytic activity of the material is between 5 and 10 times higher than zeolites, sulfated zirconias and acidic clays. Carbonacious material obtained by the thermal treatment of p-toluenesulfonic acid with d-glucose has high acidity and is a highly efficient catalyst for the esterification of succinic acid with ethanol [132]. Diethyl succinate was obtained in high yield (>99%) in 4 h at 353 K. The Brønsted acidic ionic liquid 1-(propyl-3-sulfonate)vinyl imidazolium hydrogen sulfate immobilized on silica gel exhibits high activity for a series of esterification reactions [133]. Thus, the esterification of acetic acid with 1-butanol and 1-octanol gave >99% yields of the corresponding esters at 362 K and 368 K, respectively, in 3 h. Satisfactory catalytic activity could be kept after the catalyst was recycled seven times for the synthesis of butyl acetate. The products of the esterification of glycerol with acetic acid, monoacetin (MAC, monoacetylglycol), diacetin (DAC, diacetylglycol), and triacetin (TAC, triacetylglycol) have many different applications, Eq. 6.31. The reaction of glycerol with acetic acid was studied with supported H3PW12O40 [134–136]. The reaction of glycerol with acetic acid (glycerol to acetic acid = 1:16) in the presence of HPW supported on carbon gave the glycerol conversion of 86% with product distribution of monoacetin:diacetin:triacetin:others = 25:63:11:1 at 393 K in 3 h [133].

Esterification and Transesterification

O

OH OH O O 1-monoacetin OH

OH + OH OH

O

O

OH

MAG

+ H2O

OH OH 2-monoacetin O +

O O

OH

O

+

O

+ 2H2O DAG

OH O

O O

1,3-diacetin

O 1,2-diacetin

O TAG O O O O triacetin

+ 3H2O

(6.31)

Acetylglycols can be obtained by the transesterification of glycerol with methyl acetate. SBA-15 bearing sulfo groups is an active catalyst for the transesterification [137]. The reaction of glycerol with acetyl acetate (molar ratio 1:50) at 443 K for 4 h gave the glycerol conversion of 98.2% with the product distribution of monoacetin:diacetin:triacetin ratio of 23.4:68.4 and 5.5.

6.3.3  Esterification of Acids with Alkenes

Showa Denko commercialized the process of ethyl acetate production from acetic acid and ethylene using silica-supported HPA (HSiW) catalyst [138]. The flow chart of the ethyl acetate plant is shown in Fig. 6.5. Typical reaction conditions are 400–500 K, 0.5–1.5 MPa, and C2H4/acetic acid = 5–15. Control of water partial pressure is a key point to avoid the formation of butenes. The catalyst durability of 10,000 h was achieved maintaining more than 98% selectivity. Esterification of acrylic acid with 1-butene proceeds over sulfated zirconia and also over sulfated zirconia doubly promoted with Mn and Fe [139]. Both catalysts showed similar activity and selectivity for s-butyl acrylate (90–95%), but the sulfated zirconia with promoters improved the resistance to deactivation.

413

414

Synthesis of Organic Chemicals through Solid Acid Catalysis

Figure 6.5

Flow-chart of ethyl acetate plant. Reprinted with permission from M. Misono, Catal. Today, 144, 285 (2009).

CH2=CH–COOH  +  CH2=CH–CH2CH3  CH2=CH–COO–CH(CH3)– CH2CH3

(6.32)

Silica-supported H3PW12O40 is an active catalyst for liquidphase esterification of camphene with C2, C4, and C6-carboxylic fatty acids [140].

RCOOH R=CH3, C3H7, C5H11

OC(O)R



(6.33)

The reaction afforded isobornyl carboxylates in virtually 100% selectivity and 80–90% yield at 333 K. The reactions are equilibrium controlled. The use of hydrocarbon solvent prevents HPW from leaching to allow easy catalyst recovery. The catalyst can be recovered and reused without loss of activity and selectivity.

6.3.4   Transesterification

Transesterification of ethyl acetate with alcohols or carboxylic acids proceeds over hydrous zirconia [108]. The reactions of ethyl acetate

Esterification and Transesterification

with alcohols or carboxylic acids (ester excess) were examined in autoclave and the results are shown in Table 6.8. Most of alcohols and carboxylic acids are converted into the corresponding esters with high selectivity. The transesterification of ethyl acetate with alcohols over hydrous zirconia also proceeds in vapor phase to give the corresponding acetates in high yields at 473 K. The reaction of ethyl acetate with carboxylic acids in vapor phase, however, gave the low yields of the corresponding esters. Table 6.8

Entry 1 2 3 4 5

Transesterification of ethyl acetate with alcohols or acids over hydrous zirconium oxide in autoclave

Reactant

Temperature/°C Conversion/% Selectivity/%

OH OH OH OH OH

OH

6 7 8

9

COOH COOH

COOH

150

100

66

130

96

98

150

68

68

130

18

100

150

98

100

160

4

62

180

84

85

200 200

33 75

100 100

Note: Catalyst: 2.0 g; carboxylic acid or alcohol: 10 mmol; ethyl acetate: 20 cm3; reaction for 2 h.

Transesterification of methyl salicylate (MS) and phenol to produce phenyl salicylate proceeds in the presence of ZrO2, sulfated zirconia, and Mo(VI) ion–modified ZrO2 [102]. The sulfated zirconia and Mo-modified zirconia have higher activity than ZrO2. Zirconia and Mo(VI)/ZrO2 show 100% selectivity for phenyl

415

416

Synthesis of Organic Chemicals through Solid Acid Catalysis

salicylate, but in the case of sulfated zirconia diphenyl ether was formed as a by-product. At 403 K, phenyl salicylate was obtained in a 50% yield from a 1:1 mixture of MS and phenol in 4 h over Mo(VI)/ZrO2. Transesterification of b-ketoesters can be performed in the presence of ceria–yttria catalyst [141]. O

O

OR2

R1

+ R3OH

O

Ceria-yttria based Lewis Acid Toluene, reflux



R1

O 2 OR3 + R OH

(6.34)

For example, the reaction of methyl acetoacetate with 1-butanol, 1-hexanol, cyclohexanol, and benzyl alcohol in toluene under reflux conditions gave 95% yield of the corresponding transesterification products in 8–10 h. The mechanism where Lewis acid sites activate the carbonyl group of the reactant is proposed. The reaction of carboxylic acid with dimethyl carbonate (DMC) is an easy way to obtain methyl esters of carboxylic acids.

2RCOOH + (CH3O)2C=O 

  2RCOOCH3 + CO2 + H2O (6.35)

Esterification of benzoic acid and substituted benzoic acid efficiently proceeds in autoclave in the presence of H-beta and H-ZSM-5 [142]. Thus, the reaction of benzoic acid with DMC at 423 K for 4 h afforded methyl benzoate in 98% and 99% yield in the presence of H-beta and H-ZSM-5, respectively. For disubstituted benzoic acids, H-ZSM-5 gave lower yields of the corresponding esters than H-beta because of diffusion constraint. For 3,5-dinitrobenzoic acid, the yield of the corresponding esters was 77% and 40% with H-beta and H-ZSM-5, respectively. Transesterification of triacetin with methanol proceeds over various solid acids [143–145]. For example, the reaction of triacetin with methanol (methanol/triacetin = 9) over Fe2O3-doped sulfated tin oxide at 330 K for 8 h gave triacetin conversion of 92.1% with selectivity of glycerol (63.7%), monoacetin (31.3%) and

Esterification and Transesterification

diacetin (5.0%). Generally, solid base catalysts have higher activity than solid acids [144]. Transesterification of tributyrin (glycerol tributyrate) and methyl acetate proceeds over acid or base catalysts [146]. O

O

CH2

O C

CH3

(CH2)2

CH3

O O C

(CH2)2

+

3 CH3

O C O

CH3

CH O C O CH2 O C

CH3

CH2

O

O

CH O C CH2

(CH2)2

OCH3

CH3



O

CH3

+

3 CH3

(CH2)2

C

OCH3

(6.36)

Among solid acid catalysts, a Nafion–silica composite catalyst (SAC-13) shows high efficiency. After the reaction in methyl acetate excess for 20 h at 403 K, the conversion of tributyrin was almost quantitative, methyl butyrate yield was 83%, whereas the yield of triacetin was 60%.

6.3.5  Biodiesel Synthesis

Biodiesel is an environment-friendly fuel as it is made from renewable resources. It is usually derived from the esterification of free fatty acid (FFA) or the transesterification of triglyceride with methanol or ethanol. In the industrial processes, homogeneous base catalysts such as NaOH and KOH are often used in the transesterification of refined vegetable oils. For operational reasons, heterogeneous catalysts have been extensively explored [147–149]. The first plant using a heterogeneous catalyst was built in France [149, 150]. In this continuous process using two fixed-bed reactors, the catalyst consists of a mixed oxide of zinc and aluminum. The reaction is performed at higher temperature (483–523 K) and pressure (30–50 bar) than homogeneous catalysis processes. Solid bases exhibit higher catalytic activities than solid acids. However, FFA is a strong poison of solid base catalysts and also led to the formation of soap with the resulting separation difficulties. The FFA content is low (i.e., 90%

~100%

[164]

(Continued)

419

420

Synthesis of Organic Chemicals through Solid Acid Catalysis

Table 6.10   (Continued) Yield Temperature/ (or conversion) Ref. Time

Catalyst

Fatty acid

Alcohol

Nafion/SiO2 Sulfated ZrO2

10 wt% Palmitic in sunflower oil

Methanol 333 K/24 h

Sulfated ZrO2

Myristic

Methanol   333K/7h

Ethanol

1-Prpanol   363 K/7 h Polyvinylbenzene with SO3H

Lauric

1-Butanol Ethanol

343 K/ 5 h

Zirconium sulfate/ SiO2

Oleic

Butanol

Cs2.3H0.7PW12O40

Palmitic

Methanol 333 K/ 6 h

Zirconium sulfate/ Carbon

Mesoporous ZrTiO3 Lauric

Palmitic Oleic

393 K/4 h

Methanol 333 K/16 h 333 K/13 h 333 K/18 h

~90%

[166]

98%

[167]

98%

98%

98%

99%

94.0%

[107]

[168]

91.3%

[169]

92.3%

[171]

100%

78.1% 73.2%

[170]

Sulfated tin oxide with and without doping with Fe were studied for esterification of oleic acid with methanol at 333 K [143]. The conversion of oleic acid was 73.4% and 88.9% over tin oxide non-doped and doped with Fe, respectively, in 6 h. The reusability of the catalyst was tested. After the reaction, the catalyst was separated from the reaction mixture by centrifugation, washed with methanol for several times, calcined in air at 773 K for 1 h to remove the adsorbed organic species, and then reused. After three cycles with doped catalyst, the conversion dropped from 88.9% to 74.3%. Sulfated zirconia was studied for esterification of myristic acid with methanol, ethanol, and 1-propanol [167] The reaction was catalyzed by Brønsted acid sites and the ester formation was selective. The catalyst showed decrease in activity (~28%) after five cycles.

Esterification and Transesterification

Sulfated zirconia supported on SBA-15 was prepared by a direct-synthesis method. The material was more active than unsupported sulfated zirconia for the esterification of lauric acid and palmitic acid with methanol [153]. The sulfated zirconia/SBA15 (Si/Zr = 3.01, S​O​2– 4​  ​/Zr = 0.044) gave 87.4% and 89.2% conversion of the acid in the esterification of lauric acid and palmitic acid, respectively, at ~341 K for 6 h. WO3 supported on Zr-doped MCM-41 was used for the esterification of oleic acid with methanol [154]. The materials with WO3 loading of 15–20% after activation at 973 K led to the most active catalysts. The conversion values were close to 100% at a reaction time of 24 h at 338 K and 4 h at 473 K. The catalyst with 15 wt% WO3 were reusable at least during four cycles and no leaching of tungsten species was found. Heteropolyacid (H3PW12O40, HPW) supported on niobia was tested for the esterification of palmitic acid and the mixture of the FFA and sunflower oil (SFFA) in the methanol solution under reflux conditions [157]. The HPW/Nb2O5 was most active at a loading of 25% HPW and a calcinations temperature at 750 K. The conversion of palmitic acid and SFFA was 99.1% and 97.3%, respectively. Heteropolyacid (HPW) supported on zirconia was also tested for esterification [158]. For the esterification of oleic acid with ethanol (acid:alcohol = 1:6) under reflux conditions, 20 wt% HPW/ZrO2 gave the 88% conversion of the acid in 4 h reaction. A treatment of the spent catalyst involving a sequence of washing with hexane, drying at 373 K and calcining at 573 K for 4 h, recovered conversion values as high as 70%, i.e., 80% of the original value. Heteropolyacids supported on MCM-41 or SBA-15 are effective for esterification of fatty acids with methanol [159–161]. HPW/MCM-41 was more active than H4SiW12O40/MCM-41 and H3PMo12O40/MCM-41. At 338 K, the conversion of palmitic acid, oleic acid, and stearic acid was 96%, 85%, and 78%, respectively, in their reactions with methanol for 5 h at 338 K in the presence of 7.3 wt% HPW/MCM-41 [161]. The catalysts are recyclable after simple regeneration procedures without significant loss in conversion [159–161].

421

422

Synthesis of Organic Chemicals through Solid Acid Catalysis

Amorphous carbon, which was prepared from cellulose powder and treated with sulfuric acid, is highly active for the esterification of oleic acid with methanol at 368 K [163]. The material has SO3H, COOH and phenolic OH groups. The yield of the methyl ester reached 99.9% at 4 h. The catalyst was regenerable by washing with water and drying at 403 K. No decrease in activity was observed after 10 times reuses, as long as the recovered catalyst was washed with water. The material was also effective for the transesterification of triolein with methanol [163]. The activity was much higher than Nafion/silica (SAC13), Nafion-H, and Amberlyst-15 at 403 K, maintaining high catalytic activity even in the presence of water. Mesoporous carbon prepared from a carbon-coated alumina was grafted with benzenesulfo groups by the reaction with 4-benzene-diazoniumsulfonate [164]. The material was an effective catalyst for the esterification of oleic acid with methanol. Thus, high conversion of oleic acid (90%) was attained in 4 h at 338 K. The esterification of palmitic acid in sunflower oil in the presence of SAC-13, sulfated zirconia, tungstated zirconia, and silica–alumina was reported [166]. The activity of SAC-13 and sulfated zirconia was more than one order magnitude higher than those of tungstated zirconia and silica–alumina. Sulfated zirconia could not be fully regenerated after use by cacination at 773 K because of sulfate group leaching and the formation of carbonaceous deposit derived from the decomposition of adsorbed oil. On the other hand, SAC-13 showed no significant deactivation upon reuse. Esterification of stearic acid (10%) in sunflower oil with methanol was studied over strong ion exchange resins [172]. The gelular resin type was superior to macroporous resin types. The gelular type resin EBD-100 (Rohm and Haas) gave 100% conversion of stearic acid in 24 h at 338 K. The deactivation of the resin is caused by ion exchange with traces of metal cations in the oil and the active sites can be regenerated by acid washing. Zirconium sulfate supported on silica or activated carbon showed high activity for esterification of oleic acid with 1-butanol under solvent-free conditions [168, 169] The catalytic activity of these catalysts was higher than Amberlyst-15, which showed much higher activity than Nafion-H or zeolites (H-mordenite,

Esterification and Transesterification

H-beta, H-Y, H-ZSM-5). The conversion of oleic acid was 94.0% in the reaction of oleic acid (0.1 mol) with 1-butanol (0.12 mol) at 393 K for 4 h in the presence of zirconium sulfate supported on silica.

6.3.5.2  Biodiesel synthesis by transesterification of oils with methanol

Fats and oils are primarily triglycerides that are made up of one molecule of glycerol and three moles of fatty acids (long-chain carboxylic acid). They are water-insoluble and hydrophobic. Biodiesel formation reaction is basically the transesterification of triglycerides with low molecular weight alcohols. Mono- and diglycerides can also be obtained from triglycerides by replacing two and one fatty acid moieties with hydroxyl groups, respectively. O

CH2

O

C O

R

CH

O

C O

R

CH2

O

C

R

Triglyceride

+  3R’ OH

Methanol R = Fally acid chain R = CH3

Catalyst

O 3R’

O

C

R

Fatty acid methyl ester (biodiesel)

+

CH2

OH

CH

OH

CH2

OH

Glycerol (6.37)

For the synthesis of biodiesel by transesterification of sunflower oil with methanol, WO3–ZrO2 prepared by calcination of (NH4)6H2W12O40 supported on zirconia (optimum loading: 15 wt% as WO3) was active [173]. In the presence of WO3–ZrO2 calcined at 1023 K, the conversion of the oil reached 97% at methanol/oil ratio of 20 at 473 K. The catalyst was also effective for the transesterification of other vegetable oils like mustard oil and sesame oil and the conversion was 95% and 93%, respectively. The deactivated catalyst could be regenerated by calcination without appreciable loss in activity. Tungstated zirconia–alumina (WO3/ZrO2–Al2O3) was also reported to be active for the transesterification of soybean oil [174].

423

424

Synthesis of Organic Chemicals through Solid Acid Catalysis

Tungsten oxide supported on Zr-doped MCM-41 is active for the transesterification of sunflower oil with methanol [175]. The maximum activity (82 wt% ester yield) was attained for the catalyst with 15 wt% WO3 loading. An 82% yield of the ester was obtained after 2.5 h of reaction at 473 K. The catalytic activity was maintained after three cycles of reutilization without any treatment of the catalyst. Addition of 5 wt% of water to the reactants did not retard the catalytic activity. Tantalum oxide supported on SBA-15 is active for methanolyis of sunflower oil [176]. 15% Ta2O5/SBA-15 gave 92.5% yield of biodiesel at a methanol/oil molar ratio of 12 at 473 K in 6 h. No leaching of tantalum was detected. The catalyst was able to simultaneously catalyze the esterification of FFA (oleic acid) and the transesterification of triglycerides [176]. Tin oxide supported on silica exhibits high catalytic activity for the transesterification of soybean oil [177]. The catalyst with 8 wt% SnO2 loading and calcined at 873 K gave an oil conversion of 81.7% at a methanol/oil molar ratio of 24 at 453 K. FFA (oleic acid) and water had no significant influence on the catalytic activity for the transesterification reaction. In addition, the catalysts had high catalytic activity toward the esterification of the FFA. Sulfated tin oxide is applied to the transesterification of waste cooking oil containing 2.34% of FFAs [178]. The yield of fatty acid methyl ester (FAME) of 87.4% was obtained under the conditions of 15:1 methanol-to-oil ratio and 3 h of reaction time. The activity was enhanced by adding appropriate amount of SiO2 to S​O​2– 4​  ​/SnO2. The yield of FAME was 93.5% in the presence of S​O​2– ​  / ​ SnO 2–SiO2. 4 Sulfated zirconia and sulfated zirconia supported on mesoporous silica are very active for the transesterification of various oils [179–181]. In these cases, the catalysts exhibited significant activity loss in the second run and the leaching of sulfate ions was confirmed. The blockage of the catalyst pores by the products may be another reason for deactivation. The spent catalysts were regenerable by impregnation with a 0.5 M H2SO4 solution followed by calcination [180]. Sulfated zirconia is active for the transesterification of waste cooking oil (WCO) with methanol [182]. Under the conditions of methanol/WCO molar ratio of 9:1, reaction time of 4 h, and reaction

Reactions of Epoxides

temperature of 393 K, 93.6% of biodiesel yield was obtained. No sulfate ions were found in the FFA ester after reaction. Heteropolyacid (H3PW12O40, HPW) impregnated on four different supports, hydrous zirconia, silica, alumina, and activated carbon, was examined for the biodiesel production from lowquality canola oil containing up to 20 wt% FFAs [183]. HPW supported on hydrous zirconia was found to be the most effective catalyst exhibiting the highest ester yield (90 wt%) at 473 K and at 1:9 oil to alcohol molar ratio. For regeneration, the spent catalyst was soaked in hexane and then in methanol to remove nonpolar and polar residues, respectively. The catalyst was recycled and reused with negligible loss in activity. Arenesulfonic acid tethered to mesoporous silica exhibited a high activity for the conversion of crude palm oil, which contains 5.6 wt% FFA [184]. The material showed much higher activity than ion exchange resin or Nafion/silica (SAC-13). Almost 90% of the starting oil was converted to fatty acid methyl ester after reacting 2 h of reaction under the optimum reaction conditions, temperature 433 K, methanol to oil molar ratio 30, catalyst loading 5.1 wt%. Zinc stearate immobilized on silica gel is an effective catalyst in simultaneously catalyzing the transesterification of triglycerides and esterification of FFA (15 wt%) present in waste cooking oil to methyl esters. A maximum yield of 98% could be attained at 473 K. The catalysts could be recycled and reused many times without any loss in activity [185].

6.4  Reactions of Epoxides

6.4.1  Ring Opening of Epoxides with Amines and Alcohols b-Amino alcohols are versatile intermediates for the synthesis of various biologically active natural products. One of the most straightforward synthetic approaches for the preparation of b-amino alcohols involves the reaction of epoxides with amines.

(6.38)

425

426

Synthesis of Organic Chemicals through Solid Acid Catalysis

The reactions can be performed in the presence of solid acid and bases. The selectivity of the products depends on the acid–base property of the solid surfaces. Ring-opening reaction of epoxides with use of solid bases has been reviewed [186]. Solid acids such as Amberlist-15 [187], H3PMo12O40 (HPMo) supported on alumina [188], and nanoporous aluminosilicate [189] are active for ring opening of epoxides with various amines under mild conditions. Table 6.11 shows the results of the ring-opening reactions of epoxides with amines over Amberlist-15. The reactions of epoxide (2 mmol) and amine (1 mmol) were carried out over Amberlist-15 (100 mg) in dichloromethane at room temperature. The reaction of styrene oxide with aniline gave the corresponding b-amino alcohol in 92% yield. The epoxide opening took place in a regioselective manner with the attack of nucleophile at benzylic position [187, 188]. For the same reaction, HPMo/Al2O3 also gave high yield (93%) of the b-amino alcohol in 30 min [188]. Table 6.11

Ring opening of epoxides in the presence of Amberlyst-15

Amberlist–15 catalyzed regioselective ring opening of epoxides

NH2

O a b

OH N H

NH2

O

OH N H

NH2

c

O O

d e

Reaction time (h) Yieldb

Producta

Enter Substrate Amine

O

OH

H N

H N

OH

CH3

N CH3 OH

CH3

N CH3

N H

2.0

92

2.5

89

3.0

86

2.0

90

3.5

88

(Continued)

Reactions of Epoxides

f

H N

N OH CH3

3.5

87

g

2.5

90

h

3.0

88

i

3.5

86

3.0

89

N H

3.5

86

N OH H

4.0

85

3.0

87

O

O NH2

j k l m

CH3

OH H N

OH

NH2

O

NH2

O O

O

H2N

O

N OH H

aAll

products were characterized by 1H NMR, IR, and mass spectroscopy. and unoptimized yield. Source: Reprinted with permission from M. Vijender, P. Kishore, P. Narender, B. Satyanarayana, J. Mol. Catal. A, 266, 290 (2007).

bIsolated

Glycidyl aryl ether reacts with aromatic, aliphatic, and alicyclic amines efficiently to afford the corresponding b-amino alcohols in very good yields in regioselective manner, preferential terminal attack of the nucleophile over HPMo/Al2O3 [188]. In these reactions, the product was obtained as a single isomer.

427

428

Synthesis of Organic Chemicals through Solid Acid Catalysis

Cycloalkyl epoxides such as cyclohexene epoxide react smoothly in a SN2 fashion with different aromatic, benzylic, and aliphatic amines to afford the corresponding b-amino alcohols over HPMo/ Al2O3 [188]. The stereochemistry of the ring-opening products is found to be trans from the coupling constants of the ring protons in 1H NMR spectrum. OH

O +



R–NH2 NHR



(6.39)

Monodispersed silica nanoparticles are also effective for the ring opening for the synthesis of b-amino alcohols [190]. Water can be used as solvent. With the reaction of styrene oxide, aromatic amines react in a regiospecific manner to give the corresponding b-amino alcohols with preferential nucleophilic attack at benzylic position. On the other hand, aliphatic amines such as benzyl amine, cyclohexyl amine and butyl amine give the product in good yields with preferential attack at the non-benzylic terminal carbon. [Al]-KIT-5, an aluminosilicate with a three-dimensional mesoporous networks, is very effective for the alkylation of indoles and pyrroles with styrene oxide in water at room temperature [191]. Styrene oxide undergoes cleavage at the benzylic position to selectively give the primary alcohols. In the case of substituted indoles, the 3-alkylated indole products are formed selectively (reaction 6.40). In contrast, in the case of pyrroles, the alkylation reaction results in the corresponding product mixtures of 2-alkylated and 3-alkylated pyrroles, in which the 2-alkylated pyrrole is the major one (reaction 6.41). O



N H



OH

+ N H O

N H

+

(6.40) OH

N H

OH +

N H



(6.41)

Reactions of Epoxides

Regiospecific addition of alcohols to epoxides occurs over solid acids such as sulfated yttria–zirconia [192] and nanoporous aluminosilicates [193]. The addition of methanol to styrene oxide proceeds to give 2-methoxy-2-phenylethanol selectively. The reaction of cyclohexene oxide gives the products derived from antiaddition of the alcohols.

OR

ROH

O

OH

ROH = MeOH, EtOH, iPrOH, tBuOH, and AcOH



(6.42)

Industrial production of diethanolamine [194] Ethylene oxide reacts with ammonia to produce monoethanolamine (MEA), diethanolamine (DEA), and triethanolamine (TEA) by the following successive reactions that proceed without catalyst if water is present in the reaction mixture. H



H

MEA

EO

H H2N

OH

H N HO

H2N

O

+

N

+

OH

H N

HO

O

(6.43) OH

DEA HO

OH +

O HO

N

TEA

(6.44)

OH

(6.45)

The selectivities for the three ethanolamines are dependent on the relative rates of each reaction. The product distribution may be controlled by changing the ammonia/ethylene oxide ratio in the reactant. The fraction of MEA increases when a mixture with a high ammonia/ethylene oxide is allowed to react, whereas the fraction of TEA increases with a low ammonia/ethylene oxide. However, selective production of DEA is impossible. This noncatalytic reaction is being used for the industrial production of the three types of ethanolamines.

429

430

Synthesis of Organic Chemicals through Solid Acid Catalysis

The reaction of ethylene oxide with anhydrous ammonia, on the other hand, does not proceed rapidly without catalyst and is catalyzed by cation-exchanged resins. The reaction is, however, non-selective. By utilizing the shape selectivity of zeolite, selective formation of DEA became possible. Selective production of DEA was industrialized by Nippon Shokubai in 2003. The catalyst is reported to be La3+-exchanged ZSM-5. Minimum molecular diameter of MEA is 0.47 nm, whereas that of TEA is 1.0 nm. MEA can be adsorbed on H-ZSM-5 6 times more than TEA. H-ZSM-5 could catalyze amination of ethylene oxide with a high selectivity for (MEA + DEA), TEA being produced in a small amount. La3+-exchanged ZSM-5 shows higher activity and selectivity for formation of MEA and DEA than H-ZSM-5. The high selectivity for DEA of La-ZSM-5 is presumed to be due to the transition-state shape selectivity of the catalyst, as illustrated in Fig. 6.6. The kinetic study suggests that the DEA fraction reaches more than 60 wt% and the TEA fraction is only 0.9 wt% if a part of produced MEA is recycled to the reactant mixture.

Figure 6.6

Implementation of transition state shape selectivity. Reprinted with permission from Tsuneki, Shokubai (Catalyst), 47, 196 (2005).

La3+ ions are introduced to H-ZSM-5 by the solid-state ion exchange method. Multivalent metal cations such as La3+ ion is strongly hydrated in an aqueous solution, and the diameter of the hydrated ion is large. Therefore, the hydrated La3+ ions scarcely

Reactions of Epoxides

enter the cavities of ZSM-5 and the ion exchange degree is limited to about 5% by a conventional ion exchange in the aqueous solution. The ion exchange degree can be enhanced by a solid-state ion exchange; a mechanical mixture of LaCl3 and NH4-ZSM-5 is calcined at 823 K for 5 h. In this method, the non-hydrated La3+ cations diffuse into the zeolite cavities and replace the protons to locate at the ion exchange sites. The residual LaCl3 is washed out with water. The resulting catalyst showed higher activity than HZSM-5 and the La-ZSM-5 obtained by ion exchange in an aqueous solution. Two more improvements were required for the catalyst to be used in the industrial process: improvement of plasticity and formability arising from high silica zeolite, and improvement of the regeneration method of the deactivated catalyst other than burning at a high temperature. A commercially available molded zeolite with high Si/Al normally contains an inorganic binder. With the binder, significant amounts of by-products were formed in the reaction, but without the binder, no by-products were formed. A molded catalyst without binder could be prepared by the method called “dry-gel conversion” in which alumina source, alkali, and a structure-directing agent were impregnated into the molded silica support and contacted with a high temperature steam in an autoclave. Regeneration of the deactivated catalyst by treatment with ammonia as a rinse medium is employed in the process. As the reaction temperature is 300–420 K, catalyst deactivation is not caused by coke formation but by deposition of organic substances. Burning a coke above 600 K is not necessary and extraction of the deposited organic substances is sufficient. The regeneration is performed at a temperature a little higher than the reaction temperature. Liquid state ammonia can extract the deposited organic substances causing catalyst deactivation. Utilization of one of the reactants as a rinse medium avoids inclusion of chemical substances other than the reactants. The industrial process is illustrated in Fig. 6.7. The reactor and regenerator are switched at an interval of several days. Ammonia recovered from the NH3 recovery towers and MEA separated from the products are fed back to the feed. Plural reactors are required for concurrent reaction and regeneration.

431

432

Synthesis of Organic Chemicals through Solid Acid Catalysis

Figure 6.7

Process flow diagram for diethanolamine production from ethylene oxide and ammonia. Reprinted with permission from H. Tsuneki, Bull. Chem. Soc. Jpn., 80, 1075 (2007).

6.4.2  Hydration of Ethylene Oxide

The production of monoethyleneglycol (MEG) by noncatalytic hydration of ethylene oxide (EO) is a well-known process. Considerable efforts have been made to explore efficient catalysts. Niobium oxide (Nb2O5.nH2O) has high acid strength and structural stability in water. Nb2O5 supported on a-Al2O3 shows a high activity for the hydration. The addition of tin to Nb2O5/a-Al2O3 further improved the catalytic performance [195]. In the presence of tin-promoted Nb2O5/Al2O3, the selectivity for MEG 94.0% at the EO conversion of 99.7% was attained at 423 K, H2O/EO = 22, and a liquid hourly space velocity of 10 h–1. Layered niobic acids (HxK1–xNb3O8) catalyze the selective hydration of EO [196]. The activity and selectivity depend on the value of x. The highest selectivity for MEG over 95% with EO conversion of >99% was achieved at x = 0.7 under H2O/EO ratio of 8. An in situ self-exfoliation behavior of Nb3​O​–8 ​​  is found during the EO hydration. This exposes the abundant acid sites among the Nb3​O​–8 ​​  nanosheets. The weak acidic strength is an important factor for the high selectivity.

6.4.3  Isomerization of Styrene Oxide

The isomerization of styrene oxides yields aldehydes, which are valuable for the production of fragrances, drugs, and insecticides,

Reactions of Epoxides

fungicides, and herbicides. Different catalysts have been explored for the rearrangement of styrene oxides [197]. Silica-supported heteroplyacid (H3PW12O40, HPW) is an efficient catalyst for the liquid-phase isomerization of styrene oxide to phenylacetaldehyde [198]. O



CH

CH2

CH2

CHO

(6.46)

The reaction occurs in cyclohexane as a solvent under mild conditions at 298–343 K with low catalyst loadings and without HPW leaching into the solution. At 333 K, the yield of phenylacetaldehyde reached 92% at 97% styrene oxide conversion. The catalyst could be recovered and reused. Amorphous silica–alumina is also efficient catalyst for the isomerization. The reaction in toluene at 363 K gave 100% conversion with 96% selectivity in 30 min [199].

6.4.4  Rearrangement of b-Pinene Oxide into Myrtanal

The rearrangement of b-pinene oxide into myrtanal proceeds in the presence of solid Lewis acid catalysts [200]. O

O





(6.47)

Different metals such as Zr, Sn, Ti, Nb, Ta, Al, and Ga were introduced into the network positions of zeolite beta by isomorphous substitution. Among them, [Zr]-beta gave the best selectivity. The reaction of b-pinene oxide in acetonitrile at 329 K for 2 h gave 94% selectivity and 98% conversion. In this reaction, the solvent selection is important to balance the competitive adsorption of the products. Product selectivity is enhanced with acetonitrile, and [Zr]-beta could be reused in the batch mode and in a fixed- bed reactor for several times. Leaching of the metal or crystal degradation was not

433

434

Synthesis of Organic Chemicals through Solid Acid Catalysis

detected. Brønsted acidic zeolites such as [Al]-beta gave conversions and selectivities considerably lower than [Zr]-beta. The rearrangement of b-pinene oxide to myrtanal over metalincorporated MCM-41 such as Sn-MCM-41 was also reported [201].

6.4.5  Rearrangement of a-Pinene Oxide to Camphenolic Aldehyde

Camphenolic aldehyde 12 is an important intermediate used in fragrance industry and can be obtained by the rearrangement of a-pinene oxide 11.



11

12



(6.48)

USY zeolite is active for the isomerization of a-pinene oxide [202]. After the reaction at 273 K for 2 h, the conversion was 30% with ca. 75% selectivity for camphenolic aldehyde 12. The activity of USY was increased by treatment with dilute hydrochloric acid. Under the same reaction conditions, the conversion reached 100%. The selectivity for camphenolic aldehyde increased by lowering the reaction temperature, though the rate of the reaction decreased. After the reaction at 258 K for 24 h, the selectivity was 78% with a-pinene conversion of ca. 85%. Participation of the extraframework aluminum species in the catalysis is suggested. Beta zeolite containing Ti in the framework, [Ti]-beta, is an efficient catalyst for the rearrangement in the vapor phase [203]. In the reaction of pure a-pinene at 363 K, the initial conversion of a-pinene was 93% with 63% selectivity for camphenolic aldehyde. The activity decayed rapidly: The conversion and the selectivity after 5 h were 36% and 25%, respectively. The selectivity and the stability of the catalyst increased significantly by cofeeding an organic molecule together with a-pinene oxide. When dichloroethane was fed with a-pinene oxide, 94% selectivity at 95% conversion was obtained. Alkanes such as heptane and 1,4-dimethylcyclohexane were also effective as a cofeeding molecule, which reduced the con-

Dehydration

centration of the reactant and the condensation of the oxide. The proposed mechanism includes the coordination of the epoxide oxygen atom to a Lewis acid center (Ti in the framework). The rearrangement can be also carried out in liquid phase in the presence of a supported Lewis acid. The reaction of a-pinene oxide in dichloroethane in the presence of zinc triflate supported on HMS (hexagonal mesoporous silica) for 1 h gave 69% selectivity for the desired aldehyde with complete conversion of a-pinene oxide [204]. The selectivity was 80% at 50% conversion of the reactant.

6.5  Dehydration

6.5.1  Dehydration of Glycerole to Acrolein Glycerole is a main by-product in natural triglyceride methanolysis for biodiesel production (see Section 6.3.5). The increased production of biodiesel in recent years has resulted in an increase of glycerol production. Consequently, the use of glycerol as a starting material for the synthesis of other valuable chemicals is becoming important. Acrolein is an industrially very important compound. Dehydration of glycerol to acrolein is an attractive way to use glycerole and has been studied using a variety of solid acid catalysts [205]. Recent results on this topic are summarized in Table 6.12. OH



HO

OH

–2H2O

O

(6.49)

The reaction mechanism of the dehydration over Brønsted acid sites is given in Fig. 6.8 [219]. Interaction of glycerol with a proton mainly leads to the protonation of the internal oxygen in the glycerol molecule possessing a higher negative charge compared with the terminal oxygens. Subsequent steps involve elimination of H3O+ to give 1,3-dihydroxypropene and its tautomerization to 3-hydroxypropanal. The latter undergoes further acid-catalyzed dehydration to yield acrolein [219].

435

436

Synthesis of Organic Chemicals through Solid Acid Catalysis

Table 6.12

Dehydration of glycerol to acrolein over solid acid catalysts

Time  Temperaon ture Conversion Selectivity stream Reference

Catalyst

Feed

Nb2O5

36.2 wt% in water

588 K

88%

51%

9–10 h

[206]

NbZr oxide Zr/Nb = 12

20 wt% in water

573 K

100% >90%

~65%

32 h 200 h

[208]

100%

75%

—

[210]

85.7%

76.5%

—

[212]

98.3%

86.2%

0–5 h

[214]

98%

63%

9–10 h

[216]

96.2%

82.7%

8h

[218]

97% 79%

83% 96%

1 h in N2 5 h in H2

Nb2O5/SiO2

WO3–ZrO2 WO3–ZrO2

30 wt% in water

36.2 wt% in water 20 wt% in water

20 wt% WO3/Sidoped ZrO2a in water WO3–TiO2

20 wt% in water

HSiW/SiO2c

10 wt% in water

HPW/ ZrO2a,b

36.2 wt% in water

10 wt% HSiW/ SiO2-Al2O3b,c in water

593 K

588 K 553 K 573 K 553 K 588 K 548 K 548 K

ZSM-5

35 wt% in water

593 K

Nd4(P2O7)3

36.3 wt% in water

593 K

Beta

Cs2.5PW d

28.7wt% in water 10 wt% in water

Pd/Cs2.5PWd 10 wt% in water

613 K

548 K 548 K

100% 50%

100% 100% 100% 77% 79%

100% >80%

95.2% 52.0% 100% 41%

aZrO /SiO /WO = 87.3/0.6%/12.1 (wt%). 2 2 3 bHPW: H PW O . 3 12 40 cHSiW: H SiW O supported on silica–alumina. 4 12 40 dCs PW: Cs H 2.5 2.5 0.5 PW12O40.

50–70% 2 h 10 h 48% 65% 65% 74%

69%

75% 75%

44.7% 23.0% 98% 94%

1–2 h 9–10 h 6h 97 h

6–9 h

175 h

0–2 h 10–12 h 1h 6h

[207]

[209]

[211]

[213]

[215]

[217]

[219]

Dehydration

Figure 6.8

Mechanism of glycerol dehydration on Brønsted acid sites. Reprinted with permission from A. Alhanash, E. P. Kozhevnikov, J. V. Kozhevnikov, Appl. Catal. A, 378, 11(2010).

Since glycerol is usually produced as a mixture with water, the direct use of glycerol in water is advantageous over pure glycerol for the production of acrolein. The glycerol dehydration has been studied using aqueous solutions of glycerol (10–30 wt%) with a flow reactor. Therefore, a highly water-tolerant solid acid catalyst has to be selected. The catalysts studied include niobium oxides, tungstic oxides and heteropolyacids supported on carriers and zeolites. Reaction temperature is in the range of 548–593 K. Initial conversion of glycerol was usually very high, and the selectivity for acrolein of 60–80% was obtained. The conversion and selectivity decreased with time on stream rather quickly. Therefore, the deactivation and regeneration of the catalysts are important issues for the dehydration. The catalysts are regenerated by oxidative treatment at higher temperatures. In the case of zeolites, most of the micropores are filled with carbon species at the initial stage of the reaction [217]. A variety of solid acid catalysts are active for the dehydration. Acid sites having –8.2 ≤ Ho ≤ –3.0 are most effective for the selective formation of acrolein [220]. Stronger acid sites promote coke deposition, leading to lower selectivity. Among zeolites, beta zeolite shows the highest activity and selectivity [217]. The increase in the partial pressure of water increases the selectivity for acrolein, though it does not affect glycerol conversion.

437

438

Synthesis of Organic Chemicals through Solid Acid Catalysis

H4SiW12O40 (HSiW) supported on alumina gave complete conversion of glycerol at the beginning of the reaction and the conversion was still higher than 80% at a time on stream of 175 h [215]. The selectivity for acrolein (75%) did not change with time on stream. It was suggested that the catalyst could be regenerable in situ with a small amount of oxygen in the feed at moderately higher temperature [215]. HSiW supported on SiO2 showed high selectivity for acrolein [214]. The selectivity depends on the size of mesopores, HSiW/SiO2 with mesopores of 10 nm showed the highest catalytic activity with the acrolein selectivity of >85% at 548 K. Rare earth pyrophosphates are efficient catalysts for glycerol dehydration to acrolein [218]. Nd4(P2O7)3 prepared under specified conditions are most effective. High selectivity for acrolein (80%) was attained at 593 K. The presence of weak acid sites favors the acrolein formation. The catalysts with stronger acid sites give lower selectivity because of the further reactions of acrolein. Cs salt of heteropolyacid (Cs2.5H0.5PW12O40, Cs2.5PW) is an active catalyst for the dehydration of glycerol [219]. The initial glycerol conversion amounted to 100% with 98% acrolein selectivity at 548 K, but the conversion decreased significantly with time on stream (40% after 6 h) due to the coking. Doping Cs2.5PW with Pd together with cofeeding hydrogen improves catalytic stability to deactivation. The catalyst with 0.5%Pd/Cs2.5PW gave 96% acrolein selectivity at 79% glycerol conversion at 5 h time on stream [219].

6.5.2  Dehydration of 1,2-Propanediol

Propanal is an important chemical intermediate used extensively in the manufacture of rubbers, plastics and pesticides. Propanal can be obtained by dehydration of 1,2-propanediol, which is produced by hydrogenolysis of glycerol. OH



OH

–H2O

O



(6.50)

Zeolites are highly active for the dehydration of 1,3-propanediol to propanal with high selectivity [221]. Zeolites with non-

Dehydration

intersecting, unidirectional channels gave the highest selectivity to propanal. ZSM-23 and theta-1 gave complete conversion and over 93% selectivity to propanal at 573 K. Heteropolyacid (H4SiW12O40) supported on silica shows a high activity for the dehydration [222]. Presence of water improves the activity and selectivity. At 473 K, 100% conversion was attained with propanal selectivity higher than 93%.

6.5.3  Dehydration of Fructose and Glucose to 5-Hydroxymethylfurfural

5-Hydroxymethylfurfural (HMF) is a desirable intermediate for plastics, polymers and fuels and can be produced by elimination of three water molecules from fructose [223, 224]. CH2OH

O HO H H



OH

OH OH CH2OH Fructose

OH

O

O

HMF



(6.51)

The dehydration of fructose to HMF can be performed with use of homogeneous and heterogeneous acid catalysts [223–225]. The reaction scheme of fructose conversion over acid catalysts is shown in Fig. 6.9 [225]. Franose form of fructose is dehydrated to HMF, which is further rehydrated to levulinic acid and formic acid. Humins and polymers are also formed as by-products.

Figure 6.9

Possible reaction products of the acid-catalyzed dehydration of fructose and the side reactions.

439

440

Synthesis of Organic Chemicals through Solid Acid Catalysis

NMR study using 13C labeled d-fructose revealed that the first carbon (C-1) or sixth carbon (C-6) of fructose maps onto the corresponding carbon of HMF and that the C-1 and C-6 carbon of HMF are transformed to the carbon of formic acid and methyl carbon (C-5) of levulinic acid, respectively [226].

HO OH –3H2O O 2 6 5 1 HO OH 3 4 OH



HO 6 5 O 2 4

3

O

1

O

+2H2O H

OH

+

5

4

3

O

2

1 OH

O

(6.52)

Selection of the solvent has a profound effect on the performance of the catalysts. Water is in general an environmentally benign solvent, but facilitates the rehydration of HMF toward levulinic acid and formic acid, leading to low selectivity for HMF. Dimethylsulfoxide (DMSO) prevents formation of levulinic acid and humins and gives high activity and selectivity for HMF, but the high boiling point solvent have to be separated from the product and water formed. Two-phase systems such as water-methyl isobutyl ketone (MIBK) are often used [227]. In the biphasic systems, the reaction proceeds in an aqueous phase and the product, HMF, is transferred to the organic phase. This avoids the formation of byproducts in the aqueous phase. The direct interaction of MIBK with the catalyst surface is also proposed in the reaction over zeolites [228]. MIBK fills the pore of the zeolites and facilitates the desorption of HMF. MIBK is also adsorbed on the external surface of the zeolites. These effects decrease the secondary reactions of HMF and enhance the selectivity for HMF. By removing water produced continuously under mild evacuation, very high yields of HMF (>90%) were obtained in DMSO over various solid acids such as H-beta zeolite, Amberlyst-15, Nafion, sulfated zirconia, and WO3–ZrO2 at 393 K in 2 h [229]. Zr- and Ti-based catalysts with different structures are active for fructose dehydration in water (1.1 g fructose in 18.3 mL of water) [230]. Cubic zirconium pyrophosphate (ZrP2O7) and gtitanium phosphate (Ti(PO4)(H2PO4) . 2H2O) are especially useful. In the presence of g-ZrP2O7, the selectivity for HMF was 99.8%

Dehydration

with fructose conversion of 44.4% after the reaction of 0.5 h at 373 K. Both the reaction rate and selectivity for HMF decreased with reaction time. After 2 h, the selectivity dropped to 81.4% with fructose conversion of 52.8%. Niobium phosphate is a water-tolerant catalyst. In the presence of mesoporous niobium phosphate, HMF was obtained in a 78.2% selectivity with a 57.6% conversion of fructose in 30 min at 403 K in water [231]. Among zeolites, mordenite gave high selectivity for HMF [228, 232]. The selectivity depends on the MIBK/water ratio and fructose conversion. The rate of fructose conversion decreases, but the selectivity for HMF increases with increase in the MIBK/water ratio. In the case of MIBK/water of 3–5, the selectivity for HMF was very high at low conversions, but it decreases at higher conversions. The selectivity was 63% at 85% conversion of fructose [231]. Mesoporous silica doubly modified with thioether groups and sulfo groups was examined for HMF synthesis [233]. The thioether groups promote the isomerization of fructose to the furanose form, whereas sulfo groups catalyze the dehydration of the furanose. The bifunctional mesoporous catalyst achieved a selectivity for HMF of 74% at 66% fructose conversion in MIBK/ 2-butanol at 453 K. Ion exchange resins are effective catalysts for the dehydration under mild conditions [223, 229, 234, 235]. Figure 6.10 shows the time course of HMF formation from fructose in DMF over Amberlyst-15 at 353 K and 373 K [235]. At 373 K, 66% of the fructose was converted into HMF after 15 min. After 1 h, fructose was fully converted with high selectivity (91%) to HMF. At 353 K, HMF was obtained a 77% yield with 88% selectivity after 2 h reaction. When Amberlyst-15 was powdered into the diameter of 0.15–0.053 mm, 100% yield of HMF was obtained in the reaction in DMSO for 2 h at 393 K by continuously removing water during the reaction [229]. The reaction of fructose in the presence of Amberlyst-15 in water gives levulinic acid in good yield. Levulinic acid was obtained in 52% yield with 93% conversion of fructose after the reaction of 24 h at 393 K [236]. A carbon-based solid acid, which is prepared from glucose and p-toluenesulfonic acid, is highly active for fructose dehydration

441

Synthesis of Organic Chemicals through Solid Acid Catalysis

to HMF in DMSO [237]. The yield of HMF was as high as 91.4% (conversion: ~100%) at 403 K after 90 min. The yield was higher than that over Amberlyst-15 (81.4%). 100

373 K

80

Conversion/%

442

353 K

60 40 20 0



0

0.5

1

1.5

Reaction time/h

2

2.5 

Figure 6.10 Time course of HMF formation from fructose over Amberlyst15. Reaction conditions: fructose (0.1 g), Amberlyst-15 (0.1 g), DMF (3 mL), 353 K and 373 K. Fructose conversion (353 K () and 373 K ()). HMF yield (353 K () and 373 K ()). Reprinted with permission from M. Ohara, A. Takagaki, S. Nishimura, K. Ebitani, Appl. Catal. A, 383, 149 (2010).

The reaction of glucose over Amberlyst-15 does not produce HMF at 373 K. Glucose can be isomerized to fructose in the presence of a base catalyst. Thus, 41% yield of HMF was produced from glucose by a simple one-pot synthesis using both Amberlyst-15 (solid acid) and Mg-Al hydrotalcite (solid base) in DMF at 373 K [235]. The reaction of glucose over sulfated ZrO2–Al2O3 (Zn/Al = 1) in DMSO gave 47.6% yield of HMF within 4 h at 403 K [238]. It has been suggested that the catalyst with higher acidity and moderate basicity is more favorable for the formation of HMF. Sn-W oxide (Sn/W = 2) catalyzes the dehydration of fructose [239]. A 76% yield of HMF was obtained at 100% conversion of fructose in DMSO at 353 K in 12 h. The catalyst could also catalyze the reaction of glucose. The reaction of glucose at 393 K for 18 h gave a 48% yield of HMF.

Dehydration

6.5.4  Dehydration of Xylose to Furfural Furfural is obtained by acid-catalyzed dehydration of xylose, which in turn is obtained by the acid hydrolysis of pyranose fractions of biomass. Industrially, water is used as the solvent and the common catalyst is sulfuric acid. The use of water-tolerant solid acids instead of H2SO4 is highly attractive. A variety of solid acids are tested for this transformation as listed in Table 6.13. HO



OH

OH

HO

Table 6.13

O

O d-xylose

CHO



(6.53)

Dehydration of xylose to furfural over solid acid catalysts

Catalyst

Temperature/ Furfural Solvent time Conversion yield Reference

MCM-22

Water

443 K/48 h

97%

52%

[240]

Delaminated zeolite Nu-6

Water– toluene

443 K/4 h

~80%

47%

[241]

443 K/4 h 413 K/24 h

92% 91%

SAPO-11

Water– toluene Water– toluene

453 K/4 h 443 K/16 h

Water– toluene

SO3H-MCM-41 DMSO

SO3H– mesoporous shell silica beads [Al]-TUD-1

Beta/TUD-1

443 K/16 h

Water

Water

Water– toluene

Water– toluene

92%

70%

96% 100%

51% 60%

443 K/4 h 413 K/24 h

85% 91%

66% 76%

463 K

64%

413 K/24 h

443 K/6 h

443 K/8 h

70% 75%

[242]

[243]

27%

14%

43%

[244]

91%

60%

[245]

98%

78%

[246]

(Continued)

443

444

Synthesis of Organic Chemicals through Solid Acid Catalysis

Table 6.13 Catalyst Sulfated Al-modified zirconia

(Continued)

Temperature/ Furfural Solvent time Conversion yield Reference Water– toluene

433 K/4 h

50%

22.5%

[249]

Sulfated tin oxide

Water– toluene

373 K/ 48 h

56.2%

30.5%

[248]

H3PW12O40/ mesoporous silica

Water– toluene

433 K/4 h

82%

48%

[250]

Niobium silicate

Water– toluene DMSO

Exfoliated Water– HTiNbO5/MgO toluene

433 K/6 h

413 K/4 h

433 K/ 4 h

90%

53%

92%

50%

50%

55%

[249]

[251]

Conversion of xylose in water in the presence of ZSM-5 zeolite was examined [252]. Figure 6.11 shows the concentration profile of reaction products at 373 K. Lyxose as well as furfural was the primary product from xylose. Solid product increased with reaction time and was ascribed to the polymeric materials formed from furfural. Formic acid was also formed. From the product profiles at different temperatures, the reaction scheme shown in Fig. 6.12 was proposed [253].

Figure 6.11 Concentration profiles in xylose conversion over ZSM-5 at 473 K (Xylose concentration: 10 wt% in water). Reprinted with permission from R. O’Neill, M. N. Ahmad, A. L. Vanoye, F. Aiouache, Ind. Eng. Chem. Res., 48, 4300 (2009).

Dehydration

To minimize by-products formation, water-organic solvent (toluene) is often used for xylose dehydration. The reaction of xylose takes place in the aqueous phase (where it dissolves completely) and the product furfural transfers to the organic phase. In the reaction over MCM-22 at 443 K, the yield of furfural was 52–54% at 97% conversion in water, whereas the yield was 70% at 92% conversion in water–toluene (3:2 mass ratio) [238]. Over MCM-41 tethered with SO3H, the yield of furfural was 14% at 27% conversion in water, whereas it was 66% at 83% conversion in water–toluene solvent [243]. OH

HO

OH

HO O d-xylose

O

CHO

Degradation products Oligomerization products

O

H C HO

C

H

HO

C

H

H

C

OH

CH2OH Lyxose Figure 6.12 Reaction scheme for xylose transformation into furfural.

Sulfated zirconia, persulfated zirconia, mesoporous sulfated zirconia, and (per) sulfated zirconia supported on mesoporous silica were examined for the dehydration of xylose in a water– toluene solvent at 433 K [247]. Furfural yields of up to 50% could be achieved at >90% conversion in 4 h with modified zirconia catalysts. Dehydration of xylose with sulfated tin oxide was also reported [248]. Silicoaluminophosphate molecular sieves have been tested for xylose conversion in water–toluene at 443 K [242]. Furfural

445

446

Synthesis of Organic Chemicals through Solid Acid Catalysis

yields at 4 h using SAPO-11 were 34–38%. Complete xylose conversion was reached within 16–24 h, with furfural yields of up to 65%. The selectivity for furfural increases with xylose conversion, indicating that furfural is not an immediate product from xylose and formed via an intermediate(s) species. A delaminated zeolite obtained by the swelling and ultrasonication of a layered precursor of Nu-6(2) has a high surface area of 151 m2g–1 and is active for dehydration of d-xylose [241]. The furfural yield after 4 h reaction was 47% with ~90% xylose conversion at 443 K. The catalyst could be recycled several times without loss of performance or Al leaching. Al-containing TUD-1 is active for the conversion of xylose into furfural [245]. The reaction of xylose gave 87%–96% conversion with furfural yield of 56–60% in water–toluene solvent at 333 K for 6 h. Zeolite beta nanocrystals embedded in a silicious mesoporous matrix TUD-1 (Beta/TUD-1) is effective for the production of furfural [246]. The furfural yield of 74% with high xylose conversion (ca. 98%) was obtained by the reaction of xylose in the presence of beta-TUD-1 in a water–toluene solvent at 443 K for 8 h. The selectivity for furfural was much lower when beta-zeolite was used as the catalyst. When the reaction of xylose was carried out in the presence of BEA/TUD with only water as the solvent, a conversion of 81% was obtained in 6 h, but the furfural yield was poor (25%). In the biphasic system, the reaction of xylose (insoluble in toluene) takes place in the aqueous phase, and the in situ extraction of furfural from the aqueous phase into the organic phase may enhance furfural yields by avoiding its decomposition through consecutive reactions in aqueous phase. The catalyst was regenerable by thermal treatment of the used catalyst in air. Microporous crystalline niobium silicate gave furfural yield of 46% at 85% xylose conversion in water–toluene at 433 K after 6 h [249]. Comparable yields could be obtained within 1 h at 453 K. Exfoliation of a layered oxide, HTiNbO5, gives an active catalyst for the dehydration of d-xylose [251]. After 4 h reaction xylose conversion of up to 92% and furfural yield of up to 55% were achieved at 433 K.

Dehydration

6.5.5  Dehydration of Ethanolamine to Ethyleneimine Ethyleneimine (aziridine) is an important intermediate compound to produce pharmaceuticals, various other amines and amine type functional polymers for coating papers and textiles, etc. Ethyleneimine has been produced by dehydration of monoethanolamine in liquid phase by sulfonation with sulfuric acid followed by ring closure with NaOH according to the Wenker process (Fig. 6.13). The liquid-phase process, however, has some disadvantages such as low productivity caused by the batch reactor system and formation of large amounts of sodium sulfate as by-product. OH

H2N MEA

H2SO4

H2N

OSO3H

(Liquid phase)

NaOH

N H

EI

Solid catalyst (Vapor phase) Figure 6.13 Synthesis of ethyleneimine (EI) from monoethanolamine (MEA). Reprinted with permission from K. Tanabe, W. F. Hoelderich, Appl. Catal. A, 181, 399 (1999).

Vapor-phase dehydration of ethanolamine was tried over various types of solid acid catalysts, but not successful until the late 1980s. Ethyleneimine produced was strongly adsorbed on the catalyst surfaces and underwent polymerization, which caused catalyst deactivation in a short period. In addition, because ethanolamine is very reactive, various side-reactions occurred in the presence of acid sites and/or base sites other than the dehydration. Acid sites promoted dehydrodimerization to piperazines and decomposition to ethylamine and acetonitrile, whereas base sites promoted deammoniation to acetoaldehyde. Suppression of these side-reactions was a key point in preparation of an effective catalyst for the selective dehydration. Nippon Shokubai Co. succeeded in preparation of an effective catalyst for the selective formation of ethyleneimine [253–256]. The process has been commercially operated since 1990. The

447

448

Synthesis of Organic Chemicals through Solid Acid Catalysis

strategy was to prepare the catalyst, which has both acidic and basic sites on the surface, but their strengths are weak. SiO2, P2O5, Nb2O5, and V2O5 were selected as acidic compounds, and alkaline earth metal oxides and alkali metal oxides were selected as basic compounds. Out of various combinations, Si–Ba–Cs–P–O (atomic ratio, 1/0.1/0.1/0.1/2.4) was found to show an efficient catalytic performance. The acid sites and base sites of the catalyst were so weak that acidity and basicity could not be detected by TPD of NH3 and CO2, respectively. The catalyst showed a high conversion of ethanolamine (86%) and a high selectivity to ethyleneimine (81%) with a flow type reactor at 673 K. The dehydration proceeds by acid–base bifunctional catalysis as shown in Fig. 6.14. The OH groups of ethanolamine dissociate by the cooperative action of metal cation (Lewis acid) and the adjacent O2– (base site) followed by nucleophilic attack of N atom to b-C atom to complete the formation of ethyleneimine.

Figure 6.14 Mechanism for intramolecular dehydration of ethanolamine. Reprinted with permission from M. Ueshima, H. Yano, H. Hattori, Sekiyu Gakkaisi, 35, 362 (1992).

The idea that the catalyst possessing both weak acid sites and weak base sites functions as an efficient dehydration catalyst by acid–base bifunctional catalysis was applied by Nippon

Dehydration

Shokubai in preparation of catalysts for the other two industrial processes: the productions of vinyl ether from glycol ether (reaction 6.54) [257] and N-vinyl-2-pyrrolidone from N-(2hydroxylethyl)-2-pyrrolidone (reaction 6.55) [258].



ROCH2CH2OH    ROCH=CH2 + H2O

(6.54)

O

O

N

HO

N

+ H2O

(6.55)



(6.56)

For the dehydration of 2-ethoxyethanol to ethyl vinyl ether, the SiO2 modified with Cs2O in the Cs/Si atomic ratio of 0.03 showed a maximum conversion of 95.5% with a selectivity of 80% at a reaction temperature of 693 K. A higher selectivity of 84.2% was obtained with the catalyst modified with Cs2O in the Cs/Si atomic ratio of 0.005. High activities and selectivities were observed for the other 2-alkoxyethanols such as n-propoxy-, isopropoxy-, t-butoxy-, and phenoxyethanol. The high activity and selectivity were attributed to the existence of the acid and base pair sites whose acid and base strengths were lower than H0 = +6.8 and H– = +9.4, respectively. For the dehydration of N-(2-hydroxylethyl)-2-pyrrolidone to N-vinyl-2-pyrrolidone, the SiO2’s modified with alkali metal oxides exhibited pronounced catalytic performance in the temperature range 633–673 K. Similar to the case of the dehydration of 2-ethoxyethanol, modification with a small amount of alkali metal oxides generated weak acid and weak base pair sites on SiO2 surface, which acted as active sites for the reaction by acid–base bifunctional catalysis. The process of the production of N-vinyl-2-pyrrolidone by use of the alkali metal oxide-modified SiO2 is a replacement of the wellknown liquid-phase commercial process using strong alkali and explosive reactant acetylene. O



O

NH + C2H2 2-pyrrolidone acetylene

N N-vinyl-2-pyrrolidone

449

450

Synthesis of Organic Chemicals through Solid Acid Catalysis

6.6  Conversion of Trioses into Lactates A triose is a monosaccharide or simple sugar containing three carbon atoms. There are only three possible trioses, l-glyceraldehyde (LGLA), d-glyceraldehyde (D-GLA), and dihydroxyacetone (DHA). The trioses can be obtained catalytically from glycol, which is a main by-product of the transesterification of triglycerides used to produce bio-diesel. The trioses can be converted into lactic acid or alkyl lactate in the presence of a suitable solid acid. The reaction pathway from trioses to lactates is shown in Fig. 6.15 from various evidences described below.

Figure 6.15 Reaction pathways of DHA/GLA to lactate/lactic acid (path A) and to hydrated/methylated pyruvaldehyde (path B) in methanol (R=CH3) and water (R=H). Reprinted with permission from R. M. West, M. S. Holm, S. Saravanamurugan, J. Xiong, Z. Beversdorf, E. Taarning, C. H. Chritensen, J. Catal., 269, 122 (2010).

The conversion of DHA and GLA to alkyl lactate in alcohol solvents using solid acids, namely a series of zeolites, was reported [259, 260]. Ultrastable Y (USY) exhibited the highest activity [259, 260]. Figures 6.16a and 6.16b show the reaction profiles for DHA and GLA, respectively, in methanol in the presence of USY (Si/Al = 6) [259]. Both GLA and DHA were observed when either substrate was used as a starting reagent, indicating that isomerization between these two isomers occurs and pyruvaldehyde (PA) is formed as an intermediary product. DHA reacted more quickly than GLA. In the reaction of DHA with methanol at 388 K, the yield of methyl lactate was 96% with 99% DHA conversion in 24 h, whereas in the reaction of GLA with methanol at 398 K, the yield of methyl lactate was 98% after 48 h.

Conversion of Trioses into Lactates 100

(a)

80

Carbon Balance (%)

Carbon Balance (%)

100

60 40 20 0

0

2

4

6 8 Time (Hours)

24 25

(b)

80 60 40 20 0

0

2

4 6 8 Time (Hours)

24 36 48 

Figure 6.16 Concentration profile for DHA and GLA in methanol. Other includes pyruvaldehyde (PA), dimethylacetal of pyruvic acid (PADA). (a) 1.25 mmol DHA in 4.0 g methanol at 388 K with 80 mg USY; (b) 1.25 mmol of GLA in 4.0 g methanol at 388 K with 80 mg USY. Reprinted with permission from R. M. West, M. S. Holm, S. Saravanamurugan, J. Xiong, Z. Beversdorf, E. Taarning, C. H. Chritensen, J. Catal., 269, 122 (2010).

When water was used as the solvent, lactic acid was obtained as  a main product. The reaction of DHA in water gave lactic acid in a 71% yield with >99% DHA conversion at 388 K for 24 h. In the conversion of DHA in ethanol, USY zeolite with Si/Al = 2.6 gave a 91% yield of ethyl lactate in 3 h at 393 K, the by-product being diethylacetal of pyruvic anhydride (9%). Glyceraldehyde was also converted into ethyl lactate in 76% yield at 383 K in 4 h. Following the correlation of the types and amounts of acid sites in the different zeolites, it is proposed that Brønsted acid sites catalyze the dehydration of trioses to the reaction intermediates PA, whereas Lewis acid sites further assist in the intermolecular rearrangement of the aldehyde into the desired lactate ester. The strong acid sites catalyze the intermediate pyruvic aldehyde into alkyl acetals instead of alkyl lactate [260]. The reaction of PA in CD3OD shows no incorporation of deuterium in the hydrocarbon backbone [260]. The interaction of carbonyl oxygen atom with Lewis acid center is suggested. OD

O

O



LA

+ CD3OD H

451

O H

OCD3

(6.57)

452

Synthesis of Organic Chemicals through Solid Acid Catalysis

Sn-incorporated MCM-41 shows very high catalytic activity and selectivity for the conversion of DHA [261]. The yield of ethyl lactate was 98% with 2% yield of diethyl acetal of PA in 6 h at 363 K in ethanol. The excellent catalytic performance of Sn-MCM-41 was ascribed to a combination of strong Lewis acidity and mild Brønsted acidity of the material. Titanosilicate beads catalyze the reaction of DHA in ethanol to give 47.5% yield of ethyl lactate with 97.0% selectivity [262].

6.7  Hydration of Alkenes

Hydration of alkenes to form alcohols is a reverse reaction of the dehydration of alcohols, which proceeds by a carbenium ion mechanism.



R–CH=CH–R¢ + H+

+ R–CH–CH2–R¢ + H2O 

O+ H

H

(6.58)

R–CH–CH2–R¢



O+

 

H R–CH–CH2–R¢



+ R–CH–CH2–R¢



H

R–CH–CH2–R¢ + H+ OH



(6.59)



(6.60)

Brønsted acids act as a catalyst for the hydration of alcohols. Homogeneous acids such as sulfuric acid and aqueous heteropolyacids are used for hydration of most of alkenes. Solid acid catalysts are used for a limited number of hydration reactions in industry. For industrial hydration of ethylene, isobutene, and cyclohexene, solid phosphoric acid, ion exchange resin and ZSM-5 are used, respectively. Difficulty in use of solid acid catalysts for hydration arises from the reaction conditions under which a large amount of water is present in either vapor state or liquid state. The acid sites on the surface are poisoned or weakened by water, and adsorption of the organic reactants in water on the acid sites is difficult.

Hydration of Alkenes

Industrial process of Hydration of cyclohexene to cyclohexanol [263, 264] Hydration of cyclohexene to cyclohexanol is one of the rare cases where solid acid catalysts are used in the industrial processes. Cyclohexanol is an important intermediate raw material leading to 6,6-nylon through adipic acid and to 6-nylon through e-caprolactam. Cyclohexanol is produced in industry by three different routes: oxidation of cyclohexane, partial hydrogenation of phenol and hydration of cyclohexene. The hydration of cyclohexene was developed to replace the cyclohexane oxidation process, which produces a large quantity of by-products. Homogeneous acids are known to catalyze cyclohexene hydration, but have never been used in industry. Cyclohexene hydration process was developed by Asahi-Kasei Chemicals in 1990. The company also developed a process for partial hydrogenation of benzene to cyclohexene. By combining the two processes, the company established a novel route to cyclohexanol from benzene. The hydration process uses ZSM-5 with a Si/Al ratio more than 20. The reaction is carried out in a liquid phase in which a mixture of water, cyclohexene and catalyst is stirred to make an emulsion state as illustrated in Fig. 6.17. During the reaction cyclohexene and water make separate phases; the catalyst is in the water phase as a slurry state. The reaction starts when the reaction mixture is stirred in a temperature range 373–403 K. The reaction mixture is transferred to a settler where the mixture separates into two phases, the upper phase containing unreacted cyclohexene and produced cyclohexanol, and the bottom phase containing the catalyst suspended in water. Cyclohexanol is obtained by simple distillation of the upper phase; cyclohexanol remained undistilled, and cyclohexene is distillated to feedback to the reaction mixture. Although the conversion is 10–15% due to the equilibrium limitation, the selectivity to cyclohexanol exceeds 99%. The point in designing an efficient catalyst working in the slurry state is selection of the hydrophobic catalyst. Among zeolites examined, the ZSM-5 and ZSM-11 with a high Si/Al ratio were found to work in the reaction conditions. Zeolites with a low Si/Al ratio did not work well. To be sufficiently hydrophobic, the Si/Al ratio higher than 20 is required.

453

454

Synthesis of Organic Chemicals through Solid Acid Catalysis



Figure 6.17 Hydration of cyclohexene in liquid-phase reactor using ZSM-5.

The hydrophobicity of H-ZSM-5 and mordenite was evaluated by adsorption of cyclohexanol in water [264]. The zeolites with a high Si/Al ratio could adsorb a larger amount of cyclohexanol than the zeolites with a low Si/Al ratio, indicating that the hydrophobicity increases with the Si/Al ratio of the zeolites. Cyclohexene can be adsorbed more easily on the hydrophobic zeolites in the presence of large amount of water. In addition to the hydrophobic properties, the pore size of zeolites is important factor for an efficient catalyst. ZSM-5 and ZSM-11 with a high Si/Al ratio give a high selectivity to cyclohexanol. This is due to the shape selective properties of the zeolites with 10-membered ring. Zeolites with 12-membered ring such as mordenite and ZSM-12 produce dicyclohexyl ether resulting from successive reaction of cyclohexanol with cyclohexene to a considerable extent. In the industrial process, H-ZSM-5 is adopted, because ZSM-5 is much easily prepared than ZSM-11. The industrial process was completed by establishing a proper method for regeneration of the spent catalyst. Coke formation in the catalyst pores is an inevitable issue when unsaturated hydrocarbons are allowed to react over acidic catalysts. Another cause to bring about activity decay for the catalyst is dealumination during the reaction. Dealumination occurs even at a temperature of 373–403 K in the presence of water. For regeneration, coke has to be removed. Furthermore, the catalyst must be realuminated; Al dislodged should be returned to the original framework positions.

Isomerization/Rearrangement

These problems were solved by developing unique regeneration procedures. For removal of coke, the successful method is treatment of the spent catalyst with hydrogen peroxide in wet state. Normal regeneration method of burning coke at a high temperature is not applicable for the wet spent catalyst, because much heat is required to dry the slurry catalyst and further dealumination occurs during heat treatment in a high partial pressure of water vapor. For returning the dislodged Al to the original framework position, the successful method was found to be the repeated treatment with aqueous sodium hydroxide followed by treatment with dilute nitric acid. Adopting these methods for regeneration of the spent catalyst, the process was completed.

6.8  Isomerization/Rearrangement

6.8.1  Isomerization of a-Pinene to Camphene Isomerization of a-pinene gives camphene, which is an important intermediate for the synthesis of borneol and camphor. The isomerization is catalyzed by acid catalysts. Besides camphene, different isomers are also formed.

+ Camphene

H+ a-Pinene

Tricyclene

+

+

Pinene cation Limonene



+ b-Pinene

+ Terpinolene

a-Terpinene

p-Terpinene

(6.61)

Ferrierite (Si/Al = 8.9) is active for the isomerization of a-pinene [265]. The conversion of a-pinene reached 70% in 3 h, the selectivities for camphene and limonene being 52 and 35%, respectively. The proposed mechanism of a-pinene isomerization is shown in Fig. 6.18. The selectivity for camphene decreased and the selectivity for limonene increased by the extent of dealumination. The selectivity toward camphene and limonene was, however, close to 85% for all the dealuminated samples.

455

456

Synthesis of Organic Chemicals through Solid Acid Catalysis

+H+ +H+ a-pinene

+

pinanyl cation

camphene products

+

p-menthenyl cation –H+

–H+

+

terpinolene

tertiary p-menthenyl cation

–H+

a-terpinene

–H+

isoterpinolene

limonene

g-terpinene

p-cymene

Figure 6.18 Mechanism of a-pinene isomerization. From R. Rachwalik et al., J. Catal., 252, 161 (2007).

WO3–Al2O3 (20 wt% WO3) prepared by a sol-gel method gave 73% a-pinene conversion with 55% camphene selectivity by the reaction of a-pinene for 2 h at 423 K [266]. Heteropolyacid (H3PW12O40, HPW) is an efficient catalyst for the isomerization in liquid phase [267]. The reaction of a-pinene in the presence of HPW supported on silica (20 wt%) for 60 min at 373 K gave 90% conversion with 50% selectivity for camphene and 28% selectivity for limonene as main by-products. Minor byproducts were tricyclane, a-terpinene, and terpinolene. The isomerization in gas-phase proceeds over a range of bulk and supported heteropoly compounds to yield camphene as the main product with other monoterpenes such as limonene, terpiolenes, terpinenes, b-pinene, p-cymene, etc. [268]. Bulk HPW and Cs2.5H0.5PW12 O40 possessing very strong Brønsted acid sites suffer from deactivation. Conversely, the catalysts prepared by

Isomerization/Rearrangement

supporting HPW on Nb2O5, ZrO2, and TiO2, although weaker acids, exhibit more stable as well as more selective performance. The catalysts comprising HPW supported on TiO2 gave a camphene yield of 51% at 473 K after 15 h on stream.

6.8.2  Bamberger Rearrangement

Bamberger rearrangement of phenylhydroxylamine (PHA) to paminophenol (PAP) was investigated with water as solvent over solid acid catalysts [269].

H N



NH2

OH



HO

(6.62)

The schematic diagram of the Bamberger rearrangement is shown in Fig. 6.19. The solid acid catalysts studied include beta zeolite, K-10 clay, sufonated silica, and sulfated zirconia. Sulfated zirconia, calcined at 923 K, gave the best selectivity for PAP. By the reaction of PHA in the presence of sulfated zirconia at 353 K for 4 h, PHA was fully converted with a PAP yield of 92%, the main by-product being aniline (8%). Under the same reaction conditions, 3-hydroxyphenylhydroxylamine and 3-aminophenylhydroxylamine gave the quantitative yields of the corresponding aminophenols. + NH2OH

NHOH

R H+ R H+

+ NH

NH2

NH

+ NHOH2

H2O –H+ R –H2O

R

+

R

R OH

Figure 6.19 Schematic diagram of Bamberger rearrangement. From J. Ratnam et al., Appl. Catal. A, 348, 26 (2008).

457

458

Synthesis of Organic Chemicals through Solid Acid Catalysis

6.8.3  Beckmann Rearrangement: Production of e-Caprolactam e-Caprolactam is a raw material for 6-nylon production through ring opening by hydrolysis, condensation, and polymerization. The traditional process for e-caprolactam production is shown in Fig. 6.20. Cyclohexanone oxime is produced from benzene or phenol by different routes. Cyclohexanone oxime is then converted into e-caprolactam by the Beckmann rearrangement. Cyclohexanone oxime is first treated with fuming sulfuric acid (oleum) to form the sulfate of caprolactam, which is neutralized by addition of NH3 to isolate e-caprolactam. (NH4)2SO4 is produced as a by product accordingly. The (NH4)2SO4 production counts to 1.7 times by weight as much as e-caprolactam production.

NOH

fuming sulfuric acid

NH 1/2 H2SO4 OH

NH3

NH

+  1/2 (NH4)2SO4

O



(6.63)

Photonitrosation +NOCI

+H2

Oxidation +O2 OH

+H2

+H2O

O

NOH

O Oximation +NH2OH

Beckmann rearrangement NH

+ H2SO4 + NH3 (Neutralization)

–H2

Figure 6.20 Process flow of traditional process for e-caprolactam production.

Solid acid catalysts for the Beckmann rearrangement of cyclohexanone oxime was extensively examined [270]. No efficient catalysts that could be used in the industrial process had been found

Isomerization/Rearrangement

until 1986. Many of the solid acid catalysts showed the activity for the Beckmann rearrangement, but did not have enough selectivity for e-caprolactam. Furthermore, they deactivated very quickly. Sato et al. disclosed a high-silica ZSM-5 (Si/Al > 1000) showed a high selectivity for e-caprolactam and the activity persisted for a long time. The selectivity increased further by an increase the Si/Al higher than 100,000. With this catalyst, 85% selectivity was obtained at 100% conversion [271]. Addition of methanol to the reaction system further increased the selectivity up to 95% [272, 273]. The active sites of the catalyst were examined. Because the Si/Al is higher than 100,000, the catalyst practically contains no Al in zeolite framework. TPD of NH3 showed no desorption profile for NH3, indicating the absence of strong acid sites on the catalyst. Heitmann et al. studied the silanol groups on high silica MFI zeolites after treatment with acid or base to different degree to form terminal, geminal, vicinal, and nest silanol groups in different ratios [274]. It was proposed that nest silanol groups (Fig. 6.21) act as active sites for the Beckmann rearrangement. The acidity of the nest silanol is weak so that no desorption peaks appeared in TPD of NH3 [272, 274]. The presence of the terminal silanol groups causes side reactions.

Si

Si

OH

O H H O

O

Si

HO

Si

O

O Figure 6.21 Silanol nest in MFI framework.

In the presence of methanol, the terminal silanol groups of the catalyst convert to methoxy groups.

459

460

Synthesis of Organic Chemicals through Solid Acid Catalysis

OCH3

OH

Si O

O

O

Si

+  CH3OH

O

O

O

+  H2O (6.64)

The flow diagram of the industrial process for the Beckmann rearrangement is shown in Fig. 6.22, in which the reaction takes place in the presence of methanol. Methanol is recovered at the end of the reactor and recycled.

Figure 6.22 Flow diagram for Sumitomo’s Beckmann rearrangement process.

In 1995, a new production route for cyclohexanone oxime by ammoximation was disclosed. In this process, a mixture containing NH3, H2O2, and cyclohexanone is allowed to react over TS-1 (titanosilicate with MFI structure). NH3 reacts with H2O2 to form hydroxylamine, which reacts with cyclohexanone without catalyst to form cyclohexanone oxime. NOH



NH3

TS– 1 + H2O2

NH2OH O



(6.65)

Sumitomo Chemicals Co. combined this ammoximation process with the Beckmann rearrangement process to construct a new process for e-caprolactam production from cyclohexanone (Fig. 6.23) [275]. The by-product from the new process is only H2O. This is environmentally benign process. The ammoximation step is arried carried at 353 K; cyclohexanone conversion is 99.9%, with cyclohexanone oxime selectivity of 98.2% (based on cyclohexanone), cyclohexanone oxime yield being 93.2% (based on H2O2). In the Beckmann rearrangement step carried out at 623 K, cyclohexanone oxime conversion is 99% and e-caprolactam selectivity is 95%. A fluidized bed reactor is employed for the

Acetalization

Beckmann rearrangement step because coke is accumulated on the catalyst during the reaction. The catalyst is regenerated continuously. The commercial operation started in 2003. NOH

O

+  NH3 + H2O2

TS–1

+  2H2O NH MFI zeolite

O

Figure 6.23 Production of e-caprolactam through ammoximation and Beckmann rearrangement.

6.9  Acetalization

The protection of functional groups such as aldehydes or ketones is one of the most important organic synthetic strategies for the production of multifunctionalized molecules. Acetalization offers a means of protection during manipulation of multifunctional organic molecules. Furthermore, acetals are molecules with industrial applications often involved in cosmetic formulations as fragrances, cosmetics, and polymer production. Acetal synthesis is the traditional form of protecting carbonyl compounds using alcohols, glycols or their S-substituted analogs by acids such as p-toluenesulfonic acid. O



C

CH3OH

OCH3

H3CO

C



(6.66)

Solid acids such as zeolites and mesoporous materials are applied for the acetalization. Mesoporous sulfated zirconia (202 m2 g–1) shows high catalytic activity toward protection of carbonyl compounds through acetal/ketal formation [276]. The results of acetal formation from different carbonyl compounds with methanol are shown in Table 6.14. For example, 97% conversion with 100%

461

462

Synthesis of Organic Chemicals through Solid Acid Catalysis

selectivity was obtained in the acetalization of cyclohexanone in 45 min at room temperature under solvent-free condition. The catalyst is also effective for the acetalization with ethylene glycol. Table 6.14

Substrate

Acetalization of different carbonyl compounds with methanol over mesoporous sulfated zirconia

Cyclohexanone Acetophenone

Time/min

Conversion/%

45

97

45

80

15

Benzophenone

120

4-Hydroxybenzaldehyde

90

4-Hydroxyacetophenone Benzaldehyde

25

2-Hydroxybenzaldehyde

120

4-Nitrobenzaldehyde

45

3-Hydroxybenzaldehyde 4-Methylbenzaldehyde 2-Nitrobenzaldehyde

4-Chlorobenzaldehyde Heptanal

120 60

120 25 20

85 0

97 65 0

71 78 92 90 94 45

Source: Reprinted with permission from A. Sinhamahapatra, N. Sutradhar, H. C. Bajaj, A. B. Panda, Appl. Catal. A, 402, 87 (2011).

Acetalization of heptanal with methanol over the mesoporous material, SBA-1, was reported [277]. At 373 K, the reaction of heptanal with methanol (heptanal:methanol = 1:3) for 8 h gave a 92% conversion of heptanal over SBA-1 (Si/Al = 40). The selectivity for acetal (1,1-dimethoxyheptane) was 86%, the by-products being hemiacetal (8%) and vinyl ether (6%). The proposed reaction pathway is shown in Fig. 6.24. Acetalization of aldehydes (heptanal, 2-phenylpropanal, and diphenylacetaldehyde, 3 mmol) with trimethyl orthoformate (15 mmol) proceeded in the presence of [Al]-MCM-41 to give high acetal yields at reflux temperature. A large-pore zeolite, beta, is active for the acetalization of heptanal and 2-phenylpropanal, but showed low activity for the acetalization of diphenylacetaldehyde [278].

Acetalization

Figure 6.24 Reaction pathway for the formation of hemiacetal, acetal, and vinyl ether from heptanal and methanol over [Al]-SBA15. Reprinted with permission from K. Venkatachalam, M. Palanichamy, V. Murugesan, Catal. Commun., 12, 299 (2010).

Acetalization of heptanal, benzaldehyde and cyclohexanone with ethanol or triethyl orthoformate (triethoxymethane) proceeded in the presence of alumina-grafted SBA-15 [279]. The reaction of heptanal with ethanol for 3 h at 353 K gave the corresponding acetal yield of 97%. Triethyl orthoformate was more effective than ethanol as an acetalization agent. Benzaldehyde and cyclohexanone gave the corresponding acetal (or ketal) in almost quantitative yield using triethyl orthoformate or triethyl orthoformate-ethanol mixture in 30 min. The same catalyst was effective for the protection of hydroxyl groups over the mannitol derivative (1,6-dibenzoyld-mannitol) 13 by the reaction with 2,2-dimethoxypropane. Isomers 14a and 14b were obtained in 52% and 31% isolated yield, respectively, by the reaction for 7 min at 353 K. MCM-41 is active for acetalization of heptanal with methanol [280]. The activity does not depend on the content of Al in the mesoporous silica, but depends on the pore size of the materials. The activity shows a maximum at the pore diameter of about 1.9 nm. Acetalization of pentaerithritol with aldehydes and ketones proceeds in toluene in the presence of an Al-pillard saponite under refluxing conditions. The saponite was intercalated with aluminum polycation, [Al13O4(OH)24(H2O)12]2+. The reaction of

463

464

Synthesis of Organic Chemicals through Solid Acid Catalysis



(6.67)

pentaerithritol with benzaldehyde gave 93% yield of the desired product in 2 h [281]. CHO

2

HO

OH

O

O

HO

OH

O

O

(6.68)

+

Glycerol can be acetalized with acetone to produce 2,2dimethyl-1,3-dioxolane-4-yl methanol with 5-membered ring and 2,2-dimethyl-[1,3]-dioxan-5-ol with 6-membered ring (Eq. 6.68). OH HO

O OH +

O

O + OH

O

O

OH

+ H2O (6.69)

TiO2–SiO2 mixed oxides [282], silica-included heteropolyacids [283] and niobia [284] are effective for this transformation. TiO2– SiO2 (Si/Ti = 1) was most effective when calcined at 823 K. Under the conditions of acetone/glycerol ratio 4 and reflux temperature, the glycerol conversion reached 95% in 3 h with selectivity for the 5-membered ring product of 90% [282].

Acetalization

465

The reaction proceeds over Brønsted acid sites [282, 283]. The proposed scheme for the acetalization in the presence of Brønsted acid sites is given in Fig. 6.25 [282]. The glycerol acetalization leads to formation of the hemiacetal. The dehydration yields a tertiary carbenium ion, which can be stabilized by resonance with the nonbonded electron pairs of the adjacent oxygen atom. Then, a quick nucleophilic attack of the secondary hydroxyl group easily occurs. The formation of the five-membered ring ketal is controlled by kinetics.



Figure 6.25 Mechanistic scheme of glycerol acetalization with acetone over TiO2–SiO2.

Acetalization of glycerol with furfural proceeds over [Al]-MCM41 [285]. The products were obtained as a mixture of (2-furan2-yl)-1,3-dioxolane-4-yl)methanol and 2-(furan-2-yl)-1,3-dioxane5-ol in approximately 7:3 ratio. About 80% yield was observed at a reaction time of 2 h at 373 K. O

O

H

O

OH

+ HO

OH

O

O

O

OH

O O

OH

(6.70)

Acetalization of 1,3-propanediol (1,3-PD) into the acetal could be a convenient way for recovery of 1,3-PD from a dilute aqueous solution (Eq. 6.70). Sulfated TiO2–ZrO2 is effective for the acetalization of 1,3-PD with acetaldehyde to form 2-methyl-1,3-dioxane (2-MD) [286]. HO

O



CH2

+

H3C H

CH2

HO

CH2

H+

O

H3C O

+ H2O (6.71)

466

Synthesis of Organic Chemicals through Solid Acid Catalysis

Acetaldehyde was added dropwise to the vessel containing 1,3-PD and the catalyst at 323 K. The yield of 2-MD reached 96.5% after 3 h over sulfated TiO2–ZrO2 (Ti/Zr = 4). During the reaction, sulfuric acid was leached, but the contribution of leached sulfuric acid to the reaction was small. The catalytic activity decreased with repeated reuse. After five times, the yield dropped to 86.3%. The sulfated TiO2–ZrO2 was also effective for hydrolysis of 2-MD. A 98% conversion of 2-MD was achieved in 18 h over TiO2–ZrO2 (Ti/Zr = 4) at 373 K. Thioacetalization of a variety of carbonyl compounds with 1,2-ethanedithiol efficiently proceeds in water in the presence of silica functionalized with sulfo groups at 353 K [287]. When a mixture of benzaldehyde (2 mmol), 1,2-ethanedithiol (2.1 mmol) and catalyst (300 mg) in H2O (5 mL) was stirred at 353 K for 100 min, 2-phenyl-1,3-dithiolane was produced and isolated in 97% yields after workup. Thioacetalization of ketones proceeds well to produce the corresponding 1,3-dithioacetals, but needs more time to be converted into their thioacetals. Preyssler-type heteropolyacid (H14NaP5W30O110) is a mild and efficient catalyst for protection of a variety of carbonyl compounds with 1,3-propanedithiol [288]. Thioacetals are obtained by the reaction of aldehyde or ketones (2 mmol) and 1,3-propanedithiol with Preyssler acid (0.1 mol) in CHCl3 (10 mL) at room temperature. Thus, benzaldehyde gave the corresponding thioacetal in 96% yield in 5 min, whereas cyclohexanone gave the corresponding thioacetal in 90% yield in 20 min.

6.10  Prins Reaction: Nopol Synthesis

Nopol is generally used in the agrochemical industry to produce pesticides and also in manufacturing household products such as soaps, detergents and polishes. Nopol is obtained by the Prins reaction of b-pinene and paraformaldehyde.

OH

+ (CH2O)n b-pinene

paraformaldehyde

Nopol



(6.72)

Synthesis of Xanthenes

Sulfated zirconia is very effective for the Prins reaction [289]. Sulfated zirconia synthesized by impregnation of Zr(OH)4 with a sulfuric acid solution is highly selective catalyst. The reaction of b-pinene with paraformaldehyde (1:2 molar ratio) in acetonitrile at 353 K for 12 h gave a high b-pinene conversion (>99%) with ~99% selectivity to nopol. Although both Brønsted and Lewis acid sites are active, it is suggested that the reaction over Lewis acid sites is predominant over the sulfated zirconia. Solid Lewis acids such as Sn-containing mesoporous silica were applied for the reaction [290–292]. In the reaction with [Sn]-MCM-41 and [Sn]-SBA-15, the best selectivity values were obtained in nitrile solvent such as acrylonitrile and butyronitrile, but toluene was also useful. The reaction of b-pinene with paraformaldehyde (1:2) in butyronitrile at 373 K for 8 h in the presence of [Sn]-SBA-15 (Si/Sn = 13.5), 98.4% conversion of b-pinene with 94.5% nopol selectivity was obtained [291]. The reaction of b-pinene over [Al]-MCM-41 gave low selectivity for nopol, indicating that Brønsted acid sites are not effective for this transformation [290]. [Zr]-SBA-15 composed of platelets with short mesochannels is highly active and selective for the Prins reaction [293]. The b-pinene conversion of 74% with 100% selectivity was achieved in 6 h at 353 K.

6.11  Synthesis of Xanthenes

Xanthenes and benzoxanthenes are very important class of compounds widely used as dyes in laser technology and in fluorescent materials. Furthermore, these compounds have received great attention because of their wide range of therapeutic and biological properties, such as antibacterial, antiviral, and anti-inflammatory activities. Many synthetic procedures are disclosed; many of them involve expensive reagents, strong acidic conditions, long reaction times, low yields, and use of toxic organic solvents. Therefore, to avoid these limitations, solid acids are explored for more efficient synthetic methods of preparing xanthenes. The synthesis of 14-aryl (or alkyl)-14H-dibenzo[a,j]xanthenes 15 from 2-naphthol and aryl (or alkyl) aldehydes using solid acids as eco-friendly catalysts has been reported.

467

468

Synthesis of Organic Chemicals through Solid Acid Catalysis

R OH

RCHO  + 2

O 15





(6.73)

Solid acids such as ion exchange resin (Dowex-50W), heteropolyacids (supported and non-supported), HClO4/SiO2 and SBA-15 functionalized with COOH groups are found effective for the synthesis of 15, as shown in Table 6.15. The desired products are obtained in high yields at 363–398 K under solvent-free conditions. The mechanism shown in Fig. 6.26 is proposed for the synthesis on HClO4/SiO2 [297]. R

O R

OH H

R

+

+

OH

H+

R

R

OH

OH +

OH

H+

+

R

OH

–H2O O

OH OH

Figure 6.26 Mechanism of synthesis of xanthenes from 2-naphthol and aldehydes. Reprinted with permission from M. A. Bigdeli, M. M. Heravi, G. H. Mahdavinia, J. Mol. Catal. A, 275, 25 (2007).

Xanthenediones (1,8-dioxo-octahydroxanthenes) 17 can be prepared by the condensation of cyclic diones 16 with aromatic aldehydes in the presence of solid acids. O

R

R

16

O

Ar

+ O O

Ar

O

H R

R

O

17

R

R

(6.74)

Synthesis of Xanthenes

Table 6.15

Synthesis of xanthenes from 2-naphthol and aldehydes R

OH RCHO  + 2 O 15 R

Catalyst

C6H5-

Dowex-50W

78

[294]

373

89

[295]

H14[NaP5W30O110]

30

363

98

[296]

30 10

373 393

89 95

[295] [297]

Mesoporous zirconium phosphate

180

413

94

[298]

H6P2W18O62/SiO2

90

60

373

89

[294]

H14[NaP5W30O110]

30

45

373

94

[295]

Dowex-50W

H3PW12 O40/SiO2 HClO4/SiO2

8

373 363

398

90 99

96

[295] [296]

[297]

SBA-15 with COOH

300

298

75

[299]

H6P2W18 O62/SiO2

60

373

84

[295]

4-NO2-C6H4- Dowex-50W

H3PW12 O40/SiO2 HClO4/SiO2 CH3CH2-

373

30

HClO4/SiO2

CH3-

90

H6P2W18 O62/SiO2

H3PW12 O40/SiO2

4-Cl-C6H4-

Time Temperature Yield (min) (K) (%) Reference

120 60 10

SBA-15 with COOH 240

373

373 398

298

84

84 90

82

[294]

[295] [297]

[299]

SBA-15-COOH

300

298

78

[299]

H14[NaP5W30O110]

90

363

97

[294]

HClO4/SiO2

20

398

86

[297]

469

470

Synthesis of Organic Chemicals through Solid Acid Catalysis

The reaction of benzaldehyde with dimedone (5,5-dimethyl1,3-cyclohexanedione, R=CH3) in the presence of H3PW12O40 (20 wt%)/MCM-41 in methanol gave the corresponding product in 94% yield in 5 h at 363 K [301]. Alumina tethered with sulfo groups and polyphosphoric acid/SiO2 are also effective for the synthesis of xanthenediones [301, 302].

6.12  Pechmann Condensation

The Pechmann condensation allows the synthesis of coumarins by reaction of phenols with b-keto esters. Silica gel supported zirconyl chloride octahydrate (ZrOCl2 . 8H2O) is an efficient catalyst for the synthesis of a series of coumarin derivatives under solvent-free conditions [303]. Reaction 6.75 with a series of combinations of R1 and R2, gives the corresponding coumarins.



(6.75)

For example, equimolar quantities of resorcinol (R1 = m-OH) and ethyl acetoacetate (EAA, R2 = CH3) were treated at 363 K for 40 min to give hymecromone (7-hydroxy-4-methylcoumen-2-one, 7-hydroxy-4-methylcoumarine) in 94% yield. Heteropolyacid (H3PW12O40) supported on SnO2 is an efficient catalyst for the reaction of resorcinol with EAA [304]. A 78% yield of hymecromone was achieved at 30 wt% loading of the acid on SnO2 under the reaction conditions of 393 K, reaction time of 2 h and 1:2 (resorcinol::EAA) molar ratio. Mesoporous zirconium phosphate is also effective for the condensation of phenols and EAA [305]. The mesoporous zirconium phosphate has BET surface area of 497 m2g–1 and mono-dispersed pore diameter of ~30 nm. The reaction of resorcinol with EAA gave 94% yield of hymecromone by the reaction at 433 K in 4 h. The

Friedländer Reaction

reaction of phenol with EAA gave 57% yield of 4-methylcoumarine at 433 K in 4 h, whereas the reaction of 3-aminophenol with EAA gave 100% yield of 7-amino-4-methylcoumarine at 373 K in 4 h. The difference of the reactivity among phenols can be ascribed to the electron donating ability of amino group. The reaction of resorcinol with EAA proceeds in the presence of zeolites [306]. Among the zeolites studied (mordenite, ZSM-5, Y and beta), mordenite was most active. Under the reaction conditions of resorcinol:EAA of 1:2, 333 K, and 7 h, the corresponding coumarin was obtained in 62% yield. Under ultrasound irradiation and otherwise same reaction conditions, the reaction rate was enhanced to obtain the coumarin yield of 88%.

6.13  Friedländer Reaction

The Friedländer synthesis is the reaction of 2-aminoarylketones with a-methylene carbonyl compounds to form quinoline derivatives.

R1

O R

R1

O

NH2

+ R2

R3

R3 R

N

R2



(6.76)

Ion exchange resin, Amberlyst-15, was found to be very effective for the Friedländer reaction [307]. Substituted quinolines were prepared from different 2-aminoaryl ketones and various a-methylene compounds under reflux conditions in ethanol. 2-Aminoaryl ketones include both 2-aminoacetophenone and benzophenone derivatives, whereas the a-methylene carbonyl compounds include cycloalkanones, 1,3-diketones (cyclic and acyclic) and b-ketoesters. For example, the reactions of 2aminobenzophenone (R=H, R1=C6H5) and 2-aminoacetophenone (R=H, R1=CH3) with ethyl acetoacetate (R2=CH3, R3=COOC2H5, EAA) for 2.5 h gave the corresponding quinoline derivatives in 89% and 87% yield, respectively. Among zeolitic materials, beta zeolite is most selective [308]. In the reaction of 2-aminoacetophenone (R=H, R1=CH3) with EAA

471

472

Synthesis of Organic Chemicals through Solid Acid Catalysis

in toluene, H-beta zeolite gave 73% yield of the corresponding quinoline with selectivity of 86% at 363 K in 1 h. In the reaction of 2-aminobenzophenone (R=H, R1=C6H5) with EAA in toluene, the yield of the corresponding quinoline was 86% with the selectivity of 90% in 6 h. Magnetic nanoparticles (g-Fe2O3) are a suitable support for clean recoverable catalysts systems. Sulfamic acid (–NHSO3H) heterogenized on hydroxyapatite-encapsulated g-Fe2O3 is an active and selective catalysts for a variety of Friedländer reactions [309]. By the reaction of 2-aminoacetophenone (R=H, R1=CH3) with EAA at room temperature for 3 h under solvent-free conditions, the corresponding quinolone was obtained in a 93% yield. The catalyst was recyclable at least 10 times.

6.14  Synthesis of Amides 6.14.1  Ritter Reaction

The Ritter reaction is the acid-induced nucleophilic addition of a nitrile into a carbenium ion, followed by hydrolysis to the corresponding amide. Any substrate capable of generating a stable carbenium ion is a suitable starting material. For example, acrylonitrile reacts with alcohols in the presence of an acid catalyst. Usually, sulfuric acid is used as the catalyst.

CH2=CH–CN + ROH    CH2=CH–CONH–R

(6.77)

Various solid acids were explored for the Ritter reactions [310]. For the reaction of acrylonitrile with 1-adamantanol and t-butyl alcohol, Cs salt of heteropolyacid (Cs2.5H0.5PW12O40) and ion exchange resins (Nafion, Nafion/silica, Amberlyst-15) exhibited a high catalytic performance. In the reaction of acrylonitrile with t-butyl alcohol (nitrile/alcohol = 5) at 373 K for 8 h, the yield of N-t-butylacrylamide was 80.0% based on the alcohol. On the other hand, for the reaction of acrylonitrile with 2-propanol (nitrile/ alcohol = 5) at 423 K for 24 h, ZSM-5 gave the highest yield (62.2%) of N-isopropylacrylamide. The by-products were acrylamide and diisopropyl ether. The Cs salt of the heteropolyacid and ion exchange resins showed poor activities for this reaction.

Synthesis of Amides

Various zeolites were also tested for the reaction of acrylonitrile with alcohols [311]. For the reaction of acrylonitrile with t-butyl alcohol, Y and beta zeolites showed the highest activity and selectivity at 373 K, whereas for the reaction of the nitrile with 2-propanol and s-butyl alcohol, ZSM-5 zeolite showed the best performance at 433 K. Sulfated tungsten oxide is an efficient catalyst for the Ritter reaction of various combinations of alcohols and nitriles under solvent-free conditions at 373 K. Here, alcohols used are t-butanol, 1-phenylethanol, benzyl alcohol, cyclohexanol and 1-borneol, whereas nitriles used include acetonitrile, acrylonitrile, benzonitrile, malononitrile, benzylcyanide, and 4-cyanopyridine [312]. Magnetic silica can be prepared by coating g-Fe2O3 with silica layers. HClO4 was then supported on the magnetic silica. The resultant catalyst, [g-Fe2O3@SiO2–HClO4] exhibited very high activities for the Ritter reactions of various combination of nitriles (acetonitrile, benzonitrile, and acrylonitrile) with secondary and tertiary alcohols at room temperature [313]. The magnetic property of the catalyst facilitated efficient recovery from the reaction mixture during the work-up procedure by an external magnet. ZnCl2 supported on silica (ZnCl2/SiO2) is also active for the reaction of the Ritter reaction of benzonitrile at 373 K [314]. With this catalyst, benzonitrile was converted to the amides with a variety of t-butyl cation sources such as methyl t-butyl ether, di-t-butyl ether and butyl acetate. The reaction proceeded much faster with t-butyl acetate than with t-butyl alcohol.





(6.78)

Acetonitrile, acrylonitrile, and malononitrile were also converted to the corresponding amides with t-butyl acetate in excellent yields at 373 K. The activity for the reactions is attributed to the Lewis acidity of Zn2+ ions.

473

474

Synthesis of Organic Chemicals through Solid Acid Catalysis

6.14.2  Amidation of Amines with Carboxylic Acids Formamides are valuable intermediates in the construction of various pharmaceutically important compounds. While many methods for N-formylation of amides have been reported, solid acid catalysts are found to offer a very efficient route to amides from formic acid and amines. R1

R1



N R2

O

N

H + HCOOH R2



H

(6.79)

Titanium dioxide (TiO2–P25) and sulfated titanium dioxide are effective catalysts for the N-formylation of a variety of amines with formic acid, as shown in Table 6.16 [315]. The reactions of amine (1 mmol) and formic acid (3 mmol) were carried out in acetonitrile in the presence of the catalyst (0.1 g) at room temperature. Sulfated TiO2 was more effective than TiO2. Figure 6.27 shows the reaction mechanism for the Nformylation. The mechanism involves the complexation of the catalyst with the formic acid, nucleophilic attack of lone pair of electrons present in the amine to carbonyl carbon of formic acid. Then dehydration occurs to give the N-formyl product.

Figure 6.27 Mechanism of N-formylation of aniline with formic acid over TiO2 or sulfated TiO2. Reprinted with permission from B. Krishnakumar, M. Swaminathan, J. Mol. Catal. A, 334, 98 (2011).

Metal oxides such as ZnO, CuO, NiO, CoO, Mn2O3, and Cr2O3 are active for N-formylation of amines with formic acid [316]. For example, the reaction of aniline with formic acid in the presence of NiO gave N-phenylformamide in 99% yield in 10 min.

Synthesis of Amides

Table 6.16

N-Formylation of amines with formic acid over TiO2 and sulfated TiO2

Entry

Substrate

Product

NH2

1

NHCHO

%Yielda with TiO2–P25 (min)

%Yielda with 2 TiO2–S​O​4​ ​ (min)

99.2 (45)

99.2 (30)

2

H3C

NH2 H3C

3

H3C0

NH 2 H3C0

4

CI

NH 2

CI

NHCHO

78.0 (420) 98.3 (360)

5

F

NH 2

F

NHCHO

90.0 (420) 98.5 (360)

NHCHO

6

HOOC

NH 2 HOOC

7

ON

NH 2

NHCHO

NHCHO

ON CH3

NHCH3

N–CHO

9

NH2

NHCHO

11

NH2

NHCHO

H0 CH2 CH2– NH2

H0 CH2 CH2– NHCHO

12

13 14

aYields

N–CHO

NH

H3C

NH NH 2

NH 2

H3C

90.0 (300) 99.0 (240)

NHCHO 85.0 (480) 95.0 (420)

8

10

88.3 (300) 96.4 (240)

N–CHO N

N H

82.3 (480) 92.3 (420) Trace (980) 65.2 (920)

95.0 (60)

98.0 (45)

51.5 (180) 85.0 (120)

45.0 (180) 72.4 (120) Trace (780) Trace (960)

60.0 (720) 40.0 (900)

40.0 (180) 75.0 (120)

with respect to amine, catalyzed by TiO2–P25. with respect to amine, catalyzed by TiO2–S​O​2– 4​  ​. Source: Reprinted with permission from B. Krishnakumar, M. Swaminathan, J. Mol. Catal. A, 334, 98 (2011).

bYields

475

476

Synthesis of Organic Chemicals through Solid Acid Catalysis

Treatment of aqueous formic acid (85%) with amines in the presence of sulfonic acid supported on hydroxyapatiteencapsulated-g-Fe2O3 gives the corresponding formamides in good to excellent yields at room temperature [317]. This magnetically catalytic system can be recovered using an external magnet device. Sulfated tungsten oxide is active for amide synthesis from a variety of combinations of carboxylic acid and amines [318]. The reaction of benzoic acid and benzylamine affords N-benzylbenzamide in 81% yield in 12 h under azeotropic reflux conditions.

O

O

OH + H2N

6.15  Biginelli Reaction

N H



(6.80)

The Biginelli reaction is an acid-catalyzed three-component reaction that creates 3,4-dihydropyrimidine-2(1H)-ones from an aryl aldehyde, a b-ketoester and urea.

(6.81)

The reaction mechanism of the Biginelli reaction in the presence of acid catalysts has been discussed in reference [319]. Sulfated tungsten oxide gives high yields of the desired products by this type of reactions at 353 K under solvent-free conditions as shown in Table 6.17 [320]. Thiourea can also be used to provide the corresponding dihydropyrimidin-2-(1H)-thiones in high yields [320]. Layered a-zirconium sulfophenylphosphonate [321], betazeolite [322], ion exchange resin (Amberlyst-70) [323], mesoporous zirconium phosphate [324] and metal oxides [319] also show high activity for the Biginelli reaction.

Strecker Reaction

Table 6.17

Biginelli reaction (6.81) in the presence of sulfated tungsten oxidea

Sudstrate/product R

R1

X

Yield (Z)

Time (min)

C6H5

–C2H5

O

92

60

C6H5

–CH3

O

92

60

4-CH3–C6H4

–C2H5

4-Cl–C6H4

–CH3

4-OCH3–C6H4

–C2H5

3-NO2–C5H4

–C2H5

3,4-(OCH3)3–C6H4

–C2H5

3,4-CI2–C6H4

–C2H5

2-furyl

–C2H5

C6H5

–C2H5

4-OCH3–C5H4

–C2H5

3-NO2–C6H4

–C2H5

i-C4H4

–C2H5

aReaction

O O O O O O O

90 90 96 90 84 88 86

60 60 30 60

180 80

100

S

90

60

O

62b

65

S S

97 79

30 90

conditions: aromatic aldehyde (10 mmol); b-ketoester (10 mmol); urea or thourea (15 mmol); catalyst (10 wt%); temperature 353 K. b393 K. Source: Reprinted with permission from S. D. Salim, K. Akamanchi, Catal. Commun., 12, 1153 (2011).

6.16  Strecker Reaction a-Aminonitriles serve as efficient precursors for the synthesis of a-aminoacids. The Strecker reaction is one of the most important multicomponent reactions for the direct one-pot synthesis of a-aminonitriles from aldehyde, amine, and trimethylsilylnitrile (TMSCN).

O

R

+ H

R1

R1

H N

R2

+ Me3SiCN

Ga–TUD–I solventless

R

N

R1

CN (6.82)

477

478

Synthesis of Organic Chemicals through Solid Acid Catalysis

Mesoporous silica containing Ga (TUD-1) is an efficient catalyst for the Strecker reaction under the solventless conditions [325]. The reactions give excellent yields and selectivity for the reactions of a variety of aldehydes and amines. Table 6.18 shows the results of the one-pot synthesis of a-aminonitriles from benzaldehyde, amines, and TMSCN. Table 6.18

One-pot synthesis of a-aminonitriles via condensation of benzaldehyde, amines, and TMSCN at room temperature Time (h)

Yield (%)a

C6H5NH2

0.5

95

4-Me-C6H4NH2

0.5

93

Amine

2-Me-C6H4NH2 4-Cl-C6H4NH2

4-Br-C6H4NH2

Cyclohexylamine Piperidine

Pyrrolidine

Morpholine Butylamine

0.5 0.7 1.0 0.5 2.0 2.0 1.5 1.0

89 92 90 83 88 90 83 84

Reaction conditions: 1.0 mmol benzaldehyde, 1.0 mmol amine, 1.3 mmol TMSCN, 20 mg catalyst. aIsolated yield. Source: Reprinted with permission from B. Karamakar, A. Sinhamahaptra, A. B. Panda. J. Benerji, Appl. Catal. A, 392, 111 (2011).

Magnetic Fe2O3 functionalized with sulfamic acid is also a very effective catalyst for the Strecker reactions with a series of aldehydes and amines [326]. The catalyst can be recovered easily and reused without significant loss of its catalytic activity.

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