Chemical Synthesis Using Supercritical Fluids

Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner Chemical Synthesis Using...

Author: Philip G. Jessop | Walter Leitner

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Chemical Synthesis Using Supercritical Fluids

Edited by Philip G. Jessop and Walter Leitner

Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner

Weinheim - New York * Chichester Brisbane - Singapore - Toronto

Dr. Walter Leitner Max-Planck-Institut fur Kohlenforschung Kaiser-Wilhelm-Platz 1 D-45470 Mulheim an der Ruhr Germany

Prof. Philip G. Jessop Department of Chemistry University of California Davis, California 95616 USA

This book was carefully produced. Nevertheless, the authors, editors and publisher do not warrant the information contained therein to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

Library of Congress Card No. applied for. A catalogue record for this book is available from the British Library. Deutsche Bibliothek Cataloguing-in-Publication Data: Chemical synthesis using supercritical fluids I ed. by Philip G. Jessop and Walter Leitner. Weinheim ; New York ; Chichester ; Brisbane ; Singapore ; Toronto : Wiley-VCH, 1999 ISBN 3-527-29605-0 0 WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999 Printed on acid-free and chlorine-free paper. All rights reserved (including those of translation in other languages). No part of this book may be reproduced in any form - by photoprinting, microfilm, or any other means - nor transmitted or translated into machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Composition: Hagedorn Kommunikation, D-685 19 Viernheim Printing: Strauss Offsetdruck GmbH, D-69503 Morlenbach Bookbinding: J. Schaffer GmbH & Co. KG, D-67269 Grunstadt Printed in the Federal Republic of Germany.

Preface

Reactions under supercritical conditions have been used for large scale industrial production for most of the 20th century, but the application of supercritical fluids (SCFs) in the chemical synthesis of complex organic molecules or specialized materials is only just emerging. Research in this field has been particularly active in the last decade of this century. This book is intended to introduce the reader to the wide range of opportunities provided by the various synthetic methodologies developed so far. Supercritical fluids may be alternatives to liquid solvents, but they are neither simple nor simply replacements of solvents. The experimental chemist could not modify a written synthetic method by simply crossing out the word “benzene” and replacing it with the words “supercritical carbon dioxide”. Many other modifications to the procedure would be necessary, not only because of the need for pressurized equipment but also because of the inferior solvent strength of many SCFs. On the other hand, additional degrees of freedom in the reaction parameters emerge from the high compressibility of SCFs, allowing density to be introduced as an important variable. This is only one of the reasons why the result of a chemical synthesis can sometimes be dramatically changed, often for the better, by this solvent switch. It is only fair to say that we are still far away from a detailed understanding of supercritical solvent effects. More basic research will be needed before we learn how to exploit these benefits in the most efficient way, In the meantime, it is our hope that the chemist considering using a SCF as a medium for a reaction will turn to this volume to find out both what has been done and, more importantly, how to do it. Previous monographs concerning SCFs have emphasized their use for extractions and separations, usually with a single chapter dedicated to reaction chemistry. Now we turn the tables. We have selected the chapter topics to guide the reader through the process of planning and carrying out chemical syntheses in SCFs. The subjects include equipment, safety concerns, in-situ monitoring techniques, reaction methods, and purification using SCFs. The largest part of the book is then devoted to various types of chemical reactions involving SCFs as solvents and/or reactants. The emphasis is on synthetic reactions,

VI

Preface

rather than reactions tested for the purpose of investigating near-critical phenomena or reactions involved in destructive or combustion chemistry. For coverage of reactions in SCFs with less of an emphasis on chemical synthesis and related experimental techniques, the reader will be well served by the Chemical Reviews special issue and by a book just published in Japanese (Principles and Developments of Supercritical Fluids Reactions, T. Ikariya, Ed., CMC, Tokyo, 1998). Although the present book provides the newcomer with a summary of the physico-chemical background and purification techniques using SCFs, the excellent introduction to these topics by McHugh and Krukonis (M. McHugh, V. J. Krukonis, Supercritical Fluid Extraction, 2nd edition, Butterworth-Heinemann, Boston, 1994) is highly recommended for further reading. At this point, we must offer a safety warning and disclaimer. Supercritical fluids are used at high pressures and in some cases at elevated temperatures. The chemist contemplating their use must become acquainted with the safety precautions appropriate for experiments with high pressures and temperatures. Some SCFs also have reactivity hazards. The safety considerations mentioned in chapters 1.1 and 2.1 are meant neither to be comprehensive nor to substitute for a proper investigation by every researcher of the risks and appropriate precautions for a planned experiment. The contributors to the present volume, all leading experts in the field, have given us a wide view of the types and methods of research being performed in supercritical fluid reaction chemistry. Many of the techniques that the reader will find described in these pages have been laboriously developed by these contributors and their colleagues. We gratefully thank all of the contributors for agreeing to take time out from their research schedules to write chapters for this volume, and for agreeing to share with the reader the detailed experimental methods and “tricks of the trade”. The editors also gratefully acknowledge fruitful discussions with and advice from a number of colleagues and friends, especially Prof. M. Augustine (University of California, Davis, UCD), Prof. J. Brennecke (University of Notre Dame), Prof. C. Eckert (Georgia Institute of Technology), Prof. A. Fisher, (UCD), Prof. T. Ikariya (Tokyo Institute of Technology), Dr. A. Levelt Sengers (NIST), Prof. R. Noyori (Nagoya University), Prof. T. Patten (UCD), Dr. W.-J. Richter (MPI fur Kohlenforschung) and Prof. S. Tucker (UCD). We also thank the following people and institutions for providing us with information or photographic material on the historical aspects and the industrial use of SCFs: Dr. J. Abeln (Forschungszentrum Karlsruhe), Dr. U. Budde (Schering AG), Dr. H.-E. Gasche (Bayer AG), Dr. P. Mgller (Poul Mgller Consultancy), Dr. T. Muto (Idemitsu Petrochemical), Prof. G. Ourisson and representatives of the Acadkmie des Sciences, Dr. A. Rehren (Degussa AG), M.-C. Thooris (Ecole Polytechnique Palaiseau) and representatives of Eco Waste Technology and General Atomics. Special thanks are due to Drs. Nicole Kindler and Anette Eckerle at WileyVCH for their competent help and collaboration in producing this book. Furthermore, we wish to express our sincere thanks to the members of our

Preface

VII

research groups, for their talents and their enthusiasm, which make our research efforts devoted to SCFs so much fun. Finally, and most importantly, we dedicate our own contribution to this book to our wives and families, for all their love and understanding throughout the years and especially during the preparation of this volume. October 1998

Philip Jessop and Walter Leitner

Contents

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

V

Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

IX

List of Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . .

XVII

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

1

1 1

1.1 Supercritical Fluids as Media for Chemical Reactions . . . . . 1.1.1 What is a Supercritical Fluid (SCF)? . . . . . . . . . . . . . . Practical Aspects of Reactions in Supercritical Fluids (SFRs) . 1.1.2 Motivation for Use of SCFs in Modern Chemical Synthesis . . 1.1.3 1.1.4 A Brief History of Chemical Synthesis in SCFs . . . . . . . . 1.1.4.1 Discovery of SCFs and their Use as Solvents . . . . . . . . . 1.1.4.2 Early Examples of Chemical Reactions in SCFs . . . . . . . . 1.1.4.3 Industrial Use of SCFs as Reaction Media . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6 9 13 13 20 25 30

1.2 Phase Behavior and Solubility . . . . . . . . . . . . . . . . . . 1.2.1 Basic Physical Properties of Supercritical Fluids . . . . . . . . 1.2.2 Phase Behavior in High Pressure Systems . . . . . . . . . . . 1.2.2.1 Types of Binary Phase Diagrams . . . . . . . . . . . . . . . . 1.2.2.2 Asymmetric Binary Mixtures . . . . . . . . . . . . . . . . . . 1.2.3 Factors Affecting Solubility in Supercritical Fluids . . . . . . . 1.2.3.1 SCF Solvent . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.3.2 Chemical Functionality of the Solute . . . . . . . . . . . . . . 1.2.3.3 Temperature and Pressure Effects . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

37 37 41 41 43 47 48 49 51 53

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Contents

1.3 Physical Properties as Related to Chemical Reactions . . . . . 1.3.1 Behavior of Diffusion Coefficients . . . . . . . . . . . . . . . 1.3.2 Diffusional Effects on Reactions . . . . . . . . . . . . . . . . 1.3.3 Transition-state Theory Applied to SCFs . . . . . . . . . . . . 1.3.4 Density Dependence of Two Competing Reactions . . . . . . . 1.3.5 Solvation Effects on Reactions . . . . . . . . . . . . . . . . . 1.3.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Experimental Techniques . . . . . . . . . . . . . . . . . . . .

54 55 56 58 62 63 65 65 67

2.1 High-pressure Reaction Equipment Design . . . . . . . . . . . 2.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2 Basic Equipment and Components . . . . . . . . . . . . . . . 2.1.2.1 Design of Thick-Walled Vessels . . . . . . . . . . . . . . . . . 2.1.2.2 Closures and Connectors . . . . . . . . . . . . . . . . . . . . . 2.1.2.3 Tubing and Fittings . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2.4 Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2.5 Compressors and Pumps . . . . . . . . . . . . . . . . . . . . . 2.1.2.6 Stirring and Mixing . . . . . . . . . . . . . . . . . . . . . . . 2.1.2.7 Optical Windows . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.3 High Pressure Systems . . . . . . . . . . . . . . . . . . . . . . 2.1.3.1 Single-batch High-pressure Reactors . . . . . . . . . . . . . . 2.1.3.2 View Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.3.3 Systems for Continuous Processing . . . . . . . . . . . . . . . 2.1.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

67 67 69 69 71 73 74 76 78 78 78 80 83 84 86 86

Extraction and Related Separation Techniques . . . . . . . . . General Aspects of Supercritical Fluids as Mass Separating Agents . . . . . . . . . . . . . . . . . . . . . . . . Extraction from Solids . . . . . . . . . . . . . . . . . . . . . . Basic Process Design . . . . . . . . . . . . . . . . . . . . . . Process Parameters . . . . . . . . . . . . . . . . . . . . . . . . Modeling the Extraction . . . . . . . . . . . . . . . . . . . . . Solids in Multiple Stages and Countercurrent Operation in SFE Continuous Extraction of Contaminated Soil with Supercritical Water . . . . . . . . . . . . . . . . . . . . . . . . Countercurrent Multistage Extraction . . . . . . . . . . . . . . Basic Process Design . . . . . . . . . . . . . . . . . . . . . . Phase Equilibria . . . . . . . . . . . . . . . . . . . . . . . . . Separation Analysis with Respect to Theoretical Stages . . . . Multicomponent Process Simulation . . . . . . . . . . . . . . . Determination of the Height (Length) of a Theoretical Stage . Determination of Column Diameter . . . . . . . . . . . . . . . Chromatographic Separation with Supercritical Fluids . . . . .

88

2.2 2.2.1 2.2.2 2.2.2.1 2.2.2.2 2.2.2.3 2.2.2.4 2.2.2.5 2.2.3 2.2.3.1 2.2.3.2 2.2.3.3 2.2.3.4 2.2.3.5 2.2.3.6 2.2.4

88 90 91 92 93 95 95 97 98 99 101 102 102 103 104

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XI

2.2.4.1 Design of SFC Apparatus . . . . . . . . . . . . . . . . . . . . 2.2.4.2 Methods for Scale-up of Chromatography . . . . . . . . . . . 2.2.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

105 106 106 106

2.3 Precipitation and Crystallization Techniques . . . . . . . . . . 2.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2 Thermodynamics and Phase Equilibria . . . . . . . . . . . . . 2.3.2.1 CSS, PGSS and RESS . . . . . . . . . . . . . . . . . . . . . . 2.3.2.2 GASP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3 Process Basics and Reference Schemes . . . . . . . . . . . . . 2.3.3.1 Crystallization from a Supercritical Solution (CSS) . . . . . . . 2.3.3.2 Formation of Particles from Gas Saturated Solution (PGSS) . . 2.3.3.3 Rapid Expansion of a Supercritical Solution (RESS) . . . . . . 2.3.3.4 Precipitation by a Gas or a Supercritical Antisolvent (GASP) . 2.3.4 Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.5 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

108 108 109 109 112 115 115 115 117 120 123 125 125

2.4 Microemulsions, Emulsions and Latexes . . . . . . . . . . . . 127 2.4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 2.4.2 Interfacial Tension . . . . . . . . . . . . . . . . . . . . . . . . 128 130 2.4.3 Microemulsions . . . . . . . . . . . . . . . . . . . . . . . . . 131 2.4.3.1 Phase Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . 132 2.4.3.2 UV-Vis Indicators . . . . . . . . . . . . . . . . . . . . . . . . 2.4.3.3 Electron Paramagnetic Resonance Techniques . . . . . . . . . . 133 2.4.3.4 Neutron Scattering . . . . . . . . . . . . . . . . . . . . . . . . 134 134 2.4.3.5 Enzymes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.3.6 Organic-CO2 Microemulsions . . . . . . . . . . . . . . . . . . 135 135 2.4.4 Emulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.4.1 Organic-CO2 Emulsions . . . . . . . . . . . . . . . . . . . . . 135 2.4.4.2 Water-COZ Emulsions and COz-Water Emulsions . . . . . . . 137 2.4.5 Reactions in SCF Emulsions, Microemulsions and Latexes . . 140 2.4.5.1 Phase-Transfer Reactions Between Water and C 0 2 . . . . . . . 140 2.4.5.2 In Situ Studies of Dispersion Polymerization . . . . . . . . . . 142 143 2.4.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.7 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 144 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 3

Spectroscopy of SCF Solutions . . . . . . . . . . . . . . . . .

147

3.1 3.1.1 3.1.2 3.1.2.1 3.1.2.2

Vibrational Spectroscopy . . . . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . Dilute Solutions . . . . . . . . . . . . . . . . . . . . . . . . . Infrared Spectroscopy . . . . . . . . . . . . . . . . . . . . . . Raman Spectroscopy . . . . . . . . . . . . . . . . . . . . . . .

147 147 148 148 150

XI1

Contents

3.1.2.3 Polymer Modification . . . . . . . . . . . . . . . . . . . . . . 3.1.2.4 Vibrational Spectroscopic Studies of Aqueous Microemulsions in SCFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3 Concentrated Solutions. . . . . . . . . . . . . . . . . . . . . . 3.1.4 Monitoring of Fast Reactions in SCFs using Time-resolved Vibrational Spectroscopy . . . . . . . . . . . . . 3.1.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.6 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 3.2.1 3.2.2 3.2.3 3.2.4 3.2.5

NMR Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . Some Toroid Probe Designs . . . . . . . . . . . . . . . . . . General Properties of Supercritical Media . . . . . . . . . . . Measurement of Dynamic and Equilibrium Processes in C 0 2 Rate and Selectivity Measurements Associated with Propylene Hydroformylation in C 0 2 . . . . . . . . . . . . . 3.2.6 Diffusion and Relaxation Time Measurements . . . . . . . . . 3.2.7 NMR of Quadrupolar Nuclei in Supercritical C 0 2 . . . . . . 3.2.8 Future Research . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.9 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.3 UV, EPR, X-ray and Related Spectroscopic Techniques . . . 3.3.1 UV-Vis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2 Fluorescence . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3 Electron Paramagnetic Resonance . . . . . . . . . . . . . . . 3.3.4 X-ray Absorption Fine Structure. . . . . . . . . . . . . . . . 3.3.5 X-ray and Neutron Scattering and Diffraction . . . . . . . . 3.3.5.1 Small Angle Scattering . . . . . . . . . . . . . . . . . . . . . 3.3.5.2 Wide Angle Scattering . . . . . . . . . . . . . . . . . . . . . . 3.3.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.7 Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . .

151 153 154 156 161 161 161 165 165 167 172 177

. . .

184 187 188 191 192 193

.

195 195 198 199 200 206 207 208 209 209 209

. . .

4

Reactions in SCF . . . . . . . . . . . . . . . . . . . . . . . .

213

4.1 4.1.1 4.1.2 4.1.3 4.1.3.1 4.1.3.2 4.1.3.3 4.1.3.4 4.1.4

Synthesis of Inorganic Solids . . . . . . . . . . . . . . . . . . Historical Summary . . . . . . . . . . . . . . . . . . . . . . . Experimental Techniques . . . . . . . . . . . . . . . . . . . . . Hydrothermal Chemistry . . . . . . . . . . . . . . . . . . . . . Metal Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . Phosphates and Silicates . . . . . . . . . . . . . . . . . . . . . Metal Sulfides . . . . . . . . . . . . . . . . . . . . . . . . . . Other Hydrothermal Syntheses . . . . . . . . . . . . . . . . . . Supercritical Amines . . . . . . . . . . . . . . . . . . . . . . .

213 214 216 226 226 229 232 233 234

XI11

Contents 4.1.5 Other Solvents . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

237 238 239

4.2 4.2.1 4.2.2 4.2.3 4.2.4 4.2.5 4.2.5.1 4.2.5.2 4.2.5.3

243 243 244 245 248 252 253 255

Synthesis of Coordination Compounds . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . Detailed Syntheses . . . . . . . . . . . . . . . . . . . . . . . . Preparation of C T ( C O ) ~ ( C ~.H.~ .) . . . . . . . . . . . . . . . Preparation of C P M ~ ( C O ) ~ ( ~ ~. -.H. ~. ) . . . . . . . . . . . Preparation of C P * M ~ ( C O ) ~ ( ~ from ~-H~) C P * M ~ ( C O )-HSiEt3) ~(~~ . . . . . . . . . . . . . . . . . . . . . 4.2.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.7 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

256 257 257 257

4.3 Stoichiometric Organic Reactions . . . . . . . . . . . . . . . 4.3.1 Experimental Methods . . . . . . . . . . . . . . . . . . . . . . 4.3.2 Supercritical Carbon Dioxide . . . . . . . . . . . . . . . . . 4.3.2.1 Diels-Alder Reactions . . . . . . . . . . . . . . . . . . . . . . 4.3.2.2 Reduction and Coupling . . . . . . . . . . . . . . . . . . . . . 4.3.2.3 Esterification . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2.4 Cracking and Rearrangements . . . . . . . . . . . . . . . . . 4.3.3 Superheated and Supercritical Water . . . . . . . . . . . . . 4.3.3.1 Oxidation of Methane . . . . . . . . . . . . . . . . . . . . . . 4.3.3.2 Cleavage/Hydrolysis . . . . . . . . . . . . . . . . . . . . . . . 4.3.3.3 Elimination Reactions . . . . . . . . . . . . . . . . . . . . . . 4.3.3.4 Diels-Alder Reactions . . . . . . . . . . . . . . . . . . . . . . 4.3.3.5 Rearrangement . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.4 Other Fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

259 260 260 262 265 266 266 267 267 268 271 272 273 275 276

4.4 4.4.1 4.4.1.1 4.4.1.2 4.4.1.3 4.4.1.4 4.4.2 4.4.2.1 4.4.2.2 4.4.2.3 4.4.3 4.4.3.1

. .

. .

Photochemical and Photo-induced Reactions . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . Definition of a Supercritical Fluid . . . . . . . . . . . . . . . . Solvent Properties of SCFs . . . . . . . . . . . . . . . . . . . Scope of this Chapter . . . . . . . . . . . . . . . . . . . . . . Experimental Considerations . . . . . . . . . . . . . . . . . . . Photochemical Reactions in Supercritical Fluid Solvents . . . . Geometric Isomerization . . . . . . . . . . . . . . . . . . . . . Photodimerization . . . . . . . . . . . . . . . . . . . . . . . . Carbonyl Photochemistry . . . . . . . . . . . . . . . . . . . . Photo-induced Reactions in Supercritical Fluid Solvents . . . . Free Radical Brominations of Alkyl Aromatics in Supercritical Carbon Dioxide . . . . . . . . . . . . . . . . . . . . . . . . .

280 280 280 280 281 282 285 285 285 287 289 289

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Contents

4.4.3.2 Free Radical Chlorination of Alkanes in Supercritical Fluid Solvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.5 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

290 293 294 294

4.5 Polymerizations in Dense Carbon Dioxide . . . . . . . . . . . 4.5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.2 Homogeneous Solution Polymerizations . . . . . . . . . . . . . 4.5.2.1 Free-Radical Chain Growth . . . . . . . . . . . . . . . . . . . 4.5.2.2 Cationic Chain Growth . . . . . . . . . . . . . . . . . . . . . 4.5.3 Heterogeneous Polymerizations . . . . . . . . . . . . . . . . . 4.5.3.1 Free-Radical Chain Growth . . . . . . . . . . . . . . . . . . . 4.5.3.2 Cationic Chain Growth . . . . . . . . . . . . . . . . . . . . . 4.5.4 Metal-catalyzed Polymerizations . . . . . . . . . . . . . . . . . 4.5.4.1 Ring-opening Metathesis Polymerization . . . . . . . . . . . . 4.5.4.2 Epoxide-C02 Copolymers . . . . . . . . . . . . . . . . . . . . 4.5.4.3 Oxidative Coupling Polymerizations . . . . . . . . . . . . . . . 4.5.5 Step-growth Polymerizations . . . . . . . . . . . . . . . . . . . 4.5.6 Hybrid Systems . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.8 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

297 297 300 300 303 305 305 317 319 319 319 320 320 321 321 322 322

4.6 4.6.1 4.6.2 4.6.2.1 4.6.2.2 4.6.2.3 4.6.3 4.6.3.1 4.6.3.2 4.6.3.3 4.6.4 4.6.4.1

326 326 321 327 329 33 1 333 334 336 338 341

Free-Radical Polymerization in Reactive Supercritical Fluids . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental Methods and Techniques . . . . . . . . . . . . . On-line Spectroscopy . . . . . . . . . . . . . . . . . . . . . . Kinetic Coefficients from Laser-Assisted Techniques . . . . . . Continuously Operated High-pressure Polymerization Reactors Ethene Homopolymerization . . . . . . . . . . . . . . . . . . . Propagation and Termination . . . . . . . . . . . . . . . . . . Chain-Transfer to Monomer . . . . . . . . . . . . . . . . . . . Chain-Transfer to Polymer and p-Scission . . . . . . . . . . . Ethene Copolymerizations . . . . . . . . . . . . . . . . . . . . Reactivity Ratios for Copolymerizations of Ethene with Acrylic Acid Esters . . . . . . . . . . . . . . . . . . . . . 4.6.4.2 Homopropagation and Homotermination Kinetics of the Comonomers . . . . . . . . . . . . . . . . . . . . . . . 4.6.4.3 Modeling of High-pressure Ethene Copolymerizations . . . . . 4.6.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

341 345 347 348 348

4.7 Metal-Complex-Catalyzed Reactions . . . . . . . . . . . . . . . 351 4.7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 351 4.7.1.1 Potential Benefits of Using Supercritical Fluids in Metal-Complex-Catalyzed Reactions . . . . . . . . . . . . . 351

Contents

XV

4.7.1.2 Practical Considerations . . . . . . . . . . . . . . . . . . . . . 4.7.1.3 Solubility of Metal Complexes in SCFs . . . . . . . . . . . . . 4.7.2 Hydrogenation and Related Reactions . . . . . . . . . . . . . . 4.7.2.1 Hydrogenation of C 0 2 under Supercritical Conditions . . . . . 4.7.2.2 Hydrogenation of C = C Double Bonds . . . . . . . . . . . . . 4.7.2.3 Hydrogenation of Imines . . . . . . . . . . . . . . . . . . . . . 4.7.3 Hydroformylation . . . . . . . . . . . . . . . . . . . . . . . . 4.7.4 C-C Coupling Reactions . . . . . . . . . . . . . . . . . . . . 4.7.4.1 Cobalt-catalyzed Cyclization Reactions . . . . . . . . . . . . . 4.7.4.2 Coupling Reactions Involving scC02 as Solvent and Substrate 4.7.4.3 Olefin Metathesis . . . . . . . . . . . . . . . . . . . . . . . . 4.7.4.4 Palladium-catalyzed Coupling Reactions of Aryl Halides . . . . 4.7.4.5 Cyclopropanation . . . . . . . . . . . . . . . . . . . . . . . . . 4.7.5 Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7.5.1 Peroxides as Oxidants . . . . . . . . . . . . . . . . . . . . . . 4.7.5.2 Molecular Oxygen as Oxidant . . . . . . . . . . . . . . . . . . 4.7.6 Polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7.6.1 Polymerization of Alkenes under Supercritical Conditions . . . 4.7.6.2 Polymerization Utilizing Compressed C 0 2 . . . . . . . . . . . 4.7.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

352 356 358 358 360 363 36.5 369 369 370 371 373 375 377 377 378 380 380 381 384 384

4.8 4.8.1 4.8.1.1 4.8.1.2 4.8.1.3 4.8.1.4 4.8.1.5

388 389 390 391 393 393

Heterogeneous Catalysis . . . . . . . . . . . . . . . . . . . . . Fischer-Tropsch Synthesis . . . . . . . . . . . . . . . . . . . . Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . Reaction Performance in Three Reaction Phases . . . . . . . . Diffusion Behavior of Synthesis Gas . . . . . . . . . . . . . . Diffusion and Reaction of the Products . . . . . . . . . . . . . Wax Production: Addition of Heavy Alkene to the Supercritical Phase . . . . . 4.8.2 Isomerization Reactions . . . . . . . . . . . . . . . . . . . . . 4.8.3 t-Butyl Alcohol Synthesis by Air Oxidation of Supercritical Isobutane . . . . . . . . . . . . . . . . . . . . 4.8.4 Supercritical Phase Alkylation Reactions over Solid Acid Catalysts 4.8.5 Synthesis of Fine Chemicals and Other Products . . . . . . . . 4.8.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.9 4.9.1 4.9.2 4.9.3 4.9.3.1 4.9.3.2 4.9.3.3 4.9.3.4 4.9.3.5

Enzymatic Catalysis . . . . . . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . Enzymes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Enzyme Reactors . . . . . . . . . . . . . . . . . . . . . . . . . Batch Reactors . . . . . . . . . . . . . . . . . . . . . . . . . . Recirculating Batch Reactor . . . . . . . . . . . . . . . . . . . Extractive Batch Reactor . . . . . . . . . . . . . . . . . . . . . Semicontinuous Flow Reactors . . . . . . . . . . . . . . . . . Continuous Reactors . . . . . . . . . . . . . . . . . . . . . . .

395 398 399 403 407 411 411 414 414 415 416 416 419 419 420 420

XVI

Contents

4.9.4 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . 4.9.4.1 A Compilation of Published Experiments . . . . . . . . . . . 4.9.4.2 Enzyme Stability in Supercritical Fluids . . . . . . . . . . . 4.9.4.3 The Role of Water . . . . . . . . . . . . . . . . . . . . . . . . 4.9.4.4 Pressure Effects . . . . . . . . . . . . . . . . . . . . . . . . . 4.9.4.5 Mass Transfer Effects . . . . . . . . . . . . . . . . . . . . . . 4.9.4.6 Effects on Selectivity . . . . . . . . . . . . . . . . . . . . . . 4.9.5 Downstream Processing and Costs . . . . . . . . . . . . . . 4.9.5.1 Downstream Processing Schemes . . . . . . . . . . . . . . . 4.9.5.2 Prosessing Cost Estimate . . . . . . . . . . . . . . . . . . . . 4.9.6 Summary and Outlook . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

421

. 421 . 425

428 431 434 436 . 439 . 439 441 441 443

4.10 Phase Transfer and Ammonium Salt Catalyzed Reactions . . . 446 4.10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 446 446 4.10.2 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . 449 4.10.3 Published Work . . . . . . . . . . . . . . . . . . . . . . . . . 451 4.10.4 Experimental Procedure . . . . . . . . . . . . . . . . . . . . . 4.10.5 Related Catalytic Reactions . . . . . . . . . . . . . . . . . . . 452 453 4.10.6 Future Directions . . . . . . . . . . . . . . . . . . . . . . . . . 453 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix A: Conversion Factors . . . . . . . . . . . . . . . . . . . . . Appendix B: Abbreviations and Symbols . . . . . . . . . . . . . . . .

455 455 457

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

465

List of Contributors

Olli Aaltonen VTT Chemical Technology Biologinkuja 7 Otaniemi, PL 1401 02044 VTT Finland

Sabine Beuermann Institut fur Physikalische Chemie Universitat Gottingen Tammannstr. 6 37077 Gottingen Germany

Masahiko Arai Institute for Chemical Reaction Science Tohoku University Katahira, Aoba-ku Sendai 980 - 8577 Japan

Gerd Brunner Arbeitsbereich Verfahrenstechnik 11 Technische Universitat HamburgHarburg 2 1071 Hamburg Germany

Eric Beckman University of Pittsburgh 1243 Benedum Hall Pittsburgh, PA 15261 USA

Michael Buback Institut fur Physikalische Chemie Universitat Gottingen Tammannstr. 6 37077 Gottingen Germany

Alberto Bertucco Istituto di Impianti Chimicj Universiti di Padova via Marzolo, 9 35131 Padova PD Italy

Markus Busch Institut fur Physikalische Chemie Universitat Gottingen Tammannstr. 6 37077 Gottingen Germany

XVIII

List of Contributors

Anthony A. Clifford School of Chemistry University of Leeds Leeds LS29JT UK Christy W. Culp School of Chemical Engineering Georgia Institute of Technology Atlanta, Georgia 30332-0100 USA Tammy A. Davidson Department of Chemistry East Tennessee State University Box 70695 Johnson City, TN 37614-0695 USA Janet DeGracia Department of Chemical Engineering University of Colorado Boulder, CO 80309 USA Joseph M. DeSimone Department of Chemical Engineering CB #3290 - Venable Hall University of North Carolina Chapel Hill, NC 27599 USA Charles A. Eckert School of Chemical Engineering Georgia Institute of Technology Atlanta, Georgia 30332-0100 USA Cornelis J. Elsevier J. H. Van’t Hoff Research Institute Universiteit van Amsterdam Nieuwe Achtergracht 166 1018 WV Amsterdam The Netherlands

Li Fan Dept. of Applied Chemistry, School of Engineering University of Tokyo 7-3 -1, Hongo, Bunkyo-ku Tokyo 113-8656 Japan Ralf Fink BASF AG ZKD/B 1 67056 Ludwigshafen Germany David E. Fremgen Chemical Technology Division Argonne National Laboratory 9700 South Cass Avenue Argonne, IL 60439 USA Neil Foster School of Chemical Engineering and Industrial Chemistry The University of New South Wales Sidney, NSW 2052 Australia Kaoru Fujimoto Dept. of Applied Chemistry, School of Engineering University of Tokyo 7-3-1, Hongo, Bunkyo-ku Tokyo 113-8656 Japan John L. Fulton Pacific Northwest National Laboratory PO BOX999, MS P8-19 Richland, WA 99352 USA

List of Contributors

XIX

Sander Gaemers J. H. Van’t Hoff Research Institute Universiteit van Amsterdam Nieuwe Achtergracht 166 1018 WV Amsterdam The Netherlands

Keith P. Johnston Department of Chemical Engineering University of Texas 26th and Speedway Austin, TX 78712-1062 USA

Rex E. Gerald I1 Chemical Technology Division Argonne National Laboratory 9700 South Cass Avenue Argonne, IL 60439 USA

Jenny L. King Department of Chemistry University of Nottingham University Park Nottingham NG7 2RD UK

Michael W. George Department of Chemistry University of Nottingham University Park Nottingham NG7 2RD UK

Robert J. Klingler Chemical Technology Division Argonne National Laboratory 9700 South Cass Avenue Argonne, IL 60439 USA

Steven M. Howdle Department of Chemistry University of Nottingham University Park Nottingham NG7 2RD UK

Joseph W. Kolis Department of Chemistry Clemson University Clemson, SC 29634 USA

Yutaka Ikushima National Industrial Research Institute of Tohoku 4-2-1 Nigatake, Miyagino-ku Sendai 983-8551 Japan Gunilla B. Jacobson Department of Chemical Engineering University of Texas 26th and Speedway Austin, TX 78712-1062 USA Philip G. Jessop Department of Chemistry University of California Davis, CA 95616 USA

Michael B. Korzenski Department of Chemistry Clemson University Clemson, SC 29634 USA David R. Lamb School of Chemistry Georgia Institute of Technology Atlanta, GE 30332-0400 USA C. Theodore Lee Department of Chemical Engineering University of Texas 26th and Speedway Austin, TX 7871211062 USA

XX

List of Contributors

Walter Leitner Max-Planck-Institut fur Kohlenforschung Kaiser-Wilhelm-Platz 1 45470 Mulheim an der Ruhr Germany

Jerome W. Rathke Chemical Technology Division Argonne National Laboratory 9700 South Cass Avenue Argonne, IL 60439 USA

John C. Linehan Pacific Northwest National Laboratory PO BOX999, MS P8-19 Richland, WA 99352 USA

Sandro R. P. Da Rocha Department of Chemical Engineering University of Texas 26th and Speedway Austin, TX 78712-1062 USA

Charles L. Liotta School of Chemistry Georgia Institute of Technology Atlanta, GE 30332-0400 USA

James M. Tanko Department of Chemistry Virginia Polytechnic Institute and State University Blacksburg, VA 24061-0212 USA

Frank P. Lucien School of Chemical Engineering and Industrial Chemistry The University of New South Wales Sidney, NSW 2052 Australia Carson Meredith Department of Chemical Engineering University of Texas 26th and Speedway Austin, TX 78712-1062 USA Martyn Poliakoff Department of Chemistry University of Nottingham University Park Nottingham NG7 2RD UK Theodore W. Randolph Department of Chemical Engineering University of Colorado Boulder, CO 80309 USA

Klaus Woelk Institut fur Physikalische Chemie Universitat Bonn WegelerstralSe 12 53115 Bonn Germany Matt Z. Yates Department of Chemical Engineering University of Texas 26th and Speedway Austin, TX 78712-1062 USA Clement R. Yonker Pacific Northwest National Laboratory P. 0. BOX999, MSIN K2-44 Richland, WA 99352 USA

Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

1 Introduction 1.1 Supercritical Fluids as Media for Chemical Reactions PHILIPG. JESSOPand WALTER LEITNER

1.1.1 What is a Supercritical Fluid (SCF)? What is a supercritical fluid? The definition used in this book, modified from that of IUPAC, refers to a SCF as the defined state of a compound, mixture or element above its critical pressure @), and critical temperature (T,) but below the pressure required to condense it into a solid (Figure 1.1-1). It is noteworthy that the definitions by IUPAC [l], the Oxford English Dictionary [2], and other dictionaries [3] omit the clause concerning condensation into a solid. That the melting curve extends over the supercritical region [4-61 is often forgotten even though the pressure at this curve is not always impractically high. For example, the pressure required to solidify C02 at its critical temperature is only 5 700 bar [7], but that for water is an enormous 140 000bar [8]. Some dictionaries [9, 101 omit all restrictions on pressure and only require that the temperature be greater than T,. Technically, a SCF is a gas but not a vapor. The term “gas” refers to any phase which “will conform in volume to the space available” [9]. A “vapor” is defined as “a gas whose temperature is less than the critical temperature” [9]. This was the definition proposed by Thomas Andrews [12] in 1869 and is still generally, but not universally [lo], accepted today. In practice, however, the terms gas and vapor are often used interchangeably. The conditions under which a compound is investigated are often described in terms of redttced temperature (T,) and reduced pressure @,), defined as the actual values of T and p divided by T, and p,, respectively (eqs 1.1-1 and 1.1-2). The “law of corresponding states” as introduced by van der Waals [14] implies that compounds behave similarly under the same values of the reduced variables. This allows valuable comparison of different compounds under various conditions, but deviations can be substantial in close proximity to the critical point.

T, = TIT,

(1.1-1)

Pr = PIP,

(1.1-2)

2

I Introduction

10,m

-

1OOO-

P [bar]

10 -

100

10.1

-100

I -50

I

I

0

50

100

T Wl

Figure 1.1-1The phase diagram of C 0 2 [7,11]. The critical and triple points are shown as filled circles. The inset (with a linear pressure scale) shows an expanded view of the area around the critical point; the tear-shaped contour indicates the compressible region.

The properties of SCFs are frequently described as being intermediate between those of a gas and a liquid. This Janus-faced nature of SCFs arises from the fact that the gaseous and liquid phases merge together and become indistinguishable at the critical point. Figure 1.1-2 shows how the meniscus between the phases disappears upon reaching the critical point for COz. Not all properties of SCFs are intermediate between those of gases and liquids; compressibility and heat capacity, for example, are significantly higher near the critical point than they are in liquids or gases (or even in the supercritical state further from the critical point). Although the properties of a compound may change drastically with pressure near the critical point, most of them show no discontinuity. The changes start gradually, rather than with a sudden onset, when the conditions approach the critical point. It is quite common to refer to the somewhat ill-defined region where such changes are noticable as the “near-critical” region. The synonym “para-critical” may be occasionally found in older literature. Technically, the near-critical region extends all around the critical point, but the expression is commonly used to refer to the nonsupercritical section only. The very similar expression “compressible region” refers to the area around the critical point in which the

I . 1 Supercritical Fluids as Media for Chemical Reactions

3

Figure 1.1-2 The meniscus separating liquid and gaseous COz disappears when the critical point is reached by heating liquid COz in a closed vessel. A small amount of a highly COz-soluble and brightly coloured metal complex [13] was added for better contrast.

compressibility is significantly greater than would be predicted from the ideal gas law. Although a significant portion of the compressible region lies inside the SCF section of the phase diagram, there is overlap with the liquid and vapor regions as well (inset of Figure 1.1-1). Thus, even liquids do have significant compressibility near the critical point, although they are virtually incompressible at T, << 1. Liquid phases at temperatures below but not too far below T, are called “subcritical liquids”, whereas “subcritical gases” are those at pressures below p,.

I Introduction

4 1,2

1

1

=.

I

I

I

/”

cyclohexane I

o,o! 0

. . .

..

i

loo

. .

.

1

200

.

. .

,

. . .

300

n-pentane

I

400

. . .

I

..

.L 0

500

pressure [bar]

Figure 1.1-3 The density and the solvent power (as expressed by the Hildebrand parameter) of scCOz as a function of temperature and pressure [15,16].

When working with a SCF, it is valuable to refer to a plot of the dependence of density (6)on pressure and temperature, as presented for supercritical C02 (scCO2) in Figure 1.1-3. Note that the density changes sharply but continuously with pressure in the compressible region. At higher pressures, the density changes occur more gradually. The critical density d, (i.e. d at T, and p,) is the mean value of the densities of the gas phase and the liquid phase and amounts to 0.466 g/mL for C02. The reduced density is defined in analogy to the other reduced variables (eq 1.1-3). The density data shown in Figure 1.1-3 correspond to the bulk density of the medium, but density fluctuations lead to microscopic areas of decreased and increased local densities in SCFs (local density augmentation). Because of the very large compressibility, these density fluctuations are most pronounced very near to the critical point. If the fluctations are of the same order of magnitude as the wavelength of visible light, scattering of the light leads to critical opalescence, which may be apparent as a clouding or coloration of the SCF and can be used to determine the critical point. d, = did,

(1.1-3)

Many solvent properties are directly related to bulk density and will therefore have a pressure dependence similar to the one shown in Figure 1.1-3. The best known example is the continuous variation in “solvent power” over a fairly wide range, which provides the basis for the technical use of SCFs in highly selective natural product extractions. The solvent power is a rather ill-defined property, but there have been experimental approaches to devise scales for liquid solvents. One of the most successful attempts was put fore-

I . 1 Supercritical Fluids as Media for Chemical Reactions

5

20 18-

b)

161412E

0

I 100

I 200

P [bar]

I 300

I 400

501 0

10-

0

0.5

1.0

1.5

2.0

25

dr

Figure 1.1-4 The dielectric constant of H20(4OO0C, T, = 1.04) [20,21], COz (40°C, T, = 1.03) [17], and CHF3 (30°C, T, = 1.01) [18,19], as functions of (a) pressure and (b) reduced density.

ward by Hildebrand [ 161; the so-called Hildebrand parameter for solvent power was found to be directly proportional to the density of SCFs [15], as shown in Figure 1.1-3 for COz. Some typical organic solvents are marked on the Hildebrand scale for comparison to give some indication of the tunability of the solvent power of C02. It should be apparent from the diagram that in order for a SCF to have significant solvating ability, it must usually have d, of greater than 1. The dielectric constant (E) also increases sharply with pressure in the compressible region (Figure 1.1-4a), and this behavior parallels to some extent the change in density. Thus E is more logically plotted against density (Figure 1.1-4b). The magnitude of the increase in E depends on the nature of the SCF. For COz at T, - 1, E changes from ca. 1.3 at d, = 1 to just above 1.6 at d, = 2, i.e. scCOz remains a very non-polar solvent regardless of the actual conditions [17]. In contrast, the dielectric constants of fluoroform [18, 191 and water [20,21] increase, with the same density change, from approximately 3 to 6 and from 5.4 to 13.5, respectively. The use of the tunable dielectric constant of polar supercritical fluids is a topic that has not yet received much attention, but could assist in studies of solvent effects and the polarity of transition states. The possibility of using SCFs as “tunable solvents” not only for extraction (SFEi) but also for chemical reactions (SFR) is one of the many interesting features associated with their application in modern synthesis. Before we discuss the many potential benefits in detail (Section 1.1.3), it seems appropriate to give a brief introduction to some practical aspects of the use of SCFs on a laboratory scale.

6

I Introduction

1.1.2 Practical Aspects of Reactions in Supercritical Fluids (SFRs) Inorganic and organic compounds which are frequently used as SCFs are listed, together with leading references for their volumetric behavior, in Tables 1.1-1 and 1.1-2. The listed SCFs include those that are most commonly employed for syntheses and chemical reactions and they are compared in terms of critical parameters, physical properties and approximate cost. For critical parameters and volumetric data of other SCFs, the reader is referred to several reviews [22-261 and the series of contributions by the IUPAC Commission 1.2 on Thermodynamics, Subcommittee on Thermodynamic Data [27-331. Even more detailed volumetric data for SCFs can be found in two excellent series of monographs: the IUPAC Thermodynamic Tables Project [ l l , 34-43] and the Thermodynamic Properties series by the National Standard Reference Data Service of the USSR [44-501. The purity of the SCF is an important consideration in the planning of a synthesis. Low concentrations of impurities can have noticeable effects on the volumetric and phase behavior of SCFs. For example, helium can be present in commercial C 0 2 because it is sometimes added as a "head-gas" to ensure nearly-complete delivery of the cylinder contents and this has been found to affect the use of scC02 as a solvent for analytical and preparative purposes [51-531. The He head-gas is unnecessary if a cooled pump is used for C02 delivery. Purity can also have an effect on the cost of the SCF. The prices in Table 1.1.-1 are typical for research quantities with the purity closest to 99.99 % as of early Table 1.1-1 Selected inorganic supercritical fluids.

SCF Name

Ar COz HCl HBr HI H20 NH3 N20 Kr SF6 Xe

TC

V-3

Pc (bar)

argon -122.5 48.6 carbon dioxide 31.1 73.8 82.6 hydrogen chloride 51.5 85.5 hydrogen bromide 90.0 hydrogen iodide 150.7 83 water 374.0 220.6 ammonia 132.4 113.2 nitrous oxide 36.4 72.5 krypton -63.76 54.9 sulfur hexafluoride 45.5 37.6 xenon 16.6 58.3

dc MW WmL)

p

We-

Costb References ($k)

bye)"

0.531 0.466 0.42 n.a. n.a. 0.322 0.235 0.453 0.912 0.737 1.099

39.95 44.01 36.46 80.91 127.9 18.02 17.03 44.01 83.80 146.1 131.3

0 6 0 3 20 1.08 0.82 50 0.44 n.a. n.a. 1.85 3 1.47 0.167 50 0 3000 0 50 0 4000

[46, 1921 [ l l , 1931 [24, 194, 1951 [24] [24] [196-1981 [26, 199, 2001 [201] [46] [202-2051 [46, 2061

Reference 207. 'Cost: $ per kg assuming 99.99% purity, prices from common distributors. All of these prices fluctuate, especially those of the noble gases. n.a. = not available.

a

I . I Supercritical Fluids as Media f o r Chemical Reactions

7

Table 1.1-2 Selected organic supercritical fluids.

CHF3 CH2F2 CH4 CH40 C2H4 C2H6 C2H60 C2H& C3H6 C3H8 C4H,o C4H10 C5H12 C6H6 C6H14

fluoroform difluoromethane methane methanol ethene ethane dimethyl ether ethylenediamine propene propane n-butane isobutane n-pentane benzene n-hexane

25.8 78.1 -82.6 239.5 9.2 32.2 126.9 320 91.8 96.7 152.0 134.7 196.6 289.5 234.5

48.2 57.8 46.0 80.8 50.4 48.7 54 62.8 46.0 42.5 38.0 36.4 33.7 49.2 30.3

0.525 0.424 0.163 0.273 0.214 0.207 0.242 0.29 0.228 0.220 0.228 0.224 0.232 0.300 0.234

70.01 1.65 125 52.02 1.97 n.a. 16.04 0 80 32.04 1.70 18' 16 28.05 0 30.07 0 100 46.07 1.30 15 60.10 1.99 2OOd 42.08 0.366 9 44.10 0.084 10 58.12 <0.05 15 58.12 0.132 15 72.15 n.a. 30 78.11 0 30' 86.18 n.a. 300

[45, 2081 1209, 2101 [43, 2'111 [30, 40, 2121 [27, 48, 2131 [31, 47, 2141 [231 1251 127, 381 [31, 44, 2151 131, 2151 12151 r311 [216, 2171 1311

Reference 207. bCost: $ per kg assuming 99.99% purity except for higher hydrocarbons (ethene and C3 or greater), which are 99.5 %. Prices from common distributors as of early 1998. All of these prices fluctuate, especially those of the halocarbons. n.a. = not available. 99.9 % purity. 99.5 % purity.

a

1998. However, for some materials, especially the higher hydrocarbons, the price is highly dependent on the purity and very high purities are prohibitively expensive. For these compounds the price is listed for 99.5 % purity; the major contaminants are usually other hydrocarbons of similar molecular weight. Specialized equipment is required for experiments with supercritical fluids, as described in more detail in Chapter 2.1. Figure 1.1-5 shows a photograph of typical laboratory scale equipment, which allows the investigation and visual inspection of common SCFs at temperatures up to 150"C and pressures up to 250-300 bar. A wide variety of modifications to this standard set-up is possible allowing addition of reagents or catalysts, sampling or online spectroscopic investigations. With this arsenal of techniques at hand, chemical reactions in SCFs can be studied in as much detail as in conventional solvents and many examples will be discussed throughout this book. The physical and chemical properties of the SCFs can sometimes present hazards to the experimentalist [54]. All researchers in the field should search the literature for information concerning the hazards of the materials with which they are working. The following information is presented as a brief overview only, and should not be considered a comprehensive review on the subject.

8

I Introduction

Figure 1.1-5 n p i c a l laboratory scale reactors as used at the Max-Planck-Institut fur Kohlenforschung for the investigation of chemical synthesis in SCFs. The reactors are basically window-equipped modifications of standard high pressure vessels of 225 mL and 25 mL volume, respectively. In addition to the monitoring devices for pressure and temperature, various fittings and connections for addition, sampling or online analysis can be installed on the lid of the larger design on the left.

All SCFs are compressed gases, and therefore contain a great deal of potential energy which can be released upon catastrophic failure of the equipment. As safety regulations vary from country to country and also depend on the size of the reactor and the maximum applied pressure, we can only give some general advice here. All new equipment should be pretested by filling with an incompressible liquid (water, oil, hexane) under pressures of approximately 1.5-2 times the maximum operating pressure. One of the simplest and most effective safety rules when working with SCFs is to avoid direct exposure of the operator to the pressurized vessel, for example by using strong polycarbonate or Lexan shields or similar optically transparent safety equipment. The use of angled mirrors or video equipment also allows visual inspection without direct exposure. Other than the large potential energy, most of the hazards of SCFs are related to the chemical reactivity of the gas itself. Several SCFs, such as scH20, scHCl and the other acids, corrode standard stainless steel reaction equipment, which could result in catastrophic failure. Explosive deflagration or decomposition is common with acetylene even at subcritical pressures [55]. Perfluoroethylene (scC2F4) will explode at pressures above 2.7 bar unless inhibitor is added [56]. Even seemingly stable SCFs can explosively decompose at sufficiently high temperatures (e.g. 500 "C for hexane) [57]. Runaway polymerization can be a concern when polymerizable SCFs like scC2H4 are used [58]. The

1.1 Supercritical Fluids as Media for Chemical Reactions

9

polymerization can be initiated by free radicals, 02,or metal catalysts including even stainless steel components. SCFs, like most other compounds, can be incompatible with some other materials; examples of incompatible combinations that have exploded or violently reacted in the past include NH3/ethylene oxide, HBr/Fe203, HCVAl, C2H2/Cu and many others [56]. Flammable SCFs include d l of the hydrocarbons plus others like scCH30H, scCH30CH3, scNH3, and ethylendiamine. Silane (scSi&) is particularly dangerous because it could autoignite upon leaking from the vessel, independent of any spark source [59]. The only commonly used oxidizing SCF is scN20. Mixing significant quantities of combustible material with scNzO has led to at least two explosions in the past. One of these occured when ethanol (9 vol %) was used as a cosolvent in scN20 [60,61]. while the other occured when 1 g of roasted coffee was exposed to scN20 in a 2.5 mL extraction vessel [62]. Most of the SCFs listed in Tables 1.1-1 and 1.1-2 have a comparatively low level of acute toxicity [63], but the high local concentrations which may result from use under high pressures require appropriate safety considerations, such as sufficient ventilation. Irritant poisons include ethylenediamine, NH3 and the acidic SCFs (HCl, HBr, and HI) [56]. SCFs such as scCHF3 and scN20 (laughing gas) are known to act as narcotics when at high concentrations [64]. Carcinogenic SCFs such as benzene should be replaced by other SCFs with similar properties wherever possible. Toxic compounds dissolved in SCFs can be spread throughout the laboratory if the pressurized solutions are vented outside of a fume hood. The same action can result in contamination of stock chemicals in the laboratory [65]. Those SCFs with particularly high T, values (eg. scH20) could cause thermal burns to operators if a leak were to occur. Chemical bums could result from leaks if irritants or acids are used as SCFs or are dissolved therein. These chemical risks, and the procedures for avoiding them, should be familiar to the practicing chemist utilizing SCFs on a laboratory or technical scale.

1.1.3 Motivation for Use of SCFs in Modern Chemical Synthesis Why use SCFs as solvents for chemical reactions? There are numerous advantages associated with the use of SCFs in chemical synthesis, all of which are based on the unique combination of properties of either the materials themselves or the supercitical state. Different types of reactions may benefit particularly from a specific property, and these sometimes spectacular effects will be discussed in detail in the individual chapters of the book. Here, we try to summarize briefly the various potential improvementsthat can be expected if SCFs are employed as solvents for synthetically useful chemical reactions. The advantages fall into four general categories: environmental benefits, health and safety benefits, process benefits, and chemical benefits (Table 1.1-3).

10

I Introduction

Table 1.1-3 Advantages of using SCFs rather than liquids as reaction media. Category

Advantage

Which SCFsa

Environment

do do no no

most mest CO2, H2O CO; and other volatile SCFs

Health and safety

noncarcinogenic nontoxic nonflammable

most (but not C6H6) most (but not HC1, HBr, HI, en, NH3) C02, N20, H20, Xe, Kr, CHF3

Process

no solvent residues : facile separation of products high diffusion rates low viscosity adjustable solvent power adjustable density inexpensive

C 0 2 and other volatile SCFs C 0 2 and other volatile SCFs all all all all COz, H20, NH3, Ar, hydrocarbons

Chemical

high miscibility with gases variable dielectric constant high compressibility local density augmentation high diffusion rates altered cage strength

all the polar SCFs all all all all

a

not contribute to smog not damage ozone layer acute ecotoxicity liquid wastes

Of those which appear in Tables 1.1-1 and 1.1-2.

Environmental benefits are most often cited for processes with scC02 or scH20 as the solvents. Although C02 and many of the other SCFs are greenhouse gases, the use of C02 as an industrial solvent would still be of benefit to the environment because it would allow the replacement of environmentally far more damaging liquid organic solvents. Processes involving C 0 2 as a solvent would not increase C 0 2 emissions, but rather provide an opportunity for the recycling of waste COz. Also, reactions that result in the fixation of the scCO2 would consume a small amount of waste C02 and decrease our dependence on fossil fuels as sources of carbon-containing molecules. At present, most recovered C 0 2 is generated as a byproduct of ammonia and hydrogen production, but efforts to sequester C 0 2 from flue gases of power plants are rapidly increasing [66]. Health and safety benefits include the fact that the most important SCFs, scCO2 and scH20, are noncarcinogenic, nontoxic, nonmutagenic, nonflammable and thermodynamically stable. Very few traditional liquid solvents fit this description. However, supercritical water may not be appropriate as a solvent for some organic syntheses because of its extremely high critical temperature. The foremost application for which scH20 is being tested and used is oxidative destruction of toxic wastes, which is beneficial to the environment.

I . I Supercritical Fluids as Media for Chemical Reactions

11

A detailed analysis of the environmental impact of a chemical process must also include the energy consumption during operation, which is of course also a major concern for economical reasons. This balance is not always straightforward; for example energy would be saved during removal of the solvent by releasing C02 instead of distillation of an organic solvent. On the other hand, the compression of C 0 2 is energy costly. However, it is often forgotten in these comparisons that a process operating with compressed C02 would not alternate between the pressure required in the supercritical mixture and a full expansion to ambient pressure; reducing the pressure to any value that allows separation of the product is sufficient and the required pressures may well be close to or even above the pc of pure C02. Recompression is usually achieved by cooling the :gas to a liquid and then repumping the liquid, a process which is considerably less expensive than recompressing the gas directly. Furthermore, temperature rather than pressure may be the variable of choice for many separation processes. Therefore, such comparisons require sufficiently detailed information on potential technical solutions for the process under scrutiny in order to avoid decisions based on prejudice. Process benefits derived from the physical properties of SCFs, such as high diffusivity, low viscosity, and intermediate density, make SCFs particularly suitable for continuous-flow processes. The high flow rates and fast reactions often encountered with SCFs allow the design of high-throughput reactions in relatively small scale reactors. The engineering solution for up-scaling of SFRs that have been tested in batch or semibatch reactors on laboratory scale will in most cases not involve an increase in the size of the reaction vessel, but rather the design of a continous-flow system with high space-time yields. In other words, the actual technical solution of a successful synthesis in SCFs will most likely bear more resemblance to a gas phase process, whereas the exploratory test phase can be widely similar to the screening of liquid phase reactions. One of the most obvious advantages of SCFs for chemical synthesis is their adjustable solvating power. The considerable body of knowledge concerning extraction and solubility in SCFs can be brought to bear to solve engineering problems in the separation and purification steps of industrial chemical processes in SCFs. For example, the extractive properties of SCFs may be exploited to separate products from byproducts or to recover homogeneous catalysts. The design of integrated SCF-based processes replacing wasteful and time-consuming work-up and separation schemes seems highly attractive, but has been met piecemeal at best. An elegant application of this concept is the in situ regeneration of heterogeneous catalysts by SCF extraction of wax or tar byproducts that would block pores and active sites in gas phase reactions. Some control of the molecular weights of growing polymer chains is also possible by control of the precipitation. It might also be possible to isolate intermediate reaction products by selective precipitation or extraction and prevent them from further reacting or decomposing. The volatility of many SCFs allows complete removal from the product without the need for costly or energy consuming drying processes. Thus, solvent residues

12

I Introduction

in products can be avoided; this is particularly valuable in the preparation of cosmetics, pharmaceuticals, food additives, and materials for use in electronics. In addition to making processes cleaner and more efficient, the use of SCFs as solvents can also have beneficial effects directly on chemical reactions. Many reactions that can be performed in SCFs-occur also in liquid solvents, but there is a considerable number of examples -where the use of an SCF as the solvent increase the rate of the reaction over that which would be observed in a liquid medium. Several of the unique properties of SCFs can cause such a change in rate, and it is not always easy to identify the most important contribution. For example, extremely rapid reactions that are diffusion controlled or altered by solvent cage effects can be more rapid in SCFs because of the higher diffusivity and weaker cage effects. Local solute-solute “clustering” can lead to high local concentrations of reagents, increasing the rates of reactions which are performed near the mixture critical point. Another advantageous property of SCFs is their miscibility with other gases, which can lead to high rates of reactions if the kinetics are first order or higher in the concentration of the dissolved gas. The presence of a single homogeneous phase is especially important for catalytic reactions that would be operating under mass transport limitations under two-phase liquid-gas conditions. Similarly, reactions that involve mass transfer between a liquid phase and a solid phase such as a polymer or zeolite are often mass transport limited. The rates of such reactions can be greatly enhanced under supercritical conditions. The same arguments predict that diffusionally limited reactions involving suspended enzymes in liquid organic solvents would be faster in an SCF. Optimization of the rate of a SFR can be achieved by varying the choice and concentration of cosolvent, a technique known as “cosolvent tuning”. Finally, the SCF may take part directly in the chemical reaction, for example as one of the reagents (hydrolysis, C02 fixation) or by modifying substrates or catalytically active sites. Other chemical benefits can be related to selectivity changes. Any of the above factors known to affect rates could affect selectivity by altering the rates of competing reactions. In addition, the pressure dependence of typical solvent parameters like dielectric constant of some SCFs may cause a considerable tuning effect on the selectivity of enzymatic and homogeneous catalysis. Owing to the high compressibility of the supercritical phase, the chemical potential of reactants and catalysts can also be varied widely, without changing the reaction volume. All these factors may affect the chemo-, regio-, and stereoselectivity of chemical reactions, and open additional degrees of freedom for the optimization of synthetic processes. It is possible to become too caught up in the excitement over environmentally benign processes, rate increases and other potential benefits of SFRs. If the reaction can be performed anywhere close to adequately in a relatively benign liquid solvent, then there is little motivation for the switch to SCFs, because their use is still considered to be expensive. However, in cases where chemical, process, or environmental benefits can be obtained, industrial use of supercritical conditions is economically feasible and often already a reality

I . 1 Supercritical Fluids as Media for Chemical Reactions

13

(Section 1.1.4.3). Although the costs for the implementation of high pressure equipment and for operating a SFR process are arguably higher than using an existing reaction vessel, a more detailed analysis must include the costs for all steps of the process including work-up and waste treatment and may well lead to different results. As mentioned above, it is also important to consider possible engineering solution; which are quite different from the intial screening procedure and may lead t o much lower equipment and operating costs than anticipated. Furthermore, one should bear in mind that small-scale operating units for SFR can be highly flexible and may allow the equipment to be switched from one process to another without long down times. Nevertheless, it seems fair to conclude that new applications of SFR will be most likely in the synthesis of high-value fine chemicals that are given directly to the customer, rather than for commodities which are used as intermediates for further downstream processing. One key aspect which is often neglected in this context is marketing: it might still be cheaper to produce decaffeinated coffee using CH2C12 as a solvent, but it is very unlikely that the customer would accept this product. We suspect that the market would also react favorably to food preservatives, pharmaceuticals or cosmetics which were produced using natural “carbonic acid” (scC02) instead of organic solvents.

1.1.4 A Brief History of Chemical Synthesis in SCFs 1.1.4.1 Discovery of SCFs and their Use as Solvents The interest in SFRs has seen a tremendous increase over the last few years, because the special properties of SCFs make them particularly attractive solvents for modern synthetic chemistry. We should be aware of the fact, however, that the idea of using SCFs as reaction media has been emerging ever since the discovery of this “peculiar state of matter” early in the nineteenth century by Baron Charles Cagniard de LaTour, an experimental physicist in France [67]. The experiments which led to the discovery of the critical point were prompted by the research of Denys Papin in England in 1680. He designed a high pressure vessel, his “digester” (Figure 1.1-6), and used it to prove that the boiling of water could be suppressed by the action of pressure. He demonstrated a practical application for the raised boiling temperature of water by cooking a meal in his digester for King Charles I1 [68]. In France, Baron Cagniard de LaTour (Figure 1.1-7) speculated that this suppression of boiling must have a limit, and his risky experiments to test this theory in 1822 proved the existence of the critical point [67] (see box on p. 15).

14

I Introduction

Figure 1.1-6 The “digester” made by Denys Papin in 1680 [72]. The first vessel of Cagniard de LaTour was based on this design (Oxford University Press).

Figure 1.1-7 No portraits of Baron Cagniard de LaTour were found by the authors, but we do know, from this registration form, that he had chestnut hair, grey eyes, a small mouth, long chin and was 1.73 m tall (photo courtesy of Ecole Polytechnique).

1.1 Supercritical Fluids as Media for Chemical Reactions

15

I introduced into a small Papin’s digester, built from the end of a thickwalled gun barrel, a certain quantity of alcohol at 36 degrees and a marble or sphere of flint; the liqui pied nearly a third of the interior capacity of the apparatus. Havin rved the kind of noise that the marble produced upon m y making it roll in the barrel at first cold, and then heated little by little over a fire, I arrived at a point where the marble seemed to bounce at each collision, as if the liquid no longer existed inside the barrel [69].

The same effect was observed with “l’ether sulfurique” (diethyl ether) and petroleum ether, but not with water because of the high critical temperature of water. He called this new state of matter “1’Ctat particulier”. With the use of sealed glass tubes (Figure 1.1-8) he was able to observe the transition, describing it in the following manner [67]:

n Figure 1.1-8 The glass tube design used by Cagniard de LaTour to observe the transition from a liquid past the critical point to a supercritical fluid. The internal diameter was l mm over most of the length, but the section def had a diameter of 4 mm. Mercury was introduced into section bcde and ether was put into ef. The ends a and f were then sealed and the tube heated over a fire. At the moment when the ether was transformed into a vapor, the level b of the mercury had climbed to point g. The pressure was calculated from the distances to be 37-38 bar [67].

16

I Introduction

The liquid, after approaching double its original volume, completely disappeared, and was converted into a vapor so transparent that the tube appeared entirely empty [70].

He refined his methods to allow for the determination of critical temperatures and pressures 1711. His values for diethyl ether and carbon disulfide are within 15°C and 4 bar of the values accepted today. Slowly at first, other researchers continued his work. The nature of the supercritical state and the significance of the critical point were debated by Michael Faraday, Dimitrii Mendeleev, Thomas Andrews, and others [73]. Faraday referred to the critical point as the “disliquefying point” and the supercritical state as “that beautiful condition which Cagniard de la Tour has made known” 1741 which he later shortened to “Cagniard de la Tour’s state”. Andrews (Figure 1.1-9) introduced the term “critical point” 1121 and described the true nature of the supercritical state in his thorough study of carbon dioxide (Figure 1.1-10) 175,761.

Carbonic acid at 35.5” and under 108 atmospheres of pressure, stands nearly midway between the gas and the liquid; and we have no valid grounds for assigning it to the one form of matter any more than to the other. . . . The gaseous and liquid states are only distant stages of the same condition of matter, and are capable of passing into one another by a process of continuous change. [I21

Figure 1.1-9 Thomas Andrews (1813-1885) [77].

I . I Supercritical Fluids as Media for Chemical Reactions

17

Only 4 years after these remarkable experiments, van der Waals wrote his PhD thesis [ 141 at the University of Leiden entitled Die Kontinuitut des Piissigen und gasfiirmigen Zustands (on the continuity of the liquid and gaseous f

0fr

Figure 1.1-10 The equipment used by Thomas Andrews to explore the phase behaviour of carbon dioxide. The glass tube had a diameter of 1.25 mm except in the centre section where the diameter was 2.5 mm. The tube was flushed through with C 0 2 and then a was sealed. The lower end m was put into a mercury tube. Heating or applying external vacuum caused some C02 to be expelled and then some mercury was drawn up into the tube. The glass tube was placed within a copper tube containing water, which was compressed with a steel screw to pressures up to 400 bar [12].

-

0

-

a

18

1 Introduction

states) where he developed his equation of state for nonideal gases. His theory also provided a first qualitative explanation for the critical phenomena of gases at temperatures around or above T,, but it failed for T < T,. In 1875, Maxwell introduced the equal area construction which allowed the inclusion of subcritical temperatures as well [78]. Our current Understanding of the behavior of gases is still mainly based on their work. The sometimes quite spectacular phenomenon of critical opalescence also attracted‘the interest of many physicists at the beginning of this century. The relationship between density fluctuation and light scattering formed the basis of theoretical work by von Smoluchowski [79] and Einstein [80,8 11, with important contributions devoted directly to critical opalescence from Ornstein and Zernike [82]. Following Andrews’ pioneering work, the number of experimental investigations devoted to SCFs increased rapidly in the last quarter of the nineteenth century, including for example the studies of Sajotschewsky who located the critical points for eight compounds in 1879 [83]. A journal covering the field of compressed gases, Zeitschrijit fur Komprimierte und Flussige Gase, started publication in 1897. The early phase behaviour and critical point studies were mainly devoted to pure gases [22-24,841, but the possibility of making gaseous solutions by dissolving solids in compressed gases soon became of interest, too. Experiments by Gore in 1861 had already shown that liquid C02 had the ability to dissolve compounds such as camphor, naphthalene and iodine [85]. However, the failure of other solids, including charcoal, aluminium metal and iron sulfate, to dissolve led Gore to remark rather unfairly that liquid C02 was “a very feeble solvent of substances in general.” Hannay and Hogarth undertook the first systematic solubility studies on SCFs in 1879-1880 [86,87], demonstrating that chlorophyll, KI and other solids were soluble in supercritical alcohol. They extended the work to sulfur and other reagents in supercritical carbon disulfide. Their astute observations included the comments that “a liquid has its critical point raised by the solution in it of a solid,” [87] and that a very finely divided solid could be produced by rapidly reducing the pressure of a supercritical solution. The latter phenomenon is the basis for the modern-day RESS process (Chapter 2.3). Paul Villard (Figure 1.1-11) a physicist better known for his later discovery of gamma rays [MI, was working on phase behavior in 1896 when he found that iodine, alkanes, stearic acid, and camphor could be dissolved in supercritical ethylene (scCZH4) [89,90]. He noticed that the action of the S C C ~ H ~ caused the camphor to melt before it dissolved, even though the temperature was not being raised (see the PGSS process, Chapter 2.3). Studies of solubilities and phase behavior continued through the twentieth century, and the results are described in a number of reviews [91-961. More details on the current understanding of solubility data and phase behaviour can be found in Chapter 1.2 of this book. Starting from the early 1920s, practical applications of compressed gases were recognized for extraction and purification. E. B. Auerbach from Berlin filed several international patents on a process utilizing liquid C02 in the

I . I Supercritical Fluids as Media for Chemical Reactions

19

Figure 1.1-11 Paul Villard (18601934) [97] (photo used with permission from the AcadCmie des Sciences).

years between 1926 and 1928 [98,99]. SCFs were investigated especially in the rapidly growing petrochemical industry. Shell Oil patented various processes utilizing para-critical (i.e. near-critical) hydrocarbons and other gases including COz [loo, 1011. In the mid 1950s, several technical solutions for SFE processes for coal, mineral oil and wool were proposed by Zhuze [102]. In 1955, Todd and Elgin pointed out the analogy of SCF/solid and SCFAiquid separation with conventional extraction [99]. A major technical breakthrough came with the development of natural product extraction using scC02 by Kurt Zosel (Figure 1.1-12) at the MaxPlanck-Institute for Coal Research in the early 1960s [103]. His interest in SCFs originated from accidental observations made during the preparation of higher olefins from ethene in the presence of aluminium trialkyls (Aufbau reaction), when experiments were carried out under conditions beyond T, and pc of ethene. His enthusiasm for scC02 was initially seen with some scepticism, but it soon became apparent that extraction with toxicologically benign carbonic acid was highly attractive for the food industry. The first applications of “destraction” - as the process was named by Zosel - to the extraction of caffeine from green coffee beans and of hops aroma were already implemented on a technical scale between 1975 and 1985. Today, more than 100000 t of decaffeinated coffee are produced per year world-wide using this technology. Some recent developments in this area, which is now commonly referred to as supercritical fluid extraction (SFE), and aspects related to chemical synthesis are found in Chapter 2.2 of this book.

20

I Introduction

Figure 1.1-12 Kurt Zosel (1913-1989) (photograph used with permission from the Photoarchiv MPI fur Kohlenforschung, MulheimRuhr).

1.1.4.2 Early Examples of Chemical Reactions in SCFs When was the first use of a SCF as a medium for a reaction? If we exclude the fact that Nature has been doing chemistry in scH20 in the Earth’s crust for a very long time [104], the first reactions observed in SCFs appear in the nineteenth century. It was again Baron Cagniard de LaTour who noticed as early as 1822 that near-critical water seemed particularly reactive. He observed that it was “capable of decomposing glass by dissolving its alkali” and speculated that “one could possibly obtain some other interesting results for chemistry, in multiplying the applications of this process of decomposition” [ 1051. Indeed, most of the studies of SFR during the next few decades primarily concerned H 2 0 at temperatures above T,. Pioneering studies were carried out by Gabriel-Auguste Daubrk, a geologist who held the Chair of Mineralogy and Geology at Strasbourg [106]. In 1857 he reported tests of the reactions of scH20 with glass and with minerals at 400 “C. Daubrke put the water and mineral in a glass tube, and put the sealed tube in water in an iron outer tube. The whole assembly was then heated. Thus, the glass would not break because it would experience roughly equal pressures from both inside and out. The pressures were, in his words, “enormous”, but as they were not measured, we know only that the water was supercritical in terms of temperature (ignoring the effect of dissolved minerals on the T,). Hot spring water containing potassium silicate, when heated in this manner, deposited quartz. Reaction of this water with kaolin produced crystals of feldspar. Wood was‘transformed to a material resembling anthracite under similar conditions [107].

1.1 Supercritical Fluids as Media for Chemical Reactions

21

Early research on hydrothermal reactions was also carried out by Charles Friedel, an instructor at the Ecole Normale SupCrieure [108], the same institution where Paul Villard received his education. Over many years Charles, and later his son Georges Friedel, published a series of papers on the reaction of minerals in H20 at temperatures and presumably also pressures far above critical, starting with the preparation of quartz in 1879 [log, 1101. Charles Friedel is more famous for his discovery of the Friedel-Crafts alkylation of aromatics. The works of de LaTour, DaubrCe and the Friedels together ensured that the first experimental reactions of and in SCFs were those of H20, as reviewed in great detail by Morey [l 111. Reactions in SCFs other than water started, so far as we have been able to determine, with Hannay and Hogarth’s [86] solubility experiments in 1879. They found that a blue liquid ammonia solution of elemental sodium underwent a reaction when heated past the critical temperature of the ammonia, producing a white solid and hydrogen gas. With hindsight, we would formulate the reaction as shown in eq 1.1-4. Just as unexpected was their observation that solid zinc oxide reacted with scCCb at 300°C without dissolving [87]. The products were zinc chloride and an unidentified gas. Elemental arsenic seemed to dissolve in scCS2 but when the pressure was released, reddish yellow crystals were precipitated. Hannay and Hogarth believed these to be arsenic sulfide arising from a reaction with the SCF. Villard also observed reactions during his measurements of the solubility of solids in SCFs in 1898 [89,90]. When he tried to dissolve iodine in scC2H4 at 17°C and 300 bar he found that, once dissolved, the iodine slowly reacted with the ethylene (eq 1.1-5). Over a period of 1-2 h the deep violet colour of the solution faded. The solid white product was isolated in crystalline form by pressure reduction.

(1.1-5) 300 bar A wide variety of high pressure reactions were described by Vladimir Ipatiev (Figure 1.1-13) [108], a professor at the Chemistry Lab of the Michail Artillery Academy in St. Petersburg, in articles from 1904 to 1914 (Scheme 1.1-1). Many of these reactions were at supercritical conditions, although not all of his reports accurately specified the pressures at which the reactions were performed. We now take for granted the high pressure “bomb” or autoclave, which he was the first [ 1121 to develop (Figure 1.1-14). Among his significant discoveries was the first observation that heating and pressurizing S C C ~ H caused ~ it to be non-catalytically oligomerized into liquid alkanes and cycloalkanes of a range

22

1 Introduction

Figure 1.1-13 Vladimir N. Ipatiev (18671952) in 1897, 6 years before he started his high pressure work [log].

70 bar

Ni

(CHd2C=0 (CH3)2CHOH ~ ( C H ~ ~ C H O H )

+

H2 + CH, etc.

200-325"C

0;i,2*p I

+otherproducts

l10bar

Scheme 1.1-1 Some of the reactions of supercritical fluids or mixtures studied by Ipatiev.

1.1 Supercritical Fluids as Media for Chemical Reactions

23

Figure 1.1-14 A, model of Ipatiev’s high’ pressure “bomb” [ 1081.

of molecular weights up to cyclotetradecane [113-1151. He also noticed that with ZnC12 or A1Cl3 as presumably heterogeneous and homogeneous catalysts, respectively, oligomerization occurred at much lower temperatures [ 1151. Other heterogeneously catalyzed supercritical phase reactions reported by Ipatiev included the dehydrogenation of 2-propanol [ 1161, the dehydration of 2-butanol [ 1171, the dehydrogenative coupling of benzene to diphenyl [ 1181, and the isomerization of cyclohexane to methylcyclopentane and other products [57]. Most of his work was dedicated to the development of heterogeneously catalyzed hydrogenation reactions such as the Ni-catalyzed hydrogenation of benzene in what was probably a supercritical mixture of benzene and hydrogen [ 1181. He revolutionized heterogeneous catalysis [ 1121, introducing not only the use of high pressures but also multicomponent catalysts such as Ni203/A1203. In all, 550 articles and patents were written or cowritten by Ipatiev [112]. Several other chemists were active in the field of supercritical fluid reactions at the beginning of the twentieth century. For example, Briner studied the decomposition and reactivity of supercritical fluids such as scNO and scC0 (e.g., eq 1.1-6) [119-1221. He also investigated the system N2-H2 and reported that N2 and H2 do not combine at room temperature and 900 bar [122]. At the Chemical Institute of the University of Berlin, Arthur Stiihler studied the reactions of alkyl halides such as chloroethane with scNH3 (eq 1.1-7) [123]. One of the earliest attempts to utilize SCFs for the selective synthesis of low molecular weight organic products dates to the early 194Os, when Patat at the University of Innsbruck studied the hydrolysis of aniline under supercritical conditions (eq 1.1-8) [124]. 2N0

+ 2HC1

scNO RT 300 bar

NOCl

+ H20 + hC12 + hN2

(1.1-6)

24

-

1 Introduction EtCl

+ NH3

SCNH~ 220 "C - 220 bar

[EtNH3]Cl+ [Et2NHZ]C1+ [Et,NH]Cl (1.1-7) 80 % 15 % 5%

(1.1-8) 440 "C Early mechanistic work on reactions in SCFs includes a kinetic study of the bimolecular decomposition of supercritical HI in 1928 [125]. In the following two decades, a number of researchers tried to address the question of the effect of near-critical temperatures on reaction rates, although not all of them took into account pressure and density effects [126-1291. In one of the more careful studies, Holder and Maass [I271 in 1938 described tests of the rate of the reaction between HCl and propene in liquid and supercritical mixtures at different temperatures. The density within a series of experiments was kept constant by adjusting the pressure. The rates of reaction went through a minimum at the critical temperature for three of the four chosen densities with d, > 1. The minimum was most marked, in percentage terms, when d, was close to 1. In 1946, Toriumi et al. [I291 reported that the rate of uncatalyzed oxidation of NH3 by oxygen reached a maximum at the critical temperature. Similarly, the rate of the Pt-catalyzed oxidation of SO2 by oxygen was found to be highest at the T, of SO2. Rapid diffusion at the critical temperature was offered as one possible explanation. The themes of rate changes near the critical point and the effectiveness of SCFs as solvents for heterogeneous catalysis (Chapter 4.8) have fascinated researchers ever since and have been the subject of many more recent studies. Chemical fixation of C02 under supercritical conditions was attempted for the first time in the middle of the twentieth century in the polymerization field. In a 1949 patent, Sargent suggested that C02 could be incorporated into polyethylene to give a more waxy product. This was achieved by copolymerizing S C C ~ and H ~ scCO2 at 72-88°C and 1000 bar in the presence of a free-radical initiator such as benzoyl peroxide. The resulting polymers were reported to incorporate one COz for every 29 ethylene units [130]. This contrasts with more recent studies which have demonstrated the use of scCO2 as an "innocent" solvent for free radical polymerizations (see Chapter 4.5). In 1951, Buckley and Ray [I311 patented the preparation of polymeric ureas by the treatment of carbamates with scCO2 at 200°C and 500 bar, but it is not clear to what extent any of the carbamates dissolved in the supercritical phase. Stevens patented in 1966 the copolymerization of C02 and ethylene oxide to form polycarbonates; in several of his examples the reaction mixtures were clearly at supercritical conditions [ 1321. There are many more reports and patents of reactions in SCFs than the selected examples mentioned here, especially in the period 1910-1945. Since 1945, the literature has become so extensive that it is not possible to review

25

1.1 Supercritical Fluids as Media for Chemical Reactions

it in this short chapter, but detailed coverage of individual aspects is given in other chapters of this book. Further information on the historical background of SFR is also given in the excellent reviews written by Subramaniam and McHugh (covering the years 1945 to 1985) [133], Savage et al. (covering 1986-1994) [ 1341 and Scholsky (covering polymerization from 1940 to 1990) [135]. There are also many symposia on supercritical fluids [94,136-1421.

1.1.4.3 Industrial Use of SCFs as Reaction Media Of the many industrial applications of SFR (Table 1.1-4), the first and the most famous is arguably the synthesis of ammonia, although some might not include this as an example of SFR because the reduced density is less than 1. Highpressure studies of the reaction between H2 and N2 to form ammonia started in 1901 with Le Chatelier's patent on the uncatalyzed process at up to 100 bar [143]. Fritz Haber later found heterogeneous catalysts such as osmium for this process, but using pressures as high as 177 bar [144]. The chemical and technical problems associated with the ammonia formation and the drastic conditions required were solved by Haber and Bosch. These studies led to the commercialization of the iron-catalyzed process by BASF (eq 1.1-9), with operations of the first plant starting in September 1913 at Oppau, Germany [143,145,146]. Table 1.1-4 Some industrial processes involving reactions in or with SCFs. Reaction

Processlproduct

0xidation Polymerization Hydrogenation Hydrogenation Hydration Transfer hydrogenation Hydrogenation Nitration

LDPE Ammonia Methanol Alcohols Estradione various Energetics

N2

+ 3H2

scwo

Fe 100-350 bar 400-530 "C

2NH2

SCF

Status Production Production Production Production Production Pilot plant Pilot plant Pilot plant in preparation

(1.1-9)

Modern plants for ammonia production by the Haber-Bosch process [145] operate at 100-350 bar and 40O-53O0C, well above both Tc and p c for the reaction mixture. Most notably, the largest plants use the highest pressures because this allows a greater throughput per unit volume of vessel. The actual ammonia synthesis step may not be the only part of the plant that operates at supercritical conditions. The production of H2 in the Shell and Texaco ammo-

26

I Introduction

nia processes is performed by the partial oxidation of fuels by 0 2 at 1250°C and pressures up to 80 bar, followed by the water gas shift reaction. The conditions of this partial oxidation step may well be supercritical depending on the choice of fuel and the f ~ e l - 0ratio. ~ In 1913, the same year as the first commercial ammonia process was launched, BASF began initial tests of a synthesis of methanol from pressurized H2/C0 mixtures, leading to the first plant for the production of synthetic methanol (eq 1.1-10) at Leuna in Germany in 1923 [147,148]. Patart in France later claimed prior invention [149, 1501. CO

+ 2H2

Zn/Cr oxide > CH30H 250-350 bar 320-450 "C

(1.1-10)

'

The high pressure of the process was required because of the poor activity of the catalyst, but this problem was alleviated by the introduction of better catalysts by ICI and Lurgi in the 1960s and 1970s [148]. The current process for the synthesis of methanol, as operated by ICI in England [148], requires a CO/CO2/H2 feed mixture and a pressure of between 50 and 100 bar, which may or may not be supercritical, depending on the composition and pressure. Some other industrial processes are or have been carried out under supercritical conditions even though the fact may not have been generally recognized. For example, McKee and Parker pointed out in 1928 that some oil cracking processes performed industrially at the time occurred above the critical temperature of the reaction mixtures [151]. These processes have since been replaced by lower temperature catalytic cracking methods. Other historical industrial processes which probably involved supercritical phases include the synthesis of melamine [ 1521 from dicyanodiamide, N2 and NH3, and the alkylation of aniline by methanol [ 1521. The oxidation of light alkanes by air or O2 at supercritical temperatures and pressures was explored by Standard Oil in the mid-1920s [153]. Experiments were performed at the laboratory and then semicommercial plant level. The primary products were alcohols. For example, the oxidation of pentane was performed at supercritical conditions (240"C, around 200 bar and a few mole per cent 02)and produced primarily C2-C3 alcohols and acids. However, the oxidation of heptane was performed at subcritical temperatures (225 "C) and produced primarily C6-C7 alcohols. The change in selectivity was attributed to either the difference in phase or more likely the difference in temperature. Other commercial processes for the formation of alcohol denaturants or formaldehyde were reported in the same decade [154,155], but it is unclear whether those reactions were operated at supercritical pressures. Modern processes involving alkane oxidation are heterogeneously catalyzed and operated at subcritical pressures [156]. The polymerization of ethene attracted a great deal of commercial interest two decades after Ipatiev's pioneering studies. Fawcett and co-workers, in a 1937 patent for ICI, reported a dramatic improvement in the method

1.1 Supercritical Fluids as Media for Chemical Reactions

27

Figure 1.1-15 A 9 L high pressure reactor for the production of high temperature and high pressure polyethylene at ICI at 900 bar and 200 "C [ 1601.

[157-1591. If a higher pressure of around 2000 ,ar and rigorous temperature control (-170°C) were used, solid products of molecular weights around 3500 could be obtained. Failure to control the temperature resulted in explosive production of carbon and hydrogen. Further, they found that if oxygen were present in the scC2H4, then solid products could be obtained at only 500 bar. The fortuitous introduction of oxygen in their experiment was due to a leaky vessel! Their 9 L vessel is shown in Figure 1.1-15. ICI built a plant in Cheshire (UK) which produced polyethylene, primarily as an insulator for cables which were badly needed for the wartime radar system. To increase production, the secrets of the method were passed on to American companies [161]. Immediately after the war, DuPont patented refinements to the process. Krase, the investigator at DuPont, found that stepwise reduction of the pressure of the scC2H4allowed the fractionation of the polyethylene by selective precipitation [162,163]. The system was further developed and the relevant phase behavior published by Ehrlich in a series of papers in the 1960s and 1970s [164-1671, including a major review on the subject [168]. The high pressure synthesis of low density polyethylene (LDPE) as currently practiced [ 1691 involves the polymerization of S C C ~ at H ~1000-3000 bar and at 80-300°C. Oxygen is still one of the initiators employed, others being benzoyl peroxide and azodi-iso-

28

1 Introduction

butyronitrile (AIBN). Approximately 3.5 X lo7 metric tons per annum of LDPE is produced in the USA alone, and the amount is still increasing [170]. Supercritical or - near-critical water has found technical applications for hydrothermal syntheses, as discussed in detail in Chapter 4.1. Another more recent industrial application of chemical reactiqns in SCFs is the oxidative destruction of chemical wastes in scH20 (SCWO, supercritical water oxidation). A detailed coverage of the large and prolific field of SCWO is outside the scope of this book on chemical synthesis. The extensive pilot plant activity, primarily by MODAR (now General Atomics) and Eco Waste Technologies, has recently been summarized by Schmieder [171]. The first commercial plant was opened by Huntsman Chemical in collaboration with Eco Waste. The hydration of light alkenes has often been performed with the alkene in the supercritical state. For example, the hydration of ethene to ethanol occurs when a mixture of ethene and water is fed through the reactor at 300°C and 70 bar. The conditions are kept above the dew point of the ethene-water mixture because condensed water would deactivate the catalyst, typically phosphoric acid on a silica support. This process is carried out by Shell, BP, Erdol-Chemie and Hibernia-Chemie [172,173]. Isopropanol is made from propene at ICI with tungsten oxide catalysts at 270°C and 250 bar [172] and at VEBA with a supported phosphoric acid catalyst at 180-260°C and up to 650 bar [174]. The synthesis of 2-butanol for eventual dehydrogenation into 2 -butanone (methyl ethyl ketone, MEK) is performed in an Idemitsu Petrochemical Co. plant (Figure 1.1-16) in Tokoyama, Japan by a two-phase process. Supercritical butene is bubbled through liquid water at low pH, 200°C and 200 bar. The hydration of the butene (a mixture of 1- and 2-butene) takes place in the aqueous phase, following which the 2-butanol dissolves preferentially in the SCC4Hs. The supercritical phase then exits the reactor from the top, is cooled to the liquid state, and is then separated into 2-butanol and unreacted butene. The butene is recycled. The extracting ability of the SCC4H8 is helpful in isolating the product and in preventing the build-up of polymeric residues in the reactor. Annual production of 2-butanone is 40000 metric tons, and the plant has been in production since 1985 [175,176]. The highly successful applications of SFR in the production of polymers and bulk chemicals illustrate that the technical challenges associated with reactions under high pressure and high temperature can be overcome even on a very large scale. Despite this encouraging precedence, the industrial application of SFR to the production of fine chemicals, pharmaceuticals or other more specialized products is still in its infancy. The following examples illustrate the current industrial interest in fine chemical SFR The reduction of 1,4-androstadiene-3,17-dione(AAD) as an industrial route to estradione using supercritical tetralin as both the solvent and the hydrogen donor was investigated at Schering AG in cooperation with the University of Gottingen [ 177,1781. The reaction kinetics were studied at between 350-600 "C and 50-300 bar. A pilot plant with a possible throughput of up to 30 L h-' was studied successfully over 1 year with continuous processes run up to 4 days at 575°C and pressures up to 100 bar at contact times of less than 1 s.

1.1 Supercritical Fluids as Media for Chemical Reactions

29

Figure 1.1-16 The Idemitsu Petrochemical plant for the hydration of supercritical butene to butanol, for eventual conversion to 2-butanone (MEK). Of the three tallest towers, that on the right (with the framework around it) is the hydration reactor, whereas that on the left is the MEK fractionation column (photo courtesy of Idemitsu Petrochemical).

Heterogeneously catalyzed hydrogenation processes involving gaseous hydrogen have been studied for industrial applications most extensively. Hoffman La Roche has investigated the technical feasibility of a variety of such processes [ 1791, including the heterogeneously catalyzed hydrogenation of vitamin precursors in a continuous-flow process in scC02 on a pilot plant scale. The reactor vessel has an internal volume of 40 L and an output of 800 t per annum [180,181]. Flowcharts comparing batch and continuousflow processes have been published [ 1821, but apparently no full production facility is planned for the near future. Longstanding research efforts at Degussa have revealed that the hardening (hydrogenation of the C-C double bonds) of edible oils and fatty acids and their esters can be achieved with high efficiency and selectivity in scCO2 using commercially available supported fixed-bed Pd catalysts [ 183-1861. Up to 15 times higher space-time yields were achieved and 3 times higher catalyst productivities resulted from the extended lifetime of the catalyst in scC02 compared to reactions in a trickle-bed process. Based on this technology, Thomas Swan Ltd in cooperation with the University of Nottingham has developed a miniature flow-through system that allows the hydrogenation of large amounts of unsaturated substrates in surprisingly small reactors [ 1871.

30

1 Introduction

In cooperation with the University in Grothenburg, Sweden, the Danish company Poul Mdler Consulting has developed various processes including hydrogenation of fatty acid methyl esters (FAME) to fatty alcohols and the synthesis of hydrogen peroxide using SCF technology [188,189]. The reactions benefit mostly from increased reaction rates owing to enhanced availibility of the gaseous substrates, especially H2.Three such processes are expected to enter the pilot plant stage in 1998 and 1999. Enzymatic reactions have been tested in SCFs on a pilot plant scale, and a detailed analysis of potential technical solutions and costs is given in Chapter 4.9. Finally, the synthesis of energetic materials has been suggested to be performed in liquid or supercritical COz. The Indian Head Division of the US Naval Surface Warfare Center is nbw constructing a pilot plant with a 100 L vessel for the synthesis of such energetics as MTV (magnesium/Teflon/ Viton) and poly-3-nitratomethyl-3-methyl oxetane [190, 1911. Some of the advantages associated with the use of SCFs for chemical reactions have been known for a very long time, ever since chemists started investigating these media. Nevertheless, it is only now that we are starting to fully appreciate all of the benefits, with the development of new synthetic methodologies. In many cases, the basic principles of carrying out such processes in SCFs have still to be explored and synthetic chemists as well as spectroscopists will find ample opportunity for ground-breaking studies. The challenge that lies ahead is to combine these efforts with new engineering skills based on the knowledge available from extraction and purification processes. If these innovations are given a chance, it seems likely that they can provide new solutions for economically efficient and ecologically benign synthetic chemistry.

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1.1 Supercritical Fluids as Media for Chemical Reactions

31

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32

I Introduction

[45] V. V. Altunin, V. Z. Geller, E. K. Petrov, D. C. Rasskazov, G. A. Spiridinov, T. B. Selover Jr. (Eds.), Thermophysical Properties of Freons. Methane Series, Part 1, Hemisphere Publishing-Corp., Washington, DC, 1987. [46] V. A. Rabinovich, T. B. Selover (Eds.), Thermophysical Properties of Neon, Argon, Krypton, and Xenon, Hemisphere Publishing Corp., Washington, DC, 1988. [47] V. V. Sychev, A. A. Vasserman, A. D. Kozlov, V. A, Zagoruchenko, G. A. Spiridinov, V. A. Tsymarny, T. B. Selover Jr. (Eds.), Thermodynamic Properties of Ethane, Hemisphere Publishing Corp., Washington, DC, 1987.‘ [48] V. V. Sychev, A. A. Vasserman, E. A. Golovsky, A. D. Kozlov, G. A. Spiridinov, V. A. Tsymarny, T. B. Selover Jr. (Eds.), Thermodynamic Properties of Ethylene, Hemisphere Publishing Corp., Washington, DC, 1987. [49] V. V. Sychev, T. B. Selover (Eds.), Thermodynamic Properties of Nitrogen, Hemisphere Publishing Corp., Washington, DC, 1987. [50] V. V. Sychev, T. B. Selover (Eds.), Thermodynamic Properties of Methane, Hemisphere Publishing Corp., Washington, DC, 1987. [51] A. Kordikowski, D. G. Robertson, M. Poliakoff, Anal. Chem. 1996, 68, 4436-4440. [52] M. Roth, Anal. Chem. 1998, 70, 2104-2109. [53] N. L. Porter, B. E. Richter, D. J. Bornhop, D. W.Later, F. H. Beyerlein, J. High Resolut. Chromatogr. Chromatog,: Commun. 1987, 10, 477. [54] J. Y. Clavier, M. Penut, in High Pressure Chemical Engineering: Proceedings of the 3rd International Symposium on High Pressure Chemical Engineering, Zurich, Switzerland, 7-9 October, 1996, P. R. von Rohr, C. Trepp (Eds.), Elsevier, Amsterdam, 1996, p. 627-63 1. [55] P. G. Urben (Ed.), Bretherick’s Handbook of Reactive Chemical Hazards, Butterworth/ Heinemann, 1995. [56] R. J. Lewis Sr., Sax’s Dangerous Properties of Industrial Materials, Van Nostrand, New York, 1992. [57] V. Ipatiev, N. Dovgelevich, J. Russ. Phys. Chem. SOC. 1911, 43, 1431-1436. [58] J. Albert, G. Luft, Chem. Eng. Processing 1998, 37, 55-59. [59] Encyclopedie des Gaz, L‘ Air LiquideElsevier, Amsterdam, 1976. [60]B. N. Hansen, B. M. Hybertson, R. M. Barkley, R. E. Severs, Chem. Muter. 1992, 4, 749-752. [61] R. E. Severs, B. Hansen, Chem. Eng. News 1991, 69(29), 2. [62] D. E. Raynie, Anal. Chem. 1993, 65, 3127-3128. [63] Merkblatter gefahrliche Arbeitsstoffe (Kuhn-Birett), Vof. 8, Landsberghch, 1990. [64] S. Budavari, M. J. O’Neil, A. Smith, P. E. Heckelman, J. F. Kinneary (Eds.), Merck Index, Merck & Co., Inc., Whitehouse Station, NJ, 1996. [65] Y. Fang, Y. K. Chau, Appl. Organomet. Chem. 1995, 9, 365. [66] R. Pierantozzi, in Encyclopedia of Chemical Technology, Vol. 5, John Wiley & Sons, J. I. Kroschwitz, M. Howe-Grant (Eds.), New York, 1993, p. 35-53. [67] C. Cagniard de LaTour, Ann. Chim. Phys. 1822, 21, 127-132, 178-182. [68] I. Asimov, Asimov’s Biographical Encyclopedia of Science and Technology, Doubleday, Garden City, NJ, 1982. [69] “J’ai introduit dans une petite marmite h Papin, construite avec un bout de canon de fusil trbs Cpais, une certaine quantitt d’alcool h 36 degrts et une bille ou sphbre de silex; le liquide occupait h-peu-prts le tiers de la capacit6 inttrieure de l’appareil. Ayant observt l’espbce de bruit que la bille produisait en la faisant rouler dans le canon d’abord froid, et ensuite 6chaufft peu il peu sur un brasier, je suis arrive h un point oh la bille semblait bondir h chaque percussion, comme s’il n’avait plus exist6 de liquide dans le canon.“ [671 [70] “Le liquide, aprbs &treparvenu 8-peu-prbs au double de son volume primitif, a disparu complbtement, et s’est converti en une vapeur tellement transparente que le tube semblaif &tretout-&-fait vide...‘‘ [67] [71] C. Cagniard de LaTour, Ann. Chim. Phys. 1823, 22, 410-415.

I . I Supercritical Fluids as Media for Chemical Reactions

33

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34

I Introduction

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35

[149] C. Lormand, Ind. Eng. Chem. 1925, 17, 430-432. [150]G. Patart, Ind. Eng. Chem. 1925, 17, 859. [151] R. H. McKee, H. H. Parker, Ind. Eng. Chem. 1928, 20, 1169-1172. [152] G. Bandel, E. Bocker, D. Henninger, H. Petri, in Chemische Technologie, Band 4, Organische Technologie II, K. Winnacker, L. Kuchler (Eds.), Carl Hanser Verlag, Munchen, 1972, p. 124-233. [153] P. J. Wiezevich, P. K. Frolich, Ind. Eng. Chem. 1934, 26, 267-276. [154] F. J. Carman, T. H. Chilton,-U. S . 1,697,106 1929. [155] W. P. Bitler, J. H. James, Chem. Met. Eng. 1928, 35, 156. [156] G. Franz, R. A. Sheldon, in Ullmann’s Encyclopedia of Industrial Chemistry, Vol. A18, B. Elvers, S. Hawkins, G. Schulz (Eds.), VCH, Weinheim, 1991, p. 261. [157] E. W. Fawcett, R. 0. Gibson, M. W. Perrin, J. G. Paton, E. G. Williams, Brit. 471,590 1937. [158] E. W. Fawcett, R. 0. Gibson, M. W. Perrin, U. S. 2,153,553 1939. [159] M. W. Perrin, J. G. Paton, E. G. Williams, U. S. 2,188,465 1940. [160] G. D. Wilson, in The Development of Plastics, S . T. I. Mossman, P. J. T. Moms (Eds.), Royal Society of Chemistry, Cambridge, 1994, p. 70-86. [161] M. Kaufman, The First Century of Plastics, the Plastics Institute, London, 1963. [162] N. W. Krase, U. S. 2,388,160 1945. [163] N. W. Krase, A. E. Lawrence, U. S. 2,396,791 1946. [164] P. Ehrlich, J. Polymer Sci., A 1965, 3, 131-136. [165] P. Ehrlich, R. H. Fariss, J. Phys. Chem. 1969, 73, 1164-1167. [166] P. Ehrlich, J. Macromol. Sci.-Chem. 1971, A5, 1259-1263. [167] T. Takahashi, P. Ehrlich, Macromolecules 1982, 15, 714-719. [168] P. Ehrlich, G. A. Mortimer, Adv. Polymer Sci. 1970, 7, 386-448. [ 1691 J. A. Brydson, Plastics Materials, Buttenvorth Heinemann, Oxford, 1995. [170] Chem. & Eng. News 1998, 76(26), 40-81. [171] H. Schmieder, N. Dahmen, J. Schon, G. Wiegand, in Chemistry Under Extreme or Non-Classical Conditions, R. van Eldik, C. D. Hubbard (Eds.), Wiley, New York, 1997, p. 273-316. [172] H.Bestian, H.-J. Friedrich, 0. Horn, in Chemische Technologie, Band 4, Urganische Technologie 11, K. Winnacker, L. Kuchler (Eds.), Carl Hanser Verlag, Munchen, 1972, p. 1-123. [173] N. Kosaric, A. Farkas, H. Sahm, S. Bringer-Meyer, 0. Goebel, D. Mayer, in Ullmann’s Encyclopedia of Industrial Chemistry, Vol. A9, W. Gerhartz, Y. S . Yamamoto, L. Kaudy, J. F. Rounsaville, G. Schulz (Eds.), VCH, Weinheim, 1987, p. 587. [174] A. J. Papa, in U l l m n n k Encyclopedia of Industrial Chemistry, Vol. A22, B. Elvers, S . Hawkins, W. Russey, G. Schulz (Eds.), VCH, Weinheim, 1993, p. 173-183. [175] T. Yamada, T. Muto, Sekiyu Gakkaishi 1991, 34, 201-209. [176] T. Yamada, T. Muto, K. Yamaguchi, AIChE 1987 Summer National Meeting, 1987. [ 1771 M. Buback, in Supercritical Fluids Fundamentals for Application, NATO ASI Series E, Vol. 273, E. Kiran, J. M. H. L. Levelt Sengers (Eds.), Kluwer Acad. Publ., Dordrecht, 1994, p. 481-498. [1781 A. Hanke, Anwendungsperspektiven von Oberkritischen Medien, DECHEMA, Frankfurt a. Main, 1995. [179] M. Jansen, C. Rehren, EP 0841 314 A1 1998. [180] M. Poliakoff, S . Howdle, Chemistry in Britain 1995, 118-121. [181] Roche Magazin 1992, (41, May), 2-12. [ 1821 K. H. Pickel, K. Steiner, 3rd International Symposium on Supercritical Fluids, Strasbourg, 1994, pp. 25-29. [183] T. Tacke, S. Wieland, P. Panster, Process Technol. Proc. 1996, 12, 17-21. [184] T. Tacke, Chem. Anlagen Verfahren. 1995, 18. [ 1851 T.Tacke, S. Wieland, P. Panster, 4th International Symposium on Supercritical Fluids, Sendai, Japan, 1997, pp. 511.

36

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[186] T. Tacke, S. Wieland, P. Panster, M. Bankmann, R. Brand, H. Magerlein, U. S. 5,734,070 1998. [187] M. Poliakoff, T, M. Swan, T. Tacke, M. G. Hitzler, S. K. Ross, S. Wieland, WO 97138955 1997. [188] M. H h o d , P. M~ller,WO 96/01304 1996. [189] M. H h o d , M.-B. Macher, J. Hogberg, P. Meller, 4rh Italian Conference on Supercritical Fluids and their Applications, Capri, Italy, 1997, pp. 319. [190] R. E. Farncomb, G.W. Nauflett, Waste Management 1997, 17, 123-127. [191] R. E. Farncomb, G.W. Nauflett, International ZCT Conference, Karlsruhe, 30 June 3 July, 1998, 1997 . [192] R. B. Stewart, R. T. Jacobsen, J. Phys. Chem. Re$ Data 1989, 18, 639. [193] R. Span, W. Wagner, J. Phys. Chem. Re$ Data 1996, 25, 1509-1596. [194] A. L. Horvath, Process Technol. Znt. 1973, 18, 67-69. [195] K. Mangold, E. U. Franck, Ber. Bunsenges. Phys. Chem. 1962, 66, 260. [196] G.S. Kell, in Watel; a Comprehensive Treatise, Vol. 1 , F. Franks (Ed.), Plenum Press, New York, NY, 1972, p. 363. [197] J. M. H. Levelt Sengers, B. Kamgar-Parsi, F. W. Balfour, J. V. Sengers, J. Phys. Chem. Ref. Data 1983, 12, 1. [198] H. Sato, K. Watanabe, J. Sengers, J. S. Gallagher, P. G. Hill, J. Straub, W. Wagner, J. Phys. Chem. Re$ Data 1991, 20, 1023-1044. [199]A. Harlow, G. Wiegand, E. U. Franck, Ber: Bunsenges. Phys. Chem. 1997, 101, 1461- 1465. [200] L. Haar, J. S. Gallagher, J. Phys. Chem. Ref. Data 1978, 7, 635-677. [201] E. J. Couch, K. A. Kobe, L. J. Hirth, J. Chem. Eng. Data 1961, 6, 229-237. [202] J. Berg, Z . Wagner, Fluid Phase Equilib. 1990, 54, 35-45. [203] H . Pohler, E. Kiran, J. Chem. Eng. Data 1997, 42, 389-394. [204] Z . Gokmenoglu, Y. Xiong, E. Kiran, J. Chem. Eng. Data 1996, 41, 354-360. [205] W. H. Mears, E. Rosenthal, J. V. Sinka, J. Phys. Chem. 1969, 73, 2254. [206] 0. Sifner, J. Klomfar, J. Phys. Chem. Re$ Data 1994, 23, 63. [207] R. C. Weast (Ed.), CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, Florida, 1982. [208] A. G. Aizpiri, A. Rey, J. Dlvila, R. G. Rubio, J. A. Zollweg, W. B. Streett, J. Phys. Chem. 1991, 95, 3351-3357. [209] R. Tillner-Roth, A. Yokozeki, J. Phys. Chem. Re$ Data 1997, 26, 1273-1328. [210] P. F. Malbrunot, P. A. Meunier, G.M. Scatena, W. H. Mears, K. P. Murphy, J. V. Sinka, J. Chem. Eng. Data 1968, 13, 16-21. [211] U. Setzmann, W. Wagner, J. Phys. Chem. Re$ Data 1991, 20, 1061. [212] R. D. Goodwin, J. Phys. Chem. Re$ Data 1987, 16, 799. [213] M . Jahangiri, R. T. Jacobsen, R. B. Stewart, R. D. McCarty, J. Phys. Chem. Re$ Data 1986, 15, 593-650. [214]D. G.Friend, H. Ingham, J. F. Ely, J. Phys. Chem. Re$ Data 1991, 20, 275. [215] B. A. Younglove, J. F. Ely, J. Phys. Chem. Re$ Data 1987, 16, 577. [216] P. N. Cheremisinoff, A. C. Morresi, Benzene. Basic and Hazardous Properties, Marcel Dekker, Inc., New York, 1979. [217] R. Deul, S. Rosenzweig, E. U. Franck, Ber: Bunsenges. Phys. Chem. 1991, 95, 5 15-519.

Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

1.2 Phase Behavior and Solubility FRANK P. LUCIENand NEIL R. FOSTER

The ability of supercritical fluids (SCFs) to act as solvents has been known for well over a century [l] yet the most significant developments in the application of this technology have taken place in the last few decades. Among their most recent applications, SCFs have been employed as media for chemical reactions. As a reaction medium, the SCF may either participate directly in the reaction or simply act as a solvent for the various chemical species. The physical properties of SCFs are highly dependent on pressure and temperature which makes it possible to fine-tune the reaction environment. These characteristics are unique to SCFs and provide the potential to tune the reaction environment in order to optimize reaction rate and selectivity. In view of their importance to process design, fundamental aspects of SCF technology are described in this section. Basic physical properties of SCFs are presented initially, followed by a consideration of phase behavior in high pressure systems. Lastly, the factors affecting the solubility of components in SCFs are described.

1.2.1 Basic Physical Properties of Supercritical Fluids A fluid is defined as being supercritical if it is maintained at conditions above its critical temperature and pressure (T, and pc). The critical point, as defined by T, and p c , marks the end of the liquid-vapor coexistence curve in the phase diagram for a pure substance, as shown in Figure 1.2-1. A supercritical region originating from the critical point can also be identified in this phase diagram. In general, only a single homogeneous phase can exist in the supercritical region, irrespective of the temperature and pressure. However, solidification may occur at very high pressure. Changes in phase from liquid to vapor are accompanied by abrupt changes in physical properties but, in the supercritical region, properties may be varied continuously by manipulating the temperature and pressure. It is possible to construct the path shown in Figure 1.2-1, where

I Introduction

38

I

I

I I

s

E

n Region

Figure 1.2-1 Phase diagram for a pure substance. Temperature

Tc

the progression from a liquid state (A) to a gas state (B), and vice versa, proceeds without any visual signs of boiling or condensation. The properties of SCFs vary over a wide range depending on the temperature and pressure but are generally intermediate to those of gases and liquids. Selected physical properties of SCFs are shown in Table 1.2-1 together with typical values for gases and liquids. The properties of SCFs are very sensitive to small changes in temperature and pressure in the vicinity of the critical point. This applies especially to density as shown in Figure 1.2-2. This figure represents the projection of the liquid-vapor coexistence curve in the densitypressure plane. Pressure and temperature are represented in terms of their reduced values which are simply the actual values divided by the critical values br = p/p,, Tr = T/T,). The left portion of the diagram reveals the two-phase envelope and the equilibrium between liquid and vapor phases is indicated by a vertical tie-line. The supercritical region in this view of the phase diagram is defined by the region to the right of the T, = 1.0 isotherm and for pr > 1.0. In the immediate vicinity of the critical point, the density of supercritical C 0 2 is around 0.4 g/mL,. For reduced pressures greater than 2, it can be seen that the density of supercritical C02 is comparable to that for liquid C02. It is this liquid-like density that enables many materials to be solubilized Table 1.2-1 Comparison of the physical properties of gases, liquids and SCFs [22].

Property Density (g/mL) Viscosity (Pa s) Diffusivity (cm2/s)

Gas

SCF

Liquid

10-3

0.3 lo4

10-3

10-5 0.1

10-3

1 5 x 10"

1.2 Phase Behavior and Solubility 1.2

39

Reduced

Figure 1.2-2 Variation of the density of CO2 in the vicinity of its critical point. D Reduced Pressure (plpc)

to a level which is several orders of magnitude greater than that predicted by ideal gas considerations. As density is a measure of the solvating power of an SCF, temperature and pressure can be used as variables to control the solubility and separation of a solute. For example, at T, = 1.1 in Figure 1.2-2, decreasing the reduced pressure from 3 to 1 lowers the density by around 80%. The conditions of temperature and pressure of most interest in SCF processes are usually bounded by reduced temperatures between 1.0 and 1.2 and reduced pressures greater than 1. The diffusion coefficient (or diffusivity) and viscosity represent transport properties which affect rates of mass transfer. In general, these properties are at least an order of magnitude higher and lower, respectively, compared with liquid solvents. This means that the diffusion of a species through an SCF medium will occur at a faster rate than that obtained in a liquid solvent, which implies that a solid will dissolve more rapidly in an SCF. In addition, an SCF will be more efficient at penetrating a microporous solid structure. However, this does not necessarily mean that mass transfer limitations will always be absent in an SCF process. For example, in the extraction of a solute from a liquid to an SCF phase, the resistance to diffusion in the liquid phase will probably control the overall rate of mass transfer. Stirring will therefore continue to be an important factor in such systems. The diffusion coefficient varies with both temperature and pressure and is strongly influenced by density and viscosity [2]. Density and viscosity both increase with pressure with a corresponding decrease in the diffusion coefficient. The effect is less pronounced at higher pressure because density becomes less sensitive to pressure. The diffusion coefficient generally increases with temperature at constant pressure. However, at constant density, temperature appears to have a minimal effect.

40

I Introduction

Supercritical fluids have a number of distinct advantages over conventional liquid solvents. The adjustable solvent strength and favorable transport properties have already been mentioned and it is these features which really differentiate SCFs from liquid solvents. Most SCFs are low-molecular-weight gases which have relatively low critical temperatuTes. Operations may therefore be carried out at moderate temperatures which is desirable in the recovery of thermally labile materials. Perhaps the most important advantage offered by SCFs is that after the release of pressure, components are left virtually free of residual supercritical solvent. The majority of studies involving SCFs has focused on four fluids: CO,, ethene, ethane and water. The first three fluids all have critical temperatures below 35 "C. Carbon dioxide is by far the most widely used SCF and has several advantages over the hydrocarbons in that it is nontoxic, nonflammable and readily available in high purity. Carbon dioxide also exhibits low miscibility with water and is a moderately good solvent for many low-to-medium molecular weight organics. Reported solubilities for nonpolar solids and liquids in CO;! range from 0.1 to 10 mol%. Marginally higher solubilities are obtained in ethane and ethene. Supercritical water (SCW) has a very high critical temperature and pressure in comparison with the other commonly used SCFs. Nonetheless, SCW has been investigated for more than a decade as a medium for the oxidation of organic wastes [3]. The properties of SCW are quite different to those of ordinary water. In the vicinity of the critical point, SCW behaves like a moderately polar organic liquid. The dielectric constant is reduced to the point where organic materials are readily soluble while the solubility of inorganic species is greatly reduced. These solvation characteristics make SCW an ideal medium for the oxidation of organics. The oxidation process leads to the formation of simple and nontoxic compounds such as water and carbon dioxide. The addition of a caustic solution to the process promotes the conversion of heteroatoms such as C1, F, P, and S into simple salts which precipitate from the reactor effluent. A list of other commonly used SCFs and their critical properties is given in Table 1.2-2. It is evident from this table that SCFs usually require the use of pressure in excess of at least 40 bar. It is interesting to note that linear hydrocarbons generally have a critical pressure below 50 bar and a critical temperature that increases with molecular weight. In addition, substances capable of hydrogen bonding require relatively high critical temperatures and pressures. In some applications, the pressure required for an SCF process may result in prohibitively high capital investment for process equipment. However, it is possible to increase the solvent power of a primary SCF with the addition of small amounts of cosolvents, such as methanol or acetone. The significance of this is that lower operating pressures (and temperatures) are made possible. The selectivity of the SCF for a particular component may also be improved with the addition of a cosolvent.

1.2 Phase Behavior and Solubility

41

Table 1.2-2 Critical data of supercritical fluids [23]. Solvent

Critical Temperature ("C)

Critical Pressure (bar)

COZ N20 Ethane Ethene Propane Propene Ammonia Methano1 Water CHF3 CClF3 SF6

31.0 36.4 - 32.3 . 9.2 96.7 91.7 132.4 239.5 374.0 26.2 28.9 45.5

73.8 72.5 48.8 50.5 42.5 46.0 113.5 81.0 220.6 48.5 38.7 37.7

1.2.2 Phase Behavior in High Pressure Systems The development of SCF processes involves a consideration of the phase behavior of the system under supercritical conditions. The influence of pressure and temperature on phase behavior in such systems is complex. For example, it is possible to have multiple phases, such as liquid-liquid-vapor or solidliquid-vapor equilibria, present in the system. In many cases, the operation of an SCF process under multiphase conditions may be undesirable and so phase behavior should first be investigated. The limiting case of equilibrium between two components (binary systems) provides a convenient starting point in the understanding of multicomponent phase behavior.

1.2.2.1

Types of Binary Phase Diagrams

The phase behavior of most binary systems can be described by nine types of phase diagrams that can be predicted qualitatively with the van der Waals equation of state [4]. The nine types of phase diagrams may be further grouped into five major classes designated I to V. A sixth class exists for some aqueous systems but it is much less common than the other classes and is not considered here. The phase diagrams are based on the projection of three-phase lines and mixture critical lines from pressure-temperature-composition (p-T-x) space onto the p-T plane. A detailed discussion on the three-dimensional features of the various classes of phase diagrams is given by McHugh and Krukonis [5] and by Streett [6]. It should be noted that some differences exist between the classification systems of these two authors. The system referred to here is that described by Streett [6].

42

1 Introduction

component 2

-Pure component vaporlzation curve -.----

fIn t a

Mixture critical line

c2

Two-phase LV

Figure 1.2-3 Class I binary phase diagram: (a) three-dimensional representation in p-T-x Space; (b) p-T projection. (b)

Temperature

The simplest class of binary phase diagram is class I as shown in Figure 1.2-3. The component with the lower critical temperature is designated as component 1. The solid lines in Figure 1.2-3(b) represent the pure component liquid-vapor coexistence curves which terminate at the pure component critical points (C, and C,). The feature of importance in this phase diagram is that the mixture critical line (dashed line in Figure 1.2-3(b)) is continuous between the two critical points. The mixture critical line represents the locus of critical points for all mixtures of the two components. The area bounded by the solid and dashed lines represents the two-phase, liquid-vapor (LV) region. The mixture-critical

1.2 Phase Behavior and Solubility

43

Table 1.2-3 Examples of the five classes of phase diagrams 161. Class

System

I I1

Ad&, N2/02 COz/n-octane, C02/2-hexanol ethanelmethanol, C02/H20 C02/nitrobenzene ethane/ethanol, ethaneln-propanol

111 IV V

.

line is denoted as L = V as it represents the merging of liquid and vapor phases. The mixture critical line shown in Figure 1.2-3 does not necessarily exhibit a maximum and can take on a variety of shapes. The distinctions between the various classes of phase diagrams are based mainly on the behavior of the mixture critical line. The class I1 phase diagram, like class I, also has a continuous mixture critical line between the pure component critical points. In classes IV and V, the mixture critical line originating from C2 exhibits a maximum in pressure and then intersects an additional three-phase, liquid-liquid-vapor (LLV) line. In the class I11 phase diagram, the mixture critical line which originates from C2 rises to very high pressures and can take on a number of different shapes. Examples of the different shapes of mixture critical lines in class I11 systems are given by Schneider [7]. Some typical examples for each of the five classes of phase diagrams are given in Table 1.2-3.

1.2.2.2 Asymmetric Binary Mixtures Phase diagrams for binary systems consisting of components with widely separated critical temperatures are considered to be a subset of class I11 systems [6]. This situation often arises in supercritical extraction operations in which relatively nonvolatile components are extracted. The general features of the class I11 phase diagram are shown in three dimensions in Figure 1.2-4(a) and as the two-dimensional p-T projection in Figure 1.2-4(b). The following discussion refers to Figure 1.2-4(b). The mixture critical line originating from C2 (L = V) may pass through an inflection point before rising to higher pressures. These critical lines usually end at an intersection with a three-phase, solidliquid-vapor (SLV) line (not shown). The dashed line represents a threephase, LLV line which terminates at a critical end point (open triangle). The critical end point also marks the termination of another mixture critical line (L = V) originating from C,. The points MI, M2 and M3 are located on the respective mixture critical lines or three-phase LLV line for the given temperature T I . Critical end points represent the conditions at which two of three coexisting phases merge and become identical. Both lower and upper critical end points are possible, depending on whether the end point occurs at a high temperature

44

I Introduction

component 2

\

L=L

mebphaseline

\ L=V

\

Figure 1.2-4 Class I11 binary phase diagram: (a) three-dimensional representation in p-T-x Space; (b) p-T projection. Temperature

branch of a three-phase line (UCEP) or a low temperature branch (LCEP). At the UCEP in Figure 1.2-4,a liquid and vapor phase critically merge into a single vapor phase in the presence of another liquid phase. The presence of the LLV line in Figure 1.2-4 causes the upper mixture critical line to pass continuously from L = V to L = L in the vicinity of the UCEP. At a temperature TI, between C1 and the UCEP, the p-x diagram has the characteristics shown in Figure 1.2-5. As pressure is increased fromp, to p2, a single vapor phase splits into a two-phase LV region and eventually a three-phase LLV point is reached. The horizontal line at p2 is a tie line which connects the three coexisting phases 'at a fixed temperature and pressure. It is the p-T projection of the

1.2 Phase Behavior and Solubiliry Mixture critical points

I

45

\

Three-phase tie-line

\

u \

LV region

Figure 1.2-5 p-x diagram for a class I11 binary mixture at a temperature which intersects the three-phase LLV line.

P1

pure component 1

pure component 2

Composition

tie-line which appears as one point (M3) on the three-phase LLV line in Figure 1.2-4(b). At pressures higher than p2 there are two distinct two-phase regions in the form of closed domes. The stationary point on each closed dome represents the critical point of the two-phase mixture. The critical point of the LL envelope at the right (M2) represents a point on the mixture critical line which originates from C2. The other critical point (MI) appears on the mixture critical line from C , .

Figure 1.2-6 Class III phase diagram for an asymmetric binary mixture consisting of a solid and an SCF. Temperature

46

1 Introduction

In a binary mixture consisting of a solid and an SCF, the critical temperatures of the pure components are normally far apart. The critical temperature of the supercritical solvent, in particular, lies below the triple point of the solid and there is no common range of temperature where the pure components both exist as liquids. As a consequence, the three-phase LLV line in Figure 1.2-4 becomes an SLV line which terminates at a LCEP as shown in Figure 1.2-6. A second, higher temperature branch of 'a SLV line originates from the triple point of the solid. This SLV line, which was not shown in Figure 1.2-4, intersects the L = V mixture critical line from C2 at an UCEP. At both the LCEP and UCEP, a liquid and vapor phase critically merge into a single fluid phase in the presence of a solid phase. An interesting feature of Figure 1.2-6 is that the higher temperature branch of the SLV line lies at lower temperatures than the solid-liquid coexistence curve of the pure solid. This indicates that melting point depression of the solid can occur in the presence of an SCF. As an example of melting point depression, consider the three-phase SLV line for the binary octacosane-ethane system [8]. The normal melting point of octacosane occurs at 64.5 "C whereas the UCEP occurs at 38.9 "C and 87.5 bar. For temperatures between the UCEP and the normal melting point of the solid, the phase behavior is modified by the presence of the SLV line as shown in Figure 1.2-7. As pressure is increased from p 1 to p 2 , a single vapor phase splits into a twophase SV region until the SLV line is reached as indicated by the horizontal tie-line at pressure p 2 . For pressures greater than p2 two regions of twophase behavior appear. When the overall composition of the system is less than xL (to the right of xL in Figure 1.2-7), a region of solid-vapor equilibria exists up to very high pressures. If the overall composition of the system is greater than xL (to the left), LV equilibria are established up to the stationary point at pressure p 3 which represents the critical point of the two-phase mix-

SV region

4

Mixture critical point

Threephase tie-line (SLV)

SV region

:

Vapor region

I

PureSCF

:

Pure solid

Figure 1.2-7 p - x diagram for an asymmetric binary mixture at a temperature which intersects the three-phase SLV line.

1.2 Phase Behavior and Solubility

47

ture. This point lies on the mixture critical line connecting the UCEP and C2 (Figure 1.2-6). The solids of interest in SCF processes usually have relatively low vapor pressures which results in an LCEP which is located close to the critical point of the SCF. The region between the critical point of the SCF and the UCEP generally defines t h e temperature range where unconstrained solidvapor equilibria exist, i.e. liquid phase formation is absent. However, operating at temperatures below the UCEP does not always ensure unconstrained solidSCF equilibria because the SLV line can exhibit a temperature minimum. At low pressures the SLV line begins with a negative slope, passes through a temperature minimum and then continues with a positive slope to the UCEP. Octacosane exhibits:this type of behavior under the influence of C 0 2 [8]. The complexity of the phase diagrams presented clearly demonstrates that phase behavior under high pressure conditions can vary markedly. This is particularly important for chemical synthesis in SCFs because the number and types of phases has a direct impact on the progress of reaction. A knowledge of phase behavior is therefore essential for the proper interpretation of experimental data.

1.2.3 Factors Affecting Solubility in Supercritical Fluids The solubility of compounds in SCFs has perhaps been the most extensively investigated area of SCF research. Solubility data indicate how well the SCF performs as a solvent for a particular solute and are an important starting point in the consideration of potential process applications. Although the solubility of a single solute in an SCF is not necessarily the same as that obtained in a multicomponent system [9],binary solubility data are nonetheless useful for estimating the selectivity of an SCF for a particular solute. The solubility of a component is mainly influenced by its chemical functionality, the nature of the SCF solvent, and the operating conditions. The discussion that follows is centered on the solubility of solids, although similar principles apply for the case of liquids. Referring back to Figure 1.2-6, the region between the LCEP and the UCEP defines the temperature range where unconstrained solid-vapor equilibria exist with respect to pressure. Solid solubility data available in the literature are normally measured in this range of temperature. Furthermore, a check is usually made to determine the maximum temperature at which data can be measured before the onset of melting. As stated previously, the high-temperature branch of the SLV line may exhibit a minimum with respect to temperature and this has the effect of reducing the temperature range for unconstrained solid-vapor equilibria.

48

I Introduction

1.2.3.1

SCF Solvent

The solubility (y) of a material in an SCF is usually expressed in terms of the overall mole fraction of the solute in the SCF phase. The ability of SCFs to dissolve many substances arises from the highly nonideal behavior of pure SCFs. The solubility of a component, as predicted by the ideal gas law, decreases asymptotically with increasing pressure because the solubility is simply the ratio of the vapor pressure (p""') to the total pressure (p). Under supercritical conditions, however, the solubility is enhanced by several orders of magnitude above that predicted by the ideal gas law. The solubility enhancement of a component, particularly in the vicinity of the critical point, is driven primahly by the augmentation in the density of the SCF. The extent of solubility enhancement which occurs in the SCF phase is usually expressed in terms of an enhancement factor (E) which is defined as the actual solubility divided by the solubility predicted from the vapor pressure of the solute and ideal gas considerations:

E = -Y P

(1.2-1)

PSat

The enhancement factor measures the extent of solubility in excess of that generated from the vapor pressure of the pure solid. This provides a convenient way of comparing the solubilities of solids with different vapor pressures. As the actual solubility of a solid is heavily influenced by the SCF density, the enhancement factor displays a similar dependence on density. Values of the enhancement factor ty ically vary between lo3 and lo6, although enhancement factors as high as lotg have been reported for some systems [lo]. The solubility of a given solute also depends on the type of SCF as shown in Table 1.2-4. Fluoroform has the highest mass density under the conditions shown but displays the lowest affinity for naphthalene. The variation in the solubility of naphthalene in different SCFs therefore suggests that there are varying degrees of intermolecular interaction between the solid and SCF. The different levels of intermolecular interaction can be explained in terms of solvent polarity. The overall effect of solvent polarity on the solubility of naphthalene follows the same general solubility rule in liquid extractions that "like dissolves like". Naphthalene is a nonpolar solid and is most soluble in supercritical ethane. Carbon dioxide behaves as a nonpolar solvent but less so because of its quadrupole moment [ 111. Fluoroform is the most polar solvent because of the elecTable 1.2-4 Solubility of naphthalene in various SCFs at 200 bar and 45°C [16].

Solvent

Solubility (mole fraction)

Density (g/mL)

Ethane Carbon dioxide Fluoroform

4 . 7 5 lo-' ~ 2.42X10-' 1.17xlO-'

0.39 0.8 1 0.94

1.2 Phase Behavior and Solubility

49

tron-withdrawing capability of the fluorine atoms. In general, nonpolar solvents such as ethene and ethane are the preferred solvents for aromatic hydrocarbons. For polar solids, the effect of solvent polarity is not as straightforward. Although it is true that nonpolar SCFs exhibit lower affinities for polar solutes, the maximum solubility of a polar solute does not necessarily occur in the most polar SCF. For example, C 0 2 is a better solvent for benzoic acid than fluoroform [12]. Polar SCFs may exhibit greater potential for polar molecules when they contain functional groups that increase the level of intermolecular interaction with the solvent. Fluoroform, for example, is as good a solvent as COz for 2-aminofluorene probably as a result of hydrogen bonding between the amino group and the acidic proton in fluoroform [12]. In the case of 2-cyanonaphthalene, intermolecular interactions between the solute and fluoroform lead to much higher solubilities than those obtained with C 0 2 [13].

1.2.3.2 Chemical Functionality of the Solute The difference between solid solubilities in a given SCF depends mainly on the solid vapor pressure and intermolecular interactions between the solvent and solute. The individual solubilities of solids can vary greatly although most values are well below 10 mol %. The differences between enhancement factors are less pronounced [14,15], however, which suggests that the vapor pressure of the solid exerts the primary influence on solubility. Intermolecular interactions between the solvent and solute depend on the types of functional groups present in their chemical structures. In general, the intermolecular interactions are dominated by dispersion forces, which accounts for the similar values of the enhancement factor for many solids. It was previously described how the degree of intermolecular interaction can increase when a polar SCF is used for polar solutes. As another example, consider the solubilities of naphthalene and 1,4-naphthoquinone in fluoroform shown in Table 1.2-5. The solubility of naphthalene is much larger than that for 1,4-naphthoquinone under similar conditions mainly because of the difference in their vapor pressures. With the effect of vapor pressure removed, the enhancement factor for 1,4-naphthoquinone is substantially larger than that for naphthalene. This suggests that a higher degree of intermolecular interaction occurs between fluoroform and 1,4-naphthoquinone. The additional interaction between solvent and solute may be associated with hydrogen bonding Table 1.2-5 Comparison of the enhancement factors for naphthalene and 1,4-naphthoquinone in supercritical fluoroform at 200 bar and 45 "C [16]. Solute

Solubility (mole fraction)

Enhancement factor

Naphthalene 1,6Naphthoquinone

1.17 x lo-' 3.1 1x 10-3

3 380 92 500

50

I Introduction

Y

.0 E3

-

Figure 1.2-8 Dampening effect of substituents on the solubility of indole derivatives at 35 "C I

.

I

.

I

[24,25,26].

.

i0

between the acidic proton in fluoroform and the carbonyl groups in 1,4naphthoquinone [ 161. The vapor pressure of a solid and the intermolecular interactions it is capable of are ultimately determined by its chemical structure. Differences between the solubilities of solids can also be explained in terms of structural features which limit or enhance solubility. Beginning with a parent compound, the addition of a functional group generally has the effect of reducing solubility. This applies especially to hydroxy (-OH) and carboxy (-COOH) functional groups as described by Stahl et al. [ 171. The carboxy functional group has a greater dampening effect on solubility than the hydroxy group, as shown in Figure 1.2-8. Beginning with indole as the parent compound, the solubility decreases by around 1.5 orders of magnitude with the addition of the hydroxy group (5 -hydroxyindole). The addition of the carboxy group to indole (indole-3 -carboxylic acid) decreases the solubility by a much larger amount of around 2.5 orders of magnitude. For comparison, the addition of a methyl group to indole (skatole) causes a relatively minor solubility reduction of less than 0.5 orders of magnitude. A detailed study on the effect of structural features on solubility is given by Dange el al. [18]. This study considers structural features such as chain length, branching and the number of rings in addition to the position and types of substituents. Structure-solubility relationships using C 0 2 as a solvent are illustrated for several classes of compounds including hydrocarbons, alcohols, phenols, aldehydes, ethers, esters, amines, and nitro compounds. Normal alkanes, for example, are completely miscible with COz when the carbon number is below 12. Solubility decreases rapidly beyond a carbon number of 12. For a given carbon number, the solubility of acyclic hydrocarbons is favored by unsaturation; for example, 1-octadecene is more soluble than n-octadecane.

1.2 Phase Behavior and Solubility

51

Branching leads to more favorable solubilities compared with normal alkanes. The maximum limit of complete miscibility for branched alkanes also occurs at a much higher carbon number which is between 19 and 30. Although the degree of intermolecular interaction can increase when a polar SCF is used for polar solutes, the overall solubility of polar solutes is normally much lower than that for nonpolar solutes. Organic salts will exhibit a wide range of solubility values in SCFs depending primarily on vapor pressure. Inorganic salts exhibit very low solubility in SCFs and in most cases may be considered as insoluble. Metal ions are insoluble in SCFs because of the charge neutralization requirement but, if they are bound to organic ligands, they may become soluble to an appreciable extent. A good example of this is the complexation of metal ions with tri-n-butyl phosphate (TBP) and di(2 -ethylhexyl) phosphoric acid [ 191. The choice of the organic chelating agent impacts significantly on solubility. For example, diethyl dithiocarbamate (DDC) forms stable complexes with a wide range of metals and such complexes are soluble in supercritical CO;! to varying degrees. Fluorination of the ligands, as in bis(trifluoroethy1) dithiocarbamate (FDDC), can increase the solubility of the metal complexes by several orders of magnitude [20]. It is interesting to note that fluorination of an organic structure is a useful way of increasing its solubility in supercritical C02. As another example, the technique has been used as a means of increasing the solubility of hydrocarbon polymers [21]. 1.2.3.3 Temperature and Pressure Effects Solubility data are typically presented in the form of solubility isotherms as shown for the naphthalene-C02 system in Figure 1.2-9. The characteristic feature to note in this diagram is that solubility increases rapidly at lower pres-

Retrograde

328 K

Solute vapor pressure dominant

Figure 1.2-9 Solubility of naphthalene

I

I Pressure (bar)

52

I Introduction

sures, generally near the critical pressure of the SCF, whereas at higher pressures the increase in solubility is less pronounced. The behavior of the solubility isotherm in this way simply reflects the density changes which are occurring in the SCF. The effect of temperature on solubility is more complex and involves both a consideration of the solute vapor pressure as wdl as the density of the SCF. The solubility isotherms shown in Figure 1.2-9- are typical of most solidSCF systems in that they intersect within a narrow range of pressure. For any two isotherms, the point of intersection, or crossover pressure, represents a change in the temperature dependence of solubility. The region of pressure below the crossover pressure is known as the retrograde region. In this range of pressure, solubility decreases with an increase in temperature because the density of the SCF falls sharply. The decrease in density is sufficient to overcome any increases in solute vapor pressure that would normally lead to an increase in solubility. Above the crossover pressure, the decrease in solvent density is less sensitive to temperature and so solubility increases with temperature because the vapor pressure effect becomes dominant. Solubility data provide important information on achieving separation between the solute and the SCF. The solubility data shown in Figure 1.2-9 suggest that there are two ways in which this separation might be achieved. The first separation scheme, which is perhaps the most conventional procedure, involves an isothermal decrease in pressure. This would preferably be accomplished in the vicinity of the critical pressure of the SCF where solubility is sensitive to changes in pressure. The reduction in pressure need not occur completely to atmospheric pressure because an order of magnitude reduction in solubility is easily accomplished in the steeply rising portion of the solubility isotherm. The second separation scheme involves an increase in temperature under isobaric conditions. This can be achieved in the retrograde region, and again an order of magnitude reduction in solubility can occur with a modest increase in temperature. This separation method is probably more favorable than the first method in terms of energy consumption because the first method involves a significant recompression step after separation. However, a separation based on a temperature increase requires much more specific information on the solubility behavior of the solute. In contrast, a separation based on a pressure reduction mainly involves consideration of the critical pressure of the solvent. A near 100% separation can be achieved once the pressure is reduced to below the critical pressure. Solubility data also suggest potential operating conditions for an extraction process. Referring again to Figure 1.2-9, an extraction at 200 bar requires a lower solvent-to-feed ratio than extraction at 100 bar. This advantage may be offset by the higher capital costs and a larger recompression duty. A separation at 200 bar based on a temperature decrease involves a significant reduction in solubility but the overall separation efficiency remains low in comparison with a temperature increase at 100 bar in the retrograde region. This

1.2 Phase Behavior and Solubility

53

example demonstrates that higher pressures favor the extraction step whereas lower pressures, particularly in the retrograde region, favor the separation step. The actual choice for the operating pressure is therefore a compromise between process yield (or solvent-to-feed ratio) and operating cost.

References [ l ] J. B. Hannay, J. Hogarth, Proc. R. SOC. London Sex A 1879, 29, 324. [2] K. K. Liong, P. A. Wells, N. R. Foster, Znd. Eng. Chem. Res. 1991, 30, 1329. 131 R. W. Shaw, E B. Brill, A. A. Clifford, C. A. Eckert, E. U. Franck, Chemical and Engineering News 1991, Dec. 23, 26. [4] P. H. van Konynenburg, R. L. Scott, Phil. Trans. Roy. SOC. 1980, 298, 495. [5] M. A. McHugh, V. J. Krukonis, Supercritical Fluid Extraction: Principles and Practice, Butterworths, Boston, 1986, Chapter 3. [6] W. B. Streett, in Chemical Engineering at Supercritical Fluid Conditions, M.E. Paulaitis, J. M. L. Penninger, R. D. Gray, P. Davidson (Eds.), Ann Arbor Science, Ann Arbor, MI, 1983, Chapter 1. [7] G. M. Schneider, Adv. Chem. Phys. 1970, 17, 1. [8] M. A. McHugh, T. J. Yogan, J. Chem. Eng. Data 1984, 29, 112. [9] F. P. Lucien, N. R. Foster, Znd. Eng. Chem. Res. 1996, 35, 4686. [lo] K. G. Liphard, G. M. Schneider, J. Chem. Thermodyn. 1975, 7, 805. [ll] J. M. Prausnitz, R. N. Lichtenthaler, E. G.de Azevedo, Molecular Thermodynamics of Fluid-Phase Equilibria, Prentice-Hall, Englewood Cliffs, NJ, 1986, Chapter 4. [12] W. J. Schmitt, R. C. Reid, J. Chem. Eng. Data 1986, 31, 204. [13] T. Nakatani, K. Ohgaki, T. Katayama, Znd. Eng. Chem. Res. 1991, 30, 1362. [14] J. F. Brennecke, C. A. Eckert, AZChE J. 1989, 35, 1409. [15] J. M. Dobbs, K. P. Johnston, Znd. Eng. Chem. Res. 1987, 26, 1476. [16] W. J. Schmitt, R. C. Reid, in Supercritical Fluid Technology, J. M. L. Penninger, M. Radosz, M. A. McHugh, V. J. Krukonis (Eds.), Elsevier Science Publishers B.V., Amsterdam, The Netherlands, 1985, pp 123-147. [17] E. Stahl, W. Schilz, E. Schutz, E. Willing, Angew. Chem. Znt. Ed. Engl. 1978, 17, 731. [18] D. K. Dange, J. P. Heller, K. V. Wilson, Znd. Eng. Chem. Prod. Res. Dev. 1985, 24, 162. [19] F. Dehghani, T. Wells, N.J . Cotton, N. R. Foster, J. Supercrit. Fluids 1996, 9, 263. [20] K. E. Laintz, C. M. Wai, C. R. Yonker, R. D. Smith, J. Supercrit. Fluids 1991, 4, 194. [21] D. Betts, T. Johnson, C. Anderson, J. M. DeSimone, ACS Polym. Prepx, Div. Polym. Chem. 1997, 38, 760. [22] K. Johnston, in Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed., M. Grayson, D. Eckroth (Eds.), John Wiley & Sons, New York, 1984, Supplement volume. [23] CRC Handbook of Chemistry and Physics, 75th ed., D. R. Lide, H. P. R. Frederikse (Eds.), CRC Press, Inc., Boca Raton, FL, 1994, Section 6. [24] T. Nakatani, K. Ohgaki, T. Katayama, J. Supercrit. Fluids 1989, 2 , 9. [25] S . Sako, K. Ohgaki, T. Katayama, J. Supercrit. Fluids 1988, 1 , 1. [26] S . Sako, K. Shibata, K. Ohgaki, T. Katayama, J. Supercrit. Fluids 1989, 2, 3. [27] Y. V. Tsekhanskaya, M. B. Iomtev, E. V. Mushkina, Russ. J. Phys. Chem. 1964, 38, 1173.

Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

1.3 Physical Properties as Related to Chemical Reactions ANTHONY CLIFFORD

The physical properties that are important for reactions in supercritical fluids (SCFs) are (a) phase behavior, (b) diffusion rates and (c) interactions between reagents, products and transition states with the fluid. First, phase behavior is important in bringing reagents into the same phase to allow them to react more effectively. For example light gases, such as hydrogen, are often miscible with SCF substances. Second, phase behavior can be important in separating the product of a reaction, possibly also stopping the reaction at an intermediate stage or pushing a reversible reaction to completion. The phase behavior of the reacting system must therefore be understood. At the minimum it will be necessary to know whether the reaction is in a single phase, as is usually required. This will often be a matter of being at a sufficiently high pressure. It will also be important to estimate the critical temperature, T,, of the multicomponent reacting mixture in order to know whether the system is a liquid (below T,) or an SCF (above T,). The difference in behavior in passing through T, is not dramatic, but as the temperature moves below T, the medium becomes less compressible and loses the control by pressure characteristic of a supercritical fluid [l]. Because it has been covered in chapter 1.2, phase behavior will not be further discussed in this section. Diffusion coefficients are typically higher in SCFs than in liquids. This is partly because the substances used as the solvent, such as carbon dioxide, have typically lighter and smaller molecules than organic liquid solvents and partly because the density of an SCF is typically less than a liquid. Consequently, reactions controlled by diffusion may be faster than in a liquid, giving the advantage of smaller process plant size. However, in the region of the critical point, diffusion coefficients can show an anomalous lowering, which can effect reaction rates. The behavior of diffusion coefficients is therefore discussed in Section 1.3.1 and its effect on reactions in Section 1.3.2. For reactions controlled by activation, solvation effects on the reagents and the transition state can affect the equilibrium coefficient for the formation of the transition state and therefore the reaction rate. Equilibria can also be affected by solvation effects on reagents and products. In a supercritical fluid, solvation effects can be controlled by density, and therefore at constant

1.3 Physical Properties as Related to Chemical Reactions

55

temperature by pressure, and so in Section 1.3.3 transition-state theory is adapted to the context of SCFs. The important application to the density dependence for two competing reactions is then discussed in Section 1.3.4. The effect of solvation and transition-state effects on chemical equilibria is finally treated in Section 1.3.5.

1.3.1 Behavior of Diffusion Coefficients For dilute solutions, the diffusion coefficient of a solute in an SCF has an inverse density relationship [2]. Figure 1.3-1 shows some experimental measurements [3] for the diffusion coefficient of naphthalene in carbon dioxide at 313K. The open circles represent values of D12multiplied by density, d, and then plotted against density. The solid point near zero density is the value calculated for 1 bar from rigorous kinetic theory, using an experimental measurement of the diffusion coefficient for naphthalene in air [4] to obtain the distance parameter in the intermolecular potential for naphthalene. The dotted line emanating from this point was calculated from Thorne-Enskog theory [5]. It can be seen that the product d D I 2for both the experimental and calculated values stays within a 25% band over the density range and the dashed curve shows the general trend, without having any theoretical basis. Other published measurements also fall near the same curve. A possible explanation of the rising part of the curve can be made using the concept of edgewise diffusion. This concept is used to explain that flat molecules, such as benzene, diffuse more rapidly in viscous fluids than would be predicted by hydrodynamic theories based on their behavior in more mobile fluids [6]. It is argued that when rotational diffusion of molecules becomes slow, as the density rises, translational movements persist more strongly along particular molecular axes.

Kinetic 0'

8

Figure 1.3-1 The product of the density and diffusion coefficient for naphthalene in C 0 2 at 313K, plotted versus density.

.\.. .

0

a

---

Critlcal density 600

300 Density (kg m4 )

900

56

I Introduction

Whether this explanation is correct or not, the discussion illustrates the difficulty in predicting diffusion coefficients in dense media. At constant higher density, there is undramatic behavior of the diffusion coefficient as the temperature is lowered from supercritical conditions, through the critical temperature, to liquid conditions [ 7 ] . In the region of the critical point, diffusion coefficients show a lowering effect, to an extent dependent on concentration 181. As the critical point is approached closely, the diffusion coefficient tends to zero for finite concentrations. A physical explanation of this is that, as the temperature is lowered towards the critical temperature at the critical density, a situation is being approached where two phases with two different concentrations of the solute coexist in equilibrium, and where there is no tendency to reduce the concentration difference. Some experimental observations of this decrease of the diffusion coefficient towards a critical region have been made [9].

1.3.2 Diffusional Effects on Reactions Some rapid homogeneous reactions are controlled by the rate of diffusion of reagents towards each other. In some cases, as pressure increases in an SCF, a reaction may pass from activation control to diffusion control [lo]. In the simplest analysis of diffusion control, the second-order rate coefficient in terms of mole fractions, k,., is given by the Smoluschowski equation,

where DA and DB are the binary diffusion coefficients of the reagents in the fluid, r is the distance of approach necessary for reaction and q5 is a statistical or steric parameter of less than unity which takes into account, for example, the necessary relative orientation of the two molecules for the reaction to take place. The diffusivity of a dilute solute in an SCF, somewhat removed from the critical point, is typically an order of magnitude greater than in liquid solvents at comparable temperatures. The Stokes-Einstein equation, relating the diffusion coefficient to the fluid viscosity, is used to rewrite eq (1.3-1) in terms of viscosity as

k, = 8RTBq

( 1.3-2)

where q is the coefficient of viscosity of the medium. Good agreement between experiment and the predictions of eq (1.3-2) have been obtained in SCFs [ 1 I]. According to eq (1.3-2), all diffusion-controlled second-order rate coefficients are the same in a particular solvent. Diffusion is also important for unimolecular fission. Thus, radical initiators under SCF conditions are able to escape more readily from solvent cages, and the rate coefficient for the initiation process is markedly increased. Pro-

1.3 Physical Properties as Related to Chemical Reactions

57

cesses propagated by free radicals, such as polymerization, are also rate enhanced. For unimolecular decompositions, initiated thermally or by light, decreasing the diffusion coefficient by increasing density can decrease the decomposition rate. This is ascribed to slower diffusion out of a cage of solvent molecules, giving rise to an increased rate of the geminate recombination of two molecular fragments Tormed by decomposition. In a complex reaction scheme, where only some of the steps are diffusion controlled, the course of the reaction can be changed by controlling the diffusion coefficients. The archetypal example here is polymerization. The effect of diffusion control on overall rate and the rates of the different steps is first discussed. A simplified equation, sufficient for a qualitative discussion, for the instantaneous rate of polymer formation is (1.3-3) where E is the initiator efficiency, xI and xMare the mole fractions of initiator and monomer, and ki, kp, and kt are the rate coefficients for the initiation, propagation and termination processes respectively. The factor kdkt“2, which gives the pressure dependence of the reaction if its initiation is pressure independent, is found to increase with increasing pressure. This is readily explained by termination steps becoming diffusion controlled at much lower pressures than propagation steps because the former, in general, involve two large polymer radicals, whereas the latter involve a smaller monomer molecule as one of the reacting species. However, initiation may also be affected if it is suggested that diffusion control is associated with the formation of solvent cages, as indicated in eq (1.3-4) by curly brackets around reagent molecules. A B e (Aa-B}+ A * + * B

(1.3-4)

When the molecule concerned is a radical initiator this has implications for the efficiency of radical production, related to cage escape. Under supercritical conditions, geminate recombination can be controlled by density, as described earlier. In the region of the critical point, diffusion coefficients can fall for finite concentrations, as described in Section 1.3.1. The behavior of reactions in the critical region can therefore be discussed qualitatively using this effect. However, in a more integrated approach, the methods of nonequilibrium thermodynamics [12] can be used as a basis for discussion of what effects can be expected on both the rates, including diffusion-controlled rates, and equilibrium positions of chemical reactions due to the proximity of a critical point. These arguments have been reviewed and applied to the discussion of a number of experimental studies by Greer [13]. Critical region behavior of the diffusion coefficient can have effects on product ratios. This is probably the explanation of the increase in yield of the photo-Fries enolization products near the critical density in the photochemical

58

I Introduction

-

O

IRH Scheme 1.3-1

A

reaction of 1-naphthyl acetate with 2-propanol in supercritical carbon dioxide [14], as shown in Scheme 1.3-1. The photo-Fries enolization products are formed within the solvent cage, and diffusion out of the cage is necessary to give the other product, 1-naphthol. Outside the critical region the ratio of photo-Fries enolization products to naphthol is around four, but this increases to more than 12 near the critical density. Very often catalytic heterogeneous reactions are controlled by the rate of diffusion to the catalyst surface and enhanced diffusion rates in supercritical fluids can be an advantage. An important category of such reactions are enzymatic conversions, as enzymes are insoluble in most normal organic solvents and the use of nontoxic carbon dioxide can be an advantage. Diffusion is not only enhanced in the bulk fluid, but also diffusion within the pores of particles containing enzyme molecules or other heterogenous catalysts is more rapid. Enhanced swelling of such particles, compared with that in liquid solvents can be beneficial as well.

1.3.3 Transition-state Theory Applied to SCFs During a reaction process, the reacting species, A and B, are considered to pass through a potential energy maximum in their transition from reagents to products. The minimum energy pathway on the relevant potential energy surface is termed the reaction coordinate, and the molecular configuration coyesponding to its zenith is the activated complex or transition state species, C . Although the activated complex does not have any stable existence, it is regarded as a species and treated thermodynamically. Transition-state theory assumes that the activated complex is formed in a rapid pre-equilibrium with the reagents, as indicated in eq (1.3-5) for the common case of a bimolecular reaction. A

+B

Cf

+ products

(1.3 -5)

1.3 Physical Properties as Related to Chemical Reactions

59

The theory then considers that this species breaks up by a first-order process and describes the rate of formation of one of the immediate products, P, in terms of the amoynt of the activated complex and an appropriately defined rate coefficient, k : dnp/dt = k'

(1.3-6)

nCt,

where niare numbers of moles. It is assumed throughout, unless stated otherwise, that pressure and temperature are kept constant, to simplify the notation in derivatives such as dnpldt = (&zp/at)p,T. Although transition-state theory is usually presented in terms of concentrations, in the context of SCFs it is more convenient to use mole fractions, xi. This is because changes in concentration are often dominated by volume changes, which are not of direct interest, and which cause many of the expressions to have compressibility terms. As can be easily shown by differentiation, if s is the total number of product molecules formed from the activated complex, (1.3-7) dxpldt = (lln) ( 1 - (s-l)xp} dnp/dt and with eq (1.3-6) this becomes dxpldt = k

* XC:

(1

-

(S-l)Xp}

( 1.3 - 8)

Thus, if only one product molecule is formed from the activated complex, it becomes true that drpldt = k' xct

(1.3-9)

and eq (1.3-9) is also approximately true in other cases, provided the mole fraction of any product is small, and dilute conditions of reagents and products will now be assumed throughout. The theory then assumes that the equilibrium between the reagents, A and B, and the activated complex, C , is governed by an equilibrium constant. In a near-critical fluid, the most convenient approach is to define the equilibrium in terms of fugacities, Alpe, referred to the standard state of p*, typically 1 atmosphere: ( 1.3 - 10)

The quantity KjO is an equilibrium constant for the ideal-gas state and is thus equal to that used for dilute-gas reactions. The fugacities are given in terms of the mole fractions, xi, the fugacity coefficients, &, and the pressure, P , by (1.3-11) A = 4i P xi

I Introduction

60

The substitution of eq (1.3-10) and (1.3-11) into equation (1.3-9) gives the following expression for the rate of increase of the product P: ( 1.3- 12)

In transition-state theory, the first two factors on-the right-hand side of equation (1.3-12) are modified (leaving the product of the factors unchanged in value), using the arguments of statistical mechanics given in many textbooks in physical chemistry, to produce the following more useful equation: (1.3-13) where kB and h are Boltzmann’s and Planck’s constants, respectively. The --te bar above K p indicates the modification of the equilibrium constant: it is called a pseudo equilibrium constant, but treated like a normal equilibrium constant. The rate equation in terms of mole fractions and the measured rate constant, k,, is dxpldt = k,

(1.3-14)

XA xB,

Comparison of eq (1.3 -13) and (1.3 -14) gives the following expression for the rate coefficient: (1.3 -15) Equations (1.3-14) and (1.3-15) thus give the prediction from transition-state theory for the rate of a reaction in terms appropriate for an SCF. The rate is seen to depend on: (i) the pressure, the temperature and some universal constants; (ii) the equilibrium constant for the activated-complex formation in an ideal gas; and (iii) a ratio of fugacity coefficients, which express the effect of the supercritical medium. Equation (1.3-15) can therefore be used to calcu--te late the rate coefficient, if K p is known from the gas-phase reaction or calculated from statistical mechanics, and the ratio ~ j ~ / 4estimated ~t) from an equation of state. Such calculations are rare; an early example is the modeling of the dimerization of pure chlorotrifluoroethene (T, = 105.8 “C) to 1’2-dichlorohexafluorocyclobutane (Scheme 1.3-2) and comparison with experimental results at 120°C, 135°C and 150°C and at pressures up to 100 bar [15].

>=
F 2x

F

F

-

F F#

CI

Scheme 1.3-2

1.3 Physical Properties as Related to Chemical Reactions

61

Fugacities were calculated from the Redlich-Kwong equation of state and there were differences of 30 % between experiment and prediction, which reflects the uncertainties in calculating fugacities. After taking the natural logarithm of eq (1.3-15) and using

--te

--te -

AGp +RTlnK,

( 1.3- 16)

=O

--te

where AGp is the modified standard Gibbs function change for the formation of the activated complex in the ideal gas, we obtain

[kiT)

--te ( 1.3- 17)

Ink, =In - - A G ~ +In RT

In order to make calculations from this equation, two 0: the terms are combined to give a Gibbs function change for the reaction, AG , which is still standard in terms of mole fractions, but which applies to the particular SCF conditions used, where AGt= AEiO -ln[z]4 ~ 4 ~ RT

( 1.3- 18)

In terms of AG', eq (1.3-17) becomes ( 1.3- 19)

Using the general thermodynamic relationships ( d G / d ~ =) ~V~and RT

[7 ) v i - q - RT

=

T,x

(1.3 -20)

P

are partial molar volumes under general and ideal-gas conwhere & and ditions, respectively, and the subscript x indicates that composition is kept constant for the derivative, equation (1.3-17) is transformed, after differentiation with respect to pressure at constant temperature, to

RT

[7 )

--te

=-AV

T,,

-(V,-~-VA-VB)

+ (G-q-T) (1.3-21)

p e terms on the right-hand side of equation (1.3-21) are now considered. AV is the volume of activation in an ideal gas, as normally used. (Vet V, - V,) is the rate of change of the volume of the reaction mixture with respect to the amount of activated complex formed in moles at constant pressure and temperature. In view of the large partial molar volumes possible in SCFs, this is often important, especially in the critical region. It can appropriately be given the symbol AVt. is the volume change for the

(G-e-e)

62

1 Introduction

formation of the activated complex in the ideal gas, arising from the voluyes --8 of the three species themselves. It is almost identical to the first term AV , the difference arising from the modification of the equilibrium constant in transition-state theory, as described in the last section, and indicated by the bar. If this difference between the first and third terms is ignored and they are cancelled, this yields the equation (1.3-22) Equation (1.3 -22) can be used to calculate a volume, of activation from ratecoefficient data in a supercritical fluid. It gives an activation volume which is the difference between the partial molar volume of the activated complex and those of the reagents and includes both intrinsic and solvent effects. Partial molar volumes have been discussed in Chapter 1.2 and shown to be very large and negative in the critical region. Similarly, in many cases activation volumes in SCFs in the critical region are found to be large and negative, both experimentally and in modeling studies. As an example of the latter, modeling of the dimerization of chlorotrifluoroethene discussed earlier [15] gave a value for AVt of -3290 cm3 mol-', close to the critical density and 14°C above the critical temperature. These extreme negative values are a reflection of the fact that the density of the medium is changing rapidly with pressure, and as a consequence the rate coefficient is increasing rapidly from a gas-like to a liquid-like value. They are not an indication of important effects on reaction behavior due to the proximity of the critical point.

1.3.4 Density Dependence of Two Competing Reactions The large effects due to rapid changes in density with pressure can be eliminated if two reactions are considered that have the same reagents, but produce two different products. In this case there will be two rate coefficients, ki and k,: and also other thermodynamic quantities which will be indicated by the single and double primes. The ratio of the rate coefficients will be of importance as this will indicate the ratio of products that will be obtained. By subtracting two equations of the form of eq (1.3-22), the following expression for this ratio is obtained: (1.3 -23) Because the reagents are the same in both cases eq (1.3 -23) will simplify to (1.3 -24)

1.3 Physical Properties as Related to Chemical Reactions

63

The following relationship is true for partial molar volumes at infinite dilution:

vi = v - ( a v / a p ) , ~ ( ~ p / a x i ) T , V

(1.3 -25)

Therefore, assuming that all reagents, transition states and products are at infinite dilution, the following expression is obtained from eq (1.3 -24), after the molar volume terms cancel:

The term involving the compressibility, ( ~ V I ~ Pcan ) ~now ~ , be divided out of equation (1.3-26) to give R T [ aln(ki/k:)

av

-[%I

) T,x

axc,

T,v

(1.3-27) T,V

Thus the density dependence of the ratio of rate coefficients is related to the difference in the derivatives of pressure with respect to the mole fractions of the transition states. These quantities arise from interactions between the transition states and the solvent molecules [l] and can be described as the tuning functions for the transition states.

1.3.5 Solvation Effects on Reactions Solvation can affect the equilibrium constant of a reaction in an SCF, expressed here in terms of mole fractions, Kx. The sensitivity to pressure is given by the standard thermodynamic relationship

[y) T,x

= - AV

(1.3 -28)

where AV is the difference between the partial molar volumes of the products minus those of the reagents, multiplied by their stoichiometries. Thus the dramatic values of partial molar volumes in SCFs may cause interesting behavior. If there is the same number of product molecules as reagent molecules, the molar volume terms in eq (1.3 -25) will cancel, and the rate of change of equilibrium constant with density becomes equal to differences in tuning functions. A number of studies have been made and one example is described here: the tautomerization between 2 -hydroxypypiridine and 2-pyridone in 1,l-difluoroethane and propane (Scheme 1.3-3) [16]. The reaction was monitored in situ spectroscopically, at reagent concentrations causing negligible self-association of 2 -pyridone. The equilibrium con-

I Introduction

64

O

O

H

-

H

H

0-

Scheme 1.3-3

stant was shown to be a strong function of pressure for both solvents in their respective critical regions, with stronger variation'in polar 1,l -difluoroethane. Solvation of reagents and transition states also affects reaction rates, for which the theory was provided in Section 1.3-3.A number of studies of single reactions have been made and these are characterized by rapid changes in rate with pressure near the critical density, especially when the reaction is just above the critical temperature. These rapid changes correspond to large (usually negative) values of the activation volume, on the basis of eq (1.322). An example was given in Section 1.3-3 as the equations were discussed. These effects overwhelmingly reflect the fact that, near the critical density, the density changes rapidly with pressure and the solvent is rapidly changing from gas-like to liquid-like. For two competing reactions, however, the effect of the large compressibility cancels, as shown in Section 1.3.4, and a more subtle effect of solvation, expressed by tuning functions can be observed. An experimental study on the Diels-Alder reaction between cyclopenta-1,3-diene and methyl acrylate was carried out [ 11, which produces two stereoisomeric products, the endo and exo forms (Scheme 1.3-4).

OMe

C0,Me C0,Me endo addition

ex0 addition

Scheme 1.3-4

Experiments were carried out at low concentrations to ensure that conditions were supercritical and that the limit of infinite dilution was closely approached. Results for the ratio of endo to ex0 product, plotted against density at 313K, are shown in Figure 1.3-2.The product ratio is seen to go through a maximum, although the range of values of the ratio is not different than can be obtained by changing the solvent in the liquid phase reaction [17]. However, by controlling the density, this range can be obtained in a single solvent. The curve shown is derived from Eq (1.3-27)using the van der Waals equation to obtain the difference in tuning functions and fitting to the experimental values. The maximum does not occur at the critical density and thus is not associated with clustering in the critical region. Instead, it appears that the maximum is associated with tuning of the average distance between the solute molecules and the transition states as the pressure and density vary.

1.3 Physical Properties as Related to Chemical Reactions

65

4.0

Figure 1.3-2 Experimental data, shown as points, for the ratio of endo to ex0 product from the reaction of methyl acrylate and cyclopentadiene in carbon dioxide at 3 13K, with the curves shows predictions derived from eq (1.3-27).

1.3.6 Conclusions

.

Thus there are a number of characteristics of the physical properties in supercritical fluids which may lead to advantages in carrying out reactions in the medium. The first has to do with phase behavior, which may, on the one hand, facilitate reaction by bringing reagents and catalysts into one phase and, on the other, bring about required separation of products. The second is that diffusion is typically faster than in liquids, which may speed up both homogeneous and heterogeneous reactions. Finally, because both pressure and temperature can be used to alter the compressible medium, there may be greater control of reaction pathways through both diffusion and transitionstate mechanisms.

References [ l ] A. A. Clifford, K. Pople, W. J. Gaskill, K. D. Bartle, C. M. Rayner, J. Chem. SOC.,Faraday Trans. 1998, 94,1451. [2] A. A. Clifford, Fundamentals of Supercritical Fluids, Oxford, 1998. [3] R. Feist, G . M. Schneider, Separation Science and Technology 1982, 17, 261. [4] E. Mack, J. Am. Chem. SOC. 1925, 47, 2468. [5] G. C. Maitland, M. Rigby, E. B. Smith, W. A. Wakeham, Intermolecular Forces, Clarendon Press, Oxford, 1981. [6] H. J. V. Tyrrell, P. J. Watkiss, DifSusion in Liquids, Butterworths, London, 1979. [7] H. H. Lauer, D. McManigill, R. D. Board, Anal. Chem. 1983, 55, 1370. [8] A. A. Clifford, S. E. Coleby, Proc. R. SOC.London, Ser. A 1991, 433, 63. [9] H. A. Saad, E. Gulari, Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 834.

66

I Introduction

01 H. Hippler, V. Schubert, J. Troe, J. Chem. Phys. 1984, 81, 3931. 11 J. F. Brennecke, in Supercritical Fluid Engineering Science, ACS Symp. Ser. 514, E. Kiran, J. F. Brennecke (Eds.), American Chemical Society, Washington, DC, 1993, Chapter 16. [12] S. R. De Groot, P. Mazur, Non-equilibrium Thermodynamics, North Holland, Amsterdam, 1962. 1131 S. C. Greer, Phys. Rev. 1985, A31, 3240. [14] D. Andrew, B. T. Des Islet, A. Margaritis, A. C. Weedon, J. Am. Chem. SOC. 1995, 117, 6132. [15] G. M. Simmons, D. M. Mason, Chem. Eng. Sci. 1972, 27, 89. [16] D. G. Peck, A. J. Mehta, K. P. Johnston, J. Phys. Chem. 1989, 93, 4297. [17] J. A. Berson, Z. Hamlet, J. Mueller, J. Am. Chem. SOC. 1962, 84, 297.

Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

2 Experimental Techniques 2.1 High-pressure Reaction Equipment Design RALFFINKand ERICJ. BECKMAN

High-pressure processes and equipment have been important to the chemical industry ever since the development of ammonia synthesis by Haber and Bosch. Today a wide range of industrial processes make use of high pressure technology, including the liquefaction of gases, the synthesis of low density polyethylene, and the synthesis of diamond and boron nitride. In particular, the use of supercritical fluid (SCF) solvents in separation and extractions [ 1-61, and chemical reaction processes [7-91 are described in numerous examples in the literature, and similar design and safety principles for both industrial processes and benchtop reactors (either for scientific or pilot scale purposes) can be applied. The wide variety in application and scale of high-pressure technology makes it necessary to be selective when considering the design and safety of such systems. Nonetheless, because of the numerous design examples it seems impossible to give a complete (or even partial !) description of high-pressure reaction equipment. Consequently, this chapter focuses on small-scale (benchtop) high-pressure equipment, their related safety questions, and their application to chemical reactions and extractionheparation as shown in the literature. It needs to be stressed that in-depth information is given in the corresponding literature and, and in particular, by the manufacturers of commercially available components, thus recommending the study of the specific catalogues. Most of the manufacturers mentioned in this chapter maintain pages on the World Wide Web, and further information can be obtained there.

2.1.1 Introduction The internationally accepted (SI) unit of pressure is the Pascal (Pa) and consequently for high pressure processes the megapascal. In synthetic applications it is more common to use the bar as the unit of pressure (1 MPa = 10 bar). Therefore, the unit used in this book is also the bar. Because various other units are still in use it is important to be familiar with the most important conversions.

68

2 Experimental Techniques

Table 2.1-1 Conversion factors of commonly used pressure units.

MPa 1 MPa

1

1 bar

bar

atm

10

9.8692

0.1

1

0.9869

1 atm

0.1013

1.0133

1

1 kg/cm2

9.8067X10-2 9.8067X10-' 0.9678

kg/cm2 10.197 1.0197

lb/in2 (psi)

mm Hg (tom)

145.04

7500.6

14.504

750.06

1.0332

14.696

760.00

1

14.223

735.56

1 lb/in2 6.8947~ 6.8947~ 6.8046X 7.0306X 1 51.716 (psi) 1 mm Hg 1.3332x1O4 1 . 3 3 3 2 ~ 1 0 - ~1 . 3 1 5 8 ~ 1 0 - ~1 . 3 5 9 5 ~ 1 0 - ~ 1.9337X10-2 1 (tom)

Table 2.1-1 gives conversion factors for commonly used pressure units in the literature. A list of additional useful conversions can be found in Appendix A. The literature is not entirely consistent as to the definition of high and low pressure. However, at pressures above 6000 bar the question of material choice (particularly if corrosive fluids are to be employed in the system) becomes critical and a specialized reactor design must be chosen [17,18].As such, this discussion focuses on systems designed for use at pressures below 6000 bar. The design of high pressure equipment makes it necessary to consider first some essential questions: (1) the maximum pressure at which the reactor will be operated; (2) volume and shape of the inner chamber (stirrer type and shape); (3) whether a batch process will be sufficient, or continuous feeding or extraction of products will be required; (4)temperature range of the process (control, rate of change, removal of exothermic reaction heat); (5) whether corrosive materials will be used; (6) miscellaneous factors such as fast opening, easy cleaning. In all cases conservative safety factors of at least 2:l (yield strength) or 4:1 (ultimate strength) for allowable stresses should be applied. Additionally, it should be kept in mind that repeated application of high pressure to components, such as in repetitive loading or generation of pressure, or as a result of mechanical vibration, may cause fatigue cracks that can propagate through the wall of the component so that leakage can occur. However, if the material of construction lacks toughness, the crack may lead to fast fracture and the component fails catastrophically. The numerous possible high pressure applications and their associated requirements can be satisfied by a variety of different assemblies of reaction vessels, closures, tubing, valves, pumps or compressors. First, an overview of these different components is presented, after which several complete high-pressure systems found in the literature are discussed. At this point it should be emphasized that in showing equipment from one manufacturer or another, no preference for a particular product is being expressed. Further details on equipment employed for spectroscopy at high pressure are provided in Chapter 3.

2. I High-pressure Reaction Equipment Design

69

2.1.2 Basic Equipment and Components 2.1.2.1

Design of Thick-Walled Vessels

Much information is available on the deformation and fatigue behavior of simple thick-walled cylinders [ 10-171, but it must be remembered that most process reactors will not be a simple hollow cylinder. Components such as connectors, threads and sleeves, windows, and removable closures make a complete analytical solution for a high-pressure system design problem quite involved. Useful design criteria for thick-walled vessels can be derived, however, under the,assumption that the material of which the vessel is made is isotropic and that the cylinder is long (more than five diameters) and initially free from stress. The radial and tangential stresses in the walls are then only functions of the radius coordinate ( r ) and the internal pressure. Given the outer-toinner wall radius ratio as o/i = w, and the yield point (Yo) of the material, the yield pressure (py)is p y = Yo (w-1)/(3

(2.1-1)

The yield pressure is hereby defined as the point where the material (e.g. AISI 316 steel) reaches its elastic limits, namely where any pressure increase irreversibly deforms the material. As seen in eq (2.1-1), as the wall thickness increases, the inherent strength of the material is employed less efficiently. For example, increasing the wall thickness ratio w beyond 2 does not increase the yield pressures significantly, and p y asymptotically approaches Yd3'" as w approaches infinity. One solution in order to achieve higher yield pressures is to utilize steels with higher yield points. The yield point, the typical composition, and other characteristic data of high-strength austenitic stainless steels are listed in Table 2.1-2. In particular, these steels are resistant to atmospheric corrosion and exhibit excellent resistance to stress-corrosion cracking (e.g. AISI 316 stainless steel is frequently used for high-pressure tubing and reactors). However, due to the nature of steel, acidic conditions lead to an increased rate of corrosion [17,18]. This may become a problem in COz-water mixtures, where the pH can easily drop to ca. 3.0. Clearly, use of supercritical water presents an extreme challenge in material selection. An alternative strengthening mechanism is prestressing, called autofrettage, which ensures a more uniform stress distribution under load [16-201. This intentional over-pressurizing of the vessel leads to a plastic-elastic interface that moves outward as the pressure increases. When the pressure is released, the residual stresses left as a result of radial expansion allow the vessel to be used up to the pressure at which it was subjected to autofrettage, without exceeding the yield point of the material. Easier to achieve than the autofrettage in order to increase the yield pressure is the construction of a compound cylinder. Here a second cylinder (or jacket)

70

2 Experimental Techniques

Table 2.1-2 Typical high-strength austenitic stainless steels [ 171.

Steel

Composition (wt. %)

AISI 202

AISI 301

AISI 316

Micromach (Washington Steel Co.)

USS Tenelon

0.15 max. C, 7.5-10 Mn, 17-19 Cr, 4.6 Ni 0.1 c, 2.0 max. Mn, 1.0 max. Si, 17.0 Cr, 7.0 Ni 0.08 C, 2.0 Mn, 1.0 Si, 16-18 Cr, 10-14 Ni, 2.0-3.0 MO 0.11 C, 13.5 Mn, 16.2 Cr, 4.6 Ni, 0.92 V, 0.36 N 0.08-0.12 C, 2.0 max. Mn, 1.0 max. Si, 17.3 Cr, 6.2 Ni 0.10 C, 14.5 Mn, 17.0 Cr, 0.4 N

Maximum yield strength (bar)

Maximum tensile strength (bar) ~

Elongation

Hardness

(“/.I

(Rb)

3900

7000

55

90

2800

7000

60

85

2600

6000

65

78

18 000

19 000

2

12700

14 100

14800-16900 15800-17900

3-9

is heated and then contracted around the inner cooled cylinder, such that the yield pressure becomes:

p y = 2Yo ( ~ o ~ c * - ( c ~ + ~ ~ x ~ ) / ( ~ w ~ ) ~ / ~

(2.1-2)

where o = outer radius, i = inner radius, and c = intermediate wall radius. When c = (oi)’l2

p y = 2Yo ( 0 - 1 ) / ( 3 ~ ~ ) ~ / ~

(2.1-3)

The temperature limit for the outer heat-treated cylinder, as well as for the autofrettaged vessel, is limited to 500°C; otherwise the temper is lost (Figure

2.1-1). Both high and low temperatures have a tremendous effect on the physical properties of materials used to construct high pressure vessels. With regard to very low temperatures, the toughness and impact strength of most alloy steels decrease as the temperature is reduced and one has to choose the applied materials carefully to avoid embrittlement. At high temperatures, however, the

2.1 High-pressure Reaction Equipment Design

71

Figure 2.1-1 Thick walled vessels: (a) double walled cylinder with interference fit d; (b) single walled, autofrettaged. (0 = outer radius, i = inner radius, c = interference

diameter).

yield and tensile strength of steel decreases. For most alloy steels used in construction of high-pressure equipment, at temperatures above 350 "C the stresses to the wall of the vessel are no longer independent of time. Additionally, any chemical reaction performed in such vessels that produces exothermic heat can produce a temperature gradient, possibly exceeding the temperature limitation of the vessel at the inner wall, thereby leading to creeping loss of temper and possible vessel failure. 2.1.2.2

Closures and Connectors

In general, the vessel will be tight if the seal pressure (p,) is higher than the inner pressure @i). Various closure designs have been made to provide the needed seal pressure [21-251. Closures usually consist of three basic elements: the cover, a coupling device keeping the cover in position to the vessel body, and a seal between the cover and the vessel. The sealing ring or gasket is always made of a softer material (such as copper or an elastomer such as Buna N). The seal deforms under pressure, smoothing and/or filling any irregularities on the connecting surfaces. The O-ring is a typical example (Figure 2.1-2(a)). Initial sealing is afforded by the diametric squeeze on the O-ring; then, after the internal pressure is applied, the O-ring is forced to the side of the groove. The maximum pressure, which can be applied to such seals is about 300 bar. Similar seal designs with an optimized support for the O-ring (or a soft metal flat gasket) allow significantly higher pressure ratings of up to ca. 15 x lo3 bar (Figure 2.1-2(b)). Additionally, O-rings made of Buna N, for example, tend to swell in a carbon dioxide environment, thus increasing the seal pressure but preventing the use of highly corrosive chemicals and temperatures above 90 "C. Two other draw-

72.

2 Experimental Techniques

Figure 2.1-2 Different seal designs: (a) O-ring; (b) soft gasket; (c) unsupported-area seal (Bridgeman seal).

backs to the use of elastomeric O-rings are that non-covalently bound additives can be extracted during use at high pressure, interfering with spectroscopic studies, and the swelling of O-rings can lead to their explosive failure if depressurized too quickly. The latter problem can be dealt with, however, by the use of elastomers with a higher crosslink density (thus decreasing swelling), and their low cost makes them attractive seals. Although polytetrafluoroethylene (PTFE) gaskets are popular in chemistry, as they are resistant to most chemical environments, they are known to swell in carbon dioxide (see Chapter 2.5 and 4.7). This could cause problems if PTFE is used to seal against moving parts where tolerances are tight. Pressure-energized seals (Bridgeman seals) have pressure ratings up to 20 X lo3 bar and can also be used for leak-prone gases such as helium and hydrogen. Support rings made of Teflon or a soft metal (such as copper) prevent the extrusion of the gasket during use at high pressure. These seals use the principle of unsupported area (a) in order to provide the condition ps>pi (Figure 2.1-2(c)). While the inner pressure acts over the total area ( A ) of the plug head (sometimes referred to as a mushroom plug), the seal pressure is constrained over the reduced area A-a, thus increasing the stresses in the seal ring higher than the working pressure in the vessel: (2.1-4) However, the seal pressure cannot be increased without limit. In addition to extrusion of the seal ring between the plug and the wall at pressures higher than 2000 bar, the seal is also pushed to the wall and the plug, which can lead to the deformation or failure of the wall or stem material. For corrosive materials, or when the inner fluid should not be contaminated with any kind of seal lubricants, metal-to-metal seals have been successfully employed. In this case, for example, two conical segments impact along a line, making easy opening of the vessel possible. Coupling devices to keep the closure in place are often bolted flanges. These flanges have to withstand not only the internal pressure, but also the stress, which is induced by (repeated) tightening. Flanges can be integrally forged

2. I High-pressure Reaction Equipment Design Gland

Sleeve

73 Collar

Figure 2.1-3 Tubingvessel connections for pressures up to 10000 bar makes use of threaded tubing (High Pressure Industries Co.).

to the reactor body or alternatively screwed (welded) to it. As a rule, the diameter of the bolt circle should be as close as possible to the gasket so that the coupling takes place nearby, thus reducing the thickness of the end closure. Another solution is the use of screwed closures, which are inexpensive and easy to open and close. It should be kept in mind that in case of leakage the coupling devices as well as any closure should never be loosened or tightened under pressure, because enormous stresses are developed in the flanges, especially when the pressure changes during operation. In order to perform intertubing or tubing-vessel connections, sleeve or collar type connectors are used. For pressures of up to 1000 bar and tubing diameters smaller than 3/8", taper seal connections are suitable. Here a gland nut is tightened onto the connector, ensuring the metal-to-metal seal of the sleeve. For pressure applications of up to 10000 bar the tubing is additionally threaded into the sleeve (collar), improving the tubing-sleeve connection (Figure 2.1-3).

2.1.2.3

lhbing and Fittings

In general, tubing can be seamless, welded or drawn. For the highest pressure ratings (above 1000 bar) cold-drawn, seamless tubing is preferred. Preferred materials include AISI 304 and 3 16 stainless steel which exhibit, especially at small diameters, very high pressure ratings [26-281 and are stress-corrosion resistant as long as acidic conditions are avoided. Some sizes and related ratings of commercially available tubing are given in Table 2.1-3. Despite these high (and naturally conservative) ratings, the tubing for a system should be chosen with great care. In particular, the pressure in the tubing of a reciprocating pump cycles much more than that of the system, and thus it is subject to more repetitive stressing than other parts of the system. Furthermore, any scratches or nicks derived from bending or installation under stress must be avoided. The tubing can be cut by hand or machine, although with small tubing 'care should be taken to avoid crushing [10,26-281. Tubing should always be secured to the benchtop or the frame of the apparatus.

74

2 Experimental Techniques

Table 2.1-3 Pressure ratings of typical high pressure tubinga ~

Outer diameter (in)

Inner diameter (in)

odlid ratiob

Material'

1/16 1I4 318 1I4 318 314 1I4 318

0.03 0.125 0.25 0.109 0.203 0.438 0.063 0.063

2.08 2 1.5 2.29 1.85 1.71 4 6

316 S.S. 316 S.S. 304 S.S. 316 S.S. 316 S.S. 316 S.S. 316 S.S. 304 S.S.

~~

Pressure rating (bar)

~~~~~

100 66 66 200 200 200 6600 10000

All values are approximate, based on typical data published by manufacturers. Ratings for specific tubing may differ. Temperature, use of corrosive materials, and fatigue factors should be considered sperately. od = outer diameter, id = inner diameter. ' In these cases all tubings are cold drawn, seamless, and not annealed. a

2.1.2.4

Valves

As with all other parts of a high pressure system, an initial decision has to be made as to the working conditions (pressure range, temperature, corrosive materials, size, etc.) and purpose (odoff-valve, flow rate adjustment, flow restrictions when open) a valve should fulfill [29-301 prior to selection. In some applications, such as measuring the swelling of polymers via the differential pressure method, or when employing expensive reagents, valves with low dead volume are also desirable. A great variety of commercially available designs are able to match almost any requirement. For many applications a simple ball valve (on/off) is suitable and, although these valves are usually operated manually in lab-scale systems, use of pneumatic or hydraulic actuators allows remote control. Needle valves can also be used in high-pressure applications of up to 15 x lo3 bar, particularly where greater sensitivity in flow control is desired. When closing the valve, the needle moves vertically into the valve seat. The stem tip is made of steel but soft packing can also be used. Several stem designs are available, such as from High Pressure Equipment Co. (HIP), where the bottom part of the stem is attached to the body with an internal screw thread, or in case of the rolling design where the stem floats within the outer threaded enclosure (Figure 2.1-4). The top part of the stem does not move vertically, but rather rotates and thereby moves the bottom part up or down. These designs, as is the case with most modern valves, avoid the rotating of the tip on the seat, which could lead to grinding and galling of the stem and seat parts. Check valves ensure flow in one direction only [31,32]. Their design consists of a spring which holds a piston, a disc, or a ball in place to open or close the connection at a predetermined pressure, set by the tension in the

2. I High-pressure Reaction Equipment Design

75

Figure 2.1-4 Example of two-way odoff-valve designs: (a) positive guide stem, (b) rolled style stem (taper seal, rated for ca. 650 bar, High Pressure Equipment Company).

spring, in the direction of flow only. Pressure ratings up to 10000 bar are available for ball type check valves, which renders them particularly useful in compressors (Figures 2.1-5 and 2.1-6). At considerably higher spring forces, the check valve finds its use as a safety feature - the pressure relief valve [33]. As the internal pressure increases above the preset release pressure the valve opens and closes automatically, protecting the vessel and its components from overpressure (Figure 2.1-5). Relief valves with pressure ratings of up to 2000 bar are available [10,26,34]. A relatively simple method to prevent the system from failing catastrophically due to overpressure is the use of a rupture disc. Whereas the relief valve opens and closes, releasing excess material (and thus lowering pressure) while maintaining the vessel operational, the rupture disc bursts, venting the entire contents of the vessel [35]. For further use the disc must then be replaced. The rupture disc should be used at significantly lower working pressures than the burst pressure (ca. 90% of the burst pressure). Ratings for the design shown in Figure 2.1-5 are up to 400 bar. Filters should be used when a system part (mostly a recirculating pump) needs to be protected from solid impurities. Line filters such as shown in Figure 2.1-5, with customized filter sizes and pressure ratings, should only be used where small amounts of impurities are expected, otherwise clogging will occur. Filters designed to withhold higher amounts of solids, e.g. during venting of a batch reactor, usually have a higher filter volume [26]. The design of back-pressure regulators uses a similar principle as that employed in check valves. Here the state of the valve (openklosed) is set by three factors: the spring force, and the inner and outer pressures. For example, at a given inner pressure the opening (and closing) of the valve is determined by the spring force and the outer pressure, maintaining a constant ratio between outer and inner pressure and thus allowing, for example, a controlled release under changing working conditions. Back-pressure regulators are commonly used in gas tank regulators, keeping outlet pressure constant during changing inlet pressure conditions. The pressure difference between inlet and outlet for a back-pressure regulator can either be set manually, or one can

76

2 Experimental Techniques

Figure 2.1-5 Scheme of (from the top): (a) check valve, (b) line filter, (c) rupture disc, (d) pressure relief valve (High Pressure Equipment Co.).

employ an automated version such as that manufactured by JASCo (Japan Spectroscopy Co), although the latter is significantly more expensive. Sometimes the injection of an accurately defined amount of a fluid to a highpressure reactor is necessary. Sample injection valves as used in high-performance liquid chromatography (HPLC) fulfill such needs [36]. In the loading mode a fluid fills a defined inner volume, moving into the reactor upon adjusting the valve to discharge mode. Such systems are often combined with displacement pumps of the syringe type (see Figure 2.1-6).

2.1.2.5 Compressors and Pumps The definitions of “compressor” and “pump” given in the literature are not entirely consistent [37-391. Here, compressors are referred to as units for compressing and liquefying gases, whereas pumps are used for recirculating and compression of fluids. However, both classes can be further distinguished by the basic mode of ‘operation: either a rotating blade or a reciprocating piston.

2.1 High-pressure Reaction Equipment Design

77

Piston Handle

Figure 2.1-6 (a) Small-volume reciprocating pump head. Single ball check valves are used for controlling the airlfluid flow (Superpressure Inc.). (b) Laboratory scale positive displacement pump (syringe pump; High Pressure Equipment Co.).

Reciprocating compressors make use of pistons similar to those in an automobile engine. In general, a gas or fluid is drawn into the inner chamber through an inlet check valve, compressed and expelled through an outlet check valve, all by moving a piston. As these pistons need lubrication, reciprocating compressors may have the disadvantage of the contamination of the compressed gas or fluid with such lubricants. Today the use of a diaphragm avoids this effectively [32]. Typical compression ratios of 10-2O:l are obtained for each working stage. Further important design factors are the operating pressure and the flow rate (m3/s, at standard condition). The compressor itself can be driven by hand, compressed air or a motor, yielding outlet pressures for benchtop systems of up to 2000 bar. However, because of the nature of such pistonbased systems, the fluctuating pressures during the different operational stages may be a disadvantage, especially when used for recirculation at constant pressures. Rotary type compressors give a more pulse-free output. In such a unit the fluid is sucked into rotating blades, from which it gains kinetic energy. This energy can be converted into flow or compression, although such systems are typically used as pumps for recirculating purposes. Rotary systems can be further divided into kinetic or positive displacement type. Although the design of the kinetic type is similar to that of a turbine, the positive displacement pump consists of a piston or a screw which displaces the fluid in a single con-

78

2 Experimental Techniques

tinuous working step (reciprocating systems are therefore sometimes also referred to be a positive displacement pump). Such displacement pumps can be driven manually (syringe type, Figure 2.1-6) or electrically (for example, those manufactured by ISCo), and are ideal for the metered addition of fluids to a reaction vessel. For highest accuracy of sych addition, sample injection valves as already described above can be used. 2.1.2.6 Stirring and Mixing As austenitic stainless steel is not magnetic, it is sufficient in some cases (for example in small reactors in which only low-molecular-weight liquid reagents are employed), to use a simple magnetic stirring bar. For higher volumes, very low (and very high) viscosities, and in particular for reactions containing a solid phase, more vigorous stirring is often necessary. In principle, there are two different types of stirrers: packless magnetic drives and stirrers with self-sealing packing glands. The magnetic drive offers, because the inner rotor is completely enclosed within a non-rotating housing with fixed seals, higher achievable working pressures. The upper pressure limit of such stirring assemblies is limited primarily by the type of seal used (e.g. Buna N O-ring, Figure 2.1-7). As the use of most type of seals also limits the upper reaction temperature, the stirring unit should be equipped with a cooling jacket to protect the seal from excessive temperatures [40]. 2.1.2.7 Optical Windows

Optically transparent windows for high-pressure applications are typically made of synthetic sapphire [41]. Their form is usually of a cylinder which can be sealed for high pressure applications by using, for example, two opposed O-ring seals, rated for ca. 500 bar, (Figure 2.1-8). More detail on the use of high-pressure windows to perform spectroscopy in high-pressure environments is presented in Chapter 3.1, devoted to vibrational spectroscopy. A variety of materials have been used to construct high pressure windows, and information is available in the literature regarding the relationships between material specification and the chemical environment/application, the transmittance for a specific wavelength [42], and the cell design itself [43].

2.1.3 High Pressure Systems In general, high-pressure systems can be distinguished as either batch or continuous process reactors. Each type can then be subdivided by their use for chemical reactions or for extraction/fractionation purposes. Continuous reaction processes often consist of a reaction unit followed by an extraction process in order to recover at least one of the reactants.

2.1 High-pressure Reaction Equipment Design

Figure 2.1-7 Packless magnetic drive with attached water cooling sleeve (ratec for ca. 300 bar, Parr Instrument Co.).

Figure 2.1-8 Window assembly with two opposite O-ring seals (similar to that used in Figure 2.1-9).

79

2 Experimental Techniques

80

2.1.3.1 Single-batch High-pressure Reactors The simplest type of batch high-pressure reactor is essentially a hollow cylinder with one or two end caps, often referred to as a pressure bomb. Such vessels can be purchased from a variety of manufacturers, including Pam, Autoclave Engineers, and Pressure Products Industies, to name just a few. These reactors typically are equipped with a stirring system, such as described in an earlier section, a heating jacket, and ports for introduction of a thermocouple, as well as addition or removal of samples. Sizes range typically from 50 mL up to several liters, and pressure and temperature ratings vary from one manufacturer to another. Although the reactors described above are suitable for a number of situations, it is often found beneficial in the course of performing research in supercritical fluid technology to design and construct a reactor for a specific purpose/project, completing the system through the purchase of commercially available auxiliary components (stirring assembly, pressure transducers, thermocouples, and controller). One example of such a system is shown in Figure 2.1-9,where this system has been used primarily for emulsion and dispersion polymerization in carbon dioxide [44,45]. It was felt that this approach provided more flexibility in system design at a lower cost. The high pressure reactor shown in Figure 2.1-9is made of AISI 316 stainless steel with an internal cylindrical cavity of ca. 55 mL, and has been used at pressures and temperatures up to ca. 400 bar and 80°C.The inner diameter is approximately 3 cm while, as suggested by eq (2.1-1),the wall thickness is twice as large. The reactor is equipped with two opposed sapphire windows (1.5 cm thickness) which are held in place by screwed closures as shown in Figure 2.1-8,with high-hardness Buna N O-rings used as seals. Although the elastomeric O-rings must be replaced after every polymerization experiment, their low

T

Avent

P

tt

C02-tank

-

I

Rupture disc

Variable Speed Drive

11

L7-1

II

U

- ~

L S u..w-i Valve ~

55 mi High Pressure Reactor

wlth View Wlndows

Figure 2.1-9 Scheme of a single batch high pressure view reactor.

2. I High-pressure Reaction Equipment Design

81

cost renders the economic impact of this choice negligible. The use of windows in the reactor allows visual monitoring of the reaction and also the performance of photochemistry. Three ports provide access to the inner chamber, allowing C 0 2 supply, venting, and monitoring of the temperature. A thermocouple is introduced into the_ inner chamber and sealed using a high-pressure fitting which had been enlarged to allow passage of the thermocouple. Heating of the reactor is accomplished using cylindrical elements (4 X 200 W, 240 V) pushed into holes drilled into the outer reactor walls. The reactor cap is fitted with an adapter machined to allow connection of a commercial magnetic stirrer, as high intensity mixing is vital to a successful emulsion or dispersion polymerization. Other auxiliary components (temperature controller, pressure transducers, valves, fittings, tubing) are obtained from commercial suppliers, as described previously. Various reactor outputs (temperature, pressure, torque on the stirrer) can be recorded on a PC using a standard A/D board. Although in Figure 2.1-9 the rupture disk is shown connected to the reactor via tubing, in most cases it should be connected directly to the reactor for maximum safety. A typical continuous reactor would again be a hollow cylinder with end caps, where material would be introduced at one end and removed from the other. The end caps should be fitted with filters (as in those manufactured by Thar designs, for example) to retain catalyst or other solids in the reactor, preventing contamination of the pump or compressor. It is important to note that a major stumbling block in continuous processing at high pressure, either in the lab or on a commercial scale, is that robust technology for the continuous introduction (and removal) of solids to high-pressure environments is not currently available commercially. Most fundamental kinetic quantities needed for scale-up can be obtained in batch systems, particularly if real-time spectroscopic analysis is used to measure product formation (see Chapter 3). This may often be preferred as batch systems use much smaller quantities of material. The problem of the defined addition of fluids or gases into a laboratory scale high pressure reactor can be overcome, for example, by the use of a syringetype positive displacement pump [46]. Furthermore, for small and highly accurate volumes a sample addition valve should be used. A reactor in which both have been utilized is shown in Figure 2.1-10. In this system the kinetics of a heterogeneous palladium-catalyzed hydrogenation of a C02-soluble anthraquinone derivative, the first step of a C02-based synthesis of hydrogen peroxide, have been studied. As usual, aromatic redox processes of this kind can be easily monitored using UV spectroscopy.

82

2 Experimental Techniques

6

Nltank

ve [47].

per stroke, ca. 300 bar) and the UV-cycle are filled with liquid carbon dio while T-valve 5 remains dosed, thus leaving the UV-spectrometer cycle uncontaminated from the anthraquinon by revolving the sample injection and opening valve 4;the syringe pump is used (T1 open) to pump the hydrogen through the loop into t by the changes in th none is consumed. I is equipped with an internal stain front of the spectrometer prevents any possible clogging or contamination of the UV cell with itoring of changes in almost pulse-free an pressure gear pump (Micropump, Concord, CA) was used.

v4

T1

Figure 2.1-11 Sample addition valve in loading position (Rheodyne Inc., rated for ca. 450 bar).

2. I High-pressure Reaction Equipment Design

2.1.3.2

83

View Cells

The successful implementation of processes employing near-critical or supercritical fluids as solvents depends to a great extent upon knowledge of the phase behavior of reactants and products as a function of temperature, pressure, and composition. For example, use of a process simulator such as that developed by Aspen Technology, Inc. (Aspen Plus Process Simulator, Version 10.0) to design a high pressure separation process is relatively straightforward if one has the relevant equilibrium thermodynamic information (choice of modeVequation of state, pure component and mixture parameters) and transport properties (flow rates, densities, viscosities) close at hand. Equilibrium thermodynamic infom'ation is most easily obtained through use of a variable volume, high pressure view cell, which allows one to find the equilibrium pressure or temperature of phase separation at constant composition, and also the compositions of the phases in equilibrium if samples can be withdrawn from the view cell. For the case of extractions, one usually requires equilibrium partition coefficients, i.e. the extent to which a particular substance will partition between two phases (for example, water and COs) vesus temperature, pressure, and perhaps composition. This requirement is no different than that for the design of a traditional extraction column. It should be noted that it is difficult to scale up (and analyze for economic viability) a process from purely supercritical fluid extraction (SFE) data, as such data incorporates both equilibrium and transport influences, whose relative contributions may not be easy to separate. Thus, SFE laboratory-scale systems are not examined in this chapter, although some information is given in Chapter 2.2. The simplified scheme of a variable volume view cell is shown in Figure 2.1-12 (D.B. Robinson and Associates, Edmonton, Alberta, Canada) [47,49]. The cell is rated to 200°C and 700 bar. Mixing is accomplished by mechanically rocking the entire cell. The main component is the thick-walled, hollow Oil Rerewoir

Figure 2.1-12 Scheme of a high pressure view cell (Robinson Cell, D.B. Robinson and Associates) [47].

84

2 Experimental Techniques

cylinder (Pyrex) into which a movable piston is introduced. The piston is equipped with a Bridgeman-type seal, which maintains its integrity as the piston moves vertically within the cylinder. The cylinder itself is completely surrounded by silicone oil, eliminating any pressure gradient across the cylinder wall. Besides the piston, the sample vdume is isolated from the silicone oil on the top of the glass cylinder by supported O-rings. As the glass wall and the oil are transparent, the contents of'the sample area can be viewed under high pressures through borosilicate windows on each side of the high-pressure cell. Again, the pressure and temperature can be monitored using a pressure transducer and a thermocouple respectively. In addition to the design described above, a highly successful variablevolume view cell introduced by McHugh [49] has been widely applied, where essentially a view cell is affixed to the end of a manual syringe pump, creating a variable-volume view cell. A device similar to that designed by McHugh is available commercially from Supercritical Fluid Technology (Newark, DE). In addition, a recent design by Franck and co-workers [50] differs from the preceding two in its use of water as a hydraulic fluid, which can be useful for applications where small or even minute silicone contamination needs to be avoided. Finally, visual inspection of the phase behavior in a high-pressure cell is not desirable from a safety standpoint, and thus some commercially available view cells (such as that from Thar Designs) come equipped with a video recorder with remote camera. An important feature to a high pressure view cell, particularly if polymeric samples are to be examined, is the presence of a high intensity mixer. Although any of the designs mentioned above will provide the location of phase boundaries (versus temperature and pressure), it is also important to know the compositions of the two phases in equilibrium. Note that while tie lines (lines connecting phases in equilibrium on T-x or p-x diagrams) are horizontal for simple binary mixtures, this is not true for phase separation in multicomponent systems (most notably polymer-fluid systems where the polymer sample contains chains of various lengths). Consequently, ports which allow withdrawal of samples following phase separation and equilibration are an important feature of view cells. Such ports also allow for the measurement of partition coefficients of solutes between, for example, aqueous and CO2 phases. 2.1.3.3 Systems for Continuous Processing Batch or semicontinuous processing is appropriate for many laboratory scale operations. However, continuous processes are typically used by industry to allow higher throughput with smaller equipment size. To gain scale-up information, the importance of relatively inexpensive but fully operable pilot plants (or miniplants) cannot be overestimated. Many types of continuous reactors (fixed bed, fluidized bed, continuous stirred tank CSTR) can be adapted for high pressure service.

2. I High-pressure Reaction Equipment Design

85

Krukonis et al. [511 developed a continuous-countercurrent process using scCO2 for the production of high purity esters (eicosapentanoic acid (EPA)/ docosahexanoic acid (DHA)) from urea-adducted ethyl esters derived from menhaden oil (Figure 2.1-13), able to produce ca. 4.5 kg per day EPA (at 90% yield and 90% purity). The separation from the feed is based on solubility differences of the esters in C02, mainly caused by their different chain lengths (c16, C18, C20, and C22).Consequently, the first step in the design of such a continuous-countercurrent process is the acquisition of the equilibrium partition coefficient data for the major components of the anticipated feed material. Based on these partition coefficients a decision has to be made at which pressures and temperatures it is useful (efficiency, cost) to operate. Krukonis et al. selected a temperature of 60°C and a pressure of ca. 150 bar. In Figure 2.1-13 the scheme of this pilot plant is shown. Ester-containing streams are shown with bold lines. The ester feed is introduced in the side of the first of the two columns (C-1). At the chosen reaction conditions the ester density exceeds that of the scCOz and the esters tend to move down the column, contacting the C 0 2 which is fed in continuously from the bottom. The C02 strips the shorter-chain-length materials present in the feed, whereas the esters with the higher molecular weight remain at the column bottom. At the top of C-1, the c16 and c18 esters are condensed by reducing the pressure (e.g. back-pressure regulator) or by increasing the temperature, while the depleted C02 is recycled to the column. In addition, a portion of the liquid extract is returned to the head of the column in order to provide a constant

0XhCl

COT+ makeup

I Figure 2.1-13 Scheme of the continuous extraction of esters (eicosapentanoic acid (EPA)/docosahexanoic acid (DHA)) [5 11.

86

2 Experimental Techniques

downward-flowing reflux stream in a countercurrent manner (as is the case for any distillation or liquid-liquid extraction). The C20-rich and C22-ri~hliquid phase collected at the column bottom is then fed into C-2, which operates in the same way as the first column, thus separating the fraction into C20-rich and C22-ri~hphases.

2.1.4 Summary This chapter provides an introduction to the construction of high-pressure equipment for the laboratory. Clearly, it is not possible to cover all the necessary design materiaVoptions in such’ a short chapter, yet sufficient information should be available to allow researchers to explore chemistry in high-pressure fluids, such as C02, a field which continues to show great promise.

References [ l ] E. Karmana, B. Eiler, D. Maniero, M. Bedner, R. M. Enick, Res. Cons. Recycl. 1997, 20, 143. [2] P. Peterson, S . L. Reese, G. Yi, H. Yun, A. Malik, J. S. Bradshaw, B. E. Rossiter, M. L. Lee, K. E. Markidas, J. Chrom. A 1994, 684, 297. [3] M. A. McHugh, V. J. Krukonis, Supercritical Fluid Extraction, 2nd ed., ButtenvorthHeinemann, Boston, 1994. [4] J. W. King, G. R. List, Supercritical Fluid Technology in Oil and Lipid Chemistry, AOCS-Press, Champaign (Illinois), 1996. [5] (a) M. Gottesmann, R. Prasad, R. A. Scarella, US Patent 4341804; (b) K. Zosel, Angew. Chemie 1978, 90, 748; Angew. Chemie Int. Ed. 1978, 17, 702. [6] M. L. Cygnarowicz, W. D. Seider, in Supercritical Fluid Technology, T. J. Bruno, J. F. Ely (Eds.), CRC Press, Boca Raton (Florida), 1991. [7] B. Subramaniam, M. A. McHugh, Ind. Eng. Chem. Process Des. Dev. 1986, 25, 1. [8] P. E. Savage, S. Gopalan, T. I. Mizan, C. J. Martino, E. E. Brock, Reac. Kinet. Cat. 1995, 41, 1723. [9] C. B. Wu, S. C. Paspek, M. T. Klein, C. LaMarca, in Supercritical Fluid Technology, T. J. Bruno, J. F. Ely (Eds.), CRC-Press, Boca Raton (Florida), 1991. [lo] I. L. Spain, J. Paauwe, High Pressure Technology, Vol. I , Marcel Dekker, New York, 1977.

[ l l ] J. F. Harvey, Theory and Design of Pressure Vessels, Van Nostrand Reinhold Co. Inc., New York, 1985. [12] H. H. Bednar, Pressure Vessel Design Handbook, 2nd ed., Van Nostrand Reinhold Co. Inc., New York. [ 131 W. F. Sherman, A. A. Stadtmuller, Experimental Techniques in High-pressure Research, John Wiley & Sons Ltd., Chicester, 1987. [14] W. Cross, History of the ASME Boiler and Pressure Vessel Code, ASME, New York, 1989.

[15] V. C. D. Dawson, Computer Program for a Monobloc Hollow, Closed-End Cylinder Subjected to Internal Pressure, NOLTR (US.Naval Ordnance Laboratory), White Oak (Maryland), 1970, p. 70-41. [16] V. C. D. Dawsonc in High Pressure Technology, Volume I , I. L. Spain, J. Paauwe (Eds.), Marcel Dekker, New York, 1977.

2. I High-pressure Reaction Equipment Design

87

[17] (a) D. M. Fryer, J. F. Harvey, High Pressure Vessels, Chapman & Hall, New York, 1998; (b) H. H. Bednar, Pressure Vessel Design Handbook, Van Nostrand Reinhold Co. Inc., New York, NY, 1986. [18] T. Lyman, Metals Handbook, 8th ed., American Society of Metals, 1961. [19] V. C. D. Dawson, A. E. Seigel, Reversed Yielding of Fully Autofrettaged Tube of Large Wall Ratio, NOLTR (US. Naval Ordnance Laboratory), White Oak (Maryland), 1963, p. 63- 123. [20] K. V. Raghavan, Chem. and Proc. Eng, 1970. [21] D. F. Fredrick, Technical reprint 2751, Autoclave Engineers, Inc., Erie (PA). [22] W. S. Goree, B. McDowell, G.A. Scott, Rev. Sci. Instr. 1965, 36, 99. [23] Machine Design, Reference Issue, Seals, 9th ed., Penton Publishing Co., Cleveland (Ohio), 1969. [24] B. A. Niemeier, Trans. A. S. M. E 1953, 369. [25] R. A. Rothman, in Chemical Engineering/Deskbook, McGraw-Hill, New York, NY, USA, 1973. [26] High Pressure Equipment Co. Catalogue, 12/97, Erie, PA, USA. [27] S. Crocker, Piping Handbook, McGraw-Hill Book Company, New York. [28] W. C. Mack, Chemical Engineering 1976, 83 (June 7), 145. [29] A. Brodgesell, in Chemical Engineering/Deskbook, McGraw-Hill, New York, NY, USA, 1971, p. 218. [30] Fundamentals of Valves, Petroleum Managment, The Engineer, General Section 1963. [31] A. Fleming, Instruments of Control Systems 1967, 40. [32] Superpressure Inc. Co. Catalogue, Newport Scientific, Inc./Superpressure Div., Jessop, MD, USA. [33] R. Kern, Chemical Engineering 1977, 84, 187. [34] Nupro Co. Catalogue, Nupro Company, Willoughby, OH, USA. [35] V. Ganapathy, Chemical Engineering 1976, 83 (Sep. 13), 199. [36] J. W. Hutchison, ISA Handbook of Control Valves, 2nd ed., Instrument Society of America, Pittsburgh (PA). [37] I. J. Karassik, W. C. Krutzsch, W. H. Fraser, J. P. Messina, Pump Handbook, McGrawHill Book Co., Cleveland, 1976. [38] American Petroleum Institute Publications, (a) #822-61700; Standard 61 7, Centrifugal Compressors for General Refinery Services, 2nd ed., 1973; (b) #822-61800; Standard 618, Centrifugal Compressors for General Refinery Services, 3rd ed., 1974; (c) #822-6 1900; Standard 619, Rotary Type, Positive Replacement Compressors for General Refinery Services, American Petroleum Institute, Washington, DC. [39] D. H. Newhall, Industrial and Engineering Chem. 1957, 49, 1949. [40] Parr Instruments Co. Catalogue, Parr Instrument Co., Moline, IL, USA. [41] Optical Materials for Infrared Instrumentation, U.S. Department of Commerce State of the Art Report, PB 181087. [42] W. Paul, W. M. DeMeiss, J.M. Besson, Rev. Sci. lnstr. 1968, 39, 928. [43] S . J. Gill, W.D. Rummel, Rev. Sci. lnstr. 1961, 32, 752. [44] F. A. Adamsky, E. J. Beckman, Macromolecules 1994, 27, 312. [45] (a) C. Lepilleur, E. J. Beckman, Macromolecules 1997, 30, 745; (b) R. Fink, E. J. Beckman, Macromolecules, submitted for publication. [46] A. Bertucco, P. Canu, L. Devetta, Ind. Eng. Chem. Res. 1997, 36, 2626. [47] M. S. Super, R. M. Enick, E. J. Beckman, J. Chem. Eng. Data 1997, 42, 664. [48] Y-H. Li, K. H. Dillard, R. L. Robinson, J. Chem. Eng. Data 1981, 26, 53. [49] M. A. McHugh, A. J. Seckner, T. J. Yogan, Ind. Eng. Chem. Fund 1984, 23, 493. [50] R. Diguet, R. Deul, E. U. Franck, Ber: Bunsenges. Phys. Chem. 1987, 91, 551. [51] V. J. Krukonis, J. E. Vivian, C. J. Bambara, W. B. Nilsson, R. E. Martin, in Seafood and Biochemistry: Composition and Quality, G.Flick, R. E. Martin (Eds.), Technomic Publishing Co., Lancaster (PA), 1992, 169.

Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

2.2 Extraction and Related Separation Techniques GERDBRUNNER

2.2.1 General Aspects of Supercritical Fluids as Mass Separating Agents Separation processes with supercritical gases, called supercritical fluid extraction (SFE),is a group of separation processes that applies supercritical fluids (SCFs) as separating agents in the same way as other separation processes, such as liquid-liquid extraction or absorption, make use of liquid solvents. In these processes, the solvent is a supercritical component or a supercritical mixture of components [ 1-31. SCFs may be used in the same way as other ordinary solvents taking into account their different properties and behaviors. Supercritical fluids can replace liquids solvents in many processes, such as extractions from solids (leaching), countercurrent multistage separations, chromatographic separations, and others, provided the solvent properties of the SCFs are adequate. SFE must be seen in comparison to separation processes which are well known, widely used, and sufficiently well understood. These processes comprise the various forms of distillation (rectification), evaporation, crystallization, liquid-liquid extraction, liquid-solid extraction, and stripping. SFE is used only if there are major advantages apparent at an early point of decision. In synthetic chemistry, SFE can be attractive as an alternative to conventional methods for purification of reaction products such as vitamins, pharmaceuticals and many other high-value products [4]. Its technical use is currently mainly restricted to applications in food industry for extraction from natural products and in some cases for the fractionation of the products [ 5 ] . This chapter therefore focuses on these types of production and purification processes, but all the methodologies discussed may be readily adapted to synthetic applications. Equipment for carrying out separation processes of various sizes is available from commercial suppliers, and is described in more detail in Chapter 2.1 and elsewhere [l]. Situations where SCFs may be useful emerge from the effects that supercritical fluids have on pure compounds and mixtures. SCFs can change com-

89

2.2 Extraction and Related Separation Techniques Solvent + A

r

Feed

+

Regeneration

Separation

Product P1

precipitation

Solvent Product P2

I

Make-up solvent

(compressedgas)

Figure 2.2-1 Generalized scheme of a process for applying a solvent, such as an SCF.

pound properties, phase equilibria, chemical equilibria, and therefore often the rate of related processes. A generalized scheme for a process applying a mass separating agent, or solvent, is shown in Figure 2.2-1. The characteristic features are solvent flow; cycling of solvent or release, if quantities are low; recovery of products from the solvent; removal of remaining solvent from the products (extract and raffinate). The basis for a separation process by means of SCFs is the solvent power of the dense gaseous fluid and the different solubility of compounds in this solvent. SFE processes can be carried out in different modes: single stage, multiple stage, multiple stage and countercurrently, as a chromatographic process, and with the aid of chemical reactions. The various process modes are applied primarily on the basis of the properties of the feed mixture. Solids are processed mainly in single stage or multiple stage operational mode. Fluid feed mixtures containing compounds of similar solubility in the solvent are best treated in a multiple stage countercurrent process. The separation of isomers falls within the field of chromatography or selectively catalyzed reactions. Chemical reactions, e.g. esterification of free fatty acids, may be part of the separation process. In the following, the different types of processing modes will be discussed. It is the intention to review methods for basic design. This basic design enables understanding, planning of experiments, feasibility studies, the design of pilot plant and demonstration units, and creates a database for comparison of economics.

90

2 Experimental Techniques

2.2.2 Extraction from Solids SFE from solids is carried out by continuously contacting the solid substrate with the supercritical solvent. In most cases the solid substrate forms a fixed bed. During extraction, the supercritical solvent flows through a fixed bed of solid particles and dissolves the extractable components of the solid. The solvent is fed to the extractor and evenly distributed to the inlet of the fixed bed. The loaded solvent is removed from the extractor and fed to the precipitator. The solid material will be depleted from the extractable material in the direction of flow. Concentrations of extract components increase in the direction of flow in the supercritical solvent and in the solid material. The shape of the concentration curve depends on the operating conditions, the kinetic extraction properties of the solid material, and the solvent power of the supercritical solvent, as discussed below. The transport of substances can occur within a solid and across the borders of a structure, for example plant material structure, in the extraction of natural products. Such structures may differ from substrate to substrate. Furthermore, the solid material can consist of particles of different size and form and the size distribution of the particles may vary. The bed of particles may form different geometries and can even change its geometry during the process. In some cases it may be stirred or even fluidized. The substances to be extracted may be adsorbed on the outer surface, on the surface of pores, or evenly distributed within the solid or the plant cells. Each of these different distributions has some influence on the course of the extraction. However, the integral extraction curve is usually of a relative simple form. The extraction of soluble compounds from solid plant material proceeds in several steps: 1. The plant matrix absorbs the supercritical solvent and other fluids that are deliberately added to influence the extraction process. 2. In parallel to step 1, the extracted compounds are dissolved by the solvent. A chemical reaction may occur prior to solvation. 3. The dissolved compounds are transported to the outer surface of the solid. Diffusion is the most important transport mechanism. 4. The dissolved compounds pass through the outer surface. 5 . The compounds are transported from the surface layer into the bulk of the supercritical solvent and are subsequently removed with the solvent from the bulk of the solid material. Similar sequences apply to the SFE of other solid materials.

2.2 Extraction and Related Separation Techniques

91

2.2.2.1 Basic Process Design For a laboratory experiment the goal is to achieve an extract containing the required substances. With a more technical approach, the goal is to determine the optimum process parameters, the size of the extracting equipment, and the quantity of solvent needed €or a certain extraction result. As the technical approach includes the necessary knowledge of laboratory experiments, the technical approach is now considered. This approach is partly empirical. Rigorous models, based on physico-chemical properties, thermodynamics, and related basic physical laws, are fitted to experimental results by introducing coefficients as adjustable parameters. An investigation of the extraction is carried out, determining the major influences of the most important parameters on the product specifications. These parameters include the process temperature T, process pressure p , extraction time t, amount of solvent ms, solvent ratio (amount of solvent per unit of time and amount of solid) S, conditions of extract removal (precipitation) from solvent Tp, p p , and eventual pretreatment of solid feed material. In addition, the solubility of the extracted compounds in the solvent as a function of pressure and temperature may be investigated. The course of the extraction is an unsteady process for the solid as well as for the solvent. The course of the process can be followed by determining the amount of extract against time of extraction. From these data, more information on the process can be deduced, as discussed below. The amount of extract accumulating during the course of the extraction will, in principle, follow the curve of Figure 2.2-2(a). The first part of the curve may be a straight line, corresponding to a constant extraction rate. The second part is nonlinear, approaching a limiting value which is given by the total amount of extractable substances. The gradient of the first part of the graph can be given by the equilibrium solubility of the extract compounds in the supercritical solvent. From this gradient the equilibrium solubility may be determined. However, a straight line can be caused by constant mass transfer resistance and is no proof that equilibrium solubility is obtained during the extraction. In the second part of the extraction, two effects cause a declining medium concentration of extract in the outflowing solvent: 1. The extract in the solid substrate near the solid-gas interface is depleted for most of the solid substrate. Transport of the extract within the solid to the interface then adds an additional transport resistance. 2. The length of the fixed bed containing the initial content of extract is not long enough to enable the maximum loading of the solvent.

The maximum value of extract concentration in the supercritical solvent is given by the equilibrium solubility of extractable components. Due to different transport resistances and equilibrium distribution coefficients for different compounds, they may be successively extracted at different rates. The maxi-

92

2 Experimental Techniques

Figure 2.2-2 (a) Integral extraction curve showing the total amount of extract against time of extraction. (b) Extraction rate curve showing the amount of extract in a given time interval, plotted against time of extraction or amount of solvent. time

mum concentration of extract in the supercritical solvent may then be a function of extraction time. 2.2.2.2 Process Parameters

Temperature A higher temperature often causes a higher extraction rate, if pressure is not too low. One reason for this is the dependence of solvent power on temperature. At relatively low pressures, the decrease in the density of the SCF with increasing temperature prevails, resulting in a decrease of the solvent power of the SCF and the solubility. At higher pressures, the increase in the vapor pressure of the solute with temperature prevails, resulting in higher solubility. The absolute values for low pressure and higher pressure vary for different solutes, but the general behavior is the same. Another reason for a higher amount of extract per unit of time is enhanced mass transfer rates with higher temperature. Pressure At the process conditions of SFE, the solvent capacity in general increases with pressure at constant temperature. Therefore, the amount of extract after a certain time of extraction will increase with pressure. Note that increasing pressure increases solvent power. For mixtures of extract compounds this will result in different extracted compositions at different pressures.

2.2 Extraction and Related Separation Techniques

93

Density With increasing density, the extraction rate increases at constant temperature. Density is responsible for the capacity and solvent power of a solvent, as the solubility of a compound rises with increasing density. In the extraction process, mass transfer is also of importance. Therefore, the extraction results will be different for the same density at different temperatures. Solvent Ratio The solvent ratio is the most important parameter for technical applications of SFE, once approximate values of pressure and temperature have been selected. With increasing solvent ratio the extraction rate can be enhanced more than by changing process parameters within relatively narrow limits. The influence of the solvent ratio must be discussed while also considering the economic consequences [ l ] which are beyond the scope of this chapter. Size of Solid Particles Mass transfer in extraction from solid substrates in most cases depends heavily on the transport rate in the solid phase. The length of the transport path determines mass transport in the solid phase. In general the extraction rate increases with decreasing particle size. However, mass transfer into the fluid phase has to be achieved. If the smaller particles hinder the flow of the fluid in the fixed bed, then the mass transfer rate and the amount of extract decrease with smaller particles. 2.2.2.3 Modeling the Extraction The extraction of substances from solid substrates with supercritical solvents can be analyzed and modeled in a simple way by considering only the medium values and by determination of unknown coefficients by fitting to the extraction curve and a mass balance [l]. This approach results in simple equations that can represent parts of the extraction curve sufficiently, but fail for others, especially during the first part of the extraction. If the process is to be modeled more accurately, the analysis is far more complex and beyond the scope of this chapter. Nevertheless, some parameters determining the extraction and influencing the result are listed below together with the description of a simplified model that may provide some insight into the applied methods.

Basic Considerations The extraction system involves a bulk solid phase and a fluid phase. The fluid phase comprises the supercritical solvent and the dissolved extract. The solid phase remains within the extraction vessel and the fluid phase is passed through the extraction vessel. Mass transport occurs between the two phases. Any of the following parameters that can influence the process may be of interest. 1. Fluid phase (extract phase) (a) Concentration of the extract downstream of the extraction vessel: accumulated quantity of extract;

94

2 Experimental Techniques quantity of extract per unit of time; composition of the extract as a function of time.

(b) Concentration af the extract in the extraction vessel: medium concentration over the total volume; concentration throughout the extraction vessel for plug flow; local concentrations considering radial distribution (no backmixing, but no plug flow); local concentrations considering radial and axial distributions (backmixing).

2. Solid phase (raffinate phase) (a) Concentration of the extractable substances in the bulk solid: accumulated depletion of the solid (mean value for the extraction vessel); depletion of the solid related to the remaining content of extractable substances (mean value for the extraction vessel); remaining concentration of extractable substances: radial and axial distribution. (b) Concentration of the extractable substances in single particles: mass transport by diffusion; mass transport resistance by chemical reactions and/or phase transitions; simple geometric particles, complex shape of particles; size and size distribution (polydispersity) of the solid particles.

3. Operating parameters Pressure, temperature, and density of the fluid, quantity of solvent per unit of time and mass of solid (solvent ratio); chemical composition of the extracting solvent. 4. Pretreatment of the solid size reduction and enlargement of surface; destruction of the plant cells; adjustment of the water content; chemical reactions to liberate the extracted compounds.

Simplified Model for SFE from Solids Realistically, this large number of parameters cannot be treated within a practical model. The goal of the modeling procedure is therefore to obtain a quantitative representation of the process with a simple system of equations and just a few physically meaningful parameters [l]. In such a model, the following parameters are usually considered sufficient to calculate the course and result of an extraction:

2.2 Extraction and Related Separation Techniques

95

equilibrium distribution between solid and supercritical solvent (adsorption isotherm); diffusion in the solid (effective diffusion coefficient or effective transport coefficient as defined by the transport model); mass transfer from the su-rface of the solid to the bulk of the fluid phase (supercritical solvent). Experimental extraction curves can be represented by this type of model, by fitting the kinetic coefficients (mass transfer coefficient to the fluid, effective transport coefficient in the solid, effective axial dispersion coefficient representing deviations from plug flow) to the experimental curves obtained from laboratory experiments. With optimized parameters, it is possible to model the whole extraction curve with reasonable accuracy. These parameters can be used to model the extraction curve for extractions in larger vessels, such as from a pilot plant. Therefore, the model can be used to determine the kinetic parameters from a laboratory experiment and they can be used for scaling up the extraction. 2.2.2.4 Solids in Multiple Stages and Countercurrent Operation in SFE

Multistage countercurrent contacting is the most effective mode for carrying out separation processes. It reduces the amount of solvent and makes possible the continuous production of extract. Real countercurrent contact is not easily established for solids, because special effort is necessary for moving the solid, with increased difficulties at elevated pressure. Therefore, it is easier not to move the solid material and to achieve countercurrent contact by other measures. For SFE from solids, a well known and often used configuration is to have several fixed beds in countercurrent contact with the solvent. 2.2.2.5 Continuous Extraction of Contaminated Soil with Supercritical Water

Continuous extraction of solids is not easily achieved in a high pressure environment, but can be achieved in certain cases. These favourable cases are met when the solids can be ground to a small size (below 1 mm) and a diluted suspension of the solid material in the solvent can be used. As this possibility is of fundamental importance, an experimentally verified example is treated here in some detail. The suitability of supercritical water for decontamination of soil material can be proved by several semibatch extraction experiments [6]. At 380°C and 250 bar, the extraction results of hydrocarbons from soil material are excellent, even if it has been weathered for over 20 years. This kind of soil material, often the highly contaminated effluent of a soil washing process, can not be further decontaminated by biological treatment because the hydrocarbon contamination has been exposed to microorganisms throughout the weathering

96

2 Experimental Techniques )

Id v2

exhaust

t

1

v1

8.I

extraction

preheate

I

'

C

t lb

---Pa

piping

---_-

compressed air exhaust

Figure 2.2-3 Flow diagram of apparatus for continuous extraction from solids [7]. magnetic stirrer, b feed vessel, c membrane pump, d buffer vessel, e, f preheater (28 mL), g extraction pipe (38 mL), h cooler, i flask, j COz meter, k evacuated glas

a

vessel.

process. The effluent must be incinerated or deposited. With semibatch extraction all of the initial contamination could be removed from the soil material in an extraction lasting 6 h. Extraction results are excellent, but extraction times are long. In a new apparatus suitable for continuous extraction (Figure 2.2-3), experiments with contaminated soil material and supercritical water were carried out. Figure 2.2-4 compares the results of a semibatch and a continuous extraction as a function of solvent to soil ratio. The semibatch extraction residence times can be reduced remarkably by continuous operation. For extraction under parallel (co-current) flow, solvent-to-soil-ratios must be high for sufficient dilution of contamination in the fluid phase, leading to a high concentration gradient. At low solvent-to-soil-ratios, the degree of extraction is not sufficiently high, and the solid material must be recharged. Recharging of the solid material may also be needed if the concentration of the solid increases. Experiments with a suspension of 2wt % soil material showed that after three runs of extraction the clean-up result could be improved by up to 94%. The total time of extraction was no longer than 90s.

2.2 Extraction and Related Separation Techniques

Figure 2.2-4 Extraction results from semibatch and continuous extraction from solids (1 MPa = 10 bar) 171.

0

100

200

300

solvent to soil ratio [kg,20/kg,r,

97

400 sDi,

1

This example reveals that SFE from solids is applicable to systems other than natural materials. Recent applications have been directed to dyeing fabrics, cleaning complicated and delicate mechanical structures, cleaning catalysts and stationary phases for chromatography, and desorption of compounds from adsorbents.

2.2.3 Countercurrent Multistage Extraction Countercurrent multistage extraction using SCFs may be compared to fractional distillation. Distillation uses thermal energy to transfer the components from the liquid to the gaseous state at their various boiling temperatures. SFE uses a supercritical solvent for that process. The transfer from the liquid to the gaseous state can now take place at any temperature higher than the critical temperature of the solvent. Both processes apply the approach to equilibrium as the driving force. Realizing an equilibrium between two flows results in one equilibrium stage which changes composition of the mixtures to some extent but not sufficiently. Therefore, several such equilibrium stages are added, forming a multistage separation (Figure 2.2-5). With the appropriate number of equilibrium stages such a procedure can separate a binary mixture or two fractions to a given specification. Countercurrent flow is achieved between the liquid and the gaseous phase, thus transporting the liquid and gas to the next stage. At the end of the cascades countercurrent flow must be initiated which is achieved at the bottom by the gaseous supercritical solvent and at the top by reintroducing some of the top product. Countercurrent multistage extraction is the application of SFE to the fractionation of liquid mixtures and the purification of liquid substances. SFE in the modification of countercurrent multistage operation extends the possibilities of separation processes, such as fractional distillation, absorption and liquidliquid extraction to the isolation and purification of components of low volatility. The separation of volatile liquid components is also possible at tempera-

2 Experimental Techniques

98

Enriching Section Supercdtical

Solvent

e=+ 5

4

6P

Cycle .'7 Pump or Compressor

Stripping Section

, & = 7-w I

Bottom Product

(Low P)

Figure 2.2-5 Flow diagram of continuous countercurrent

multistage extraction.

tures below their normal boiling points. Furthermore, SFE enables the separation of components with very similar properties if used in the countercurrent mode. Process temperatures are determined by the critical temperature of the solvent and not, as in the case of distillation, by the liquid-vapor transition of the feed mixture. Compared to liquid-liquid extraction, gas extraction makes it possible to operate a two-cascade separation column, applying a stripping and an enriching section. Combined, these possibilities allow SFE to be operated at very moderate temperatures and as a separation process for difficult separations. 2.2.3.1 Basic Process Design Multistage countercurrent separation is usually considered for laboratory applications. As the principles apply to laboratory size equipment as well as to large-scale installations, the procedure given below should also be used for laboratory experiments.

2.2 Extraction and Related Separation Techniques

99

The goal of a design procedure for a countercurrent multistage separation is to determine the number of theoretical stages needed for a separation, the height of a separation column, the capacity of the mass transfer equipment, and the diameter of a separation column. For technical purposes, these considerations on the separation equipment are completed by the design of product recovery and gas cycle [l]. The design procedure is outlined in the following discussion. Knowledge on the phase equilibrium is needed first and can be obtained from laboratory experiments. The phase equilibrium yields the extent of the two-phase region, the mutual solubilities - in particular the solubility in the gaseous phase - the separation factor and, if possible, its concentration dependence. The separation factor is the ratio of the distribution coefficients of compounds between the gaseous and the liquid phase. A separation factor greater than 1 indicates that a separation is possible. In SFE, separation factors as low as 1.3 can lead to effective separations, but separation factors are usually higher. With the equilibrium and the separation factor known, an initial simplified determination of the number of theoretical stages can be carried out using standard chemical engineering methods [8]. This analysis is carried out on a quasi binary system (key components) or with pseudo components and provides essential insight into the separation process. Limiting values for the number of theoretical stages and the amount of top product to be reintroduced to the separation are determined at this point. Neglecting these limiting values will lead to failure of the separation. Now, separation experiments are carried out in a laboratory column, ensuring the accuracy of the separation factor as determined by phase equilibrium measurements and providing initial data on the height (length) of a theoretical stage. In addition, some material of different composition to the feed is produced for further phase equilibrium measurements in order to determine the concentration dependence of the separation factor. A multicomponent simulation of the separation process can be carried out using a commercial process simulator. The simulation for technical purposes can include different methods and conditions of product recovery and gas cycles. Experiments in a column of higher capacity (3070 mm diameter) than normal laboratory installations (15-25 mm diameter) can be carried out. Material is produced for further evaluation, as samples for customers, or for product development. In parallel, simulation experiments serve to define the plant design and the optimal operation of a column. An economical evaluation will conclude the developmental stage of the process design.

2.2.3.2

Phase Equilibria

Information on phase equilibria can be obtained from measurements of mutual solubilities which reveal the extent of the two-phase region with respect to pressure (Figure 2.2-6). Solubilities over a certain range of pressure and temperature have to be determined. Most important for the separation process are

2 Experimental Techniques

100

,

Liquid phase

Vapor phase

,,

I

Figure 2.2-6 Phase equilibria of orange peel oil - carbon dioxide (1 MPa = 10 bar) [9]. 94

92 w

o

96.98

l o (D

co,

the distribution coefficients for the individual compounds and the separation factors determined from that data. For example, the separation factor of squalene for a separation from a tocopherol-sterol mixture is shown (Figure 2.2-7). It is also necessary to determine the dependence of the separation factor on the concentrations of the key components because these vary substantially along the separation column. It is not always easy to acquire data on the concentration dependence of the separation factor because only one feed mixture is available for equilibrium measurements. Therefore, mixtures of other composition must be produced by mixing or from the products of experimental separations (which are also neeeded to determine the height of theoretical stages). Experimental determination of all the properties and their dependences on the operating conditions is tedious and expensive. Computational thermodySqualene-a-Tocopherol 5 -

0

&

26.0MPa

;o

8

'I h, -

j 3c3 2l

T

29.1MPa

\

23.0MPa

A

20.0 MPa

p-655ks4nS

\ '\

2-

A

'---. --.--&\

1-

I

I

I

I

I

Figure 2.2-7 Separation factor for squalene from a tocopherolsterol mixture (1 MPa = 10 bar) [lo].

2.2 Extraction and Related Separation Techniques

10 1

namics provides methods for correlating phase equilibria using equations of state. These models are powerful tools and make it possible to correlate binary and multicomponent phase equilibria by adjusting the interaction parameters to fit the measured data. However, for complex mixtures of similar compounds of low volatility, their applicability is somewhat limited. The extent of the twophase area can be correlated, well away from the critical region of the system, with sufficient accuracy for technical purposes. To date, no method is able to correlate the distribution coefficients and the separation factors with sufficient accuracy for design purposes. Solubilities and separation factors provide the necessary basic data for carrying out an analysis of the separation process on the basis of theoretical stages. This analysis i s independent of the type of separation equipment used.

2.2.3.3 Separation Analysis with Respect to Theoretical Stages The basic model for a countercurrent separation process needs to consider equilibrium and kinetic relationships, as given in chemical engineering textbooks. For SFEi processes, equilibrium relationships include phase equilibria, mass balances, and energy balances, whereas kinetic relationships refer to mass transfer. The number of theoretical stages is first determined by well-known shortcut methods and then by a multicomponent simulation of the separation process. Both parts can and will be used for analyzing the separation process with respect to the effect of separation factor, reflux ratio (the amount of top product reintroduced to the separation), degree of separation etc. on the number of theoretical stages. There, computational methods provide insight into the separation process without actually carrying out the separation. According to the McCabe-Thiele method, the system for separation is considered on a quasi-binary basis. In this approach, it must be possible to neglect the influence of the solvent, which is acceptable if the phase boundary lines (solubilities) do not change much with concentration during separation. In this case, the number of theoretical stages, the minimal reflux (ratio), the minimum number of theoretical stages, and their mutual dependence can be determined. If the mutual solubility changes significantly with concentration, a stage to stage calculation can be carried out according to the method of PonchonSavarit by including the solvent in the calculation and by using a quasi-ternary system. The graphical representation is displayed on a Janecke diagram, which is similar to the enthalpy-concentration diagram in distillation with enthalpy being replaced by the solvent ratio for SFE. These short-cut methods provide a good model of the separation process without any experimentation. The difference in the theoretical stages for the two methods can be appreciable: it is small if the number of theoretical stages is high, and vice versa.

102

2 Experimental Techniques Figure 2.2-8 Simulation of concentration profiles with ASPEN+ in a multistage countercurrent separation (13.5MPa = 135 bar) 1111. - A set of equilibrium data, . determined experimentally, in

13.5 MPa

O

2.2.3.4

2o

30

40

+ Composition [wt.-%.]

6o

70

combination with mass and energy balances enables the calculation concentration profiles along a separation device operated in the countercurrent mode for multicomponent mixtures. ASPEN+ is a commercially available process simulation program.

Multicomponent Process Simulation

A better insight into composition of phases along the separation process is provided by multicomponent process simulation as it can be carried out with commercial process simulating programs, such as ASPEN+. As usual, the process is separated into theoretical stages. Normally, ASPEN+ provides thermodynamic models and calculates thermodynamic properties such as the distribution coefficients and separation factors. As the accuracy of these results is not sufficient for a design analysis in many cases, distribution coefficients (and if necessary solubilities) can be provided by a user-defined module which uses empirical correlations for these values. The simulation procedure is then as follows: The starting value for the number of theoretical stages is taken from short-cut methods. Distribution coefficients are provided from external correlations. Then, concentrations for the individual (pseudo) components are calculated with algorithms known from multicomponent multistage processes (Figure 2.2 - 8). In order to determine the height of a column, the height (length) of a theoretical stage must be known. 2.2.3.5

Determination of the Height (Length) of a Theoretical Stage

The height of a theoretical stage is the size of an equipment in which just one theoretical stage, i.e. equilibrium, can be achieved. For a countercurrent column this size is given by the length (height) of the column in which one equilibrium stage can be obtained. The height of a theoretical stage (HETS) can be calculated from the number of theoretical stages, determined from a separation experiment, and the height of the separation column, used for the experiment:

2.2 Extraction and Related Separation Techniques HETS = h/nth

103 (2.2-1)

Most literature data on the HETS are determined this way. For fatty acid ethyl esters, the average HETS was found to be 0.22m in a laboratory column (17.5 mm diameter) and 0.27-m for the pilot plant column (70 mm diameter) using wire mesh packing (Sulzer EX and Sulzer CY, respectively). Therefore, the laboratory column can be taken as the first guess for the HETS, at least for this system. The HETS is strongly dependent on the type of system (and of the type of separation equipment). According to our experiments, the HETS for the squalene-tocopherol-sterol system is in the range of 2.5-0.7 m. The HETS decreases with increasing liquid loading of the column, and consequently increases with increasing gas loading. The main reason for the different HETS values for fatty acid ethyl esters and squalene-tocopherol-sterol mixtures is, most probably, the very different viscosities of the gas-saturated liquid phase. The viscosity of the liquid phase of the ethyl ester system is 60 X 10-5Pas (at 140 bar, lOOOC), and for the squalene-tocopherol system is 400 x 10-5Pas (300 bar, l0OOC). ''

2.2.3.6 Determination of Column Diameter

The column diameter for a countercurrent separation for a given amount of feed depends essentially on the maximum allowable gas loading of the mass transfer equipment. For general orientation, an operating area can be defined. It is limited at high densities by the density difference between phases being too low; 0.1 g/mL is selected as the lower limit. At low densities, solubility in the supercritical solvent is not high enough. The lower temperature limit is set by the onset of solidification, low transport coefficients, and low concentration in the gaseous phase. The higher temperature limit is set by the stability of the compounds. These limiting values are somewhat arbitrary. For operating countercurrent columns, the pressure drop and flooding point are important values. In gas extraction, pressure drop is not as important as in other separation processes, but from the pressure drop, the mass transfer regime can be concluded. Furthermore, it characterizes the type of mass transfer equipment for flooding point calculations. The flooding point in countercurrent flow driven by gravity (the liquid phase flows downward due to its higher density) is the limiting amount of gas that can flow through a column. No countercurrent two-phase flow is possible at the flooding point. An example is given in Figure 2.2-9. Flooding points, as well as pressure drop curves, may be determined in the separation column, but it is better to exclude mass transfer effects and use a special experimental set-up. Countercurrent multistage separation using supercritical gases is a powerful tool for separating mixtures that are currently separated by high vacuum processes. Furthermore, separations that are otherwise impossible can be achieved

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2 Experimental Techniques

.. .. .. .. .. .. ... .

... .. . .. . .. ... . . . . .

Packing: Sulzer CY 35 mm

Llnlen+Symbol

MellaPak 250 Y RaluPak 250 CY

B)

b 8

V

v

0,Ol

W

MontrE41-300 MonlrE41-200 Montz81-100

. . . . ... ... ... ...

..

..

1

091

Flow parameter

mo

Figure 2.2-9 Flooding point diagram for countercurrent gas-liquid flow [ 121 after Billet [13]. The lines mark the upper limit of gravity driven countercurrent flow. Under usually operating conditions, 80% of that limit can be used.

because the process temperatures are low, degrading of components is negligible, and the number of stages is not limited. Applying a separation column with two separation cascades (enriching and stripping section) makes it possible to obtain high purity and high yield at the same time.

2.2.4 Chromatographic Separation with Supercritical Fluids In a chromatographic separation, a mixture of substances is transported by a carrier, the mobile phase, over a surface, the stationary phase. Between the two phases mass transfer processes take place, which lead to different transport velocities along the surface of the stationary phase for different components of the mixture. The components reach the end of the stationary phase at different times and can be detected and collected separately. Supercritical fluid chromatography (SFC) uses supercritical gases as a mobile phase [14-171. The solvent power of supercritical gases is determined by density (see above) which can be easily modified by pressure [18]. Furthermore, separation of the eluted compounds is easily achieved by pressure release. Liquid solvents may be only applied in minor quantities as modifiers. Therefore, SFC has good potential for scale-up to preparative and production scale chromatography [ 15-17,19,20].

2.2 Extraction and Related Separation Techniques 2.2.4.1

105

Design of SFC Apparatus

An apparatus for chromatographic separations (Figure 2.2-10) with a supercritical gas consists of (1) the separation column as the central part in a temperature controlled environment; (2) the reservoir for the mobile phase; (3) a unit for establishing, maintaining, and controlling pressure; (4) an optional unit for adding a modifier; - ( 5 ) the injection part for introducing the feed mixture; (6) a measuring device (detector) for determining concentration of the eluted substances; (7) a sample collection unit; (8) a unit for processing the mobile phase; and (9) a unit for processing data and controlling the overall apparatus. The flow of. the SCF under pressure is maintained by long-stroke piston pumps, reciprocating piston pumps, or membrane pumps which deliver the mobile phase in liquefied form. The fluid is then heated to supercritical conditions before entering the column. Pressure and flow rate must be kept as constant as possible in order to maintain constant conditions for separation and to achieve a stable baseline in the chromatogram. Oscillating pumps can therefore have three heads, which deliver at different times, or a pulsation dampener in order to minimize pulsation. In preparative and more so in productive SFC, the quantity of the mobile phase is not small and it must be recycled. To avoid backmixing, the recycled mobile phase must be totally free from any dissolved substances. Concentrations of the dissolved substances are low. In most cases they will be of the order of 0.1 % or even 0.01 %. Then, separation methods for the dissolved substances from the mobile phase become important.

Gas

Treatment of Mobile Phase (8)

Detector (6)

---I

I

- - - - - - - - - - - - - - - - --- - - - -- - - - - - --- Sample

I

Expansion

-

Figure 2.2-10 Flow diagram of apparatus for supercritical fluid chromatography (SFC).

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2 Experimental Techniques

2.2.4.2 Methods for Scale-up of Chromatography

Chromatography can be scaled up by several means: increasing size of the chromatographic separation column, employing more than one (many) chromatographic separation columns in parallel, and @creasing the amount of substance for one injection. Increasing the size of a tube is a straightforward process, but maintaining chromatographic conditions is not. The larger the diameter of the column, the more difficult it becomes to maintain plug flow. Pressure drop becomes a problem mainly in liquid chromatography, as the forces in large diameter columns are high. Therefore, particle diameters for larger columns are about a factor of 10 greater than for analytical packing, about 50ym as compared to 3-5 pm in analytical applications. Large size packing of small diameter particles is difficult to produce without inhomogeneities.

2.2.5 Conclusion SCFs provide an excellent tool for carrying out separations. Although to date mainly the extraction from solids has been applied in commercial processes, some countercurrent separations are also in operation and more will come soon. Chromatographic separation with supercritical gases is being investigated intensively for scale-up. Given the advantages of easy solvent handling, SFC should appear in commercial processes in a short time. For scientists working in laboratories, all the methods above described are available on a small scale. Particularly interesting applications will emerge if conventional methods are combined with SCF techniques.

References [ I ] G. Brunner, Gas Extraction. An Introduction to Fundamentals of Supercritical Fluids and their Application to Separation Processes, Steinkopff Darmstadt, Springer New York, 1994. [2] M. McHugh, V. Krukonis, Supercritical Fluid Extraction, Butterworth-Heinemann, Boston, 2nd ed., 1994. [3] E. Kiran, J. F. Brennecke (Eds.), Supercritical Fluid Engineering Science: Fundamentals and Applications, ACS Symp. Ser. Vol. 514, American Chemical Society, Washington, DC, 1993. [4] T. L. Chester, J. D. Pinkston, D. E. Raynie, Anal. Chem. 1998, 70, R301. [ 5 ] K. Zosel, Angew. Chem. 1978, 90, 748; Angew. Chem., Int. Ed. Engl. 1978, 17, 702. [6] K. Nowak, Thesis, TU Hamburg-Harburg, 1995. [7] A. Firus, Thesis, TU Hamburg-Harburg, 1996. [8] R. H. Perry, D. W. Green, J. 0. Maloney (Eds.), Perry’s Chemical Engineers’ Handbook, 6th ed., McGraw-Hill, New York, 1996. [9] M. Budich, Thesis, TU Hamburg-Harburg, 1999.

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107

[1O] C. Saure, Thesis, TU Hamburg-Harburg, 1996. [1I] V. Riha, Thesis, TU Hamburg-Harburg, 1996.

[12] J.-T. Meyer, Thesis, TU Hamburg-Harburg, 1998. [I31 R. Billet, Packed Towers in Processing and Environmental Technology, VCH, Weinheim, 1995. [14] E. Klesper, A. H. Corwin, D, A. Turner, J. Org. Chem. 1962, 27, 1962. [15] M. Yoshioka, S. Parvez, T. Miyazaki, H. Parvez, (Eds.), Supercritical Fluid Chromatography and Micro-HPLC, (Progress in HPLC Vol 4.), VSP, Utrecht, The Netherlands, 1989.

[ 161 B. Wenclawiak (Ed.), Analysis with Supercritical Fluids: Extraction and Chromatogra-

phy, Springer, Berlin, 1992. [17] M. Saito, Y. Yamauchi, T. Okuyama (Eds.), Fractionation by Packed-Column SFC and SFE: Principles and Applications, VCH, New York, 1994. [18] J. C. Giddings, M. N. Myers, L. McLaren, R. A. Keller, Science 1968, 162, 67. [19] L. T. Taylor, J. Chromatogr: Sci. 1997, 35, 374. [20] K. L. Williams, L. C. Sander, J. Chromatogr: A 1997, 785, 149.

Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

2.3 Precipitation and Crystallization Techniques ALBERTO BERTUCCO

2.3.1 Introduction The potential of supercritical fluids (SCFs) as media suitable to develop new processes for the production of crystals as well as amorphous powders has been outlined and studied since the mid-1980s. The precipitation or crystallization of a compound by a SCF can be performed according to different techniques, four of which will be considered here. They are: crystallization from supercritical solution (CSS); particles from gas saturated solution process (PGSS); rapid expansion of a supercritical solution (RESS); precipitation by a gaseous or a supercritical antisolvent, referred to in the literature as GAS or SAS or PCA (Precipitation with a Compressed Antisolvent), depending on different applications of the same idea. Here we call it the process using a gas as the antisolvent (GASP). The CSS technique is usually aimed at obtaining large crystals, and can be treated as an extension of classical thermal crystallization to the case of a solution where the solvent is a SCF. However, PGSS, RESS and GASP are new alternatives to conventional techniques for particle size reduction. In fact, both mechanical (crushing, milling, grinding, jet-milling) and thermal (crystallization from solution, spray-drying, freeze-drying) processes can have major drawbacks, especially when applied to specialized products, such as pharmaceuticals, electronic materials, or organometallics, which may undergo thermal or chemical degradation. In addition, they do not allow particles with a narrow size distribution to be obtained. All of the techniques considered in this chapter are new. CSS was developed, among others, by Tavana and Randolph [l] in the late 1980s. The development of PGSS, due to Weidner et al. [2], is dated 1994. RESS was first proposed by Krukonis in 1984 [3] to comminute thermally labile organics and pharmaceu-

2.3 Precipitation and Crystallization Techniques

109

ticals, and to modify the morphology of polymer particles. In 1989 Gallagher et al. [4] invented the use of a SCF as an antisolvent to produce fine crystals of high explosives starting from a solution of the compounds of interest in an organic solvent. CSS is the least used and _more classic one, so it will be discussed only briefly. Owing to lack of freely available information, PGSS can be addressed only as far as its physical background is concerned, and with reference to a qualitative apparatus scheme. Therefore, RESS and GASP are the main topics in this chapter as far as details about apparatus and procedure are concerned. It is noteworthy that, although many SCFs have been investigated as precipitation media in all of the techniques defined above, C 0 2 is the one most often considered and used, because of its particularly favorable properties.

2.3.2 Thermodynamics and Phase Equilibria It is interesting to see that all precipitation processes with SCFs rely on some kind of special thermodynamic behavior of mixtures at high pressure. Also, it is essential to understand phase equilibrium phenomena to appreciate any process development in this field. 2.3.2.1

CSS, PGSS and RESS

Let us consider first the solubility of a heavy compound (HC) in a SCF as a function of pressure at three increasing temperatures. The case of a dyestuff in C02 is presented in Figure 2.3-l(a). This figure can be qualitatively applied to any system consisting of C02 and a nonpolar molecule of molecular weight of 200 g/mol or above. A number of peculiarities are shown by the curves: At high pressure the solubility tends asymptotically to a value which is temperature dependent but very low. Generally, the solubility of solids in SCFs is well below 1 % w/w, and most often it ranges from 0.01 % w/w downward. In particular, there are very few compounds of molecular weight in the thousands that can be dissolved in scC02 to a detectable extent. The solubility isotherms cross one another. Namely, at pressures roughly ranging from 40 bar to 170-180 bar (zone B of the plot) a retrograde behavior is seen, where the solubility is decreased by a temperature increase, whereas above the crossover pressure the usual dependence is encountered (zone C of the plot). At low pressure, up to 30-40 bar (zone A of the plot), the solubility is essentially zero. In this region, the mole fraction of the HC can be represented qualitatively by a hyperbole branch, as dictated by the ideal-gas law: PSa' y= P

(2.3-1)

2 Experimental Techniques

110

- .

50

0

200

150

100 Pressure,bar

-.

@'E 300-

0 v)

100

0

l

'

l

'

l

'

l

'

l

'

l

'

l

'

l

'

l

'

Density, g/cm3

Figure 2.3-1 (a) Solubility of 1,4-bis-(n-propylamin0)-9,lO-anthraquinone(a dyestuff) in C 0 2 as a function of pressure at different temperatures. (b) Solubility of 1,4-bis(n-propylamin0)-9,10-anthraquinone in C 0 2 as a function of density at different temperatures. Adapted from Wagner et al. [ 5 ] .

2.3 Precipitation and Crystallization Techniques

111

where y is the compound's mole fraction in the SCF, pSatits sublimation pressure and p the total pressure. However, at increasing pressure, instead of tending to zero as predicted by eq (2.3-1), the solubility shows a minimum and raises up again to its asymptotic value. This solubility enhancement can be represented by the corrected isofugacity equation y = -PSa' E P where E, the so-called enhancement factor, is expressed by

[ ---sat)]

E=qOJ: exp vs ( P

(2.3-2)

(2.3-3)

(P"

In eq (2.3-3) q0."and 9" indicate the HC fugacity coefficients in the reference state (pure vapor) and in the vapor phase, respectively, vs is the molar volume of the solid phase, T the temperature and R the universal gas constant. As q0."can be reasonably set equal to 1 and the exponential term (known as the Poynting correction) is not markedly greater than 1 for pressures below 500 bar, the enhancement effect is almost completely due to q", that is to the nonideality of the supercritical solution. With reference to the above points, it is important to note that: In the literature, values of E as high as lo6 are reported (for example, see Brennecke and Eckert [6]). However, this does not necessarily mean high solubility, as E is just a measure of solubility relative to the ideal-gas behavior. In general, the solubility of HCs in SCFs is extremely low. Especially for dilute solutions far from the critical point, it is usually correct to assume that the volumetric properties of the supercritical phase are those of the SCF, i.e. the solute effect can be neglected. Pressure is not the best independent variable with which to represent solubilities in SCFs. The quite confused representation of Figure 2.3-l(a) can be improved if the same solubility isotherms are plotted against the vapor density (i.e. the SCF density), as in Figure 2.3-l(b). This smooth dependence is embedded in a simple equation proposed by Chrastil [7] and recently modified by Mendez-Santiago and Teja [8] to correlate solubility of solids in SCFs: T In E = A

+ Bd

(2.3-4)

where d is the SCF density and A, B are two adjustable parameters. We recall that eq (2.3-4) is not thermodynamically correct, yet it is extremely useful when fitting solubility data in density ranges that are not too broad. The approach now summarized to represent solid-fluid equilibria can be profitably used in the development of both RESS and CSS processes. Basically, RESS takes advantage of the solubility dependence on SCF density, and that, in the supercritical region, the sensitivity of density on pressure

112

2 Experimental Techniques

(i.e. the compressibility) is much higher than the ideal-gas one. It is then possible to achieve high supersaturation by a relatively small but fast pressure change. However, if the supersaturation is obtained by slowly changing the temperature and/or the pressure, a CSS can be performed. In both RESS and CSS processes it is not necessary to involve components other than the SCF and the one to be precipitated; i.e. the HC. A co-solvent is often used to obtain higher solubility, but it is not needed in principle. This is also the case for PGSS, where the SCF is applied to lower the melting temperature of a solid, or a solid solution, in order to finally obtain solid particles as a result of the subsequent pressure reduction. The difference is that, here, the solubility of the SCF in the HC is needed, rather than the opposite. Data of this kind are still scarce, but it is well known that the amount of SCF soluble in a HC can be significant. For instance, we recall the plasticizing effect of C02 on many polymers, which lowers the polymer glass transition temperature, so that a liquid solution can be formed, provided that a suitable pressure is applied. The content of C02 in the condensed phase is usually in the range 5-50% w/w. No thermodynamic model is available yet to perform such calculations; note that in the application of eq (2.3-3) as outlined above a pure solid phase was always assumed, which is not valid for PGSS.

2.3.2.2

GASP

In antisolvent processes at least three components are involved, because an organic solvent (0s) is always present. It is then important to characterize the behavior of the ternary system SCF-OS-HC. Of the three binary systems SCF-OS, SCF-HC and OS-HC, the first is the most relevant to understanding the principle at the base of the process. In Figure 2.3 -2(a) the vapour-liquid equilibrium curves for the system C02toluene is shown at T = 311 K. The liquid phase is represented by the boiling point locus, the vapor phase by the dew point locus; experimental data are also reported in the figure. It is clear that: In the liquid phase the C 0 2 content at equilibrium can be varied from zero to around 95 %, depending on the pressure applied. The amount of toluene in the vapor phase is no more than a few percent, regardless of the pressure. Although the temperature is higher than the critical temperature of C02, the binary system cannot be referred to as being supercritical while the pressure is within the vapour-liquid range. At higher pressure, a homogeneous fluid phase is obtained which could also be defined as supercritical, but there is no practical reason for doing so. The relevant issue here is that the composition of the liquid phase can be modified from pure toluene to almost pure C 0 2 by simply increasing the pressure. If the HC is soluble in toluene, as often hap-

1 13

2.3 Precipitation and Crystallization Techniques

"0

Figure 2.3-2 (a) Vapor-liquid equilibrium of the system C02-toluene at 311 K. Experimental bubble points (0) and dew points ( 0 ) are shown as a function of the pertinent composition. The bubble and dew curves are calculated with the Peng-Robinson equation of state. (b) Percent volumetric expansion of the liquid phase in the system C02-toluene at 298 K as a function of pressure. The curve was calculated with the Peng-Robinson equation of state.

0.2

0.4 0.6 MOLE FRACTION of C02

b)

0.8

1

2ool

400 c 0 m B . 9 (

-g

I

so0

100

0

0

10

20

SO

40

SO

60

Pressure (bar)

pens, but it is not in COz, as it is usually the case, there must be a composition of the mixed solvent where precipitation occurs. A dramatic volumetric expansion is associated with the dissolution of CO2 in the liquid phase. This is reported in Figure 2.3-2(b) for the same system and slightly different conditions. The volumetric expansion is defined by

V-VO AV= (2.3-5) VO where and Vare the liquid volumes at ambient and variable pressures, respectively. The features demonstrated for the system C02-toluene can be extended with good approximation to all binaries between C 0 2 and any 0s. This means that one can achieve the almost complete dissolution of COz in the liquid phase, with the corresponding volumetric effect, by applying a suitable pressure. This value depends on the 0s and on the temperature, but is usually lower than 100 bar. The extension of such a phase equilibrium behavior to systems containing SCFs other than C02 is not straightforward, as the specific properties of the SCF may lead to different phase diagrams.

114

2 Experimental Techniques

To model the binary vapor-liquid equilibria at high pressure accurately, an equation-of-state approach is required. For instance, to calculate the curves in Figures 2.3-2(a) and (b) a Peng-Robinson equation with one adjustable parameter was used. This point was examined by Sandler [9]. Regarding the ternary system SCF-OS-HC, th_e most important information to look for is the pressure dependence of the HC solubility in the liquid phase. Again, this can be obtained either experimentally or by a suitable thermodynamic model, which is difficult to develop. However, Chang and Randolph [lo] proposed a simple and approximate expression which is independent of the HC: vos

(2.3 - 6)

s(T, p , X ) = s(T, p = 1, x = 0)'-

4s

In eq (2.3-6) s is the HC solubility, x the mole fraction of the SCF in the liquid phase and the terms v and vo are the partial and pure compound molar volumes of the 0s in the liquid phase, respectively. In Figure 2.3-3, the solubility curve is presented for the system CO2-toluene-B-carotene. Results obtained from a rigorous model and the simplified one of eq (2.3-6) are reported. It can be seen that eq (2.3-6) does not represent the experimental points well; however, it does provide a fair idea of the pressures required for total precipitation. In summary, a good thermodynamic understanding of the system behavior is absolutely essential to develop and design CSS, PGSS, RESS and GASP applications. Note that we have not addressed the problem of the experimental determination of thermodynamic properties; this is discussed, for example, by Bruno [ll]. However, a more detailed simulation of phase equilibria of the systems related to these techniques is given by Kikic et al. [12].

0

P (bar)

Figure 2.3-3 Mole fraction ( x ) of p-carotene in the liquid phase for the system C02-toluene-pcarotene at 298K. Comparison between experimental data and calculations performed by models of Chang and Randolph [lo] and of Kikic et al. [12]. Adapted from Kikic et al. Proc. of The 4th Int. Symp. on Supercritical Fluids, Sendai, Japan, 1997, p. 42-45.

2.3 Precipitation and Crystallization Techniques

115

2.3.3 Process Basics and Reference Schemes To understand and compare the advantages and drawbacks of the techniques considered, a simple description of the related processes is given in the following paragraphs. For RESS and GASP, details on apparatus and procedure are also provided. 2.3.3.1 Crystallization from a Supercritical Solution (CSS) CSS is similar to conventional batch crystallization, where the crystals are obtained by slowly cooling down a saturated solution according to an optimal cooling policy. This allows the desired supersaturation level to be maintained, as well as a constant crystal growth rate. If the solvent is a SCF, high pressure is required; therefore, not only temperature but also pressure can be used to trigger nucleation and growth of crystals. An additional advantage is that crystals can be obtained that are completely free of organic solvents. The interest in CSS comes from a number of attractive features: the possibility of tuning the crystal size distribution [l], of producing large crystals [13], and of purifying solid materials from impurities [14]. In this respect, we quote also a patent by Shlichta [15]. A batch CSS experiment can be carried out in a simple apparatus, essentially a stirred pressure-resistant vessel. First, the supercritical solution is prepared by loading into it the proper amounts of SCF and HC. Then, the stirrer is switched off, and the temperature and/or pressure of the solution are varied according to the policy adopted for the process, until the crystals are formed. Operating temperatures can be close to ambient, if a suitable SCF (such as C02) is used; the pressure value will be that needed to get the required concentration of the supercritical solution. Various crystal morphologies and sizes can be obtained by changing the main operating variables, such as the concentration of the starting solution and the T or p gradients applied. However, due to the low solubility of the HC in the SF, more than lo4 kg of SCF per kilogram of HC are usually needed. This fact, together with the long cooling and/or expansion times are two serious disadvantages for the development of CSS on a large scale. On a laboratory scale, simple versions of CSS may be useful for the generation of high quality crystals for X-ray crystallograPhY*

2.3.3.2 Formation of Particles from Gas Saturated Solution (PGSS) PGSS is a new process about which little information is available, because most is covered by patents [16]. PGSS has been applied successfully to the micronisation of glycerides [ 171 and polyethyleneglycols [ 161.

116

2 Experimental Techniques

This technique takes advantage of the fact that the solubility of gases in HCs (either liquid or solid) can lower significantly the melting temperature of the HC . In a PGSS experime,nt, the SCF is first dissolved in the HC by increasing the pressure until a melt is obtained. The gas-saturated solution is then expanded through a nozzle where it is cooled simultaneously by evaporation and Joule-Thompson effects. In this way supersaturation conditions are reached, and solid particles are formed. Again, the product is completely free of organic solvents. The amount of SCF needed is low, between 0.2 and 0.6 kg per kilogram of HC. According to the apparatus diagram in Figure 2.3-4, three pieces of equipment are required: a thermostatted autoclave V for the dissolution of the SCF in the HC; a spray-tower autoclave PR equipped with nozzles through which the gassaturated mixture is pumped and where particles are formed; a system for the separation and collection of the solid particles - both cyclones (CY) and electrofilters (EF) may be used, connected in series. Operating pressures are between 100 and 200 bar, and temperatures around 373K or more are usually needed. For a given system, the particle size of the final particles, which is correlated to the surface tension and the viscosity of the melt, can be adjusted by selecting the nozzle diameter (which is usually larger than 0.5 mm), p and T in the autoclave and p in the spray tower. Depending on the process conditions, three classes of particles were obtained: fibres, spheres and sponges [17].

Figure 2.3-4 Diagram of a PGSS process: V = high pressure saturation vessel; PR = precipitation vessel; CY = cyclone separator; EF = electrofilter; P1, P2, P3 = solid products; C 0 2 = SCF vent. Adapted from Weidner et al. [16]

2.3 Precipitation and Crystallization Techniques

117

Apart from the unavailability of detailed technical information, at present there are two major disadvantages for the development of PGSS: the relatively high operating temperatures and the problem of nozzle clogging. In addition, data on the solubility of SCFs in HCs, essential for process design, are usually neither available nor predictable.

2.3.3.3 Rapid Expansion of a Supercritical Solution (RESS)

RESS was the first technique devised to exploit SCFs as precipitation media. It received wide attention from the chemical engineering community especially in the late 1980s and early 1990s. Two important review papers are a general one by Tom and Debenedetti [l8] and a second, tailored to pharmaceutical processes, by Phillips and Stella [19]. Basically, RESS allows production of small particles of solid materials dissolved in an SCF by rapidly expanding the supercritical solution through a suitable nozzle. Due to the fast pressure reduction, very high supersaturation values can be achieved. Additionally, as the pressure change travels at the speed of sound, uniform conditions are guaranteed within the solution and a large number of nuclei that have no time to grow are formed. Organic solvents are not needed, so no contamination occurs. Therefore, RESS is extremely attractive: in principle, very fine crystal and powders should be obtained with a narrow size distribution. The basic RESS process is simple, as sketched in Figure 2.3-5. Three main units are needed: a high-pressure saturation vessel S to dissolve the HC in the SCF; a device to reduce the pressure, PRV; a precipitation vessel E where the crystals can be collected. Note that RESS is a semibatch operation, as the SCF is flowing continuously through the apparatus. The size and morphology of the product can be tuned by changing the process parameters, namely the concentration of the supercritical solution, the temperature of the expanding orifice, and the temperature and pressure of the expansion vessel. Details on the effects of these variables are extensively discussed by Reverchon [20].

Figure 2.3-5 Diagram of the RESS process: PV = high pressure pump; S = saturation vessel; PRV = device for pressure reduction; E = precipitation chamber; COZ = SCF feed.

co2

118

2 Experimental Techniques

Although the literature about RESS is rich, it mainly consists of scientific papers rather than industrial applications. This can be explained by a number of reasons: To obtain particles, the HC must be dissolvedby the SCF. Many interesting materials do not show detectable solubility in whatever SCF is used as the solvent. The solubility must be high enough. If this is not the case, higher pressure has to be used, calling for a special apparatus with much larger plant and operating costs. Alternatively, solubility can be enhanced by a cosolvent, but then the process is no longer solvent-free, and the cosolvent may interfere during the expansion step. The particle sizes and size distributions found experimentally are much less favorable than the expected values, probably due to strong coalescence effects during expansion. The material to be processed by RESS must be of high value, as, at best, lo4 kg of SCF are needed to obtain 1 kg of product. Recycling can be used to reduce the loss of solvent, but the economics of the process is unfavorable. '

Apparatus and Procedure The equipment consists of two main units (one for dissolution and the other for precipitation) plus a number of other parts, as reported in Figure 2.3-6. To perform a standard RESS experiment one needs:

Figure 2.3-6 Apparatus for RESS: A = SCF reservoir; PV = high pressure volumetric pump; H1, H2 = heat exchangers; V, V1, V2, V3 = on-off valves; S = saturation vessel; EN = expansion nozzle; E = expansion vessel; F = flow meter; G = rotameter; PI = pressure indicator; TI = thermometer.

2.3 Precipitation and Crystallization Techniques

119

A high pressure pump PV for the SCF, able to deliver the desired flow rate at the selected pressure. The SCF must be subcooled in H1 before entering PV. Note that an additional pump is needed when using a cosolvent; A preheater H2 (either a coil or a heat exchanger) to set the SCF temperature; A thermostatted stainless &eel saturation vessel S filled with the HC to be dissolved in the SCF (glass wool or beads can be used as packing). It must be equipped with a safety system (rupture disk, check valve). A line to bypass S both to start up the experiment and to dilute the supercritical solution before expansion. A pressure reduction device (EN), such as a capillary, a laser-drilled nozzle, or even a micrometric valve. For best results, pinpoint orifices of 20-40 pm, with 0.2-0.4 nun thickness, are suggested. The nozzle temperature must be controlled accurately, for instance with a variable resistance electric heater. An expansion vessel E containing a suitable system for the collection of particles (such as a plate, a paper filter, a glass beaker). If only partial expansion, i.e. not all the way to atmospheric pressure, is carried out, this vessel must be a stainless steel chamber, and a valve has to be added to the exit line. A number of pressure resistant on-off valves. Pressure gauges (PI) and temperature sensors (TI). A temperature control system for the pressure reduction device EN. A flow-meter F and a wet or dry rotameter G in the vent line. Note that the part of the apparatus under high pressure must be designed for operation up to 400 bar, so that special tubing, valves and fittings are required. Note also that it is important to keep the lines between the two main units at the same temperature. A standard operating procedure can be as follows: 1. The saturation vessel S is pressurized and heated to the required conditions with valves V1 and V2 closed. 2. A flow of pure SCF is started through a line which bypasses S and is set to a constant flow rate at the same pressure and temperature as S. To do this, both valves V2 and V3 are closed, and valve V1 is opened. The pure SCF flow goes on until the temperature of the expansion device is adjusted to its setpoint. 3. Valve V1 is closed and valves V2 and V3 are opened simultaneously, so that the supercritical solution is allowed to flow through the orifice EN, and the solid can be precipitated in the expansion vessel E. Note that now the bypass line can be used to reduce the HC concentration in the SCF before entering the nozzle. Note also that the regulation of the preexpansion temperature is essential to obtain the desired product. 4. The process is continued until the required amount of particles has been collected. 5 . The system is depressurized.

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2.3.3.4 Precipitation by a Gas or a Supercritical Antisolvent (GASP) Using the SCF as an antisolvent can be more attractive, as the antisolvent effect is more generally applicable. For example, C02 can be dissolved in any HC-OS system. In this way, the properties of OSs are modified by C02 so that they are no longer able to keep the HC in the solution, and a complete precipitation of the HC can be obtained. Pressures lower than 100 bar are usually enough to do this job; in addition, the precipitation often takes place within a narrow pressure range. For a given SCF-HC-OS system the phenomenon is a function of pressure and temperature or, more precisely, of the mixed (SCF+OS) solvent density, which accounts for both effects. However, during and/or after the formation of the solid particles, it is necessary to remove completely both the mixed-solvent liquid phase and the 0s which remains adsorbed and trapped in the solid (usually a few percent). Product purification can be achieved by stripping 0s residuals with the same SCF used as a solvent. Applications of GASP have been proposed for different materials: polymers, foods, superconductors, pharmaceuticals, dyestuffs, and catalysts. The published literature on this topic is increasing exponentially, and many patents have been or are being issued, proof of the enormous interest in these techniques by industry. For an up-to-date review one can refer to Reverchon [21]. An interesting paper concerning particle formation using compressed C02 in pharmaceutical processing, and dealing especially with antisolvent precipitation, was written by Subramaniam [22]. The development of biocompatible materials and products with GASP is a field of great potential [23,24]. GASP operations are currently used to carry out coprecipitation for impregnation purposes and fractional crystallization for the recovery of pure solids from blends and mixtures [25]. In the literature there is no agreement about nomenclature concerning gas and supercritical antisolvent processes (the most recent proposal is by Bungert et al. [26]). Therefore, they are classified here by the mode of operation: 1. In the GAS (SAS) process, the OS-HC solution is first loaded into the auto-

clave, and then the pressure is increased by feeding the SCF up to the value required for precipitation. At this point, three phases are present in the vessel: the solid product, the liquid mixed-solvent and a vapor phase, essentially SCF. GAS (SAS) batch is a completely batch operation, very simple to carry out. However, as the system undergoes all pressure conditions, between the initial and final, the product obtained depends also on the pressure profile during pressurization. 2. If the 0s-HC solution is injected into the vessel already pressurized with the SCF, this is called PCA (Precipitation with compressed antisolvent). Note that the 0s is completely vaporized into the supercritical phase during injection and the specific amount of SCF required to get precipitation is much larger than in GAS (SAS) batch. Therefore, PCA is more useful during development than production.

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Zs Figure 2.3-7 Diagram of a GASP process:

P = precipitation vessel; E = expansion

vessel; PRV = valve for pressure reduction; SOL = liquid solution feed; C02 = SCF feed.,

3. When both the OS-HC solution and the SCF are fed continuously into the vessel, so that one can decide the exact operating pressure, this is known as continuous GAS (SAS), even though the solid phase accumulates in the vessel. Depending on the flow rate ratio, a liquid phase may also accumulate in the vessel. In this case, the liquid has to be withdrawn continuously. A typical GASP process is represented schematically in Figure 2.3-7. Three main units are required: a precipitation autoclave P, equipped with a stirrer, where the particles are formed and collected; a pressure reduction valve PRV; a low pressure expansion vessel E to collect the 0s. The line for recycling the SCF is also indicated in Figure 2.3-7. Knowledge of the volumetric expansion of the liquid phase with pressure and of the precipitation pressure at the selected temperature are essential for performing any supercritical antisolvent precipitation experiment. The concentration of HC in the starting organic solution also affects the shape and dimensions of the product obtained. In summary, GASP techniques offer the possibility of obtaining new products with a quite efficient process. At best, the consumption of CO2 ranges from 10 to 100 kg per kilogram HC (for PCA, from 1000 to 10000 kg per kilogram HC). Furthermore, a relatively low-pressure apparatus is required. In comparison to RESS the apparatus is also simpler, but the procedure is slightly more complicated, as outlined below. Of course, the presence of the OS, from which the final product has to be purified, may be a substantial drawback.

Apparatus and Procedure As there are only minor differences between the three operating modes outlined above, only the simplest GAS batch mode will be described. The apparatus is shown in Figure 2.3-8 and comprises:

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2 Experimental Techniques v3 H1

h

f l

U

I

PV

H2

V v3

Figure 2.3-8 Apparatus for GASP batch: A = SCF tank; H1,H2 = heat exchangers; PV = high pressure volumetric pump; P = precipitation vessel; E = expansion vessel; V = o d o f f valves; V3 = three-way valves; PRV = valve for pressure reduction; F = flow meter; G = rotameter; PI = pressure indicator; TI = thermometer.

A high pressure pump PV, able to deliver the desired flow rate at the selected pressure. The SCF must be subcooled in H1 before entering PV. To perform PCA and continuous GAS experiments, a second pump is needed. A preheater H2 (either a coil or a heat exchanger) to set the SCF temperature. A stainless steel precipitation vessel P, possibly with a glass window, designed to work up to 120 bar. It has to be thermostatted by either a jacket, or a heating tape. It must be equipped with a safety system (rupture disk, check valve) and possibly a magnetic stirrer. A system to obtain a fine distribution of the SCF when entering P - either a capillary nozzle, of the same type used for RESS, or a metallic filter. To perform PCA and continuous GAS experiments, a second distributor is needed for the liquid solution. A number of pressure-resistant odoff valves. Two three-way valves V3 to select up-flow or down-flow of the SCF through P. A metering valve PRV for pressure reduction after the vessel. It must be heated to compensate for Joule-Thompson effects during expansion. An expansion vessel E to collect the 0 s after pressure reduction. This can be operated near atmospheric pressure. Pressure gauges (PI) and temperature sensors (TI). A flow-meter F and a wet or dry rotameter G in the vent line. Note that, in the high pressure section, standard tubing, valves and fitting can be used. The following experimental procedure is suggested.

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123

1. The HC-OS solution of required composition is loaded into vessel P and the desired temperature is reached. Valve PRV is closed. 2. The SCF is fed to P, which is pressurized at constant temperature at the desired (constant) rate (usually between 1 and 20 barhin); 3. The precipitation is carried out at constant pressure and temperature (the values depending on the material to be processed and on the product to be obtained). 4. The mixed-solvent liquid phase is washed out of P, either from the top or the bottom. To do this, it is necessary to open valve PRV between P and the expansion vessel E, and to operate valves V3. 5 . The particles are purified at constant pressure (the same or greater than that used at step 3.) by a continuous flow of SCF. This step requires some time to reach the target composition of 0s in the particles. 6. The vessel is depressurized and the solid product is recovered.

Note that, when running PCA experiments, no liquid phase is present in P so that step 4 is not necessary. Note also that, for the continuous GAS operation, special care must be taken to prevent solid precipitation in the feed line of the liquid solution.

2.3.4

Example

In order to show the potential of RESS and GASP for the manufacturing of pharmaceuticals, and to demonstrate the improvements they give with respect to conventional methods, the micronization of a polysaccharide is now considered [23]. The problem is the production of microspheres of a biocompatible and biodegradable polymer, a poly(hya1uronic acid, benzylic ester) abbreviated HYAFF. This polymer, loaded with drugs, is used as a controlled-drug delivery device. The production of HYAFF is currently performed by a solvent emulsion precipitation method, which involves many steps and requires a number of organic solvents. Both RESS and GASP were investigated as alternative methods. The SCF was C 0 2 and the organic solvent for GASP was dimethylsulfoxide (DMSO). The RESS experiment was performed in the apparatus shown in Figure 2.3-6. The saturator S was filled with HYAFF powder, and the procedure was as listed in Section 2.3.3.3. Several temperatures and pressures were investigated; the best results were obtained at 333 K and 250 bar, and are shown in Figure 2.3-9. Spherical particles of average diameter around 8-9 pm were obtained, with a qualitatively acceptable size distribution. However, the quantity of microspheres produced was extremely low, because the solubility of HYAFF in scCOz is negligible. No substantial improvements were achieved by applying cosolvents. Enhanching the solubility by increasing the temperature was not possible,

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Figure 2.3-9 SEM photograph of HYAFF microspheres formed by RESS at 333K and 250 bar with pure COz using a 40 pm i.d. nozzle (bar = 10 pm, ~ 1 8 5 0 ) .(Copyright John Wiley and Sons; reproduced with permission from Reference 23).

because fibres were produced instead of spheres. It was concluded that RESS was not suitable for this application. A GAS batch experiment was then considered and the apparatus was built up according to Figure 2.3-8. The internal volume of the precipitation chamber P was about 0.3 L. Different solutions of HYAFF in DMSO were prepared; they were loaded into P (usually 0.03 L) and then pressurized with C02 up to 100 bar, at temperatures ranging between 308 K and 333 K. The washing out of the mixed liquid solvent after precipitation and the purification of the particles were carried out according to the procedure described in Section 2.3.3.4. The best results obtained are shown in Figure 2.3-10, being nanospheres with an acceptable particle size distribution and average size 350 nm. Note that this value is 50 times smaller than that provided by the solvent emulsion process. In addition, all the HYAFF loaded at the beginning of the run, as DMSO solution, was recovered quantitatively as nanospheres at the end. The DMSO content in the particles was between 1 % and 3 %, but could be lowered further under a suitably prolonged flow of C02.

Figure 2.3-10 SEM photograph of HYAFF microspheres formed by GAS batch at 3 13 K and 100 bar with a C 0 2 flow rate of 8.0 glmin; The concentration of HYAFF was 1.0% w/w in DMSO (bar = 1 pm, x 10 000). (Copyright John Wiley and Sons; reproduced with permission from Reference 23).

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125

2.3.5 Concluding Remarks A number of crystallization and precipitation techniques using SCFs have been considered, with special respect to the rapid expansion of a supercritical solution method (RESS)and to the SCF antisolvent processes (GASP). Knowledge of the phase behavior of the systems involved was shown to be of utmost importance for the development of these processes. Therefore, data on related binarykernary systems under pressure must be known with respect to any possible application. The advantages and drawbacks of the techniques considered were discussed. Details were plpvided about the experimental set-up and procedure for RESS and GASP. An example of their application to the production of microspheres of a polysaccharide was presented.

References [l] A. Tavana, A. D. Randolph, AIChE J. 1989, 35, 1625-1630. [2] E. Weidner, Z. Knez, Z. Novak, Proc. of the 3rd Int. Symp. on Supercritical Fluids, 1994, Strasbourg (F), T3, 229-234. [3] V. J. Krukonis, Proc. Annual AIChE Meeting, AIChE, San Francisco (USA), 1984, paper 140f. [4] P. M. Gallagher, M. P. Coffey, V. J. Krukonis, N. Klasutis, in Supercritical Fluid Science and Technology, ACS Symp Sel: 406, K. P. Johnston, J. M. L. Penninger (Eds.), American Chemical Society, Washington, DC, 1989, 334-354. [S] B. Wagner, C. B. Kautz, G. M. Schneider, Proc. 8th Int. Con$ on Properties and Phase Equilibria, Noordwijkerhout (NL), 1998, 139. [6] J. F. Brennecke, C. A. Eckert, AIChE J. 1989, 35, 1409-1427. [7] J. Chrastil, J. Phys. Chem. 1982, 86, 3016-3021. [8] J. Mendez-Santiago, A. S. Teja, Proc. 8th Int. Con$ on Properties and Phase Equilibria, Noordwijkerhout (NL), 1998, 123. [9] S. Sandler, in Supercritical Fluids: Fundamentals for Applications, NATO ASI Series E, Vol. 273, E. Kiran, J. M. H. Levelt-Sengers (Eds.), Kluwer Academic Pub., Dordrecht (NL), 1994, 147-175. [lo] C. J. Chang, A. D. Randolph, AIChE J. 1990, 36, 939-942. [ 111 T. J. Bruno, in Supercritical Fluid Technology, Reviews in Modern Theory and Applications, T. J. Bruno, J. F. Ely (Eds.), CRC Press, Boca Raton, 1991, 293-325. [12] I. Kikic, M. Lora, A. Bertucco, Ind. Eng. Chem. Res. 1997, 36, 5505-5515. [13] C. Y. Tai, C . 3 . Cheng, AIChE J. 1995, 41, 2227-2236. [ 141 H. Freund, R. Steiner, in High Pressure Chemical Engineering, P. R. von Rohr, C. Trepp (Eds.), Elsevier, Amsterdam, 1996, 21 1-216. [15] P. J. Shlichta, U.S. Patent 4,512,846, April 23rd, 1985. [16] E. Weidner, R. Steiner, Z. Knez, in High Pressure Chemical Engineering, P. R. von Rohr, C. Trepp (Eds.), Elsevier, Amsterdam, 1996, 223-228. [17] Z. Knez, Proc. 5th Meeting on Supercritical Fluids, Nice (F), 1998, T I , 13-19. [IS] J. W. Tom, P. G. Debenedetti, J. Aerosol Sci. 1991, 22, 555-584. [19] E. M. Phillips, V. J. Stella, Int. J. Pharmaceutics 1993, 94, 1-10. [20] E. Reverchon, G. Della Porta, R. Taddeo, P. Pallado, A. Stassi, Ind. Eng. Chem. Res. 1995, 34, 4087-4091.

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[21] E. Reverchon, Proc. 5th Meeting on Supercritical Fluids, Nice (F), 1998, T I , 221-236. [22]B. Subramaniam, R. A. Rajewski, K. Snavely, J. Pharm. Sci. 1997, 86, 885-890. [23] L. Benedetti, A. Bertucco, P. Pallado, Biotech. Bioeng. 1997, 53, 232-238. [24] M. A. Winters, B. L. Knutson, P. G. Debenedetti, H. G . Sparks, T. M. Przybycien, C. L. Stevenson, S. J. Prestrelski, J. Pharm. Sci. 1996, 85, 586-594. [25] M. Lora, I. Kikic, A. Bertucco, AZChE J. 1998, 44, 2147-2158. [26] B. Bungert, G . Sadowski, W. Arlt, Fluid Phase Equil: 1997, 139, 349-359.

Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

2.4 Microemulsions, Emulsions and Latexes KEITHP. JOHNSTON, GUNILLA B. JACOBSON, C. TEDLEE,CARSON MEREDITH, SANDRO R. P. DA ROCHA,MATTZ. YATES,JANETDEGRAZIA and THEODORE W. RANDOLPH

2.4.1 Introduction As compressed carbon dioxide is a nonpolar molecule with weak van der Waals forces (low polarizability per volume), it is a relatively weak solvent [ 13. Thus, many interesting separations and chemical reactions involving insoluble substances in C02 can be expected to take place in heterogeneous systems, for example, microemulsions, emulsions, latexes and suspensions. Microemulsion droplets 2-10 nm in diameter are optically transparent and thermodynamically stable, whereas kinetically stable emulsions and latexes in the range from 200nm to lOpm are opaque and thermodynamically unstable. The first generation of research involving colloids (micron-sized or smaller dispersions of particles or droplets) in supercritical fluids (SCFs) addressed water-in-alkane microemulsions, for fluids such as ethane and propane [ 1-41 as reviewed elsewhere [4-71. A lattice fluid (LF) self-consistent field theory was developed to describe steric stabilization of microemulsions, emulsions, and latexes in supercritical fluids as a function of the extension of the surfactant tails [5,6,8]. According to LF self-consistent field theory and computer simulation [9], these colloids flocculate (form loose aggregates) at the same density where the stabilizer moiety of the surfactant phase separates from the SCF in bulk solution. This principle has been verified experimentally [ 10,111 and is useful for designing surfactants for supercritical fluids. Applications of surfactants and colloids in supercritical fluids include dry cleaning, reactions and separations involving hydrophiles such as proteins and ions, dispersion [ 121 and emulsion polymerization [ 131, and formation of high-surfacearea materials including nanoparticles. Several important criteria must be satisfied to stabilize a colloid in an SCF [5,6,8,9]. The surfactant must adsorb at the interface, and the surfactant tails must be solvated and long enough to provide steric stabilization, where the force between the droplets or particles is repulsive. Near-critical propane can solvate the tails of many hydrocarbon-based surfactants which are also utilized to form microemulsions in alkanes such as hexane, for example, bis-2-

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ethylhexyl sodium sulfosuccinate (AOT), and nonionic ethoxylated alcohols such as C12E6, where E indicates an ethylene oxide repeat unit [4,7]. For C02, which has a -much lower polarizability per volume, surfactant tails with lower cohesive energy densities (e.g. as characterized by the surface tension) are usually required. Thus, the C02-philicity of a surfactant tail decreases as its surface tension increases, for example in the order poly( 1,l-dihydrodecafluorooctyl acrylate) > poly(perfluoroethers) > poly(dimethylsi1oxane) (PDMS) > poly(propy1ene oxide) = poly(buty1ene oxide) > paraffin wax [ 141. To date, water-in-C02 (wlc) microemulsions have been formed with surfactants containing fluorinated ether or alkane tails and ionic head groups, as discussed below. To form these microemulsions with the nonionic surfactant pentaethylene glycol n-octyl ether ir' was necessary to add significant amounts of n-pentanol as a cosurfactant [ 151. In contrast, waterloil microemulsions may be formed in SCF alkanes without cosurfactant [ 161. However, kinetically stable wlc emulsions have been formed with ionic and nonionic surfactants containing fluoroethers [17], PDMS, and poly(buty1ene oxide) [ 181, and organic-in-C02 (oh) emulsions have been formed with all of the tails listed above except alkanes. The next section describes measurements of interfacial tension and surfactant adsorption. The sections on wlc and olc microemulsions discuss phase behavior, spectroscopic and scattering studies of polarity, pH, aggregation, droplet size, and protein solubilization. The formation of wlc microemulsions, which has been achieved only recently [19, 201, offers new opportunities in protein and polymer chemistry, separation science, reaction engineering, environmental science for waste minimization and treatment, and materials science. Recently, kinetically stable wlc emulsions have been formed for water volume percentages from 10 to 75, as described below. Stabilization and flocculation of wlc and olc emulsions are characterized as a function of the surfactant adsorption and the solvation of the C02-philic group of the surfactant. The last two sections describe phase transfer reactions between lipophiles and hydrophiles in wlc microemulsions and emulsions and in situ mechanistic studies of dispersion polymerization.

2.4.2 Interfacial Tension The adsorption of a surfactant at an interface between C02 and a second fluid, such as water, may be determined directly from measurement of the interfacial tension (change in Gibbs free energy with surface area), y, versus surfactant concentration. A novel tandem variable-volume pendant drop tensiometer has been developed to measure equilibrium and dynamic values of y as a function of T p and time (Figure 2.4-1) [21]. An organic [21] or aqueous phase [18] is preequilibrated with C02 in the first variable-volume cell (drop-phase cell). A droplet of this liquid is injected into the second variable-volume cell, with two windows at 180O mounted on a diameter, containing either pure C02 or C 0 2 and surfactant.

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Rotary Val

I

Measurement Cell

X Rcssure High Syringe Pumps

Figure 2.4-1 Tandem variable-volume view cell tensiometer for measuring the interfacial tension by the pendant drop technique: TC = temperature controller; PG = pressure gauge.

The drop image is recorded with a video camera and digitized in order to calculate y from the Young-Laplace equation [13]. Independent measurements of the densities of the two phases are made with a vibrating tube densitometer. In Figure 2.4-2 y at the CO2-water interface is plotted as a function of the concentration of the surfactant, CF30(CF2CF(CF3)0)3CF2COO-NH4+,(PFPE COO-NH4+) [181 (MW = 740 g/mol). The y value decreases from 20 (without surfactant) to values below 2 mN/m, indicating that the surfactant adsorbs strongly at the COz-water interface. A discontinuity in the slope, observed wt fraction, indicates the formation of w/c microemulsion drojust below plets at the critical microemulsion concentration (cpc) [22]. Above this cpc, additional surfactant forms more microemulsion droplets and the magnitude of the slope decreases. Below the cpc, the area per surfactant molecule may be calculated as -[( URT) dyldlncl-' (reciprocal of the surfactant adsorption). In Figure 2.4-2 the surface coverage reaches 100 A2 per molecule, which is sufficient for microemulsion formation at and above the cpc. This technique has also been used to determine y for the C02-polyethylene glycol (600 molecular weight), CO2-polystyrene (M,, = 1850), and C02-2-ethylhexylacrylate (M,= 92 K) interfaces, and to determine the adsorption of poly(fluoroacry1ate)

-

-

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2 Experimental Techniques

Figure 2.4-2 Reduction of the interfacial tension between COZ and water due to adsorption of PFPE-COO-NHd+.

and PDMS-based block copolymers at the polystyrene-C02 interfaces. This knowledge of surface coverage plays an important role in understanding the stability of emulsions and latexes in CO2.

2.4.3

Microemulsions

A variable-volume view cell [51] containing a piston and sapphire window, along with a computer-controlled syringe pump (ISCO) shown in Figure 2.4-3 plays a central role in many of the experiments by the authors. It offers the ability to maintain a constant pressure during reaction or while sampling or injecting solutions. The temperature and pressure may be varied independently in either direction as many times as desired at constant composition. The vessel should be shielded with polycarbonate, and the window should be pointed sideways and viewed from the front with a mirror at 45". A variety of auxiliary equipment may be attached to the mother variable-volume view cell. For microemulsions, the type A (without a capillary) system offers a means to inject known amounts of water, aqueous buffer dye solution, with a sample loop in a rotary HPLC switching valve (Valco) to a system already containing C 0 2 and surfactant. Multiple injections may be used to study incremental water concentrations. The solution may be recirculated with an HPLC pump or a gear pump (Micropump). The flow cell in the recirculation loop may be designed for UV-Vis, fluorescence, FTIR or EPR spectroscopy. An alternative technique is to load a given concentration of each of the components into a fixed volume cell. Next, C 0 2 is added in increments to study a series of pressures. Another approach is to mount two windows at 180 on a diameter in a variable-volume view cell and to measure spectra directly in this cell [34, 401.

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131

Ressure Generator

A View

cell Reversible Mini-svrianc

B View Cell

Figure 2.4-3 Apparatus based upon a variable-volume view cell with a computer-controlled ISCO syringe pump. Type A: recirculation mode for adding or removing solutions, emulsification through a capillary, and/or optical access, Type B : reciprocating pump for turbidimetry studies of latexes or suspensions.

2.4.3.1 Phase Behavior The phase behavior of a microemulsion may be measured with the apparatus shown in Figure 2.4-3 (type A). At high pressure, after seconds to hours of mixing depending upon the system, the mixture becomes transparent, often with an orange tint due to light scattering by the microemulsion droplets. The pressure is lowered slowly until a distinct cloud point is observed visually or with a laser and detector. After studying p versus T phase boundaries for a given water loading, another increment of solute is injected and the process is repeated. Figure 2.4-4 shows cloud-point data for PFPE COO-NH4+ surfactants in C 0 2 with similar molecular weights. Although the surfactant dissolved in less than 5min, in some cases it took 2 h or more to dissolve a loop of water. For a given composition, the pressure must be raised as the temperature is increased in order to keep the density of C 0 2 high enough to solvate the surfactant tails. For a given amount of surfactant, the pressure must be increased to raise the water-to-surfactant ratio, W,. As W, and thus the droplet size increase, the interaction between droplets becomes stronger, ultimately leading to precipitation of surfactant and water [23]. As the surfactant concentration increases from 1.4 to 5 wt% at a given W, (droplet size), the cloud point

2 Experimental Techniques

132

400,

P

e

300 250 200

1

1

. . . . . . . . . . . . . . . . . . . . . . . . . .

Wo = 14.7 W o = 19.4 0 0 0

0 0

0

u 70 60

Figure 2.4-4 Cloud-point data for PFF'E-COO-NH4+ w/c microemulsions. Open symbols = 1.4 wt% surfactant, 740 MW [29]; filled symbols = 5 wt% surfactant, 667 MW [17] The system is in the high-density one-phase region above each curve and in the low-density two-phase region below each curve.

pressures also increase. Here the number of micelles increases which increases micelle-micelle interactions. 2.4.3.2

UV-Vis Indicators

The polarity and the acidity of microemulsion droplets, which is of great interest for solubilization and reactions, may be characterized by methyl orange, a sensitive solvatochromic probe. As increasing amounts of water are added to the microemulsion, the visible absorption maximum of methyl orange shifts to higher wavelength indicating greater polarity [20]. To probe the acidity of PFPE COO-NH4+ reverse micelles with methyl orange, it was first necessary to understand and calibrate the spectral shifts for the relevant solutions. Spectra were obtained for methyl orange dissolved in pure water, liquid water saturated with CO;! and dry PFPE COO-NH4+ reverse micelles with no added water. The spectrum of methyl orange in a w/c microemulsion is shown (Figure 2.4-5), and it is clear that this spectrum is a composite of the three individual spectra as described elsewhere [24]. Most noticeable is a long-wavelength tail which indicates the presence of carbonic acid in the microemulsion. From this

Figure 2.4-5 UV-Vis absorption spectra (solid line) of methyl orange probe at 35°C and 241 bar in a w/c microemulsion (W,= 17): (a) interfacial region away from the polar water; (b) bulk water environment; (c) carbonic acid environment.

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spectra, and appropriate calibration, the apparent pH of water in the microemulsion is 3, a value comparable to the pH of aqueous solutions saturated with CO2 at elevated pressures [25,26]. With the addition of 500mM 2-[Nmorpholinolethane sulfonic acid as a buffer, the pH increases to about 6, as measured by the indicator -methyl red. Acidity in microemulsion droplets may be avoided by using ethane or propane as the solvent. 2.4.3.3 Electron Paramagnetic Resonance Techniques Electron paramagnetic resonance (EPR) spectroscopy is a technique that has proved particularly useful for studying the microstructure of both supercritical fluids [27,28], and supercritical fluid microemulsion and emulsion systems [20,29]. EPR spectroscopy does not require samples that are transparent to visible wavelengths, and it can therefore be used not only for microemulsions but also for emulsions. A requirement of EPR spectroscopy is the presence of an unpaired electron. This requirement may be met by using stable nitroxide free radicals or by using transition metals ions such as Mn2+. SuchEPR active moieties may be incorporated directly in the surfactant, or added as a probe molecule, such as TEMPOL (4 -Hydroxy-2,2,6,6-tetramethyl-piperidin1-oxyl) [20]. Examples of the former include Mn(PFPE)z, formed by reacting PFPE-COOH surfactants with MnClZ,and PFPE-TEMPO, generated by reacting 4 -hydroxy-2,2,6,6-tetramethyl-piperidin-l-oxyl with the acid chloride of PFPE-COOH [30,31]. For convenient operation at moderate pressures, fiberglass or poly(ether ether ketone) (PEEK) tubing may be used as sample containers. To provide mixing and temperature control, solutions from a thermostatted exterior reservoir can be pumped through the EPR cavity by using a recirculating gear pump (micropump). As shown in Figure 2.4-6, COz is first passed through a scrubber to remove trace oxygen. Addition of COz is controlled by a manually operated syringe pump. For studies requiring precise control of water content, COz is Themnostatted Bath

To C02

Figure 2.4-6 High pressure closed-loop recirculating cell used for EPR studies of supercritical fluid microemulsions and emulsions: RD = rupture disk; P = pressure transducer; T = thermocouple.

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passed over a P2O5 drying column before entering the recirculation system. Data from EPR experiments can be analyzed to provide information on the polarity of the local environment surrounding the EPR-active compound, such as of water -droplets in a microemulsion as the water-to-surfactant ratio changes. In addition, the local concentration of the EPR-active compound may be determined to describe surfactant aggregation to form micelles. 2.4.3.4 Neutron Scattering The size of w/c microemulsion droplets has been measured by neutron scattering for a di-chain hybrid surfactant (C7HI5)-(C7Fl5)CHSO4-Na+ [32], 667 g/mol PFPE-COO-NH4+ [33], and for a partially fluorinated di-chain sodium sulfosuccinate surfactant [34]. For the PFPE-COO'NH4+ surfactant, the droplet radius increases from 20A to 36A for W, values of 14 and 35, respectively. For the di-chain sodium sulfosuccinate surfactant, droplet radius varied linearly from 12 to 36 8, as W, increased from 5 to 30. This linear relationship has also been shown for AOT reverse micelles in organic solvents [7]. In each of these studies for a one-phase microemulsion, droplet size and W, were found to be only a weak function of pressure, unless the pressure is reduced to the phase boundary where droplets aggregate. This trend was calculated theoretically [6,23] and has been measured in AOT w/o microemulsions in supercritical propane [35,36]. 2.4.3.5 Enzymes The high specificity and high room-temperature catalytic efficiencies of enzymes have made them tantalizing targets for inclusion in supercritical fluid emulsion or microemulsion systems. Enzymes have been known for more than a decade to be catalytically active in supercritical COz [37], and their use with COz-based microemulsions has been patented [38]. Recently, proteins have been solubilized in the water interior of reversed micelles in the water/CO2/PFPE-C00-NH4+ system [20]. Two large obstacles to practical use of enzymes in supercritical fluid microemulsions have been the low pH of COz-saturated water and the challenges of cofactor regeneration. deGrazia [39] found that the pH of the aqueous phase of the C02/water/h4n(PFPE)2 emulsion system increased to 4.75-5 with the addition of 150mM sodium phosphate buffer, as determined by visual observation of an ethyl red colorimetric pH indicator. Although there are no reports of cofactor regeneration in supercritical fluid systems, the recent advances in forming COZ-water emulsions may enable more complicated reaction schemes than were previously possible.

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135

2.4.3.6 Organic-C02 Microemulsions In addition to w/c microemulsions, o/c microemulsions may be formed for systems with strong surfactant adsorption. The area occupied by PFPE-COO-NH4+ at the interface between 600 molecular weight polyethylene glycol (PEG) and C02 is 440 A2 per molecule based upon measurement of the interfacial tension versus surfactant concentration [213. This surface coverage is sufficient for microemulsion formation as was verified with phase behavior measurements. Only 0.55 wt% of 600 molecular weight polyethylene glycol is soluble in COz at 45 "C and 300 bar. With the addition of 4 wt% PFPE-COO-NH4+ surfactant, up to 1.8 wt% is solubilized. The additional PEG resides in the core of the microemulsion droplets, consistent with the prediction from the adsorption measurement.

2.4.4

Emulsions

2.4.4.1 Organic-C02 Emulsions Many volatile low-molecular-weight organics are completely miscible with carbon dioxide at relatively modest temperatures and pressures. However, nonvolatile compounds or those with higher molecular weights, especially polymers, are often insoluble. Insoluble liquid compounds may be dispersed into C 0 2 with the aid of appropriate surfactants to form a kinetically stable o/c emulsion [ 10,111. Stable emulsions are important in separation processes, heterogeneous reactions and materials formation processes, such as precipitation with a compressed fluid antisolvent [40]. These emulsions are the precursors to solid latex particles in dispersion polymerization. Stabilization of o/c emulsions has been studied in-situ to understand surfactant design for polymerization [ 10,111. Emulsion droplets undergoing Brownian motion can flocculate from Van der Waals attraction. Larger droplets formed by flocculation and/or coalescence will sediment at a faster rate than smaller droplets, leading to a smaller dispersed phase volume fraction. The stability of an emulsion may be monitored by measuring changes in droplet size or dispersed phase volume fraction over time by light scattering. The hydrodynamic diameter may be determined from dynamic light scattering (DLS), or fluctuations in intensity of scattered light due to Brownian motion [41]. DLS [ l l ] and turbidimetry [lo] measurements have been used to study stabilization and flocculation of o/c emulsions. Figure 2.4-3 (type A) shows an apparatus for studying emulsion formation and stability at pressures up to 345 bar and temperatures up to 80°C. The emulsion is formed by introducing a liquid into a CO2/surfactant solution with a six-port rotary valve (Valco) and shearing the solution into small droplets by recirculation through a lOOpm i.d. silica capillary with an HPLC pump. The optical cell for DLS contains three windows at right angles and

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2 Experimental Techniques

is inserted in a goniometer (Brookhaven model BI-200) [11,42] to measure light scattering at a 90" angle, whereas the turbidity cell with an aperture of 11/16in contains two collinear sapphire windows [ 101. DLS has been used to measure droplet diameters for dilute poly(2-ethyl hexyl acrylate) emulsions in C02 stabilized with poly ( 1,l-dihydroperfluorooctylacrylate) (PFOA) and PFOA-b-poly(styrene). In this experiment the C02 density was lowered in increments by decreasing the pressure at a constant temperature of 25 "C. At the higher COz densities (0.896-0.838 g/mL), the droplet size remains relatively constant at about 500nm. However, as the density is decreased to 0.824g/mL, rapid flocculation of the emulsion occurs as is evident by a large increase in the particle diameter to 1000nm. This critical flocculation density (CFD) of the emulsion [9,10,23] is somewhat analogous to critical flocculation temperatures observed for emulsions in conventional liquid solvents [43]. Near the CFD, very small changes in C02 density can result in dramatic changes in emulsion stability. It was found that the CFD is strongly related to the phase behavior of the C02-philic part of the surfactant. Figure 2.4-7 shows turbidimetry results for a 3 wt% PEHA emulsion plotted as the logarithm of the change in turbidity with time (dddt) versus C02 density at 25°C. The more negative the value of log(dddt), the more stable the emulsion is to sedimentation. The results show that the PEHA emulsion without surfactant is only modestly stable at all densities. PFOA homopolymer and PS-b-PFOA surfactants increase the stability at high C02-densities, but do not work below the CFD. The emulsions stabilized by PS-b-PFOA may be redispersed by raising the density above the CFD, whereas those stabilized by PFOA flocculate irreversibly. These results may be further explained by measurements of interfacial tension reduction by adsorption of the surfactant. The surface coverage by PFOA is limited, and it can bridge between two droplets, whereas bridging is negligible for the strongly adsorbing block copolymer PS-b-PFOA [5, 91. The CFDs determined by turbidimetry agree with those determined from DLS [lo, 111. The CFD 3.0 wt% PEHA, 0.25 wt% surfactant

Figure 2.4-7 Stability of 3 wt% PEHA emulsions in COZ at 45°C stabilized by 0.25wt% PS-b-PFOA (4.5 W24.5 K) (M,,IM,,) or PFOA (M,, = lo6).

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137

results at various temperatures can be used to map the temperature and pressure regimes where surfactants are effective in stabilizing colloids in COz. 2.4.4.2

Water-C02 Emulsions and C02-Water Emulsions

Conductivity and Dielectric Measurements The conductivity and high-frequency dielectric constant of an emulsion are both indicators of which phase (water or C02) is the continuous one. If C02 is the continuous phase in an emulsion, the emulsion conductivity will be extremely low. In contrast, if water is the continuous phase, substantial conductivity will be observed, provided that ionic solutes are present. Likewise, the dielectric of a C02-continuous emulsion should be similar to that of C02 (ca. 1.2), whereas the dielectric of a water-continuous emulsion should approach that of water (ca. 80). Emulsions were created by using a high-pressure emulsifier (Emulsiflex, Avestin) to provide high-shear mixing for systems containing C02, water, and Mn(PFPE)2 surfactant as shown in Figure 2.4-8. To measure dielectric constants, a capacitor was built by using two squares of platinum, 12.5mm on each side. The plates were 3 mm apart. Each plate was attached to two pins of a five-pin vacuum electrical feedthrough (Insulator Seal) using gold pins (J.B. Saunders Electronics). The capacitor was inserted into a large tee that attached to a 28mL view cell (Jerguson gage). The tubing into the tee was adjusted so that flow into the system passed through the two plates. Dielectric measurements were taken with a potentiostat (Model 283, E G & G Instruments). Conductivity measurements were made in the same apparatus, by using a multimeter to directly measure the resistance between the two plates. ThermostattedBath

To CO,

potentiostat

Figure 2.4-8 Apparatus for measurement of dielectric constants and conductivities for COz-water and water-Cop emulsions.

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2 Experimental Techniques

60

80

loo

140

120

160

180

200

220

240

Pressure (bars) 0 0

ro(surlectsnMndwater=.m

ro(cKlrfadMtlmd~=.oo10 T mOlWrfectsnthndWater~.W11

Figure 2.4-9 Conductivities in a ~ a t e r / C O ~ h i n ( P F P Eemulsion )~ with a water content of 50% by volume. The aqueous phase contains 1 5 m M NaCl. Conductivities are normalized by the conductivity of C02-saturated 15mM salt water at each pressure.

Remarkably, the C02/water/Mn(PFPE)2 emulsion system is C02-continuous up to 50% water by volume. As shown in Figure 2.4-9, conductivities for the entire emulsion are as low as six orders of magnitude less than those of the aqueous phase, clearly suggesting that the emulsions are C02-continuous. Dielectric data support this assertion as well. The measured dielectric constants for the emulsion remain not far above those of C02, from 1.6 to 1.7, even at 50% by volume water. These opaque emulsions look like white milk. Stability and Droplet Size The stability of w/c emulsions, defined as the time required for the volume of the emulsion to settle from 100 % to 90 % based upon visual observation, has been measured for PFPE-COO-NH4+ surfactants with molecular weights ranging from 667 to 7500 [17]. Figure 2.4-10 shows the stability of emulsions formed by the above microfluidizer for equal weights of water and C 0 2 and 1.3 wt% of 2500 g/mol PFPE-COO-NH4+. For each experiment where nonflocculated emulsions were present during shear, the specific conductivity was less than 0.1 pS/cm, indicating water droplets in a C 0 2 continuous

2.4 Microemulsions. Emulsions and Latexes

Figure 2.4-10 Stability of water-in-C02 emulsions formed from equal weights of water and COz with 10.7mM PFPE-COO-NH4+ (MW = 2500) in the COz phase and 0.01 M NaCl in the water phase. The lines are isostability contours with times given in minutes.

139

.

10

20

30 40 50 Temperature ("C)

60

70

phase. The average droplet size was approximately 2-5pm, on the basis of optical microscopy through a 100pm i.d. by 350 pm 0.d. fused silica capillary tube. After stopping shear, the formation of 20-100pm flocs was observed by optical microscopy at a magnification of 900; however, primary droplets were visible indicating that the droplets did not coalesce. Thus, the loss of the emulsion is a result of flocculation and sedimentation of water droplets in the lowviscosity C02. Emulsion stability is a maximum, greater than 60 min, at low temperature and high pressure and decreases with either an increase in temperature or a decrease in the pressure. Both of these changes reduce the density of C 0 2 which can reduce stability for various reasons. A decrease in the density and viscosity of the continuous phase will each raise the sedimentation velocity. Also, a less viscous thin film between water droplets drains more readily leading to flocculation. As the chains collapse with a decrease in density (loss in solvation), the steric repulsion between droplets diminishes, and the droplets flocculate due to the van der Waals attractive forces. On the basis of LF self-consistent field theory [5,6,9], the flocculation of the sterically stabilized emulsion may be expected to occur at the critical solution density of the surfactant tail in the supercritical solvent, neglecting kinetic factors. The shaded region in Figure 2.4 - 10 corresponds to emulsions that were observed both visually and by optical microscopy to be highly flocculated even during shear. We found that the transition from non-flocculated to flocculated emulsions corresponds to the cloud-point of the surfactant in C02, which is consistent with the theoretical prediction.

140

2 Experimental Techniques

2.4.5 Reactions in SCF Emulsions, Microemulsions and Latexes 2.4.5.1 Phase-Transfer Reactions Between Water and COz Reactions in supercritical fluids with phase transfer catalysts are discussed in detail in Chapter 4.10. Wlc microemulsions and emulsions may be used as novel media for phase transfer reactions without requiring toxic organic solvents or phase transfer catalysts. The high interfacial area may be expected to facilitate phase transfer reactions between hydrophilic nucleophiles and hydrophobic, C02-soluble, inorganic or organic substrates. Relatively few reactions have been reported in water-in-oil emulsions, with the exception of heterogeneous polymerizations, due to the difficulty in breaking the emulsion to isolate the reaction products. A wlc emulsion may be broken simply by reducing the pressure as shown above. It is likely the pressure may be manipulated to fractionate products and to recover C02 and surfactant. Clarke et al. [24] reacted water-soluble inorganic salts in PFPE-COO-NH4+ wlc microemulsion droplets with gaseous H2S or SO2 dissolved in C02 and followed the reactions in situ with UV-Vis spectroscopy. The reaction of a C02-soluble hydrophobic organic substrate, benzyl chloride, with a hydrophilic nucleophile, KBr, has been accomplished in a PFPE-COO-NH4+ wlc microemulsion. The yield, based upon direct measurement of benzyl bromide relative to an internal standard, is an order of magnitude higher than in a conventional waterloil microemulsion for similar reaction conditions [44], most probably because of the low microviscosity in the interface. The microemulsion containing KBr in the water droplets was formed as described above. Benzyl chloride was injected into the microemulsion solution with a lOOpL sample loop. Periodic samples were obtained with the sample loop, expanded and recovered with ethanol, and analyzed by GC. Figure 2.4-11 shows the conversion of benzyl chloride to benzyl bromide in a wlc microemulsion with W, = 9.7 (corrected for water in bulk C02). An advantage of this approach was that benzyl alcohol was not formed, unlike the case when the reaction is performed in a mixture of water and an organic solvent. The relatively large interfacial area in wlc emulsions, as well as the large amounts of both phases, can be advantageous for phase transfer reactions. The hydrolysis of benzoyl chloride has been studied in a wlc emulsion formed with Mn(PFPE)2 surfactant and equal weights of water and C02. The emulsion was formed as described above and then benzoyl chloride was injected through a 90pl sampling loop. The reaction was sampled and analyzed by GC in the same manner as above for the wlc microemulsion. Figure 2.4-12 shows the conversion of benzoyl chloride to benzoic acid versus time, indicating that nearly complete conversion was achieved after one hour. The reaction was also performed in a two-phase water-C02 system without surfactant, which was stirred and sheared in the same manner as the emulsion. As shown in Figure 2.4-12, the presence of surfactant increased the reaction

2.4 Microemulsions, Emulsions and Latexes

141

50

40

I

130

d

20

c)

Ei

1

Figure 2.4-11 Conversion of benzyl chloride to benzvl bromide at 65°C and 276 gG in a wlc microemulsion formed with 1.4 wt% PFPE-COO-NH4+ at various initial concentrations of KBr in water and benzyl chloride in COz.

&

10 0 0

5

10

0%

00

Figure 2.4-12 Hydrolysis of benzoyl chloride to benzoic acid in a wlc emulsion formed with 7 mM Mn(PFPE)2 surfactant and equal amounts of water and C 0 2 at 25°C and

81

1 a

15

20

25

Reaction Time (a)

6

80

-

0

-

40-

0

o

no surfactant

0

200

-

b o

B

0

-

. ~ . . l . ' . ~ . . ' . ~ . . ' . . . ~ ' ' . . . ' . . . . ' . . ' '

rate markedly, illustrating the effect of the high interfacial area for an emulsion.

Nucleophilic substitution in w/c emulsions Reactions in microemulsions and emulsions have been performed in a stainless steel variable-volume view cell (2in. (5.1 cm) o.d., 11/16 in. (1.7 cm) i.d., 35.2mL total volume), equipped with a piston and a sapphire window (1 in. (2.5 cm) diameter by 3/8in. (0.95 cm) thick) [44,55]. Water, nucleophile (e.g. KBr), surfactant (0.5 wt. %), and an internal standard were added to the cell. The cell was sealed with the sapphire window, pressurized with a known amount of COz from an ISCO pump, and heated to the desired temperature. An emulsion was formed as discussed above by recirculating the contents of the vessel (mixed with a teflon-coated magnetic stir bar) through a 0.254mm diameter by 5cm long steel capillary tube with a

142

2 Experimental Techniques

reciprocating HPLC pump (Thermo Separations Products) with a maximum flow rate of 460 d / h . During recirculation, the mixture was withdrawn from the bottom -and sprayed from the top through the upper C 0 2 phase. The organic reactant, benzyl chloride, was injected with a loop in a sixport valve. The reaction was monitored by taking out aliquots with this loop, trapping in ethanol and analyzing by GC. For reactions in microemulsions, similar procedures were used, but the capillary tube was not needed, as microemulsions are formed without shear,

2.4.5.2 In Situ Studies of Dispersion Polymerization Polymer latexes in scCOz may be produced from soluble organic monomers through dispersion polymerization of styrene [45], methyl methacrylate [12,46-481, 2-ethyl hexylacrylate [49], and vinyl acetate [50]. A key challenge is to design inexpensive surfactants that adsorb to growing polymer nuclei and provide steric stabilization against coalescence. At the early stages of the reaction, the coalescence of particles, which is strongly influenced by the surfactant composition and concentration, 'is crucial in determining the final particle size of the latex [47]. Particle nucleation and growth may be followed by scanning electron microscopy (SEM) for sizes above 200 nm if dry powder can be recovered from the reactor. This approach is not possible during the crucial early stages of particle formation because residual monomer will dissolve the latex particles during depressurization. Also, particle sizes cannot be determined for liquid polymers or highly C02-plasticized polymers such as poly(viny1 acetate) (PVAc), because the particles will coalesce during depressurization. Recently, an in situ turbidimetry technique was developed to measure particle size and number density in order to characterize the mechanism of steric stabilization, particle nucleation and growth (Figure 2.4-3, type B) [47,48]. The apparatus consisted of a variable-volume view cell, small path length (0.29 mm) optical cell, and a custom built reciprocating 1 mL minisyringe pump with a 1/8in plunger. The reciprocating pump was chosen due to difficulties in pumping solid latex particles through check valves in HPLC pumps. The ISCO syringe pump was used to maintain constant pressure as the reciprocating pump pulled latex particles back and forth through the optical cell. Turbidity spectra were collected versus wavelength and time with a UVVis spectrophotometer (Beckman Model DU-40). The turbidity of a monodisperse colloid is given by z = (UZ) In (ZJZ) = (3$K)/(2D), where q5 is the dispersed phase volume fraction, D is the particle diameter, and K is the scattering coefficient [51]. According to Mie theory, the scattering coefficient is a function of particle diameter, wavelength, and particle and continuous phase refractive indices [52]. The ratio of turbidities at two different wavelengths for a monodisperse colloid is given by zl/z2 = K1/K2.An apparent average particle diameter may be calculated from this turbidity ratio and then used to determine q5 [53].

143

2.4 Microemulsions, Emulsions and Latexes

-PVAc(30.9k)-b-PFOA(53.Sk) -PDMS

-WA~(l3.Ok)-b-PDMS(26.8k)

g

I

2

Figure 2.4-13 In situ measurement of particle number density of poly(viny1 acetate) particles in COz in the presence of PFOA- and PDMS-based sta-

bilizers.

rs,

109

lo*

lo' 10

15

20

25

30

35

40

TLme (min.)

The turbidity ratio technique was used recently to evaluate a variety of surfactants for the dispersion polymerization of vinyl acetate in scCOz at 345 bar and 65 "C [50]. Figure 2.4-13 shows the particle number density (calculated by dividing the measured dispersed phase volume fraction by the volume per particle) obtained with four different surfactants. The surfactants include a PDMS homopolymer, PDMS-b-PVAc, a poly( 1,l-dihydropeffluorooctylacrylate)homopolymer (PFOA), and PFOA-b-PVAc. The numbers in parentheses in the caption give the M,, of each block. The measured particle number densities after 40min agreed with those for diluted samples at the completion of the reaction, suggesting that the number density is fixed early in the reaction [19]. The results show that the PDMS-based surfactants produce larger particles than the PFOA-based surfactants. Higher particle number densities correspond to smaller particle size and greater latex stability at the completion of the reaction, in agreement with visual observations of the polymerizations. The block lengths of PFOA and PVAc were changed to investigate the effect of anchor-soluble balance on latex formation [50]. It was found that PVAc(30.9 k)-b-PFOA(53.5 k) gave the smallest particle size and the highest particle number density for the surfactants studied. The surfactant's anchorsoluble ratio of 0.58 is within the range recommended by Barrett for optimum surfactant performance in liquid solvents [54].

2.4.6

Conclusions

With recent theoretical and experimental advances in the understanding of colloid and interface science of SCF systems, it is becoming possible to design surfactants for microemulsions, emulsions, and latexes on a rational basis. The-

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2 Experimental Techniques

ory, simulation and experiment indicate that emulsions and latexes are stable to flocculation when the density is above the cloud point for the surfactant tail in the bulk fluid, when the tail is sufficiently long. The adsorption of surfactants at SCF interfaces and the stability of emulsions and latexes may be evaluated with complementary interfacial tension and static and dynamic light scattering measurements. Future work is needed to identlfy additional types of surfactant tails that are solvated by COz and to explore a variety of surfactant architectures for low-molecular-weight and polymeric surfactants. Thermodynamically stable microemulsions and kinetically stable emulsions may be utilized to bring water and nonvolatile hydrophilic substances, such as proteins, ions, and catalysts, into contact with a SCF-continuous phase (e.g. COz) for separation, reaction and materials formation processes. Reactions between hydrophilic and hydrophobic substrates may be accomplished in these colloids without requiring toxic organic solvents or phase transfer catalysts. COz and aqueous phases may be mixed together over a wide range in composition in w/c and c/w emulsions. The emulsion is easily broken by decreasing the pressure to separate the water and COz phases, facilitating product recovery and COz recycle. Reaction rates can be enhanced due to the considerably lower microviscosity in a w/c as compared to a water-in-alkane microemulsion or emulsion. Surfactants may be utilized in COz to stabilize latexes produced in dispersion polymerization and inorganic materials including nanoparticles produced by reaction and separation processes. In many of these examples reaction and phase separation mechanisms may be manipulated, but significant research is needed to understand how to design surfactants for these novel interfaces. For example, in dispersion polymerization, in situ measurements of particle size in the early stages of particle formation and growth may be used to develop strategies for achieving the final desired morphology.

2.4.7 Acknowledgments We acknowledge financial support from the Separations Research Program at the University of Texas, the National Science Foundation, the Department of Energy (DE-FG03- 96ER 14664) and the Texas Advanced Technology Program.

References [ l ] K. O’Shea, K. Kirmse, M. A. Fox, K. P. Johnston, J. Phys. Chem. 1991, 95, 7863. [2] J. L. Fulton, R. D. Smith, J. Phys. Chem. 1988, 92, 2903-2907. [3] K. P. Johnston, G. J. McFann, R. M. Lemert, in Supercritical Fluid Science and Technology, ACS Symposium Series 406, K . P. Johnston, J. M. L. Penninger (Eds.), American Chemical Society, Washington, DC, 1989, p. 140-164.

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[4] G. J. McFann, K. P. Johnston, in Microemulsions: Fundamental and Applied Aspects, P. Kumar (Ed.), 1999, in press. [5] J. C. Meredith, K. P. Johnston, in NATO Adv. Study Institute on Supercritical Fluids, E. Kiran (Ed.), 1999, in press. [6]D. G. Peck, K. P. Johnstan, J. Phys. Chem. 1993, 97, 5661. [7] K. A. Bartscherer, M. Minier, H. Renon, Fluid Phase Equilibria 1995, 107, 93-150. [8] D. G. Peck, K. P. Johnston, Macromolecules 1993, 26, 1537. [9] J. C. Meredith, I. C. Sanchez, K. P. Johnston, J. J. de Pablo, J. Chem. Phys. 1998, 109, 6424-6434. [lo] M. L. O’Neill, M. Z. Yates, K. P. Johnston, S. P. Wilkinson, D. A. Canelas, D. E. Betts, J. M. DeSimone, Macromolecules 1997, 30, 5050-5059. [ l l ] M. Z. Yates, M. L. O’Neill, K. P. Johnston, S. Webber, D. A. Canelas, D. E. Betts, J. M. DeSimone, Macromolecules 1997, 30, 5060-5067. [12] J. M. DeSimone, E. E. Maury, Y. Z. Menceloglu, J. B. McClain, T. J. Romack, J. R. Combes, Science 1994, 265, 356. [13] A. W. Adamson, A. P. Gast, Physical Chemistry of Surfaces, 6th ed., New York, 1997, pp. 784. [14] M. L. O’Neill, Q. Cao, M. Fang, K. P. Johnston, S. P. Wilkinson, C. D. Smith, J. L. Kerschner, S. H. Jureller, Znd. Eng. Chem. Res. 1998, 37, 3067. [15] G. J. McFann, K. P. Johnston, Lungmuir 1993, 9 , 2942. [16] G . J. McFann, S. M. Howdle, K. P. Johnston, AIChE 1994, 40, 543. [17] C. T. Lee, K. P. Johnston, J. deGrazia, T. Randolph, in preparation. [18] S. R. P. da Rocha, K. L. Harrison, K. P. Johnston, Langrnuir 1999, in press. [19] K. Harrison, J. Goveas, K. P. Johnston, E. A. O’Rear, Langmuir 1994, 10, 3536. [20] K. P. Johnston, K. L. Harrison, M. J. Clarke, S. M. Howdle, M. P. Heitz, F. V. Bright, C. Carlier, T. W. Randolph, Science 1996, 271, 624. [21] K. L. Harrison, K. P. Johnston, I. C. Sanchez, Langmuir 1996, 12, 2637-2644. [22] R. Aveyard, B. P. Binks, S. Clark, P. D. I. Fletcher, J. Chem. Tech. Biotechnol. 1990, 48, 161-171. [23] D. G. Peck, K. P. Johnston, J. Phys. Chem. 1991, 95, 9549-9556. [24] M. J. Clarke, K. L. Harrison, K. P. Johnston, S. M. Howdle, J. Am. Chem. SOC. 1997, 119, 6399-6406. [25] J. Holmes, D. Steytler, G. D. Rees, B. H. Robinson, Lungmuir, 1998, 14, 6371. [26] J. Holmes, D. Steytler, K. P. Johnston, in preparation. [27] T. W. Randolph, C. Carlier, J. Phys. Chem. 1992, 96, 5146-5151. [28] C. Carlier, T. W. Randolph, AIChE J. 1993, 39, 876-884. [29] M. P. Heitz, C. Carlier, J. deGrazia, K. Harrison, K. P. Johnston, T. W. Randolph, F. V. Bright, J. Phys. Chem. 1997, 101, 6707. [30] M.-Kupfer, R. Stosser, S. Schramm, D. Prescher, W. Damerau, Z. Chem. 1989, 29, 175-176. (311 P. Yazdi, E. J. Beckman, J. Muter. Res. 1995, 10, 530-537. [32] J. Eastoe, D. C. Steytler, Z. Bayazit, S. Martel, R. K. Heenan, Lungmuir 1996, 12, 1423. [33] R. G. Zielinski, S . R. Kline, Kaler, E. W. Rosov, N. Lungmuir 1997, 13, 3934-3937. [34] J. Eastoe, B. M. H. Cazalles, D. C. Steytler, J. D. Holmes, A. R. Pitt, T. J. Wear, R. K. Heenan, Langmuir 1997, 13, 6980-6984. [35]G. J. McFann, K. P. Johnston, J. Phys. Chem. 1991, 95, 4889. [36] E. W. Kaler, J. F. Billman, J. Fulton, R. D. Smith,J. Phys. Chem. 1991, 95, 458. [37] T. W. Randolph, D. S. Clark, H. W. Blanch, J. M. Prausnitz, Science 1988, 238, 387-390. [38] T. W. Randolph, H. W. Blanch, C. Wilke, J. M. Prausnitz, US 4,925,790 1990. [39] J. deGrazia, Thesis, U. of Colorado (Boulder), 1998. [40] S. Mawson, M. Z. Yates, M. L. O’Neill, K. P. Johnston, Lungmuir 1997, 13, 1519-1528. [41] B. Chu, Laser Light Scattering: Basic Principles and Practice, 2nd ed., Academic Press, Boston, 1991, p. 343.

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[42] K. Otake, S. E. Webber, P. Munk, K. P. Johnston, Langmuir 1997, 13, 3047-3051. [43] D. H. Napper, Polymeric Stabilization of Colloidal Dispersions, Academic Press Inc., New York, NY, 1983. [44] G. B. Jacobson, C T. Lee, K. P. Johnston, J. Org. Chem. 1999, 64, 1201-1206. [45] D. A. Canelas, D. E. Betts, J. M. DeSimone, Macromolecules 1996, 29, 2818. [46] C. Lepilleur, E. J. Beckman, Macromolecules 1997, $0, 745-756. [47] M. L. O’Neill, M. Z. Yates, K. P. Johnston, Macromolecules 1998, 31, 2848-2856. [48] M. L. O’Neill, M. Z. Yates, K. P. Johnston, Macromolecules 1998, 31, 2838-2847. [49] J. J. Shim, M. Z. Yates, K. P. Johnston, Znd. Eng. Chem. Res. 1999, submitted. [50] D. A. Canelas, D. E. Betts, J. M. DeSimone, M. Z. Yates, K. P. Johnston, Macromolecules 1998, 31, 6794. [5 1J M. A. McHugh, V. J. Kmkonis, Supercritical Fluid Extraction Principles and Practice, 2nd ed., Butterworths, Stoneham, MA, 1994. [52] P. W. Barber, S. C. Hill, Light Scattering by Particles: Computational Methods, 1st ed. World Scientific Publishing Co., Teaneck, NJ, 1990. [53] L. H. Garcia-Rubio, in Particle Size Distribution: Assessment and Characterization, ACS Symp. Sel: 332, T. Provder (Ed.), American Chemical Society, Washington, DC, 1987, p. 161-178. [54] K. E. J. Barrett, Dispersion Polymerization in Organic Media, John Wiley & Sons, New York, 1975. [55] G. B. Jacobson, C. T. Lee, S. R. P. daRocha, K. P. Johnston, J. Org. Chem. 1999, 64, 1207-1210.

Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

3 Spectroscopy of SCF Solutions 3.1 Vibrational Spectroscopy STEVENM. HOWDLE,MICHAEL W. GEORGEand MARTYN POLIAKOFF

3.1.1 Introduction Supercritical fluids (SCFs) have proved to be versatile media for a wide range of chemical processes [ 11 from stereoselective organic chemistry [2] through catalytic hydrogenation [3], polymer synthesis [4] and polymer modification [5] to the preparation of novel inorganic materials [6] and organometallic complexes [7].IR and Raman spectroscopy have played a significant role [8] in many of these developments. Chemists traditionally use spectroscopy to monitor the progress of reactions, thereby gaining insights into mechanisms and kinetics. In conventional solutions, aliquots of a reaction mixture can be withdrawn at various time periods for spectroscopic analysis. Alternatively, monitoring may be performed on-line through windows or by using transparent vessels. Because of the higher pressures required to maintain supercritical conditions, the withdrawal of aliquots of solution is much harder to achieve [9]. Thus, in situ monitoring is preferable. Most SCF syntheses are performed in sealed metallic autoclaves, usually with thick walls, which need to be adapted for spectroscopic analysis [lo]. The sections in this chapter (3) show that, despite the technical difficulties, all of the spectroscopic methods with which synthetic chemists are familiar (NMR, EPR, mass spectrometry, UV-Vis, Raman, IR) can be applied to monitoring reactions in SCF solutions, even under quite extreme conditions of temperature and pressure. An additional complication with SCF processes is the possibility of complex phase behavior. A thorough understanding of the phase behavior of each reaction mixture is desirable, but not always possible. There is an extensive literature devoted to the investigation and prediction of phase behavior of SCFs [ll], and the subject is introduced in chapter 1.2. Spectroscopic monitoring can provide at least an indication of phase behavior; for example, which compounds are dissolved in which phase, as well as a method for monitoring the progress of reactions and identifying intermediates and products. It is best to resist the temptation to treat a supercritical reactor as a black box, and simply

148

3 Spectroscopy of SCF Solutions

to analyze final products. Indeed, there are already examples of reactions which were subsequently found probably not to have taken place in an SCF at all, but in a reactant-rich liquid phase at the bottom of the vessel. There are already several excellent reviews and multiauthored books that describe various designs for high pressure spectr-oscopic vessels [ 10,12-161. This chapter demonstrates the use of vibrational spectroscopy for in situ monitoring of chemical reactions in SCFs. A key consideration in vibrational spectroscopy is to ensure the effective delivery of the IR beam to the SCF and the subsequent collection of the light. This involves choosing the correct window materials and ensuring that the spectroscopic features of interest are not so weak that they cannot easily be detected, nor so strong that they :become totally absorbing. This chapter is divided into three parts beginning with the very simplest case of dilute supercritical solutions near ambient temperatures, for example supercritical carbon dioxide, scC02. These are processes which may be followed simply by Fourier transform IR (FTIR) or Raman spectroscopy. In practice, most synthetic reactions are performed in rather more concentrated supercritical solutions which may even become turbid as the reaction proceeds, thus necessitating the use of reflectance or scattering techniques for monitoring. Finally we describe the use of fast time-resolved ZR spectroscopy (TRIR) using pulsed lasers, to detect and identify very short lived intermediate species generated in supercritical solution. Each section provides a brief introduction to the spectroscopic methods used, and gives a range of examples to illustrate the possibilities for reaction monitoring.

3.1.2 Dilute Solutions 3.1.2.1 Infrared Spectroscopy One of the key advantages of supercritical solvents over conventional liquids is their unique combination of gas-like and liquid-like properties. Thus, reactant gases such as H2 or N2 can, under the correct conditions of temperature and pressure, be totally miscible with SCF solutions. At Nottingham, this unique property has been exploited to synthesize a range of novel organometallic complexes with metals bonded directly to dinitrogen [17] and dihydrogen [7,18], as shown in eq (3.1.1). These experiments are outlined in more detail in chapter 4.2. CpMn(C0)3 + H2

uv

420

CpMn(C0)2(q2-H2) (3.1-1)

Initially, photochemical studies were performed in a miniature spectroscopic cell which was designed with a low volume (ca. 1 mL), to minimize stored potential energy and to permit safe operation on the open bench. This modular

3.I Vibrational Spectroscopy

149

design [ 191 facilitates rapid assembly, disassembly and cleaning of the equipment between experiments. Subsequently, flow reactors were developed to enable synthesis of realistic quantities of new organometallic species [7]. The key to the successful operation of the flow reactors, such as those shown in Section 4.2.5 was the incorporation of one or more of these miniature cells to allow both spectroscopic monitoring by IR and also the photochemical initiation of the reaction (Figure 3.1-1). The main pressure seal in this cell is formed between the stainless steel threaded window holder and the cell body through a Teflon O-ring (FTFE). Spectroscopic windows are sealed into the window holder and the cell body either by an epoxy resin, or by use of a thin indium metal gasket. The advantage of this sealing geometry is that no mechanical compression is exerted on the windows and, when the cell is pressurized, the force exerted by the fluid on the inside surface of the window reinforces the seal on the outer surface. The pathlength can be altered for different experiments, simply by using a series of different threaded window holders in which the window is recessed to different depths. A long pathlength would be required for very dilute solutions, whereas a shorter one may be more appropriate for more concentrated solutions. High pressure valves and tubing, thermocouple and pressure transducer may be attached via the threaded ports around the circumference of the cell. The choice of window material is determined by the type of reaction to be undertaken, the temperature and pressure, and the particular region of interest in the FTIR spectrum or the wavelength of the Raman excitation source. Calcium fluoride (CaF2) windows have become the material of choice, as they transmit across the UV and visible spectrum. However, information below 1100cm-' in the FTIR spectrum is obscured by the spectroscopic cut-off of the windows. In general, observation of the region below llOOcm-' of the FTIR spectrum is not crucial in most experiments. By contrast potassium bromide (KBr) will transmit down to 400cm-', but has considerably lower mechanical strength, thus reducing the maximum pressure range which can be used during spectroscopic measurement. In addition, the water sensitivity

Figure 3.1-1 Cross-sectional view of a microscale spectroscopic vessel for monitoring reactions in SCFs C = CaF2 window; P = port; S = stainless steel window holder; and T = Teflon O-ring (FTFE).

YYdb

S

150

3 Spectroscopy of SCF Solutions

of KBr means that care is needed to prevent clouding by atmospheric moisture, although this problem can be rectified by carefully repolishing the windows. Several other window materials are available, each having advantages and disadvantages. Othe; alkali metal halides have been used for supercritical studies, but suffer from lower mechanical strengths (e.g. NaCl or CsI). Sapphire is mechanically strong but the spectroscopic cut-off i's rather hi h (ca. 1900cm-') and usually it displays strong v(0H) bands above -3000cm- f, obscuring much of the important information contained in the FTIR spectrum. KRS-5 contains a mixture of bromide and iodide salts of thallium which has a very low wavenumber cut-off, allowing observation of features down to ca. 220cm-'. However, the mechanical strength of KRS-5 is substantially lower than CaF2 and the material is both highly toxic and water soluble. Furthermore, the red coloration of KRS-5 precludes photochemical initiation. Other materials including ZnSe and ZnS are also used, and recently diamond has been demonstrated as an effective window material for FTIR monitoring of processes in scH20 at high temperatures and pressures [20-231.

3.1.2.2

Raman Spectroscopy

Centrosymmetric molecules such as H2 and N2 are IR inactive but can be detected by Raman spectroscopy. A series of miniature vessels has been designed [24,25] for Raman spectroscopy including a microscale version based upon a short length of silica capillary (Figure 3.1-2), which provides a convenient method for monitoring Hz in the photochemical flow reactor. For effective synthesis of the organometallic dihydrogen complex (eq 3.1-1) it was important that the H2 was properly mixed with scCO2. The linewidth of the Raman-active rotational lines of H2 are a very good indicator of the immediate environment of the H2 molecules [26]. When solvated in scC02, the rotational lines are broad, whereas, for non-solvated H2 in the gas phase, the lines are very sharp under the same conditions, even at high pressure, 530prn id capilliary

Figure 3.1-2 Schematic diagram of capillary cell for FT-Raman spectroscopy. The design is based upon a short length of chromatography capillary from which the polyimide coating has been removed to allow laser light to enter and scattered light to leave. To maximize the collection of scattered light the cell is mounted in a commercial sample holder (Perkin Elmer) with concave mirrored surfaces. The capillary is connected to conventional 1/16 in tubing by use of PEEK tubing connectors. Adapted from S. M. Howdle, K. Stanley, .V. K. Popov, V. N. Bagratashvili, Applied Spectroscopy 1994 48, 214.

151

3.1 Vibrational Spectroscopy Figure 3.1-3 On-line FT-Raman spectra of scC02/H2mixtures. The initial spectrum is a homogeneous COz/H2 mixture at 74 bar. The arrows show the effect due to a 50% increase in the output pressure of the H2 compressor: the mixture is now no longer homogeneous and is incapable of dissolving CpMn(C0)3. Note that the rotational bands of H2 are broader in the spectrum with less H2. This is because it is dissolved in COz and therefore in a much denser environment. The COz-rich fluid contains approximately 40% H2, and the H2-rich fluid, about 6 times more H2 than C02. The inset is FTRaman spectra showing the effect of scCOp density on the linewidth of the Sl(0) rotational lines of H;z. At higher densities (corresponding to higher C02 pressure) the H2 molecules are more strongly solvated and the linewidth increases.

200 bar

;

580

'\

I

\

I I I

I I I

t

I

I

9

F. 2'

Hp

I I

I

900

I

300

Wavenumber cm-1

(Figure 3.1-3). Thus, the transition from narrow to broad linewidth (i. e. from gas phase to solvated) provides a qualitative check that the mixing of scCO2 and H2 in the flow reactor is effective, and allows optimization of the photochemical synthesis of the desired product [7].

3.1.2.3

Polymer Modification

Because of their gas-like transport properties, SCFs are efficient media for impregnation and extraction of polymers and these properties have been exploited to impregnate organometallic species into polymer hosts [S, 27301. The IR carbonyl stretching vibrations (v(C0)) of organometallic species, such as CpMn(C0)3, have relatively narrow linewidths and very high molar extinction coefficients, leading to very intense bands in the FTIR spectrum [31]. Moreover, the position of the v(C0) bands is very sensitive to their solvent environment, thus providing an effective probe with which to monitor partitioning between the polymer and the SCF solvent (Figure 3.1-4). This partitioning has been shown to be highly dependent upon the density of the scCO2 [27,32,33], and can be modified dramatically by addition of antisolvents such as N2 gas. These experiments were performed in a cell similar to that in Figure 3.1-1, in which IR radiation is passed through the polymer film and the scCO;? solution at the same time. For probe molecules that do not contain metal carbonyl groups, it is found that IR bands are generally broader and do not show the

152

3 Spectroscopy of SCF Solutions

I

?ado

2030

1010

Wavenumber / cm-1

Figure 3.1-4 A series of infrared spectra which show in siru monitoring of impregnation of c ~ M n ( C 0 from ) ~ the COz solution into a film of ultrahigh-molecular-weight polyethylene (UHMWPE). A solution of c ~ M n ( C 0 )in~ heptane (8 pL, ca. 0.1 M) was injected into the FTIR cell. After evaporation of the solvent, the cell was opened and an UHMWPE disk (250pm X 5mm diameter) was placed inside. The cell was then fitted with a 5mm pathlength window and purged of air. The cell was heated to ca. 35 "C and then pressurized with C 0 2 (preheated to ca. 35 "C) to 75 bar. Spectra were recorded at a resolution of 1 cm-' over a period of about 1 h.

same sensitivity to their solvent environment as do v(C0) bands. Thus, in order to monitor in situ the impregnation and extraction of other species from polymeric films, Kazarian et u1. have extended the technique by design of an elegant double beam spectroscopic cell [34] in which the polymer and scC02 solution can be monitored independently. This apparatus has been used to monitor the drying of poly(methylmethacry1ate) PMMA [35], the interaction of PMMA [36] with scCO2, the impregnation of dyes [37], and the effect of cosolvents on poly(dimethylsi1oxane) stationary phases for chromatography [38]. The application of FTIR to extraction, impregnation and drying has also been reviewed recently [39]. Impregnation of reactive species into a host polymer provides a method for polymer modification via a chemical reaction. This work has been extended by showing that photolysis of W(CO)6 in polyethylene (PE) can lead to isomerization of the C=C double bonds, an interesting variation of Wrighton's work on catalytic alkene isomerization [40]. The method can be used to prepare isomerized PE because the W(CO)6 can be removed entirely after the isomerisation has taken place [5,41]. Ultraviolet photolysis of Fe(C0)s in PE under a pressure of H2 leads to reduction of up to 80% of the C=C bonds [41], while photolysis under an atmosphere of 0 2 generates an oxide, most probably Fe203, within the PE matrix [41].

3.I Vibrational Spectroscopy

153

3.1.2.4 Vibrational Spectroscopic Studies of Aqueous Microemulsions in SCFs Ionic and polar species are not generally soluble in the more common SCFs such as ethane, propane and C02, and this has limited the range of reactions possible in SCF solution. Hence, there has been considerable interest in developing methods for solubilizing such species by the use of aqueous microemulsions. The key problem is to find surfactants that are able to support microenvironments of “bulk” hydrogen-bonded water into which ionic species can be dissolved. Elegant studies by several groups have shown that surfactants such as bis(2-ethylhexyl) sodium sulfosuccinate (AOT) and others can form thermodynamically stable microemulsions of water in supercritical alkanes and in supercritical xenon, and crucial evidence in some of these studies was provided by IR spectroscopy [42-561. However, dispersion of water in scC02 proved to be elusive, despite attempts with more than 150 surfactants [57, 581. Recently, it has been shown that an ammonium carboxylate perfluoropolyether (PFPE) was able to support water-in-scC02 (see p. 154) microemulsions [59], (Chapter 2.4). Again, the key evidence was provided by IR spectroscopy which confirmed the formation of microenvironments of bulk water, as well as revealing the presence of traces of free (monomeric) water dissolved in the scCO2 solvent (Figure 3.1-5). It was necessary to use D20 rather than H 2 0 to shift the crucial IR bands of water into regions of the spectrum that were not obscured by the bands of scC02 itself. Great care had to be exercised in these

2800

2700

2600

2500

Wavenumber cm -1

Figure 3.1-5 Trace (a) is the FTIR spectrum of D20 in scC02 microemulsions (156 bar and 32 “C). The very broad v(0D) band in the region 2700-2500 cm-’ demonstrates the presence of bulk DzO. Trace (b) is the spectrum of a saturated solution of monomeric D20 dissolved in scC02 (156 bar and 32°C) in the absence of PFPE surfactant. Note that the broad band of D-bonded water is absent. Only the antisymmetric and symmetric vibrations of free D20 at 2761 and 2653cm-’ can be seen. These same bands are also discernible in (a). The band marked * indicates the presence of a small quantity of HDO,formed by proton exchange with the NH4+ counterion of the surfactant. Adapted from M. J. Clarke, K. L. Harrison, K. P. Johnston, S. M. Howdle, J. Am. Chem. SOC. 1997, 119, 6399.

154

3 Spectroscopy of SCF Solutions

2160

2025

1890

Wavenumberl cm-1

Figure 3.1-6 FTIR difference spectrum showing the changes which occur after addition of H2S to aqueous sodium nitroprusside in water-in-scCOz microemulsions. The bands of nitroprusside (Naz[Fe(CN)s(NO)]) (v(CN) 2143 cm-' and v(N0) 1939cm-') decrease, with the concomitant generation of product bands at 2086 and 2051 cm-'. At the same time, the solution was observed to change in color from red to pale yellow-green, indicating that the HzS had readily reacted with the sodium nitroprusside in the micelle core. Adapted from M. J. Clarke, K. L. Harrison, K. P. Johnston, S. M. Howdle, J. Am. Chem. SOC. 1997, 119, 6399.

experiments to ensure that the bulk water detected by FTIR was not merely the result of splashes of water adhering to the inner surfaces of the spectroscopic windows. Further proof that these water-in-scC02 microemulsions really do contain bulk water can be obtained by dissolving ionic species within the micelles, and by monitoring the subsequent reactions of these ions using IR spectroscopy. The inorganic species dissolved in the aqueous microenvironment will react rapidly with a reactant gas present in the bulk scC02 solvent [60] (Figure 3.1-6). Other groups are now beginning to exploit the high concentration of added gases which may be obtained in scC02 to enhance the efficiency of enzyme reactions within the aqueous microenvironment [6 11.

3.1.3 Concentrated Solutions All of the examples described above have involved dilute solutions, where transmission IR spectroscopy is relatively easy to use, and there were sufficient windows in the spectrum of the supercritical solvent to permit the detection of the molecules of interest. However, this is not always the case. For example the IR spectrum of scCO2 is totally absorbing between ca. 3700-3500 and

3.1 Vibrational Spectroscopy

155

2600-2100 cm-'. Hence, for the water in scC02 microemulsions study, the bulk of the work was performed with D20, because the important vibrational lines of H20 were obscured by fundamental modes of scCO2 [60]. However, such isotopic substitution is not always feasible. With more concentrated solutions, the IR spectrum may well become totally absorbing, allowing little or no spectroscopic information to be obtained. This problem may be Overcome by monitoring a higher wavenumber region where the overtone and combination modes of the molecules of interest occur, or by use of near-IR spectroscopy (NIR). These techniques work because the molar absorption coefficients of the first overtone/combination are approximately one order of magnitude less than for the fundamental modes. Similarly, the coefficients for second overtone/combinations are a further order of magnitude less. For concentrated solutions, it is usually also necessary to use a very short pathlength (ca <100pm), to ensure that IR bands are not totally absorbing. The difficulty, however, is to ensure adequate mixing within such narrow films of solutions and indeed to predict, in advance, the size of pathlength required for a particular experiment. An elegant solution to the problem has been devised by Buback [15] in which the pathlength of the cell may' be varied mechanically while the cell is under pressure. Such a design solves many of the problems associated with measuring the spectra of concentrated solutions, the only drawback being the rather bulky nature of the vessel. A similar variable pathlength design suitable for IR and Raman spectroscopy of supercritical water solutions has also been devised [62]. Attenuated total reflectance (ATR) is a rather different method for obtaining spectra from very concentrated or turbid solutions. In this case, a rod-like crystal (e.g. ZnSe) is sealed into a high pressure autoclave so that the two ends protrude. IR radiation is focused on one of the protruding ends and is then transmitted by bouncing diagonally between the internal surfaces of the crystal. At each reflection, a small portion of the IR radiation penetrates a short distance into the surrounding SCF. Hence, the spectrum can be recorded with an effective pathlength of only a few microns. Using this method, the problem of totally absorbing bands may be avoided. Kazarian and colleagues [36] used a cylindrical attenuated ATR cell to monitor the intermolecular interaction of C 0 2 with a poly(dimethylsi1oxane) film deposited on the surface of the ATR crystal. Others have shown that similar techniques can be applied to monitoring the diffusion of hydrocarbons in zeolites [63a] and the progress of cataIytic reactions in situ [63b,c]. Polymerization reactions in scCO2 often begin with a concentrated homogeneous solution of monomer and initiator, which quickly becomes turbid as the polymer particles grow and then precipitate. Such a reaction is very difficult to monitor by transmission FTIR because the precipitated particles scatter the IR radiation strongly. Recently, preliminary results have shown that a commercially available ATR-FTIR fiber optic probe can be modified to allow the successful monitoring of precipitation polymerization reactions in scC02 (Figure 3.1-7).

156

3 Spectroscopy of SCF Solutions

0.6

8 c Q

$ 0.4 2 0.2 I

I

I

1600

1400

1200

Wavenumber (cm-')

Figure 3.1-7 A series of FTIR spectra obtained from a modified ATR-FTIR probe inserted into a high pressure autoclave. The spectra were coll'ected over a period of 40min and show the depletion of acrylic'acid monomer in a concentrated (30% by volume) scC02 solution at 70°C and 180 bar in the presence of AIBN initiator. The bands observed correspond well with those observed previously for acrylic acid in supercritical ethene [64].

3.1.4 Monitoring of Fast Reactions in SCFs using Time-resolved Vibrational Spectroscopy UV-Vis flash photolysis is the most widely used technique with which to follow fast photochemical reactions. However, although flash photolysis provides excellent kinetic information about excited states and reaction intermediates, UV-Vis spectra in solution are often broad and featureless and provide little structural information. Moreover, in experiments where several photoproducts are produced, overlapping of the broad absorption bands can make it very difficult to elucidate the mechanism. Transient vibrational spectroscopy provides more detailed structural information to probe photochemical reactions especially since Raman and IR bands of large molecules in solution are generally much narrower than UV-Vis bands. The most widely used vibrational spectroscopic technique is time-resolved resonance Raman spectroscopy (TR3) [65]. This has been used successfully to obtain structural information about organic excited states in scC02. McGarvey and co-workers probed the excited triplet state of anthracene in scCO2 [66]. However, TR3 experiments involve data collection over many laser pulses, with all of the problems associated with secondary photolysis. These problems have prevented TR3 being used effectively to follow chemical reactions apart from highly photoreversible processes. To our knowledge, TR3 has not yet been used to follow chemical reactions in SCFs. Recently, however,

3.I Vibrational Spectroscopy

157

Tahara reported [67] the design of a flow system for obtaining ultrafast measurements, including posecond-TR3, in SCFs. Such flow systems should allow wider use of TR to monitor chemical reactions under supercritical conditions. Time-resolved infrared spectroscopy (TRIR) has been outstandingly successful in identifying reactive intkrmediates and excited states of both metal carbonyl [68,69] and organic complexes in solution [70-721. Some time ago, the potential of TRIR for the elucidation of photochemical reactions in SCFs was demonstrated [73]. TRIR is particularly suited to probe metal carbonyl reactions in SCFs because v(C0) IR bands are relatively narrow so that several different species can be easily detected. Until now, TRIR measurements have largely been performed using tunable IR lasers as the IR source and this has restricted the application of TRIR to the specialist laboratory [68]. However, recent developments in step-scan FTIR spectroscopy promise to open up TRIR to the wider scientific community [74]. The Nottingham tunable IR laser-based TRIR apparatus has been described in detail elsewhere [68]. Briefly, a pulsed Nd:YAG laser (Quanta-Ray GCR-11; 266 nm or 355 nm) is used to initiate the photochemical reaction and a continuous-wave infrared source (Mutek IR diode laser) monitors the changes in infrared transmission following the UV-Vis pulse. The IR diode laser is repeatedly tuned to different IR frequencies after monitoring the changes in IR absorption of a reaction solution following a UV laser pulse, and IR spectra are built up on a point-by-point basis (an example is given in Figure 3.1-9(b)). Although IR lasers can cover all of the mid-IR region, this pointby-point approach to TRIR restricts the IR range that can be studied due to the long acquisition time required for a complete spectrum. Indeed, recording a spectrum covering a range of 300 cm-’ spectrum is a whole days work on this apparatus! However, many problems can only be solved by obtaining data over a much wider IR range. In principle, these extra data can be obtained by step-scan FTIR, a technique which benefits from spectral multiplexing, increased IR through put, and fast data acquisition. C02 complexes of transition metals [75] are of interest in the context of recycling and removing C 0 2 from industrial emissions, and the potential of using C 0 2 as a feedstock for fine chemicals. Efforts to convert C 0 2 to useful chemicals have largely centred on transition metal catalysts, although none is yet efficient enough to be commercially viable [76]. When a molecule of C02 coordinates to a single metal centre, there are three possible adducts [75-771 which depend on the nature of the metal as well as steric and electronic factors, as shown in Scheme 3.1-1:

M + O=C=O

-

0” M-C( a+,r- or

\.

6”

(1)

@

or

M=c=o

k0

(11)

(W

Scheme 3.1-1. Possible coordination modes for metal/COz adducts.

158

3 Spectroscopy of SCF Solutions

Both ql-C (type I) and $-CO (type 11) COz complexes have been isolated, and the mode of coordination of CO2 has been confirmed by crystallography. By contrast, stable q l - 0 (type 111) C 0 2 complexes have not yet been unambiguously characterized [76]. They have, however, been predicted theoretically or deduced from spectra of low temperature matrices [78]. W(CO),(CO,) was shown to have a linear oxygen-bound C 0 2 ligand in lowtemperature matrices [79] and has been observed- in the gas phase at room temperature [801. We have recently reported [81] the first tentative observation of an organometallic ql-0 (type 111) C 0 2 complex in solution at or above room temperature following the photolysis of W(CO),j in scC02. This evidence was based on the v(C0) frequencies of W(CO),(CO2) in the TRIR spectrum [81]. Furthermore, TRIR studies in scC02 suggest that the coordination modes of C 0 2 are different in the two complexes CpMn(C0)2(C02) and CpRe(C0)2(C02), but these con-

t

?--I

...... step-scan FTIR (Nlcolet 860)

6

InterferometerDisplacement

Figure 3.1-8 (a) The Nottingham nanosecond-step-scan FTIR set-up: B = beamstop; D = MCT detector; E = external bench; P = pulse generator (Stanford Research DG535);S = supercritical cell. (b) A series of time-dependent interferograms showing the step-scan principle. In step-scan FTIR, the event of interest is completed long before the mirror has time to travel the distance required for an ordinary scan. In the step-scan mode, the moving mirror in the interferometer is stopped at regular intervals along its travel. At each stop, the reaction is initiated and the time-dependent IR signal is measured, with a time resolution limited only by the photovoltaic detector. After the chemical system has relaxed, the mirror is stepped to a new position and the measurement is repeated. The .computer builds up a series of time-dependent interferograms and the Fourier transformation of these data yields time-resolved IR spectra.

3.1 Vibrational Spectroscopy

159

clusions were necessarily based on TRIR measurements over a restricted wavenumber range (2100-1850 cm-') [82]. Confirmation of the mode of COz co-ordination will require observation of the v(OC0) vibrations of co-ordinated C02 throughout a much wider frequency range (2300-1100 cm-') [78], which cannot easily be covered using an IR laser. We believe the combination of step-scan FTIR and SCFs will provide new insight into bonding of C02'to metal centres. The use of step-scan FTIR for TRIR spectroscopy in SCFs has not been described previously, and a detailed description will be published elsewhere [83]. Briefly, the Nottingham apparatus (Figure 3.1- 8(a)) comprises a commercially available step-scan FTIR spectrometer (Nicolet Magna 860) equipped with a l00MHz 12-bit digitizer and a 50MHz MCT detector interfaced to a Nd:YAG laser (Spectron SL850). Synchronization of the Nd:YAG laser with data collection was achieved using a pulse generator (Stanford DG535). Like laser-based TRIR [68], step-scan FTIR uses repetitive UV photolysis to build up the data. The difference is that the photolysis and data collection in step-scan FTIR is fully automated. Therefore a wider spectral range can be covered much more rapidly than with laser-based TRIR. The difference between laser based and step-scan TRIR measurements is shown in Figure 3.1-9, where data from the flash photolysis of W(CO)6 in scXe are illustrated. The two sets of illustrated data cover approximately the same wavenumber range. Although the signallnoise ratio is clearly better in the step-scan experiment (Figure 3.1-9(a)) and data were collected in 30 min (compared to 4 h for Figure 3.1-9(b)), the most important difference is that the step-scan experiment provided data over the whole wavenumber range of 8000-llOOcm-' (the lower limit being set by the CaF2 windows used in these experiments). It should be noted the these

Figure 3.1-9 TRIR data obtained following irradiation of W(CO)6 in scXe in the presence of CO. (a) A series of TRIR spectra (in 1 ps increments) obtained using the step-scan FTIR set-up displayed in Figure 3.1-8. (b) A TRIR spectrum obtained 1 ps after photolysis using the IR laser based TRIR. In (b) * represent the data points and the solid line is a curve fit through these data. In both (a) and (b) negative peaks indicate loss of W(CO)6 and positive peaks indicate formation of W(CO),(Xe). The step-scan data show that W(CO),(Xe) decays to reform W(CO)6 and to form a new species that can be assigned to W(C0)5(H20). This demonstrates that trace impurities in supercritical fluids may have a dramatic effect on the course of a reaction.

60

3 Spectroscopy of SCF Solutions

Moo

2100

1900

1W

oo

Moo

1900

1soO

Wavenumber I em"

WavenumberI cm" I

10

BmelZ

0

5

Time 10 I m t

15

20

Figure 3.1-10 Step-scan FTIR spectra obtained (a) 1 ps after irradiation (532nm) of tr~ns-[CpMo(CO)~]~ in scC02. The TRIR spectrum shows formation of the [CpMo(CO)3]' radical formed by cleavage of the Mo-Mo bond. (b) The [CpMo(CO)J radical decays by second-order kinetics at a diffusion-controlled rate to form tr~ns-[CpMo(C0)~]2 and the unstable ga~che-[CpMo(CO)~]~ (see spectrum (c) obtained 20 ps after irradiation). (d) The unstable isomer decays via first-order kinetics to form the more stable trans[ C ~ M O ( C O )isomer. ~]~

TRIR spectra represent the first evidence for organometallic noble gas complexes in solution at room temperature [81]. Figure 3.1-10 illustrates how step-scan FTIR can be used to monitor more complicated photochemical reactions in SCFs. Visible photolysis of trans[CpMo(CO)& in scC02 generates [CpMo(CO)J radicals which recombine at a diffusion controlled rate to form the stable trans and unstable gauche forms of [CpMo(CO)312. Gauche-[CpMo(CO)& slowly isomerizes to trans[CpMo(CO)& (Scheme 3.1-2). Using step-scan FTIR, we can obtain spectra of both transient species, the [CpMo(CO)J radical and gauche-[CpMo(CO)3]2. In addition, we can measure the reaction kinetics of both the radical recombination and the gauche to trans isomerization. In the near future, this type of monitoring is likely to make a major contribution towards understanding the mechanistic aspects of synthetic chemistry in SCFs.

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Scheme 3.1-2 The photochemisg of truns-[CpMo(CO)& in scCOz following visible photolysis.

3.1.5 Conclusions The field of SCF chemistry is advancing rapidly. Increasingly, chemists are searching for improved selectivity, greater efficiency and optimization of yields. Spectroscopic monitoring will play a crucial role in leading chemists to these goals. This chapter gives a brief summary of the current state of the art for vibrational spectroscopy in SCFs. A number of examples of Raman and IR monitoring which have already been important in optimizing synthetic reactions are cited. The pace of developments is accelerating and improved techniques will bring this monitoring to a wider range of temperatures, pressures and concentrations. The coupling of spectroscopy with studies of phase behavior will provide a better characterization of complex reaction mixtures. The new technique of time-resolved step-scan FTIR will give chemists a deeper level of understanding of chemical processes in SCFs.

3.1.6 Acknowledgments We thank all our colleagues and collaborators for their contributions. We are particularly grateful to Nicolet Instruments Ltd for loan of an ATR-FTIR probe and the EPSRCAXutherford Appleton Laboratory Lasers for Science laser loan pool. We thank the EPSRC, the Royal Society, the EU TMR Programme, the Royal Academy of Engineering and Nicolet Instruments Ltd. for support.

References [ l ] P. E. Savage, S. Gopalan, T. I. Mizan, C. J. Martino, E. E. Brock, AIChE J. 1995, 41, 1723. [2] M. J. Burk, S. Feng, M. F. Gross, W. Tumas, J. Am. Chem. SOC. 1995, 117, 8277. [3] M. G. Hitzler, M. P. Poliakoff, Chem. Commun. 1997, 1667. [4] J. M. DeSimone, Z. Guan, C. S. Carlsbad, Science 1992, 257, 945. [5] M. J. Clarke, S. M. Howdle, M. Jobling, M. Poliakoff, J. Am. Chem. SOC. 1994, 116, . 8621-8628.

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[6] J. E. Jerome, P. Wood, W. T. Pennigton, J. W. Kolis, Inorg. Chem. 1994, 33, 1733. [7] J. A. Banister, P. D. Lee, M. Poliakoff, Organometallics 1995, 14, 3876. [8] S . G. Kazarian, S. M. Howdle, M. Poliakoff, Angew. Chem. Int. Ed. Engl. 1995, 34, 1275. [9] P. D. Lee, J. L. King, S. Seebald, M. Poliakoff, Organometallics 1998, 17, 524. [lo] R. Whyman, in Laboratory Methods in Vibrational Spectroscopy, H. L. Willis (Ed.), John Wiley & Sons Ltd., London, 1987, p. 281. [ l l ] M. A. McHugh, V. J. Krukonis, Supercritical Fluid Extraction, Butterworth-Heinmann, Boston, MA, 1994. [ 121 W. F. Sherman, A. A. Stadtmuller, Experimental Techniques in High-pressure Research, John Wiley & Sons Ltd, London, 1987. [13] M. Poliakoff, S. M. Howdle, M. W. George, in Chemistry under Extreme and NonClassical Conditions, Chapter 5, R. Van Eldik (Ed.), Spektrum, Heidelberg, 1996, p. 189-218. [14] M. Buback, Angew. Chem. Int. Ed. Engl. 1991, 30, 641-653. [ 151 M. Buback, H.-D. Ludemann, in High-pressure Technology in Chemistry and Physics, W. B. Holzapfel, N. S. Isaacs (Eds.), Oxford University Press, New York, USA, 1997, p. 151. [ 161 M. Buback, in NATO Advanced Study Institute Supercritical Fluids - Fundamentals for Application, Vol. 273, E. Kiran (Ed.), Kluwer Academic Publishers, Dordrecht, 1994, p. 499-526. [17] S. M. Howdle, M. A. Healy, M. Poliakoff, J. Am. Chem. SOC. 1990, 112, 4804-4813. [18] S . M. Howdle, M. Poliakoff, J. Chem SOC., Chem. Commun. 1989, 1099. [19] S. M. Howdle, M. Poliakoff, in NATO Advanced Study Institute Supercritical Fluids Fundamentals f o r Application, Vol. 273, E. Kiran (Ed.), Kluwer Academic Publishers, Dordrecht, 1994, p. 527-537. [20] A. J. Belsky, T. B. Brill, J. Phys. Chem. A 1998, 102, 4509-4516. [21] P. G. Maiella, T. B. Brill, Inorg. Chem. 1998, 37, 54-458. [22] J. W. Schoppelrei, T. B. Brill, J. Phys. Chem. A 1997, 101, 8593-8596. [23] J. W. Schoppelrei, T. B. Brill, J. Phys. Chem. A 1997, 101, 2298-2303. [24] S. P. Best, S. M. Howdle, J. Raman Spectrosc. 1993, 24, 443-445. [25] S . M. Howdle, K. Stanley, V. K. Popov, V. N. Bagratashvili, Appl. Spectrosc. 1994, 48, 2 14-218. [26] S. M. Howdle, V. N. Bagratashvili, Chem. Phys. Lett. 1993, 214, 215-219. [27] A. I. Cooper, S. M. Howdle, J. M. Ramsay, J. Polymer Sci. Polymer Phys 1994, 32, 541-549. [28] Y. P. Kudryavtsev, E. N. Said-Galiev, 0. L. Lependina, L. N. Nikitin, V. K. Popov, A. L. Rusanov, M. Poliakoff, S. M. Howdle, Chemical Physics (Russ). 1995, 14, 190. [29] E. N. Sobol, V. N. Bagratashvili, A. E. Sobol, S. M. Howdle, Doklady Akademii Nauk 1997, 356, 777-780. [30] E. N . Sobol, V. N. Bagratashvili, V. K. Popov, A. E. Sobol, E. E. SaidGaliev, Zhurnal Fizicheskoi Khimii 1998, 72, 23-26. [3 11 P. S. Braterman, Metal Carbonyl Spectra, Academic Press, London 1975. [32] S. M. Howdle, L. Qun, V. N. Bagratashvili, V. K. Popov, E. N. Sobol, Proceedings of 4th. Internat. ConJ: on Supercritical Fluids, 1997, 259-262. [33] P. B. Webb, S. M. Howdle, V. K. Popov, E. N. Sobol, 1999, in preparation. [34] S. G. Kazarian, M. F. Vincent, C. A. Eckert, Rev. Instrum. Sci. 1996, 67, 1586. [35] M. F. Vincent, S. G. Kazarian, C. A. Eckert, AIChE J. 1997, 43, 1838-1848. [36] S . G. Kazarian, M. F. Vincent, F. V. Bright, C. L. Liotta, C. A. Eckert, J. Am. Chem. SOC. 1996, 118, 1729-1736. [37] S . G. Kazarian, N. H. Brantley, B. L. West, M. F. Vincent, C. A. Eckert, Appl. Spectrosc. 1997, 51, 491-494. [38] M. F. Vincent, S. G. Kazarian, B. L. West, J. A. Berkner, F. V. Bright, C. L. Liotta, C. A. Eckert, J. Phys. Chem. B 1998, 102, 2176-2186.

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[39] S. G. Kazarian, Appl. Spec. Rev. 1997, 32, 301. [40] M. S. Wrighton, Chem. Rev. 1974, 74, 401. [41] M. J. Clarke, P. B. Webb, A. I. Cooper, M. Poliakoff, S. M. Howdle, 1999, in preparation. [42] J. L. Fulton, R. D. Smith, J. Phys. Chem. 1988, 92, 2903. [43] J. P. Blitz, J. L. Fulton, R. 4.Smith, Appl. Spectrosc. 1989, 43, 812. [44] J. L. Fulton, J. P. Blitz, J. M. Tingey, R. D. Smith, J. Phys. Chem. 1989, 93, 4198. [45] R. W. Gale, K. L. Fulton, R. D. Smith, Anal. Chem. 1987, 59, 1977. [46] K. Jackson, L. E. Bowman, 3. L. Fulton, Anal. Chem. 1995, 67, 2368-2372. [47] E. W. Kaler, J. P. Billman, J. L. Fulton, R. D. Smith, J. Phys. Chem. 1991 458. [48] R. D. Smith, J. P. Blitz, J. L. Fulton, in Supercritical Fluid Science And Technology, ACS Symposium Series, Vol. 406, K. P. Johnston, J. M. L. Penninger (Eds.), ACS, Washington DC, 1989, pp. 1-550. [49] R. D. Smith, J. L. Fulton, J. P. Blitz, J. M. Tingey, J. Phys. Chem. 1990, 94, 781-787. [50] G. G. Yee, J. L. Fulton, J. P. Blitz, R. D. Smith, J. Phys. Chem. 1991, 95, 1403-1409. [51] P. Yazdi, G. J. McFann, M. A. Fox, K. P. Johnston, J. Phys. Chem. 1990, 94, 7224. [52] G. J. McFann, K. P. Johnston, S. M. Howdle, AZChE J. 1994, 40, 543-555. [53]G. J. McFann, K. P. Johnston, J. Phys. Chem. 1991, 95, 4889. [54] J. Zhang, F. V. Bright, J. Phys. Chem. 1992, 96, 5633. [55] K. A. Bartscherer, M. Minier, H. Renon, Fluid Phase Equilib. 1995, 197, 93. [56] Y. Ikushima, N. Saito, M. Arai, Journal Of Colloid And Interface Science 1997, 186, 254. [57] K. A. Consani, R. D. Smith, J. Supercrit. Fluids 1990, 3, 51. [58] A. Iezzi, R. Enick, J. Brady, Supercritical Fluid Science and Technology, ACS Symposium Series 406, American Chemical Society, Washington DC, 1989. [59] K. P. Johnston, K. L. Harrison, M. J. Clarke, S. M. Howdle, M. P. Heitz, F. V. Bright, C. Carlier, T. W. Randolph, Science 1996, 271, 24-626. [60] M. J. Clarke, K. L. Harrison, K. P. Johnston, S. M. Howdle, J. Am. Chem. SOC. 1997, 119, 6399. [61] F. V. Bright, E. D. Niemeyer, J. Bonzagni, Presentation at 215th National ACS Meeting 1998.

[62] Y. E. Gorbaty, G. Bondarenko, Rev. Sci. Instrum. 1993, 64, 2346. [63] (a) M. G. Suer, Z. Dardas, Y. Lu, W. R. Moser, Y. H. Ma, AIChE J. 1997, 43, 1717; (b) 2. Dardas, M. G. Suer, Y. H. Ma, W. R. Moser, J. Catal. 1996, 162, 327; (c) S. Kainz, A. Brinkmann, W. Leitner, A. Pfaltz, J. Am. Chem. SOC. 1999, 121, in press. [64] M. Buback, F. 0. Mahling, J. Supercritic. Fluids 1995, 8, 119. [65] S. E. J. Bell, Analyst 1996, 121, 107-R. [66] J. N. Martin Hegarty, J. J. McGarvey, S. E. J. Bell, A. H. R. Al-Obaidi, J. Phys. Chem. 1996, 100, 15704. [67] M. Kayam, T. Tahara, Annual Review of the Institute f o r Molecular Science 1997, 139. [68] M. W. George, M. Poliakoff, J. J. Turner, Analyst 1994, 119, 551-560. [69] J. J. Turner, M. W. George, E P. A. Johnson, J. R. Westwell, Coord. Chem. Rev. 1993, 125, 101-114. [70] X. Z. Sun, I. G. Virrels, M. W. George, H. Tomioka, Chern. Lett. 1996 1089-1090. [71] M. W. George, C. Kato, H. Hamaguchi, Chem. Lett. 1993 873-876. [72] T. Yuzawa, C. Kato, M. W. George, H. 0. Hamaguchi, Appl. Spectrosc. 1994, 48, 684-690. [73] M. Poliakoff, S. M. Howdle, M. Jobling, M. W. George, Proc. of 2nd Internat. Con$ on Supercritical Fluids, Boston, 1991, p. 189. [74] Whole issue, Appl. Spectrosc. 1997, 51, No. 4. [75]D. H. Gibson, Chem. Rev. 1996, 96, 2063. [76] W. Leitner, Coord. Chem. Rev. 1996, 153, 257. [77] K. K. Pandey, Coord. Chem. Rev. 1995, 140, 37.

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Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

3.2 NMR Spectroscopy JEROME W. RATHKE, ROBERT J. KLINGLER, REXE. GERALD 11, DAVID E. FREMGEN, KLAUS WOELK, SANDER GAEMERS and CORNELIS J. ELSEVIER

This chapter describes high-pressure NMR studies conducted in supercritical fluid (SCF) reaction media. Most of this research was performed in the various research programs of the several authors of this review, namely, the Fluid Catalysis and Ion Transport Programs at Argonne National Laboratory, the In Situ NMR Spectroscopy and Rotating-Frame Microscopy Program at the University of Bonn, and the NMR and Catalysis in Supercritical Media Program at the University of Amsterdam. Although much of the described research was performed with metal toroid probes, or with single-crystal sapphire NMR cells at high pressure, which together represent only a small subset of the available probe designs, the cited experiments are intended to provide at least a representative sampling of the great diversity of measurements that have already been attempted in supercritical media. This review is less focused on probe design (for which a number of high-quality reviews have recently appeared [ 1-31), but instead it concentrates on design for only two device types (namely, toroid probes and sapphire NMR cells) and then focuses more on the type of measurements and observations that synthetic chemists might encounter if they choose to use NMR to explore reaction chemistry or to measure reaction parameters in supercritical fluids. The unusual properties of supercritical fluids that make them attractive reaction media can sometimes also manifest themselves in unusual (and surprising) NMR effects that can be extremely useful in the hands of the wary, and confounding in the hands of the unwary, researcher.

3.2.1 Background The first high-pressure NMR probe was developed by Benedek and Purcell in 1954 [4]. Their probe, like many of those in use today, was constructed from beryllium-copper alloy (Berylco 25) and was used in hydrostatic pressure experiments at pressures up to 10000 bar. As research in high-pressure NMR spectroscopy progressed, a large variety of increasingly specialized probes have appeared. Thus, in 1979 de Fries and Jonas reported the first

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probe for supercritical water studies [5]. This probe was constructed from an Imperial Metal Industries titanium alloy (IMI- 680), selected for its ability to withstand the extreme corrosiveness of supercritical water (T, = 374 "C at 221 bar [6]). The probe had a remarkable operating temperature range of 25700 "C at pressures up to 2000 bar. Employing a different probe design of smaller pressure range (1-400 bar) but comparable temperature range (25-600 "C) [7], Hoffmann and Conradi recently determined the degree of hydrogen bonding in supercritical water [8] and supercritical lower alcohols (methanol and ethanol) [9] from 'H chemical-shift analyses. By 1987, the diamond anvil cell (DAC) was in use in NMR spectroscopy [lo]. Use of DACs soon led to measurements at pressures as high as 100000 bar [ll]. Also in 1987, Vander Velde and Jonas [12] described an NMR probe for homogeneous catalysis studies that incorporated an innovative external solenoid-driven plunger to achieve gas-liquid mixing. Unusually compact pressure probes that used toroid detectors first appeared in 1989 [13]. In that same year, Merbach et al. [14] constructed the first high-pressure beryllium-copper probe for use at 400 MHz. In contrast with the metal pressure vessel and DAC probes so far mentioned, NMR pressure cells have also been developed that fit within standard commercial probes, and thus conveniently make use of a commercial probe's electronic circuitry and variable temperature capabilities. Because of their ease of construction and use, and their lack of need for specialized equipment (such as wide-bore magnets and probe-tuning instrumentation), NMR pressure cells have been particularly popular with synthetic chemists (who often have only occasional use for pressure investigations, and set-up time and equipment investment are, therefore, a major concern). This is not to say that the pressure cells do not have other intrinsic advantages, perhaps the foremost of these being their capability to operate at the higher frequencies (500 MHz and above) associated with modern spectrometers and their capability to achieve the best possible resolution since the cells are, in most cases, spinnable. The simplest cells of this type are made by sealing off commercial Pyrex and quartz glass tubing (5 mm o.d., 0.5-3 mm i.d.). The heavier-walled ones have been used at pressures as high as 300 bar [15], even though use of such sealed tubes is probably (and most certainly for the thinner-walled ones) commonly reserved for pressures below 20 bar. Jameson et al. [16] have routinely used specially pulled Pyrex capillaries in NMR studies of Xe gas at pressures up to 200 bar. Today's widely used single-crystal sapphire NMR cells with rotatable titanium alloy heads were first developed by Roe in 1985 [17], with later improvements by Horvith and Ponce [18] that allowed for larger (10 mm) tubes and provided Teflon O-ring protection for the epoxy-glue sealant. The 5mm sapphire tubes were judged to have a reasonable margin of safety for pressures up to 130 bar. Kinrade and Swaddle [19] used a similar apparatus, but constructed from Vespel (a high-strength polyimide plastic), to study alkaline silicate solutions at routinely used conditions of 16 bar and 200°C. More recently, Yonker et al. [20] used a highly innovative design based on fused silica capillary tubing, bent repeatedly to permit multiple passes into standard 5 mm and 10 mm NMR tubes. Pressures up to 4000 bar have been

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employed with a high degree of safety with this new design, which even allows sample spinning.

3.2.2 Some Toroid Probe Designs One might visualize specific uses for each of the above-mentioned NMR devices in exploring synthetic chemistry in supercritical fluids. However, it has been found that simple designs based on toroid detectors often work well in this kind of research. Toroids are intrinsically more sensitive than other coil types [21] and can be operated close to the internal walls of a metal pressure vessel without significant degradation of the NMR signal [l 1,22,23]. Both of these features stem from the internal confinement of the magnetic flux within the torus, as shown in Figure 3.2-1. As is evident from this figure, the magnetic flux in the commonly used Helmholtz and solenoid detectors extends well beyond the dimensions of the coils. To avoid signal losses due to magnetic coupling of the pressure vessel with these coils, Hoult [24,25] recommends that they be located at least a distance equal to their largest dimension from nearby conductors (in this case, the walls of the vessel). This recommendation essentially triples the inside diameter, height, and wall thickness of the pressure vessel required to achieve optimal performance from solenoids or Helmholtz detectors. These coils not only use up additional magnet space, which may not be available, but tend to lead to cumbersome cells with large internal volumes. When measurements with gases are involved, large internal volumes can be hazardous because of the large amount of energy that can be stored in compressed gases. In contrast, because of the toroid coil’s confined magnetic flux, it can be operated close to the walls of a much smaller vessel without suffering magnetic coupling losses. A compact pressure probe design that utilizes a four-turn torus is shown schematically in Figure 3.2-2.

Figure 3.2-1 Relative compactness of metal pressure probes with various detectors.

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3 Spectroscopy of SCF Solutions

GAS CONNECTIONS

s/* A.f"

TORUS REACTOR BODY (1 l/E" O.D.) PROBE CIRCUIT

BOTTOM PLUG

* -

R.F. FEEDTHROUGHS

EXPLODED VIEW OF FEEDTHROUGH

Figure 3.2-2 Pressure probe design using a toroid NMR detector.

The internal confinement of the magnetic flux within a torus leads to high efficiency because both the magnetic flux and the sample are located within the coil. Thus, essentially all of the flux in the torus is available for sample excitation. Glass and Dorn [26] have shown that toroids are 3-5 times more sensitive than Helmholtz detectors. The high sensitivity allows observation of I3C within pressure probes without sample enrichment. As shown in Figure 3.2-3, a high-quality spectrum with a signal-to-noise ratio (SLN) of 445 is obtained with one pulse on a standard I3C test sample (ASTM test). It should also be noted that toroids perform exceptionally well on extremely insensitive nuclei (e.g. Io3Rh) that have very low gyromagnetic ratios. In these cases, the ability of the torus to achieve high inductances in limited space is highly beneficial in detection of the low frequencies required to observe these nuclei. The first direct observation I o 3 R h NMR spectrum to be recorded in a pressure probe is shown in Figure 3.2-4 [22].

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169

S/N = 445

Figure 3.2-3 ASTM 13csensitivity test at 75.6MHz with a sample volume of 1.6 mL.

The pressure vessel of Figure 3.2-2 has an internal volume of 8.0 mL and was machined from Be-Cu alloy with a tensile strength of 13000 bar. Incorporation of an SP-I (i.e. DuPont VespeP) clamp in the rf feedthrough ensured that the system was leak-free at a hydrostatic test pressure of 1722 bar. The total feedthrough capacitance is only 6pF. With use of the hybrid probe circuit in the figure, the four-turn torus was double-tuned to allow computer switching between 'H (fH = 300.5MHz) and 59C0 or 13C (fx = 71.1MHz or 75.6 MHz, respectively). Thefx part of the hybrid circuit consists of the inductor and C1 and C2 in a standard tapped parallel tuned circuit arrangement. To achieve the maximum sensitivity required for these nuclei, the capacitor box housing C1

Rh(C0bcac (0.1 M in C&) Torus: 12 turns Volume: 2.2 ml NA: 1000 LB: 1.0 Hz SIN: 55

spectrum at 9.46 MHz.

1800

iiso

iioo

1650

rioo

PPM

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3 Spectroscopy of SCF Solutions

and C2 is located within the magnet close to the pressure vessel, and these capacitors must be adjusted with long extension handles attached to them. The 3/4 wavelength transmission line in the fH part of the circuit is long enough to bring the capacitor box housing C4 and C5 conveniently outside of the magnet. Hence, this box can be removed or altered during the course of an experiment, if required. The transmission- line is needed to transform the toroid circuit, which is normally capacitive at the relatively high proton frequency, to an inductive one, which can then be tuned and impedance matched to 300.5MHz with capacitors C4 and C5. Probe heating is accomplished by means of a chilled-water-jacketed electrical furnace that fits snugly around the pressure vessel. The heating element consists of 50 turns of bifilar (i.e. noninductively) wound constantan wire powered from the spectrometer's temperature controller. Additional capacitance filtering was necessary to remove voltage spikes from the controller's output. Temperatures up to 250°C could be achieved in this way with temperature control to within & 0.1 "C. Early in the high-pressure NMR studies, there were difficulties in tuning to the very high frequencies required for the observation of protons. After all, toroids are the most efficient inductors [21], and even the inductance of a very small 3- or 4-turn torus is too large to allow tuning to 300MHz when combined with the inductance and capacitance of the rf feedthrough in the pressure vessel. The tuning problem can be surmounted by the use of transmission lines, but their use usually results in sensitivity losses. Owing to its extremely low inductance, the toroid-cavity detector in Figure 3.2-5 can easily be tuned to

O.D.: 16.3mm

length: 25.4 mm volume: 2.57 ml

I

Central conductor O.D.: 0.775 mm

L, = L, In

Figure 3.2-5 A toroid cavity resonator (O.D. = outer diameter, L, = total inductance, Li = inductance of individual turn, n = number of turns).

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171

well above 300MHz and is probably a preferable solution to the tuning problem in cases where a single nucleus probe for 'H observation is required. As depicted in the inset in Figure 3.2-5, the low inductance of the cavity can be rationalized by considering it as a toroid coil in which the individual turns are connected in parallel and each additional turn of inductance Li actually contributes to a decrease in the total inductance L,, in contrast with the usual series configuration wherein each additional turn adds to it. As shown in Figure 3.2-5, slots were machined into the detector to ensure complete filling or draining of the cavity when used with fluid media. The location of the toroid cavity detector within the pressure vessel is shown in Figure 3.2-6. The cavity resonator described here differs from the gap-reentrant cavity resonator reported by Alderman and Grant [27] in that the new cavity is tuned externally. The probe is tuned with a capacitor box mounted close to the pressure vessel in the same fashion as used for toroid coils. Compared with coils, the cavities possess the additional advantages of being more rugged and more reproducible in design. In our experience, they also achieve better resolution on protons (about 0.7Hz for the best-performing cavities versus 2.5Hz for the best coils for measurements on chloroform at 300 MHz).

Figure 3.2-6 A compact probe design using a toroid cavity.

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3 Spectroscopy of SCF Solutions

3.2.3 General Properties of Supercritical Media Much of the research with high-pressure NMR has been associated with an attempt to develop a supercritical version of the C~~(CO)~-catalyzed 0x0 process for the hydroformylation of alkenes. For this purpose, the gas manifold shown in Figure 3.2-7 was used in conjunction with the toroid probe of Figure 3.2-2. In this set-up, remotely controlled pneumatic valves and a computercontrolled, high-pressure syringe pump were used to inject liquid C02 into the pressure probe. The pressure probe was heated slightly above the critical temperature of C 0 2 (T, = 31 "C) so that liquefaction within the probe could not occur and the amount of C 0 2 added could be monitored via pressure measurements. The heating bath shown near the outlet of the syringe pump in Figure 3.2-7 was used to heat a small ballast volume above the critical temperature of C02. The gas in the heated ballast volume served as a compressible cushion to avoid the large pressure surges that would otherwise accompany the pressurization of liquid C02, which is relatively noncompressible. From the beginning of working with the 0x0 catalyst system, it was apparent that the properties of supercritical fluids, apart from the anticipated benefits for catalysis, are also of great value in high-pressure NMR experiments. As only one phase is present in the supercritical system, stirring to achieve gas-liquid mixing is unnecessary. Figure 3.2-8 indicates that this is not the case for a gas (H2, 101 bar) and a liquid (benzene-d6, 7.0 mL) when the former is admitted above the liquid-gas interface and provisions for stirring are not present. The data for the figure were obtained by integrating the 'H NMR signal of dissolved H2 (6 = 4.45) versus the calibrated signal of the residual protons in benzene-d6 (6 = 7.15) using the pressure vessel in Figure 3.2-2. It is readily apparent that the hydrogen dissolution rate (tl12 = 5.2h at 22°C) is far too slow for all but the most sluggish of hydrogen-consuming reactions. It is also noteworthy that the hydrogen concentration in the vapor phase (4.1 M) To Vent

k-

Temperature Control Bath

NMR Probe (resistance heater, Dewar, and water jacket not shown) To Capacitor Box

Figure 3.2-7 Manifold for pressurization of the NMR probe.

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173

.*-*-*-*-

0.8

It121, M

[H21,,,=

0.2

4.1 M

IH21,0,= 0.31 M 0.1

Figure 3.2-8 Dissolution of H2 at 101 bar in benzene-d6 (7.0 mL) in the NMR probe at 22°C.

0.0

f,,2= 5.2 h

b

I

0

16

I

rime, h

80

4s

is more than one order of magnitude higher than the value measured in the liquid phase (0.31 M) when equilibrium is finally reached. In a supercritical fluid, the high gas-phase concentration of hydrogen would be immediately available for reaction. The comparisons just cited make supercritical fluids attractive media for fast reactions that involve gases and obviate the need for special stirring mechanisms which would otherwise be required and, however cleverly designed [12,28], are difficult to implement in NMR applications. Supercritical fluids, even when compressed to achieve liquid-like densities, have gas-like viscosities that are frequently more than one order of magnitude lower than those of typical liquids. At its critical point, C 0 2 has a viscosity of only 0.025 cP, whereas benzene at 25 "C has a viscosity of 0.61 CP [29]. Figure 3.2-9 illustrates that the lower viscosities have a beneficial line-narrowing effect on quadrupolar nuclei stemming from the increased transverse relaxation times, T2, that accompany decreased viscosity [30]. For 59C0, with an electric quadrupole moment Q of 0.4 x 10-28cm2, the lines are sufficiently broad in normal liquids to cause (1) partial overlap, even for peaks having substantially

Figure 3.2-9 59C0 NMR spectra of C O ~ ( C O ) ~ , 0.04 M,comparing widths at half height for supercritical C 0 2 (density = 0.5g/mL) and liquid benzene solutions.

174

3 Spectroscopy of SCF Solutions

different chemical shift values, and (2) digitization problems for peaks having widths comparable to the largest available spectral window [31]. As shown in Figure 3.2-9, the width at half height for C O ~ ( C Oin ) ~benzene-d6 at 25°C is 30 kHz, whereas in supercritical C 0 2 at 32 "C and at a density of 0.5 g/mL, the line width at half height (HHW) is only 3.1 kHz,In a related problem, use of a supercritical fluid can surmount digitization troubles associated with spectra of nuclei such as Ig7Re, which have even less favora%le quadrupole moments than 59C0. In benzene solution, the Ig7Re NMR signal from Re2(CO)lo decays too rapidly to be observed at all by standard high-resolution NMR equipment. However, as shown in Figure 3.2-10, a useful spectrum is obtained in supercritical C02. Perhaps an even more useful line-width effect is the near temperature independence of line widths of quadrupolar nuclei in supercritical fluids. In liquids, line widths of quadrupolar nuclei decrease with increasing temperature because of strongly temperature dependent changes in viscosity. These temperature dependent line-width variations in liquids often obscure variations that occur through chemical exchange processes and make such processes difficult or impossible to quantify or sometimes even to identify. However, chemical exchange processes in supercritical fluids are easily measured because, in the absence of chemical exchange, line-width variations in these media are small and frequently negligible. The molecular correlation time that is responsible for spectral line widths depends mainly on the viscosity of the medium and, for supercritical fluids, the viscosity only varies as the square root of the temperature [30]. For example, as shown in Figure 3.2-11, 59C0 line widths of C O ~ ( C Oin ) ~ supercritical C 0 2 are nearly invariant for over a 100°C temperature range from below 32°C to above 140°C. Hence, the chemical exchange process involving the dissociation of Co2(CO)* to produce two Co(C0); radicals is easily discerned by the line broadening at temperatures above 160°C.

a

T = 200 O C P = 145 atm Q = 2.6 x 1O9*cm2 HHW = 7.2 KHz

3.2 NMR Spectroscopy

1

Figure 3.2-11 Temperature dependence of the 59C0 NMR line widths (HHW) for coZ(c0)8*

175

[CMCO),] = 4.8 mM [CO]=7.1 M 0.5 g/ml

d,=

0 0

100

200

T, "C

The solubility of the metal complexes in the SCF is a major concern for their use in NMR spectroscopic studies. Unless some favorable chemical interaction exists, solubilities in supercritical fluids are governed mainly by solute volatility [32]. However, some surprisingly strong solvation interactions can occur. For example, when compared with other gases, COz is particularly soluble in perfluorocarbon solvents. This same strong interaction is apparently responsible for the fact that fluorinated acrylic polymers with molecular weights greater than 250 000 g/mol can dissolve in supercritical COz to make 25 % w/v solutions [33]. The high solubility of perfluorinated compounds in supercritical C 0 2 might be based on specific solute-solvent interactions as is evident from recent NMR investigations [34]. It has been used successfully to record 31P NMR spectra of phosphine complexes in the supercritical state [35]. Further, metal carbonyls dissolve easily in COz, probably mostly as a consequence of their high volatility, but perhaps also due to favorable solvation. Stemming from the internal confinement of their magnetic flux, toroids are particularly useful in making solubility measurements. As any sample external to the torus contributes very little to the observed signal, placement of the torus in the gas phase in a supercritical system affords a simple method of measuring dissolved species while discriminating against unwanted signal from undissolved solids. This approach was used to obtain the solubility plot in Figure 3.2-12, wherein the solubility of C O ~ ( C O )in~ COz (containing CO) at 180°C was determined to be 0.22M [36]. This is nearly two orders of magnitude more soluble than required for 0x0 catalysis. More recent research has investigated a stirring device which can easily be adapted to toroid cavity probes, as shown in Figure 3.2-13. The device comprises a copper coil form wound with several turns of fine Teflon-coated cop-

176

-20

In Conc., -2.6

\

3 Spectroscopy of SCF Solutions

At Conc. I80'=C: 0.22 M

4.0

*\

*\*

4.s

Figure 3.2-12 Solubility of Co2(CO)8 in COz at a density of OSg/mL.

1rrx1000

per wire, which is mounted loosely over the narrow cylinder just beneath the cavity in Figure 3.2-13. When energized with an alternating current at 20-100 Hz, the coil undergoes a vigorous rocking motion in the intense Bo field of the NMR magnet. Without stirring, complete dissolution of solids in supercritical CO2 often requires several hours. It is hoped that when the device of Figure 3.2-13 is perfected, equilibration of these solutions can be achieved much more quickly.

nn

Toroid cavity

Gas inlet

-

tube (Be/Cu)

M

Figure 3.2-13 Stirrer for dissolving solids in a toroid cavity probe.

3.2 NMR Spectroscopy

177

3.2.4 Measurement of Dynamic and Equilibrium Processes in COZ In the research on a supercritical hydroformylation process, all the important dynamic and equilibrium steps in the catalytic system had to be measured for comparisons with conventional media. The hydrogenation of dicobaltoctacarbonyl C O ~ ( C O+) ~H2

2 HCo(C0)4

(3.2-1)

is a key step in hydroformylation catalysis; this reaction must work well in any solvent system aimed at improving the catalytic process. Equilibrium constants for the reaction in supercritical C 0 2 were measured [36] by signal integration using a combination of 59C0 and 'H NMR spectra to define all of the species in eq (3.2-1). Representative spectra are shown in Figures 3.2-14 and 3.2-15. The van't Hoff plot in Figure 3.2-16 yielded the enthalpy and entropy changes for the reaction, (4.7 f 0.2) kcaYmol and (4.4 k 0.5) caV(Kmol), respectively. The results in C 0 2 agree closely with those of UngvAry [37] for measurements in liquid n-heptane solution, indicating that from this point of reference at least, supercritical C 0 2 is comparable to a typical hydroformylation medium. It is noteworthy in Figure 3.2-14 that extensive broadening and the onset of

L A -

Figure 3.2-14 59C0 NMR spectra for the reaction of C O ~ ( C O(0.048M) )~ with H2 (1.4M) in supercritical COs.

1eo'c

SO'C

60.C I

I

I

-1WO

a00

5500

PPm

3 Spectroscopy of SCF Solutions

178

H2

-10.0

-12.0

-11.0

-13.0

-14.0

ppn

I

H20

HWCO14

\

1 I

I

0.0

2.0

I

-5.0

I

-12.0

PPm

Figure 3.2-15 'H NMR spectrum at 180°C. Conditions are those of Figure 3.2-14.

\

--

AH 4.7 i 0.2 koal/mol AS 4.4 f 0.6 cal/(Kmol)

O

-3.6

In(Kaql

-4.6

0

I

-6.6

2.1

2.4

2.7

lfr, x 1000

3.0

Figure 3.2-16 Van't Hoff plot for the hydrogenation of C02(C0)8 in supercritical C 0 2 .

merging of the 59C0 signals for C O ~ ( C O and ) ~ HCO(CO)~occur at the upper temperature measurements. Significantly, the process responsible for this effect does not broaden the 59C0 NMR signal of C O ~ ( C O ) , even ~, at 2OO0C, as shown in Figure 3.2-17, nor the 'H NMR signals of H2 or HCO(CO)~at 180°C, as is evident in Figure 3.2-15. If the process in eq (3.2-1) were

3.2 NMR Spectroscopy

-1400

-1900

-2100

,'

179

-3100

wm Figure 3.2-17 59C0NMR spectrum of a mixture containing C O ~ ( C O )C, ~O, ~ ( C O and )~, HCO(CO)~at 200°C in supercritical COz.

rapid on the NMR timescale, the 59C0 signals for C O ~ ( C Oand ) ~ HCO(CO)~ would coalesce, as observed in Figure 3.2-14. However, this process is not nearly fast enough and, in addition, would result in broadening and merging of the 'H NMR signals of H2 and HCO(CO)~,which definitely does not occur. A process involving dissociation of Co2(CO)8 into two Co(C0); radicals, followed by hydrogen atom transfer from HCO(CO)~to the radical, is depicted in eqs. (3.2-2) and (3.2-3), respectively. C O ~ ( C Oe ) ~ 2 Co(C0); HCo(C0)4

+ Co(C0);

(3.2-2) Co(C0);

+ HCo(C0)4

(3.2-3)

This process would interconvert the 59C0 signals without affecting the 'H signals and is believed to be responsible for the observations. An activation barrier of (15.3 0.4)kcaVmol was determined for the process by using NMR line-shape analysis [38]. In a related activity, thermodynamics were determined for the hydrogenation of dimanganese decacarbonyl [39].

*

Mn2(CO)lo + H2 e 2 HMn(C0)s

(3.2- 4)

During the course of the experiments, the presence of C O ~ ( C Owas ) ~ found to strongly catalyze this reaction, which is otherwise extremely sluggish below 165°C. Use of the catalyst allowed equilibration of the reaction in eq (3.2-4) at temperatures as low as 80°C. The van't Hoff plot for data with and without the catalyst is shown in Figure 3.2-18. As seen from the figure, all of the data fit well to the same straight line, yielding enthalpy and entropy changes for the reaction of (8.7 0.3) kcaVmol and (8.5 k 0.8) cal/(K mol), respectively.

*

180

3 Spectroscopy of SCF Solutions

-4.6

-6.6

In K, -6.6

-7.6

0 [Mnz(CO),o]l= 0.0387 M

E [Mn,(CO),o] I= 0.0153 M, [C0z(CO),]l=0.0174 M

Figure 3.2-18 Van’t Hoff plot for the hydrogenation of Mn2(CO)lo in supercritical C 0 2 .

-8.6

1.0

26

2.2

2.8

llT, x 1000

Figure 3.2-19 shows typical 59C0 and 55Mn NMR spectra used in measurements involving the catalyzed reaction. These spectra show the various species under conditions in which both the hydrogenation of C O ~ ( C Oin ) ~eq (3.2-1) and the hydrogenation of Mnz(CO)lo in eq (3.2-4) are at equilibrium. Examination of the spectra in Figure 3.2-19 indicates that the equilibrium constant for eq (3.2-1) is significantly larger than that for eq (3.2-4), because the ratio of HCO(CO)~to Co2(CO)8 is larger than the ratio of HMn(CO)5 to Mn2(CO)lo under conditions where the initial concentrations of C O ~ ( C O ) ~ and Mn2(CO)lo are nearly equal. The more favorable equilibrium constant for Co2(CO)8 stems mainly from the more favorable enthalpy change for eq (3.2-1). Thus, the difference in enthalpy between the two systems is 4.0kcal/mol, whereas at 100°C the difference in the entropy term (TM) is only 1.5 kcal/mol. The NMR measurements of the type shown in Figure 3.2-19 also allowed determination of the equilibrium constants for the formation of the mixed dimer, (CO)5Mn-Co(C0)4, in accord with the equilibrium of eq (3.2-5). Co2(CO)g

+ Mn2(CO)lo S

(3.2 - 5 )

2 (CO)5Mn-Co(C0)4

The van’t Hoff plot in Figure 3.2-20 yielded the enthalpy and entropy changes for the reaction to be (0.8 0.3) kcal/mol and (0.7 0.8) cal/(K mol), respectively. This indicates that the heterobimetallic bond formation process in eq (3.2-5) is nearly thermoneutral, and that there is little ionic character to the Mn-Co bond in (CO)5Mn-Co(C0)4.

*

*

3.2 NMR Spectroscopy

181

M%(CO)lO

Figure 3.2-19 59C0 and "Mn spectra for the hydrogenation of Mn2(CO),,, (0.018M) and C O ~ ( C O(0.018 )~ M) at 100 "C in supercritical C 0 2 @(Hz)=

C ~ ~ ~ n c O C C O ~

Mn(cO)e

A

1

1.6

0.6

K,(lOO°C)

= 0.7

In K , -0.6 0

AH = 0.8 f 0.3 kcaVmol

-1.6

AS = 0.7 f 0.8 caV(K mol)

Figure 3.2-20 Van't Hoff plot for the redistribution reaction of C O ~ ( C Oand )~ Mnz(CO)lo in supercritical C02.

-2.6

1.0

2.2

2.6

lIT, x loo0

2.8

182

3 Spectroscopy of SCF Solutions

The bond energy (BDE) value of 68kcal/mol for the H-Mn bond has recently been determined using a combination of electrochemical and acidity measurements [40,41]. In addition, the bond energy for the Mn-Mn bond in Mn2(CO)lo has been investigated by a variety of techniques [42], yielding a particularly uninformative range of values from-20 to 40 kcaVmo1. However, the most recent determinations [43-451 seem to favor the high end of this range, (38 & 5 ) kcal/mol. The determination of the standard enthalpy change for the hydrogenation of Mn2(CO)lo in eq (3.2-4) allows an independent comparison to be made between these BDE values for the H-Mn and Mn-Mn bonds. The resultant calculation using 104 kcaVmol for the BDE of H2 predicts the enthalpy change for eq (3.2-4) of (6 & 5) kcaVmo1, corresponding well with the value of (8i7 & 0.3) kcal/mol determined in supercritical C02. In contrast to the Mnz(CO)lo system, the results for bonding in C O ~ ( C O ) ~ and HCO(CO)~are not as well defined. Thus, the difference in enthalpy of hydrogenation between eq (3.2-1) and eq (3.2-4) is only 4 kcaVmo1, while the reported [40,41] H-Co and H-Mn bond energies differ by only 1kcaV mol. These results imply that the Co-Co bond energy differs, at most, from the Mn-Mn bond energy by 6kcal/mol and indicate a value of at least 32kcal/mol. Such a high Co-Co bond energy seemed inconsistent with radical processes apparent in high-pressure NMR studies, such as the hydrogen atom transfer process of eq (3.2-3), and is much higher than the 15 kcal/mol determined by mass spectral measurements [46,47] of the equilibrium constants for the homolytic dissociation of C O ~ ( C O in ) ~ eq (3.2-2). For these reasons, it was decided to undertake high-pressure NMR measurements of the magnetic susceptibilities of solutions containing C O ~ ( C Oat) ~elevated temperatures. If the Co-Co bona were as strong as 32kcaVmo1, no paramagnetism would, be measurable under any conditions achievable in the pressure prdbes, whereas if the bond energy were as low as 15kcaVmo1, strongly paramagnetic solutions would be obtained even at relatively low temperatures. Thus, thermolysis of the Co-Co bond in C O ~ ( C Ovia ) ~ eq (3.2-2) changes the magnetic susceptibility of its solutions due to the production of (CO)&o. radicals. The extent of the dissociation was measured [38] by determining the temperature dependence of the volumetric susceptibility for a solution of C O ~ ( C Oin ) ~ compressed, i.e. supercritical carbon monoxide (T, = -140 "C, p c = 34.9 bar [48]) at a constant density of 0.279g/mL ([CO] = 9.95M). As might be anticipated, based on the results for supercritical C02, CO*(CO)~is also reasonably soluble in CO when it is compressed to a relatively high density. The requisite volumetric susceptibilities were measured by a modification of Evan's method [49-511 using methane in a small glass cylindrical capillary within the probe as reference. The susceptibilities were calculated from the chemical-shift difference between the 'H NMR signals of methane inside and outside of the capillary. Note that the gas-phase measurements utilized are advantageous in that unlike measurements in liquids, corrections for temperature dependent gas-liquid partitioning and liquid

3.2 NMR Spectroscopy

183

-4

CO,(CO), + 2 (CO),Co*

-6

In K -8

-10

AS = 29 f 4 eu

Figure 3.2-21 Van? Hoff plot for Co-Co bond homolysis in Co2(CO)8 from magnetic susceptibility measurements.

-12

1.0

23

2.1

2.6

1 I T x 1000

expansion are not necessary. The concentrations of the radical and, hence, the equilibrium constants for eq (3.2-2), were calculated [51] from the magnetic susceptibilities using Curie’s law and the spin-only magnetic moment of 1.73 BM for Co(C0);. The van’t Hoff plot for data measured in the range 120°C to 225°C is shown in Figure 3.2-21. The measured enthalpy and I

250

v. l-lz

150

50

Figure 3.2-22 13C chemical shift for CO in the absence (background) and presence of C O ~ ( C O(0.117 )~ M) in mesitylene solution.

I

2.1

2s

I

I

1

2.s

2.7

2.0

llTxl000

I

8.1

I

8.a

184

3 Spectroscopy of SCF Solutions

entropy changes for the reaction in eq (3.2-2) are (19 f 2) kcaVmol and (29 f 4)caV(Kmol), respectively. Although the new value for the Co-Co bond of (19 2) kcal/mol is not much higher than the mass spectrally determined 15 kcaVmol ialue, it at least allows a measurable concentration of radicals to exist, whereas the aforementioned value of 32 kcal/mol does not. The 19kcal/mol value is also in excellent agreement with an independent determination [38] of the bond energy by analysis of the contact-shift data shown in Figure 3.2-22. The large shifts for the 13C NMR resonance of CO in a mesitylene solution containing C O ~ ( C O )compared ~, with those in its absence, are believed to stem from CO exchange with Co(C0);. The contact shifts can be used to calculate the mole fraction of the radical at various temperatures. This calculation gives the value (19 2) kcaVmol for the bond energy, in perfect agreement with the magnetic susceptibility result. The values of (19 f 2) kcaVmol for the BDE of C O ~ ( C O )(8.7 ~, 0.3) kcaVmol for the enthalpy change for the hydrogenation of C O ~ ( C Oin ) ~eq t.0. (3.2-l), and 104kcaVmol for the H-H BDE in dihydrogen lead to a Co-H bond energy of (59 f 1)kcaVmol for the Co-H bond strength in HCo(C0)4. This value also seems reasonable in that an upper limit of 63 kcaVmol for this bond energy was estimated based on the activation parameters for hydrogen atom transfer to styrene [52].

*

*

*

3.2.5 Rate and Selectivity Measurements Associated with Propylene Hydroformylation in COZ In continued research on the hydroformylation of propylene in supercritical C02, it was necessary to measure the overall hydroformylation rate, the product selectivity, and the steady-state concentrations of catalytic intermediates in supercritical CO;! for comparison with values obtained in the conventional liquid-phase process. The high-pressure NMR measurements have shown that propylene hydroformylation proceeds cleanly in supercritical C02 (density = 0.5 g/mL), yielding only the expected n- and isobutyraldehyde products, which were measured by integrating their 'H NMR signals near 9.6ppm and 9.8ppm, respectively [53]. Figure 3.2-23 shows the catalytic conversion of propylene (0.14 M) to butyraldehyde in the supercritical medium in the presence of C O ~ ( C O (0.017M) )~ at 80°C using H2 and CO pressures of 42.5 bar each. At the relatively low temperature used, reduction of the alkene to produce alkane or further reduction of the aldehydes to produce alcohols, even after prolonged reaction (-6 half-life periods), is insignificant, and in situ 'H and 13C spectra did not indicate other products. Cobalt-containing intermediates were easily monitored by means of 59C0 NMR spectra collected alternately with the 'H NMR data. A typical 59C0 NMR spectrum at 80°C and 22.2 bar is shown in Figure 3.2-24. The small peak near -2030 ppm corresponds closely with the chemical shift

3.2 NMR Spectroscopy

0.16

I

185

I

c-

.Q

E8

0.10

8

0.08

c

Figure 3.2-23 Formation of butyraldehydes from propylene hydroformylation:in supercritical COz at 80 "C.

0.00 0

$0

16

Time, h

value obtained for a tetrahydrofuran solution of an authentic sample of n-C3H7C(0)Co(C0)4, prepared by reaction [54] of n-C3H7C(0)C1 with N~CO(CO)~. As judged from the asymmetry of the small peak in Figure 3.2-24, the resonance might stem from both the normal and the is0 derivatives. Measurable quantities of C O ~ ( C Owere ) ~ ~ not detected. The only species observed were C3H7C(0)Co(C0)4, Co2(C0)*, and HCO(CO)~,as is consistent with Mirbach's infrared study of the reaction in liquid methylcyclohexane solution [55]. As shown in Figure 3.2-25, the cobalt complexes reach a nearly steady-state condition early in the hydroformylation, which persists during the 15 hour period that the alkene is still present at significant levels. During this period,

CO,(CO),

/ 2m(c0)4 C~H~C(O)CO(CO)~

3 Spectroscopy of SCF Solutions

186

0.015

0.010

0.005

0.000 0

15

SO

Time, h

Figure 3.2-25 Concentrations of catalytic intermediates during propylene hydroformylation in supercritical COz at 80°C.

HCO(CO)~is held below its equilibrium value for the reaction in eq (3.2-1), which is only achieved after the alkene is fully consumed. The results for propylene hydroformylation in supercritical C 0 2 have been compared 1531 with those of Mirbach [55] for the reaction of I-octene in methylcyclohexane solution. Under comparable conditions, the steady-state concentrations of the intermediates do not differ greatly (i.e. by little more than a factor of three), and the overall hydroformylation rates are quite similar, d[aldehyde]/dt = 1.2 X M-' s-l and 0.77 x M-' s-', for the methylcyclohexane and C02 systems, respectively. Although different alkenes were used in the two studies, the comparisons are believed to be meaningful, as Wender et al. [56] have shown that hydroformylation rates for a wide range of straight-chain terminal alkenes vary only slightly with chain length for cobalt catalysts. The comparisons just cited indicate that under conditions where reaction chemistry controls the rates (namely, at temperatures near 80 "C), supercritical C 0 2 does not alter the measured parameters much from those obtained for a typical hydroformylation-type medium. Thus, one can anticipate that at higher temperatures, where mass transport across the liquid-gas interface normally controls the rates, the supercritical medium would achieve higher rates than expected for any liquid solvent. Perhaps of greater importance than reaction rate in hydroformylation catalysis is the selectivity to linear versus branched aldehyde product. The ratio of n-butyraldehyde to isobutyraldehyde products in supercritical COz solution, obtained by integration of the aforementioned proton NMR signals, is 7.2. This value is appreciably higher than has previously been achieved with conventional solvents; measured values [57,58] for these vary from 3.8 to 4.6.

3.2 NMR Spectroscopy

187

3.2.6 Diffusion and Relaxation Time Measurements A toroid cavity NMR detector generates a unique, gradient radio-frequency field (B1) that is confined to the inside of the torus. The cavity’s B1 field is mathematically defined by the simple equation B1 = A/r [59], where r is the radial distance from the center axis of the torus, and A is a proportionality constant, the so-called torus factor. With rotating-frame imaging technology [60], the nonuniform but strong B1 gradient can be used to resolve radial distances without the application of gradients in the main magnetic field. Accordingly, the chemical-shift information is not sacrificed for imaging purposes, and two-dimensional spectra are obtained in which the chemical shift is plotted versus nutation frequency or, equivalently, versus radial distance [61]. In the vicinity of the central conductor, structures can be resolved down to a few micrometers. The imaging capabilities of toroid-cavity resonators have been further developed to determine diffusion coefficients of liquids and fluids. Again, a method that is based on the B1 gradient, that is, the magnetization-grating rotating-frame imaging (MAGROFI) technique [62], rather than the standard pulsed-gradient spin-echo (PGSE) technique, is used to obtain precise and reproducible results for diffusion coefficients [63]. During a MAGROFI experiment, a single hard preparation pulse generates a spatially irregular z-magnetization grating along the nonuniform B1 gradient, where the grating is tight close to the central conductor and widens as the radial distance increases. Figure 3.2-26 (left) shows a 13C NMR rotating-frame image of a typical z-magnetization grating as obtained from a preparation pulse applied to supercritical COz. After an evolution time that might range from milliseconds up to several seconds, and during which parts of the grating decay because of diffusion and T1 relaxation, the same standard rotating-frame imaging method can be used to sample the remaining z-magnetization grating. Figure 3.2-26 (right) shows such a partially decayed grating in supercritical C 0 2 obtained after an evolution time of 100 ms. A three-parameter computer fit to either an analytical approximation equation [63] or a complex finite-difference analysis [64]

Figure 3.2-26 I3C image of a z-magnetization grating in a sample of supercritical carbon dioxide before (left) and after (right) an evolution time of looms.

188

3 Spectroscopy of SCF Solutions

reveals both the T I relaxation time and the diffusion coefficient of the fluid to a high precision. Diffusion coefficients in supercritical C02 are usually between and 10-9m2s-'; thus, they range between the coefficients of liquids and gases. As diffusion in supercritical C 0 2 strongly depends on dissolved substrates, knowledge of the coefficients is of great interest for mass transport processes in supercritical fluid extraction and, likewise, in supercritical fluid chromatography. The T I relaxation time is also obtained from the three-parameter computer fit of the theoretical approach to the experimental MAGROFI data. It can be compared with relaxation times derived from conventional inversion-recovery experiments. However, to compensate for the strong B1 gradients of toroid cavities, and to maximize the dynamic range of the measurements, the standard 90" and 180" pulses of the inversion-recovery sequence should be substituted by especially designed composite pulses [65] that transfer more than 98 % of the equilibrium magnetization phase-correlated into the transverse plane (x-y plane) of the rotating frame. This enabIes more than 99% inversion of the same magnetization, even if the strongest B1 field inside the toroid cavity is about 10 times the weakest. The T I relaxation times of liquid C 0 2 are around 1-4 s, as opposed to supercritical COz, where they reach values between 10 s and 14s, depending on density and temperature [66]. The relaxation time jump that coincides with the transition from liquid to supercritical C 0 2 can easily be used to verify whether a system under investigation is actually supercritical or still liquid. This information is of special interest if working with C02 solutions using conditions close to the critical point, because both the critical temperature and the critical pressure may easily shift, depending on the dissolved substrates. Additional information about the physical properties of supercritical C02, is provided in review articles in the literature [67] and in Chapter 1 of this book.

3.2.7 NMR of Quadrupolar Nuclei in Supercritical CO2 Supercritical media have very promising features for NMR spectroscopy. Because of their low bulk viscosity and the associated small molecular rotational correlation times of solutes in these media [68], relaxation times of quadrupolar nuclei increase significantly and, hence, their line widths (HHW) will be considerably reduced compared to those in common solvents [30,69,70]. This feature may be advantageously employed for the detection of NMR resonances of quadrupolar nuclei, because measurements in the liquid state are often troubled due to extremely broad line widths. An illustrative example involving 59C0 was described in Section 3.2-3. There is a host of organic, organometallic, and coordination compounds containing quadrupolar nuclei of interest. These systems are amenable to NMR anal sis by investigating not only the metal center, but also, for instance, ' the 74 N resonance of nitrogen donor ligands (which are increasingly used in homogeneous catalysis both in supercritical and classic media). However, char-

3.2 NMR Spectroscopy

189

acterization of compounds containing nitrogen and the collection of direct structural information about the coordination environment by means of 14N and 15N NMR are hindered by the quadrupolar moment of the I4N isotope and the low (0.36%) natural abundance of the I5N isotope. The use of supercritical media such as supercritical C02 may partially alleviate the problem of quadrupolar line broadening,-as shown by Robert and Evilia [30] who were the first to report significant narrowing of NMR lines of 14N in N20 in supercritical C02 as the solvent. Experimental procedures include the use of sealed glass tubes, which puts severe restrictions on the applied pressures. In some cases, extremely long acquisition times were required [713. Sapphire tubes introduced by Roe [17] paved the way for more routine applications of highLpressure NMR involving gas-liquid interphases. Systems similar to the original one have been extensively used for numerous applications [2,18-20,72,73]. For applications with supercritical media such as C02, CHF3, and CF3C1, a modified cell equipped with a pressure sensor and a 10-mm sapphire tube has recently been described (Figure 3.2-27) [74]. For measurements of solutes in supercritical media, the phase behavior is always complicated by the presence of two or more components. Although the critical point can be estimated from the equation of state, this estimate is not always accurate. An ideal and fast probe for the experimental determination of the state of C 0 2 is to monitor the chemical shift and intensity of its 170resonance at natural abundance. Once the fluid reaches its supercritical state, the intensity of the 170resonance decreases concomitant with a change K 9

v, N

Figure 3.2-27 Modified cell for high-pressure NMR measurements with supercritical fluid: A, titanium valve assembly; B, titanium pressure sensor; C, needle valve; D, sapphire tube; E, glue clearance; F, copper wire; G, Viton O-ring; H, Vespel O-ring; I, Teflon O-ring; J. Viton O-ring; K, gas inlet. The dimensions are in millimeters.

3 Spectroscopy of SCF Solutions

190

0.0

-0.2

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

-1.6

-1.8

77.4

--

72.7

--

71.7

--

69.9

--

64.8

--

-2.0

P P

Figure 3.2-28 The change of the I7O resonance of COz as a result of changes in density upon heating a sample of liquid COz from 25 "C to well above the critical temperature (40°C). The pressures as measured in the Ti-sapphire tube with pressure sensor are indicated for each spectrum. The final density of the supercritical state is 0.40 g/mL.

of the chemical shift of COz (see Figure 3.2-28). What is in fact observed is the change of density of the initially liquid solvent present between the coils going to the supercritical state. The effect is real and very clear at densities between 0.4 and 0.7g/mL. At higher densities the change is less pronounced. In practice, the sample is heated until the 1 7 0 resonance indicates that the supercritical state has been reached, and the nucleus of interest is then measured. This simple method drastically reduces the time required for the experimental determination of the state of the solvent, as no intermittent or prior visual verification of the supercritical state outside the spectrometer is required. Verification of the phase transition may also be obtained by checking the increase in the T I time for 14N by a standard inversion-recovery sequence in the temperature region where the transition from the liquid to the supercritical state occurs. Concomitantly, the 14N line width decreases appreciably. It has been found that the increase of T I with temperature is largest at or slightly after the phase transition [74]. This finding emphasizes the importance of employing a fast and reliable method with which to experimentally determine the critical point [75]. Experimental and calculated values of line widths are usually in good agreement. The appropriateness of the method has been demonstrated by measuring the 14N line widths of a series of tertiary amines and a-diimines in supercritical COz [75]. The line widths are reduced 3-5 times upon going from benzene to supercritical COz as the solvent. For instance, for (t-Bu-N=CH), the I4N

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191

line width in benzene-d6 is 305Hz, which is reduced to 100 Hz in liquid C02 and 82Hz in supercritical C02. The resulting gain in resolution is demonstrated by the distinct separation of peaks in the 14N spectrum at 7.05T of a mixture of amines in supercritical C02 containing equimolar amounts of NMe3, NEt3 and NPr3; this gain is not observed in common solvents. Jonas and co-workers have pointed out that often a compromise must be found between sensitivity and resolution in NMR spectroscopy [76]. Line narrowing is optimum in regions of low supercritical fluid density (where the viscosity is low), but then the solubility of compounds is also low. Sometimes, admixtures with small amounts of low-viscosity solvents such as acetone may be tried to obtain a reasonable concentration of the compounds studied, i.e. coordination compounds such as (R-N=CH)~MO(CO)~.However, 14N line widths for this compound decrease by a factor of about four to six when comparing benzene-d6 solutions to supercritical C02 (with 8 % acetone-d6). The dispersion of the nitro en chemical shift ensures identification of coordinated ligands by using "N NMR, in the above molybdenum complex, where A6(14N) = -36 ppm) 1751. Transition metal nuclei such as 55Mn,53Cr,61Ni,"Ru, and '87Re are of interest, but broad to very broad NMR lines are usually observed in common solvents. For instance, line narrowing by a factor of five has been observed for 55Mn in CH3Mn(CO)5by Jonas and co-workers [76]. It has been found that 53CrNMR of a 0.05 M solution of (CO),Cr=C(NH2)(Ph) in supercritical C02 (with 5 % acetone-d6) resulted in a line width of HHW = 432Hz (T = 50°C) after 14h [75]. Importantly, at the same sample concentration, no signal could be obtained in pure acetone-d6 after 14 hr (a sample at 0.4 M in acetone-d6 had HHW = 1150Hz) [77]. Even for more symmetric compounds such as (CO)5Cr(CNt-Bu), the effect is appreciable; the 53Cr line width is HHW = 90Hz in acetone-d6 (T = 22"C), and HHW = 35Hz in supercritical C 0 2 (T = 70°C). This demonstrates the aptness of the methodology for measuring NMR of transition metal complexes at relatively low concentrations in supercritical COz.

3.2.8 Future Research The in situ studies of supercritical fluid chemistry discussed here often involved lengthy experiments (some lasting more than 1 week) with the probe enduring pressures up to 500 bar at 250°C for prolonged reaction periods. Although these conditions may seem severe, many industrial processes, and perhaps many more supercritical ones, require similarly high pressures at much higher temperatures. For example, supercritical water which has critical conditions of 221 bar at 374°C [6], is receiving considerable interest as a solvent for oxidative degradation reactions. In situ spectroscopic investigations of such processes are often needed to identify intermediates or measure highpressure equilibrium or kinetic processes that can't be measured by conventional sampling techniques. In these investigations, safety considerations that

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3 Spectroscopy of SCF Solutions

are mindful of the large amounts of energy stored in compressed gases (in contrast with the more benign situation that prevails with hydrostatic experiments with liquids, even at much higher pressures) dictate the use of compact probes with reduced sample volumes. In addition, managing excess heat in high-temperature experiments, particularly those requicng prolonged measurement times, is greatly facilitated with small pressure vessel designs in which more of the internal space within the magnet can be allocated to heat exchangers and thermal insulation. As was shown here, toroids are particularly beneficial when compactness and sensitivity are key issues, and future efforts with toroids will use even more compact probes with higher temperature capabilities. Recent work has been experimenting with a toroid cavity design in which the pressure vessel itself comprises 'the outer walls of the cavity. High resolution and sensitivity have been obtained with cavity probes based on this approach for designs having an internal volume of 2.0mL. These probes are much smaller than those shown in Figures. 3.2-2 and 3.2-6 (internal volumes of 8.0mL) and are aimed at achieving considerably higher pressures and temperatures. However, although reductions in probe size can lead to improved temperature and pressure capabilities, the rf feedthrough is a remaining potential weak spot. Plastic or ceramic materials are frequently used in feedthrough construction, and these materials tend to be the source of leaks at high temperatures and pressures. In an effort to alleviate the feedthrough problem, cavity probe designs that have their electrical connections on their outer surface have been tried. Thus, future probe designs may solve the feedthrough problem by simply avoiding them altogether. At the same time, the application of sapphire tubes will prevail as a useful technique with which to investigate chemical reactions in SCFs at moderate pressures and temperatures. In particular, their potential for investigations in the near-critical region will increase further with the development of devices allowing precise pressure control during the NMR experiments. Application of SCFs in sapphire tubes to NMR of biomolecules may become an important technique as it has recently been shown that, while keeping the structure of the protein intact, appreciable decrease of rotational and translational diffusion leading to increasing T2 of protons can be achieved for middle-large proteins, using invers micelles in supercritical media [78]. In general, the large variety of high pressure NMR equipment ensures that NMR spectroscopy will continue to help explore the principles of chemical reactions in SCFs.

3.2.9 Acknowledgments The authors acknowledge the assistance of Dr T. R. Krause and J. M. Emsting, who coauthored several of the cited articles. We thank professor J. Halpern for helpful discussions. Support for this work was provided by the Office of Basic Energy Sciences, Division of Chemical Sciences, US Department of Energy,

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under Contract W-3 1-109-ENG-38, by the German Research Fondation (DFG) under program WO 613/2-1, and by the Conncil for Chemical Sciences of the Netherlands Organization for Scientific Research (CW-NWO).

References [ l ] W. B. Holzapfel, N. S. Isaacs, High-Pressure Techniques in Chemistry and Physics, Oxford University Press, New York, 1997. [2] I. T. HorvAth, J. M. Millar, Chem. Rev. 1991, 91, 1339. [3] P. Diehl, E. Fluck, H. Gunther, R. Kosfeld, J. Seelig (Eds.) High-Pressure NMR, Vol. 24 of NMR, Bask Principles Qnd Progress, Springer-Verlag, New York, 1991. [4]B. Benedek, E. M. Purcell, J. Chem. Phys. 1954, 22, 2003. [5] T. H. de Fries, J. Jonas, J. Magn. Reson. 1979, 35, 111. [6] J. M. H. Levelt Sengers, B. Kamgar-Parsi, F. W. Balfour, J. V. Sengers, J. Phys. Chem. Ref. Data 1983, 12, 1. [7] M. M. Hoffmann, M. S. Conradi, Rev. Sci. Instrum. 1997, 68, 159. [8] M. M. Hoffmann, M. S. Conradi, J. Am. Chem. SOC. 1997, 119, 3811. [9] M. M. Hoffmann, M. S. Conradi, J. Phys. Chem. 1998, 102, 263. [lo] S. H. Lee, K. Luszynski, R. E. Norberg, M. S. Conradi, Rev. Sci. Instrum. 1987, 58, 415. [ l l ] R. Bertani, M. Mali, J. Roos, D. Brinkmann, Rev. Sci. Instrum. 1992, 63, 3303. [12] D. G. Vander Velde, J. Jonas, J. Magn. Reson. 1987, 71, 480. [13] J. W. Rathke, J. Magn. Reson. 1989, 85, 150. [14] U. Frey, L. Helm, A. E. Merbach, High Pressure Research 1990, 2, 237. [15] S . Gordon, B. P. Daily, J. Chem. Phys. 1961, 34, 1084. [16] A. K. Jameson, C. J. Jameson, H. S. Gutowsky, J. Chem. Phys. 1970, 53, 2310. [17] D. C. Roe, J. Magn. Reson. 1985, 63, 388. [18] I. T. HorvAth, E. C. Ponce, Rev. Sci. Instrum. 1991, 62, 1104. [19] S . D. Kinrade, T. W. Swaddle, J. Magn. Reson. 1988, 77, 569. [20] C. R. Yonker, T. S. Zemanian, S. L. Wallen, J. C. Linehan, J. A. Franz, J. Magn. Reson. 1995, 113, 102. [21] D. I. Hoult in Topics in Carbon-13 NMR Spectroscopy, G.C. Levy (Ed.), Vol. 3, John Wiley, New York, 1979. [22] J. W. Rathke, US 5045793, 1991. [23] R. J. Klingler, J. W. Rathke, Progr. Inorg. Chem. 1991, 39, 113. [24] D. I. Hoult, R. E. Richards, J. Magn. Reson. 1976, 24, 71. [25] D. I. Hoult, P. C. Lauterbur, J. Magn. Reson. 1979, 34, 425. [26] T. E. Glass, H. C. Dorn, J. Magn. Reson, 1983, 52, 518. [27] D. W. Alderman, D. M. Grant, A Cavity Techniquefor Use in High-Field Superconducting Probes, Abstracts of the 21st Experimental NMR Conference, Tallahassee, FL. 1980. [28] K. Woelk, J. Bargon, Rev. Sci. Instrum. 1992, 63, 3307. [29] M. McHugh, V. Krukonis, Supercritical Fluid Extraction, Buttenvorths, Stoneham, MA, 1986.

[30] J. M. Robert, R. F. Evilia, J. Am. Chem. SOC. 1985, 107, 3733. [31] R. Kidd, R. Goodfellow, in NMR and the Periodic Table, R. K. Harris and B. E. Mann (Eds), Academic Press, New York, 1978. [32] K. P. Johnson, in Supercritical Fluid Science and Technology, ACS Symp. Sex 406, K. P. Johnson and J. M. L. Penninger (Eds.), American Chemical Society, Washington DC, 1989, p. 4. [33] J. M. DeSimone, Z. Guan, C. S. Elsberd, Science 1992, 257, 945. [34] A. Dardin, J. M. DeSimone, E. T. Samulski, J. Phys. Chem. B 1998, 102, 1775.

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[35] S. Kainz, D. Koch, W. Leitner, in Selective Reactions of Metal-Activated Molecules, H. Werner and W. Schreier (Eds.) Vieweg, Wiesbaden, 1998, p. 151. [36] J. W. Rathke, R. J. Klingler, T. R. Krause, Organometallics 1992, 11, 585. [37] F. J. Ungvhy, J. Organornet. Chem. 1972, 36, 363. [38] R. J. Klingler, J. W. Rathke, J. Am. Chem. SOC. 1994, 116, 4772. [39] R. J. Klingler, J. W. Rathke, Znorg. Chem. 1992, 31,804. [40] M. Tilset, V. D. Parker, J. Am. Chem. SOC. 1989, 11, 6711. [41] M. Tilset, V. D. Parker, J. Am. Chem. SOC. 1990, 12; 2843. [42] J. A. Martinoho SimBes, J. L. Beauchamp, Chem. Rev. 1990, 90, 629. [43] J. L. Goodman, K. S. Peters, V. Vaida, Organometallics 1986, 5, 815. [44] J. R. Pugh, T. J. Meyer, J. Am. Chem. SOC. 1988, 110, 8245. [45] J. A. Martinoho SimBes, J. C. Schultz, J. L. Beauchamp, Organometallics 1985, 4, 1238. [46] D. R. Bidinosti, N. S. Mcintyre, J. Chem. SOC. Chem. Commun. 1967, 1. [47] D. R. Bidinosti, N. S. Mcintyre, Can. J. Chem. 1970, 48, 593. [48] J. F. Mathews, Chem. Rev. 1972, 72, 71. [49] D. F. Evans, J. Chem. SOC. 1959, 36, 2003. [50] S. K. Sur, J. Magn. Reson. 1989, 82, 169. [51] T. H. Crawford, J. Swanson, J. Chem. Educ. 1971, 48, 382. [52] D. C. Eisenberg, J. R. Norton, Zsr. J. Chem. 1991, 31, 55. [53] J. W. Rathke, R. J. Klingler, T. R. Krause, Organometallics 1991, 10, 1350. [54] R. Heck, D. S . Breslow, J. Am. Chem. SOC. 1962, 84, 2499. [55] M. F. Mirbach, J. Organornet. Chem. 1984, 265, 205. [56] I. Wender, S. Metlin, S. Ergun, W. Sternberg, H. Greenfield, J. Am. Chem. SOC.1956, 78, 5401. [57] P. Pino, F. Piacenti, M. Bianchi, in Organic Synthesis via Metal Carbonyls, Vol. 2, I. Wender and P. Pino, (Eds.), John Wiley & Sons, New York, 1977. [58] J. W. Rathke, R. J. Klingler, US 5,198,589, 1993. [59] K. Woelk, R. J. Klingler, J. W. Rathke, J. Magn. Reson. A 1994, 109, 137. [60] D. I. Hoult, J. Magn. Reson. 1979, 33, 183. [61] K. Woelk, R. J. Klingler, J. W. Rathke, J. Magn. Reson. A 1993, 105, 113. [62] R. Kimmich, B. Simon, H. Kostler, J. Magn. Reson. A 1995, 112, 7. [63] K. Woelk, R. E. Gerald 11, R. J. Klingler, J. W. Rathke, J. Magn. Reson. A 1996, 121, 74. [64] K. Woelk, B. L. J. Zwank, J. Bargon, R. J. Klingler, R. E. Gerald 11, J. W. Rathke, in Spatially Resolved Magnetic Resonance: Methods and Applications in Material Science, Agriculture and Biomedicine, B. Bliimich, P. Blumler, R. Botto, and E. Fukushima (Eds), Wiley-VCH, New York, 1998, Chapter 8. [65] K. Woelk, J. W. Rathke, J. Magn. Reson. A 1995, 112, 7. [66] S. Bai, Ch. L. Mayne, R. J. Pugmire, D. M. Grant, Magn. Reson. Chem. 1996, 34, 479. [67] U. v. Wasen, I. Swaid, G. M. Schneider, Angew. Chem., Int. Ed. Engl. 1980, 19, 575. [68] R. F. Evilia, J. M. Robert, S. L. Whittenburg, J. Phys. Chem. 1989, 93, 6550. [69] D. M. Lamb, S. T. Adamy, K. W. Woo, J. Jonas, J. Phys. Chem. 1989, 93, 5002. [70] I. P. Gerothanassis, C. G. Tsanaktsidis, Con. Magn. Res. 1996, 8, 63. [71] J. M. Robert, Thesis, New Orleans, 1987, UMI order number 8808115. [72] C. J. Elsevier, J. Mol. Catal. 1994, 92, 285. [73] A. Cusanelli, U. Frey, D. T. Richens, A. E. Merbach, J. Am. Chem. SOC., 1996, 118, 5265. [74] S. Gaemers, H. Luyten, J. M. Emsting, C. J. Elsevier, Magn. Reson. Chem. 1999, 37, 25. [75] S. Gaemers, C. J. Elsevier, Chem. SOC. Rev. 1999, in press. [76] D. M. Lamb, D. G. Vander Velde, J. Jonas, J. Magn. Reson. 1987, 73, 345. [77] A. Haffner, L. S. Hegedus, G. deWeck, B. Hawkins, K. H. Dotz, J. Am. Chem. SOC. 1988, 110, 8413. [78] S. Gaemers, C. J. Elsevier, A. Bax, Chem. Phys. Lett. 1999, in press.

Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

3.3 UV, EPR, X-ray and Related Spectroscopic Techniques CLEMENT R. YONKER,JOHNC. LINEHAN and JOHNL. FULTON

The techniques described in Chapters 3.1 and 3.2 provide information on a molecular level about solute structure, solution dynamics and chemistries in SCFs. The goal of this chapter is to briefly discuss other spectroscopic techniques applied to SCFs which can be used to determine solution structure or the effects of pressure and temperature on reactions in these solvent systems. An overview of this type, by its nature, can not be considered inclusive, but it is hoped to give future practitioners in the field a reasonable expectation of the type of molecular-level information that different spectroscopic techniques provide under SCF conditions.

3.3.1 UV-Vis UV-Vis spectroscopy of compounds dissolved in SCFs has been used to determine solvent structure [l-61, solute solubilities [7,8], solid matrix changes [9, lo], and reaction kinetics [ll-161. UV-Vis is often the spectroscopic technique of first choice because many solute molecules of interest have an observable absorption band which can be used for identification and quantitation. In the simplest UV-Vis SCF experiment a pressure vessel (stainless steel or other high pressure metal alloy) is fitted with an inlet (and an outlet) connection for high-pressure tubing and high pressure windows (quartz, sapphire, or diamond). The relative strength of the window materials is diamond > sapphire > quartz. The pressure limits of the cell design depend on the window material and the area of the unsupported window exposed to the fluid pressure. The typical UV cutoff wavelengths for these materials are: sapphire 1500A, quartz 2100 A, and diamond type I1 2600 A. Many different cell designs have been reported in the literature. A typical high pressure cell design is shown in Figure 3.3-1. The pressure seal between the sapphire window and the cell body is made with a gold plated metal V-ring. The path length is typically 2.5 cm and the cell volume is 3-4 mL. The sapphire windows are 1 cm thick and 2.5 cm in diameter. Common, high-pressure UV-Vis spectroscopy

-

-

-

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3 Spectroscopy of SCF Solutions

Figure 3.3-1 A diagram of a high pressure UV-Vis spectroscopic cell for the study of supercritical fluid solutions used in the authors’ laboratory.

cells for SCFs are usually rated from 200 to 700 bar [2,3,6,9,16]. More complex high-pressure UV-Vis systems will have temperature control of the cell (heaters and chillers) and may have a third window for photolysis (or fluorescence spectroscopy) of the solution. In photolysis experiments, a flow-through system with a large reservoir of the solute dissolved in the pressurized fluid is used. The high pressure spectroscopic cell is loaded with the sample and the fluid is introduced at a known density - controlled by pressure and temperature. An alternative is to control the density of the SCFs in the cell, based on the cell volume and weighing the introduced SCF. For solubility studies, the solute is added to the pressure vessel which is then sealed and pressurized with the solvent of choice. Solution equilibrium is obtained by either stirring the sample or recirculating the fluid using a high pressure pump. Tubing, vessels and fittings rated for high pressure work are commercially available. As with all high pressure experiments, prudent safety practices must be observed when using this equipment. UV-Vis spectroscopy can be used to determine the solubility of a solute in SCF solutions, which is particularly useful for SFE and is a prerequisite for studying reactions. This technique can be used for both organic and inorganic compounds, depending on their absorption maxima and wavelength [7,8]. In situ determination of the solute concentration is accomplished by monitoring the intensity of the spectral bands in the spectrophotometer using Beer-Lambert’s law (A = E ~ C ) There . are some pitfalls using this methodology of which one must be aware. Variations in the spectral absorption properties of the solute (such as molar absorptivity changes) and its band shape can hinder quantitation. Adsorption of the solute on the cell windows can further complicate quantitative results. The assumption of constant molar absorptivity of the solute (E) over all density ranges used for solute extraction may not be valid in all cases. Solute molar absorptivity can differ significantly between SCFs and liquids. The molar absorptivity for anthracene and pyrene in scCOz was shown to be density dependent and systematically increased 30-170 % as COz density increased from 0.3 to 0.9g/mL [71. UV-Vis spectroscopy can also be used to further characterize the physicochemical properties of the solution. Solution structure in SCFs has been investigated through the. use of solvatochromic probe molecules in which the n+n* or n+n* electronic transition of a probe molecule responds to changes in its

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solvation environment [ 1-61. The Kamlet-Taft IT*solvent polarity scale [ 171, &(30) scale 1181, and other solvent polarity scales are based on a linear solvation energy relationship relating peak position of the UV-Vis absorption maximum of a solute in a liquid solvent with values of solvent polarity-polarizability (SPP) effects, hydrogen-bond donor acidities (HBD), and hydrogen-bond acceptor basicities (HBA). The solvent-solute interactions were related to the UV absorption maxima of select probe (solute) molecules (4-nitroanisole and N,N-dimethyl-4-nitroaniline) in a set of 229 different liquid solvents to evaluate the IT* scale of solvent effects (HBD, HBA, SPP) [17]. The IT*solvent polarity-polarizability scale has also been applied to SCFs based on the solvatochromic shift of the absorption peak maximum. Measurement of the solvatochromic shift of the probe molecule directly interrogates the cybotactic region of the solute (i.e. the region around the solute where the structural order of the solvent molecules has been influenced by the solute), providing information on the solute-solvent intermolecular interactions in this solvation shell. This technique has been applied to both pure and binary SCF solvent systems. It has been demonstrated that the cybotactic region becomes enriched for both a pure solvent (clustering) and a binary modifier (co solvent augmentation) as determined from UV-Vis spectroscopic studies of SCF solvents [l-61. Added cosolvents can improve solute solubilities by a factor of 2-5. High pressure UV-Vis spectroscopy has proved itself particularly amenable to such investigations. The local solvation environment about a solute molecule demonstrated a net enrichment of a binary solvent modifier as compared to the bulk composition caused by specific interactions such as hydrogen bonding [4,5]. This was demonstrated by an investigation of the cybotactic region about the polar probe molecule, 2-nitroanisole, in a scC02-2-propanol binary solvent system. The cybotactic region was enhanced with 2-propanol at low pressures and the local concentration was dependent on both temperature and pressure. As pressure increased, the local concentration of the organic cosolvent in the cybotactic region decreased for the binary fluid mixture [4]. These types of specific solute-cosolvent interactions need to be well understood before they can be used to advantage during chemical synthesis, SCF extractions, and reactions. UV-Vis spectroscopy has been used to monitor polymer plasticization and dye impregnation [9,10]. The Tg of glassy polymers is reduced in scCOz which leads to enhanced mobility in the polymer. The enhanced mobility has been used to facilitate the impregnation of dopant molecules into the polymer. Even though the solubility of the dopant molecule was low in scCO2, impregnation rates were very high in the polymer due to the large partition coefficient of the dopant molecule between the solvent and the polymer. This process was monitored in situ by UV absorption by following the solvatochromic effect of the dopant molecule in the polymer matrix. Laser flash photolysis with UV-Vis detection has been used to study various photoinduced reactions in SCFs. There has been a tremendous wealth of information produced from studies of the effect of density and cosolvents on reac-

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3 Spectroscopy of SCF Solutions

tions in SCFs [ 11-13]. Investigation of hydrogen abstraction from 2-propanol or 1,4-cyclohexadiene by triplet benzophenone in scCOz revealed that the bimolecular rate constants increased rapidly with a decrease in pressure near the critical point [f2]. This was attributed to local clustering of the substrate (1,4-~yclohexadiene) about the triplet benzophenone at lower pressures. In contrast, the bimolecular rate constants for the reaction of triplet benzophenone with oxygen increased only slightly with increasing pressure for both scCO2 and scCHF3 [ll]. This was interpreted as demonstrating that 2-propanol or 1,4-cyclohexadiene have stronger intermolecular interactions with benzophenone than with oxygen. The triplet-triplet annihilation reaction of benzophenone was investigated by laser flash photolysis for both scCO2 and scC2H6. The authors reported no evidence of enhanced cage effects either near the critical point or removed from the critical point due to SCF solvent clustering or solute-solute interactions [13]. Photoinduced ligand substitution reactions of metal hexacarbonyls have also been studied by UV-Vis spectroscopy to investigate the coordination ability and solvation effects on the displacement mechanism using pure and binary SCF mixtures [14,15]. Activation volumes of 7000cm3 per mole were found just above the critical point for both scC02 and scC2H6 for these ligand substitution reactions. The huge repulsive contribution to the activation volume was interpreted as being due to the strong dependence of the isothermal compressibility of the fluid and its effect on the dynamics of solvation on the ring closure reaction for the metal carbonyl-ligand system in eq (3.3-1) [14,15]. M(CO)~+ L-L -+ M(co)&-L-L) + co -+ M(co),(~~-L-L) +

co

(3.3-1)

where M is the metal and L-L is the bidentate ligand, bipyridine. The ring closure reaction is between the two nitrogen groups on the bipyridine ligand and the metal. Therefore, experimental measurement of the absolute reaction kinetics have shown how the degree of local enhanced solvation can influence reaction rates of kinetically controlled reactions in SCFs.

3.3.2 Fluorescence Studies of fluorescent molecules in SCFs have done much to improve the understanding of solute-fluid and solute-solute interactions. The experimental spectroscopy cells are similar to those used for UV-Vis spectroscopic studies [19-21].-A novel cell design has been described by Bright et al. [22] which is constructed from a standard stainless steel fitting known as a cross. Many of the experimental studies of solute-solute and solute-solvent clustering by fluorescence spectroscopy in SCFs have used pyrene as the probe molecule [23-251 because the liquid-phase behavior of this molecule is well under-

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199

stood [23]. In SCFs, Brennecke et al. [24] first observed excimer-like emission for pyrene which was attributed to solute-solute clustering. Time-resolved fluorescence demonstrated that the excimer-like emission was due to the formation of an excited state dimer and was diffusion controlled in SCFs [25]. These investigations probed the extent of density augmentation about pyrene and showed a maximum enhancement in the range of reduced densities of 0.5-0.6. Above this range, the density enhancement decreased until the local density surrounding pyrene and the bulk density were similar. Recently, it has been determined that the extent of local density augmentation for pyrene differed between the ground state and the excited state. The density enhancement in CO2 for the excited state was determined to be 1.5 times greater than that of the: ground state, which matched the ratio of electrostatic interaction energies between the excited and ground state of pyrene [23]. Intramolecular excimer formation has also been studied in scCOz and the effect of solute-solute clustering for local concentration augmentation in SCFs has been investigated [26]. The method of dynamic fluorescence (luminescence) quenching has been used to explore the exchange kinetics between microemulsion droplets formed in near-critical propane using the surfactant didodecyldimethyl ammonium bromide (DDAB) [21,27]. In the DDAB-propane microemulsion, the intermicellar exchange rate increased as the density is reduced. The increase in the intermicellar exchange rate at lower densities is much greater than would be expected due to reduced viscosity effects alone. However, strong interdroplet attractive interactions occur for most of the near-critical and supercritical microemulsion systems studied and the magnitude of the attractive forces depends strongly upon the density of the fluid solvent. These strong attractive forces drive the formation of large micellar clusters. Thus, the formation of large micellar clusters significantly increased the exchange rates. These changes in the secondary structure lead to important and unexpected consequences for conducting chemical reactions in supercritical and near-critical microemulsions.

3.3.3 Electron Paramagnetic Resonance Electron paramagnetic resonance (EPR) spectroscopy is a sensitive technique that allows the detection of solutes at concentrations as low as mole fraction in SCFs. The solute-solvent and solute-solute interactions of paramagnetic organic or inorganic molecules dissolved in SCFs can be probed through the determination of rotational correlation times and the Heisenberg spin exchange rate, respectively, as measured by EPR spectroscopy [28-301. The rotational correlation times of a probe molecule, T, allows the determination of the hydrodynamic radius of the molecule or, alternatively, the enhanced local density of the solute molecule arising from interactions with the solvent. Heisenberg spin exchange, electron spin exchange between two molecules with

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3 Spectroscopy of SCF Solutions

antiparallel spin states, is a rapid process dependent upon the collision frequency and the reaction cross-section (or reaction probability). Recent studies of Cu(F0D) (FOD is 2,2,3 -trimethyl-6,6,7,7,8,8,8-heptafluoro-3,5-0ctanedlbnate) in scC02 demonstrated local solvent density enhancement about the Cu(F0D) which decreased with increasing bulk density. In particular, one interpretation of the rotational correlation time for Cu(F0D) in scCO2 gave a local density higher than the bulk-density of the solvent [29]. The local density enhancement in the cybotactic region of the organic free radical increased the time the radicals are in contact with one another, which increased the probability of spin exchange [31]. There has been a recent report using time-resolved EPR spectroscopy to study pulsed, photoinduced transient free radical generation in SCFs [32]. A flow-through quartz EPR cell was devised to hold pressures up to 200 bar and necessitated the design of two microwave cavities to accommodate the quartz tube. Microwave coupling, flow optimization, and laser light coupling were investigated for C02(l) and scCO2 solutions of stable and transient free radicals [32]. Time-resolved EPR spectroscopy was used to investigate the reaction mechanism using a free radical initiator in the synthesis of tetraethylene-based fluoropolymers in scCO2.

3.3.4 X-ray Absorption Fine Structure Often, the unique and unusual molecular structure in SCF solutions profoundly affects the reactivity in these systems. Thus, the elucidation of this structure is one of the first requirements for developing a predictive capability for reaction rates and pathways. A powerful spectroscopic technique that has only recently been used for in situ characterization of the molecular structure in supercritical fluid solutions is X-ray absorption fine structure (XAFS). XAFS provides detailed structural information about the number of nearest-neighbor atoms, bond distances, and bond strengths (from the Debye-Waller factor). The application of XAFS to a wide range of SCF solutions provides another powerful tool to explore the detailed structure of SCFs. The basis of the XAFS technique is as follows. The X-ray energy is scanned through a region that excites the core electron into the continuum. The ejected photoelectron is backscattered by the atomic cores that surround the excited atom. Thus the wavefunction of the scattered photoelectron interferes with the initial wavepacket and this interference gives rise to oscillations in the absorption on the high-energy side of the absorption edge. A Fourier transform of the extracted oscillations gives a radial structure function that is closely related to a radial distribution function. Standard methods for extraction of the structural parameters from the XAFS spectra have been extensively described [33-381. Because the XAFS scattering process is rapidly damped at longer distances, only the immediate vicinity of the absorbing atom is probed. For SCF studies, small, high-pressure XAFS diamond windows can be chosen that efficiently transmit the X-rays into the sample with little attenua-

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tion. Thus three-quarters of the elements (atomic number > potassium) on the periodic table can be selected as the central absorbing atom. Backscattering atoms are typically all of the elements other than hydrogen or helium (which have too few electrons for efficient backscattering). Since the mid 1990s the theoretical standards [37] that are used in the analysis of the XAFS spectra have reached a very high level of refinement. In addition, automated programs for background correction [36] and for fitting to theoretical standards [38] make the technique much more available to researchers who need key structural information about the molecular structures of reactive species. XAFS spectra can be acquired at any of a number of different synchrotron facilities throughout the world. Synchrotrons provide a tunable, high-intensity, highly collimated X-ray beam having dimensions small enough to direct into and through the high pressure solution. Access to these facilities typically involves little more than a brief request for beam access. In contrast to the tour de force techniques such as neutron diffraction, XAFS provides similar information with a much lower investment of time and resources and a much wider range of applicable systems including systems undergoing chemical reactions. A summary of the characteristics of XAFS spectroscopy with respect to SCF solutions are shown below. Measures local structure in amorphous solutions. Provides local structure out to 2-3 atomic shells - limited to short-range structure. Determines interatomic distances to f 0.005 A. Provides some information on the chemical identity of nearest neighbors. Provides information on the angular correlation of neighboring atoms from multiple scattering effects. About three-quarters of the elements in the periodic table are addressable by XAFS with equal atomic resolution. XAFS has the ability to work at extremely low concentrations, e.g. < 1 mM. Oxidation state and the concentrations of metal ions in solution can be directly determined. Transformed results are closely related to a radial distribution function. Direct comparison to a molecular dynamics simulation is possible. The comparable techniques are neutron and X-ray diffuse diffraction. To date there have been only a handful of studies of fluids but there are a wide range of potential areas where XAFS may provide key structural information necessary to elucidate reaction rates and pathways. Only a few of the potential uses of XAFS for determination of SCF structure and reactions have thus far been explored. A few possible applications of the technique include: (1) supercritical water hydration, (2) supercritical water ion pairing, (3) redox chemistry of inorganic species under supercritical conditions, (4) in situ characterization of organometallic structures, ( 5 ) binding of solutes or solvents to organometallic species, and (6) weak chemical interactions of the solute or solvent with other solutes.

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3 Spectroscopy of SCF Solutions

There are two different experimental techniques for acquiring XAFS spectra of SCF solutions. One involves the use of small diamond windows (or a similar low-atomic-number material such as boron carbide) to provide efficient X-ray transmission through the high pressure solutions [39,40]. A schematic of the XAFS cell, data collection, and the analysis technique is shown in Figure 3.3-2. The cell is machined from a block of high-nickel alloy, Hastelloy C-22, containing two diamond windows for the transmission of the X-ray beam and a single sapphire (A1203) view port window. Diamond windows having thicknesses in the range from 0.25 to 1 mm and having diameters of about 3 mm are appropriate. The diamonds can either be single crystal or polycrystalline material formed by chemical vapor deposition. The selection of the specific diamond type depends upon the characteristics of the beam, including its diameter and the energy range of interest. Downstream transmission detectors are well suited for these diamond window cells; however, for solutions at low concentrations, fluorescence detectors [41] can be used. The X-ray path length of the cell is 5.8mm and its volume is -5mL. The second XAFS technique uses a fused silica capillary [42] for the pressure cell. A schematic of this configuration is shown in Figure 3.3-3. Use of the capillary cells mandates a more highly focused beam - ideally a rectangular shaped beam having dimensions of approximately 100 pm by 2-10 mm for transverse illumination. There are many styles of X-ray fluorescence detectors that provide highly efficient detection for these types of cells. With the use of capillary cells and the standard pumps and mixing vessels that are typically used for SCF studies, any researcher can assemble the necessary equipment with minimum effort and cost. In contrast to the more complicated techniques such as neutron diffraction, XAFS provides similar information with a much lower investment of time and resources and a much wider range of applicable systems including systems undergoing chemical reactions. The first a plication of XAFS to SCFs was to explore the extent of ion hydration (Sr ) in scH20 [39]. In later studies, solutions containing different

f+

Figure 3.3-2 Schematic of a supercritical fluid XAFS cell and the data analysis technique.

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Figure 3.3-3 A quartz capillary cell for XAFS studies of supercritical fluid solutions. ’

cations (Rb+, Ni2+) and one anion (Br-) were explored [41,43,44].For all systems studied thus far, results show a large reduction in the extent of hydration as one approaches and exceeds the critical temperature. The results to date are also consistent with more recent neutron scattering results on chemically similar systems [45].The most recent XAFS results have explored the structure and extent of ion pairing in scH20 [44].These results show that there is concurrent reduction in the extent of hydration as the ion-pair forms. The relevance of these results with respect to chemical reactions is very high. Obviously for homogeneous catalytic reactions involving metal ions, the dramatic change in the hydration and the formation of ion pairs will play a major role in defining the kinetics and reaction pathways. For reactions involving ionic reactants or products - including ionic organic species - the increasing importance of the electrostatic effects may dominate the reaction sequence in near-critical and SCF solvent systems. An example of the type of information that can be derived from XAFS is shown in Figure 3.3-4 which presents a radial structure plot (RSP) for an 2.5

I

“I2+] c

g0 c a

L

2.0

-

‘ W T = 25°C P = l bar

= 0.2 molal

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3 Spectroscopy of SCF Solutions

Table 3.3-1 Structural parameters derived from XAFS for the hydration and contact ion pair of Ni2+ under ambient and supercritical conditions. The number of nearest neighbor water molecules (NHZO) and their distances (RHZo)% the number of bromide contact ions (NBr-)and their distances (RB~-)are reported. Estimated errors are shown in parenthesis.

25

1.05

5.9 (0.3)

2.Q62 (.004) 0.0

325 425

0.81 0.65

4.9 2.6

2.08 2.09

0.2 (0.2) 1 .o

-

2.47 (0.4) 2.41

ambient, a subcritical, and a scH2O solution containing Ni2+ ion. The vertical axis represents the probability of finding an atom at a certain distance from the central scattering atoms. In Figure 3.3-4, the RSP for scH20 shows the emergence of a new local shell for the contact-ion pair (CIP) at a distance that is just beyond the first water hydration shell. In this case the Ni2+ counterion is Br-. The parameters (and the associated error estimates) that are derived from the fits to the theoretical standards are given in Table 3.3-1. The RSP plots in Figure 3.3-4 include an XAFS backscattering phase shift that gives an apparent positional shift of about -0.3A, whereas the data in Table 3.3-1 have been corrected for this phase shift and are accurate to within about f0.01A. The structural parameters given in Table 3.3-1 demonstrate the high degree of structural information that can be derived from the XAFS technique. Under supercritical conditions the number of nearest Br- neighbors is approximately 1.0 f 0.3 and the Ni-Br distance is 2.41 A. The identity of the Br- nearest-neighbor can be confirmed from the unique set of XAFS amplitude- and phase-shift functions that are used to fit the oscillations in the XAFS spectra. The Debye-Waller factor (a measure of the positional disorder) measured for the CIP indicates that the Br- anion is tightly bound to the Ni2+ cation, the strength of this binding approaches that of the remaining H 2 0 in the first solvent shell [44]. The measured distance of the Ni2+/0 first hydration shell is 2.08 8, which is just slightly larger (0.02A) than the distance measured under ambient conditions. A more recent application of XAFS involves exploring the molecular structure of a manganese organometallic compound, cymantrene (CpMn(COh, Cp = q'-cyclopentadienyl), in scCO2. Figures 3.3-5(a) and 3.3-5(b) show XAFS spectra of the cymantrene dissolved in liquid hexane and in scC02. The oscillations in the spectra arise from the backscattering from the oxygen and carbon atoms on the three carbonyls and from carbon atoms on the cyclopentadiene. Both spectra were acquired in a diamond-window highpressure cell, although similar spectra have been acquired in a capillary cell [42]. Figure 3.3-6(a) and 3.3-6(b) give the radial structure plots for cymantrene in both hexane and in scC02, respectively. The doublet at

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205

10 1

-8

I

1 0.2 M, 25.OoC, 1 bar

_".

-

.

2

0

.

.

4

-

,

6

.

.

.

.

.

.I

8

10

12

8

10

12

k (A-')

Figure 3.3-5 XAFS spectra for 0 . 2 M cymantrene (CpMn(C0)3): (a) in liquid hexane under ambient conditions; (b) in scCOz at 60°C and 129 bar. The oscillations in the spectra arise from backscattering of the photoelectron by neighboring C and 0 atoms. 0

2

4

6

k (A-')

about 1.6A (distance not corrected for XAFS phase shifts) corresponds to carbons on the carbonyl and the cyclo entadiene whose distances are different by about 0.3A. The peak at 2.4 corresponds to the oxygens on the carbonyls. As one might expect for this solvent comparison, there are no differences in the interatomic distances within the experimental resolution of k0.005A. In addition there are no differences in the spatial disorder (Debye-Waller factor) of the carbons or oxygen. One of the goals of the study was to look for evidence of weak binding of the C 0 2 solvent molecules near the metal center or carbonyls. Such evidence would be most prevalent as changes in the XAFS spectra (Figure 3.3-5) in the region from 0 < k < 3A-l. Again, there is no indication of a significantly different solvent environment in scCOa than that found for hexane.

8:

3 Spectroscopy of SCF Solutions

206

I a)

0.2 M,25.0°C, 1bar

1

6.0

r

3 2 4*0

-x Y

2.0

0.0 0.0

1.0

3.0

20

RC 8.0

4.0

,'

5.0

(5.0

h

. b)

0.0

0.2 M,60.0°C, 128 bar

1.0

2.0

3.0

R

(4

4.0

5.0

6.0

Figure 3.3-6 An XAFS radial structure plot (k2 weighted) for 0.2M cymantrene (CpMn(C0h): (a) in liquid hexane under ambient conditions; (b) in scCO2 at 60°C and 129 bar. The doublet at 1.6 A is from the carbons on the carbonyl and the cyclopentadiene moieties. The peak at 2.4A is for the carbony1 oxygens. The plotted distances are not corrected for XAFS phase shifts.

3.3.5 X-ray and Neutron Scattering and Diffraction The use of X-rays and neutrons having wavelengths of the order of 1 A permits the study of fluid structure from about SOOnm down to atomic resolution (characteristic distance h/(2sin(8)). Small angle scattering (SAXS and SANS) focuses on the region at low angle to the incident beam and thus resolves the structure of macromolecular species such as polymers and microemulsions having dimensions from about 5 to 500nm. In contrast, the wide-angle X-ray or neutron diffuse diffraction (DD) region provides information about much shorter distances appropriate for discerning the spatial distribution of small solutes and solvent molecules. DD is suitable for measuring the full pair distribution functions of solutes and solvent in a supercritical solution. Because of the limited availability of these high-energy X-ray and neutron sources, none of these scattering techniques can be used for routine analysis; however, the power of these methods goes far beyond any other spectroscopic method for resolving the microstructure of solutions. For this reason, X-ray and neutron scattering have unlocked many of the mysteries about the atomic, molec-

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ular and macromolecular structure of SCF solutions and have provided a basis upon which further descriptions by other spectroscopic techniques have evolved.

3.3.5.1 Small Angle Scattering Both small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) have been used for obtaining detailed structural information about macromolecular species such as micelles or polymers in supercritical solutions. A variety of different microstructures have been identified in SCFs. In addition, changes in the fluid density have been shown to not only affect the primary structure, but also the secondary structure involving the spatial distribution of micelles or polymers in the continuous-phase solvent. This can have dramatic effects on reaction rates and pathways. In the method of small angle scattering, the radiation is scattered to low angles because of inhomogeneities in the properties of the micelles or particles relative to those of the solvent [46-481. The dimensional scale probed by these methods can resolve the spacing between individual surfactant molecules in the interfacial region, the geometry and size of individual microemulsion droplets, and the larger-scale secondary, macromolecular structure or micelle clustering. Results for numerous near-critical and supercritical systems have been reported [49-561. These studies have shown that a variety of microstructures exist in compressible fluids and that changes in the density of the fluid can affect both the primary structure and the secondary structure (including the spatial distribution of microemulsion droplets). For SAXS and SANS, the theoretical methods required to interpret the scattering information are virtually identical, except for the basis of the scattering contrast. For SAXS, inhomogeneities in the electron densities give rise to coherent, excess scattering, whereas in SANS, inhomogeneities in the nuclei’s scattering cross-section give rise to excess neutron scattering. Both methods require a high-pressure cell with beam-transparent windows which are typically single-crystal diamonds for X-ray studies and sapphires for neutron studies. For examining small molecular clusters, neutron scattering perhaps holds an advantage because the scattering contrast can be finely controlled by changing the percentage deuteration of the solute or solvent. (Protons and deuterons have very different scattering cross-sections.) For studies of organic liquid solutions, SAXS is generally not the preferred method because of the poor electron-density contrast between the solute and solvent. For SCFs, however, this problem is eliminated as the lower-density SCF phase has good scattering contrast (i.e. a large difference in the electron densities) with the higher-electron-density solute. Further, for systems containing fluorocarbon surfactants (very high electron density) in scC02 (very low electron density), the scattering contrast is one of the highest available by either SAXS or SANS. Clearly, both SAXS and SANS are powerful techniques with which to probe structure in SCF solutions, and each has strengths in probing different regions of the microemulsion structure.

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3 Spectroscopy of SCF Solutions

In two early SANS studies, [49,50] the structure in a near-critical propane microemulsion formed with sodium bis(2-ethylhexyl) sulfosuccinate (AOT) was explored. These studies conclusively demonstrated that, as the continuous-phase density was adjusted, the micelle size remained constant for systems containing ionic surfactants in the single-phase region under low-to-moderate volume fractions of the dispersed phase. The most recent scattering studies were of reverse micelles and microemulsions formed in SCCOZ.In the SAXS study by Fulton et al. [53], the aggregation of three different amphiphiles was explored. Of significance, the results showed that a surfactant based upon the graft copolymer poly(ethy1ene oxide)-g-poly(1,l -dihydropeffluorooctylacrylate) (PEO-g-PFOA) [57-601 formed large aggregates in SCCOZ.The diameter of these highly monodisperse structures was quite large, about 25nm, and before this discovery, it was not clear that structures this large could be stably suspended in scCOz. The overall structure of these microemulsion droplets was later confirmed in a SANS study [54]. Recently, Eastoe et al. [55], following the initial work of Harrison et al. [61], used SANS to study the structure of another scCOz-surfactant system. Finally, the polymer structure of perfluorooctylacrylate in scCOz was determined by SANS [62]. The solubilization of polymers that have very low solubility can be accomplished with the use of block copolymers which incorporate COz-philic blocks. Recent SANS and SAXS studies [63] have been used to determine their solution microstructure as a function of pressure and temperature in SCCOZ.

3.3.5.2

Wide Angle Scattering

Almost all of the papers dealing with diffuse diffraction (DD) or wide angle scattering have been for studies of pure SCFs such as COz, xenon and water. Studies of inert gases and their structure at various densities and pressures [64,65] have provided a basis for the understanding of more complicated fluids having quadrupolar, dipolar or hydrogen-bonding interactions. Near the critical point, X-ray diffraction provides clear evidence of the well-known long-range density fluctuations [65]. There is also evidence of local solvent depletion, arising from repulsive interactions at densities above the critical density [65]. X-ray scattering studies of scC02 suggest the possibility that higherorder phase transitions occur in the supercritical region [66]. Similar observations were obtained for scCHF3 [67]. There is a great deal of interest in understanding the structure of strongly hydrogen-bonding fluid solutions such as scHzO. One area of particular interest is to determine the changes in the extent of intermolecular hydrogen bonding for supercritical water. There have now been several neutron scattering studies of this system and, after several years of controversy and reanalysis, a consistent picture has emerged [68-701 (in agreement with measurements by other techniques such as NMR [71]) in which there is an appreciable amount of hydrogen bonding in supercritical water. As the experimental and data analysis methods for wide angle scattering mature, the technique will be invaluable for elucidating the structural details

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of binary or multicomponent systems that are currently the subject of interest from other techniques such as NMR, electronic or vibrational spectroscopy.

3.3.6 Conclusions The information ascertained from the spectroscopic techniques discussed in this chapter includes solubility determination, local solvent concentrations about the solute molecule, and molecular structure as a function of pressure and temperature in the supercritical fluid solution. Reaction kinetics have been investigated using these various spectroscopic techniques, coupled with an increase in the current understanding of solute-fluid and solute-solute interactions in supercritical fluids. UV-Vis and fluorescence are mature techniques in the area of supercritical fluid applications. There are some very interesting time-resolved studies being performed in supercritical fluids investigating reaction kinetics as a function of pressure and temperature. The X-ray spectroscopies are gaining greater attention as a result of atomic and molecular level structural information which can be obtained from XAFS and SAXS. EPR continues to contribute to the understanding of local solvent dynamics in supercritical fluids and investigation of reaction mechanisms. Spectroscopic studies of the physicochemical properties of supercritical fluid solutions and reactions are still in an initial growth stage. It is anticipated that there will be continued interest in UV-Vis, fluorescence, EPR, XAFS, X-ray and neutron scattering spectroscopies, as applied to supercritical fluid solutions, in the coming years.

3.3.7 Acknowledgment Work at the Pacific Northwest National Laboratory was supported by the Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division of the US Department of Energy, under Contract DEAC076RLO 1830.

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[34] E. A. Stem, S. Heald, in Handbook of Synchrotron Radiation, D. E. Eastman, Y. Farge, E. E. Koch (Eds.), North Holland, Amsterdam, 1983. [35] D. C. Koningsberger, R. Prins (Eds.), X-ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS and XANES, John Wiley & Sons, New York, 1988. [36] M. Newville, P. Livins, Y. Yacoby, J. J. Rehr, E. A. Stern, Phys. Rev. B. 1993, 47, 14126- 14 13 1 . [37] S. I. Zabinsky, J. J. Rehr, A. Ankudinov, R. C. Albers, M. J. Eller, Phys. Rev. B 1995, 52, 2995-3009. [38] M. Newville, R. Ravel, D. Haskel, J. J. Rehr, E. A. Stem, Y. Yacoby, Physica B 1995, 208 & 209, 154-156.

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[39] D. M. Pfund, J. G. Darab, J. L. Fulton, Y. Ma, J. Phys. Chem. 1994, 98, 13102-13107. [40] J. L. Fulton, D. M. Pfund, Y. Ma, Rev. Sci. Instrum., 1996, 67(CD-ROM Issue), 1-5. [41] S . L. Wallen, B. J. Palmer, D. M. Pfund, J. L. Fulton, M. Newville, Y. Ma, E. A. Stem, J. Phys. Chem. A 1997, 101, 9632-9640. [42] S. L. Wallen, D. M. Pfund, J. L. Fulton, C. R. Yonker, M. Newville, Y. Ma, Rev. Sci. Instrum. 1996, 67, 2843-2845, [43] J. L. Fulton, D. M. Pfund, S. L. Wallen, M. Newville, E. A. Stem, Y. Ma, J. Chem. Phys. 1996, 105, 2161-2166. [44] S . L. Wallen, B. J. Palmer, J. L. Fulton, J. Chem. Phys. 1998, 108, 4039-4046. [45] T. Yamaguchi, M. Yamagami, H. Ohzono, H. Wakita, K. Yamanaka, Chem. Phys. Lett. 1996, 252, 317-321. [46] 0. Glatter, 0. Kratky, Small Angle X-ray Scattering, Academic Press, New York, 1982. [47] L. A. Feigin, D. I. Svergun, Structure Analysis by Small Angle X-ray and Neutron Scattering, Plenum Press, New York, 1987. [48] L. J. Magid, in Nonionic Sur$actants, M. J. Schick (Ed.), Marcel Dekker, New York, 1987. [49] J. Eastoe, W. K. Young, B. H. Robinson, D. C. Steytler, J. Chem. SOC. Faraday Trans. 1990, 86, 2883-2889. [50] E. W. Kaler, J. F. Billman, J. L. Fulton, R. D. Smith, J. Phys. Chem. 1991, 95, 458-462. [51] J. Eastoe, D. C. Steytler, B. H. Robinson, R. K. Heenan, J. Chem. SOC. Faraday Trans. 1994, 90, 3121-3127. [52] D. M. Pfund, J. L. Fulton, in Proceedings of the Third International Symposium on Supercritical Fluids, M. Perrut (Ed.), Institut National Polytechnique de Lorraine (Vandoeuvre), Strasbourg, France, 1994, p. 235-240. [53] J. L. Fulton, D. M. Pfund, J. B. McClain, T. J. Romack, E. E. Maury, J. R. Combes, E. T. Samulski, J. M. DeSimone, M. Capel, Lungmuir 1995, 11, 4241-4249. [54] D. Chillura-Martino, R. Triolo, J. B. McClain, J. R. Combes, D. E. Betts, D. A. Canelas, J. M. DeSimone, E. T. Samulski, H.D. Cochran, J. D. Londono, G. D. Wignall, J. Mol. Struct. 1996, 383, 3-10. [55] J. Eastoe, Z. Bayazit, S. Martel, D. C. Steytler, R. K. Heenan, Langmuir 1996, 12, 14231424. [56] J. B. McClain, D. E. Betts, D. A. Canelas, E. T. Samulski, J. M. DeSimone, J. D. Londono, H. D. Cochran, G. D. Wignall, D. Chillura-Martino, R. Triolo, Science 1996, 274, 2049-205 1. [57] J. M. DeSimone, E. E. Maury, Y. Z. Menceloglu, J. B. McClain, T. J. Romack, J. R. Combes, Science 1994, 265, 356-359. [58] K. A. Shaffer, J. M. DeSimone, Trend in Polym. Sci. 1995, 3, 146-153. [59] Z. Guan, J. M. DeSimone, Macromolecules 1994, 27, 5527-5532. [60] E. E. Maury, H. J. Batten, S. K. Killian, Y. Z. Menceloglu, J. R. Combes, J. M. DeSimone, Polym. Prepr. (Am. Chem. Society, Div. Polym. Chem.) 1993, 43, 664. [61] K. Harrison, J. Goveas, K. P. Johnston, E. A. O’Rear, Lungmuir 1994, 10, 3536-3541. [62] J. B. McClain, D. Londono, J. R. Combes, T. J. Romack, D. P. Canelas, D. E. Betts, G. D. Wignall, E. T. Samulski, J. M. DeSimone, J. Am. Chem. SOC. 1996, 118, 917-18. [63] J. D. Londono, R. Dharmapurikar, H. D. Cocharn, G. D. Wignall, J. B. McClain, D. E. Betts, D. A. Canelas, J. M. DeSimone, E. T. Samulski, D. Chillura-Martino, R. Triolo, J. Appl. Crystallogr. 1997, 30, 690-695. [64] J. D. Londono, V. M. Shah, G. D. Wignall, H. D. Cochran, P. R. Bienkowski, J. Chem. Phys. 1993, 99, 466-470. [65] D. M. Pfund, T. S. Zemanian, J. C. Linehan, J. L. Fulton, C. R. Yonker, J. Phys Chem. 1994, 98, 11846- 11857. [66] T. Morita, K. Nishikawa, M. Takematsu, H. Iida, S. Furutaka, J. Phys. Chem. B 1997, 101, 7158-7162. [67] K. Nishikawa, T. Morita, J. Phys. Chem. B 1997, 101, 1413-1418.

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[68] M. Bellissent-Funel, T. Tassaing, H. Zhao, D. Beysens, B. Guillot, Y. Guissani, J. Chem. Phys. 1997, 107, 2942-2949. [69] P. Postorino, R. H. Tromp, M. Ricci, A. K. Soper, G. W. Neilson, Nature 1993, 366, 668. [70] A. K. Soper, F. Bruni, M. Ricci, J. Chem. Phys. 1997, 106, 247-254. [71]M. M. Hoffmann, M. S. Conradi, J. Am. Chem. SOC, 1997, 229, 3811-3817.

Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

4 Reactions in SCF 4.1 Synthesis of Inorganic Solids JOSEPHW. KOLISand MICHAEL B. KORZENSKI

Supercritical fluids (SCFs) are fascinating media for the synthesis of inorganic compounds [l]. This is due in part to the tremendous versatility of these fluids, especially in providing access to unusual and kinetically stabilized solid phases. It should not be assumed, however, that either the techniques or the concepts are particularly new. In fact, most of the current techniques are really just derivatives of the methods developed in the last 50-100 years. There is broadening of interest in this field as inorganic chemists continue to search for new synthetic routes to unusual compounds. Several outstanding earlier reviews summarized the field to date and inspired new workers to enter the field [2-41. However the most recent of these reviews is now over 12 years old and substantial new results justify a more current review. This review summarizes the current state of the field, with the major emphasis on results obtained since the mid 1980s. In addition, this is a techniquedriven field, so some detail is provided to aid workers in the design of new experiments where appropriate. Given the constraints of space, attention is generally focused on compounds prepared in solutions nominally above the critical temperature of the solvent. Thus less attention is given to the large body of work regarding reactions done in water below 200 “C. This is a rapidly moving field and deserves its own review. It is well understood that the ideal critical point of a solvent in a real solution has little meaning, and a gradual onset of critical properties with increasing temperature is normally observed, rather than a dramatic change in behavior at the magic critical point. In aqueous phases, the term “hydrothermal” applies to any reaction done in water above lOO”C, whereas reactions done above the critical point of 374°C are said to be supercritical. An older term, favored mostly by geochemists, for aqueous fluids above the critical point, is “pneumatolytic”, but this has not received general usage in the mainstream chemical literature.

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4 Reactions in SCF

4.1.1 Historical Summary The first deliberate man-made hydrothermal chemical reaction was probably performed by Bunsen in the 1830s when he investigated the growth of barium carbonate from superheated water in a thick walced glass tube [ 5 ] . This work was followed by that of desenarmont, who investigated the synthesis of various crystalline solids in superheated water sealed in glass tubes and counterpressured in welded rifle barrels [6].The early work was quite productive, and over 80 types of known minerals were prepared from various recipes in water above its boiling point. A classic article by Morey and Niggli not only establishes the theoretical underpinnings of synthesis in superheated water, but also provides a critical description of then-available techniques, as well as abstracts of all inorganic synthesis in supercritical water to that point [7]. This work can be regarded as the cornerstone for modem hydrothermal chemistry research. Further systematic work by Morey and others mostly concerned investigations of the chemistry of known minerals, particularly quartz, in hydrothermal environments [8]. Most of this early work was located securely in the realm of geochemistry, and workers were able to grow a wide variety of known minerals, including oxides, silicates, phosphates and sulfides, under hydrothermal conditions [9]. During this period, two important technical achievements occurred. One was the development of the Morey vessel, which allows hydrothermal fluids to be contained in autoclaves lined with an inert metal, such as silver, gold or platinum, at pressures up to 800 bar [8]. The other was the invention of the Bridgeman seal by the great Percy Bridgeman, which allowed solutions to be contained at much higher pressures, up to 7 kbar [lo]. Both of these inventions are used in much the same form today. The major turning point in this field was the explosion of work in microwave communications during and after World War 11, which led to unprecedented demands for ultrapure single crystals of a-quartz. Previously the radio market could be served by less pure naturally occurring single crystals. However, most of the highest quality quartz crystals came from Brazil, and the British blockage of German ports, as well as German submarine activity off the American and British coasts, made this a tenuous supply for both sides during the war. In addition, the growing demands for large amounts of materials of increasing purity made it unlikely that the natural supply would be sufficient after the war. Postwar microwave applications demanded quartz of very low dielectric loss, and it is rare to find naturally occurring minerals of any type with such low levels of contaminants. All this led to substantial efforts to grow large single crystals of a-quartz in the laboratory [Ill. The problem with conventional melt or flux crystal growth methods is that a-quartz is only stable below 5 8 0 ° C and silicate melts are so viscous that they form glasses long before crystallization occurs. Thus methodology was developed for controlled crystal growth of a-quartz under hydrothermal conditions. The first attempt at systematic crystal growth using hydrothermal techniques was by Nacken in Germany [12]. However, shortly after WW 11, workers at Bell

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215

Labs were able to make dramatic improvements in the methodology and consistently grow single crystals up to one kilogram in size [13]. currently there are several dozen companies throughout the world that produce a-quartz commercially, and over 500000 kg are produced each year. The successful commercialization of quartz synthesis encouraged several groups to examine the growih of other compounds under hydrothermal conditions, and several other types of crystals were subsequently prepared commercially using hydrothermal techniques, including A1PO4, KTiOP04 and emeralds. Also, groups led by Laudise at Bell Labs [14-181, and Rabenau at Phillips Electronics [ 19,201 began more wide ranging investigations of other solids. However, in most of these cases, the emphasis was on the crystal growth of known materials for electronic applications, and exploratory inorganic synthesis was not a primary goal. l b o other groups deserve mention for their work during this period, namely those of Somiya in Japan [21-221 and the Russian group, led by Lobachev and Demianets [23,24]. Both groups did beautiful and extensive work on a wide variety of mostly naturally occurring minerals, particularly oxides. However, since neither group published extensively in western journals they did not get much of the credit that they deserved. In addition, until recently, much of the Russian work was not easily accessible to western workers. Fortunately, reviews of many of the older papers from the Institute of Crystallography at the Academy of Sciences in Moscow have been collected in two sets of English translations and are a mine of information [23,24]. Even a casual perusal of these papers makes it clear that the lack of single crystal data available to these workers limited their ability to characterize products, so a great many new and undoubtedly interesting phases lay undiscovered. During this postwar period, geochemists were by no means inactive, and some elegant investigations on the transport and deposition of important mineral phases from geothermal fluids were reported. In particular, these workers demonstrated that geothermal brines, namely aqueous fluids containing high concentrations of mineralizers such as halides, carbonates or hydroxides could solubilize and transport normally intractable compounds [9]. Thus solids such as PbS and HgS have solubilities up to several millimolar in hydrothermal brines, sufficient for reactivity and transport [25]. Again, this work focused mainly on naturally occurring phases, and the isolation of any new phases was considered a sideshow. The detailed equilibrium behavior of minerals in hydrothermal fluids has been summarized in several monographs [26-281. Another historically important area involving inorganic chemistry in hydrothermal fluids is related to the synthesis of zeolites and other microporous solids. It was recognized early on that naturally occurring zeolites (or boiling stones) are the result of natural hydrothermal activity. Thus, a fairly intense effort to prepare new zeolites from hydrothermal solutions in the laboratory was mounted, particularly in the petrochemical industry. Much of the early work is summarized in an excellent monograph by Barrer [29]. Most of the useful microporous solids are prepared at relatively low temperatures

216

4 Reactions. in SCF

( 4 8 0 ° C ) and hence fall out of the range of this article, but some aspects of this work are discussed where appropriate. In the mid 1980s two seminal review articles appeared in mainstream chemical journals highlighting the enormous potential of SCFs in inorganic synthesis [2,3]. Both articles appeared at a time when materials chemistry was beginning to expand rapidly as a new field, and the ready availability of X-ray diffraction made structural characterization of new compounds a relatively simple matter. Thus as chemists began to seek new preparative routes to novel solid materials, the use of SCFs as an exploratory synthetic medium became self evident. Indeed, the use of SCFs as solvents for the synthesis of new compounds has undergone a rapid expansion since the late 1970s, and shows no signs of abating.

4.1.2 Experimental Techniques Most technology in this field was developed many years ago. To the knowledge of the authors, no significant new technical developments have been added by the more recent inorganic entrants. However, the rapid expansion and widespread use of any technique inevitably leads to useful innovations, and this process is now underway in the mainstream chemical community. The primary experimental challenge in this field is obviously the containment of pressure. The choice of technique is subject to a number of parameters, mostly temperature and pressure. Ordinary thick-walled borosilicate glass or quartz tubing has the advantage of simplicity and low cost, but cannot contain high pressures safely. Some workers claim that properly constructed quartz vessels can contain pressures up to 60 bar [30], but this is clearly not a desirable approach. Ammonia is routinely contained at room temperature (10 bar) in quartz tubing without incident, but that pressure is never intentionally exceeded without other containment. It should also be noted that quartz or glass cannot contain hydrothermal fluids with even small concentrations of OH- or F, as either of these attack silica at elevated temperatures. Because both of these molecules turn out to be important mineralizers for inorganic synthesis, this is a real limitation. The next level of containment is probably the fluoropolymer-lined digestion bomb. These autoclaves are sold by a number of vendors and are inexpensive (ca. $300 each), easy to handle, and require no special auxiliary equipment [31]. They are the vessels of choice for a number of workers doing some remarkable chemistry [32]. They also have the important advantage that they are inert to aqueous base and fluorides, making them a workhorse for the zeolite industry. However, they have the limitation that the fluoropolymer liner will defonn and weaken above about 240°C. Also, the autoclaves have simple screw top seats meaning they can rarely contain pressures exceeding 140 bar. Thus they cannot contain true supercritical aqueous fluids. Nonetheless their convenience makes them the vessel of choice for the large amount of hydrothermal work done at 200°C and below.

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To contain fluids at higher temperatures and pressures (>3OO0Cand 200 bar) metal seals are required [33-351. There are three basic designs available. The so-called Morey vessel is probably the oldest of these, and was designed by George Morey early in the twentieth century as a modification of an even older design. It consists of a thick walled metal vessel with a flat lip on which a soft metal cap is $aced. This cap is backed by a sturdy plunger held down by a screw cap. Thus the ultimate pressure containment is determined by the brute force with which the screw cap can be tightened down. During heating, the whole apparatus is placed in the furnace. The ultimate temperature is determined by the material of construction. Typical stainless steel can withstand temperatures around 450-500 "C before oxidation and creep begin to occur. Other readily available but more expensive alloys such as Stellite, Inconel or Rene 41 can withstand temperatures up to 800 "C. An important advantage of Morey vessels is that they can be lined relatively easily with an inert metal such as silver or platinum. Thus, if the capping disk is also an inert metal, the fluid is completely contained in the inert vessel. This attribute has made them important containers for the synthesis of electronic crystals where the purity demands are extremely high. However, these vessels also suffer from several disadvantages. They are not readily available from common commercial vendors, and generally must be custom designed. Also they require some nontrivial auxiliary equipment, such as large(!) vises and wrenches, and ready access to a metal lathe to rework the metal lip after runs. Thus they are not recommended for the neophyte. Perhaps most importantly, the ultimate pressure is determined by the ability to tighten the cap onto the plunger. It is the authors' experience that this provides an upper pressure limitation of about 800 bar, which is sufficient for most aqueous solution work between 350 and 450 "C, but insufficient for more specialized investigations. The next type of containment is the Bridgeman seal developed in the 1920s by Percy Bridgeman [8]. There have been considerable modifications of this design up to the present day but the basic principle remains unchanged (Figure 4.1-1). The seal consists of a plunger that makes an original seat using a screw driven assembly. Behind the plunger is a a deformable metal wedge sandwiched between the plunger and a strong backing. As the internal pressure of the fluid increases during heating the plunger is driven back up into the backing, squeezing the deformable metal wedge into the walls of the vessel, making the seal. Thus the seal is called self-energized, in that the seat becomes more secure as the pressure increases. This is a versatile and useful design that works remarkably well and is used commonly today in various adaptations in many commercial designs [36,37]. The design has a number of advantages. It can contain virtually any pressure that the autoclave body can (up to 7 kbar in extreme cases). It is also reliable and reusable many times. One disadvantage of this design is that there is no way to line the seal with an inert metal, so a truly clean and fully contained experiment is usually not possible. However, commercial designs are available wherein the body is lined with an inert metal and the plunger is covered with an inert metal disk [37]. Thus, even though the solution will flow around the disk to the wedge seal, the area is

218

4 Reactions in SCF adapter to hlgh pressure gauge

T

expandable wedge

2 1I 2

1q

Figure-4.1-1 Autoclave with Bridgeman self-energized seal. The plunger absorbs the pressure generated within the autoclave. It is forced against the expandable wedge which is -usually made of a soft deformable metal such as nickel, copper or silver. This wedge is held immobile by a strong backing. Thus it expands against the side of the vessel creating a tight seal. The tightness of the seal only increases as the pressure in the autoclave increases, justifying the term "self-energized".

sufficiently isolated to inhibit contamination of the bulk fluid with metal from the autoclave. The final major seal design is the so-called Tuttle cold seal named after its designer (Figure 4.1-2). This design was developed in the late 1940s for geochemists investigating solubility equilibria in hydrothermal fluids [38-391. It is very simple and consists of a coned plunger with a strong backing. The plunger is driven into a coned female receiver which is cut at a slightly smaller

llh -connection

to valve

1 conical seal screw cap

- Rene 41

lo.

113 1 4 ' 4

body

Figure 4.1-2 Tuttle cold-seal autoclave. The conical seal is screwed into place by the screw cap, creating a line seal with the body.

4.1 Synthesis of Inorganic Solids

219

angle than the cone of the plunger by a threaded nut. A line seal is made which is reliable and can be broken and remade many times without damage. The autoclave is usually long and thin so that the area containing the seat is not actually inserted into the furnace. Thus, the seal area is somewhat cooler than the body of the vessel, which prevents the metal around the seat from distorting under pressure. This-is the origin of the name “cold seal” which is somewhat of a misnomer as the occasional beginner will learn. Like the Bridgeman seal, the seat can be cooled even further if desired by wrapping the area around the seat with copper tubing connected with circulating water, but this is rarely necessary except for extreme conditions. These designs are readily available commercially, and are often machined from advanced alloys such as Rene 4:1, which allows them to withstand pressures exceeding 3 kbar at 700°C. Their chief advantages are simplicity and reliability of use, their commercial availability [40] and their toughness under relatively extreme conditions. Their primary disadvantage is that they are impossible to line with an inert metal. Other more specialized and extreme designs for more rigorous conditions are available in the literature [9,34-351, but they are beyond the needs of the average investigator. The pressures are normally generated autogenously, meaning the ultimate pressure is generated by the internal expansion of the fluid upon heating, rather than any external force. The fluid is placed in the vessel at a low temperature with a degree of fill such that the fluid will expand to fill the chamber at the reaction temperature. Thus the key parameter in these reactions, regarding pressure, is the degree of fill. The ultimate pressure will be determined by the initial room temperature fill, and can be calculated at the desired reaction temperature using P-V-T tables (Figure 4.1-3) [41]. It is typically found that fills between 50 % and 75 % of the available volume at ambient conditions generate a fluid which is sufficiently dense to provide good solubility, but keeps the pressure below the container limits. Fluids at high temperatures and pressures are often quite corrosive, so considerable thought must be given to isolating the reaction fluid from the autoclave. Unfortunately, the autoclave type most amenable to lining with a noble metal is the Morey vessel, which has several other inherent limitations. The most common solution to this problem is the so-called floating liner. In this case the reaction solution is sealed in an inert ampoule at ambient conditions. The ampoule cannot normally contain the high internal pressures at the reaction temperature, so appropriate counter-pressure is generated by an inert fluid placed in the autoclave. As long as the pressure inside the ampoule never exceeds that of the surrounding fluid in the autoclave, the ampoule will not burst. The counterpressure fluid can be generally anything that is inert to the autoclave walls, including water, C02, Ar or N2. The pressure of the surrounding fluid can be generated using an air-driven gas or liquid-booster pump available from local equipment dealers, or an inexpensive hand pump [42]. The ampoule can be made of a variety of materials, most commonly quartz or noble metals such as silver or gold. Quartz has the advantage of low cost and easy sealing using a hot gas flame. It is rigid so the internal pres-

220

4 Reactions in SCF x 102

0

200

400 600 TEMPERATURE r C )

000

1ooo

Figure 4.1-3 Diagram of pressure as a function of temperature for pure water using the filling factor (degree of fill) as a parameter as determined by Kennedy [41]. Each line represents a degree of fill of the chamber by water under ambient conditions. As more water is added to the chamber, the pressure increases faster with temperature. v p i c a l fills for growth of inorganic single crystals is between 65 and 8 5 % . The dotted line below the critical point is the equilibrium line of vapor and liquid.

sure is determined by the degree of fill. It is reactive toward even slightly basic water and fluoride, but is inert to most other solutions, including strong acids, chlorides and sulfide solutions up to 600°C. If quartz is insufficient, ampoules made of inert metal tubing can be used (usually silver, gold or platinum). The tubing can be purchased at market price and cut into small lengths (usually 3-5 cm). The ends are crimped and arc welded using a small inert gas electroarc welder. The tubing is usually thin enough to collapse under the counterpressure. Thus the reaction pressure is generated by the counterpressure fluid, and can be read directly from the gauge on the autoclave. The advantage of these systems is that the solution is contained in a truly inert environment. The disadvantages are the expense of the precious metal tubing and the need for a specialized arc welder, the operation of which requires some degree of craftsmanship. In general, we have found that small quartz ampoules (7 mm outer diameter, 8-10 cm long) and silver ampoules (6.4 mm o.d., 3.8 cm long) are sufficient for exploratory inorganic synthesis, and a number of commercial autoclaves can be chosen such that up to 10 ampoules can be placed in each one during each particular run, allowing for the rapid screening of reaction conditions. It should be noted that the concept of the floating liner can be scaled for large commercial crystal growth. A platinum can is fabricated and welded

4.1 Synthesis of Inorganic Solids

221

shut after all ingredients are added. The internal pressure is calculated very carefully and the external counterpressure is matched closely to prevent any collapse or expansion of the can during growth [43]. It should be noted that this is the frontier of technology and craftsmanship in this field. Such work is obviously beyond the budget and experience of the average experimenter and usually restricted to corhnercial applications. A word about connections is in order. All joints must be considered potential leaks at high temperatures and pressures [44]. Swage-style fittings are excellent for low pressure applications, but are rarely adequate above 400-700 bar. The most convenient high pressure connections are the coned and threaded type. This style was designed in the 1920s and originally manufactured by Autoclave Engineering for the Army. They are commercially available in standard sizes from a number of vendors [36,42]. They are simple, inexpensive, reliable, versatile, and interchangeable, and can be reused many times. They can be fitted to a number of types of valves, and can handle pressures up to 7,000 bar. An interesting side use for this style is as an autoclave seal. Several vendors sell medium pressure connectors in large sizes (up to 2.5 cm) which are rated to 2,000 bar [42]. This is sufficient for most aqueous chemistry below 500 "C. Thus simple, inexpensive autoclaves for hydrothermal experiments can be prepared using two endcaps and a nipple of any length, or an endcap, nipple adapter and valve (Figure 4.1-4). The components for these designs can be purchased for less than $300. If used as containment vessels for quartz tubes, they can provide a low cost entry into exploratory synthesis in SCFs. To illustrate some of the procedures for synthesis of inorganic compounds, several detailed preparations are included. The following is a detailed experimental procedure for the hydrothermal synthesis of tetrahedrite. 12 CU + 2 SbZS3

+ 7 s H20/375 "C/240bar c u 12Sb4S 13

(4.1-1)

However, these experimental procedures can be used for any reactions run under the parameters of supercritical water. Hydrothermal synthesis of tetrahedrite Copper powder (0.0520 g, 0.8 18 mmol), antimony(II1) sulfide (0.0442 g, 0.130 mmol), elemental sulfur (0.0150 g, 0,467 mmol) and calcium chloride (0.0300 g, 0.270 mmol) are added to a quartz ampoule (7 mm o.d., 5 mm i.d.) approximately 9 cm in length inside an argon-filled glove box. The ampoule is then filled with distilled water until approximately 35-40 % full and fitted to a vacuum line using an Ace-Thred tube adapter (model 5027, #7). A Dewar filled with liqN2 is placed on a lab jack and slowly raised until the water in the bottom tip of the ampoule begins to freeze. The stopcock to the vacuum line is then slowly opened to allow for the evacuation of the ampoule. The Dewar is gradually raised until all the water is frozen. Once the solution is completely frozen, the ampoule is sealed at a position

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4 Reactions in SCF

High pressure valve

I Adapter (9116' to: 114') (Coned and threaded)

Tubing

GlandNut

-

Figure 4.1-4 (a) HiP autoclave with coned and threaded fittings. Use of this apparatus in an actual procedure is detailed in the text. The end cap and adapter are attached by standard cone and thread joints. The reactor body is medium pressure tubing purchased from HiP Co [42]. (b) Detail of standard cone-type seal.

2/3 higher than the water level using an oxygen torch. A hot flame is used to soften the quartz in a triangular three-point fashion, avoiding concave portions in the sealed end of the ampoule. These areas tend to be thinner and therefore prone to breakage during reaction. Once the three sides are fused together, the flame is directed at each of the three comers for about 5-10 s while the tube is pulled downward with a pair of stainless steel (s.s.) tweezers. Once the sealed end is cool to the touch, the ampoule is placed in a beaker filled with cold tap water to thaw out. The ampoule is then placed inside a 316 s.s. cone and thread fitted HIP medium pressure 9/16 in (1.4 cm) i.d. tubing with screw-on end caps. The autoclave is counter-pressured with water at a 7 0 % fill mark. The end caps are then screwed on and tightened with an adjustable wrench. The autoclave is placed in a tube furnace set at 375°C for 3-5 days. Upon completion

4.1 Synthesis of Inorganic Solids

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of the reaction, the autoclave is vented inside a hood and opened to remove the quartz tubes. Caution Safety goggles and gloves should be worn throughout this step in case any residual pressure builds up during the reaction. These reactions have the ability to produce poisonous gases such as H2S as byproducts, so opening of ampoules and filtering procedures should be done in a well-ventilated hood. The ampoule is frozen once again and scored in the middle using a glass cutter. Using thick gloves or thickly folded paper towels the ampoules are opened and placed in a schlenk flask to thaw. The products are air stable and can be isolated and washed in a normal manner. In cases where the Parr 4740 autoclave is used as the pressure containment device, multiple quartz reaction ampoules can be used in each run. These autoclaves have the advantage that up to eight quartz ampoules can be run at once. In this case, pressurization can be most conveniently accomplished by hooking up the autoclave directly to a new cylinder of inert gas. Most commercial gas suppliers supply gases in cylinders which are pressurized to 150-170 bar and the Parr autoclave can easily contain these pressures. Direct inlet adapters can be purchased from standard compressed gas dealers. Swage type and NPT connectors are suitable for these connections but any tubing must be medium walled copper or steel. Caution Plastic tubing or thin walled metal, such as flexible steel tubing, is not sufficient to contain these pressures. Upon heating to 375 "C, the pressure in the autoclave will increase to ca. 240 bar, which is well within the operating capabilities of the autoclave, but SUEciently high to contain the pressure within the quartz tubes at the fill levels described here. At the conclusion of the reaction, the autoclave is vented in a hood and opened in the normal fashion, and the quartz tubes examined for reactivity and worked up as described above. Synthesis in SCNHJ Reactions involving the use of anhydrous liquid ammonia, as in the isolation of [Mn(NH&$b6S lo]. can employ techniques as described above. The reagents are loaded into fused silica ampoules in an argon filled atmosphere dry box in the desired elemental ratios. The reaction tubes are then evacuated on a vacuum line tom) and NH3 distilled into the quartz ampoules until the liquid reached a level approximately 40% of the total volume of the ampoule. The ampoules are sealed after freezing the solvent and thawed under warm water. The reactions tubes are placed inside a Parr autoclave as described above and counterpressured with approximately 170 bar of argon. The autoclaves are typically heated at 170°C for 3 days, followed by slow cooling to room temperature under a flow of nitrogen gas. After visual inspection of the ampoules using an optical microscope, the solvent was frozen down inside the ampoules using liquid nitrogen and subsequently

224

4 Reactions in SCF

opened using a glass cutter. 'i'he ampoules are immediately placed inside an evacuated Schlenk flask, and the ammonia allowed to evaporate under argon. Wne of the most important aspects of the use of SCFs as a medium tor inorganic synthesis is the mineralizer. It should be berne in mind that the fluids themselves generally do not solubilize reactants yery well compared to the liquid state, because the density of the fluid is lower than the ambient liquid [45,46]. Thus, to increase the solubility of reagents to levels sufficient for reactivity in a reasonable time, a mineralizer is usually introduced. The mineralizer is a small soluble molecule, usually ionic, which attacks the bulk starting material. This attack generates discrete soluble molecules which also have some solubility in the fluid. It is important to note that the concentration of this intermediate does not have to be high. Usually one or two percent is sufficient for observable reactivity because of the kinetic enhancement of the lowviscosity fluid. Transport of the small mobile species and precipitation of the product leads to dissolution of more reactant. This low steady state concentration of intermediate is credited for the well formed crystals which are often observed in SCFs. A good illustration of this behavior is the role of hydroxide in quartz synthesis. The solubility of quartz or silica in scHzO is so low as to lead to virtually no reactivity under any conditions. However, the presence of even a small amount of hydroxide leads to formation of small silicate anions in concentrations of 1-3 % [47].

(4.1-2) These anions can migrate through the fluid to a growing crystal site. Through careful control of temperature gradients, a supersaturation condition is established at the growing crystal site and the above equilibrium is reversed, with deposition of Si02 on the crystal face (Figure 4.1-5). It is believed that the identity and concentration of the mineralizer is the single most important factor in inorganic synthesis in supercritical fluids. Generally the most effective mineralizers are small nucleophiles such as OH-, halides, sulfides and polychalcogenides, and carbonates and bicarbonates. Normally these are added as alkali metal salts but often ammonium salts are used as well. It is our qualitative experience that ammonium salts, when feasible, are superior to alkali metal cations. A dramatic example of the systematic investigation of the mineralizer for optimum reactivity is the investigation of the hydrothermal crystal growth of potassium titanyl phosphate for nonlinear optical applications. Previous results led only to crystal growth under conditions which were too inefficient and expensive for commercial development of the material. However, a systematic investigation of a wide variety of mineralizers [48,49] has led to the discovery that, with the appropriate amount of KH2PO4 and KPO3 as mineralizer, high quality KTiOP04 crystals can be grown under relatively mild hydrothermal conditions. This investigation paved the way for the commercialization of hydrothermal crystal growth of this important material.

225

4.1 Synthesis of Inorganic Solids

____---CLWGER

*‘‘THRUST

--SEAL

Figure 4.1-5 Modified Bridgeman seal autoclave showing the temperature gradient crystal growth onto seed crystals [17]. The nutrient is held at a higher temperature than the seed crystals, thus leading to supersaturation conditions in the seed region. The transport is facilitated by the large convection currents in the low viscosity fluid. The baffle is to prevent turbulence of the nutrient and ensure complete saturation in the nutrient zone.

3cum(

WASHER

RING

--- NUTRIENT

CHAMBER

As another example, Kuznetsov and co-workers have studied the mineralization of the very refractory Group IV oxides [50]. They found that they could get excellent transport and reactivity under relatively mild conditions, and that ammonium fluoride provides the most effective transport. A variety of interesting new solids have been prepared this way. These examples provide an excellent illustration of the value of the mineralizer in this chemistry.

226

4 Reactions in SCF

4.1.3 Hydrothermal Chemistry By far the supercrifical solvent of most interest for inorganic synthesis is water. It has critical values of 374" and 221 bar. It is well known that the dielectric constant of water decreases dramatically as it approaches the critical point. However the fluid remains sufficiently polar that most metal oxyanions are soluble to some degree (> 1 %). Thus aqueous base is often the first choice of solvent for most metal oxide-based preparations. Supercritical water is quite reactive towards most types of steel, so some care must be taken in autoclave design. A major driving force for the investigation of water as a hydrothermal solvent is its role in natural mineral synthesis. A large number of the well-formed mineral crystals, including many of the silicates, aluminates, phosphates, carbonates, oxides and sulfosalts were probably formed in some form of hydrothermal fluid. As a result, many geochemists have devoted an enormous amount of time and effort to the chemistry of metal complexes in SCH~O [9].

4.1.3.1

Metal Oxides

The crystal growth of a-quartz is probably the best studied chemical reaction under hydrothermal conditions. It has been inordinately successful work, and over half a million kilograms of high quality single crystals of quartz are prepared each year. Due to the explosion of popularity of cellular phones, the demand for quartz is growing. This work has been the subject of a number of excellent reviews [14-181. Following on the heels of the success of quartz, several other metal oxides have been prepared as well-formed single crystals from hydrothermal solution, including A1203 and ZnO. Most of the oxides are amphoteric and thus can be prepared using recipes similar to that for quartz. These reactions typically use hydroxide as a mineralizer, and temperatures around 400"C, with a temperature gradient of 40-80°C and a degree of fill between 60 and 80%. These fills generate pressures of about 700-800 bar. The solubility of most of these amphoteric metal oxides is a linear function of concentration of mineralizer (Figure 4.1-6) [47]. One advantage of quartz synthesis over other metal oxides is that, in the presence of NaOH, the soluble silica reacts with the iron of the reactor walls to form acmite, NaFeSi04 and related compounds, which passivate the autoclave walls. For preparation of other oxides for electronic applications, reactors lined with noble metals are often required, which increases the cost significantly. Most of these syntheses have been reviewed in detail [14-181. Due to their interesting magnetic and electronic properties, the hydrothermal chemistry of transition metal oxides has been studied to a considerable extent. It appears that iron oxides are especially amenable to hydrothermal synthesis, and their chemistry is quite rich and promising. The simplest iron oxides such as magnetite and hematite can be prepared [51], as well as substantially more

4.1 Synthesis of Inorganic Solids

0

227

2 4 6 6 l O 1 2 H S Y 4 0 2 4 1

YOLALITY (OH)

Figure 4.1-6 Solubility data as a function of mineralizer concentration in hydrothermal systems [ 171.

complex iron oxides. A specific area of interest is the synthesis of barium ferrites which have applications as memory devices [52-541. In addition, a number of other iron oxides, such as AgFeOp [54] and several interesting alkali iron titanium bronzes [55-571 have been isolated from supercritical fluids. Driven by the same considerations, a number of manganese oxides have also been synthesized hydrothermally [58]. Another related group of compounds whose hydrothermal synthesis has been studied extensively is the garnets. These compounds have had a wide variety of optical and magnetic applications and are normally made from fluxes, but several hydrothermal routes have also been developed. A variety of garnets can be synthesized from aqueous base at temperatures above 400 "C [59,60].

228 Y2O3

4 Reactions in SCF

+ A1203 450 "C/6MNa2C03/H20(800bar)>

Y3Al5OI2(YAG)

(4.1-3)

The hydrothermal synthesis of these and other related magnetic oxides has been reviewed in detail [61]. Oxides of the early d-block metals can ilso be grown hydrothermally, and this work is beginning to attract more atkntion due to the importance of these materials in a variety of technical applications [62]. In contrast to the later transition metals, the crystals of Group IV oxides are often best grown using fluoride as the mineralizer [63,64]. A number of these systems have been studied in detail and some elegant theoretical models have been developed for their phase stability €651. One of the more intriguing applications of hydrothermal chemistry is the laboratory synthesis of gems. It is well understood that most minerals commonly recognized as gems were formed from natural geothermal fluids. Thus it is not unusual that such materials could be prepared in the laboratory under presumably similar conditions. Probably the first report of an actual designed hydrothermal synthesis of a gem was the report of the synthesis of ruby in aqueous base above 425°C [66]. Although there is no doubt that high quality crystals of rubies and sapphires can be prepared in this fashion, the much lower-cost methods of melting alumina powders in hot gas flames (Verneuil process [67]) makes it unlikely that the hydrothermal methods will ever be used for any but the most specialized applications for these gems. The most obvious use of hydrothermal fluids for cultured gem synthesis is for the quartz-based gems such as topaz, citrine and amethyst. However, the lower cost and relative abundance of natural materials makes their wholesale hydrothermal synthesis somewhat uneconomical. One gem that has been prepared commercially by hydrothermal methods is emerald, along with its relative, beryl. It has long been speculated that the famous Chatham emeralds synthesized by the Chatham family in San Francisco are made hydrothermally. Although the family is understandably reluctant to reveal its process, which is unprotected by patent, careful investigation suggests that they are probably prepared by a flux method rather than a hydrothermal one [66]. In contrast, commercial emeralds were prepared hydrothermally first by Lechleitner in about 1960 [67], and then by Linde Corp in 1965 [68], using aqueous media with HC1 as the mineralizer:

-

450 "C HCVH20

Be3(AlrCr)2Si601*

Although an interesting field, the cost of the technique, coupled with the decreased perceived value of synthetic gems as compared to real ones, rarely makes hydrothermal synthesis of gems economically feasible [69]. Perhaps the most dramatic example of hydrothermal synthesis of gems is the recent synthesis of diamond from graphite in scH20 at 800°C and 1700 bar

4.1 Synthesis of Inorganic Solids

229

[70]. Although the growth is somewhat slow, and far from practical at this point, it stands as the state of the art hydrothermal synthesis [71]. 4.1.3.2 Phosphates and Silicates A logical extension of hydrothermal chemistry from metal oxides is the use of simple building blocks, such as phosphates and silicates. Phosphates are especially amenable to hydrothermal synthesis. They can be mineralized under both acidic and basic conditions and the polyanionic tetrahedral building block can lead to an almost infinite number of coordination environments. Even the most subtle change in reaction conditions can lead to formation of different products. Thus a tremendous amount of chemistry over the last several decades has focused on metal phosphates. A vast majority of this work has taken place under conditions somewhat below supercritical values, usually around 200°C. A wide variety of truly spectacular metal phosphates have been prepared under these conditions, and many compounds defy credulity in their complexity and beauty [32,72]. They are reminiscent of a number of naturally occurring metal phosphates which also have elaborate structures [73,74]. Recently, metal complexes of more complicated building blocks, such as borophosphates, have also been prepared this way [75]. Much of this work is driven by the desire to prepare new microporous solids for catalytic purposes. As a result, a great deal of effort has been focused on the use of organic cations as templates leading to open-framework structures. The templates are usually quaternary ammonium ions and, after isolation of the solid, they are normally burned out, hopefully leaving an intact microporous solid. This particular type of chemistry again is inherently limited to relatively low temperature (<250 "C) synthesis because of the instability of the organoammonium ions above this temperature. Nevertheless a very rich chemistry of metal phosphates prepared above 350°C has been developed and this will be the focus of this section. It should be noted that in the early 1980s, a series of microporous aluminum phosphates were also prepared using hydrothermal methods [76]. Due to their similarities to silicates, these materials aroused intense interest, but again they were prepared at somewhat lower temperatures. The simplest phosphate to be studied in any detail is AlP04. This work was originally driven by the desire to prepare high quality single crystals for electronic applications. It has been quite successful for the most part although, for a number of reasons, commercial hydrothermal synthesis of AlP04 is not very common. However, conditions have been developed whereby aluminum phosphate feedstocks can be mineralized in nearly supercritical H20, leading to well formed large single crystals of AlP04 [77].

AlP04 (powder) H3P04

(6

OC>

Alp04 (crystals)

(4.1-5)

Another metal phosphate important for electronic applications is KTP (KTiOP04), used as a frequency doubler for solid state lasers. Again sub-

230

4 Reactions in SCF

stantial effort was needed to develop the appropriate conditions, but this and related compounds are now produced commercially by hydrothermal methods [78]. Since the late 1980s, other metal phosphates have been synthesized in scH2O for more exploratory reasons. Recently several gallium phosphates have been prepared, presumably to explore the link to AlP04. These require the use of sealed gold tubes and temperatures exceeding '500"C [79,80]. Transition metal phosphates are desirable because, if they could be made microporous, catalytically active metals would be present in the pore without further treatment, leading to a new class of highly active catalysts. Indeed a number of exciting alkali metal cobalt phosphates were recently synthesized at relatively low temperatures (<250"C), along with a new sodium cobalt phosphate prepared at 500 "C [81-831. Many of the transition metal phosphates usually contain alkali metal or alkaline earth cations. The presence of s-block cations generally creates low-dimensional open frameworks with the metal cations located in channels or between layers [84-861. An iron phosphate was even prepared containing ammonium as the counterion [87]. The majority of these compounds were prepared at 40O-50O0C, which necessitated the use of quartz or gold ampoules and very high counterpressures (>1,400 bar). Our group has also begun investigating the chemistry of transition metal phosphates in scH20. As we are working at higher temperatures, organic templates are not an option and the work is focused on alkali and alkaline earth derivatives. A surprising find was that, despite the enormous amount of work already in this field, it is still quite rich, and many compounds remain undiscovered. Thus a wide variety of new alkaline and alkaline earth iron phosphates with interesting structural types have been prepared [88]. These include several low dimensional materials, such as layered compounds and metal phosphate chains (Figure 4.1-7). In an attempt to explore the role that temperature plays in the isolation of products, we performed a number of identical reactions at 200°C versus 400°C versus 600°C and found that indeed, the chemistry changes completely, with entirely new products formed in each case [89]:

BaFeP207 or SrFe3(P04)3 (M = Sr, Ba)

(4.1-6)

Ba3Fe2(HP04)6or SrFe(HP04)(P04)

(4.1-7)

Similarly, the reactions are all extremely sensitive to reaction conditions and, thus far, none can be made using classical dry reactions. Changing the mineralizer or the iron source also leads to different products [90]:

4.1 Synthesis of Inorganic Solids

231

Figure 4.1-7 Polyhedral unit cell representation of S T F ~ ~ ( P as O~)~ viewed down the a axis showing the condensed Fe-0 framework. The Fe06 polyhedra are lined, whereas the PO4 tetrahedra are dotted. The strontium atoms are circles [89].

NaBaFe4(HP04)3(PO& -H20

NaOH

+ BaHP04 + 3Fe0 H20/H3P04/400 "C> BaFe3P207

(4.1-8)

(4.1-9)

Many of these compounds are low-dimensional solids, with either layered or channeled structures (Figure 4.1-8). None are true microporous solids but many have other interesting topotactic properties such as ion exchange. The extreme sensitivity to reaction conditions, together with the fact that these compounds are not easily made using conventional ceramic methods, indicates that a hydrothermal medium is important for their preparation. The large number of compounds prepared in a very short time using only iron as the transition metal, also suggest that the potential for this field is quite high. Metal silicates have long been of interest, mostly for their potential use as microporous solids. Most classical microporous materials are prepared at temperatures below 200 "C. The traditional theory is that increasing temperatures led to more condensed structures, and indeed there is no evidence that a microporous silicate or zeolite has ever been prepared above 300°C [29]. Although still prepared at lower temperatures, several spectacular crystals have recently been isolated using superheated nonaqueous solids, suggesting that this field is also not exhausted [91,92].

232

4 Reactions in SCF

Figure 4.1-8 Polyhedral unit cell representation of NaBaFe4 (HP04)3(P04)3.HZ0showing the stacking of layers which run parallel to the a axis. The sodium (small crosshatched circles) and barium (large cross-hatched circles) atoms lie between the layers along with the isolated water molecules which are shown as open circles. The Fe06 polyhedra are lined and the PO4 tetrahedra are dotted [90].

Several metal silicates have been prepared in true scH20 above 400 "C, mostly for electronic purposes. Thus, Laudise and co-workers prepared an extensive series of alkali metal silicates of the rare earths [93]: NaOH

+ Si02 + Y203 H20/500 "C/830bar > Na3YSi6015

(4.1-10)

A number of other intriguing new metal silicate phases have also been reported in the Russian literature, but many of their structures are new and have not been determined [94]. Recently, several metal titanium silicates were also prepared in hydrothermal solutions in gold tubes at 750°C and 2000 bar as ion exchange materials [95]. During the 1960s and 1970s Demianets and co-workers prepared a variety of metal germanates [96]. Most of these contained rare earth metals, but several had d-block transition metals. Most of the germanates were prepared under alkali conditions, or using KF as mineralizer, around 5OO0C, and most could be grown as large crystals several millimeters in size under the appropriate hydrothermal conditions. Their chemistry awaits further exploration.

4.1.3.3

Metal Sulfides

Surprisingly, a large number of metal sulfides are stable towards hydrolysis in scH20, so reaction chemistry can be done in hydrothermal fluids. Once again,

4.1 Synthesis of Inorganic Solids

233

the seminal work in this area has been done by geochemists interested in the origin and mechanism of transport of various sulfide-based minerals, such as bornite (Cu5FeS5), chalcopyrite (CuFeS2) and tetrahedrite (Cu12Sb4S13) [97]. Most of these sulfosalts can be solubilized under hydrothermal conditions, and transport occurs best using HS- or C1-, which emulate naturally occurring brine fluids. Many of these complex equilibria and solubility parameters have been worked out in considerable detail [98]. A number of these compounds, as well as several new metal sulfosalts, can be grown in the laboratory as large high-quality single crystals using hydrothermal brines [99,100]. Like the oxides, several metal sulfides have also drawn the attention of crystal growth experts. Thus important compounds like ZnS can be grown in electronic-grade quality in supercritical water [ 101,1021. Hydrolysis is occasionally observed under certain conditons, but is not generally a problem. A variety of other related compounds, such as Bi2S3, Ag3AsSe3, and CdTe, have been prepared hydrothermally, but their growth chemistry has not been studied in great detail [ 1031. 4.1.3.4 Other Hydrothermal Syntheses

A number of miscellaneous compounds have also been prepared hydrothermally and, while space precludes a comprehensive discussion, a number of older examples are listed in reviews [2]. Interestingly, metal fluorides can be prepared under hydrothermal conditions without hydrolysis, using fluoride as the mineralizer. Thus compounds such as K[MF4], K2[MF5] (M = rare earth), and [NH4][FeF4] can be prepared in scH2O [104]. In addition, a number of less common fragments can be employed to generate new solids. Molybdates and tungstates are excellent building blocks, and a number of new phases have been prepared using halides or hydroxides as mineralizers [105,106]: Sr(OH)2 + Na2W04 K2MoO4

+ Mo

H20/LiCl/400-500 "C

H2OKOW500-700 "C

3

SrWO4

' K2MO8016

(4.1-11) (4.1- 12)

It should be noted that vanadates and molybdates have a very rich chemistry in hydrothermal fluids below 250 "C, but their chemistry in higher temperature fluids has not been extensively investigated. The library of building blocks has been extended by a series of new transition metal tellurites with interesting layered structures [ 107-1091: 2C0304 + 9 TeOz

H~O/NbCl/375 OW690 bar

CO2Te308

(4.1-13)

H3Fe2(Te03)4Cl

(4.1-14)

234

4 Reactions in SCF

Finally, a class of compounds which is gaining attention because of their photorefractive properties is the sillenites, Bi12M020 (M = Si, Ge, other metal ions). They can be prepared by a variety of methods but, for several derivatives, the most satisfactory route is a hydrothermal one. Thus, single crystals of these compounds may soon be prepared for commercial purposes [1101.

4.1.4 Supercritical Amines Ammonia is an intriguing medium for inorgaaic synthesis in high pressure fluids. It is less polar and less protic than water but is still able to solubilize many inorganic reagents. It is a fascinating solvent because it provides access to a whole range of compounds that are unstable in water for one reason or another. Furthermore it is almost unexplored, so it is a very exciting area for chemists seeking new synthetic venues. Ammonia provides its own set of experimental challenges, and is somewhat more difficult to handle than water. Most obviously it is a gas at room temperature and generates considerably higher pressures than water at comparable temperatures and fill ratios. In contrast, its critical conditions are lower than those of water (T, = 132 “C,pc = 113.5 bar). The original work on ammonothermal inorganic synthesis, by Juza and Jacobs, was the preparation of several amides and Be3N2 at 400 “C and 280 bar [ 111-1 131. This work was subsequently followed up more thoroughly by Jacobs and coworkers [114]. They were able to prepare a wide variety of metal amides, imides and nitrides in supercritical ammonia (scNH,), including a wide variety of pure amides of extremely oxophilic metals. These include such fundamentally important compounds as MNH2 (M = alkali metal) and M’(NH2)z (M’ = alkaline earth metal), as well as some more exotic species such as M3[M’(NH2)6] (M = alkali metal, M’ = lanthanide metal). They were also able to prepare single crystals of several metal nitrides which were unknown by any other route, including Eu3N2 and Mn3N2 [I 15,1161. These reactions were generally performed at fairly extreme conditions (400-600°C and 6000 bar). It should be noted that temperatures higher than this are not generally acceptable for ammonia due to the onset of the Haber equilibrium. The initial attraction to this field for the authors was the early series of reports on the use of scNH3 to prepare alkali metal sulfide derivatives. As it is well known that anionic main group clusters are soluble and stable in ammonia and ethylenediamine, but reactive with protic solvents [ 1171, this seemed an ideal place to begin investigations in supercritical amines. Supercritical ethylenediamine and ammonia are excellent solvents for the synthesis of new transition metal chalcogenide phases. An extensive series of novel phases were prepared of the general formula A,M,Q, (A= alkali metal, M = Cu, Ag, Au; Q = S, Se, Te). In general the compounds can be viewed as a product of oxidation of the transition metal by the Q-Q bond in the anionic polychalcogenide (Figure 4.1-9) [118-1201:

4.1 Synthesis of Inorganic Solids

235

Figure 4.1-9 Unit cell of KAg& viewed down the z axis. The silver atoms are represented as

cross-hatched circles, sulfur atoms are shaded circles, and the potassium atoms are open circles. K2S4

+ Ag + KAgsS3

(4.1-15)

In all cases the metal ion was monovalent and the compounds contain no Q-Q bonds. In most cases the compounds are valence precise, diamagnetic, narrow bandgap semiconductors or semimetals. A much more extensive series of compounds can be prepared by including mixed group 15/16 compounds as the starting materials. This leads to quaternary compounds, A,,,M,E,,Q, (A = K, Rb, Cs, T1; M = Cu, Ag; E = P, As, Sb, Bi; Q = S, Se, Te) with tn- and pentavalent atoms creating additional linkages and greater structural complexity. Thus were isolated and characterized an enormous series of structurally fascinating quaternary phases [ 121-1231: 3Cs2C03

+ 4Ag + 3Sb2S3 + S g

NH3/175 "C/240 bar Cs3Ag2Sb3Sg

(4.1-16)

Product formation is extremely sensitive to reaction conditions and, with the large number of reaction parameters available, there appears to be no end to the number of new phases that can be made. However, in virtually all cases, the compounds are valence precise, diamagnetic semiconductors with bandgaps between 1.8-2.8 eV. Thus their electronic properties are of minimal interest. For all preceding chalcogenide compounds prepared in supercritical amines, the metal centers are always monovalent. This includes both the alkali metal and the transition metal. In attempts to make more electronically interesting compounds, the inclusion of polyvalent metal ions with open shells in the chalcogenide framework was attempted. This was done by replacing the alkali metal cations with divalent cations or trivalent rare earth cations. However,

236

4 Reactions in SCF

the introduction of higher valent cations invariably leads to formation of homoleptic ammine complexes of the metal cation [124-1261: (4.1-17) This preferential coordination of ammonia to ihe metal center occurs with alkaline earth cations as well: (4.1-18) These are the first well formed examples homoleptic ammine complexes of felement cations, but they are extremely unstable outside of the ammonia environment, and further chemistry is difficult. Replacement of the monovalent coinage metals with higher-valent earlier transition metal cations such as Mn", Fe" and Co" was also attempted. It was hoped that if the open shelled transition metals could be incorporated into the main group framework like the coinage metals, a whole new class of magnetically and electronically interesting solids could be prepared. Again, very high quality crystals were obtained but they consist of well separated Werner complexes in a complex main group matrix: NH3/170 "C MnC03 + Sb2S3 A [ M ~ ( N H ~ ) ~ ] [ S ~ ~ S I O ] (4.1-19) Thus the ammonia acts to effectively sequester the open shelled transition metal ion, creating a relatively uninteresting classical metal complex that acts as a counterion for a number of complicated sulfosalt frameworks [127,128]. In general, the ammonia solvent will complex to harder di- and trivalent metal centers preferentially to sulfide ligands. In order to inhibit the complexation of amines to the higher valent metal centers, it appears that acidic solutions and higher temperatures are required. Thus NH41 is an excellent mineralizer for the crystal growth of SrS in ammonia above 300°C [129]: NH3/400 "c SrS + NH41 SrS (crystals) (4.1-20) The use of acidic scNH3 at higher temperatures is a relatively unexplored area and is ripe for development. One interesting and potentially important use of scNH3 is in the preparation of metal nitrides, particularly the Group 3 nitrides AlN and GaN. The compounds are useful for a number of applications, most immediately as substrates for blue diode lasers [130]. These nitrides, although easy to make as powders, are notoriously difficult to prepare in single crystal form, and only a few extremely specialized routes are currently available [131-1331. Several workers have demonstrated that microcrystalline gallium and aluminium nitride can both be prepared in scNH3 [134,135]. Recently, a systematic investigation has begun of the factors necessary for the growth of high quality single crystal of the nitrides in scNH3. The original idea was that amide in ammonia is

4.1 Synthesis of Inorganic Solids

237

exactly analogous to hydroxide in water. Thus it was hoped that, just as hydroxide attacks Si02 to form soluble silicate anions, amide might attack metal nitride feedstock leading to soluble metal amide anions. These soluble species deposit as large crystals of the nitrides: AlN(powder) + NH2' -

+ 2NH3 + Al(NH2)4- + AlN(crysta1) + NH2- + 2NH3

(4.1-21)

However, the situation is substantially more complicated, and it appears that more complex chemistry occurs in the mineralization stage [136]. This complexity is highlighted by a very unusual observation by Purdy who was able to grow single crystals of the unstable cubic form of GaN (as opposed to the thermodynamically stable hexagonal polymorph) under essentially the same conditions, using ammonium iodide as the mineralizer [137]. The importance of the mineralizer is emphasized by the fact that the cubic form only occurs in the presence of iodide, as opposed to the ammonium salt of any other halide.

4.1.5

Other Solvents

One solvent that has not received much attention for inorganic synthesis is supercritical methanol. This is somewhat surprising in that it is reasonably benign and has a reasonable critical point (240 "C and 8 1 bar). It is somewhat less polar than water but still able to solubilize a variety of inorganic compounds. Furthermore it does not attack quartz under basic conditions as readily as water does. Sheldrick and co-workers have used superheated methanol as a medium for the synthesis of a variety of alkali metal main group sulfides and selenides [138]. In general, they use an alkali metal carbonate to induce disproportionation of a neutral main group chalcogenide, leading to formation of a series of complex polymeric main group chains or layers [139,140]. Strictly speaking they rarely use truly supercritical methanol, usually working 50-75 "C below the critical point, but they see reactivity and crystallization behavior typical of a supercritical fluid. It should also be noted that supercritical COz has recently been shown to be an effective medium for the synthesis of several inorganic and organometallic complexes (Chapter 4.2). Supercritical C 0 2 has also been used as a solvent for inorganic aerogels [141]. Thus far pure C02 has not found much use for strictly inorganic synthesis, given that it is so nonpolar that it cannot solubilize many inorganic compounds. However a potentially intriguing solvent is a mixture of waterKO*. This of course will lead to carbonic acid, and the mixture should be polar enough to solubilize a variety of reagents. Several metal carbonates have been prepared this way [142,143], but the area is for all practical purposes unexplored. Rabenau and co-workers have done some very interesting chemistry using strong hydrohalic acids (5-15 M) as solvents [72]. Interestingly, they have

238

4 Reactions in SCF

been able to solubilize and recrystallize a number of pure elements such as gold, silver and platinum [103]. These elements often occur in nature as pure crystals, and -the ability of these fluids to transport them suggests that this is the method of their natural formation [145]. The ability of these concentrated hydrohalic acids to dissolve noble metals leads to a number of interesting ternary metal chalcogen halides (Figure 4.1-10) [145,146]: HI( 1 OM)/350 "C/700 bar > AuTe21 (4.1-22) Au + Te + I2 This chemistry is simplified by the fact that quartz is unattacked by these acids up to 5OO0C, making it a convenient reaction vessel. Indeed, Rabenau has reduced the floating insert technique to a very simple and useful synthetic method [ 1451.

Figure 4.1-10 View of AuTezI nets shown perpendicular to the a axis. Gold atoms are represented as cross-hatched circles, tellurium atoms as shaded circles, and iodine atoms as open circles [147].

4.1.6 Conclusions The use of SCFs in inorganic synthesis has enormous potential. Most of the early groundwork was laid by geochemists and crystal growers. These groups have performed enormous amounts of very detailed and elegant work. The equipment designs in use today, and the fundamental understanding of high pressure solutions resulted from this early chemistry. However, most of the traditional work concentrated on known compounds, and the search for new phases was not a primary goal. Recently this focus has begun to change and

4.1 Synthesis of Inorganic Solids

239

more investigators are beginning to exploit the unique properties of SCFs for the synthesis of new compounds. One of the driving forces for this work is the search for new optoelectronic and microporous materials, and new exploratory synthetic investigations are underway in several laboratories. Many of the new phases are solids which are stabilized at relatively low temperatures and, as such, do not-appear on classical phase diagrams, so there are few predictive guidelines for future syntheses at this point. However, it is clear that the chemistry is very rich and large numbers of new compounds await discovery. The ability of SCFs to solubilize and transport low concentrations of reactive intermediates is leading to a wide variety of unexpected and exciting compounds. One of the strengths of the technique is that the experimentalist has the same chemical control as that of a solution chemist. Thus, factors like acidity, concentration, relative stoichiometry, solvent polarity, reaction time and temperature can be varied in the SCF, just as they can in the flask. This enormous chemical control makes this technique one of almost unlimited promise in inorganic synthesis.

References [l] JWK would like to dedicate this article to the memory of Bob Laudise; friend and mentor (Deceased August 20, 1998). [2] A. Rabenau, Angew. Chem. Int. Ed. Engl. 1985, 24, 1026. [3] R. A. Laudise, Chem. Eng. News, 1987, Sept. 28. [4] L. N. Demianets, A. N. Lobachev, Curr. Top. in Muter. Sci. 1981, 7, 485. [5] R. Bunsen, Ann. 1848, 65, 70. [6] H. de Senarmont, Ann. Chem. Phys. 1851, 32, 129. [7] G. W. Morey, P. Niggli, J. Am. Chem. SOC. 1913, 35, 1086. [8] G. W. Morey, J. Am. Ceram. SOC. 1953, 36, 279. [9] R. Roy, 0. F. Tuttle, in Physics and Chemistry of the Earth, Vol. 1, L. H. Ahrens, K. Rankama, S . K. Runcorn (Eds.), Pergamon, New York, 1956, p. 138. [lo] P. W. Bridgeman, The Physics of High Pressure, Bell, London, 1949. [ 111 R. A. Laudise, A. A. Ballman, in Kirk-Othmer Encyclopedia of Chemical Technology 2nd Ed. John Wiley and Sons, New York, 1969, Vol. 18, p. 105. [12] R. Nacken, Chem. Z. 1950, 74, 745. [13] E. Beuhler, A. C. Walker, Ind. Eng. Chem. 1950, 42, 1369. [14] R. A. Laudise in Crystal Growth: An Introduction, P. Hartman (Ed.), North-Holland, Amsterdam, 1973. [I51 R. A. Laudise, J. W. Nielson, Solid State Physics 1961, 12, 149. [16] R. A. Laudise, Prog. Inorg. Chem. 1962, 3, 1. [17] A. A. Ballman, R. A. Laudise, in The Art and Science of Growing Crystals J. J. Gilman (Ed.), J. Wiley and Sons, New York, 1963, p. 231. [18] R. A. Laudise. Crystal Growth of Electronic Materials, E. Kaldis (Ed.), Elsevier Science, Amsterdam, 1985, Ch. 13. [19] A. Rabenau, H. Rau, Phillips Technical Review, 1969, 30, 89. [20] A. Rabenau, Physics and Chemistry of the Earth, 1981, 13/14, 361. [21] S . Somiya (Ed.), Hydrothermal Reactions for Materials Science and Engineering, Elsevier Applied Science, London, 1989. [22] T. Morioshi, 2nd Int. Con$ on Solvothermal Reactions, Tokamatsu, Kagawa, 1996.

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4 Reactions in SCF

[23] A. N. Lobachev (Ed.), Crystallization Processes under Hydrothermal Conditions, Consultants Bureau, New York, 1973. [24] A. N. Lobachev (Ed.), Hydrothermal Synthesis of Crystals, Consultants Bureau, New York, 1971. [25] H. L. Barnes Physics and Chemistry of the Earth, 1981, 13/14, 321. [26] D. T. Rickard, F. E. Wickman (Eds.), Chemistry and Geochemistry of Solutions at High Temperatures and Pressures: Physics and Chemistry of the Earth 13/14, Pergamon Press, Oxford, 1981. [27] H. C. Hegelson, Complexing and Hydrothermal Ore Deposition, Pergamon Press, Oxford, 1964. [28] H. L. Barnes, Geochemistry of Hydrothermal Ore Depositss, 2nd ed, Wiley & Sons, New York, 1979. [29] R. M. Barrer, Hydrothermal Chemistry of Zeolites, Academic Press, London, 1982. [30] R. Speed, A. Filice, Am. Mine,: 1964, 49, 1114. [31] Parr Equipment, 211 53rd St., Moline, IL 61265, USA. [32] R. C. Haushalter, L. A. Mundi, Chem. Mate,: 1992, 4, 31. [33] For more detailed information regarding these techniques see Ref 15. [34] G. C. Ulmer, H. L. Barnes (Eds.), Hydrothermal Experimental Techniques, Wiley Interscience, New York, 1987. [35] G. C. Ulmer, Research Techniques for High Pressures and High Temperatures, Springer, New York, 1971. [36] Autoclave Engineers, 2930 W 22nd St., Box 4007 Erie, PA 16512, USA. [37] LECO Corp/Tem. Pres. Division Blanchard St. Extension, P.O. Box 390, Bellefonte, PA 16823, USA. [38] 0. F. Tuttle, Am. J. Sci. 1948, 246, 628. [39] 0. F. Tuttle, Geol Sci. Am. Bull. 1949, 60, 1729. [40] LECO Corp./Tem. Pres. Division. [41] G. C. Kennedy, Am. J. Sci. 1950, 248, 540. [42] High Pressure Equipment, 1222 Linden Ave., Erie, PA 16505, USA. [43] R. A. Laudise, P. M. Bridenbaugh, T. Iradi, J. Crystal Growth 1994, 140, 51. [44] The old adage that a chain is only as strong as it’s weakest link is never more true than in this type of chemistry. Thus only the finest available valves, gauges and connectors should be used. The word “rig“ has no place in the high pressure fluid laboratory. [45] E. U. Franck, Endeavour 1968, 27, 5 5 . [46] E. U. Frank, Fluid Phase Equilibria 1983, 10, 211. [47] R. A. Laudise, E. D. Kolb, Endeavour 1969, 28, 114. [48] R. A. Laudise, R. J. Cava, A. J. Caporaso, J. Crystal Growth 1986, 74, 275. [49] R. A. Laudise, W. A. Sunder, R. F. Belt, G.A. Gashurov, J. Crystal Growth 1990, 102, 427. [50] V. A. Kuznetsov, in Crystallization Processes under Hydrothermal Conditions, A. N. Lobachev (Ed.), Consultants Bureau, New York, 1973, p. 43. [51] E. D. Kolb, A. J. Caporaso, R. A. Laudise J. Crystal Growth 1973, 19, 242. [52] H. Hebst, Angew Chem Int. Ed. Engl. 1982, 21, 270. [53] S . Okamoto, H. Sekizawa, S . I. Okamoto, J. Phys. Chem.Solids 1973, 36, 591. [54] S . Okamoto, S. I. Okamoto, T. Ito, Acta Cryst. 1972, B28, 1774. [55] S. Hirano, M. G. M. U. Ismail, S. Somiya, Mat Res. Bull. 1976, 11, 1023. [56] M. G. M. U. Ismail, S. Somiya, in Proc. 1st Int. Symp. Hydrothermal Reactions, S. Somiya (Ed.), Okayama, Mageru, Japan, 1983, p. 669. [57] M. Yoshimura, K. Yamasawa, S. Somiya, Proc. Int. Mtg. on Chemical Sensors, Kodanshi Japan, 1983, p. 198. [58] T. Endo, S. Kume, M. Shimada, M. Koizumi, Mine,: Mag. 1974, 39, 559. [59] E. D. Kolb, R. A.Laudise, J. Crystal Growth 1975, 29, 29. [60] R. A. Laudise, J.. Crystal Growth 1972, 13/14, 27. [61] L. N. Demianets, Cryst.: Growth, Prop., Appl. 1978, I , 97.

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Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

4.2 Synthesis of Coordination Compounds JENNIFER L. KINGand MARTYN POLIAKOFF

4.2.1

Introduction

Historically, interest in supercritical fluids (SCFs) has been cyclical or, perhaps more accurately, sinusoidal. Currently, research into SCFs is undergoing a substantial resurgence, largely driven by possible applications in so-called “green” chemistry, where supercritical C 0 2 (scCO2) offers considerable scope as an environmentally more acceptable replacement for conventional organic solvents. However, the emphasis here is rather different because the synthesis of new organometallic or coordination compounds in SCFs does not have such obviously. green applications as homogeneous catalysis (see chapter 4.7). Nevertheless, these compounds occupy an important place in SCF chemistry because they represent a large number of the new compounds which have been prepared in SCF solution. The use of coordination compounds in SCFs is rather larger than one might initially expect [I]. The major applications can be divided into two broad categories: (1) the transport of metals, which includes selective extraction, deposition of metal and oxide films and impregnation of metals as a route to composite materials; and (2) synthesis and reactions, which covers a wide range of topics from the synthesis of oxides and other solid state materials to the reactions of organometallic compounds. This chapter focuses on the synthesis of organometallic compounds because this is the area where the majority of the new experimental techniques have been developed. The chapter complements chapter 3.1 and 3.2 which cover, respectively, vibrational and NMR spectroscopy, because IR spectroscopy, and to a lesser extent NMR, have been key tools in the exploration of this chemistry. The chapter is divided into four parts: Strategy, which explains the rationale for carrying out these reactions in SCF solution; Chemistry, which outlines what the reactions are; Equipment, which describes some of the key components and introduces a modular approach to high pressure experiments; and Synthesis, which gives details of how to make particular compounds, including the use of flow reactors and semiflow reactors. It concludes with a brief summary and outlook.

244

4 Reactions in SCF

4.2.2

Strategy

Carrying out chemical reactions in SCF solution will always be harder than in conventional solution because of the relatively high pressures involved with SCFs [2]. Therefore, the use of SCFs must bring Some real chemical advantage to justify the added inconvenience. In the area of organometallic chemistry, this advantage lies in the preparation of compounds with weakly, or extremely weakly, coordinated ligands, "L. Such compounds are important because they include many of the co-ordinatively saturated species implicated in catalytic cycles as well as a range of unusual but simple ligands (N2, C02, alkanes, nobles gases) which are inherently interesting. There are three problems associated with compounds containing weak ligands: (1) how can the compounds be generated from more stable precursors: (2) once formed, how can they be stabilized; and (3) how can they be recovered from solution without decomposition. In general, the compounds are generated from precursors by substitution of a ligand, X, more strongly bonded than "L, as shown in eq (4.2-1). ML,X

+ "L

-

ML,"L

+X

(4.2-1)

This normally precludes a direct thermal route from ML,X to ML,"L, because the M-X bond is harder to break thermally than M-"L. Therefore, the conversion is usually most easily achieved photochemically. Indeed, all of the syntheses described below involve a photochemical step, either in SCF solution or immediately prior to the use of the SCF. Stabilization of ML,"L generally involves using a high concentration of wL, which reduces the probability of recombination with the ejected ligand, X, and slows down other possible decomposition pathways. It is here that SCFs offer a real advantage because an SCF solvent enables very high concentrations of many typical "LS to be achieved. Either the SCF can be used as both solvent and reactant (e.g. S C C ~ used H ~ as a solvent for preparation of C2H4 complexes) or a chemically inert SCF can be used as a solvent for reactions with permanent gases (H2, N2, etc.) which are completely miscible with the SCF. This miscibility gives rise to concentrations of dissolved gases up to an order of magnitude higher than in conventional solvents under similar pressures of the gas. In addition, the high diffusivity and low viscosity of SCFs can aid mass and heat transport. The high concentration of gases has, in turn, led to the photochemical generation of a whole range of previously unknown dihydrogen and dinitrogen complexes [3-51, some of which have been isolated by the procedures outlined below [6,7]. Even when the compounds were previously known, the supercritical preparation has sometimes proved to be chemically much simpler [3] than the original route. It is often difficult to isolate compounds with weak ligands from convention solvents, because the heat or vacuum which removes the solvent also removes the weakly bound ligand. Here again, SCFs offer an advantage because the sol-

4.2 Synthesis of Coordination Compounds

245

vent power of the fluid can be manipulated, merely by changing the applied pressure and hence the density of the SCF. The mechanism by which solids dissolve in SCFs is beyond the scope of this chapter but, very briefly, solubility of a compound depends on its vapor pressure (which is comparatively high for nonpolar organometallic compounds) and on the density of the SCF. In the limit, rapid expansion of an SCF leads to precipitation of all of the material dissolved in it. This so-called RESS (rapid expansion of supercritical fluid solution) technique has been widely applied to the preparation of finely divided materials and particles and is treated in more detail in chapter 2.3 [8-111. The important point here is that RESS precipitation avoids both heating or use of vacuum, the factors which normally render compounds with weak ligands so hard to isolate.:All of the syntheses described below use RESS precipitation to recover the solid products. Most synthetic chemistry involves small-scale exploratory experiments followed by scale-up of the more successful experiments. Conventionally, exploratory experiments might be carried out in NMR tubes, with scale-up in Schlenk tubes. In SCFs, the exploration has mostly been done by IR [5,12,13], largely because it is experimentally simpler than NMR [14-191, (Figure 4.2.-1). Scale-up has involved the use of miniature flow reactors or semiflow reactors, neither of which have obvious analogs in conventional synthesis. Figure 4.2-1 Schematic views of (a) the miniature IR cell and (b) the high pressure capillary NMR apparatus [17-191, which have both been used for small-scale exploration of organometallic reactions. Further details of the IR cell are given elsewhere [12,13]. P indicates the ports for filling the IR cell. Note that the two parts of the Figure are not drawn to the same scale.

4.2.3

(a)

Chemistry

The use of SCFs for organometallic synthesis follows a tradition set by Mason and Ibers, who attempted to synthesize COz complexes in liquid COz (they were thwarted by residual H20 in the COz) [20] and by Turner and co-workers who developed the use of liquefied noble gases as solvents [21,22]. Table 4.2-1 lists most of the organometallic compounds that have been synthesized in SCFs. The majority involve one of three ligands (H2, N2 or CzH4) and all share a common synthetic route from the corresponding metal carbonyl complex.

L,M(CO),

uv L,M(CO)n-lWL + CO + "L + (wL = Hz, Nz,or CzH4)

(4.2-2)

Alkene

Mono(N2)

BisWz

TMNz)3

HzM

Related

Table 4.2-1 Organometallic compounds that have been prepared or generated in SCF solution; those shown in bold have been isolated as solids (Cp = CsHs; Cp’ = C5H4Me; Cp* = C5Mes).

h

m

4.2 Synthesis of Coordination Compounds

247

This route was derived from the successful methods developed by Turner, Poliakoff and co-workers for the synthesis of similar complexes in cryogenic liquid noble gas solution [23]. Under those conditions, the low temperatures of the liquid gas solvent helped to stabilize complexes [24], such as Ni(C0)3N2, which would have been very short lived at ambient temperatures. Although this cryogenic stabilization is lost in SCF solution, the loss is compensated at least in part by the high concentration of dissolved H2 or N2, which increases the lifetime of these complexes by reducing the apparent rate of ligand dissociation. There are a number of important points which should be made about the compounds listed in the Table. Nearly all of the compounds were synthesized photochemically, which imposes a number of quite strict limitations on larger-scale preparations of these compounds. The primary, or at least the initial, identification of these compounds has been via IR spectroscopy. (The sole exception has been Cp*Re(CO)(C2H4)2which was tentatively identified by NMR [ 181). In most cases, characterization of the metal carbonyl moiety via v(C-0) IR bands is definitive, particularly if library spectra are available from liquid noble gas [23] or matrix isolation experiments [4]. This is because these bands are sharp and intense, and the C-0 stretching vibrations are largely uncoupled from other vibrations of the molecule. Furthermore, the precise wavenumber of the bands are extremely sensitive to the oxidation state of the metal center as illustrated, for example, by Kazarian et al. in their study of hydrogen bonding to metal centers [25]. IR identification is also very effective for coordinated N2 groups via the v(N-N) bands, which are easily detectable, although inherently somewhat less intense than v(C-0) absorptions. The situation is slightly more complicated in the case of (qn-C,H,)M(C0)3-,(N2)x complexes. For a range of metals, there is a near-overlap of the v(N-N) bands of the three possible dinitrogen complexes ( x = 1-3). Once understood, however, this overlap did not present a serious problem to identification [s]. Many dihydrogen complexes have been identified by the observation of a broad band (ca. 3000 cm-') due to the v(H-H) vibration of the q2-H2 ligand [26,27]. Even in the most favorable cases, the v(H-H) band is weak and it becomes increasingly weaker as the number of CO groups is reduced. Therefore, there has been no success in observing the v(H-H) bands of any of the compounds listed in the table. Given the lack of v(H-H) bands, the presence of the q2-H2 ligand had to be deduced from indirect IR evidence. Briefly, this evidence consisted of the similarity of the wavenumber and relative intensities of the v(C-0) bands of the dihydrogen and corresponding dinitrogen complexes [5]. These deductions were fully supported in those cases [6,7] where the compounds were finally isolated and characterized by 'H NMR. The IR characterization of dihydrides and alkyl hydrides are aided by three factors, comparison with literature spectra, the comparatively high wavenumber of the v(C-0) bands caused by the oxidation of the metal center relative to the metal carbonyl precursor [3] and in many cases direct observation of a weak IR band due to v(M-H) vibrations [28,29]. As with dihydrogen compounds, these

248

4 Reactions in SCF

IR assignments were confirmed by 'H NMR in those cases where the compounds were subsequently isolated. Although C2H4 ligands do have characteristic IR bands, the relevant regions of the spectrum we% always obscured by the absorptions of the scCzH4 solvent used for the synthesis of the ethene complexes listed in the table. These were, therefore, identified by comparison with literature'data and by 'H NMR, either before [18,19] or after isolation of the compound [6,30]. There is a lower limit on the lifetime of molecules that can be observed in SCF, by use of photochemistry and conventional FTIR. The precise limit depends on the optical arrangement but is usually a few seconds. Nevertheless, much shorter-lived complexes, such as CpRe(COhXe, can be detected in SCF by use of timeresolved IR spectroscopy [31] (see chapter 3.1). The table also shows that SCFs allow the multiple substitution of CO by N2 to occur [4,5,32]. This is perhaps one of the most intriguing features of this chemistry (Scheme 4.2-1). The immediate reason for this multiple substitution is obvious. UV irradiation of the intermediate compounds, e.g. CpM(C0)*(N2), leads to photoejection of the CO group with a quantum yield which must be comparable to (or at least only slightly smaller than) that for loss of N2. The very high concentration of N2 in the reaction mixture ensures that loss of CO is effectively irreversible, whereas loss of N2 is almost instantaneously reversed. As yet, unfortunately, there are no ready answers to the deeper questions of why the reaction does not proceed in scCOz or why some metals/compounds undergo multiple substitution while others do not. The various possible explanations have been discussed in some detail in a recent review [l].

Scheme 4.2-1

4.2.4

Equipment

All of the synthetic reactions described involve weak ligands, "L, and most involve photochemistry. These factors impose limitations on the isolation of the product and on scaling-up from a scoping experiment to a preparative scale [6,7,33]. The strategy, outlined in Section 4.2-2, is to isolate the products by RESS, using the rapid expansion of the SCF to deposit the solid material from solution in a relatively gentle manner, thereby minimizing decomposition. Our approach has been to use a flow system, which enables the production of the compound to be scaled up, without increasing the size of

4.2 Synthesis of Coordination Compounds

249

-5obar

I -r/ Figure 4.2-2 Schematic diagram of the pneumatic pump (NWA GmbH, Module PM101) for pumping condensable gases. Gas from a cylinder is condensed into the reservoir which feeds the small cylinder. The liquid is then compressed by applying a compressed air supply to the large face of the piston. (Modified from J. A. Banister, Ph. D. Thesis, University of Nottingham, UK, 1994.)

UsuM

c w i coih /

>> non-returnvalves

the photolysis cell or reactor. The flow (or semiflow) system is then combined with spectroscopic monitoring to allow the yield to be optimized in real time. Apart from the IR cell (Figure 4.2-l(a)), which has been described elsewhere [12,13] three pieces of apparatus are crucial to the construction of the flow reactors: a pump for the SCF, a variable-volume view-cell, and a backpressure regulator (BPR). There are several possible commercial sources for the pump and BPR, and many designs for view-cells. Here we describe briefly those which we have used. They are not necessarily the best equipment but they have features that are extremely helpful for such experiments. It is extremely important to remember that, whatever equipment is chosen for a particular experiment, it must be suitably rated for the high pressure conditions of the experiment. Furthermore, the entire apparatus must comply with all the relevant local safety regulations which apply to the laboratory. Figure 4.2-2 shows a schematic view of a pump suitable for any gas which condenses (under pressure) at temperatures above -20°C.The pump is pneumatically driven so that it can be safely used with flammable gases as well as with COz. The maximum flow rate is ca. 50 mL of fluid per min, far higher than is needed for these experiments. The disadvantage is that there is a momentary pressure pulse as the pump refills but this pulse is not a serious problem in the experiments described here and can be removed by using a modest ballast volume as a pulse dampener (see Figure 4.2-7). Figure 4.2-3 shows the variable-volume view-cell. This cell is used as a general purpose mixing vessel, reservoir and reactor, depending on the particular reaction under study. It has a piston sealed with an elastomer O-ring, which can be moved by applying a pressure of COz. The original version of this

250

4 Reactions in

SCF

Carbonyl + SCF

Pure coz

-

L

I

, I

I

I

Piston

\ t b

Sapphire Wndow

Figure 4.2-3 Schematic cross-section of a variable-volume view-cell based on a design by McHugh [2]. Pure COz is used to drive a piston which forces the solution of metal carbonyl in scCOz (or other SCF) out through the port in the main chamber. The sapphire window allows the solution to be viewed. (Caution:Such cells should never be viewed directly but with a mirror, through a thick polycarbonate screen, or via a CCTV camera.) Solid carbonyl compounds are loaded by removing the sapphire window. Liquids or solutions can be added via the “Carbonyl + SCF” port and the solvent can then be evaporated as in a Schlenk tube. The cell can be stirred by a magnetic flea (also loaded through the window aperture) and an external magnetic stirrer; the cell body is nonmagnetic. (Diagram modified from J. A. Banister, Ph. D. Thesis, University of Nottingham, UK, 1994.)

cell, usually used water rather than C 0 2 to drive the piston, to achieve much higher pressures than used here [34]. C 0 2 is favored here, particularly because of the risk of contaminating the reaction mixture with H20. Most RESS experiments involve expansion through a nozzle following the original design of Smith and co-workers [35]. Unfortunately nozzles can easily become blocked especially when using near-saturated solutions. This can be overcome by the use of a backpressure regulator (BPR) incorporating an electronically-pulsed needle valve, which rarely blocks. A BPR of the type shown in Figure 4.2 -4(a) has the added advantage that it will hold pressure without allowing any fluid to pass through it. As soon as the pressure rises again, an appropriate amount of fluid is released to return the pressure to its preset level. This feature enables flow-reactors to be operated intermittently, without loss of supercritical conditions. Although a mechanical BPR should, in principle, also be able to maintain pressure without flow, the experience has been that they operate less well in reactors than an electronic BPR. RESS expansion transforms material dissolved in the SCF into an extremely fine powder. This means that careful thought is needed to design a receiving vessel which can trap these powders efficiently. Figure 4.2-4(a) includes one successful design and Figure 4.2-4(b) shows a variation intended to collect material for NMR spectroscopy. These large components, pump, BPR and view-cell, need to be linked to build a reactor. We have developed a technique for constructing such equipment relatively rapidly. This involves mounting most of the minor components onto magnets and assembling the reactor on a magnetic table. The components

4.2 Synthesis of Coordination Compounds

25 1

Figure 4.2-4 (a) View of the back pressure regulator (Jasco Model 880-81), BPR, as used for isolating compounds. GT is a glass tube (a modified Schlenk tube) for collection of the solid product, P. The other parts are labeled as follows: IN, inlet pipe from the flow-reactor; E, epoxy resin seal; ST, stainless steel tube; GS, ground glass stopper; X,exhaust for waste gases. (Reproduced with permission from J. A. Banister, P. D. Lee, M. Poliakoff, Organometallics 1995, 14, 3876; 0 American Chemical Society). (b) The adapter for collection of product and immediate transfer to an NMR tube. Additional parts are labeled as follows: grJ, greaseless joint; SS, suba-seal; YT, Youngs greaseless tap, fitted to the top of the NMR tube. Note that the stainless steel tube deposits the product as close a possible to the top of the Youngs tap. In this way, only a minimum amount of solvent is need to flush the product into the NMR tube, which is evacuated prior to the experiment so that the solution is sucked into the tube when the tap is opened.

can then be placed in the required position where they are held by the magnets. Then it is merely necessary to link them with high pressure capillary tubing. In this way, quite complicated systems can be assembled without incurring a large dead volume of piping. Figure 4.2-5 shows the principles of a complete reactor for the synthesis of ethene complexes. A supercritical solution of the precursor metal carbonyl complex is held in the view-cell. The solution is then driven through a UV cell where the reaction is carried out and is eventually forced through the BPR where the product is recovered. The key feature of this approach is that the photolysis arrangements are essentially identical to those used in the preliminary experiments carried out in the IR cell shown in Figure 4.2-1. Thus, the amount of material produced in the reactor depends on the run time of the reactor rather than its volume, although obviously the reserve of starting material in the view-cell will eventually be exhausted. In earlier designs of such reactors [30], the solid carbonyl compound was dissolved directly into the flowing stream of scC2H4. However, the dissolution was difficult to control and there were frequent blockages. The use of the view-cell removes these problems [7,36]. Section 4.2.5 describes how such a reactor can be used to synthesize and isolate C T ( C O ) ~ C ~ H ~ . The next section describes how this reactor can be converted for the photochemical synthesis of CpMn(C0)2(q2-H2) where two gases, H2 and scCO2, are

252

4 Reactions in SCF

P Figure 4.2-5 Schematic view of a reactor for the synthesis of an unstable ethene complex in scCzH4. The parts are labeled as follows: BPR, backpressure regulator (as in Figure 4.2-4);IR, a spectroscopic cell for IR monitoring of the reaction (see Figure 4.2-l(a));P, product precipitated by RESS; R, view-cell (as shown in Figure 4.2-4)containing the carbonyl precursor dissolved in scC2H4; scC02, high pressure/supercritical C 0 2 for driving the piston of the view cell (it is convenient to pump the C02 with a modified HPLC pump which allows the fluid to be delivered at a constant flow rate; UV, high pressure cell for UV photolysis.

needed. We have also investigated possible thermal routes to related dihydrogen compounds. After investigating several precursors, we found C~*Mn(c0)~(q~-HSiEt3) to be the most effective precursor [6]. This compound is sufficiently stable to be isolated from conventional solvents but still reactive enough for H2 to displace the HSiEt3 ligand. Unfortunately, the corresponding Cp complex is too unstable to be isolated and therefore this route cannot be used for C ~ M n ( c o ) ~ ( q ~ - H The ~ ) ethyl . vinyl ether complex, Cp*Mn(C0)2(H2C=CHOEt), also underwent interesting reactions [6]. Although thermally too stable to be a useful precursor to Cp*Mn(C0)2(q2-H2), it provided an efficient route to Cp*Mn(C0)2(N2). A particularly interesting result of this investigation [6] was that SCFs provide considerable freedom for manipulating the chemistry of these labile complexes. Indeed, the concentrations of the various ligands can be varied over a sufficiently wide range that almost any weak ligand can be made to displace any other. This ability to manipulate reactions is likely to play a significant role in future studies of homogeneous catalysis in supercritical solution.

4.2.5 Detailed Syntheses This section describes three reactors, with related but slightly different designs, which have been used for the synthesis of alkene and dihydrogen complexes. Their common feature is the use of on-line FTR to optimize the conversion of reactant to product. With care, the reactors can be adjusted to give almost complete conversion so that little or no subsequent purification is needed. This is particularly important because conventional manipulation of such compounds with labile ligands often leads to rapid decomposition through ligand loss. It is also important to bear in mind that, when these reactors were first used, it

4.2 Synthesis of Coordination Compounds

253

was not known whether these compounds would be sufficiently stable to be isolated.

4.2.5.1 Preparation of Cr(C0)5(C2H4) (Note that this procedure is different, and improved [36] from that published previously [7,33]). The apparatus is set up as in Figure 4.2-6 but with an adapted Varian Bond Elut (a standard filter cartridge used in chromatography and extraction) as a collection vessel for the initial stages of the flow reactor synthesis. The view-cell, R, is filled with Cr(C0)6 (> 100 mg) and evacuated

Figure 4.2-6 Layout of the reactor for the synthesis and isolation of C T ( C O ) ~ ( C ~ H ~ ) from Cr(C0)6 in scC2H4. (Note that this layout is illustrated schematically in Figure 4.2-5 and is slightly different from that described by Banister et a1 [7], because recent work has shown the layout in this figure to be more effective). The components are labeled as follows: BPR, back pressure regulator; C1 and C2, control valves (SSI or similar HPLC valves); C2H4, ethene cylinder; IR, infrared cell; P, solid product, Cr(CO)S(C2H4);PP, pneumatic pump; R, variable-volume view-cell containing a solution of Cr(C0)6 in C2H4; SP, syringe pump (Brownlee Lab Microgradient; this particular pump is no longer manufactured but similar units are available, e.g. from Isco. Depending on the internal pipework of the pump, it may be necessary to install an extra valve between the pump and R to prevent loss of pressure in R, when SP is refilled) containing scC02 to drive the piston of the variable volume cell; T, pressure transducer (RDP Electronics); UV, photolysis cell; W, sapphire window of the variable volume cell; X, exhaust vent. It is important to understand why there are two C02 pumps: PP provides pressure while SP can deliver COz at a specific (programmable) flow rate. In this reactor, SP could be used without PP, provided that it could be cooled for filling with C02. However, this would not be possible in the reactors, shown in Figures 4.2-7 and 4.2-8 because there, PP is also needed to provide C02 for the reaction mixture. Safety is always of paramount importance and the design of any reactor must foresee possible pressure rises in excess of safe limits. This reactor is not explicitly shown with a pressure-rupture (bursting) disc because the pumps PP have been modified to incorporate pressure release valves and the read-out of the pressure transducer, T, is connected to a trip which automatically stops the pumps in the event of an over-pressure (note that this and the subsequent figures are not drawn to scale).

254

4 Reactions in SCF

and argon filled (3 times) on a vacuum line to remove all traces of air from the cell. The cell is left under an argon atmosphere and plumbed into the reactor. The flow reactor system is flushed for 1 h with C2H4 to ensure air-free conditions and a background FTIR spectrum is recorded at 140 bar. Valve C2 is closed while R is filled with C2H4 to a pressure of 140 bar. SP is then used to pressurize the back of R with COz. C2 is %-opened and the Cr(C0)d scC2H4 mixture is driven out of the cell into the-reactor. SP, C1 and C2 are now adjusted to give a flow rate of ca. 200 mL gas per min, measured at atmospheric pressure with a flow meter, connected between BPR and X (not shown). For C2H4, this flow rate corresponds to a usage of ca. 0.5 mol h-' or 14 g h-'. The concentration of Cr(C0)6 is monitored by FTIR and adjusted as necessary by SP and the control valves. The UV lamp is switched on and the extent of the photolysis reaction is monitored by FTIR and optimized. As soon as the conditions are such that optimum conversion is achieved, the UV lamp, SP and gas flow are switched off and an evacuated and argon filled glass vessel is substituted for the Bond Elut and attached to the BPR. The gas flow, UV lamp and SP are restarted and crude C T ( C O ) ~ ( C ~ H is ~collected, ) typically at 40 mg h-'. The reactor is not truly continuous in operation because the stock of Cr(C0)6 will eventually run out. The longest continuous operation has been ca. 2 h, but there is no reason why it should not be run longer because very little deposit builds up on the UV window. The collection vessel is then detached from the BPR and product is removed and worked up in a glove box or a conventional Schlenk line. The procedure for isolation of C T ( C O ) ~ ( C ~ leads H ~ ) to a product with some contamination (ca. 10-15 %) by C T ( C O ) ~ ( C ~ Hwhich ~ ) ~ , is hard to separate subsequently. In this reaction, strong IR absorption by scC2H4 effectively obscures most of the region needed for monitoring v(C-0) bands [7, 12,301. This means that FTIR monitoring has to be carried out in the v(C-0) overtonekombination region (4100-3800 cm-'). A very similar approach can be used to sythesize and isolate Cr(CO)5(propene) [37]. This is quite surprising because the T, of propene (96.4"C)is considerably above ambient temperature. Nevertheless, Cr(CO)5(propene) can be isolated (ca. 50 % purity) by rapid expansion of liquid propene at room temperature. An extension of the apparatus shown in Figure 4.2-6 can be used first to synthesize Cr(C0)5(C2H4) and then to impregnate the compound directly into polyethylene or into porous inorganic media (alumina or silica) without isolating the solid compound [38]. Not all metal-alkene complexes are as labile as C T ( C O ) ~ ( C ~ and H ~ ) some such as Cp*Mn(C0)2(C2H4) can be synthesized thermally from a suitable precursor [6,39], using a reactor similar to that in Figures 4.2-5 and 4.2-6, where the UV irradiation cell is replaced by a heated metal coil. The overall reaction sequence is summarized in eq (4.2-3).

4.2 Synthesis of Coordination Compounds

255

4.2.5.2 Preparation of CpMn(CO)z(q'-Hz) [7] Figure 4.2-7 illustrates the layout of the reactor. The variable volume cell R is loaded with ca. 130 mg of solid c ~ M n ( C 0 )and ~ is sealed (by replacing the window). R is then purged with H2 and a pressure of 14 bar is left in the cell, which is filled with scC02 and the resulting solution of c ~ M n ( C 0 in )~ H2/C02 is stirred magneticalIy. The COz pump PP is now set to ca. 210 bar, the H2 compressor PC to ca. 200 bar, and the BPR to 170 bar. The dosage unit, DU, is set up to switch every 0.70 s. The flow of scC02/H2 is then allowed to stabilize with the BPR pulsing regularly, and the FTIR interferometer is started scanning repetitively (ca. one spectrum per minute). The syringe pump SP is now used to pressurize the variable-volume cell behind the piston, driving the solution of c ~ M n ( C 0 into ) ~ the flowing stream of C02/H2. Once the pressure output from SP stabilizes, the software limits the flow rate of c ~ M n ( C 0 to ) ~a preset value (normally 100 pL min-'). The UV lamp is turned on as soon as the v(C-0) bands of c ~ M n ( C 0 appear )~ in the ITIR spectrum. The flow parameters can then be adjusted to give optimal conversion of CpMn(C0)3 to CpMn(C0)2(q2-H2), P, which collects in the collection vessel attached to the BPR. The efficiency of collection can be improved by cooling the vessel with ice. The reactor is then run until the efficiency of photolysis drops or the solution in R is exhausted. The reaction of metal carbonyls with H2 eventually fails because of a build up of opaque deposits on the window used for UV irradiation. The deposits are carbonates formed by reaction with residual H20 in the scC02, the same problem which dogged Ibers and Mason [20]. In an attempt to increase the amount

Figure 4.2-7 Layout of the reactor for the synthesis and isolation of CpMn(C0)2(q2-H2) from CpMn(C0)3 and H2 in scC02. The components are labeled as in Figure 4.2-6 with additional items as follows: C, control valve; DU, gas dosage unit (NWA); H2, hydrogen cylinder; IR, infrared cell; P, solid product, CpMn(C0)2(q2-H2); PC, pneumatic compressor (NWA Model CU10.5); R, variable volume view-cell containing a solution of CpMn(C0)3 in an H2/scC02 mixture; S, mixer with magnetic stirrer (Kontron M491). (Reproduced with permission from J. A. Banister, P. D. Lee, M. Poliakoff, Organornetallics 1995, 14, 3876; 0 American Chemical Society).

256

4 Reactions in SCF

of dihydrogen complex which could be generated, a detailed exploration of thermal routes [6] to c ~ * M n ( C o ) ~ ( q ~ was - H ~carried ) out. The study focused on finding isolable precursors which had ligands sufficiently labile to be displaced by H2 thermally and was therefore carried out with the Cp* derivatives. 4.2.5.3 Preparation of Cp*Mn(CO)2(q2-Hz)from Cp*Mn(C0)2(q2-HSiEt) [6] The reaction sequence for the synthesis of C P * M ~ ( C O ) ~ ( ~ by ~-H thermal ~) ligand replacement in scC02 is outlined in eq (4.2-4). The experimental procedure is applicable to the reaction of both Cp*Mn(CO)2(H2C=CHOEt)with N2 and c ~ * M n ( C o ) ~ ( H s i Ewith t ~ ) HZ, the only significant difference being the reaction temperatures (20°C for HSiEt3 and 38°C for H,C=CHOEt). Cp*Mn(CO)3

W

HS1Et3

Cp*Mn(CO)2(HSiEt3)

-co

A scco2m2 -HSiEt3

Cp*Mn(C0)2(q2-H2) (4.2-4)

The semiflow reactor is set up as in Figure 4.2-8. A flow of scCOz/H2 is used to purge the system and to record a background FTIR spectrum. 70 mg c ~ * M n ( C o ) ~ ( H s i Eist ~added ) to the front compartment of R, as above. With the BPR fixing the overall pressure at 180 bar, the C 0 2 pump PP and H2 compressor PC are both set to 210 bar. The precise switching rate of the dosage unit DU is not critical because it only determines the rate at which R is filled (conveniently, it is set to 1 Hz) and scC02/H2 is flowed until the pressure in R

BPR

Figure 4.2-8 Layout of the semiflow reactor for the synthesis and isolation of Cp*Mn(C0)2(qZ-Hz) from Cp*Mn(C0)2WL(wL = labile ligand) and H2 in ScCO2. The reactor is very similar to that shown in Figure 4.2-7 but without the UV photolysis cell. Here the variable volume view-cell R is used as a thermal reactor. All other components are labeled as in Figures 4.2-6 and 4.2-7 (reproduced with permission from P. D. Lee, J. L. King, S. Seebald, M. Poliakoff, Organometallics 1998, 17, 524; 0 American Chemical Society).

4.2 Synthesis of Coordination Compounds

257

reaches 180 bar. SP is then used to pressurize the rear compartment of R so as to drive small aliquots of the solution into the on-line IR cell. Regular FTIR scans permit the progress of the reaction to be monitored until it reaches completion, whereupon the contents of R are rapidly driven through BPR and isolated as a yellow solid in an Ar-filled glass collection vessel (typical yield of Cp*Mn(C0)2(q2-H2)is 40 mg, ca. 80% yield).

4.2.6 Conclusions This chapter gives an outline of the techniques which have been devised over the past few years for the synthesis and isolation of new organometallic complexes from SCF solution. Frequently, progress has been slower than one would have wished. However, it has to be remembered that these techniques have been developed with few precedents to serve as a guide. Now that they have been developed, the field is ripe for exploitation and major advances are to be expected in the near future.

4.2.7 Acknowledgments We thank the EPSRC for Grant Nos. GR/H95464 and GWJ95065 and for a Fellowship to MP. We also thank the EU Human Capital and Mobility and TMR Programmes, the Royal Academy of Engineering, Perkin Elmer Ltd and Zeneca Plc for support. We are grateful to our colleagues, co-workers and collaborators for their help and advice in the near future.

References [l] J. A. Dan; M. Poliakoff, Chem. Rev. 1999, 99, 495. [2] M. A. McHugh, V. J. Krukonis, Supercritical Fluid Extraction, Butterworth-Heinmann, Boston, 2nd ed., 1994. [3] S . M. Howdle, M. Poliakoff, J. Chem SOC., Chem. Commun. 1989, 1099. [4] S. M. Howdle, P. Grebenik, R. N. Perutz, M. Poliakoff, J. Chem. SOC., Chem. Comm. 1989, 1517. [5] S. M. Howdle, M. A. Healy, M. Poliakoff, J. Am. Chem. SOC. 1990, 112, 4804. [6] P. D. Lee, J. L., King, S. Seebald, M. Poliakoff, Organometallics 1998, 17, 524. [7] J. A. Banister, P. D. Lee, M. Poliakoff, Organornetallics 1995, 14, 3876. [8] R. -D, Smith, U.S. Patent 4,582,731, 1986. [9] R. D. Smith, U.S. Patent 4,734,451, 1988. [lo] R. D:Smith, U.S. Patent 4,734,227, 1988. [11] D. W:Matson, J. L. Fulton, R. C. Peterson, R. D. Smith, Znd. Eng. Chem. Res. 1987, 26, 2298.

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[ 121 S. M. Howdle, M. Poliakoff, in Supercritical Fluids - Fundamentals f o r Applications,

NATO ASI Series, Vol. 253, E. Kiran, J. M. H. L. Levelt-Sengers (Eds.), Kluwer Academic Publications, Dordrecht, 1994, p. 527. [13] S. G. Kazarian, S: M. Howdle, M. Poliakoff, Angew. Chem. Int. Ed. Engl. 1995, 34, 1275. [14] R. J. Klingler, J. W. Rathke, J. Am. Chem. SOC. 1994, 116, 4772. [15] J. W. Rathke, R. J. Klingler, T. R. Krause, Organometallics 1991, 10, 1350. [16] J. W. Rathke, R. J. Klingler, T. R. Krause, OrganomeYallics 1992, 11, 585. [17] D. M. Pfund, T. S. Zemanian, J. C. Linehan, J. L. Fulton, C. R. Yonker, J. Phys. Chem. 1994, 98, 11846. [18] C. R. Yonker, S. L. Wallen, J. C. Linehan, J. Microcolumn Separations 1998, 10, 153. [19] J. C. Linehan, C. R. Yonker, J. T. Bays, S. T. Autry, T. E. Bitterwolf, S. Gallagher, J. Am. Chem. SOC. 1998, 120, 5826. [20] M. G. Mason, J. A. Ibers, J. Am. Chem. SOC. 1981, 104, 5153. [21] J. J. Turner, M. B. Simpson, M. Poliakoff, W. B. Maier, 11; M. A. Graham, Inorg. Chem. 1983, 22, 911. [22] W. B. Maier, II; M. Poliakoff, M.B. Simpson, J. J. Turner, J. Chem SOC., Chem. Commun. 1980, 587. [23] M. Poliakoff, J. J. Turner, in Molecular Cryochemistry; Vol. 23, Ed.: M. 0. Bulanin, John Wiley; London, 1995, p. 275. [24] J. J. Turner, M. B. Simpson, M. Poliakoff, W. B. Maier, I1 J. Am. Chem. SOC.1983, 10.5, 3898. [25] S. G. Kazarian, P. A. Hamley, M. Poliakoff, J. Am. Chem. SOC. 1993, 115, 9069. [26] R. K. Upmacis, M. Poliakoff, J. J. Turner, J. Am. Chem. SOC. 1986, 108, 3645. [27] R. K. Upmacis, G. E. Gadd, M. Poliakoff, M. B. Simpson, J. J. Turner, R. Whyman, A. F. Simpson, J. Chem SOC., Chem. Commun. 1985, 27. [28] M. Jobling, M. A. Howdle, M. Poliakoff, J. Chem SOC., Chem. Commun. 1990, 1762. [29] J. A. Banister, A. I. Cooper, S. M. Howdle, M. Jobling, M. Poliakoff, Organometallics 1996, 15, 1804. [30] J. A. Banister, S. M. Howdle, M. Poliakoff, J. Chem SOC., Chem. Commun. 1993, 1814. [31] X. Z. Sun, D. C. Grills, S. M. Nikiforov, M. W. George, M. Poliakoff, J. Am. Chem. SOC. 1997, 119, 7521. [32] J. A. Banister, M. W. George, S. Grubert, S. M. Howdle, M. Jobling, F. P. A. Johnson, S. L. Morrison, M. Poliakoff, U. Schubert, J. Westwell, J. Organomet. Chem. 1994,484, 129. [33] J. A. Banister, M. Poliakoff, J. Supercritical Fluids 1993, 6, 195. [34] F. Rindfleisch, T. P. DiNoia, M. A. McHugh, J. Phys. Chem. 1996, 100, 15581. [35] R. C. Petersen, D. W. Matson, R. D. Smith, J. Am. Chem. SOC. 1986, 108, 2100. [36] J. L. King, PhD Thesis, University of Nottingham, UK, 1998. [37] J. L. King, M. Poliakoff, 1998, unpublished results. [38] J. L. King, T. F. Nolan, M. Poliakoff, in Proc. 4th Con$ on Supercritical Fluids and Their Applications, Capri, Italy, 1997, p 357. [39] M. Poliakoff, S. M. Howdle, M. W. George, in High Pressure Chemical Engineering, P. Rudolph von Rohr, C. Trepp (Eds.), Elsevier, Netherlands, 1996, 67. [40] J. A. Banister, Ph. D Thesis, University of Nottingham, UK, 1994. [41] J. C. Linehan, S. L. Wallen, C. R. Yonker, T. E. Bitterwolf, J. T. Bays, J. Am. Chem. SOC. 1997, 119, 10170. [42] M. P. Poliakoff, M. W. George, J. Webster, 1998, unpublished results. [43] M. Jobling, S. M. Howdle, M. A. Healy, M. Poliakoff, J. Chem SOC., Chem. Commun. 1990, 1287.

Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

4.3 Stoichiometric Organic Reactions YUTAKA IKUSHIMA and MASAHIKO ARAI

Supercritical fluids (SCFs) have been used as solvents for various organichnorganic, homogeneousheterogeneous chemical reactions [11. The physicochemical properties of SCFs may be manipulated by relatively small changes in operating pressure and temperature [2-51. The advantages of SCFs as solvents have led to their increased use in chemical reactions because it is possible to tune solubilities, mass transfer, solvent strength, and reaction kinetics of reacting species present in SCF solvents [6,7]. Increasing interest in global environmental problems requires the reduced utilization of organic solvents and their replacement with environmentally friendly alternative media. Much effort has been thus made to avoid organic solvents in chemical reactions and to perform organic syntheses in SCFs. One can use various SCF solvents, such as supercritical carbon dioxide (scC02) and supercritical water (scH20), that are widely different in physical properties. The appropriate supercritical solvent can be feasibly selected by considering reaction systems and conditions. ScC02 and scH20 are very attractive solvents for industrial applications because these reaction media are nontoxic, nonflammable and inexpensive. The former solvent is nearly inert although scCOz can react with the bound water around proteins [8] or interact chemically with proteins, forming carbamates with free amino groups on the surface of the protein [9-111, whereas the latter can be reactive depending on reaction conditions employed [ 121. The solute-solute, solute-solvent, and solvent-solvent interactions in SCFs are of fundamental importance because small changes in pressure or temperature near the critical point may bring about great changes in reactivities, selectivities, and separations of products, reactants, and solvents [ 13-15]. In addition, the solvent strength of the SCFs was found to vary widely near the critical region [6, 16-18]; this sensitivity could be applied to the control of chemical reaction processes without the addition of any catalysts. The use of SCF solvents may overcome the disadvantages of using strong acid or base catalysts and of short lifetimes of catalysts and they will contribute to the development of environmentally friendly and practically effective alternative routes for organic syntheses.

260

4 Reactions in SCF

This chapter describes several examples of organic reactions in scC02, scH20, and other fluids (e.g. ethane, ethene, xenon, trifluoromethane), for which a few detailed descriptions of experimental procedures are included.

4.3.1 Experimental Methods Some equipment is necessary to carry out experiments of chemical reactions in SCF solvents at high pressures and/or high temperatures. One experimental apparatus is that of Ikushima et al., who fabricated a high-pressure in situ FTIR spectroscopy system for the study of chemical reactions in scC02 [19,20]. Figure 4.3-1 shows this system, which can be operated in batch and flow modes. The reactor (cell) has two IRTRAN 2 windows made from crystalline ZnS, which possesses a high modulus of rupture and a wide usable mid-IR region (800-5000 cm-'). When using 5 mm thick windows, the reactor can withstand a pressure of 300 bar and a temperature of 100°C. This system is combined with a UV-Vis spectrometer for examining solvent properties. Ikushima et al. [21,22] have developed a similar system with Raman spectroscopy for the study of properties and chemical reactions in scH20, the critical temperature and pressure of which are even higher than those of C02. Experiments in scH20 face severe problems such as corrosion leading to the formation of salts and mineral acids at high temperatures. The choice of materials for these reaction systems is very important. These authors use a monoblock of Inconel 718 for the reactor cell, which has sapphire windows sealed to the cell using gold-plated metal foil [21]. In addition to in situ spectroscopic examination, it is important to monitor the phase behavior of the reaction mixture because it may be different from that of pure SCFs. Observation with the naked eye can be made through some windows made of transparent materials like sapphire; a good example is described in the work of Renslo et al. [23]. Polycarbonate shielding between the window and the observer is recommended.

4.3.2 Supercritical Carbon Dioxide Carbon dioxide is one of the most promising SCF solvent, owing to its physical properties as described above. Diels-Alder reactions are typical examples of organic syntheses that have been investigated by many workers using scC02 as a reaction medium. These addition reactions are of practical significance in the production of organic chemicals such as fragrances, plasticizers, and dyes. In addition, Diels-Alder reactions are good test reactions for fundamental studies because their reaction features are well known [24].

4.3 Stoichiometric Organic Reactions

261

Hwted

T

r

D -@ -0 t

-@

t

; :

Gui

Figure 4.3-1 High-pressure in situ spectroscopy apparatus for chemical reactions in SCFs. (a) flow chart: 1, C 0 2 cylinder; 2, C 0 2 pump; 3, preheater; 4, injector; 5, three-way valve; 6, separation column; 7, extractor; 8, UV detector; 9, on-off valve; 10, FTIR; 11, pressure transducer; 12, pressure regulator; 13, FID detector; 14, gas sampler; 15, recorder. (b) IR flow cell: 1, cell; 2, gasket; 3, window; 4, weight made of Delrin; 5, weight made of SUS 303; 6, bolt.

262

4 Reactions in SCF

4.3.2.1 Diels-Alder Reactions Several workers have reported on the effects of pressure on the rate of reaction and product distribution in Diels-Alder reactions in scCO2. Paulaitis and Alexander carried out the cycloaddition of maleic anhydride and isoprene in COz at pressures of 80-430 bar and at three temperatures of 35, 45, and 60°C [25]. They observed that the rate constant increased greatly fiear the critical pressure, and at 200 bar or above it was similar to that obtained in a solution of ethyl acetate. Kim and Johnston studied the parallel Diels-Alder reaction of methyl acrylate and cyclopentadiene, producing the endo and ex0 adducts (Scheme 4.3-1) [26]. They showed that the ratio of the endo to ex0 rate constants changed slightly with pressure in the range '100-300 bar and it was a little larger at 35 "C than at 45 "C. The ratio in nonpolar fluid CO2 is smaller than in polar organic solvents such as acetone [24] and pyridine [27]. The logarithm of this ratio for C 0 2 at 300 bar and at 35°C and 45°C is around 0.46, similar to that of the nonpolar solvent cyclohexane [27]. Ikushima et al. investigated the Diels-Alder reaction of isoprene and maleic anhydride by using high-pressure FTIR spectroscopy to monitor the change of reacting species in pressurized COz (Scheme 4.3-2) [28]. Their FTIR observations at 75.5 and 79.5 bar and at 33 "C strongly suggest that some intermediates are formed, and so the cycloaddition reaction proceeds through a two-step mechanism - in contrast to one-step mechanisms under ordinary conditions [29]. The same authors also studied the Diels-Alder reaction of isoprene and methyl acrylate in C02 at pressures of 50-200 bar and at 50°C [14]. The rates of both Diels-Alder reactions were found to increase with pressure. In the latter reaction, they reported an unusual pressure effect on the reaction selec-

+

\

H

transition state

exo

Scheme 4.3-1 Diels-Alder reaction of methyl acrylate and cyclopentadiene in scC02.

263

4.3 Stoichiometric Organic Reactions

A

P

Scheme 4.3-2 Diels-Alder reaction of isoprene and maleic anhydride in scCOp.

tivity near the critical pressure, at which the ortho product was more favorably formed than the para product. The selectivity to the formation of the ortho product is 33,61, and 29% at 49.6,75.5, and 119.3 bar, respectively. They suggested that the unusual: product distribution could be attributable to the aggregation of the solvent molecules, leading to some steric constraints to the reacting species.

In situ monitoring of a Diels-Alder reaction The experimental procedures of Ikushima et al. are described here as a typical example for scC02 experiments [ 14,281. They use a high-pressure system for reaction and characterization as illustrated in Figure 4.3 -1. The stainless steel reactor has a volume of 16 pL. Liquid C 0 2 is charged into a high-pressure syringe pump and compressed to the desired pressure, which is controlled within 0.1 bar by a back pressure regulator. For example, 0.05 pL of isoprene and 0.05 pL of methyl acrylate are simultaneously injected from injection valves. The reaction is conducted in batch mode at temperatures up to about 100°C, and the reaction mixture is analyzed in situ through windows (as described in section 4.3.1) by spectroscopic techniques or is sampled from an outlet valve and analyzed by gas chromatography. Renslo et al. examined the reaction selectivity in some Diels-Alder reactions in scCOz and conventional solvents [23]. They showed that the product distribution in scC02 at pressures of 49-118 bar and at temperatures of 50 "C and 150°C was very similar to that obtained in conventional solvents such as toluene. This is different from the previous observations under similar conditions [14] and Renslo et al. pointed out the importance of the phase behavior when sampling COz reaction mixtures for results. Isaacs and Keating carried out the Diels-Alder reaction between p-benzoquinone and cyclopenta- 1,3diene in C 0 2 at 25-40°C [30]. It was shown that the reaction effectively occurred throughout liquid and supercritical ranges with no discontinuity, and the rates of reaction were about 20 % greater than those obtained in diethyl ether. Weinstein et al. studied the Diels-Alder reaction of cyclopentadiene and ethyl acrylate in C02 from 38 to 88 "C and from 80 to 210 bar [31]. The rate of reaction was shown to increase with pressure (or density) for the whole range examined at a constant temperature. These authors also carried out the same reaction in methylene chloride, tetrahydrofuran, and hexane at 25 "C and 1 bar. The reaction rates in these solvents are slightly higher, but within a factor of two of those obtained in C02 as solvent. Weinstein et al. presented a good temperature-independent correlation between the density and the rate constant

264

4 Reactions in SCF

normalized to that at the same temperature and a fixed density (0.5 g/mL) [31]. They also showed that similar correlations were possible for the other DielsAlder reactions of Paulaitis and Alexander [25] and Ikushima er al. [32]. These results demonstrate the practical significance of using scCO2 as reaction media for organic syntheses. In addition to the reaction studies, it is important to examine the reaction kinetics and mechanism in scCO2 solvent at the molecular level and to find good parameters that can be used to correlate and predict the reaction rate and selectivity. Several phy sicochemical parameters may be used to describe the solvent properties of compressed C02, including density, dielectric constant [33], E~(30)parameter of Dimroth [34], and the Z values of Kosower [35]. Although bulk properties such as density are important, local properties around reacting species can be very different from bulk properties. The ET(30) scale and other solvatochromic parameters are obtained from the transition energy of selected dyes, which depends in turn on the media surrounding the indicators. These parameters are useful for the microscopic analysis of chemical reactions in scCO2. An example is that of Kim and Johnston for the Diels-Alder reaction of methyl acrylate and cyclopentadiene in C02 [26]. The logarithm of the ratio of the endo to ex0 rate constants at pressures of 99-300 bar and at 30°C and 45 "C is well correlated with the solvatochromic ET(phenol blue) parameter of the solvent, which ranges from 53.5 to 54.6 kcal/mol (224 to 229 kJ/mol). Another example is that of Ikushima er al. for the Diels-Alder reaction of isoprene and methyl acrylate in C02, at pressures of 50-200 bar and at 50 "C. They showed a linear relationship between the logarithm of the rate constant at 50 "C and the ET(30)value of the solvent ranging from 31.3 to 31.8 kcaVmol (131 to 133 kJ/mol) [14]. In addition, Ikushima er al. estimated the bulk ET(30) value of C02 from the dielectric constant and refractive index according to Bekarek er al. [36]. They indicate that the ET(30) value of the dye-organized cybotactic region is larger than the bulk ET(30)value and this difference decreases with increasing pressure up to 200 bar, above which there is no difference between the bulk and cybotactic values [14]. They also estimated the E~(30)value at 45°C and 80°C and showed its temperature dependence to be small. Recently Eberhardt et al. [37] experimentally determined the ET(30)value for C 0 2 using solvatochromic betaine dyes. They showed that at 40°C the ET(30) value is 28.5 kcaVmol (119 kJ/mol) at 150 bar and it increases up to 32.4 kcal/mol (136 kJ/ mol) with increasing pressure to 550 bar. The results at 40 "C, 60 "C, and 80 "C indicate that the ET(30)value depends little on temperature. For kinetics analysis, transition state theory has been used in several studies [14,25,26,31,38] and some data are reported for the activation energy and the activation volume. Also, the aggregation number of C 0 2 molecules about the activated complex has been estimated by the method of Debenedetti [39]. For the cycloaddition between isoprene and methyl acrylate in C02 at 50"C, Lkushima er al. gave a maximum aggregation number of about 10 near the critical pressure, whereas this was about 2 at 50 bar and at 150 bar or above [14]. They also estimated an activation volume of -700 cm3/mol near the critical point. Such large negative values were reported for other reactions in SCFs near their critical point [40].

265

4.3 Stoichiometric Organic Reactions

For the parallel Diels-Alder addition of methyl acrylate and cyclopentadiene, Kim and Johnston estimated the difference in the partial molar volumes of the endo and ex0 transition states in scC02, and found that the selectivity was related to the ET(30)polarity parameter of the solvent [26]. Large negative partial molar volumes were measured near the critical point for several systems by Eckert et al. [41]. Furthei work is needed to understand the unusual reaction rates and selectivities that can appear near the critical point. 4.3.2.2 Reduction and Coupling Hadida et al. have studied several radical reactions with alkyl and fluoroalkyl tin hydride reagents in s c C 0 2 [42]. For example, the reduction of bromoadamantane (0.05 M) to adamantane was conducted at 90°C and 276 bar using tributyltin hydride (Bu3SnH) and tris(perfluorohexy1)tin hydride ((C6F13CH2CH2)3SnH) with AIBN. The reactant is soluble in scC02, whereas the product has poor solubility. Organofluorine compounds are known to be highly soluble in scCOz [43] whereas the mixture with the alkyl tin hydride is not homogeneous. In the case of tributyltin hydride, it reacts with C02 forming tributyltin formate. For the fluorous tin hydride, such a reaction does not occur and it can be recovered unchanged. The reduction was shown to proceed in both reactions, and after purification procedures the product from tributyltin hydride provided adamantane in 88 % yield, whereas the reaction with fluorous tin hydride provided adamantane in 90 % yield. Hadida et al. also investigated similar reductions of a primary iodide and a steroidal iodide, bromide, and phenyl selenide, and they showed that the corresponding reduced products were obtained in high yields. In contrast, reduction of 9-iodoanthracene with the fluorous tin hydride provided anthracene in 71 % yield along with 10 % yield of 9-anthracenecarboxylic acid. They have thus pointed out that there is some potential to carboxylate reactive radicals in scC02 and carboxylation could presumably increase with C 0 2 density. Hadida et al. further studied the reactions in scC02 given in Scheme 4.3-3.

&r

t

87%

Ad-I

1 equiv

&

Ph

Ph

+

-CN 5 equiv 1.5 equiv

7%

sc c02 80'C

81% 70%

Scheme 4.3-3 Reduction of 1,l -diphenyld-bromo-1-hexene in scCO2 forming the 5-ex0

cyclized product along with the reduced product (upper). Addition of iodoadamantane to acrylonitrile in scCOz (lower).

266

4 Reactions in

SCF

Kudo et al. reported the synthesis of oxalate from CO and C 0 2 at 111 bar and at 380°C using various carbonates M2CO3 (M = Li, Na, K, Rb, Cs) [44]. They showed that Cs2C03 was effective for the formation of the coupling product (oxalate) with small amounts of CsHC03 and Cs02CH. This carbon-carbon bond formation from the two simple and basic inorganic compounds, CO and C02, as shown in Scheme 4.3-4, may be significant for organic syntheses. c02

co

t

co2cs

I

t

co2cs

cs2co3

t

1 1

380°C

.

H20

Cs02CH

t

CsHCO

Scheme 4.3-4 Synthesis of oxalate from C 0 2 and CO under supercritical conditions.

4.3.2.3 Esterification Ellington et al. studied the pressure effect of esterification of phthalic anhydride with methanol in scCO2 (Scheme 4.3-5) [45,46]. They conducted the reaction at pressures up to 172 bar and at 50°C [45], and showed that the rate of reaction is enhanced when it is conducted in the compressible region, due to increased local concentration of the methanol around the phthalic anhydride. The bimolecular rate constant was found to drop sharply with pressure at 100-120 bar and to change little at higher pressures.

0:)- 8c”

C- OMe

‘

F

MeOH

sc cop 50°C

0

$-OH

0

Scheme 4.3-5 Esterification of phthalic anhydride with methanol in scC02.

4.3.2.4 Cracking and Rearrangements Karakus et al. studied the reactivity of C 0 2 during thermal cracking of heavy alkanes at supercritical conditions [47]. They conducted the cracking of n-hexadecane at temperatures of 450-600°C and examined the influence of pressure and temperature on product distribution. Higher fractions of both alkanes and carboxylic acids were obtained at higher pressures. They concluded that bimolecular interactions of C 0 2 and hydrocarbons were more significant than the interactions of alkyl radicals with alkanes in a system with a high C02 partial pressure. As a result, fatty acids were obtained in rather high selectivity at high pressures and moderate temperatures. At high temperatures carboxylic acids were shown to decompose, increasing alkene selectivities. Olesik et al. studied the decomposition of allyldiisopropylamine oxide in scC02 [48]. Sigman et al. investigated the decomposition of bis(isobutyry1) peroxide in scCO2 and supercritical CC14 and CHC13 [49].

4.3 Stoichiometric Organic Reactions

267

Rearrangements in scC02 have been studied, such as the cis to trans relaxation of 4 -(diethylamino)-4’-nitrobenzeneby Sigman and Leffler [50], tautomerization of 2,4-pentanedione by Yagi et al. [51], tautomeric equilibria of acetylacetonate @-diketonesby Wallen et al. [52], and geometrical isomerization of 1,l-difluoro-2,3 -diphenylcyclopropane in scCOz with its ring opening by Roth et al. [53]. Yagi et hl measured infrared spectra of 2,4-pentanedione in C 0 2 at 45 “C and at 89-271 bar, and showed that the keto-to-enol ratio increased with the C02 density.

4.3.3 Superheated and Supercritical Water Much attention has been drawn to organic and inorganic reactions in superheated and scH20 due to water’s low cost and its environmental friendliness [54-571. Water has a critical temperature of 374°C and a critical pressure of 221 bar [58]. In this section the term “superheated water” is supposed to imply very hot water at temperatures over the boiling point but below the critical temperature. The physicochemical properties of water are known to change widely with pressure and temperature [59,60], and are different from those of C02. For example, the static dielectric constant of water is about 80 for the liquid phase, and decreases to the range 3-20 in the superheated and supercritical regions [611. Therefore, nonpolar organic compounds are very soluble or miscible in superheated H 2 0 and scH20 [62]. Transport properties and the miscibility are important parameters that determine the rate and homogeneity of reactions. High diffusivity and low viscosity, together with complete miscibility with materials that are insoluble at ambient conditions, result in homogeneous and rapid reactions. In superheated H 2 0 and scH20, solvent-efficient aqueous-phase organic syntheses are thus expected with or without catalyst. In addition, the increased ion product of water at high pressures and temperatures (e.g. pK, = 11.1 at 300°C and 253 bar [63]) suggests that acid- or base-catalyzed reactions that cannot occur at ambient conditions might be accelerated. The organic reactions conducted in superheated H20 and scH20 include oxidative destruction of organic wastes [64-661, pyrolysis and hydrolysis of coal and biomass compounds [67-701, and geochemical reactions [71]. The very few organic syntheses that have been carried out in such a reaction medium have been concerned mostly with the partial oxidation of methane [72,73] and the dehydration of alcohols [74-761. 4.3.3.1 Oxidation of Methane Partial oxidation reactions have been examined to oxygenate hydrocarbons such as methane in the absence of catalyst in scH20, whereas in scC02, Xe, and Kr, partial oxidations using heterogeneous metal catalysts have been studied by Knopf et al. [77] , Suppes et al. [78] and McHugh et al. [79]. Non-cata-

268

4 Reactions in SCF

lytic partial oxidation in scH20 was first reported by Franck 1801, who found that methane oxidation in scHzO yields methanol with a methane conversion of 15-20 % at a constant temperature of 380 "C and at pressures of 300 and 600 bar. Savage e t a l . [81] also examined the partial oxidation of methane to methanol without catalyst in near-critical and supercritical water. The methanol selectivities ranged from 4 to 75 %, with *e highes't selectivities occurring at the lower conversions of < 0.05 %. The highest yield was 0.7 %. Abraham et al. [82] used a Cr203 catalyst for the same partial oxidation in scH20 at 450"C, where the highest methanol yield was about 4 % at a methanol selectivity of ca. 40 %. 4.3.3.2 Cleavage/Hydrolysis Superheated and scH2O have also been used as reaction media for the production of high-value chemicals by the degradation of compounds of higher molecular weight such as biomass [83-881. For example, a phenolic polymer such as lignin could be depolymerized to phenols and other single-ring aromatics in scH20. Funazukuri et al. demonstrated that much higher oil yields were obtained with scHzO than with superheated water or with pyrolysis in an argon atmosphere [89]. In addition, condensed polymers such as cellulose and polyethylene terephthalate (PET) can be readily hydrolyzed to glucose and oligosaccharide in the absence of acid or base catalysts [90]. Thus, the production of chemicals from the treatment of biomass and biomass-like polymers in superheated and scH20 can be regarded as an organic synthesis. The conversion of cellulose is important because about half of biomass material is cellulose. Arai et al. [91-931 studied the hydrolysis of cellulose, glucose, and cellobiose in the temperature range of 300-400 "C, and at short residence times (0.04-2 s) in superheated and scH20. Figure 4.3-2 shows the temperature dependence of the rate constant of hydrolysis of cellulose as well as the rate constants of the degradation of glucose and cellobiose in superheated and scH20. They found that the rate constant of hydrolysis of cellulose significantly increased near the critical temperature, whereas in the superheated region the rate constants of the degradation of the products were higher than that of cellulose, leading to high conversion of cellulose in the scHzO region. Glucose and cellobiose decomposition kinetics and products in superheated and scH20 were further studied at 300-400°C, pressures of 250-400 bar, and residence times between 0.02 and 2 s [92,93]. The direct glucose decomposition and epimerization to fructose, followed by subsequent decomposition to other products, were presented as the main pathways [911, whereas cellobiose was found to decompose to glucose, erythrose, and glycolaldehyde via hydrolysis of the glycosidic bond and to decompose to glycosylerythrose and glycosylglycoaldehyde via pyrolysis of the reducing end [93]. Mok et al. [94] also reported the conversion of cellulose to glucose in superheated water at 345 bar. They found that hydrolysis, catalyzed by low acid concentration, increased glucose yields to 55 % at 220°C. Yu et al. [95] carried out the gasification of glucose as a model carbohydrate in biomass in scH20, where

4.3 Stoichiometric Organic Reactions

100

=c

4y

350 I

269

300 I

I

Glucosc(monomer) Cellobiose(dimer)

0.07""""""""""'""~

1.45

1.50

1.55

1.60

1.65

1.70

1.75

1.80

1OOom [ l K ]

Figure 4.3-2 Temperature dependence of the rate constant of the hydrolysis of cellulose in scH;O.

the gasification efficiency reached ca. 90 % at 600 "C and 350 bar, with a residence time of 30 s. These studies suggest that scHzO could be a useful medium for synthesizing higher-value products in the conversion of biomass materials. Kabyemela et al. [92] studied the degradation of dihydroxyacetone and glyceraldehyde, the products of glucose decomposition, at 300-4OO0C, 250-400 bar, and at residence times of 0.06-1.7 s in superheated and scH20. Their motivation was to obtain pyruvaldehyde, an important ingredient in organic synthesis (e.g. for the preparation of vitamin A) during the degradation. They found that isomerization between dihydroxyacetone and glyceraldehyde occurred and was followed by their subsequent dehydration to pyruvaldehyde, as shown in Scheme 4.3-6. In related work, Lawson et al. [96] examined the influence of the addition of water on guaiacol pyrolysis at 383 "C. No addition of water resulted in the production of catechol and char as the chief products, as well as trace amounts of methanol, phenol, and 0-cresol, whereas the addition of water increased the yields of catechol and methanol and reduced the formation of char. They concluded that the additional water provided a hydrolysis route for guaiacol in addition to the pyrolysis route. Water acts as a reactant in the hydrolysis, and the hydrolysis was promoted by increasing the amount of water; however, they did not report any specific reactivities near the critical point of pure water. Penninger [97] also demonstrated that water functions as a significant reactant affecting the decomposition pattern of di-n-butylphthalate in the temperature range from 305 "C up to 390 "C and water densities up to 0.31 g / d . It favors hydrolytic decomposition, which results in o-phthalic acid, butanol and butene, whereas in the absence of water the thermolysis results in only

270

4 Reactions in SCF

::::

+

H-c-c-cn~

HOHZC

p yruvaldehyde

::

- c -CH20H

dihydrcixyar

H-

::

C - y

P"-cn*OH

H glyceraldehyde

f CH20H

H

H on glucose

Ha

J

OH

H OH 1,6-nnhydrciglucose

erythruse

' -Rc - H

HOH~C

glyeolaldehyde

Scheme 4.3-6 Glucose decomposition pathway.

butene and a mixture of benzoic acid and benzene. Polycondensation of phenyl species was found to be completely suppressed in scH20. Klein et al. 1121 gave evidence of the participation of scH2O solvent in hydrolysis using H2I80 labeled water, indicating that the H20 molecules were incorporated into products. This reaction is a nucleophilic substitution which involves a saturated carbon with a heteroatom-containing leaving group as shown in Scheme 4.3 -7. This hydrolysis reaction proceeded in parallel with free-radical pyrolysis decomposition routes. Neither 1,2-diphenylethane (PhCH2CH2Ph) nor 1,3-diphenylpropane (Ph(CH2)3Ph)underwent the hydrolysis that the heteroatom-containing analogues did [98]. However, these hydrocarbon molecules decomposed easily by free-radical pathways at around 400°C [99,100]. It was thus suggested that hydrolysis did not occur through a free-radical route, but

Scheme 4.3-7 Pathway for hydrolysis by scH20; LG = leaving group.

4.3 Stoichiometric Organic Reactions

271

rather through an ionic (polar) route such as the nucleophilic substitution shown in Scheme 4.3-7. The influence of scH20 on reactions of organic chlorides such as 1-chloro-3-phenylpropane, 2-chlorotoluene, and 4 -chlorophenol has been further examined [loll. ScH20 did significantly increase the rate of consumption of the aromatic chlorides over that of dry pyrolysis at 450 and 500°C. Metal chlorides were evident in the water layer and HC1 was not detected in the reaction. The reaction of organic chlorides and/or the HCl produced from them with the reactor walls (Inconel or stainless steel) may promote the reactivity. In order to gain an understanding of hydrolytic mechanisms in a superheated/scH20 medium, in situ fiber-optic Raman spectroscopy has been used for the hydrolysis of acetic anhydride to acetic acid [102]. Acetals and ketals are very reactive to aquathermolysis, undergoing this reaction in high yield without any side or secondary reactions by base catalysts. At 254 "C, hydrolysis of benzaldehyde diethyl acetal in aqueous KOH solution at an acetallbase molar ratio of 4 was followed by a Cannizarro disproportionation [ 1031, where benzyl alcohol and benzoic acid were predominantly formed, whereas at 80°C the hydrolysis decreased to 33% even in the presence of basic barium oxide [103]. Tsao et al. [lo41 demonstrated the feasibility of a Cannizarro reaction of this aldehyde catalyzed by NH3 in scH20. Furthermore, in the hydrolysis of p-nitrophenyl acetate in room temperature aqueous solution, sonolytic catalysis was verified to accelerate the rate by two orders of magnitude [105]. During cavitational bubble collapse caused by ultrasonic irradiation, high temperatures and pressures over the critical values of water occur in the vapor phase of the cavitating bubbles and at the interfaces between the hot vapors and the cooler bulk aqueous phase. The formation of transient scH20 appears to be an important factor in the acceleration of the chemical reaction in the presence of ultrasound. None of the products formed in the reactions described above is considered to significantly change the reaction rates and product distributions by acting as a catalyst. However, Siskin et al. [lo61 reported that in the hydrolysis of methyl 1-naphthoate at 343 "C naphthalene was predominantly formed because the potential for autocatalysis arises. They concluded that decarboxylation of naphthoic acid, the main product at a lower temperature of 250°C led to the formation of naphthalene at 343 "C; the reaction is catalyzed by the generated carbonic acid. In the hydrolysis of typical fl-keto esters, ethylacetoacetate was completely converted to acetone, ethanol, and C02 within 30 min at 250°C. Under the same conditions, t-butyl acetate degraded into a bright red, insoluble mixture of unidentified products resulting from polymerization of isobutylene [ 1071.

4.3.3.3 Elimination Reactions Kuhlmann et al. [lo31 reported that the treatment of a-ethyl-4-methoxy- and d,l-4 -chloro-a-propylbenzyl alcohol in pure superheated water at 277 "C resulted in elimination, wherein neither cleavage of the para substituents nor

272

4 Reactions in SCF

polymerization of the products was detected by 'H or 13C NMR measurements. Furthermore, cyclohexanol and methylcyclohexanol derivatives were exclusively dehydrated in pure superheated water at 250-300 "C. The treatment of cyclohexanol at 300 "C led to 33 % conversion to cyclohexene, whereas at 278 "C an 85 % yield of the cycloalkene was obtained. The addition of sulfuric or hydrochloric acid promoted the conversion and nearly complete dehydration (99 %) was achieved at 250 "C [108]. The dehydrations of 1- and 2-propanol in scH2O were reported by Antal et al. [109]; the addition of acid ranging in low concentrations from 1 to 25 mM significantly promoted the dehydrations at 374 "C. Antal et al. [75,110-1121 demonstrated the possibility of dehydration reactions by selectively converting ethanol to ethylene, ethylene glycol to acetaldehyde, glycerol to acrolein, and lactic acid to acrylic acid by using acid catalysts in scH2O. Furthermore, Parsons [ 1131 reported that cyclohexanol dehydration was catalyzed by an acid or a base; for example, the isomerization of cyclohexanol or cyclohexene to 1-methylpentene was catalyzed in the presence of acid. Without the catalysts no reactions were observed, and scH2O alone was not considered to be sufficiently reactive to induce the dehydration processes. Cis and trans isomers of 2-methylcyclohexanol were used by Kuhlmann et al. [lo31 to probe whether the eliminations occurred via an El or E2 mechanism. That is, higher reactivity of the cis isomer could indicate the bimolecular E2 pathway because only in this isomer could the leaving group assume the required antiperiplanar conformation. Equal conversion rates would be expected for El reactions. Cis- and trans-2-methylcyclohexanolwere eliminated to 1-methylcyclohexanol in low yield but high selectivity (100%) in pure superheated water at 300°C [103]. However, the treatment of the trans isomer at 270°C yielded a mixture of methylcyclohexenes at a conversion of about 70 %. Similar results were obtained for cis-2-methylcyclohexanol;however, l -methycyclohexene was more predominant than double-bond migration products. These results are still not sufficient to elucidate the mechanism or the function of water in the dehydration. Dehydration of neopentyl alcohol or pentaerythritol, concomitant with the carbon-bond migration, did not occur within 60 min at 250-300°C. None of the alcohols examined were dehydrated to ethers. 4.3.3.4 Diels-Alder Reactions The Diels-Alder reaction is the most widely used synthetic method for the production of polycyclic ring compounds. Ikushima et al. [ 14,281 examined Diels-Alder reactions in scC02 and found specific changes in the isomer distribution and in the rate of reaction near the critical point. In the early 1980s Breslow et al. [114,115] and Grieco et al. [116,117] reported that the rates of Diels-Alder reactions were greatly promoted by using water instead of conventional organic solvents as media. However, nonpolar compounds have poor solubility in the aqueous phase at ambient conditions. Kolis et al. [118] have reported the possibility of performing Diels-Alder reactions in super-

4.3 Stoichiometric Organic Reactions

273

0 1

2

3

m2Et

0 1

0 1

6

7

Scheme 4.3-8 Diels-Alder reactions using scHzO as the reaction media.

heated and scH20 due to the unique properties of scH20 [62]. The reactions tested were the cycloadditions of cyclopentadiene (1)with the unsaturated carboxylic acid derivatives diethyl-fumarate (2) and diethyl-maleate (4) using scHzO as a solvent in the absence of any catalysts (see Scheme 4.3-8). They obtained yields of 10% and 86% of 3 and 5, respectively, after 1 h. Although the yield of the endo/exo-2,3 -diethylester-5-norbornene (3) was low, equal amounts of both isomers of 5 were formed in good yield from the cis diene. Using an ethanol-water cosolvent with bakers' yeast as the catalyst resulted in yields of 74 % and 78 % of 3 and 5, respectively, after 48 h at 37 "C. Furthermore, without catalysts in scH20 the norbornene derivative 7 was synthesized in approximately 80% yield after 0.5 h (see Scheme 4.3-8), whereas in a 5 M KC104 and diethyl ether solution its yield was 93 % after 5 h at room temperature [119]. With regard to the isomeric ratios, the former showed an almost 1:l endo-exo ratio, compared to an 8:l endo-exo ratio of the latter. Thus, it was demonstrated that using scH20 as a reaction medium for Diels-Alder reactions led to high yield in a short time. 4.3.3.5

Rearrangement

Rearrangements of pinacol, 1,l '-dihydroxy-1,1'-dicyclopentyl, and 1,l '-dihydroxy-1,1'-dicyclohexyl to the corresponding ketones, with negligible alkene formation, occurred in superheated D2O in 60 min at 275°C [103]. Although a classical method for the pinacolone from pinacol required boiling in 25 %

274

4 Reactions in SCF

H2S04 for 3 h [120], in superheated water pinacolone was obtained with high reaction efficiency. The Beckmann rearrangement of cyclohexanone-oxime into E-caprolactam is commercially important for the production of synthetic fibers. Sat0 et al. [121] reported the Beckmann rearrangement of cyclohexanone-oxime in the absence of acid catalysts in scH20 and its interesting reactivity in the near-critical region of pure water. Rearrangement of cyclohexanone-oxime into &-caprolactam The Beckmann rearrangement in superheated and scH20 was performed at temperatures of 250-400°C at fixed densities of 0.35 and 0.50 g/mL. The experiments were conducted using a batch reactor system. The reactor vessel was made from a piece of SUS 316 tubing, providing an internal volume of 10 cm3. A predetermined amount of reactant solution (cyclohexanone-oxime of 0.44 mmol and water of 0.20 or 0.28 mol) was loaded into the reactor in N2 atmosphere. The reactor vessel was immersed and vigorously shaken in a fluidized molten salt bath. The heating time to raise the reactor temperature from 20°C to 400°C was within 30 s and the temperature was controlled within k 2°C. After a preselected reaction time of 3 min, the reactor was removed from the bath, and then quenched in a water bath. Figure 4.3-3 shows the change in the rate of reaction with temperature at a constant density of 0.35 g/mL. The residual cyclohexanone-oxime decreased to about 20%, and the rate of reaction went up-to a maximal value at the critical temperature, and then decreased with temperature. In Figure 4.3 -3 the deviation of the Raman frequency of the OH symmetric stretching mode (vl) of water, Aflr), from the frequency of the monomer structure (3657 cm-

30 260

280

300

320

340

360

3x0

400

Temperature( “c)

Figure 4.3-3 Relationship between the deviation, Af (O), of the maximum frequency relative to the monomer frequency and the rate of reaction (0)as a function of temperature at a fixed density of 0.35 g/mL. The reaction time is 3 min.

4.3 Stoichiometric Organic Reactions

275

[122]) is also shown [123], where the value of Af implies the strength of the hydrogen bonding. At the critical temperature , the Af value shows a minimal value and the extent of the hydrogen bonding weakens uniquely in the near-critical region, where dimers or monomers are predominant [ 123-1251. Monomers, in part, are further broken into protons due to large fluctuations of the structure of water [21]. The ion product of bulk scH20 near the critical point is similar to the value at ambient conditions (lO-I4) [lo], and microscopically dynamic behavior attributable to the structure fluctuations would predominantly influence the reactivity.

4.3.4

Other Fluids

There have been several reports of reactions in SCFs besides scCO2 and scH20. Suppes et al. [78] and McHugh et al. [79] studied the partial oxidation of cumene to cumene hydroperoxide in scXe and scKr at 110 "C. Ikushima et al. [126] used scC2H6 as a continuous phase for micelles (see Chapter 2.4) and studied a chemical reaction in such a H20/AOT/C2H6 microemulsion, selecting the alkaline fading of crystal violet. The rate of reaction could be adjusted over three orders of magnitude with the molar ratio of H20 to AOT and operating pressure. Furthermore, intermolecular electron transfer reactions between ionic and neutral compounds were investigated in scCHF3 [127] and S C C ~ H ~ [ 127,1281; the reactivities were influenced by preferential aniodcation solvation. Aka0 et al. [ 1291 investigated the keto-enol rearrangement of acetylacetone in CHF3 near the critical temperature. As the solvent density increased, the equilibrium constant, K=[enol]/[keto], decreased rapidly in the low density region ( ~ 0 . 2g / d ) and less rapidly in the density region of 0.3 to 0.8 g/mL. The density dependence of the equilibrium constant around the critical density (0.315 g/mL) was small, and no remarkable anomaly was detected. Suzuki et al. [ 1301 performed Friedel-Crafts reactions, alkylations, and etherifications at 350"C, 152 bar, and residence times of 120 min in the absence of any acid catalysts in scCH30H. Several kinds of addition reactions have been examined in various supercritical fluids. The dimerization of acrylic and of methacrylic acid in scC2H4 was confirmed by the C = O stretching fundamentals with FTIR spectroscopy [131]. Fox et al. [132] demonstrated that the rate of the Michael addition of piperidine to methyl propionate (Scheme 4.3-9) in scCHF3 and scC2Hd depends on fluid density. In fluoroform, the rate constant was linearly related to pressure above 84 bar, with a smaller nonlinear increase at lower pressure, showing a minimum at 84 bar; in ethane, the rate was linear with pressure except near the critical point where the rate was significantly promoted. These critical anomalies were attributed to solvent-solute clustering in fluoroform and to solute-solute clustering in ethane. Furthermore, Metzger et al. [ 1331 examined the addition of cyclohexane (8) to phenylethyne (9) in the temperature range from 260 to 340°C in supercritical cyclohexane (ratio 1OOO:l). The addition

276

4 Reactions in SCF

proceeds via 2-cyclohexyl-1-phenylethenyl radical (10) to provide 1-cyclohexyl-2-phenylethene (11) as shown in Scheme 4.3-10. The radical chain is initiated by a bimplecular reaction of cyclohexane with phenylethylene to give a cyclohexyl radical and a 1-phenylethenyl radical; however, no effect on the reaction rate constant near the critical point was observed.

Scheme 4.3-9 Pathway for the Michael addition of piperidine to methyl propiolate in SCCHF~. PhCECH

+

c-C~H~Z

8

WPh

- c6H1H

9

10

c-c6H11*

+

H

Q - 11

‘-c6H11< H

03 - 11

Scheme 4.3-10 Pathway for the addition of cyclohexane to phenylethyne (ratio 1000:1) in supercritical cyclohexane.

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Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

4.4 Photochemical and Photo-induced Reactions JAMES M. TANKO

4.4.1

Introduction

4.4.1.1 Definition of a Supercritical Fluid The critical point on a phase diagram designates the pressure (pC)and temperature (Tc) at which the vapor and liquid phases of a substance become indistinguishable. By definition, a supercritical fluid (SCF) is above p c and Tc. Generally, the physical properties of an SCF (density, viscosity, and dielectric constant) are intermediate between those of a liquid and a gas, and these properties vary dramatically as a function of temperature and pressure [1,2]. Because of these unique features, there is enormous interest in the use of SCFs as solvents for chemical reactions [3-61. Another reason for the increased interest in SCFs as reaction solvents is that many common SCFs are essentially nontoxic and environmentally benign (e.g. H20 and C02) [7].By virtue of the fact that these are naturally occurring substances, it is unlikely that their use in chemical synthesis and manufacture will result in unanticipated environmental damage. Also, from the standpoint of public perception, there is less likelihood of opposition to the use of these naturally occurring (and recognizable) substances.

4.4.1.2 Solvent Properties of SCFs There are several potential advantages which may be realized with the use of supercritical fluids as solvents for chemical reactions from the standpoint of reactivity and selectivity. As many of the examples discussed in this chapter illustrate, the unique features of SCFs can be exploited to control the behavior (i.e. kinetics and selectivity) of many chemical processes in a way not possible with conventional liquid solvents. Changes in reaction rates arising from direct effect of temperature and pressure on the kinetics of a reaction are governed by transition state theory, and

4.4 Photochemical and Photo-induced Reactions

28 1

the same considerations pertain to both reactions in SCF media and in conventional solvents [8-111. However, a unique feature of SCFs is that solvent properties, such as polarity (dielectric constant), viscosity, and solubility parameter, vary dramatically with temperature and pressure, and changes in these properties may alter reaction rates. (The polarity of the reaction medium will exert an effect on the rate of a chemical reaction if the polarity of the reactants and transition state are different. Solvent viscosity will exert its influence on reactions that are diffusion controlled or on reactions in which cage effects are important). Thus, control of these solvent properties (via manipulation of temperature and pressure) provides a way of adjusting the kinetics of a chemical process unique to the SCF medium [8-lo]. However, there are additional factors which may affect reactivity. Numerous studies have shown that, for SCFs in the compressible region of the phase diagram, the local solvent density about a solute is often enhanced relative to the bulk solvent density [ 12,131. The term “solvent-solute clustering” has been coined to describe this phenomenon. Because of enhanced local solvent density, the rotational and translational motion of a solute may be restricted (attributable to increased local viscosity). Reaction rate and selectivity will be perturbed only when the reactions are extremely rapid (diffusion controlled), or if cage effects are important. There is also evidence that in some cases, solutesolute clustering may be important, [ 14-17] and conceivably reaction rates could be affected because of locally higher concentrations. In general, clustering is most important near the critical point, and there is considerable interest in what effect this phenomenon has on reaction rates and selectivities [18,19]. 4.4.1.3

Scope of this Chapter

This chapter summarizes the literature to approximately mid-1998 pertaining to photochemical and photoinduced (organic) reactions in SCF solvents. Photochemical reactions involving organometallic compounds in SCF solvents have been extensively studied by Poliakoff [20], and are discussed in Chapter 4.2. In this chapter the emphasis is on reaction chemistry in which stable products are formed and isolated, and how the unique features of the SCF medium influence reaction yields, rates, and/or selectivities. What is not discussed at length are photophysical phenomena in SCF solvents (e.g. fluorescence quenching, triplet-triplet annihilation, charge transfer, and exiplex formation) which have been extensively used to probe SCF properties, in general, and have been especially informative regarding the existence of clusters (solvent-solute and solute-solute) and their effect on reactivity. Absorption and fluorescence spectroscopy (both steady state [21-33] and time resolved [34-401, vibrational spectroscopy [4 1-44] pulse radiolysis, [45] and EPR [46,47] have all been utilized in this regard.

4 Reactions in SCF

282

4.4.1.4 Experimental Considerations

Photochemical reactions conducted under supercritical conditions require high pressure reaction vessels equipped with a window which permits light to enter, and the necessary hardware and plumbing to generate high pressures and maintain constant temperatures. A typical reaction vessel [48] used for photochemical reactions conducted in supercritical carbon di'oxide (SCCOZ)is shown in Figures 4.4-1 and 4.4-2. It is fabricated from a strong (inert) alloy such as stainless steel or Hastelloy, and is equipped with a sapphire window. Sapphire is especially suited for high pressure work and is optically transparent in the region 150-6000 nm. (CaF2 and quartz have also been used.) Provisions for stirring may also be included; in many cases a simple magnetic stir bar suffices. Temperature control is typically achieved through the use of a resistive heater, thermocouple, and a temperature controller. Utilizing such a reactor, pressures up to 700-1000 bar can be achieved. A complete system for generating scCOz (Figure 4.4-3) [48] requires a device to generate high pressures (compressor, high pressure piston, or HPLC pump), a pressure transducer, and the necessary plumbing. Often, a high pressure release valve (rupture disk) is used to ensure that pressures in the reactor do not exceed specifications. Because many photochemical and free radical reactions require the exclusion of oxygen, provisions can be made for purging the system with an inert gas such as argon. Solid and liquid samples can be added to the reactor (under argon backflush); volatile liquids or gases can be introduced in glass vials or ampoules (which rupture when the reactor is pressurized).

L

threaded end-cap

copper washer sapphire window (0.5" thick) Teflon o-ring

-

inletloutlet (1 of 2) magnetic stir bar

0.88"

Figure 4.4-1 Cross-section of scCOz reactor.

4.4 Photochemical and Photo-induced Reactions

283

Figure 4.4-2 Photograph of scCOz reactor.

eJ

Figure 4.4-3 Schematic diagram of apparatus for generating scC02.

Generally, light sources typically used for photochemical reactions in conventional solvents (e.g. medium- or high-pressure mercury lamps) are used for reactions in SCF solvents. Because many popular SCFs used for photochemical experiments do not absorb UV-Vis light (e.g. COz, CHF3, low-molecular-weight alkanes), and because these experiments are usually conducted at low substrate concentrations, heating of the reactor (and the concomitant increase in pressure) is generally not a problem. Occasionally, heating may occur because of the heat given off by the lamp itself, but this can be avoided by separating the lamp and reactor and providing ample ventilation.

284

4 Reactions in SCF

After irradiation, the reactor must be vented so as to bring the system to ambient pressure. Products that are solids often precipitate from solution as the pressure is lowered, and are readily recovered. Separation of volatile compounds from an S C F is often more challenging, and usually entails bubbling the contents of the reactor into an organic solvent. Another strategy, applicable to SCFs that are not gases at room temperature ouch as H20) is to cool the reactor to room temperature where the system is no longer pressurized. Two procedures that illustrate some of the experimental protocols associated with these experiments are highlighted below: Photodimerization of isophorone in scCOz [49] This was accomplished, first by pressurizing (to ca. 4 bar) and depressurizing the reactor with C 0 2 several times to purge the system of air [49]. The reagent was introduced into the reactor via syringe under a positive pressure of COz. Subsequently the reactor was sealed, brought to the desired temperature and pressure, and irradiated with a 450 W medium pressure mercury lamp. At completion of the reaction, the reactor contents were bubbled into methylene chloride, the reactor and lines were rinsed with solvent, and the combined solutions analyzed by GC. Chlorination of cyclohexane in scCOz The following procedure was employed [50,51]. The appropriate volume of cyclohexane was placed in a 1 mL ampoule. The ampoule was degassed by several freeze-pump-thaw cycles (freezing to -198 "C, evacuating to less than 0.01 torr (0.01 mbar), and warming to room temperature), sealed under vacuum, and placed in the reactor. A second sealed ampoule containing the appropriate amount of C12 (similarly degassed) was added to the reactor. The reactor was then sealed, covered with aluminum foil (to prevent premature initiation of the reaction via action of ambient laboratory light), and brought to 40 "C (the desired reaction temperature). Following several argon purges, the reactor was pressurized with C02 and allowed to equilibrate at 40°C for several minutes. The aluminum foil was removed and the reactor was illuminated with a 450 W mercury arc lamp. Following illumination, the contents of the reactor were bubbled slowly into hexanes cooled to 0°C. An internal standard was added, and direct analyses by GC were performed to assess product yields.

285

4.4 Photochemical and Photo-induced Reactions

4.4.2 Photochemical Reactions in Supercritical Fluid Solvents 4.4.2.1

Geometric Isomerization

Aida and Squires examined the photoisomerization of E-stilbene (Scheme 4.4 -1) in conventional organic solvent (cyclohexane) and scCO2. This system was selected for study because the solvent effect on the isomerization was already documented; increased viscosity facilitates the E + Z conversion [52]. For liquid C02 at 25"C, a change in pressure from 83 bar to 214 bar (corresponding to a change in viscosity from ca. 0.07 to 0.1 cP) changes the Z E ratio from 5.5 to 6.8. For scCO2, where an analogous pressure variation changes the viscosity from 0.02 to 0.08 cP, the effect is more dramatic with Z E changing from 1.4 to 7.0 [52]. This study provides one of the first examples of how the outcome of a photochemical reaction can be altered by varying the solvent properties of the SCF via manipulation of pressure.

hv

P h A P h

Scheme 4.4-1

4.4.2.2

PhAPh

+

Ph L p h

Photodimerization

In 1989, Fox, Johnston, et al. [49] studied the [2+2] photodimerization of isophorone (Scheme 4.4-2) in scCO2 (38 "C) and scCHF3 (34.5 "C). Three dimers were produced: a head-to-head dimer (H-Hanri), and two diasteromeric head to tail dimers (H-Tanrj and H-TSyn). (In conventional solvents, Chapman found that more polar solvents favor production of the more polar product: the ratio H-Hmti :H-Ttotal was 1: 4 in cyclohexane versus 4 :1 in methanol [53]). Analogous results were observed in SCF solvents: the more polar product (H-Hanti) was a major product in the more polar solvent (CHF3, where the H-H: H-Ttotal ratio varied from 0.75 to 1.0 with increasing pressure), and only a minor product in C02 (in which the H-H :H-Ttotd ratio was essentially 0.10, independent of pressure) [49]. These observations are explicable on the basis that, over the range of pressures examined, the dielectric constant (a measure of solvent polarity) varies more for CHF3 (from 2.5 to 8.4) than it does for C02 (from 1.34 to 1.54).

$, hv

a+&+ 0

Scheme 4.4-2

H-HM (p = 5.08 D)

H-T, (p= 1.03 D)

0 H-T, (p= 1.09 D)

286

4 Reactions in SCF

4.5 4

8 z..

0

3.5

0

3

On0

0

0 0

c;i 2.5

0

0

0 0

I

2

0 CHFS

I3 co,

1.5 0

200 400 Pressure (bar)

600

Figure 4.4-4 Anti:syn ratio for the head-to-tail dimers formed in the dimerization of isophorone in scCHF3 (34.50C) and scCOz (38°C) as a function of pressure (data taken from Reference 49).

An unanticipated result of this study was that for the head-to-tail dimers, the anti :syn ratio varied with pressure (Figure 4.4-4).In conventional solvents, both are formed in approximately equal amounts. The authors suggested that “differential solvent reorganization” was responsible (i.e., that more desolvation must occur to form the syn isomer compared to the anti, Scheme 4.4-3) [49]. Thus, at higher pressures (higher solvent densities), solvent reorganization was important thereby favoring the anti isomer.

0-0 0 . . 0 0 0

/

anti

Scheme 4.4-3

Sun, et al. reported that in scC02, the quantum yield for anthracene dimerization (Scheme 4.4-4)was (a) 10 times greater in scCO2 compared to conventional liquid solvents (at comparable anthracene concentrations), and (b) pressure dependent, with the yield decreasing at higher pressures [54]. The key hv

Scheme 4.4-4

,

4.4 Photochemical and Photo-induced Reactions

287

step in the dimerization process involves formation of an eximer via diffusioncontrolled reaction of anthracene in its ground-state and singlet excited-state, A and 'A*, respectively (eq. 4.4-1). In conventional liquid solvents, the rate constant for a diffusion controlled reaction is on the order of 10" M-'s-'. However, because of the higher diffusivity of the SCF medium, the limit for diffusion-control is higher (ca. 10" M-ls-'). Thus, because the quantum yield for anthracene dimerization is directly proportional to bm,a 10-fold increase in efficiency is achieved in SCF media. (The pressure effect arises because C 0 2 viscosity increases with increasing pressure) [54].

'A*

+A

% '(AA)*

(4.4-1)

4.4.2.3 Carbonyl Photochemistry The Norrish Type I photocleavage represents a classic process in organic photochemistry, and has been extensively studied in conventional solvents. In 1991, Fox extended this reaction to SCF solvents: In an attempt to probe for cage effects (and possibly enhanced cage effects attributable to solvent-solute clustering), the photolysis of an unsymmetrical dibenzyl ketone was examined in scCOz and scC2H6 [ 5 5 ] . (Dibenzyl ketone photolysis had been shown to lead to cage effects in conventional solvents.) The rationale behind this experiment is outlined in Scheme 4.4-5. Photolysis leads to [A'CO'B],,,,. In-cage coupling is expected to yield exclusively to cross-coupling product A-B, whereas cage escape will lead to all possible coupling products, A-A, A-B, and B-B, in a statistical ratio of 1: 2 :1. Over a pressure range from just above p c to 300 bar, this reaction yielded only a statistical distribution of products [55]. Thus, no cage effect (enhanced or otherwise) was observed for this reaction.

Scheme 4.4-5

A-A + A-B

+ B-B

288

4 Reactions in SCF

This problem was recently discussed by Chateauneuf, Brennecke and coworkers who examined the decarbonylation of the phenylacetyl radical PhCH,C'=O

+ PhCH2' + CO

(4.4-2)

by laser flash photolysis [37]. These workers found no evidence for a cage effect (enhanced or otherwise) in SCF solvent. Moreover, to explain the absence of a cage effect in these reactions, they went on to suggest that the integrity of the cage is maintained for only a few picoseconds, whereas decarbonylation occurs in the time regime of a few hundred nanoseconds (i.e. the cage disintegrates long before in-cage coupling can occur) [37]. In order to address this issue, a process which involves a much shorter-lived radical pair needed to be examined. Toward this end, Weedon, et al. examined the photo-Fries rearrangement of naphthyl acetate (Scheme 4.4-6) in scCO2 at 35 and 46°C [56]. Photolysis of 1 leads to caged pair [2/3]; reaction in-cage yields the photo-Fries products, 2- or 4-acetylnaphthol (4 and 5). However, cage escape, followed by hydrogen abstraction (isopropanol was present as a hydrogen atom donor) leads to a-naphthol (6).

1

2

in-cage, followed by enolization

1

RH/

&

COCH3

Scheme 4.4-6

d 6

(4 : 1,2-; 5: I&)

A plot of the product ratio (4+5):6 as a function of pressure is presented in Figure 4.4-5. This plot exhibits a dramatic spike at pressures near the critical pressure which the authors attribute to the onset of solute-solvent clustering; disintegration of the caged pair is inhibited because the viscosity at the molecular level is much greater than the bulk viscosity.

4.4 Photochemical and Photo-induced Reactions

289

2

1 150

Figure 4.4-5 Ratio of products produced from photolysis of a-naphthyl acetate in COz (data taken from Reference 56).

50

100

Pressure (bar)

4.4.3 Photo-induced Reactions in Supercritical Fluid Solvents 4.4.3.1 Free Radical Brominations of Alkyl Aromatics in Supercritical Carbon Dioxide

In 1994, Tanko and Blackert reported that the free radical bromination of alkylaromatics (e.g. toluene) could be carried out in scC02 [57]. This reaction is photoinitiated, and proceeds via the chain process outlined in Scheme 4.4-7 [58]. Reaction yields were analogous to those observed using conventional solvents (e.g. CC14). Via competition experiments, the relative reactivity of the secondary hydrogens of ethylbenzene versus the primary hydrogens of toluene on a per hydrogen basis, r(2"/1"), were assessed. Within experimental error, selectivity did not vary over a pressure range from 75 to 423 bar (r(2"/1") = 30 2) at 40°C [57]. In retrospect this result is reasonable because (a) the hydrogen abstraction step is insensitive to solvent polarity effects, and is well below the diffusion controlled limit so that viscosity effects are unimpor-

*

Br' PhCHp*

PhCHs

+ +

Br2

-

HBr

+

PhCH2Br

PhCH2* +

Br-

I

pmpagation

4 Reactions in SCF

290

tant, and (b) the difference in the volume of activation for hydrogen abstraction from toluene versus ethylbenzene is small (ca. 4.8 cm3/mol), so that over the range of pressures examined, the selectivity change would be of the same magnitude as experimental error [48,57]. The observed selectivity in scC02 is nearly identical to that found in conventional organic solvents: r(2"/1°)= 35 k 1, 34 f 1, and 29 f 1 for CC14, Freon 113,and CH2C12, respectively, at 40°C [48,57].These results confirm the role of Br' as the chain carrier in these experiments as depicted in Scheme 4.4-7,and suggest that Br' selectivity is not altered by complexation to C02. It is noteworthy that Br' forms a complex with CS2 (which is isoelectronic to C02) and that this complex exhibits enhanced selectivities in hydrogen atom abstractions [59]. With molecular bromine (Br2) as 'the brominating agent, a small amount of p-bromotoluene is formed, arising from the competing electrophilic aromatic substitution process. However, with the use of N-bromosuccinimide (NBS) as the brominating agent in direct analogy to the classical Ziegler reaction (Scheme 4.4-8),the EAS side product is completely eliminated. Reaction yields and selectivities are identical to those observed in CCL, the solvent most widely used for the Ziegler reaction [48,57]. Competition experiments (ethylbenzene versus toluene) confirm the role of Br' as chain carrier in the Ziegler bromination in scCOz [57]. The role of NBS in this reaction is to maintain a low, steady-state concentration of Br2 (Scheme 4.4-9).

ocH3- aCHzBr {NBr

+

hv

+

0

0

(NW Bra PhCH,* NBS

PhCH3

+

Br,

+

+

HBr

{NH

-

(SH) HBr

+

PhCHzBr SH

+

Scheme 4.4-8

PhCH,. + Br,

Br*

Scheme 4.4-9

4.4.3.2 Free Radical Chlorination of Alkanes in Supercritical Fluid Solvents The free radical chlorination of alkanes represents a classic procedure for the functionalization of alkanes R-H

+ C12 + R-C1 + HCl

(4.4-3)

Many of the details of this reaction have been understood for more than half a century [60]. In the laboratory, this reaction is initiated by action of visible

29 1

4.4 Photochemical and Photo-induced Reactions

Scheme 4.4-10

RH

+

CI.

R.

+

CI2

-

RRCI

+ +

HCI CI-

light (C12 + hv + 2 Cr), with product formation occurring via the propagation steps outlined in Scheme 4.4-10. Chlorine atom abstracts hydrogen from the alkane yielding an alkyl radical and HC1. The alkyl radical subsequently reacts with molecular chlorine yielding the product alkyl chloride and regenerating chlorine atom. The chlorine atom is a highly reactive species, and exhibits low selectivity in hydrogen abstractions. In solution, 3" C-H (4.2) > 2" C-H (3.6) > 1" C-H (l.O), on a per hydrogen basis (25 "C) [58], absolute rate constants for hydrogen abstraction are just slightly below the diffusion-controlled limit [61]. The chlorine atom cage effect, first discovered by Skell [62] in 1983, has been the subject of numerous investigations [63-651. Put briefly, for the chlorine atom abstraction step in the free radical chlorination of an alkane (RH2), the geminate RHCVC1' caged pair is partitioned between three pathways (Scheme 4.4-1 1): diffusion apart &iff), abstraction of hydrogen from RH2 comprising the cage walls (kRHZ), and a second in-cage abstraction of hydrogen from the alkyl chloride (RRHC~). Although the k d i and ~ kRH2 steps result in the formation of monochloride (RHC1, M), the kRHClstep results in the formation of polychlorides (P)via RCl'

+ Cl2 + RClZ + C1'

(4.4-4)

In conventional solvents, the ratio of mono- to polychlorinated products (M/P) has been shown to depend on solvent viscosity [65]. Tanko, et al. utilized the chlorine atom cage effect as a highly sensitive probe for studying the effect of SCF viscosity and the possible role of solvent clusters on cage lifetimes and reactivity [50,5 11. These experiments were conRH2 + CI* RH. + CI2

,

-

-

(RHCI/CI*),,

(RHCI I CI*),,

f

RCI9 + HCI

RHCl + CI.

(W Scheme 4.4-11

HCI + RH.

RHCl + RH- + HCI (M)

292

4 Reactions in SCF

ducted in scC02 (40 "C at various pressures), with parallel experiments in conventional solvents and in the gas phase. Cage effects are typically quantified in terms of the Noyes model, which predicts that the efficiency of cage escape should vary linearly with the inverse of viscosity (l/q) [66]. In Figure 4.4-6, M / P ratios observed in the chlorination of 2,3-dimethylbutane, neopentane, and cyclohexane are plotted as a function of l/q for the experiments conducted in scCO2 and in conventional solvents. Overall, these plots are linear over a range of viscosities spanning 1.7 orders of magnitude (from conventional solvents to scCO2) and provide no indication of an enhanced cage effect (unusually low observed M/P ratio) near the critical pressure. It is also worth noting that the best straight line through the solution phase results successfully predicts the SCF phase results [50,5 I]. Based upon these observations, there was no indication of an enhanced cage effect near the critical point in scCOz solvent. The magnitude of the cage effect observed in scCOz at all pressures examined is well within what is anticipated, based upon extrapolations from conventional solvents. It was suggested that for instances where enhanced cage effects have been observed attributable to solute-solvent clustering, this enhancement may be unique to the specific systems studied [SO, 5 11. The experiments with 2,3 -dimethylbutane (23DMB) provided insight into the extent that C1' selectivity varies as a function of pressure. In the gas phase (40°C), the relative reactivity of the 3" and 1" hydrogens of 23DMB (r(3"/1")) is 3.97. In the condensed phase (neat 23DMB, 40"C), r(3"/1") = 3.27. In scC02, r(3"/1") varies with pressure and falls between the gas and solution phase values. The fact that r(3"/1") is so close to the solution and

25

1

2o 15

I

02-6

0

Oneopentam Ocydohexane

M/P

10

5

0

0

20

40 1hl

Figure 4.4-6 Ratio of mono- to polychlorides produced in the free radical chlorination of 2,3-dimethylbutane (23DMB), neopentane, and cyclohexane in conventional and supercritical fluid solvents as a function of inverse viscosity at 40°C (data taken from References 50 and 51).

4.4 Photochemical and Photo-induced Reactions

293

gas-phase values suggests that C1' selectivity is not altered by complexation to C02 [51]. The slight variation in r(3"/1") can be explained as follows. The rate constants for lo, 2", or 3" hydrogen abstractions by C1' from alkanes are nearly diffusion-controlled in conventional solvents. Consequently, the intrinsic selectivity of C1' is diminished in conventional solvents because of the onset of diffusion control. In the gas phase, selectivity is slightly higher because the barrier imposed by diffusion is eliminated. The viscosity of a supercritical fluid (a) lies between that of conventional fluid solvent and the gas phase and (b) varies with pressure. Because of the low viscosity of supercritical fluids, bimolecular rate constants greater than the 10" M-'s-l diffusion-controlled limit cari be realized in SCF and, as a consequence, enhanced selectivity is achieved. Consistent with this interpretation is the observation that the plot of r(3"/1°) versus inverse viscosity is approximately linear (Figure 4.4-7) [51]. 4.1

Figure 4.4-7 Chlorine atom selectivity in scCOz solvent at 40°C (data taken from Reference 51).

4.4.4

2.5

{

I 0

40

20

80

IIq (cP-')

Conclusions

The examples discussed demonstrate that the unique nature of SCFs provides a means of dialing up the selectivity of a chemical process in a manner that is impossible using conventional solvents (i.e. by manipulation of temperature and pressure). SCF solvents such as C 0 2 and H20, the latter of which has not yet been exploited as a solvent for organic photochemistry, are especially attractive as they are environmentally benign alternatives to a number of classical solvents which pose hazards to either health or the environment. Coupled with the tunable properties of a supercritical fluid, these solvents emerge not only as viable alternatives to conventional organic solvents, but in some cases at least, superior alternatives. Finally, a point which has not been fully emphasized in this chapter is that SCF solvents are superb tools for probing

294

4 Reactions in SCF

solvent effects in chemical processes, as it is possible to vary (via manipulation of pressure) pertinent solvent properties (such as viscosity and polarity) without changing the molecular functionality of the solvent.

4.4.5 Acknowledgments Support from the National Science Foundation (CHE-9524986) is acknowledged and appreciated.

References [ 11 M. McHugh,

V. Krukonis, Supercritical Fluid Extraction, Principles and Practice, Butterworths, Boston, 1986, pp. 1-11. [2] K. P. Johnston, in Supercritical Fluid Science and Technology, ACS Symp. Ser. 406 K. P. Johnston, J. M. L. Penninger (Eds.), American Chemical Society; Washington, DC 1989, pp. 1-12. [3] B. Subramaniam, M. A. McHugh, Ind. Eng. Chem. Process Des. Dev. 1988, 25, 1-12. [4] B. C. Wu, S. C. Paspek, M. T. Klein, C. LaMarca in Supercritical Fluid Technology: Reviews in Modern Theory and Applications, T. J. Bruno, J. F. Ely, (Eds.), CRC Press, Boca Raton. 1991, DD. 511-524. [5] P. E. .Savage, S. Gopalan,-?. I. Mizan, C. J. Martino, E. E. Brock, AIChE J. 1995, 41, 1723-1778. [6] C. A. Eckert, B. L. Knutson, P. G. Debenedetti, Nature 1996, 383, 313-318. [7] (a) I. Amato, Science 1993, 259, 1538; (b) R. E. Wedin, Todays Chemist at Work 1993, 2 , 16; (c) D. Illman, Chem. Eng. News 1993, 71 (May 29). 5. [8] S. Kim, K. P. Johnston, in Supercritical Fluids, ACS Symp. Sel: 329, T. G. Squires, M. E. Paulaitis. (Eds.), American Chemical Society, Washington, DC 1987, pp. 42-55. [9] S. Kim, K. P. Johnston, Chem. Eng. Comm. 1988, 63, 49-59. [lo] K. P. Johnston, C. Haynes, AIChE J. 1987, 33, 2017-2026. [ l l ] B. C. Wu, M. T. Klein, S. I. Sandler, Ind. Eng. Chem. Res. 1991, 30, 822-828. [12] S . Kim, K. P. Johnston, AIChE J. 1987, 33, 1603-1611. [13] K. P. Johnston, G. J. McFann, D. G. Peck, R. M. Lemert, Fluid Phase Equilibria 1989, 52, 337-346. [14] J. B. Ellington, K. M. Park, J. F. Brennecke, Ind. Eng. Chem. Res. 1994, 33, 965-974; C. B. Roberts, J. Zhang, J. E. Chateauneuf, J. F. Brennecke, J. Am. Chem. SOC.1995, 117, 6553-6560, J. Zhang, D. P. Roek, J. E. Chateauneuf, J. F. Brennecke, J. Am. Chem. SOC. 1997, 119, 9980-9991. [15] C. E. Bunker, Y.-P. Sun, J. Am. Chem. SOC. 1995, 117, 10865; C. E. Bunker, Y.-P. Sun, J. R. Cord, J. Phys. Chem A. 1997, 101, 9233. [16]T. A. Rhodes, M. A. Fox, J. Phys. Chem. 1996, 100, 17931-17939. [17] P. C. Debenedetti, I. B. Patsche, R. S. Mohamed, Fluid Phase Equilibria, 1989, 52, 347-356. [18] T. W. Randolph, J. A. O’Brien, S. Ganapathy, J. Phys. Chem. 1994, 98, 4173-4179. [19] S . C. Tucker, M. W. Maddox, J. Phys. Chem. B. 1998, 102, 2437-2453. [20] M. Poliakoff, J. J. Turner in Molecular Cyrospectroscopy, R J. H. Clark and R. E. Hester (Eds.), John Wiley and Sons; New York, 1995, pp. 275-306. [21] M. E. Sigman, S.’M. Lindley, J. E. Leffler, J. Am. Chem. SOC.1985, 107, 1471-1472.

4.4 Photochemical and Photo-induced Reactions

295

[22] C. R. Yonker, S . L. Frye, D. R. Kalkwarf, R. D. Smith, J. Phys. Chem. 1986, 90, 30223026; C. R. Yonker, R. D. Smith, J. Phys. Chem. 1988, 92, 2374. [23] T. Okada, Y. Kobayashi, H. Yamasa, N. Mataga, Chem. Phys. Lett. 1986, 128, 583-586. [24] S . Kim, K. P. Johnston, Ind. Eng. Chem. Res. 1987, 26, 1206-1213. [25] 0. Kajimoto, M. Futakami, T. Kobayashi, R. Yamasaki, J. Phys. Chem. 1988, 92, 13471352. [26] J. F. Deye, T. A. Berger, A. 6. Anderson, Anal. Chem. 1990, 62, 615-622. [27] J. F. Brennecke, D. L. Tomasko, J. Peshkin, C. A. Eckert, Ind. Eng. Chem. Res. 1990, 29, 1682-1690. [28] R. M. Lemert, J. M. DeSimone, J. Supercritical Fluids 1991, 4, 186-193. [29] Y.-P. Sun, G. Bennett, K. P. Johnston, M. A. Fox, J. Phys. Chem. 1992, 96, 1000110007; Y.-P. Sun, M. A. Fox, K. P. Johnston, J. Am. Chem. SOC. 1992, 114, 11871194; Y.-P. Sun, M. A. Fox, J. Am. Chem. SOC. 1993, 115, 747-750. [30] Y. Ikushima, N. Saito, M. Arai, J. Phys. Chem. 1992, 96, 2293-2297. [31] Y.-P. Sun, C. E. Bunker, N. B. Hamilton, Chem. Phys. Lett. 1993, 2 1 0 , 111-117; Y.-P. Sun, C. E. Bunker, Ber: Bunsenges. Phys. Chem. 1995, 99, 976-984; H. W. Rollins, R. Dabestani, Y.-P. Sun, Chem. Phys. Lett. 1997, 268, 187-193. [32] J. B. Ellington, K. M. Park, J. F. Brennecke, Ind. Eng. Chem. Res. 1994, 33, 965-974. [33] K. Takahashi, K. Abe. S . Sawamura, C. D. Jonah, Chem. Phys. Lett. 1998, 282, 361-368. [34] C. Gehrke, J. Schroeder, D. Schwarzer, J. Troe, F. Voss, J. Phys. Chem. 1990, 92, 48054816. [35] T. A. Betts, J. Zagrobelny, F. V. Bright, J. Am. Chem. SOC. 1992, 114, 8163-8171; J. Zagrobelny, T. A. Betts, F. V. Bright, J. Am. Chem. SOC. 1992, 114, 5249-5257; M . P. Heitz, F. V. Bright, J. Phys. Chem. 1996, 100, 6889-6897. [36] C. B. Roberts, J. E. Chateauneuf, J. F. Brennecke, J. Am. Chem. SOC. 1992, 114, 84558463; C. B. Roberts, J. Zhang, J. F. Brennecke, J. E. Chateauneuf, J. Phys. Chem. 1993, 97, 5618-5623; C. B. Roberts, J. Zhang, J. E. Chateauneuf, J. F. Brennecke, J. Am. Chem. SOC. 1995, 117, 6553-6560. [37]C. B. Roberts, J. Zhang, J. E. Chateauneuf, J. F. Brennecke, J. Am. Chem. SOC. 1993, 115, 9576-9582. [38] R. M. Anderton, J. F. Kauffman, J. Phys. Chem. 1995, 99, 13759-13762. [39] 0. Kajimoto, K. Sekiguchi, T. Nayuki, T. Kobayashi, Ber: Bunsenges. Phys. Chem. 1997, 101, 600-605. [40] Q. Ji, C. R. Lloyd, E. M. Eyring, R. van Eldik, J. Phys. Chem. A 1997, 101, 243-247. [41] S. Akimoto and 0. Kajimoto, Chem. Phys. Lett. 1993, 209, 263-268. [42] R. S. Urdahl, K. D. Rector, D. J. Myers, P. H. Davis, M. D. Fayer, J. Phys. Chem. 1996, 105, 8973-8976. [43] J. N. M. Hegarty, J. J. McGarvey, S. E. J. Bell, A. H. R. Al-Obaidi, J. Phys. Chem. 1996, 100, 15704-15707. [44] X.-Z. Sun, M. W. George, S . G. Kazarian, S. M. Nikiforov, M. Poliakoff, J. Am. Chem. SOC. 1996, 118, 10525-10532. [45] K. Takahashi, C. D. Jonah, Chem. Phys. Lett. 1997, 264, 297-302. [46] T. W. Randolph, C. Carlier, J. Phys. Chem. 1992, 96, 5146-5151. [47] S . Ganapathy, C. Carlier, T. W. Randolph, J. A. O’Brien, Ind. Eng. Chem. Res. 1996, 35, 19-27. [48] J. M. Tanko, J. F. Blackert, M. Sadeghipour, in Benign By Design. Alternative Synthetic Design f o r Pollution Prevention, ACS Symp. Ser. 577, P. T. Anastas, C. A. Farris (Eds.), American Chemical Society, Washington, DC 1994, pp. 98-1 13. [49] B. J. Hmjez, A. J. Mehta, M. A. Fox, K. P. Johnston, J. Am. Chem. SOC. 1989, 111, 2662-2666. [50] J. M. Tanko, N. K. Suleman, B. Fletcher, J. Am. Chem. SOC. 1996, 118, 11958-11959. [51] B. Fletcher, N. K. Suleman, J. M. Tanko, J. Am. Chem. SOC. 1998, 120, 11839-11844.

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[52] T. Aida, T. G. Squires, in Supercritical Fluids: Chemical and Engineering Principles and Applications ACS Symp. Sel: 329, M. E. Paulaitis, (Ed.), American Chemical Society; Washington, DC 1987, pp. 58-66. [53] 0. L. Chapman, P, J. Nelson, R. W. King, D. J. Trecker, A. Griswold, Rec. Chem. Prog. 1967, 28, 167. [54] C. E. Bunker, H. W. Rollins, J. R. Gord, Y.-P.Sun, J.-Org. Chem. 1997, 62, 7324-7329. [55] K. E. O’Shea, J. R. Combes, M. A. Fox, K. P. Johnston, Photochem. and Photobiol. 1991, 54, 571-576. [56] D. Andrew, B. T. Des Islet, A. Margaritis, A. C. Weedon, J. Am. Chem. SOC. 1995, 117, 6 132-6 133. [57] J. M. Tanko, J. F. Blackert, Science 1994, 263, 203-205. [58] G. A. Russell, J. Am. Chem. SOC. 1958, 80, 4997-5001. [59] M . Sadeghipour, K. Brewer, J. M. Tanko, J. Org. Chem. 1997, 62, 4185-4188. [60] K. U. Ingold, J. Lusztyk, K. D. Raymr, Acc. Chem. Res. 1990, 23, 219-225. [61] N. J. Bunce, K. U. Ingold, J. P. Landers, J. Lusztyk, J. C. Scaiano, J. Am. Chem. SOC. 1985, 107, 5464. [62] P. S. Skell, H. N. Baxter 111, J. Am. Chem. SOC. 1985, 107, 2823-2824. [63] K. D. Raner, J. Lusztyk, K. U. Ingold, J. Am. Chem. SOC. 1988, 110, 3519-3524. [64] J. M. Tanko, F. E. Anderson 111, J. Am. Chem. SOC. 1988, 110, 3525-3529. [65] D. D. Tanner, H. Oumar-Mahamat, C. P. Meintzer, E. C. Tsai, T. T. Lu, D. Yang, J. Am. Chem. SOC. 1991, 113, 5397-5402. [66] See T. Koenig, H. Fischer, H. in Free Radicals, Vol. 1, Kochi, J. K. (Ed.), Wiley, New York, 1973, pp. 157-189.

Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

4.5 Polymerizations in Dense Carbon Dioxide TAMMY A. DAVIDSON and JOSEPH M. DESIMONE

4.5.1

Introduction

The scientific community has been striving to become more environmentally conscious, especially in reducing aqueous waste streams, minimizing the emissions of volatile organic compounds (VOCs), and attempting to completely phase out the use of chlorofluorocarbons. These concerns provide the primary motivation to develop carbon dioxide-based technologies, especially in the polymer industry. Recently, there has been great interest in utilizing dense C02 (liquid and supercritical) as a continuous phase for various polymerization reactions. Supercritical fluids (SCFs) possess unique properties, such as tunable densities and the ability to plasticize glassy polymers, making them attractive solvents which have been relatively unstudied until quite recently. This chapter focuses on recent efforts that have explored the use of supercritical C02 (scC02) as a polymerization solvent. The reader is also referred to two recent reviews on polymerizations in liquid and supercritical 0 2 [1,21. Due to their unique properties, SCFs present an interesting medium in which to conduct chemical reactions. An SCF is defined as a substance or mixture at conditions which exceed the critical temperature (T,) and pressure (p,) of the sample (Chapter 1.1). The properties of SCFs are intermediate between those of liquids and gases. For instance, an SCF can have a liquid-like density while exhibiting a gas-like viscosity. A significant advantage of employing an SCF as a reaction medium is that the solvent strength and other typical solvent properties (e.g. dielectric constant) can be manipulated simply by varying the pressure and temperature of the system [3-61. The scientific community has begun to realize that carbon dioxide represents an attractive continuous phase for many applications, especially as it is naturally occurring and readily available. Carbon dioxide can be harvested from abundant natural reservoirs, and recycled C02 can be recovered from the waste streams of power plants and industrial plants that produce ammonia, ethanol, hydrogen, and ethylene oxide [7]. In addition,

298

4 Reactions in SCF

the critical point of C 0 2 is easily accessible at a T, of 31.1 "C and a p c of 73.8 bar [8]. The benefits associated with utilizing carbon dioxide as a solvent include that it is inexpensive, nontoxic, nonflammable, and is easily recycled. Since the late 1970s, a considerable amount of research has focused on the advantages which stem from utilizing C 0 2 in polymer processing. The greatest benefit can be attained most efficiently when the polymer is actually synthesized in the supercritical medium. For example, as the density of an SCF can be adjusted drastically just by varying the pressure of the system, mixtures of polymers with different molecular weights can be easily fractionated [9-121. Supercritical C 0 2 can also be used to extract unreated monomer, solvent, or catalyst from a polymerization product [5]. Employing C02 as a polymerization solvent can eliminate the need for energy intensive drying processes, as polymerization products isolated after venting C 0 2 are completely dry. As scC02 sorption by a polymer causes a reduction in solution and melt viscosities [ 13-15], the resulting polymer morphology can be controlled by supercritical drying [16] or foaming [17-191. In addition to these advantages, the low T, of carbon dioxide makes it useful as a solvent in applications involving heat-sensitive materials, including pharmaceuticals [20], flavors [2 11, enzymes [22,23], and highly reactive monomers [24]. For these reasons, carbon dioxide represents a reaction medium which may allow new advances in polymerization techniques, as well as for synthetic chemistry in general. When considering compressed C 0 2 as a polymerization medium, its solvency and plasticization effects on the resulting polymeric products are of primary importance. When these capabilities are considered along with the fact that C 0 2 may be able to reduce the use of much more expensive and hazardous solvents, the importance of studying C 0 2 as a continuous phase for polymer synthesis and processing can be easily seen. The solubility behavior of small molecules in compressed C 0 2 has been well studied. Carbon dioxide has a low dielectric constant, with reported values ranging from 1.01 to 1.45 for gaseous C 0 2 and 1.60 to 1.67 for liquid C02 [25]. Other solubility parameters for C 0 2 show even more pronounced dependence on the pressure of the system [26]. Carbon dioxide behaves essentially like a hydrocarbon solvent with respect to its ability to dissolve small molecules, and many monomers that are soluble in hexane have been found to be soluble in C02 [27]. Although it has a relatively low dielectric constant, C 0 2 is capable of dissolving some polar molecules, such as methanol, due to its considerable quadrupole moment [28]. However, other polar molecules, including amides, ureas, urethanes, azo dyes, and water, display very limited solubility in C 0 2 [27]. In the late 1920s, Lowry and Erickson studied the solubility of water in C 0 2 and discovered that less than 0.05 wt% of water dissolves in liquid C 0 2 over a range of temperatures [29]. More recently, King et al. completed a detailed investigation of the C02-H20 binary system under both liquid and supercritical conditions [30]. Additionally, the solubilities of many compounds of low volatility have been studied in supercritical C02, and tabulated by Bartle et al. [31].

4.5 Polymerizations in Dense Carbon Dioxide

299

Contrary to the generalizations for small molecules, carbon dioxide is an exceedingly poor solvent for most high molecular weight polymers. For example, a moderate molecular weight sample of poly(methy1 acrylate) (PMA, M, = 10600 g/mol) required a temperature of 80°C and a pressure of over 2000 bar to obtain a- homogeneous solution (2 wt%) in C 0 2 [32]. In fact, amorphous fluoropolymers and silicones are the only classes of polymers that exhibit appreciable solubility in carbon dioxide under relatively mild conditions (below 100°C and below 350 bar) [9,26,33-361. Although the factors that govern the solubility of polymeric materials in C02 are not fully understood, there have been many studies that have examined the various solute-solvent interactions [37-421. Regardless of the nature of these interactions, the limited solubility of most polymers in C 0 2 at reasonable temperatures and pressures requires that heterogeneous polymerization techniques be employed for the synthesis of most industrially important hydrocarbon-based polymers. When polymerizations are conducted in carbon dioxide, plasticization of the polymer results at certain temperatures. This plasticization by C02 is instrumental in both the diffusion of unreacted monomer into the swollen polymer phase [43-451 and the transport of additives into the polymer matrix [44, 46-48]. The effects of compressed C 0 2 on the glass transition temperature (T,) and mechanical properties of polystyrene (PS) have been studied by Wang et al. and significant plasticization of PS by C 0 2 was observed [49]. A few years later, Chiou e f al. utilized differential scanning calorimetry to estimate the glass transition temperatures of several polymers, including poly(methy1 methacrylate) (PMMA), PS, polycarbonate (PC), poly(viny1 chloride) (PVC), poly(ethy1ene terephthalate) (PET), and blends of PMMA and poly(viny1idene fluoride) (PVF2), under C 0 2 pressures up to 25 bar, and substantial reductions in Tg of up to 50 "C were observed [50]. Polymer swelling [5 13 and creep compliance, as a function of time [52], were measured by Wissinger and Paulaitis to determine the Tg values of PMMA, PS, and PC in carbon dioxide at increased pressures. In addition to these early studies, a considerable amount of research has focused on the plasticization of polymeric materials by dissolved carbon dioxide [53-621. It can be seen that the ability of compressed C 0 2 to plasticize polymers is an integral part of many polymerization processes that are currently in development. Although the polymer industry has just begun to utilize the advantages of carbon dioxide, the food industry has been exploiting these techniques for many years, emphasizing that supercritical fluids present an economical alternative to hazardous solvents. As an example, the coffee industry now utilizes scC02 to decaffeinate coffee on an industrial scale, rather than dichloromethane as was used previously [63,64]. Actually, more than 90% of SCFbased separations that are conducted today employ carbon dioxide as the eluent [65]. Certainly, the many advantages of utilizing C 0 2 as a solvent make it an attractive alternative for the polymer community. Polymerization reactions are typically carried out in our laboratory according to the general procedure outlined below.

300

4 Reactions in

SCF

General Polymerization Procedure Polymerizations are carried in 10 or 25 mL cylindrical 316 stainless steel high pressure view cells equipped with 1 cm thick sapphire windows that allow for visual observation of the reaction mixture [66]. The cell is sealed with Teflon O-rings, and the contents are mixed using a magnetic stirbar and an external stirplate. Polymerizations are usually run as 10% (w/v) solutions in C 0 2 . In a typical reaction, the reactor is charged with initiator and, if applicable, the desired amount of surfactant. The reactor is purged with a flow of argon and the monomer is then added to the system under argon via syringe. An Isco Model No. 260D automatic syringe pump is used to pressurize the reactor with the desired amount of C 0 2 , and the reactor is heated with external heating tape. At the end of the reaction, COz js slowly vented from the cell, and the polymeric products are removed from the cell.

4.5.2

Homogeneous Solution Polymerizations

Many high-molar-mass fluoropolymers have extremely poor solubility in most common organic solvents, which leads to difficulties in synthesis and processing of these materials. It has been determined that chlorofluorocarbons (CFCs) and carbon dioxide are the best solvents for amorphous fluoropolymers. Because of the environmental issues associated with using CFCs as solvents, the scientific community is turning toward more benign alternatives with which to replace CFCs, including hydrochlorofluorocarbons and hydrofluorocarbons. However, these alternative solvents present environmental problems as well, most notably the accumulation of trifluoroacetic acid in the atmosphere [67]. Therefore, carbon dioxide is an ideal inert alternative solvent for the polymerization of highly fluorinated monomers.

4.5.2.1 Free-Radical Chain Growth High-molar-mass amorphous fluoropolymers have been synthesized using freeradical initiators in scCOz [34, 68-72]. The solutions remained homogenous throughout the polymerization due to the high solubility of the resulting fluoropoly mers in CO2. Several fluorinated acrylate monomers have been polymerized in this manner to give high yields of polymer. As an example, the polymerization of 1,l-dihydroperfluorooctyl acrylate (FOA) is shown in Scheme 4.5-1. Other fluorinated acrylate polymers that have been generated from the same methodology include poly [2 -(N-ethy 1perfluorooctanesulfonamido)ethy1 acrylate], poly[2-(N-ethylperfluorooctanesulfonamido)ethyl methacrylate], and poly[2-(N-methylperfluorooctanesulfonamido)ethyl acrylate] [73]. Another class of monomers that has been polymerized via homogeneous solution poly-

4.5 Polymerizations in Dense Carbon Dioxide

301

59.4 "C CO2 212 bar

48h

Scheme 4.5-1 Homopolymerization of FOA in COz. *Contains ca. 25% -CF3 branches [34].

merization in scCOz includes styrenes substituted in the para position with perfluoroalkyl side chains, such as p-perfluoroethyleneoxymethylstyrene (STF) [68]. The synthesis and polymerization of STF is given in Scheme 4.5-2. As expected, the polymer generated from solution polyfiierization in C 0 2 is identical to that obtained from solution polymerization in 1,1,2-trichloro1,2,2-trifluoroethane (Freon-1 13), which indicates that C 0 2 can be used with excellent results as a replacement solvent for CFCs in this type of reaction. Additionally, this method has been used in the synthesis of statistical copolymers of fluorinated monomers with various hydrocarbon monomers, including methyl methacrylate (MMA), butyl acrylate (BA), ethylene, and styrene, as shown in Table 4.5 -1. Carbon dioxide-soluble polymeric amines have also been synthesized via homogeneous solution copolymerization of FOA with 2-(dimethy1amino)ethylacrylate or 4 -vinylpyridine [74]. Table 4.5-1 Statistical copolymers of FOA with vinyl monomers. Polymerizations were conducted at (59.4 f 0.1) "C and (348.5 f 0.5) bar for 48 h in C02. Intrinsic viscosities were determined in 1,1,2-trifluorotrichloroethane(Freon-113) at 30 "C [ 191.

Copolymer

Feed ratio

Incorporation

Intrinsic viscosity (dug)

Poly(FOA-co-MMA) Poly(F0A-co-styrene) Poly(FOA-co-BA) PoMFOA-co-ethy lene)

0.47 0.48 0.53 0.35

0.57 0.58 0.57

0.10 0.15 0.45 0.14

-

302

4 Reactions in SCF

0

'+

F(CF2)&H&HrOH

AIBN

co2

353.0 bar, 60°C 3 days

Scheme 4.5-2 Synthesis and polymerization STF [68].

CHZOCH~CH~C~F~, of

We have also studied the kinetics of free radical initiation in C02 using azobis(isobutyronitri1e) (AIBN) as an initiator [35]. These experiments were accomplished using high pressure UV spectroscopy, and illustrated that AIBN decomposes more slowly in C 0 2 than in traditional hydrocarbon solvents, yet the initiator efficiency is much greater in C 0 2 due to the reduced solvent cage effect in the low viscosity supercritical medium. The main conclusion drawn from this work was that C 0 2 can therefore be employed effectively as a solvent for free radical polymerizations and remains an inert solvent even in the presence of highly electrophilic hydrocarbon radicals. A significant advantage of conducting polymerization and oligomerization of fluoroalkanes in carbon dioxide rather than other solvents is the absence of chain transfer to COz. Radicals generated from fluoroalkene monomers such as tetrafluoroethylene (TFE) are quite electrophilic, and will undergo facile chain transfer to virtually any hydrocarbon that is present in the system. Moreover, highly reactive monomers such as TFE can be handled more safely as

4.5 Polymerizations in Dense Carbon Dioxide

303

c4F01

Scheme 4.5-3 Telomerization of TFE in supercritical C 0 2 [75].

F2C=CF2

____)

c4F,-(cF2-cF2k

I

C 0 2 (solvent)

mixtures with COz [24]. An advantage of conducting polymerizations of these highly reactive monomers in CO2 is that the monomer mixture can be used directly without removal of the COz. The free radical telomerization of TFE in supercritical COz makes use of the high solubility of low molecular weight perfluoroalkyl iodides to homogeneously synthesize the telomers [75]. In these reactions, perfluorobutyl iodide was employed as the telogen and reactions were conducted:in both the presence and absence of the thermal initiator AIBN. When AIBN was used, the results were inconsistent and not reproducible, prohably because of chain transfer to initiator. However, when the telomerizations were conducted in the absence of initiator (Scheme 4.5 -3), the resulting perfluoroalkyl iodides had controlled molecular weights and narrow molecular weight distributions, as described in Table 4.5-2. The key aspect of fluoroalkene polymerization in carbon dioxide is whether the resulting polymer is soluble in CO2. Fluoropolymers that have a high degree of crystallinity are inherently solvent resistant. In order to generate a fluoroalkene polymer in COz via homogeneous polymerization, one needs to run these reactions at conditions where the resulting fluoropolymer is in the amorphous state. This amorphous morphology can be readily obtained by lowering the crystalline melting point of the fluoropolymer through the synthesis of copolymers, or by eliminating crystallization alltogether with high levels of comonomers. Using this methodology, polymerizations can be conducted at temperatures that exceed the melting temperature of the resulting polymer, hence the reaction will be homogeneous [76]. 4.5.2.2 Cationic Chain Growth Several fluorinated monomers have been polymerized via homogeneous cationic polymerization in compressed carbon dioxide. For example, vinyl ethers with fluorinated side chains were polymerized in SCCOZat 40°C using adventitious water initiation with ethylaluminum dichloride as the Lewis acid coinTable 4.5-2 Experimental results from the telomerization of TFE in supercritical COz. Perfluorobutyl iodide was employed as the telogen [75].

[monomer]/[telogen]

Yield (%)

Mn(glmol)

MJMn

1.6 1.5 1.8 2.2

88 87 86 78

570 590 630 650

1.35 1.38 1.38 1.44

304

4 Reactions in SCF 1) EtAIC12 ethyl acetate

co

H 2 - 7

3580 bar, 40 “C

PR

+2-yH)n

2) sodium ethoxide

PR Scheme 4.5-4 Homogeneous cationic polymerization of fluorinated vinyl ethers in supercritical COz [77].

-

R = -CH2CH2(CF,),CF3 ; n = 5 7

R = -CH&HZNSO&aF,,

I C3H7

1) BF3-THF

COP

296 bar 0 OC

2) NaOH CH20CH&F&F&F,

*

YH3

H-(O-CH2-C-CH,)OH

I

n

CH2OCH&F&F&F:

(FOX-7)

Scheme 4.5-5 Homogeneous cationic polymerization of FOX-7in liquid COa [77].

itiator, as depicted in Scheme 4.5-4 [77]. Both the monomer and coinitiator were soluble in C02, and the solution remained homogeneous throughout the polymerization as expected. Typical conversions for these reactions were approximately 40%, and the number average molecular weight (M,)for the polymer bearing a fluorinated sulfonamide side chain was 4.5 X lo3 g/mol with a polydispersity index (PDI) of 1.6. In addition to the vinyl ethers, a fluorinated oxetane, 3 -methyl-3’-[( 1,1-dihydroheptafluorobutoxy)methyl]oxetane (FOX-7), was homogeneously polymerized in liquid C 0 2 at 0°C and 296 bar for 4 h, as shown in Scheme 4.5-5 [77]. For this system, trifluoroethano1 was employed as the initiator and boron trifluoride tetrahydrofuranate as the coinitiator. The resulting polymer was obtained in 77% yield with an M,, of 2.0 X lo4 g/mol and a PDI of 2.0. A control experiment was conducted in Freon-1 13 and resulted in a polymer with comparable yields and molecular weights, indicating once more that CO2 represents a suitable alternative to CFCs as a reaction medium. The results with these systems also indicate that cationic chain growth mechanisms can be effectively utilized for the homogeneous polymerization of fluorinated monomers in C02.

4.5 Polymerizations in Dense Carbon Dioxide

305

4.5.3 Heterogeneous Polymerizations 4.5.3.1 Free-Radical Chain Growth Precipitation Polymerizations Much of the early work that'focused on the synthesis of industrially important polymers in carbon dioxide was dominated by precipitation polymerizations. Hagiwara et al. examined the polymerization of ethylene in C02 using gamma radiation and AIBN as free radical initiators at a pressure of 445 bar and temperatures ranging from 20 to 45°C [78]. Carbon dioxide was chosen as the polymeri'zation solvent due to its high stability to ionizing radiation. Analysis of the polymers by infrared spectroscopy revealed that utilizing a C 0 2 continuous phase had little effect on the polymer structure. They also examined the kinetics of initiation, propagation, and termination for the gamma radiation induced ethylene polymerization in liquid C02 [78]. In these experiments, it was found that the initiation rate increased with increasing C 0 2 concentration, which was attributed to the effects of the electron density of the reaction medium on the absorption of the gamma radiation. Although C02 was shown to be inert to growing oligomeric radicals at low dose rates (9.0 X lo2 radh), higher dose rates produced oxygen as a byproduct of irradiation, causing the formation of carboxyl terminated polymer chains. The study also examined the effects of various alkyl halides on radiation induced polymerization of ethylene in C02 [79], and a patent was obtained for continuous polymerization of ethylene in COz using a tubular reactor [80]. It was determined that, although the ethylene was initially soluble in the C 0 2 continuous phase, the polyethylene that formed was insoluble in C 0 2 and was isolated in powder form and could easily be removed from the reactor. Precipitation polymerizations typically produce powder products, and an advantage of utilizing C 0 2 as a solvent is the dryness of the resulting polymer. The polymerization of vinyl monomers in liquid and supercritical C02 has been studied extensively. Patents were issued in 1968 to the Sumitomo Chemical Company [81] and in 1970 to Fukui et al. [82] for the preparation of homopolymers of polystyrene, poly(viny1 chloride), poly(acrylonitri1e) (PAN), poly-(acrylic acid) (PAA), and poly(viny1 acetate) (PVAc), as well as the random copolymers PS-co-PMMA and PVC-co-PVAc. Additionally, a patent was issued in 1995 to Bayer AG [83] for the preparation of styrene/acrylonitrile copolymers in scCO2. In 1986, the BASF Corporation was issued a Canadian patent for the precipitation polymerization of 2-hydroxyethylacrylate and various N-vinylcarboxamides in compressed carbon dioxide [84]. In 1988, Terry et al. attempted to homopolymerize ethylene, 1-octene, and 1-decene in scC02 for the purpose of increasing the viscosity of C02 for enhanced oil recovery [85]. These reactions utilized free-radical initiation with benzoyl peroxide and t-butylperoctoate at 71 "C and 100-130 bar for 24-48 h. Although the resulting polymers were not well characterized, they were found to be relatively

306

4 Reactions in SCF

insoluble in the C 0 2 continuous phase, and therefore ineffective as viscosity enhancers. It should also be noted that a-alkenes are known not to yield high polymer via free-radical polymerization due to substantial chain transfer to monomer. The precipitation polymerization of acrylic acid- in COz has been well studied, probably because the reaction has a fast rate of propagation that enables the formation of high-molecular-weight polymer 'even though it has limited solubility in the continuous phase. Several patents have been issued for the preparation of poly(acry1ic acid) in C 0 2 [86,87]. Also studied was the polymerization of acrylic acid in COz, which demonstrated effective molecular weight control by employing ethyl mercaptan as a chain transfer agent [88]. Other experiments demonstrated that the molecular weight of the PAA can be controlled through manipulation of reaction pressures and temperatures [891. The precipitation copolymerization has been reported of tetrafluoroethylene (TFE) with peffluoro(propy1 vinyl ether) (PPVE) and with hexafluoropropylene (HFP) in supercritical C 0 2 using bis(peffluoro-2 -propoxy propiony1)peroxide as an initiator at 35°C (below the melting temperature of the polymer which is approximately 310 "C) and pressures less than 135 bar [90,91]. Polymerization in the C 0 2 system resulted in good yields of high-molarmass copolymers (>1O6 g/mol) and offers three significant advantages over the conventional aqueous process. First, the highly electrophilic propagating radicals derived from the fluoroalkene monomers do not chain transfer to the C 0 2 reaction medium. Second, the aqueous process typically results in the production of materials with reactive carboxylic acid and acid fluoride end groups that can cause difficulties in processing and inferior polymer performance; however, in the carbon dioxide-based system, these detrimental end groups do not form to any great extent. The third advantage involves the safety issues associated with the handling of highly reactive monomers such as TFE. The use of stable free-radical polymerization techniques in COz represents an emerging new area of research. Ode11 and Hammer have demonstrated the use of reversibly terminating free radicals generated by systems such as benzoyl peroxide or AIBN and 2,2,6,6,-tetramethyl- 1-piperidinyloxy free radical (TEMPO) to polymerize styrene at a temperature of 125 "C and pressures of 245-280 bar in C 0 2 [92]. At low monomer concentrations (10% by volume), the polymerization resulted in low conversions of PS with an M, of about 3000 g/mol and a narrow molecular weight distribution (PDI c 1.3). NMR analysis of the resulting polymer confirmed that the precipitated polystyrene chains are predominantly end-capped with TEMPO. Additionally, the polymer could be isolated and later extended by the addition of more monomer under an inert argon blanket. It was also determined that the precipitated PS could be extended while still in the C02 continuous phase simply by increasing monomer concentration in the reactor.

4.5 Polymerizations in Dense Carbon Dioxide

307

Emulsion and Dispersion Polymerizations Heterogeneous polymerization has been widely discussed in the literature, yet there is often inconsistency in the terminology used to discuss these processes, and some clarification is warranted here. In any heterogeneous polymerization process, either the monomer or the resulting polymer, is insoluble in the reaction medium. The four common heterogeneous processes - suspension, precipitation, emulsion, and dispersion - are distinguished on the basis of the initial state of the polymerization mixture, the mechanism of particle formation, the kinetics of polymerization, and the shape and size of the resulting polymer particles [93]. Precipitation polymerizations in carbon dioxide were discussed earlier in this chapter. The other two heterogeneous polymerization methods that have been studied in C 0 2 are emulsion and dispersion polymerization. In an emulsion polymerization, the monomer has poor solubility in the continuous phase, and the reaction begins as a heterogeneous mixture. The initiator is preferentially dissolved in the continuous phase and not in the monomer phase, as is necessary to take advantage of desirable Smith-Ewart kinetics [94]. As a result of the unique kinetics of an emulsion polymerization, highmolecular-weight polymer can be produced at very high rates of polymerization. The polymer particles that result from an emulsion polymerization are spherical and typically smaller than 1 pm in diameter. Nevertheless, most vinyl monomers exhibit high solubility in COZYsuggesting that emulsion polymerization in C 0 2 is unlikely to be a practical process for synthesis of commercially important polymers. Unlike emulsion polymerization, dispersion polymerizations are initially homogeneous due to the solubility of both the initiator and the monomer in the continuous phase. When the growing oligomeric radicals reach a critical molecular weight, phase separation occurs. The polymer particles are stabilized as a colloid, and consequently the reaction continues to higher degrees of polymerization than those obtained from the analogous precipitation polymerization in the absence of a stabilizer. Dispersion polymerizations do not follow SmithEwart kinetics because the initiator and monomer are not segregated or compartmentalized, yet enhanced rates of polymerization can be observed due to autoacceleration within growing polymer particles. As for emulsion polymerization, spherical particles are also obtained from dispersion polymerization; however, these particles usually range in size from 100 nm to 10 pm [93]. As many small organic molecules are readily soluble in compressed COz, dispersion polymerization represents the optimal method that has been found to produce high-molecular-weight hydrocarbon polymers in a C 0 2 continuous phase. Design and Synthesis of Stabilizers Coagulation or flocculation of the growing polymer particles in a colloidal dispersion can be prevented by using electrostatic, electrosteric, or steric stabilization, and the use of these methods in conventional liquid solvents has been extensively reviewed [95-971. Steric stabilization offers several advantages over the other two methods because it allows for large variations in reac-

308

4 Reactions in SCF

tion conditions. Polymeric stabilizers are often effective for use in solvents with low dielectric constants, making steric stabilization a logical choice for carbon dioxide-based systems. Steric stabilization works in a heterogeneous system by the attachment of the stabilizing molecule to the surface of the growing polymer particle, either by grafting or physical adsorption. The polymeric stabilizer exists preferentially at the polymer-solvent interface and prevents aggregation of the polymer particles by coafing the surface of each particle and creating long-range repulsions between them. These repulsions must be great enough to overcome long-range van der Waals attractions [96]. A lattice fluid self-consistent field theory was developed by Peck and Johnston to describe the surfactant chain at an interface in a compressible fluid [98]. This development allowed traditionid colloid stabilization theory [95] to be applied to SCF continuous phases. Amphiphilic polymeric stabilizers have long been recognized for their effectiveness in traditional aqueous systems. The term “hydrophilic-lipophilic balance” (HLB) has been widely used in the literature to describe the relative solubilities of the hydrophilic and lipophilic segments of the molecules in aqueous and organic media, respectively. However, the use of the HLB term would be unsuitable for carbon dioxide systems because of its unique solvation properties. A more appropriate term with which to describe the amphiphilic character of stabilizers in C 0 2 is the “anchor-soluble balance” (ASB), which qualitatively describes the relative proportions of the soluble and insoluble constituents of the stabilizer [96]. An amphiphilic polymer must have an appropriate ASB in order to function effectively as a stabilizer. For carbon dioxide applications, the amphiphilic polymer is comprised of a segment that has high solubility in carbon dioxide, termed the “C02-philic” segment [99,100], as well as an anchoring segment which has very limited solubility in carbon dioxide, termed the “CO2-phobic” segment [ 1011. The C02-phobic segment may be either hydrophilic or lipophilic, as dictated by the nature of the monomer to be polymerized. Additionally, a range of compositions and architectures can be explored when designing stabilizers for use in carbon dioxide, varying from homopolymers to random, block, and graft copolymers. Many surfactants that are commercially available were designed for use in an aqueous continuous phase, and are consequently completely insoluble in C02. Consani and Smith [lo21 have studied the solubility of more than 130 surfactants in C02 at 50°C and 100-500 bar, and have concluded that commercial surfactants form microemulsions much more readily in other low-polarity supercritical fluids such as xenon and alkanes rather than in carbon dioxide. The authors also confirmed the compatibility of fluorinated hydrocarbons and C02 that had been previously suggested by McHugh and Krukonis [ 5 ] . A great deal of work has gone into the design and synthesis of polymeric materials that are suitable for stabilizing the polymerization of water-soluble monomers or dispersing large amounts of water in COz. Two general approaches to accomplishing this task are discussed below. An initial approach targeted the design and synthesis of C02-philichydrophilic block and graft copolymers to be utilized in the dispersion of hydrophi-

4.5 Polymerizations in Dense Carbon Dioxide

309

lic monomers in C02. Also described has been the use of the macromonomer technique to synthesize an amphiphilic graft copolymer with a CO2 -philic poly ( 1,l-dihydropeffluorooctyl acrylate) (PFOA) backbone and hydrophilic poly(ethy1ene oxide) (PEO) grafts [1031. Solvatochromic characterization was utilized to show that the PEP grafts allowed the hydrophilic dye molecule to be solubilized in scC02. Characterization of this graft copolymer by small angle X-ray scattering revealed that spherical micelles were formed in the presence of water in a C 0 2 continuous phase [104]. These results were significant, as amphiphilic molecules which are effective as stabilizers for heterogeneous systems tend to form micelles in solution. Additionally, this was the first direct confirmation that micelles could form in compressed CO2. A second approach to aqueous-C02 stabilization utilizes the high solubility of fluorinated and siloxane-based polymeric materials to bring an ionic headgroup into a CO2 continuous phase. As C 0 2 has a low dielectric constant and low polarity, it was predicted that the ionic headgroups would associate in the presence of polar molecules, such as water and charged metal species, allowing these polar materials to be evenly dispersed in the nonpolar C02 continuous phase. Beckman and co-workers have examined the effect of the polarity of the hydrophilic head group on the phase behavior of silicone-based and fluoroether-functionalized amphiphiles in supercritical CO2 [36,105]. The fluoroether-functionalized amphiphile was shown to allow the extraction of thymol blue from an aqueous solution into C02. Johnston and co-workers reported the formation of a one-phase microemulsion consisting of the hybrid fluorocarbon-hydrocarbon surfactant C7FI5CH(OSO3-Na+)C7H15and water in carbon dioxide with a water-to-surfactant ratio as high as 32:l at 25 "C and 236 bar [106]. Using this type of surfactant was shown to increase the amount of water dissolved in C 0 2 by an order of magnitude. More recently, Johnston et al. have disclosed the formation of an aqueous microemulsion in a CO2 continuous phase by employing an ammonium carboxylate peffluoropolyether surfactant [ 1071. These aqueous microemulsions have been studied using various spectroscopic techniques, and are discussed in greater detail in two recent reviews [108,109] and in Chapter 2.4. A considerable amount of work has focused on the design and synthesis of macromolecules for use as emulsifiers for lipophilic materials and as polymeric stabilizers for the colloidal dispersion of lipophilic, hydrocarbon polymers in compressed C02. It has been shown that fluorinated acrylate homopolymers, such as PFOA, are effective amphiphiles as they possess a lipophilic acrylate-like backbone and CO2 -philic, fluorinated side chains, as indicated in Figure 4.5-1 [loo]. Furthermore, it has been demonstrated that a homopolymer which is physically adsorbed to the surface of a polymer colloid precludes coagulation due to the presence of loops and tails [110]. These fluorinated acrylate homopolymers can be synthesized homogeneously in C02 as described in an earlier section. The solution properties [ 111,1121 and phase behavior [45] of PFOA in scC02 have been thoroughly examined. Additionally, the backbone of these materials can be made more lipophilic in nature by incorporating other monomers to make random copolymers [34].

310

4 Reactions in SCF

.-

Lipophilic, acrylic-like anchor

>

C02-philic,fluorinated side chain *

~i~~~ 4.5-1 Structure of amphiphilic polymeric stabilizer, PFOA. *Contains ca. 25 % -CF3 branches [loo].

In addition to the use of amphiphilic polymers as stabilizers for dispersion polymerizations, a variety of copolymerizable materials can be employed for the stabilization of COZ-phobic polymer colloids [ 1131. Unlike amphiphilic materials which adsorb to the surface of the growing polymer particle, this type of lyophilic stabilizer chemically grafts to the surface of the particle. These copolymerizable materials must possess a long C02-philic tail in order to be effective, and can be in the form of macroinitiators, macro-chain transfer agents, and macromonomers. Using this method, a block or graft copolymer that behaves as a stabilizer is formed in sifu. A significant advantage of this type of approach is that a very small amount of stabilizer is needed to be effective [ 1141. A copolymerizable polydimethylsiloxane (PDMS) monoacrylate is also effective at stabilizing the dispersion polymerization of MMA in carbon dioxide (Scheme 4.5-6) [113]. Although other researchers have examined the behavior of polysiloxanes in COz [ll, 12,141, these experiments represent the first successful use of PDMS stabilizers in COz. The polymerizations were conducted in either liquid COz at 30°C and 76 bar or in supercritical CO2 at 65°C and 344 bar using azo initiators, which do not chain transfer to the PDMS backbone, and resulted in the formation of spherical PMMA particles. Control reactions were run without stabilizer or with PDMS homopolymer (which does not have a polymerizable group), yielding lower-molecular-weight polymers and conversions than those reactions run in the presence of the PDMS macromonomer. It was found that, although the PDMS macromonomer could copolymerize with MMA to stabilize the polymer colloid, only a small amount of the PDMS macromonomer actually reacted with MMA, and washing the final polymer with either hexanes or C 0 2 facilitated the removal of the unreacted PDMS macromonomer. A series of block copolymer surfactants has been designed and synthesized for use in COz-based applications. Due to the amphiphilic nature of these

4.5 Polymerizations in Dense Carbon Dioxide

311

PDYS macromonomer

MYA

m130

AlBN

%,

345 bar 4 hOUM

+ & - X

Scheme 4.5-6 Dispersion polymerization of MMA in C 0 2 using PDMS macromonomer as a copolymerizable stabilizer [ 11 31.

7’0 OCH3

PMMA

copolymers, it was predicted that these materials would self-assemble in a C02 continuous phase to form micelles with a C02-phobic core and a CO2-philic corona. Fluorocarbon-hydrocarbon block copolymers of PFOA and PS have been synthesized using free radical techniques [ 1011, and self-assembly of these molecules into multimolecular micelles in solution has been confirmed by small angle neutron scattering [115]. A schematic representation of the micelles formed from these amphiphilic block copolymers is given in Figure 4.5 -2. In addition to the fluorocarbon-based amphiphiles, the siloxane-based block copolymer stabilizer PS-b-PDMS has been shown to be effective at

,

‘C0,-phobic’ core

I ‘C0,-philic’

Figure 4.5-2 Schematic representation of micelle formed by amphiphilic diblock copolymer.

corona

312

4 Reactions in SCF

H3c-CH2-CHfCH2Hu7w0k$ kc H 3 structures of Figure 4.5-3 Chemical 'iH3

y

3

y

3

CH3 CH3

the amphiphilic diblock copolymeric stabilizers, PS-b-PFOA (top) and PS-b-PDMS (bottom) [116].

the stabilization of colloidal particles [116,117]. Figure 4.5-3 shows the chemical structures for both the fluorocarbon-based and the siloxane-based amphiphilic diblock copolymers. Both these classes of surfactants contain lipophilic anchoring segments that form the cores of the micelles in C02, and may find uses in a variety of applications including polymer processing and separations. Polymerization of Lipophilic Monomers

The first successful dispersion polymerization of a lipophilic monomer (MMA) in a supercritical fluid continuous phase was reported in 1994 [loo]. These experiments were conducted in C 0 2 at 65 "C and 210 bar using AIBN or a fluorinated derivative of AIBN as the initiator. The homopolymer PFOA was employed as a stabilizer, generating PMMA at high conversions (>90%) and high degrees of polymerization (>3000) in scC02. Table 4.5-3 illustrates the results of the polymerizations initiated with AIBN. The polymer was isolated from these dispersion polymerizations as spherical particles merely by venting the C 0 2 from the reaction mixture. Scanning electron microscopy conTable 4.5-3 Results of MMA polymerization with AIBN as the initator in COz at 210 bar and 65°C; stabilizer is either low molecular weight (LMW) or high molecular weight (HMW) PFOA [ 1001.

Stabilizer (w/v a)

Yield (%)

Mn ( x 1o - g/mol) ~

MwIMn

Particle size (P)

0% 2% 4% 2% 4%

39 85 92 92 95

149 308 220 315 321

2.8 2.3 2.6 2.1 2.2

1.2 1.3 2.7 2.5

LMW LMW HMW HMW

(kO.3) (*0.4) (*0.1)

(kO.2)

4.5 Polymerizations in Dense Carbon Dioxide

313

firmed that the product was spherical in nature and in the micrometer size range with a relatively narrow particle size distribution. As a comparison, control reactions were conducted in C 0 2 in the absence of the PFOA stabilizer. These reactions produced polymer of considerably lower molar masses and conversions, as well as a random morphology as compared to the spherical particles generated with the added stabilizer. These results indicate that the amphiphilic PFOA macromolecule was integral to the stabilization of the growing PMMA colloid particles. It has recently been determined that very small amounts of the PFOA stabilizer (0.24 wt%) are sufficient to achieve a stable dispersion of PMMA particles in C02 [45,118]. The stabilizer was subsequently removed from the polymer product by, extraction with C 0 2 , an important aspect as the stabilizer is expensive and residual amounts could have adverse effects on polymer performance. Also studied were the effects of reaction time and pressure on conversions, polymer molecular weight, and particle size. It was determined that a gel effect occurs in the growing PMMA particles between one and two hours of reaction time, a result which is also seen in a typical dispersion polymerization in organic media after 20-80% conversion [96]. Additionally, as C02 is able to plasticize the growing polymer particles, diffusion of monomer into the particle is increased, causing autoacceleration which permits the reaction to proceed to higher conversions. The phase behavior of PFOA in C02 has been thoroughly studied, and these experiments have indicated lower critical solution temperature (LCST) behavior and have confirmed the greater solubility of fluorinated acrylate polymers in C02 than in other hydrocarbon analogs [451. Amphiphilic molecules have been used as stabilizers in COz for the synthesis of polystyrene in high degrees of polymerization (>800) and high conversions (>95 %) [I 171. The stabilizers used in these experiments with PS were diblock copolymers of PS, the anchoring block, and PFOA, the soluble block [loll, as initial attempts at using PFOA homopolymer as a stabilizer gave inadequate results. This observation indicates that the C02-phobic nature of the PS block has a significant affinity for the polymer-solvent interTable 4.5-4 Results of styrene polymerization with AIBN as the initiator in COz at 207 bar and 65 "C; stabilizer is either PFOA homopolymer or PS-b-PFOA copolymer [117].

Stabilizer

Yield (%)

M, ( X lo3

M,/M,

Particle size (Pm)

Particle size Distribution

2.3 2.8 3.6 3.1 3.0

none none 0.40 0.24 0.24

none none 8.3 1.3 1.1

g/mol) none PolY(FOA) (3.7 W16 K)" (4.5 W25 K)" (6.6 W35 K)"

22.1 43.5 72.1 97.7 93.6

3.8 12.8 19.2 22.5 23.4

" The notation indicates molecular weights of PS and PFOA blocks, respectively, meric stabilizers.

in copoly-

314

4 Reactions in SCF

Figure 4.5-4 Scanning electron micrographs of PS synthesized in C 0 2 (a) without stabilizer and (b) with PS-b-PFOA stabilizer [117].

face, thus better stabilizing particle formation. Table 4.5 - 4 describes the results from these polymerizations. The PS generated from these dispersion polymerizations was isolated directly from the reaction vessel as a dry, white, free-flowing powder. Figure 4.5 - 4 shows scanning electron micrographs of PS synthesized in COz (a) without stabilizer and (b) with PS-bPFOA stabilizer. The diameter and size distribution of the polystyrene particles was controlled by varying the anchor-soluble balance of the polymeric stabilizer. Additional studies with block copolymer stabilizers have focused on the silicone-based diblock copolymer PS-b-PDMS [ 1171. Utilizing the PS-b-PDMS stabilizer offers several advantages over the fluorinated stabilizers, including relatively low cost of materials, ease of synthesis and characterization, and the use of living anionic polymerization techniques in the synthesis of the stabilizers, which allows for narrow molecular weight distributions in the blocks. This silicone-based stabilizer was effective for the polymerization of both styrene and methyl methacrylate in supercritical CO;! at 65°C and 350 bar, producing high yields (>90%) of fine, dry, white polymer powders (Figure 4.5-5). The PMMA particles isolated from these polymerizations had an M , of 1.8 X lo5 g/mol and an average particle diameter of 0.23 pm, whereas the PS particles had an M , of 6.5 X

315

4.5 Polymerizations in Dense Carbon Dioxide

Figure 4.5-5 Scanning electron micrographs of PS particles synthesized via dispersion polymerization in: C02 using PS-b-PDMS stabilizers [ 1171.

lo4 g/mol and an average diameter of 0.22 pm. It is interesting to note that the PMMA particles generated when the PS-b-PDMS is utilized as a stabilizer are nearly ten times smaller than those that are produced when PFOA homopolymer is used as a stabilizer. As a comparison, the analogous PDMS homopolymer was ineffective at stabilizing these systems, further indicating that the interfacial interaction of the PS anchoring segment is integral to the efficacy of these stabilizers.

Polymerization of Hydrophilic Monomers The feasibility of conducting an inverse emulsion polymerization of acrylamide in C 0 2 has been examined by Adamsky and Beckman [119]. They polymerized acrylamide in the presence of water, a cosolvent for the monomer, in COz at 350 bar and 60 "C using AIBN as an initiator. Polymerizations were conducted both in the presence and absence of an amide-capped poly(hexafluoropropylene oxide) surfactant, as shown in Figure 4.5-6.Table 4.5-5shows a summary of the results obtained from these polymerizations. When no stabilizer was used in these reactions, the precipitation polymerization of acrylamide resulted in a single mass of high-molecular-weight polymer in high conversion. When the amide-capped poly(hexafluoropropy1ene oxide) stabilizer was used, the reaction mixture had a milky white appearance, which is indicative of latex formation. However, no significant increase in polymerization rate with added surfactant was reported, and no micrographs of the particles were included to support the hypothesis that the surfactant was effective at stabilizing the growing polymer particles in a reverse microemulsion.

Figure 4.5-6 Amide end-capped poly(hexafluoropropylene oxide) used for the inverse emulsion polymerization of acrylamide in C02 [119].

0 II

F+icF2-ofFF-c-NH2 CF3 14

316

4 Reactions in SCF

Table 4.5-5 Results of acrylamide polymerization in CO2 at 60°C [119].

Concentration of fluorinated polyether

Polymer property

intrinsic viscosity (dug)

M" x lod Huggins constant Yield (wt%)

0%

1%

2%

11.60 6.61 0.310 91.4

12.28 7.09 0.505 99.8

9.15 4.92 0.479 99.8

Polymer Blends Supercritical CO2 is effective at swelling and plasticizing solid polymer samples. In 1991, the concept of infusing a polymer with a monomer carried in an SCF was patented by Sunol for the preparation of polymer-laden wood [120]. Watkins and McCarthy have employed this idea to develop a route to synthesizing polymer blends [43,44,121]. They infused a scCOz swollen solid polymer substrate with styrene monomer, which was then polymerized in situ, as shown in Figure 4.5-7. Either AIBN or t-butyl perbenzoate was used as an initiator, and the solid polymer matrices were composed of high density polyethylene (HDPE), bisphenol A polycarbonate, poly(ch1orotrifluoroethylene) (PCTFE), poly(4-methyl-1-pentene) (PMP), or nylon-6,6 -poly(oxymethylene). Transmission electron microscopy and energy dispersive X-ray analysis confirmed that the polystyrene exists as discrete phase-segregated regions throughout a PCTFE matrix. Thermal analysis of the blends indicated that radical grafting reactions are minimal. These experiments signify that COz plasticization can be a useful technique for the transport of small molecules into polymeric phases, allowing preparation of polymer blends or the incorporation of additives.

heat

infusion

v

solid polymer 8Ub8tMte

decornpm!

SCF/8tyrenelinltiator 8WOllen8Ub8tratO

8ub8tratdpoiy8tyrene blemd

Figure 4.5-7 Schematic of novel route to composite polymer materials [43,44].

4.5 Polymerizations in Dense Carbon Dioxide

317

4.5.3.2 Cationic Chain Growth In addition to the free radically initiated polymerization systems, cationic polymerization represents another method in which both liquid and supercritical C02 have been successfully employed as continuous phases for heterogenous polymerizations. Supercriticd C02 is particularly interesting in that the dielectric constant of the medium can be varied to some extent simply by changing the temperature or pressure of the system [122], and thus the intimacy of the propagating ion pair can be adjusted. This methodology may offer a significant advantage over conventional systems which are limited by chain transfer to monomer, as the “livingness” of the cationic species could be readily controlled in C02..Initial experiments with heterogeneous cationic polymerization have centered on the polymerization of formaldehyde [123-1 261, ethyl vinyl ether [82], oxetanes [77], and isobutylene [127] in liquid C02, in addition to the polymerization of isobutylene [128-1311, isobutyl vinyl ether [77,132], and epoxides [133] in scC02. Only those polymers generated in a scC02 continuous phase are discussed in further detail here. Nearly 500 million pounds of butyl rubber are produced annually in the USA via cationic copolymerization of isobutylene and small amounts of isoprene [134]. The industrial methods for polymerizing isobutylene are plagued by two major drawbacks - the use of toxic chlorinated hydrocarbon solvents and the need to carry out these polymerizations at very low temperatures. Each of these drawbacks may be circumvented through the use of carbon dioxide as the continuous phase for polymerization. The first example of the synthesis of polyisobutylene in compressed carbon dioxide was reported in 1960 by Biddulph and Plesch using titanium tetrachloride or aluminum bromide as a catalyst in liquid C02 at -50°C [127]. More recently, the polymerization of isobutylene in scC02 with added cosolvents has been examined by Pernecker and Kennedy [128-1311. They reported the synthesis of polyisobutylene (7-35 % conversion) in C02 with methyl chloride as a cosolvent at 32.5”C and about 120 bar utilizing 2 -chloro-2,4,4-trimethylpentane (TMPC1)/SnCl2 and TMPCVTiC14 as initiators. They note that this is the highest temperature at which isobutylene has been successfully polymerized to yield reasonably high-molecular-weight polymer (M,in the range of 1000-2000 g/mol and PDIs from 1.5 to 3.4), results which they attribute to the limited amount of chain transfer in the supercritical solvent as compared to traditional systems [1281. Cationic polymerization in scCO2 could be advantageous over conventional systems which must be cooled to -20 “C to -100 “C in order minimize chain transfer reactions which serve to lower the molecular weights of the resultant polymers [130]. These authors have also reported the synthesis of tert-C1-terminated isobutylene via mixed Friedel-Crafts acid initiating systems [ 1281, the determination of the ceiling temperature of isobutylene polymerization in scCO2 [ 1291, and the synthesis of poly(isobuty1ene-co-styrene) [ 1301. Vinyl ethers are another class of monomers which have been successfully polymerized in C02. As described earlier, fluorinated vinyl ether monomers

318

4 Reactions in SCF

can be polymerized via homogeneous solution polymerization in carbon dioxide due to the high solubility of the resulting amorphous fluoropolymers in C02. In contrast, polymerization of hydrocarbon vinyl ethers in C02 occurs through a heterogeneous precipitation process. Recently studied are the polymerization of isobutyl vinyl ether (IBVE) in scC02. These experiments were conducted in the temperature range 30-60°C and at 350 bar using an initiator comprised of the adduct of acetic acid and IBVE, ethyl aluminum dichloride as the Lewis acid coinitiator, and ethyl acetate as the Lewis base deactivator [77, 1321. This initiating system has been employed by Higashimura for the living cationic polymerization of vinyl ethers in conventional solvents [ 1351. The polymerizations conducted in C02 became heterogeneous upon formation of the insoluble hydrocarbon polymer, yet high molecular weights and conversions were obtained. As a control, analogous polymerizations were conducted in cyclohexane, yielding comparable molecular weights and conversions, but with more narrow molecular weight distributions. It is known that C 0 2 can act as a monomer in some cationic polymerization processes initiated by Lewis acids [136], and it was therefore crucial to show that the scC02 reaction medium remained inert throughout the polymerization and did not become incorporated into the polymer backbone. Characterization of the polymers by 'H and 13C NMR, as well as by infrared spectroscopy, confirmed that the polymers generated in scCO2 were no different structurally than those synthesized in hexane, proving that the C 0 2 continuous phase is indeed inert to cationic polymerization [132]. Another class of monomers that have been polymerized via heterogeneous cationic polymerization in C 0 2 is strained cyclic ethers, such as epoxides and oxetanes. Early work in this area focused on the ring-opening polymerization of epoxides including ethylene oxide, propylene oxide, and styrene oxide [ 1331, using catalyst systems such as triethyl aluminum, aluminum bromide, diethyltin-water, and titanium tetrachloride. The products of these polymerizations were easily isolated after venting of the C 0 2 as dry, free-flowing powders in 32-93 % yields. Additionally, supercritical fluid extraction with C02 facilitated the removal of unreacted monomer. Recent work has examined the precipitation polymerization of 3,3'-bis(ethoxymethy1)oxetane (BEMO) in liquid COz (-10°C and 290 bar) using BF3 as an initiator [77]. Similar results were obtained when the polymerization of BEMO was carried out in dichloromethane, further confirming that compressed C 0 2 is an inert reaction medium for cationic polymerization. One final example of cationic polymerization in compressed C 0 2 is the preparation of polydiorganosiloxanes, as reported by Rhone-Poulenc Specialty Chemicals [ 1371. These reactions employed trifluoromethane sulfonic acid as an initiator for the polymerization of octamethylcyclotetrasiloxane (D4) in gaseous, liquid, and supercritical C02. The polymerizations in scC02 resulted in good yields of high-molecular-weight polymer (M,, in the range of 8.3 X lo4 g/mol to 2.2 X lo5 g/mol). These results also serve to verify the possibility of conducting cationic polymerizations in carbon dioxide.

4.5 Polymerizations in Dense Carbon Dioxide

319

4.5.4 Metal-catalyzed Polymerizations 4.5.4.1 Ring-opening Metathesis Polymerization

Although a detailed discussion of metal-catalyzed polymerizations in carbon dioxide is also presented in Chapter 4.7, a brief mention of metal-catalyzed polymerizations is warranted here. A ring-opening metathesis polymerization has been conducted in compressed carbon dioxide [138,139]. Bicyclo[2.2.1]hept-2 -ene (norbornene) was polymerized using [Ru(H~O)~](TOS)~ as the initiator (where Tos = tosylate) in both C02 and COZ/methanol mixtures, as described in Scheme 4.5 -7. Polymerizations were conducted at 65 "C and pressures ranging from 60 to 350 bar, resulting in polynorbornene with similar conversions and molecular weights as found with other solvents. Proton NMR confirmed the structure of the resulting polymer, and illustrated that the cis/ trans ratio of the polymer is affected by the methanol concentration in the system. When pure methanol or methanoVCO2 was used as a solvent, the polynorbornene had a very high trans-vinylene content, whereas the polymer generated in pure COz was primarily comprised of cis-vinylene geometry. This variation in double bond geometry arises from the syn and anti propagating carbene rotomers of the catalytic species, and is very susceptible to solvent effects [140]. These initial experiments with norbornene give insight into the possibility of conducting other metal catalyzed polymerizations in compressed C02. In addition to these experiments, other examples of metathesis reactions in C 0 2 have been recently presented utilizing well-defined carbene complexes as initiators [141]. Scheme 4.5-7 Ring-opening metathesis polymerization of norbornene in COz [138,139].

compressed co, 60 350 bar, 65°C

-

mJ(H,OM(Tos),

4.5.4.2 Epoxide-COz Copolymers

Thus far, the discussion of polymerizations conducted in carbon dixiode has centered on systems where C 0 2 acts only as a solvent for the polymerization. However, there are also examples of polymerization systems where COz acts as a comonomer. Most notable among these in the context of this chapter is the coploymerization of C 0 2 and epoxides. The copolymerization of propylene oxide and carbon dioxide was conducted in scCO2 using a heterogeneous zinc catalyst [ 1421. Additionally, Beckman and co-workers have shown that a soluble, fluorinated ZnO-based catalyst can be effectively utilized to promote the copolymerization of C 0 2 and cyclohexene oxide [143]. These examples indicate that supercritical carbon dioxide can be viable as both a solvent comonomer in polymerization reactions.

320

4 Reactions in SCF

4.5.4.3 Oxidative Coupling Polymerizations

Poly(2,6-dimethylphenylene oxide) (PPO) has been synthesized in a C02 continuous phase using an oxidative coupling polymerization method [74]. These polymerizations were conducted at 350 bar and room temperature or 40°C for 20 h using a CuBr/amine/oxygen catalyst, as shown in Scheme 4.5-8. The resulting PPO was isolated in high yield and exhibited number average molecular weights as high as 1.7 x lo4 g/mol. The effects of the amine structure on the polymerization were examined, and it was determined that polymeric amines were able to stabilize the growing PPO particles as a colloid in addition to catalyzing the reaction. Additionally, when a polymeric stabilizer such as PFOA or PS-b-PFOA was added, :polymer yields and molecular weights increased. As PPO is plasticized by C 0 2 [68],stabilization as a dispersion results in higher yields and conversions than the analogous precipitation polymerizations in the absence of stabilizer.

CuBr/amine/02

Scheme 4.5-8 polymerization Oxidative coupling of

@ok

@OH COZ CH3

CH3

2,6-dimethylphenol in COZ [74].

4.5.5 Step-growth Polymerizations Throughout this chapter, the examples of polymerizations in compressed COz have been primarily for chain growth polymerization processes. However, stepgrowth methods represent an area of new interest for SCFs. Initial experiments in this area include the synthesis of aromatic polyesters such as poly(ethy1ene terephthalate) (PET) in scC02 as illustrated in Scheme 4.5-9 [144]. An advan-

HOCHzCHzOH supercritical COz extraction

f

HO C H 2 - C H 2 - 0 - 8 u 8 - ~ ~ ~ - C H * - O H

Scheme 4.5-9 Synthesis of PET by scCOz extraction u441.

4.5 Polymerizations in Dense Carbon Dioxide

321

tage when employing scCOz as a reaction medium is the facile removal of the small molecule condensate via supercritical fluid extraction. The removal of ethylene glycol in this case is hastened by the plasticization of the polymer by C 0 2 . Removal of the ethylene glycol byproduct by extraction with C02 results in polymerization to higher molecular weights at reaction temperatures considerably lower than those employed industrially. We plan to apply the technology developed for the' synthesis of PET in compressed C02 to other step-growth polymerization systems, and we are currently investigating the use of supercritical C 0 2 in polycarbonate synthesis.

4.5.6

Hybrid Systems

Hybrid polymerization methodology based on two-phase mixtures of water and C02 have been developed for the polymerization of highly reactive monomers such as tetrafluoroethylene [ 145,1461. These techniques allow for the synthesis of high-molar-mass polymers by exploiting the compartmentalization of monomer, polymer, and initiator based on the solubility of each. Highly exothermic reactions can be partially controlled, as a result of the high heat capacity of the water phase, which is an added advantage of this system. Water-soluble persulfate initiators were employed at 75 "C with sodium perfluorooctanoate as a stabilizer to produce high-molecular-weight polymer (M,= 1 x lo6 g/mol) in good yields (80-90 %). Redox-initiated polymerizations using sodium sulfate and an iron(I1) salt gave similar results. These initial experiments indicate that the hybrid water-C02 system holds promise as a polymerization medium for a variety of systems.

4.5.7

Conclusions

Dense carbon dioxide represents an excellent alternative reaction medium for a variety of polymerization processes. Numerous studies have confirmed that C 0 2 is a potential solvent for many chain growth polymerization methods, including free-radical, cationic, and ring-opening metathesis polymerizations. Carbon dioxide has also been demonstrated to be an effective solvent for step-growth polymerization techniques. Advances in the design and synthesis of surfactants for use in C02 will allow compressed C 0 2 to be utilized for a wide variety of polymerization systems. These advances may enable carbon dioxide to replace hazardous VOCs and CFCs in many industrial applications, making C02 an enviromentally responsible solvent of choice for the polymer industry.

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4.5.8 Acknowledgments The authors gratefully acknowledge financial support from the National Science Foundation through a Presidential Faculty Fellowship (JMD: 19931997), the United States Environmental Protection Agency, Alfred P. Sloan Research Fellowship (JMD: 1998-2001), and the Kenan Center for the Utilization of Carbon Dioxide in Manufacturing, sponsored by Air Products, Atochem, BFGoodrich, BOC Gases, Dow Chemical, DuPont, Eastman, H. B. Fuller, Japan Synthetic Rubber Co., MiCELL Technologies, Nalco Chemical, Oxychem, Phasex, Praxair, Rohm and Haas, and Solvay.

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Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

4.6 Free-Radical Polymerization in Reactive Supercritical Fluids SABINE BEUERMANN, MICHAEL BUBACK, and MARKUS BUSCH

4.6.1

Introduction

Carrying out chemical processes in the supercritical fluid (SCF) phase may be highly advantageous for a variety of reasons: (a) the availability of an extended supercritical pressure @) and temperature (T) range allows selection of optimal process conditions; (b) solvent properties may be continuously tuned, which provides considerable freedom for choosing p and T conditions such that either homogeneity (e.g. for reaction) or inhomogeneity (e.g. for the subsequent separation step) are achieved; and (c) heat and mass transfer processes can be very efficient under supercritical conditions. In free-radical polymerization, where product properties are kinetically controlled, a further important advantage of reaction in the SCF phase is the potential to tune polymer properties just by continuously varying polymerization conditions. It is occasionally overlooked that, according to these arguments, the high-pressure ethene polymerization is the archetype of a SCF phase process that takes advantage of all the benefits offered by reaction in the SCF phase. In contrast to what is seen with other SCF reactions, p and T conditions of ethene homo- and copolymerizations, which may be as high as 3000 bar and 300"C, respectively, are far above the critical temperature, T, = 9.5"C, and critical pressure, p c = 50.5 bar, of ethene. A particularly attractive feature of these polymerizations is that the reacting monomer itself establishes the SCF medium. This is referred to as a reactive SCF phase. Propene is not polymerized by a high-pressure free-radical reaction because of the relatively large stability of the ally1 free radical, which results in enhanced termination and chain transfer activity. Other examples of reactive SCF processes are the novel synthesis of estrone in sc tetralin [ 13 (T, = 448 "C) and polymer modification reactions in supercritical ammonia [2] (T, = 132°C). The arguments for conducting polymerizations in the SCF phase are strong enough to induce interest in free-radical polymerizations being carried out in inert SCF phases. Particularly attractive are polymerizations in scCOz, with additional interest arising from a significant reduction in environmental concerns (Chapter 4.5). Scholsky [3] provided a general overview on polymeriza-

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tion under supercritical conditions with information about copolymerizations of carbon monoxide, of carbon dioxide (using catalysts), of fluorinated alkenes, and of a few other materials. This chapter focuses on recent advances in high-pressure ethene homo- and copolymerization that have emerged from applying pulsed-laser-assisted techniques to the measurement of rate parameters, from using on-line spectroscopic monitoring of polymerization under extreme supercritical conditions, and from introducing simulation tools into data analysis. The classic paper on high-pressure ethene polymerization (and associated copolymerizations) is Ehrlich and Mortimer’s review [4]. Although these processes have been carried out industrially for several decades, there remains much interest in this field. This is motivated by a strong interest in detailed modeling of kinetics and polymer properties of the high-pressure low-density polyethylene (LDPE) process. About 16 million tons of LDPE is produced annually worldwide. Moreover, the wide tunability of the high-pressure supercritical process allows the production of polyethylene with properties that are not readily obtained within the HDPE (high-density polyethylene) and LLDPE (linear low-density polyethylene) low pressure processes [ 5 ] . In addition to homo- and copolymerization kinetics, addressed in some more detail below, the thermodynamic aspects are described with special interest in the measurement and in the modeling of cloud point curves for copolymerization systems [6-101. Section 4.6.2 illustrates the experimental procedures that have recently been applied toward the study of high-pressure free-radical polymerization processes. Section 4.6.3 presents results of propagation, termination, chain-transfer (to monomer and to polymer), and p-scission rate coefficients for ethene homopolymerization. Recent results from experiments and modeling investigations into high-pressure copolymerizations (with ethene being one of the monomers) are reported in Section 4.6.4, together with information on homopolymerization rate coefficients of the comonomer species.

4.6.2 Experimental Methods and Techniques 4.6.2.1 On-line Spectroscopy Kinetic studies on SCF phases are mostly associated with carrying out experiments under unusual and extreme conditions of pressure and temperature. It is therefore desirable to have on-line techniques that can provide reliable quantitative analysis of the major reaction species throughout an experiment, rather than measure final concentration(s) after a preselected reaction time. Infrared (IR) and near-infrared (NIR) spectroscopy are perfectly suited for this purpose as almost all chemical substances have relatively narrow characteristic IIUNIR absorption bands. Fourier transform spectroscopy allows the simultaneous measurement of the IR and NIR spectral range within seconds.

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4 Reactions in SCF

Figure 4.6-1 Optical high-pressure cell: S, bolt; F1, flange; H, heater; ST,sheathed thermocouple; W,window; SR, steel ram. By using optical cells of different length, the internal volume is easily varied between 0.5 and 10 mL.

As molar absorption coefficients decrease by orders of magnitude in going from fundamentals to higher overtone and combination modes, analysis of a single IR/NIR spectrum may allow for quantitative analysis over a concentration range covering several orders of magnitude. The enormous dynamic range for concentration measurement that becomes accessible, by simultaneously recording the IR and NIR spectral regions, may be used for almost all chemical substances. In free-radical polymerization, for example, the very low initiator concentrations may be monitored by their characteristic IR fundamental modes, whereas monomers and polymers are quantitatively measured in the NIR [l]. Several types of high-pressure optical cells for operation at typically up to 3500 bar and 350°C have been developed [ll]. Investigations into rate coefficients, reactivity ratios, and the phase behavior of high-pressure free-radical polymerizations have been carried out in autoclaves equipped with optical windows made from synthetic sapphire, which is unrivaled as a window material in the spectral range from 2000 to 50000 cm-'. (Below 2000 cm-', windows of polycrystalline silicon may be used at reduced pressure and temperature conditions [ 111.) In cells equipped with sapphire windows, in addition to quantitative on-line IR/NIR analysis of the reacting system, UV excimer laser pulses may be applied for initiation of the polymerization reaction. As shown in Section 4.6.2.2, this is extremely helpful for the detailed kinetic analysis of these processes. A basic type of high-pressure cell suitable for monitoring reactions up to 3500 bar and 350°C is shown in Figure 4.6-1. The IR/NIR probing light and, if required, UV laser pulses penetrate the cell along the cylindrical axis [ l l , 121.

4.6 Free-Radical Polymerization in Reactive Supercritical Fluids 4.6.2.2

329

Kinetic Coefficients from Laser-Assisted Techniques

Reliable rate coefficients of free-radical polymerizations are accessible from experimental techniques in which UV pulsed lasers are used to initiate the reaction. Olaj and co-workers [13,14] showed that pulsed laser polymerization (PLP) using evenly spaced pulse sequences yields polymeric product with a well-structured molecular weight distribution (MWD). The position of characteristic points of inflection of the MWD allows for the determination of the propagation rate coefficient, kp. Figure 4.6-2 demonstrates the features of this technique: Applying an evenly spaced sequence of laser pulses, with pulse repetition rate v , to a monomer-photoinitiator mixture results in a periodic free-radical concentration versus time profile, as given in Figure 4.6-2(a). The almost instantaneous production of free radicals by each laser pulse is associated with an enhanced termination probability for radicals produced by the preceding pulse(s), which yields the characteristic structure of the MWD (Figure 4.6-2(b)) by a weight fraction, w, versus loglo M distribution (from PREDICP (Polyreaction Distributions by Countable System Ingetration) [ 151 simulation). The propagation rate coefficient kp is derived from a characteristic degree of polymerization, Lo, according to = kPcMv-l,where cM is the monomer concentration. The identification of Lo with the point of inflection on the low-molecular-weight side of the MWD peak is discussed in detail elsewhere [16,17]. Performing such PLP experiments in conjunction with MWD analysis of the resulting polymer by size-exclusion chromatography (SEC) constitutes the so-called PLP-SEC technique, which is the method of choice for kp measurement as recommended by the IUPAC Working Party “Modeling of Polymerisation Kinetics and Processes” [ 181. For homopolymerizations of methyl methacrylate and of styrene, benchmark value data sets have already been put forward [ 19,201 and critical data evaluation, by this working party, of kp data for other alkyl methacrylates, functional methacrylates, and alkyl acrylates is underway. Values of kp for the ethene high-pressure polymerization have not yet been derived from PLP-SEC

im

-2 il .

3

8

0

0.1

0.2 tls

0.3

3.5

4.0

4.5 5.0 log,, M

:

5

Figure 4.6-2 Characteristic features of the PLP-SEC measurement: (a) the pulsed laserinduced free-radical concentration (cR) versus time profile, where v is the pulse repetition rate; (b) the resulting (simulated) polymer molecular weight distribution, w(Iog10 M).

330

4 Reactions in SCF

because of problems associated with the high chain-transfer to monomer rate in this system, and with the significant axial broadening in high-temperature SEC (required for measuring the MWD of polyethylene). PLP-SEC experiments are carried out in the initial stage of a polymerization, typically at monomer conversions below 3 %. The resulting propagation rate coefficient is, however, valid up to much higher conversions. The propagation step, a free-radicalmolecule reaction, is relatively slow compared to termination and thus is insensitive towards the transport properties of the polymerizing medium up to fairly high degrees of monomer conversion [18]. Information about propagation and termination (k,)rate coefficients during a polymerization reaction are obtained from pulse sequence (PS)-PLP and single pulse (SP)-PLP experiments [2 1-23].: In the latter technique, monomer conversion is induced by a single excimer laser pulse typically of 20 ns width and is recorded by on-line vibrational spectroscopy with time resolution in the microsecond range. A typical monomer conversion versus time profile obtained for an ethene polymerization [24] at 190°C, 2550 bar, and at 9.5 wt% polyethylene (from preceding polymerization) is shown in Figure 4.6-3. Fitting the measured conversion versus time trace to eq (4.6- 1) yields the parameters ktcRo and kdk, for narrow conversion intervals, usually well below 0.5%. (4.6-1) where cMois the monomer concentration at t = 0 (when the laser pulse is applied), cR0is the concentration of primary free radicals that is, almost instantaneously, generated by a single laser pulse, and kt refers to a chain-length averaged termination rate coefficient (see below). Eq (4.6-1) is derived on the basis of ideal polymerization kinetics taking into account only initiation, propagation, and chain-length-independent termination reactions [23]. As shown in Section 4.6.3, additional reaction steps need to be considered for the modeling of both kinetics and polymer properties. In contrast to the PLP-SEC experiment, an SP-PLP measurement may be carried out at any time during the course of a polymerization and k,lk, may be Y

0.08

E

9 I

0

I

I

10. 20 rlms +

Figure 4.6-3 Monomer conversion vs. time profile measured during an SP-PLP experiment for an ethene polymerization at 190 "C, 2550 bar, and 9.5 wt. % polyethylene [24].

4.6 Free-Radical Polymerization in Reactive Supercritical Fluids

33 1

mapped as a function of monomer conversion up to high polymer contents. From kdk,, the termination rate coefficient kt is obtained as a function of monomer conversion if kp is known from an independent experiment such as PLP-SEC. The SP-PLP technique is particularly well suited for studies of systems with a relatively large dsgree of monomer conversion occurring per laser pulse, such as acrylate or high-pressure ethene polymerizations. Slowly propagating monomers such as stytene or some of the methacrylates may be better studied by the PS-PLP technique in which monomer conversion is induced by laser pulse sequences and is monitored without any time-resolved measurement after applying one or several such pulse sequences [21,22]. 4.6.2.3 Continuously Operated High-pressure Polymerization Reactors

In addition to the above investigations, free-radical high-pressure polymerizations should also be studied in continuously operated devices for three reasons. (1) Because of the wealth of kinetic information contained in the polymer properties, product characterization is mandatory. Sufficient quantities of polymer, produced under well defined conditions of temperature, pressure, and monomer conversion, are best provided by continuous polymerization, preferably in a continuously stirred tank reactor (CSTR). (2) Copolymerization of monomers that have rather dissimilar reactivity ratios, such as in ethene-acrylate systems, will yield chemically inhomogeneous material if the reaction is carried out in a batch-type reactor up to moderate conversion. To obtain larger quantities of copolymer of analytical value, the copolymerization has to be performed in a CSTR. (3) Technical polymerizations are exclusively run as continuous processes. Thus, in order to stay sufficiently close to the application and to investigate aspects of technical polymerizations, such as testing initiators and initiation strategies, fundamental research into these processes should, at least in part, be carried out in continuously operated devices. A high-pressure high-temperature set-up [25-271 for operation up to 3000 bar and 300°C with two continuously stirred tank reactors, CSTRl and CSTR2, aligned in series is shown in Figure 4.6-4. The two autoclaves [28] are virtually identical. Each is equipped with a sapphire window of 20 mm aperture for control of homogeneity of the reaction system and/or for photochemical initiation of the polymerization, such as UV laser light, as required by the detailed kinetic studies. The set-up contains multiple facilities for generating pressure and for injecting liquids (initiators, comonomers, chain-transfer agents, cosolvents, etc.) at pressures between 200 and 400 bar in front of the third compressor stage or directly into CSTRl or CSTR2 at even higher pressures. Overall, mass flows up to 5 kg/h can be reached with the set-up shown in Figure 4.6-4. The backpressure control valves are specially designed to allow for expansion of the reaction mixture under conditions of huge pressure gradients and of small flow rates [25]. Using the device in Figure 4.6-4 with only one continuously stirred tank reactor, CSTRl , is sufficient to investigate homo- and copolymerization

332

4 Reactions in SCF 270 bar

UV radiation I video control

Figure 4.6-4 Scheme of a continuously operated device for high-pressure ethene (co)polymerizations: Ex, exhaust; RD, rupture disk; BPV, backpressure control valve.

kinetics in an extended SCF range. The properties of polymer products may be varied over a wide range, including those of technical LDPEs and ethene copolymers. The second vessel, CSTR2, allows the study of cascade (n = 2) performance in high-pressure polymerizations. Another attractive feature of the set-up, when operated with both CSTRs, is the possibility of producing copolymer in CSTRl and of mapping out, in CSTR2, the cloud point behavior of a particular monomer/comonomer/copolymer mixture that is produced for many hours under stationary conditions in CSTR1. The temperature in CSTR2 is kept well below that of CSTR1, to prevent any polymerization in CSTR2. The thermodynamic investigations in CSTR2 are performed on technically relevant copolymerization systems that contain both monomers. To study the cosolvent

4.6 Free-Radical Polymerization in Reactive Supercritical Fluids

333

quality of the comonomer, additional comonomer may be introduced into the mixture just before entering CSTR2. The details of such cloud point measurements are described elsewhere [29]. It goes without saying that such thermodynamic studies may also be carried out for ethene homopolymerizations. The high-pressure CSTRs have been designed with special emphasis on ideal mixing. A technique for the characterization of residence-time behavior under high-pressure conditions and during ethene polymerization has been developed [30]. Effective mixing ensures isothermal conditions within the CSTR which largely facilitates the quantitative kinetic analysis of polymerization reactions carried out in such a reactor.

4.6.3 Ethene Homopolymerization The kinetic scheme that needs to be considered for modeling the kinetics and the product properties of high-pressure ethene polymerization encompasses, at least, the reaction steps of initiation, propagation, termination, chain transfer to monomer, to polymer and to chain-transfer agents, as well as back-biting and p-scission. Among these processes, summarized in Scheme 4.6-1, initiation, propagation and termination steps determine monomer conversion versus time behavior, whereas the entire set of reactions controls molecular weight distribution and architecture of the polymeric product. initiation primary propagation event propagation termination by combination termination by disproportionation chain transfer to monomer chain transfer to chain-transfer agent chain transfer to polymer p-scission propagation of a branched free radical Scheme 4.6-1 Kinetic steps in ethene homopolymerizations: I = initiator; E = ethene; R = free radical; P = polymer chain; CTA = chain transfer agent. The subscripts are: l,r,s = chain length; sec = secondary, LCB = long-chain branching.

It goes without saying that modeling of ethene high-pressure copolymerizations requires a significant number of additional reaction steps to be included.

334

4 Reactions in SCF

4.6.3.1 Propagation and Termination Chain growth and free-radical termination are particularly important processes. To model the overall polymerization kinetics, these quantities need to be known as a function of pressure, of temperature, and of the amount and type of polymer that is present in the reacting system. The dependence of instantaneous kinetics on the history of the preceding polymerization is due to the diffusion control of reaction steps, in particular of the radical-radical termination step, but also of initiation being influenced by cage and out-of-cage processes. At very high degrees of monomer conversion even propagation kinetics may run into diffusion control. A particular example of history dependent polymerization are reactions which are carried out under conditions that lead to the formation of very high-molecular-weight material and/or highly branched polymer of poor solubility. In cases where polymerization proceeds in a heterogeneous phase, enormous consequences for both kinetics and polymer properties must be expected. Taking the physical properties of the medium into account is the key towards the detailed understanding of free-radical polymerization kinetics. Therefore, viscosity and diffusion data should also be known. Fortunately, under typical polymerization conditions, the influence of transport processes is not all-invasive and does not modify the entire set of individual reaction steps - it primarily affects the termination rate. This dependence can, however, be very complicated and may significantly vary with monomer conversion due to changes in the mechanism of diffusion control of termination during the course of a freeradical polymerization. An active field of research, for example, addresses the question as to whether and to what extent chain-length dependent termination needs to be taken into account. Beyond the interest in determining rate coefficients in the SCF state, which provide the basis for effectively tuning kinetics and thus polymer properties, such studies, by extended variation of p and T, are helpful in identifying the mode of diffusion control of the termination rate coefficient k,, for example by inspection of the associated activation parameters. From the laser techniques that are described in Section 4.6.2, only the SPPLP method has been successfully applied to ethene polymerization [31]. The microsecond time-resolved monomer conversion versus time SP-PLP profile of an ethene homopolymerization at 190°C, 2550 bar, and at about 9.5 wt% polymer content has already been shown in Figure 4.6-3. Several such single-pulse experiments may be carried out during one polymerization reaction, allowing the conversion dependence of kt to be mapped out. Without any special time-resolved analysis of the SP-PLP signal (which may provide access to chain-length-dependent k, [3l]), the termination rate coefficients available from SP-PLP are chain-length-averaged quantities [2 13. The primary parameters derived from the SP-PLP signal are the coupled quantities kdk, and k,Qi (and thus also k,ai) where Qi is the initiating quantum efficiency. The quantitiy ktai is obtained immediately from ktcRo,the kinetic parameter in eq (4.6-1) [31]. Figure 4.6-5 shows ktai data obtained during an ethene polymerization at 230 "C and 2550 bar for monomer conversions up to about 75 %.

4.6 Free-Radical Polymerization in Reactive Supercritical Fluids

335

0 ,.Q

monomer Figure 4.6-5 conversion Rate parameter measured ktmi viaasSP-PLP a function during of ethene homopolymerizations at 230 "C and 2550 bar

PI].

l 105 @ 0.2 0.4 0.6 monomerconversionI per cent

An enormous change in ktQi, by about three orders of magnitude, is seen. There is a clear decrease of ktQi in the initial conversion range followed by a very weak decay in the 20-50% conversion region. Towards still higher polymer content, k& is again significantly reduced upon further polymerization. It is more than likely that this variation with conversion is primarily due to changes in k,, with at best a minor influence from Qi. This finding is suggested by an inspection of the conversion dependence of kpQi, the second parameter from fitting the SP-PLP signal. This quantity, as shown in Reference [31], turns out to be independent of monomer conversion up to about 50 % (which is well above the range of technical conversions). The constant value of kpQi over such an extended range of conversions, where bulk viscosity varies by several orders of magnitude, strongly suggests that the two individual quantities, kp and Qi, are also independent of monomer conversion. No argument is seen as to why this constant value of Qi should be significantly different from unity. In support of this assumption, kpQi is found to be almost identical to the kp value derived from the propagation rate coefficient reported by Lim and Luft for 200 "C and 1750 bar [32] and extrapolated to 230 "C and 2550 bar (via the activation parameters from Reference [3 11). However, the variation of k, with monomer conversion is assigned to successive diffusion control by translation diffusion and by reaction diffusion without and with diffusion-controlled propagation [33]. From an extended set of SP-PLP experiments carried out at temperatures between 190 and 230°C and at pressures between 1950 and 2900 bar, Schweer [24] derived expressions for the temperature, pressure, monomer conversion (X), and viscosity ( q ) dependence of termination and propagation rate coefficients for the ethene homopolymerization, eqs (4.6-2) and (4.6-3).

336

4 Reactions in SCF (4.6-3) k: I+ 1.13 - 10lo qr

In these equations qr = q/qo is the relative bulk'viscosity, with qo referring to the pure monomer viscosity at identical p and T. The terms k: and :k refer to the termination and propagation rate coefficients at very low conversion, respectively. Also from SP-PLP experiments, these two quantities are found to be: 553 -:;:p/ bar (4.6- 4) k:/(Lrnol-'~-~) = 8.11 X 108exp

(-

% / ( L m o l - ' ~ - ~=) 1.88 X 107exp (-4126 ; : F p

/ bar

)

(4.6-5)

A detailed discussion of these results, including the procedure of estimating relative bulk viscosity is given in Reference [24]. Eqs (4.6-2) to (4.6-5) have been extensively used for modeling high-pressure ethene polymerizations during recent years.

4.6.3.2 Chain-Transfer to Monomer In ethene polymerization the molecular weight is essentially determined by transfer reactions of free radicals differing largely in size to either monomer, polymer, solvent, or to a chain-transfer agent (see Scheme 4.6-1). The chaintransfer to monomer reaction is assumed to proceed according to eq (4.6-6).

R, + E % P,

+ R,

(4.6-6)

As in Scheme 4.6-1, R, and P, refer to free radicals and to polymer of degree of polymerization s, respectively; E is ethene and R, is a small free radical which is similar in size to ethene. Although most research groups favor the kinetic picture suggested by eq (4.6-6), there is an alternative view on the chain-transfer to monomer reaction: Lorenzini et al. [34] and Goto et al. [35] assume this process in ethene homopolymerization to occur as a back-biting reaction with subsequent p-scission of the branched macroradical, yielding a polymer species together with a free radical of chain length below six. It should, however, be noted that eq (4.6-6) is more frequently used in the literature. Transfer reactions to solvents or to chain-transfer agents are also modeled by schemes analogous to eq (4.6-6).

4.6 Free-Radical Polymerization in Reactive Supercritical Fluids

Figure 4.6-6 Arrhenius plot of the inverse average degree of polymerization, El, determined by either viscosimetry or sizeexclusion chromatography for polyethylenes produced at 2000 bar and various temperatures the initiation and thus

337

; \

- -8.5 2 . 2 -9.01 ' c

-9s -10.0

. 0 viscosimetty A SEC

-10.5 :

.-

fit (Eq. 7)

-11.0 - . . , . . . . , . . . . , . . . . , . . - - , - - . . , .

According to the Mayo procedure [36], for polymerizations at low initiation and thus also at low termination levels, the inverse (average) degree of polymerization, P may be identified with Ctr,M,the chain-transfer to monomer constant (P,,-'= c,,, = ktr,M/kp).A plot of In P,,-' against inverse temperature (Figure 4.6-6) shows an Arrhenius-type behavior of C t , M for ethene homopolymerizations carried out at 2000 bar under conditions of very low initiation rate, by so-called thermal initiation [37,38]. A linear dependence is also observed for the logarithm of In Pn-' versus pressure at constant polymerization temperature. A combined fit to the experimental Pn-' data measured as a function of p and T, results in the following expression for C t r , [37,38]: ~

-',

lnCtr,M= 2.90 - (T/K)-' x (5524 + 0.257 (@/bar) - 2000))

(4.6-7)

According to the definition of the chain-transfer to monomer constant, the formal activation energy associated with Ctr,M, E A ( C ~ , Mas ) , obtained from the slope to the straight line in Figure 4.6-6 or from eq (4.6-7), corresponds to the difference in the activation energies of the transfer rate coefficient, EA(~~,M and ) , of the propagation rate coefficient, EA(kp).With EA(kp)being known from Schweer's work [24], i.e. eq (4.6-5), the experimental value of EA(Ctr,M) yields EA(k,, ) = (74 k 8) kJ mol-I which is remarkably close to E A ( ~ ~ ,=M83 ) kJ mol- estimated by Heuts et al. [39] from ab-initio quantum-mechanical calculations for a transfer reaction of an ethyl free radical with ethene. This agreement further supports the view that the kinetic picture underlying this equation provides an adequate representation of the chaintransfer to monomer step.

P

338

4 Reactions in SCF

4.6.3.3

Chain-Transfer to Polymer and b-Scission

To model the properties of LDPE, in addition to chain-transfer to monomer, also the so-called back-biting reaction, and, paricularly important, transfer to polymer and 0-scission reactions need to be co-nsidered. In the back-biting chain-transfer step, the free-radical chain end reacts with a methylene group on the same chain, preferably in the &-positionto the free-radical functionality, resulting in short-chain branches, mostly n-butyl side groups. The back-biting reaction has been thoroughly studied by Goto et al. [35] and rate coefficients are available as a function of pressure and temperature. Highly important for LDPE properties are chain-transfer to polymer reactions by which secondary macroradicals .'are formed which may either propagate, resulting in long-chain branches, or undergo 0-scission reactions, where the C-C bond in the @-positionto the free-radical site is broken and a polymer molecule together with a free radical are produced. Unfortunately no generally accepted view of the transfer to polymer process has yet emerged. Feucht et al. [40] treated this process as a single reaction step characterized by the rate coefficient ktr,p. The increasing probability for chain transfer of a larger molecule is taken into account by multiplying ktr,p with r, the chain length of a particular polymer molecule, P, that reacts with a free radical of size s, R,. The assumption of a linear increase in chain-transfer to polymer rate coefficient with r results in a significant overestimation of M , (weight average molecular mass) at high conversions. M, may even diverge under conditions where neither gelation nor network formation is experimentally observed. This is taken into account by considering the chain-transfer reaction as being composed of chain-length-independent and chain-length-dependent parts [40]. The parameter a in eq (4.6-8) indicates the contribution of chain-length independent transfer to the overall chain-transfer to polymer reaction. (4.6-8)

ps -IRr,LCB

a

E

[0

... 11

An alternative description of the transfer to polymer process has been suggested by Goto et al. [35]. They also assume the transfer to Dolvmer rate, eq (4.6-9), to be proportional to the chain length of the reachng polymer. By introducing the @-scissionstep, eq (4.6-lo), for branched intermediate macroradicals R,, the steep rise in M , toward high conversion is avoided. (4.6-9)

In contrast to the kinetic picture of independent transfer to polymer and 0-scission reactions underlying eqs (4.6-9) and (4.6-lo), Lorenzini et al. [34] assume that the transfer to polymer process takes place as a reaction sequence

4.6 Free-Radical Polymerization in Reactive Supercritical Fluids

339

starting with the formation of a secondary macroradical being followed by either propagation or p-scission, eqs (4.6-11) - (4.6-13).

Rr,sec

-

M kP Rr+ 1,LCB

(4.6-11) (4.6-12) (4.6-13)

Both the Goto et al. [35] and the Lorenzini et al. [34] models are well suited to fit the experimental long-chain branching index ZLCB (number of branches per 1000 chain .carbon atoms), the measured M , and also the full MWD. In LDPE modeling, the scheme of Goto et al., eqs (4.6-9) and (4.6-lo), is frequently favored because of a reduced stiffness of the differential equations associated with this approach. If the type and the concentration of branched macroradicals need to be specified, as is desirable in copolymerization modeling, the scheme of Lorenzini et al. [34] appears to be more suitable. The chain-transfer and p-scission rate coefficients are difficult to deduce from experimental quantities such as number and weight averages of molecular mass and long-chain branching indices (ZLCB) because the kinetic processes contribute in a complex manner to these measurable polymer characteristics. Modeling of polymer MWD and of branching structure via the PREDICI~ program is extremely helpful toward identifying the influence of chain-transfer and p-scission rates on M , M , (number average molecular mass), and ZLCB. Simulation shows that M , is essentially determined by ktr,Mat low p-scission rates, a situation that applies in polymerization at very low conversion. The contributions of ktr,pand of p-scission to M , and ZLCB as obtained via P R E D I C I ~ simulation are illustrated in Figure 4.6-7. The rate coefficients that are used with the Lorenzini et al. [34] model underlying these calculations are given in the legend to this figure. M , and ZLCB have been simulated for different degrees of monomer conversion during an ethene polymerization at 200°C and 2000 bar carried out in a CSTR. The P R E D I Ccalculations I~ have been performed for the given initial values of ktr,p and k, as well as for ktr,P and k, numbers being arbitrarily changed by 15 and 50%, respectively. The data in Figure 4.6-7 show that ktr,paffects both M , and ZLCB, whereas k, primarily influences M,. These results suggest the following procedure for ktr,p and k, determination. The experimentally observed variation of ZLCB with monomer conversion is first used to fit ktr,p. k, is subsequently varied to obtain a best fit of the measured MWD. Using the full MWD rather than looking at M, is recommended because M , is sensitive to even slight inaccuracies in measuring the amount of very high-molecular-weight species. According to the outlined procedure, ktr,p and k, values for extended ranges of temperature and pressure may be derived from carefully characterized polymer that has been produced under precisely known conditions. As mentioned above, the Goto et al. [35] scheme is frequently used for modeling ethene homopolymerization, and the associated ktr,P and k, coefficients

4 Reactions in SCF

340

4.5 1.2.105

14.0

1.1.105

2

8

0

1.o.105

13.5

0.9.105

1 3.0

0.8.105 0.7.105 0.6.105

8

0.5.10'

1 1.5

0.4.105

11.0

0.3.105

1 0.5 :...I....~o.o

0.2.105 0.1.105

1 . v. , . . . . , - . . I , . . . , . . . . , . . . . , 0 2 4 6 8 10 12

14

16

monomer conversion I per cent

Figure 4.6-7 Weight average, M,,,, of the MWD and long-chain branching index, ILCB. calculated for polyethylenes produced at 200°C and 2000 bar in a CSTR at different conversions. The rate coefficients used for this simulations are ktr,P = 256 Lmol-' s-' and kp = 1976 s-'. Values for kp and kt are taken from Schweer [24] and k V , ~is from Reference [38].

are available even for temperature and pressure variation. Lglmmel [41] used this approach in combination with PREDICPsimulation for the modeling of high-pressure ethene polymerizations. Using the ktr,p and ke values deduced from experiments at monomer conversions up to 5.5 % and assuming these numbers to apply also at higher conversions, he obtains bimodal MWDs. Typical results are shown in Figure 4.6-8. It should be noted that this bimodality is

1-.....-

2 %conversion

-

20 % conversion

-c.

I

h

8M

. s

0.5

0

M

s

x 0.0

3

4

5 6 log,, (MIg.mol-')

7

Figure 4.6-8 Simulated (via PREDICI") MWDs of polyethylenes produced at 200°C and 2000 bar at different levels of monomer conversion varying between 2 and 20% in steps of 2 % [41].

4.6 Free-Radical Polymerization in Reactive Supercritical Fluids

341

not due to non-ideality of the system, but is estimated for a well-mixed CSTR with the rate coefficients being derived from experiments at lower conversion where no such bimodality is seen.

4.6.4 Ethene Copolymerizations Vinyl acetate, acrylic acid esters, and (meth)acrylic acid are important monomers for high-pressure copolymerizations with ethene. Introducing these functional monomers enormously influences polymer properties. Continuously varying p and T in the SCF phase may be additionally used to tune copolymer properties. Being aware of the difficulties encountered in the detailed analysis of fluid phase ethene homopolymerizations, it comes as no surprise that copolymerization reactions in the supercritical phase with ethene as one of the monomers are far from being fully understood. Considerable effort has been spent toward improving the knowledge about copolymerization kp [42], reactivity ratios, modeling of copolymerization, and about cloud point behavior [6]. From this large and active field, only the issues of reactivity ratios for ethene copolymerization with acrylates (acrylic acid esters), of homopolymerization rate coefficients for the acrylate comonomers (which are required for simulation), and finally of modeling of copolymerization kinetics and product properties are addressed. To reduce scope and complexity, only binary copolymerization systems are considered. Regarding high-pressure polymerizations with more than two monomers, the ternary systems ethene-vinyl acetate-acrylonitrile and ethene-vinyl acetate-methyl acrylate are described in References [43] and [44], respectively. 4.6.4.1 Reactivity Ratios for Copolymerizations of Ethene with Acrylic Acid Esters Reactivity ratios q are defined by ri = kii/kij, the ratio of homopropagation, kii, to cross-propagation, kij, rate coefficients, where kij refers to the addition of monomer j to a free-radical chain-end terminating in species i. Under ideal polymerization conditions the mole fraction of monomer units i contained in the copolymer Fi is given by eq (4.6-14), which holds for both the terminal and the implicit penultimate unit models (see Section 4.6.4.3) [45]. (4.6 - 14) In eq (4.6-14)h is the mole fraction of monomer i in the reacting monomer mixture. This equation is used to derive ri and rj = kji/kji from the measured

4 Reactions in SCF

342

composition of the monomer mixture and of the copolymer,J and Fi, respectively. A survey of reactivity ratio data for high-pressure ethene copolymerizations is contained in Ehrlich and Mortimer's review [4] and in Scholsky's article [3]. As has been shown with the ethene-methyl acrylate data taken as an example [46], an enormous scatter is seen on the reported reactivity ratio data which may at least partly be due to inhomogeneity of the reacting system in several of the earlier studies. The experimental data (that will be illustrated below) have recently been measured in continuously operated devices, such as in the set-up illustrated in Figure 4.6-4 with the homogeneity of the copolymerizing system being monitored visually and/or spectroscopically by passing the polymerizing system through an optical high-pressure cell. The high-pressure copolymerization systems of particular technical interest are: ethene (E)vinyl acetate (VA), E-methyl acrylate (MA), E-butyl acrylate (BA), E-2-ethylhexyl acrylate (EHA), and E-(meth)acrylic acid ((M)AA)). The system E-VA shows almost ideal copolymerization behavior, with both reactivity ratios being close to unity [43,47,48]. This system will not be considered in the remainder of this section which addresses ethene copolymerization with highly polar comonomers such as the acrylates. The systems E-MAA and E-AA are not included as reliable reactivity ratio data for these copolymerizations are still in the process of being carefully worked out [49]. Plotted in Figure 4.6-9 is the copolymer mole fraction F M A versus the monomer mole fractionfMA for E-MA copolymerizations carried out in a continuously operated reactor [46] at 2000 bar and temperatures of 220, 250, and 290°C. Total monomer conversion in these reactions is very small, mostly below 1 %. As can be seen from the figure, copolymerization leads to a significant increase in MA content (in going from the monomer mixture to the polymer). From pairs of F M A and fMA values measured at identical pressure and temperature, the two reactivity ratios, rE and rMA, are obtained via eq (4.6- 14). That the two reactivity ratios are correlated may be taken into account by calculating 95 % confidence intervals for these quantities according to the proFigure 4.6-9 Methyl acrylate (MA) content of the copolymer, F M A (in mol%), plotted against the MA content of the monomer mixture, f'A. for E-MA copolymerizations at 220, 250, and 290°C and 2000 bar. The curves are obtained by nonlinear fitting of the

70 60

8

so

.

40

<

4'

30 20 10

0 0

5 .

10 fMA

I mol%

15

(4.6-14).

343

4.6 Free-Radical Polymerization in Reactive Supercritical Fluids

Figure 4.6-10 95 % joint confidence intervals for ethene and acrylate reactivity ratios r~ and rA, respectively, in copolymerizations of E-MA, E-BA and E-EHA at 220°C and 2000 bar.

E 0.040 L

l

T

l

3

2

'

l

5

4

~

,

3

6

7

A '

cedure suggested by van Herk [50]. Such confidence intervals are plotted in Figure 4.6-10 for copolymerizations of E-MA, E-BA, and E-EHA at 220°C and 2000 bar. The size of the individual confidence ellipsoids reflects the accuracy of the underlying composition data and also the number and range of such FMAand fMA values. The ellipsoids for the ethene and acrylate reactivity ratios of E-BA and E-EHA are overlapping which means that a statistically adequate analysis of 5 and Fi data for the two systems yields reactivity ratios that cannot be distinguished (on the basis of these particular composition data). However, the corresponding data for E-MA differ from the E-BA and E-EHA values in a statistically significant manner. Irrespective of the coupling between the ethene and acrylate reactivity ratios, individual Arrhenius plots of these parameters have been made. Figure 4.6-11 shows this data for E-MA, E-BA, and E-EHA copolymerizations at 2000 bar. The ethene reactivity ratios are rather similar for the three systems, whereas the

Figure 4.6-11 Temperature dependence of (a) rE and (b) rA in copolymerizations of E-MA (triangles), E-BA (squares) and E-EHA (circles) at 2000 bar; the E-BA and E-EHA data are fitted by a single (dashed) line [461.

2.5 -

2

.

1.5 1.0OS!

1.6

-

I

1.8

.

E-BA I E-EHA

I

.

I

2 2.2 T J I 10-3 K-I

.

. 2.4

1

2.6

1

344

4 Reactions in SCF

acrylate reactivity ratio (rA) for E-MA is slightly, but significantly, different from the corresponding values for the E-BA and E-EHA systems. The temperature dependence at 2000 bar of the reactivity ratios in the E-MA system is represented by eqs (4.6-15) and (4.6-16) [46]. In rE(MA) = -0.0202-1516

(T / K)-'(p = 2000 bar; 220°C 5 T 5 2900C)

In rMA = -3.170-2406

X

X

(T / K)-'

(4.6-15)

(4.6-16)

(p = 2000 bar; 220°C 5 T 5 290°C)

The corresponding expressions for the combined systems E-BA and E-EHA are given in eqs (4.6-17) and (4.6-18) [27]. In ~E(BA/EHA)= -0.0834-1431 x (T / K)-'

(4.6- 17)

(p = 2000 bar; 150 "C 5 T 5 250 "C)

In rA(BA/EHA) = -4.135-2670 (p = 2000 bar; 150 "C 5

(T / K)-' T 5 250 "C) X

(4.6-18)

Using the simplifying terminal model, cross-propagation rate coefficients kij may be deduced from ri and from the associated homopropagation rate coefficients kii (according to the defining equation ri = kii/kij). This approach, which includes the approximations that the terminal model is applicable and that the homopropagation rate coefficients (which are determined at much lower temperature) may be extrapolated via Arrhenius relations to the copolymerization temperatures, has been used in Reference [5 11. The analysis of cross-propagation rate coefficients, kAE, (refemng to the addition of an ethene molecule to a chain end terminating in acrylate comonomer unit A) shows that the activation energies, EA(kAE), are rather similar, which indicates the major influence of the type of monomer molecule on EA(kA~).The preexponential factor, A(kAE), on the other hand, varies largely and thus demonstrates the control of this quantity by the terminal unit on the free radical. Consistent with this latter finding is the observation that A(kEA), the pre-exponential of cross-propagation rate coefficients of free radicals terminating in an ethylene unit, varies comparatively little upon changing the type of monomer molecule. The variation of reactivity ratios with pressure is small. The activation volume, A f i ( c ) = -(dln ri/ap)=RT, serves as a quantitative measure of this dependence. To derive reliable, accurate Afi(ri) values, experiments over an extended pressure range are required. Unfortunately, the pressure range of ethene-acrylate copolymerizations is not easily extended (to induce larger changes in ri) as polymerization pressure is limited towards high p by the equipment and towards low p by inhomogeneity of the reaction mixture.

4.6 Free-Radical Polymerization in Reactive Supercritical Fluids

345

Because of the relatively large uncertainty in acrylate reactivity ratio (TA) determination there is no easy access to reliable A f l ( r A ) values. A few experiments [27,46] have, however, been carried out to estimate A v ( r E ) . The numbers obtained for the E-MA and E-EHA systems are: A f l ( r E ) = -(7.4 & 4.1) cm3 mol-' and A f l ( r E ) = -(8.2 k3.5) cm3 mol-I, respectively. A recently reported value of A v ( r E ) for the ethene-butyl methacrylate system is -(6.0 & 3.9) cm3 mol-' [5 13. Negative A v ( r E ) ,a s observed for all ethene-acrylate systems, says that rE, which is well below unity, increases with pressure. The terminal model approach may also be used to investigate the pressure dependence of cross-propagation rate coefficients kij. As pressure-dependent studies are restricted to rE, only the activation volume of kEA, A v ( k ~ , ) , may be estimated from experimental data:

where Av(kEE) is the activation volume of the ethene homopolymerization measured by Schweer [24,31] to be -27 cm3 mol-I. The analysis via eq (4.6-19) in Reference [51] suggests that, to a good approximation, the activation volume AV(kEA) may be calculated as the arithmetic mean of the associated homo-propagation values, A v ( k E E ) and A v ( k A A ) . Assuming this to be true also for Afl(kAE) offers the possibility of deriving a number for A F ( r A ) , from the homopropagation activation volumes, which is otherwise not that easily accessible.

4.6.4.2 Homopropagation and Homotermination Kinetics of the Comonomers As described in Section 4.6.4.1, homopropagation data of the comonomers is required to derive copolymerization rate coefficients such as ~ E Aand kAE. It goes without saying that, in addition to propagation, extended information about termination and chain-transfer for homopolymerization of the comonomers is needed to model copolymerization kinetics. The experimental techniques described in Section 4.6.2.2 have been used to deduce reliable kinetic coefficients for free-radical polymerization of acrylic and methacrylic acid esters. PLP-SEC has been successfully applied to a large number of methacrylate monomers with different types of ester groups (linear, branched, and cyclic alkyl groups partly containing functional moieties such as glycidyl, benzyl or hydroxy) [52,53] over wide pressure ranges and at temperature variation. For some of these monomers the technique was also applied to study depropagation kinetics at higher temperature [54]. For acrylate PLP-SEC work only limited temperature ranges are available because of the high chain transfer to monomer activity of these substances [55-591. The pressure dependence has, however, been investigated up to technically relevant conditions (2000 bar) [55]. Experimental results for acrylates and methacrylates are summarized in Figure 4.6-12. The variation of kp with

346 4.5

4 Reactions in SCF 5.0

-

4.0 -

.

%.

MA BMA

3.5 -

.' 1.5

I

I

I

I

3.0

3.5

4.0

4.5

T-1

I

.

lo00

0

I 10-3 K-1

.

.

,

2000

plbar

Figure 4.6-12 Variation of the propagation rate coefficient kp with (a) temperature at ambient pressure and (b) pressure at 30°C for the following monomers: methyl methacrylate (MMA), butyl methacrylate (BMA), and dodecyl methacrylate (DMA); methyl acrylate (MA), butyl acrylate (BA), and dodecyl acrylate (DA); for references see text.

pressure and with temperature shows a pronounced family-type behavior for each class of monomers. The temperature dependence (Figure 4.6-12(a)) is associated with an activation energy of (22 f 2) kJ mol-' for the methacrylates and of (17 2) kJ mol-' for the acrylates. The pressure dependence of kp (Figure 4.6-12(b)) is characterized by the following (family-type) activation volumes: AV = -(16 2) cm3 mol-' for the methacrylates and AW = -( 11 f 2) cm3 mol-' for the acrylates. Termination rate coefficients, k,, of several acrylate monomers have been determined via SP-PLP experiments. Figure 4.6-13 shows the dependence of kt on monomer conversion for free-radical polymerizations of methyl (MA), butyl (BA), and dodecyl acrylate (DA) at 40°C and 1000 bar. A significant reduction in kt with increasing size of the ester group is observed. In addition, the type of conversion dependence of kt is largely influenced

*

*

OMA

-

8.0 a0 BDA A

h

; t P

I

E

1

1.0 I

I

I

I

I

6.0

0

20 40 60 monomer conversion I per cent

8o

Figure 4.6-13 Variation of kt with monomer conversion for bulk polymerizations of methyl (MA), butyl (BA), and dodecyl (DA) acrylate at 40°C and 1000 bar; the data are from SP-PLP studies [60,61].

4.6 Free-Radical Polymerization in Reactive Supercritical Fluids

347

by the ester size: for DA, k, stays constant up to about 80% monomer conversion. MA exhibits the strongest variation of k, with conversion; after being constant up to about 15% monomer conversion, k, shows a significant decrease. For BA a small initial increase of k, is seen up to 10% monomer conversion, followed by a modest decrease in kt toward higher conversion. These variations of k, with monomer conversion may be explained by taking segmental, translational and reaction diffusion control into account [33]. It is beyond the scope of this article to address these mechanistic aspects in any detail. An activation energy of (7 f 2) kJ mol-’ and an activation volume of (17 f 3) cm3 mol-I have been determined for acrylate k, at monomer conversions of about 10% [60,61]. By applying the PS-PLP technique, k, values as a function of monomer conversion, pressure, and temperature have also been derived for methacrylate monomers and for styrene [21,62].

4.6.4.3 Modeling of High-pressure Ethene Copolymerizations The analysis of copolymer composition in Section 4.6.4.1 has been carried out using the terminal model which assumes that radical reactivity is solely determined by the terminal unit on the free-radical chain. The terminal model has been successfully applied to represent the monomer and copolymer compositions for a wide variety of systems, mostly studied at ambient pressure. This model is, however, not capable of describing both copolymer composition and copolymerization kinetics with a single set of reactivity ratios [63]. As was demonstrated by Fukuda et al. [64], the failure of the terminal model to represent copolymerization kinetics is not due to an inadequate consideration of the termination process, but to the influence of the penultimate unit on the free-radical chain propagation kinetics. Fukuda et al. [45] successfully used the penultimate model, first formulated by Merz et al. [65], for the representation of propagation kinetics in copolymerization. For an excellent brief introduction into the aspects of copolymerization modeling see also the paper by Hutchinson et al. [42]. Assuming free-radical reactivity to be influenced by the penultimate unit increases the number of relevant propagation rate coefficients for a binary copolymerization system from four to eight and results in four reactivity ratios. The studies into copolymerization modeling carried out to date demonstrate that only for a few systems the entire set of eight (penultimate) propagation rate coefficients needs to be considered [42]. According to the phrase coined by Fukuda et al. [64], these systems are showing an “explicit penultimate unit effect”. The majority of copolymerization systems studied so far can by represented well by the implicit penultimate unit effect (IPUE) model, where the two radical reactivity ratios, s1 = k 2 1 1 / k l land l s2 = k122/k222, are introduced as additional parameters, to account for the influence of the penultimate unit on homopropagation. Within the IPUE model, no penultimate unit effect is considered for the reactivity ratios r l l = r21 and r22 = r I 2 .Despite the remarkable

348

4 Reactions in SCF

success of the IPUE model to interpret copolymerization kinetics, there are continued efforts to further improve the understanding of copolymerizations. The status of the modeling of termination processes in free-radical copolymerization is presented3 the paper by Fukuda e f al. [66].Obviously a lot of work remains to be done, in particular on termination, initiation, and transfer processes in copolymerization. It should be noted that PREDICI" simulation of kinetics and of polymer properties is extremely valuable for fitting measured copolymerization data and for extracting the full information contained within the experimental data. Moreover, PREDICI" simulation effectively supports attempts to identify reaction conditions that are best suited to the unambiguous and sensitive determination of kinetic parameters [67].

4.6.5 Conclusions The advent of laser-assisted techniques has enormously improved the quality of measuring propagation and termination rate coefficients of ethene homopolymerization. The rate coefficients for chain transfer to monomer, to polymer, and to chain-transfer agents as well as for back-biting and p-scission reactions are available from careful MWD analysis of polyethylenes produced at quite different reaction conditions. Modeling of kinetics and polymer properties for ethene homopolymerizations within extended ranges of temperature and pressure may be easily carried out using commercial programs such as PREDICI". Ethene copolymerizations have been thoroughly studied during recent years. A lot of work remains to be done in this area, however, in particular on termination and chain-transfer processes. The progress in these investigations is correlated to improvements achieved in the quality of copolymer MWD analysis.

References [ 11 M. Buback, in Supercritical Fluids-Fundamentals for Application, E. Kiran, J. M. H. Levelt-Sengers (Eds.), Kluwer Academic Publishers, Dordrecht, Boston, London, 1994. [2] H. Brackemann, M. Buback, F. Rindfleisch, S. Rohde, Angew. Chem., Int. Ed. Eng. 1991,

30, 1654. [3] K. M. Scholsky, J. Supercrit. Fluids 1993, 6, 103. [4] P. Ehrlich, G. A. Mortimer, Adv. Polym. Sci. 1970, 7 , 386. [5] L. L. Bohm, H.-F. Enderle, M. Fleissner, F. Kloos, Angew. Makromol. Chem. 1997, 244, 93. [6] H.-S. Byun, B. M. Hasch, M. A. McHugh, F.-0. Mahling, M. Busch, M. Buback, Macromolecules 1996, 29, 1625. [7] B. Folie, C. Gregg, G . Luft, M. Radosz, Fluid Phase Equilibria 1996, 120, 11. [8] S.-H. Lee, M. A. McHugh, Polymer 1997, 38, 1317.

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[9] G. Luft, R. Wind, Chem. Ing. Tech. 1992, 64, 1114. [lo] B. Folie, M. Radosz, Ind. Eng. Chem. Res. 1995, 34, 1501. [ l l ] M. Buback and C. Hinton, in High-pressure Techniques in Chemistry and Physics: a Practical Approach, Isaacs, Holzapfel (Eds.), Oxford University Press, 1997. [12] M. Buback, H. Lendle, Z Naturforsch. 1979, 34a, 1482. [13] 0. F. Olaj, I. Bitai, F. Hinkelmann, Makromol. Chem. 1987, 188, 1689. [14] 0. F. Olaj, I. Schniill-Bitai, Eur. Polym. J. 1989, 25, 635. [15] M. Wulkow, Macromol. Theory Simul. 1996, 5, 393. [16] R. A. Hutchinson, M. T. Aronson, J. R. Richards, Macromolecules 1993, 26, 6410. [17] M. Buback, M. Busch, R. Lammel, Macromol. Theory Simul. 1996, 5, 845. [18] M. Buback, R. G. Gilbert, G. T. Russell, D. J. T. Hill, G. Moad, K. F. O’Driscoll, J. Shen, M. A. Winnik, J. Polym. Sci. Polym. Chem. Ed. 1992, 30, 851. [19] M. Buback, R. G. Gilbert, R. A. Hutchinson, B. Klumperman, F.-D. Kuchta, B. G. Manders, K. R O’Driscoll, G. T. Russell, J. Schweer, Macromol. Chem. Phys. 1995, 196, 3267. [20] S. Beuermann, M. Buback, T. !F Davis, R. G. Gilbert, R. A. Hutchinson, 0. F. Olaj, G. T. Russell, J. Schweer, A. M. van Herk, Macromol. Chem. Phys. 1997, 198, 1545. [21] S . Beuermann, M. Buback, K. Matyjaszewski (Ed.), in Controlled Radical Polymerization, ACS Symp. Ser. 685, American Chemical Society, Washington DC, 1998. [22] M. Buback, B. Huckestein, U. Leinhos, Makromol. Chem.. Rapid Commun. 1987, 8, 473. [23] M. Buback, H. Hippler, J. Schweer, H.-P. Vogele, Makromol. Chem., Rapid Commun. 1986, 7, 261. [24] J. Schweer, Ph. D. Thesis, University Gottingen, 1988. [25] M. Buback, M. Busch, K. Lovis, F.-0. Mahling, Chem.-1ng.-Tech. 1995, 67, 1652. [26] M. Buback, M. Busch, K. Lovis, F.-0. Mahling, Macromol. Chem. Phys. 1996, 197, 303. [27] M. Buback, T. Droge, A. v. Herk, F.-0. Mtihling, Macromol. Chem. Phys. 1996, 197, 4119. [28] M. Buback, M. Busch, K. Lovis, F.-0. Mlhling, Chem.-Ing.-Tech.1994, 66, 510. [29] M. Buback, M. Busch, H. Dietzsch, T. Drage, K. Lovis in High Pressure Chemical Engineering, Ph. R. von Rohr, Ch. Trepp (Eds.), Elsevier Science, 1996. [30] M. Buback, M. Busch, K. Panten, H.-P. Vdgele, Chem.-Zng.-Tech.1992, 64, 352. [31] M. Buback, J. Schweer, Z. Phys. Chem. N. F. 1989, 161, 153. [32] P. Lim, L. Luft, Makromol. Chem. 1983, 184, 849. [33] M. Buback, Makromol. Chem. 1990, 191, 1575. [34] P. Lorenzini, M. Pons, J. Villermaux, Chem. Eng. Sci. 1992, 47, 3969; P. Lorenzini, M. Pons, J. Villermaux, Chem. Eng. Sci. 1992, 47, 3981. [35] S. Goto, K. Yamamoto, S. Furui, M. Sugimoto, J. Appl. Polym. Sci.: Applied Polymer Symposium 1981, 36, 21. [36]F. R. Mayo, J. Am. Chem. SOC.1943, 65, 2324. [37]M. Buback, C.-R. Choe, E . 4 . Franck, Makromol. Chem. 1984, 185, 1685. [38]M. Buback, C.-R. Choe, E . 4 . Franck, Makromol. Chem. 1984, 185, 1699. [39] J. P. A. Heuts, Sudarko, R. G. Gilbert, Macromol. Symposia 1996, 111, 147. [40] P. Feucht, B. Tilger, G. Luft, Chem. Eng. Sci. 1985, 40 , 1935. [41] R. Lammel, Ph. D. Thesis, University of Gottingen, 1997. [42] R. A. Hutchinson, J. H. McMinn, D. A. Paquet, Jr., S. Beuermann, C. Jackson, Ind. Eng. Chem. Res. 1997, 36, 1103. [43] M. Buback, K. Panten, Makromol. Chem. 1993, 194, 2471. [44] G. Luft, F. Stein, M. Dorn, Angew. Makromol. Chem. 1993, 211, 131. [45] T. Fukuda, K. Kubo, Y.-D. Ma, Prog. Polym. Sci. 1992, 17, 875. [46] M. Buback, T. Droge, Macromol. Chem. Phys. 1997, 198, 3627. [47] M. Ratzsch, W. Schneider, D. Musche, J. Polym. Sci. 1971, Part A, 9, 785. [48] F. E. Brown, G. E. Ham, J. Polym. Sci. A. 1964, 2 , 3623.

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[49] L. Wittkowski, Ph. D. Thesis, University of Gottingen 1999. [50] A. M. v. Herk, J. Chem. Educ. 1995, 72, 138. [51] M. Buback, T. Droge, Macromol. Chem. Phys. 1999, 200, 256. [52] M. Buback, C. H. Kurz, Macromol. Chem. Phys. 1998, 199, 2301. [53] R. A. Hutchinson, S. Beuermann, D. A. Paquet, Jr., J. H. McMinn, C. Jackson, Macromolecules 1998, 31, 1542. [54] R. A. Hutchinson, D. A. Paquet, Jr, S. Beuemann, J. H. McMinn, Ind. Eng. Chem. Res. 1998, 37, 3567. [55] M. Buback, C. H. Kurz, C. Schmaltz, Macromol. Chem. Phys. 1998, 199, 1721. [56] S. Beuermann, D. A. Paquet, Jr., J. H. McMinn, R. A. Hutchinson, Macromolecules 1996, 29, 4206. [57] R. A. Lyons, J. Hutovic, M. C. Piton, D. I. Christie, P. A. Clay, B. G. Manders, S. H. Kable, R. G.Gilbert, Macromolecules 1996, 29, 1918. [58] B. Manders, Pulsed Initiation Polymerization, Universiteitsdrukkerij TUE, Eindhoven, 1997.

[59] M. Busch, A. Wahl, Macromol. Theory Simul. 1998, 7, 217. [60] C. H. Kurz, Luserinduzierte radikalische Polymerisation yon Methylacrylat und Dodecylacrylat in einem weiten Zustandsbereich, Verlag Graphikum, Gottingen, 1995. [6 11 C. Schmaltz, Radikalische Polymerisation von Butylacrylat in fluidem Kohlendioxid bis zu hohem Druck, Klaus Bielefeld Verlag, Friedland, 1998. [62] M. Buback, E-D. Kuchta, Macromol. Chem. Phys. 1997, 198, 1455. [63] J. Schweer, Makromol. Chem. Theory Simul. 1993, 2, 485. [64] T. Fukuda, Y.-D. Ma, K. Kubo, H. Inagaki, Macromolecules 1991, 24, 370. [65] E. Merz, T. Alfrey, G. Goldfinger, J. Polym. Sci. 1946, I , 75. [66] T. Fukuda, N. Ide, Y.-D. Ma, Macromol. Symp. 1996, 111, 305. [67] Ph. Becker, M. Busch, Macromol. Theory Simul. 1998, 7, 435.

Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

4.7 Metal-Complex-Catalyzed Reactions PHILIP JESSOPand WALTER LEITNER

4.7.1 Introduction 4.7.1.1 Potential Benefits of Using Supercritical Fluids in Metal-Complex-Catalyzed Reactions Revolutionary developments in liquid-phase homogeneous catalysis since the 1960s have provided the synthetic chemist with a wide variety of highly efficient and selective methodologies on both the laboratory and industrial scale [l]. However, other than a few early examples (see Chapter l.l), the general applicability of supercritical fluids (SCFs) as solvents for metal-complex-catalyzed reactions was not recognized until the early 1990s [2], and the vast majority of examples discussed in this chapter dates from this decade. As with any other chemical reaction, a metal-complex-catalyzed process may benefit from being carried out as a supercritical fluid reaction (SFR). The special properties of SCFs, such as high diffusion rates, absence of gas-liquid phase boundaries, compressibility, tunability of solvent properties, and in some cases environmental friendliness, are often advantageous in metal complex catalysis. In addition, there are some unique features that make the combination of transition metal complex catalysts and reactive SCFs particularly attractive. Reactive SCFs can alter the catalytic cycle, resulting in different activities and selectivities than those observed in conventional solvents. Some SCFs, such as H20, NH3 and MeOH, have obvious reactivity and coordination ability, but even the supposedly inert SCFs, such as C02 and CHF3, have the potential to interact with catalytically active intermediates. CO2 is known to form a variety of stable coordination compounds [3], and it inserts readily into metal-hydride, -alkyl, -alkoxide, or -amide bonds [4]. This may change the resting state of the catalytic cycle, but it is also the basis of the highly attractive concept of using C02 as a solvent and a feedstock at the same time [ 5 ] . Another possible advantage arising from the reactivity of some SCFs is their ability to form protecting groups by reacting with some functional groups in the substrates or products [6]. However, there is no guarantee

352

4 Reactions in SCF

that any of these interactions will actually improve catalyst performance. For example, COz can also be reduced to CO, a very strong ligand which may act as a catalyst poison in certain cases. Water or alcohol dissolved in scCOz can lead to the formation of catalytically inactive metal carbonates [2b]. In biphasic scCOz/H20 mixtures, the aqueous phase has a pH of around 3 [7], which can cause catalyst deactivation. Supercritical fluoroform has a slightly acidic proton, which prevents the use of very strong bases as reagents or catalysts. Nevertheless, for many reactions the possible benefits of SCFs clearly surpass their potential shortcomings. One of the greatest opportunities for SCFs in metal complex catalysis relates to the design of integrated processes, allowing for the separation of products and the recovery of catalysts in active form. The recovery of the precious metal compound, including often highly specialized and expensive ligands, is one of the key problems for the implementation of homogeneous catalysis on an industrial scale. Currently, the most widely discussed approach to solving this problem with conventional solvents is the use of so-called “biphasic systems” which rely on the separation of two immiscible liquid phases [8]. Supercritical fluids clearly offer an attractive alternative approach, because the rich phase behavior of supercritical multicomponent mixtures opens a variety of possibilities for the combination of homogeneous reaction mixtures with subsequent selective separation stages. The combination of homogeneous catalysis and supercritical fluid extraction (SFE) using scCOz has been established recently as a general approach; these processes are referred to as “catalysis and extraction using supercritical solutions” (CESS) [9,11,12]. Retrograde crystallization, a SFE technique that involves a temperature change to separate the catalyst from the product, has also been suggested as an option [lo]. Before discussing individual examples of metal catalysis in SCFs in detail (Sections 4.7.2-4.7.6), it is appropriate to outline briefly some experimental approaches to carrying out a metal-complex-catalyzed SFR.

4.7.1.2 Practical Considerations A typical experimental set-up for the investigation of a metal-complex-catalyzed SFR is given in Figure 4.7-1. Most of the individual parts of the equipment are described in detail elsewhere in this book (Chapter 2.1). The central part is a stainless steel reactor equipped with an appropriate safety device (burst disk, safety valve etc.), a thermocouple, a pressure transducer and two thick-walled glass or sapphire windows for visual control. Standard reactor sizes are 10-250 mL. Mixing with magnetic stirring bars is generally sufficient with SFRs on this scale. (For photographs of some typical examples see Figure 1.1-5 of this book.) The SCF is introduced with a pump or a compressor. Reactant gases are fed’either directly from the container or via a compressor, depending on the pressure required. If the whole system is installed in a

4.7 Metal-Complex-Catalyzed Reactions

353

Figure 4.7-1 An experimental apparatus for the investigation of metal-complex-catalyzed SFRs. 'Ifipical components include a high pressure reactor, an Isco 500D or JASCO PU-980 pump with cooling jacket, an Isotemp 1016s cooling recirculator for cooling the SCF pump, a Fisher Acuflow Series 1 pump for the liquid wash solvent and a Rheodyne 7125 injection valve equipped with a sample loop. The reactor (e.g. a 160 mL Parr N4772 general purpose bomb of T316 stainless steel) is modified to include a rupture disk or a similar safety device, thermocouple, gas release valve, Ashcroft K2 pressure transducer and an extra port. Diametrically opposed sapphire or thickwalled glass windows can be placed in the body of the vessel. The transducer and thermocouple outputs are displayed by Omega DP25-S and DP25-TC meters and can be connected to a computer via standard AID interfaces. The transducer and display are frequently calibrated against an Omega DPG602-2K digital pressure gauge with 0.25 % FS accuracy and 2000 psi (136 bar) full scale, which is recalibrated annually. The heat source could be a water bath, an electric heater, or an oven. For larger reactors (> 100 mL) the temperature can be effectively controlled by heating elements incorporated in the outer wall or by heating mantles. It is best to use two thermocouples, one measuring the temperature of the heating source and one controlling the temperature inside the reaction mixture. The abbreviations are: B = burst disk, P = pressure transducer, R = reagent addition port, T = thermocouple. Other suppliers provide similar equipment and the choice depends on the individual requirements of the project.

fixed manner, a device for rinsing the tubing and valves after each run is necessary. There are various ways to charge the reactor with solid or liquid reagents or catalysts. In the simplest case, all materials are placed together inside the reactor under a flow of argon or in a glove box. If some of the reagents or catalysts have to be kept separate before the supercritical state is reached, they can be contained in different compartments (a glass beaker or Eppendorf vial) fixed inside the reactor in such a way that the meniscus of the liquid phase stays below the top of this compartment at T < T,. Glass ampoules can also be used, although it is often difficult to control the conditions under which they rupture. An additional port for the addition of liquids via a syringe pump is highly recommended. This port can also be used to connect small containers via a ball valve, which can be filled with catalysts or substrates and pressurized with an inert gas, such as argon, to about 20-25 bar above the reaction pres-

354

4 Reactions in SCF

sure. Opening the ball valve leads to injection of solid or liquid materials into the reaction medium. The connection should be made with tubing of sufficient width (at least 1/8’ pr 3 mm inner diameter for solids) and the container should not exceed more than 10% of the reactor volume in size. The additional valve is also required as an outlet when the reaction is-combined with an extraction stage in CESS. In the simplest case, the reaction is maintained at the desired temperature T > T, for a certain amount of time and then cooled to T < T, before careful venting. The gas stream should always be passed through a cold trap held just above the condensation temperature of the SCF, in order to avoid any loss of volatile substrates or products. The contents of the trap and the reactor can be collected as the neat liquid or solid or with the aid of a solvent if the amounts are too small. Conventional methods are used for analysis. To obtain more insight into the course of reactions, a sample loop has proved useful, which allows the withdrawal of samples for off-line analysis under operating conditions. The dead volume of the loop and the size of the reactor must be chosen in a way that even repeated samplings do not cause a severe pressure drop. In a more advanced set-up, the sample loop can be operated automatically and connected directly to a gas chromatograph for on-line analysis [ll]. The use of an in situ high pressure reflectance probe for on-line FTIR has also been described, whereby the probe was connected to the reactor via a specially designed lid [12]. Various designs of high pressure equipment for NMR spectroscopy have been used for investigations of metal-catalyzed SFRs. More details on in situ spectroscopy are found in Chapter 3 of this book. The phase behavior of the reaction mixture and the solubility of the catalyst in these phases is a prime consideration for researchers using metal complexes for catalysis under supercritical conditions. There are three distinct approaches to carrying out metal-complex-catalyzed SFRs (Figure 4.7-2). In the most obvious strategy, the solvent, all reactants and the dissolved catalyst form a single supercritical phase (Type I). This truly homogeneous catalysis provides the ideal opportunity to gain all potential benefits arising from the specific

Substrates Products

Substrates Products Substrates Products

U Catalyst

liquid

.__

SCF

Catalyst

Figure 4.7-2 The three different types of metal-complex-catalyzed reactions in SCFs. Type I: fully homogeneous single-phase reaction. Type 11: single-phase reaction medium with an insoluble metal complex as the catalyst. Type 111: two-phase catalysis with (a) the catalyst in the liquid phase and the products and/or substrates in the SCF, and (b) vice versa.

4.7 Metal-Complex-Catalyzed Reactions

355

properties of the supercritical fluid (as discussed above), especially when combined with a second separation stage. In the second approach, there is again only one supercritical phase consisting of solvents and reactants, but the metal complex is now insoluble in this medium and the reaction occurs-at the solid-SCF interface (Qpe 11). The insolubility may be an intrinsic property of the metal complex or may arise from anchoring the metal species' to an insoluble support. The arguments that make the use of SCFs attractive in these cases are similar to those discussed for classical heterogeneous catalysis (see Chapter 4.8). The third possibility comprises a two-phase system made up from a liquid phase and a supercritical fluid. The liquid phase may be considerably swollen by dissolved gas, as is for example observed with mixtures of organic liquids and compressed C 0 2 [13,14]. This can alter some physical properties, such as the solubility of gases in the liquid phase, to some extent. Type I11 can be further differentiated into two extremes, depending on the location of the catalyst. In Type IIIa, the catalyst is dissolved in the liquid phase only, whereas the substrates and/or products are mainly in the SCF. Q p e IIIb refers to a system with the catalyst separating exclusively in the supercritical phase. Both systems are very similar to those used in conventional biphasic homogeneous catalysis [8]. The differentiation of these three types of reaction systems is crucial for the design and success of metal-catalyzed SFRs. Indications for Q p e I1 or Type I11 processes usually come from careful visual inspection of the reactor, although small amounts of liquid or solid material can sometimes be quite hard to detect. The proof of a homogeneously dissolved catalytically active species is more difficult, even if a window-equipped reactor is used. For example, the bulk material of the catalyst may remain apparently insoluble (indicating a Type I1 process), but the catalytic cycle can still be carried by small amounts of a dissolved metal species (Qpe I reaction). Fortunately, there are several straightforward control experiments which unambiguously prove the presence of a homogeneously dissolved catalytically active species, as exemplified below for reactions in scCO2. Jessop et al. demonstrated the solubility of the Ru complex used in their studies of the catalytic hydrogenation of scCOz [14]. They dissolved a solid sample of the complex into the SCF, passed the solution through a line filter and a back pressure regulator, and collected the precipitating complex directly in an NMR tube. The spectrum of the precipitated material was shown to be identical to that of the starting material, proving that the complex was indeed dissolved in its original from. Another very similar test involves connecting a second reactor after the filter, instead of the NMR tube, and testing for catalytic activity of the SCF after filtration [15]; if the catalysis occurs, then this method is unambiguous. If the reaction fails, however, great care has to be taken to prove that dissolved species did not precipitate in the tubing because of pressure or temperature gradients.

356

4 Reactions in SCF T c Tc

T > Tc

T c Tc

substrate

T c Tc

T > TC

sccop substratel cat. gas

liqCO2 + substrate

Figure 4.7-3 A simple procedure to determine the nature of the active species in metalcomplex-catalyzed SFRs. Experimental details and alternative methods are described in the text.

Another simple control experiment avoiding this ambiguity is depicted in Figure 4.7-3[12]. The procedure is exemplified for a catalytic reaction involving a liquid or solid substrate and a reaction gas in scCOz, but can be generally applied. The catalyst or catalyst precursor is placed together with a little stirring bar inside the reactor in a different compartment separate from the mixture of substrate(s) and liquid C02 as described above. The inner compartment should be deep enough to prevent mechanical extrusion of solids when stirring. Alternatively, it can be closed with a piece of filter paper or a plug of glass wool. When heated beyond T, the supercritical reaction medium gets into contact with the metal complex and will eventually dissolve the active species. Upon venting, all dissolved species, including the catalyst, precipitate everywhere in the reactor. If possible, the product can be extracted quantitatively by SFE at this point. After taking out the original container of the catalyst, the reactor is recharged with substrate and the experiment is repeated. Conversion in the second run unambiguously proves the homogeneous nature of the catalysis. If no reaction occurs, another set of two consecutive reactions must be carried out without initially separating the catalyst to make sure that no deactivation occurred during work-up. This procedure allows the determination of the homogeneous or heterogeneous nature of the catalysis ( Q p e I versus Type II/III) on the basis of no more than four single catalytic runs. 4.7.1.3 Solubility of Metal Complexes in SCFs

The solubility of metal complexes is a key factor for the design and operation of a metal-catalyzed SFR. The largest body of solubility data is available for scCOz as a solvent. In 1982, Ibers published a study on the behavior of a wide variety of metal complexes with relevance for catalysis in liquid C02 and concluded that most of them were insoluble [16]. A few complexes led

4.7 Metal-Complex-Catalyzed Reactions

357

to nonproductive decomposition reactions, and known C 0 2 complexes were formed in isolated cases. A more recent study by Walther and co-workers has identified certain catalytically relevant complexes that are stable and reasonably soluble in liquid and supercritical C02, but in general the same severe limitations were encountered-[151. However, the large amount of data on transition metal complex solubility in relation to toxic metal extraction [17] indicates that the problem of solubility can be overcome by the correct choice of a C02-philic ligand framework including, for example, the utilization of perfluoroalkyl, fluoroether, or siloxyl substituents [ 181. Indeed, the proper choice of the metal complex has been a major theme for homogeneous catalysis in SCFs from the start. In their pioneering studies, Rathke and co-workers used the catalyst precursor [CO~(CO)~]. a very nonpolar compound of fairly high volatility [19]. Noyori’s team took advantage of the higher solubility of trialkyl phosphines compared to triaryl phosphines in their work on C02 hydrogenation [14]. They also suggested the use of peffluorinated alcohols as additives to increase the solubility of catalysts [20]. In a joint effort, the groups of Burk and Tumas showed that even cationic complexes can have sufficient solubility in scCO2 if the lipophilic anions triflate (triflate = trifluoromethylsulfonate) or BARF (BARF = tetrakis-(3,5-bis(trifluoromethyl)-phenylborate) are used [21]. Leitner and co-workers put forward the concept of designing C02-philic phosphine ligands by directly attaching perfluorinated alkyl groups, especially of the type (CH2),(CF2),F [22]. These chains can be attached to the aryl rings of triarylphosphines in a highly flexible synthesis [23,24] and do not interfere with the properties of the active metal center if a suitable substitution pattern is maintained [22]. At the same time, these substituents provide solubilities that are large enough not only for catalysis, but also for NMR spectroscopic investigations [24]. These studies, together with the data on the organometallic chemistry of coordination compounds in SCFs (Chapter 4.2), enable the factors that contribute to high catalyst solubility in scC02 to be summarized as follows: ligands should be unreactive to C02, contain few to no aromatic rings, and have perfluoroalkyl, siloxyl, or branched alkyl groups; complexes should be hydrophobic, fairly volatile, nonpolar, uncharged, andor have as many CO2-philic ligands as possible; charged complexes should contain C02-philic counterions such as the BARF anion or tetraalkylammonium cations. It should be noted, however, that the presence of substrates and/or products in preparatively useful concentrations can alter the solubility properties of scC02 considerably and may lead to sufficient solubility even for a nonideal catalyst [12]. For other non-polar SCFs, similar rules will apply, whereas polar SCFs may require quite different designs. Unfortunately, a sufficiently large body of data is currently not available to allow more general conclusions for these media.

358

4 Reactions in SCF

4.7.2

Hydrogenation and Related Reactions

The complete miscibility of SCFs with other gases is one of the outstanding features of these reaction media, especially for hydrogenation reactions. In contrast to the situation in an SCF, H2 has limited solubility in most liquids; as a result, hydrogenation rates are ofteri H2-mass-transfer-controlled and H2starved conditions are soon reached. However, H2 and scC02 are miscible in all proportions above 31 "C [25]. A scCO2 solution containing the amount of H2 corresponding to a partial pressure of 80 bar at 50°C is 3 M in H2, a concentration that cannot be reached in liquid benzene except at an H2 pressure of approximately 1000 bar [26]. A price must be paid for this high H2 concentration, and that is the reduction in the ability of the SCF medium to dissolve substrates and catalysts. Added H2 acts as an antisolvent, mainly by reducing the density of the SCF at comparable pressure and temperature values. Thus, the solubility of catalysts in H2/C02 mixtures can be poor, especially at high H2 mole fractions. New ligands are now being designed to enhance the solubility of homogeneous catalysts in scC02, and nowhere will this be more important than in reactions involving H2 as one of the reactants. 4.7.2.1

Hydrogenation of C 0 2 under Supercritical Conditions

In the presence of a suitable homogeneous metal catalyst, C 0 2 itself can be hydrogenated to yield formic acid or derivatives thereof [27]. This reaction represents an interesting approach to the use of C02 as a chemical feedstock on a large industrial scale, and it is therefore not surprising that the use of scC02 has found one of its first applications in homogeneous catalysis in this reaction. The solubility problem encountered with traditional triarylphosphine-containing catalysts was overcome by using trialkylphosphine complexes, namely cis-[Ru(H)2(PMe3)4] (1) and trans-[Ru(C1)2(PMe3)4] (2). In view of the thermodynamics of the C02 hydrogenation [13,27], triethylamine was added to the reaction mixture as a scC02-soluble base to produce the stable formic acid-triethylamine (ca. 2: 1) azeotrope. The result was outstanding rates for C02 hydrogenation at 50 "C (Scheme 4.7-l), especially in the presence of cocatalysts or cosolvents such as water, methanol or DMSO [5,14]. Importantly, the phase behavior of the reaction mixtures has a strong effect on the rate of this reaction. The reaction is initially homogeneous and turns heterogeneous later due to the precipitation of the liquid HCOOH-NEt3 product. However, if too much NEt3, water or methanol is added, the reaction mixture is heterogeneous from the start, leading to a dramatic decrease in rate [5,14]. Preparation of a 2:l adduct of HC02H-NEt3 2.7 pmol of complex 1 [28], is placed into an oven-dried 50 mL reaction vessel in an argon-filled glove bag. The vessel is then connected via an adapter to a gadvacuum manifold, evacuated for 10 min under high vacuum and

4.7 Metal-Complex-Catalyzed Reactions NEb T = 50'C/

COz

+

Hz

-

a

PMe, ,.H

"h

Me,PO

he3 1

HCOzWNEk (23)

/

Ru-mt. HCOzH

T > Tc 80 bar HZ ~ 2 0 bar 0 total

Me,P.,/

NEb'MeOt HCOzMe+ HzO T = 80%

\

HNMT = 100°C

HC(0)NMe + HzO

,.PMe2

CI Me,P.,,i;.PMe, Me,P'

359

b

.PMe3

2

Si(OEt), 3

3

4

I

Si(OEt),

Scheme 4.7-1 Synthesis of formic acid and its derivatives by hydrogenation of C02 under supercritical conditions. Species 1-4 are some ruthenium complexes that have been used as catalysts or catalyst precursors.

refilled with argon. Methanol (0.54 mL, 13 mmol) and NEt, (0.7 mL, 5.0 mmol) are injected into the reactor against a positive argon pressure through a threaded opening in the top, which is plugged at all other times. The reactor is then detached from the g a s h c u u m manifold, connected to the SCF equipment (Figure 4.7-1). filled with 4 0 bar of H2 and heated to 50°C. After temperature equilibration, the pressure of H2 is increased to 80 bar and then C 0 2 is added to a total pressure of 200 bar. After a reaction time of 15 h, the reactor is half-submerged in a bath of acetone which is subsequently cooled by addition of dry ice. When the pressure has reached a steady low value, the H2 gas is vented and the reactor slowly warmed, the CO2 venting as it sublimes. The liquid product contains 9.1 mmol HCOzH plus 0.4 mmol HC02Me.

The C 0 2 hydrogenation can be coupled in situ with subsequent reactions to prepare derivatives of formic acid (Scheme 4.7-1). For example, thermal esterification gives high yields of methyl formate when scCOz is hydrogenated at 80°C in the presence of large amounts of methanol [29,14]. Surprisingly, under these conditions the production of formic acid and methyl formate was found to be faster if liquid methanol was present than if the reaction was in the homogeneous supercritical medium. Using secondary dialkyl, rather than tertiary trialkyl, m i n e s allows the subsequent dehydration of the [NH2R2][O2CH] product to occur, giving N,N-dialkylformamides [30,14]. In particular, N,N-dimethylformamide (DMF) was produced at 100 "C in 99 %

360

4 Reactions in SCF

yield and 99% selectivity. Turnover numbers (TON, mole product per mole metal center) as high as 420000 were observed. A liquid phase is always present during this reaction. At the start of the reaction, the liquid phase is dimethylammonium- carbamate (dimcarb), formed from Me2NH and C02. Towards the end of the reaction, the liquid phase is primarily water, while the DMF product is partitioned in the liquid andsupercritical phases [14]. Baiker and co-workers reported that the complex truns-[Ru(Cl)2(dppe)2] (3) (dppe = 1,2-bi~(diphenylphosphino)ethane) catalyzes the formation of DMF very efficiently, despite the insolubility of the complex in scC02 [31]. They were even able to use a heterogenized silica-grafted ruthenium complex, which was obtained from the five-coordinated precursor 4 and tetraalkoxysilanes in a sol-gel process [32]. With these catalysts, the reaction occurs in the liquid dimcarb phase. The fact that high rates can be achieved with such heterogenized metal complexes makes them attractive materials for other processes involving SCFs or compressed gases. The hydrosilylation of scC02 with dimethylethylsilane to give the corresponding silylformate has also been described briefly [2d]. The catalytic efficiency of complex 1 in scCO2 was good (62 TON) compared to literature reports of comparable catalysts in liquid solvents [33]. However, the overall yield of silylformate was low (2 % based on hydrosilane after 63 h), suggesting that attempts to perform hydrosilylations of organic substrates in scCO2 should not meet with significant interference from the reaction of C02 with hydrosilanes. Up to now, however, there have been no reports of the hydrosilylation of alkenes or ketones in SCFs. 4.7.2.2

Hydrogenation of C =C Double Bonds

The successful C02 hydrogenations marked a breakthrough in SFR research because they indicated that SCFs could be applied to the large class of reactions catalyzed by homogeneously dissolved phosphine metal complexes. They also suggested that interesting effects might be expected for reactions that are sensitive to variations in H2 concentrations, as for example the asymmetric hydrogenation of prochiral alkenes using the chiral catalyst [Ru(OAc)2 (BINAP)] (OAc = 02CCH3; BINAP = 2,2'-bis(diphenylphosphino)-l,l 'binaphthyl, (5)) [34,35]. Initial investigations of the asymmetric hydrogenation of tiglic acid (Scheme 4.7-2) revealed, however, that the solubility of 5 and its metal complex in scCOz was far too low for efficient catalysis. This problem was ameliorated by using a partially hydrogenated ligand (S)-H8BINAP (a), which led to a smooth reaction in the supercritical phase and yielded an enantiomeric excess (ee) comparable to liquid organic solvents [20,36].

Asymmetric hydrogenation of tiglic acid 1.5 mmol of CF3(CF2)&H20H, 0.70 mmol of tiglic acid, and 4.5 pmol of [37] are placed into an oven-dried 50 mL reactor in an argon[RU(OAC)~(~)] filled glove bag. The reactor is closed, flushed with C02, and pressurized to

4.7 Metal-Complex-Catalyzed Reactions

Scheme 4.7-2 Asymmetric hydrogenation of tiglic acid in scC02. Typical results are listed in Table 4.7-1.

(S)-BINAP, (S)

(s)-H$Iwp,

361

(6)

20 bar using the equipment shown in Figure 4.7-1. After temperature equilibration to 50 "C, the pressure of C 0 2 is increased to 180 bar and then H2 is added to a total pressure of 185 bar. After 15 h, the reaction is stopped by the acetone/dry ice method. (S)-2-Methylbutanoic acid is obtained in essentially quantitative yield with 89 % enantiomeric excess. Note that the above procedure requires a compressor to supply H2 at a sufficiently high pressure for most of the experiments summarized in Table 4.7-1. In the absence of such equipment, the H2 could be introduced after the temperature equilibration and immediately before adding the remainder of the C02. As long as the C 0 2 is pumped in very quickly thereafter, little hydrogenation would take place during the pumping time. This technique could not be used if the reaction were ( t 1 / 2 < 30 min). The reaction depicted in Scheme 4.7-2 shows a different dependence of the enantioselectivity on H2 pressure in different solvents (Table 4.7-1) [20,38]. In methanol, the ee decreases with increasing H2 pressure, but in scC02 the ee increases with increasing H2 pressure. As a result, at high H2 pressures, scC02 and methanol are equally good solvents for this reaction, but at low H2pressure the enantioselectivity in methanol is superior. A fluorinated alcoTable 4.7-1 Effect of pressure and solvent on the asymmetric hydrogenation of tiglic acid with [Ru(OAc)2(6)] catalyst [20, 361.

ee

(a)"

Solvent

Low pressure H2

High pressure H2

Hexane Methanol scco2 SCCO~/CF~(CF~)~CH~OH SCCHF~

74 95 71 89 90

73 82 81 76

~~~~

a

b

Substrate-to-catalyst ratio (S/C) = 150, T = 50°C. Reactions at low pressure H2 had 5-11 bar of H2, whereas reactions at high pressure H2 had 30-40 bar. The @)-configured product was preferred in all cases. Not available.

362

4 Reactions in SCF

hol dissolved in scC02 caused an immediate increase in the ee compared to that obtained in pure C02, and made the SCF more methanol-like in that the enantioselectivity then decreased with increasing H2 pressure. Finally, scCHF3 was found-to be a better solvent than pure scC02 for this reaction at lower H2 pressures. The cationic chiral catalyst [Rh(cod)(7)]+showed high activity and selectivity in the presence of CO2-philic counterions such as the BARF or triflate anion (Scheme 4.7-3) [21]. The DuPHOS-type ligand 7 is a fairly nonpolar alkyl phospholane and is especially useful in asymmetric hydrogenation of dehydroamino acids in conventional solvents [38]. The enantioselectivities in scC02 compared well to those in methanol for most substrates, and the asymmetric induction was considerably higher for P,p-disubstituted substrates (Table 4.7-2). R' R2~

[Rh(cod)(7)]BARF (J(H;

H2

*~o2,4ooc 14 bar H,

R'

$02Me

R ~ ' N ( H ) A ~

330 bar total

= (R~R)-Et-DuPHoS* Et

Scheme 4.7-3 Asymmetric hydrogenation of dehydroamino acids in scCOz using cationic rhodium complexes. Representative results are listed in Table 4.7-2.

Table 4.7-2 Asymmetric hydrogenation of dehydroamino acids with [Rh(cod)(7)]BARF as catalyst in scCO2 [21].

Substrate R'

a

R2

MeOH

ee (%)" Hexane

scco;!

H Et Me

98.7 98.7 62.6 81.1

96.2 96.8 69.5 76.2

99.5 98.8 84.7 96.8

Substrates as in Scheme 4.7-3. S/C = 500, T = 40°C. Reactions in scCO2 were carried out at 14 bar of H2, whereas reactions in liquid solvents had only 4 bar. Products with @)-configuration were favored in all cases.

In contrast to the studies on enantioselective hydrogenation, there has been very little work on the use of homogeneous hydrogenation in SCFs for the synthesis of achiral products. The hydrogenation of a substituted cyclopropene with [MnH(CO)=J in scCO2 has been reported. Although the reaction can be run catalytically under H2/C0 pressure, the experiment was performed stoichiometrically to probe the effect of the anticipated weak cage effect of scC02 on the selectivity of the radical mechanism [39]. Another system that

363

4.7 Metal-Complex-Catalyzed Reactions

has been investigated is the chemoselective hydrogenation of isoprene to give a mixture of isomeric isopentenes using the C02-philic catalyst [Rh(hfacac)(3-H2F6-dppe)] (hfacac = hexafluoroacetylacetonate; 3-H2F6-dppe = R2PCH2CH2PR2with R = m-C61&(CH2)2(CF2)6F)[24]. The reaction resulted in high selectivity for the hydrogenation of the conjugated diene with very little overreduction to the fully saturated hydrocarbon, but the rate of reaction was inferior to that of related catalysts in organic solvents.

4.7.2.3 Hydrogenation of Imines The hydrogenation of C=N double bonds is an important synthetic strategy for the synthesis of secondary amines. Chiral iridium catalysts allow the hydrogenation of prochiral imines to be carried out with high enantioselectivity in conventional liquid solvents. Such a process has already found industrial application in the preparation of (S)-metolaclor, a herbicide produced by Novartis in Switzerland [40]. Recent research at the Max Planck Institute for Coal Research has demonstrated that reactions of this type can be carried out in scCO2 with the same level of enantioselectivity and with enhanced catalyst efficiency [ 121. A series of iridium complexes 10-12 was synthesized based on the chiral phosphanodihydrooxazole ligands introduced for enantioselective imine hydrogenation in CH2C12 [41] and was screened in scC02 for the test reaction shown in Scheme 4.7-4. Compared to the known catalyst 10a, the new complexes had lipophilic anions and/or perfluorinated side chains in the periphery of the Ar2P moiety. Both the side chains and the lipophilic anions increased the solubility, but the choice of the anion also had a dramatic effect on the enantioselectivity (Figure 4.7-4). Only the complexes containing the BARF anion allowed for an asymmetric induction of up to 81 % ee (12a) which was fully comparable with the reaction in CH2C12 and the related industrial process. Furthermore, the substituent at the nitrogen in substrate 8 and product 9 was found to play a crucial

rP phKC%

10, I 1 or 12

+

H2

30 bar Hz, 40'C

8. R = P h

Scheme 4.7-4 Asymmetric hydrogenation of imines in scC02 using chiral iridium catalysts. Representative results are summarized in Figure 4.7-4.

d o 2 ( d = 0.75 g/mL)

L

10 x = pF6 a R=H l1 = Bpb b R = (CH2)2(CF&F 12 X = BARF

Np PhAC& 9s R = P h

364

4 Reactions in SCF

60

-

70 80

:50 0

40

30 20 10 0

1Or

lob

llr

llb

catalyst

: 12a

12b

Figure 4.7-4 Influence of perfluoroalkyl substituents (compounds a or b) and of anions X- on the enantiomeric excess in the hydrogenation of imine 8a using chiral iridium catalysts 10-12 [12].

role for this SFR. High pressure on-line IR and NMR investigation revealed that the higher reactivity of the product 9b, compared to 9a, to form the corresponding carbamic acid [2b,42] probably accounts for the fact that 8b could not be hydrogenated to more than 30% in scCOz. It should be noted, however, that carbamic acid formation can also have beneficial effects in catalysis, as outlined in Section 4.7.4.3. Hydrogenation of phenyl-(l-phenyl-ethy1idene)-amine(8a) A window-equipped 100 mL stainless steel reactor with magnetic stirrer is charged with the substrate 8a (820 mg, 4.2 mmol) and catalyst 12a (9.5 mg, 6.0 x mmol, S/C = 700, 0.14 mol% catalyst) under an argon atmosphere. A weighed amount of C 0 2 is introduced using a compressor to obtain the desired COz density of 0.75 g/mL. The reactor is heated to 33°C and hydrogen is introduced to obtain a partial pressure of 30 bar at this temperature (the actual Hz pressure required must be determined from a calibration curve). The temperature is increased quickly to 40°C and the mixture kept at that temperature for a standard reaction time of 20 h. For work-up, the reactor is cooled to room temperature and vented carefully. The reactor content is collected by washing with CH2C12. The solvent is removed in vacuo and the product isolated by Kugelrohr distillation to give analytically pure 9a with typically > 98 % recovery and 81 % ee of the (R) isomer. Alternatively, the reaction can be carried out as a CESS process, whereby the product is collected by extraction from the reactor with CO:! at 40 "C and 110 bar. With substrate 8a, the use of scCOz leads to a significant improvement of the performance of the catalyst compared to the reaction in CHzClZ. Hence, significantly lower amounts of iridium complex are required for efficient catalysis in the supercritical medium and turnover fre uencies (TOF, mole product per mole metal center per hour) above 2,000 h-9 were achieved at catalysts loadings as low as 0.014 mol%. It is important to note that this effect is not related to a simple increase in reaction rate, but rather to a change in the overall kinetic behavior of the catalytic system.

4.7 Metal-Complex-Catalyzed Reactions

365

Owing to the low solubility of 12a in scC02 in the absence of imine 8a, the product 9a could be isolated in pure form by simply venting the reactor and subsequently extracting the product from the residue by SFE under conditions similar to those used during the reaction. The catalyst remaining in the reactor could be reused seven times with less than 10% decrease in ee. The activity started to decrease after the fourth run, but quantitative conversion was still possible after prolonged reaction times. This is the first successful combination of catalysis and extraction using supercritical fluids (CESS) in asymmetric synthesis with chiral metal complexes. This study, and the related investigations described in Section 4.7.2.2, demonstrate that scC02 provides a useful alternative reaction medium for asymmetric hydrogenation reactions, which can lead to higher rates and catalyst efficiencies as well as sometimes higher enantioselectivies. With asymmetric hydrogenation becoming increasingly important for the preparation of chiral pharmaceuticals, food additives and other biologically active compounds, toxicologically and environmentally benign C02 may often be the solvent of choice even if the level of enantioselectivity does not exceed that obtained in conventional solvents.

4.7.3

Hydroformylation

The addition of CO and H2to a C=C double bond to yield aldehydes or, with subsequent reduction, alcohols is referred to as hydroformylation or the oxoprocess. This reaction is one of the most important processes catalyzed by homogeneous organometallic catalysts on an industrial scale [43].Hydroformylation catalysts are classified according to the metal used, with cobalt and rhodium-based catalysts being by far the most successful systems. These can be divided into the so-called unmodified and modified systems. In unmodified systems, the catalytically active species is a hydrido metal carbonyl complex formed in situ from a suitable precursor in the absence of any additional ligand. In modified systems, the active intermediates contain additional ligands to control the activity and the chemo-, regio- and stereoselectivity. The most commonly used ligands in the modified systems are organophosphorus compounds. All of these hydroformylation catalysts can be adapted for use in scC02; Table 4.7-3gives an overview of the present state of the art. In a seminal study, Rathke and co-workers provided the first example of the utilization of scC02 in a typical homogeneous metal-catalyzed reaction, the hydroformylation of propene using [ C O ~ ( C O )(13) ~ ] as a catalyst precursor (Scheme 4.7-5)[19]. Although this investigation was largely devoted to the detection of catalytically active intermediates by 59C0 NMR spectroscopy, most of the potential benefits that are now often quoted for homogeneous catalysis in SCFs were explicitly addressed. The cobalt species detected by NMR under turnover conditions were identical to those observed in liquid solvents, indicating that no major change in the intermediates of the catalytic pathway

[Rh(4-H%'-TPP)3C1]

(P:Rh=lO:l) 45

14/4-HZF6-TPOP 2175 (P:Rh = 1O:l) [Rh(CO)z(acac)]/4-~-TPP 0.77

i

0.62

70

80

65

65 65

'

27

69

>98

98

95

88 2

92 93

>98 34

20 18

15 3.5

2.4

3.7

8.5

1.6 5.6

g

7.3

79

115

1375 500

h

1.3

Time Yield' n:isod T O P (h) (8) (h-')

20

20

20 20

f

60

0.62 0.62

113.7

80

0.5 0.72

Pressure H2/COb (bar)

Temperature (C)

COz (g/mL)

a Molar ratio of substrate to metal center. bHz/CO = 1:l. Yield of aldehydes. Ratio of linear aldehyde to all other isomeric aldehydes. Maximum turnover frequency in mole aldehyde per mole of metal center per hour. No additional synthesis gas. g M ~ ~ tcis l y isomer. Data not available. ' Total pressure 180 bar. j Estimated from graphics in original paper. Total pressure 250 bar.

1-Hexene

1-0ctene

1-Hexadecene

1-Octene

2650 2100

[Rh(cod)(hfacac)] (14) 14/3-HZF6-TPP (P:Rh = 1O:l)

cyclopropene 1-0ctene 1-Octene

SM'

25 0.3

Catalyst

[CO~(CO)SI (13) 3,3-Dimethyl-l2-diphenyl- [MnH(CO)5]

Propene

Substrate

Table 4.7-3 Transition metal-mediated hydroformylation in scCOz.

[461

WI

[ll]

[ll] [ll]

[19a] [391

Reference

3

t-.

s.

2

k

Q

o\

o\

w

367

4.7 Metal-Complex-Catalyzed Reactions

Scheme 4.7-5 Cobalt-catalyzed hydroformylation of propene in scCOz [ 191. Conditions and typical results are listed in Table 4.7-3.

occurred. Despite the mechanistic similarities, an enhanced selectivity for the linear product n-butyraldehyde at comparable catalytic activity was observed by changing from a conventional liquid solvent to scC02. A kinetic investigation of the Co-catalyzed hydroformylation in a semibatch reactor [44] showed that the pseudo-first-order rate constant for the reaction is a function of pressure if the rate law known from liquid organic solvents is used for data analysis. However, a nonlinear relationship between the reaction rate and the catalyst concentration was observed and is not in agreement with the simple kinetic scheme. From a practical viewpoint, the large catalyst loadings and the relatively forcing conditions are still severe limitations of this catalytic reaction. As in conventional solvents, rhodium-based systems are generally much more active than cobalt catalysts in scC02 (Scheme 4.7-6; Table 4.7-3). Various alkenes have been hydroformylated in scCOz with [Rh(hfacac)(cod)] (cod = 1,5-cyclooctadiene; 14) as the catalyst precursor without additional ligands at substrate:Rh ratios as high as 2600:l [ll]. The reaction rate was found to be considerably higher in scCOz than in liquid organic solvents,

scCO2,40-65'C 20-60 bar C O / Y (1:1)

RJ

'

Rd

H -)

0

R' = c ~ H ,R~ ~ ,= H, i-octene; R' = R~= ~ 2 ~ trans-ahexene; 5 . R' = Ph, R2 = H, styrene; R1= CH~OAC, R2 = H, ally1 acetate

Scheme 4.7-6 Rhodium-catalyzed hydroformylation in scCO2 [ 111. The notation x-HYFZ used here and in the text defines perfluorinated alkyl substituents of general formula (CH2),(CF2),F with the x giving the substitution position relative to phosphorus [22]. TPP and TPOP are used as acronyms for triphenylphosphine and triphenylphosphite, respectively. Typical results are listed in Table 4.7-3 and represented in Figure 4.7-5.

368 1600 1400

' IL

4 Reactions in SCF

-* --

1000 800-600-400

--

200 -0, 14

14/3-H2F%-TPP 14JlPP in

toluene

14/4-H2F%TPOP

Figure 4.7-5 Comparison of activities and n:iso ratios for various catalyst systems in the rhodium-catalyzed hydroformylation of 1 -octene in scCOz (Scheme 4.7-6) and in toluene under otherwise identical reaction conditions. Data taken from Reference [ 111.

the effect being most pronounced for internal alkenes such as trans-3-hexene. The course of the reaction was investigated using an on-line GC reactor. A maximum TOF of 1345 h-' for the hydroformylation of 1-octene was observed under a standard set of reaction conditions. The hydroformylation of 1-octene has been carried out as a CESS process with a phosphine-modified catalyst consisting of 14 and the ligand 4-H2F6TPP (Scheme 4.7-6), resulting in rhodium contents in the product as low as 0.7 ppm [ll]. A direct comparison between the systems 14/3-H2F6-TPP in scC02 and 14/TPP in toluene under identical reaction conditions revealed that the SCF system exhibits a slightly higher rate and significantly higher selectivity towards the desired linear aldehyde (n:iso ratio). A remarkably high intrinsic selectivity for the linear aldehyde leading to an overall n i s o ratio of about nine was observed for the perfluoroalkyl-substituted phosphite ligand 4 -H2F6-TPOP(Figure 4.7-5). Furthermore, double bond isomerization, which is a typical side reaction for phosphite-based hydroformylation catalysts in conventional solvents, was effectively suppressed in scC02. The latter observation was attributed to the absence of a mass transfer limitation for CO in the SCF. Hydroformylation of 1-octene and separation of product and catalyst A window-equipped 275 mL stainless steel reactor is char ed with the catalyst components (14 26.7 mg, 0.64 x mol; 4-H5F6-TPP 1.65 g, 1.26 X mol) and I-octene (20.0 mL, 0.13 mol) under an argon atmosphere. The mixture is pressurized at room temperature with CO:H2 (1: 1) and CO2 is introduced using a compressor to obtain a density of 0.62 g mL-'. After heating to 65°C for 20 h, the reactor is cooled below T, to 20°C and gaseons components are vented until a pressure of 55 bar is reached. The reactor is then heated again to 65°C resulting in a pressure of 110 bar. The mixture consists now of a colored liquid phase and a color-

4.7 Metal-Complex-Catalyzed Reactions

369

less gaseous, presumably supercritical phase. The two isomeric aldehydes can be extracted quantitatively from this mixture at 65 "C and 110 bar with 500 g C 0 2 and the remaining catalyst shows no apparent loss in selectivity and activity in at least four subsequent runs. An n:iso ratio of 3.7 is obtained under these conditions. A preliminary report on the use of a peffluoroalkyl substituted aryl phosphine ligand lacking the ethylene spacer (4 -@F6-TPP) presented very promising results for the hydroformylation of long-chain alkenes up to C16 [45]. Erkey found that even the introduction of a CF3 group into the para position of TPP increases the solubility sufficiently to allow hydroformylation activity [46]. Using the 'Wilkinson-type catalyst [Rh(4 -H°F'-TPP)3Cl] as a precursor, an n:iso ratio of 2.4 was achieved with 1-octene as substrate. Although the directly attached perfluoro groups without the spacer also provide enough solubility for the use of aryl phosphines in scC02, the corres onding ligands are considerably more electron deficient than TPP or the H2Ft -TPP ligands [47]. This must be kept in mind for applications in reactions that are very sensitive to electronic variations. Trialkylphosphines, such as PEt,, have also been used successfully for 1-hexene hydroformylation in scC02 by Bach and Cole-Hamilton [48]. Naturally, these ligands differ quite dramatically from TPP in their electronic and steric properties [47]. Asymmetric hydroformylation provides viable routes to important antiinflammatory drugs starting from simple vinyl arenes. Rhodium catalysts bearing the chiral phosphine/phosphite ligand (R,S)-BINAPHOS allow very high levels of enantiocontrol [49]. Investigations with a catalyst made up from 14 and (R,S)-BINAPHOS revealed that the ligand-bound rhodium species have insufficient solubility in the supercritical phase [SO]. Even moderate asymmetric induction could only be obtained at low C 0 2 densities, when an additional liquid phase was present at some stage of the reaction [50,51]. Preliminary results with perfluoroalkyl-substituted BINAPHOS derivatives indicate that the solubility problem can be solved again by this methodology [52]. Taken together, hydroformylation is probably the best-investigated reaction using metal complex catalysts in scC02 and in SCFs in general. Although a new C-C bond is formed in this process, it was therefore treated separately from other reactions of this type, which are discussed in the following section.

4.7.4 C- C Coupling Reactions 4.7.4.1

Cobalt-catalyzed Cyclization Reactions

The catalytic cyclotrimerization of alkynes to give substituted benzenes was investigated in supercritical water by the groups of Parsons [53a] and Dinjus [53b] using [ C ~ C O ( C O ) ~ as] the catalyst. The reaction proceeded smoothly with 1-alkynes and phenylacetylene at T = 374 "C and p = 250 bar using substrate to catalyst ratios up to 35:l. The ratios of isomeric trisubstituted ben-

370

4 Reactions in SCF

zenes were similar to those observed in organic liquids. Several organometallic complexes related to the catalytic cycle established in conventional solvents could be isolated from the reactions carried out in scHaO, some of which were characterized by X-ray diffraction [53b]. Cyclotrimerization of 1-hexyne and acetonitrile to give substituted pyridines u-sing the same catalyst was unsuccesful because of rapid hydrolysis of the nitrile function [53a]. The Pauson-Khand reaction (i.e. the three-component coupling of an alkyne, an alkene and CO) is one of the most powerful synthetic strategies for the assembly of cyclopentenone rings. It has found widespread application in organic synthesis including the synthesis of biologically active compounds [54]. Jeong and co-workers demonstrated the feasibility of conducting Pauson-Khand reactions in scCO2 using the dimeric cobalt carbonyl complex 13 as a catalyst as shown for an enyne as a test substrate in Scheme 4.7-7 [55].

13 EtO2

30 bar CO 198 bar total

Scheme 4.7-7 Cobalt-catalyzed Pauson-Khand reaction in scCOz [ 5 5 ] .

Both intra- and intermolecular cyclizations worked well, and greater than 80% yields could be obtained using 5-10 mol% cobalt [55]. Only the ex0 isomer was isolated from the intermolecular reaction of phenylacetylene and other alkynes with norbornene and CO. No direct comparison of rates and selectivities in scC02 and conventional solvents are yet available, but problems with insolubility of very polar substrates were noted. Rhodium and ruthenium carbony1 complexes are much less effective catalysts for the Pauson-Khand reaction in scC02 than they are in organic solvents [56]. This is largely due to the high temperatures required for the activation of these systems, resulting in pressures that are too high for the use of standard equipment.

4.7.4.2 Coupling Reactions Involving scCO2 as Solvent and Substrate In the presence of phosphine complexes of Ni', two alkyne molecules can be coupled with C02 to give 2-pyrones [57,58]. The attractive option of using scCO2 as a solvent and substrate in such processes (Scheme 4.7-8) was recognized by Reetz et al. who investigated the formation of tetraethyl 2-pyrone from 3-hexyne and C02 using a catalyst generated in situ from [Ni(cod)*] and the chelating diphosphine 1,4-diphenylphosphinobutane (dppb) [59]. The TON and selectivity for pyrone formation was moderate, but almost identical to that reported in organic solvents with the same catalyst. Although the phase behavior of.the low density reaction mixture and the solubility of the metal complex were not reported, these results demonstrated that Ni catalysts

4.7 Metal-Complex-Catalyzed Reactions

-2PR3 Ph2P(CH&PPh2 2 PMe3 2 PM83

TON

Ref.

102’c

7

[59l

95°C

11 18

PCI

T

51“C

37 1

1151

Scheme 4.7-8 Nickel-catalyzed formation of tetraethyl 2-pyrone from 3-hexyne and C02 at T > T, and p >pc of pure COz.

allow catalytic C -C coupling reactions with C 0 2 under conditions beyond T, and p c . Replacing dppb with a monodentate trialkyl phosphine leads to a considerably more effective catalytic system, and the homogeneous nature of the reaction mixture has been visually confirmed in this case [2c, 151. The higher solubility of the PMe3-based catalyst has been used to explain the absence of an induction period, which was observed with the dppb ligand [2c]. Using PMe3, tetraethyl 2-pyrone could be formed with > 98 % selectivity at temperatures as low as 50°C [15]. These data compare well with results obtained with the same system in acetonitrile/THF, the mixture of conventional solvents that was found to be best suited for this reaction after laborious optimization. However, the catalyst has a shorter lifetime in scC02 than in the liquid solvent (TON ca. 20 and 60, respectively), because nickel-mediated oxygen transfer from C 0 2 to phosphine results in catalyst deactivation and the formation of nickel carbonyl species and R3P0 [15]. Attempts to use scCO2 as solvent and substrate in the Pd-catalyzed coupling with butadiene to form &lactones have been hitherto unsuccesful [60]. 4.7.4.3

Olefin Metathesis

The mutual exchange of alkylidene units is known as olefin metathesis, a process for which a variety of heterogeneous and homogeneous metal catalysts are known [61]. The major application of this reaction type has long been the production of higher olefins and ring opening metathesis polymerization (ROMP; see Section 4.7.6.2). More recently, the process has also found application in C-C coupling reactions for organic synthesis [62]. In particular, the so-called ring closing metathesis (RCM) has been used as a key step in a number of complex syntheses of biologically active materials [63]. The molybdenum complex 15 [64] and the ruthenium carbene 16 [65] are the two most prominent examples of catalyst precursors used in such reactions (Scheme 4.7-9).

372

4 Reactions in SCF

15

Scheme 4.7-9 Carbene complexes used as catalyst precursors for olefin metathesis.

16

RCM is typically performed in chlorinated solvents or aromatic hydrocarbons and requires high dilution conditions for medium and large rings, which are the most attractive target molecules for pharmaceutical or cosmetic applications. Complex 15 acts as a highly soluble and efficient homogeneous catalyst for olefin metathesis in scCOz [6]. Upon introduction of substrate, the homogeneous reaction mixture shows the typical color change observed for the conversion of 15 into the chain-carrying carbene intermediate in conventional solvents. In contrast, 16 was found to have very low solubility in scCOz and the bulk material remained insoluble after substrate introduction. Nonetheless, the RCM of various dienes proceeded smoothly in the presence of 16 to give the corresponding cyclic products, which could be isolated in pure form by SFE with scCOz in many cases (Scheme 4.7-10). The reaction is believed to occur with a small amount of homogeneously dissolved catalytically active ruthenium species, rather than as a Q p e I1 process. As shown in Scheme 4.7-10, the RCM process differs remarkably from all previously discussed reactions in scCOz in that a gaseous compound, ethene, is produced in this reaction, rather than consumed as in the other examples. At first sight, it seems unreasonable to carry out such a reaction under high pressure, but the equilibrium of the reversible RCM process is of course only influenced by the partial pressure of the components, whereas the total pressure is mainly due to the supercritical solvent. There is, however, a remarkable influence of the total pressure on the selectivity of the metathesis reaction: the RCM product is obtained in high yields at high pressure, whereas mainly

I 5 or I 6

+

hC=Ch

R

R' = H, (X) = N-Ts, 03%

0

Rl - - ~ (x ) ,=

d

o

R'=H.(X)=

d o - o -

-

88%

0 87%

0

Scheme 4.7-10 Selected examples for ring closing metathesis (RCM)in SCCO~ [6].

4.7 Metal-Complex-Catalyzed Reactions

373

1

1

0

80

Figure 4.7-6 Influence of density on the distribution of products obtained from RCM (macrocycle) or ADMET (oligomers) during the metathesis of an unsaturated ester in scCOz using catalyst 16. Data taken from Reference [ 6 ] .

.

. (%.) 4020

-

.

_ _ _ _ _------d _--

0I

0.55

0.80

.

,

.

0.85

,

.

0.70

d (grnL-')

,

.

0.75

,

0.80

.

,

0.85

oligomers are formed at low pressure via acyclic diene metathesis (ADMET) [6]. This behavior can be rationalized in terms of density, rather than pressure, as shown in Figure 4.7-6. Owing to the high compressibility of the SCF, an increase in pressure results in a considerable increase in density, and hence in a decrease of the molar fraction of the substrate. Thus, the compressibility of the SCF allows the chemical potential of the substrates to be changed at constant volume and high density favors the formation of the cyclization product over the oligomer. The resulting dilution-effect resembles qualitatively the Ziegler-Ruggi principle [66]. Although complex 16 tolerates a wide variety of functional groups, it is deactivated by secondary or primary amines in conventional solvents. The metathesis approach to the natural product epilachnen therefore requires protection of the NH group in the diene precursor. In scC02, however, the unprotected diene reacted to give the aza macrocycle in 74 % yield (Scheme 4.7-11). This can be attributed to the reversible formation of the corresponding carbamic acid, and provides the first example of the use of scCO2 as a solvent and as a reversible protecting group [6]. 16

sccq,40'C Scheme 4.7-11 C02 as a reversible protecting group for N-H functionalities during RCM in scC02.

4.7.4.4 Palladium-catalyzed Coupling Reactions of Aryl Halides .

Palladium-catalyzed coupling reactions of aryl halides or triflates with vinylic compounds are of paramount importance for the generation of sp2-sp2 carbon

374

4 Reactions in SCF

bonds in organic synthesis [67]. Generally, the reactions are classified according to the leaving group LG in the vinylic position. The Heck (LG = H), Stille (LG = SnR3) and Suzuki (LG = B(0R)z) couplings are most widely used. The reactions are generally [67] but not universally [68 believed to proceed via a Pdo intermediate, generated in situ either from Pd or Pd" precursors. Phosphine ligands play a crucial role in stabilizing the active intermediates and controlling the selectivity of the reaction. Selected examples from recent efforts towards Pd-catalyzed couplings of this type in scCOz are summarized in Scheme 4.7-12. Tumas and co-workers investigated the Stille coupling of PhI with vinyl(tributy1)tin using [Pdz(dba)3](dba = di(benzy1idene)acetone)as the Pdo source in the presence of various phosphine ligands [69]. They found that P[C6H3-3,5-(CF3)~13 ( 3 3 -H°F'-TPP) yielded a soluble and active catalyst, but initial rates were approximately two times lower than in toluene. The Heck reaction using palladium(I1) acetate with two equivalents of the same ligand gave > 90 % conversion and > 90 % selectivity for the coupling of PhI with acrylic acid and styrene. Pressures were quite high in these reactions (345 bar), owing to the Felatively high reaction temperatures (90 "C) and the fairly high densities required to dissolve the catalyst. Similar results were obtained in the Heck coupling by Holmes using isolated complexes of the formula [PdLzXz] where L = PhP[(CHz)z(CFz)6F]zand X = C1 or OAc [70]. At 100 "C,a 91 % isolated yield of methyl cinnamate from PhI and acrylic acid was achieved using 5 mol% of acetate complex. The Suzuki coupling with aryl boronic acids using the chloride complex gave lower yields (50%). Coupling of PhI with alkynes was also possible using CuI as an additional cocatalyst (Sonogashira coupling), with up to 62 % yield. Unfortunately,

d

Pd-cathase

1

scc0~,90-10~c

A

R

Pd-cat./base sccop, IOO'C

scco2, 90°C Pd-cat.lCul/base / I ! scc02, IOO'C

/

R

I-\

Scheme 4.7-12 Palladium-catalyzed coupling reactions in scCOz [69,70].

4.7 Metal-Complex-Catalyzed Reactions

Scheme 4.7-13 The Heck reaction in superheated or supercritical water 1711.

0 -

375

Pd-catalyst

+

@ "

(NHd][HCOd*

superheated H20 (260'C)

25% yield

supercritical H20 (400°C) 15% yield

no data on the density of the supercritical medium or the total pressure during these reactions are available. The Heck reaction is also one of the very few metal-catalyzed organic syntheses that have been studied in SCFs other than CO2. Supercritical and especially superheated water have been found to be suitable media for this reaction using various palladium complexes without phosphine ligands (Scheme 4.7-13) [71]. Colloidal Pd' or small particles of metallic palladium were assumed to be responsible for the catalytic activity. The reaction was found to be very sensitive to the nature of the aromatic halide or pseudo halide. Chlorobenzene, a usually less reactive substrate, could be coupled successfully in superheated water, whereas p(trifluoromethy1)phenyl iodide was unreactive, despite its high reactivity under classical Heck conditions. Ammonium bicarbonate was found to be better suited as an HX scavenger than the organic bases used in conventional solvents. The yields of coupling products were below 30 %, even under optimized conditions, but this was not due to uncontrolled decomposition. Unreacted starting materials in addition to small amounts of side products, were recovered in most cases. There was no significant difference between operation at superheated (260 "C) or supercritical (400 "C) conditions, except for a larger tendency for side reactions such as hydrogenolysis at the higher temperature. 4.7.4.5 Cyclopropanation Supercritical fluoroform (scCHF3) is yet another SCF that provides highly interesting opportunities for metal-catalyzed reactions. Although probably less suited for practical large-scale applications, the remarkably broad range of variation of the dielectric constant (E) with pressure (see Figure 1.1-4, Chapter 1.1) makes this solvent particularly interesting for the tuning of catalytic processes and for mechanistic studies. Note that, in liquid solvents, pressure effects on enantioselectivity are observed generally only at inert gas pressures above several kbar [72]. Wynne and Jessop chose to study the effect of pressure on the enantioselectivity of homogeneous catalysis in various SCFs including scCHF3 [73]. Rhodium-catalyzed asymmetric cyclopropanation was investigated because it exhibits a marked selectivity dependence on soivent polarity in liquids [74,75]. For example, the cyclopropanation of styrene with methyl phenyldiazoacetate catalyzed by the dimeric rhodium(I1) carboxylate complex 17 proceeds with substantially higher enantioselectivity in nonpo-

376

4 Reactions in SCF

Ph

P

17

SCCHFS,29°C 50-150 bar

Me

1;

17 =

Bu

'

4

Scheme 4.7-14 Rhodiumcatalyzed asymmetric cyclopropanation in scCHF3. Representative results are summarized in Figure 4.7-7.

lar than in polar liquid solvents [74b]. Indeed, a strong dependence of enantioselectivity on pressure was observed when the same reaction was performed in scCHF3 at various pressures (Scheme 4.7-14) [73]. A plot of ee and E as a function of pressure (Figure 4.7-7(a)) shows the reverse relationship between the polarity of the medium and the enrichment of the (R,S)-enantiomer. The increase in asymmetric induction with decreasing pressure is fully in line with its increase with decreasing E in liquid solvents, as demonstrated for the enantiomeric ratio in Figure 4.7-7(b). This new result demonstrates the possibility of dielectric tuning of catalytic reactions in SCFs. 8

80 70

-

12

80-

O0

b)

10-

t =(%)

-

8-

408-

30-

-3

20 -

-2

10-

-1

0-0

0

20

40

-

60 80 100 120 140 180 180

P ( W

4-

2" I

I

0

2

I 4

I 8

I 8

I 10

E-b

Figure 4.7-7 (a) Dependence of the enantiomeric excess (ee) obtained in rhodium-catalyzed cyclopropanation (Scheme 4.7-14) and of the dielectric constant E of the medium on the pressure p in scCHF3. (b) Dependence of the enantiomeric ratio on E for both scCHF3 and liquid solvents [73].

4.7 Metal-Complex-Catalyzed Reactions

4.7.5

377

Oxidation

4.7.5.1 Peroxides as Oxidants

Oxidation reactions are among the most important processes for introducing functional groups into hydrocarbons. Many of these reactions are catalyzed by transition metal compounds [76]. SCFs are well known as highly efficient reaction media for destructive oxidation in waste treatment. For example, supercritical water oxidation (SCWO) is already an important industrial process based on SCF technology (see Chapter 1.1). In contrast, selective oxidation in scH20 has met :with very limited success [77]. For synthetic purposes, compressed hydrocarbons and carbon dioxide have probably the greatest potential as reaction media. It is hoped that the hydrocarbons will act as solvents and reactants, opening an attractive approach to the utilization of new chemical feedstocks. Supercritical C02 is of special interest because of its inertness towards oxidation, thus providing additional safety and avoiding side products from solvent oxidation. Catalytic oxidation in a two-phase water-C02 medium has been investigated for the synthesis of adipic acid from cyclohexene [2b]. The substrate and products were dissolved in the supercritical fluid while the oxidant (e.g. NaI04) resided in the aqueous phase. The catalyst Ru02 was oxidized to Ru04 in the aqueous phase which in turn oxidized the substrate, presumably at the liquid-supercritical interface. This is an example of a 'Qpe IIIa process. Unfortunately, catalyst efficiency was fairly low (five catalytic cycles), probably due to deactivation by formation of carbonates in the aqueous phase. The oxidation of olefinic substrates with tert-butylhydroperoxide (TBHP) and various metal catalysts in scCOz has been investigated [15,77-791. For simple cyclic alkenes, [Mo(CO)~](18) proved by far the most active catalyst (Scheme 4.7-15), one reason being the high solubility of the complex. Quantitative conversion and high selectivity for the epoxide was observed in case of cyclic alkenes such as cyclooctene or cyclohexene with 2 mol% of catalyst when TBHP solutions in water or hydrocarbons were used at temperatures below 90 "C [15,79]. With aqueous TBHP solutions at temperatures above 90°, the reaction was found to produce the trans-1,2-diols via hydrolysis of

/ ( b C 0

Scheme 4.7-15 Molybdenum-catalyzed oxidation of alkenes using TBHP as the oxidant in scC02 [15,76,78].

n = 2,4

+ 'BuOOH

aqueous condiiions

\

TBHP scc02, T < 90% aqueous or anhydrous conditions

378

4 Reactions in SCF

the epoxides [77,79]. The reaction mixtures were reported to be fully homogeneous even if 70% aqueous TBHP was used as the oxidant [77]. Only a small fraction of the carbonyl complex 18 was found to be involved in the oxidation, indicating a very high activity of the true catalytically active species [15]. The catalyst showed continuous activity when olefin was added to the reaction mixture several times [77]. Catalyst 18 was also used in a brief study on epoxidations with cumene hydroperoxide as the oxidant [2d, 801. The epoxidation of allylic and homoallylic alcohols with TBHP occurs smoothly in liquid C02 using oxovanadium(V) tri(isoprop0xide) [VO(OPr')3] as a catalyst [79]. Employing Sharpless' procedure [81] using [Ti(OPI)4] in the presence of di(isopropy1)tartrate (DIPT) as a chiral catalyst allowed asymmetric epoxidation of (E)-hex-3-en-l-01 (Scheme 4.7-16) [79]. The use of the isopropyl groups in the metal components and the ligand was crucial for sufficient solubility and hence catalytic efficiency. The optical induction was found to be highly temperature dependent and a remarkable 87 % ee was reported at 0 "C. A vanadium catalyst with a salen-type ligand [81] gave fairly high diastereoselectivities for the epoxidation of homoallylic alcohols under supercritical conditions [78]. These results indicate that compressed C02 may be a promising reaction medium for asymmetric epoxidation, although rates and stereoselectivities are still somewhat lower than those obtained in chlorinated solvents [81].

OH

[Ti(O'Pr),YDIPT

IiqCOz, 0°C

ee = 87%

Scheme 4.7-16 Asymmetric epoxidation of a homoallylic alcohol using di(isopropy1)tartaric acid (DIPT) as chiral ligand in liquid C 0 2 [79].

4.7.5.2 Molecular Oxygen as Oxidant One of the biggest challenges in oxidation catalysis is the use of molecular oxygen as an efficient and selective oxidant. Supercritical C 0 2 appears to be an ideal solvent for such processes, because its complete miscibility with 0 2 provides high concentrations of the oxidant and avoids mass transfer limitations. Furthermore, C 0 2 is inert to oxidation by O2 and consequently offers additional safety compared to common solvents. Note that exothermic oxidation reactions in an SCF can cause very sudden and very large pressure rises. However, SCFs provide heat transport capacities that are orders of magnitude higher than gaseous phases, allowing for more effective temperature control than gas phase oxidation. An interesting, but hitherto widely neglected feature of C02 in this context is its ability to chemically interact with metal-oxygen complexes, leading to transition metal peroxocarbonates [4,82]. These complexes are able to transfer one oxygen atom to oxophiles such as phosphines, alkenes and active methylene

4.7 Metal-Complex-Catalyzed Reactions

379

groups [83], mimicking the reactivity of monooxygenases. The participation of transition metal peroxocarbonates has been invoked to explain the influence of gaseous C02 on the rhodium-catalyzed oxidation of tetrahydrofuran (THF) to y-butyrolactone in conventional solvents using an 02: C 0 2 (1 :1) gas mixture as oxidant [83]. This oxidation also occurs’in scC02 despite the fairly low solubility of the catalyst [(dcpe)Rh(hfacac)] -(dcpe = 1,2-bis(dicyclohexylphosphino)ethane (Scheme 4.7-17) [84]. The TONS and the amount of by-products (mainly formic acid, 2-hydroxy-THF and dihydrofuran-2,5-dione) were found to depend strongly on the temperature and the partial pressure of 02.The highest TON obtained for the formation of y-butyrolactone was 135, with a maximum TOF of 15 h-l. NMR investigations showed that the phosphine ligand is rapidly oxidized under these reactions conditions, but its presence was crucial for the reaction to occur. Although the use of different catalysts and fairly different conditions prevents a quantitative comparison, the efficiency of the SCF system is not dramatically different from the one utilizing gaseous C 0 2 only. Koda and co-workers [85] studied the catalytic oxidation of cyclohexane with O2 in liquid and supercritical COz in the presence of acetaldehyde (cyclohexane:acetaldehyde = 4 :1) and an iron porphyrin catalyst bearing pentafluorophenyl groups. The main oxidation products were cyclohexanone (2.9 % yield) and cyclohexanol (2.3 % yield) with the optimum yields being achieved slightly below the estimated critical pressure of the reaction mixture. Although the field is clearly still in its infancy, it becomes evident from these examples that a wide variety of oxidation reactions can be carried out in SCFs and in particular in liquid and supercritical C02.

+

other oxidation products

3-30 bar 021C02

15 h-’

Scheme 4.7-17 Oxidation of THF using molecular oxygen in the presence of O2 and a hypothetical peroxocarbonate intermediate.

380

4 Reactions in SCF

4.7.6

Polymerization

4.7.6.1 Polymerization of Alkenes under Supercritical Conditions Polymerization of alkenes under supercritical conditions is an exciting and industrially important subject that has been studied for most of the twentieth century (see Chapters 1.1 and 4.6). Historically, this is also the area where the first uses of metal catalysis in SCFs were attempted. As early as 1913, Ipatiev tested various metal compounds as catalysts for the polymerization of SCC~H [86]. ~ The heterogeneous catalyst ZnCl2 allowed oligomerization at 275°C and 70 bar, giving a fair yield of liquid products. With high purity AlC13 as a presumably dissolved catalyst, oligomerization occurred even at room temperature with greater selectivity for acyclic products, the product being described as a brightly colored fluorescent liquid. Several patents deal with the use of Ziegler-type or metallocene catalysts for the polymerization of supercritical ethene and propene [87]. Exxon operates its metallocene-catalyzed low-density polyethylene process under supercritical conditions [88]. A patent assigned to Exxon describes the use of bis(n-butylcyclopentadieny1)zirconium dichloride with a large excess of methylalumoxane (MAO) as cocatalyst in scC2H4 at 1500 bar and temperatures above 150°C. This gives polyethylene at very high rates with weight-averaged molecular weights M, of 80000 to 200000 and molecular weight distributions M,/Mn (polydispersities, PDI) of 1.8 to 2.2 [87c]. The advantage of using such high temperatures is that lower A1:Zr ratios could be used without significant loss in activity. Luft et al. compared the activity of a homogeneous catalyst (an unspecified zirconocene with MA0 as cocatalyst) with that of a heterogeneous catalyst (Ti/ MgC12/tetraisobutyl dialuminoxane) in scC2H4 at 1500 bar and 150-230 "C [89]. They found that the homogeneous catalyst had four times higher activity and yielded a narrower molecular weight distribution (PDI = 2 compared to 5-10 for the heterogeneous catalyst). More recently they developed supported metallocene catalysts for the polymerization of scC2H4[90]. Olonde ef al. reported that in situ alkylation of a neodymium complex [Cp*2Nd(C1)2Li(OEt2)2]with dialkylmagnesium generates an active catalyst for the polymerization of scCzH4 that does not require activation with MA0 [91]. After the reaction at 200°C and 1200 bar, the product was a linear, vinyl-terminated polymer with an M, of 46000 and a lower PDI (M,/Mn = 2) than obtained with comparable commercial heterogeneous catalysts. The copolymerization of ethene with a variety of other alkenes or dienes was also studied. The copolymerization of supercritical mixtures of ethene and propene (120-220°C and 1000-1500 bar) was catalyzed by the silyl-bridged bis(tetrahydroindeny1)zirconocene catalyst 19 and MA0 at a metallocene concentration of 6 X mole fraction and an A1:Zr ratio of 22000 [92]. With a 5050 mixture of scC2H4 and S C C ~ Hthe ~ , resulting polymer had only 8 % incorporation of propene. Increasing concentrations of propene resulted in

4.7 Metal-Complex-Catalyzed Reactions

38 1

Scheme 4.7-18 Different polymerization pathways for the coplymerization of ethene and 1,5-hexadiene using the zirconocene precursor 19; P = polymer chain [94].

shorter chain lengths, due to the lower reactivity of propene end groups and the increased rate of chain-terminating transfer to propene. Copolymerization of scCzH4 with longer 1-alkenes gave even less incorporation of the longer alkene and resulted in copolymers of decreasing crystallinity [93]. The copolymerization of ethene and 1,5-hexadiene was catalyzed by 19 and MA0 at 1500 bar and 180°C [94]. The mole fraction of diene was varied from 0 to 1, but the reactions at high mole fractions were subcritical (the T, of 1,5hexadiene is 234 "C [95]). Again, the activity decreases greatly with increasing comonomer concentration. Incorporation of the diene occurred at both terminal double bonds at low mole fractions. At high mole fractions (subcritical) only one of the two double bonds was incorporated into the polymer, leaving a dangling unsaturated group (Scheme 4.7-1 8). Overall, these Zr catalysts were found to be highly active, giving 4500 kg of polyethylene homopolymer per gram of zirconium. 4.7.6.2 Polymerization Utilizing Compressed C02 Compressed liquid or supercritical carbon dioxide has been recognized as a useful alternative reaction medium for radical and ionic polymerization reactions (see Chapter 4.5). Many of the benefits associated with the use of scCOz in these processes apply equally well to polymerizations relying on a metal complex as the chain-carrying species. However, the solubility of the metal catalyst and hence the controlled initiation of chain growth add to the complexity of the systems under study. Furthermore, many of the environmental benefits would be diminished if subsequent conventional purification steps were needed to remove the metal from the polymer. Nevertheless, the interest in metal-catalyzed polymerizations is increasing, and some promising systems have been described. One interesting aspect of applying metal catalysts to polymerizations in scCO2 is the incorporation of C 0 2 into the polymer. The copolymerization of epoxides with COz is an example of such a process, and has been studied

382

4 Reactions in SCF catalyst nbe

-

Catalyst

45°C

Scheme 4.7-19 Metal-catalyzed ring opening metathesis (ROMP) of norbornene (nbe) in compressed scC02.

under supercritical conditions using both heterogeneous [96] and homogeneous [97] zinc-based catalysts. The homogeneous catalysts were obtained from zinc carboxylates bearing perfluoroalkyl groups. A polycarbonate with a molecular weight M, of 180000 and with over 90 % carbonate linkages was formed in 69% yield from 1,2-epoxycyclohexane and scCO2 at 110°C and 136 bar [97]. The heterogeneous catalyst was a zinc glutarate, which gave polypropylene carbonate with similar characteristics [96]. The ring opening metathesis polymerization (ROMP) of norbornene and other cyclic alkenes has been studied in liquid and supercritical CO2 (Scheme 4.7-19). Polynorbornenamer (Norsorex@)has very poor solubility in C 0 2 and the reactions proceed as precipitation polymerizations. The ruthenium salt [RU(H~O)~](TOS)~ is also virtually insoluble in pure C02 and the results with this catalyst proved difficult to reproduce because of variations in catalyst purity and reaction mixture agitation [98]. The catalytic performance could be greatly improved by working in C02/MeOH mixtures. The microstructure of the polymer was found to be almost identical for samples prepared in neat norbornene or in C 0 2 and greatly differed from that obtained in MeOH or C021 MeOH mixtures [99]. In contrast, the carbene complexes 15 and 16 could be used very efficiently in pure liquid or supercritical CO2, giving high yields of polynorbornenamer with very similar cis:trans ratios of the double bonds to those obtained with the same catalyst in CH2C12 [6]. More recent studies showed that the robust complex 16 can be used at substrate to catalyst ratios up to 5300 without deactivation [ 1001. Fairly broad molecular weight distributions were observed when 16 and norbornene were placed together in the reactor, but narrow distributions could be achieved if the catalyst was injected as a solid or in solution to the supercritical mixture of alkene and C02. The polymer morphology was very similar to samples prepared with 16 under conventional conditions.

4.7 Metal-Complex-Catalyzed Reactions

383

Ring opening metathesis polymerization in scCOz A window-equipped 27 mL, stainless steel high pressure vessel with thermocouple, manometer, a needle and a ball valve is charged with norbornene (407 mg, 4.32 mmol) under argon. A dosing unit containing 16 (5.50 mg, 0.67 X mmol) is connected to the reactor through the ball valve and pressurized with argon to 150 bar. The reactor is filled through the needle valve with CO2 (15.3 g, density = 0.58 g mL-') using a compressor. A colorless homogeneous phase is formed upon heating to 43 "C (93 bar) under stirring. After temperature equilibration, the solid catalyst is introduced into the reaction vessel by opening the ball valve. After 14 h the reactor is cooled to room temperature, vented and the crude polymer (341 mg) is isolated. Purification of the polymer by conventional methods gives a white tacky material (316 mg, 78 %), with a molecular weight M, = 1.9 x lo5, a PDI of 2.6 and a cis:trans ratio of 25:75. The polymerization of phenylacetylene gives an unsaturated polymer which can exist in principle in four different isomeric forms; the properties of polyphenylacetylene (PPA) depend strongly on the microstructure [1011. Rhodium catalysts give predominantly cis-configurated polymer chains with high selectivity for the cis-transoid isomer in polar solvents such as THF [102].In contrast, mainly the cis-cisoid form was obtained when the polymerization was carried out in compressed C 0 2 (Scheme 4.7-20)[103].The cis-cisoid form is also preferred in n-hexane, but the rate of polymerization and the stereoregularity of the polymer were found to be higher in liquid or supercritical C02. PPA was obtained in 72% yield with a molecular weight M, = 45000, a cistransoidxis-cisoid ratio of 24:76 and a cis content of 81 % in the cis-transoid isomer by using a highly soluble catalyst formed from [Rh(acac)(nbd)] (acac = acetylacetonate, nbd = norbornadiene) and 4-H2F6-TPP in the presence of quinuclidine in scC02 at 41 "C and 187 bar.

H [Rh(acac)(nWl Ph

H

4-H2F6-TPP COz I arnine 26-43'C 94-150 bar

\

Ph H

Ph

I

I

\

H\

/c=c

f=yc=d c=c\ c =2, ~

+

4

\

Ph

4

bh

cis-transoidal PPA

PkCH II H'

c, c,' c-

,Ph \C/H II C 'Ph \

Pi cis-cisoidal PPA

Scheme 4.7-20 Polymerization of phenylacetylene using a soluble rhodium phosphine catalyst in compressed COz [103].

384

4 Reactions in SCF

4.7.7 Conclusions The various exampfes discussed in this chapter demonstrate the considerable potential of both SCFs and liquefied gases as alternative media for metal-complex-catalyzed reactions. Some additional examples utilizing transition metal compounds to mediate the synthesis of organic compounds under presumably supercritical conditions have been reported. These include the Pd-mediated acetoxylation of propylene in scCO2 to form a mixture of vinyl and ally1 acetates [104].The acetoxy transfer occurs with stoichiometric amounts of palladium(II)acetate and additional acetate ions were provided in C02-soluble form as bis(tripheny1phosphine)-iminium (PPN) acetate. Attempts have been made to produce dialkyl carbonates in the presence of various metal complexes at temperatures and pressures above the critical data of COz [105].The cobalt diacetate-mediated dehydrogenation of cyclohexanol has been studied in scH20 with in situ Raman spectroscopy [106]. In addition, a large number of main group metal compounds have been investigated as catalysts or mediators for organic synthesis in SCFs (see Chapter 4.3).Heterogeneous catalysis (Chapter 4.8)provides another large and prospering area of related interest. Collectively, these studies illustrate the broad range of possible applications of SCFs in metal-mediated organic synthesis, and the challenge is now to make efficient use of these methodologies. Investigations towards the understanding of coordination chemistry in SCFs (Chapter 4.2)will stimulate the elucidation of metal-complex-catalyzed reactions. However, it should be evident from this chapter that the field of complex-catalyzed SFRs is far from being complete, and much remains to be done. The potential of the technique has been hinted at, but many new ways to exploit the special properties of SCFs in metal complex catalysis are still to be discovered.

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Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

4.8 Heterogeneous Catalysis LI FANand KAORUFUJIMOTO

Different uses of supercritical fluid (SCF) solvents in chemical separation processes have been of considerable research interest since the 1970s. The basic principles of SCF extraction engineering and a number of applications for this technology are described in several review papers [ 1,2]. As a new field related to SCF technology, the application of supercritical solvents as reaction media attracts increasing attention, especially for catalytic reactions. In such processes, the SCF may either actively participate in the reaction or function solely as the solvent for the reactants, catalysts, and products. SCFs exhibit several characteristic features to be used in developing new chemical reaction processes. First, the solubility properties guarantee a promising solvent system. Pressure change alone can control formation of a homogeneous phase or a heterogeneous phase, and the transfer between a homogenerous phase and a heterogeneous phase. Many solid organic compounds can dissolve in SCFs to form a homogeneous reaction phase. Second, the high diffusion coefficient can enhance the reaction rate in the diffusion-controlled reaction regime. Third, intrinsic properties of SCFs, such as density, polarity or viscosity, can be altered continuously, and at times dramatically. Wide variations in density and viscosity are possible with small changes in pressure and/or temperature. Further, the cage effect, derived from solvent-solute as well as solute-solute intermolecular forces, or the formation of clusters between solute and solvent can determine the reactivity and selectivity of the target reaction [3,4]. This chapter reviews the field of heterogeneous catalytic reactions in SCFs [5-81. By exploiting the unique solvent properties of SCFs, it may be possible to enhance reaction rates while maintaining or improving selectivity. The following benefits can be expected.

4.8 Heterogeneous Catalysis

389

1. Gaseous reactants such as hydrogen are able to mix well with an SCF at high concentrations. Reaction rates dependent on gas concentration will be enhanced. 2. Organic compounds become soluble in SCFs to form a homogeneous reaction phase, leading to high reactivity and selectivity. 3. Through pressure control; separations between the catalyst and reactant/ product are easily realized. This property has a strong effect on reaction equilibrium shift, suppresses side reactions and assists in the extraction of precursors to catalyst poisons. 4. The high mass transfer efficiency and effective heat transfer capacity of SCFs simplify process control. Based on the unique properties of SCFs, not only can known chemical reactions be improved, but also new reactions may become possible due to the use of an SCF medium. Most such new reactions will need a catalyst.

4.8.1 Fischer-Tropsch Synthesis The Fischer-Tropsch (F-T) reaction, which is conducted as a solid-catalyzed gas-phase reaction, and which is commercially operated in several countries, is inevitably accompanied by local overheating of the catalyst surface as well as by the production of heavy wax (alkanes higher than C ~ O Local ). overheating of the catalyst may lead to catalyst deactivation and also to an increase in methane selectivity. Heavy wax may plug micropores of the catalyst and the catalyst bed itself, also resulting in catalyst deactivation. The slurry-phase F-T process, in which a slurry, composed of fine powdery catalyst and mineral oil, is used as the reaction medium, has been developed to overcome disadvantages of the gas-phase process [9]. However, the diffusion of synthesis gas into the micropores of the catalysts is so slow in the slurry phase that the overall reaction rate is markedly lower than that in the gasphase reaction [ 10,111. Further, the concentration of solid catalyst particles in the slurry medium is limited to low levels (<20 wt%) in order to maintain slurry fluidity. Other disadvantages of the slurry reactors are the accumulation of high-molecular-weight products in the reactor during operation, and the in situ separation of fine catalyst particles from the heavy products. Fujimoto and Fan developed an F-T synthesis in the supercritical phase and compared its reaction performance to that in the liquid phase and the gas phase. The supercritical phase F-T reaction, as described here, shows unique characteristics such as rapid diffusion of reactant gas, effective removal of reaction heat, and in situ extraction of high-molecular-weight hydrocarbons (wax).

390

4 Reactions in SCF

4.8.1.1

Experimental

Alumina-supported ruthenium catalysts were prepared by impregnating alumina (Aerosil, 200 m2/g) with ruthenium chloride from its aqueous solution. The catalysts composition was Ru:A1203 = 2:98 by weight. The catalyst precursors were dried overnight at 120 "C in an air oven, and were then calcined at 450 "C for 2 h to form a supported metal oxide -[12,13]. The catalysts were reduced in a hydrogen flow at 150°C and 300°C for 1 h each, and at 400°C for 2 h in series and then passivated. They were reduced again at 400°C for 2 h in situ before the catalytic reaction. Silica-supported cobalt catalysts were prepared from cobalt nitrate (Co(NO3)2), lanthanum nitrate (La(N03)3) and commercially available silica gel (Fuji Davison, ID gel, 270 m2/g) using conventional methods of impregnation [14]. The composition of the catalyst was Co:La:SiOz = 20:6:87 by weight. The catalyst precursor was dried in air at 120°C and then calcined at 450 "C for 3 h to form supported metal oxides. It was then exposed to hydrogen at 400°C for 12 h. The mean pore diameter of the catalyst was 8.7 nm. The configuration of the reactor for the supercritical-phase reaction was similar to that of a conventional pressurized fixed-bed flow reactor system. The only difference was that a vaporizer and an ice-cooled high pressure trap were set upstream and downstream of the reactor, respectively, as shown in Figure 4.8-1. To compare characteristic features of the gas-phase, liquid-phase and supercritical-phase reactions, all three kinds of reactions were conducted in the fixed bed reactor. The liquid-phase reaction was operated in a downflow-type trickle bed. The balance materials were nitrogen for the gas-phase reaction, and n-hexadecane and nitrogen for the liquid-phase reaction [15-171. The reaction temperature was defined as the highest temperature of the catalyst bed, measured by a thermocouple placed along the center of the catalyst bed. Temperature profiles of the catalyst bed varied within 1-2°C in the supercritical phase reaction.

(for gas phase) Nitrogen

Draft

Nitrogen High pressure pumps

Figure 4.8-1 Reaction apparatus for Fischer-Tropsch synthesis.

4.8 Heterogeneous Catalysis

391

Detailed analytical methods were as previously described [ 181. Gel permeation chromatography (GPC)and capillary columns were employed for the precise analysis of the products. After each reaction, residual products remaining within the catalyst and the reactor were extracted by supercritical n-hexane at 250 "C (10 "C higher than the reaction temperature) and 35 bar, with additional N2 pressure of 10 bar. The supercritical fluid extraction after the reaction was conducted for 1 h, after which the products were also determined by chromatography [ 191. The catalyst effectiveness factor, determined by experiment, was defined as the ratio of catalyst activity of specified pellet size to that of well-crushed catalyst (pellet radius 0.1 mm) [20]. The SCF was: selected by the following criteria: 1. The critical temperature and pressure should be slightly lower than the typical reaction temperature and pressure. 2. The SCF should not poison the catalysts and should be stable under the reaction conditions. 3. The SCF must have a high affinity for aliphatic hydrocarbons to extract wax from catalyst surface and reactor. n-Hexane (C6H14),for which the critical temperature and critical pressure are 233.7 "C and 30.1 bar [21], was chosen as the supercritical solvent. The standard reaction conditions were T = 240"C, p(tota1) = 45 bar, p(CO+H2) = 10 bar, p(ba1ance gas) = 35 bar, CO:H2 = 1:2, contact time W/F(CO+H2) = 10 g of catalyst h mol-'. Argon was used as the internal standard with a concentration of 3 % in the feed gas. Breman et al. studied the phase behavior of CO and H2 in light hydrocarbons at these reaction conditions [22].

4.8.1.2 Reaction Performance in Three Reaction Phases The results of F-T syntheses using the Ru/A1203 catalyst in three different reaction phases are summarized in Table 4.8-1. The rate of CO conversion in the supercritical phase reaction was higher than that in the liquid phase reaction, Table 4.8-1 Phase effect on the F-T synthesis using the Ru/A1203 catalyst. Reaction phase

Gas

Supercritical

Liquid

CO conversion (%) Effluent products (C-mmoYg-cat.h) Extracted products Residual ratio (%) Chain growth probability Carbon balance (%)

44.7 10.8 3.0 22 0.94 90

39.0 12.8 0.2 2 0.95 99

28.0 8.82 0.5 5 0.85 96

Ru/AI2O3, standard reaction conditions. Residual ratio is the % of extracted products in the total products (extracted and effluent). "C-mmol" refers to mmol of C atoms.

4 Reactions in SCF

392

0.8

r

O E f f l u e n t products tracted products

0.4 0

0.8

1

Gas p h a s e

t"

-1

SupercritY.ca1 p h a s e

0.4

0 0.8 0.4

0

I

Liquid phase

polox 0

0 I

5

10

15

20

25

30 3 5

40

1

Figure 4.8-2 Hydrocarbon distribution of the F-T product in various

reaction phases. Ru/A1203 catalyst, standard reaction conditions.

Carbon Number

but lower than that in the gas phase reaction. This suggests that the diffusion of synthesis gas in the supercritical phase was faster than in the liquid phase, but slower than in the gas phase. The amount of product extracted from the catalyst bed after the supercritical phase reaction was much smaller than in the gas phase reaction, but similar to that in the liquid phase reaction. Furthermore, the carbon chain growth probability in the supercritical phase or in the gas phase reaction was higher than in the liquid phase. This revealed that the CO:H2 ratio inside the catalyst pores in the supercritical phase reaction was similar to that in the gas phase reaction, because of the effective molecular diffusion in the supercritical phase (which is discussed later). Figure 4.8-2 shows the carbon number distribution of the products. In the supercritical phase reaction and the gas phase reaction, the carbon number distribution of the total products (including the effluent and the extracted products) extended to more than 40, which is a characteristic feature of the Ru catalyst. In contrast, the product distribution in the liquid phase reaction was limited to a carbon number of 35. The carbon number distributions of the extracted products were flat and independent of the carbon number for both the supercritical phase reaction and the liquid phase reaction, whereas the products extracted from the gas phase reaction was characterized by its high carbon number compared with its effluent products. This should be regarded as an effect of the in situ extraction of the products which occurred in the supercritical and liquid phase reactions.

393

4.8 Heterogeneous Catalysis 4.8.1.3 Diffusion Behavior of Synthesis Gas

The data in Figure 4.8-3, which shows the Arrhenius plots for cobalt-catalyzed reactions in the three reaction phases, reveal that the reaction rate fell in the order gas phase > supercritical phase > liquid phase. The apparent activation energies were 23 kcaVmol for the gas phase reaction, 21 kcaymol for the supercritical phase reaction and 17 kcaVmol for the liquid phase reaction. These results lead to the conclusion that the mass transfer efficiency in the supercritical phase reaction is higher than that in the liquid-phase reaction and is close to that in the gas phase reaction. The lower catalytic activity and the smaller apparent activation energy in the liquid phase reaction should be attributed to the lower rate of synthesis gas diffusion inside the catalyst particles compared to the rate of the surface reaction, because the activation energy of diffusion should be smaller than intrinsic reaction activation energy. As a result, the reactant concentrations inside the catalyst particles were so low that the effectiveness factor of the catalyst dropped. Temp. ( Oc 1 280 I

20-

250 I

I

I

234 I

220 I

-

,

Supercritical phase Gas phase

10

P

I

0 ,

64-

1 7 kcal/mol

A 21.80

I

I

1.90

I

I

-

2.00

x lo-.' (WT) Figure 4.8-3 Arrhenius activation energy of the F-T reaction in various phases. Co-La/ Si02 catalyst, standard reaction conditions, W/F(CO+H2) = 3 g-cat.h/mol.

4.8.1.4 Diffusion and Reaction of the Products In the F-T synthesis, 1-alkenes are produced as the primary products and are subjected to various secondary reactions such as hydrogenation, C-C chain propagation and hydrocracking [23,24]. Figure 4.8-4 shows a comparison of the alkene content in the products obtained in each reaction phase over a Ru/A1203catalyst. The alkene content of

4 Reactions in SCF

394

Gas phase

800 600 400

200 0

1

3

5

7

9

11

13

15

17

19

21

23

25

loooT

Supercritical phase

1

3

5

7

9

11

13

15

17

19

21

23

25

23

25

Liquid phase

800 600

400 200

n 1

3

5

7

9

11

13

15

17

19

21

Carbon Number paraffin

olefin

Figure 4.8-4 Distribution of hydrocarbon products of the F-T reaction in various phases. Ru/A1203 catalyst, standard reaction conditions.

the products decreased in each reaction phase, with an increase in the carbon number of the products. The decrease in the alkene content with increasing carbon number should be attributed to the increase in the hydrogenation rate relative to the diffusion rate with increasing carbon number. This is due to the longer residence time on the catalyst surface, derived from the slower diffusion rate of these alkenes. Although the mass transfer rate in the supercritical fluid is lower than that in the gas phase, the alkene content in the hydrocarbons produced in supercritical

4.8 Heterogeneous Catalysis

395

phase reaction was much higher than in the gas phase reaction. This means that the fine balance between the rates of product desorption from the catalyst surface and its transfer from the catalyst pores is the characteristic feature of the effective overall transfer of the products in the supercritical phase reaction. The alkenes, therefore, were extracted and transported by the SCF out of the catalyst particles quickly, minimizing readsorption and hydrogenation. In the gas phase reaction, however, the primary products, especially those of high molecular weight, hardly desorbed to the gas phase flow medium and were subjected to hydrogenation, resulting in low alkene content. In the liquid phase reaction, the low alkene content was mainly due to the slow diffusion of these products inside the liquid-filled catalyst pores. The slow diffusion enhanced the possibility of successive hydrogenation, even if these primary alkenes desorbed fast enough from the catalyst surface. As described above, the SCF had an effect not only on the diffusion of the reactant, but also on the desorption of adsorbed species. Figure 4.8-5 shows the influence of catalyst particle size on the alkene content in the supercritical phase reaction. It was found that a change of catalyst particle size had little effect on the alkene content in the product. This suggests that the supercritical fluid had a more obvious effect on the desorption of the produced alkenes. The diffusion rate may have been higher than the adsorption rate of the alkenes, at least under these experimental conditions. Bukur et al. conducted the F-T reaction on an iron-based catalyst with supercritical phase propane [25-271, and similar conclusions were obtained, indicating the above analysis to be independent of the catalyst itself and the SCF. 80

Figure 4.8-5 Catalyst pellet size influence on the alkene content in the supercritical phase F-Treaction. Conditions: Ru/A1203 catalyst, R, = pellet radius, 240 "C, p(n-hexane) = 35 bar, p(CO+H2) = 10 bar, W/F(CO+Hp) = 10 g-cat.h/mol.

4.8.1.5

h

-8

CI

60

V

4 to

A

40

A

Wax Production: Addition of Heavy Alkene to the Supercritical Phase

F-T wax is a highly interesting material, combining a high melting point, high hardness value, and low viscosity. Furthermore, it is free of aromatics and nitrogen- or sulfur-containing compounds. It can be used directly in fine chemical applications such as cosmetics, packing materials and adhesives, and as a

396

4 Reactions in SCF

Carbon Number

0

Figure 4.8-6 Addition of heavy alkenes to SCF F-T reactions gives an anti-ASF distribution. Conditions: Co-LdSiOZ catalyst, 220 "C, p(n-pentane) = 35 bar, p(CO+Hz) = 10 bar, W/F(CO+Hz)= 9 g-cat-Wmol, added alkene: 4 % based on

co.

feedstock for cracking reactions to produce various hydrocarbon fractions such as kerosene and aviation fuels. It is difficult to selectively synthesize waxy hydrocarbons through an F-T reaction. The main reason is that the F-T products follow the AndersonSchultz-Flory (ASF) distribution [23,24]. Development of a new type of F-T reaction, free from ASF constraints on product selectivity, is of great importance for wax production, as modification of the F-T catalyst alone is not enough to increase wax selectivity significantly. Addition of a small amount of heavy 1-alkene into supercritical phase F-T reaction can significantly promote the chain growth and greatly enhance the selectivity of waxy products. As a matter of interest, this phenomenon does not occur in the gas phase reaction [28-301. Shown in Figure 4.8-6 is the F-T product distribution profile after a reaction with 4 mol% (based on CO) addition of 1-tetradecene or 1-hexadecene in SCF n-pentane (T, = 196.6"C; p c = 33.7 bar). It is clear from the figure that the product distributions in the alkene-added systems are very flat, in marked contrast to the supercritical phase F-T reaction without addition of alkene. The selectivities for hydrocarbons lower than CI4 were higher in the F-T reaction without addition of alkene. The reverse was true, however, for heavy products with carbon numbers higher than 14; the selectivity to waxy products was remarkably enhanced in the alkene-added reactions. Another remarkable phenomenon was the suppression of methane formation in the alkene-added F-T reactions. Similar behavior was observed with addition of 1-heptene or 1,7octadiene to the supercritical phase reaction [29,30]. The characteristics of the alkene-added reaction systems are summarized in Figure 4.8-7. Methane selectivity in any alkene-added reaction was lower than half that in the same reaction without addition of alkene. CO conversion was also higher in the 1-alkene added reactions, except for 1,7-octadiene. However, the C02 selectivity in all F-T reactions with heavy alkene addition was lower than that in the absence of added alkene.

4.8 Heterogeneous Catalysis

-60

6

E

,

.i?

Q

v

8 .-

p

4

.2

C

s

B8 Figure 4.8-7 Reaction performance of the supercritical phase F-T reaction with addition of various long-chain alkenes (Co:La/SiOp = 25 :3/ 100). Reaction conditions as for Figure 4.8-6,

397

0

U

2

0

none

a-C7'

a-C,,=

a-C,,=

1.7-Cg"

selectivity for C O ~ selectivity for methane

0co conversion

Compared with that in the conventional supercritical phase reaction, the carbon chain growth was accelerated by the addition of alkenes with long carbon chains into the accompanying fluid. The essential prerequisite for this process is the rapid diffusion of these heavy added alkenes inside the catalyst pores to reach the metal sites, as well as effective diffusion of the heavy products produced from the interior active sites to the outer catalyst surface. Both diffusion processes are readily achieved in the supercritical phase. The added 1-alkenes reach the metal sites aided by the SCF and adsorb onto the active sites as alkyl radicals to initiate carbon chain growth; the resulting chains are indistinguishable from other carbon chains formed directly from synthesis gas. These new alkyl radicals consume additional methylene units to initiate new carbon chain propagation processes. Thus the selectivity for methane, which is formed mainly from methylene hydrogenation, decreases. CO adsorption and cleavage of CO to carbide on the metal site, as well as hydrogenation of carbide to methylene species, are both accelerated. This is attributed to increased consumption of the adsorbed methylene species. Experimentally, the CO conversion increased with addition of 1-alkene. This acceleration may contribute to the suppressed C02 selectivity as well in the allcene-added reaction, as C 0 2 is the byproduct from CO in the water-gas shift reaction. It should also be noted that the increase in CO conversion was more noticeable for addition of 1-alkenes with shorter carbon chains (e.g. heptene), whereas enhanced chain growth occurred more effectively with addition of longer-chain 1-alkenes (Figure 4.8-7). If the added alkene has a long carbon chain, it would be expected to adsorb more strongly on the metal site and to cover a larger active area of the catalyst, thereby inhibiting CO adsorption onto the metal site. This may be the reason for the lower CO conversion with 1,7-octadiene addition and the relatively low CO conversion with the addition of l-CIbH32.

398

4 Reactions in SCF

4.8.2 Isomerization Reactions Tiltscher and Hoffman conducted 1-hexene isomerization on a low-activity, macroporous a-A1203 catalyst and found that the initial ratio of cis/trans2-hexene could not be influenced by temperature in the gas phase. There was a modest effect of both temperature and pressure on this ratio for reactions in the liquid phase. There was a more obvious effect in the supercritical phase [31]. Tiltscher et al. also showed that catalyst deactivation occurred during the gas phase reaction as hexene oligomers with low volatilities were deposited on the catalyst surface [32]. Performing the reaction at the same temperature, but above the critical pressure, prevented the deposition of these oligomers and the ensuing catalyst deactivation. Thus, one advantage of supercritical media for heterogeneous catalysis is the possibility of achieving insitu extraction of coke precursors. Subramaniam and co-workers extended this work on 1-hexene isomerization by conducting the reaction on an active, microporous Pt/y-A1203 catalyst and using C 0 2 as a diluent in the reactor [33,34]. They analyzed the phase and reaction equilibria to establish the thermodynamic constraints in this system, and they reported new reaction data from both batch and continuous reactors. The conversions achieved in the batch experiments confirmed the equilibrium analysis. The continuous runs were used to assess catalyst deactivation in hexene-C02 mixtures. At a subcritical pressure, catalyst activity decreased, whereas at a nearly identical temperature but supercritical pressure no loss of catalyst activity was observed. Saim et al. [35] attributed this maintenance of catalyst activity to the solvent power of the dense SCF, which presumably prevented the deposition of higher-molecular-weight oligomers in the catalyst pores. These oligomers were thought to be coke precusors, and their extraction by the SCF was supported by the brownish color of the reactor effluent at supercritical conditions as opposed to the clear effluent obtained at subcritical conditions. These results highlight again the possible advantage of SCF technology over conventional processes in some heterogeneous catalytic applications. Saim and Subramaniam [36] observed that the end-of-run isomerization rates decreased with isothermal increase in pressure in the subcritical region, but increased with pressure in the supercritical region. In sharp contrast to the activity maintenance observed by Tiltscher and co-workers in a macroporous catalyst, the microporous Pt/y-Alz03 catalyst used by Saim et al. deactivated even at supercritical conditions. A significant portion of the catalyst activity was lost due to the build-up of unextractable coke in the catalyst pores during the subcritical phase of reactor fill-up. In a related work, Manos and Hofmann [37] concluded that the complete in situ reactivation of a microporous zeolite catalyst by an SCF is impossible. This conclusion was based on coke desorption rates and the solubilities of model coke compounds in the SCF. The catalyst deactivation rate can be reduced at supercritical conditions, however, because freshly formed coke precursors can be dissolved by the SCF reaction medium.

4.8 Heterogeneous Catalysis

399

Saim and Subramaniam [38] and Ginosar and Subramaniam [39] also found that the in situ extraction of the coke compounds by near-critical or supercritical reaction mixtures prevents pore plugging that otherwise occurs at subcritical (gas-like) conditions. Although the coke laydown decreased at supercritical (liquid-like) conditions, the isomerization rates were lower and deactivation rates were higher due to pore diffusion limitations in the liquid-like reaction mixtures. It was therefore concluded that near-critical reaction mixtures provide an optimum combination of solvent and transport properties that is better than either subcritical (gas-like) or dense supercritical (liquid-like) mixtures for maximizing the isomerization rates and for minimizing catalyst deactivation rates. These findings indicate that catalytic reactions which require liquid-like reaction media for coke extraction and heat removal, yet gas-like diffusivities for enhanced reaction rates, can benefit from the use of near-critical reaction media. Amelse and Kutz [40] described a patented catalytic process to isomerize a mixture of xylenes to a product stream enriched in p-xylene at conditions over the critical point of the mixture. Supercritical operation provided enhanced catalyst activity compared to the conventional liquid phase catalytic process that operated at lower temperatures. Moreover, operating at supercritical conditions reduced catalyst deactivation compared to the gas phase reaction. Reduced catalyst deactivation allowed elimination of the H2 and related compression equipment that was required to maintain catalyst activity in the gas phase process. '

4.8.3 t-Butyl Alcohol Synthesis by Air Oxidation of Supercritical Isobutane Few organic oxidation reactions in SCFs have been studied in which air or oxygen is used directly as the oxidant. It is also of interest to investigate the reaction behavior of oxygen species such as peroxides in SCFs. t-Butyl alcohol (TBA) is the starting material in the production of methyl t-butyl ether (MTBE) production and methyl methacrylate (MMA) synthesis, where MTBE is a high-octane-number gasoline additive and MMA is a resin material [41,42]. TBA can also be simply converted to isobutene by dehydration reactions. Commercial production of isobutene from isobutane through direct dehydrogenation needs high reaction temperatures around 500-600 "C, at which the catalyst is deactivated quickly. Fan et al. reported a new synthesis for TBA where air was used directly as the oxidant to convert isobutane to TBA [43,44]. This reaction could be conducted efficiently if isobutane was in the supercritical state, over selected catalysts.

Oxidation of supercritical isobutane A typical flow-type fixed-bed reactor was employed where isobutane was fed by a high-pressure pump. Isobutane has a critical temperature of 135 "C and a critical pressure of 36.4 bar. The catalysts included commercially avail-

400

4 Reactions in SCF

able amorphous Si02-Ti02 (Fuji Silysia Chemical Co.). Na2W04/Si02 (10 wt%) and Na2Mo04/Si02 (10 wt%) catalysts were prepared by impregnating Na2W04 or Na2Mo04 aqueous solution on Si02 gels (Fuji-Davison), followed by drying at 120°C for 12 h. PdCeOz (4wt%) and Pd/C (2.5 wt%) catalysts were prepared by impregnating an acidic solution of PdCI2 on Ce02 (Soekawa Chemicals, 99.99 %) or active carbon (Shirasagi, Takeda Pharmacy Co.) and were dechlorinated in flowing hydrogen at 400°C for 3 h. The standard reaction conditions were as follows: mole ratio of isobutane/ air = 3/1; W/F (total) = 10 g.h/mol; catalyst weight = 0.5 g; reaction temperature = 153 "C. The total pressure was 44 bar for the gas phase reaction and 54 bar for the supercritical phase reaction. The partial pressure of isobutane was 33 bar and 41 bar, respectively. Table 4.8-2compares the reaction performances of five catalysts and the noncatalytic case in supercritical and gas phases. In all cases, changing the isobutane from the gas phase (44bar) to the supercritical phase (54bar) resulted in remarkably enhanced conversions of isobutane and oxygen. Generally, selectivity for the target products (TBA and isobutene) increased slightly on changing from the gas phase to the supercritical phase. As an exception, the P d C catalyst gave tremendous improvement in both conversions and selectivities of the reaction with the phase change. When the total pressure was increased from 44 bar to 54 bar, the total yield of TBA and isobutene on the PdC catalyst was enhanced from 0.31 % to 2.70%. Similarly, the Table 4.8-2 Results of the catalytic oxidation of isobutane by air in the supercritical phase or the gas phase. Conditions: isobutane/air = 3/1, W/F = 10 g.h/mol, catalyst weight 0.5 g, temperature 153 "C. Catalyst

Total pressure (bar)

i-C.JIIO conversion

(%I

conversion

TBA selectivity

(%)

(%)

0 2

i-C4HB selectivity

(%I

~

None None Si02-Ti02 Si02-Ti02 Si02-Ti02 Si02-Ti02a Pd/C PdIC Pd/Ce02 Pd/Ce02 Na2W04/Si02 Na2WOJSi02 NazMoO4/SiOz Na2Mo04/Si02

44 54 44 54 12 54= 44 54 44 54 44 54 44 54

0.3 1.2 2.9 4.9 0.0 0.1 0.5 3.1 1.8 3.2 2.1 5.6 6.7 7.0 ~

a

55.0 58.1 59.0 61.2 0.0 55.5 61.2 64.8 36.5 56.5 48.1 51.8 25.3 31.7

2.5 9.9 24.0 40.6 0.0 1.1 4.2 25.6 17.1 28.4 19.9 55.5 61.0 65.8 ~~

~

~

Liquid-phase reaction where the reaction temperature was 130 "C

TBA and i-C4Hs yield

(%I

~~

7.0 8.1 5.2 6.3 0.0 7.7 2.1 20.1 17.4 3.7 0.6 0.8 6.1 4.1

0.2 0.8 1.9 3.3 0.0 0.1 0.3 2.7 1 .o 1.9 1 .o 2.9 2.1 2.5

401

4.8 Heterogeneous Catalysis

Na2W04/Si02 catalyst caused an increase in the TBA and i-C4H8 yield from 1.0% in the gas phase (44 bar) to 2.9% in the supercritical phase (54 bar). From the high isobutene selectivity on the Pd/C catalyst in the supercritical phase, it was inferred that the TBA formed was dehydrated rapidly on the acidic site of this catalyst. All the catalyzed reactions showed remarkably higher activities than the noncatalytic reactions, which proved the promotional role of the catalysts in the reaction. As shown in Table 4.8-2 for the Si02-Ti02catalyst, if the total pressure of the gas phase reaction was as low as 12 bar, the reaction did not proceed. Similarly, the reaction rate, was very low for the liquid phase reaction over SiOz-Ti02. Generally, the‘main byproduct of the reaction was acetone, but methanol was also formed. The combustion product, C02, only formed in very small amounts, and CO was not detected. Lighter hydrocarbons ( C 4 3 ) formed but in very tiny amounts, over 98% being methane. The total amount of all C1 species (methanol, C02, CH4) was equal to that of acetone, which indicates the decomposition of C4 to C3 species (acetone) and C1 fractions. No formic 1 4

!

40

3.5

35

3

-c

30

g

20

5

V

2.5

25

I!

15 1

10

0.5

5

0

0

127

132

137

142

147

152

157

Temp. (“C)

I0

Figure 4.8-8 Comparison of the reaction performance in supercritical phase and liquid phase. Conditions: SiO2-Ti02 catalyst, 54 bar, W/F = 10 g.h/mol, catalyst weight 0.5 g, i-isobutanelair = 311.

-

50

-

.--40 h 8 30 20

10

supercritical phase

-

-

127

&

4

-

0

132

137

142

147

Temp.(“c)

152

157

4 Reactions in SCF

402

acid or formaldehyde was detected. As clearly indicated in Table 4.8-2, this reaction depended greatly on temperature and pressure, which implies some effect of the reaction phase. Figure 4.8-8 compares the reaction performance over the Si02-Ti02 catalyst at 54 bar while the reaction temperature was varied around the critical point (135 "C). It is clear that isobutane and oxygen conversions were enhanced remarkably when the isobutane was changed from the liquid phase to the supercritical phase. Consequently, TBA and isobutene yield was enhanced in the supercritical phase. Concerning the selectivity of the products, as also shown in Figure 4.8-8, TBA selectivity and acetone selectivity were enhanced and isobutene selectivity was suppressed in the supercritical phase reactions over the Si02-Ti02 catalyst. Figure 4.8-9 shows the sudden rise of the activity of the SiOz-Ti02 catalyst around the critical pressure when the reaction system changed from gas phase to supercritical phase. The total yield of TBA and isobutene increased sharply from 2.2% to 3.6%, while the total pressure was slightly enhanced from 47

-2

50

5

45

4.5

40

4

35

3.5

3 30 8

3

-

c

25

2.5 9

20

2

15

1.5

40

42

44

46

48

50

52

54

3

56

Total Pressure (atm)

70 60-

A

A i supercritical phase

s2 40 ..-> -d 30 20

.

-

40

42

44

46.

48

50

52

TO~A ~ressure'(am)

54

56

Figure 4.8-9 Comparison of the reaction performances in the supercritical phase and the gas phase. Conditions: SiOz-TiOz catalyst, 153 "C, W/F = 10 g.h/mol, catalyst weight 0.5 g, isobutane/air = 3/1, (1 atm = 1.013 bar).

4.8 Heterogeneous Catalysis

403

bar to 49 bar, indicating the obvious critical phenomenon around the critical point (48 bar). Correspondingly, the conversions of 0 2 and isobutane were .enhanced to a great extent at this slightly enhanced pressure. Interestingly, a further increase in the total pressure in the supercritical phase did not improve the reaction over the Si02-Ti02 catalyst. It is clear that the activity at 54 bar was lower than that at 49 bar. For the oxidation mechanism in an SCF, it is speculated that dioxygen can attack the most active hydrogen of isobutane to form t-butyl hydroperoxide (TBHP, (CH3)3COOH). TBHP is well established as an oxygen donor in the epoxidation of alkenes [45]. It is inferred that TBHP can form in the supercritical phase containing isobutane and dioxygen. This auto-oxidation step can proceed without,'a catalyst [46] and probably occurs during the induction period at the initial stage of the reaction. TBHP can decompose homolytically, resulting in a t-butoxy radical and a hydroxide radical. The t-butoxy radical may abstract a hydrogen from another isobutane molecule to form TBA. TBA can then be dehydrated at the acidic sites of the catalyst, leading to an increase in isobutene selectivity. The main byproduct, acetone, could be derived from decomposition of t-butoxy radicals, accompanied by the formation of C1 compounds such as CH30H, CO2 or CH4. The role of the catalyst is to improve the oxidation of isobutane with TBHP, resulting in the observed enhancement of the conversion.

4.8.4 Supercritical Phase Alkylation Reactions over Solid Acid Catalysts Due to the Clean Air Act, increasing attention is paid to the production of alkylates, which is a very clean burning fuel and has a high MON (motor octane number) with a low octane sensitivity and moderate vapor pressure. Commercially operated alkylate production uses a liquid acid catalyst such as H2SO4 or HF, resulting in problems associated with cost, apparatus and the environment [47]. New synthetic methods utilizing solid acid catalysts have been developed but no commercial process has emerged due to fast catalyst deactivation [48]. As outlined above, application of SCFs to heterogeneous catalysis is of great interest [49]. Selected supercritical fluids can extract high-molecular-weight hydrocarbons from catalyst micropores to avoid catalyst deactivation [50] or to reactivate spent catalyst [5 11. This section describes the application of SCFs to alkylation reactions on solid acid catalysts [52,53]. Alkylation using solid acid catalysts Commercially available H-USY (Catalyst & Chemical Ind., Si02/A1203 = 8.6) catalyst was mainly utilized with pellets of 20-40 mesh. Before reaction it was calcined in flowing air for 3 h at 450°C in situ [54].A continuous

~

404

4 Reactions in SCF

flow type fixed-bed reactor equipped with upstream preheater was utilized. A tailor-made reactant mixture with different composition (alkene/alkane ratio varied between 1/5 and 1/70) was pressurized into the high pressure pump before the reaction. The products were determined by GC-MS (Shimadzu GCMS QP1 100EX). Quantitative analysis was conducted by online gas chromatography. The critical pressures and critical temperatures of the fluids used were: propane, 96.8 "C and 42 bar; 2-methyl-propane (isobutane), 135 "C and 36 bar; 2-methyl-butane, 188°C and 33 bar. With the exception of propane, the alkanes acted here as both reactant and SCF. Fig. 4.8-10 compares the durability results of the 2-methyl-propene and 2 -methyl-propane reaction in the gas phase, liquid phase and supercritical phase. Very high initial activity, with yields of alkylate (2,2,4,-trimethylpentane) as high as 70%, was observed in the liquid phase reaction (50"C, 35 bar). However, the activity decreased rapidly, and no activity was observed when the accumulated feed amount of olefin reached 20 mmol/g-cat. Similarly, rapid catalyst deactivation was observed in the gas phase reaction. Alkylate yield dropped to near zero at an accumulated alkene feed of 20 mmol/g-cat. For the supercritical-phase reaction, the catalyst deactivation was suppressed by the SCF. Even when the accumulated alkene feed reached 35 mmol/g-cat (after 5.6 h), the alkylate yield was still higher than 10%. Although the yield of alkylate decreased with time-on-stream, 2-methyl-propene conversion was almost 100%. If the reaction was carried out in the liquid phase at conditions (125 "C, 50 bar) only slightly different from those of the SCF reaction, the deactivation behavior was similar to that of the gas phase reaction. It should be mentioned that supercritical phase and gas phase reactions were implemented at 140"C, but liquid phase reactions were conducted at 50°C or 125 "C. For the gas-phase reaction, high reaction temperatures favored side

P

50 40

Supercritical Phase 140 T, 50 atm

30

20 10

Liquid Phase at High Temp.

n 140%, 3 a -0 10

125 "C, 50 atm

.20

30

40

50

Accumulated alkene feed / mrno1.g-cat-'

Figure 4.8-10 Phase effect on the alkylation reaction. Conditions: isobutenelisobutane = 1/50: W P = 40 g h/mol; 450°C-calcined H-USY ( 1 atm = 1.013 bar).

4.8 Heterogeneous Catalysis

Figure 4.8-11 Alkylation reaction in different supercritical solvents. Conditions: isobutenel isobutane = 1/50; W/F = 40 g ldmol; 450 "C-calcined H-USY (1 atm = 1.013 bar).

405

. s

"0

5 10 15 20 25 30 35 40 Accumulated alkene feed / mmol cat-g-'

45

reactions and secondary reactions, deactivating the catalyst. Consequently, the initial alkylate yield was only about 30% under these conditions, rather lower than that for the supercritical phase reaction. C5-C7 hydrocarbons formed in relatively high selectivity in the gas phase or supercritical phase reactions, attributable to the high reaction temperature. In the liquid phase reaction at 50 "C, C I 2 alkane formed in high selectivity, especially after catalyst deactivation. This can be attributed to slow diffusion of the reactant and alkene oligomer products inside the liquid phase reaction medium. The residence time of the alkenes in the catalyst bed is therefore prolonged in the liquid phase reaction, compared with the supercritical phase reaction. Supercritical fluids exhibit quite different extraction capacities if their molecular weights are different. Fig. 4.8-11 shows the durability of alkylation reaction activity where 2-methyl-butane, 2-methyl-propane and propane in the supercritical state were utilized as the reaction medium. It should be mentioned that 2-methyl-butane and 2 -methyl-propane were reactants as well. For the 2 -methyl-butane case, hardly any deactivation was observed, but the initial yield was low (20%). In contrast, a high initial yield of about 60% was reached in the reaction in supercritical propane while deactivation was the most pronounced in this SCF. The reaction conducted in 2 -methyl-propane showed intermediate results, compared to the reactions in 2-methyl-butane or propane. The reason for the low alkylate yield in the reaction in 2-methyl-butane was most likely the high reaction temperature. High reaction temperatures favored side reactions, reducing the selectivity of alkylate. Indeed, C5-C7 hydrocarbon products formed in high selectivity in the high temperature reaction conducted in supercritical 2-methyl-butane. It is possible that hydrocracking occurred as a side reaction at the same site where the alkylation reaction proceeded. The temperature of the reaction with propane was low and the side reactions were effectively suppressed. The deactivation of this reaction is probably due to the poor extraction capacity of the propane medium, especially at the low reaction temperature used here. The low solubilities of catalyst poisons in supercritical propane at these reaction conditions deactivated the catalyst. The different supercritical fluids mentioned above were used to extract the deactivated catalyst in order to measure their regeneration capacity

406

4 Reactions in

n "0

I0

$

2

20

30

60

50-

SCF

s

0-

40

0.84

40

9.2 18.5

40-

0

20

30-

v 20-

8 2

10-

\

0

0

10

20

30

10.8 20.8 30.8

40 so

30

-

20

-

10

I

0

0

-

10

20

30

40

0.84

10.8 20.8 30.8

Accumulated alkene feed / m o l cat-g-'

Figure 4.8-12 Extraction ability of different supercritical solvents. Conditions: 450 "C-calcined H-USY (60 atm = 61 bar).

(Fig. 4.8-12). After 2-methyl-butane was used to extract the spent catalyst for 1 h, the initial activity of alkylation reaction was restored to about 70%. A similar effect was observed upon extraction with 2-methyl-propane. However, if propane was used to regenerate the deactivated catalyst, no significant improvement was obtained and the catalyst still showed low alkylate yield and high oligomer yield. These results are in good accord with those reported in Fig. 4.8-11. It is clear that catalyst deactivation is closely related to the extraction capacity of the SCF. In situ extraction of catalyst poisons determines catalyst lifetime. Furthermore, failure of the extraction of these catalyst poisons favored the oligomerization reaction of the alkene. It was found that if the content of the hydrocarbons extracted by supercritical 2-methyr-butane were analyzed, no hydrocarbons other than CI2 alkene (two isomers) were detected. It is clear that CI2 alkene, derived from oligomerization reactions, deposited onto catalytic sites and deactivated the catalyst. More specifically, high molecular weight alkenes such as CI2, which have high electron density, might combine strongly with Lewis acid sites inside

4.8 Heterogeneous Catalysis

407

the catalyst framework, to block the participation of the Lewis acid sites in the alkylation reaction. In situ extraction of CI2alkenes from these acidic sites was critical to retaining catalyst activity. Recently, Clark and Subramaniam conducted a similar reaction, 1-butenelisobutane alkylation, in scC02 with USY zeolite as the catalyst [55]. The utilization of scC02 was considered mainly to lower the reaction temperature, as the high reaction temperatures in other supercritical phase systems could have increased the cracking and coking reactions.

4.8.5 Synthesis of Fine Chemicals and Other Products Poliakoff et al. introduced the supercritical phase to the Friedel-Crafts alkylation reaction by using scC02 or by making propene, one of the reactants, the supercritical fluid [56]. This heterogeneous supercritical phase reaction was conducted continuously and the selectivity was very high if a solid acid Deloxan catalyst was utilized. Vieville et al. conducted the esterification of oleic acid with methanol in scC02 using a cation exchange resin [57]. They reported ester yields of about 50% for reactions at 40°C and 160 bar, as shown in eq (4.8-1). They also found that the reaction rate was limited by transportation of the reactants to the external surface of the resin. RCOOH

+ CH30H.

scco2 acidic resin

RCOOCH3

(R = CH~(CH~)~CH=CH(CH&J)

+ H20 (4.8-1)

Functional group transformations in supercritical water were examined for a series of cyclohexane derivatives including cyclohexane, cyclohexene, cyclohexanol, cyclohexanone, benzene and phenol [58]. No reactions were observed in the absence of a catalyst, but hydration-dehydration, isomerization, and hydrogenation-dehydrogenation transformations did occur in the presence of an acid, base or metal catalyst. Dehydrogenation included the ketonization of cyclohexanol and the aromatization of cyclohexanone, cyclohexene, and cyclohexane. The dehydrogenation required a late transition metal catalyst, such as Pt/C, and proceeded even in the absence of a sacrificial hydrogen acceptor. Cyclohexanol dehydration was catalyzed by either acid or base, whereas cyclohexene hydration was only observed in reactions involving both PtO2 and an acid or base. Isomerization was an acid-catalyzed process. These reactions demonstrate that organic functional group transformations in supercritical water can be achieved in the presence of appropriate catalysts. Catalyzed reactions between supercritical water and quinoline were studied by Li et al. [59] as a possible approach to removing nitrogen from organic compounds. The reaction of quinoline with supercritical water was hetero-

408

4 Reactions in SCF

geneously catalyzed by ZnC12. The rate was described by an equation first order in quinoline and inverse first order in water. This was from a Langmuir-Hinshelwood bimolecular mechanism with competitive adsorption between quinoline and water. The product distribution’s dependence on reaction time indicates that the initial step in the reaction was the rupture of the C-N bond in quinoline followed by fragmentation of the hydrocarbon side chain to provide species for ring alkylation. Higher temperature and higher catalyst loading gave more complete nitrogen removal in the form of ammonia. The enantioselective hydrogenation of ethyl pyruvate to (R)-ethyl lactate has been studied by Minder et al. [60] using SCFs as reaction media. The catalyst was Pt/alumina modified with cinchonidine. In supercritical ethane, the reaction time could be reduced by a factor of 3.5 compared to toluene under similar conditions, without any loss in enantioselectivity. A further advantage of ethane was that the enantioselectivity remained high, even at high catalyst/ reactant ratios, allowing the application of a continuous fixed-bed reactor for this reaction. Strong catalyst deactivation was observed in scCO2, due to the reduction of CO2 on Pt. Hybrid catalysts derived from cocondensation of Group 8 metal-chloro complexes with Si(OEt)4 via a sol-gel process were highly active for the synthesis of N,N-dimethylformamide from C02, H2 and dimethylamine under supercritical conditions, affording turnover numbers up to 100 800 at 100 % selectivity [61]. The activity of the catalysts, containing methylphosphine ligands, decreased in the order Ru > Ir> Pt,Pd > Rh. It seemed that the high activity of silica matrix stabilized ruthenium complexes was due to the formation of an active hydride intermediate by hydrogenolysis of the Ru-Cl bond. C 0 2 + H2 + Me2NH + Me2NC(0)H

+ H20

(4.8- 2)

From the viewpoint of reaction engineering, some studies provided interesting insights into the role of SCFs in heterogeneous catalysis. Poly-siloxanesupported noble metal catalysts were used for hydrogenation of a wide range of organic functional groups by H2, where supercritical C 0 2 or propane was used in a flow-type reactor [62]. Because many reaction condition parameters such as temperature, pressure or feed ratio could be adjusted independently and continuously, the selectivity and yield of a particular product could be optimized. Similar catalysts have been used very successfully for the hydrogenation of fatty acids and their derivatives by Tacke et al. and Moller et al., as described in Section 1.1.4.3. Bertucco et al. investigated the effect of scC02 on the hydrogenation of unsaturated ketones catalyzed by a supported Pd catalyst, by using a modified internal-recycle Berty-type reactor [63]. A kinetic model was developed to interpret the experimental results. To apply this model to the multiphase reaction system, the calculation of high-pressure phase equilibria was required. A Peng-Robinson equation of state with mixture parameters tuned by experimental binary data provided a satisfactory interpretation of all binary and ternary vapor-liquid equilibrium data available and was extended to multicomponent

4.8 Heterogeneous Catalysis

409

calculations. The kinetic model was able to reproduce the experimental results on the basis of the calculated compositions in the liquid phase. The influence of the vapor-liquid equilibrium on the reaction kinetics accounted for some otherwise unexplainable behavior. A continuous hydrogenation trickle-bed reactor was employed to check the feasibility and advantages of the same reaction in scC02 [64]. The reaitor was modeled in the framework of a process simulator. A two dimensional reactor model was developed to account for radial temperature profiles. No adjustable parameters were used, indicating a totally predictive model. Calculated results agreed reasonably with experimental data obtained from a pilot unit. Dooley and Knopf conducted the oxidation of toluene by air in scCO2 [65]. If a redox or acid catalyst was used, toluene was converted to benzaldehyde effectively. It was revealed that an A1203-supported COO catalyst, partly oxidized to Co3+, was the most active and selective catalyst, due to the Co3+/Co2+redox couple. However, if this reaction was conducted in the liquid phase, COz formed in large amounts and the selectivity for partial oxidation was much lower. Supercritical water was used as a solvent for the partial oxidation of methane to methanol on a Cr2O3 catalyst [66]. The presence of water in high concentration inhibited methane conversion but increased the methanol yield. The methane oxidation was observed to be of slightly negative order in oxygen concentration; increasing the oxygen concentration resulted in a slight decrease in methane conversion. On the other hand, the oxidation of methanol was positive order in oxygen concentration; thus, increasing oxygen concentration reduced the methanol yield dramatically. A consistent set of reaction pathways was developed, and rate constants were calculated that accurately modeled the experimental results. Adschiri et al. removed 70 wt.% of the nitrogen from coal-derived pitches by catalytically hydrotreating the pitch in a supercritical toluene-tetralin mixture at 450°C [67]. These pitches could be used for the production of highquality electrode carbon. The more conventional process for this reaction involves catalytic hydrotreating of the pitch in the liquid phase. The higher diffusivities in the SCF resulted in higher reaction rates. Moreover, catalyst coking could be reduced effectively due to the increased pressure and hence the solvent power of the SCF. Suppes and McHugh studied the effects of different surfaces on the decomposition of cumene hydroperoxide in supercritical krypton, xenon, C02, propane, and CHFzC1. They reported that the observed first-order rate constants were strongly dependent on the metals present. Gold and 316 stainless-steel surfaces gave larger rate constants than did Teflon-coated surfaces [68]. The authors also observed that the different SCF solvents influenced the reaction rate. Collins et al. studied the disproportionation of toluene over a ZSM-5 zeolite catalyst at conditions near the critical point of toluene (equ. 4.8-3) [69]. This reaction is used commercially to produce p-xylene. Byproducts include benzene and 0- and rn-xylene isomers. The experimental results showed that the

410

4 Reactions in SCF

selectivity for p-xylene was highest near the critical pressure (42 bar) while the temperature was higher than the critical temperature. The selectivity decreased at pressures that were either lower or higher: (4.8-3) It was suggested by the authors that clustering occurring near the critical point could suppress the secondary reaction of the p-xylene. Cluster formation may happen in dilute supercritical mixtures in the vicinity of the critical point; the cluster size was defined as the excess number of solvent molecules surrounding each solute molecule. However, chemical effects of cluster formation are still restricted to a limited number of cases and need further clarification. Gabitto et al. [70] reported on the PbO-catalyzed dehydrogenation of toluene to dibenzyl and stilbene at supercritical conditions. They found that at 47.6 bar, dibenzyl was the major product at a supercritical temperature of 370"C, but stilbene was the major product at the subcritical temperature of 180°C. The rate of dibenzyl production at 370°C increased as the pressure was increased from subcritical to supercritical values. The same reaction conducted in the gas phase at 577 "C and at atmospheric pressure produced primarily stilbene, with dibenzyl and benzene as byproducts. These results showed that the catalyzed reaction could occur at temperatures lower than those used previously if the pressure was high.

+ Hz

(4.8-4)

Low studied the hydrotreating of heavy oil under supercritical conditions [711. He reasoned that there might be advantages to upgrading the supercritical extracts of oil shale or coal while the materials are still at supercritical conditions. The results demonstrated the feasibility of supercritical upgrading for a number of different heavy hydrocarbon feeds. It was found that an aliphatic solvent was better than an aromatic solvent as the latter can be hydrogenated and consume expensive H2. He concluded that supercritical hydrotreating was superior to conventional liquid phase hydrotreating because supercritical conditions led to a better product spectra with less gas, and less coke on the catalyst. Dardas et al. developed an in-situ version of the cylindrical internal reflection infrared (CIR-FTIR) technique to monitor the catalytic behavior of alkane cracking under supercritical conditions [72,73]. The results showed that the stretching frequency of the C-H bonds was altered in supercritical heptane, probably due to intermolecular hydrogen bonding. IR data also demonstrated an increased heptane concentration within the pores of a commercial Y zeolite cracking catalyst during catalytic cracking at supercritical conditions. Kinetic measurements showed that the activity of the catalyst was substantially higher

4.8 Heterogeneous Catalysis

411

during supercritical cracking, suggesting stabilization of the catalyst against rapid deactivation. The dense supercritical reaction medium continuously removed the newly formed coke by solubilization from the zeolite supercages, pore mouths, and the external surface. This mechanism was verified by the report of Niu et al. [74]

4.8.6 Conclusions Heterogeneous catalysis takes place in multiphase systems. If a new phase, the supercritical phase, is selectively introduced, remarkable change can be expected. The effects caused by the introduction of a supercritical phase depend on many parameters, such as fluid properties, reaction conditions, and affinity. Successful supercritical phase heterogeneous catalytic reactions can be realized, as long as these parameters are carefully controlled. With greater activities, catalyst lifetimes, or selectivities than for gas phase or liquid phase reactions, some supercritical phase reactions have industrial potential.

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[18] K. Yokota, Y. Hanakata, K. Fujimoto, Stud. Surf: Sci. Card 1991, 61, 289-294. [19] L. Fan, K. Yokota, K. Fujimoto, AIChE. J. 1992, 38, 1639-1648. [20] L. Fan, K. Yokota, K. Fujimoto, Topics in Catal. 1995, 2, 267-283. [21] E. Knapp, Vapor Liquid Equilibria for Mixtures of Low Boiling Substances, DECHEMA Chemistry Data Series, Vol. 6, 1982. [22] B. B. Breman, A. Beenackers, E. Rietjens, R. Stege. J. Chem. Eng. Data 1994, 39, 647-666. [23] R. B. Anderson, The Fischer-Tropsch Synthesis, Academic Press, Orlando, FL, 1984. [24] G. Henrici-Olive, S. Olive, The Chemistry of the Catalyzed Hydrogenation of Carbon Monoxide, Chapter 9, Springer-Verlag, Berlin, 1984. [25] X. Lang, A. Akgerman, D. Bukur, Ind. Eng. Chem. Res. 1995, 34, 72-77. [26] D. Bukur, X. Lang, A. Akgerman, Z. Feng, Ind. Eng. Chem. Res. 1997, 36, 2580-2587. [27] K. Snavely, B. Subramaniam, Ind. Eng. Chem. Res. 1997, 36, 4413-4420. [28] K. Fujimoto, L. Fan, K. Yoshii, Abstracts of the 207th National Meeting, American Chemical Society, San Diego, March, 1994. [29] L. Fan, K. Yokota, K. Fujimoto, Topics in Catal. 1995, 2, 259-266. [30] K. Fujimoto, K. Ohdan, K. Yoshii, Jpn. Kokai Tokkyo Koho 181293 1992,138534 1993, 138535 1993. [31] H. Tiltscher, H. Hofmann, Chem. Eng. Sci. 1987, 42, 959-964. [32] H. Tiltscher, H. Wolf, J. Schelchshorn, Bel: Bunsenges. Phys. Chem. 1984, 88, 897-903. [33] B. Subramaniam, B. J. McCoy, Ind. Eng. Chem. Res. 1994, 33, 504-511. [34] S . Saim, B. Subramaniam, Chem. Eng. Sci. 1988, 43, 1837-1843. [35] S . Saim, D. M. Ginosar, B. Subramaniam, in Supercritical Fluid Science and Technology, ACS Symp. Sex 406, K. P. Johnston, J. M. L. Penninger (Eds.), American Chemical Society, Washington DC, 1989, p. 301-321. [36] S. Saim, B. Subramaniam, J. Supercrit. Fluids 1990, 3, 214-219. [37]G. Manos, H. Hofmann, Chem. Eng. Tech. 1991, 14, 73-80. [38] S . Saim, B. Subramaniam, J. Catal. 1991, 13I, 445-455. [39] D. M. Ginosar, B. Subramaniam, in Catalyst Deactivation: Proc. Int. Symp. On Catal. Deac., B. Delmon, G.F. Froment (Eds.), Elsevier, Amsterdam, 1994. [40] J. A. Amelse, N. A. Kutz, US Patent 5,030,788, 1991. [41] European Chemical News, 1993, Feb. 15. [42] Chemical Week, 1993, Feb. 17. [43] L. Fan, T. Watanabe, K. Fujimoto, Appf. Catal. 1997, 158, L41-L46. [44] L. Fan, Y. Nakayama, K. Fujimoto, Chem. Comm. 1997, 1179-1180. [45] N. Emanuel, E. Denisov, Z. Maims, Liquid Phase Oxidation of Hydrocarbons, Plenum Press, New York, 1967. [46] A. Bielanski, J. Haber, Oxygen in Catalysis, Marcel Dekker, New York, 1991, p. 452. [47] J. A. Cusmano, CHEMTECH, 1992, 48, 2-4. [48] A. Corma, A. Martinez, Catal. Rev. Sci. Eng. 1993, 35, 483-504. [49] P. E. Savage, S. Gopalan, E. Brock, AIChE J. 1995, 41, 1723-1775. [50] S . Baptist-Nguyen, B. Subramaniam, AIChE J. 1992, 38, 1027-1026. [51] G. Madras, C. Erkey, A. Akgerman, Ind. Eng. Chem. Res. 1993, 32, 1163-1172. [52] L. Fan, I. Nakamura, S. Ishida, K. Fujimoto, Ind. Eng. Chem. Res. 1997, 36, 1458-1463. [53] L. Fan, I. Nakamura, S. Ishida, K. Fujimoto, Ind. Eng. Chem. Res. 1998, 37, 298-299. [54] I. Nakamura, S. Ishida, K. Fujimoto, Preprints Div. of Petro. Chem. ACS 1995, 40, 5 12-516. [55] M. Clark, B. Subramaniam, Ind. Eng. Chem. Res. 1998, 37, 1243-1250. [56] M. Hitzler, F. Smail, S. Ross, M. Poliakoff, Chem. Comm. 1998, 359-360. [57] C. Vieville, Z. Mouloungui, A. Gaset, Ind. Eng. Chem. Res. 1993, 32, 2065-2071. [58] R. Crittendon, E. Parsons, Organometallics 1994, 13, 2587-2591. [59] Z. Li, T. Houser, Ind. Eng. Chem. Res. 1992, 3I, 2456-2459. [60] B. Minder, T. Mallat, K. Pickel, K. Steiner, A. Baiker, Catal. Lett. 1995, 34, 1-9. [61] 0. Krticher, R. Koppel, A. Baiker, Chem. Comm. 1996, 1497-1498.

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[62] M. Hitzler, M. Poliakoff, Chem. Comm. 1997, 1667-1668. [63] A. Bertucco, P. Canu, L. Devetta, A. Zwahlen, Ind. Eng. Chem. Res. 1997, 36, 2626-2633. [64] L. Devetta, P. Canu, A. Bertucco, K. Steiner, Chem. Eng. Sci. 1997, 52, 4163-4169. [65] K. Dooley, F. Knopf, Znd. Eng. Chem. Res. 1987, 26, 1910-1916. [66] C. Dixon, M. Abraham, J. SBpercrit. Fluids 1992, 5, 269-273. [67] T. Adschiri, S. Takaki, K. Arai, Fuel 1991, 70, 1483-1484. [68] G. J. Suppes, M. A. McHugh, Ind. Eng. Chem. Res. 1989, 28, 1146-1152. [69] N. A. Collins, P. G. Debenedetti, S. Sundaresan, AIChE. J. 1988, 34, 1211-1224. 1701 J. Gabbito, S. Hu,B. J. McCoy, J. M. Smith, AIChE. J. 1988, 34, 1225-1233. [71] J. Y. Low, ACS Div. Fuel Chem. Prep. 1985, 30, 161-165. [72] Z. Dardas, M. Suer, Y. Ma, W. Moser, J. Catal. 1996, 159, 204-211. [73] Z. Dardas, M. Suer, Y. Ma, W. Moser, J. Catal. 1996, 162, 327-338. [74] F. Niu, H. Hofman, Can. J. Chem. Eng. 1997, 75, 346-352.

Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

4.9 Enzymatic Catalysis OLLIAALTONEN

4.9.1 Introduction Enzymes, isolated from their source organisms, are used industrially as homogeneous or immobilized catalysts in the textile and leather, detergent, starch, food, beverages, paper and pulp industries, as well as in bioremediation. Traditionally, enzymes are used as homogeneous catalysts in aqueous solutions. The findings in the mid-1980s that enzymes are also active catalysts in nearly anhydrous environments expanded their potential use considerably. Oils and fats, for example, are now processed with enzymes. Enzymes are not widely used as catalysts in the fine chemicals industry. Among the reasons for this are their poor stability in aqueous environments and low efficiency as heterogeneous catalysts in liquids when compared with inorganic catalysts. However, the possibility of using enzymes as heterogeneous catalysts in supercritical media opens up new possibilities for chemical synthesis. One of the expected benefits from using enzymes in supercritical fluids (SCFs) is that mass transfer resistance between the reaction mixture and the active sites in the solid enzyme should be greatly reduced if the reactants and products are dissolved in an SCF instead of running the reaction in a liquid phase. It is expected that the high diffusivity and low viscosity of SCFs will accelerate mass-transfer controlled reactions. Because enzymes are not soluble in SCFs it should be possible to disperse free enzyme in the SCF and recover the enzyme without the need to immobilize it on a support. As SCFs are often hydrophobic, many water-insoluble compounds may be processed in one phase, instead of the two-phase systems involved with using enzymes as homogeneous catalysts in aqueous solutions. For the same reason traditional hydrolysis reactions may be reversed and esterifications may be accomplished in SCFs with hydrolytic enzymes. Because ordinary .gases, such as O2 or H2 are miscible with SCFs, the limitation of poor gas solubility in liquids is removed. Therefore enzyme-catalyzed

4.9 Enzymatic Catalysis

4 15

oxygenations and hydrogenations should occur much more rapidly and with better control in SCFs than in liquid media. Downstream processing after enzymatic conversions in SCFs may be expected to be much simpler and more economical than after enzymatic steps in either aqueous solutions or in organic, liquid solvents. The first reports of enzyme activity in SCFs were published almost simultaneously in 1985 and 1986 [l-31. Great enthusiasm emerged and since this pioneering work more than 200 papers on enzymatic catalysis in SCFs have appeared. The subject has been reviewed in 1991 [4] and in 1995 [ 5 ] . This chapter first explains enzyme nomenclature, describes enzymatic, supercritical reactor configurations, and gives a compilation of published experimental results. The. most important topics concerning enzymatic reactions in SCFs are then covered. These are: factors affecting enzyme stability, the role of water in enzymatic catalysis, and the effect of pressure on reaction rates. Studies on mass transfer effects are also reviewed as are factors that have an effect on reaction selectivities. Finally, a rough cost calculation for a hypothetical industrial process is given.

4.9.2

Enzymes

Enzymes are classified according to the types of reactions which they catalyze. The major classes are [6]: 1. Oxidoreductases - catalyze oxidation-reduction reactions; 2. Transferases - catalyze group transfer reactions; 3. Hydrolases - catalyze hydrolytic reactions; 4. Lyases - catalyze reactions involving a double bond; 5 . Isomerases - catalyze reactions involving isomerization; 6. Ligases or synthetases - catalyze reactions involving joining two molecules coupled with the breakdown of a phosphate bond. Each enzyme has a four digit number (the EC number) which is based on this classification and a systematic name to identify the catalyzed reaction. More than 2000 enzymes have been identified. A few hundred are available in isolated, purified form. Within each class the isolated enzymes are more accurately identified by their source organism. Lipases, a subclass of hydrolases, have been isolated from yeasts and fungi (Cundidu, Mucor, Pseudomonus) and from the pancreas (porcine pancreatic lipase, PPL). The enzyme is usually identified more precisely by naming the exact species of the source organism. For example, a lipase may origin from either Cundidu cylindruceu or Cundidu unturcticu yeasts. In this chapter the enzymes are named using their systematic name (lipase etc.) together with the complete name of the source organism (Mucor miehei etc.). Trivial names for enzymes are used

416

4 Reactions in SCF

when they are well established. An example is subtilisin Carlsberg for the protease produced by Bacillus subtilis. Lipases, which belong to the major class of hydrolases, are an important subclass relevant to reactions in SCFs because they catalyze reactions involving esters, which are generally soluble in scCOz and other SCFs. Lipases isolated from several source organisms have been used in- a number of SCF reaction studies as outlined in more detail in Section 4.9.4; Gunnlaugsdottir recently formulated a good description of lipase structure and reaction mechanism [7]:“A characteristic feature among many lipases is a ‘lid’, in the shape of a loop or a small a-helix blocking access to the active site. . . . the activation step has been shown to involve a conformational change in which the lid moves to uncover the active site. This movement of the lid buries hydrophilic amino acid residue on the outside surface of the lid and exposes the active site and a large hydrophobic surface, believed to be the binding site for lipids. Theoretical studies of the lid opening in Mucor rniehei have indicated that a low dielectric constant in the medium is required for opening the lid.” However, a more recent experimental study showed that the solvent’s immiscibility with water and its apolarity are themselves irrelevant to the enzymatic activity of lipases and proteases [8].

4.9.3

Enzyme Reactors

Batch, recirculating batch, extractive semibatch, semicontinuous flow, continuously stirred tank (CSTR) and continuous packed bed reactors have all been succesfully tested as enzyme reactors for SCFs (Figure 4.9- I). References to helpful descriptions for designing small-scale reactors for enzymatic studies are collected in Table 4.9-1. 4.9.3.1 Batch Reactors Batch, mechanically mixed pressure vessels are suitable for preliminary screening enzymatic reactions in supercritical fluids. They are cheaper and much more easily controlled than the various flow reactor types. However, to obtain suitable reaction rate data for up-scaling, it is necessary to run experiments in a flow reactor. At VTT Chemical Technology, the following procedure has been used for screening enzymes and reaction conditions in SCFs in a batch reactor. Figure 4.9-2 describes the batch reactor configuration. First the enzyme and one of the reactants are weighed in a 40 mL or 200 mL pressure vessel, a magnetic bar is added and the vessel is closed with a lid which has three high pressure connections. The desired amount of water is weighed in a pressure tube, a feeding tube, which is then connected to the inlet. The reactor assembly with the tubing, valves and the feeding tube is

4.9 Enzymatic Catalysis

8 Batch

SCF Substrates Enzyme

Extractivesemibatch

417

Continuous flow

Enzyme

Recirculating batch

Semicontinuousflow

Substrates

Enzyme

Figure 4.9-1 Operating principles of enzymatic reactor types for SCFs.

Table 4.9-1 Constructional descriptions of enzyme reactors for SCFs. Reactor type

Organization

Batch

University of Maribor, Slovenia

Batch

VTT Chemical Technology, Finland

Batch

Tokyo Metropolitan University

Batch

E.N.S.I.C. Nancy

Batch

Massachusetts Institute of Technology

Batch view-cell

Universidade Nova de Lisboa

Recirculating batch

Cornell University

Recirculating batch

AFRC Inst. of Food Research, Norwich, UK

Extractive semibatch

Lund University

Semicontinuous flow

University of California at Berkeley

Continuous, stirred tank (CSTR)

Wageningen Agricultural University, Netherlands

Continuous packed bed

The University of Tokyo

Continuous packed bed

E.N.S.I.C. Nancy

Continuous packed bed

Nat. Center for Agric. Utilization Research, Peoria

Continuous packed bed

I.N.S.A. Toulouse

Reference

[I51

418

4 Reactions in SCF

placed in a thermostatted box. The inlet and outlet are connected, and a thermocouple, pressure indicator and a rupture disc assembly is installed. Next, the thermostatted box is heated to the desired reaction temperature with an electric heater which has a fan to circulate the heated air inside the box. The reactor content is mixed by starting a standard laboratory magnetic stirrer which is placed immediately under the reactor bottom. Then, the reactor is pressurized with CO:!to approximately 20 bar below the desired reaction pressure by letting CO:!flow through the feeding tube. The desired amount of water is injected into the reactor this way. The feeding tube is depressurized and refilled with the other reactant. The reaction is started by pressurizing the reactor to the final reaction pressure by letting COz flow into the reactor through the feeding tube. Alternatively, only the enzyme is weighed into the reactor and the reactant mixture and water is injected into the reactor through the feeding tube in one batch. The reaction is followed by taking samples from the pressurized reactor into sampling tubes. The sampling tubes are 20 cm long high-pressure tubes with a needle valve at one end. The volume of the sampling tubes is 0.5 mL. Samples are taken by first opening the reactor side valve and then slightly opening the outlet needle valve to fill the tube with a representative aliquot. Both valves

W Magneticstirrer ...................................................................

Depressurization

Rinsing

J

Air heated chamber

Figure 4.9-2 Batch reactor set-up at VTT Chemical Technology for screening enzymatic reactions in SCFs.

4.9 Enzymatic Catalysis

4 19

are closed and the sampling tube is disconnected from the reactor. A short piece of capillary tube is connected to the needle valve, with the end of the capillary tube dipping into a vial which contains a suitable organic solvent, such as hexane. The needle valve should be opened with great care so that the contents of the sampling tube start bubbling through the liquid. The depressurization may take some hours. After the sampling tube has reached atmospheric pressure the tube is finally rinsed with an organic solvent. The composition of the reaction mixture can then be analyzed from the obtained solution. Because the depressurization is so slow, several sampling tubes are used simultaneously: one is connected to the reactor, one is being depressurized, and one is being rinsed. The small (40: mL) pressure vessels are made from AISI 3 16 steel. AISI 3 16 steel is nonmagnetic and allows using a magnetic stirrer through the vessel bottom. The lid has fluoroelastomer (Viton) or Nitrile Buna N (NBR) O-ring seal. Both polymers absorb scC02 and they have to be discarded after each reactor opening. High-pressure tubing (0.25 in) with coned fittings (Nova Swiss or SITEC) is used. The pressure in the reactor is regulated with a Tescom pressure regulator which maintains the preset pressure during sampling. For pressurization, a metal diaphragm compressor (Nova Swiss) is used. The high-pressure T-connections, closing valves and needle valves are from Nova Swiss and from SITEC.

4.9.3.2 Recirculating Batch Reactor In a recirculating batch reactor the enzyme is packed into a column and the supercritical reaction mixture is recirculated through it. A magnetically coupled centrifugal pump can be used for recirculating the reaction mixture [9]. At VTT Chemical Technology an air driven gas booster (Resato GmbH) is used for circulation [lo]. The gas booster is a reciprocating differential area piston pump. It uses a relatively large air drive piston connected to a smaller high pressure gas piston. The volumetric pumping rate is adjusted by changing the air-side pressure. To obtain kinetic data in a recirculation reactor, the flow rate has to be measured or a calibrated piston pump has to be used. One benefit is that changes of the reaction mixture flowrate through the enzyme column provide data about the effect of external mass transfer. A disadvantage is that pumping a supercritical reaction mixture can be hampered by the fact that precipitations may occur in the check valves or other components of the circulation loop.

4.9.3.3 Extractive Batch Reactor Gunnlaugsdottir and Sivik used a two-phase reactor for continuous extraction of the reaction product from a liquid substrate-enzyme mixture. They adjusted the pressure and temperature of the scC02 in the reactor so that the fatty acid

420

4 Reactions in SCF

ethyl esters, which were the reaction products, were extracted with flowing C02 from the substrate triglycerides which remained in the reactor liquid phase. Removal of-the reaction products continuously shifted the transesterification towards completion [ 113. 4.9.3.4 Semicontinuous Flow Reactors A batch of substrates is dissolved in the supercritical carrier stream which is then fed continuously through the enzyme bed. In their pioneer work, Randolph et al. deposited cholesterol on glass wool which was then placed in a saturation chamber. A constant flow of scCOz was led through this saturation chamber and then through a packed enzyme bed [12]. Hammond et al. used a similar configuration. They placed the substrate-laden glass wool upstream of the enzymes which were deposited on glass beads. The glass wool and the immobilized enzyme were placed in the same reactor tube [2]. This reactor configuration has subsequently been used in many investigations. The pre-enzyme-bed saturator can also be used to saturate the SCF with water in a continuous reactor. C 0 2 may be bubbled through a partly water filled saturator tube before leading the flow to the reactor [13] or moistened cotton may be put in the water saturation chamber [14]. The advantage of a semicontinuous flow reactor is that only pure SCF needs to be pumped. Standard HPLC pumps can be used without fear of clogging the check valves. As in all flow reactors it is necessary to sub-cool the fluid to avoid cavitation and loss of pumping capacity. With C 0 2 it is usually enough to cool to approximately -5 "C at the pump head. A cooling jacket around the pump head is needed; experience suggests that the temperature of the cooling liquid should be about -20°C. The disvantage of semicontinuous flow reactors is that substrate concentration in the SCF is always close to saturation. Therefore, substrate concentrations and reaction conditions cannot be chosen independently. Small changes of pressure or temperature of the saturated fluid may cause precipitation of substrates in the enzyme bed. This may distort the results.

4.9.3.5 Continuous Reactors Substrates and the SCF are independently pumped together to obtain a single-phase mixture. The mixture is then led through the enzyme reactor. The reactor may be either a tube packed with enzyme or a continuously stirred tank (CSTR). The concentrations, reaction conditions, flow rates, and residence times can be set independently. HPLC pumps are used for pumping the substrates into the SCF, provided that the viscosity of the liquid is low [13,. 151. The SCF and substrates are equilibrated in a separate vessel prior to leading the mixture to the enzyme reactor [16]. Pulse-

4.9 Enzymatic Catalysis

42 1

less, single syringe pumps (Isco, Inc.) are used for pumping the reaction mixture [ 171. Complete enzymatic reactors with downstream separators for high-pressure fluids are available from Separex (Nancy, France) from NWA (Lorrach, Germany) and from Chematur Ecoplanning Oy (Tampere, Finland).

4.9.4 Experimental Results 4.9.4.1 A Compilation of Published Experiments

A list of reported, successful enzymatic reactions in SCFs is given in Table 4.9-2. For process feasibility calculations it is important to know not only that the conversion occurs, but also to have rough estimates on the obtainable productivities. Examples of reported maximum initial reaction rates in selected Table 4.9-2 A compilation of enzyme-catalyzed reactions in supercritical fluids. Enzyme source or commercial name

Esterifications Candida cylindracea lipase

SCF

Substrates

Conditions Reference

(+)citronello1 + oleic acid, n-valeric acid

69-245 bar [59,24] 31-41 "C 140 bar [61,66] 35 "C

+

Porcine pancreatic lipase

racemic glycidol butyric acid

Mucor miehei lipase Candida lipase B

butanethiol + oleic acid butanol + lauric acid

Fusarium solani cutinase

hexanol

+ hexanoic acid

Mucor miehei lipase

ethanol

+ myristic acid

125 bar 50 "C

Mucor miehei lipase

ethanol

+ myristic acid

Mucor miehei lipase

propanol

130 bar [13,21, 50 "C 351 100-250 bar [lo, 221 36-62 "C

Mucor miehei lipase

isopropylideneglycol + fatty acid

Rhizomucor miehei, Mucor miehei lipases

oleyl alcohol

+ R,S-ibuprofen

+ oleic acid

[671 300 bar 40 "C 130 bar 45 "C

[54] [31] [55,56]

78-220 bar [52] 40 "C 8 0 4 5 0 bar [ 16,341 40-80 "C

422

4 Reactions in SCF

Table 4.9-2 (continued). Enzyme source or commercial name

SCF

Substrates

Candida cylindracea lipase

C02

ethanol

+ oleic-acid

Mucor miehei lipase

co2

ethanol

+ oleic acid

Pseudomonas sp. lipase

200 bar [621 (*)3-penten-2-01, 2 3 4 0 "C (*)6-methy1-5-hepten-2-01, (+)2-octanol + acetic anhydride Including: interesterifications, alcoholysis, acidolysis co2 D,L-menthol + 100 bar [30,68] isopropenylacetate 50 "C

Conditions Reference

-

Transesterifications Lipase AY30, Novozym 435-lipase, Esterase EPlO Lipase PS Candida cylindracea lipase

C02

Candida cylindracea lipase Mucor miehei lipase

co2

Mucor miehei lipase

co2

Mucor miehei lipase Mucor miehei lipase Recombinant: Aspergillus oryzae Candida antarctica Thermophilic lipases Candida antarctica lipase

C02

Candida antarctica lipase

C02

Candida antarctica lipase

C02

Rhizopus arrhizius lipase

C02

C02

[36] [9,32]

(+) 1-phenylethanol,

D,L-menthol Fluoroform, Ethane, SF6, Propane C02

140 bar 40 "C 130 bar 40 "C

+ tnacetin

methyl methacrylate + 2-ethyl- 1-hexanol

methyl methacrylate + 2-ethyl- 1-hexan01 nonanol + ethyl acetate amyl alcohol, nonanol + ethyl acetate geraniol + propyl acetate ibuprofen ethyl ester + amyl alcohol (k)1-phenylethanol + vinyl acetate

100 bar [30,68] 50 "C 40-260 bar [18,51] 45-60 "C

100 bar [18,20] 40-60 "C 130-200 bar [44] 60 "C 100 bar [42] 60 "C 140 bar [19] 40 "C 150 bar [lo] 50 "C 200 bar [63] 40 "C

triglycerides from palm and 170-280 bar [69] soybean oil (randomization) 65 "C triglycerides from corn oil + 240 bar [ 171 methanol 50 "C triglycerides from cod liver 500-900 bar [70] oil + ethanol 40 "C trilaurin + palmitic acid 150 bar [71] 40 "C

423

4.9 Enzymatic Catalysis Table 4.9-2 (continued). Enzyme source or commercial name

SCF

Substrates

Rhizopus arrhizius lipase

COz.

trilaurin

Rhizopus arrhizius lipase

C02 Ethane COZ

Mucor miehei lipase

Subtilisin Carlsberg protease C 0 2 Aspergillus and subtilisin Carlsberg proteases Rhizopus delemar Rhizopus japonicus Alcaligenes sp. Mucor miehei lipases Thermolysin

Fluoroform

co2

Conditions Reference

+ myristic acid

80-110 bar [14] 35 "C trilaurin + palmitic acid 100-300 bar [51] 40 "C 100-300 bar [41] triolein + ethyl behenate 40-70 "C N-acetyl-L-phenylalanine 150 bar [29] chloro ethyl-ester + ethanol 45 "C N-acetyl-(L,D)-phenylahine 70-350 bar [49,64] ethyl-ester + methanol 50 "C triolein + stearic acid 290 bar [46,471 50 "C

C02

aspartic acids + phenylalanine ester

300 bar 20-40 "C

Hydrolysis Mucor miehei lipase

COZ

Cellulase

COZ

racemic 3-(4-Methoxypheny1)glycidic methyl-ester microcrystalline cellulose

Alkaline phosphatase

co2

disodium p-nitrophenyl phosphate

130 bar [45] 40 "C 30-140 bar [33] 35-46 "C 100 bar [l] 35 "C

fluoroform

bis(2,2,2-trichloroethyl)adi- 80-370 bar [72] pate + 1,4-butanediol 50 "C

COz,

cholesterol

Polymerizations Porcine pancreatic lipase Oxidations Streptomyces sp., Norcardia sp., Pseudomonas and Gloeocysticum chrysocreas cholesterol oxidases Polyphenol oxidase

coz +

+ O2

[581

100 bar 35 "C

[12,731

350 bar 34-36 "C

[2]

cosolvents

COZ fluoroform

p-cresol, p-chlorophenol O2

+

enzyme/fluid/reaction systems are collected in Table 4.9-3. It is important to note that the enzyme concentration, reactor type and other parameters have varied considerably from paper to paper. Many enzymatic reactions in supercritical fluids follow Michaelis-Menten kinetics. The Michaelis constants and maximum reaction velocities as well as inhibiting substrates have been identified in several enzymatic reactions in

424

4 Reactions in SCF

Table 4.9-3 Examples of reported maximum initial reaction rates in SCFs.

Enzyme source

Enzyme SCF concentration (gW

Mucor miehei 5.3 Mucor miehei Porcine pancreatic lipase Mucor miehei Candida cylindracea Fusariun solani 20 Candida cylindracea 20 Candida cylindracea 20 Candida cylindracea 20 Candida cylindracea 420 Mucor miehei Mucor miehei Mucor miehei 1.4 Esterase EPlO

Reaction type

Maximum Referinitial rate ence mmoY (g enzyme x h)

6 esterification 0.01 esterification 11 COZ esterification COZ transesterification 0.04 0.3 COZ esterification 0.07 COz : esterification propane alcohol.ysis 500 50 coz alcoholysis 200 fluoroform alcoholysis 6000 SF, alcoholysis 0.6 COZ alcoholysis 19.2 coz esterification 10.2 COZ transesterification 0.01 COZ transesterification

COZ

coz

SCFs [14,18-211. Figure 4.9-3 shows how kinetic parameters can be obtained from a Lineweaver-Burk plot. The example is from the esterification of ibuprofen with propanol in scC02 using Mucor miehei lipase (Scheme 4.9-1) [22]. In this case the Michaelis constant was K , = 2.8 mmol ester per mole of mixture and the maximum reaction velocity V,,, = 360 g ester per kg enzyme per hour. The kinetic parameters are specific to the reaction system, reactor type, enzyme and substrate concentrations as well as to reaction conditions. l/v I (kg enzyme 'h I' kg product)

Figure 4.9-3 A Lineweaver-Burk plot for determining Michaelis-Menten kinetic parameters for ibuprofen esterification using it. lipase in scCO2 according to Scheme 4.9-1 [22].

I

2

1

- 0 m 5 T

- 1 /K,

o

1 /IS1

(mol mixture / mmol product)

4.9 Enzymatic Catalysis

425

Mucor miehei lipase

R-(-)-Ibuprofen

+

HO-

__c

COP S-(+)-Ibuprofen propyl ester in rnax. 70%, ee

S-(+)-Ibuprofen

:

Scheme 4.9-1 Enantioselective esterification of racemic ibuprofen in scCOz [22].

Although supercritical C02 is by far the most extensively used supercritical medium for enzymes, there are other possibilities. Kamat succesfully tested supercritical fluoroform, ethylene, ethane, propane and sulfur hexafluoride as dispersants for Cundidu cylindruceu lipase in alcoholysis. Interestingly, sulfur hexafluoride, an anhydrous inorganic supercritical solvent, was the best solvent for the tested compounds (2-ethylhexanol and methylmethacrylate) [ 181. Two parameters that have no relevance in the classical use of enzymes in aqueous solutions need to be considered in nonaqueous, supercritical reaction systems: the water activity and the density of the reaction mixture. Their effects are reviewed in the following subsections. 4.9.4.2 Enzyme Stability in Supercritical Fluids

Many enzymes are stable and catalyze reactions in supercritical fluids, just as they do in other non- or microaqueous environments. Many enzymes are more stable in supercritical fluids than in aqueous solutions. However, enzymes are not soluble in SCFs; therefore enzymatic catalysis in SCFs is always heterogeneous. Enzyme stability and activity depend on the enzyme species, the supercritical fluid, the water content of the enzyme/support/reaction mixture, and on the pressure and temperature of the reaction system. One would expect that basic free amino groups in enzymes could react with acidic supercritical carbon dioxide to form carbamates. Free amino groups exist in lysine, histidine, and arginine. Kamat et al. [20] obtained direct evidence that carbon dioxide forms carbamates with subtilisin, which is a protease with nine lysine groups. They used laser desorption mass spectrometry (LD-MS) to accurately measure the molecular weight increase when subtilisin was placed in “dry ice”. Subtilisin’s molecular weight increased by 176 atomic mass units: the weight of four C02 molecules. The formation of carbamates was reversible. As the dry ice sublimed under vacuum,

426

4 Reactions in SCF

subtilisin's original molecular weight was restored. From these results it was concluded that the observed inhibiting effect of scCO2 on Cundidu cylindruceu lipase-catalyzed alcoholysis is the result of enzyme changes by carbamate formation. Investigations under a wide range of reaction cpnditions for Cundidu lipase failed to identify conditions where this enzyme was active in scCO2. This was unexpected because Mucor miehei lipase, for example, is highly active under the same conditions. Recently, Ikushima and co-workers reported about an extremely narrow pressure range, 77-87 bar at 3 1 "C, where Cundidu cylindruceu lipase takes up a large number of C 0 2 molecules [23,24]. As a result, the enzyme's conformation changes so that the active site becomes accessible for substrate molecules. Thus, it seems that near-critical C 0 2 is a trigger to the activation of Cundidu cylindruceu lipase. Furthermore, they found that the activation was stereospecific. De Carvalho et ul. compared Subtilisin Curlsberg protease-catalyzed transesterification in propane and in C 0 2 and concluded that scCO2 had a direct effect on the protein resulting in a decrease of activity [25]. Concerns about effects of supercritical carbon dioxide on food quality has prompted related investigations on the possible reactions of amino acids with carbon dioxide and water. Weder found that moist supercritical carbon dioxide at 80°C did not react with any of the examined amino acids including lysine. Weder also found that Lysozyme lipase unfolded and partially oligomerized in moist scC02. He concluded that denaturation was caused by heating the protein in the presence of water and not by interactions with-scC02 r26i The possibility of using high pressure carbon dioxide as a sterilizing agent for food has sparked a considerable number of further investigations. It was found that yeast and bacteria cells were indeed sterilized by treatment with scC02 at 35 "C. However, sterilization occurred only when the microorganisms contained 70-90 % water [58]. Dry cells (2-10 % water) were not sterilized. It was also found that none of the nine tested commercial enzymes lost its activity when treated in dry or water-containing (0.1 wt%) scCO2 at 35 "C and 200 bar. A two thirds loss of enzymatic activity was observed when the enzyme contained 50 wt% water [27]. For industrial production, it is important that the enzyme catalyst retains its activity for a considerable period of time. Hammond et ul. carried out oxidations using polyphenol oxidase according to Scheme 4.9-2 [2]. The enzyme was stable in the SCFs containing oxygen but was inactivated by the phenolic reactants. Randolph et ul. noted that cholesterol oxidases from different source organisms can exhibit very different stabilities in scC02 [12]. They oxidized cholesterol according to Scheme 4.9-3 and noted that cholesterol oxidase from Gloeocysticum retained its activity for 3 days, whereas the enzyme from Streptomyces sp. lost its activity in about one hour. Lozano et ul. noted an interesting pressure effect on a-chymotrypsin stability in scCO2. A pressure increase (from 80 to 150 bar) appeared to increase the half-life of immobilized a-chymotrypsin [28].

g R

427

4.9 Enzymatic Catalysis

-

Polyphenol oxidase

+On

$0:

Cop or HCF,

R R = CH3 or CI

oz

-

Polyphenol oxidase Co, or HCF,

R

Scheme 4.9-2 Oxidation of p-cresol and p-chlorophenol to o-benzoquinones in scCOz and in supercritical fluoroform [2].

Scheme 4.9-3 Oxidation of cholesterol to cholest-4-ene-3-onein scCOz [ 121.

The thermal stability of enzymes seems to be generally better in microaqueous media, including SCFs, than in buffered water solutions. Pasta et al. reported increasing reaction rates for subtilisin protease-catalyzed reactions up to 80 "C in supercritical carbon dioxide [29]. Michor found that the residua1 activity of Pseudomonas marginata, Esterase EP 10, was even higher after 21 h in scCO2 at 70°C than initially. In contrast, the enzyme lost 21-39% of it's initial activity in a buffered water solution at 37 "C [30]. A 10 % activity decrease was found for cutinase and for an immobilized lipase during six days in scC02 at about 40°C and 130 bar [31,32]. Zheng and Tsao noted that it is possible to prevent the activity loss upon depressurization if the pressure is decreased gradually [33]. They noted a 10% decay in cellulase activity during 5 days at scC02 at 35°C. No significant loss of activity was found for an immobilized Mucor miehei lipase during 64 h in scCO2 [lo]. Habulin et al. kept an immobilized Rhizomucor miehei lipase in a continuous SCF reactor for one month. The conversion of their esterification reaction decreased from approximately 70% to about 66% during that time [34]. Dumont et al. reported that their Mucor miehei lipase retained 96% of its original activity after 6 days in scC02 [35]. Yu et al. measured the stability of Candida cylindracea lipase and compared it with the previously reported stability data of Mucor miehei lipase [32]. In scC02 at 136 bar and 40"C,both lipases lost about 10% of their activity in 4 days [36]. As one might expect, water has a dramatic effect on enzyme stability in SCFs. Lozano et al. found that the half-life time of a-chymotrypsin decreased exponentially in scC02 with increasing water content from 0 to 15 wt% [28]. Kashe et al. found that a-chymotrypsin, trypsin and penicillin amidase partially unfolded during pressure reduction in humid scC02. They suggested that

428

4 Reactions in SCF

enzyme denaturation occurs when COz is rapidly released from the enzymebound water during depressurization [37]. Yang and Yang report similar findings for porcine pancreatic lipase, a-amylase and glucoamylase [38]. Enzymes with disulfide bridges are more resistant to unfolding during pressure reductions. Chulalaksananukul et al. measured the residual activity of a lipase from Mucor miehei after one day in scCOz at 40-100 "C at various water concentrations. As the temperature rises, the enzyme molecule at first unfolds reversibly and then undergoes one or more of the following reactions: formation of incorrect or scrambled structures, cleavage of disulfide bonds, deamination of trypsine residues, and hydrolysis of peptide bonds. Each process requires water and is therefore accelerated with increadng water concentration [ 191. The role of water on the performance of enzymes in SCFs is described in more detail in Section 4.9.4.3 Although many enzymes are stable in supercritical fluids one should pay considerable attention to finding the correct reaction conditions for each substrate/enzyme/SCF system. Very sharp optima for water content and pressure have been found for some enzymes in scCOz and they can be easily missed. Although successful reactions have been reported with subtilisin Carlsberg protease and Candida lipases in scCOz, there is also evidence for their instability [18,20,25] or the existence of a narrow pressure range of activity [23,24]. These enzymes are fairly stable in other SCFs such as fluoroform, ethylene, ethane, propane, and sulfur hexafluoride [ 181. Immobilized Mucor miehei lipase appears to be very stable in scCOz. Mucor miehei lipase is a monomeric enzyme with three stabilizing disulfide bonds [39] which may play a role in maintaining its activity in SCCOZ. Enzyme stability generally decreases with increasing water concentration, whereas activity requires some water to be present. Therefore, the water content has to be optimized to find the best balance between enzyme life and activity.

4.9.4.3

The Role of Water

Enzymes are not catalytically active if water is completely absent. The often cited explanation is that at least a monolayer of water per enzyme molecule is necessary to keep the enzyme active [40]. Apparently, the essential noncovalently bound water maintains the enzyme's native protein structure. In an enzymatic reaction under supercritical conditions, the water partitions between the enzyme, the enzyme support and the reaction mixture. In an essentially nonaqueous system, the existing water partitions preferrably to the solvent with increasing hydrophilicity. If there is little water in the system and if the solvent is relatively hydrophilic, the solvent may strip the essential water from the enzyme, making it inactive. When Zaks and Klibanov first noted that enzymes were more active in hydrophobic solvents than in hydrophilic organic solvents,

'

4.9 Enzymatic Catalysis

429

they proposed that the differences in water partitioning were responsible for this effect. Experiments with enzymes in SCFs revealed very early that scCO2 could strip the essential water from the enzyme. Randolph et al. found that damp immobilized enzyme lost its activity when exposed to bone-dry carbon dioxide. The activity was quickly regained when they injected a small amount of water into the system [12]. Dumont also reported results from an immobilized lipase/C02 system where the conversion rate decreased when the enzyme was in contact with a dry C02/substrate flow. Reaction rates were restored completely when they directed the C02 flow through a water saturator [ 131. Kamat et al. also support this hypothesis [18]. They compared lipase-catalyzed transesterification rates in supercritical carbon dioxide, fluoroform, ethylene, ethane, propane, and sulfur hexafluoride as well as in several conventional liquid solvents of different polarities. The reaction rates increased with increasing hydrophobicity of solvent within the SCFs and also within the liquid solvent group. Because the solvent’s immiscibility with water and its apolarity, by themselves, are irrelevant to enzymatic activity [8], it appears that the activity loss is the result of the enzyme losing essential water. Although scCO2 is generally considered to be a hydrophobic solvent, it is more hydrophilic than fluoroform or hexane and capable of stripping essential water from the enzyme in an essentially nonaqueous environment. In aqueous environments, enzymatic activity is sensitive to the pH of the bulk solution. One may therefore suspect that the scCO2, which is dissolved in the microwater layer of an enzyme in an essentially nonaqueous system, would change the pH of that layer and affect enzyme activity. Kamat et al. clearly showed that this effect is negligible [5]. Increasing the C02 pressure by a factor of 100 decreases the pH of bulk water by only one unit in an unbuffered system. Enzymes are normally lyophilized from a buffered solution prior to their use as catalysts in SCFs. In lyophilization, the enzyme is first dissolved in water where the pH is adjusted for maximum enzymatic activity. The enzymehuffer salt solution is then freeze-dried under vacuum to remove almost all of the water. In a typical phosphate buffer solution, the pH may be 7.8. Kamat et al. calculated that at 100 bar COz pressure the new pH of the buffer solution would be 7.75, and at 1010 bar the pH would be 7.66. Lyophilization increases the buffer salt concentration in the residual water in the enzyme considerably. Thus, the effect of C02 on the pH of the remaining microaqueous layer in enzymes becomes even smaller. The water content has probably the strongest effect in reaction rates of all process parameters of the supercritical reaction system. The water concentrations in the fluid phase and in the enzyme support phase depend on the adsorption isotherms between the fluid and solid phases. The form of the adsorption isotherm depends on the fluid, the enzyme support material, and on the substrates in the reaction mixture. Water partitioning data for SCFs and enzyme supports are available from several papers.

430

4 Reactions in SCF

For example, Yoon et al. report that the water adsorption isotherm for an immobilized lipase (Lipozyme IM) and pure scCOz follows eq (4.9-1) where w is the water concentration in the enzyme support (wt%), C is the water concentration in COz (mM) and C, is the water solubility in COz (mM) under system pressure and temperature [41]. Marty et al. report water adsorption isotherms (33 "C, 40 "C, 50 "C) between a microporous anionic resin support and ethanol containing SCCOZat three pressures (110, 130 and 170 bar) [9]. Water partitioning data for an anionic resin support and pure scC02 are also available [42,43].

w = 14.4 (C / C0)0.753:

(4.9-1)

A controlled amount of water may be maintained in the reaction system in several ways. In a batch reactor the water may be weighed directly in the reactor initially with substrates and enzyme [41] or injected into the reactor with the solvent SCF [22]. Many groups have used a saturator placed upstream of flow reactors [13,14]. Water may also be pumped into the fluid continuously with substrates [15,16,44]. A compilation of reported optimum water concentrations for maximum enzymatic reaction rates in scCOz is given in Table 4.9-4. When the solubility limit of water in the fluid is approached or exceeded, the reaction rates drop drastically [13,21]. The liquid water in the enzyme particles apparently forms a mass-transfer barrier. It has also been noted that enzyme particles tend to stick together when the optimum water concentration is exceeded [45]. Obviously this reduces the enzyme area that is accessible to substrates. In contrast, Chi et al. did not find an optimum water content in lipasecatalyzed hydrolysis or transesterification in C02 [46]. They put up to 20 wt% water in the enzyme carrier without encountering an optimum. Nakamura Table 4.9-4 Optimum water concentrations for enzymatic reactions in SCFs. Reaction

Enzyme support

Optimum water concentration Reference in SCF

Esterification Alcoholysis Hydrolysis Transesterification Transesterification Esterification Esterification Esterification Esterification Transesterification Esterification

0.7 m L L anionic resin anionic resin anionic resin anionic resin anionic resin anionic resin free enzyme hydrophilic gel, Sephadex hydrophilic gel, Bio-gel P-6 anionic resin 4-12 mM anionic resin 0.13 wt%

in solid (wt%)

10 >3 >2 5 10 9 22 20-25

4.9 Enzymatic Catalysis

43 1

reports 4-12 mM as optimum water concentrations in scCOz for hydrolysis and acidolysis reactions [47]. . Halling suggested that the thermodynamic activity of water rather than water concentration is the key parameter [48]. Activity is related to concentration according to

a, =

YXW

(4.9-2)

where a, is the water activity, y is the activity coefficient and x, is the water concentration. Water activity should be considered separately for the fluid and the solid phase. At equilibrium, the water activities in the fluid and solid phase are equal. Although the activity coefficients of water in organic solvents at atmospheric pressure can be estimated, Condoret et al. suggest that conventional methods are not satisfactory for SCFs. They propose a simplified method which has been described and successfully tested [43]. Inorganic salt hydrates can be used to maintain controlled water activities in the enzyme support and in the fluid phase. Chaudhary et al. used sodium pyrophosphate (Na4P207 X 10H20) in fluoroform to adjust the water activity and to maximize the enzyme activity. They put the salt hydrate inside the batch reactor together with substrates and enzyme. They observed that the salt concentration had an effect on reaction rates, and therefore had to be optimized [49]. Condoret et al. [43] and Sereti ef al. [31] equilibrated the enzyme and the substrates with saturated salt solutions prior to reaction experiments. They placed salt solutions of known water activity in a sealed container with substrates and enzyme. The equilibration took place in the vapor phase for 24-48 h. The optimum water activity for cutinase-catalyzed esterification in scC02 was found to be 0.8. The water activity of pre-equilibrated substrates and enzymes may change when they are put in contact with the SCF; for example, scC02 may take up water from the enzyme. Furthermore, if water is consumed or produced in the reaction, the water activity of the reaction system will change. Putting salt hydrates in contact with the reaction mixture should buffer the water activity. Suitable pairs of solid salt hydrates may be used to control the water activity more accurately. Examples of salt hydrates and saturated solutions are given in Table 4.9-5. 4.9.4.4

Pressure Effects

Apart from the direct conformational changes in enzymes, which may occur at very high pressures, pressure affects enzymatic reaction rates in SCFs in two ways. First, the reaction rate constant changes with pressure according to transition stage theory and standard thermodynamics. Theoretically, one can predict the effect of pressure on reaction rate if the reaction mechanism, the activation volumes and the compressibility factors are known. Second, the reaction rates may change with the density of SCFs because physical parameters, such

432

4 Reactions in SCF

Table 4.9-5 Water activities of selected salt hydrates and saturated salt solutions [31,43]. Medium

Water activity a,

Solid salt hydrates Na2HP04 x 7H20/Na2HP04x 2H20 Na&2' 07 X 10H20/Na&07 NaBr x 2H20/NaBr Na+(CH3COO)-x 3H20/Na+(CH3C00)Na2HP04 X 2H20/Na2HP04

0,73 0.59 0.43 0.35

Saturated solutions KZCr207 K2S04 at 0 C KCI NaCl Mg(NO& at 0°C Mg(N03)2 6H20 Mg(NO& at 25°C MgClz x 6H20 MgC12 LiCl

0.21

0.98 0.97 0.85 0.75

0.61 0.53 0.53 0.33 0.32 0.11

as the Hildebrandt solubility parameter and the dielectric constant, change with density. These changes may indirectly influence enzyme activity. In conventional solvents the changes in dielectric constant have a significant effect on protein flexibility. The dielectric constant of supercritical fluoroform, for example, changes from a value of 1.5 to 8 (which resembles the value for liquid methylene chloride) when the pressure increases from 55 to 276 bar. A condensed treatment of the effect of pressure on the physical parameters of SCFs is available in the excellent review by Kamat et al. [5] and details are also found in Chapters 1.1 and 1.2. Theoretical predictions are, however, difficult because the activation volumes of reaction steps and the compressibilities of SCFs change with pressure. A further complication is that, by changing pressure, one simultaneously changes the density-dependent physical parameters of the supercritical fluid. Effects of mass transfer are also always present to some extent. Therefore, only apparent activation volumes have been measured for enzymatic reactions in SCFs. The reaction mechanisms of enzymatically catalyzed reactions are often not known. Kamat et al. found that the initial transesterification rate with Candida cylindracea lipase decreased markedly upon increasing the pressure from 80 to 120 bar [5]. They ran the reaction in fluoroform at 50°C and calculated the apparent activation volume of the reaction from initial reaction rates at different pressures. The apparent activation volume showed a maximum near the critical point of fluoroform. As the pressure increased from 60 to 180 bar the apparent activation volume approached zero and the reaction rate decreased to one tenth.

4.9 Enzymatic Catalysis

433

They also obtained data indicating that in the Candida cylindracea lipasefluoroform system the enzyme activity depends on fluoroform’s dielectric constant. As the pressure rises from 59 bar to 280 bar the dielectric constant of fluoroform increases from 1 to 8 and the initial reaction rate decreases rapidly

POI.

However, Vermue et al. found that pressure-induced changes in Hildebrandt solubility parameter had only a marginal effect on Mucor miehei lipase-catalyzed transesterification rates in scCO2 [44]. Yoon ef al. noted similar nonsensitivity to pressure in their transesterification study of Mucor miehei lipase in SCCOZ[41]. Thus, there is ,experimentalevidence that changing pressure actually changes enzymatic reaction rates at constant substrate concentrations (moVvolume). If, in a constant volume reactor, the substrate concentration (moVvolume) is kept constant and pressure rises, the mole fraction of substrates decreases. In almost all reported cases the pressure increase parallels the reaction rate decrease and the moVmol concentration decrease. Erickson et al. studied the effects of pressure in a lipase-catalyzed transesterification using the same mol per volume substrate concentration at all fluid densities. Because they obtained a similar pressure effect in C02 and in ethane, they concluded that the changes in reaction rate are due to changes in the supercritical fluid phase and not in the immobilized enzyme phase, such as pH changes. They found that the decrease of rate with increasing pressure could be modeled accurately by using the mole fractions of the substrates instead of moVvolume concentrations in the rate law [51]. Saito et al. made the same observation; the reaction rate decreased with the decrease of substrate mole fraction due to increasing pressure and density of C02 [52]. Investigations of Mucor miehei lipase-catalyzed esterifications in scC02 showed that a pressure increase from 100 to 250 bar reduced the initial reaction rates from 23 to 8 pmol per gram enzyme and per hour, parallelling the decreasing mole fraction of substrates (Scheme 4.9-1) [22]. Miller et al. report a decrease of pseudo-first-order rate constants with incresing pressure. They examined a Rhizopus arrhizius lipase-catalyzed transesterification in scC02 [14]. Barreiros et al. similarly report decreasing catalytic efficiency of subtilisin Carlsberg in compressed propane, supercritical ethane and scC02 with increasing pressure [53]. Steytler appears to be the only author who reports increasing reaction rates with increasing pressure at constant substrate concentration (moVvolume). They obtained higher conversion rates with increasing pressure from 150 bars to 300 bar in a Candida lipase B-catalyzed esterification in C02 [54]. Further pressure increase to 500 bar did not result in any further rate increase. They speculate that the lower reaction rate at 150 bar may be due to adsorption of the ester on the enzyme bed.

434

4 Reactions in SCF

4.9.4.5

Mass Transfer Effects

Two types of mass transfer can be distinguished for catalysis with heterogeneous catalyst particles. External mass transfer refers to molecular transport between the bulk reaction mixture and the surface of the enzyme particle through a boundary layer. Internal ma$s transfer is the molecular transport inside the solid enzyme phase. Internal mass transfer occurs within the pores of the catalyst particle to and from the particle surface. Figure 4.9-4illustrates the definitions of external and internal mass transfer. The possible effect of external mass transfer has been investigated by changing flow rates through the enzyme .bed in a flow reactor and by changing agitator speed in a batch reactor. The external mass transfer rate is inversely proportional to the thickness of the stagnant fluid film, the boundary layer, on the enzyme particle surface. As the flow rate of the fluid passing the enzyme particle is increased, the boundary layer becomes thinner and the concentration of substrates on the enzyme surface increases, leading to an increase in reaction rates. If this is observed, the enzymatic reaction rate is limited by external mass transfer. External mass transfer between the SCF phase and the enzyme particle surface is usually not rate limiting. External diffusion Fluid flow

....... ; ; ;B

(-

Substrate

Internal diusion

/

\

Figure 4.9-4 External and internal mass transfer on a heterogeneous enzyme catalyst.

4.9 Enzymatic Catalysis

I

435

Miller et al. studied external mass transfer by using three reactors of different lengths to maintain constant residence time (42 s) while the velocity was .varied [14]. They varied the superficial velocity from 0.6 to 2.4 m d s through a packing where enzymes were immobilized on porous glass beads. The observed interesterification rate. did not change with flow rate, indicating that there were no external mass transfer limitations. Randolph et al. varied the flow rate of cholesterol-saturated scCOz through a constant length enzyme reactor which was packed with cholesterol oxidase on glass beads [12]. They obtained a linear correlation between a 4.6-fold increase in flow rate and enzymatic oxidation rate (mom), and concluded that there was no mass transfer limitation. Van Ejs et al. also obtained a linear correlation between flow rates (4-15 mm/s) and enzymatic esterification rates using a lipase immobilized on an anionic resin [42]. They concluded that the reaction was limited by external mass transfer. The contradictory conclusions of Randolph and Van Ejs from the same experimental observations shows that chemical kinetics cannot be excluded from mass transfer studies. The oxidation reaction was apparently so fast, or the enzyme column was so long, that the reduced residence time due to increased flow rate in a constant-length enzyme column did not decrease conversion. The esterification reaction was apparently slower, so that the conversion decreased due to reduced residence time. A considerable reaction rate increase with increasing flow rate in a recirculating batch reactor has been observed [lo]. The correlation was not linear. The production rate was defined as millimoles product obtained per kilogram enzyme per hour contact time in the enzyme bed with the same enzyme as used by van Ejs et al. [42]. Much higher flow rates were applied, namely 50-300 m d s , for which the pressure drop along the enzyme column was considerable. Pressure drops were of the order of 20 bar. It was speculated that part of the flow is forced through the pores of the enzyme support at high flow rates and pressure drops. More active surface becomes available to the substrate and the reaction rate per kg enzyme increases. Batch reactor studies with different agitator speeds have shown that there was no evidence for external mass transfer limitation on reaction rates in the examined cases when the agitator speed was beyond 200 rev/min [9,19,21,55]. Internal mass transfer occurs within the pores of the enzyme and enzyme support. Internal diffusion becomes rate limiting at some point when reaction rate increases andor diffusion length (i.e. particle diameter) grows. Possible internal mass transfer limitations can be investigated comparing overall reaction rates with enzyme particles of different sizes. Steytler et al. studied this using different-sized glass beads [54]. They coated the same amount of enzyme on 1 mm and 3 mm diameter glass beads. When the enzyme column was packed with the same volume of beads of different diameters, the surface area of the smaller beads was nine times the area of the larger beads. The esterification rates in C 0 2 were the same for both sizes of beads. As the amount of enzyme in the reactor was the same with both bead sizes, there was apparently no internal diffusion limitation. Bernard et al. measured esterification rates with different-sized immobilized lipase particles, which they ob-

436

4 Reactions in SCF

tained by sieving a commercial enzyme product. They used particle sizes from 180 to 460 pm and found that the esterification rate decreased to about one half with increasing particle size [55,56]. The Thiele modulus, calculated from reaction rate data, diffusion length, and diffusion coefficient, can be used to evaluate internal mass transfer effects. Expressed with the Michaelis constant and maximum reaction velocity, the Thiele modulus c$ is given by

(4.9-3) where R is the radius of the enzyme particle, V,,, is the maximum reaction velocity, D A B is the substrate diffusivity in the pores, and K , is the Michaelis constant. Kamat et al. have calculated the dependence of the Thiele modulus on particle size and enzyme activity in scCOz [57]. For a reaction that obeys Michaelis-Menten kinetics, any combination of particle radius and V,,,/K, that gives a Thiele modulus greater than about 1 will be limited by internal mass transfer. The authors provided graphs for determining whether the specific combination of particle size and enzyme activity will result in internal or external diffusional limitations. Erickson et al. [51], Bernard and co-workers [55,56], and Miller et al. [14] studied diffusion limitations by calculating the Thiele modulus from reaction rate data obtained for lipases in scC02. The calculated Thiele moduli ranged from to 0.92, indicating either no or intermediate internal .mass transfer control. The strongest est effect of internal mass transfer was observed for esterification in scC02 with a Mucor miehei lipase immobilized on the largest examined anionic resin beads of 460 pm diameter [%I.

4.9.4.6 Effects on Selectivity Chaudhary et al. found that the pressure of fluoroform had an effect on the substrate specificity of subtilisin Carlsberg protease in transesterifications. Specificity towards methanol over propanol increased considerably with a pressure increase from 75 to 200 bar. This is clear evidence that nucleophilic specificity can be altered by changing the SCF conditions [49]. Ikushima et al. studied the pressure and temperature effects on the stereoselectivity of (+)-Citronello1 esterification at the initial stage of reaction (up to 5.8% yield; Scheme 4.9-4). Generally, the optical purity of the product decreased with increasing reaction rate and increasing yield. They found a spectacular temperature effect for the Candida cylindracea-catalyzed esterification. At constant pressure (84.1 bar) and constant yield (3.4-3.6%), a temperature decrease of only 4°C caused the optical purity (enantiomeric excess, ee%) to increase 'from a mere 4.1 % to almost complete stereoselectivity, 98.9 % [59]. In later papers they report a strong but slighly less dramatic tem-

4.9 Enzymatic Catalysis

437

Oleic acid

co,

Scheme 4.9-4 (*)-Citronello1 esterification with oleic acid and Candida cylindracea lipase in SCCOZ[59].

I

Candida cylindracea lipase

(S)-(-)-oleic acid 3,7-dimethyl-B-octenyl ester in max. 99 % optical purity

perature effect [24,60]. Pressure had also a strong effect and it appears to have an optimum, with almost complete stereoselectivity around 84 bar. They suggested that the clustering of C 0 2 around the enzyme made the enzyme stereoselective. In contrast, the reaction was completely nonstereoselective in watersaturated cyclohexane. An immobilized Mucor miehei lipase has been used in stereospecific esterification [lo, 221 (see Scheme 4.9-1) and hydrolysis [45] (Scheme 4.9-5) experiments in scC02. The reaction was never completely stereoselective. Even at low conversion levels, a fraction of the less-reacting stereoisomer was participating in the reaction. The water concentration, substrate concentrations, pressure and temperature were varied. Only substrate concentration and temperature had an effect on reaction selectivity. Maximum stereoselectivity was obtained at the lowest tested temperature of 35°C. The highest enantiomeric excess was 87 %, obtained for the hydrolysis of 3-(4-methoxyphenyl)glycidic ester at 53 % total conversion. Because enzymatic reactions are usually not completely selective, it is important to note that end-point analysis of the reaction mixture does not reveal enzyme selectivity if the reaction has proceeded to high conversion. The selectivity can only be determined by analyzing the concentrations of the products as the conversion of substrates proceeds. An example is given in Figure 4.9-5.

4 Reactions in SCF

438

8 -

CH30

(~S,3R)-methylmethoxyphenyl glycidate

(PS,BR)methyl methoxyphenyl glycidic acid in max. 87% ee

Scheme 4.9-5 Enantioselective hydrolysis of racemic 3-(4-methoxyphenyl)glycidic methyl ester in SCCOZ1451.

(PR,3S)-methyl methoxyphenyl glycidate

ee, %

75

25

c

7

t, Figure 4.9-5 Enantiomeric excess and conversion (for the reaction shown in Scheme 4.9-1) [22].

25

50

75

Conversion, %

Martins et al. reported 100 % enantiomeric purity for the first few percent of the conversion in esterifying glycidol according to Scheme 4.9-6 [61]. At 25-30 % conversion the ee decreased to 83 96. They used porcine pancreatic lipase in scC02. Cernia e f al. studied the stereoselectivity of esterifications of five racemic secondary alcohols with acetic anhydride in scCO2. They used an immobilized crude lipase of Pseudomonas sp. and obtained ee values from 76 96-90 9% at conversions ranging from 23 % to 53 % [62]. Later [63] they report 100 % ee for (+)-1-phenylethanol transesterification with vinyl acetate in scC02. They used recombinant lipases for catalyzing the reactions and obtained complete selectivity at least up to 32% conversion. Michor et al. investigated four lipases and an esterase in the stereospecific transesterification of racemic (+)-menthol and (+)-citronello1 in scCOz. The transesterification of racemic menthol was reasonably fast and gave high

4.9 Enzymatic Catalysis

439

- H/c\/.

cJ-4 Porcine pancreatic lipase

(R)glycidol +

LA-

+

HO

coo (S)-glycidyl butyrate

/CHflH

H/c;\JH10

in max. 100% ee '

(S)-glycidol

Scheme 4.9-6 Enantioselective esterification of racemic glycidol in S C C O[61]. ~

enantiomeric excess, whereas the reaction with citronellol was nonstereoselective [30]. Kamat et al. found a pressure effect on the stereoselectivity of transesterifications in supercritical fluoroform [64]. The stereoselectivity of subtilisin Carlsberg and Aspergillus proteases increased, while the activity decreased with increasing fluoroform pressure from 66 to 352 bar. They conclude that the stereoselectivity of these proteases increases because fluoroform becomes more hydrophilic as the pressure rises. This finding is in accordance with a previously presented hypothesis for substrate binding in enzymes. Thus, stereoselectivities from 0 % to 100 % have been reported for enzymatic reactions in SCFs. These results show that enzymatic stereoselectivity can be altered by adjusting the properties of the SCF. However, only very qualitative rules exist at present with which to predict how reaction selectivities might change with conditions for a new substrate/enzyme/SCF system.

4.9.5 Downstream Processing and Costs 4.9.5.1 Downstream Processing Schemes The tunability of solvent properties of SCFs in the reaction stage and also in the downstream stage provides possibilities for elegant processing schemes. Imagine a process where the substrates are dissolved in an SCF; after the single-phase reaction mixture is brought in contact with the enzyme catalyst, the pressure of the post-reactor fluid is reduced so that either the less soluble compounds or all solutes precipitate from the SCF. Further fractionation of the solutes can be achieved by extracting the mixture with the same supercritical solvent that was used in the reaction step, but under different pressure and/ or temperature. It may even be possible to crystallize the product from the SCF in the desired particle size and form by rapid expansion of the supercritical solution (RESS). Alternatively, purification of the product may be accom-

HZO

440

4 Reactions in SCF

plished via supercritical fluid chromatography (SFC), possibly using the same SCF as eluent. In an ideal case, the same single-component SCF could be used for dissolving the starting materials, for carrying out the reaction step, and for purifying the final product. If SCF modifiers are used- in any of the processing steps the process becomes much less elegant. The product is then obtained as a solution in an organic solvent and final purification requires conventional evaporation or distillation steps for recovering the product and solvent. The organic solvent has to be recycled separately from the SCF. Marty et al. have reported a complete pilot-plant reactorheparator system for carrying out enzymatic reactions in SCFs. The plant comprises a continuously operated tubular reaction vessel followed by four cyclone separators. The separators may be put in descending pressure order by adjusting needle valves between each vessel. Liquid products are recovered from the bottom of each vessel discontinuously [ 151. Schemes of possible downstream processing steps using SCFs are shown in Figure 4.9-6. Stepwise precipitation

Product 1

Product 2

Fractionating extraction

Product 1

Figure 4.9-6 Downstream processing schemes for enzymatically catalyzed reactions using SCFs as solvents for reaction and purification.

4.9 Enzymatic Catalysis

441

4.9.5.2 Prosessing Cost Estimate .The total production costs of manufacturing 5000 kg of fine chemical per year in one enzymatic reaction step using only scC02 throughout the process has been calculated. The calculations are based on a continuously operating tubular enzyme reactor where 70% of the fluid leaving the reactor is recycled back to feed flow. It was also assumed that the portion of the reaction mixture that is withdrawn from the reactor is completely precipitated from COz and then fractionated and purified in a two-stage SFE process [4]. The production costs are sensitive to the concentration of substrates in the SCF, to enzyme lifetime and, most notably, to the productivity of the enzyme catalyst. As is evident from Table 4.9-3, not even the order of costs for a new substrate/enzyme/SCF system can be predicted from published data. Experimental optimization of parameters is necessary in each case prior to evaluating the production costs. Pressure variation (70-300 bar) or temperature variation (35-60°C) do not have a strong effect on the production cost. The costs of a specific example are presented, together with a set of assumptions, in Table 4.9-6. The processing costs are within the range for producing fine chemicals, most probably pharmaceuticals or their intermediates.

4.9.6

Summary and Outlook

The experimental data that have accumulated since ca. 1992 have greatly increased the understanding of the interactions between enzymes and SCFs. It is now known that enzymatic reaction rates, substrate selectivity and enantiomeric selectivity can be tailored by altering the type and the density of the SCF. Although C 0 2 would be the most desirable industrial SCF as reaction medium, it is not suitable for all enzymes. It is an inhibitor for some proteases and lipases [20] but there are other common enzymes, including lipases, which are stable and active in scC02. Supercritical hydrocarbons and fluoroform are good and tunable solvents for enzymatic reactions. Heterogeneous enzymatic reactions in liquids are often limited by diffusion rates, but there are usually no diffusion limitations in SCFs. Consequently, many enzymatic reactions are considerably faster in SCFs than in liquids. The selectivity of enzymes, especially enantiomeric selectivity, can be fully utilized in SCFs. High selectivity can usually be obtained at slightly elevated temperatures when conditions are optimized and conversion is limited to less than the maximum achievable level. The use of extremely low (down to -90 "C) temperatures, often needed for high selectivity in conventional catalysis, may thus be avoided. The simplicity and cost efficiency of downstream processing is a major benefit of enzymatic synthesis in SCFs, compared with enzymatic reactions in aqueous or organic solutions.

442

4 Reactions in

SCF

Table 4.9-6 Production cost estimate for a one-step enzymatic conversion in scCOz. Assumptions Production rate Enzyme productivity Running time Enzyme replacements Solutes concentration in C 0 2 Conversion per pass: Recycle: Total yield of product:

5000 kglyear

150 6000 20 3 10 70 80

g product/(kg enzyme hourslyear batcheslyear wt% %

X

h)

% % of theoretical

Resulting process Required amount of enzyme in reactor Volume of reactor tubes COz flowrate in reactor COz flowrate in extraction

6 kg 15 liters 232 kghour 69 kghour

Equipment cost Reactor module Extraction module Total equipment COz make-up Enzyme cost Electricity Manpower

salary side costs overhead

Insurances Service, repairs Waste treatment

Capital cost

0.7 MEUR 0.3 MEUR

-

1.0 MEUR Production costs 0.5 500 0.08 2000 21 60 0.6 2 1

kEUWyear

6 20 25 3

EUR/kg EUFUkg EURlkWh EUWmonth

kgh batcheslyear kW shiftslday

% % % of equipmenuyear % of equipmenuyear

EURlkg

1.3 k g h

Total operating cost

kEUWyear EURlkg product

7 % interest of equipment Total production cost

1 EUR E 1.1 USD

18 60 12 72 15 52 6 20 8 263 53 70

kEUWyear 333 EURkg product 67

4.9 Enzymatic Catalysis

443

Even in SCFs, the enzymatic reaction rates are low compared with inorganic catalysis. Solute concentrations in SCFs also remain lower than in organic solvents. It is thought that enzymatic reactions in SCFs might be commercially utilized in fine chemical synthesis where the high specificity of enzymatic catalysis is required and where the production is limited to a few tons per year. Pharmaceutical producers are the most probable users of the technique. Reactions involving ester .bonds have been extensively explored using lipases as catalysts in SCFs. However, enzymes catalyze a variety of reactions and synthetically much more useful reactions may be found. There are only two papers describing enzymatic reactions in SCFs which involve gases as substrates (Table 4.9-2). Enzymatic oxidations with molecular oxygen are clearly an interesting application because of the miscibility of oxygen with SCFs (see Schemes 4.9-2 and 4.9-3). Considering the good miscibility of ordinary gases in SCFs, researchers may find interesting applications and great benefits with other gases as well. Another unexplored area appears to be the use of SCFs as diluents and viscosity reducers in the enzymatic treatment of viscous substances, such as fats and oils. Micromicelles have been used to overcome the solubility limitation of polar compounds in SCFs (see Chapter 2.4). The technique involves using SCFsoluble surfactants. Using enzymes in micromicelles for processing hydrophilic substrates in SCFs has not yet been extensively explored and holds considerable promise for future developments.

References [ l ] T. W. Randolph, H. W. Blanch, J. M. Prausnitz, C. R. Wilke, Biotechnol. Letters 1985, 7 , 325-328. [2] D. A. Hammond, M. Karel, A. M. Klibanov, V. J. Krukonis, Appl. Biochem. Biotech. 1985, ZI, 393-400. [3] K. Nakamura, Y. M. Chi, Y. Yamada, T. Yano, Chem. Eng. Commun. 1986, 45, 207212. [4] 0. Aaltonen, M. Rantakyla, Chemtech 1991, 21, 240-248. [5] S. V. Kamat, E. J. Beckman, A. J. Russell, Critical Reviews in Biotechnology 1995,15, 41-71. [6] Enzyme Nomenclature Recommendations (1978),Academic Press, London, 1979,3-26. [7] H. Gunnlaugsdottir, Lipase-Catalyzed Lipid Modifications in Supercritical Carbon Dioxide, Dissertation, Lund University, 1997. [8] V. S. Narayan, A. M. Klibanov, Biotechnology and Bioengineering 1993,41, 390-393. [9] A. Marty, W. Chulalaksananukul, R. M. Willemot, J. S. Condoret, Biotechnology and Bioengineering 1992,39, 273-280. [lo] 0 . Aaltonen, M. Rantakyla, Proc. 2nd Int. Symp. Supercrit. Fluids, Boston, 1991, 146149. [ll] H. Gunnlaugsdottir, B. Sivik, J. Am. Oil Chem. SOC.1995, 72, 399-405. [I21 T. W. Randolph, H. W. Blanch, J. M. Prausnitz, AtChE J. 1988,34, 1354-1360. [13] T. Dumont, D. Barth, M. Penut, J. Supercrit. Fluids 1993,6, 85-89. [14] D. A. Miller, H. W. Blanch, J. M. Prausnitz, Ind. Eng. Chem. Res. 1991,30, 939-946.

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4 Reactions in SCF

[15] A. Marty, D. Combes, J.-S. Condoret, Biotechnology and Bioengineering 1994, 43, 497504. [16] Z. Knez, V. Rizner, M. Habulin, D. Bauman, J. Am. Oil Chemists SOC.1995, 72, 13451349. [17] M. A. Jackson, J. W. King, J. Am. Oil Chemists SOC.1996, 73, 353-356. [18] S. Kamat, J. Barrera, E. J. Beckman, A. J. Russell, Biptechnology and Bioengineering 1992, 40, 158-166. [19] W. Chulalaksananukul, J.-S. Condoret, D. Combes, Enzyme Microb. Technol. 1993, 15, 691-698. [20] S . Kamat, G.Critchley, E. J. Beckman, A. J. Russell, Biotechnology and Bioengineering 1995, 46, 610-620. [21] T. Dumont, D. Barth, C. Corbier, G.Branlant, M. Permt, Biotechnology and Bioengineering 1992, 39, 329-333. [22] M. Rantakyla, 0. Aaltonen, Biotechnoi. Letters 1994, 16, 825-830. [23] Y. Ikushima, N. Saito, M. Arai, H. W. Blanch, J. Phys. Chem. 1995, 99, 8941-8944. [24] Y. Ikushima, M. Saito, K. Hatakeda, 0. Sato, Chem. Eng. Sci. 1996, 51, 2817-2822. [25] I. B. de Carvalho, T. C. de Sampaio, S. Barreiros, Proc. 3rd Symp. Supercrit. Fluids, Strasbourg, 1994, 155-160. [26] J. K. Weder, Food Chem. 1984, 175-190. [27] M. Taniguchi, M. Kamihira, T. Kobayashi, Agric. Biol. Chem. 1987, 51, 593-594. [28] P. Lozano, A. Avellaneda, R. Pascual, J. Iborra, Biotechnol. Letters 1996, 18, 13451350. [29] P. Pasta, G. Mazzola, G. Carrea, S . Riva, Biotechnol. Letters 1989, 11, 643-648. [30] H. Michor, R. M m , T. Gamse, T. Schilling, E. Klingsbichel, H. Schwab, Biotechnol. Letters 1996, 18, 79-84. [31] V. Sereti, H. Stamatis, F. N. Kolisis, Biotechnol. Techniques 1997, 11, 661-665. [32] A. Marty, W. Chulalaksananukul, J. S. Condoret, R. M. Willemot, G.Durand, Biotechnol. Letters 1990, 12, 11-16. [33] Y. Zheng, G.T. Tsao, Biotechnol. Letters 1996, 18, 451-454. [34] M. Habulin, V. Krmelj, Z. Knez, Proc. High Pressure Chemical Engineering, Ph. Rudolf von Rohr, Ch. Trepp (Eds.), Elsevier, Amsterdam, 1996, 85-90. [35] T. Dumont, D. Barth, M. Permt, Absrr. Handbook 2nd Int. Symp. High Pressure Chem. Eng., Erlangen, 1990, 65-70. [36] Z.-R. Yu, S. Rizvi, J. A. Zollweg, Biotechnol. Prog. 1992, 8, 508-513. [37] V. Kasche. R. Schlothauer, G. Brunner, Biotechnol. Letters 1988, 10, 569-574. [38] J. C. Yang, X . M. Yang, Proc. 4th Int. Symp. Supercrit. Fluids, Sendai, 1997, 139-141. [39] B. H. Jensen, D. R. Galluzzo, R. G.Jensen, Lipids 1987, 22, 559-565. 1401 A. Zaks, A. M. Klibanov, J. Biol. Chem. 1988, 263, 3194-3201. [41] S.-H. Yoon, 0. Miyawaki, K.-W. Park, K. Nakamura, J. Ferment. Bioeng. 1996, 82, 334-340. [42] A. M. M. van Eijs, J. P. L. de Jong, H. J. Doddema, D. R. Lindeboom, Proc. Int. Symp. Supercrit. Fluids, Nice, 1988, 933-942. [43] J. S . Condoret, S. Vankan, X. Joulia, A. Marty, Chem. Eng. Sci. 1997, 52, 213-220. [44] M. H. Vermue, J. Tramper, J. P. J. de Jong, W. H. M. Oostrom, Enzyme Microb. Technol. 1992, 14, 649-654. [45] M. Rantakyla, M. Alkio, 0. Aaltonen, Biotechnol. Letters 1996, 18, 1089-1094. [46] Y. M. Chi, K. Nakamura, T. Yano, Agric. Bid. Chem. 1988, 52, 1541-1550. [47] K. Nakamura, in Supercritical Fluid Technology in Oil and Lipid Chemistry, J. W . King, G. R. List (Eds.), AOCS Press, 1996, 306-320. [48] P. J. Halling, Enzyme Microb. Technol. 1994, 16, 178-206. [49] A. K. Chaudhary, S. V. Kamat, E. J. Beckman, D. Nurok, R. M. Kleyle, P. Hajdu, A. J. Russell, J. Am. Chem. SOC.1996, 118, 12891-12901. [50] S. J. Kamat, B. Iwaskewyck, E. J. Beckman, A. J. Russell, Proc. Natl. Acad. Sci. USA 1993, 90, 2940-2944.

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445

[51] J. C. Erickson, P. Schyns, C. L. Cooney, AZChE J. 1990, 36, 299-301. [52] N. Saito, 0. Sato, Y. Ikushima, K. Hatakeda, S. Ito, Proc. 3rd Symp. Supercrit. Fluids, Strasbourg, 1994, 179-184. - [53] S . Barreiros, N. Fontes, M. E. Elvas, T. C. de Sampaio, Proc. 4th Znt. Symp. Supercrit. Fluids, Sendai, 1997, 131-133. [54] D. C. Steytler, P. S. Moulson, J. Reynolds, Enzyme Microb. Technol. 1991, 13, 221-226. [55] P. Bernard, D. Barth, Biocatalysis and Biotransformation 1995, 12, 299-308. [56] P. Bernard, D. Barth, M. Perrut, High Pressure Biotechnol. 1992, 224, 451-455. [57] S. Kamat, E. J. Beckman, A. J. Russell, Enzyme Microb. Technol. 1992, 14,265-271. [58] M. Kamihira, M. Taniguchi, T. Kobayashi, Agric. Biol. Chem. 1987, 51, 407-412. [59] Y. Ikushima, N. Saito, T. Yokoyama, Chem. Letters 1993, 109-112. [60] Y. Ikushima, Adv. Colloid Zntegace Sci. 1997, 71-72, 259-280. [61] J. F. Martins, I. B. de Carvalho, T. C. de Sampaio, S . Barreiros, Enzyme Microb. Technol. 1994, 16, 785-790. [62] E. Cernia, C. Palocci, F. Gasparrini, D. Misiti, N. Fagnano, J. Mol. Catalysis 1994, 89, L11-L18. [63] E. Cernia, C. Palocci, S. Soro, Proc. 5th Meeting Supercrit. Fluids, Nice, 1998, 921924. [64] S. V. Kamat, E. J. Beckman, A. J. Russell, J. Am. Chem. SOC. 1993, 115, 8845-8846. [65] H. Noritomi, M. Miyata, S. Kato, K. Nagahama, Biotechnol. Letters 1995, 17, 13231328. [66] J. F. Martins, T. C. Sampaio, I. B. Carvalho, M. Nunes da Ponte, S. Barreiros, High Pressure Biotechnol., 1992, 224, 411-415. [67] M. Caussette, A. Marty, D. Combes, J. Chem. Tech. Biotechnol. 1997, 68, 257-262. [68] H. Michor, T. Gamse, R. Marr, Chemie Zngenieur Technik 1997, 69, 690-694. [69] M. A. Jackson, J. W. King, G. R. List, W. E. Neff, J. Am. Oil Chem. SOC. 1997, 74,635639. [70] H. Gunnlaugsdottir, M. Jaemo, B. Sivik, J. Supecrit. Fluids 1998, 12, 85-93. [71] J. C. Erickson, Thesis, Massachusetts Institute of Technology, 1988. [72] A. K. Chaudhary, E. J. Beckman, A. J. Russell, J. Am. Chem. SOC. 1995, 117,37283733. [73] T. W. Randolph, D. S. Clark, H. W. Blanch, J. M. Prausnitz, Science 1988, 238, 387390.

Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

4.10 Phase Transfer and Ammonium Salt Catalyzed Reactions CHARLES A. ECKERT, CHARLES L. LIOTTA,CHRISTY W. CULP, and DAVIDR. LAMB

4.10.1 Introduction Phase transfer catalysis (PTC) is an important industrial method for carrying out reactions between two or more reactants which would normally be immiscible [l-61. Traditionally, polar aprotic solvents have been used to dissolve the reactants into a single phase where reactions may be carried out homogeneously. However, these solvents are frequently expensive, environmentally undesirable, and difficult to remove from the reaction products. PTC eliminates the need for these solvents and replaces them with less exotic media. With a less polar organic solvent, there are two phases present and a phase transfer catalyst is employed to transfer one of the reacting species from one phase into a second phase where reaction can occur. The special properties of supercritical fluid (SCF) solvents [7,8]for PTC reactions bring substantial environmental and economic advantages. First, they permit the use of totally benign solvents, especially C02, and the solvent separation from product becomes quite facile. Moreover, since PTC processes always involve mass transfer, the lower viscosity and higher diffusivity of SCFs significantly enhance transport. With the recent growth of research in this area, the general understanding of PTC systems is becoming well-defined. One result of the collaboration between chemists and engineers has been the realization that PTC can occur at the interface between a solid and an SCF; this combination of techniques is a new frontier in chemistry and its potential has yet to be realized.

4.10.2 Background Before discussing this new area of chemistry, a brief introduction to traditional PTC systems is presented; a more thorough discussion is available elsewhere [6]. The technique of PTC is a powerful one in which two or more normally immiscible reactants are brought together with the help of a dual action catalyst. In other words, the term “catalyst” in this context is a bit broader than the

447

4.10 Phase Transfer and Ammonium Salt Catalyzed Reactions

1

Figure 4.10-1 Schematic of a general PTC system, where X = C1, Y = CN, M = Na, and Q = N(Cdd4- for the reaction of chloroalkane with NaCN (Q’ X- = quartemary ammonium salt).

I

Interface = U “ l lW l l L W

I

I

traditional definition of a catalyst. In this context the catalyst does not simply provide a lower :energy path along the reaction coordinate. The catalyst creates a new reaction pathway with a lower energy barrier both by transferring a species from one phase to another, and then activating that species for reaction. Figure 4.10-1 shows the general scheme for the operation of a phase transfer catalytic process. The purpose of the catalyst is to ion-pair with the nucleophilic anion and carry the anion into the organic phase, where it reacts with the electrophile to produce the product. In order to understand fully the process it is necessary to discuss the individual steps. Step ( 1 ) Transfer of nucleophilic anion from the aqueous to the organic phase: As organic solvents used in phase transfer catalysis are usually media of low polarity, salts have limited solubilities. As a consequence, anionic nucleophiles must be ion-paired with organophilic cations, such as quaternary ammonium cations, in order to transfer them into the organic phase. This is easily accomplished by the presence of long alkyl chains attached to the quaternary nitrogen. For instance, the tetrabutylammonium cation provides enough organic structure to facilitate the transport of many anionic species into the organic phase. Step ( 2 ) Nucleophilic reaction in the organic phase: The mere transfer of an anion into the organic phase does not necessarily mean that subsequent reaction with an organic electrophile will ensue. The nucleophilic anion must be transferred in an activated state for reaction to take place successfully. To achieve such activation it is necessary for there to be charge separation between the center of positive charge on the quaternary cation and the center of charge on the anion. In principle, the greater the distance the more reactive the anion. This is another reason that the lengths of the alkyl groups attached to the quaternary nitrogen are of such importance, the first reason being the organophilicity of the cation for solubility purposes. Step ( 3 ) Transfer of the product anion from the organic to the aqueous phase: To complete the cycle in the PTC process it is necessary to transfer the product anion to the aqueous phase from the organic phase so that the cationic catalyst can ion-pair with another nucleophilic anion to continue the catalytic process. If this does not occur the reaction process will come to a

448

4 Reactions in SCF

halt and only a few percent reaction will be realized. This is termed catalyst poisoning and is usually observed when the product anion is organophilic (e.g. tosylate, iodide). Therefore, it is important to the success of the phase transfer catalytic process that the appropriate leaving group be chosen on the electrophilic species. The classic example of a PTC reaction (Figure 4.10-1) depicts the reaction of a 1-chloroalkane and aqueous sodium cyanide [9]. In the absence of any catalyst, there is no reaction between the substrates after 1 or 2 days even if the system is heated. However, in the presence of only 1 mol % of a quaternary ammonium salt such as tetrabutylammonium chloride, displacement of the chloride by cyanide ion occurs rapidly to give 100% 1-cyanoalkane in only a few hours. This example is powerful in that it shows the ability of PTC systems to lower the energy barrier to reaction by providing an alternative reaction pathway. In summary, the example in Figure 4.10-1 defines the basic steps that are required in PTC systems. First, a reactant (usually an anion) is complexed with the catalyst and transferred across the phase boundary into the other phase. Second, the intrinsic reaction must take place to produce the desired product, and the byproduct anion then complexes with the catalyst. Third, the byproduct anion, with the aid of the catalyst, is transferred back across the phase boundary where, in the fourth step, it is exchanged for the reactant anion. The first, third, and fourth steps all deal with the transfer of anions and are generally termed “anion transfer”. The second step deals only with the intrinsic reaction that occurs between reactants and is therefore termed “intrinsic reaction”. It is important to distinguish between these two stages of reaction. There are many variables that can affect the rate of a PTC reaction and can be categorized as affecting either the anion transfer or the intrinsic reaction. When the relative speeds of anion transfer and intrinsic reaction are equal then the PTC system is at its maximum rate. However, if the transfer step is slower then the system is limited by the rate of transfer. If the reaction rate is the slower of the two, then the system is reaction rate limited. In either case, there are many parameters that can be manipulated to increase the overall rate of reaction: catalyst structure [ 101, catalyst concentration [ 111, agitation [ 121, temperature [13], water content [ 141, solvent choice [ 151, inorganic anion [3] and cation [16], and even cocatalysts [17] can be used. The parameters that are important in traditional PTC systems may also be important in SCFPTC systems. In many PTC systems, a third phase can form where reaction occurs and rates are enhanced. This third phase can form in many multiphase systems, and has been termed the “omega phase” [18]. The first example of an omega phase was discovered in the PTC reaction of cyanide ion on a benzyl halide in the presence of a crown ether as phase transfer catalyst. This liquid-solid PTC system was found to have rates dependent on the amount of water present. It was determined that in the absence of either water or the

4.10 Phase Transfer and Ammonium Salt Catalyzed Reactions

449

solid phase the catalytic rate constant decreased. In this three-phase system the amount of catalyst in the organic phase decreased as the amount of water . increased, showing that the distribution of the PTC across the phases has an important effect on rate. The omega phase phenomenon is not limited to solid-liquid PTC; a specific example of a solid-SCF PTC reaction is discussed below.

4.10.3 Published Work Most reactions 'that have been investigated using PTC in supercritical fluids have been solid-SCF systems, not liquid-SCF. The first published example of PTC in an SCF is the displacement reaction of benzyl chloride 1 with potassium bromide in supercritical carbon dioxide (scC02) with 5 mol% acetone, in the presence of tetraheptylammonium bromide (THAB) [ 19-20] (Scheme 4.10-1) to yield benzyl bromide 2. The effects on reaction rate of traditional PTC parameters, such as agitation, catalyst type, temperature, pressure, and catalyst concentration were investigated. The experimental technique is described below. PTC appeared to occur between an SCF phase and a solid salt phase, and in the absence of a catalyst the reaction did not occur. With an excess of inorganic salt, the reaction was shown to follow pseudo-first order kinetics. The reaction between benzyl chloride 1 and potassium cyanide in scC02 in the presence of tetraheptylammonium chloride (THAC) to yield benzyl cyanide 3 (Scheme 4.10-2) showed interesting results [21]. Similar first-order kinetics were found; however, the authors propose that the reaction occurred in a highly reactive third phase, the omega phase. The low catalyst solubility in the fluid phase combined with the linear increase in rate constant (two orders of magnitude above the solubility limit of the catalyst) suggest the presence of the

Scheme 4.10-1 First literature example of a PTC reaction in an SCF solvent [19].

Scheme 4.10-2 Displacement of chloride ion with cyanide under PTC/SCF conditions

WI.

0""' +

THAB sccq

KBr

w

KCI +

10-71 OC 130-200 bar

1

2

0""' +

1

WBr

KCN

THAC scCO2 10-71OC 130 bar

0"

'CN

KCI

~

+

3

4 Reactions in SCF

450

Catalyst Phase

Figure 4.10-2 Three-phase PTC system with a catalyst-rich surface phase under dynamic conditions [201.

omega phase on the salt surface, and that the reaction occurs in this catalystrich phase. Figure 4.10-2 depicts the proposed three-phase system, and the concentrated catalyst phase where the reaction is believed to occur. In the presence of acetone as a cosolvent, the reaction rate decreased, perhaps due to the increased solubility of the catalyst in the SCF and the dilution of catalyst in the omega phase. It was also shown that the reaction occurred with a completely insoluble polyether PTC. Although it is customary to add cosolvents to SCFs to increase solubilities, in this case the increased solubility appears to be detrimental to reaction rate. Another reaction involving an SCFPTC system is an esterification reaction [22] where the primary role of the SCF is to solubilize an intermediate product to prevent the overreaction to an unwanted byproduct. In this system (Scheme 4.10-3) an insoluble aromatic carboxylic acid 4 with a second reactive functional group is esterified at elevated temperature in supercritical dimethyl ether (scDME) with ethylene oxide 5, which is soluble in the fluid phase, in the presence of a thermally stable and insoluble phase-transfer catalyst. When esterification occurs, the product ester 6 is then soluble in the SCF and is pulled away from the site of reaction and trapped before the second functional group can be altered. Experimental data for this work were obtained using a modified Hewlett-Packard supercritical fluid extractor. This is an example of a PTC reaction where an intermediate product is desired, and the SCF system is designed to obtain only that intermediate.

g-y R

+

A

-PTClSCF-

o<

R

OH

-

O I O H

4

5

6

Scheme 4.10-3 Esterification reaction in a PTC/SCF flow system [21].

4.10 Phase Transfer and Ammonium Salt Catalyzed Reactions

451

4.10.4 Experimental Procedure The phase transfer-catalyzed displacement reactions [19-21] that have been performed in supercritical fluids to date have been reactions between insoluble solid salts and soluble organic liquids, in the presence of solid catalysts. A diagram of the apparatus for such an experiment is shown in Figure 4.10-3. The reaction vessel was a stirred autoclave reactor with an electric heating mantle. Before the reaction was started, the solid catalyst and the salt were stirred together for several hours so that the solids were crushed into uniform particle sizes. This is a common procedure for making PTC kinetic measurements. Next the reactor was heated, and the solvent was added using a high-pressure syringe pump. When the desired temperature and pressure were achieved, the reaction was initiated by adding the liquid organic reactant from a manual pressure generator thraugh a fitting in the top of the reactor. The sampling technique used involved a two-way sampling loop, through which the reactor was vented in order to isolate a sample. Samples were very small compared to the reactor volume, so any upset of the system by the procedure was minimized. Once the sample was isolated, the loop was depressurized into a solvent, so that the organic compounds dissolved and could be analyzed by standard techniques.

The substitution reaction between benzyl chloride and potassium cyanide This reaction is catalyzed by tetraheptylammonium chloride. The solvent is supercritical carbon dioxide, with acetone as a cosolvent.

7 r

Magnetic

T-

Pressure

High Pressure Syringe Pump

Reactor

coz Supply Figure 4.10-3 Diagram of FTC reaction experimental apparatus [20].

452

4 Reactions in SCF

Because the presence of minute amounts of water can have large effects on the behavior of a PTC system, extreme efforts were made to eliminate water from the reaction system. After the reactor was cleaned with acetone and water, it was dried overnight in a vacuum oven at 60°C. The reactor was then heated for 2 h at 100°C and under 70 bar of C02 pressure. A11 lines from the reactor were vented several times during this period. The 300 mL stainless steel reactor was loaded in a glovebox under a nitrogen atmosphere to exclude atmospheric water. Placed in the reactor were 5.09 g of KCN, 0.175 g of tetraheptylammonium chloride, and 9.75 mL of acetone. These quantities correspond to a salt amount equal to five times that of the benzyl chloride on a molar basis, a catalyst amount of 2.5 mol % of the benzyl chloride, and 5 mol% acetone based on the solvent. The reactor was then sealed, and the contents were stirred overnight at 200 rpm at 35 "C and atmospheric pressure. Before charging the system with the benzyl chloride, the reactor was heated to 75°C and pressurized to 138 bar with C02. Using a manual high-pressure syringe pump, 1.80 mL of benzyl chloride was introduced to the reactor, yielding an initial concentration of 0.055 M. Throughout the reaction, stirring was maintained at 200 rpm. Samples were taken using a 250 pL sample loop, through which the reactor was vented slowly for 30 s. The loop was then isolated from the reactor and its contents were vented into 3 mL of cold acetone in a 5 mL volumetric flask. Finally, 2 mL of additional acetone was flushed through the loop to rinse it. The samples were taken in triplicate. Analyses were performed using gas chromatography.

4.10.5 Related Catalytic Reactions Another study exists which the authors term a PTC/SCF reaction [23], where some characteristics differ somewhat from the usual type of PTC reaction. Cyclohexene was oxidized to adipic acid in the presence of an aqueous Ru04 catalyst, in scCOz at 40°C and 160-270 bar. The reaction was run in a viewcell, but the authors do not report the number of phases present. From the description, it seems likely that the CO2 and the cyclohexene might be miscible and there would possibly be some solubility of the catalyst in this phase to run the reaction. Most investigators regard PTC reactions as involving ion-pairing of the amphiphilic PTC with a reactant ion to transport the ion into the less polar phase and to activate it [6]. This oxidation reaction is an important example of the use of scC02 as a reaction solvent, and it does have interphase transport of a catalyst; however, it does differ a bit from what is normally thought of as PTC, as there is no ion transport or activation.

4.10 Phase Transfer and Ammonium Salt Catalyzed Reactions

453

4.10.6 Future Directions PTC in SCF solvents offers a plethora of opportunities. Certainly the technique will be both applicable and useful for a wide variety of reactions. With opportunities to tune selectivity, enhance transport, and to remove solvents, all in an environmentally benign medium, this emerging area of research has great potential. With the recent surge of interest in developing technologies for a sustainable society, PTC/SCF systems can play an important role in industrial ecology.

References [ l ] E. V. Dehmlow, Angew. Chem., Int. Ed. Engl. 1977, 16, 493. [2] W. P. Weber, G. W. Gokel, Phase Transfer Catalysis in Organic Synthesis, SpringerVerlag, Berlin, 1977. [3] C. M. Starks, C. L. Liotta, Phase Transfer Catalysis; Principles and Techniques, Academic Press, New York, 1978. [4] H. H. Freedman, Pure Appl. Chem. 1986, 58, 857. [5] E. V. Dehmlow, S . S . Dehmlow, Phase Transfer Catalysis, 3rd edn, VCH, Weinheim, 1993.

[6] C. M. Starks, C. L. Liotta, M. Halpern, Phase-Transfer Catalysis, Chapman & Hall, New York, 1994. [7] C. A. Eckert, B. L. Knutson, P. G. Debenedetti, Nature 1996, 383, 313. [8] C. A. Eckert, K. Chandler, J. Supercritical Fluids 1998, 131, 187. [9] C. M. Starks, J. Am. Chem. SOC, 1971, 93, 195. [lo] N. A. Gibson, D. C. Weatherburn, Anal. Chim. Acta 1972, 58, 159. [ l l ] D.-H. Wang, H.-S. Weng, Chem. Eng. Sci. 1988, 43, 2019. [12] V. Ragaini, G.Colombo, P. Barzaghi, E. Chiellini, S . D’Antone, Ind. Eng. Chem. Res. 1988, 27, 1382. [13] N. Kahana, A. Deshe, A. Warshawsky, J. Polym. Sci., Chem. Ed. 1985, 23, 231. [14] C. L. Liotta, E. Burgess, C. C. Ray, E. D. Black, B. E. Fair in Phase-Transfer Catalysis: New Chemistry Catalysts and Applications, ACS Symp. Sex 326, C. M. Starks (Ed.), American Chemical Society, Washington, DC 1985, p. 15. [15] C. M. Starks, R. M. Owens, J. Am. Chem. SOC. 1973, 95, 3613. [16] 0. Arrad, Y. Sasson, J. Am. Chem. SOC. 1988, 110, 185. [17] 0. Bortolini, V. Conte, F. DiFuria, G. Modena, J. Org. Chem. 1986, 51, 2661. [18] C. L. Liotta, E. M. Burgess, C. C. Ray, E. D. Black, B. E. Fair, in Phase-Transfer Catalysis: New Chemistry, Catalysts, and Applications, ACS Symp. Sel: 326, C. M. Starks (Ed.), American Chemical Society, Washington, DC 1987, p. 367. 1191 D. L. Boatright, D. Suleiman, C. L. Liotta, C. A. Eckert, Solid-Supercritical Fluid Phase Transfer Catalysis, ACS 207th National Meeting, San Diego; CA, March 15, 1994.

[20] A. K. Dillow, S. L. J. Yun,D. Suleiman, D. L. Boatright, C. L. Liotta, C. A. Eckert, Ind. Eng. Chem. Res. 1996, 35, 1801. [21] K. Chandler, C. W. Culp, D. R. Lamb, C. L. Liotta, C. A. Eckert, Ind. Eng. Chem. Res. 1998, 37, 3252. [22] H. P. Lesutis, J. S. Brown, D. Bush, D. R. Lamb, C. L. Liotta, C. A. Eckert, J. S. Hurley, D. Schiraldi, ‘Interrupting a Reaction Sequence by Supercritical Fluid Extraction,’ AIChE Annual Meeting, Miami Beach, FL, November 20, 1998.

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4 Reactions in SCF

[23] D. A. Morgenstern, R. M. LeLacheur, D. K. Morita, S. L. Borkowsky, S. Feng, G. H. Brown, L. Luan, M. F. Gross, M. J. Burke, W. Tumas in Green Chemistry: Designing Chemistry for theJnvironment, ACS Symp. Sel: 626, P. T. Anastas, T. C. Williamson (Eds.), American Chemical Socienty, Washington, DC 1996, pp. 132-151.

Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

Appendix A: Conversion Factors Exact conversions are indicated by italics.

Table A.l Pressure [I]

I atm

bar

kg/cm2

MPa

psia

tom

1.0332 1.0197 1x10”

0.101 33

14.696 14.504 0.014 223 14.223 0.145 04 145.04

760 750.06 0.735 56 735.56 7.5006 7500.6 51.715

1 atm

1

1.0133

1 bar 1 g/cm2 1 kg/cm2 1 kPa 1 MPa 1 psia 1 tom

0.9869 9.678X lo4 0.967 84 0.009 869 2 9.8692 0.068 046 0.001 315 8

1

9.807X lo4 0.980 66 0.01 10

0.068 948 0.001 3332

I 0.010 197 10.197 0.070 307 0.001 360

0.1

9.807X 10” 0.098 066 0.001

1

0.006 894 8 1 1 . 3 3 ~ 1 0 ‘ ~ 0.019337

1

~

Note: 1 a I = 14.696 psia = 0 psig; psia = pounds per square inch absolute pressure; psig = pc ids per square inch gauge pressure.

Table A.2 Density [ l ]

1 g/mL 1 kg/m3 1 1wft3 1 Ib/in3

g/mL (g/cm3)

kg/m3

lb/ft3

lb/in3

1 0.001

1000 I 16.018 27 680

62.428 0.062 428

0.036 127 3.6127X lo-’ 5.787X lo4 I

0.016 018 27.68

1

I728

456

Appendix

Table A.3 Volume [ l ] ~

ft’ 1 ft’ 1 gal (UK) 1 gal (US liq.) 1 in3 1L 1 m’ 1mL

in’

I 1728 0.16054 277.42 231 0.133 68 5 . 7 8 7 0 ~ 1 0 ~1 0.035 3 15 61.024 35.315 61024 3.5315X 0.061 024

~

L

m’

m~ (cm’)

28.3 17 4.5461 3.7854 0.016 387 1

0.028 3 17 4.5461 x lo-’ 0.003 785 4 1.6387~10-’ 0.001 1 1x10“

28317 4546.1 3785.4 16.387 I000 1X l O 6 1

1000 0.001

Table A.4 Energy [ l ] 1 kcal (US National Bureau of Standards) = 4.1840 kJ

References [ l ] R. C. Weast (Ed.), CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, Florida 1982.

Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

Appendix

457

Appendix B: Abbreviations and Symbols Some chapters define other symbols, such as those for various types of volumes, not included in this list. The reader should note the definition given in the chapter for such'symbols.

2,3 -dimethylbutane pre-exponential factor (kinetics) or absorption (spectroscopy) or torus factor (NMR) water activity a, 1,4-androstadiene-3,17-dione AAD acetylacetonate acac analog/digital AID acyclic diene metathesis ADMET 2,2'-azobis(isobutyronitri1e) AIBN bis(2-ethylhexyl) sodium sulfosuccinate AOT anchor-soluble balance ASB Anderson-Schultz-Flory distribution ASF attenuated total reflectance ATR path length (spectroscopy) b magnitude of magnetic field B butyl acrylate BA tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anion BARF bond dissociation energy BDE 3,3 '-bis(ethoxymethy1)oxetane BEMO 2,2'-bis(dipheny1phosphino)- 1,l '-binaphthyl BINAP BINAPHOS 2-(diphenylphosphino)-l,1'-binaphthen-2'-yl-l, 1'-binaphthen2,2'-diylphosphite butyl methacrylate BMA back-pressure regulator BPR molar concentration C concentration (spectroscopy) C. chain-transfer to monomer constant CW,M critical micelle concentration CPC closed-circuit television CCTV catalysis and extraction using supercritical solutions CESS chlorofluorocarbon CFC critical flocculation density CFD

23DMB A

458

Appendix

cylindrical internal reflection Fourier transform infrared 1,5-cyclooctadiene q5-cy clopentadienyl q5-pentamethylcyclopentadienyl crystallization from supercritical solution css continuously-stirred tank reactor CSTR density d critical density dc reduced density dr diffusion coefficient or particle diameter (micelles) D octamethy lcyclotetracy cloxane D4 dodecyl acrylate DA diamond anvil cell DAC di(benzy1idene)acetone dba didodecyldimethyl ammonium bromide DDAB diethyl dithiocarbamate DDC di(isopropy1)tartrate DIPT dynamic light scattering DLS dodecyl methacrylate DMA dimethyl ether DME N,N-dimeth ylformamide DMF dimethylsulfoxide DMSO 1,2-bis(diphenylphosphino)ethane dPPe 1,2-bis(2,5 -dialkylphospholano)benzene DuPHOS ethylene (polymerization) E enhancement factor (solubility) or trans E (“entgegen“, organic chemistry) activation energy EA enantiomeric excess ee ethylhexyl acrylate EHA ethylenediamine en electron paramagnetic resonance EPR Reichardt solvent polarity parameter ET(30) Et-DuPHOS 1,2-bis(2,5 -diethylphospholano)benzene fugacity or mole fraction of monomer or resonance frequency f (NMR) mole fraction of copolymer F fatty acid methyl esters FAME bis(trifluoroethy1) dithiocarbamate FDDC 1,l-dihydroperfluorooctylacrylate FOA 2,2,3 -trimethyl- 6,6,7,7,8,8,8-heptafluoro-3,5 -0ctanedioneate FOD 3 -methyl- 3 ’-[(1,l-dihydroheptafluorobutoxy)methyl]oxetane FOX-7 Freon- 113 l,l-trichloro-l,2,2-trifluoroethane full scale FS F-T Fischer-Tropsch Fourier-transformation (spectroscopy) FT CIR-FTIR cod CP CP*

Appendix

-

AG GAS GASP GC GCMS h h Hg-BINAP HBA HBD HC HDPE HETS hfacac HFP H-H HHW HiP HLB HPLC H-T hv HYAFF IBVE id IPUE IR IUPAC k kB

K Km

KTP L L-L LCEP LCST LD-MS LDPE LF-SCF LG LLDPE M

459

Gibbs free energy (process for precipitation by) gaseous anti-solvent gaseous anti-solvent precipitation process gas chromatography gas chromatography/mass spectrometry hour (unit) Planck’s constant 2,2’-bis(diphenylphosphino)-5,5’,6,6‘,7,7‘,8,8’-octahydro1,l ’-binaphthyl hydrogen bond acceptor hydrogen bond donor hkavy compoundhydrocarbon high density polyethylene height of theoretical stages 1,1,1,5,5,5-hexafluoroacetylacetonate hexafluoropropy lene head-to-head dimer line width at half height High Pressure Equipment Company hydrophilic-lipophilic balance high performance (or high pressure) liquid chromatography head-to-tail dimer irradiation (photochemistry) poly(hya1uronic acid, benzylic ester) isobutyl vinyl ether inner diameter implicit penultimate unit effect (model for copolymerisation) infrared (spectroscopy) international union of pure and applied chemistry rate constant Boltzmann’s constant equilibrium constant or scattering coefficient (colloids) Michaelis constant KTiOPO4 liquid or ligand or liter potentially bidentate (chelating) ligand lower critical end point lower critical solution temperature laser desorption-mass spectrometry low density polyethylene lattice fluid self-consistent theory leaving group linear low-density polyethylene metal atom (inorganic chemistry) or monomer (polymer chemistry) number-average molecular weight

460

Mw

Appendix

MA MAA MAGROFI MEK min MMA MON MPI MTBE MTV M.W. MWD n n.a. NA nbd nbe NBR NIR diso NMR NPT OAc olc od

0s

P Pc Pr Psat

PY

P Pn PAA PAN PC PCA PCTFE PDI PDMS PE PEEK PEG PEHA

weight-average molecular weight methy lacry late methacrylic acid mangnetization-grating rotating-frame imaging methylethyl ketone (2-butanone) minute(s) methyl methacry late motor octane number Max-Planck-Institute methyl tertiary butyl ether magnesiumReflonNiton molecular weight molecular weight distribution (see also PDI) number of moles not available Avogadro’s number norbornadiene norbornene nitrile buna rubber near-infrared (spectroscopy) linear to branched hydroformylation product ratio nuclear magnetic resonance (spectroscopy) national pipe thread acetate organic-in-scC02 emulsion outer diameter organic solvent pressure critical pressure reduced pressure (saturated) vapor or sublimation pressure yield pressure polymer chain degree of polymerisation poly(acry1ic acid) poly(acrylonitri1e) pol ycarbonate precipitation with a compressed antisolvent poly(chlorotrifluoroethy1ene) polydispersity index of the molecular weight distribution (MJMn) poly(dimethylsi1oxane) polyethylene poly(ether ether ketone) poly(ethy1ene glycol) poly(2-ethylhexyl acrylate)

Appendix

PEO PET PFOA PFPE PGSE PGSS PH PLP PMA PMMA PMP PPA PPL PPN PPO PPVE PS PTC

PTFE PVAc PVC PVF2 r r R R Rf RCM RE RESS rf

ROMP RT S

S SANS SAS SAXS SIC sc SCF SCW

scwo sec

poly(ethy1ene oxide) poly(ethy1ene terephthalate) poly( 1,l-dihydroperfluorooctylacrylate) perfluorinated polyether pulsed-gradient spin-echo particles from gas-saturated solution negative decadic logarithm of H+ concentration pulsed laser polymersiation poly(methy1 acrylate) poly(methy1 methacrylate) poly(4 -methyl-1-pentene) poly(phenylacetylene) porcine pancreatic lipase bis(tripheny1phosphine)iminium cation poly(2,6-dimethylphenylene oxide) perfluoro(propy1 vinyl ether) polystyrene (polymer chemistry) or pulse sequence (laser experiment) phase transfer catalysis poly(tetrafluoroethy1ene) poly(viny1 acetate) poly(viny1 chloride) poly(viny1idene fluoride) chain length (polymerization) radius (vessel design) or distance or reactivity ratio (polymerization) radical or alkyl substituent ideal gas constant fluorinated alkyl substituent ring-closing metathesis rare earth rapid expansion of supercritical solution radio frequency ring-opening metathesis polymerization room temperature second(s) solid small-angle neutron scattering (process for precipitation by) supercritical anti-solvent small-angle X-ray scattering substrate to catalyst ratio supercritical supercritical fluid supercritical water oxidation in supercritical water secondary

461

462

Appendix

SEC SFC SFE SFR SLV S M SIN

SP SPP ss STF t

T TI T2 Tc T&7 Tr

TBA TBHP TBP TEMPO TFE THAB THAC THF TMPCl TOF TON Tos TPOP TPP TR3 TRIR UCEP UHMWPE

uv V

V Afl VA

VF2 Vis Vmm

size exclusion chromatography supercritical fluid chromatography supercritical fluid extraction supercritical fluid reactions (reactions involving a supercritical phase) solid-liquid-vapour molar ratio of substrate to metal centre signal to noise ratio single pulse (laser experiment) solvent polarity/polarizability stainless steel p-perfluoroethy leneoxy methy lstyrene metric ton (unit) or time (parameter) or tertiary (organic chemistry) temperature longitudinal relaxation time transverse relaxation time critical temperature glass transition temperature reduced temperature t-butyl alcohol t-butyl hydroperoxide tri-n-butyl phosphate 2,2,6,6-tetramethyl-l-piperidinyloxy free radical tetrafluoroethylene tetraheptylammonium bromide tetraheptylammonium chloride tetrahydrofuran 2 -chloro-2,4,4-trimethylpentane turnover frequency turnover number tosylate tripheny lphosphite tripheny lphosphine time-resolved resonance raman (spectrosocopy) fast time-resolved infrared (spectroscopy) upper critical end point ultra high molecular weight polyethylene ultraviolet (spectroscopy) velocity (reaction rate) vapor or volume activation volume vinyl acetate vinylidene fluoride visible (light or spectroscopy) maximum reaction rate (Michelis-Menten kinetics)

Appendix

voc WO

WIC WE X

X

X

XAFS YAG

Z

z-H"FY

volatile organic compound water-to-surfactant ratio water-in-scCO2 emulsion contact time (mass of catalystlflow rate) mole fraction substituent conversion x-ray absorption fine structure Y3A150 12 cis ("zusammen", organic chemistry) substituent of type (CH2)x(CF2)yFin position(s) z of an aryl group

Greek Characters activity coefficient or interfacial tension Hildenbrand solvent parameter or chemical shift heat or change & dielectric constant E molar absorptivity viscosity (physics) or hapticity (coordination chemistry) rl A wavelength CI dipole moment TI* Kamlet-Taft solvent polarity parameter T rotational correlation time 4 Thiele modulus quantum yield or fugacity coefficient Q, rp fugacity coefficient Y 6 A

463

Chemical Synthesis Using Supercritical Fluids Edited by Philip G. Jessop and Walter Leitner © WILEY-VCH Verlag GmbH, 1999

Index Illustrations are indicated in italics. Procedures are indicated in boldface.

Abbreviations 457ff Absorptivity (UV-Vis) 196 Acetylene, hazards of 8f Acid catalysts, heterogeneous 403ff, 407 Acidity, see pH Acrylamide,polymerizationin C02 3 15f Acrylates, (co)polymerizationof 142, 300f, 305,309ff, 329,34lff, 345ff, 346 Acrylic acid, polymerizationin C02 306 Activation controlled reactions 54,58 Activation energy, of F-T synthesis 393 of polymerization 337ff, 344ff Activation volume 61,198,264f, 344ff, 431ff Activity of catalysts, effect of SCF on 402f Adipic acid, synthesis of 377,452 ADMET, see Metathesis Adsorption, on catalyst surfaces 395,397, 408 -,of surfactants 129,135 Aerogels, synthesis and drying of 237 Aggregation number 264 AIBN 27f, 302f, 305f, 312f, 316 Air, as an oxidant 399ff, 409 Alcohols, dehydration of 23,267,271f, 407 -,dehydrogenationof 23 -,hydrogen bonding in 166 -,synthesis by hydration of alkenes 28,29 -,synthesis by oxidation of alkanes 26,399, 399ff Aldehydes, as co-oxidant 379 -,from hydroformylation 365ff, 368 Alkanes, chlorination of 284,290ff -,complexes of 244

-,cracking of 266,410 -,oxidation of 26,379,399ff, 399,409 -,as SCF 7,23,26,308,404ff, 391,425, 428f Alkenes, copolymerizationwith s c C 327, ~ 341,380f -,F-T reaction product distribution 394 -,hydration of 28,29,407 -,hydroformylationof 365ff, 368 -,isomerization of 267,272,285,398 -,metathesis, see Metathesis -,oxidation of 377f, 452 -,polymerization in C02 305,317,319, 381ff Alkyl halides, hydrolysis of 140 -,reaction with N H 3 23 -,substitution reaction 14Of, 447ff, 451f Alkylaromatics,bromination of 289f Alkylation, acid catalyzed 403ff, 403 -,ofaniline 26 -,Friedel-Crafts 21,275,407 Alkynes, cyclotrimerizationof 369f -,cyclotrimerizationwith Co;! 370f -,cyclotrimerizationwith nitriles 369f -,polymerizationof 383 Allylic alcohols, oxidation of 378 Alumina, as heterogeneouscatalyst 398 Amides, synthesis of metal 234,237 Amines, synthesis of 363f, 364 Amino acids, interactions with C02 425ff -,synthesis of 362 Ammine complexes 236 Ammonia, hazards of 9 -,properties of 6

466

Index

-,reaction with alkyl halides 23 -,reaction with sodium 21

-,as solvent 223f, 234ff, 326 -,synthesis of 25f Anderson-Schultz-Flory distribution 396 Aniline, alkylation of 26 -,hydrolysis of 23 Anion transfer 448 Anthracene, dimerization of 286f -,excited state structure in scC& 156 Anthraquinone, hydrogenation of Slf, 82 Antisolvent precipitation, see Precipitation Antisolvents, H2 358 -,SCCOZ losf, 112ff, 120ff, 124 AOT, see Microemulsions Aquathermolysis 27 1 Argon, properties of 6 Aromatization by dehydrogenation 407 Aryl halides, coupling reactions of 373ff Aspen process simulator 83, 102 Asymmetric catalysis, see Enantioselective reactions ATR, see FllR Autocatalysis 27 1 Autoclave, see Pressure vessel Autofrettage 69f Back biting 333,336,338 Backpressure regulator 75f, 76,25Of, 251 BARF 357,362,363f Batch reactor, see Pressure vessels; Reactor systems Beckmann rearrangement 274 Beer-Lambert law 196 Benzene, reactions of 23 -,properties of 7 -,synthesis of derivatives 369f Benzoquinones, from enzymatic oxidation 426f Betaine dyes, see Solvatochromic probes BINAP 360f BINAPHOS 369 Binary mixtures, phase behavior of 41ff Biomass conversion 268,269 Biphasic catalysis 352,354f Boiling, suppression of 13 Bond dissociation energies 182ff Bridgeman seal 72, 72,214,217f, 218,225 Bromination, of hydrocarbons 289f Bubble point locus 112,113

Butanes, properties of 7 iso-Butane, see Isobutane Butene, hydration of 28,29 iso-Butene, see Isobutene 2-Butano1, synthesis from butene 28 t-Butyl alcohol, synthesis from isobutane 399ff t-Butyl hydroperoxide 377f, 403 Cagniard de la Tour’s state 16 Caffeine, extraction of 19,299 Cage effects 12,56f, l98,287ff, 291ff, 302, 334,362 Cannizarro disproportionation 27 1 Carbamates 24,360,425f Carbamic acids 364,373 Carbon chain growth 392f, 396f Carbon dioxide, as catalyst poison 352,371, 377,425ff -,coordination compounds of 157ff, 157, 244ff -,copolymerization of 24,3 19,38If -,coupling with alkynes 37Of -,density of 5 -,dielectric constant of 5 -,drying of 134 -,effect on enzyme reactions 425ff -,as feedstock 24,35 1,358f, 370f, 38 1 -,as greenhouse gas 10 -,Hildebrand parameter of 5 -,hydrogenation of 355,358ff, 358 -,hydrosilylation of 360 -,interaction with homogeneous catalysts 35 1 -,liquid as solvent 18,245,297ff -,nature of the supercritical state 16 -,phase diagram of 2 -,properties of 5f -,as protecting group 373 -,reactions of 24, 157ff, 244ff, 266, 351f, 355,358ff, 358,37Of, 377,381 -,sources of 10,297 Carbon disulfide 18,21 Carbon monoxide, decomposition of supercritical 23 -,hydrogenation of 26,389ff -,as SCF 182 -,substitution of 245,248ff Carbon number distribution of F-T products 392

Index Carbon tetrachloride 2 1,266 Carbonates,see also Dialkyl carbonates; Polycarbonates -;synthesis of metal 237 -,formation during catalyst deactivation 352 Carbon-carbon bond forming reactions, see also Carbon chain growth; Diels Alder; - Cyclopropanation;Coupling; Polymerization 266 Carboxylation,of radicals 265 Catalysis, see also Enzymatic reactions; Heterogeneouscatalysis; Homogeneous catalysis; Phase transfer catalysis; -,early use in SCFs 22 - 26 Catalyst effectiveness factor 391 Catalytic intermediates, detection by NMR 184ff, 185,189 Cation exchange resin catalyst 407 Cells, see Pressure vessels; Spectroscopic cells Cellulose conversion 268f CESS process 352,365,368,372 Chain transfer 333ff -,to C02 solvent 302f, 305f, 3 17f Chain-transferto monomer constant 337 Chemical exchange, NMR study of 174, 177ff Chiral catalysts, see Enantioselective Chlorination, of hydrocarbons 284,290ff Chloroform,as SCF 266 Chlorotrifluoroethane,dimerization of 60 Cholesterol 426f Chromatography 89,104ff, 105,439 Chromium catalysts, heterogeneous 409 Chromium complexes 25 lff, 253 Cloud point 83, 131f, 139, 143,332f Clustering, see also Local density augmentation -,cosolvent-solute 197,266 -,effect on reactions 410 -,solute-solute 12, 198, 275 -,solvent-solute 197,263,275,281, 287ff Coal extracts 410 Coalescence of latex particles 142 Coalescence, NMR 179 Cobalt catalysts, heterogeneous 390,393, 3965 409 Cobalt complexes 172ff, 179ff, 183,357, 365ff, 369f, 384 Coke, see also Heterogeneouscatalysts 398

467

Colloids, as catalysts 375

-,stabilization of 127,307ff

Competingreactions, density effects on 62,373 Complexes,see Coordination compounds Compressibility 2f, 111f Compressibleregion 2f, 2,, 266 Compressors 76ff Conductivity 137f Connectors 72,73,221,222,418 Contact ion pair 204 Continuous flow processes 11,84ff, 85, l96,390,399,403,408,417,420f, 434f, 44Of,440 Continuously stirred tank reactor, see Pressure vessels Conversion factors 68,455f Coordination ability of solvents 198 Coordination compounds,see also Homogeneous catalysis; Ligands, Metal carbenes; Metal carbonyls; entries for individual metals; 243ff -,of alkanes 244 -,Of C02 157ff, 157,244ff, 351 -,as EPR probes 133 -,of ethene 244ff, 25 lff, 252,253 -,as heterogenized catalysts 360 -,of H2 244ff, 247,25 lf, 255,256 -,inorganic support impregnation with 254 -,ofN2 244ff -,of noble gases 159f, l59,244ff, 248 -,photochemical synthesis of 244ff -,polymer impregnation with 152,254 -,solubility of 5 1, 245, 356ff -,table of 246 Copolymerization,see also Polymerization; Copolymers;Telomerization -,for in siru stabilization 310 -,modeling of 34Off, 341,342, 343 -,reactivity ratios 341ff, 343 Copolymers,epoxides-C02 24,3 19,381f -,of ethene 24,327ff, 33 1,332,340ff. 38Of -,hydrocarbon-fluorocarbon 301,31 lf, 313f -,hydrocarbon-siloxane 3 1If, 3 14f -,of isobutylene 3 17 -,of styrene 305,317 -,as surfactants 309ff -,of TFE 306 Copper catalysts 320,374 Cosolvent, i05,440

468

Index

-,substrates as 365 -,effect on ROMP 382 Cosolvent tuning 12 Countercurrent multistage extraction 97ff, 102f Cracking of alkanes 266,410 Critical data of SCFs 6f, 41,404 Critical end point 43 Critical flocculation density 136 Critical line, of binary mixtures 42 Critical microemulsion concentration 129 Critical opalescence 4,18 Critical point, of binary mixtures 37 definition of 1 -,determination of 16,18,189f, 190 -,discovery of 13 -,of pure substances 37 Crystallization 108, 109, 115, 214f, 224, 233 CSS 108,109,115 Cut-off wavelength, of window materials 149f Cybotactic region 197,200,264 Cyclic alkenes, polymerization of 319,382, 383 Cycloaddition, see Diels-Alder reaction Cyclopentadiene, Diels-Alder reaction of 64, 262ff, 273f Cyclopentenones, synthesis of 370 Cyclopropanation, enantioselective 375f Cyclotrimerization of alkynes 369ff Cymantrene 204

-.

Debye-Waller factor 204 Decarboxylation 27 1 Decomposition see also Hazards -,of glucose 269 -,of organic compounds 266,269 -,of peroxideshydroperoxides 266,403, 409 Degree of fill 2 19,220 Degree of polymerization, see Molecular weight control Dehydration, of alcohols 23,267,271f, 407 Dehydrogenation, see also Aromatization -,of alcohols 23,407 -,of toluene 410 Dehydrogenative coupling 23 Denitrogenation 407.409 Density,see also Local density augmentation

-,critical (table) 6f -,dependence on pressure 3f, 38,39 -,effect on reactions 64,65,373,433 -,reduced 4 -,units of 455 Desorption,-from catalyst surface 395,398 Destraction . 19 Dew point locus 112,113 Dialkyl carbonates, synthesis of 384 Diamond, synthesis of 228f Dibenzyl ketone, photolysis of 287f Dielectric constant 5,5,264,267 -,apparatus for measuring 137 -,effect on enzyme reactions 432 -,effect on reactions 317,3755 376,432, 433,439 -,of emulsions 137 Dielectric tuning 376 Diels-Alder reaction 64,65,260, .262ff, 263,272f Dienes, copolymerization with ethene 38Of -,hydrogenation of 363 -,metathesis of 372f Diffuse diffraction 206, 208f Diffusion, 12, 38f, 54ff -,diffusion controlled reactions 12,39,54, 56ff, 199,281ff -,in enzymatic catalysis 58,434ff, 434, 446 -,in extraction 90,93,95 -,in heterogeneous catalysis 58,391,393, 395,397,405,407,434 -,in homogeneous catalysis 358,360,368 -,of H2 in benzene 173 -,IR monitoring of 155 -,NMR monitoring of 173 -,in polymerization 57,330,334ff, 347 Diffusion coefficients 54 -,in critical region 56 -,density effect on 55 -,fromNMR 187 -,product ratios and 57 -,viscosity and 56 Difluoromethane, properties of 7 Digestion bombs, see Pressure vessels Dihydrogen complexes 244ff, 247,25 1f, 255,256 Di(isopropy1)tartrate 378 Dilution principle, see Ziegler-Ruggli DrinciDle Dimeriz'ation, of acrylic acids 275

Index -,of anthracene 286f -,of chlorotrifluoroethane 60 -,of isophorone 284,285f Dimethyl ether, as SCF 7,450 Dinitrogen complexes 244ff, 248,252,256 Diols, from olefin oxidation 377f Displacement reactions, see Substitution Disproportionationof toluene 409 DMF, synthesis of 359f DuPHOS 362 Dye, see Solvatochromicprobes; Polymer impregnation Dynamic fluorescence quenching 199 Dynamic light scattering 135f Edible oils, hardening of 29 Electron microscopy, for polymer analysis 312ff, 314,315,316 Electron paramagnetic resonance, see EPR Electron transfer reactions 275 Electronic materials 12, 215, 229 Eliminationreactions 27 If Emeralds, synthesis of 215,228 Emulsifier 137 Emulsions, see also Microemulsions; Micelles; Surfactants -,applications of 135 -,conductivity of 137f -,dielectric constant of 137 -,flocculation of 127, 128, 135, 138f, 143 -,formation of 135 -,microscopy of 139 -,organic-COz 135 -,pH of 134 -,sedimentationof 136,139 -,stabilization or stability of 128, 135, 138f, 143 Enantiomericexcess, as function of conversion 437f, 438 -,as function of H2 pressure 361 -,as function of SCF pressure 375f, 437, 439 -,as function of temperature 436f Enantioselectivereactions -,cyclopropanation 375f -,esterification 421f, 436ff -,hydroformylation 369 -,.hydrogenation 36Off, 360,363f, 364,408 -,oxidation 378 Endo-exo ratio 64f, 262ff, 273

469

Energetic materials, synthesis of 30, 109 Enhancement factor 48,49, 111 Environmentalimpact of SFRs lOf, 267 Enynes, carbonylation of, see Pauson-Band reaction Enzymatic reactions, in emulsions 134 -,general procedures for 416ff -,industrialuse 30,441ff -,initial rates of 424 -,table of 421ff Enzyme stability 425ff Enzymes, classification of 415 Epimerizationof glucose 268 Epoxides, copolymerizationwith C02 24, 319,381f -,from olefin oxidation 377f -,polymerizationof 317f EPR 199f -,flow cell for 130,133,200 Equations of state 113f, 408 Equilibriumconstants, from NMR 177ff Equilibrium stages, in countercurrent extraction 97 Equipment, see Pressure vessels; Reactor systems; Spectroscopiccells; View cells; entries for specific components Esterification 266,359,407,42Off, 450 Estradione, synthesis of 28 E~(30)solvent polarity scale 197,264 Ethane, as SCF 7,408,425,428f Ethanol, as SCF 18,166 Ethene, see also Ethene polymerization -,complexes of 244,248,25 lff, 252,253, 254 -,copolymerizationof 24,327ff, 331f, 332, 340ff, 380f -,explosive decompositionof 27 -,hydration of 28 -,iodination of 21 -,properties of 7 -,as solvent for enzyme reactions 425,428f Ethene polymerization 326ff, 333ff, 380f -,inCO2 305 -,early history of 21ff, 26ff, 27, -,kinetic model 333ff -,runaway polymerization 8f Etherification 275 Ethyl red indicator 134 Ethylene, see Ethene Ethylenediamine,as SCF 7,234

470

Index

Excimer laser 199 Explosives and energetics 30, 109 Extraction,see also Countercurrent multistage extraction; CESS; Retrograde crystallization -,of caffeine 19 -,of contaminatedsoil 95f -,continuous 85,95ff -,early history 18f -,in enzymatic reactions, 420,439f -,of esters 85f, 85, 103 -,fixedbed 90 -,in heterogeneouscatalysis 389,395,398f, 403,405ff, 41 1 -,of liquids 97ff -,model approach to 93ff -,of natural products, 19,88ff -,in polymerization298,310 -,process description 85,89ff, 96, 98, -,separation analysis of 101 -,from solid material 90ff -,with scHzO 95f, 96 Extraction curves 9 1f, 92 Fatty acid esters, extraction of 85f, 85, 103,419f -,hydrogenation of 30 Filter 75, 76,355 Fischer-Tropschsynthesis 389ff, 390 Fixed bed reactor, see Pressure vessels Flanges 72f Flash photolysis 156, 197f Floating insert or liner 2 19ff, 238 Flooding point 103 Fluidized bed reactor, see Pressure vessels Fluidized molten salt bath 274 Fluorescence spectroscopy 198f Fluorides, synthesis of 233 Fluorination, solubility increase by 5 1, 128ff, 175, 300ff, 356ff Fluoroform,dielectric constant of 5 -,enzyme catalysis in 423f, 425,428f, 432f, 4362 -,homogeneous catalysis in 375f -,as narcotic 9 -,properties of 7 Fluoropolymers 128ff, 300ff. 309ff FOA, see Poly(1,l-dihydroperfluorooctyl acrylate) Foodadditives, preparation of 12

Formaldehyde,polymerizationof 3 17 Formic acid, synthesis of 358f, 359 Free-radical, see Radical Friedel-Crafts alkylation 21,275,407 Fries enolization, see Photo-Fries reaction FTIR i48ff -,in alkane cracking 410 -,of aqueous microemulsions 153f, 153 -,cells for 130,131, 149, 245, 260,261, 3275 328 -,in coordination chemistry 245ff -,for diffusion processes 155 -,in homogeneous catalysis 155,354,364 -,of organic reactions 262,275 -,of overtone and combinationmodes 155, 328 -,in polymer modification 151f -,in polymerization 155,156, 327f -,reflectance techniques for 148, 155, 156, 354,410 -,time-resolved, see TRIR; Step-scan FTIR Fugacity 59, 111 GAS(P) losf, 112ff, 12Off, 124 Gaseous products, formation in SCFs 372 Gaseous reagents, diffusion into liquids 173 -,miscibility with SCFs 12, 148, 172f, 244, 358 GC, for on-line reaction monitoring 354 Gems, synthesis of 228f Geochemistry 20,214,215,226,233,267 Geothermal fluids 215,226,233 Glass transition temperature 197, 299 Glucose 268 Greenhouse potential of SCFs 10 Haber-Bosch process 25,234 Halogenation,of hydrocarbons 284,289ff Hazards of SCFs 8f, 68 Health benefits of SFRs 10 Heating, of pressure vessels 81, 170,353, 418 Heck reaction 374f Heisenberg spin exchange rate 199 Helium head-gas 6 Heterogeneouscatalysts, acid catalysts 403ff, 403,407 -,acidic sites in 406f -,coke deuosition on 398 -,deactivation of 389,398,405

Index

-,in-situ regeneration 11,389, 398,403, 41 1 metal complexes as 354f, 354,360 -,overheating of 389,399 -,plugging of pores 389,399 . -,preparation of 120 -,regeneration, by SFE 405f n-Hexane, as SCF 7f, 391 High pressure, definition of 68 Hildebrand parameter 4f History, of hydrothermal synthesis 214ff -,of SCF research 1.3ff -,ofSFRs 20ff Homoallylicalcohols, oxidation of 378 Homogeneous catalysis, see also Metal-complex catalysis; entries for individual reactions 35 Iff -,catalyst recovery l1,352,364f, 368,372 -,general procedure for 353f -,IR monitoring of 155,354,364 -,N M R monitoring of 184ff. 185,186, 364, 365 -,phase behavior in 354f, 354,358ff -,polymerization 319ff, 38Off -,proof of 355f, 355 -,bysalts 24 Homopolymerization,see Polymerization Hydration, of alkenes 28,29,407 -,of solutes in scH20 202ff Hydride complexes 177ff, 247,351,3585 362,365 Hydrocracking,during F-T synthesis 393 Hydrodynamic radius or diameter 135, 199 Hydroformylation 184ff, 365ff, 368 -,enantioselective 369 -,NMR monitoring of 184ff, 185,186 -,table of 366 Hydrogen, as antisolvent 358 -,detection by Raman 15Of, 151 -,diffusion into benzene I73 -,miscibility with SCFs 148, 172f, 244,358 -,pressure effect 361 -,solubility in liquid solvents 173,358 Hydrogen atom abstraction 179, 198 Hydrogen bonds, in supercriticalalcohols 166 -,in scH2O 166,208,274f Hydrogen bromide, as SCF 6, 9 Hydrogen chloride, as SCF 6.9.24 Hydrogen complexes, see Dihydrogen -+

'

471

complexes Hydrogen iodide, as SCF 6,9,24,238 Hydrogen peroxide, synthesis of 8 1 Hydrogen sulfide, reactions of 140 Hydrogenation,see also Transfer hydrogenation -,of anthraquinone 8 1f, 82 -,of C02 358ff,359 -,continuous reactor system for 82 -,of cyclopropene 362 -,of dienes to monoenes 363 -,enantioselective 360ff. 360,363f, 364, 408 -,of fatty acids and derivatives 29f -,during F-T synthesis 393 -,heterogeneouslycatalyzed 23,29f, 81f, 408f -,of imines 3635 364 -,inhibition by C02 408 -,of nitrogen 25 -,of polyethylene double bonds 152 -,of a&-unsaturated acids and esters 360ff, 360 Hydrolysis, of acetals and ketals 27 1 -,of aniline 23 -,of cellulose 268f of nitriles 370 -,of polyethylene terephthalate 268 Hydroperoxides 266,377f, 403,409 Hydrosilylation,of scCO2 360 Hydrothermal synthesis 20f, 213,214f, 221ff, 227 Hydrotreating 409,410

-.

Ibuprofen 369,424f Imines, hydrogenation of 3635 364 Impregnation,of inorganic supports 254 -,of polymers 151f. 197,254,299 Indoles, solubility in scC@ 50 Industrial applications of SFR, cost of 12f, 441f -,examples of 25ff, 215,224,228,230,234 Initiators, see Polymerizationinitiators Interface, catalyst-SCF 355,377 -,enzyme-SCF 434 -,polystyrene-C02 130 Interfacial tension 128 Ion pairing 203f Ion moduct of water 267.275 IR siectroscopy, see FHR; NIR;TRIR;

472

Index

Step-scan FTIR Iridium complexes 363f Iron catalysts, heterogeneous 25,395 Iron complexes 379 Isobaric separation (retrograde crystallization) 52 Isobutane, oxidation of 399ff Isobutene, polymerization of 3 17 Isomerization, see also Photo-Fries reaction; Tautomerization; Rearrangments -,ofalkanes 23 -,of alkenes 267,272,285,368,398 -,of complexes 160f -,of cyclopropanes 267 -,of glyceraldehyde 269 -,of polyethylene double bonds 152 -,of stilbene 285 -,of xylenes 399 Isophorone, dimerization of 284,285f Isothermal separation 52

Liquids, swelling by dissolved gases 113,355 Liquid-vapor equilibria 42ff, 46 Local density augmentation 4, 199,200, 281,287ff, 291ff Long-chainbranching index 333,339f Lower critical end point 43f, 45

Macrocycles 373 MAGROFI 187 Manganese complexes 137, 179ff, 180,36 MA0 380 Marketing 13 Mass transfer, see Diffusion Materials synthesis 12,215,216,224,234 Matix isolation 247 McCabe-Thiele method 101 Melamine, synthesis of 26 Melting point lowering 18,46, 116 Metal carbenes 37 lf, 382f Metal carbonyls, catalysis with 357,362, Joints, see Connectors 365ff, 3695 377f -,photochemistry of l58ff, 159,160,198, Kamlett-Taft solvent polarity scale 197 244ff, 253,255 Keto-enol equilibria 267,275 -,reactions of 174, 177ff Kinetic model, for copolymerization 347f -,solubility of 175,357 -,for ethene polymerization 333ff, 333 Metal complexes, see Coordination Kinetics, see also Rate coefficients; Rates of compounds; reaction Metal-complex catalysis, classification of -,of enzymatic reactions 423f 3545 354 -,of hydroformylation 367 Metal hydrides 177ff, 247,351,358, 362, Krypton, as SCF 6,267,409 365 Metallocenes 380 Lactones 371,379 Metathesis, of acyclic dienes 372 Latexes 127, 130, 135, 142ff -,ring closing 371ff Lattice fluid self-consistentfield theory 127, -,ring opening polymerization 3 19,371, 139,308 382f, 383 LD-MS 425 Methane, partial oxidation of 2675 409 LDPE, see Polyethylene -,properties of 7 Ligands, see also enties for specific Methanol, properties of 7 compounds -,as SFX medium 237,275 -,solubility of 357 -,synthesis of 26,409 -,substitution of 245ff Methyl acrylate, see also Polymerization -,weakly coordinating 244ff -,Diels-Alder reaction of 64 Lineweaver-Burk plot 424 Methyl formate, synthesis of 359f Linewidth, in Raman spectroscopy Methyl methacrylate polymerization 142, 150f, I51 310,312ff, 329,345 -,of quadrupolar nuclei in NMR 173f, 173, Methyl orange 132 174,175, 188f Micelles, see also Emulsions; Lipase 416,424ff Microemulsions; Surfactants

Index

-,during dispersion polymerization 311

-,for NMR of biomolecules 192 Michael addition 275 Michaelis-Mentenkinetics 423f Microemulsions,see also Emulsions; Surfactants 127ff -,dispersion polymerizationin 308ff -,enzymatic reactions in 154 -,FTIR spectroscopyof 153 -,inorganic reactions in 154 -,intermicellar exchange 199 -,neutron scattering 134 -,organic-COz 135 ' -,pH of 132f -,polarity of 132 -,size of droplets of 131 -,stability of 144 -,structure of 199,207f -,in supercritical alkanes 308 -,in supercritical xenon 308 Microscopy, of emulsions 139 Mineralizers 215,216,224 -,fluoride 225,228,232,233 -,hydroxide 224,226 -,iodide 236,237 Minerals, solubility and transport of 233 -,synthesis and reactions of 20f. 214f, 221ff, 226ff Miscibility of SCFs with other gases 12, l48,172f, 244,358 Mixer, see Stirrer Mixtures, see Phase; Solubility Mole fraction 59 -,effect on enzyme reactions 433 -,effect on selectivity 373,381 Molecular weight control 298,303,306, 310,317f, 321,336f, 339f, 380,382 Molecular weight distribution, simulation of 329,339f, 341,348 Molybdates, synthesis of 233 Molybdenum catalysts, heterogeneous 400 Molybdenum complexes 372,377f, 382f Morey vessel, see Pressure vessels Multicomponent process simulation 102 Naphthalene solubility 49,51 NaphthaleneKO2diffusion coefficient 55 1,4-Naphthoquinone,solubility in scCHF3 49 NBS 290

473

Near-critical region, definition 2

-,effect on reaction rates 24, 197,266,276,

281,287ff Near-critical fluids, extraction in 19 Nearest neighbor atoms 200 Neodynium complexes 380 Neutron diffuse diffraction 206,208f Nickel, as heterogeneous catalyst 22f Nickel complexes 37Of Nickel ions in scH20 203f NIR 155,327f Nitricoxide 23 Nitrides, synthesis of 234,236f Nitrogen, hydrogenation of 25f -,removal from organic compounds 407, 409 Nitrogen complexes, see Dinitrogen complexes Nitrous oxide as SCF 6.9 NMR imaging 187f, 187 NMR spectroscopy 172ff -,apparatus 172 -,of biomolecules 192 -,I3C 168f, 183,187 -,capillary cell for 166,245 -,cavity detector 170,171, 187 -,chemical exchange 174,177ff -,59Co 172ff, 173,177,179,181, 184f, 185, 365 -,in coordination chemistry 165ff, 245f -,53Cr 191 -,critical point determinationby 189f, I 9 0 -,detector principle 167 -,for diffusion measurements 173, 187 -,'H 169ff, 178,192 -,at high frequencies 166, 17Of -,line-narrowing 173f, 173, 174, 188f, 191 -,5%4n 180ff, 181, 191 -,monitoring of reactions 177ff, 184ff, 185, 186,189,364,365,379 -, '% 189ff -, 170 189f, 190 -,31P 175,357 -,probe designs 165f -,probe heating 170 -,of proteins 192 -,of quadrupolar nuclei 173ff, 173, 174, 175, 188ff, 190 -, 187Re 174, 191 -,relaxation times 173, 187f, 19Off

474

Index

-,resolution 171, 191 -, 169 -,sapphire tubes 166,189 -,sensitivity 169, 191 -,stirrer for 176 -,thermodynamic data from 177ff, 178, 180,181,183 -,toroid probes 167ff, 168,171, 187 Noble gases, coordination compounds of 159f, 159,244ff -,as liquid solvents 245,247 -,as SCFs 159,248 Non-linear optical materials 224 Norbomene, polymerization of 3 19,382f Nomsh photo-cleavage 287 Norsorex, see Polynorbomenamer Nucleophilic substitution 270,447ff, 45 If O-rings, see Seals Oil, upgrading of 410 Oil cracking 26 Oil shale extracts 410 Olefins, see Alkenes Olefin metathesis, see Metathesis Omega phase 448 Opalescence 4,18 Organometallic compounds, see Coordination compounds Oxalate, synthesis of 266 Oxetanes, polymerization of 304,3 17f Oxidation, see also SCWO -,by air 399ff, 409 -,of alkanes 26,2675 379,399ff, 3 W ,409 -,of alkenes 3775 452 -,of allylic alcohols 378 -,of ammonia 24 -,enantioselective 378 -,enzyme catalyzed 426f, 443 -,formation of metal peroxocarbonates 378f -,metal-catalyzed 377ff -,of methane 267f, 409 -,with 0 2 26, 378f, 399ff, 409,426 -,with peroxides 377f -,ofS02 24 -,in water-scC02 377 Oxides, synthesis of 226ff Oxiranes, see Epoxides 0x0 process, see Hydroformylation Oxygen, see also Oxidation -,as initiator of polymerization 27

Palladium, colloidal catalysts 375 -,complexes 371,373ff, 384 -,heterogeneous catalysts 29,375,400,408 Papin's digester, see Pressure vessel Para-critical, see Near-critical Particles -,from dispersion polymerization 141ff, 312ff, 314,315 -,from Gas Saturated Solution (PGSS) 18, l09f, 115ff -,size effect on extraction 93 Pauson-Khand reaction 370 PCA l08f, 112ff, 12Off, 124 PDMS, see Poly(dimethylsi1oxanes) Peng-Robinson equation of state 113f, 408 Penultimate unit effect 347f Perfluorinatation, see Fluorination Perfluorinated alcohols as additives 357, 36 1 Perfluorinated ligands, see also Phosphorus ligands Perfluoroethylene, explosion of 8 Peroxocarbonates 378f PFOA, see Poly(1,l -dihydroperfluorooctyl acrylate) PGSS process 18, lWf, 115ff pH, effect on enzyme reactions 134,429 -,of microemulsions 132ff Pharmaceuticals 12, 117, 120, 123f, 124 Phase behavior, see also Solubility -,of binary systems 41ff -,during extraction 99ff -,importance of 260 -,of microemulsions 131 -,of orange peel oil and C02 100 -,simulation of 83 -,view cells for observation of 83f, 83, 130 Phase diagram, of binary mixtures 41ff. 42, 44,45 -,forCO2 2 -,mixture critical line 42 -,for pure substances 38 Phase separation 52 Phase transfer catalysis 14Of, 446ff Phenols, enzymatic oxidation of 426f -,polymerization in C02 3 19 Phenylacetylene, polymerization of 383 Phosphates, synthesis of 229ff Phosphorus ligands, alkyl substituted 357f, 361f, 366,369,371f, 379,382f ~

Index

-,aryl substituted 357,36Of, 363,369,370 -,fluorinated 357,363,366ff, 374,383 nomenclatureof fluorinated 367 -,solubility of 3575 360,363,369ff, 374 Photochemicalreactions, 57f, 149, 196ff, 244ff, 280ff, 284 Photo-Fries reaction 57f, 288 Photolysis 196, 197f -,of dibenzyl ketone 287f -,of metal carbonyls 245,248ff, 253f,255 -,of naphthyl acetate 288,289 Phthalic anhydride, esterification of 266 Physical properties of SCFs vs. liquids and gases 2ff, 38 Pitch, hydrotreating of 409 Platinum catalysts, heterogeneous 398,407, 408 Pneumatolyticreactions 213 Polarity, see Dielectric constant Poly(acry1amide) 3 15f Poly(acry1ic acid) 156, 305f Poly(acrylonitri1e) 305 Polycarbonate,plasticization of 299, 3 16 -,synthesis of 24,319,381f Polycondensation 270,320 Poly( 1,l-dihydropeffluorooctylacrylate), phase behavior in C02 136,313 -, as surfactant 128f, 136, 143ff, 31Off, 320 -,synthesis of 300f Poly(2,6-dimethylphenylene oxide) 320 Poly(dimethylsiloxanes), micelle formation 31 1 -, as surfactants 128f, 143,310ff -, synthesis of 3 11 Polyester, hydrolysis of 268 -, plasticization of 299, 321 -, synthesis of 320 Polyethylene, see also Ethene polymerization 326ff -,isomerization of double bonds 152 -,low-density 27f, 327,380 -,plasticization of 3 16 -,reduction of double bonds 152 Poly(ethy1ene terephthalate),hydrolysis of 268 -,plasticization of 299, 321 -,synthesis of 320 Poly(2-ethyl hexyl acrylate)emulsions 136 PolvMuoroacrvlate) 129 PoG(F0x-7) 504 '

-.

475

Polyisobutylene 3 17 Polymer, see also Copolymer -,blends 316 -,drying 152,298 -,extraction 152,298,313 -,fractioning 27,298 -,impregnation 151f, 152,197,254,299 -,modification 151f, 316,326 -,particle number density 143 -,particles 123f, 124, 142f, 307ff, 314,315 -,plasticization 142, l97,298f, 313f, 316, 32 1 -,processing 27,298,327 -,solubility in COz 136, 142, l55,299ff, 307ff -,structure in scCO2 142,208,309,31 If -,swelling 151,299,316 Polymerization,see also Copolymerization; Telomerization;entries for specific monomers and polymers -,of aromatic esters 320 -,cage effects in 57,302,334 -,in C02 297ff, 300 -,dispersion 142f,307ff -,emulsion 142f,307 -,of fluorinated monomers 300ff -,free-radical 142f, 300ff, 305ff, 326ff, 331f -,homogeneous 300ff, 326ff -, of hydrophilic monomers 3 15f -, IR monitoring of 155,156,327f, 328 -, kinetic model for 333ff -, of lipophilic monomers 142f, 3 12ff -, metal-complex-catalyzed 3 19f, 38Off,

383

-, precipitation 27,305ff, 382

-,rate coefficients of 329f, 333ff -, of reactive monomers 298,302f, 306,321 -,in reactive SCFs 326ff -, reactors for 27,80, 131, 331ff, 332 -,UV monitoring of 302 Polymerizationinitiators -,AIBN 27f, 302f, 305f, 312f, 316 -,alkyl peroxides 305f -,benzoyl peroxide 27,305f, 316 -,for cationic chain growth 303,317f -,gamma radiation 305 -,laser pulses 328ff, 329, 330 334ff, 345ff metal comDlexes 319f. 38Off -,oxygen 27

-.

476

Index

-,TEMPO 306 -,water-soluble 321 Poly(methy1 methacrylate) 142,299,310, 312ff, 329,345 Polynorbornenamer 3 l9,382f, 383 Poly(perfluoroethyleneoxymethy1styrene) 301f Poly(phenylacety1ene) 383 Polysiloxanes, see also Poly(dimethylsi1oxanes) 3 18 Polystyrene, copolymers of 305,3 11, 3 17 -,with perfluorinated side chain 301 -,plasticization of 299 -,polymer blends of 316 -,synthesis of l42,305f, 313ff Poly(tetrafluoroethy1ene) 302f, 306 Poly(viny1 acetate) 142f. 305 Poly(viny1 chloride) 299, 305 Poly(viny1 ether), copolymers of 306 -,synthesis of 303f, 317f Poly(viny1idene fluoride) 299 Ponchon-Savarit method 101 Porphyrin ligands 379 Poynting correction 111 Precipitation, see also Extraction -,with antisolvent l08f, 112ff, 12Off, 121, 122,135 -,by pressure reduction 11, 18,27,52, losf, 117ff. 117,245,250,251,439f, 440 -,by temperature increase 52,352 PREDICI simulation 329,339f, 340,348 Pressure, critical (table) 6f, 41 -,reduced 1,38 -,units of 68f, 455 Pressure drop during extraction 103 Pressure effect, on enzyme reactions 43 Iff -,on rate 64,l98,262f, 266,281,287ff -,on selectivity 62f, 262f, 266,285ff, 361, 373,375f, 402,410,436ff Pressure vessels, see also View cell; Reactor systems; Spectroscopiccells -,of Andrews I7 -,Bertytype 408 -,with Bridgeman seal 214,217f, 218,225 -,of Cagniard de LaTour 14,15, 16 catalysis by steel walls 409 -,chemical resistance 8,69,71ff, 217,226 -,compound cylinder 69f, 71 -,design of 68ff . -,digestion bombs 2 16

-.

-,fixed bed flow type 84,390,399,403,408 -,fluidized bed 84 -,glass tubes 15, l5f, 17,216 -,for hydrothermal synthesis 216ff, 218, 222,225 -,Ipatiev’s bomb 2 1,22 -,Morey vessel 214,217,219 -,Papin’s digester 13,14, 15 -,for photochemical reactions 282 ,282ff -,for polyethylene synthesis 27 -,quartz tubes 216,220,221,238 -,for SFRs 7 , 8 -,trickle-bed 390,409 -,Tuttle cold seal 218,218f -,windows for 78, 79, 130, 149f, 149, 195f, 202,282,282ff, 328 Process benefits of SFRs 9ff Process schemes, see Reactor systems Process simulation 83, 102 Product separation, see Extraction; Precipitation; Phase separation Propane, as SCF 7,199,395,404ff, 425, 428f Propanol, synthesis from propene 28 Propene, alkylation of 407 -,hydration of 28 -,hydrochlorination of 24 -,properties of 7 -Protecting group, scCO2 as 373 Proteins, NMR of 192 -,denaturation of 425ff Pseudo equilibrium constant 60 Pulsed laser polymerization 329ff. 329, 330, 3358 345ff Pumps, compressors 76ff -,cooling of 6,353,420 -,pneumatic 249 -,problems with 142,419 -,reciprocating 76ff, 77, 142,419f -,for recirculation 419 -,syringe 76ff, 77 2-Pyridone 63 Pyrolysis 270 Pyrones, synthesis of 370f Quartz, synthesis of 21,214f, 224,226 Radial distribution function 200 Radial structure plot 203f Radical, see also Cage effects;

Index Polymerization Radical reactions 57,265,270,276,287f, .289ff, 362 -,monitored by step-scan FTIR 16Of, 160 Raman spectroscopy 148, 150ff, 260,274f -,capillary cell for 150 -,of scCOz-Hz mixtures 15Of, I51 -,time-resolved 156f Rate, see also Activity Rate coefficients, for acrylate polymerization 345ff, 346 -,for copolymerization 344ff -,for ethene polymerization 3295 333ff Rate equation, from transition-state theory in SCFs 60 Rates of reaction, see also Near-critical effects -,increases in 24,307,358,3635 367f -,pressure effects 24,64, l98,262f, 266, 281,287ff -,viscosity effects 281, 287ff, 290ff RCM, see Metathesis Reactive SCFs Sf, 326 Reactivity ratio 341ff, 343,347 Reactor systems, see also Pressure vessels; Spectroscopiccells 78ff -,batch-type SOff, 80,352ff, 353,416ff, 417,418 -,for continuous extraction 84ff, 85,96, 98 -,for continuous or semi-continuous reactions 8 1f, 82, 25 1ff, 252,253,255, 256,33lff, 332,390,399,403,408f, 417, 420f -,for enzymatic reactions 416ff, 417,418 -,for Fischer-Tropschsynthesis 390 -,for homogeneous catalysis 352ff, 353 -,for hydrogenation 82,408,410 -,for phase transfer catalysis 451 -,for photochemical synthesis of coordinabion compounds 25 lff, 252,253, 255 -,for polymerization 80,131,331ff, 332 -,forSFC 105 -,for thermal synthesis of coordination compounds 254,256 Rearrangements,see also Isomerization; Photo-Fries reaction; Tautomerization 273f, 274 Recompression,energy costs of 11 Reduction, see also Hydrogenation

477

-,of wastelemission 9ff, 297 Refractive index 264 RESS 18, losf, lI7,117ff, 123ff, 124,245, 250,251,439 Retrograde behavior 52, 109 Retrograde crystallization 52,352 Reverse micelles, see Emulsions; Micelles; Microemulsions Rhodium complexes 362f, 366ff, 370,375f, 379,383 ROMP, see Metathesis Rotating-frameimaging 187 Rotational correlation times 199 Rupture disk 74f. 76 Ruthenium catalysts, heterogeneous 390ff. 408 Ruthenium complexes 319,358ff. 370, 371ff. 377,382f Safety, benefits of SFRs 10 -,hazards of SCFS Sf, 68 -,pressure relief devices 74f, 76,418 -,shields 8, 250 Salen-type ligands 378 Salt hydrates 43 If Sample additiodwithdrawaltechniques 76, 82,82,131,142,284,353,418 SAS l08f, 112ff, 12Off, 124 SCWO 10,28,40,377 Seals, Bridgeman 72, 72,214,217f, 218 -,cone 218,22 1,222 -,metal-to-metal 72 -,O-ring 71f, 72,419 -,soft gasket 71, 72 -,Tuttle cold 218,218f Selectivity, effect of density 62f, 65, 373 -,effect of pressure 62f, 262,266,285ff, 361,373,375f, 402,410,436ff -,by extraction of kinetic product 450 -,factors affecting 12 -,in F-T synthesis 396 -,in homogeneous catalysis l86,319,361f, 367f, 372f, 375,381,383 Semiflow reactor, see Reactor systems Separation factor 99f, 100 Separation, see CESS; Chromatography; Extraction; GAS; Phase; Precipitation; RESS -. general scheme of 88f, 89 SFC, see Chromatography

478

Index

Silanes 9,360 Silicdtitania heterogeneous catalyst 399ff Siloxanes, polymerization of 3 10,318 Size-exclusion chromatography 329 Small angle neutron scattering (SANS) 206ff Small angle X-ray scattering (SAXS) 206ff Smith-Ewart kinetics 307 Smoluchowski equation 56 Solid-liquid-vapor equilibria 46 Solubility,see also entries of individual compounds and classes of compunds -,dampening of 50 -,density and 48 -,enhancement factor 48,49, 111 -,equilibrium 91 -,functional groups and 49 -,influence of reactant gases on 358 -,influence of substrates on 365 -,intermolecularinteractions and 49 -,measurement of 196 -,pressure and 5 1, 109ff -,solvent polarity and 48 -,temperature and 51, loSf, 110 -,tuning of 4, 11,89f, 104 -,vapor pressure and 49 Solute-solute interactions 198, 199 Solvation 197, 198 -,of ions in SCFs 275 Solvation effects, on equilibria 54,63 -,on transitions states 54 Solvatochromic probes 196f, 264 Solvent polarity, see Dielectric constant Solvent power 4, 11, 89f, 104 Solvent residues 1If Solvent-solute interactions 197, 198, 199 Sonication 27 1 Sonogashira coupling 374 Spectroscopiccells, see also Pressure vessels; Reactor systems -,for EPR spectroscopy 130,131,200 -,for fluorescence spectroscopy 130,131, 198 -,for FTIR spectroscopy 130,131,149, l57,158,245,260,261,327f,328 -,for NMR 166ff, 168,171,172,189,245 -,pathlength, variation of 149,155 -,for UV-Vis spectroscopy 130,131, 195f, 196,260,261 -,windows, see Pressure vessels, windows

forXAFS 202 Stabilizer for dispersion polymerization, see Surfactants Steel, catalysis by 409 -,chemical resistance of 8,69,71ff, 217, 226-,properties of 69f Step-&an FTIR 157ff -,apparatus for 158 -,comparison with TRIR 159 -,detection of noble gas complexes 159f, 159 -,reaction monitoring by 160f, I60 Stilbene, isomerization of 285 Stifle coupling 374f Stirring 39,78,79, 173,176,418f Stokes-Einsteinequation 56 Structural parameter 200 Styrene, cyclopropanation of 375f -,polymerization of l42,302,305,313ff, 329 Subcritical gases 3 Subcritical liquids 3 Substitution,organic reactions 14Of, 270, 447ff, 451f -,of hazardous solvents and compounds 10f -,of ligands 244ff, 253ff Sulfides, synthesis of 232f, 234ff, 237 Sulfur dioxide 140 Sulfur hexafluoride 6,425,428f Supercriticalfluids, definition 1 -,physical properties 2ff, 38ff -,prices of 6 Supercriticalwater oxidation, see SCWO Supersaturation 112 Surfactants,see also Emulsion; Microemulsions; Micelles -,adsorption of 129,135 -,aggregation of 134,307 -,amide-capped 315 -,anchor-soluble balance of 308, 3 14 -,AOT 134,153,275 -,for biomolecules 134, 154, 192,443 -,cloud point 131f, 139, 143 -,design of 127f, 307ff -,di-chain sodium sulfosuccinate 134,309 -,for dispersion polymerization 142ff, 307ff, 312ff, 314,315,320 -,hydrophilic-lipophilic balance of 308 -,lattice fluid self-consistentfield

Index description 308

-,in metal-catalyzed polymerizations 320 -,perfluoroalkyl-based l34,309f, 314

-,PFPE-based 131ff, 134f, 137f, 14Of, 153 -,polymer adsorbed 307ff

-, polymer-based 128, 142f, 309ff;31 I

-,polymer grafted 3 10 -,spectroscopy of 153,207f -,surface tension of 128ff, 128,129 Suzuki coupling 374f Swelling, of catalysts or enzymes 58 -,of liquids 113,355 -,of polymers 151,:299, 316 Synchrotron 201

Tautomerization 63,267,275 Telomerization 302f Temperature, critical (table) 6f, 41 -,reduced 1,38 TEMPO 133,306 Tensiometer, pendant drop 128 Tetrafluoroethene, polymerization of 302f, 306 Tetrahedrite, synthesis of 221ff Tetralin, supercritical 28,326 Theoretical stages, see also Equilibrium stages 99 -,height of 99f, 102f THF, oxidation of 379 Thorne-Enskog theory 55 Three-phase lines, LLV or SLV 43,44,46 Tie line 39,45 Tin hydrides 265 Titanium complexes 378,380 TR3 156f Transfer hydrogenation 28 Transition metal complexes, see Coordination compounds Transition state theory 55, 58, 264 Transport phenomena, see Diffusion Triphenylphosphine, perfluorinated derivatives of 367ff, 383 Triphenylphosphite, perfluorinated derivatives of 367 Triplet-triplet annihilation 198 TRIR 148,157ff -,apparatus for 157 -,comparison with step-scan FTIR 159f, 159 -,detection of C02 complexes 158f

479

-,detection of noble gas complexes 159f. 159 Tubing 73f, 418f Tungsten catalysts, heterogeneous 400 Tungsten complexes 158,159 Turbidimetry 135, 142 Ultrasonic irradiation 271 Units, conversion factors for 455f Upper critical end point 43,44 Ureas, polymeric 24 UV-Vis flash photolysis 156 UV-Vis laser pulse, see Pulsed laser polymerization UV-Vis spectroscopy, cells for 130, 195f, 196 -,monitoring of reactions 82, 140, 195ff, 302 Valves, see also Backpressure regulator -,ball 74 -,check 74f, 76 -,injection 76,82,82 -,needle 74f, 75,419 -,pressure relief 75, 76 Vanadates, synthesis of 233 Vanadium complexes 378 Van3 Hoff plots 177ff, 178,180,181, 183 Vapor, definition 1 Vibrational spectroscopy, see FTIR; Raman Video cameras, for use with view cells 84 View cell, variable volume 83f, 83, 130, 141,250 Vinyl acetate, dispersion polymerization of 142 Vinyl arenes, hydroformylation of 367, 369 Vinyl ethers, polymerization of 303f, 306, 317f Vinylic couplings 373ff Viscosity, diffusivity and 56 -,effect on linewidth in Nh4R 173f, 173, 175 -,effect on polymerization rates 330,334ff, 347 -,effect on reactivity 281, 287ff, 291ff -,of SCFs (compared to liquids and gases) 38 Vitamin precursors, hydrogenation of 29 Volume, units for 456 Volumetric data for SCFs 6f, 2 19,220

480

Index

Volumetric expansion of liquids 113 Volumetric susceptibility, from NMR 182 Waste oxidation, see SCWO Water, corrosion by 219f, 226 -,decomposition of glass by 20, 216 -,dielectric constant 5,267,226 -,effect on enzyme reactions 425ff. 430, 432 -,hydrogen bonding in 166,208,274f -,ion product 267,275 -,metal-complex catalysis in 3695 375 -,NMR 166,191 -,properties 6,41 -,reactions of minerals in 20,214ff Water-in-scC02 microemulsions, see Microemulsions Water-to-surfactant ratio 13 1 Wax production, by F-Tsynthesis 395ff Windows, see Pressure vessels, windows for

Xenon, coordination compounds of 159f, 159,248 -,microemulsions in 308 -,properties of 6 -,as SFR medium 159,248,267,409 X-ray crystallography 1 15 X-ray diffuse diffraction 206,208f Xylenes, isomerization of 399 -,synthesis of 409f Yield pressure 69 Young-Laplace equation 129 Z values 264 Zeolites, as catalysts or supports 398,407, 409f -,synthesis of 215,216 Ziegler catalysts 380 Ziegler reaction 290 Ziegler-Ruggli principle 373 Zinc complexes 380,382 Zirconium complexes 380