Advances in Catalysis, Volume 32

ADVANCES IN CATALYSIS VOLUME 32 Advisory Board M. BOUDART Stanford, California M. CALVIN Berkeley, California V. B...

Author: D. D. Eley | Herman Pines | Paul B. Weisz

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ADVANCES IN CATALYSIS VOLUME 32

Advisory Board

M. BOUDART Stanford, California

M. CALVIN Berkeley, California

V. B. KAZANSKY Moscow, U.S.S.R.

G . A. SOMORJAI Berkeley, California

P. H. EMMETT Portland, Oregon

A. OZAKI

G.-M. SCHWAB

Tokyo, Japan

Munich, Germany

R. UGO Milan, Italy

ADVANCES IN CATALYSIS VOLUME 32

Edited by

D.D. ELEY TIw Uniivrsity Nottinghatn. Englerncl

HERMANPINES Northit~t~.srern Uniiwsity Ei,trnsion,

Illinois

PAULB. WEISZ Mohil Rr.serirc,h crnd

DcJidoptnc,nr Cotportit ion Princeton. NeM' Jcwev

1983

ACADEMIC PRESS A Subsidiary of Harcourt Brace Jovanovich, Publishers

New York Paris

San Diego

San Francisco

London

Sao Paulo Sydney Tokyo Toronto

COPYRIGHT @ 1983, BY ACADEMIC PRESS,INC. ALL RIGHTS RESERVED. NO PART OF THIS PUBLICATION MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM O R BY ANY MEANS, ELECTRONIC OR MECHANICAL, INCLUDING PHOTOCOPY, RECORDING, OR ANY INFORMATION STORAGE AND RETRIEVAL SYSTEM, WITHOUT PERMISSION IN WRITINQ FROM THE PUBLISHER.

ACADEMIC PRESS,INC. 111 Fifth Avenue, New

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United Kinadom Edition oitbhlred bv ACADEMiC PRESS, INC. ( L O N D O N ) LTD. 24/28 Oval Road, London NWI

1DX

LIBRARY OF CONGRESS CATALOG CARDNUMBER:49-7755 ISBN n-12-007832-5 PRINTED I N THE UNITED STATES O F AMERICA 83 84 85 86

9 8 7 6 5 4 3 2 1

Contents CONTKIBUK~KS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . P u E r ~ r.t. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

iX

xi

Characterization and Reactivity of Molecular Oxygen Species on Oxide Surfaces M. CHLA N D A. J . TENCH

I. 11.

Ill. IV. V. VI. VII. VIII.

Introduction ............................................... Neutral Oxygen Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . characterization of Charged Dioxygen Species. . . . . . . . . . . . . . . . . . . . . . . . . . Formation and Stability of Charged Diatomic Species. . . . . . . . . . . . . . . . . . . . Oxygen Ions Containing More Than Two N Reactivity of Molecular Ions . . . . . . . . . . . . . The Relation of Mononuclear Surface Oxyg Spectroscopic and Catalysis Studies . . Comparison of Oxygen Species and Appendix A. Summary of g,, Value Appendix B. The Experimental "0 Diatomic Oxygen Species ( 0 2and ROO) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . f Oxygen Species by Infrared Spectroscopy Appendix Reference ....................................... Note Added in Proof . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 3 X 36 X2 98

109 Ill 123 12x

130 134

148

Catalysis by Alloys in Hydrocarbon Reactions VLADlMlK PONEC

I. 11. 111. IV. V. VI.

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Particle Size Effects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanism of Hydrocarbon-Hydrogen Reactions . . . . . . . . . . . . . . . . . . . . . . . Hydrocarbon Reactionson Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

149

151 159 162

186 205 206

Modified Raney Nickel (MRNi) Catalyst: Heterogeneous Enantio-Differentiating (Asymmetric) Catalyst YOSHIHARU IZUMI 1. 11.

What Is MRNi? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 History of Discovery and Development of MRNi.. . . . . . . . . . . . . . . . . . . . . . . 218 V

vi

CONTENTS

111. IV.

V.

v1. VII. VIII. IX. X.

Profile of MRNi in Hydrogenation.. , . . Profile of MRNi in Stereo-Differentation . . . . . . . . . . . . . . . . . . . . . . . ..................... Other Profiles . . . . . . , . . . . . . . . . . . . . . . . Surface Conditions. . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanism of Enantio-Differentiation. .................... Characterization of Catalyst by Modify .................................. TA-NaBr-MRNi . . . Other Investigations. , . . . . . . . . . . . . . . . . . . . . . . . . . . . .................................. References . . . . . . . . .

224 229 248 249 254 262 264 267 269

Analysis of the Possible Mechanisms for a Catalytic Reaction System JOHN

I. 11.

Ill. IV.

V.

VI. VII.

HAPPEL A N D PETERH. SELLERS

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

274 278 General Formulas for Mechanisms and Reactions . . . . . . . . . . . . . . . . . . . . . . 283 287 A Procedure for Finding Every Direct Mechanism Systems with a Simple Overall Reaction.. . . . . . . . . . . . . . . . . . . , . . . , . . . . . . . 29 I 300 Overall Reactions with a Multiplicity Greater Than One.. . 317 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320 List of Symbols . . . . . 32 1 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.

Homogeneous Catalytic Hydrogenation of Carbon Monoxide: Ethylene Glycol and Ethanol from Synthesis Gas B. D. DOMBEK 1.

II. 111.

1V.

V.

VI. VII.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326 Cobalt Catalysts.. . . . . . . . . . . . . . . . . . . . . . . . Rhodium Catalysts. . . . . . . . . . . . . . . . . . Unpromoted and Carboxylic Acid-Promoted Ruthenium Catalysts. . . Lewis Base-Promoted Ruthenium Cata Other Catalysts.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . 408 References .......................................................... 410

Cyclodextrins and Cyclophanes as Enzyme Models IWAOTABUSHI AND YASUHISAKURODA 1. 11.

Introduction.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 417 Basic Principles of Molecular Recognition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 420

CONTENTS 111 .

IV . V.

vii

Enhancement of Binding and Catalysis by Host Design . . . . . . . . . . . . . . . . . . 436 Enhancement of Binding and Catalysis by Guest Design . . . . . . . . . . . . . . . . . 456 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 461 462 References and Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

AUTHOR IN1)EX

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

INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SUBJECT OF P~iiviousVOLUMES .............................................. CONTENTS

467 494 509

This Page Intentionally Left Blank

Contributors Numbers in purentheses indicute the puges on which the authors' contributions begin.

M. CHE,Laboratoire de Chimie des Solides, ER 133, C N R S , UniversitP Pierre et Marie Curie (Paris V l ) , 75230 Paris Cedex 05, France ( 1 )

B . D. DOMBEK,Union Carbide Corporation, South Charleston, West Virginia 25303 (325) JOHN HAPPEL,Department of Chemical Engineering and Applied Chemistry, Columbia University, New York, New York 10027 (273) YOSHIHARUIZUMI, Institute for Protein Research, Osaka University, 3-2 Yamadaoka, Suita, Osaka 565, Japan (215) YASUHISAKURODA,Department of Synthetic Chemistry, Kyoto University, Kyoto 606, Japan (417) VLADIMIR PONEC,Gorlaeus Laboratoria, Rijksuniversiteit Leiden, 2300 R A Leiden, The Netherlands (149) PETERH . SELLERS,The Rockefeller University, New York, New York I0021 (273) IWAO TABUSHI,Department of Synthetic Chemistry, Kyoto University, Kyoto 606, Japun (417) A. J . TENCH.*Chemistry Division, Atomic Energy Research Establishment, Harwell, Oxfordshire OX11 ORA, United Kingdom ( 1 )

*Deceased.

ix

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Catalysis embraces a variety of fields, and it is the aim of the Editors to see that each volume of Advances in Catalysis contains articles spanning a broad spectrum of interest. The opening contribution, by M. Che and A. J . Tench, is a survey of work on adsorbed molecular species and their role in oxidation reactions. This article, together with its companion piece in Volume 31 by the same authors, will stand as a memorial to A. J . Tench, who died on March 17, 1983 from Hodgkin’s disease, two days after submitting the contribution presented here. The importance of catalysis by alloys is well recognized in the petrochemical industry. By means of alloying. dramatic changes can be achieved in the stability and selectivity of metal catalysts. The last decade has witnessed a renaissance in alloy research, and the review by V. Ponec gives a comprehensive survey of this active field. Great strides in developing highly selective enantio-differentiating (asymmetric) catalysts have been made by modifying Raney nickel. Y. Izumi. a pioneer in this area of endeavor, surveys this field. Catalytic reactions proceed through a network of intermediates that are connected by elementary reactions. To explain a catalytic reaction it is necessary to consider how steps may be combined in appropriate proportions. The article by J . Happel and P. H . Sellers reviews methods to achieve it. Hydrogenation of carbon monoxide by heterogeneous catalysts has been studied for decades: it was surveyed in Volume I of this publication. The use of homogeneous catalysts for this type of reaction is, however, of a more recent vintage, and opens new synthetic feasibilities. Conversion of carbon monoxide to two carbon atom compounds is reviewed by B. D. Dombek. Cyclodextrins, also called cycloamylases, doughnut-shaped oligosaccharides, have attracted much attention as enzyme models. Although this area of research was surveyed in Volume 23, much subsequent progress in this field through multifunctionalization of cyclodextrin necessitates a new review. This contribution was written by I . Tabushi and Y. Kuroda. active researchers in this area.

HERMANPINES xi

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ADVANCES I N CATALYSIS. VOLUME 32

Cha racte r izat io n a nd Reactivity of Molecular Oxygen Species on Oxide Surfaces M . CHE

.

Laboratoire de Chimie des Solides ER 133. CNRS UniuersirC Pierre et Marie Curie (Paris V I ) Paris. France AND

A . J . TENCH* Chemistry Division Atomic Energy Research Establishment Harwrll. Oxjordshire. United Kingdom

I . Introduction . . . . . . . . . . . . I1 . Neutral Oxygen Species . . . . . . . . . A . Triplet Oxygen . . . . . . . . . . B . Singlet Oxygen . . . . . . . . . . I11. Characterization of Charged Dioxygen Species . . A . TheO; Ion . . . . . . . . . . . B . The 0: Ion . . . . . . . . . . . C. The 0:-Ion . . . . . . . . . . . D . The 0:- Ion . . . . . . . . . . . IV . Formation and Stability of Charged Diatomic Species A . Ionic Oxides . . . . . . . . . . . B . Transition Metal Oxides . . . . . . . C. Aluminosilicates . . . . . . . . . . D . Supported Metals . . . . . . . . . E . Dioxygen Adducts . . . . . . . . . V . Oxygen Ions Containing More Than Two Nuclei . . A . The 0 ; Ion . . . . . . . . . . . B . The 0;Ion . . . . . . . . . . . VI . Reactivity of Molecular Ions . . . . . . . A . Exchange Reactions . . . . . . . . . B. Oxidation Reactions . . . . . . . . C . Photo-Induced Reactivity . . . . . . .

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

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

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

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

. 2 . 3 . 3 . 6 . 8 . 10 . 33 . 34 . 35 . 36 . 36 . 44 . 51 . 74 . 78 . 82 . 82 . 95 . 98 . 98 . 100 . 105

Deceased March 1983.

I

.

Copyright 0 1983 by Academic Press Inc . All rights of reproduction in any form reserved . ISBN 0-12-007832-5

2

M. CHE AND A. J . TENCH VII. The Relation of Mononuclear Surface Oxygen Species to Electron Spectroscopic and Catalysis Studies . . . . VIII. Comparison of Oxygen Species and Their Role in Catalytic Reactions . . . . . . . . . . . A. Charicterization. . . . . . . . . . . . B. Reactivity . . . . . . . . . . . . . C. Future Directions . . . . . . . . . . . D. Conclusions . . . . . . . . . . . . . Appendix A. Summary of gzz Values for 0;on Surfaces . Appendix B. The Experimental "0 Hyperfine Parameters (in gauss) of Diatomic Oxygen Species (0; and ROO') . . . . . . . . . Appendix C. Characterization of Oxygen Species by Infrared Spectroscopy . . . . . . References . . . . . . . . . . . . . . . Note Added in Proof. . . . . . . . . . . .

1.

. . .

109

. . . Ill . . . Ill

. . . .

.

. . . . . . .

.

116

. 121 123 123

. 128 I30 I34

.

148

Introduction

Oxidation and oxidative dehydrogenation reactions over oxide catalysts have been widely studied in recent years. The precise role of oxygen in these reactions remains elusive, but slowly a more detailed picture is emerging which suggests that both oxide ions of the lattice and oxygen species on the surface can play an important role (1,2). The surface oxygen species can conveniently be divided into two broad classes, i.e., mononuclear and molecular. The mononuclear species such as 0-, 0;;(lattice ions in low coordination), and M=O have recently been reviewed by Che and Tench (1).These are now well characterized and their role in simple and in some more complicated reactions is now better understood. Molecular oxygen species are also formed on the surface and there has been considerable progress since these were last reviewed by Lunsford (3).The characterization of the adsorbed species has improved markedly as isotopic labeling with "0 has become more widely used. Some novel forms of molecular oxygen species have been reported and, in particular, the reactivities of species such as 0; and 0; have been studied. Molecular oxygen species have been also identified as intermediates in some biological reactions and are important as oxygen adducts in natural and artificial oxygen carriers ( 4 , 5 ) . The purpose of this review is to survey the work on adsorbed molecular oxygen species and to show how recent developments point the way toward an understanding of the role that they and the mononuclear forms of oxygen may play in oxidation reactions. The coverage is restricted to those papers where there is direct evidence on the nature of the oxygen species concerned.

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

II.

3

Neutral Oxygen Species

The ground state of the oxygen molecule is a triplet 'C,- state with two unpaired electrons, and at slightly higher energy there are two low-lying electronically excited states, the singlet 'A, and 'C,' levels (6) (Fig. 1). A. TRIPLET OXYGEN The electron paramagnetic resonance (EPR) spectrum of ground-state oxygen has been characterized both in the gas phase and in the solid state. In the gaseous state, the coupling of the spin angular momentum with the end-over-end molecular rotation angular momentum gives rise to an EPR spectrum with many lines covering more than 10 kG (7-9). In the solid state, oxygen has been observed as an impurity in solid N,, CO, Ar, and CD, (10-12). The lines are very broad with g1 = 2.02, gll 0.7, and a zero field splitting of 108 GHz (11).Kon ( 1 2 4 observed an isotope effect between the EPR spectra of l 6 0 l 6 Oand ' s O ' 8 0 ( S = 1) at temperatures below 10 K, which was explained in terms of torsional oscillation of 0, in the matrix around the equilibrium position. In similar work, 0, molecules trapped in NaClO, and KClO, single crystals show well-resolved isotope shifts (I2b). Model calculations reveal that the discrepancy between the spin Hamiltonians of the trapped and the free molecule originates from the angular librations of the trapped molecule.

-

FIG.1 . n, orbital occupancy and energies ( 6 ) of triplet and singlet dioxygen.

4

M. CHE AND A. J. TENCH

It might be expected that effects due to adsorption on the surface would be observable as a change in the EPR spectrum of gas-phase oxygen. Clarkson and Turkevich (13)have adopted this approach and used the variation in linewidth of one of the EPR lines of gas-phase oxygen to follow the adsorption of oxygen on porous Vycor glass at 78.3 K. A best overall fit was obtained using a BET plot, but at low pressures there was evidence for an initial adsorption step of higher energy. Takaishi et a/. (14)have shown that oxygen adsorbed on mordenite gave the same spectrum as that of gaseous oxygen and was freely rotating and translating above 200 K, whereas Seymour and Wood (15) reported two new EPR lines for oxygen adsorbed on carbon corresponding to physisorbed and chemisorbed oxygen. More recently, Lemke and Haneman (16)have carried out a careful study and shown that no changes were observed in the free oxygen lines when crushed silicon was exposed to oxygen in an ultra-high-vacuum (UHV) system at 100 K, although it is known to adsorb on the surface. No new lines were observed and no oxygen lines remained after the system had been evacuated, while maintaining the silicon sample at 100 K. The authors suggest that adsorption on the surface will split the ni and energy levels, and the two electrons would pair up in the lower of the two states to give a nonparamagnetic adsorbed state (Fig. 1). For weak interactions, the splitting may not be large enough for the electrons to be paired all the time and an EPR signal may be observed which decreases as the interaction increases. It should be kept in mind that the crushed silicon used in the work of Lemke and Haneman (16)is relatively low-surface-area material compared to the catalyst supports commonly used and this may account for the different results. The broadening of an EPR signal from a surface species when exposed to oxygen has been known for many years (17). Such broadening is brought about by the magnetic interaction occurring on collision of oxygen with the surface species. Because of the exchange interaction, such broadening of the EPR spectrum is commonly referred to as exchange broadening and is also known in solution (180). Busca (18b) has evaluated the literature values for infrared bands attributed to coordinated and adsorbed dioxygen species. He concludes that it is very difficult to deduce the nature of the dioxygen coordination from measurements of the frequency shift, Avo,, with respect to the stretching frequency of the free molecule. It needs to be stressed that it is also difficult to distinguish between mononuclear and molecular species from measurements of voo, and this can only be achieved by careful interpretation of experiisotopic mixtures. The absence of such experiments ments using 160/'80 very often accounts for the conflicting attributions in the literature which are discussed in later sections. Griffiths et al. ( 1 9 4 have investigated the adsorption of oxygen on cr-Fe,O,

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

5

and observed infrared (IR) bands at 1350 and 1270 cm-', which they assigned to 0, and 0; species. In a later report, Al-Mashta et al. (19b) have reinvestigated the same system and have suggested that both bands at 1350 and 1270 cm-' should be reassigned to perturbed 0; species as discussed in Section III,A,A Davydov et a/.(20a,b)have reported low-intensity absorption bands in the IR on adsorbing oxygen on a range of high-surface-area systems, including TiO,, SnO,, V,O,/SnO,, MoO,/AI,O,, MoO,/MgO, and NiY zeolite. They suggest that bands in the range 1600-1700 cm-' can be attributed to adsorbed molecular oxygen in a neutral state, possibly as singlet oxygen, and the IR transitions being allowed, due to coupling with the lattice. Many likely impurities, such as water and oxides of carbon and nitrogen, absorb in this same region, but the authors argue that these may be eliminated because other associated IR bands are not observed. For comparison, Raman measurements on triplet oxygen gas show a band at 1555 cm-' (21) and it is not clear why such a band would be induced to move to higher energies when oxygen is adsorbed at the surface. According to Al-Mashta et al. (196),a possible explanation is the partial electron withdrawal from the antibonding orbitals of the oxygen by a polarization induced by the cations which act as the adsorption sites. In fact, this explanation is difficult to understand since the polarization arises from a purely electrostatic effect of the charge on the cation whereas the electron withdrawal from antibonding orbitals will be related to the availability of suitable empty orbitals on the cation. Eberhardt et al. ( 2 2 4 have studied the photoemission of oxygen physisorbed on graphite at 10 K. The photoemission spectra exhibit vibrational structure in the 27c band. From calculations based on Franck-Condon factors, the authors conclude that on the graphite surface the equilibrium distance of the oxygen nuclei is decreased by 0.065A relative to the gas phase. This would also be consistent with a partial electron withdrawal from oxygen antibonding orbitals into available orbitals in the graphite. Long and Ewing (22b) have reported IR evidence for the formation of bound (O,), dimers in the gas phase at 90 K characterized by two narrow bands at 1586.1 and 1596.6 cm-' superimposed on the broad collisioninduced IR spectrum of oxygen. The energy of formation of the dimer was found to be -530 2 70 cal/mol, indicating a van der Waals type complex. Dimerization of oxygen to form O4 has been reported (22c,d)on y-alumina. Magnetic susceptibility studies show a significant decrease (about 25%) in paramagnetic susceptibility of oxygen at 77 K over a small pressure change. This is taken as evidence for a dimerization equilibrium on the surface 2 0 , * 0,

with very weak bonding between the oxygen molecules. Making use of

6

M. ('HE AND

A. J . TENCH

parallel observations of 0; labeled with "0 and of gas-phase oxygen labeled with "0, Tanaka and Kazusaka (22e)postulate that 0, is the intermediate for the homomolecular oxygen exchange reaction on ZnO at 77 K. Anufrienko et ul. (22f) suggest that 0, complexes can also be formed on SnO, at temperatures between 100 and 130 K .

B. SINGLET OXYGEN The possibility that singlet ( 'Ag) oxygen could play a role in reactions at oxide surfaces has not been considered seriously, because the energy level is 22.64 kcal above that of ground-state triplet oxygen. The EPR spectrum of ('A,) oxygen in the gas phase has been investigated by Miller (241); Wilkinson and Brummer (24b) have collected rate constants for the decay and reactions of singlet oxygen in solution. Kearns (23a)has suggested that decomposition of 0; might yield singlet oxygen, and Khan (23b) has observed the ( ' A g ) 0 2 emission spectrum at 1.29 pm from the reaction of KO, with water. The reactions of singlet oxygen with organic molecules have recently been reviewed (24c and references therein) and the study of this chemistry is made possible due to the lifetime of the singlet states (since transition to the triplet ground state is forbidden). Tsyganenko et al. (24d,e) have investigated the low-temperature adsorption of oxygen on NiO and C r 2 0 3 using 160/'80 mixed isotopes to check the presence of two oxygen atoms in the surface species. They detected IR bands at 1500 cm-' on NiO and 1460 cm-' on C r z 0 3 ,which they assigned to singlet oxygen because of the closer proximity of the bands to the frequency of gas-phase singlet oxygen at 1483 cm-' (241') than to that of gas-phase triplet oxygen at 1555 cm-' (21).There are a number of factors which can influence the voo frequency of adsorbed oxygen, as discussed in Appendix C. This assignment needs to be verified using reactions specific to singlet oxygen. Recently, Slawson and Adamson have shown (25) that films of linolenic acid on silica gel undergo an autoxidation which is accompanied by a chemiluminescence. The emission spectrum contains two components and the low-energy component close to 630 nm is attributed to bimolecular reaction of two ('Ag) oxygen molecules. In a subsequent paper, Slawson el al. (26a)report that heating 2,Sdiphenylfuran in air on a silica or titanium dioxide surface results in its conversion to cis-dibenzoylethylene, which is characteristic of reaction with singlet oxygen in the homogeneous phase. A singlet oxygen quencher inhibited the reaction which was not affected by a free radical scavenger. The authors suggest that for adsorbed oxygen, the

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

7

normal orbital degeneracy is removed and the singlet configuration may become the one of lowest energy. This is similar to the suggestion of Lemke and Haneman (16)on the state of adsorbed oxygen. It has been proposed that singlet oxygen is formed from 0; on transition metal oxides (26b) and is the active form of oxygen which interacts with olefins. Dmuchovsky et al. (26c) have proposed that singlet oxygen is important in the oxidation of benzene to maleic anhydride over vanadiamolybdena catalysts, whereas Khan (26d) has reported singlet oxygen is formed on hot tungsten filaments. Lipatkina et al. (26e) report an unusual EPR signal with g1 = 1.95 and g l l = 1.92 when oxygen is adsorbed on chromium oxide catalysts containing Cr5 ions. They suggest that a surface complex [ C r 5 + 0 2 ] is formed since the EPR spectrum is characteristic of Cr5+,indicating that the oxygen must be in the singlet state. The presence of oxygen in the complex was confirmed by "0 labeling; the change in line shape was thought to be consistent with a total hyperfine interactions of 10-15 G, which can be compared with the 140-150 G observed for 0; (see Section III,A,2). Guillory and Shiblom (26f) have used the reaction of rubrene to form an endoperoxide to detect the presence of singlet oxygen formed in a flowing gas stream over a range of catalysts. Positive results were obtained only with a lithium-tin-phosphorus catalyst, but the results were irreproducible. Munuera et al. (269) have adopted an approach based on the use of chlorinated TiO, to produce singlet oxygen on surfaces. Chloride'ions on the surface of these samples are thought to be transformed into C10- by ultraviolet (UV) irradiation in the presence of oxygen (26h). If the TiO, surface is highly hydroxylated, the irradiation also produces H,O, (26h), which further reacts with the C10- ions in the classical reaction seen in aqueous solution (26i)to form singlet oxygen: +

(CIO-),

+ H202+H20 + Cl; + ( ' A g ) 0 2

The presence of singlet oxygen was shown by a specific reaction with a sulfonic acid (269).It might be expected that surface oxygen would show the same reaction chemistry as singlet oxygen does in homogeneous media. The proposals that singlet oxygen is involved in heterogeneous catalytic reactions have not yet been explored fully. The methods used for the detection of the excited singlet state of oxygen need to be improved, and an approach based on the detection of the emission from ( 'Ag)O, as observed by Khan (23b) will present a significant advance if it can be applied to heterogeneous systems. It is clear that more quantitative work is required and in this respect the evidence that 0; can react with water to form ( ' A e ) 0 2 (23b)could have considerable mechanistic interest.

M. CHE AND A. J. TENCH

8 111.

Characterization of Charged Dioxygen Species

Several kinds of charged dioxygen species have been reported on surfaces, including O l , O , , O:-, and O ; - . All of these, with the exception of Oz-, would be expected to be paramagnetic and to give an EPR signal. In addition, the optical and IR absorption bands are known for some of these species and can also be used for characterization (Appendix C). Table I summarizes the properties of the dioxygen species relevant to this paragraph, whereas Table I1 is concerned with the thermodynamics of processes involving dioxygen species. Since no values are available for the species adsorbed on the surface, we have given the gas-phase values in Table I1 and these should be taken only as a general guide. Inspection of Table I shows that the dioxygen bond length becomes progressively larger on going from 0;to 0;- and this increase is accompanied by a decrease in the dioxygen bond strength. These facts can be explained

TABLE I Properties of’ Dioxygen Species“ ~~

~

Db ‘0-0

Species

Example

(4

O,PtF, (27) Gas Gas Gas LiO, NaO, KO2 HO2 Gas NazO, Rb,OZ BaO, H,OZ ROOR‘

1.17 (27) 1.123 (28) 1.207 (28) 1.216 (28) I .33 ( 4 ) 1.33 (29) 1.28 (30) 1.3 (3Ia) 1.34 (32) 1.49 (33a) 1.54 (336) 1.49 (30) I .49 (34)

(kcal/mol)’

(kJ/mol)’

149 (28)

623 490

I I7 (28) 95 (28)

64 (31b)

268

49

Bond ordeP 2.5 2.5 2 2 1.5 I .5 I .5 I .5 1.5 1 1.1

1.1

51 (34) 38 (35)

213 159

1.1 1.1 0.5

* References appear in parentheses.

D denotes dissociation energy. Conversion factors used are as follows: 1 eV = 23.060 kcal/mol, 1 cal = 4.184 J . Defined as N = (n - n*)/2, where n and n* are the numbers of electrons in the bonding and antibonding molecular orbitals, respectively, of the corresponding dioxygen species ‘ R = alkyl.

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

9

TABLE I1 Entlmlpy of Processes Involving Dioxygen Species in rhe Gas Phase" AHb Process

I. 2. 3. 4. 5. 6. 7. 8. 9. 10.

11. 12. 13. 14.

(kcal/mol)

0, + e - -0; 0, + 2e- -0;0; + e- -0;0, 0; + e0, - 2 0 0; -0 + 00;- - 2 0 20; -0, + 0:0; + e - - 2 0 0, + e - -0- + 0 0, + 2e- - 2 0 0; -0 + o+ 'Zgg0, ' A g0, 'Zg- 0, ' Z l 0,

- 10.15

154.5 164.65 278.45 118 94.37 - 104.06 174.8 60.59 83.02 50.44 153.63 22.64 31.73

-

--

(kJ/mol)

Ref.

-42.47 646.4 688.9 1165.0 493.7 394.8 -435.4 731.4 253.5 347.4 21 1.0 642.8 94.7 157.9

32 36 c

37 28

d d e d 40 d

.f 6 6

All reactants and products (except those of processes 13 and 14) are assumed to be in the ground state. Conversion factors used are 1 eV = 23.060 kcal/mol, lo3 c m - ' = 2.859 kcal/mol (1 c m - ' = 1.2398.10-4eV) when original values are given in eV or cm- and I cal = 4.184 J. ' Calculated from processes I and 2. Calculated using 0 + e - +O-, A H = - 33.78 kcal/mol, as given in Ref. 38, and a thermochemical cycle, a s described by Tuck (39). Calculated from a thermochemical cycle involving processes I and 2. Calculated using 0 -0' + e - , A H = 314.08 kcal/mol, as given in Ref. 41, and a thermochemical cycle, similar to that described by Tuck (39) for 0; -0 + 0 - ; the value 153.63 kcal/mol is to be compared with the spectroscopic value of 149 kcal/mol (28).

',

by reference to the molecular orbital energy level diagram (Fig. 2 ) . In the case of O,, the gg and nu bonding orbitals are fully occupied and the two additional electrons reside in the degenerate ng antibonding orbitals, giving a bond order of 2. Removal of an antibonding electron from 0, to give 0: will increase the bond order to 2.5 and lead to a shortening of the 0-0 bond, while the formation of 0; and 0;- from 0, requires that electrons be added to the antibonding orbitals, leading to a decrease in bond order and a lengthening of the 0-0 bond. There is no data available for O:-,

M. CHE AND

10

?:E : 0

A. J. TENCH

7G r;

+

e-

0,

+e-

r::

0;

*:

FIG.2. The simplified energy level diagram for 0:. 0 , ,and 0 ; in their ground state. When a crystal field is present, the n, and nu levels are not degenerate.

but this species would be expected to have a bond order of 0.5 and a weak 0- 0 bond. Of the reactions listed in Table 11, the only process that leads to a decrease of the energy of molecular oxygen is the formation of the free superoxide ion, 0; ( - 10.15 kcal/mol). The superoxide ion would therefore be expected to be the dioxygen species most commonly formed on oxide surfaces and in fact it is the species most studied, both in the bulk of various matrices and on surfaces. The other species (0; and 0;-)are not stable in the gas phase, although they can be stabilized in the solid state (Table I) due to the additional coulombic stabilization from the lattice. Nearly all the data in the literature refers to the characterization of 0; on various surfaces and this is discussed in detail in the following sections. A. THEO; ION

By far the most commonly reported species on oxides is the 0; (superoxide) ion, which has been characterized mostly by EPR using the g, hyperfine, and superhyperfine tensors. The EPR signals have only been seen in the case of 0; adsorbed on nonparamagnetic ions since if the ion at the adsorption site is paramagnetic, there will be a strong interaction between the unpaired electrons leading to line broadening. The absence of an EPR signal does not necessarily mean that the oxygen is in a nonparamagnetic form such as 0 2 - as assumed by some authors. In these situations other techniques

MOLECULAR OXYGEN SPECIES O N OXIDE SURFACES

11

such as IR. although of much lower sensitivity, become the major source of information. The usually accepted approach is to adopt an ionic model for the superoxide ion on the surface. In this model, an electron is transferred from the surface to the oxygen to form O;, and there is an electrostatic interaction between the cation at the adsorption site and the superoxide ion. A calculation of the CJ tensor based on this model (Section III,A,I) accounts for nearly all the data from adsorbed 0; and is consistent with the evidence that the spin density on both oxygen nuclei is the same (Section III,A,2). However, there are examples of oxygen adsorbed on the surface where the g values do not fit the predictions of the ionic model (Section IV,E) and also a few cases where the spin density on the two oxygen nuclei is found to be different. In these situations it seems likely that a covalent model in which a 0 bond is formed between the cation and the adsorbed oxygen, is more relevant. These two approaches are considered in the following sections. 1.

The g Tensor

a. The Ionic Model. The 0; ion is formed by adding an electron to one of the degenerate x g orbitals of the oxygen molecule to give the electron ~ ( with a 'n ground configuration ( I aJ2( 1a,)2(2a,)2(2a,)2(3a,)2( l ~ , ) lzJ3 state. Interaction of the free ion with the matrix either in the bulk or on the surface removes the degeneracy of the highest occupied x g orbital, splitting it into two components with a separation A (Fig. 2). Kanzig and Cohen (42) have derived theoretical expressions [Eqs. (1)-(3)] for the y tensor of 0; assuming an ionic model :

where the x axis is chosen along the ng orbital containing the unpaired electron and z is along the internuclear axis. To prevent ambiguity, all the results discussed in this review are presented with this convention. The energy level separations A and E are defined in Fig. 2, and 1 is the spinorbit coupling constant of oxygen, generally assumed to be 135 cm-' (4.3~). The parameter 1 is a correction to the angular momentum about z caused by the crystal field and is normally found to be close to unity.

12

M. CHE A N D A. J . TENCH

The properties of these equations for the g tensor can be seen more clearly if they are simplified by assuming I = 1,1 < A << E, and neglecting secondorder terms. This is quite reasonable since for many instances 1 < 0.1OA. The equations then take the form gxx = ge

+ 2l/E = ge + 2VA

(4)

gyy = ge

(5)

gzz

(6)

Thus, g, will be a sensitive reflection of the environment of the 0; ion and a plot of glL versus the oxidation state of the nearest cation shows a good correlation (Fig. 3 and Ref. 3). From Eqs. (4) and ( 5 ) it is clear that g x x is expected to have the smallest value close to g e , with gyg slightly larger; this will give the appearance of axial symmetry for large linewidths. From the above, it is clear that the magnitude of the gzz component can be used as a probe of the oxide surface since it gives a measure of the cation charge at the adsorption site. For example, it has been used to establish that 0; can be adsorbed at both A13+ and Mo6+ ions on the Mo0,/Al,03 system (see Section IV,B,4). However, Fig. 3 must be used with care because, in some cases, a wide range of g,, values are observed when only one charge state of the cation is expected, for example on TiOz (20b) and MgO (see Section IV,A,I). In these systems, the various gzz values must represent adsorption sites where the effective crystal field is changed either as a result of different crystal planes and/or of different local coordination. Theoretical

2112.16. 215.

.

2.1 4 213. 2.1 2 . 2.1 1 210 '

.

209. ZDB-

2Dl. 206.

2135-

mc203. 202. 2131.

2001

1

2

3 L 5 6 oxidation s t a t e o f the m e t a l ion

FIG.3. The variation of gz. of the 0;ion with the charge of the stabilizing cation.

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

13

calculations also show that the gzzvalue decreases as the cation-0; distance is shortened (4%). The effect of the surface on the adsorbed ion can be seen by assuming a situation similar to the alkali metal superoxides (M' O;), which are known to have highly ionic structures with C l v symmetry (44). This assumption is consistent with most of the hyperfine data (see Section III,A,2). The presence of a nearby M"' ion will split the uppermost occupied ng orbital into an in-plane component (y) and an out-of-plane component (x), where the reference plane refers to the MOO plane (Fig. 4). For a simple ionic model, the cation will interact more strongly with the in-plane orbital, which should lie lower in energy than the out-of-plane orbital. This would indicate that the unpaired electron is in the out-of-plane orbital, i.e., $, and this corresponds to the gxx component of the g tensor. The model is oversimplified, but it is consistent with much of the data and in many cases the oxygen nuclei are found to have equivalent spin density (Section III,A,2). Refinement of the ionic model to include a small covalent mixing of the oxygen x g orbitals with suitable electron orbitals of the cation explains the observation of superhyperfine interactions (Section 111,A,3) and also can lead to changes in the nXg to 71; separation, A, since only one of the orbitals may interact and in some cases actual inversion of the x and y levels may occur (45, 46a). It is interesting to note that, although the actual directions of the g tensor cannot be extracted from the experimental data, the considerations outlined above suggest that Fig. 4 gives a reasonable picture for those situations to which the ionic model can be applied. Y

I

/

M nr

FIG.4. The coordinate axis system of 0 ; interacting with M"' on the surface. The x axis and the internuclear axis I of 0; lie parallel to the surface, while they axis is perpendicular to it. The unpaired electron is in a molecular orbital built from the two shaded atomic orbitals.

14

M. CHE A N D A . J . TENCH

A list of reported .L/ values for 0, species on different supports is given in Appendix A and has been used to construct Fig. 3. This can be a useful guide for determining the site of adsorption of the 0; provided that the simple ionic model of Eq. ( 6 ) is appropriate. The superhyperfine tensor (Section III,A,3), where available, also confirms the nature of the adsorption site. The symmetry of 0, on the adsorption site can also be mono- or triclinic. In a field of triclinic symmetry, the g value expressions differ from those of orthorhombic symmetry and calculations show that both gxx and gPycan exceed the free electron g value. This case has been considered by Miller and Haneman ( 4 7 )for 0; adsorbed on elemental semiconductors. b. The Covalent or Spin Pairing Model. The electrostatic model for adsorbed 0; is successful for many oxide systems, where the anisotropy of the g tensor depends on the charge of the adsorbing site and labeling with 1 7 0 shows the oxygen nuclei to be equivalent (Section III,A,2), consistent with side-on adsorption of the oxygen ion (Fig. 4).There are, however, several examples (Section III,A,2) of oxygen-containing complexes with significantly nonequivalent oxygen nuclei, indicating that covalent bonding between the adsorption site and the oxygen species cannot be neglected. A different approach is then necessary and this has been made by Tovrog et al. (48)in a discussion of a series of cobalt dioxygen adducts, where " 0

0/ O

I

B

Q

FIG.5. A restricted molecular orbital description of the essential spin pairing interaction involved in dioxygen binding to a cobalt (U-base (B) adduct. Reprinted with permission from Ref. 48. Copyright 1976 American Chemical Society.

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

15

labeling shows the oxygen nuclei are inequivalent [60 and 40% spin density on the terminal and middle oxygens, respectively (49)l. In this model (Fig. 5), the bonding interaction involves a spin pairing of an unpaired electron in an antibonding ( n g )orbital of oxygen with the unpaired electron in the dZ2orbital of cobalt (11) to form a CT molecular orbital containing two electrons. The second x g orbital of the oxygen is orthogonal to the CT molecular orbital and contains the unpaired electron. The orbitals can be written in the form

+ B(=J

$1

="42)

$2

=

=g

$3

=

P(dZ2) - c 1 ( q

where p = ( 1 - u 2 ) 1 / 2There . will be an effect on the g tensor due to delocalization. To first order, deviations from the free electron g value come solely from spin density on the superoxide ion, so that first-order g value expressions derived by Kanzig and Cohen (42) for the ionic model can be modified to include delocalization of spin density from superoxide onto the cation (46b).These are shown below in the case of axial symmetry:

where 1,A, and 1 have the same meanings as earlier. When c1 = 0, the complex is equivalent to Co(1II)O; (i.e., the ionic model) since and t j 3 contain only contributions from 0, and Co, respectively. When c1 = /3, the complex can be written Co(II)O,, whereas when fl = 0, the complex is equivalent to Co(1)0:. From ''0labeling experiments, it is found (49)that the unpaired electron resides mainly on the oxygen. According to Tovrog et al. (48),this, however, does not necessarily mean that an electron has been transferred from the Co(l1) ion to the 0, to form 0;. The extent of electron transfer depends on the relative energy levels of the d,, orbital of the cobalt and the ng orbitals of the oxygen. As the ligand field strength around the cobalt increases, dZ2 is raised in energy relative to the ng orbital of the oxygen and the complex becomes more ionic in character. A decreasing ligand field lowers the cobalt dZ2orbital energy and causes it to approach the energy of the ng orbital, resulting in an increased cobalt character in the molecular orbital. The oxygen is regarded as forming a nonlinear Co-0-0 complex, where the C o - 0 - 0 angle is 126" (50). This model accounts for observed data on the cobalt-oxygen adducts, including cobalt in zeolite (51) (Section IV,E).

16

M. CHE AND A. J. TENCH

A different approach has been taken by McCain and Palke (52),who have reconsidered the EPR parameters of radicals containing two oxygen nuclei i.e., ROO. The g values are shown to depend mainly on the energy level spacings for the various orbitals, which are determined by the nature of the R group. In this approach most peroxy radicals are intermediate between the two extreme ionic cases 0; and 0;. In O:, the unoccupied ne orbital (Fig. 2) is considered as an electron acceptor which acts as a Lewis CT acid whereas the filled level of 0; is an electron donor or Lewis CT base. In this picture, primarily covalent bonding of peroxy radicals can be considered to result from Lewis acid-base reactions between R groups and 0; or 0, radicals. The open shell (singly occupied) ng molecular orbital in either 0; or 0; can act as a Lewis n acid or base toward p orbitals on R, where n and (T Lewis acid-base characters are defined according to Pearson (53).Thus, comparing one R group with another, McCain and Palke (52) found that the better CT donor increases the isotropic ( 9 ) value of ROO whereas the better n donor has the opposite effect. Chuvylkin et al. (54) have used this approach to discuss EPR signals arising from weak R 0 2 surface complexes in a number of systems where the g tensor does not fit the pattern expected [Eq. (6) and Fig. 31 from the ionic model. This is not discussed quantitatively, but they conclude that the appearance of covalently bonded oxygen is impossible without a favorable orientation of appropriate electronic orbitals. A similar covalent bonding approach has been considered theoretically for the chemisorption of oxygen on silicon surfaces (55). Examples of weakly bonded oxygen are given in Section IV,E. c. Motion. The superoxide or peroxy species is normally regarded as stationary on the surface, but there is evidence that a limited degree of motion is possible in some cases. Studies of the relaxation time T , of 0, on supported silver surfaces as a function of temperature indicate changes in the nature of the adsorption and degree of surface mobility (56).Measurements using an EPR saturation transfer technique on the same system sec indicate that the correlation time for reorientation varies from at 173 K to sec at 273 K (57). A more direct method of studying rotation is to look for variation in the g or hyperfine (Section 111,A,2) tensors as the temperature is changed. For certain peroxy radicals in polymers, considerable changes in the g tensor occur which can be correlated with both the onset of rotation of the polymer chain and also rotation about the C - 0 bond with increasing temperature (58, 59). Similar changes in the g tensor have been observed for 0; ions formed by irradiation in frozen alcohols (60).Kazusaka et al. (61) have observed changes from an orthorhombic g tensor g1 = 2.0266, g2 = 2.0097, g3 = 2.0042 at 77 K to an axial g tensor with q l 1= 2.007 and g1 = 2.018 at

-

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

17

418 K for 0; on W0,/Si02, whereas in the same temperature range, there was no indication of any change in the hyperfine tensor. The authors concluded that the results could be explained by an anisotropic motion of the 0; ion about a y axis assumed to be perpendicular to the plane of the surface. Analysis taking the choice of axis (Fig. 4) used throughout this review leads to the conclusion that the results are consistent with a tumbling motion about the x axis parallel to the surface rather than the spinning motion suggested by the authors. For 0; on T h o , (62),the g tensor data are consistent with rotation about the z direction, but there is some problem with the hyperfine tensor data (Section III,A,2). Recently the species (CO,--O,)- was identified on MgO at 77 K (63)and assigned an orthorhombic y tensor, 2.040, 2.0072, and 2.0015. Schlick and Kevan (64) have proposed that the additional radical with g , , = 2.0015 and g1 = 2.025, which is observed at the same time, is in fact a signal from the same species undergoing jumps of the 0-0 group around the C-0 bond. A computer simulation shows that the overall spectrum can be fitted with such a model with a correlation time of 0.08 p e c . In this case, no additional data were available at other temperatures. A more detailed analysis has been carried out by the same authors (65a) on the g anisotropy associated with molecular motion in the triphenylmethyl peroxy radical, in which 120" jumps of the peroxy group about the C-0 bond are thought to occur. The authors comment that, in general, the possibility of some dynamic processes occurring on the surface which may give rise to additional peaks should always be considered before invoking new species. The motion of peroxy radicals is now well documented and has been studied in detail in various matrices such as urea-polyethylene complex (65b),urea-n-tetracosane (65c), methacrylate copolymers and polypropylene (65d), and polytetrafluoroethylene (65e). The motional dynamics of 0; adsorbed on Ti supported surfaces has been analyzed over the temperature range 4.2-400 K in a recent paper by Shiotani et a!. (66).Of the several types of O,, a species noted as 0; (111),and characterized by g.rx = 2.0025, g y y = 2.0092, gzz = 2.0271 at 4.2 K, exhibited highly anisotropic motion. While g.y.xand gzz varied with increasing temperature and were accompanied by drastic line shape changes, grr was found to remain constant. This observation indicates that the molecular motion of this 0; can be described by rotation about the y axis perpendicular to the internuclear axis of 0; and perpendicular to the surface with the notation given in Fig. 4. The EPR line shapes were simulated for different possible models and it was found that a weak jump rotational diffusion gave a best fit of the observed spectra below 57.4 K, whereas some of the models could fit the data above this temperature. The rotational correlation time was sec (263 K), while the found to range from sec (below 14.5 K) to

18

M. CHE AND A. J. TENCH

activation energy for rotational diffusion was estimated to be 0.5 kcal/mol above 100 K. 2. The Hyperfine Tensor The g tensor can give only limited information on the nature of oxygen species; however, further details can be obtained from the hyperfine tensor and the use of oxygen isotopically enriched in 1 7 0 (natural abundance 0.04%, I = 5) has been particularly valuable in their characterization. It was first used by Tench and Nelson (67) in 1966 to study oxygen adsorbed on neutron-irradiated MgO. They could not observe any hyperfine structure but were able to show from the intensity of the signal that a species having at least two oxygen nuclei was involved. Two years later, Tench and Holroyd (68) reported a well-resolved spectrum of 0; formed by adsorption of oxygen onto surface F-type centers and this has been followed by reports of 7O labeled species on many different oxides. These oxides are tabulated in Appendix B and only particular examples will be discussed in the following sections. a. Dioxygen Species with Equivalent Oxygen Nuclei. The nucleus of I7O has a spin 1 of 5, so that a diatomic species could give 6 x 6, i.e., 36 lines (21 + I for each nucleus) for every principal direction of the hyperfine tensor. This would be too complex to resolve fully in a powder spectrum. However, the spectrum for 0; on MgO (Fig. 6 ) turned out to be consistent with a much simpler situation where both oxygen nuclei are equivalent. The adsorbed oxygen was enriched to 58 atom % with a composition of 18.47; l 6 0 l 6 O ,48.5% I7Ol6O,and 33.1% 170170. For equivalent oxygen nuclei, ( 1 7 0 1 6 0 ) - should give 6 lines and (1701’0)1 I lines for each principal direction of the hyperfine tensor. The spectrum (Fig. 6 ) can then be analyzed to give a hyperfine tensor of the form A,, = 77 f 2, A,, = 0, and A,, = 15 & 2 G, where the x, y, and z axes are defined as shown in Fig. 4. No information on the sign and absolute direction of the hyperfine tensor can be obtained from a powder spectrum and this has led to some confusion in the literature on the choice of A,, and A,,,; however, in some special situations where a detailed temperature dependence of the hyperfine tensor can be measured (69a),it may be possible to determine the sign of the tensor components. In this review, the largest hyperfine value is defined to be along A,,, which is then the orbital containing the unpaired electron and lies in the same direction as g,, (see Section III,A, Fig. 4). To prevent ambiguity, all the results are presented with this convention. The form of this tensor is in agreement with the solid-state work on 0; in KCl (A,, = 67.5, A,, = 0, and A,, = 19.7 G) (69b).The sign of A,, will be taken as negative since aiso must be negative from the negative value of yN for 1 7 0 .

MOLECULAR OXYGEN SPECIES O N OXIDE SURFACES

19

FIG.6. The EPR spectrum at 77 K of " 0 ; on the surfiace of high-surface-area MgO. The gain has been reduced by 5 times for the central line; a small portion of the high field spectrum is overmodulated to show the outermost lines. N o attempt has been made i n this diagram to insert levels for the lines around y, (68).

On most oxides, only the A,,y splittings are resolved for "0; and some typical values are given in Table 111, with a full tabulation in Appendix B. The lines are generally fairly broad because of the heterogeneity of sites on the surface. I t is striking that very often for 0; on oxide surfaces only one value of A,, is observed. The two oxygen nuclei are usually equivalent and this has been interpreted to mean that the oxygen is adsorbed in an ionic form with the internuclear axis ( z direction) parallel to the surface (68). Theoretical calculations indicate that this is a stable conformation (43b). The A,, values are found to be in a narrow range, nearly all between 74 and 80 G, indicating that the localization of the unpaired electron on the oxygen is largely independent of the support. The hyperfine tensor can be used to estimate the spin density on the oxygen atoms. Assuming that the unpaired electron is in an axially symmetrical n; orbital, the axial hyperfine tensor can be separated into an isotropic part (aiso)and an anisotropic traceless tensor in the form A , , All) = aiso

+ (-B,

- B , 2B)

where aiso

= (2'41

+ All)/3.

20

M. CHE AND A. J. TENCH

The isotropic and anisotropic terms are then given by aiso= A,c:, and B = B,c:,, where cis and c,; are the spin densities of the unpaired electron in the 2s and 2p orbitals of the oxygen atom, and A, and B, the hyperfine constants for pure 2s and 2p oxygen orbitals. A range of values for A, and B, has been used in the past and in this review all calculations have been made using A , = - 1651 and B, = -51.38 G (77).In fact, since the 2s spin density arises from a spin polarization mechanism and because of the small values involved, only the 2p populations need be calculated. The total spin density figures in Table 111 have been calculated for the two oxygen atoms using only A,, ( = A , , ) and assuming the A,, ( = A , ) and A,, ( = A,) components are zero and also that no motion is taking place, which is not always true even at 77 K (66).In general, the total spin density values are very close to unity, indicating that the electron is almost completely localized on the two oxygen atoms. The aimvalues are rather high, giving about - 51 G for the two nuclei summed together. This should be compared with the solution data (Appendix B), where the ''0 isotropic hyperfine coupling constants for inorganic and organic peroxy K radicals are found to follow the relationship a, = Q;,po, where QZ, = -41 k 3 G and p o is the total spin density (78). If the hyperfine tensor is analyzed in more detail for MgO with A,, = -77 G and A,, = 15 G, this leads to a value for aimof - 20.7 G for one oxygen atom. The sum for the two nuclei is then -41.4 G, in good agreement with the solution data. The dipolar tensor is then no longer axial and can be analyzed as the sum of two axially symmetrical tensors, indicating some contribution from orbitals other than ni (79),or more fully, in terms of an unrestricted Hartree-Fock approach (696) as for 0; in KCl. It seems

+

TABLE 111 Selected ''0 Hyperfine Constants and Total Spin DenxitiexJor Diatomic Oxygen Species with Equivalent Oxyyen Nuclei

Axxa

System MgO CaO SrO CeO,/SiO,. CeO, SnO, TiO, (rutile) WOJSiO, NaY zeolite

(G)

Ayy

Azz

(G)

(G)

- 17 - 71 - 16

0

+I5

-

-

-

-

- 15

-

-

-80.5 - 76 - 14 - 16

-

-

-

-

-

~

-

-

aima (G)

- 20.1 - 25.1 -25.3 - 25 -26.8 -25.3 - 24.1 -25.3

Total spin densityb 1.00

1.00 0.98 0.97 1.04 0.98 0.96 0.98

Ref. 68 70, 71 72 45, 73 74 75 61

76

~~

Assumed to be negative (see text). Calculated using only A,,y = A , , (see text) and assuming no motion is taking place.

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

21

probable that unresolved components from A,, are present in all the systems reported. For example, for 0; on ZnO (80) the A,, component has been resolved and the hyperfine tensor is ( - 80,0, + 15) G. So far, we have considered only those systems in which the two nuclei are equivalent. This is not the case for 0; on MoOJSiO, (81, 82) and some y-irradiated zeolites (81, 83a), where the oxygen nuclei are found to be inequivalent. This case is considered in more detail in the next section. b. Dioxygen Species with Inequivalent Oxygen Nuclei. When the oxygen nuclei are found to be inequivalent, the spectrum is then more complex (Fig. 7) and the analysis as shown later is not always straightforward. For all these species only the A,, components can be resolved and these values, one for each oxygen nucleus, have been used to calculate the total spin densities following the method described earlier. The total spin densities given in Table IV are close to unity, but some minor variations exist, probably due to the difficulty of obtaining reproducible values from the spectra. Comparison with those organic peroxy radicals for which data on the hyperfine tensor are available (58,84,92,93)shows similar values. For all these radicals, the sum of the aimvalues for the two oxygen nuclei seems to be close to 50 G and, as for the case of equivalent oxygen nuclei, this would be consistent with an unresolved nonzero contribution from A,, of opposite sign to A.y.v. It is also possible that for some of those systems given in Table I11 and Appendix B, the oxygen nuclei are not exactly equivalent but the differences are small (66,82).

FIG.7. The EPR spectrum of 0; ion on MoO,/SiO, at 77 K showing the hypertine interaction with two inequivalent oxygen nuclei (84).

M. CHE AND A. J. TENCH

22

TABLE 1V Selected " 0 HyperJine Constants and Total Spin Densities for Diatomic Oxygen Species with Nonequivalent Oxygen Nuclei

Axxa( G )

System MoOJSiO, HY zeolite y-irradiated HY. HZ zeolite y-irradiated RuY zeolite MgO/(COz-OJ CrO,/SiO,/(CO,-O,) MoO,/SiO,/(CO,-0,) TiO,/(CO -0,)HY zeolite/(CO-0,)TiO,/RCH,-OO. TiOJRR'CH-00 Co(ll)NH, complex Si-0-0' (bulk) Tetralin peroxy Polytetrafluoroethylene peroxy Polypropylene peroxy

-85, -72 -82, -69 -82, -63 -84.5, -64.2 -80, -67 -100. -50

-98, -42 -104, -40 -104, -42.5 -107, -37 -95, -35 -94, - 36 -80, -60 -101.7, -43.2 -88, -60 -107, -46 -98, -60

aisoY(GI

Spin density Total oneach spin oxygen' density'

Ref. 82,84,85

-28.3, -24

0.55, 0.47

I .02

-21.3, -21 -28.2, -21.4

0.53, 0.41 0.55. 0.42

0.94 0.97

-26.7, - 33.3. - 32.7, -34.7, -34.7, -35.7, -31.7, -31.3, - 26.7, -33.9, -29.3, -35.7.

0.52, 0.43 0.65, 0.33 0.64, 0.27 0.68, 0.26 0.68, 0.28 0.69, 0.24 0.62, 0.23 0.61, 0.23 0.52, 0.38 0.66, 0.28 0.57, 0.39 0.70, 0.30

0.95 0.98 0.91 0.94 0.96 0.93 0.85 0.84 0.91 0.94 0.96 I .oo

83b 63 86 87 88 89 90 90 51

0.64.0.39

I .03

84

-22.3 - 16.7 - 14 - 13.3 - 14.2 -12.3 -11.7 - 12 - 20 - 14.6 -20 - 15.3

-32.7, -20

81 81 83a

91

92 58. 93

Assumed to be negative (see text). = A,, for inequivalent oxygen nuclei (see text) and assuming no motion is taking place.

' Calculated using only A,,

What sort of picture is appropriate for these adsorbed species? As pointed out earlier, the analysis of the spectrum observed when two inequivalent oxygen nuclei are present is not straightforward. In most cases, two hyperfine splittings are obtained from an analysis of the hyperfine lines for the species (the unlabeled oxygen can containing only one labeled oxygen, i.e., ("00)be either l6O or "0, but since both nuclei have zero nuclear spin, this does not affect the present reasoning). Two explanations for this observation are possible (Fig. 8). One is that these splittings refer to oxygen nuclei in two 0; ions in different surface sites (case 1); the other is that they refer to two oxygen nuclei in the same 0;(case 2) (82, 85). In general, if two different types of 0; are present, they should lead to two sets of g tensors. It is also likely that they will respond differently in their rates of formation, reactivities, saturation behavior, or thermal stabilities (81,82,85).Furthermore, a careful analysis of the hyperfine structure (82, 85) allows these two cases to be

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

23

O II @< & ' '/ + I

I

SITES 1.2

I

SITE 1 Case 1

SITE 2

Equivalent nuclei,ditferent sites

mh7 SITES 1.2

0; with onelabeled nucleus

I

SITES 11.21

I

Case 2 Equivalent sites. different nuclei

I I

0; with both nuclei labeled

I

I FIG 8

Configuration for inequivalent oxygen nuclei in the 0, ion (85)

distinguished. This argument has been criticized recently and it was suggested that either mobility or second-order effects in hyperfine couplings could alter the validity of the analysis (66). Taking into account the secondorder effects and using spectra recorded at 36 K, Shiotani er al. (66) have simulated the EPR spectra of 0; adsorbed on Ti supported on porous Vycor glass. They could not demonstrate which one of the two cases gave a better fit but found it reasonable to attribute the nonequivalent 7O hyperfine splittings to two nuclei in the same 0; being nonequivalent, i.e., the same conclusion that had been reached for the MoO,/SiO, system (82).Any second-order effects in the spectrum have been shown to be small by com85). Recently (84, paring and analyzing X- and Q-band EPR spectra (83~1, this problem has been reinvestigated and high-resolution spectra were obtained exhibiting hyperfine lines due to (I7Ol7O)- (Fig. 7) which showed unambiguously that, for the MoO,/SiO, system, the previous analysis was correct i.e., two nuclei in the same 0; being nonequivalent (82). A careful inspection of the EPR spectra due to 0; ions with nonequivalent oxygens reveals that the peak heights of the doublet relative to the same value of rn, ( I = $ for 1 7 0 ) are not equal (Fig. 7). Simulations of the spectra have been carried out by Shiotani et a/. (66) on two different assumptions. One is based on a different line width for each line of the doublet (the larger linewidth being associated with the larger hyperfine splitting). The other is that the intensity (or radical concentration) of the 0; giving the smaller hyperfine splitting is twice as large as that of the 0; with the larger hyperfine splitting. This latter assumption contradicts the model of two inequivalent

24

M. CHE AND A. J . TENCH

nuclei in the same 0; discussed above (case 2; Fig. 8). The simulation carried out by Shiotani et a / .(66)is in better agreement with the experimental spectra using the first assumption of different linewidths. This difference may be due to a significant isotope effect on the motion of O,, a possibility which could imply an important tunneling mechanism (94a) as suggested by Shiotani et al. (66).This isotope effect for 0; is very similar to that observed for trapped 0, molecules, as discussed in Section 11. The similarity of 0; with nonequivalent oxygen nuclei to the organic and inorganic peroxy radicals (Appendix B) has lead to the suggestion that, in those systems with inequivalent oxygen nuclei, the molecule should be regarded as adsorbed with one oxygen nucleus closer to the adsorption site (85).In this situation, the outermost oxygen (B in Fig. 8) is assumed to have the largest spin density by analogy with the results from linewidth studies (94h) and selective isotopic labeling (95) for the organic peroxy radicals. The reasons for this kind of adsorption are not clear and they probably depend on two main factors: the topology of the surface and/or the nature of the orbital at the adsorption site available to overlap with the n: orbitals of oxygen. The first factor has been favored by Che et al. ( 8 3 , who find that 0; changes from nearly parallel to the mean plane of the surface toward a more perpendicular orientation on going from MoO,/AI,O, to MoO,/SiO, and suggest that this is due to steric hindrance on a zigzag surface. 0; is then adsorbed on MoO,/SiO, as follows:

0

/ \

(0 0

Mo

/ \

It has been postulated (96)that the origin of the zig zag surface stems from a move into the surface of the molybdenum ion due to a short Mo=O bond (97).The second factor, i.e., the nature of the orbital available at the site where oxygen is bonded, is supported by recent analysis of the oxygen carriers, where both end-on (48)or side-on (98)geometries have been found, depending on the orbitals available. It is probable that both the topology of the surface and the nature of the orbitals contribute to producing 0; with nonequivalent oxygen nuclei. For the MoO,/SiO, system, it is also possible that changes in the local environment could alter the energy levels of the d orbitals relative to the ng orbitals of O;, leading to an increase in the covalent contribution to the bonding of the 0; with the surface. In some cases, O2 is not adsorbed on the metal cation. For example, in zeolites (83a) it is thought that 0; is formed on a V-type center (a hole localized on an oxygen atom of the lattice) (Section IV,C,3), whereas in V,O,/SiO,, 0, is adsorbed on 0 - (99) (Section V).

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

25

A different class of 0; species is formed when oxygen is adsorbed on a surface which has previously been exposed to CO, or C O at low temperature. An anionic coadsorbate is formed with 70,which shows a hyperfine pattern characteristic of nonequivalent oxygen nuclei. On MgO (63)a complex ion (C0,-0,)- is formed with A,, of 100 and 50 G. A similar species (C0-0,)is formed with CO on TiO, (88),on T h o , (62),and also on y-irradiated HY zeolite (89).It seems likely that 0, is adsorbed on top of the adsorbed CO, or CO but that the unpaired electron is localized very largely on the adsorbed 0,. Calculations by Ben Taarit et al. (63) indicate a good agreement with experimental data when a C-0,-0,, bond angle of about 130" and a C - 0 , distance close to 1.5A are assumed for (C0,-O,)-. A similar species has recently been reported on the CrO,/SiO, system with A,, of 98 and 42 G (86)and on the MoO,/SiO, system with A,, of 104 and 40 G (87) (Fig. 9). These species have similar hyperfine parameters and also appear to be very similar to the organic peroxy radicals and SiOO in the bulk (Table IV). There are at present few examples in this class of adsorbed species formed by secondary reactions (96). It is relevant here to mention that I7O has been used to study related dioxygen radicals in the biochemical field. For example, Bray et a / . (100) reported that a dioxygen species, formed enzymatically in aqueous solution and then cooled to 77 K, had equivalent oxygen nuclei with A,, = 72 G, in good agreement with the data in Table 111. Similarly, y-irradiated protein exposed to 1702also forms peroxy radicals but with a poorly resolved spectrum. The oxygen nuclei are not equivalent (A,.y of 46 and 68 G ) and spin densities of 0.29 and 0.45 were estimated for the inner and outer oxygens, respectively (101). The total spin density of 0.74 probably reflects some rotation or torsional oscillation of the peroxy group (Section III,A,2c). c. Motion. The 170hyperfine structure can also give information on the motion of the adsorbed oxygen. This approach has been used particularly in the case of polymer peroxy radical (58, 59). There are, however, some difficulties in this approach because lower signal intensity, measured usually as the height of the signal, is observed when a hyperfine structure is present, and this is often accompanied by a line broadening when motion is taking place, i.e., when the temperature is raised. In view of this, it is not surprising that various types of motion have been observed and rotations about all three reference axes of 0; (Fig. 4) have been reported. Warming of 0; adsorbed on WO,/SiO, (61) broadens the hyperfine lines characteristic of equivalent oxygen nuclei but leaves A,, unchanged. This is consistent with rotation about the axis of the orbital containing the unpaired electron, i.e., the x axis parallel to the surface plane using the notation of Fig. 4. This explanation differs from the original one proposed by Kazusaka et u1. (61), who appear not to have made the correct choice for the axis system.

]:

N

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

27

Vedrine et al. (102a) have reported a different type of motion for 0; in a Ce-X zeolite which is characterized by unusual parameters: y, = 2.0242, g2 = 2.0208, 9, = 2.01 12, and A , = 24, A , = 66, A , = 12 G . However, inspection of the published spectra indicates that A , and A , are difficult to estimate. The two oxygens are apparently equivalent and the spectrum is tentatively interpreted in terms of a rotation of 0; about an axis perpendicular to the internuclear axis at 77 K, i.e., they axis in Fig. 4. This is difficult to reconcile with the observed orthorhombic g and A tensors, which are not averaged. A rotation of 0; around the internuclear axis has also been reported by Breysse ef al. (62)for 0; with equivalent nuclei adsorbed on T h o , . The g.,x and g,,,. components, which are distinct at 77 K, are averaged to gL = (gA, + g,,)/2 at 298 K and this is explained in terms ofa rotation around the internuclear z axis. The 1 7 0 hyperfine tensor, however, is not completely averaged. If one assumes that A,.,,is zero at 77 K, as for most oxides (Appendix B), then A,, and A,, should average to A , = ( A x x+ A,.,.)/2 = 37.5 G at 298 K. This value is substantially different from the experimental one of 65 G reported by the authors (62). Obviously there are limitations in the use of the 1 7 0 hyperfine tensor to derive information on the motion. When both tensors are available, the y tensor seems to give more detailed information on the type and dynamics of the motion than the hyperfine tensor (59, 66). In some cases the situation may be more complicated if the axes of the g and A tensors do not coincide, but this is difficult to measure for powder systems. When spin densities are calculated, it is important to remember that motion of the 0; may substantially decrease the values below unity, and caution must be taken not to draw conclusions on the presence of covalent bonding between oxygen and the surface unless it is clear that motion is not present. In this respect, the direct observation of inequivalent oxygen nuclei and of superhyperfine interactions (Section 111,A,3) is a better guide. There is also the question as to whether the observation of two equivalent oxygen nuclei in 0; can be explained by the rotation of 0; having two inequivalent oxygens. The presence of an orthorhombic g tensor, together with the observation that the calculated total spin density on the two oxygens is very close to unity are strong arguments against this possibility. Apparently equivalent oxygens (102b)may also be caused by the 0; ion jumping between two equivalent bent conformations as reported by Getz et al. (49)for cobalt oxygen complexes in solution

co

co

28

M. CHE AND A. J. TENCH

While the two oxygens appear equivalent in solution (102b),they become inequivalent when the solution is frozen (49). The discussion of the hyperfine tensor has indicated clearly that the analysis of the hyperfine pattern has been very valuable in developing an understanding of the adsorbed oxygen ion. On balance, in the oxide systems it would seem preferable to use the term superoxide rather than peroxide or peroxy for 0;. Overall, the picture is largely consistent with an ionic model for 0; on surfaces.

3. The SuperhyperJine Tensor When 0; is formed near a cation which has a nonzero nuclear spin, then a superhyperfine interaction may be observed if some of the unpaired electron spin is delocalized onto this cation. This is observed in the solid state (44,60)but has only been seen in a few of the surface systems: zeolites (103105a,h), transition metal oxides (56, 106-110), oxide supports ( 1 1 1 , 112), elemental conductors (47, 113, 114),and some oxygen adducts (51, 115, 116) (Table V). In all cases but one (115; see Section IV,E), the number of lines observed confirms that the interaction is with a single cation nucleus as expected from either the ionic or covalent model. Considerable attention has been paid to the O;/V20,/SiO2 system (106),and calculations based on this and on more extensive data from frozen aqueous solutions (117-119) indicate that the superhyperfine interaction arises through a spin polarization mechanism which involves the ng orbitals of the 0; ion and those s, p, and d orbitals of the cation which have the correct symmetry. The results of the calculations are fairly sensitive to the model and geometry adopted but confirm that a direct dipole interaction cannot explain the observed superhyperfine splitting, and it is necessary to invoke a spin polarization mechanism. The origin of the superhyperfine splittings has also been discussed for the cobalt dioxygen carriers. It has been shown (48)that the cobalt superhyperfine splitting could not be attributed to direct delocalization of the unpaired electron from the oxygen on cobalt, as believed earlier (46b), because the sum of the spin densities derived from the cobalt and 1 7 0 anisotropic hyperfine coupling parameters would suggest more than one unpaired electron in a system which contains only one. Thus, the cobalt hyperfine coupling must arise via an indirect mechanism, and Tovrog et al. (48) have proposed that the spin interaction of the unpaired electron in t+b2 (mainly oxygen p orbital) (Fig. 5 ) produces a polarization of the pair of electrons in the bonding molecular orbital t+bl (see Section III,A,l,b) and an actual electron transfer is not necessarily involved. The superhyperfine tensor is primarily used to confirm the nature of the adsorption site. This is clearly seen for 0; on enriched 95M003/A1203

29

MOLECULAR OXYGEN SPECIES O N OXIDE SURFACES

TABLE V Some Superhyperjine Consrantsfor 0;Adsorbed at a Cation Site on the Surface ~~~

System Al HY zeolite Sc Y zeolite La Y zeolite HY zeolite NaX zeolite WHY zeolite V,O,/SiO, V,O,/P,O,/SiO, MoO,/SiO, MoOJSiO, Mo0,/AI,03 77 K 300 K

COO in MgO AgJSiO, y-Al,O, ,O, >)-A1 AlSb GaAs

GaAs Co ammonia adducts in Y zeolite Co amine adducts in Y zeolite

s*z

g,,

2.038 2.030 2.044 2.038 2.033 2.024 2.023 2.022 2.01 76 2.017

2.009 2.009 2.009 2.009 2.008 2.01 I 2.01 I 2.01 1 2.0105 2.010

2.010 2.009 ? ? 2.0279 2.0089 2.038 2.006 2.040 2.008 2.041 2.005 2.035 2.007 2.046 2.009 2.084 2.000 2.017 2.039

{ :::::

2.01 1.998

A*, (G)

A,, (G)

6.5 2.003 5.7 2.002 2.005 12 6.5 2.003 ? 2.002 ? 2.003 9.7 2.004 ? 9.6 2.0050 2.0 1.2 2.004

4.8 4.4 9 4.7 7.4 ? 6.8 6.9 1.9 1.8

g xx

? 5.4 37 32 3.04 ? 4.9 6.4

2.004 2.004 1.987 1.993 2.0041 2.006 2.004 2.002 2.004 2.006 2.000

24.5" 17.8

2.00 1.992

17.8 20

15

3.3 3.6 ?

? 4.84 3.6 3.8 4.4 25 39" 12.5

12 10

A,,

(G)

Nucleus

5.7

103 i03 103

5.1

8 5.7 6.1 15 5.9 ? 1.0

104 105a 1056

106 107

108 109

21

? 5.2 17.5 15.0 13.2 ? 3.7 4.4 2.5 10" 12.5

12.5 13

Ref.

108 108 110 56

111 112 47 113 114 51

cyco Y o

115 116

" Note g and A tensors do not have the same principal axes-consult original article

(108)and y5M003/Si0,(108,109) (Fig. 10). On the former system, formation

of the ion at 77 K gives an EPR signal with gz, = 2.017 and a resolved set of six hyperfine lines about qyy,whereas a sample that is not enriched shows no superhyperfine structure. This is consistent with adsorption at a Mo6+ site. At 300 K, a new 0; signal appears on both samples with gzz = 2.039 and a superhyperfine structure of A,, = 5.4,Ay,v = 3.6, and A,, = 5.2 G. This corresponds to adsorption at an A13+ site. On "Mo03/Mg0, there is no indication of Mo6+ sites available either at 77 K or at 300 K. The 0; ions are adsorbed on Mg2+ sites (108). The superhyperfine tensor has also been used to derive the amount of spin delocalization on the cation leading to the superhyperfine structure. In view of what has been said above on the origin of the superhyperfine interaction, the result must be handled with caution. Thus, the unpaired electron

30

M. CHE AND

A.

J. TENCH

i 10 Oe

42

1 I Ill I I 91

I I Ill I I FIG.10. The EPR spectrum of 0;ion on 9SMo0,/Si0, at 77 K showing the superhyperfine interaction with the Mo ion ( I O Y ) .

of 0; is 5% localized in the Al atom orbital on AlSb (47)and 23% localized in the Ga atom orbital on GaAs (113, 114), whereas on oxide surfaces the figures are generally smaller (118). These latter figures are consistent with the spin densities obtained from the "0 hyperfine tensor (Tables I11 and IV). In addition to the nature of the cation at the adsorption site, the superhyperfine tensor can also give information on neighboring atoms further away. For example, 0; adsorbed on MgO exhibits a superhyperfine tensor ascribed to the presence of a nearby proton, presumably as a hydroxyl group (68) and this has been confirmed by isotopic labeling with deuterium (see Section IV,A). Superhyperfine tensors indicating the presence of nearby protons have also been reported for 0;adsorbed on ferrocene deposited on porous Vycor glass (PVG) (120)and for alkylperoxy radicals supported on TiO, (90). The information which is obtained from the superhyperfine tensor is important and much effort has been aimed at obtaining this parameter.

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

31

Where naturally occurring isotopes of the cation are not suitable, then enrichment of the surface cations with nonzero-nuclear-spin isotopes is a powerful technique, e.g., 9 5 M 0 0 3 on A1203, S i 0 2 , or MgO (108). Often, the presence of a superhyperfine interaction leads to spectra of low resolution and it is useful to increase both the intensity of the spectra and the resolution. Second or even higher derivative spectra can be used to enhance the resolution and in some cases secondary reactions (121)have been shown to increase the intensity.

4. Optical Properties An absorption band in the UV results from a transition between the o g and the 7cg orbitals of 0; (Fig. 2). For 0; in the alkali halides, this band is centered at about 5 eV ( f 2 2 , 123) and detailed measurements of the stress dependence have been carried out (69h). A yellow luminescence in alkali halides has been known for many years (124) but was not identified as originating from 0; until 1961 (122). At low temperature, the emission spectrum in both alkali halides (125)and sodalites (126)shows a number of sharp zero phonon transitions between 400 and 600 nm; the spacing of these lines corresponds to the ground-state vibrational frequency of the 0; ion. The Ag'O; complex has been observed (127a)in a matrix experiment and gives an absorption at 275 nm. Although well documented in the solid state, no optical absorption or luminescent spectra have been reported for 0; on an oxide surface even where EPR has shown the ion to be present on, for example, zeolites (127h) or the alkaline-earth oxides (128, 129). This may arise because absorption from surface oxide ions in low coordination occurs at about the same energy as the optical absorption for 0; in the oxides such as MgO (f30,131). The 0; ion is not normally expected to be active in the IR but laser Raman studies on the crystalline alkali metal superoxides have led to the assignment of frequencies between 1137 and 1164 cm-' to the 0-0 stretching vibration (21, 132). A number of 0; complexes with the alkali metals (133a, b, c) and transition metals (127a and references therein) have been studied as matrix isolated species. There are no observations of 0; on the surface using Raman spectroscopy but there are now several reports on 0; by IR spectroscopy. Davydov et al. (20h) have reported a band at 1180 cm-' on TiO,, which they assign to a molecular species such as 0;. If correct, this means that the surface must perturb the adsorbed oxygen sufficiently to make the molecular ion infrared active. This is unexpected, since the EPR data (Section III,A,2,a) show that the oxygen nuclei are equivalent in this system (75); however, it is possible that different species

32

M. CHE AND A. J . TENCH

are being observed. Conflicting results have been also obtained for oxygen adsorbed on cr-Fe,O,. Griffiths et al. (19a) observed bands at 1350 and 1270 cm-' and assigned them to adsorbed 0, and 0; species, respectively. On heating Fe,O, in oxygen, Davydov et al. (134a) observed two strong bands at 965 and 918 cm-' and assigned them, together with weaker bands at 890, 835, and 797 cm-', to the vibrations of bonds between the surface cation and oxygen produced as a result of dissociative adsorption of oxygen on cation sites of different coordination. Al-Mashta ef al. (19b) have reconsidered the case of Fe,O, and tried to rationalize the previous results on the following basis. As indicated in Appendix C, dioxygen species are known to absorb at 1550 cm-' (O,), at ca. 1150 cm-' (O;), and at ca. 800 cm-' (O:-). These wavenumbers relate to formal bond orders of 2, 1.5, and I , respectively (Table I), but intermediate situations are possible (134b).Al-Mashta et al. (19b)have suggested that such species perturbed by the strong electrical forces of the quasi-ionic solid would give bands of higher wavenumbers than the above values, due to partial electron withdrawal from antibonding orbitals. In this discussion, the possibility of backbonding from metal orbitals to the antibonding orbitals of oxygen (134c),which will tend to decrease the voo frequency of the dioxygen species, has been neglected. They have suggested that all reported bands on m-Fe2O3 between 1350 and 1250cm-' should be assigned to a perturbed 0; species, intermediate between 0, and O;, and absorption between 1100 and 900 cm-' to perturbed 0:- species, intermediate between 0;and 0;-. However, mononuclear species such as M=O also absorb in the region I 100 to 900 cm-' (Appendix C) and the assignment needs to be confirmed by '60/'80 experiments. On the basis of isotopic studies on Cr,03 (134d), Sheppard and co-workers have revised their original attribution of the bands to 0:- ions and now conclude that they are more consistent with a mononuclear species such as Fe=O. In the case of diluted MgO-Coo solid solutions, Zecchina et al. (1344 have made labeling experiments and assigned 0-0 stretching frequencies in the 1160-1015 cm-' range to adsorbed 0; superoxide ions. These results are in line with those obtained with oxygen carriers where absorption in the range 1120- 1140 cm- has been observed and assigned to coordinated molecular oxygen in agreement with the approximate representation Co(II1)-0; (134s).

'

5. Photoelectron Spectroscopy

Gopel et al. (135a)have reported ultraviolet photoemission spectra (UPS) of the interaction of 0, with the (lOT0) face of a single crystal of ZnO. Between 300 and 600 K, chemisorption of oxygen is observed on "stoichiometric" ZnO (1010) surfaces and UPS difference spectra indicate peaks at

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

33

4,6.5, and 10.5 eV below E, (valence band edge). By comparing with EPR data and using the notation of Fig. 2, the bands were thought to arise from O,, with the peak at 6.5 eV tentatively attributed to the cglevels of 0; and the broader peak at 4.0 eV to the n,(,, levels. The electronic states of the adsorbed 0; are almost decoupled from the ZnO states, confirming that it can be regarded as a localized surface complex. It is also interesting to note that X-ray photoelectron spectroscopy (XPS) has also been applied to the study of dioxygen cobalt adducts. Burness et al. (13.56)have obtained the binding energies of the cobalt 2p electrons in the parent cobalt (11) complexes and the cobalt 2p and oxygen 1s1,2 binding energies in the dioxygen adducts and interpreted their results in terms of a formulation of the dioxygen adducts as Co(111)-0;. They were also able to measure the electron transfer from cobalt to oxygen and found a value of 86%, in good agreement with the value derived from EPR data (46b). B. THE0; ION The 0; ion has only a single electron in the ng orbitals, compared to three for the 0; ion (Fig. 2), and is isoelectronic with NO. The presence of the surface will break the degeneracy of the ng orbitals in the same way as for O;, but in this case an unoccupied molecular orbital is formed slightly higher in energy than the ng orbital since this must now contain the unpaired electron and the EPR signal is expected to have a negative g shift. In the solid state, EPR signals have been observed from a series of fluoride complexes, e g , 0; -AsF; (136);two of the g tensor components were between 1.96 and 2.00 and the third was between 1.73 and 1.76. This is in agreement with an analysis based on crystal field theory assuming an ionic model, which to first order gives gzz

= ge

- 2AlA,

gxx = g e -

21IEy

gyy

= ge

where the symbols have the meaning given in Fig. 2 and ni is assumed to contain the unpaired electron. The parameter 1is the spin-orbit coupling constant of oxygen. Certain preparations of Ti02 after heating in oxygen show complex EPR signals which have been assigned successively to 0; (137), coordinated oxygen (138), solid-state defects (139),and (TiO)3+(140).These assignments were based on an analysis of the g tensor believed to be orthorhombic. Repetition of experiments in X- and Q-bands has shown that the earlier interpretation of the Q-band spectra (137) was erroneous since two of the g tensor components were in fact hyperfine features due to 14N ( I = 1) (141).The EPR signals are formed when ammonia introduced by the pre-

34

M. CHE AND A. J. TENCH

parative method (142) is catalytically oxidized according to the Ostwald process (141): 2NH,

+ $0,- 2 N O + 3H,O

Nitric oxide can then be detected alone or in interaction with an oxide ion to form NO:- (141, 143). These processes suggest that the catalytic activity of T i 0 2 samples is likely to depend on both preparation and heat treatment (144). From the work discussed above, there is no clear-cut evidence at present that the 0;ion can exist on oxide surfaces. In this connection, Davydov (204 has investigated the adsorption of oxygen on the Mo0,/A120, and MoO,/MgO catalysts by IR spectroscopy. Broad absorption bands were observed in the 1500-1700 cm-' range which disappeared on heating to about 100°C. Davydov assigned these bands to adsorbed molecular oxygen; he explained the increase of voo on adsorption by a transfer of oxygen to its singlet state 'Ag. This explanation is doubtful since voo for the gas-phase singlet oxygen at 1483 cm-' ( 2 4 4 is lower than that for gas-phase triplet oxygen at 1555 cm-' (21). It is more likely, as indicated in Section 11, that a partial electron withdrawal from the antibonding orbitals of the oxygen molecule occurs to form an adsorbed species with some 0;character. C. T H E O ~ION -

The 0:- ion is normally referred to as the peroxide ion (33b),which should be distinguished from the covalent peroxy radical (ROO.). It has been previously treated in an earlier review by the present authors (1) as a dimer 0 - species. Although well known as a bulk peroxide (33b), this ion is difficult to characterize on the surface because it is diamagnetic and would be expected to be infrared inactive. Peroxides are associated with a broad optical absorption at about 260 nm (245a,b),which is very similar to 0;(Section III,A,4). Andersen and Baptista (146) have reported 0:-in KCl crystals, characterized by an optical absorption at 260 nm and distinguished from 0; by the absence of an EPR signal. Yao and Shelef (147) report a new EPR signal when oxygen is admitted to 12% Re/y-A120, catalyst after previous reduction in hydrogen. No EPR parameters are given but the signal is attributed to Re2+ and therefore taken as evidence that 0:- is formed on the surface. The arguments are not very convincing and the state of the oxygen on the surface is not well defined. Studies of metal-dioxygen complexes show that the peroxide-like complexes have IR bands voo in the range 800-932 cm-' (148).These data, taken together with the Raman work described below, indicate that the frequency

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

35

range 1061-1088 cm-' (quoted on p. 97 of Ref. 1 ) previously assumed as characteristic of the peroxide ion in the alkaline-earth oxides (21)is incorrect and the bands probably arise from a superoxide impurity. Davydov et al. (149a) have reported IR observations of a negatively charged oxygen species formed when oxygen is adsorbed on chromic oxide absorbing at a frequency of 985 cm-'. The molecular nature of this species was confirmed by isotopic labeling and it was found to convert to an atomic form when the reaction temperature was increased to 200°C. The molecular species was not identified by the authors, but comparison with Raman data on bulk peroxides (149b) where the stretching frequency is near 800 cm-' suggest that the 0-0 bond is very weak and if it is a dioxygen species, it would correspond to 0;- on the surface. However, isotopic studies by Sheppard and co-workers (1344 do not confirm Davydov's original suggestion of dioxygen species and are more consistent with the presence of a mononuclear species such as Cr= 0. Conductivity and chemical methods (2) of measuring the charge on the oxygen do not distinguish between 2 0 - and O;-. It is therefore not possible to obtain direct evidence on the nature of the oxygen species and these approaches are not discussed further.

D. THEO:- ION Symons (150) has suggested that the 0- species on the surface would be better described as the species O;-, which is isoelectronic with F;. However, an experiment performed with MgO enriched in 1 7 0 showed that the interions was not measurable (151).Ben Taarit et al. (152) action of 0- with 702have observed an oxygen species on Pd(1) zeolite with g values of g1 = 2.050 and gl, = 1.99. The g values are inverted from those expected for 0, (i.e., g1 < gll) and this could be accounted for by rotational averaging. Alternatively, the authors speculate that the signal may be due to 0;- ions with the unpaired electron in a CT* orbital, since the g values would then be as expected from analogy to Cl, and F; (153). In support of this, approximate measurements of the intensity of the signals indicate that three Pd(1) ions are lost to form one oxygen species, as would be expected for O i - . At the present time, the argument is open and more evidence is needed to support the existence of 0;- on surfaces. Even in the solid state there are few examples of O i - . Pure crystals of CeO, UV-irradiated at 77 K exhibit several paramagnetic centers with orthorhombic g tensors; a typical g tensor is gxx = 2.0175, g y p = 2.0054, and gzz = 2.0317. Several possible models for these centers are proposed, one of which involves an 0;- molecular ion near a stabilizing impurity ion (154).

'

36

M. CHE AND A. J. TENCH

IV.

Formation and Stability of Charged Diatomic Species

The adsorption of oxygen on an oxide surface depends on the method of pretreatment and this can be divided into three main types :

(i) The method most generally used in studies of oxygen species is slight reduction, by thermal treatment in uucuo or in a reducing atmosphere, at a few hundred degrees Celsius. This cleans the surface and produces a slight nonstoichiometry or valence change in the metal oxide so that the surface adsorbs oxygen readily. (ii) For stoichiometric oxides, UV or y irradiation has often been used after thermal treatment to provide excess electrons at the surface. The oxygen species are produced by adding oxygen to the samples after irradiating either in uucuo or in a reducing atmosphere; alternatively, by contacting the thermally treated sample with oxygen and then irradiating it in oxygen. (iii) Lastly, the oxygen species can be produced by secondary reactions. In these cases, the thermally treated surface can be activated by pretreatment with a reactive molecule ( H 2 , CO, etc.), normally at room temperature, followed by contacting with oxygen to produce the oxygen species. The adsorption of the reactive molecule is viewed as the primary reaction, whereas the formation of the oxygen species is the secondary one (96). In the following paragraphs, in order to help the discussion on the formation of the oxygen species, the various oxide surfaces have been divided somewhat arbitrarily into groups. Most of the discussion refers to the 0; species since, although there is much evidence to show that this is not the only dioxygen species, there is essentially no direct information on the nature of the other oxygen species. A.

IONIC OXIDES

1. Alkaline-Earth Oxides and Their Solid Solutions with Transition Metal Oxides Ultraviolet or y irradiation of MgO (68,155) in uucuo or in hydrogen forms electrons trapped on the surface, which will react with oxygen to form 0;. A typical g tensor is gzz = 2.0777, g y y = 2.0089, gxx = 2.0018 with a range of gzzvalues indicating the presence of several sites on the surface. The superoxide ion is stable at room temperature for several months. The identification was confirmed by using 1 7 0 2 and the two oxygen nuclei were found to be equivalent, indicating adsorption parallel to the surface (68). Irradiation in the presence of oxygen leads to a more complex spectrum, indicating the

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

37

presence of several oxygen species (68,156). Superhyperfine interactions can also be observed with nearby protons under some conditions (68,159). The formation of 0; by adsorption of oxygen has been reported on MgO (157)and CaO (71,158)which have been thermally activated in uucuo at high temperature. This has been confirmed by 1 7 0 2 (71) adsorption on CaO, but 0; is only formed when the CaO is made by thermal decomposition of hydrated CaCO, in uucuo after heating to 800°C and is not formed on CaO prepared from Ca(OH), in the same way. These observations can probably be best understood in terms of the influence of hydrogen-containing impurities (see next paragraph) in activating the surface. A similar effect would account for the observations with MgO. Derouane and Indovina (157) have attempted an analysis of the variations in the g tensor for the 0; on different crystal planes of MgO but the analysis does not seem justified, particularly since there is great difficulty in obtaining accurate values for gyy and gxx because of overlapping signals in a polycrystalline sample. It has been shown recently, that the alkaline-earth oxide surface can be activated for the formation of 0; by either preadsorbed gases (159) or by transition metal ions (110). Indovina and Cordischi (159) reported that exposure of a MgO surface to H,, CO, or C,H, after thermal activation followed by subsequent exposure to oxygen leads to a strong EPR signal from 0;. Preadsorption of H, (160) gave a multicomponent gzz feature ranging from 2.0895 to 2.0623, similar to that seen on irradiated samples (68, 155). The 0; signal was completely destroyed after heating at 300°C and different thermal stabilities were obtained for 0; in the different sites. Activation of the surface by H, was thought to occur via the homolytic dissociative adsorption of H, onto a pair of adjacent surface 0- ions originally formed by dehydrogenation of the surface under vacuum : H+ H, + 0,. . 0,

I

-+

Ht

I

0;; ' .Ot,

where 0;; refers to ions in low coordination on the surface ( I ) . The (0;;-H') entity then acts as the active center for electron donation to form 0; adsorbed at a nearby Mgz+ ion. However, neither the pair of 0- ions nor the active center could be seen directly by EPR, although they would be expected to be paramagnetic. A similar effect has been reported in CaO (158) and other work has shown that preadsorption of pyridine on MgO (72,161,162), CaO (70),and SrO (72,129) followed by adsorption of oxygen leads to the formation of 0; on the oxide surface. For SrO, measurements of spin concentration have shown that the 0; ions are produced by electron

38

M. CHE AND A. J . TENCH

transfer from a dipyridyl anion radical formed by adsorption of pyridine (129). Garrone et al. (163) have rationalized all these results by proposing the following mechanism for the adsorption of hydrogen and alkenes. Oxygen ions in positions of low coordination on the surface abstract protons from the adsorbed molecules to form OH, ions and a carbanion: R H + 0:; h O H , + R R - + 0, ‘ 0 ; + R . 2R. + 0, + R O O R

These carbanions react with oxygen to form O;, whereas the radical forms a bridged peroxide or dimerizes. In the case of hydrogen, adsorption leads to heterolytic dissociation at sites of low coordination on the surface (164a,b) to give(02--Hf)and(Mg2+-H-); the (Mg2+-H-)complex is then thought to act as the electron donor on oxygen adsorption, ultimately forming H, and 0; (165). This mechanism does not involve the formal transfer of electrons from the surface, although the overall reaction appears the same. A new form of adsorbed 0; has been reported by Ben Taarit et al. (63) which is formed by adsorbing oxygen at low temperature onto a MgO surface containing CO;. The resulting complex is assigned to (CO,-O,)with a g tensor of gzz = 2.040, gyy = 2.0072, gxx = 2.0015 and a hyperfine tensor obtained using I7O, which shows that the oxygen nuclei are not equivalent, with A,, = 100 and 50 G. On warming to room temperature, this species is transformed into 0; with the normal spectroscopic parameters. These hyperfine values indicate that the two oxygen atoms must be bonded end-on, at an angle to the CO,, in a peroxy-type linkage (Section III,A,2). Added transition metal ions can also induce the formation of 0; and adsorption of oxygen onto 1% Mn ions in MgO gives a poorly resolved signal centered about g = 2.007, which has been attributed to 0; (166). The Coo-MgO system has been studied in some detail, covering concentration ranges of 0.05-5 Co atoms per 100 Mg atoms (110).Two kinds of 0; ions adsorbed at Mg2+ and Co3+ sites can be identified (Fig. 1 l ) , together with some evidence for 0;. The O;-Co3+ complex is characterized by a g tensor of 2.124, unresolved, and 1.987. The large value (gzz) is high for adsorption at a 3 + cation (Fig. 3, Section III,A,I), but a superhyperfine interaction of 37, unresolved, and 17.5 G was observed and confirms that the adsorption site is a cobalt ion. It would seem that the bonding in the (Co” . . . 0;) complex is more D type than n type and the ionic model is not suitable. No I7O2 work has been reported. Adsorption of oxygen at temperatures above -70°C gives O;, thought to be adsorbed at a Mg2+

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

39

I )

0;IMg2')

FIG. 1 1 . The EPR spectra at 77 K of 0; on COO-MgO samples. Spectra (a) and (b) were recorded after evacuation of oxygen at 298 K, (c) and (d) in the presence of a small amount of oxygen. Spectra (a) and ( c ) refer to a 0.2%COO-MgO sample, whereas spectra (b) and (d) refer to a 5", Coo-MgO sample (the cobalt concentration is expressed as Co atoms per 100 Mg atoms) (110).

site (gzz= 2.098-2.062). The (Co3+. . . 0;) complex disappears entirely at 25°C and is thought to form Oi-, although there is also an increase in the concentration of 0; adsorbed on Mg2+ ions. Zecchina et a!. (134e)have also studied oxygen adsorption on the surface of CoO/MgO solid solutions in the same composition range using IR spectroscopy. They conclude that -85% of the adsorbed oxygen is in an undissociated molecular form at 77 K, probably as 0; characterized by stretching frequencies in the range 1160-1015 cm-'. There was some indication of a bridged superoxide structure on the surface. The EPR data (110) indicates that 0; is adsorbed at both Mg2+ and Co3+ sites at fairly similar concentrations at 273 K, whereas for 0; formed at 77 K the Co3+ site predominates. This would suggest that the IR data at 77 K refer to the oxygen ion adsorbed at Co3+ rather than Mg2+. 2. Zinc and Cadmium Oxides The formation of oxygen species on ZnO has been of interest for some time (17). Thermal activation of ZnO at about 500°C in U ~ C U Ogives an EPR signal at g = 1.96 which is thought to arise from a donor species such as Zn+ ions (155,167-170). Adsorption of oxygen decreases the signal at 1.96,

40

M. CHE AND A . J. TENCH

and a new signal is formed corresponding to 0; with gzL in the range 2.052'2.042 (155, 167, 170-1740). The range of gLZvalues indicates that there are several sites on the surface which are all in reasonable agreement with adsorption at a cation charge 2 +. The signal is broadened reversibly by excess (80)indicates equivalent oxygen oxygen in the gas phase. Adsorption of 702 nuclei with a hyperfine tensor of A,, = 80, A,, = 0, and A,, = 15 G corresponding to oxygen adsorbed parallel to the surface. A report by Codell et 01. (174b) with different hyperfine splittings is inconsistent with the rest of the data and appears to be due to a misinterpretation of the EPR results. Some exchange of 0; with the lattice ions has been observed after heating at 200°C (80),which is slightly above the limit of its stability ( 180- 19OOC)on the surface as inferred from thermal desorption studies (170, 173). Oxygen adsorbed on ZnO with preadsorbed hydrogen was also identified as 0; by EPR, but it desorbed at 1 10- 120°C, indicating a marked reduction in adsorption strength for 0; on the hydrogen-preadsorbed sample (173).A similar species is seen after y or UV irradiation of ZnO (173, 174c, 175-179) and possibly also on Be0 (179) in the presence of oxygen. A signal with gZz= 2.045 has been attributed to 0; adsorbed on a ZnO (1010) face (180). Adsorption of CO or C,H, at 25°C on samples with preadsorbed oxygen does not change the EPR parameters of O;, but adsorption of NH, at - 30°C leads to an initial increase in g,, to 2.069, rising to 2.109 for excess NH, (181).The increase in gzz may reflect a change in bonding from n to o type or it may just reflect a decreased effective charge seen by the 0; ions. No work with I7O2 has been reported to check these two possibilities. From water adsorption experiments on ZnO, surfaces, it is suggested that 0; ions can be reversibly produced from 0;- ions (1744 according to the reaction 30:-

+ 2H,O

#

20;

+ 40H-

Setaka and Kwan (182) have investigated the CdO/Al,O, system and reported g values of 2.039, 2.009, and 2.002 for 0;. Although the authors did not discuss the adsorption site, the gzz value suggests that the 0; ion is adsorbed on A13+ rather than on CdZ+,in agreement also with the line shape of the signal, which indicates probable broadening due to 27AI nuclei. 3. Tin Oxide Thermal activation of SnO, at 500°C in

U ~ C U Ogives

an EPR signal at

g = 1.896 which has been attributed to donor electrons (183). Oxygen adsorption gives a complex signal (183-186). A major triplet with a g tensor of

2.024, 2.009, and 2.0036 has been assigned to 0; adsorbed at Sn4+ sites (74, 184) and both oxygen nuclei are found to be equivalent with an "0 hyperfine splitting A,, = 80.5 G (74). Two minor triplets with g = 2.028,

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

41

2.008, 2.002 and g = 2.00, 2.00, 1.9984 have been assigned to 0- and O:, respectively, by Mizokawa and Nakamura (183).This seems unlikely in view of the fact that only the major species shows a hyperfine splitting with "0. Furthermore, the assignment of the latter triplet to 0; seems to be in disagreement with theory, which predicts gYY= ge and gx,, gzz < ge (see Section 111,B). Separation of the peaks into triplets gives ambiguous results without work at more than one frequency; for example, Meriaudeau et al. (74) find another triplet with g = 2.034, 2.004, and 1.994, the origin of which is uncertain. It is clear from measurements of adsorption and conductance (183) that 0; is not the only species formed, and nonparamagnetic oxygen species must also be formed on the surface. Parallel thermodesorption and EPR experiments indicate that 0; species adsorbed at Sn4+ ions are stable up to 150°C (170). The complex EPR spectrum of 0; adsorbed on SnO, has received another interpretation by Anufrienko et a!. (22f,187), who assign the various peaks

FIG. 12. Analysis of the spectrum of 0;stabilized on Sn2+ showing hyperfine lines due to 16% naturally abundant 115-117*119Sn(I =

wn.

42

M. CHE AND A. J . TENCH

to superhyperfine lines due to interaction of the unpaired electron with l15Sn, "'Sn, and '19Sn, all with I = 3 (all have about the same magnetic moment and a total natural abundance of 16%)(Fig. 12). The 0 , species are believed to be adsorbed either on Sn4+ ions with g , = 2.025, y 2 = 2.009, y3 = 2.0036 and A , = 27.5, A 2 = 34, A 3 = 25.5 G , or on Sn2+ ions with g , = 2.049, g2 = 2.009, g3 = 2.0028 and A , = 47, A , = 58, A 3 = 47 G, the former disappearing at lower temperatures (200°C) than the latter (300°C).

I al

50 G

F-----i

Ibl

FIG.13. The EPR spectra at 77 K of 0 ; ion on SnO, pretreated at 400 C (a) in (b) in hydrogen (188).

L'IICUO or

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

43

Recent experiments using thermal treatment in uucuo and hydrogen (Fig. 13) and a Q-band spectrometer suggest that the hypothesis of Anufrienko et al. is not correct and that the complex spectrum is best explained by the presence of 0; stabilized on Sn4+ ions in different environments (188). 4. Thorium Oxide Thermal treatment of T h o , at 450°C in uacuo or hydrogen gives no EPR signal, but subsequent adsorption of oxygen gives a complex signal with gzz = 2.0589, 2.0540, gyy = 2.0098, 2.0092, and gxx = 2.0073, 2.0042 (189). This is consistent with the formation of 0; in two different sites on the surface. The nature of the cation at the adsorption site is not clear, but the gzz values are more consistent with a cation charge of 2+ than 4-t. The signal from 0; gradually decreases in intensity with increasing temperature of annealing and disappears at about 300°C; this process is thought to involve conversion to other oxygen species such as 0-,but there is no evidence for such species from the EPR data. T h o , is one of the cases (see alkaline-earth oxides and ZnO) where the effect of preadsorbed gases on the formation of 0; has been studied (62, 190). Preadsorption of C O leads to an EPR signal with g1 = 1.998 and gII = 1.981 (190).An additional new signal appears on adsorption of 0, at 77 K which has been attributed to two species; species A with g values of 2.019, 2.008, 2.002 and species B with g values close to the preceding values, except that gzz is between 2.088 and 2.040 (62).After warming to 298 K, just one species remains with a gzz of 2.048. Species B is also formed by adsorption of oxygen at 298 K followed by hydrogen. Adsorption of 1 7 0 2 (62) shows that species B is 0; with two equivalent oxygen nuclei ( A x x= 75 G), whereas species A appears also to be 0; but with inequivalent oxygen nuclei ( A x , = 95 and 65 G). Comparison of the EPR spectra for 0; (B) at 77 and 298 K indicates that rotation is occurring about the gzz axis at the higher temperature to give an axially symmetric g tensor while the hyperfine splitting decreases with increasing temperature. It was suggested that the bonding of the 0; with the surface varies with temperature, but it seems more likely that the effects on A,, arise from the rotation (Section III,A,2). 5. Rare-Earth Oxides, Including Scandium and Yttrium Oxides Thermal activation of either pure (191, 192) or silica-supported CeO, (45, 191) at 500°C in uacuo gives a signal at gav = 1.963 which has been attributed to Ce3+ ions or to electrons in the solid. The adsorption of oxygen on pure CeO, gives a poorly resolved signal with gll = 2.0312 and g1 = 2.0137 (192),whereas on the Si0,-supported system an orthorhombic g tensor is reported with gzz = 2.028, gvv = 2.0109, and gxx = 2.0158 (45). Adsorption

44

M. CHE AND A. J. TENCH

of 702 on either system (45, 73) indicates that both oxygens are equivalent with A,, = 75 G. This spectrum gives rise to an unusual situation because the hyperfine structure is not centered about the smallest g value as usually expected, but about the middle value of the g tensor, indicating that the nf and n: orbitals may be inverted (45). This gives rise to an inconsistency between the g and the A tensors, since both the smallest value of g and the largest value of A are expected to lie in the direction of the orbital containing the unpaired electron. In the g tensor above, the A,, component has been taken so as to correspond to the orbital containing the unpaired electron and used to label the g value of 2.0158 as gx,. The g tensor is in reasonable agreement with values obtained by Dufaux et al. (191) and by Setaka and Kwan (182)for CeO, on A l , 0 3 . A large giWvalue is found also for 0; on UO,/AI,O, (182). Probably the simple ionic model is not suitable for these systems, which have an unpaired electron in extended f orbitals. Steinberg and Eyal (193)assign an EPR signal with g = 2.010, 2.060,and 2.12to 0; on Y20,.Although the nature of the 0; species was confirmed by 170labeling experiments, the assignment to an orthorhombic y tensor would appear to be in disagreement with the line shape of the EPR signal, which gives a better fit to an axial g tensor with gIl = 2.060and g1 = 2.010. It is likely that 0 ; adsorbed at a second site with gll = 2.12 and gl = 2.010 is also involved. This would bring the results of Steinberg and Eyal in line with those of Loginov et a/. (194),who found three types of 0; ions on Y , O J , two of which, with g z z = 2.055, g,,,. = 2.007, gAx= 2.003 and gI1= 2.121, g1 = 1.9995, are in reasonable agreement with our new assignments. Loginov et ul. (1Y4) have looked at 0; on a number of rare-earth oxides which have preadsorbed gases such as H, and CO. The gZLvalue is shown to vary considerably with the type of pretreatment, from 2.035 to as high as 2.121. The large g shifts were interpreted in terms of considerable 0 bonding with the cation at the adsorption site. These ideas are not completely consistent with evidence from 0; on the supported CeO, system ( 4 9 , where the two oxygen nuclei are known to be equivalent. In the case of lanthanum oxide (La,O,), a small superhyperfine splitting was observed, confirming a lanthanum cation adsorption site. More information on 0; adsorbed on rare-earth-exchanged zeolites can be found in Section IV,C,3.

B. TRANSITION METALOXIDES I.

Titanium Oxide Thermal activation of TiO,, either as anatase or rutile, at 300-5OO'C

in uucuo or in a reducing atmosphere gives a slightly reduced solid with a paramagnetic signal which has been attributed to Ti3+ centers (137, 138,

45

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

195, 196); thermal activation in air at 500°C produces complex EPR signals which have been assigned successively to 0; (137), coordinated oxygen (138), solid-state defects (13Y), and TiO: (140).These complex EPR signals are in fact due to adsorbed NO and NO:- species (141, 143), as discussed in Section II1,B. Exposure of the slightly reduced samples to oxygen leads to a new complex signal which has been attributed to various forms of coordinated oxygen and/or o-, 0; (138,185,197) or to 0; ions adsorbed at different surface sites (I74c, 196). The situation was clarified by the work of Naccache et al. (75), who were able to show, using "O,, that the signals observed on anatase or rutile (Fig. 14) should be attributed to 0; adsorbed at different sites on the surface, but in all cases the oxygen nuclei were equivalent (Table VI), indicating that 0; is adsorbed parallel to the surface. This has been confirmed by theoretical calculations (43b).The gzzvalues are reasonably consistent with adsorption at a Ti4+ site and the A,, values show only a small variation. In addition to the 0; signals, a symmetrical signal was observed at giso= 2.003, which was seen also by Van Hooff (185). The origin of this signal is not clear, since no hyperfine splitting could be detected using 70-enriched oxygen.

Y

:

10

Ti3'

n I

100 G

L

160170

170 170

L

I I

I

I

I

1

I

I I

I

I I

I

I

I

I

FIG.14. The EPR spectra at 77 K of 0 ; ion on reduced TiO, showing the hyperfine interaction with two equivalent oxygen nuclei (75).

46

M. CHE AND A. J . TENCH

TABLE VI Spectroscopic Constantsfor 0;Adsorbed on Thermally Activated TiO,"

TiO,

8rz

Rutile (species I unstable at 25°C) Rutile (species I1 fairly stable at 25°C) Anatase Ih (species I stable at 25°C) Anatase Ilh(speciesI 1 unstable at 25°C)

2.030 2.020 2.025 2.024

g ,,

2.008 2.009 2.009 2.009

Yxx

A,, (G)

2.004 2.003 2.003 2.003

76 72 77 77

From Ref. 75.

' Anatase I was prepared by flame hydrolysis of the chloride, whereas anatase I1 was prepared by precipitation.

In their study of oxygen adsorption on TiO, by temperature-programmed desorption and EPR, Iwamoto et al. (170)have shown that several types of 0; ions with different gzz values and thermal stabilities could be detected. They found that 0; ions with yzz values of 2.019, 2.023, and 2.026 were related to desorption temperatures of 125, 250, and 190°C, respectively, and suggested that variations in g z z values and thermal stabilities for 0; were due to differences in coordination number of the Ti4+ adsorption centers (see Section III,A,l). Similar results are obtained for thermally activated bulk or supported TiO, systems. Shvets and Kazansky (198)found that two types of 0; could be observed at 77 K on TiO, supported on silica, with yzz values of 2.026 and 2.020, their relative intensities depending on the TiO, content. They assigned the gzz value at 2.026 to 0; adsorbed on tetrahedrally coordinated titanium ions formed at low TiO, content, whereas the yzzvalue at 2.020 was related to 0; adsorbed on titanium ions in square pyramidal coordination prevailing at higher concentration. In a later study on TiO, supported on porous Vycor glass (PVG), Shiotani et al. (66) reported two triplets at 77 K with yzzvalues of 2.0237 and 2.0305, which they assigned, by analogy with the results obtained by Naccache et al. (75) for unsupported TiO,, to 0;adsorbed on anatase and rutile, respectively. Che and Naccache (199)have studied the kinetics of 0; formed on slightly reduced anatase using EPR. They found that the adsorption could be explained on the basis of different formation rates for 0; adsorbed at different sites, with zero- and first-order kinetics for the oxygen and Ti3+ concentrations, respectively. Using the same approach, Hauser (200) has extended this work and proposed different models to explain the kinetics based on the formation of O;, 0,-, and O f -ions for which activation energies around 1 kcal/mol were obtained. Nikisha et al. (201) have studied the oxygen adsorption kinetics using EPR, conductivity, and volumetric measurements.

-

N

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

47

They concluded that the initial fast step involved the localization of electrons by oxygen, but significant amounts of 0; were formed more slowly. The amount of oxygen adsorbed exceeded the concentration of 0; by an order of magnitude or more in the case of highly reduced TiO,. Infrared studies (20b) indicate that oxygen is adsorbed in three forms: a neutral molecular form which absorbs in the range 1600-1700 cm-', a second molecular species absorbing at 1180 cm-' which is assigned to O;, and a dissociated form which is characterized by a metal-oxygen bond vibration in the range 700- 1000 cm-'. Presumably the surface perturbs the adsorbed oxygen sufficiently to make the molecule infrared active as discussed in Section III,A,4. With increasing reduction of the sample, the amount of the neutral molecular form became progressively less and most of the oxygen was adsorbed in a dissociated form. A wider range of gzz values (2.0213-2.0330) for 0; was observed for the more highly reduced sample. Calculations using extended Huckel theory (202) suggest that these changes are to be expected with an increasing degree of reduction. In the presence of oxygen atoms, 0; is not formed on rutile but subsequent exposure of the sample to molecular oxygen gives 0; with y,, = 2.019 (203). Many papers covering oxygen photoadsorption on Ti 0, have been published [see, for example, Refs. 204,205 and the references quoted therein, and also the review by Bickley (206)l.The subject is complex, but there is general agreement that the hydroxyl groups at the surface participate in the photoadsorption of oxygen by TiO, (207). Ultraviolet irradiation of TiO, in the presence of oxygen at 77 K can lead to a number of paramagnetic oxygen-containing species depending on the outgassing conditions of the solid prior to UV irradiation. 0 - (or O:-), HO,., O,, O,, and 0;- have been reported (88, 205, 208), but unambiguous assignment has proved difficult (I; see also Sections II1,D and V,A). If the sample is warmed to room temperature, only the 0; species remains visible. Meriaudeau and Vedrine (88)have used "0, labeling to study the species produced by photolysis at 77 K in oxygen on TiO, dehydrated at 450°C. A normal 0; is formed with g values of 2.021,2.009,2.001 and a hyperfine splitting of 77 G with equivalent oxygen nuclei. Two other species were observed with g = 2.014,2.009, 2.003 and gIl = 2.008, g1 = 2.001 which were attributed to 0; and O:-, respectively, but no hyperfine structure was seen. However, the species attributed to 0;- (see Section V) readily reacts with C O at 77 K to give a new species identified as 0,-O,,-CO- with a g tensor of 2.0465, 2.006, 2.001 and with A,, = 104 and 42.5 G for 0, and O,,, respectively. This is a peroxy-type radical with nonequivalent oxygen nuclei, in which all the unpaired spin resides on oxygen atoms I and I1 originating from the gas-phase molecular oxygen, and is thought to be formed by the reaction 0:-

+ co -+O& + 0-0-co

48

M. CHE A N D A. J . TENCH

Supported T i 0 , systems have also been used for photoadsorption studies. Shiotani et ul. (66) have reported that UV irradiation of the TiO,/PVG system in oxygen gave rise to various oxygen species. One species was unambiguously identified as 0; by means of 70-enriched oxygen. Measurement of the hyperfine tensor at low temperature showed that two slightly inequivalent oxygens were present in the same 0; with A,, values of 74.9 and 80.3 G at 36 K, in good agreement with earlier data obtained for anatase (82). 2.

Vanudium Oxide

Because of the superhyperfine interaction which arises when the 0; ion is formed on a cation with nonzero nuclear spin (see Section 111,A,3) vanadium pentoxide, with 100% naturally abundant "V isotope (1 = i),has been of considerable interest. However, the presence of a superhyperfine splitting has created some difficulty in the assignment of the signals. V,O, cannot be prepared with large surface area and most of the data refer to supported V,O, systems. Silica-supported V,O, is generally activated by thermal treatment at ca. 500°C in an atmosphere of oxygen followed by hydrogen. This procedure leads to the formation of tetrahedrally coordinated V4+ ions (209) and subsequent adsorption of oxygen gives a complex EPR signal. This was initially thought to be from 0; (210) or a combination of 0; ions (211) and was then reinterpreted in terms of a mixture of 0; and 0 - (106)with the following parameters for 0; : gzz = 2.023, yyy = 2.01 1, glx = 2.004 and A,, = 9.7, A,, = 6.8, A,, = 5.9 G. The superhyperfine interaction arises from 51V and confirms that the adsorption site is a vanadium cation. Calculations of the electronic structure and superhyperfine parameters indicate that the 0; ion is donor of n and B electrons to the metal ion and give reasonable agreement with the experimental values (117, 118). Spectra ob) not sufficiently well resolved to give the tained using I7O2 ( 2 1 2 ~were unpaired electron distribution between the oxygen nuclei. The thermal stability is dependent on the experimental conditions. Shvets et ul. (106) reported that the 0; ion was stable at temperatures up to 300°C in an oxygen atmosphere, whereas Yoshida et al. (212b)observed that heating for 15 min at 150°C caused a decrease of 0; by 80%. These results are to be compared with those reported by Iwamoto et ul. (170),who observed a broad desorption peak ranging from 100 to 500°C and assigned it mainly to 0; by comparison with earlier EPR results. Fricke et ul. (107) have studied the formation of 0; and 0 - on silicasupported V20,-P,0, catalysts. The 0; and 0 - formed are stabilized on vanadium ions, but the amount decreased with increasing fraction of P,O,.

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

49

However, the maximum selectivity for butene oxidation to maleic anhydride occurs at a P/V ratio of 2/1, where the concentrations of 0- and 0; are much reduced. This was taken to indicate that the predominant role of 0; and 0- in this system is in nonselective oxidation. ZrO,, MgO, and Al,O, have also been used as supports (198,213, 214). 0; ions with g values of 2.032,2.009, and 2.003 can be formed after adsorption of oxygen at room temperature on slightly reduced V,O,/ZrO,. The gzzvalue of 2.032 coincides with that obtained for 0; adsorbed on the ZrO, support and is therefore characteristic of Zr4+ adsorption sites (198).After admission of 0, at room temperature on thermally reduced V,O,/MgO (213, EPR signals with gzZ = 2.070 and 2.080 are formed which are consistent with adsorption of 0, at Mg2+ sites; these ions are stable up to 150°C. Adsorption of oxygen at 77 K resulted in a more complex situation and gzz values were observed at 2.080 and 2.090 originating from 0; on Mg2+ sites together with a new signal with g values of 2.026, 2.009, and 2.003. This latter signal did not exhibit any superhyperfine structure from ,'V and disappeared on warming to room temperature. The authors suggested that this signal also was due to 0 , because it did not disappear on contact with H,, and that lattice 0,- ions or pairs of vanadium ions were involved as the adsorption sites. In the case of V,O,/AI,O, it was necessary to adsorb oxygen at 77 K to detect a signal at gZz= gII= 2.024 and g1 = 2.008 which disappeared on heating to room temperature. The assignment was similar to that given for the signal at 2.026 in the V,O,/MgO system (213,214). Khalif et a!. (215) have carried out adsorption, microcalorimetric, and EPR studies of oxygen chemisorption on V,05/Mg0 and V,05/Al,03 to determine heats of adsorption. The interpretation of this type of measurement is difficult because oxygen is adsorbed in more than one form. For V,O,/MgO a comparison of the adsorption isotherm for oxygen and the EPR data for 0, showed that 0, only appeared in the spectrum after adsorption of about half the oxygen, and it was assumed that the heat of adsorption of oxygen at the last adsorption point corresponded to the heat of 0, formation. This gives a value of 18-24 kcal/mol, which agrees well with the heat of 0; formation on MoO,/MgO and MoO,/AI,O, (216). The oxygen adsorbed during the first half of the isotherm was thought to be in the form of 0- ions because it reacted with C O with a heat of reaction of 60 kcal/mol, whereas the oxygen adsorbed in the second part did not react. The 0- ions are not visible by EPR because their association with paramagnetic ions leads to a strong exchange interaction. The heat of adsorption of the oxygen in the first half is 40 kcal/mol, but adsorption of other molecular forms of oxygen is thought to reduce the observed value from the expected 60 kcal/mol observed for MoO,/MgO (216). On V205/ Al,03, oxygen is adsorbed with a heat of adsorption larger than 60 kcal/mol;

50

M. CHE AND

A . J . TENCH

there is no EPR signal from oxygen species and almost no reaction with CO. It seems likely that in this case oxide ions are predominantly formed on the surface. This work is an interesting illustration of how microcalorimetric data can be used in conjunction with other techniques to obtain direct information about the thermodynamics of adsorbed species, but the identification of the adsorbed species is not always certain. 3. Chromium Oxide

Despite their importance in olefin polymerization reactions, little attention has been paid to the nature of the adsorbed oxygen species on supported chromium oxide systems. The formation of 0; ions has been reported (217) for supported chromosilicate catalysts after reduction at 500°C with carbon monoxide and subsequent exposure to oxygen. However, the values yII = 2.007 and yI = 2.004 are quite different from what would be expected for 0, on this system on the basis of the ionic model [Eq. (6), Section III,A,l]. Doi (218) has reported different 0; species for CrO,/SiO, catalysts; reduction with ammonia gave a signal with g1 = 2.020, g2 = 2.009, and g3 = 2.004 together with a line at 2.027, whereas reduction in hydrogen gave only the line at 2.027; signals attributed to 0; were also observed. Howe (219) has used a different method of preparation based on the decomposition of Cr(CO), on silica, and tentatively identified a poorly resolved EPR signal as due to 0; with yzz= 2.01 7, yyy = 2.010, and g,, = 2.010. The gzZvalues in the range 2.017-2.020 would not seem unreasonable for 0; on Cr6+, but this system is not fully understood at present and experiments using 170need to be carried out. A difficulty arises with Cr ions because several oxidation states (from + 2 to +6) can be stabilized on the surface, depending on the thermal treatment, and a range of yzzvalues is possible. The thermal stability of these 0; ions is also unusual, since their EPR signals disappear on evacuation at room temperature and can be restored by subsequent reexposure to oxygen (217,219). Shvets ef ul. (86) have recently reported both 0; and a CO; + 0, adduct on CrOJSiO,. The CO; + 0, adduct is described as CO, and has a y tensor of 2.046,2.006,2.001 with 7O hyperfine interactions corresponding to two inequivalent oxygen nuclei (98 and 42 G). This is very similar to the adduct on MgO (63) and MoO,/SiO, (87) (Table IV). On warming, the adduct decomposes, giving off CO, and forming 0; with a y tensor of 2.070, 2.006, 2.001 while the hyperfine tensor remains the same. When observed at 300 rather than 77 K, the EPR signal is isotropic with giro= 2.022 and an isotropic hyperfine interaction of 30 G is observed, indicating considerable rotational freedom on the surface. From IR studies, Davydov et al. (1494 have reported an absorption from

51

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

oxygen at 985 cm-I on Cr,03 which was attributed to a molecular oxygen species such as O:-. Subsequent work using isotopic labeling with I6O/l ' 0 by Sheppard and co-workers (134d) has not confirmed this assignment and it is more consistent with a mononuclear species such as Cr=O. 4. Molybdenum Oxide

A number of very important reactions such as selective oxidation (220), olefin metathesis (221), and hydrodesulfurization (222) are catalyzed by systems involving molybdenum. Because of this interest, the adsorption of oxygen on supported molybdenum oxide has been investigated by many authors. Usually MOO, is supported on SiO, or Al,O,, but MgO and TiO, have also been used as supports. The molybdenum is activated for oxygen chemisorption by thermal treatment in uucuo at 500-600°C or by reduction in hydrogen to give EPR signals assigned to Mo5+(191,223).The adsorption of oxygen at 77 K on MoOJSiO, and MoO,/AI,O, was first studied by Dufaux et a/. (191) and subsequently by a number of other workers (81,82, 84, 85, 198, 213, 219, 224-228). The EPR lines from the adsorbed oxygen species are broad, particularly on Mo03/A1,0,, leading to some variation on reported g values, but generally accepted values are y1 = 2.016-2.0175, 9, = 2.0098, and y3 = 2.0042.These values are consistent with 0; adsorbed at a cation site with high charge such as Mo6+ (191).The departure of the smallest g value from that of the free electron is probably indicative of some covalent bonding rather than a purely ionic interaction (117). Krylov et d. (213) and Howe and Leith (226) have used the yzzvalue of the y tensor to show that adsorption of oxygen at 300 K on MoO,/MgO and Mo03/A1,03 leads to oxygen adsorbed at Mg2+ and A13+ cation sites, respectively. Electron transfer from one adsorption site to another (Fig. 15) was proposed by Krylov et ul. (213)where the original 0; adsorbed on Mo6+ was formed by adsorption of oxygen at 77 K. Similar evidence is also available from other work (227). Since the grr value can only give an indication of the charge at the adsorption site, it is more informative to study the superhyperfine interaction from the cation (see Section III,A,3). For this purpose, 95Mo-enriched catalysts

I

Mo6'-0

&

l

l

M o 6 L 0 - A13'

-

I

Mo6*-0-A13'

FIG. 15. Electron transfer occurring at the surface of MoO,/AI,O, according to Krylov e r a / . (213).

52

M. CHE A N D A . J . TENCH

have been prepared (109, 229) and oxygen adsorption investigated (108, 109).Che et al. (108)have reported a superhyperfine interaction with A,, = 2, A,, = 1.9, and A,, = 1 G, for 0; at 77 K and 300 K on MoO,/SiO,, arising from interaction with a Mo nucleus ( I = $). For MoO,/Al,O,, the situation was more complicated, and on warming to 300 K both a g,, value and a superhyperfine structure characteristic of adsorption at AI3+ were observed. This is clearly not a simple transfer process, since the total concentration of the different 0; ions increases on warming and 0; stabilized on Mo6+ can still be observed if the sample is recooled to 77 K. Thus, formation of 0; stabilized on A13+ has been attributed to electron transfer not from 0; adsorbed on Mo6+ but from reduced molybdenum sites not available at 77 K. The electron transfer can then be envisaged from Mob'' ions located in the bulk (Fig. 16). This is not unexpected, since it is known that molybdenum ions deposited on the support surface can migrate at moderate temperatures into the bulk of the matrix, e.g., MoO,/TiO, (230) and MoO,/SnO, ( 2 3 1 ~ )The . results obtained for oxygen adsorption on MoO,/AI,O, and MoO,/MgO suggest that a similar migration into the bulk occurs for alumina and magnesia as supports. This is a good example which demonstrates the difficulty of ascertaining the environment of the adsorbed oxygen and the complexity of the processes on the surface. "0, studies on both MoO,/SiO, and MoO,/Al,O, (82,84,85)confirm a diatomic adsorbed species; for MoO,/Al,O, the oxygen nuclei are nearly equivalent, with A , , = 77 and 80 G, but for MoO,/SiO, they are clearly different, with A , , = 72 and 85 G (see Section III,A,2). Che et d.(8.5)attributed this difference to a particular geometry at the surface, probably depending on the energy levels of the d orbitals of the Mo ion relative to those of 0; (see Section lIl,A,2). Using "0 enriched oxygen, Giamello et ul. (231b) have observed both equivalent and inequivalent oxygens in various types of adsorbed 0; depending on the Bi/Mo ratios of the bismuth molybdates supported on silica. The reason for these observations is not clearly

I

Fiti 16 (ION)

I I I

l

i

\e-

1

I II

Electron transfer occurring at thc surface ol' MoO,/AI,O, according to Che r t

ctl

MOLECULAR OXYGEN SPECIES O N OXIDE SURFACES

53

understood. Balistreri and Howe (2314 have irradiated MoO,/SiO, catalysts at 77 K with 306-nm light in the presence of both 0, and H, to form 0; and OH radicals. Warming above 77 K gives new signals which the authors tentatively assign to 0- and HO,. Irradiation in the presence of 0, enriched with 1 7 0 gave the normal hyperfine pattern for 0; but none for the OH signal, suggesting that the OH was formed from the lattice oxide ions. The 0; ion is thermally stable in oxygen for MOO, on Al,O, and MgO up to 150°C (213).Khalif et al. (216)have measured the heat of formation of 0; as 20 kcal/mol independent of support and in good agreement with work on V,O,/MgO (215), and suggest that other forms of oxygen such as Oi-, 0-, and 0,- which have higher heats of formation (viz., 60-80 kcal/mol) are also present. It is not always clear at which site the oxygen is adsorbing, but for low MOO, concentration the gzz value indicates that the adsorption site is A13+, and at higher concentration of MOO, the grr value of 2.0234 is rather larger than expected for a Mo6+ site. Akimoto and Echigoya (232) have studied the reactivity of 0; on supported MOO, in the catalytic oxidation of butadiene and this is discussed in Section VI. 5. Tungsten Oxide WO, supported on MgO or Al,O, can be activated by thermal treatment at 600°C in onc'uo or reduction in hydrogen to give weak EPR signals in the range y = 1.76-1.82 which have been attributed to W5+ (233).Adsorption of oxygen at 77 or 300 K with Al,O, as a support gives 0; stabilized at A13+ sites characterized by gzz = 2.040 and at W6+ ions with gZL= 2.019. For MgO as a support, adsorption of oxygen at 300 K leads to 0; with g,, = 2.070 characteristic of Mg2+ adsorption sites, whereas at 77 K three gII values were reported at 2.070, 2.080, and 2.026 (233).While the 2.070 and 2.080 values were indicative of Mg2+ sites by comparison with results obtained with the MoO,/MgO system (21.9, it is not clear from the discussion given by Spiridonov et nl. (233) what is the assignment for the 2.026 value. For silica-supported tungsten prepared by decomposition of various organometallics containing tungsten, Howe (219) reported an EPR signal with g values of 2.025, 2.01 I , and 2.004 after oxygen adsorption which was assigned to 0; formed on tungsten. The gzZvalue is larger than expected for adsorption at a W6+ site and may indicate stabilization of 0; on the support ions (234). The molecular nature of the species was confirmed later by Kazusaka et nl. (61). Adsorption of 170-enriched oxygen gave a resolved spectrum at 77 K, indicating that both oxygen nuclei are equivalent with A , , = 74 G (i.e., adsorption parallel to the surface). Raising the temperature leads to broadening of the hyperfine lines and an averaging of the g tensor,

54

M. CHE AND A. J . TENCH

consistent with a restricted motion on the surface (Section III,A,I,c). The 0; ions were stable at 145°C but disappeared after outgassing at 200°C. More information on 0; adsorbed in tungsten-exchanged zeolites can be found in Section IV,C,3. 6. Iron Group Oxides

There are only a few spectroscopic studies on the adsorption of oxygen on the iron group oxides (COO,NiO, FeO, and MnO) which give direct evidence on the nature of the adsorbed oxygen species. This is because these oxides are difficult to prepare with a large surface area and also they are easily oxidized or reduced to form higher oxides or metal particles. In addition, the superoxide ion 0; cannot be observed by EPR if it is adsorbed on cations which are paramagnetic (Section IV) or on superparamagnetic or ferromagnetic particles. The papers dealing with the iron group elements exchanged in zeolites will be discussed in Section IV,C,3. a. Iron Oxides. Using IR spectroscopy, Griffiths et al. (1%) have studied the adsorption of oxygen on Fe,O, previously degassed at room temperature. After adsorption at 350°C two bands were observed at 1350 and 1270 cm-' and assigned to adsorbed 0, and O;, respectively. These assignments have been criticized by Davydov et al. (235), who suggested that the two bands were due to carbonate-carboxylic species. The problem has been reconsidered in a later study by Al-Mashta et al. (19b),and a number of bands have been observed and classified into A-bands ( I 350- 1250 cm- ') and B-bands (1100-900 cm-I), which were assigned to perturbed 0; and 0;- species, respectively, by comparison with absorption frequencies of model dioxygen compounds. However, subsequent isotopic labeling experiments have shown that the type B-bands should be reassigned to mononuclear oxygen groups of the general type Fe=O (1344, in agreement with the expected pattern (Appendix C). The frequencies of the A-bands lie between those of superoxides and gas-phase oxygen (Appendix C), making the original assignment of perturbed 0; (intermediate between 0, and 0;) reasonable. The chemisorption of oxygen on FeO prepared by decomposition of the oxalate has been investigated by Dyrek (236).This author reported a change in the EPR spectrum of Fez+ at g 3.0 into that of Fe3+ at g 2.0 as chemisorption proceeds. No oxygen EPR signal is observed and this was interpreted to mean that oxygen is chemisorbed as 0,-, in agreement with results obtained using the iodometric analysis described by Bielanski and Najbar (237). It is difficult to assess the validity of this latter method since we know of no example where it has been checked for oxides in which there was also independent spectroscopic evidence for the existence of 0 - or 0;

-

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MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

55

adsorbed on the surface. By the nature of the method, the results are likely to be ambiguous except where the oxygen is adsorbed either as 0’- or 0; ions, as the only species. On the iron oxides, there is no E P R evidence for the electron transfer between Fez+ ions and oxygen, although it is well known in biological systems such as hemoglobin (238).However, it has been reported for some inorganic systems; for example, Imai and Habgood (239)have shown that, in Y-type zeolites, the formation of 0; was increased by adding small amounts of Fez+ ions which could act as electron donors: Fez+ +Fe’+

+

P-

Similar results have been obtained by Ismailov et al. (240) in Y-type zeolites containing iron impurities (Section IV,C,3). The formation of 0; on the ferrocene/porous Vycor glass system has been observed by Vanderspurt et al. (120) with a y tensor of 2.0300, 2.0100, 2.0020 and a superhyperfine sextet centered on each y component. These results were interpreted in terms of 0; adsorbed on the cyclopentadienyl ring of ferrocene and, assuming an ionic model, the yzzvalue of 2.0300 is broadly consistent with a 3 charge at the adsorption site. b. CohuIt Oxide. No E P R signal was observed by Dyrek (236) from COO prepared by decomposition of the carbonate, presumably because of a very fast relaxation of the Co2+ ions, and it was not possible to follow the oxidation of these ions on adsorption of oxygen. The results obtained by the iodometric method of Bielanski and Najbar (237) lead to the conclusion that oxygen is adsorbed as 0’- ions, similar to the FeO system discussed earlier. Using solid solutions of Coo-MgO with low COOcontents, Tabasaranskaya et al. (241) have observed the formation of an EPR signal 2.07 which was assigned to ions adsorbed on Mg” from 0; ions with y, sites. In a later more comprehensive study, Dyrek (242)found that for COO or its concentrated solid solutions (100-43.1 atom % Co), oxygen is adsorbed at room temperature as diamagnetic 0 2 -ions, whereas for moderately concentrated solid solutions (30.9-15.3 atom 7; Co), a poorly resolved E P R spectrum was assigned to superoxide ions 0; with approximate yzz values of 2.025-2.028 corresponding to Co3+ adsorption sites. On diluted solid solutions (10.4-3.0 atom % Co), 0; was adsorbed on Mg” sites with gzz = 2.07. Cordischi et al. (110)have extended the studies on the Coo-MgO system by impregnating magnesium hydroxide with low contents of COO (0.05-5 atom 7; Co) and showed unambiguously that oxygen is adsorbed on Co3+ at 77 K by the observation of a superhyperfine structure due to interaction of the unpaired electron of 0; with the nuclear spin of Co ( I = f). At higher temperatures of adsorption, the 0; ion is adsorbed on Mg” in agreement

+

-

56

M. CHE AND A. J . TENCH

with earlier results. Isotopic labeling experiments with " 0 indicate that in the latter case, the 0; lies parallel to the surface (243).In order to determine the role of the cobalt dispersion in the adsorption properties of the COOMgO solid solutions, Dyrek and Sojka (244a) have plotted the EPR signal intensity of 0; radicals adsorbed at room temperature as a function of the COO concentration. The curve passes through a maximum at 3.00 mole COOcorresponding to the maximum concentration of isolated Co2+ ions in tetrahedral coordination with trigonal distortion, which might imply that such ions are the adsorption sites. For higher COO concentrations, the number of Co2+ ions in clusters increases and this is thought to control the . the experiments form in which oxygen is adsorbed (0;or 0 2 - )However, were not designed to obtain information on the actual coordination of the surface Co2+ ions and how changes in the coordination would affect the adsorption of oxygen. Moreover, any contribution from the support ions to the adsorption properties of these solid solutions is not considered. Zecchina et al. (134e) conclude, from IR work, that surface Co2+ ions in a square pyramidal coordination can adsorb oxygen, but this is not substantiated by earlier results obtained by Hagan et al. (244b,c), who showed that surface Co2+ ions in Coo-MgO solid solutions were in tetrahedral coordination. c. Nickel Oxide. There is very little published work describing dioxygen species on NiO. Tsyganenko et al. (24e) have detected bands at 1500, 1140, and 1070 cm-' in the IR when oxygen is adsorbed at 77 K on NiO obtained from decomposition of the hydroxide in uacuo at about 550°C. Labeling experiments using various l 6 0 / l *O isotopic mixtures indicate that these bands correspond to dioxygen species. The band at 1500 cm- was assigned to neutral adsorbed oxygen, whereas the bands at 1140 and 1070 cm-' were attributed to O,, in reasonable agreement with the data in Appendix C. d. Manganese Oxide. Dyrek (236, 245) has investigated the adsorption of oxygen on MnO prepared in uacuo by decomposition of the carbonate in order to avoid any oxidation to Mn3+ or Mn4+ ions. A plot of the intensity of the single EPR line at g 2, due to Mn2+ ions, as a function of the amount of oxygen adsorbed shows a linear decrease suggesting oxidation of M n Z + into Mn3+.From results obtained independently by the iodometric analysis of Bielanski and Najbar (237), Dyrek concluded that oxygen was adsorbed in the form of diamagnetic 0'- ions. This author has also studied oxygen adsorption on MnO-MgO solid solutions (166)prepared by decomposition in uacuo of the parent coprecipitated carbonates. The solutions did not contain any Mn3+ or Mn4+ detectable by the iodometric method. Chemisorption of oxygen at room temperature on solutions containing 100-3.72 atom Mn was found to give rise to diamagnetic 0 2 -ions, whereas on more diluted solutions the oxygen gave an EPR signal with a g value of 2.007, thought to be from 0; or 0- ions. However, it is difficult to study

-

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

57

these signals since at low concentration the Mn2+ ions give rise to a hyperfine structure, due to the interaction with the nuclear spin of Mn (I = ;), which overlaps with the oxygen signal. Since most 0 - ions are known to exhibit a characteristic EPR line shape (I),we would associate the line shape for the oxygen signal observed by Dyrek (166)with 0;rather than 0 - ,but this has not yet been confirmed by studies using " 0 . The results obtained by Dyrek (166) differ from those obtained earlier on the adsorption of oxygen on MnO-MgO solid solutions. Cordischi et al. (246), on MgO doped with 235 atom ppm of Mn, and Yamamura et al. (247),on MnO-MgO with low contents of Mn ( <3 wt. %), did not observe any EPR signal characteristic of adsorbed oxygen. It is suggested (166)that the preparation method used by these authors resulted in partial oxidation of Mn2+ ions, thereby changing the adsorption properties of the solid solutions. Thermal desorption btudies of preadsorbed oxygen on a number of transition metal oxides have been carried out by Iwamoto (170) and others who observed several desorption peaks for each oxide. In this type of work, reliable assignment of the desorption peaks to particular oxygen species is only possible when parallel spectroscopic studies are carried out. This is difficult for many of these oxides where only material of low specific surface area is available.

C . ALUMINOSILICATES Alumina, silica, acd the aluminosilicates, whether amorphous or crystalline in the form of zeolites, play an important role in catalysis because they are used as supports in a large number of industrial catalysts. In addition to its role in dispersing the catalyst, the support is known to play a significant part in the chemistry of the surface reactions and this is illustrated by the electron transfer mechanism described earlier (Fig. 16). For these reasons, it is important to study the adsorption of oxygen on the support itself. 1.

Alumina

Gezalov et al. (248)have shown that the adsorption of oxygen on y-Al,O,, previously outgassed at 480°C and then y-irradiated in oxygen, gave an EPR signal with g ,I = 2.034 and gL = 2.008 which was assigned to 0; ions. These ions were not observed after oxygen adsorption on y-Al,03 outgassed between 200 and 500°C. Eley and Zammitt (111)have observed that the adsorption of oxygen at room temperature on a-Al,O, outgassed at 900°C gave a broad EPR signal with gI1= 2.0167 and g1 = 2.0005 which was reversibly broadened by excess oxygen. This signal was attributed to chemisorbed oxygen. When y-Al,03

M. CHE AND A. J. TENCH

58

is outgassed at 550"C,y-irradiated in uucuo, and then exposed to oxygen, an EPR signal develops with gI1= 2.038 and gl = 2.006 and a hyperfine sextet with a splitting of 3.6 G which was assigned to interaction with a neighboring 27Al nucleus. Assuming an ionic model, a gII value in the range 2.034-2.038 is characteristic of 0; adsorbed on an A13+ site, whereas the value of 2.0167 obtained on thermally activated u-A1203 must represent 0; adsorbed at an impurity site on the surface with high cation charge, unless the gl1feature belongs to an EPR signal arising from impurities (Fe, Cu) in the a-A1203 (111). Che et ul. (108) have reported EPR parameters for 0; adsorbed on y-A1203 which has been y-irradiated in oxygen with g, = 2.039, g, = 2.009, gxx = 2.004 and A,, = 5.4, A,, = 3.6, A,, = 5.2 G. These parameters are very close to those obtained for 0; on decationated Y-type zeolite, where the adsorption site is also on A13+ ion (104). In a later study, Losee (112)has formed 0; using a different procedure. The y-Al,03 was outgassed a t 600°C and treated in a stream of N,O at the same temperature, then cooled to room temperature in the flowing gas, and finally evacuated at 195 K. The EPR signal was characterized by gI1= 2.040 and g1 = 2.006 with no apparent hyperfine structure. The spectrum was simulated and the best fit was obtained when the following hyperfine constants were included: A,, = 4.9, A,, = 3.8, A,, = 3.7 G, with a Gaussian linewidth of 13.6 G. Since the y - A l z 0 3 used contained an iron impurity content of 0.05%, further work is required to clarify the possible involvement of the impurity iron as an electron donor to oxygen adsorbed after only a thermal treatment. It is likely, however, that y irradiation is necessary for the formation of 0; if transition metal impurities are absent. Oxygen chemisorption to form 0; on support ions has been reported on various oxides supported on A120, via the electron transfer mechanism depicted in Fig. 16. Depending on the temperature of oxygen adsorption, 0; can be stabilized either on supported oxide ions or on support ions. In addition, we wish to stress that the absence of EPR signals due to 0, adsorbed on the supported oxide ions is still consistent with the adsorption of oxygen on these ions in a diamagnetic form, i.e., 0;- or 02-. The temperature of thermal treatment can also influence the site where 0; is adsorbed. For example, oxygen chemisorption to form 0; has been reported (249,250)on the phthalocyaninato cobalt (II)/y-A1203system. Thermal pretreatment at 300°C favors 0; formation at cobalt centers, whereas after pretreatment at higher temperatures A13+ centers are favored. In both cases, the 0;is thought to be formed by electron transfer from the cobalt ion. It was suggested that this behavior was related to the extent of dehydration of the A1203and this is consistent with the loss of surface OH- groups to expose the A13+ ions which are strong adsorption sites.

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MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

59

2. Silica Friebele et al. (91) have identified a bulk defect in neutron-irradiated silica as -Si-O-O' with a g tensor of 2.067, 2.0074, and 2.0014. Neutron irradiation of the silica doped with "0 gave an EPR signal with hyperfine splittings 101.7 and 43.2 G, confirming that the oxygen nuclei are inequivalent (Section III,A,2,b). Neither the g nor the hyperfine tensors are consistent with an ionic model of 0; but indicate that a peroxy radical with covalent bonding is formed. It is interesting to note here that a peroxy radical model has been proposed for the chemisorption of oxygen onto silicon surfaces (55). Similar species are formed when quartz, which has been crushed in uucuo or under helium, is subsequently exposed to oxygen (251).The g tensor at 77 K was 2.0719, 2.0082, and 2.0057 and at higher temperatures a reversible change of the line shape was observed. This was interpreted in terms of a rotational averaging of the g tensor to give gav= 2.024. A smaller EPR signal was also observed with a g tensor of 2.0318, 2.0109, and 2.0057; this did not undergo changes in line shape with temperature. Exposure of silica gel, which has been y-irradiated in uacuo, to oxygen leads to an EPR signal with a different g tensor with gav= 2.015 (252) or gzz = 2.0250, gru = 2.0095, and gxx = 2.0031 (253). The largest g value of this surface species is in good agreement with the predictions of the ionic model for 0; adsorbed at a cation of charge 4 + . It is interesting to note that similar results have been obtained on silica-alumina (254). Exposure of silica-alumina, which has been y-irradiated in uacuo, to oxygen gives rise to a well-resolved EPR signal with gzz = 2.024, grv = 2.0068, and gsx = 2.0022 attributed to 0; adsorbed at a Si4+ cation site and leads to the conclusion that silica-alumina contains some areas of pure silica on the surface. Adsorption of oxygen labeled with "0 on silica, which has been y-irradiated in uacuo (255), gives a hyperfine structure consistent with equivalent oxygen nuclei in 0; and confirms the ionic picture of 0; adsorbed on the silica surface. This is in contrast with the model of a peroxy species in the bulk or at the fractured surface of silica described in the preceding paragraphs. Irradiation of porous Vycor glass with UV light ( < 330 nm) in the presence of oxygen (256)leads to photoadsorption and an EPR signal with g = 2.0310, 2.0109, and 2.0053; this is again consistent with an ionic 0; on the surface. 3. Zeolites a. Structural Aspects. Both natural and synthetic zeolites are crystalline aluminosilicates (257). The basic structure consists of an array of SiO, and A104 tetrahedra linked at their vertex oxide ions. Depending on the conditions of preparation, a variety of synthetic zeolites, characterized by different

60

M. CHE AND A. J. TENCH

P

FIG. 17. A line drawing of the structure (a) of zeolite A and (b) of the faujasite structure (zeolites X and Y).

arrangements of the tetrahedra, can be prepared which possess cavities, tunnels, and channels of various shapes and sizes. For example, zeolite A and the faujasite structure are represented in Fig. 17, and the composition and characteristics of the zeolites of interest are given in Table VII. The replacement of Si4+ by A13+ ions in the tetrahedra generates a deficit of one positive charge per aluminum ion, which must be compensated by the incorporation of extrinsic cations in the zeolite structure. The sodium or calcium ions which are most commonly found in natural or synthetic zeolites can be exchanged with other alkali, alkaline-earth, rare-earth, or transition metal ions. The zeolite open structure can accommodate not only the extraframework cations, but also various molecules provided that their size is smaller than the zeolite apertures. A key feature of cation-exchanged zeolites is the local electrostatic field associated with the cations. This has led to the view of zeolites as solid solvents (258 and references therein). In X- and Y-type zeolites which exhibit the faujasite structure, the cations can reside in four basically different types of site which are located on the threefold axes of the cubic faujasite structure (Fig. 18). Using the accepted notation, the S, site (I6 per unit cell) is located at the TABLE VII Composition and Characteristics of Some Zeolites

Type

Unit cell composition

Void volume (ml/ml)

Pore diameter

(4

Si/AI ratio

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

hangond sodmllts prlsm

61

sup.rc.g*

c.vlty

FIG. 18. The different cation sites in the faujasite structure (zeolites X and Y) (258).The figure has been simplified. but the oxygen ions of the windows are not equivalent (see text).

centers of the hexagonal prisms connecting two sodalite units. The cations at these sites are surrounded by six lattice oxide ions (three from each hexagonal window on either side of the cation), giving an octahedral coordination. The S,. site (32 per unit cell) is located in the sodalite unit just on the other side of the shared hexagonal face from the S, site. The cations at the S,. sites are surrounded by three lattice oxide ions and usually an extraframework atom. The s,,.site (32 per unit cell) is located on the sodalite unit side of the unshared hexagonal face but is displaced within the sodalite cage. At S,,. sites, the cations are bonded to three lattice oxide ions and possibly an extraframework atom. The type S,, site ( 3 2 per unit cell) is located on the unjoined hexagonal face of the sodalite cage and is slightly beyond the plane of the hexagonal face within the supercage units. These sites are usually accessible to a wide range of molecules. Cations at these S,, sites, for example, can bind enough water molecules to form hexa-aquo complexes which can then tumble freely in the center of the supercage. The dehydrated zeolites exchanged with various cations have been of catalytic interest in many reactions, among which cracking (259)and shapeselective catalysis (260) are most important. Other reactions include oxidation, carbonylation, and related reactions (261)as well as other nonacid catalytic reactions (262). b. General Features q f 0 ; Species. Many papers have dealt with the nature of oxygen adsorbed on various types of zeolites. In the majority of cases, after pretreatment of the zeolite, the 0 ; species is formed by y or UV irradiation of the zeolites either in uuciio and then adding oxygen or irradiating in the presence of oxygen. In addition, thermal activation above 300’C followed by adsorption of oxygen has been shown to lead to the formation of 0; and it has also been produced chemically by introduction of alkali metal into alkali-exchanged zeolites followed by adsorption of oxygen.

62

M. CHE AND A. J . TENCH

Because of its size, the oxygen molecule cannot reach all the zeolite cavities. For instance, in faujasite-type zeolites, oxygen can penetrate neither the hexagonal prisms nor the sodalite units and is confined in the supercages (257). In the latter cavities, gas-phase oxygen can interact with the adsorbed species and lead to line broadening (see Section II,A) in contrast to what is observed when the adsorbed species (whether oxygen or exchanged cations) are located in hexagonal prisms or sodalites. This line broadening has been used to establish the type of cavity containing the adsorbed species. This picture is changed at high pressures and temperatures since the oxygen can then enter the sodalite units (239). If the temperature is rapidly lowered after this treatment, the oxygen remains trapped in the sodalite units. c. Influence of' Experimental Conditions on the Stabilization of Oi Ions. The identification of the adsorption sites of oxygen in zeolites has been and still remains the major problem since the nature of these sites depends on a large number of parameters. The complexity of the problem is outlined by a few examples given below which illustrate the influence of each parameter on the EPR spectrum of 0;. i. Type and condition of irradiation. Wang and Lunsford (263) have investigated the spectra of 0; formed by y and UV irradiation of the alkalineearth Y zeolites. For CaY zeolite, the 0; species do not have the same g components after UV and y irradiation in oxygen at room temperature for the same pretreatment of the sample prior to irradiation. For UV-irradiated samples, the major site was found at g,. = 2.046, but for the y-irradiated sample, gzz = 2.064 for a radiation dose of 1.4 x lo6 rads. However, when the y dose was increased to 10' rads, the major site was found at gzz = 2.046. After the CaY zeolite had been y-irradiated in uucuo, three different types of defect centers were produced. One was an F center which reacted with oxygen to form the 0; species, whereas the other two were V-type centers. It is interesting to note that one V center which was not broadened by oxygen had a strong feature at gI1= 2.046. After contact with oxygen and outgassing to remove excess molecular oxygen, the EPR spectrum of an 0; center appeared exhibiting several different gzz values, with the strongest peak at 2.046. In all the preceding cases, the 0; spectra obtained had distinctly different line shapes. The results show that the nature of the adsorption sites characterized by their g,, values depend on both the type and the total dose of radiation and also on whether it is carried out in oxygen or in uacuo followed by contact with oxygen. It is possible that some features in the spectra might belong to V-type centers which are not broadened because of their location in sites not accessible to oxygen. This is clearly the case of the V-type center at g II = 2.046 observed by Wang and Lunsford (263). To add to the complexity of the problem, other oxygen species such as 0; can also be formed by irradiation (Section V,A).

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

63

The temperature of formation is also an important parameter. Kasai has observed that 0; with gzz = 2.1 13 can be produced upon y irradiation of NaY in oxygen at room temperature (127h).When NaY containing adsorbed oxygen was irradiated by X rays at 77 K, the 0; spectrum showed the most prominent component at gzz = 2.08 (264). When this sample was warmed to room temperature for several minutes, the component at gzz = 2.08 decayed, and several new components appeared at gzz = 2.10-2.15. The spectrum characterized by gZz= 2.08 represents the 0; attached to the Na’ ions and other spectra are created by diffusion of the originally formed neutral Na+-O; complexes (264). ii. Pretreatment conditions. The zeolite is normally pretreated before it is irradiated. This pretreatment is not neutral and, depending on the conditions, can drastically affect the structure of the zeolite. For example, pretreatment of NH,Y zeolite at 600°C in a deep bed geometry (265)leads to the so-called “ultrastable” zeolite in which some aluminum ions have been removed from the lattice structure by the action of both water vapor and ammonia, leaving the solid. The mechanism of aluminum migration involves the formation of interstitial aluminic acid in a mono or dimer form and a reorganization of the lattice leaving no aluminum vacancies (266). Exposed trigonal aluminum ions can also be present on the surface of zeolites at the oxygen-deficient sites of the framework produced by dehydroxylation of a decationated zeolite at temperatures above 450°C (Section IV,C,3,d).

-

H+

H+

Thus, both interstitial and exposed trigonal aluminum ions can, in principle, function as adsorption sites for 0;. We have just seen that there is evidence that aluminum ions can migrate under thermal pretreatment and this is also observed with sodium ions whose migration is favored by the presence of water (76). These processes are important since clusters of sodium ions can be formed and become efficient traps for electrons to produce F centers under irradiation, i.e.,

64

M. CHE AND A. J . TENCH

which can be reversed by thermal annealing. These F centers can then easily transfer their electron to form 0; ions. It is believed that several different types of F centers can be formed, depending on the experimental conditions, leading in turn to 0; adsorbed at different sites (267). An additional complexity arises from the dissociation of water molecules which occurs when alkaline-earth-exchanged zeolites are thermally activated since several modes of dehydroxylation are possible. This problem has been extensively investigated by IR spectroscopy, in particular by Ward (268,269) and Uytterhoeven et al. (270),and by X ray (271).They concluded that the electrostatic field associated with the cation causes dissociation of adsorbed water to produce acidic hydroxyl groups. The dissociation reaction may occur according to the following reactions : M 2 + ( H z O )+ M(OH)+

+ Hi

or 2M2+

+ H,O +(M-0-M)"

+ 2H'

After mild dehydration, the formation of the alkaline-earth oxide MO (269) or M+-O-M+ (270) has been suggested. It is also probable that some Mz+ species are also produced due to the dehydroxylation of M(OH)+ entities. Summarizing, we can say that there is good evidence for the existence of a number of different adsorption sites for O;, such as M2+, M(0H2);', M(OH)+,MO, and M+-O-M+. For the alkaline-earth series, MgY, Cay, and SrY lead to the dissociation of water but BaY does not; this is consistent with the observation of the simplest EPR spectrum for 0;in BaY by Kasai (127b)and by Wang and Lunsford (263). The manner in which oxygen is introduced during the pretreatment of the zeolite also effects the 0; ions, and Imai and Habgood (239)have observed that different types of 0; could be produced after adsorption of oxygen on thermally activated Y-type zeolite. When oxygen was adsorbed prior to irradiation at room temperature under 760 Tom, the 0;formed could be line-broadened by gas-phase oxygen at room temperature, while when oxygen was adsorbed at 500°C under 760 Torr, the 0; subsequently formed was not broadened by excess oxygen and its intensity was decreased only by 10-200/, after the sample had been evacuated at 500°C overnight. It was concluded that the 0;ions of the first type were located in the supercages, whereas those of the second type were located in the sodalite units (239). Finally, chemical activation has also been used to produce 0; ions. When NaY heated previously in sodium vapor was exposed to oxygen, the red material became instantly white, the EPR spectrum of Na:' centers disappeared, and a new signal appeared with gzz= 2.077 which was identified as 0; adsorbed at a Na+ ion (272).This is in contrast to 0;produced in NaY

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

65

by ionizing radiation as seen earlier, where several different gzzvalues can be obtained depending upon the experimental conditions. The number of 0; ions formed by irradiation is usually small ( - 10" spins/g) corresponding to one ion per 100 supercages; in this situation, the neutral Na+-O; pairs will almost all be trapped at the most stable sites. When a much larger amount of 0; is produced ( - 10'' spins/g), for instance, by alkali atoms, the resulting 0; spectrum is more likely to be dominated by the Na+-O; complexes located at the normal sites (264). iii. Starting materials. A characteristic influence of subtle changes in the starting material on the stabilization of 0; can be observed for NaX and NaY zeolites. Both these zeolites have a faujasite structure (Fig. 17) and the same cation sites (Fig. 18) and differ only by their Si/AI ratio (Table VII). The electrostatic field due to the Na' ions at the same type of site has been calculated to be larger for NaX than NaY (273). The energy splitting A of the n* levels of the 0; is proportional to this field and can be calculated from the experimental gzz values of 2.080 and 2.162 for NaY and NaX, respectively, using Eq. (6) (Section III,A,l,a), assuming an ionic model. This indicates that the local electrostatic field is indeed larger for NaY than for NaX, in agreement with the theoretical calculations. There are several examples where irradiation is not necessary to produce 0; ions. In such cases, a thermal activation is sufficient because of the presence of transition metal ions which can easily transfer one electron to oxygen. Iron is the most common impurity found in zeolites and the formation of 0; depends very much on the iron content (239,240).Transition metal ions can also be exchanged in zeolites and this will be discussed later. There is also some indication that the types of 0; can be influenced by the level of exchange. When cations of different valence states are involved in the exchange, an incomplete exchange will leave two types of cations present, creating the possibility of at least two types of adsorption sites. This has been observed for both Mg and CaY zeolites (263). iu. Conclusion. The preceding examples show that the situation is complex, especially for 0; formed in irradiated zeolites, where there is a persistent problem of reproducibility between the different laboratories. The EPR spectra of irradiated zeolites are very sensitive to trace impurity cations, and also the temperature at which the sample is irradiated and maintained after irradiation. Kasai and Bishop (264) have noted that simpler and more reproducible spectra are often obtained when extra-pure laboratory-synthesized zeolites are used and irradiated at low temperature (77 K). Also, very frequently species other than dioxygen can be formed by irradiation in the presence of oxygen. In many cases, the ozonide ion 0; is present in the reported EPR spectra and is probably formed by reaction of oxygen with accessible 0- ions (V centers) produced by irradiation (105a, 263,266,267).

66

M. CHE AND A. J. TENCH

Finally, it is clear from the examples given above that the type of thermal pretreatment can dominate the results by changing the possible adsorption sites for 0;. This has not been realized by many authors. d. Formation qf00; Species. In the following paragraphs, in order to help the discussion on the formation of dioxygen species, the zeolites have been divided into four groups according to the nature of the exchanged cation and not to the type of zeolite. As in the case of the oxides, most of the discussion refers to the 0; species since there is little direct information on the nature of the other oxygen species. i. Decationated zeolites. We start by considering decationated zeolites since they do not contain any metal ions extrinsic to the silica-alumina framework. This type of zeolite is obtained by pretreating, above 350"C, a NH,Y zeolite prepared by exchanging the sodium form of a Y-type zeolite with ammonium ions. Ammonia is evolved leaving a decationated or HY zeolite:

-

NHaY- + H t Y -

+ NH,(,,

y or X irradiation of activated NH,Y in U ~ C U Oproduces several types of defect centers, depending upon the activation process used before the irradiation. When NH,Y is activated at 400°C to form the decationated or HY zeolite, two types of V centers are observed after y irradiation in U ~ C U O(274). Both types give a rather broad EPR line with the same g tensor, g1 x 2.045, g2 = 2.005, and g3 = 2.002, but only one exhibits a six-line hyperfine structure ( A , not determined, A, = 8.0, A3 = 7.5 G) due to an interaction with 27A1 nuclei ( I = 1) which is superimposed on the broad EPR line. These V centers are attributed to a positive hole (denoted by the symbol *) trapped either on an oxygen adjacent to both an A1 and a Si atom (V,) or on an oxygen adjacent to two Si atoms (V,).

V, center

V, center

After exposure of these V centers to oxygen, new signals appear characterized by the following g tensors: gZz x 2.032, g y y = 2.0015, gxx = 1.9995 and gzz z 2.022, g y y = 2.0015, gxx = 1.9975 (83a). Both signals have been identified as due to peroxy radicals weakly bonded to the 0 atom of the V , and V, centers, respectively. This assignment was based on the nonequivalency (84.5 and 64.2 G) of the oxygen nuclei of the adsorbed dioxygen using 70-enriched oxygen (83a). Evacuation at room temperature removes the

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

67

oxygen signals and restores the original V-type centers. When the H Y zeolite was irradiated directly in oxygen, the peroxy signals were 10 times more intense. Similar results were obtained when H mordenite zeolites were used (83a). Different results are obtained when NH4Y is activated at higher temperatures (600°C). Vedrine er al. (266)showed that y irradiation in uucuo of such an activated zeolite leads to two types of signals. The signal with gI1= 2.0125, g1 = 2.0030, and a 12-line hyperfine structure with All N A , N aiso= 10.0 G was attributed to a positive hole (V center) trapped on an oxygen bridging two aluminum atoms: * 0

\

Al

/-\/

/ \

A1

0

/ \

0

the other with giso= 2.0048 and a 6-line hyperfine splitting of aiso= 9.5 G was assigned to a positive hole (V center) trapped on an oxygen near an aluminum atom. The V centers are associated with interstitial aluminum species removed from the lattice by the action of water and ammonia. When the V center associated with two aluminum atoms is exposed to oxygen, its 12-line hyperfine structure disappears and gives rise to an oxygen signal with gZz= 2.0575, g,). = 2.0085, and ,y, = 2.0026, stable at 250°C. This oxygen signal, which did not exhibit any superhyperfine interaction with aluminum atoms, was assigned to an 0; species (266).Comparison with the results observed in the case of the NH4Y zeolite activated at 400°C is more consistent with the formation of peroxy radicals (264).This needs to be confirmed by labeling experiments using the 7 O isotope. Wang and Lunsford (104) have observed different results for a NH,Y zeolite which was also pretreated at 600°C. When this zeolite was y-irradiated in uucuo, only a weak single EPR peak was observed at g = 2.0017; however, y irradiation in oxygen gave an EPR signal, stable to 250°C, with gzz = 2.038, gYy= 2.009, g,, = 2.003 and a 6-line hyperfine structure with A,, = 6.5, A , , = 4.7, A,, = 5.7 G due to interaction with 27Al ( I = ;). This signal was assigned to 0; (104),but later experiments with 1 7 0 showed that the two oxygen nuclei were inequivalent (82 and 63 G), indicating a peroxy-type character (81).The adsorption site was thought to be trigonal aluminum ions formed by dehydroxylation of the decationated zeolite. In view of the probable migration of aluminum ions during the activation of the N H 4 Y zeolite, as shown by Vedrine er al. (266),Kasai and Bishop (264)suggest that it is more likely that the peroxy-type 0; ions are associated with interstitial aluminum species. This suggestion is consistent with the results obtained by

68

M. CHE AND A. J . TENCH

Wang and Lunsford (103), who observed an identical spectrum of 0; in Al-exchanged HY, activated at 400°C with gzz = 2.038, gYy= 2.009, gxx = 2.003 and a 6-line hyperfine structure with A,, = 6.5, A,,,, = 4.8, A,, = 5.7 G (103). These parameters are very close to those reported for 0; on y-Al,O, (Section IV, C , 1). It is interesting to note than an 1 I-line hyperfine structure with aiso= 10 G was observed at giso= 2.007 when the Al-exchanged HY zeolite was irradiated in uacuo. This hyperfine structure was reported to be reversibly broadened by oxygen (103). Although the hyperfine structure appears to be similar to that observed by Vedrine et al. (266) for the V center associated with two aluminum atoms, it does not lead to the formation of 0; on adsorption of oxygen. The g tensors and hyperfine constants of the paramagnetic centers observed in irradiated NH,Y zeolites after various activation treatments are given in Table VIII. Despite the abundance of experimental results, many of the structures proposed for these centers should be regarded as suggestive rather than definitive, as previously noted by Kasai and Bishop (264).Neither the axially symmetric g tensor of the V center associated with two aluminum atoms nor the isotropic g tensor of the V center associated with one aluminum atom reported by Vedrine et al. (266)is consistent with the symmetry of the respective models proposed above. Ono et al. (121) have used the secondary reaction between preadsorbed SO, and oxygen to produce 0; in NH,Y or H mordenite. The EPR parameters of 0; are found to be 2.038, 2.009, 2.002 and 2.040, 2.010, and 2.002, respectively, and the hyperfine structures indicate that aluminum ions are involved at the adsorption sites. ii. Alkali and alkaline-earth zeolites. In an early study, Pickert et al. (275) proposed that the electrostatic field associated with the exchanged cations was responsible for the catalytic activity of the zeolites. A number of attempts have been made to detect these unusually large fields experimentally and to show that they varied in a systematic way. Kasai (127b) produced the first evidence that the energy splitting of the ne levels of the superoxide ion 0; was exactly twice as large with divalent BaY as with monovalent Nay. The energy splitting was calculated from Eq. (6) (Section lII,A,l,a) using the gzzvalue of the g tensor of O ; , which was found to be 2.1 13 for NaY and 2.057 for BaY zeolite. Although the diatomic nature of the species was confirmed with I7O-enriched oxygen (76),later experiments indicated that the situation was more complicated and several adsorption sites were detected in the case of NaY zeolite. For example, Wang and Lunsford (263)reported gzz values of 2.098, 2.074, and 2.040 (major site) for 0; adsorbed in NaY zeolites, whereas Ben Taarit et al. (76)observed a different set of gzz values at 2.095, 2.058, and 2.073 (major site).

TABLE VIII EPR Parameters of Radiation-Induced Centers in NH, Y and Their Oxygen Adducts

Zeolite

Pretreatment temperature ('C)

A("A1) (G)

g tensor

Primary center

gyy

g,

Oxygen adduct

9x1

A:,

A,,

A,,

Ref.

gzz

9yy

g ,,

Ref.

* 0

/ \ HY

400

Al

Si

2.045

2.005

2.002

-

8.0

7.5

274

2.032"

2.0015" 1.9995"

83a

Si

2.045

2.005

2.002

_

_

_

274

2.022"

2.0015" 1.9975"

83a

_ - _

103 104

2.038'

2.0575d 2.0085'

* 0

/ \ HY

400

Si

* 0

/ \ A1 HYh Dehydroxylated HY

400 600

A1

A1 ?

= 2.007 giso = 2.0017

g,i

aim = 10 G

-

_ 2.009'

2.002'

103 104

2.0026'

266

* 0

/ \ Dehydroxylated HY

600

A1

*

Al

2.0125 2.0030 2.0030

airo= 10 G

266

Si

yisO= 2.0048

aiso = 9.5 G

266

0

/ \ Dehydroxylated HY

600

A1

Inequivalent oxygens, oxygen species not stable at room temperature. This zeolite is given for comparison. Inequivalent oxygens, see Ref. 81, oxygen species stable at 250 C. Oxygen species stable at 250'C.

266

M. CHE AND A. J. TENCH

70

In a later study, Wang and Lunsford (263) used the alkaline-earth zeolites to determine whether any systematic change in the crystal field effect on the adsorbed 0; species could be detected as the cation is varied from Mg2+ to BaZ+.The results show that three or more different adsorption sites are present on each of the cationic zeolites and that there is no significant trend in the energy splitting of the ng levels of the 0; ion as one goes from Mg2+ to BaZ . Kanzaki and Yasumori (267) have attempted to rationalize the change of the g values observed for 0; in various cation-exchanged X zeolites after y irradiation in oxygen and their results are summarized in Table IX. The gzz values are classified into four groups (A, B, C, D) corresponding to (Nan)"+,mono-, di-, and trivalent adsorption sites, respectively, by comparison with ranges of gzzfor 0; observed on different systems and listed by Lunsford (3).The adsorption sites of 0;can be assigned to the exchanged M t ion and the lattice aluminum cation in monovalent cation-exchanged zeolites (LiX, NaX, KX, RbX, CsX); to M Z + ,M(OH)+ and M+-O-M+ in divalent cation-exchanged zeolites (MgX, CaX, SrX, BaX); and to M 3 + in a trivalent cation-exchanged zeolite (Lax, given for comparison) (267). The adsorption site (Nan)"+was also suggested for NaX zeolite. This classification gives a reasonable account of the data except for BaX zeolite found in group B (monovalent adsorption sites). Barium zeolite does not lead to the heterolytic dissociation of water and is not likely to produce monovalent adsorption sites, in contrast to the other alkaline-earth zeolites. The results for 0; in alkali and alkaline-earth zeolites are collated in +

TABLE IX g,: Valuesfor 0;on Cation-Exchanged

X Zeoliies y-Irradiated in the Presence 01'0,"

Exchanged Cation Li Nah K Rb cs Mg Cd

Sr Ba La

(2,) 72 I00 81 67 56 67 80 81 85 76

B

A

C 2.059 2.059

2.158 2.074 2.074 2.068 2.071 2.070 2.062 2.062

2.057 2.054 2.059

= 2.007-2.009, qxx = 2.000-2.003, Ref. 267. Ref. 10.5~.

OH,,

D 2.038 2.033 2.031 2.031 2.033

2.046 2.048 2.048 2.034

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

71

Table X. The alkali and alkaline-earth zeolites have been arranged together at the end of Table X using the approach of Kanzaki and Yasumori. The range of the gz, values is shown clearly by a comparison of the results for the NaY and NaX zeolites. Since the migration of Na+ ions is related to the presence of water (76),it is likely that the type of precursor (Na4)4+(H,O), complex formed after a proper degree of dehydration (278) will be strongly dependent on the pretreatment conditions. This will be reflected in the gzzvalues of the 0; produced during y irradiation by electron transfer from the precursor (278).It is also likely that the 0; can migrate after its formation as shown by Kasai and Bishop (264). These authors (272) have detected a superhyperfine interaction from Na nuclei ( I = i) in the EPR spectrum of 0; formed in Na-reduced NaY zeolite and characterized by g z z = 2.1 13. This value is very close to those observed for alkalisuperoxides trapped in krypton matrices (Ref. 44, Appendix A). This table shows that it is difficult, even in a model system, to present a simple view of the nature of the adsorption site because of the number of different parameters involved in the stabilization of 0;. For zeolites the problem is apparently more difficult than for oxides, since not only do the framework ions and the exchanged cations form two distinct types of adsorption sites but the latter can migrate within the zeolite structure. It is difficult to obtain a full description of the coordination of the exchanged cations and so far there has been no systematic study on this point. iii. Zeolites exchanged with transition metal ions. In the first row, scandium-, titanium-, cobalt-, and nickel-exchanged zeolites have been the most studied. Cobalt-exchanged zeolites are discussed in Section IV,E since they lead to oxygen adducts on adsorption of oxygen. There are several cases where copper and particularly iron ions are found as impurity cations which affect the oxygen adsorption properties of the zeolite. When ScY zeolite, pretreated at 5OO0C, is y-irradiated in the presence of oxygen and then heated at 15O"C, an EPR spectrum is observed with g values of 2.030, 2.009, and 2.002 which is characteristic of 0; adsorbed at Sc3+ ions, assuming an ionic model. This assignment was confirmed by the observation of an eight-line superhyperfine structure with A,, = 5.7, A,, = 4.4, and A,, = 5.1 G characteristic of the interaction with 4sSc ( I = ); (103). TiY zeolite is one of the few examples where 0; can be formed in zeolites without the need of irradiation. The TiY zeolite is prepared by cation exchange with an aqueous solution of TiCI,. After thermal activation, an EPR signal due to Ti3+ is observed which is destroyed by adsorption of oxygen and a spectrum characteristic of 0; appears at 2.0195, 2.0089, and 2.0031 (279).These results have been confirmed by Kuznicki et al. (280)in a subsequent study on TiY and TiA zeolites. The 0;ion was observed with a signal

72

M. CHE AND A. J . TENCH

TABLE X yz, Values of0; in Alkali and Alkaline-Earth Zeoli!es

-g Zeolite

No Y No Y No Y Na Y NaY b) NaY c) No Y,Nai No Y,Noi No X NoXfl Na Xql NaX,No I NaA NaAf) Li A L i Y h) Li x Na X K X Rb X cs x

Mq x Mq Y Ca A Co X Cay i) Sr X Sr Y i) Bo X BOY iI Ba Y

27t 263 16 276 264 261 16 261

216

2,l5//2.11

2p9

210

2p8 2,07 2P6 2p5 I

I

-

I

I

I

I I

I

I I

1051 051

261 261 277 277 271 239 267 051 267 2 61 2 67 261 263 217 267 263 267 263

I

I

I

I

I

I I

I

Ie)

I I

I

I

I I

I

I I

I

I

I I

I

I

I

I I

l

I

I I

261

I

I

l I

I

267 127

Zp4 2,03

I

I

1

Unless otherwise stated, the zeolite has been y-irradiated in oxygen at 298 K. y-Irradiated in oxygen at 77 K. Sample b warmed to 298 K. Treated with Na vapor before contact with oxygen at 298 K. ' Superhyperfine structure due to "AI present. y-Irradiated in vacuo before contact with oxygen. X-ray irradiated in vacuo before contact with oxygen. Thermally treated only before contact with oxygen. y,, values taken from Table I1 and Fig. 3 of Ref. 263 for samples y-irradiated in oxygen. j Kanzaki and Yasumori (105a) assume that in the M + - O - M + complex, which is usually located in the sodalite, one of the M C can be found in the supercage; this accounts for the monovalent sites available to oxygen.

' '

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

73

at 2.0197,2.0107,and 2.0054 in TiY zeolite, whereas in TiA zeolite, 0; could be observed only after illumination with visible light in the presence of oxygen. The g values of 2.040, 2.017, and 2.003 are thought to be more consistent with a 3 + adsorption site (Fig. 3) than the 2.0195 value reported for TiY zeolite (280).The published EPR spectra are, however, not well resolved and from the line shape it is likely that the signal is not from 0; only, and other oxygen species may be present. Nickel has been studied in NiCaY zeolite, where Ni' ions can be produced in the absence of metallic nickel after a reducing treatment in hydrogen at about 200°C (281,282).The EPR signal of Ni+ ions is destroyed by oxygen, and a new spectrum is obtained with g values of 2.16, 2.09, and 2.055 which has been assigned to a Ni(0,): complex. After desorption of oxygen at 100°C, a much weaker spectrum was observed with gzz values of 2.052 and 2.045, with similar values of 2.009 and 2.0015 for the other two components (281);this was assigned to 0; adsorbed on Ni2+ ions. In fact, for the 0; ions to be observed, the NiZ+ions would need to be nonparamagnetic, but there are no experimental data to substantiate that Ni2+ ions are the actual adsorption sites. There are also other possible adsorption sites when Caz ions are present as in the NiCaY zeolite. Using '70-enriched oxygen, it has been possible to show that the dioxygen species had equivalent oxygens both in 0;and Ni(0,): for the NiCaX zeolite (283). Imai and Habgood (239)have investigated the formation of 0; by thermal activation in alkali Y zeolites containing iron and/or copper ions as impurities or as exchanged cations. Two types of radical are observed. The first is O;, thought to be adsorbed on alkali metal ions (the gzz value could not be determined in the case of sodium zeolite; it was at 2.054 for lithium zeolite). The second radical with gI1= 2.074 and g1 = 2.008, observed after higher temperatures of pretreatment in oxygen, is probably located in the sodalite unit since the EPR signal is not broadened by gas-phase oxygen. This radical is probably an 0; ion and is formed only on sodium and lithium zeolites which contained impurity transition metal ions such as iron and copper. The concentration of the oxygen radical was increased by adding a small amount of Cuz+ or Fez+ ions. These ions were thought to release electrons (239),i.e., +

+ eFez+ -+Fe3+ + e

cu2++ C u 3 +

this latter process is common but the former is rather unusual. A similar mechanism has been invoked also by Ismailov et al. (240)in the formation of 0;in NaY containing impurity iron ions. The 0; ion is characterized by an EPR signal with g values of 2.073, 2.009, and 2.001. It was not observed when a purer NaY zeolite was used. Among the transition metal ions belonging to the second row, only ruthenium (83b),rhodium (284),and palladium (152,240)have been investi-

74

M. CHE AND A. J . TENCH

gated. They all lead to oxygen adducts on adsorption of oxygen and are treated in Section IV,E. In the last row, only lanthanum and tungsten ions have been studied. When LaY zeolite, degassed at 500"C, is y-irradiated in oxygen, an EPR spectrum is observed with g values of 2.044,2.009, and 2.005, which from the ionic model are characteristic of 0; adsorbed at La3+ ions. This is confirmed by the presence of an eight-line superhyperfine structure with A,, = 12, A,, = 9, and A,, = 8 G characteristic of the interaction with '39La ( I = 3). For both ScY and Lay, Wang and Lunsford propose that the adsorption sites Sc3' and La3+ are located within the sodalite units (103).However, it would seem more reasonable that these sites are located in the supercages since oxygen does not significantly penetrate in the sodalite units below 200°C (239).Similar results were obtained for 0; in L a x with g values of 2.034, 2.007, and 2.002 (267) or 2.033, 2.004, and 2.000 (285);but no superhyperfine structure due to I3'La was observed in the 0;spectrum. The decomposition of W(CO), adsorbed in the supercages of HY and NaY zeolites has been investigated by Abdo et ul. (105b). In HY, the decomposition is accompanied by the oxidation of the tungsten to W5' located in the supercages, where they can adsorb oxygen as 0; with g values of 2.024,2.011, and 2.003. The EPR parameters were different for 0; in WNaY, with g values of 2.060, 2.008, and 2.002. Tungsten atoms were thought to be the electron donor centers (Section IV,D). Isotopic experiments with 1 8 3 W and I7O confirmed that tungsten was involved at the adsorption site and showed that the 0; ion had equivalent oxygen nuclei. iu. Rnre-earth-exchanged zeolites. Cerium zeolite is the only system which has been investigated. When a CeX zeolite is activated in uacuo and then in oxygen at 200°C, and is finally outgassed at temperatures below 200"C, an EPR spectrum can be observed with g values of 2.037, 2.012, and 2.010 characteristic of 0; adsorbed at Ce4+ ions (285).The reactivity of 0; toward hydrocarbons and CO lead Krzyzanowski to suggest that the 0; ions are located in the supercages probably at S,, sites. Experiments with 1 7 0 confirmed the molecular nature of 0; (1024. A different EPR signal was observed, with a high intensity, when oxygen was introduced at 77 K onto the sample. The g values obtained were 2.0242, 2.0208, and 2.01 12. Using I7O, Vedrine et ul. (1024 showed that two equivalent oxygen were involved and attributed the signal to 0; undergoing rotational motion. This has been discussed in Section III,A,2,c.

D. SUPPORTED METALS

A general discussion of the oxygen species reported on metal surfaces is outside the scope of this review, but this section covers a few cases where

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

75

there is some direct evidence for the nature of the adsorbed species. The supported silver system is discussed in detail because of its importance as a catalyst in the oxidation of ethylene, and there has been considerable interest in the form of the oxygen on the surface. Normally, EPR is not useful to study supported metal systems because of the interference from the conduction electrons of the metal. However, Clarkson et al. (56, 286, 287) have reported the formation of 0; after the adsorption of oxygen on Ag supported on porous Vycor glass, with g values of 2.004, 2.009, and 2.028-2.030. Computer fitting of the spectrum was used to confirm an interaction with the two isotopes on silver and the authors suggest that the adsorption site is a silver ion. However, the simulated spectrum does not agree well with the observed spectrum. Assuming an ionic model, the quoted gzzvalue of 2.028-2.030 would indicate adsorption at sites of charge between 3 and 4+ (Fig. 3), consistent with electron transfer from the silver to oxygen adsorbed on the support; this possibility has not been discussed by the authors. The poorly resolved doublet about gzz could arise either from 0; in two different sites or from a hyperfine splitting, but no work at the Q-band is reported to distinguish between these two possibilities. Clarkson and McClellan (56) refer to unpublished EPR spectra obtained from adsorbed oxygen labeled with "0 as indicating inequivalent oxygen nuclei, but no spectroscopic parameters are quoted. It seems likely that this species together with that reported by Shimizu et al. (288) with g values of 2.002, 2.010, and 2.040 arise from 0; adsorbed on the support, the silver acting as a source of electrons. It is interesting to note that work by Abou-Kais et al. (289)using Cab-0-Sil as a support gave no EPR signal from oxygen species unless the system was irradiated and then 0; in a variety of sites was seen. This is consistent with the presence of sites with charges of 2 +, 3 +, and 4 + , which were attributed either to aggregates of silver ions or via an electron donation from the silver to give 0; ions on, for example, Si4+ ions. EPR measurements of oxygen on A g o (290) gave a strong, poorly resolved line broadly consistent with 0; in a variety of sites. However, it is not clear from the spectra which shoulders arise from y values and which from possible hyperfine interactions. Clarkson and Kooser (57)have studied the motion of 0; on a supported Ag system (see Section III,A,l,c). In all these studies, there is general agreement that silver is the source of the electrons, but the identity of the adsorption site of the oxygen is not clear. Since electron transfer mechanisms of the type described earlier (Fig. 16) can occur, silver ions are not necessarily the adsorption site and the gzz values would also be consistent with adsorption on the support. However, the degree of dispersion of the silver will be very dependent on the method of preparation and clusters of Ag atoms or ions of different sizes are possible. This leads to an alternative explanation of the low gzz values, since these

+

M. CHE A N D A. J. TENCH

76

small aggregates could form adsorption sites with higher positive charges. Particularly relevant to these arguments is the chemical (291) and optical (292)evidence from silver-exchanged zeolites which provides strong support for the formation of small charged clusters of silver at specific sites in the zeolite framework. High silver loadings favor the formation of clusters such as Ag:, Ag:', and Ag:'. The direct evidence for the number of atoms in and the precise charge of these clusters is not completely certain yet and it is interesting to note that no EPR signals have yet been seen in these systems, although a number of measurements have been attempted. A convincing and elegant demonstration of the presence of a diatomic oxygen involved in the ethylene/oxygen complex on a supported silver surface has been reported by Kilty et al. (293)using IR spectroscopy. Adsorption of 1 6 0 2 followed by adsorption of ethylene gave a complex with a band at 870 cm-' (Fig. 19), whereas adsorption of '*02 gave a band of 848 cm-'. Adsorption of an equilibrated isotopic mixture of oxygen gave an extra band at 859 cm-', whereas an unequilibrated mixture gave only the two bands at 848 and 870 cm-', confirming that a diatomic species is involved and both the oxygen atoms originate from the same oxygen molecule. The initial oxygen species is thought to be 0; adsorbed on a Ag' site with end-on geometry, but there was no direct evidence on the nature of this oxygen species. The results can be discussed in terms of 0; or O ; - , but the transfer of two electrons was thought to be unlikely.

I

890

I

880

I

1

870 860 Wave number (cm-1)

I

850

I

840

FIG. 19. Infrarcd spectra (890 840 cm ') obtained aftcr the adsorption of ethylene at 95 C o n Ag,;1-Al2O3previously exposed to oxygen: (a) '"02; (b) I8O2;(c) I h 0 2+ 2'"O"'O + IHO2:(d) If'02 t lXOl(293).

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

77

The IR absorption frequencies observed in the experiment by Kilty et al. (293)for the ethylene/oxygen complex are quite different from those reported from matrix isolation experiments on the Ag/02 system (127a, 294) where bands observed in the region 1030-1 100 cm-' are attributed to 0;. The oxygen nuclei were first thought to be equivalent (127a),but later work by Tevault et al. (294) at higher resolutions indicates that the Agoz molecule is nonsymmetric. The lower stretching frequency in the ethylene complex observed by Kilty et al. (293) is consistent with the 0-0 bond becoming weaker on the adsorption of ethylene to form a complex with peroxide character (Appendix C). Information on oxygen species adsorbed on silver has also been obtained from mass spectrometric analysis of thermally desorbed gases (295).Isotopic scrambling experiments with mixtures of ' *O, and 1 6 0 2 were used to differentiate between mono and dioxygen species. A desorption peak at 380 K was associated with a dioxygen species and one close to 500 K with an atomic species. Only 16% of adsorbed oxygen was thought to be in the dioxygen form, and broadly similar results were obtained for UHV studies on Ag as a single crystal and also for Ag supported on Al,O,. Related work has been reported on alkali-doped Ag single crystals (296,297). There are a wide range of papers using electron spectroscopic techniques to study the interaction of oxygen with silver and other metal systems (298), but it has proved difficult to differentiate between the various oxygen species and also OH- using these techniques (Section VII). This is likely to improve in the future as the understanding of the detailed analysis of the bands is refined. In addition to silver, there is some evidence for dioxygen complexes on other metal systems. EPR data for 0; adsorbed on a number of supported metals have been reported (299).However, there seems to be some doubt as to whether the 0; ions are adsorbed on a supported metal surface. The observed gz2 values lie in the range 2.026-2.037, which, on the basis of the ionic model, indicate that 0; is associated with cations of charge 4+ arising either from a several-step oxidation of the metal or by electron transfer from the metal to oxygen adsorbed on the support. The formation of 0;on a metal surface is more clearly shown in the work of Abdo et al. (105b),who report that zerovalent tungsten can be formed by thermal decomposition in uacuo of W(CO), adsorbed on Y-type zeolite. This W(o) reacts with oxygen to form 0;with gZz = 2.060. Isotopic labeling with 18,W broadened the EPR signal, indicating an interaction with tungsten sites, while the gzz value was thought to be consistent with a low charge, probably + 1, of the cation site Wo + 0, + W + lo;. This conclusion should be regarded with caution since the identification of the adsorption sites is a particularly difficult problem in zeolites (Section IV,C,3). The presence of

78

M. CHE AND A . J . TENCH

0; was confirmed using I7O;the oxygen nuclei appear to be equivalent with a hyperfine splitting of 70 G. Ismailov et al. (240) have reported the formation of 0; ions from Pd(o) in Y-type zeolites (see Section IV,C,3). Matrix IR measurements of Rh/O, complexes (300)indicate that a series of compounds can be formed with the simplest, a Rho, species, characterized by a band at 908 cm-'. Shubin et al. (301)have studied the Rh/SiO, system formed by impregnating SiO, with a solution of RhCI, and adsorption of oxygen after thermal treatment under vacuum (200-500°C) leads to the formation of an oxygen complex characterized by y, = 1.997. Katzer et al. (302) have reported 0; adsorbed on Pt/AI,O, catalysts with g values of 2.141,2.010, and 1.963. The spectrum is complex and it is suggested that 0 is also present, but no 1 7 0 data is available to substantiate the assignment at present. In neither of these cases are the g values fully consistent with the ionic model. In a very different type of experiment, Ono et al. (303) have formed 0; by passing an aqueous solution of HzOz through a column containing Pt, Pd, or Ag catalysts supported on A1203 and rapidly freezing the effluent. The EPR specta were characteristic of 0; in a crystalline ice matrix with g,, = 2.1 1. E. DIOXYGEN ADDUCTS

No review on oxygen species is complete without a mention of dioxygen adducts because of their general importance as oxygen carriers (4, 148,304) and their close relation to some of the surface oxygen complexes on oxides. For example, a reversible M0,-0, complex on a silica surface has now been reported (305). In general, to be classified as an adduct or oxygen carrier the reverse reaction involving the dissociation of the dioxygen complex should be readily observable by lowering the partial pressure of O,, or by gently heating the complex, or by the addition of a ligand capable of replacing the bound 0,. The general equilibrium is given by nM(L)

+ 0, * [M(L)ln(02)

where n = 1 or 2. Tovrog et al. (48)and others (46b, 134j; 306-308) have investigated a series of cobalt dioxygen adducts in frozen solution where cobalt is complexed with various organic ligands such as diphenylglyoximate. In frozen solution, the EPR spectra of these adducts show y, values ranging from 2.065 to 2.103 and a superhyperfine interaction with the cobalt nucleus ranging from 15 to 3 3 G for A,,, the magnitude of both these parameters depending on the nature of the ligands. Labeling of the oxygen with 7O gave a spectrum with a poorly defined hyperfine structure. The wings of the spectrum were used

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

79

to obtain a hyperfine tensor with A,, = 88 and 60 G, indicating inequivalent oxygen nuclei (49).These results are consistent with a bent end-on structure, called the Pauling model, for the adsorbed oxygen with 60% of the spin density on the terminal oxygen. Drago et al. (48,304) interpret the results in terms of a model in which the bonding interaction arises from a spin pairing of an unpaired electron in an antibonding orbital of 0, with an unpaired electron in a d,, orbital of cobalt (11) (see Section III,A,l,b). In many cases, the degree of electron transfer to the oxygen is small (48,309).There has been some discussion in the literature on whether Co(III)-O, is a correct description for the complex, but it appears that the discrepancy arose from differing definitions of oxidation states (148). This formalism makes no attempt to define the amount of electron transferred but just assigns the shared electron pair to the more electronegative element (310). Oxygen adducts of a completely different character have been reported with other transition metal ions, including iron (148) where it is biologically of great importance. The titanium (111) porphyrins (311,312) form dioxygen adducts in which 170labeling shows that there is almost no spin density on the oxygen nuclei. Similar characteristics are reported for the dioxygen adducts of the manganese porphyrins (98,313,314). In both these cases, the oxygen is thought to adopt a side-on geometry, called the Griffith model, which for manganese is consistent with a Mn(IV)-O$- formalism (314). This is supported by IR measurements (315) in an argon matrix, where an I6O2 stretching frequency is observed at 983 cm-' (see Appendix C and Fig. 28). Similar dioxygen adducts are well known in many systems in solution (4,418).

Surface analogs of the adducts described in the paragraphs above have + been reported. In zeolites, the formation of low-spin ( C O ~ + L , O ; ) ~complexes has been studied (51, 115, 116), where L represents ammonia or an amine. This is essentially a surface phenomenon since the complexes are prepared from Co(I1) Y zeolites by adsorption of ammonia or an amine followed by oxygen and the process is easily reversible. The thermal stability is much lower than would be expected from systems where the oxygen adsorption can be described by the ionic model. With ammonia as the ligand, the EPR spectrum is axially symmetric with gI1= 2.084 and g1 = 2.000, and shows a superhyperfine interaction from the cobalt with A , , = 17.8 and A , = 12.5 G. Adsorption of isotopically labeled 1 7 0 2 gives a hyperfine tensor with A,, = 80 and 60 G, showing that the oxygen are nonequivalent. This is consistent with a bent Co-0-0 structure, where the unpaired electron is localized on the oxygen, but it is not necessary to invoke electron transfer to the oxygen to form an ionic 0; since the observed spectra can be fully understood in terms of the spin pairing model discussed in Section III,A,I ,b.

80

M. CHE AND A. J . TENCH

In addition to the monomeric Co adduct described above, Howe and Lunsford (115) have also described a dimeric adduct (CO~+L,O~-CO~+L,)~+, where L = NH3 or CH3NH,. This peroxodicobalt species can be oxidized to give a superoxodicobalt species (CO~+L,O;CO~+L,)~+ which is paramagnetic and gives characteristic sets of 15 lines in the EPR spectrum due to superhyperfine interaction with two equivalent 59C0nuclei (Fig. 20). Winscom et a/.(316)have studied the influence of hydration and oxidation on bis(dimethylg1yoximato)-Co(I1) complexes in a cobalt-exchanged NaX zeolite. Evidence for both six- and fourfold complexes is presented. They conclude that increasing water coordination destabilizes the half-filled d,, orbital with respect to d,, and dyzrand eventually its energy will exceed that of the unoccupied n* molecular orbital of 0, to make the formation of the Co(II1)-0;adduct energetically favorable. The formation of an adsorbed oxygen species on the y-Al,O,/phthalocyaninato COW)system has been reported by Mercati et a / . (250). The

9,

= 2.072

FIG. 20. EPR spectra of [Co(Ill)(NH,),O;Co(Ill)(NH,),IS~ in COY: (a) experimental spectrum; (b) simulated spectrum. Reprinted with permission from Ref. 115. Copyright 1975 American Chemical Society.

MOLECULAR OXYGEN SPECIES O N OXIDE SURFACES

81

spectra are not very intense, but there is evidence of a Co superhyperfine splitting about gzz = 2.098, which is consistent with end-on adsorption of oxygen at a cobalt ion. The gzzvalue is higher than that reported for a similar complex in solution (317). Cordischi et al. (110) have reported the EPR spectrum of an adduct of oxygen with a dilute solid solution of COOin MgO. The adduct is stable at 77 K and shows a superhyperfine interaction with the cobalt ions. Zecchina et al. (1344 have studied the same system using IR spectroscopy (Sections IV,A,I and IV,B,6). When oxygen is adsorbed onto Cu2+ supported on silica, the EPR signal from Cu2+ pairs disappears and a new signal with g1 = 2.10, 9, = 2.00, g3 = 1.80 and A = 5, A, = A , = 65 G appears ( 3 1 8 , 3 1 9 ~The ) . reaction occurs even at 77 K, but the oxygen is very weakly bound and evacuation of the sample at - 100°C restores the original state. The thermal stability is much less than would be expected from oxygen bound as 0, in the conventional ionic model. The results can be explained on the basis of a weakly bound complex of the form (Cu . . . O,)", with S = b, but the EPR data does not ) shown that a spin pairing model fit an ionic model. Lumpov et al. ( 3 1 9 ~have based on a distorted side-on oxygen complex which takes into account the influence of the field of the ligands and also includes a small admixture of the 5s to the 3d orbitals of the copper ion forming the 0 bond is consistent with the observed EPR data. Oxygen complexes are also formed on Rh2+ ions supported on SiO, with gav= 1.997(301),on Rh2+ions in RhY zeolite with gI1= 2.01 5 and gI = 1.931 (319b),on rhodium ion pairs in RhNaY zeolite with gI1= 2.014 and g1 = 1.943 (284),on Pd+ ions in either PdNaY zeolite with g1 = 2.208, g2 = 2.069, and g3 = 2.042 (240) or Pd mordenite with g1 = 2.05 and gI1= 1.99 (152). These oxygen complexes are all unstable on evacuation at room temperature to give the original metal ions. The molecular nature of the oxygen has been confirmed only in the last case with "0-enriched oxygen and the two oxygen nuclei were found to be equivalent (152). Quantitative experiments have been performed to find the stoichiometry of oxygen complexes on surfaces. Oxygen also adsorbs reversibly on a Fe(I1) porphyrin attached to the imidazole groups of a silica gel that is adequately treated to give a 1 : 1 stoichiometry of the Fe-0, adduct (320). Gustafson et al. (83b) have reported formation of an oxygen adduct with Ru3+ in a Y-type zeolite. The monocarbonyl complex of Ru3+ is formed first and then reacts reversibly with oxygen to form a (Ru(CO)O,)~+complex. At least two forms of dioxygen are present, characterized by gzz values of 2.083 and 2.056 with similar values of 2.006 and 2.001 for the other two components. Oxygen labeled with "0 gave a hyperfine tensor with A,, = 80 and 67 G, indicating inequivalent nuclei. The two high gzz values are not consistent with an ionic model in which adsorption occurs at a 4 + site and a spin pairing model is likely to be more suitable in this case also.

M. CHE AND A . J. TENCH

82

Kellerman er a/. (321) have observed the formation of a reversible I : 1 oxygen adduct with C r Z +ions in Cr A zeolite using reflectance spectroscopy. Molecular oxygen forms a reversible coordination complex with these metal ions which is probably similar to that described above for the cobalt-oxygen adducts. On adsorption of oxygen at room temperature the magnetic moment decreases from 5.0 to 3.7 Bohr magnetons, indicating spin pairing between the chromium ion and the oxygen radical anion. The change in the electronic spectrum is interpreted as a transformation from a d4 to a d3 ion with lower symmetry. In general, all these cases of adduct formation are characterized by a low thermal stability consistent with a weaker interaction with the adsorption site than is found where an electron transfer occurs to form a largely ionic bond.

V.

Oxygen Ions Containing More Than Two Nuclei

The 0;(ozonide) ion is the only well-established species containing more than two oxygen nuclei. Two types of 0; have been reported and have been characterized mainly by EPR. The evidence for other species is weak and their existence has not been substantiated by direct observation. A. 1.

THEO; ION

The Normal Ozonide 0; Ion

a. Chmicterizution. The 0; ozonide ion has been observed in a number of oxygen-containing inorganic salts (322 and references therein) and is reported to be stable at room temperature. It is usually formed by the action of high-energy irradiation and has been studied in single crystals of different symmetries. In irradiated single crystals of KCIOJ (323-325) and lithium sulfate (326),the g tensor is anisotropic whereas in crystals of higher symmetry such as strontium and barium nitrates (327) and sodium bromates (328, 330), the y tensor is axially symmetric. In most cases, the average y value agrees with that measured for solutions of potassium ozonide dissolved in liquid ammonia (331). The 0; ion is a 19-electron radical and is isoelectronic with AB,-type radicals such as SO, and NO:- which have been observed on surfaces. In these ions, the energy levels are well separated (Fig. 21) and because they are not significantly perturbed by the surface crystal field, the g tensor can be used to “fingerprint” the species (96). Comparison between the y tensor

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

83

components for 0; observed in the bulk or on the surface shows good agreement (Table XI) even though the g components of surface radicals were obtained from EPR powder spectra and the bulk values from single crystal studies. The g tensor for 0 5 is discussed below on the basis of Walsh's diagram (336)for a 19-electron radical. The 0; ion is expected to be a bent radical with the odd electron occupying the 2b1 molecular orbital (MO) (332)which has the form (322)

12b,)

=

-c,(2pyb)

+ cZ(2pp + 2 ~ $ ~ ) / 2 ' "

with the 2p, atomic orbitals (AO) perpendicular to the molecular plane in the following coordination system: z

t O /b -\

0,

Y

0,

Thus for a 2B, ground state, ( la2)2(2b2)2(3a,)2(2bl)1 and neglecting the effect of d orbitals, the shifts in the g tensor are as follows (322): Agxx = 0

(7)

Aggy = 21(c1c3

+ c2c4)2/(E2bl

-

E4al)

(8)

where 1 is the spin-orbit coupling constant of oxygen, ci's are the MO coefficients used by Schlick (322),and the EMoenergies refer to the MOs depicted in Walsh's diagram (332) (Fig. 21). As is found generally for an electron in a bl orbital which is composed of p, AOs, Ag,, is zero if d orbitals are neglected (332).Since excited states are formed by the promotion of an electron from filled orbitals to a half-filled orbital, both AS),,,and Agz, will be positive (337).Ag,, arises from the mixing of a B2 state with the ground B1 state. The configuration . . . ( la2)2(2b2)1(3a,)2(2bl)2,'B, is generally closelying and its admixture will produce a positive shift which is generally smaller than Ag,, , which arises from admixture with the higher-energy configuration . . . (la2)2(2b2)2(3a1)1(2bl)2, 2A,. From these arguments, it is expected that for O;, Ag,,, > Ag,, > Ag,, 2: 0. The 0; ion on MgO was first reported by Tench and Lawson (338),with a g tensor in agreement with the theoretical arguments, and confirmed by Williamson et al. (339). Further proof of the identity of this species comes from measurements using the "0 isotope to give a hyperfine interaction (334).When the 0; ion is formed by the reaction 0-

+ 0,'0;

FIG.21. Correlation diagram for AB, radicals with 19 and 21 electrons (332).

TABLE XI Moynrlic Paramrrers,for O j on MgO and in KCIO, and,for 0-on My0 " 0 hyperfine A tensor

(G) Species"

Principal direction

y tensor

0,

0,

xx

2.002 2.0148 2.0121 2.0014 2.0172 2.0100 2.0035 2.0187 2.0123 2.042 2.042 2.001 3

h h h 26 <5 <5 43.6 -5 -5 19.5 19.5 103.2

108 15 15 82 <5 <5 82.6 -8 -8

0 ; on MgO (P)

YY

(major site)

--7

0; on MgO ( p )

?'$'

IS

_-

'A

xx

O j in KCIO, (sc) (site C)

YY

0 - on MgO (p)

SYY'

-_ -7

.YSL

2 zc

Oc

Ref.

1 :-: 1 1 :7

333

10

334

<5 43.6 -5 -5

322

~

-

In parentheses. p denotes powder and sc denotes single crystal. The 0, was not labeled in this work. 'The axis system is not the same a s that taken for 0;; for 0 - , the 2 axis is perpendicular to the MgO surface and the I and y axes are in the plane of the MgO surface (I). a

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

85

it can be labeled in two different ways. Reaction of '70-with natural molecular oxygen gives a new spectrum (Fig. 22a) of six lines with a hyperfine splitting of 26 G centered on gxx = 2.0014. The splittings in the other two principal directions were estimated from the linewidth to be less than 5 G. In contrast, when I 6 0 - ions are reacted with 70-enrichedmolecular oxygen, a complex spectrum was observed (Fig. 22b) with three sets of six lines centered on gXx= 2.0014 with hyperfine splittings of 26, 82, and 65 G, respectively. The two extreme lines in the spectrum are thought to arise from ozonide ions containing two 7O nuclei. No hyperfine interaction is detected along the y and z directions and it is estimated to be less than 5 G. Studies of the thermal stability (334)indicate that 0; ions prepared from 1 7 0 - and natural oxygen decompose to form O;, which does not contain l7O. On the other hand, most of the 0; ions prepared from l60and 1 7 0 enriched oxygen decompose to form 0; with significant amounts of 1 7 0 with two equivalent oxygen and a hyperfine splitting of 77 G. This evidence indicates that in the 0; ozonide ion, the three oxygen atoms are in different environments as shown by the following model:

1

0;

r

in which 0, is the original 0- ion. The ozonide ion 0; is markedly distorted so that the "lattice" oxygen 0, retains most of the negative charge. This pushes the unpaired spin onto the other two oxygen atoms and mostly onto the outermost atom, 0,. In comparison, for 0; in bulk KC103, where the more symmetrical ion has a CZvsymmetry (Table XI), the central atom 0, retains about the same spin density (-0.5) in both cases. The dramatic decrease in the hyperfine splitting of 1 7 0 - (103.2 G) (335)to that obtained for 1 7 0 , in 0, (25 G) (Table XI) confirms that the unpaired electron has largely moved away from the surface. Tench (333)has independently reported EPR spectra for 0; with hyperfine structure following reaction of l60- with 170-enriched oxygen. The spectra were analyzed in terms of one tensor of 108, 15, and 15 G due to 160,'70,'60; and another tensor of 70, 10, and 10 G corresponding to 160,'60,'70c-. Tench suggested that the similarity between the hyperfine and 1 7 0 - or the V; center in bulk MgO (340)was patterns for 160170160coincidental. These results confirm the nature of the ozonide ion 0; and its dissociation into 0; on heating to 130°C. Analysis of the intensities of the hyperfine lines due to this 0; indicates that some exchange must occur in the 0; between 0-and added oxygen, at this temperature.

M. CHE AND A. J . TENCH

86

,

L

#

1

"

I

,

A%

,

IA:x

,Akx

FIG.22. EPR spectrum of 0, produced by reaction of (a) " 0 - (71.9",, enrichment) with "'02: (b) "0-with 170-enrichedoxygen (44.5",, enrichment) (334).

The conditions of preparation and activation of the MgO are very different between the work of Tench et ul. (333, 338) and Lunsford et al. (334, 339). Although the resolution is higher in the case of Tench et al. (333,338).it is difficult to see specific reasons for the difference observed in the 0; spectra and more work is required to resolve this question. Several other techniques have been used to characterize the 0; ion. For example, two strong bands in the IR spectrum are observed at 844 and 826.5 cm-', after matrix reactions of Mg with 0,. By the use of various isotopic mixtures, Andrews et al. (341) assigned both these bands to the antisymmetric 0, stretching mode (v,); the band at 844 cm-' corresponded to a more thermally stable Mg' 0; configuration. This assignment agrees with that given by Jacox and Milligan (342), who reported a band near 800cm-' for 0, formed by reacting 0- with 0, in an argon matrix. Infrared spectroscopy has been used by very few workers to study solid ozonides (343) and not at all for adsorbed 0;;this area requires more attention in the future. A strong resonance Raman fundamental absorption at 1023 cm-' has been reported for 0; formed by matrix reactions in the Mg-0, system (344). This is in agreement with bands at 1017 and 1010 cm-' observed for polycrystalline ozonides KO3 and CsO,, respectively (132), and at 1038 cm-

'

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

400

do

6b0

BbO lib0 12b0 Wavelength (nm)

14b0

16bO

87

1/00

FIG.23. The reflectance spectra of the MgO surface with (a) F:(H) defects, (b) addition of N,O, (c) subsequent addition of O,, and (d) in uacuo (353).

for 0, trapped in fluorites (345). There are, however, no Raman data for 0; adsorbed on surfaces. In contrast, optical absorption has been used to characterize adsorbed 0;. Parallel EPR and optical experiments have shown that where oxygen is added to the 0-adsorbed on MgO, the EPR spectrum changes to that of 0;and at the same time an absorption band at 394 nm attributed to 0is lost and a new band at 420 nm appears (Fig. 23) which can be assigned to adsorbed 0; (333).This is consistent with the known data on the optical absorption bands of the ozonides which are reported at about 450 nm in liquid ammonia (346-348) and at 430 nm for polycrystalline ozonides (331). Similar results have subsequently been obtained on CaO. When oxygen is adsorbed at room temperature on CaO previously outgassed at temperatures in the range 500-8OO0C, a light pink color appears and a band at 23,500 cm(425 nm) develops which was tentatively attributed to 0, (131).Cordischi et al. (158)were able to observe a typical EPR signal (2.0023,2.0095,2.0185) due to 0;ion in addition to an EPR signal from 0;.No optical absorption has been reported for 0;on an oxide surface even where EPR has shown the ion to be present (see Section 111,A,4),so that the assignment of the band at 425 nm to 0;made by Zecchina et al. (131)seems to be correct and is consistent with the earlier data on MgO. The intense absorption band observed in the 425 nm region for polycrystalline ozonides and assigned to the 2B,-+ ( la2)' (2b2)2(3a,)2(2b1)2,2A2transition for 0;(331)has led Symons

M. CHE AND

88

A . J. TENCH

to suggest (150) that this optical band can be a most useful aid in distinguishing between 0; and 0; ions when both may be present in a system and the EPR spectra is complicated. In view of all data available now, we would assign the absorption in the 425 nm region observed by Nelson et al. (349) on MgO y-irradiated in the presence of oxygen to 0; rather than 0; and the intense features (2.0151, 2.0102) detected in the EPR spectrum to O;, in good agreement with the earlier results on MgO (333,334).The optical band at 230 nm observed by Nelson et a!. (349)is likely to be due either to 0, produced in large concentration by y irradiation of MgO in oxygen or to 0:; ions on the surface (350,Section I11 of Ref. I ) . b. Formation and Stability. In view of the electron affinity of ozone (351) 0,+

e-

+

0;. A H

=

-45.24 kcal/mol

it is, in principle, possible to form the ozonide ion 0; directly by adsorbing ozone on an electron-rich surface like an insulating oxide possessing F: centers or a nonstoichiometric transition metal oxide. However, in view of the difficulty of preparing pure ozone, the 0, has been usually produced by the secondary reaction (for definition see Section IV) between 0 - ions and oxygen. From the thermodynamics of the following processes : 0 + e-

do-.

0, 0,+ 0, +

AH = - 3 3 78 kcal/mol (38) A H = f 2 3 . 0 6 kcal/mol(352)

and from the above value for the electron affinity of ozone, it is possible to show that the secondary reaction is favored: 0- + 0,do;,

A H = -34.52 kcal/mol

in agreement with Tench’s conclusion (333). Tench and Lawson (333, 338, 353) and Lunsford et a/. (334, 339) have reported independently the formation of 0; on MgO using this secondary reaction, where the 0- ions are first produced by reaction of N,O with F: centers (1). In the early work of Williamson et al. (339),it was believed that adsorption of H,, CO, C O , or additional N,O on 0- ions could promote the reaction 3 0 - -0i

+ 2e-

as shown by the appearance of an EPR spectrum assigned to 05. These results were probably due to oxygen impurity contained in the gases used (334)and show that oxygen is far more reactive with 0- than H,, CO, C O , , or N 2 0 . There are also cases where the 0- ions involved in the secondary reaction leading to 0; are not directly observed. When CaO is outgassed at 900°C

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

89

and then contacted with oxygen at room temperature, a complex EPR signal develops which contains components due to both 0; (2.000, 2.007, 2.10) and 0; (2.0023, 2.0095, 2.0185). The origin of the 0- ions is unknown but it is possible that the 0 - ions are present as clusters, rather than as isolated ions, since no isolated 0- ions are detected by EPR after the outgassing pretreatment at 900°C (158). The authors invoke the presence of pairs and triangular arrays of 0 - ions and suggest that the failure of CaO to adsorb oxygen as 0; ions, when the surface is previously pretreated at 298 K in H,, could be interpreted as due to the reaction between 0 - and H, as O-...O-

+ H2+OH-...0H

However, it must be borne in mind that in previous work, H, did not react with a triangular array of 0- ions to form OH- ions (354).If such a reaction with H, occurred, then the 0-ions would no longer be available for 0; formation. Moreover, the reaction of pairs of 0- ions with oxygen should lead to pairs of 0; ions which would have an abnormal EPR spectrum if they can be seen at all. In fact, the g tensor is as expected for isolated 0; ions. The Coo-MgO system behaves as CaO for the formation of O;, i.e., via invisible 0 - ions. The ozonide ions characterized by a three-g-value EPR signal (2.0025, 2.012, 2.01 7) do not exhibit any superhyperfine interaction with cobalt nuclei, suggesting that they are adsorbed on Mgz+ ions (110). Depending on the system (MgO, CaO, Coo-MgO) and the experimental conditions, the ozonide ion 0; disappears irreversibly between 25" and 130°C. In the case of MgO (333,334),0; ions are formed when 0; ions are destroyed, whereas for CaO (158) and Coo-MgO (110) the evidence is not clear. Ozonide ions can also be produced from reaction of oxygen with V centers. When MgO is y-irradiated in oxygen (333) or neutron-irradiated in uucuo and then exposed to oxygen (67),an EPR signal due to 0; is formed with g values of 2.003-2.002, 2.0180-2.0150, 2.01 12 and 2.0003-2.0002, 2.01 50, 2.01 1, respectively, showing that different sites are involved. It is possible to distinguish between 0; formed from 0- or V - centers (333). On irradiated MgO (in the absence of hydrogen) the 0; exists close to a cation vacancy and is formed by the reaction of oxygen with a V- center (Fig. 24a). A distinctly different situation occurs when 0- is produced by the reaction of N,O with F centers formed by y irradiation of MgO in the presence of hydrogen; here the 0- probably occupies a normal anion lattice position; the addition of oxygen leads to the formation of 0; characterized by a small interaction with a nearby proton (333) (Fig. 24b). The differences in g factors for the various types of 0;could represent differences in bond

90

M. CHE AND A. J. TENCH

rmM

i’

0’-U 0- M M I --c

02-M 02-M 0’-

MI

M

\

0 2 - U 0 Mrm

02-M

02-m 0‘-

(a)

Y’

o--..o0

.w02-M 0 - M M 0’-M

02--,

w02-M

0‘-M

M

H

\

O

0 M 0”rm

0’- M 0 2 - M

(b)

FIG.24. Different sites for 0;on the (100) surface of MgO: (a) 0;formed by reaction of a V - center with oxygen; (b) 0; formed by secondary reaction of 0-with oxygen (333).

angle of the 0; produced by the polarization of the oxygen, depending on the presence or absence of neighboring Mg2+ sites. The bond angle is generally close to 110”(342). 0, ions can also be formed on systems UV-irradiated in oxygen (355,356). Iwamoto and Lunsford (355) suggest that on MgO UV-irradiated in oxygen, 0; ions with gyy = 2.015-2.017 can be formed from 0; as intermediates. The authors suggest that these ions are photodissociated to produce 0- ions, which then react with oxygen to form 0; ions. However, it must be remembered that 0- ions in the form of V- centers are readily produced on the surface by irradiation of MgO in uacuo or low pressures of 0,(356),and the fresh 0- ions formed in this way at cation vacancies etc. can react to form 0;. In addition, the observation of highly resolved optical absorption and photoluminescence spectra in several systems, such as sodalites (l26), argues against this excitation being associated with a photodissociation process. 0; ions have also been formed on TiO, UV-irradiated in oxygen (88, 205) and it is likely that they may be formed from 0- ions generated directly by UV irradiation (see Section III,B in Ref. 1). On TiO, (88, 205) the 0; ion was not stable at 300 K. 0;can also be formed as an electrochemical intermediate on silver during anodic oxygen evolution in concentrated KOH solution (357, 358). While 0- ions are again involved in the formation of 0; in this system (357,358),Iwamoto and Lunsford (355) have shown that it was also possible to produce the ozonide ion from reaction of 0; with N,O at 100°C on a MgO surface. Apart from reactions involving 0, or N,O, it has been suggested that atoms produced from a microwave discharge could lead to 0; ions (359), but the mechanism involved is not clear.

91

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

TABLE XI1 Some g Tensors /or !lie Normal Ozonide Ion 0;on Surfaces 9::

Ref.

2.0003-2.0002 2.002

2.0148 2.0172 2.01 7 2.0185 2.015 2.017 2.0151 2.0180-2.0150 2.0142

2.0121 2.0100 2.010 2.0095 2.015 2.012 2.0102 2.0112 2.008

333 334 360, 361 158 362 110 349 333 266

2.001

2.015

2.010

105a

2.0003-2.0002

2.0150 2.015-2.017 2.014 2.014 2.0181

2.01 1

67, 333

2.017 2.0131 2.016

2.010 2.0101 2.007

Matrix MgO/O- (major site) MgOjO MgO/OCaO ZnO Coo-MgO MgO ;f-irradiatedin 0, MgO y-irradiated in 0, H Y zeolite y-irradiated in 0, NaX zeolite y-irradiated in 0, MgO n-irradiated MgO UV-irradiated in 0, TiO, UV-irradiated in 0, TiO, UV-irradiated in 0, KOH/metal anode Electrochem N,O MgO/O; MgO/atoms GaAs ~

+

gr,

2.0020 2.0014 2.00 I8 2.0023 2.004 2.0025 -.

-

2.003 2.006 2.0019 -

2.0020 2.000

%S

-

2.009 2.009 2.01 12

355 88

205 357,358 355

359 363

The values of the g tensor for the ozonide ion stabilized on some surfaces are given in Table XII. The consistency of these values for the various systems shows that the g tensor may be used with some confidence to identify the 0; ions. There are, however, a few unusual cases where three oxygen atoms may be involved, but the normal properties of the ozonide ion are not observed. These “so-called” 0; ions are considered in the next section. 2. Related TrilPolynuclear Complexes There are several cases where the nature of the oxygen species is not clear. We have followed the original choice of the authors as to the nature of these species, but we feel it is arbitrary in some cases. a. The “So-called” 0; Ion. Shvets and Kazansky (198)have shown that oxygen adsorption at room temperature on reduced V , 0 5 supported on silica leads to the appearance of EPR spectra which can be ascribed to 0, and 0- radicals, with hyperfine structures due to interaction with ”V nuclei. The lines belonging to 0- disappear on oxygen adsorption at 77 K (364), leading to a new signal at g1 = 2.001 and g,, N 2.017 without any hyperfine structure. This new signal was assigned to 0;radicals on the basis of the

92

M. CHE AND A. J. TENCH

reactivity of 0 - with oxygen. An increase in temperature from 77 to 150 K and simultaneous evacuation resulted in the disappearance of the new signal and restoration of the original spectra due to 0- and 0; ions. However, the parallel feature was not clearly resolved in the spectrum of 0; ions because of the superposition of lines due to the presence of unreactive 0; species. Because of this, the Q , ~value of 2.017 (364) or 2.015 (365) was derived by substracting the EPR spectra of 0;. Analysis of both X-and Q-band (Fig. 25) spectra together with the use of 170-enriched oxygen lead to the following parameters g1 = 2.007, g, = 2.002, 9, = 1.998 with A , = A, N 0 and A, = 78 G (99).The hyperfine structure is consistent with the presence of two equivalent oxygen nuclei. These results can be interpreted assuming that a weak covalent bonding between the n electrons of the oxygen molecule and the free electron in 0 - takes place, i.e., a species of the type

o”Toc 0;

v5+ is formed after adsorption of oxygen. In such a species, the unpaired electron would mainly reside on Ob and 0,, as shown by the hyperfine constants which are exactly as expected from 0;. This structure could explain both the absence of hyperfine structure from 51V of the V5+0; complex and also the absence of any 1 7 0 hyperfine lines for the ‘bob

I

V ~ + ( ~ ~ O , - - - )‘60,

complex. The negative charge on the 0; in the V 5 + 0 ; complex would be attracted toward the V 5 + adsorption center while the spin density of the unpaired electron is forced to move in the opposite direction toward 0, and 0,. The thermal instability of the complex is in agreement with the model. The analysis of the g tensor of this new 0; species is more difficult since it is nearly isotropic, in contrast to the classical ozonide ion (Table XII). Shelimov et al. (99) have discussed the Ag shifts qualitatively and shown that a decrease of the apex angle 000 had to be expected from the value of 1 lo” generally found for normal ozonide 0; ions in order to account for the g tensor values. Theoretical calculations have been performed by Mikheikin et al. (366) and later by Lipatkina et al. (367) in order to account for the magnetic parameters of the ozonide ion either trapped in KCIO, (322) or adsorbed on surfaces (99,334).A good agreement could be obtained for the former case (322)as well as for 0;adsorbed on MgO (334, but not for the “so-called” 0; ion adsorbed on V,O,/SiO, (99) or on CrO,/SiO, with

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

93

- I gp2.00250 G

n

FIG.25. Q-band EPR spectrum recorded at 77 K after I6O2adsorption at 77 K on a reduced V20,/Si02 sample (99). gll = 2.008 and g1 = 2.003 (367). For these two systems, a change in the

000 angle and/or some rotational freedom were believed to be present. In view of the poor agreement of the latter examples, it is important to consider whether 0; can form complexes with oxygen. In a recent work, Byberg (368)has shown that 0; ozonide ions exchange-coupled to molecular oxygen can be formed in X-irradiated KC104 at room temperature, but are only observable below 160 K due to a strongly temperature-dependent linebroadening mechanism starting at 120 K. The interaction between 0;and 0,is well represented as an isotropic exchange coupling and the g tensor of the [O;, O,] complex with ,g, = 1.9996, gyy = 1.9995, and gzz = 2.0057 is very similar to that of the anomalous 0;ion observed on V,O,/SiO, (99) or on CrO,/SiO, (367).Although Lipatkina et al. (367)reject this possibility, additional experiments need to be performed in order to test for the presence of such complexes on surfaces. Very similar EPR spectra can be observed when TiOz (gl = 2.001, gI1= 2.008) (369)or TiO, supported on SiO, (gl = 2.002, gll = 2.01 1) (198) are UV-irradiated. First attributed to 0-centers (198, 369), these spectra were later reassigned to 0;(370). For the V205/Si02system (99),the 0; ion has been found to disappear between 200 and 300 K. In most cases only

M. CHE

94

A N D A . J . TENCH

the g tensor and the poor thermal stability are available to identify the species. Table XI11 gives a list of the systems on which this anomalous molecular complex has been suggested. The gis0value is close to the free electron value of 2 with a nearly isotropic g tensor and is completely different from the g tensors of 0; in the solid state. There are only two systems where ”0-enriched oxygen has been used: V,Os/SiO, (99)and CrOJSiO, (367). Whereas the EPR experiments indicated the presence of two oxygen nuclei in the oxygen complex for the former case (YY), it was not possible to draw a firm conclusion on the number of oxygen nuclei involved in the complex for the latter case (367).Because of the similarity of the g values and of the poor thermal stability with those of the anomalous 0; complex, we have included the TiO, system (88, 205) for which an 0:-species was invoked. This has been discussed in a previous review ( 1 ) . There are no optical or other spectroscopic data to complement the characterization of these anomalous 0; ions by EPR. Since the 0- ion readily reacts with oxygen to form 0; in a process which is often easily reversible, it has been suggested (364, 365) that this process could provide a suitable pathway for exchange reactions to occur. This exchange reaction has been studied on V205/Si0, (364, 365) as well as on UV-irradiated Ti02/Si02 (369, 370, 372) and SiO, catalysts (371, 372) (see Section V1,A). In conclusion, it appears that the classical ozonide 0; ion can be regarded as a well-characterized species on oxide surfaces. In contrast, the situation on the characterization of the “so-called” anomalous 0, complex is quite different. It is not possible at present to provide a clear theoretical explanation of the observed g values on the basis of an 0; species and the available ”0 data is consistent with interaction with only two oxygen nuclei. The identity of this species must be regarded as uncertain at this stage and on the whole is more consistent with a complex adduct species, which may involve

TABLE XI11 Some g Tensorsfor the “So-Called” 0;Complex Adsorbed on Surfaces

SiO, TiO,/SiO, V,0,/Si02 Cr03/Si0, TiO, TiO,

2.008 2.008 2.007 2.008 2.008 2.007

2.0045 2.001 2.002 2.003 2.001 2.001

2.003 2.001 1.998 2.003 2.001 2.001

2.005 2.003 2.002 2.004 2.003 2.003

371 3 70 99 36 7 88 205

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

95

more than three oxygen nuclei. Much more work is required to properly characterize these systems involving EPR and other techniques. b. Other Species. The other species which is thought to exist on oxide surfaces is the 0;-ion, which is a 21-electron radical. The 21st electron enters a cr bonding orbital whose energy tends to favor a rather linear geometry (Fig. 21). There is only one radical, F:-, known in this family and it has not been fully characterized since its g tensor is not known (373).This species has been discussed in detail in the previous review and the conclusion is that the evidence for this species is not clear ( I ) . Finally, it has been suggested that a sulfate-like oxygenate ion 0:-(374) species may exist on the surface. This species has only been studied by theoretical calculations which indicate that it would be stable with respect to 0,and 0;-. If this is so, it might be possible to form it by adsorbing ozone on a crystal of BaO,.

B. T H E O ION ~ Evidence for the existence of neutral dimers of oxygen on surfaces has been discussed earlier (Section 11) and it is now appropriate to consider the Although a number of reports have been published corresponding anion, 0;. on the existence of O;, very few present direct evidence for its identification. The 0; species is a 25-electron radical (375).The first 24 electrons are in orbitals for which a planar configuration is most stable, but the 25th electron must enter an antibonding orbital of the planar molecule. Some admixture of s character in this orbital leads to a lower overall energy and the molecular ion distorts to become pyramidal. The g tensor should reflect the axial symmetry of the anion. The g shifts for the perpendicular direction would be due to the excitation from the . . . (le)4 (5a,), (2e)4 (3e)4(la,), (6a1)', 'A, ground state to the ,E state, either as . . . (le)4 (5a1)' (2e), (3e)4(la,)* (6a,)' or a s . . . (le)4 (5a,), (2e)4(3e)3(la,), (6a1),. For the parallel direction the g shift would be due to the excitation from the ground state to the. . . (le)4 (5a1)' (2e)4(3e)4(la,)' (6a1),, ,A2 state. In terms of energy, the 6a1 orbital lies well separated from the 2e, 3e, and la, orbitals; hence only a small departure of the principal g values from the free electron value is expected. Although the geometry of the ion (uide infra) might alter this conclusion, it is predicted from these arguments that for 25-electron radicals gI1 g1 2 2.0023. This is supported by comparison with the isoelectronic ion SO,, which has an isotropic g tensor at 2.0036 in K,CH,(S03), (376), ClO,, which exhibits an axial g tensor (gl = 2.008, gI1= 2.007) in NH4C10, (377), and PO:- (gl = 2.001, gI1= 1.999) in Na,HP03.5H,0 (378). However, on surfaces there are only two reports of 25-electron radicals adsorbed on surfaces. SO; adsorbed at the surface of MgO gives an isotropic signal with

-

96

M. CHE AND A . J. TENCH

2.0034 (379)and in the titanium-exchanged A zeolite, a sharp symmetric g = 2.0090 and a linewidth of 7 G (280). In the solid state, 0; has been observed in y-irradiated single crystals of strontium, barium, and lead nitrates with g = 2.0051, gll = 2.0087, and g1 = 2.0062, respectively (380). On surfaces, the presence of 0; has been suggested to account for results obtained in oxygen isotopic exchange experiments (381).It has also been invoked to explain EPR data involving adsorbed 0; ions in isotopic exchange. Che et al. (382) used the hyperfine lines from isotopically labeled 70;on MgO to monitor the exchange of the adsorbed 0; with gas-phase oxygen at 300 K. Although the results showed that oxygen isotopic exchange did occur at 300 K, there was no evidence of a new EPR signal which could be assigned to 0;. Attempts have been made by various groups to detect EPR signals from this radical ion. On ZnO, a symmetric EPR spectrum with g = 2.003 and AH N 3 G can be observed on adsorption of oxygen. When 170-enriched oxygen is used to produce this EPR spectrum, no hyperfine structure is observed (3).It is possible that this spectrum can be assigned to 0; which is undergoing a rapid exchange reaction:

g

=

EPR spectrum is observed with

(OiA

+ (0dg=(oil*

The rapid exchange would then account for the absence of "0 hyperfine lines. Naccache et al. (75)have also reported a symmetric EPR line on TiO, which is not split in the presence of "0-enriched oxygen. This spectrum, originally assigned to the localization of conduction electrons in the lattice by adsorbed oxygen, might also be due to 0; ions. Griva et al. (383) have further studied the TiO, system by investigating the effect of the oxygen pressure on the broadening and integrated intensity of the 0; EPR spectrum in the 200-300 K temperature range. These authors showed that the equilibrium above was reversibly shifted toward the formation of 0; while the integrated intensity of 0;decreased. Here again, there was no indication of an EPR signal due to 0,. The authors suggested that the spin-lattice relaxation time was too short for 0;to be observed. The enthalpy of formation of the 0; complex was found to be 2.2 kcal/mol as compared to the gas phase value of - 13.55 kcal/mol obtained by mass spectrometry experiments (384).When oxygen is adsorbed at 77 K on 0;first produced by oxygen adsorption at 200 or 300 K on supported chromium oxide catalysts, two types of EPR spectra can be observed at gll = 2.008 and g1 = 2.002 and at g1 = 2.006, 9, = 2.002, and g3 = 1.999, which Doi (218) has attributed to 0; ions. Howe (219),however, did not succeed in reproducing Doi's results on supported chromium oxide catalysts which were prepared in a different way. This seems to indicate that the experimental conditions are critical for the formation and the observation of 0;.

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

97

It is likely that for a given system, both pressure and temperature are important in the formation of 0;. Since the reaction 0;

+ o2

+

0,

involves a decrease in the number of molecules and is exothermic, the formation of 0; should be favored by high pressures and low temperatures. This has been explored in a study by Kuznicki et al. (280)using titaniumexchanged A and Y zeolites. When oxygen under a pressure of 10 Torr is contacted at 25°C with Ti(II1) A zeolite in the presence of visible light, an EPR signal with g1 = 2.040, g, = 2.017, and g3 = 2.003 developed which has been assigned to Ti(IV)O;, in reasonable agreement with the work of Ono et al. (279). When the pressure of oxygen was increased to 750 Torr, the EPR spectrum of Ti(1V)O; disappeared, while a sharp symmetric signal developed at g = 2.0090 with a line width of 7 G and its formation was independent of temperature up to 400°C. No hyperfine structure was observed when "0-enriched oxygen was adsorbed and this was explained by the rapid exchange reaction between 0; and 0,. The existence of Ti(1V)O; formed by secondary reaction of oxygen with Ti(1V)O; was consistent with ESCA experiments which indicate an O/Ti ratio of 3.7 ( f 10%).The pressure range used in these experiments was larger than that covered in the work of Griva et al. (383).Moreover, it is likely that the cavities within the zeolite framework will increase the frequency of collisions between gas phase and species adsorbed on the walls (385). It is interesting to note that no work has been reported by Kuznicki et al. (280)to show whether the formation of 0; from 0; is reversible. The details of the process are not certain, but the type of zeolite appears to be an important parameter, since Ti(1V)O; in Y zeolite does not seem to lead to the formation of Ti(1V)O; at atmospheric pressure. It is possible that measurements at 4 K might slow the exchange rate sufficiently to permit the observation of 1 7 0 hyperfine structure for the 0; in the A zeolite. The most reliable data on the existence of 0; comes from IR observations of the matrix reactions of oxygen molecules with metal atoms. In the cesium/ ) observed a strong band at 1002 oxygen system, Andrews et a!. ( 1 3 3 ~have cm- I, showing the isotopic splittings expected for a species containing two 0, molecules which they assigned to Cs'O;. The band corresponds to an antisymmetric mode involving out-of-phase stretching of the two O2 parts of the M 0 4 species. This is in good agreement with Jacox and Milligan (342), who assigned a band near 1000 cm-' to 0; in the sodium/oxygen system. Matrix reactions with transition metal atoms have also been studied and a Ag'O; complex has been observed by McIntosh and Ozin ( 1 2 7 ~ ) with an associated IR band at 1025 cm-'. This Ag'O; complex exhibits an UV absorption at 290 nm. Raman experiments have confirmed the existence

M. CHE AND A. I. TENCH

98

of the M' 0;species and in the case of Rb' 0;and Cs+0;two strong bands were observed at 298 and 287 cm-', respectively, associated with the intermodes (386). molecular oxygen-oxygen ( O2 c,0 2 ) stretching A number of possible geometries have been considered qualitatively for the Na'O; system (1.334 such as M-OO,-O,, 0 M

/02

\

,

\ M-0, /

O,-M-O,.

0

0 2

or

/ M \

0-0,

0

and the most favored candidate was a puckered five-membered ring structure /O\O

M\

I 0/"

It is probable that such structures have surface analogs which are likely to exhibit differences in paramagnetic behavior and also in activity toward oxygen isotopic exchange reactions depending on their geometry. The EPR results discussed in this section are suggestive rather than definitive for the existence of 0;on oxide surfaces. However, it is clear from the preceding discussion that IR spectroscopy has proved to be a powerful technique to study 0;as a matrix-isolated species and the use of IR, Raman, and optical absorption together with EPR is likely to prove a very effective approach in elucidating the nature and properties of these complex oxygen ions on the surface.

VI.

Reactivity of Molecular Ions

In the following sections the behavior of the molecular oxygen species is discussed with- respect to exchange, oxidation, and photo-induced reactivity. A. EXCHANGE REACTIONS

Isotope-exchange reactions of oxygen on oxides have been reviewed by Novakova (387) and this section is restricted to those papers containing spectroscopic information about the adsorbed oxygen species. In general, there is very little direct evidence in the literature on the type of surface complexes involved in the exchange.

99

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

The l 6 0 - l 8 O exchange reactions on an oxide surface can occur via several different processes (388)which may occur simultaneously : (1) The exchange of oxygen atoms between gaseous the oxide:

-

1 8 0 , + 160:-

180160

+

I80,

and 160,2-from

1802-

(2) The exchange between gaseous "0,and a pair of I60H-ions from the oxide: 180,

+ 2160;-

+21"02-

+

160

(3) The exchange between 1 8 0 , and 1 6 0 , , both in the gas phase and also in contact with the oxide surface. Very little is known about the surface oxygen species which may be involved in processes ( 1 ) and (2), but process (3) can occur via a known adsorbed oxygen species in some systems. There are two possible mechanisms by which exchange can take place between adsorbed species and gas-phase oxygen : place exchange or isotopic exchange. Place exchange is the process whereby a gas-phase molecule is adsorbed at the surface with displacement of the preadsorbed molecule as molecular 0,. This mechanism does not involve the breaking of any 0-0 bond but only the transfer of an electron; in contrast, isotopic exchange proceeds via an activated intermediate species which changes the isotopic distribution. In some cases, it is likely that both processes occur simultaneously. Nikisha et al. (364)have studied the isotopic exchange reaction of molecular oxygen on silica-supported V, O5 at low temperatures. Although the exchange reaction involving 0-and 0;ions was dominant, there was some evidence of an exchange involving 0; in which the molecular nature of the adsorbed species was preserved (place exchange) : ('6O160),ds

+ 1s0180+ 886

('80'80),d,

+ 160160 889

without dissociation of the 0-0 bond. Che et al. (382)used the hyperfine lines from isotopically labeled I7O; on MgO to monitor the isotopic exchange at 300 K with gas-phase oxygen. The ratio of the intensity of the lines arising from ( 1 6 0 1 7 0 ) - was found to vary with time when the sample was exposed to gas-phase oxygen of normal isotopic composition, indicating that a scrambling of the oxygen atoms was occurring. This is characteristic of the existence of an intermediate species such as (O,),,,, but no new EPR signal was observed:

+

(170170)ais 160160,~~

+(

+ 160170

0 ~+; (~ 160170)~~

gas

For place exchange, no change in the relative intensity of the lines would be

M. CHE AND A. J. TENCH

100

expected. Exchange with gas-phase oxygen has also been reported for 0; adsorbed on NaY zeolite (76)at 300 K. Tanaka and Kazusaka (224 have reinvestigated the exchange process over ZnO at 77 K, where an (Oi)ads intermediate had previously been proposed (381) to explain the exchange. Both the gas phase and the adsorbed oxygen (0;) composition were monitored using the "0 and "0 isotopes, respectively, No significant decrease of the 0;hyperfine lines was noticed by EPR even with a great excess of "OZ in the gas phase, while there was evidence for oxygen scrambling from mass spectrometry. This rules out an associative mechanism via an 0; intermediate and the authors suggest that a neutral dimer of oxygen, 0,, could account for the results. Measurements by Tanaka et al. (389) using preadsorbed l8OZshowed that 0;on ZnO was inactive for exchange reactions with CO and COz as well as for the catalytic oxidation of co. The overall evidence indicates that an isotopic exchange involving adsorbed 0;does not occur at low temperatures; whereas at room temperature the exchange via 0;becomes important. The 0; ion plays an important role in exchange reactions over oxides through the reaction of gas-phase oxygen with 0-on the surface: (160-)d*

+

(1802)1.8

=

(03)is

*

(180-)ad,

+ (1601Bo)p~s

This has been discussed in the previous review by Che and Tench (1). For this mechanism to be confirmed by EPR,it is necessary to use "0labeled oxygen to be certain that the Oa;, actually exchanges and this has been seen in only one case (212a) to date. Alternatively, such an exchange could involve an intermediate (0; -02)complex; the possible existence of such complexes has been discussed in Section V,A. This may also be a major mechanism in the photo-induced exchange processes (see Section V1,C).

B. OXIDATION REACTIONS 1. Reactivity

of' the 0; lon

The 0;ion on MgO does not react with CO or alkanes at 77 K but the

EPR signal disappears slowly at room temperature (361).Similarly, on ZnO (390)it reacts only slowly with propylene at room temperature and not with CO, Hz,or ethylene. A slow reaction with propylene is also observed for 0; on VzO,/MgO at room temperature (391). Yoshida et al. (392) have studied the reactivity of adsorbed oxygen with olefins on the V205/SiOz system. Adsorption of propylene destroyed the signal from 0; slowly at room temperature and the reaction products, aldehydes with some acrolein, were desorbed as the temperature was raised to 150°C. More quantitative

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101

measurements (212b) indicated that the amount of oxidized products was similar to the initial concentration of 0; on the surface. In the most thorough study available to date, Iwamoto and Lunsford (393)have shown that a stoichiometric reaction (1 : 1) occurs between 0; on MgO and simple hydrocarbons (alkanes and alkenes). The 0; ion was produced on the surface by adding oxygen to a sample of MgO which contained electrons trapped at the surface (68, 155). The 0; ions disappeared slowly at 25°C in the presence of C , to C4alkanes or alkenes, but the reaction was accelerated at higher temperatures and most of the ions reacted with propylene in 2 hr at 175°C. No new EPR signal was formed and the different gzz components (corresponding to 0; in different sites) reacted at the same rate. Although, the 0;ions were much less reactive than either 0- or O;, several types of oxygen-containing products were formed at 175"C, as well as other hydrocarbons and CO, . Reaction with propylene gave acetaldehyde and methanol, which were desorbed at higher temperatures, and reaction with propane gave some acetone in addition. With I-butene as a reactant, 2-butanol was formed together with methanol, acetaldehyde, and acrolein above 300°C. Infrared spectra of surface intermediates indicate that the reaction of 0; with propylene at 175°C resulted in the simultaneous formation of formate (bands at 1606, 1387 and 1354 cm-l) and acetate (bands at 1585 and 1422 cm-') ions, which are consecutively converted to carbonate (bands at 1648 and 1326 cm-') ions at higher temperatures (Fig. 26). The authors suggest that hydrogen atom abstraction is the first step, and for propene, this results in the formation of an allyl radical:

+ 0;4CH2-CH.-CHz

CH,=CH-CH,

+ HO;

At the reaction temperature the allyl radical apparently reacts with the oxide ions of the surface, forming acetate and formate ions as follows: 0-

CH,.=CH.;-CH,

+ 50;-

-+

CH,-C

/ \.

0

0-

/ + HC \.

+ OH- + 7 e 0

Carbonate ions can be formed from the slow reaction of formate with surface oxide ions:

/ HC \.

0-

+ 20;-

-+CO:-

+ OH- + 2e

0

From the amount of hydrocarbon reacted, a very small proportion (probably <1%) of the surface oxide ions are involved in the above reactions. The electrons resulting from these reactions were thought to be trapped at oxide

102

M. CHE AND A. J. TENCH

ion vacancies in the surface but no EPR measurements were made to confirm this. In order to account for the significant amounts of CH3CH0 and CH,OH formed, the authors suggest that at least one of the atoms from the superoxide ion adds to a surface intermediate to form propylene oxide or a similar compound, since when propylene oxide was adsorbed on the surface, significant amounts of acetaldehyde and methanol were observed on heating to 450" and 600"C,indicating decomposition to give these compounds. Somewhat analogous reactions would be expected for the reaction of ethylene with 0; ions but the observed reaction rate is lower than for propene, suggesting that the reaction pathway may be controlled by the C-H bond energies. For reactions of propane and 1-butene with O ; , oxygenated compounds of the same carbon number as the reactants were produced. The initial step is thought to involve a hydrogen atom abstraction from a secondary carbon atom. There is fairly strong evidence that 0; is the source of oxygen in the very important epoxidation reaction of ethylene over supported silver. The IR studies of Kilty et al. (293)show that a diatomic oxygen complex with ethylene is present on the silver surface and that it readily forms ethylene oxide. However, the original form of the oxygen on the surface is not certain, although there is evidence from EPR that 0; on Ago reacts with ethylene at 15O'C (290) to give ethylene oxide with a selectivity of 60% (see Section IV,D) and also with propylene at 100°C (394). The reaction products in this latter case were acetone, C O , , and H,O. Geenen et al. (394) studied the oxidation of ethylene and propylene over Ag/Au alloys as a function of composition. The oxidation of these two molecules showed radically different behavior. For propylene, alloying the silver with gold changed the product composition from propylene oxide to acrolein (and CO, in both cases), whereas for ethylene the selectivity to form ethylene oxide decreased sharply as the gold content was increased. The authors suggest that these results can be explained if the oxidation of propylene to acrolein occurs at isolated Ag atoms, where hydrogen is readily abstracted from the adsorbed molecule by 0;.In contrast, ethylene oxide can only be formed if the abstraction reaction is inhibited by adjacent adsorbed species. These proposals are in agreement with the ideas proposed by Iwamoto and Lunsford (393) that the 0; will normally abstract a hydrogen atom as the first step. The reaction of 0; with propene adsorbed on ZnO has been studied by EPR and IR spectroscopy (395,396). The II allyl intermediate is thought to react with 0;to give a hydroperoxide (not detected) which forms glycidaldehyde, which is postulated as an intermediate in the formation of acrolein on the surface. Krylov and others (397,398) have also studied the reaction of II allyl complexes with 0;. Akimoto and Echigoya (232) have studied the oxidation of butadiene over

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

103

supported molybdena catalysts and concluded that both 0;and 0 2 -were required for the production of maleic anhydride. The catalytic activity was found to increase with increasing 0;concentration on the surface and it was suggested that the intermediate formed by the addition of oxygen was rapidly destroyed to give maleic anhydride or C 0 2 . The same authors (399) have proposed that 0;ions play an important role in the oxidation of furan to maleic anhydride over titanium or molybdenum catalysts. Comparison of the activities of the two catalysts modified by the addition of Group VA oxides such as antimony, bismuth, phosphorus, and arsenic lead the authors to suggest that, over titania, the active species was the 0;ion whereas over molybdena, the 0;initially formed was immediately transformed to Mo=O, which then acted as the oxidizing species. Fricke et al. (107) have studied the concentration of oxygen radicals adsorbed on silica-supported V20,-P,O, catalysts as a function of the V/P ratio and compared this with the selectivity of the catalyst for butene oxida-

I

iaoo

I

1

1600 1400 Wave number ( cm-1)

I

1200

FIG. 26. Infrared spectra following the reaction of propylene with 0;: (a) MgO background; (b) after the reaction at 175°C; (c) after thermal treatment under vacuum at 300°C; (d) 450°C; (e) 550°C; ( f ) difference spectrum between (b) and (a); (g) difference spectrum between (c) and (a) (393).

104

M. CHE AND A. J. TENCH

tion to maleic anhydride. They conclude that the predominant role of both 0-and 0;is in nonselective oxidation. 2. Reactivity of the 0, Ion

The normal ozonide ion does not appear to have a high reactivity. Gasphase studies carried out by Parkes (400) show that the upper limits of the rates of reactions of 0, with hydrocarbons were more than two orders of magnitude lower than that of 0-. Similarly, Naccache and Che (361) have reported that 0, on MgO did not react at - 196°C with molecules such as carbon monoxide, ethylene, or propylene. On MgO it is reported to decompose on warming to give 0; (333, 3 3 4 , but this appears to be related to the presence of hydrogen (401).In the absence of hydrogen it decomposes to give 0-, though not in stoichiometric amounts. Takita and Lunsford (401) have studied the stoichiometric oxidation of alkanes by 0;on MgO. The ozonide ion was prepared on the surface of MgO by UV irradiation in the presence of NzO. The alkane was then circulated over the sample for 2-3 hr and the reaction products were analyzed. Ozonide ions react with C, to C, alkanes at 25°C with half-lives of between 1.7 and 5.2 min with a stoichiometry of one alkane molecule reacted per 0; ion. During the reaction, a weak EPR signal, possibly from alkoxy radicals, was visible as the 0; ions were destroyed. On heating the samples, the main desorption products were the corresponding C, to C, alkenes, although the yields were less than observed in the corresponding reactions with 0-(402). The IR spectra of the surface complexes for the reaction of ethane at 25°C indicate carbonate and ethoxide are formed; at higher temperatures, the ethoxide ions decrease in concentration and acetate ions are formed. The initial step in the reaction of 0;ions with alkanes is thought to be hydrogen atom abstraction to give an alkyl radical. The alkyl radical then either reacts with a lattice oxide ion to give an alkoxide or with gas-phase oxygen to form an alkyl peroxy radical:

+ 0; -.C,H; + OH- + 0,

C,H, C,H;

+ 0'- +C,H,O- + e -

C,H;

+ 0, + C 2 H 5 0 0 '

The ethoxide ion decomposes above 300°C to give ethylene and acetate ions; the transient peroxy radicals are expected to be unstable and decompose to form carbonate ions :

+ OH+ 0, -.CH,COO- + H,O

C2HsO- +C,H, C,H50-

Overall, it is thought that a direct reaction occurs between 0; and the

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

105

alkane, but the selectivity to alkene is small and is decreased by the presence of oxygen, which forms alkylperoxy radicals. Takita et al. (403) have carried out a similar study on the oxidation of alkenes by the 0; ion at 25°C. In all cases, the 0, ions reacted stoichiometrically with the alkene with a half-life of about 5 min. Reaction with ethylene and propylene gives CO, and CH, as major products on heating the sample. The thermal desorption patterns and IR spectra of surface complexes suggest that carboxylate ions are intermediates in the formation of CO, and CH,, and peroxy radicals are the intermediates in the formation of oxygen-containing organic molecules. By analogy with the reactions between 0, and simple alkanes, it is thought that reaction with alkenes is initiated by hydrogen atom abstraction and for ethylene the following series of reactions are possible: C2H4 + 0; -+CH,=cH CH,=cH

+ 02-

+

CH2=c-

+ O H - + 0, . HO-

The CH,=C- . * . HO- complex has been identified by Ben Taarit et al. (4044 from the reaction between 0-and C2H, on MgO. In the present experiments, this complex is not stabilized at the higher temperatures, but reacts further with molecular oxygen and oxide ions to give formate, acetate, and carbonate ions. In general, it seems that reactions between 0, ions and alkenes on MgO generally lead to nonselective oxidation. Finally, concerning O,, apart from the exchange reaction (see Section VI,A), there has been only one report on the water splitting in titaniumexchanged A zeolite (404b).

C. PHOTO-INDUCED REACTIVITY Photo-induced oxidation reactions on oxide surfaces have been discussed in reviews by Bickley (206, 405) and by Formenti and Teichner (406).It is characteristic of these reactions that the energy of the irradiating light needs to be not less than that of the absorption band of the oxide. Under these conditions both electrons and holes are produced, which can then react with molecules adsorbed on the surface. Since in some cases the surface lattice ions can absorb light at an energy less than the bulk oxide ( I ) , photo-oxidation may be observed at lower wavelengths than expected from the bulk absorption band. This section is concerned with photo-induced oxidation reactions involving molecular species. The formation and reactivity of holes trapped at

106

M. CHE AND A. J. TENCH

oxide ions at the surface to form 0-ions have been discussed in the earlier review on mononuclear oxygen species (1).Only the main features of the photo-oxidation reactions are discussed since the topic has been reviewed recently (206,405,406)and also because there is little direct evidence on the nature of the oxidizing species.

1. Photoadsorption and Desorption Ultraviolet or y irradiation of a number of oxides in the presence of excess oxygen leads to the adsorption of oxygen and the formation of 0;and in some cases 0, ions. A variety of techniques have been used, such as thermal desorption, isotopic exchange, and conductivity measurements, but the principal evidence comes from EPR studies. Both the formation and stability of these species are discussed in the sections dealing with the appropriate oxide, but the overall picture is summarized below. Irradiation of MgO at room temperature in the presence of oxygen leads to formation of 0;and 0;(338,355,407),which can be readily observed by EPR. This process occurs at UV energies much less than the bulk band gap, showing that surface lattice ions are involved which have a reduced band gap ( I , 356). On TiO,, 0; is the only stable species at room temperature, but irradiation in the presence of oxygen at 77 K (66,88,205,207,208) leads to 0; as well as 0 - .The situation on TiO, is very complicated (Section IV,B,l) and depends on the degree of hydroxylation of the surface (204, 205,207,408). On ZnO (9,173,174c,175-179,409) and alumina (108,248)only 0;ions are seen, but on the porous Vycor glass (256,410)0-, O;, and 0;are observed after irradiation at 77 K. Iwamoto and Lunsford (355) suggest that the adsorbed 0;can undergo a photolysis reaction on MgO to form 0, and 0-. This has been discussed in Section V,A. 2. Photo-lnduced Exchange Reactions

Tanaka (411)has studied the isotopic exchange of a mixture of " 0 , and 1602 on TiO, (rutile) under UV irradiation. On the basis of thermal desorption data and mass spectrometric analysis, the author concludes that the exchange probably proceeds via weakly adsorbed 0; intermediates both in this case and also over ZnO (412).In both these systems, the absence of isotopic scrambling when strongly adsorbed oxygen was thermally desorbed indicated that it was unlikely that the more stable diatomic oxygen species such as 0;were involved. Kubokawa et al. (410)have studied the exchange reaction over porous Vycor glass, where a rapid exchange is completely inhibited by the presence of 1-butene, and they conclude that both photo-

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

I07

exchange and photo-oxidation occur via an 0; intermediate but the lattice oxygen does not participate in the exchange. 3. Oxidation of Alkunes and Alkrnes

The work of Teichner and others on the photo-oxidation of a number of alkanes over TiO, has been reviewed recently in detail (206,405,406). Apart from methane, which appears to be inactive, the alkanes undergo photooxidation to give aldehydes and ketones. The presence of 0; is not sufficient since, although the ion is formed by irradiation at 520 nm in oxygen, the photo-oxidation reaction is only promoted for wavelengths < 380 nm, corresponding to the absorption band of T i 0 2 .The suggested mechanism is that the 0; formed initially on the surface traps a positive hole to give a neutral or ionic mononuclear species which then attacks the hydrocarbon. The presence of this latter species is consistent with isotopic exchange (411,413) and photoconductivity (414, 415) experiments, which indicate 0 - can be formed on the surface. However, they do not necessarily confirm the above mechanism and it may be regarded as more likely that 0 - can be formed by the trapping of positive holes on lattice oxygen ions of low coordination. However, lattice ions are not thought to be involved in the initial oxidation step of alkanes. The oxidation of alkanes to form alcohols over TiO, has been reported (416,417).This mild oxidation under irradiation is thought to involve lattice oxide ions, whereas oxidation to form acetone, which is also observed, required an adsorbed oxygen species. There is no clear indication of the actual forms of the oxidizing species. Kubokawa and co-workers have studied the interaction of alkenes with porous Vycor glass (410) and TiO, (418)under irradiation. The hole center, 0 - ,is thought to be involved in the breaking of the C=C bond over T i 0 2 , but the mechanism is not clear (418). On the Vycor surface (410)it is suggested that the species 0; formed at sites of low coordination is the oxidizing species in the photo-oxidation of olefins. The olefin is thought to be dehydrogenated by the short-lived 0; species to form a n-ally1 species, which is then either oxidized further or dehydrogenated to give a diene. The g values of this species suggest that it probably is not a normal ozonide-type ion but may be a more complex species. This is supported by the observation that the lattice oxygen does not exchange, although the 0; ion apparently promotes the exchange of gas-phase oxygen. The oxidation mechanism is similar to that suggested by Takita et al. (403) in the preceding section for the thermal oxidation of olefins by 0;. Preliminary evidence indicated that a transitory and reactive 0; could be active in photooxidation over ZnO and possibly also T i 0 2 . However, the oxidation pro-

108

M. CHE AND A. J. TENCH

ducts are not usually the same for TiO, and the extent of hydroxylation of the surface is important. 4. Oxidation of Alcohols

The mechanism of photo-oxidation of alcohols over TiO, is not certain. Bickley (206, 405) postulates that the oxidation of isopropanol occurs via attack by 0; to form an aldehydic intermediate, which is then further oxidized by H202or OH. to form acetone; whereas Formenti and Teichner (406)suggest that oxidation occurs via a mononuclear oxygen species which at higher temperatures may involve activated lattice ions (417).This latter suggestion is very similar to the work on UV formation of 0-ions on the surface in a variety of oxides which have been reviewed previously (1).

5. Miscellaneous Reactions The photo-oxidation of CO to form CO, has been reported over a number of oxides (405,406 and references therein) and the adsorbed CO is thought to react with 0-on the surface. The 0-ions are probably formed by the reaction of positive holes with lattice 0'- ions. Alternatively, as discussed earlier (Section VI,C,3), they may arise from the reaction of 0, with the positive holes. Volodin et al. (419) suggest that the photo-oxidation of CO over TiO, goes through an 0;intermediate to form a C0;-type complex. There must be some doubt as to the actual species involved since the g values of g1 = 2.046 and g2 = 2.008 are not typical for the 0;species as discussed in Section V,A. 6. Conclusions

In the majority of cases, the oxidizing species involved in the photooxidation reactions is not well known because the nature of the species is inferred from indirect experiments. The 0; ion has been invoked as the precursor of the oxidizing species in a number of reactions because it is often the only species observed at room temperature using EPR. However, a variety of other species such as 0-and 0;have been identified on surfaces when the low-temperature photoreactions are observed by EPR. In addition, 0- formed on the surface may have a very short lifetime (as discussed in Ref. I, p. 93) and can only be detected by its reactivity. With these points in mind, we conclude that 0-, and particularly 0;in the presence of excess oxygen, may play a much more important role in photocatalysis than has yet been generally realized. In this connection, the paper of Kubokawa et al. (410) is of particular importance; but the use of "0-labeled oxygen is necessary to confirm the nature of the species involved. In order to explore

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

109

the photo-induced reactivity more thoroughly, future experiments should be designed so that the photoreactions of the adsorbed species can be followed in situ by spectroscopic techniques, such as EPR or reflectance spectroscopy, during the irradiation.

VII.

The Relation of Mononuclear Surface Oxygen Species t o Electron Spectroscopic and Catalysis Studies

In principle, XPS offers the possibility of distinguishing adsorbed oxygen in different forms on the surface but in practice this information has proved difficult to obtain because of the relative insensitivity of the O(ls) binding energy to the environment of the oxygen. Oxygen adsorbed on metals to form lattice 0’-ions is characterized by O(1s) values of about 530 eV depending on the substrate (298, 420); for example, on Ag it is 528.3 eV (420,421)and on Zn (OOOl), 530 eV (422).It would seem likely that there are some differences between the bulk 0’-ions and those formed by dissociation on a metal surface but they are not easy to quantify (420).The evidence for 0-from this type of study is much more doubtful, although it has been suggested on a number of systems (422, 423). The difficulty is that peaks close to 532 eV which might be attributed to oxygen ions of lower charge will coincide with those from hydroxyl groups, as shown by UPS studies of the interaction of wet and dry oxygen with Zn and Cd surfaces (424). Haber (409) has also reported more than one oxygen peak from ZnO samples. The formation of molecular oxygen ions on an oxidized Ag surface has been suggested (421), with an O(1s) binding energy of 532 eV. In view of the difficulties described above, it is clear that hydroxylation of the surface is a major problem in all these studies. XPS has been used by Inoue and Yasumori (4251)to study the surface of MgO, CaO, and BaO after exposure to water vapor followed by in situ dehydration at various temperatures. Signals attributed to 0’-lattice ions were observed after all treatments with peaks at 531.6 (MgO), 529.6 (CaO), and 528.5 (BaO) eV. Immediately after hydration, a second series of peaks at 533.7 (MgO), 532.2 (CaO), and 532.1 (BaO) eV were observed, which progressively disappeared as the dehydration temperature increased; these were assigned to OH- ions. In addition, peaks at 532.1 (CaO) and 530.6 (BaO) eV which appeared during dehydration were assigned to 0-ions formed from the following reaction: 2 0 H - + 0-. . . 0-+ H*

However, although the assignments to 02-and OH- are reasonably clear, the assignment to 0-seems much less certain. For CaO, the peak cannot be

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M. CHE AND A. J. TENCH

clearly distinguished from that of OH- and at higher temperatures of dehydration the hydrogen-bonded OH- groups have largely been desorbed, leaving the more isolated groups; these could well be characterized by a slightly different O(1s)binding energy. In the temperature range 833- 1063 K, lattice 0’- ions in sites of low coordination become increasingly prevalent ( I ) ; these will also contribute to the XPS spectrum and could show a slight shift from the normal 0’- lattice ions because of their lower lattice stabilization energy. At the highest temperatures the peaks attributed to OHbecome very small for MgO and CaO, but this is accompanied by a substantial decrease in the specific surface area, which must also lead to a decrease in the number of surface groups. In conclusion, although both the 0,- lattice and the OH- on the surface are well characterized by XPS,the evidence for 0- from XPS and UPS must be treated with considerable caution since it has proved very difficult to correlate the details of the O(1s) binding energy unambiguously with the charge and environment of the adsorbed oxygen and to distinguish clearly when OH- may be present as a contaminant. An important aspect of the reactivity of oxide surfaces is the recent observation that oxidation reactions can be structure sensitive. Using small crystallites of bulk or supported a-MOO,, Tatibouet et al. (42%-d) have shown that the (010) face which catalyzes the dehydrogenation of the C I and C , alcohols into aldehydes in presence or absence of oxygen exhibits Mo=O bonds pointing outwards. The (100) face is found to be bifunctional and promotes both dehydrogenation and dehydration of methanol leading to formaldehyde and methylal, respectively. These reactions take place on 0’ions, which can easily take up a double bond character, and on Lewis acid cations in low coordination, respectively. Similar results have been obtained on the (001)face of V 2 0 5 which contains V=O bonds associated with Lewis acid centers (425e). The oxidation of olefins has also been investigated on a-MOO,supported on carbon: the mild oxidation of propene into acrolein takes place mainly on the (100) face of a-MOO, while total oxidation occurs on the (010) face (425’g). Similar results have been obtained for the oxidative dehydrogenation of 1-butene into butadiene (425h). These results indicate that the same crystalline face does not necessarily exhibit the same catalytic properties with different molecules. Thus, the (010)face of a-MOO, is selective for the formation of aldehydes from alcohols while it promotes essentially the deep oxidation of olefins. It is expected that the studies on structure-sensitive reactions will be made more quantitative using recent methods to determine the number of surface M=O species (42%-j). It should be noted that the earlier observation on the specificity of MOO, crystalline faces in propylene oxidation has been obtained on oriented Moo,-graphite catalysts (425k).Non-structure-sensitive reactions have also been reported (425k).

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

VIII.

111

Comparison of Oxygen Species and Their Role in Catalytic Reactions

In this final chapter, the main themes that have been developed in this and the preceding review on mononuclear oxygen species ( 1 ) are drawn together. The present situation on the characterization and reactivity of the various oxygen species is summarized and emphasis is placed on how these developments are reflected in an improved understanding of the role of the various oxygen species in simple reactions. Of particular importance is the extension of this better understanding to what actually occurs in the catalysis of selective oxidation reactions. In addition, these oxygen species are relevant to the behavior of biological and synthetic oxygen carriers, biochemical oxidation processes, and photo-oxidation reactions. The catalytic role of the oxide surface can be seen in terms of forming or providing oxygen in an activated state, which then permits a new reaction pathway characterized by a lower energy barrier, with the other reactants either in the gas phase or as an adsorbed species on the surface. Such reactions may modify both the electronic levels and the surface structure of the oxide, but it should be kept in mind that for a catalyst such modification will reach a dynamic equilibrium in which restoration of electrons and replenishment of vacancies by oxygen must balance their removal by reaction products. In this sense, many of the model systems studied are unrealistic since the changes to the surface are irreversible.

A. CHARACTERIZATION

The identity of a number of different oxygen species has been discussed during the course of the two reviews. In general, the main body of direct evidence on their nature has come from experiments which have been designed to stabilize the various species in a well-defined environment; for example, at 77 K on an MgO surface. This is far removed from the situation in many catalytic reactions which occur above 300 K on complex oxides. However, oxygen species have also been identified under conditions closer to the real situations. The different oxygen species will now be discussed in three main groupings, mono-, di-, and poly-oxygen species. Both their identification and the nature of their interaction with the surface will be considered. 1 . Mono-oxygen Species The mono-oxygen species on the surface fall into two main types: 0-ions formed either by adsorption or from surface oxide ions of the lattice and 02-

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ions of the lattice either as lattice ions in low coordination and/or bonded to transition metal ions (M=O). In general, the 0- ions are not very stable, although they have been identified after treatment at 300°C in oxygen on the V,0s/Si02 system (106). All the direct evidence for 0-comes from EPR studies and both g tensor and hyperfine data are consistent with an unpaired spin density of close to 1.0 on the oxygen ion. In a number of systems (Table I1 of Ref. I), an additional interaction with a single cation at the adsorption site is also observed. These data conclusively establish that 0-exists on oxide surfaces. It is likely that additional experimental information on 0-in systems not suitable for study by EPR, such as NiO and other transition metal oxides, will emerge from the application of UPS and XPS. It has been suggested that a related species 0:-can be formed on the surface by 3 0 - ions in a triangular array (354). This is speculative and the evidence for an oxygen ion complex of this structure is very weak. Oxide ions, 02-,exist as a bulk lattice species where they are stabilized by the Madelung energy. Chemical and IR evidence (Section VI of Ref. I), together with optical absorption and excitation data for the alkaline-earth oxides, is consistent with the presence of oxide ions in low coordination on the surface. Although some approximate calculations for ions on different crystal faces (426, 427) support these assignments, no good theoretical treatments are yet available. In light of this, we conclude that the existence of these low-coordination ions on the surface is strongly suggested but not yet conclusively proven. A related species M=O has been widely characterized using IR spectroscopy (1).Selective oxidation has been associated with particular regions of bond strength but this approach would seem to be a simplification. Indirect information about the M=O species is given by EPR studies of the reduced cation (M) or by irradiation to form the 0ion. There would appear to be strong evidence for the existence of the M=O species on the surface, however, its exact characterization and the relation between the IR stretching frequency and bond character appear to be qualitative at present (Appendix C). At this time, there is considerable room for an improved theoretical treatment of the interaction with the surface to provide a more exact interpretation of the IR data.

2. Dioxygen Species A range of different dioxygen species have been considered as adsorbed species on the surface, including triplet oxygen, singlet oxygen, 0;, and O;-. Of these, by far the best characterized is 0,.There is no evidence so far for the presence of the 0:-and 0;species on the surface. Triplet oxygen has not been well characterized on the surface, although it is generally accepted as the form in which gaseous oxygen is physisorbed.

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It seems likely that physically adsorbed oxygen has the same EPR spectrum as gaseous oxygen (see Section 11) and there is some indication of a dimerization to give 0, at low temperatures. The formation of singlet oxygen has been suggested on some oxide surfaces, but the evidence is indirect and not definitive. The possible formation of singlet oxygen on surfaces during catalytic reactions has not been considered seriously by workers in the field. This is a relatively new area which deserves more study during the next few years since singlet oxygen, because it is an excited state, is expected to be more reactive than triplet oxygen and to lead to a different stereochemistry because of its two outermost electrons in the same orbital (Fig. 1). The superoxide ion, O;, has been identified by EPR on over 30 different oxide systems and is the best characterized of the surface dioxygen species. In the majority of systems, the EPR data can be described by a simple ionic model (Section III,A,l,a) where an electron is transferred to the adsorbed dioxygen; in the systems the value of gz, can be used to give information on the magnitude of the local crystal fields at the adsorption site. The ionic model corresponds to a situation where energy of the bonding orbital of the cation is much higher than that of the corresponding orbitals in the dioxygen and an electron is donated to the dioxygen. However, if the energy of the orbital of the donor cation which contains the unpaired electron is close to that of the 2pn orbitals of the dioxygen, then a spin pairing model gives a better description of the bonding (Section III,A,l,b). This approach has been developed to explain the EPR data for the cobalt adducts and other related systems and corresponds to a fractional electron transfer. The dioxygen nature of the species has been confirmed using ”0 and it exists on the surface in two different orientations: either side-on, i.e., the “Grifith” model, or end-on, i.e., the “Pauling” model. In the former, the two oxygen nuclei are equivalent and in the latter, inequivalent (Section 111,A). In all cases, apart from some cobalt oxygen adducts, the dioxygen is adsorbed at a single cation site. Infrared measurements have now been carried out on a number of oxides both supported or unsupported and there is increasing evidence that the adsorption perturbs the oxygen molecule sufficiently to make it infrared active (see Section 11). However, there are very few systems where both IR and EPR data are available. Earlier attempts to use IR spectroscopy to identify adsorbed dioxygen species by comparison with solid-state and gas-phase data for O i , 0; , and 0:- have led to a number of conflicting assignments in the literature. These arise for two principal reasons; first, the IR bands for the dioxygen species adsorbed on the surface often lie at frequencies intermediate between those expected for the individual species on the basis of the solid-state and gasphase data, and the data for different systems tend to cover a range of frequencies (Appendix C ) ;second, there is considerable overlap between the

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114

IR bands expected from M=O species about 900-1100 cm-' and those from 0;at about 640-970 cm-'. Monatomic and diatomic species can be distinguished in principle by 160/'80 isotopic studies, but this has been applied only to a few of the systems and in practice, the results are not always easy to interpret because of other changes in the spectrum. The observed frequency shifts have led a number of authors to assign the bands to perturbed oxygen species, although the selection of which species is perturbed is often arbitrary (Section IV,B). In our opinion, it would seem more realistic to consider that a continuous range of dioxygen species can exist on the surface, since bands are observed from 640 to about 1700 cm-' corresponding to dioxygen species ranging from 0;-to those with some 0: character. This can be made more quantitative by considering a plot of the stretching frequencies obtained from the solid state for the various dioxygen species versus the bond order (Fig. 27). A comparison of the

't

I800 m r

200

FIG.27. A plot of the voo bond frequency for dioxygen species in the solid state as a function of the bond order. The value for 0, refers to the gas phase value for oxygen in the triplet state (21).

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

1 I5

frequencies attributed to surface diatomic species with this plot indicates that many of these are intermediate between the well-defined oxygen ions observed in the solid state. It is likely that those systems where the EPR data is consistent with the spin pairing model would also correspond to oxygen species with an intermediate bond order. In this view, the distinction between the various species becomes blurred and the actual form of oxygen present will reflect the coordinative environment and may correspond to dioxygen with a fractional charge. These ideas may be of great significance in understanding the mechanism of catalytic reactions. 3. Polyoxygen Species

The ozonide ion 0; has been clearly characterized by EPR and reflectance spectroscopy. Labeling experiments with "0 indicate that the 0; species contains three inequivalent oxygens forming a bond angle of about 110" and that it decomposes slowly at room temperature to form 0;. A second type of species has been reported as 0; but has very different characteristics, since it is stable only at low temperatures and labeling experiments with "0, which indicate two equivalent nuclei, are difficult to interpret; the balance of the evidence points toward a more complex polyoxygen species (see Section V,A). The data for 0, indicates that it is likely to exist on the surface under special conditions and we expect to see this confirmed by further studies. 4. Summary

In summary, the 0 - , O,, and the normal 0;species can be regarded as very well characterized. The evidence for O&, M=O, and 0;- is less comprehensive, but in each case there are sound reasons for accepting that these species are now sufficiently characterized on the surface to be used with confidence in mechanistic arguments. In addition, it is becoming clear that some polynuclear forms of the oxygen ions can also exist on the surface. However, the case for the presence of some oxygen species such as O:, 0:-, and 0;- is very weak and these species should not be invoked in mechanistic arguments at our present level of understanding. The IR data is consistent with a more complex model of the surface dioxygen rather than the clearly distinct species assumed above. In this paradigm a continuous gradation of species exists between 0: and 0;and the species observed will depend on the specific environment at the adsorption site. Dioxygen species with fractional charges commonly occur on the surface. This, taken with the fact that nearly all spectroscopic measurements have been made below 100°C, lends support to the idea that some

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of the species may either not be readily distinguishable from each other or may easily transform at the temperature of catalytic reactions. B. REACTIVITY In this review, we have concentrated on the surface oxygen species which may be formed by y, photo-, or thermal activation. In general, the simple reactions that have been studied are once-through reactions, in which the oxygen species are consumed but not renewed. This is in contrast to what must happen in a catalytic oxidation reaction, where the oxygen must be replenished continuously. In the following paragraphs, the reactivity of the different oxygen species is discussed in terms of those systems where the ionic model is applicable; little evidence is available for systems where the spin pairing model is more appropriate but, in general, the oxygen is held less strongly. 1. Simple Reactions-Qualitative

Studies

The 0-ion is much more reactive than either 0;or 0;and the M=O species reacts as 0-when y- or photo-activated (see sections on reactivity in Ref. I). Hydrogen reacts rapidly and irreversibly with 0-even at low temperatures to form OH- groups on the surface and also with 0:; on some systems, but not with 0;. Oxygen reacts with 0-to form 0;on many different oxides in a reversible reaction (I). This reaction is thought to account for isotopic exchange reactions in which the isotopes can be scrambled among all three nuclei. Carbon monoxide and carbon dioxide react with 0-to form CO; and CO;, respectively, and also react readily with Of;(see Sections V and VI in Ref. I); the reaction products have not been well defined but are presumably carbonate species. 0-reacts readily with alkanes and 0;reacts slowly at room temperature, whereas 0;does not (see Sections V and VI in Ref. I and Section VI in this article). 0- can react with ethylene via hydrogen abstraction or oxygen addition depending on the surface. On MgO, the radical CH,=C'- . . . HOhas been identified as a reaction product, whereas on MoO,/SiO,, CH,CH,O- is formed and on W03/Si02, the formation of a cyclic . (CH,CH,O-) has been suggested. This brings out clearly that the reaction may be controlled by the environment of the surface when the reactants are the same. In contrast, the 0;species reacts slowly with propylene at room temperature but not with ethylene. On MgO, 0:; reacts with alkenes to abstract a proton forming the corresponding carbanion.

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TABLE XIV Stoichiometric Reactions of Oxygen Ions with Hydrocarbons on MgO”

Ion

Reactant

Intermediate

Major products

0-

C,-C, alkanes C,, C, alkenes C, alkenes C,-C, alkanes C,, C, alkenes C, alkenes C, alkane

Alkoxide ionsb Carboxylate ionsb Alkoxide ions: carboxylate ionsh Alkoxide ions,b peroxy radicalsb Carboxylate ions‘ Alkoxide ions,” peroxy radicalsb Alkoxide ionsh

Alkenes, CO, CH4, C,H4, CO2 Butadiene, CH,, CO, Alkenes, CH,, CO, CH,, CO, Butadiene, CH,, CO, Alkenes, CH,, acetone, acetaldehyde,

C, alkene C, alkene C, alkenes

Carboxylate ions‘ Carboxylate ions,‘ (epoxide)b Carboxylate ions,‘ (epoxide)*

CH4, CO2 CH,, acetaldehyde, methanol, CO, CH,, 2-butanol, CO,, butadiene, acetaldehyde

0; 0;

co2

From Ref. 393. The carbon number is the same as that of the reactant. The carbon number is smaller than that of the reactant because of scission reactions.

2. Simple Reactions-Stoichiometric

’

Studies

A thorough comparative study of the reactivity of 0 - , O;, and 0; toward alkanes and alkenes on MgO has been carried out by Lunsford and co-workers (see Sections V and VI in Ref. 1 and Section VI in this article). In these stoichiometric studies, a known concentration of oxygen species on the surface was reacted with a known amount of hydrocarbon in a recirculation reactor and the products were analyzed. In all cases, a 1 : 1 stoichiometry was observed. The most significant observation is the large differences in reactivities of the three forms of oxygen ions, with 0 - >> 0, >> 0;. This is well illustrated by the reaction with ethylene where 0- ions react readily at -60°C 0, ions react at 25°C with a half-life of ca. 5 min, whereas only one-third of the 0; ions react after 2 hr at 175°C.The authors propose a number of surface intermediates (Table XIV) in the oxidation reactions based on analysis of the desorption products and IR studies. A number of generalized comments on the reactivity can be made for the MgO system. (a) In all cases, the principal initial reaction appears to be the abstraction of a hydrogen atom from the hydrocarbon by the oxygen, followed by subsequent surface reactions which may involve oxide ions of the surface.

1 I8

M. ('HE AND A. J . TENCH

(b) The intermediates for the oxidation of the alkanes always include alkoxide ions independent of the oxygen species involved. This probably reflects the stability of the alkoxide ion on the MgO surface. (c) Carboxylate ions are thought to be the intermediates in the reactions of C2 and C3 alkenes but the type of carboxylate ion formed with 0is different from that formed with 0; and 0; ; in the latter case, there is a scission of the C=C bonds following the initial step of hydrogen abstraction.

No similar comparative studies have been carried out using other supports and so the reaction behavior cannot be assumed to be general and probably, in part, it is controlled by the specific properties of the MgO surface. 3. Photo-oxidation Reactions

The 0; ion appears to play an important role in a number of photooxidation reactions (see Section VI,C); for example, the photo-oxidation of alkenes over Ti02. However, it seems likely that 0; is not, in many cases, active in the oxidation step but further conversion occurs to give a mononuclear species, not detected directly, which then oxidizes the adsorbed hydrocarbons. Photo-oxidation of lattice oxygen in the M=O systems (e.g., V 2 0 , supported on PVG) gives rise to an excited charge transfer state such as V4+-O-. This excited state can react as 0- either by addition to a reactant molecule or by an abstraction reaction (see Section V of Ref. I ) . In the presence of oxygen, 0; is formed which then reacts further with organic molecules.

4. Catalytic Reactions Selective oxidation by heterogeneous catalysis is of great industrial importance and accounts for no less than 21% of the major organic chemicals produced via reactions involving catalysis (428-431). The oxidation reactions include allylic oxidation to give aldehydes, nitriles, and acids; aromatic oxidation to give acids and anhydrides; epoxidation of olefins; methanol oxidation to give formaldehyde; and to a lesser extent, paraffin oxidation to give anhydrides (429-432). Because of this importance, there has been considerable effort to obtain a better understanding of the mechanisms of such reactions. However, there is only limited knowledge of the way in which oxygen is involved in the overall process. It is generally assumed that in, for example, allylic oxidation, an intermediate formed on the surface is oxidized by a specific type of lattice oxygen of the catalyst rather than an adsorbed oxygen species to form the reaction products such as acrolein (431). However, in both cases the adsorption of oxygen on the surface is of

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vital importance since the oxygen consumed in the reaction, whether from the lattice or from an adsorbed species, must be rapidly replenished. In heterogeneous catalysis, the oxide lattice can have a variety of different functions, but for this review we are concerned with its ability to provide oxygen in a suitably activated state to oxidize the reacting organic molecules to form the required products. From the evidence in the preceding sections on characterization it is probable that the adsorbed dioxygen can lie within a continuous range of species from electron deficient to electron rich, depending on the nature of the oxide and the reaction conditions. and On this basis, a division of oxygen species into electron rich (02-) electron deficient (e.g., 0;) is limited since it assumes that the species can be considered as entities separate from their environment. This approach is contrary to the picture that has emerged from the preceding discussion, where it became increasingly obvious that the nature of the species depends very much on its environment at the surface and that the formal description of a species as 0; does not necessarily accurately reflect its actual charge and bond order when adsorbed on the surface. In addition, it seems that many oxides can form a variety of different oxygen species on the surface depending on the reaction conditions. Bielanski and Haber (2) have divided the metal oxides into three main groups depending on their interaction with gaseous 0, : (a) p-type semiconducting oxides (NiO, MnO, etc.) which form electron-rich species (0-, 02-), (b) a group including n-type semiconductors (ZnO, TiO,, V,O,, etc.) and also dilute solutions of transition metal ions in diamagnetic matrices (e.g., COO in MgO) which form 0; and 0-,and (c) binary oxides where the lattice oxygen is present as 02-in well-defined oxyanions, e.g., Bi203.MOO,, and which do not form adsorbed oxygen species but only 02-ions. This would seem to be an oversimplification, since it seems likely that a range of oxygen species can be observed on all these oxides given the right conditions, although the thermal stability is likely to vary considerably. Oxygen can be involved in oxidation reactions in three distinct ways, more than one of which may be operative in any reaction mechanism. The first is the abstraction of a hydrogen or proton from an adsorbed organic molecule to give a radical or carbanion on the surface; the second is the attack on the organic species by a negatively charged oxygen ion whether lattice oxygen or an adsorbed oxygen; and the third is the replenishment of lattice oxygen which has been used in a direct oxidation reaction. The abstraction reaction appears to be very common and the preceding evidence shows that it occurs with Of;, 0-, O;, and 0; ions but the reaction with 0- is particularly fast. An exception to this, which at the same time provides strong evidence for the participation of a molecular oxygen species, is the selective oxidation of ethylene over silver catalysts to form

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ethylene oxide (Section IV,D). A dioxygen ethylene complex is formed on the surface but it is not certain whether the precursor is O;, 0;-, or an intermediate between these two forms. This is the most direct evidence available for the insertion of oxygen into a C=C bond via a molecular oxygen species. Comparison with the reactivities measured by Iwamoto and Lunsford (393) on MgO would suggest that addition reactions to alkenes are consistent with 0; as the oxidizing species but hydrogen abstraction also occurs. For ethylene, only CO, was expected as a reaction product but the rate of reaction was slower than with propylene. This may indicate that the precursor of the ethylene complex on silver is a more electron-rich oxygen species such as O : - , where insertion to form a bridged structure is more likely because of the weaker bond strength. But there is no good evidence on the reactivity of this species. The interesting reactions are those involving a highly selective partial oxidation (428, 431), whereas unselective oxidation reactions are of interest only in very limited situations, for example, in the oxidation of car exhaust gases. The selective oxidation reactions, apart from the formation of ethylene oxide discussed earlier, are thought to occur via the Mars-van Krevelen (433) mechanism; the oxidizing agent is an 0,- ion from the lattice which, as it is incorporated into the hydrocarbon, donates its electrons to lattice cations. Such a mechanism is thought to be well established for the oxidation of olefins on Bi, O,/MoO, catalysts ( 4 3 4 , the ammonoxidation of propylene on Bi,O,/MoO, (435) and USb3OIo(436),and the oxidation of methanol on Fe,O,/MoO, (437a). For these reactions to occur the metal-oxygen bond energy of the active oxygen ions, under the reaction conditions, must be in a range where both removal and replacement can occur readily. Reoxidation by oxygen is assumed to lead to the formation of lattice 0,- ions via the simultaneous transfer of four electrons from the cations : 0,

+ 4e-

-+202-

However, in light of the earlier discussions, a stepwise transfer of electrons involving O,, Oi-, and 0- as intermediates might be expected; in this respect, surface potential measurements have proved useful (437b). For example, reoxidation studies on bismuth molybdate catalysts by Brazdil et al. (438)are consistent with a mechanism of oxidation where adsorption and dissociation of the dioxygen occurs before the rate-limiting incorporation step. It seems likely that these processes might be observed directly on this type of oxide if experiments were carried out under the correct conditions, but detailed investigations of these reactions have not been made. In these reactions which involve oxygen ions of the lattice, the actual nature of the intermediate species is not clear. The oxide structure appears to be associated with special defect arrays and there is evidence the lattice

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121

oxygen in different environments is responsible for the different steps in the process (439). There is also the possibility of a thermally activated charge transfer process leading to a reactive 0-species (see Section VII of Ref. 1 ) where the oxygen has a low coordination: o2-0 I1/

0-0

I/

0-M"' -0

O-M(n-l)+-

0

0

/

/

0

At lower temperatures the Mars-van Krevelen mechanism no longer applies. Sancier et al. (440) studied propylene oxidation in the presence of "0,over bismuth molybdate and found that the acrolein product contained '*O and not exclusively l60from the oxide lattice in contrast with results obtained by Keulks and co-workers (441, 442) at higher temperatures. This lower-temperature oxidation must involve adsorbed oxygen in some form but the nature is not clear. It is now accepted that not all these oxidation reactions do involve lattice oxygen (442,443). There are a number of other types of reactions where adsorbed oxygen species rather than lattice oxygen ions are thought to be the principal oxidizing agent. Tagawa et al. (444) have studied the oxidative dehydrogenation of ethylbenzene and concluded that gaseous oxygen forms 0-on the surface which abstracts a j?-hydrogen from the adsorbed complex. Akimoto and Echigoya (232,399,445)have given evidence that 0;ions are involved in the oxidation of butadiene to maleic anhydride over supported molybdena catalysts (Section VI). The authors suggest that the reactivity of the 0; ion and Mo=O are very similar, and that the Mo=O may behave like 0- during the oxidation reaction as indicated above. There is also evidence that 0; and 0-species take part in CO oxidation reactions on V 2 0 5 (365).Yoshida et al. (392)have reported the reaction of 0;on V 2 0 , with propylene and butene to form aldehydes while the lattice oxygen show little reactivity below 150°C. Fricke et al. (107) have concluded from a study of V,O5/P2O5 catalysts that both 0-and 0;lead to nonselective oxidation of butene in this system.

C . FUTURE DIRECTIONS Electron paramagnetic resonance has played a major role in the characterization of adsorbed oxygen species and the use of ''0,has enabled a major advance to be made in the understanding of the nature of the various oxygen species and how they can be bonded to the surface. The use of IR spectroscopy as a technique has tended to be neglected because of the

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difficulty of unambiguous assignment of the bands to the various oxygen species. However, it is applicable to a variety of systems, particularly bulk transition oxides such as the iron group oxides, which EPR cannot easily probe. IR spectroscopy is also capable of providing information on nonparamagnetic species such as O:-. With an improved understanding of how the surface can perturb the adsorbed molecules/ions, IR, EELS, and possibly also resonance-enhanced Raman spectroscopy are likely to play an important role in the future, both in defining the nature of dioxygen ions of fractional charge and in the characterization of polynuclear species. Optical studies will grow in importance, e.g., for the 0;ion, and the related technique of photoluminescence spectroscopy is likely to be applied more widely. Both XPS and UPS have considerable potential but very careful experiments are necessary to improve the interpretation of the spectra. Solid-state NMR of adsorbed oxygen species labeled with "0 has not been reported up till now. If sufficient sensitivity can be obtained, this technique has potential in characterizing adsorbed species and their environment for nonparamagnetic oxides. Of the different oxygen species, the main interest has been in 0-, O;, and 0;.Relatively little attention has been paid to the characterization and reactivity of singlet oxygen, Oi-, lattice or adsorbed 02-species, and most importantly polynuclear species. The work on dioxygen species is likely to be related to the studies of oxygen carriers. Features of special interest in the future on the characterization side are likely to be the detailed geometry of the adsorption site, how the oxygen species is bonded to the surface, and its mobility. The majority of the work covered in the two reviews is concerned with studies at low temperature, whereas the catalytic oxidation reactions occur at elevated temperature. There are some indications that the different oxygen species may not be so clearly differentiated at higher temperature or that interconversion may occur readily. This is a particularly interesting area which deserves further exploration. In order to understand the mechanisms of catalytic oxidation, particularly those where selectivity is good, more work needs to be done with model systems. The stoichiometric reactivity experiments should be extended to other systems using IR spectroscopy as well as EPR to follow the oxygen species. Dynamic experiments at higher temperature on model systems are also required. In the future, emphasis needs to be placed on quantitative experiments where the kinetics are followed so that it is clear what reactions are being studied. Of particular interest in this area is the mechanism by which oxide ions of the lattice are replenished from the gaseous oxygen and how oxygen species are recreated by redox reactions following the oxidation reaction.

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MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

D. CONCLUSIONS In this article we have tried to make a realistic assessment of the present level of knowledge on the characterization and reactivity of various oxygen species on the surface. However, the form of a particular oxygen species in a specific oxidation reaction is yet to be conclusively established. Part of the reason for this is the need to break down any given reaction into its parts, which has often been necessary because of the limitations of the techniques employed, but speculation has all too frequently been accepted as fact. However, particularly the results from careful IR work point toward the need for a more flexible approach in understanding the nature of the species involved and are likely to lead to change in the paradigm presently adopted by many workers in the field. Above all, however, there is a need for a holistic approach to the reaction mechanism which combines both a study of the intermediates by a variety of techniques coupled with an overall analysis of the reaction pathway. It is difficult to combine a general semiempirical approach with specialized characterization but such a synthesis is most likely to lead us to a better understanding of the complex surface phenomena.

Appendix A.

Summary of g,, Values for 0; on Surfaces

~~

Systems

Bulk' NaO, KCI Surfacesd: Oxides MgO

CaO

SrO

gzz

2.175 2.436

2.0623-2.0895 2.0733-2.0779 2.077 2.0777 Range' 2.0623-2.0895 2.089-2.098 2.093 2.10 Range' 2.089-2.10 2.100 2.102 Range' 2.100-2.102

A (eV)*

Reference

0.16 0.06

446 42,69b

0.47-0.32 0.39-0.37 0.37 0.37 0.47-0.32 0.32-0.29 0.31 0.29 0.32-0.29 0.29 0.28 0.29-0.28

160 IS7 72, ISS 68 71 70 IS8 129 72

(Continued)

M. CHE AND A . J . TENCH

124

APPENDIX A (Confinued) Systems ZnO

CdO/A1,03 SnO,

Ce0,/AI,03 CeO,/SiO,

La203

sc,o, TiO,

TiO,/SiO, TiO,/PVG V,Os/SiO, V,O,/P20,/~iO, V,O,/MgO

OZZ

a

A (ev)”

2.042-2.051 0.71-0.57 2.0424-2.0519 0.70-0.56 0.68 2.0436 2.045 0.66 0.60 2.049 2.05 1 0.57 2.052 0.56 Rangee 2.042-2.052 0.71-0.56 2.039 0.76 2.024 1.29 2.026 1.18 2.0265 1.16 2.028 1.09 2.029 I .05 2.033 0.91 Range‘ 2.024-2.033 1.29-0.91 2.054-2.0589 0.54-0.49 2.0185 1.73 2.0312 0.97 2.0246 1.26 2.030 1.01 2.0266 1.15 2.028 I .09 Range‘ 2.0185-2.0312 1.73-0.97 2.035 0.86 2.060-2.12 0.49-0.24 2.055-2.121 0.53-0.24 2.063-2.093 0.46-0.31 2.047 0.63 2.019 I .68 2.020 1.58 2.021 1.so 2.0216 I .45 2.0225 I .39 2.023 1.35 2.0237 1.31 2.024 1.29 2.025-2.030 1.23- 1.01 2.020-2.026 1.58- I . I8 2.0223-2.0305 1.4-0.99 Range’ 2.019-2.0305 I .68-0.99 2.022 1.42 2.023 1.35 2.022 1.42 Range‘ 2.022-2.023 1.42- 1.35 2.026-2.090 1.18-0.32 2.07-2.09 0.41-0.32

Reference 167 80 174c 180 171, 172, I74a 155, 186 I 70 182 74 2?f 184 183, 186 I 70 185 189 191 73, 192 191 182 191 45 182 193 194 194 194 171, 186, 203 90,143 88 137 206 I 70 174c 75, 197 75 198 66

212h 106, 198 107 213, 214 198

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

125

APPENDIX A (Continued) Systems

gzz

V2Os/AI@3 V,Os/ZrO, ZrO, Cr(CO),/SiO, CrOJSiO, CrOJSiO, Mo(CO),/SiO, MoOJSiO,

Range'

Range'

a-Al,O, Y-A1203

Range' Si02-AI,0, SiO, PVG Range'

A (eV)b

Reference

2.024 2.032 2.032 2.017 2.020 2.070 2.017 2.016 2.017 2.0173 2.0176 2.018 2.016-2.01 8 2.017 2.0155 2.0170 2.019 2.0155-2.019 2.039 2.035 2.039 2.070 2.025 2.0266 2.019 2.040 2.026-2.080 2.0300 2.025-2.028 2.062-2.098 2.070 2.124-2.138 2.0167 2.034 2.038 2.039 2.040 2.034-2.040 2.024 2.0250 2.0318 2.0310 2.0250-2.031 8

1.29 0.94 0.94 1.90 1.58 0.41 I .90 2.04 1.90 I .87 1.83 I .78 2.04-1.78 1.90 2.12 1.90 I .68 2.12- I .68 0.76 0.86 0.76 0.41 I .23 1.15 I .68 0.74 1.18-0.36 I .01 I .23- 1.09 0.47-0.29 0.41 0.23-0.21 1.94 0.88 0.78 0.76 0.74 0.88-0.74 1.29 1.23 0.95 0.98 1.23-0.95

2.022 2.032 2.038

1.42-0.94 830 0.78 121

213 I 98 171, 182 219 218 86,447 219,226 224 109 191 85, 108 81. 198 226 191 85. 108 213 226 213 108 108,213 219 61 233 233 233 120 242 110 24 I 110 111 248 111 108 112 254 252. 253 25 I 256

Zcolircs

HY

126

M. CHE A N D A. J. TENCH

Systcnis

ci:'i

A (eV)"

Rcference

AlHY H Mordenik Dehydroxylated I IY

103 I21 104 266

SCY TiY

2.038 0.78 2.040 0.74 2.038 0.78 2.0575 0.51 Alkali and alkalinc earth zeolites: See Tables IX and X 1.01 2.030 I.63 2.0195 I .6l 2.0197 0.74 2.040 0.66-0.56 2.045-2.052 0.9 1 2.033 2.034 0.88 0.67 2.044 1.29 2.024 0.49 2.060* 2.035- 2.0242 0.86- 1.28

103 279 280 280 281 285 26 7 103 105b

1.09- 1.01 I .09-0.50 0.74 0.77-0.19 0.96 1.16 0.82 0.20

56 289 288 290 299 299 299 302

TiA NiCaY Lax

La Y WHY WNaY CeX

105b 102u

Support14 metuls

2.028-2.030 2.028-2.058 2.040 2.0389-2.148 2.03 I6 2.0264 2.0366 2.141

Ag/PVG Ag/SiO, Ag/SiO, Ago Au/PVG RhjPVG Pt/PVG Pt/AI,O, Adducts'

[ CO"~(NH,),O;] ~~:C~Y [C~"'(NH,),~O;CO"'(NH,),]~~:COY [Co"'(CH,N H2),0;]' :COY [CO"~(P~NH,),O;]~+ :Coyu [Co"'(en),0;]2 :CoyB [Ru(C0)O,l4 :RuY Phthalocyaninato Co/AI 0 , y Coadsorbared MgO(C0,-0,) ThO,(CO-0,)A B TiO,(CO-0,)CrO,/SiO,(CO,-0,)MoO,/SiO,(CO,-O,)~ MoO,/SiO,(CO,-0,)HY(CO-0,)Orher systems' AlSb GaAs GaAs NaY +

+

,

2.084 2.072 2.075 2.079 2.084 2.056-2.083 2.098 2.040 2.019 2.040-2.088 2.046 2.046 2.0486 2.047 2.069 2.041 2.035 2.046 2.113

~

-

-

-

0.72 0.86 0.64 0.25

51 115 115 115

I16 83b 250 63 62 62 88 86 87 44 7 89

47 I13 I14 I276

127

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES APPENDIX A (Continued) Systems NaO, in krypton matrices KO, in krypton matrices RbO, in krypton matrices CsO, in krypton matrices SiOO' (bulk)' Polytetrafluoroethylene peroxy' Polypropylene peroxyf

Szr"

2.1106 2.1184 2. I227 2.1069 2.070 2.038 2.035

A (ev)" 0.26 0.24 0.23 0.27 -

-

Reference 44 44 44 44 91 58, 93 84

For each system, the order is given with increasing gzLvalues. Calculated using the simplified equation (6) (Section III,A,I,a); 1 has been taken equal to 0.014 eV (1276) so that comparison with earlier results can be made (3); A has been calculated for 0; ions only. ' Given for comparison. The systems are arranged using the same order as in Section IV. Some systems appear twice whenever the 0; ion can be stabilized either on the supported ion or on the support. g, values used to construct Fig. 3. The 0; ions on these systems do not fit the ionic model. Pr = propyl, en = ethylenediamine.

The choice of gzzvalues used to construct Fig. 3 has been restricted : 0 to oxides (bulk or supported oxides and zeolites) described by the ionic model 0 to gzz values which could be safely assigned to specific adsorption sites on the basis of superhyperfine interactions (see Table V and Section

111,A,3) 0 when superhyperfine interactions were absent, to gzz values which were confirmed independently by several laboratories 0 to gzz values relative to slightly reduced oxides. Stronger reduction usually results in a number of gzz values which are thought to be due to adsorption sites of various low oxidation states, different local coordinations, and/or different crystal planes (Section III,A, 1,a). This has been observed, for instance, in the case of T i 0 2 (20b, 170).

The gzz values were assigned to a given oxidation state of the adsorption site on the basis of spectroscopic and chemical evidence. For transition metal ions, the oxidation state was deduced from reactions of the type M'n+ O2-P Mn+O;, which were ascertained by a decrease in the EPR signal of M'"- l ) + ions and a parallel increase in that of 0; (Section IV). For nonreducible ions, the usual oxidation state has been taken. For the + 1 oxidation state observed only in alkali zeolites there is a large range of gzz values: 2.054-2.166 (Table X),which has been used in Fig. 3. It appears,

M. CHE AND A. J . TENCH

128

however, that the earlier value of 2.1 13 obtained in X-irradiated NaY zeolite (1276) and confirmed in Na-reduced NaY zeolite (272) corresponds to 0; adsorbed on Na' ions because of the presence of superhyperfine interaction due to the nuclear spin of Na (I = 3). Similar gzz values of 2.1 106, 2.1 184, 2.1227, and 2.1069 have been reported for the alkali superoxides NaO,, KO2, RbO,, and CsO,, respectively, trapped in krypton matrices (44).Thus the narrower range of gzz values 2.1 106-2.1227 for the + 1 oxidation state should be preferred. From Appendix A and Fig. 3, it is possible to deduce the oxidation state of the metal ion at the adsorption site and conclude whether the superoxide ion 0; is adsorbed on the supported oxide (or metal) or on the support by comparing gzz values relative to the supported system and to the support.

Appendix B. The Experimental '"0 Hyperf ine Parameters (in gauss) of Diatomic Oxygen Species (0; and ROO')

Systems

Bulk KCI K,S,O* SiOO'

A,,

Ayya

a

61.5 75.7 101.7, 43.2

-

A,,"

'isa

Reference

19.7 14.0 9,9.5

69b 448 91

0

15

-

-

~

9, 9.5

Sur/a:fbces

MgO y-irradiated MgO y-irradiated MgO (Pyridine + 0,) CaO CaO (Pyridine + 0,) SrO (Pyridine + 0,) ZnO SnO, CeO, CeO,/SiO, TiO, (anatase) TiO, (rutile I ) TiO, (rutile 11) TiO,

v,o,/sio, MoO,/AI,O, MoOJSiO, MoO,/SiO, Bi,O,, 3MoOJSi0, Bi,O,, MoOJSiO, IOBi,O,, MoO,/SiO,

71 11 76 17 76 76

~

~

-

-

-

-

-

-

68 334 72 71 70 72

15

80

80

0

80.5 75 75 77 76 72 80.3, 74.8 Notresolved 80.77 85,72 82,69

-

81 85,12 85.12

-

-

74 73 45 75 75 75

-

-

66

-

-

2120 85 82, 84, 85 81 231b 231b 231b

~

-

-

-

-

-

-~

-

-

~

-

~

~

-

129

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES APPENDIX B (Continued) Systems WO,/SiO, SiO, HY y-irradiated HY, HZ y-irradiated NaY NiCaX WHY WNaY CeX CeX Co ammonia adducts in Y zeolite Co amine adducts in Y zeolite RuY Pd mordenite MgO(CO2 + 0 2 ) ThO,(CO + 0,) A ThO,(CO + 0,) B TiO,(CO + 0,) TiO,(RCH,OO') TiO,( RR'CHOO') CrO,/SiO,(CO, + 0,) MoO,/SiO,(CO, + 0,) MoO,/SiO,(CO, + 0,) HY y-irradiated (CO + 0,) Organic peroxy Tetralin peroxy 1

I1 Triphenyl methyl peroxy 120 K 300 K Polytetrafluoroethylene peroxy (chain radical) 77 K 300 K (propagating radical) 71 K 300 K Polypropylene peroxy Enzyme Protein y-irradiated

FOO' CF,C000' (CH,),COO' ROO' C,H,(CH,),COO'

4,"

Ayya

aiaaa

Reference

74 77 82,63 84.5, 64.2 76 80 83 75 78 66 80.60 72 80,67 77 100, 50 95,65 75 104.42.5 95.35 94, 36 98,42 104,40 101,39.5 107, 37

61 255 81 83a 76 283 105b lO5b 102a 102a 51

13, 5

116 83b I52 63 62 62 88

-

90 90 86 87 447 89

87,59 88,60

1

91.61 40.29

}

449

107.46 89,40

}

58,93

107,46 98.60 72 68.46

26.5, 13

22.17, 14.5 23.3, 14.0 21.8, 16.4 23,18 21.8, 16.4

1

92

j8

84 100 101 450 45 1 946 452 94b

(Continued)

130

M. CHE AND A. J. TENCH

APPENDIX B (Continued) ~~~

Systems

Ax,

Inorganic peroxy' Bu'O(Ph,),AsOO' 93 K Bu'O(Me),AsOO' frozen solution Bu'O(OMe), POO' frozen Bu'O(Ph),POO' frozen 153 K Co oxygen carriers 71 K

A,,"

4,"

alsoa

Reference

91.61 86.5, 64.4 24.4, 18.6 76.2, 69.3 85.70 24.5, 15.5 88,60

300 K

4530

21.6

49 1026

~~

Only the absolute value of the hyperfine tensor is given; for the problem of the sign and the possible presence of a motion refer to original papers and discussion in Section III,A,2; for equivalent oxygen nuclei only one value is given. The systems are arranged using the same order as in Section IV. Bu' = tert-butyl; Ph = phenyl; Me = methyl.

Appendix C.

Characterization of Oxygen Species by Infrared Spectroscopy*

Although EPR has turned out to be the most important technique used so far in the characterization of adsorbed oxygen, there are a number of cases where it cannot be applied, for instance when paramagnetic oxygen radicals lead to linewidths broadened beyond detection or when the species are diamagnetic. In such cases IR has proved very useful, although the identification of the adsorbed species is not straightforward since the IR frequency can vary over a wide range. The various vibrational frequencies involving adsorbed oxygen are listed in Table XV together with those related to various model systems (gas phase, oxygen carriers, solid state and matrix isolated species), while Fig. 28 gives the frequency ranges of the oxygen species observed for the various sytems. The coordination of oxygen to transition metal ions which occurs mostly in the side-on fashion on surfaces (Section III,A,2 and Appendix B) can be described following the model of acetylene-metal complexes (467). Both nu and ng orbitals of molecular oxygen have proper symmetry to interact with the bonding set of s, p, and d orbitals on the metal. The bonding orbitals are shown in Fig. 29.

* See Ref. 4536.

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

TABLE XV Selecird Vihrarional Frequencies ,for Species lnuolving O.xy(jrti Bods

Species Gas phase ('AJO, ("z, )O,

voo (cm

Reference

~

1483.5 1555 1586.I - 1596.6 1876

21 226 454

780-884

148

800-932

148

M O (superoxide-like) 0

1075-1122

148

0

1130-1 I95

148

738-794 800-900 900-1100 1137-1164 1825-1864

1496 1 and Refs. therein 1 and Refs. therein 21,132,455 456,457

(OdZ

0: Oxygen carriers O M

/ \ / M O (peroxide4 ke) 0 0

24f'

\ / M (peroxide-like) O M

/ k. /

9

I M (superoxide-like) Solid state

0: ~

M-0-M M=O 0; 0: Matrix isolated species 0 0

983

315

990-11 I5

127a, 133a-c, 458

\ / M

(peroxide-like) 0; (alkali and silver superoxides) Adsorbed species 0;M-0-M M=O 0; "Neutral" 0,

640-970 750-900 900-1 100 1015-1180 1460-1700

459-462 463-465 24e, 134d, 465 19a,b, 206, 24e, 134e, 463 20a,24d,e, 466

131

M. CHE AND A. J. TENCH

132 adsorbed oxyqen

‘

M-0-M

;-

1

MzO

L

1

0-

0

I

p e r t u r b e d 05 i

1

matrix i f o l a t e d l

n

I

n e u t r a l o2 1

02

species

1

I

M-0-M

solid

M= 0

’

i

oi

0;

oxyqen corriers M

(solution)

9-P

FIG.28. Infrared frequencies for species involving oxygen bonds (4536).

A D bond is formed by transfer of electron density from a filled dioxygen nu bonding orbital to s, p, and d orbitals of appropriate symmetry on the metal and two n bonds are formed by transfer from filled metal d orbitals into unfilled n g antibonding orbitals of dioxygen. This synergic bonding mechanism involves the drift of metal electrons (referred to as “n back-bonding”) into oxygen orbitals, thus making 0, as a whole negative, and at the same time the drift of electrons to the metal in the D bond, thus making O2 positive. The combination of both the D donor and n acceptor effects (relative to 0,) may lead to a large range of IR frequencies since the simple donation from dioxygen to metal will increase the voo stretching frequency as it does for CO and NO on Lewis acid centers, while the n back-bonding from metal to dioxygen will decrease the voo frequency (18b).

L FIG.29. The orbitals involved in the bonding of dioxygen to transition metal ions.

MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

133

The energy of the metal orbitals determines the extent of electron transfer between metal and oxygen (134c) and depends on the oxidation state of the transition metal ion, its coordination number, and the donor properties and strength of bonding of its ligands. For instance, the 71 back-bonding is expected to decrease when the oxidation state of the metal increases, e.g., when there is a decrease in the number of d electrons available (186).Similarly, the II back-bonding decreases on decreasing the coordination of the metal bonded mainly to 0 - donor ligands (468).It also decreases when the electron acceptor character of the ligand attached to the metal increases (1344. The adsorption of molecular oxygen on an oxide involves (in most cases) an oxidation of the metal with a concomitant reduction of the adsorbed oxygen. This arises from two factors: (1) Oxygen adsorption on oxides is observed mainly for oxides with metals in a reduced oxidation state, and (2) there are readily accessible reduced oxidation states for oxygen, i.e., 0;and 0; -. Thus, depending on the number of electrons involved in the adsorption, the nature of the oxide, and the relative energies of the metal valence s, p, and d orbitals and the dioxygen II, and n, orbitals, it is possible to envisage the transfer of none, one, two, or more electrons from the reduced oxide to the coordinated dioxygen moiety leading to the formation of O,, O i - , or to dissociative adsorption giving rise to M=O or M-0-M species. The IR data observed for oxygen adsorbed on oxides depend on a number of factors, such as the nature of the oxide (204. the pretreatment conditions (20b),and the temperature of oxygen adsorption ( 1 4 9 4 and this results in a wide range of frequencies. Although Drago has pointed out that the IR data should be used with caution (309),it is possible to give a reasonable assignment of the IR data for adsorbed oxygen by comparison with IR frequencies observed for the model systems given in Fig. 28. However, the similarity of the frequency values for example for M=O bonds and 0-0 bonds in certain dioxygen species shows that it is difficult to distinguish between mononuclear and molecular species. The nature of adsorbed oxygen can only be inferred from careful interpretation of experiments using 160/'80 isotopic mixtures. It is also important, whenever possible, to associate other techniques such as UV-visible or EPR in order to unambiguously identify the nature of adsorbed oxygen species. The data given in Table XV and Fig. 28 for adsorbed oxygen have been confirmed by experiments using l60/l8O isotopic mixtures in a few cases only (24d,e, 134d,e, 464) and should be used with caution. For instance, bands observed at 960-1200 cm-' were reported on SnO, and assigned to adsorbed 0;(469),but later experiments with "0-enriched oxygen did not confirm this assignment (464).Similarly, a band at 985 cm-' on Cr,03 was attributed to 0;- ( 1 4 9 ~ )Subsequent . work using isotopic labeling with

I34

M. CHE AND A. J. TENCH

160/’80 (134d)has not confirmed this assignment and it is more consistent with a species such as CFO. The vibrational data of adsorbed dioxygen have recently been reviewed by Busca ( f 8 h ) .

ACKNOWLEDGMENTS The authors acknowledge the facilities provided by AERE Harwell and UniversitC de Paris VI during the writing of this review; A. J. Tench acknowledges an appointment as Associate Professor at the Universitt de Paris VI and M. Che Vacation Associate appointments at Harwell. They are also very grateful to a number of people for helpful discussions, and in particular to Dr. C. B. Amphlett for encouragement and comment. Finally, they appreciate very much the help and support of their families during the writing of this review. The authors wish to dedicate this review to the memory of Jiiri Kukk, Estonian Professor of Chemistry, who died in a Soviet labor camp on March 27, 1981 at the age of 40.

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MOLECULAR OXYGEN SPECIES ON OXIDE SURFACES

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398. Lyubimova, 0. l., Kotov, A. G., and Pshezhetskii, S. Ya., Kinet. Katal. 13, 1603 (1972). 399. Akimoto, M., and Echigoya, E., J. Chem. SOC.Faraday Trans. 173,193 (1977). 400. Parkes, D., J. Chem. SOC.Faraduy Trans. 168,613 (1972). 401. Takita, Y., and Lunsford, J. H., J. Phys. Chem. 83,683 (1979). 402. Aika, K., and Lunsford, J. H., J. Phys. Chem. 81, 1393 (1977). 403. Takita, Y., Iwamoto, M., and Lunsford, J. H., J. Phys. Chem. 84,1710 (1980). 404a. Ben Taarit, Y., Symons, M. C. R., and Tench, A. J., J. Chem. SOC.Faraday Trans. I73, 1149 (1977). 4046. Kuznicki, S. M., and Eyring, E. M., J . Am. Chem. SOC.100,6790 (1978). 405. Bickley, R. I., Catalysis (London) 5,308 (1982). 406. Formenti, M., and Teichner, S. J., Catalysis (London) 2, 87 (1978). 407. Breakspere, R. J., and Hassan, L. A. R., Aust. J. Chem. 30,971 (1977). 408. Munuera, G.,Gonzalez-Elipe, A. R., Soria, J., and Sanz, J., J. Chem. SOC.Faruday Trans. 176, 1535 (1980). 409. Haber, J., Kosinski, K., and Rusiecka, M., Discuss. Faruday SOC.58, 151 (1974). 410. Kubokawa, Y., Anpo, M., and Yun, C., Proc. Inr. Congr. Catul. 7th, 1980 B, 1170 (1981). 411. Tanaka, K., J. Phys. Chem. 78,555 (1974). 412. Tanaka, K., and Miyahara, K., J. Phys. Chem. 78,2303 (1974). 413. Courbon, H., Formenti, M., and Pichat, P., J. Phys. Chem. 81,550 (1977). 414. Herrmann, J. M., Disdier, J., and Pichat, P.,Proc. Int. Vuc. Congr., 7th. Int. Conf. Solid Surf, 3rd 2,951 (1977). 415. Herrmann, J. M., Disdier, J., and Pichat, P., J. Chem. SOC.Faruday Trans. 177, 2815 (1981). 416. Formenti, M., Juillet, F., and Teichner, S. J., BUN. SOC.Chim. pp. 1031, 1315 (1976). 417. Djeghri, N., and Teichner, S. J., J. Catal. 62, 99 (1980). 418. Yun, C., Anpo, M., Kodama, S., and Kubokawa, Y . , Chem. Commun.p. 609 (1980). 419. Volodin, A. M., Cherkashin, A. E., Zakharenko, V. S., React. Kinet. Catal. Lett. 11,277 (1979). 420. Wandelt, K.,Surf Sci. Rep. 2, 1 (1982). 421. Roberts, M. W., Sci. Prog. (Oxford) 68,65 (1982). 422. Au, C. T., Roberts, M. W., and Zhu, A. R., Surf. Sci. 115, LI 17 (1982). 423. Brundle, C. R., and Metcalf, L. P., J. Chem. SOC.Faruday Trans. I1 75, 1030 (1979). 424. Law, D. S., Lee, E. P. F., and Potts, A. W.,J. Chem. SOC.Furuduy Trans. 1178, 2101 ( I 982). 425a. lnoue, Y., and Yasumori, I., Bull. Chem. SOC.Jpn. 54, 1505 (1981). 4256. Tatibouet, J. M., and Germain, I. E., J. Chem. Res. ( S ) p. 268 (1981). (M) 3070 (1981). 425c. Tatibouet, J. M., Germain, J. E., and Volta, J. C., J. Catal. 82,240 (1983). 425d. Tatibouet, J. M., and Germain, J. E., J. Catal. 72,375 (1981). 425e. Tatibouet, J. M., and Germain, J. E., C.R. Acad. Sci. Ser. 11296,613 (1983). 425’ Volta, J. C., Desquesnes, W., Moraweck, B., and Tatibouet. J. M., Proc. Int. Congr. Catal., 7rh, 1980, E, 1398 (1981). 4258. Volta, J. C . , Forissier, M., Theobald, F., and Pham, T. P., Discuss. Furaday SOC.72,225 (1981). 425h. Phichitkul, C., Tatibouet, J. M.. and Germain, J. E., to be published. 425i. Murakami, Y., lnomata, Y.,Miyamoto, A., and Mori, K., Proc. Int. Congr. Catul., 7th. 1980, E, 1344 (1981). 425j. Miyamoto, A., Ui, T., and Murakami, Y., J. Carul. 80, 106 (1983). 425k. Volta, J. C.. and Morawek, B., Chem. Commun.p. 338 (1980). 4251. Marques, A. R., Davignon, L., and Djega-Mariadassou, G., J. Chem. Soc., Faraday Trans. 178,598 (1982). 426. Levine, J. D., and Mark, P., Phys. Rev. 144,751 (1966).

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427. Garrone. E.. Zecchina. A., and Stone, F. S.. Philos. M o q . 42, 683 (1980). 42X Gates. B C.. Kat7er. J . R., and Schuit. G. C . A , , “Chemistry of Catalytic Processes.”

p 325. McGraw-Hill, New York. 1979. Grasselli. R . K.. Cherii. Em/. Nrii..s 56, 49 (1978). Grasselli. R. K.. A . A . A . S . h r i u . M w i . . Jtrn. 4/h ( 1980). Grasselli. R. K.. and Burrington. J. D.. A h . Cu/u/.30, 133 (1981). Cullis. C. F.. and Hucknall. D. J.. Coral. (London) 5, 273 (1982). Mars. P.. and van Krevelen. D . W.. Chem. Enq. Sci. Suppl. 3,41 (1954). Sachtler. W. M. H.. and de Boer, N . H.. Proc. In/. Conqr. Ca/ul,.3rd. 1964. I , 252 (1965). Aykan. K.. J . Cu/irl. 12, 281 (1968). Grasselli. R. K.. and Suresh, I). D.. J . Card. 25. 273 (1972). Jiru. P., Wichterlova. B., and Tichy. J., Proc. Inr. Corryr. Coral.. 3 r d 1064, I , 199 (1965). Libre, J . M.. Barbaux. Y., Grzybowska, B.. and Bonnelle. J . P., to be published. Brazdil. J . F., Suresh, D. D.. and Grasselli. R. K.. J . Caral. 66, 347 (1980). Haber. J., and Witko, M., Act,. Chem. Res. 14. I (1981). Sancier. K. M.. Wentrcek. P. R.. and Wise, H.. J . C a r d 39, 141 (1975). Keulks. G . W.. J. Cord. 19, 232 (1970). Keulks. G . W.. and Krenzke. L. D.. Pro(,.In!. Congr. Carol., 6/11.1Y76. 2,806. 814( 1977). 443. Hoefs, E., Monnier. J. R., and Keulks. G . W., J. Cu/al. 57, 331 (1979). 444. Tagawa. T.. Hattori. T.. and Murakami, Y., J . Cnml. 75.66 (1982). 445. Akimoto. M.. and Echigoya, E.. J. Cum/. 35,278 (1974). 446. Bennett. J . E.. Ingram, D. J. E., and Schonland. D.. Proc. Phys. Soc. 69A, 556 (1956). 447. Lipatkina. N. I., Shubin. V. E., Shvets. V. A,, Chuvylkin, N . D.. and Kazansky. V. B.. Kina/. k’u/u/.23, 670 (1982). 448. Reuveni. A.. Luz, Z.. and Silver. B. L.. J . Mir~qnReson. 12, 109 ( 1973). 44Y. Melamud. E . . and Silver. B. L.. J. M a p R c w n . 14, I12 (1974). 450. Adrian. F. J .. J . Clitwr. P h j : ~46, . 1543 ( 1967). 451. Fessenden. R. W., J . Chon. Phjx. 48. 3725 (1968). 452. Fessenden. R. W.. and Schuler. R. H.. J. Chcrn. Phys. 44,434 (1966). 453a. Howard, J. A.. and Tail. J. C., Can.J. Chem. 56, 2163 (1978). 4536. Che, M., Kermarec, M.. Dyrek, K., and Tench. A. J.. Reo. Chim. Miner., in press. 454. Herzberg, G., “Molecular Spectra and Molecular Structure,’’ p. 560. Van NostrandReinhold, Princeton, N.J.. 1950. 455. Creighton, J. A,. and Lippincott, E. R.. J. Chem. Phys. 40, 1779 (1964). 456. Shamir. J.. Binenboym. J.. Claasen, H. H.. J. Am. Chem. Sor. 90, 6223 (1968). 457. Edwards. A. J., Falconer, W. E., Griffiths, J. E., Sunder, W. A,, and Vasile, M. J.. J. Chem. Soc., Dulron Trans. p. I129 (1974). 458. Andrews. L.. J. Phys. Chem. 73,3922 ( 1 969). 459. Metcalfe, A,, and Ude Shankar. S., J . Chem. Soc. Faraday Trans. 176,630 (1980). 460. Gland, J . L., Sexton, B. A., and Fisher, G. B., Surf: Sci. 95, 587 (1980). 461. Sexton, B. A,, and Madix. R. J., Chem. Phvs. Lett. 76,294 (1980). 462. Backx, C., de Groot. P. P. M., and Biloen, P.. Sur/: Sci. 104, 300 (1981). 463. Howe, R. F., Liddy, J. P.. and Metcalfe, A,, J. Chem. Soc. Faradav Trans. 168, 1595 (1972). 464. Harrison, P. G . , and Thornton. E. W., J. Cham. Soc. Faraday Trans. I74.2597 (1978). 465. Zecchina, A,, Coluccia, S., Cerruti, L., and Borello, E., J. Phys. Chem. 75, 2788 (1971). 466. Forster, H., and Schuldt, M., J. Chem. Phys. 66,5237 (1977). 467. Greaves, E. 0.. Lock, C. J. L., and Maitlis, P. M., Can. J . Chem. 46, 3879 (1968). 468. Kozuka, M., and Nakamoto, K., J . Am. Chem. Soc. 103,2162 (1981). 469. Gundrizer, T. A., and Davydov. A. A,. React. Kinet. Caral. Lett. 3, 63 (1975). 42Y. 430. 431. 432. 433. 434. 435. 436. 437a. 4376. 438. 43Y. 440. 441. 44-7.

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NOTEADDEDIN PRCOF The nature of the active species in the heterogeneous epoxidation of ethylene is still the subject of active debate. Using a UHV chamber linked to a high-pressure reactor cell, R. B. Grant and R. M. Lambert [Chem. Commun. p. 622 (1983)] have investigated this reaction on the (1 11) face of a silver single crystal. They conclude that chemisorbed atomic oxygen is the crucial surface species which selectively oxidizes ethylene to ethylene oxide, whereas adsorbed dioxygen plays no direct role in this reaction. This conclusion differs from that obtained in particular by Kilty e/ a/. (293) in the case of supported silver (see Sections IV,D, VI,B, VII, and VII1,B). The EPR spectra of the molecular 0;ion have now been obtained by reacting MO, + 0, (M = Na, K, Rb. Cs) in rare gas and nitrogen matrices [D. M. Lindsay, D. R. Herschbach, and A. L. Kwiram, J. Phys. Chem. 87,2113 (1983)]. Both the g and alkali hyperfine tensors suggest a dominantly ionic product M '0;. The EPR data are interpreted in terms of a model (0, - 0,)structure in which a relatively weak bond connects two equivalent 0, moieties. The EPR spectra do not allow one to distinguish between cis- and /rans-O;, but symmetry restrictions may preclude formation of the cis isomer. The g tensor of 0, differs substantially from those observed for the isoelectronic 25-electron radicals SO;, CIO,, and PO:- (see Section V,B). Finally, the origin of adsorbed oxygen on iron oxides has been further investigated by Borello and co-workers [C. Morterra, C. Mirra, and E. Borello, Chem. Commun. p. 767 (1983)l. It is found that an I R band observable at 1140 cm-' on a-FeOOH (goethite) which undergoes several reversible splittings and shifts on dehydration of the sample is the precursor of similar bands previously observed on a-Fe,O, (haematite) and assigned to adsorbed molecular oxygen (see Sections 11. 1V.B. and Appendix C).

ADVANCES IN CATALYSIS. VOLUME 32

Catalysis by Alloys in Hydrocarbon Reactions VLADIMIR PONEC Gorlaeus Laboratoria Rijksuniversiteit Leiden Leiden, The Netherlands

. . A. Electronic Structure of Alloys: Experimental Aspects . B. The Texture and Surface Composition of Alloys . . . C. Progress in the Theory of Alloys . . . . . . .

. . . . . D. Effects of Alloying o n the Chemisorption Bond Strength , . 111. Particle Size Effects. . . . . . . . . . . . . . I . Introduction.

11. Alloys

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A. Electronic Structure ol' Small Particles: Experimental Aspects and Theory . . . . . . . B. Effects of Particle Size on Chemisorption Behavior . . IV. Mechanism of Hydrocarbon-Hydrogen Reactions . . . . A. Introduction . . . . . . . . . . . . . B. Chemisorption Complexes of Hydrocarbons on Metals . C. Mechanism ol' Skeletal Reactions and Selectivity of Metals D. Particle Size EKects in Catalysis of the H C Reactions. . V. Hydrocarbon Reactions on Alloys . . . . . . . . A. Classification of the Reactions on Metals and General Description of Alloying EKects . . . . . B. Some Particular Alloy Systems . . . . . . . . C. Open Problems in Catalysis by Alloys . . . . . . D. A Speculative Model of Hydrocarbon Reactions on Metals and Alloys . . . . . . . . . . . VI. Conclusions . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . .

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Introduction

Catalysis by metals and alloys plays an important role in industry as well as in laboratory-scale preparations. Catalyzed reactions are usually run at lower temperatures than the noncataiyzed ones and they are also more I49 Copyright 6 1983 by Academic Press. Inc. All rights 01reproduction in any form reserved. ISBN 0-12-007832-5

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selective.’ It is mainly the selectivity aspect that makes catalysis so important, but nevertheless, until recently, very few studies were devoted directly to selectivity. This was so because of the general belief that the problems of selectivity will be elucidated automatically one day, when the problems of activity have been solved. However, this appears to be too simplified an idea. In metal catalysis, for example, the activity of a given metal for a certain reaction is very often determined by the selectivity for the main reaction and for the side reaction of self-poisoningof the metal, and not by its activity in a given reaction only. Research on alloys as catalysts has recently contributed very much to the identification of the factors which determine the selectivity and, by that, the activity of metals. This progress, in particular in the field of hydrocarbon reactions, will be reviewed below. Research on alloy catalysts started in the 1950s with attempts to investigate the role in catalysis of the electronic structure of metals. This research was initiated by several papers of Dowden which, measured by their response in the literature, rank among the most important papers ever written on catalysis. However, it appeared later (for reviews, see 1-5) that two basic ideas, on which the so-called “electronic theory of catalysis” was built up, were not correct. These ideas were as follows : The rigid band theory (RBT) of solids according to which there is a considerable transfer of electrons among the alloy components ; an alloy surface is then a structureless plane with atoms indistinguishable for gas molecules. (2) The idea that to activate a molecule, an electron has to be transferred to or away from it, the main function of the catalyst being to mediate the transfer of electrons among reaction components. Due to the frequent use of inadequate techniques in the preparation and characterization of alloys, very controversial results were obtained. This, together with the failure of the above-noted ideas has led to a certain crisis in alloy research (see 1-5) and a loss of interest in this kind of investigations. About 10 years ago, several sources brought about a renaissance in alloy research : (1) A renewed industrial interest in catalysis by alloys or more generally

by bimetallic catalysts (see, e.g., 6). (2) Considerable progress achieved in the quantum theory of alloys and in the theory predicting the surface composition of alloys (see, e.g., 5 for a review).

’ Note: The ucrioity of a catalyst is usually defined as a rate per unit surface area or per site, measured at standard experimental conditions. The selectiuity is then a (normalized) ratio of rates or product concentrations in different reactions running simultaneously.

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(3) Fundamental investigations demonstrated that by alloying, dramatic changes can be achieved in the selectivity of metal catalysts (1-5, 7-9). It is not our intention to repeat all these results, including those already reviewed (1-3, and therefore only the most relevant and the most recent results will be discussed below. II. A.

Alloys

ELECTRONIC STRUCTURE OF ALLOYS : EXPERIMENTAL ASPECTS

From the various methods to be used to investigate the electronic structure of metals, probably the ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) methods brought forth the information most relevant for catalysis and surface science. These methods are best suited to monitor the changes in characteristics parameters of the d-bands by alloying, and since the most catalytically active metals are transition metals where d-orbitals are the frontier orbitals (Fermi level is cutting the d-band), the interest in these methods is not incidental. Since the pioneering paper by Seib and Spicer (10) convincingly demonstrated that the RBT did not hold, many other papers confirmed this conclusion and helped to create a new picture of the electronic structure of alloys. The main points may be summarized with the help of Fig. 1 as follows: (a) For endothermically formed alloys as well as for weakly exothermic cases, the position of the d-band does not change by alloying (Fig. 1). Due to the physics of the photoemission process, it is approximately the density of states N(E), in the d-band which is reflected by the distribution of the photoemit ted electrons I( E ) . (b) With these alloys the main change by alloying is the narrowing of the d-band (lower 6 ) . This indicates the decreasing overlap of the d-orbitals when the neighboring positions around a given metal atom are occupied by another component of the alloy. This effect-leading to an increase of the local density of states in a certain energy range-may influence the phenomena sensitive to the electron shift from the adsorbates to the metal and vice versa (21). This is, of course, a second-order effect as compared to the effects caused by the changes in position of the d-band (there, where they occur). Notice also that effects due to band narrowing are “collective” or “band” effects (in contrast to “local” or “ligand” effects) in the variations of the electronic structure of alloys. The above-listed conclusions are best demonstrated by the results of several

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EF.O

E

FIG. I . Photoemission from the valence bands of metals and alloys. Intensity of emission as a function of energy. E, is the Fermi energy; 6 is the bandwidth (schematically). Reprinted from Ref. 21.

papers (this is a selective, not a full list of references) which should be mentioned in this context (10, 12-16). It has also been signalized that in systems like Pt-Cu (17), where the position of the d-band does not change by alloying too much and where evidently no transfer of electrons from one component to the other takes place (by the way, there is no unequivocal evidence for such a transfer with any alloy of the group of alloys just discussed), alloying can cause certain rehybridization on one (or both) of the alloy components (Cu). Redistribution of electrons between the orbitals of predominantly s or d character is, of course, observed quite frequently (see Pd-Au, Pd-Ag) (12- 14).

A third conclusion is the following:

(c) Some intermetallic compounds reveal more pronounced changes due to alloying [Ni-A1 (18), Ni-Ga (18), Pd-Zr (19), etc.]: The position of d- (or p-) bands shifts by alloying and simulates changes, to be observed in the spectra of atoms while forming a chemical compound. If the data, conclusion (a) and (b) are considered, it is actually not that surprising that atoms of the solution alloys [discussed under (a) and (b)] preserve most of their individual chemisorption and catalytic properties also in alloys (for review, see 4, 5, 20, 21). It is, of course, more interesting what

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will happen with the intermetallic compounds. Unfortunately, it is too difficult to make these alloys in such a way that these alloys are homogeneous and any clustering of the Group VIII metal is excluded. But if the authors of Ref. 22 succeeded, indeed, in achieving that, their paper would then show that even formation of an intermetallic compound does not completely suppress, e.g., the ability of a Group VIII metal to form single and multiple metal-carbon bonds-a very essential feature of the formation of hydrocarbon chemisorption complexes. The same conclusions with the same remark of caution can also be drawn from the papers on the catalytic behavior of Pt-Sn (without carrier) alloys (23, 24).

B. THETEXTURE A N D SURFACE COMPOSITION OF ALLOYS Very small bimetallic particles on carrier are often X-ray diffraction amorphous and it is not easy to gain any information on their composition and structure. In spite of these difficulties, very important information on the texture of bimetallic particles has been obtained by extended X-ray absorption fine structure (EXAFS) (25-28). Results obtained by Sinfelt et al. (25,26) demonstrated that a very detailed picture can be obtained of, e.g., Ru-Cu catalysts by using this advanced technique, which may shortly appear to be the most important for the study of multicomponent catalysts. In the field of alloy surface composition, both theory and experimental determination achieved much progress in recent years. The present “state of the art” does not, unfortunately, allow one to predict quantitatively the surface composition from the bulk concentrations, but calculations on models allow one to estimate various effects and to make interesting conclusions and sometimes even semiquantitative predictions. The calculations are rather easy and have already been performed for models like (1) the ideal solution model where enrichment is always confined to the outmost layer (29),(2) the ideal or regular solution model with onelayer enrichment, taking into account the difference in atomic radii (strain energy) (30-32), (3) the regular solution model with enrichment spread over n (up to 4) layers (33), and (4) intermetallic compounds (37). For complicated systems semiempirical rules based on the phase diagrams (34) or data on the diffusion coefficient (35)might sometimes be quite useful. The equations binding bulk and surface concentration together have been derived by applying classical or statistical thermodynamics (33, 36, 37), kinetic considerations (5), or the Monte Carlo technique (38). The energy gain due to surface enrichment has usually been calculated by the “broken” model assuming additivity of bond enthalpies per bond pair. However,

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recently a quantum-mechanical estimate was also performed for this parameter and these calculations then allow one to go beyond the approximation of pairwise bonding (see, e.g., 39, 40). The early papers usually assumed that surface concentration can be estimated from the normalized ratio of the Auger peaks (41,42). However, soon it appeared that this had led to incorrect conclusions on, e.g., Ni-Cu (43) or Pd-Ag (44) alloys. In particular, the progress with pure Ni-Cu alloy systems suffered very much from the uncertainties caused by improper procedures. The main point to keep in mind in this respect is that the ratio of peak intensities for metals A and B in alloys related to those in the pure metals, as seen by Auger electron spectroscopy (AES), is given by (45, 47) the following equation :

or, when enrichment is confined to the first layer only, by

where Ni is the fraction of the total signal originating in the ith layer, Nrnetal is the number of atoms in one layer of a pure metal, and X is the A.B molar fraction. Fraction Ni can be calculated either by using a discontinuous Gallon model or a model of continuous attenuation of the signal (45, 47). When applying Eq. (l),independent information on the bulk depth profile of the enrichment is necessary ; or when another method (chemisorption, ion scattering) supplies the information on the surface concentration, Auger spectrometry can serve to make some estimate on this depth profile (43). Sometimes, a form of the depth profile is assumed and roughly checked by Auger spectral measurements at two different electron energies or at two different escape angles (46).A full discussion of these and related problems, as well as a review on the methods available for surface composition determination, is published elsewhere (47). Development in the field of measurements on metals without carrier can now be considered as satisfying and there is still some progress going on. However, the problem of a reliable determination of the surface composition of alloys on carriers is still too far from being solved. In particular, problems like the detection of small amounts of unalloyed active metals on carrier and the question of homogeneity in distribution of the active metals in an inactive matrix have not been solved yet, and just such problems are most likely responsible for some controversies in the results on alloys. More work has to be done in the future in this field.

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C. PROGRESS IN THE THEORY OF ALLOYS The most essential progress from the point of view of application of this theory in catalysis and chemisorption has actually been achieved by the very first papers (48-50), where the so-called coherent potential approximation (CPA) was developed and applied. By means of this, photoemission data were explained in a quite satisfying way and the catalytic research got full theoretical support for some of the ideas introduced in catalysis earlier on only semiempirical grounds ( 3 ) ; namely, individual components are distinguishable for molecules from the gas phase and the alloy atoms preserve very much of their metallic individuality also in alloys-something that was impossible according to the RBT and the early electronic theory of catalysis. It might be of some interest for the catalytic research that in the course of time the CPA theory was further modified and developed and also some alternative approaches were suggested. The most sophisticated versions of theory now also comprise effects like short- and long-range ordering, clustering, etc. This improves the agreement between theory and experimental data on the electronic structure (mainly UPS and XPS data), but does not change in any way the main conclusions mentioned above. The development of the alloy theory is best demonstrated by a selection of papers (51-57) or by a review (58). How was this development reflected by the theory of catalysis on alloys? An early and very important paper (9) discussed the selectivity and activity effects fully in terms of the old electronic theory of catalysis. Another paper (a), which appeared simultaneously with (9), turned attention to the fact that one must also consider effects other than only the changes in the electronic structure. The results on alloys should be rationalized on the basis of two aspects of alloying (8):

( I ) By alloying a metal A is dispersed (more or less, it depends on the type of alloys) in a metal B. If a certain reaction requires a big ensemble of contiguous atoms A in the surface of alloys, this reaction will be suppressed strongly by alloying. This may lead to selectivity changes, if other potential reactions in the system can occur on smaller ensembles or even individual atoms. This is true for systems when B is much less active than A. If both components are active, one has to consider also the possibility that a big ensemble required can be formed by a mixture of A and B. In some cases (Pt/Ir, Pd/Ni, Pt/Re, . . . ?) the mixed ensembles may even be suspected to be more active than the one-component ensembles. In the literature, this kind of effect is called an “ensemble size” effect (1-5). (2) In spite of the fact that the electron transfer among alloy components is much less frequent (or much less pronounced) than envisaged by the RBT,

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and that the individuality of components is preserved to a high extent also in alloys, small effects of alloying in the electronic structure of metals must always be considered. The question to be analyzed carefully is the following : How important are these (often just postulated) changes for (a) chemisorption and (b) a catalytic reaction?

To stress the localized character of chemisorption (a term surface pseudomolecules was introduced at that time), Sachtler introduced for the alloying effects discussed in paragraph (2) a term “ligand effect” (3). It was then a task for an experimentalist to establish how important-relatively-the effects (1) and (2) were. A general consensus now is that effect ( 1 ) is more essential than (2) in any case, but the discussion is still going on, on the reliability of some pieces of evidence which have been presented in the literature in favor of a role for effect (2).

D. EFFECTS OF ALLOYING ON THE CHEMISORPTION BONDSTRENGTH The simplest case to study is hydrogen. Earlier papers, where heats of adsorption were measured calorimetrically (59, 60), reported a decrease of the heat of adsorption when Ni was alloyed with Cu. The qualitatively same result was obtained later in a very detailed study by Prinsloo and Gravelle (61), although the decrease of the heat was less pronounced here. However, the thermal desorption studies revealed that on a pure Ni surface several states of Hadsare formed, each characterized by a peak maximum or a peak shoulder in the thermal programmed desorption (TPD) spectra, and when Ni is alloyed with Cu, the various states stay where they were on Ni and only their populations change. This has been found actually first for Pt/Au (62), but later also for Ni/Cu (63). The studies on monocrystal Ni/Cu planes (64-66) lead to the conclusion that the changes in the binding strength of individual states are either negligible or of a moderate size (10-15%; i.e., of a similar order of magnitude to variations in the heats of adsorption with the crystallographic planes of the small metal). It is a fact worth noting that the effect on the hydrogen heat of adsorption of a coadsorbed CO and the effect of replacing the Ni atom by a Cu atom are both the same. This is as if just dilution of the hydrogen layer caused a decrease in heats of adsorption. This is an effect one would expect if lateral attractive forces existed in the adsorbed layer and were suppressed by alloying or coadsorption. Another gas easy to measure is CO. This does not mean that better agreement has already been achieved here. One group of authors (67) did not find any systematic variation in the CO heat of adsorption with alloy composition of evaporated metal films; other authors (68) found a moderate

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variation (similar to that of hydrogen). A more pronounced variation was reported by Yu et al. (69), but this is in part due to the fact that Ni and Ni/Cu alloys were measured with different samples and in different apparatuses. However, the following remark must be made here. Those who worked with monocrystals, prepared varying compositions by first sputtering Cu away and then, by successive anneals, bringing it back into the surface. Since it is known (70) that the presence of defects increases the heat of adsorption of CO and brings about new states [found, indeed, in (69)], the data with sputtered and annealed monocrystals have to be discussed with some caution. Nevertheless, the conclusion can be made, very similar to that with hydrogen, that the variations in heats of adsorption with alloying are not very pronounced. This is also the conclusion of a most recent and very detailed study by Eley and Moore (71), who stress that the lowering of the heat of adsorption of CO with alloying (Pd/Au) is less pronounced than the observed decrease in the extent of adsorption. Before a conclusion is made that the above-mentioned findings evidence the variation by alloying in the chemisorption bond strength, the following must be considered. The heat of adsorption is an overall effect which comprises several contributions, including the heat of mutual interactions. Now, it has been shown recently that adsorption of CO and H, are accompanied by attractive and repulsive lateral interactions (72-74). With hydrogen, these two are in better balance; with CO the repulsive interactions (mainly, electrodynamic and electrostatic interaction of dipoles) clearly prevail. The cooperative action of both attractive and repulsive forces leads to the formation of ordered (i.e., observable by LEED) domains at rather low coverages. When a layer of adsorbed hydrogen is diluted by CO or by empty Cu sites, the measured heat may be lower if the attractive interactions are suppressed by it more than the repulsive ones. With CO, the opposite assumption would explain the observations. Sachtler and Somorjai (75) studied this question in great detail, with Pt/Au alloys. When an Au layer was epitaxially grown on Pt, the heats of various types of CO adsorption were independent of alloy composition. However, when Au was spread in and over the surface of Pt by annealing, the heats of CO adsorption were higher. Evidently, in the first case clusters of Pt atoms were sufficiently big to allow CO-CO interactions (leading to the clustering of CO) to occur freely; in the second case dilution of Pt in Au kept CO molecules a distance from each other, and at the same CO dosage heats of CO adsorption were higher. It is interesting to note at this point that also the selectivity effects were observable only with the “annealed” and not with the “epitaxial” alloys. It is questionable whether the heat measurements (calorimetric or by TPD) are sensitive enough to detect changes in the binding strength due to

158

VLADIMIR PONEC

alloying. Infrared (IR) spectra of adsorbed CO are usually mentioned as being better suited for the detection of small "ligand" effects of alloying (i.e., of small localized changes in the electronic structure due to the alloying). Indeed, numerous papers (results are summarized and analyzed in 11) exist showing that the frequency v(M/CO), where M stands for a Group VIII metal, decreases with increasing amount of the second alloy component (Group VIII or IB metal). This has been explained in the literature (76) and elsewhere as the consequence of an electron shift in the following sense: 0

111

YC/

CU~Pt+CU

However, since the data on v(M/CO) usually concern the situation where O(CO)+ 1 on M but atoms of Group IB metals either are unoccupied or bear a CO molecule vibrating with a frequency v(IB/CO) different from v(M/CO), the mutual interactions of CO vibrating dipoles are lower on alloys than on pure metals at standard experimental conditions. As a consequence, the v( M/CO) should be lower on alloys (dipole-dipole interactions cause a blue shift) than on pure Group VIII metals, when O,(CO) + 1. To decide between these two explanations, one has to perform experiments with 2CO/' 3 C 0 mixtures (77-79). Each "CO molecule functions as a free site since its dipole does not feel or cause the resonance-type interaction with the l 2 C 0 dipoles. If alloying causes an electronic structure or ligand effect, in the above-mentioned sense, the effect of alloying must be the same on pure l2CO as on mixed '2CO/'3C0 layers; the curves v vs '2CO/'3C0 composition must run parallel for a pure Group VIII metal and for an alloy. If the effect of alloying is purely a dilution effect in the dipole-dipole interactions, these two curves should converge into the same point, when '2CO/13C0 approaches the limit of zero. It has been found with Pt/Cu alloys (80) that the two curves indeed converge. No ligand effect could be detected in this way. It can be reasonably expected that the same conclusion holds also for other alloys which are formed with a smaller exothermic effect (Pd/Ag, Pd/Au, etc.) or which are formed endothermically (Pt/Au, Ni/Cu, etc.). Therefore, the authors (81) turned their attention to Pt/Pb alloys, where they found that, indeed, a part of the shift in v due to alloying might be caused by other than dilution effects. Preliminary experiments with Pt/Re and Pt/Sn alloys show that also with these alloys the contribution of other than dilution effects is very small (82). Those who prefer speculations on the ligand effects of alloying in hydrocarbon reactions may object that what is true for CO or H2 is not necessarily

'

CATALYSIS BY ALLOYS IN HYDROCARBON REACTIONS

159

true for hydrocarbon chemisorption complexes. This is in principle true, but with the data available today a statement seems to be justified that the effects of alloying should be primarily rationalized by the ensemble-size and ensemble-compositioneffects, since there are data supporting this idea, whereas for alloys studied to date, solid-state physics, heats of adsorption, and IR data offer very little support for speculation on an essential role of ligand and other electronic-structure (population of d-bands etc.) effects in hydrocarbon (and actually also other) reactions. Another objection is the following: “Is it actually reasonable that no effects due to electronic-structure changes (ligand effects) are found?” Is that not an artifact? However, there is a rational explanation for this (83). Baerends et al. calculated the binding strength of CO to a metal cluster of varying size and discovered that even second shell cluster atoms contribute considerably to the heat of adsorption. However, these atoms contribute to it mainly by their s-electrons. Now, when, for example, Pt is placed into a matrix of Cu (or Pd in Ag, etc.) it does not matter much whether the s-electrons of the next neighbors are supplied by Pt or by Cu atoms; these s-electrons are delocalized anyway. This leads, in our opinion, to the absence of any pronounced ligand effects in alloys of moderate exothermicity or those formed endothermically.

111.

Particle Size Effects

STRUCTURE OF SMALL PARTICLES : A. ELECTRONIC EXPERIMENTAL ASPECTS AND THEORY Most of the industrial metallic catalysts are metals on carrier. The main purpose of using a carrier is, of course, to achieve high dispersion of the metal component and to stabilize this form of metal against a spontaneous sintering. However, in important reactions (like reforming of hydrocarbons) a metal support is not inert and the overall reaction is actually an interplay of the two functions: that of the metal and that of the catalytically active “carrier.” Moreover, some other effects may also play a role: (1) Highly dispersed metals expose atoms to the gas phase in unusual geometric arrangements, which leads to particular low coordination (unsaturated) etc. (84-93). (2) Very small particles expose atoms of a lower coordination and therefore with a population of d-electrons different from that of atoms on flat

160

VLADIMIR PONEC

surfaces. Also the local density of states at the Fermi level is different on these atoms (94-97). (3) Small metal particles are usually anchored into the supporting surface by their own ions, which are built into the support surface; the smaller the particles are, the bigger is the expected effect of this anchoring (98). (4) Small metal particles reveal a not fully developed valence band (they have a system of discrete levels rather than a quasi-continuaus metallic-like band), which effect influences the binding energy as determined by XPS and might be, in principle, important also for chemisorption and catalysis (99, 100). ( 5 ) Small metal particles have a higher ionization potential and electron affinity, and both converge only slowly to the value of the work function (101, 102).

(6) Small metal particles have a compressed crystallographic structure. This effect is very well documented in the literature (103-107). (7) Small metal particles are frequently expected (however, the evidence is sometimes questionable) to experience an electron transfer with the carrier, which modifies the adsorption and catalytic properties of the metal particles [sometimes called the “Schwab” effect (108-116)]. In other cases, by special conditions under preparations of the catalysts, a so-called strong metal support interaction effect (SMSI) (117-12]) was evoked. In particular, with zeolites as carriers, there are pieces of experimental evidence reported (115, 116) in support of the existence of such transfer (for remarks on those conclusions, see 122, 123). The arguments of the pure theoretical predictions (94-97, 99-102) are very convincing. However, what are the relevant experimentally observed phenomena and their explanation? Important in this respect are the data obtained by XPS. Small particles (thin layers, or other atoms of lower coordination) reveal (a) a narrowing of the valence (d-) band; (b) a shift of the binding energy (BE) to higher values [Ekine, (measured) = hv - BE]; (c) disappearance of the effects like spin-orbital splitting of the band, etc. This is schematically shown in Fig. 2, which summarizes the data of various papers (123-135). These are all changes which the theory of metal bonding would predict for the transition of metal- small cluster+ isolated atoms (94-97, 99-102, 136). However, before we ascribe the higher BE of small particles to bonding effects, we have to consider the following. The observed BE value is influenced by relaxation and screening phenomena which effectively decrease the BE when going from a free atom to condensed matter (e.g., a metal). When these effects cannot operate on a full scale because the valence conduction band is not fully developed, the ob-

CATALYSIS BY ALLOYS IN HYDROCARBON REACTIONS

161

3

Group Vm- metal in an alloy

Group VIII- metal small particles

BE

€F

IE=Ol

FIG.2. Photoemission from a transition metal (e.g., Pd) in the state of a bulk crystal (top) or as a small cluster on a support (bottom). Comparison with photoemission of the same metal when dissolved in a matrix of a Group IB metal (middle).

served BE should be higher, as is indeed found experimentally. Whereas the bonding effects are expected to be influenced by the support used and should be different for different orbitals being ionized (i.e., for different initial states), the final state effects (relaxation and screening) should not show this sensitivity. The fact that the shifts in BE due to the particle size variations are mainly features independent of the support and are the same for various energy levels (see, e.g., 123-135) indicates strongly that the final state effects are most likely just those which are decisive for the observed BE shifts. A small cluster or an atom of, for example, Pd (see Fig. 2) in an alloy has the same position of the d-band centroid as the full developed band of pure Pd; this in contrast to the behavior of the small Pd particles. Such a difference can be rationalized if one assumes that due to the absence of a sufficient number of s-electrons in a small particle and due to their low mobility the screening is imperfect and thus the BEs are higher, whereas a hole on a Pd atom in a matrix of, say, silver can always be screened (extra-atomically) by s-electrons of the matrix. Photoemission experiments with flat surfaces revealed that atoms of lower coordination may have a different population of d-orbitals and a different local density of states (138-140). These effects have been also predicted and analyzed theoretically (94-97, 136, 137), and should be always considered. The only question is whether they manifest themselves in the chemisorption and catalytic behavior. In any case, the impression is that by making metal particles small in size, one can cause the electronic structure of a certain fraction of the metal atoms to vary more than by making a bulk “solution” alloy.

162

VLADIMIR PONEC

B. EFFECTS OF PARTICLE SIZEON CHEMISORPTION BEHAVIOR Important information on this problem has been obtained by Grunze (141). It appears that after CO chemisorption on Pd, the d-band photoemission (UV) is attenuated (differential spectra show a “negative” band) and two new bands appear due to the chemisorbed CO [(5a %)-band and (4a)-band]. A decreasing particle size causes an increase in the apparent BE of all three bands-the shift is almost the same for all three bands. This again indicates that the final photoemission effects could be responsible for the shifts observed. One may argue that UPS and XPS are not sensitive enough to detect subtle changes in bonding with varying particle size and that, e.g., IR spectra of adsorbed molecules might be a better tool. Therefore, the authors (142,143) compared the IR data for big particles, monocrystals, and sintered films with those obtained for very small particles (Pt, Ir, Cu). It appeared that the small particles behave differently, and since possible side effects due to the CO-CO interactions were excluded, the authors concluded that the reason for the different behavior must be sought in one of the effects listed on p. 159 [most likely (2) since the authors exclude explanation by effects (3) and (5)] or in the possibility that CO can approach small particles slightly closer than flat surfaces, which would enhance back donation effects on Pt or Ir, or direct donation effects on Cu. Although this effect is already more pronounced than anything ever observed by IR as a ligand effect of alloying it is still questionable whether this is important indeed for catalysis and whether other effects do not finally overshadow the subtle effects of the electronic structure variations with the particle size. We shall turn to this point later, in Section 1II.D.

+

IV.

Mechanism of Hydrocarbon-Hydrogen Reactions

A. INTRODUCTION One of the most important technological advances of the postwar period was when platinum/alumina (i.e., a “metallic”) catalyst was introduced in oil refining (144, 145). From the point of view of the mechanism of the reforming reactions, it was suggested in the very early stages of research on Pt reforming that under the industrial conditions (Pt-O.1- 1 .O wt.%/Al,O, (pure or modified); T = 470-530°C; pressure 10-30 atm) the catalyst is actually bifunctional (146-150) : Pt is mainly responsible for various dehydrogenation reactions, whereas the carrier (modified eventually by C1-

CATALYSIS BY ALLOYS IN HYDROCARBON REACTIONS

163

or F- ions) performs what is known as “catalysis by solid acids” (150, 151). This idea was confirmed in the meantime by many pieces of evidence and today the only modification of it is that the Pt or a Pt-alloy component is supposed to catalyze also some reactions other than dehydrogenation-the latter being nevertheless still considered to be the most important function of the metallic component. Authors in different countries (see, e.g., 152-154) studied Pt catalysts which were support-free (films) or which were prepared with such carriers that the acidic function of them could be neglected (active carbon, inert SiO,). They found that these Pt catalysts catalyzed almost all reactions which are known to occur with Pt/Al,O, catalysts under reforming. On the other hand, it became also clear that under the industrial conditions the surface of Pt is covered by sulfur, carbonaceous residues, coke, etc. to such an extent that most of the reactions are severely slowed, and only the simplest reaction of dehydrogenation (leading possibly to aromatization) can still proceed. These facts lead to the conclusion formulated above. Reactions of hydrocarbons on Pt, and to a lesser extent on other Group VIII metals as well, have already been the subject of three excellent reviews in this series (155-157), each review reflecting the views of the particular author(s). It is not this author’s intention to repeat information which is available elsewhere (155-157), but rather to focus on particular points; namely, those which help us to rationalize the data obtained with alloys, or vice versa, those which have been established by studies with alloys. Of course, the selection of data presented below, or the evaluation of discussions which have already taken place in the literature, is again unavoidably influenced by the author’s personal views. It is practical to discuss certain groups of reactions separately. Conveniently, the subdivision of reactions as presented in Table I may be used. A notation “3C-, 5C-” has been used in Table I to indicate the number of carbon atoms which form the essential part of the transition state complexes of the reactions mentioned. A more detailed definition of other terms will be given below. The three types of C-C bond fission concern the following reactions : (1) Terminal fission:

cccccc +ccccc + c +cccc + 2c +ccc + 3c (2) Internal fission :

cccccc ccc + ccc cccccc cc + cccc +

+

(3) Multiple fission : CCCCCC

+

6C

164

VLADIMIR PONEC

TABLE 1 Reactions of HCIH, Mixtures A. Hydrogenation, dehydrogenation, double bond isomerization, HC/D,“ exchange (all “nonskeletal” reactions)

9. Skeletal reactions

1. Nondestructive rearrangements

a. lsomerization via the 3C-intermediatesb b. Isomerization via the SC-intermediatesb c. Dehydrocyclization into a 5-ring d. Dehydrocyclization into a 6-ring e. Aromatization via a ring enlargement

2. Destructive reactions (hydrogenolysis or “hydrocracking”) a. Terminal fission b. Internal fission c. Multiple fission

HC denotes hydrocarbon. See the text for explanation of these terms

Information on the intermediates operating in the reactions of HC/H2 mixtures has been mainly obtained by the three following ways:

( I ) Exchange reactions in HC/D2 mixtures. In particular, the bonding metal-hydrocarbon fragment is conveniently studied by these reactions. The basis of these studies has been established by Kemball (see, e.g., 158, 159), Burwell (e.g., 160), Tamaru (161), Bond (162), and others. ( 2 ) Reactions of molecules with labeled carbon (13C, 14C). Gault and his co-workers pioneered (see, for review, 157) the use of 13C-labeled molecules in experiments by which the operation of adsorbed complexes with either three or five C atoms involved could be tested (see below). I4C has been repeatedly used in problems concerning the dehydrocyclization and aromatization reactions (157,163-166). The authors (163-166) in their research combined the use of labeled molecules with the third method, the study of “archetype” molecules. (3) The study of “archetype” molecules. This method has been proposed and widely used by Rooney, Burwell, Anderson, and others (see, for review, 155, 156, 160). In this method a molecule is used which can form an archetype of chemisorbed complex (“caged” molecules as derivatives of adamantane or ethane in its hydrogenolysis, neopentane in exchange with D2 or in reforming reactions, etc.) or which can form several complexes, but the contribution of these complexes to the overall mechanism is easily derived from the product spectrum [as is the case, for example, with neohexane (167, 168)].

CATALYSIS BY ALLOYS IN HYDROCARBON REACTIONS

165

It is a serious but frequently neglected problem that the analysis of the data obtained with the method (2) or (3) above is only straightforward when each molecule undergoes only a one-step reaction upon one adsorption sojourn on the catalyst surface. If several consecutive reactions (e.g., isomerization combined with hydrogenolysis or two isomerization steps in combination) follow each other before the molecules leave the surface, useful information is still gained (167, 168), but the discussion of data is more complicated. Metals like Pt or Pd do not seem to be a problem in this respect, as is the case with other metals at the lowest possible reaction temperatures. However, metals like Ir or Rh are apparently very active in performing several consecutive steps during one residence of the molecules on the surface, and at temperatures above 200°C it is difficult to avoid the multiple reactions (167).

B. CHEMISORPTION COMPLEXES OF HYDROCARBONS ON METALS Let us discuss first the binding of hydrocarbon molecules to the surface. 1. Metal- Carbon Single Bond

It seems beyond debate that when an exchange reaction of a hydrocarbon (HC) with D2 is observed and the initial product distributions are binomial (random distribution of D atoms), single a-metal-carbon bonds are being formed. Nevertheless, this conclusion was puzzling in the period when virtually no homogeneous alkyl-metal complexes were known and the stability of alkyl-metal complexes was doubted for “principal” reasons (see, e.g., 169). However, it appeared that these complexes can be rather stable when one blocks a very fast and easy elimination of one of the H atoms in the /?-position, which step decomposes the alkyl-metal bond into an olefin and a bound hydrogen atom (170,171). On the other hand, this means that the transition H

(where M denotes metal) must be considered in the schemes of catalytic reactions as a very easy running step, wherever the concentration of bound H and olefin is sufficiently high and this reaction is not blocked by other ligands on the same M atom.

2. Metal-Carbon Multiple Bonds The existence of such bonds was inferred first from the initial product distributions of the CH4/D, reactions. Kemball(158,159,172) reported that

166

VLADIMIR PONEC

with metals like Rh or Ni, even at lowest conversions, when no repeated desorption/readsorption process could be expected, distributions were observed with a very high content of the d4- and dJ-products. Kemball (172) suggested the following mechanism to explain it :

This mechanism employed the postulated multiple bonds. It might be that with some metals and at higher temperatures the dehydrogenation is deeper and the multiplicity of bonds is even higher (e.g., that HC-M is also formed). In spite of this uncertainty, the “multiple” exchange of CH4 became a very good diagnostic tool for the multiple metal-carbon bonds. It is an important question whether with higher hydrocarbons the ULY or uuu multiple bonds can exist as well. The fact that multiple exchange (i.e., more than one D atom enter the molecule during its one sojourn on the surface) of ethane and its homologs takes place at much lower temperatures than that of methane (158) indicates strongly that, wherever possible, the aP complexes (or its alternative, n-complexed olefins) are indeed formed. Moreover, they are formed more easily than the cia and related complexes. Nevertheless, the initial distributions obtained with mononuclear homogeneous complex (173-176) show very clearly that in this situation (an isolated center), the asymmetry of the C2H6/D2 exchange is high (clear maxima at d 3 and d6); in other words, carbene-like structures can be formed also with higher hydrocarbons than methane. Most likely, the correct conclusion is that the two types of multiple reactions-i.e., via the uu and the a/? complexes-run in parallel (177). The question arises whether also other double-site multiple bonds, like ay or ad, are equally possible. The answer seems to be a negative one (see, for review, 157-162): When the hydrocarbon chain is interrupted by a heteroatom (ethers etc.) or by a quarternary carbon (neopentane, neohexane, etc.), the exchange proceeds separately on both sides of this obstacle and does not go easily over to other parts of the molecules, so that one can conclude that the formation of cry (and analogous) complexes is always more difficult than the formation of the aP complexes. Even the formation of the ap complexes seems to be subject to certain limitations. Investigations with sterically well-defined and distinguishable hydrogen atoms (like, e.g., hydrogens of adamantanes) showed that only the mutually “eclipsed” hydrogen atoms can undergo the UP complex formation (160, 178, 179). Let us mention here another relevant fact : Formation of multiply bound complexes of the uu or UP type is not substantially altered (see below) when an active metal is diluted by alloying in a matrix of an inactive metal (e.g.,

CATALYSIS BY ALLOYS IN HYDROCARBON REACTIONS

167

Ni in Cu). This indicates that the formation of the “UP,’complexes is possibly a “one”-site process. This would point strongly in the direction of the alternative (exactly the same results should be obtained from reactions running through these two types of complexes) for the ap two-site complex, namely, a n-complexed, one-site-bound intermediate. M

/

CHz-CH \z

++

M

CHz+CHz

M

M

(This complex also involves the two a/? carbons in the metal-HC bonding.) However, it is also not possible to suggest the n complexes as the only form of the aP complexes. Burwell and Shrage (179) studied the exchange reactions of bicyclo-[3.3.1]-nonane, with which molecule one can reasonably expect some suppressing of the n-complex formation. Nevertheless, the authors found that the multiple exchange proceeded easily and was spread over the whole molecule, so that there was most likely a mechanism other than n-complex formation which allowed it. The question of the exact structure of complexes which are bound to the metal surface through two carbons is thus still open, and it is not impossible that both alternatives exist side by side. 3. Two Carbon Atom Complexes Formed upon OfeJn Chemisorption Reforming reactions comprise dehydro-/hydrogenations and olefins might also be intermediates of other reactions-such as the above-mentioned exchange reactions. The two forms of associatively adsorbed olefins have been already mentioned : n complexes and ap two-o-bonded complexes. The questions posed are as follows: (a) Do dissociative forms of olefins also exist; (b) are any of these forms reactive enough to be an intermediate of hydrogenation/dehydrogenation reactions? The answer to the first question is undoubtedly a positive one. The classical papers by Beeck et a f . , Rideal er al., and others have shown that ethylene disproportionates upon chemisorption into ethane and carbonaceous (adsorbed) residues (see 162). This disproportionation takes place at relatively low temperatures: at room temperature and lower (see 162 for review). Moreover, the intensity analysis of LEED data has shown that upon chemisorption of ethylene, ethylidyne structures are formed. Similar structures are also formed by dissociative adsorption of higher olefins (181,182). There is thus no doubt with regard the first question. The second question is, however, still being discussed (see, e.g., 180)Gault et al. studied the butenes/D, exchange and came to conclusions which were supported by the data of mass and microwave spectrometries (183-185)namely, the dissociative adsorption produces on some metals intermediates

168

VLADIMIR PONEC

which are comparable in activity with those of “associative” adsorption : a x-complexed olefin and an crP two-a-bonded olefin. It is a known fact that in many heterogeneous reactions propylene is more reactive than ethylene. The allylic hydrogen is labile and the dissociative (in C-H) adsorption in the allylic position is promoted by that. Possibly, the higher olefins can thus be adsorbed by two dissociative adsorptions: through the vinylic or the allylic position (186). The ease of dissociative adsorption of multiple exchange and of P-H elimination suggest that the transition

7

C

4

c

H C

M

M

I

II

should also be easy. This is a point to consider in suggestions on the mechanisms. 4.

Complexes Involving Three Carbon Atoms (3C, cry Complexes)

Exchange reactions of neopentane have already lead to the conclusion (155, 158, 162) that 3C complexes, bound to the surface by the ay carbon

atoms, may be formed on some metals (e.g., Rh or Pt). However, it was evident from those experiments that 3Ccry complexes are formed by metals much more reluctantly than the 2CaP or the crcr-bound complexes. It means that their formation can only be studied at (much) higher temperatures than those suited for the study of the HC/D2 exchange reactions. In this case one can advantageously use the skeletal reactions of neopentane themselves as evidence for the formation of 3C complexes. When neopentane is being isomerized or split into C1 and C3 fragments, 3C complexes are certainly

C

C

I c-c- c-c I ap

Ir.Ni Rh

I c-c-c I

+

c

C

C

aP FIG.3. 2C complexes from hydrogenolysis. Illustration of the experimental evidence available on their existence, the known (!) and unknown aspects (?) of 2C complex formation are also indicated.

CATALYSIS BY ALLOYS IN HYDROCARBON REACTIONS

169

being formed-there is even no other alternative (155). It appeared that under the temperatures of reforming-type skeletal reactions, neopentane reacts in parallel toward various products (155-157, 180, 187). Unfortunately, there is no way at the moment to establish how the 3C complexes are bound to the surface : as aay, aayy, or cry metallocyclobutane-like binding? We have already discussed the arguments which lead to the conclusion that under the conditions of the HC/D, exchange reactions, the 2CaP complexes are being formed. It is probable that also at higher temperatures this is the complex most easily formed. Nevertheless, it is important that we also have support for this statement in the form of other results. When a molecule like ethane undergoes hydrogenolysis into methane, at certain stages both carbons are bound to the surface, i.e., 2C complexes are formed. An alternative would be a radical-like fission with activation energy of 80-90 kcal/mol, i.e., almost two times higher than is found experimentally. Another molecule which demonstrates the same point is neohexane (167, 168 and references therein). When neopentane and methane are formed with this molecule, one can trust that 2CaP complexes had been formed. Again, it is not certain at which stage of dehydrogenation the splitting of a C-C bond takes place-sag, - act-@, or aaa-/?/?p?Therefore, the designation “2Ca/? complexes” must be understood (unless otherwise specified below) in a broad sense, i.e., as all complexes where two carbons are involved, irrespective of the number of bonds with the surface. This holds true not only for this review but also for most of the cited literature. The most relevant facts regarding the 2C and 3C complexes are once more summarized in Figs. 3 and 4. The still unsolved question: “How many bonds are formed upon chemisorption between the molecule and the surface?” is closely related to the problems of the detailed mechanisms. Various mechanisms have been suggested and for most of them good arguments have been made, but nevertheless, because of the above-mentioned uncertainty, they remain speculative. However, even speculative mechanisms may sometimes be helpful and therefore we shall turn to the problem of mechanisms in a separate section.

FIG.4. 3C complexes, the existence of which can be seen in experimental evidence from exchange [neopentane Rh (Pt)] and from hydrogenolysis and isomerization (neopentane, neohexane). As in Fig. 3, the known (!)and the speculative aspects (?) of the 3C complex formation are indicated.

I70 5.

VLADlMlR PONEC

Complexes Involving Five Carbon Atoms

Gault et al. noticed in their early papers (157)that the product pattern of methylcyclopentane (MCP) hydrogenolysis is sometimes surprisingly similar to that of hexane or methylpentane(s) isomerizations. They suggested that isomerization proceeded via a cyclic, methylcyclopentane-like intermediate. Later it appeared that the similarity was not always found, but an important idea was already born and, more importantly, was brilliantly confirmed by later papers from the laboratory of Gaults. The idea of the evidence is rather simple and can be elucidated by means of the following experiment. Let us consider, for example, a molecule of 2-methylpentane labeled in a branched position by 13C 2-methyl-I3C(2)pentane. If the consecutive reactions in the adsorbed state are with a given metal of low extent, and this is certainly true for Pt or Pd, then the appearance, among the product, of 3-methyl-13C(3)-pentane is very strong evidence of the operation of the 5C (cyclic) intermediates. Only via a ring closure at one place and an opening at another place of the molecule can a label move simultaneously with the branch. On the other hand, when the branch and labeled atom become separated by isomerization, this is evidence of the operation of the 3Cay complexes (see Fig. 5). Because of historic reasons, the mechanism employing the 3Ccry complexes is often called a “bond shift mechanism” and the mechanism with 5C complexes-“a cyclic mechanism.” However, both mechanisms involve cyclic intermediates at certain stages and for both mechanisms bonds are shifted. Therefore, notation specifying the number of carbon atoms involved seems to be preferable. As with 3C complexes, it is not clear how many bonds are formed with the metal and how many metal atoms are involved, when 5C complexes are formed. Some suggestions will be discussed below. The “state of the art” is summarized in Fig. 5 . The use of a labeled molecule is the only exact way to determine quantitatively the contribution of the 5C complexes to the overall isomerization. However, a rough estimate of it can, in favorable cases, also be made by comparing the isomerization of pentane (3C only) and hexane (both 3C and 5C complexes are possible) on the same catalyst and under the same conditions. For Pt and Pt/Cu alloys both methods have led to the same conclusions (188). It should be briefly mentioned that not only 2-methylpentane (2MP), but also other molecules can be used to establish the proportions of the 5C/3C mechanisms. With Pt, various molecules have lead to a similar result (157). However, with other metals the discrepancies are quite substantial (189). This can be rationalized either by assuming (189) that the 4Cu6 complexes

CATALYSIS BY ALLOYS IN HYDROCARBON REACTIONS

F‘*

-

c-c-c-c-c 2MP

171

cI *

c-c-c-c-c 3MP

3MP

/c\

c-c

*

c

c-c

/c\ ..

*

*

c

/\

*

*

C

FIG.5. 5C complexes. The most important piece of evidence for their existence is indicated in the reaction scheme. The known ( !) and speculative (?) aspects of the complex formation are indicated.

are also formed, or by admitting that more than one rearrangement of a molecule is possible during each sojourn on the surface of some very active metals. An inspection of the data on exchange reactions and on the reactions with “template” molecules (archetypes of certain intermediates) shows that the formation of various complexes takes place easily and the increasing difficulty in the formation can be indicated as follows: a

--ca/l --caa --c3Cay

-+

5Cac

From this comparison, the existence of the 4Ca6 complexes does not seem probable at low temperatures. However, it is not completely excluded either. It has been observed with Pt that molecules of 2,2,3,3-tetramethylbutane undergo hydrogenolysis mainly into two molecules of isobutane. This could be evidence of the formation of 4CaS complexes (190) on Pt. 6 . How Do Metals Difler in the Formation of DifSerent Complexes?

Investigations of the H C / D , exchange reactions have led to the following conclusions (158, 159, 162, 177, 191): Ni, Ru, Rh, and C o are the best catalysts for the formation of the aa complexes; Pt, Pd, and Ir are worse in this respect. On the other hand, the 2Caj complexes of exchange reactions are most easily formed on Pt, Pd, and Ir. Pt seems to be also the best metal to show ay binding (155, 156, 192). Results on reactions of neohexane and neopentane confirmed that Pt and, to a less extent, Pd are able to form 3Cay-type complexes rather than the

172

VLADlMlR PONEC

2CaP complexes of hydrogenolysis (this particular point will be discussed later). On the other hand, all other Group VIII metals showed a preferential formation of products which can be related to the 2CaP complexes. Only at higher temperatures or when a surface was (se1f)poisoned by carbonaceous, firmly adsorbed species did the 3Cay complexes show up (167, 168, 193-1 95).

By studying different metals and, with the same metal, catalysts with different particle sizes, various authors have shown that one has to assume at least two different mechanisms involving different 3Cay complexes (195198). Also, the work on alloys (see below) leads to such a conclusion. It is obvious that metals would differ in contributions by the respective mechanisms, but at the moment a generalization in this respect is not yet possible. Low temperatures and the simultaneous availability of a large number of contiguous sites (big ensembles)-the conditions usually met with “massive” carrier-free metals and with those metals which allow a fast removal of carbonaceous deposits from the surface-are the conditions which favor the 3Cay mechanisms (of isomerization, and possibly of hydrogenolysis as well). Relatively higher temperatures, smaller particle size, and alloying with an inactive metal all seem to promote the formation of 5C complexes. As an example, small Pt particles show almost exclusively 5C complex formation, whereas massive Pt, Ni, or Ni/Cu alloys show a prevailing 3Cay complex formation (157, 197-199). A definite theoretical explanation of this behavior is not available. It is important to realize that the preference of a metal for 3C as opposed to 2C complexes or for 5C as opposed to 3C complexes may be either intrinsic or induced by adsorption of less reactive carbonaceous fragments and carbon (for simplicity, we shall refer to both of these as “carbon”) on the metal (alloy) surface. Also, the choice of the reaction conditions (apparent contact time, poisoning or self-poisoning of the catalyst, etc.) influences the temperature range in which the catalysts can be tested, and since the selectivity in various complex formations is also temperature dependent, one must always analyze which aspects of the product distributions are intrinsic properties of a metal and which are induced by often unavoidable side reactions. C . MECHANISM OF SKELETAL REACTIONS AND

SELECTIVITY OF METALS

1 . Isomerizarion and 5-Ring Dehydrocyclization It can be now considered as well established that isomerization involves formation of (various) 3C and 5C complexes. However, all other details of

CATALYSIS BY ALLOYS IN HYDROCARBON REACTIONS

173

mechanisms discussed below (or in other papers) should be considered as no more than useful speculations. Results obtained with alloys (alloying causes variations in the distribution of the ensembles according to size) and with metals of varying particle size (due to the geometry of the curved surfaces and due to the deposition of “carbon,” the same effects are expected to operate here as in alloys) have lead to the conclusion that the various 3C and 5C complexes might differ in the size of ensembles which are required for the formation or the steadystate binding of the complexes. The “small” and “big” ensembles are, in the following, schematically represented by “one-site’’ and “two-site’’ ensembles. Figure 6 presents two suggestions from the literature for the possible pathways in the conversion of the 3Cay two-site complexes. Figures 7 and 8 present the suggestions for the 3Cay one-site complexes (155-157, 198). According to the mechanism in Fig. 6, isomerization is induced by dissociation of at least three C-H bonds; according to the other mechanisms, two or even one CH bond dissociation would be enough. Mechanisms like those in Fig. 7 were suggested (in various alternatives) when it became known that a 3C isomerization in a mixture with D, (instead of H,) produces only

FIG. 6. 3C complexes as intermediates of isomerization. A “two-site” (large ensemble) mechanism via a bond shift (left) or cyclopropane ring (right), as suggested by various authors (see text). Except for the number of C atoms involved, all other aspects of the mechanisms are speculative. The same remark holds for Figs. 7-9.

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VLADIMIR PONEC

FIG.7. 3C complexes of isomerization-a the literature.

metallocarbenium intermediate as suggested in

d,-products. For the sake of simplicity, all reactions are shown with neopentane, but this does not mean that the suggestions are limited to this molecule. It has been already mentioned in passing that indications exist in the literature showing that the 3C isomerization can take place by formation of at least two different 3C complexes, having different activation energies of isomerization, different particle size effects, different responses to alloying, etc. (157, 195-198). The suggestions presented above offer a choice of different complexes for further speculations. However, a definitive description of isomerization mechanisms under different conditions (H, pressure, temperature, etc.) and with different catalysts (pure metals, alloys, etc.) is not yet possible. There is, of course, always something which supports one of the suggestions in Figs. 6-8. For example, the suggestion in Fig. 6 is supported by the fact that the results with alloys (see below) can only be rationalized when

FIG. 8. 3C complexes in isomerization. Various pathways, as suggested in the literature (see text), for the "one-site" (small ensemble) conversions of metallocyclobutane rings.

CATALYSIS BY ALLOYS IN HYDROCARBON REACTIONS

175

a “big ensemble” mechanism is assumed in addition to an “one site” mechanism. The suggestion in Fig. 8 is an analogy of the olefin metathesis (150), and the experiments with “caged” molecules support the suggestion in Fig. 7. However, it seems very probable that none of the mechanisms alone can explain all the data. Moreover, the stability of some intermediates (branched ally1 complex, cyclopropane highly strained ring) can be doubted. With one pathway of the mechanism in Fig. 8, there is also some additional trouble: With some metals or alloys the selectivity for isomerization might be very high, sometimes nearing 1007;. This implies that the reconstitution of the original alkane molecule should be 100% even under conditions (those of alkane skeletal reactions running) when the fragments would be thermodynamically more stable than the reconstituted molecule. Also, the free rotation of n-complexed olefin (the bond to the metal must not be too weak, otherwise olefins would desorb) raises some questions. In summary, probably all conversions in Figs. 6-8 should be considered when speculating on mechanisms, but caution is always called for. Isomerization via the 5C cyclic intermediates and the 5-ring closure can be discussed together. Figure 9 shows different one- and two-site intermediates which have been suggested in the literature for these two reactions (155-157, 198-201). Isomerization consists of a ring closure in these intermediates and a ring opening, both of which take place at different spots of a molecule. Upon dehydrocyclization, a desorption follows the ring closure. The Hungarian and Russian schools seem to prefer cyclization with five carbon atoms flatly lying on the surface (see, for review, 201). This is only possible when no more than one a-bond per carbon atom is formed toward the underlying metal atoms, since multiple bonds would probably lift the molecule away from the surface. However, the a-bonds are usually well localized (of course, one does not know for certain when metals are involved

*

*

*

*

*

*

FIG.9. Possible one- and two-site intermediates of reactions involving 5C complexes.

176

VLADIMIR PONEC

in this bonding) and unreactive, so that one would not expect a recombination of two 0- metal-C bonds into a new C-C bond of a cycloalkane. Another argument against such a recombination is the fact that C-C bond fission in alkanes and cycloalkanes is most likely preceded by C-H dissociation, which step automatically leads to the formation of one or more multiple metal-C bonds before or during C-C splitting. Thus, also with 5C complexes there remain open questions. 2. Hydroyenolysis Ethane can be hydrogenolyzed by all Group VIII metals, i.e., all these metals can form the 2C complexes. With all metals except Pt and Pd, the neohexane assay shows that the 2C hydrogenolysis is easier than the 3C splitting. However, neopentane is also hydrogenolyzed by all Group VIII metals, so that the difference in the ease of formation of the 3C and 2C complexes is not prohibitive for one or another mode of fission. Actually all 2C, 3C, and even 5C complexes can, at least in principle, be a starting point of C-C bond splittings. At the moment it is impossible to assess quantitatively the contribution of various complexes to the overall hydrogenolysis. A speculation in this respect will be presented at the end of this review. At that time we shall need the following information. At low temperatures, the splitting of hexane is mainly of the “internal” type for metals which are also good for isomerization, as is Pt or Ir (202204), but is of the terminal type for good hydrogenolytic catalysts such as Ni, Co, Rh, and Ru. Palladium stands between these two groups. When the temperature is increased, the selectivity for isomerization increases and that for hydrogenolysis decreases. The increase in temperature causes also a shift in the hydrogenolytic selectivities : from the internal type to the terminal splitting. The changes in isomerization and hydrogenolysis selectivities are almost mirror-like. This has been frequently observed (157, 198, 199), with various catalysts by F. Gault, who called the internal splitting “a frustrated isomerization.” Obviously, if such a close relation exists, the complexes leading to the internal hydrogenolytic splitting should be of the 3Ccry type. 3. Aromatization From the practical point of view, this is probably the most important reaction related to the metallic component of the reforming catalysts (despite the fact that a part of aromatization is acid catalyzed). There are certainly several pathways which can, at least in principle, lead to the aromatic products. Let us mention here the most relevant facts on aromatization of hexanes and higher hydrocarbons. Several authors (150, 151, 205-208) studied the formation of aromates

CATALYSIS BY ALLOYS IN HYDROCARBON REACTIONS

177

from pentanes which were substituted in such a way that the 5- but not the 6-rings could be closed in one step. Formation of aromates is then evidence that aromatization via a ring enlargement is in principle possible. However, Davis et a/. have shown, by also using labeled molecules (163-166), that wherever a direct 6-ring closure is possible, it is faster than the pathway via the 5-ring closure and ring enlargement. It has also been established that alloying, promotors, carriers, reaction conditions, etc. influence the aromatization and the 5-ring closure quite differently. This suggests that these two are, indeed, different pathways. The question now is: In which respect are they different? It is not excluded that the carbon 6-ring can be closed in a way analogous to that of 5-ring closure (see Figs. 5 and 9), i.e., via a metallocycloheptane or dimetallocyclooctane intermediate. However, the expected (283) lower stability of these rings does not make this idea very attractive. Another possibility is that aromatization is actually a consecutive dehydrogenation. Hexatriene, once it is formed, does not need any help from the catalyst to form cyclohexadiene (and benzene in the next step). Temperatures at which industrial aromatization (in the framework of reforming) takes place are high enough to make a sufficient concentration of olefins thermodynamically possible. This mechanism has been suggested and it is generally accepted for the first generation of reforming catalysts-namely, the oxides Cr,O,/MoO,/Al,O, etc., which operate at low pressures of hydrogen. However, the typical Pt/Al,O, catalysts (or Pt/Ir, Pt/Re, etc.) operate at 10-30 atm of total pressure, and under these conditions the presence of a sufficiently high concentration of olefins was doubted in the literature. Nevertheless, there are some data (201, 209, 211) which indicate that the mechanism of aromatization might also be on metals, as just described, a consecutive dehydrogenation. There is, moreover, one indirect indication for such a mechanism. As we shall see in Section V, from all possible reactions of reforming, the dehydrogenation/hydrogenation reactions seem to be able to proceed on the smallest ensembles of active sites; possibly they can be catalyzed even by a single atom. Therefore, these reactions are least affected by deposited “carbon,” by sulfur (always present in traces under industrial conditions), by alloying of Pt with inactive elements (Sn, Au), etc. Also, aromatization by the metallic component of the reforming reactions shows these features and this strengthens the belief that the metal-catalyzed aromatization, apparently always present under reforming conditions (212), is indeed a consecutive dehydrogenation and cyclization of trienes. Evidently, the “carbon” and sulfur deposits decrease the surface concentration of hydrogen, which effect leads to an increased dehydrocyclization and aromatization (see, e.g., 204). This might make the dehydrogenative pathway of aromatization feasible also at rather high total pressures.

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VLADIMIR PONEC

On the other hand, hydrogen may have an accelerating effect (213) (positive order in P H I )because it keeps-by a continuous removal of carbon and sulfur-a small part (small ensembles) of the metal surface working. 4. Activity and Selectivity of Metals

The simplest situation is with ethane, which can be only hydrogenolyzed. A rather complete collection of data exists on this reaction due to Sinfelt and co-workers (214,215) and available reviews on this subject make profitable reading. The distinct features of ethane hydrogenolysis are the highly negative order in hydrogen pressure and the very low activity of Pt and Pd. The active metals are 0 s > Ru > Ni > Rh, Ir. Of these metals, Ru, Ni, and Rh are already known to form multiple carbon-metal bonds rather readily and this might be one of the factors favorably influencing their hydrogenolytic activity (159, 191). Cyclopropane hydrogenolysis to propane is a reaction which reminds one of the hydrogenation of olefins (216). This is due to the specific electronic structure of this molecule (217, 218). This hydrogenolysis is at low temperatures and is accompanied by hydrocracking into ethane and methane. At higher temperatures a multiple hydrocracking into three methane molecules may also take place. It is interesting to note that the propensity of metals to break the C-C bond is apparently closely related to the degree

60-

50-

LO302010-

0'

I

200

I

300

I

LOO

I

T(OC)

500

FIG. 10. Selectivities in hexane conversions versus temperature for benzene formation (Be), hydrogenolysis (Hy), methylcyclopentane formation (MCP), isomerization (ISOM), and dehydrocyclization (Dehy) (9 wt. % Pt on inert SO,).

CATALYSIS BY ALLOYS IN HYDROCARBON REACTIONS

179

to which the hydrocarbon molecule is dehydrogenated upon its chemisorption (216). The lower the ratio H/C of adsorbed species is, the higher the selectivity for hydrogenolysis. Hydrocarbons higher than C , can also undergo isomerization on Pt, running in parallel to hydrogenolysis. With butane or isobutane the selectivity for isomerization is rather low, also on Pt, but the higher hydrocarbons show more of isomerization reactions. With higher hydrocarbons some other metals (Ir, Pd) also show some isomerization selectivity. The following point should be noted. Guczi et al. (219) reviewed the data on the kinetics of hydrocarbon skeletal reactions and summarized the results as follows: (a) Log rate as a function of log pH2shows a maximum; at low hydrogen pressures the slope is positive, at higher pressures it becomes negative. (b) The maximum of the above-mentioned function shifts to higher pH2when the number of C atoms increases; skeletal reactions of heptane show already a positive order in pH,near atmospheric pressure. (c) This behavior is most likely related to the deposition of “carbon” on the metal surface. When this process is more extensive (molecules like heptane and bigger ones), the selectivity for isomerization is higher and that of hydrogenolysis lower than with smaller molecules. The same parallelism is found when different metals are compared with the same hydrocarbon molecule. Starting from C , molecules, dehydrocyclization (into cyclopentane and derivatives of cyclopentane) is also possible. From C6 on up, aromatization also occurs. These two reactions comprising a dehydrogenation step are only observable at temperatures which on most metals are higher than the region where hydrogenolysis (hydrocracking) is first observed.

3oi IS0

10-

MCP I

200

300

FIG. 11. Selectivities in hexane conversions versus temperature for hydrogenolysis (Hy), isomerization (ISO), benzene (Be), and methylcyclopentane (MCP) formations (1% I r on inert SiO,).

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VLADIMLR PONEC

Thus, these reactions are only observable with metals which show a low intrinsic activity for hydrogenolysis (possibly Pt, Pd). Otherwise, the hydrogenolytic activity of a metal had to be suppressed by “carbon” deposits or by alloying with inactive metals (see Section V), by promoters and other modifiers (sulfur), etc. The last effects (e.g., “carbon” deposition) are also important with metals like Pt or Pd. The overall behavior of hexane/H, mixtures in contact with Pt or Ir on inert carriers is shown in Figs. 10 and 11. The data shown are derived from Refs. 202-204. These data have been collected by using different feed rates, and this is the reason why the Pt data do not form one smooth curve. A longer contact time leads, even at the lowest conversions, to a decrease in the C, dehydrocyclization and an increase in isomerization. This demonstrates the close relation (via the 5C intermediates) between these two reactions. Tables 11-IV demonstrate the behavior of various hydrocarbon molecules on different metals. The order of metals in their activities is also known for some reactions other than ethane hydrogenolysis. Maurel and Leclercq (220) found the following order for cyclopentane hydrogenolysis: Ru > Rh, Ir > 0 s > Ni > Pt > Pd > Cu, Fe. Various Co catalysts showed activity between that of 0 s and Pt (most likely the influence of an uncomplete reduction). Carter et a/. (221) found the following order for heptane hydrogenolysis : Ru > Rh, Ir >> Pt > Pd. The common features of these orders in activities are evident. The known data allow also a comparison of the selectivities in nonthe order usually found (decreasing Sndr) is destructive reactions (Sndr); Pt > Pd > Ir >> Co, Ru, Rh, 0 s . A comparison made by Davis and TABLE I I Product Distribution in n-Pentunc, Riwctions w i ~ hH , “

Molar percentage

Catalyst PtiSiO, (16 wt. ‘I,) Ni/SiO, (9 wt. y o )

(

pH2 Ppcnt C) (atm) (atm)

312 346 350 350 350

0.9 0.9

0.1 0.1

2.5 5.0 5.0

0.5 0.5 2.0

iso-

C, 5 6

C,

C,

17 15 20 18 85.9 6.6 4.7 77.0 8.5 8.3 52.0 0.5 3.0

C,

C,

c-C,

3 4 8.1 5.6 4.4

52 43 0 0 0

6 7 0 0 0

S,,” SC,d (‘lo)

(‘I<,)

67

8

59 0 0

9 0 0

0

0

“ iso-C, = isopentane; c-C, = cyclopentane; Sis,+ S,,,, + Ccrackinp = I (cracking = hydrogenolysis); Experiments performed in an open-flow apparatus under the conditions indicated, by respective authors (see 5).

CATALYSIS BY ALLOYS IN HYDROCARBON REACTIONS

181

TABLE I11 Selectivity in the n-Hexane Reactions with H,”

Pd

co Fe Pt Ni

405 233 250 295 250

17.5

42.7

0 0

0

39.1

0

0 45.0 0

Terminal Multiple Multiple In the middle Multiple (terminal)

39.8 100 100 15.9 100

a See Ref. 5. Catalysts: supported metals, 0.8-5%. Apparatus: pulse reactor, at atmospheric pressure of H, . Multiple splitting: hydrocarbon is split off into C , pieces (CH,) before it leaves the catalyst surface. Terminal splitting: always one C, fragment is split off during one adsorption sojourn on the surface.

Somorjai offers the picture presented in Table V. Although the order of Sndris rather well established experimentally, there is no definitive theoretical

explanation of it. However, we have already mentioned two factors, probably not independent of each other, which might be of importance here. A high hydrogenolytic selectivity seems to be related to the following factors: ( 1 ) the ease with which a metal forms multiple bonds (15Y) and (2) the depth of the dehydrogenation of adsorbed intermediates at the given temperatures (216).The second factor is also supported by data reviewed by Tetenyi (222). Most likely, a third factor can also be added to the list: (3) the extent and structure of the “carbon” layer on the surface. This third factor will be discussed further in Section V . TABLE IV Selectivity in ti-Heptane Reactions nitli H , ”

Pd Pt Rh Ru Ir

300 275 113 88 125

90.5 37.4 93.0

92.5 87

6.2 46.8 7.0 7.5 13

3.1 15.8 0 0 0

“ See Ref. 5. All catalysts: pure metal powders. Experiments performed in a flow apparatus under comparable conditions for all metals; hydrogeniheptane mole ratio of 5 : I . From M . S. Davis and G . A. Somorjai, in “The Chemical Physics ofSolid Surfaces” (D. A. King and D. P. Woodruff. eds.), Vol. 4, Elsevier, Amsterdam.

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VLADIMIR PONEC

TABLE V An Estimate of Fractional Selectivities for Alkane Isomerization Reactions over Metals' lsomerization selectivity (mole %) Reactant hydrocarbon

Pt

Pd

Ir

Rh

Ru

n-Butane lsobutane n-Pentane N eopentane n-Hexane 2-Methylpentane 3-Methylpentane n-Heptane

15

I 3-5 10-25 0 6

2 -

2

30-80 15-70 5-97 30-70 20-90 40-90 47

-

0 0

13 -

I 0 8

16

-

-

6

1 2 13 7

a

1

Re 1

7 -

-

-

-

-

-

3

4

-

-

-

7

-

-

0

-

Ni

-

-

From Davis and Somorjai (181).

D. PARTICLE SIZE EFFECTS IN

CATALYSIS OF THE

HC REACTIONS

Boudart (223) suggested that all reactions might not be equally sensitive to the geometric arrangements in various metal surfaces or to the differences in the electronic structure of sites in different geometric environments (coordination). Boudart divided the reactions into two groups : (I) structure insensitive and (11) structure sensitive. The operational criterion of structure sensitivity is the specific activity (the rate per unit surface area) or, the turnover numbers (TONs) (the rate per site): TONs should differ by more than a factor of 5-10 when the dispersion D is varied sufficiently. Bond (224) formulated similar ideas and also suggested several reasons why the variations of TONs with D can monotonically decrease (antipathic), monotonically increase (sympathetic), or show a maximum. Since then, the group of structure-insensitive reactions has been very well documented by experimental data. This can be seen in several reviews (181, 223-225). It seems to be reliably established that reactions of simple molecules such as H,, CO, or SO, oxidations, HC/D, exchange, and others, are mostly structure insensitive. Sometimes, the insensitivity is quite surprising, as with di-tert-butylacetylene hydrogenation (226). The structure-sensitive reactions present a more serious problem. First, one must always make sure that the observed sensitivity is not an experimental artifact. Then when the sensitivity is established reliably, it is not always easy to rationalize it. With regard to the first problem, it is sometimes difficult to exclude all potential difficulties entailed in the determination of the metal surface area, which uncertainty can lead to apparent variation of TONs with D or to a

CATALYSIS BY ALLOYS IN HYDROCARBON REACTIONS

183

too large scatter of data. For example, the gas used to determine the metal surface area may be adsorbed also by the carrier. Another complication may be that during the sample preparation, contaminations can migrate from the carrier to the metal ( H 2 0and Fe) or be transported via the gas phase; long and rigorous reduction may cause an SMSI effect, etc. The rationalization of the structure sensitivity of a certain reaction might also be a problem. First, one has to answer the following question: Is the structure sensitivity observed inherent for the reaction studied and therefore related to the varying exposure of different sites to the gas phase, or is the sensitivity induced by side reactions which themselves (and not the reaction followed) are structure sensitive? The latter problem was first formulated by Katzer (227),who also demonstrated that an apparent structure sensitivity may be a consequence of a structure-sensitive self-poisoning (228). It should be pointed out that the HC reactions are always accompanied by self-poisoning by the deposited “carbon.” A working surface is under steadystate conditions, almost always covered by a monolayer of “carbon.” Only the ledges, kinks, etc., which are difficult to cover by a continuous layer, keep their intrinsically high activity in the H-H and C-H bond breaking, serving as portholes for molecules arriving at the surface from the gas phase and for molecules desorbing from the surface (181,229,230). Davis and Somorjai (181)also report the following : (1) Metals like Ni, Rh, and Ir are covered by smaller fragments than Pt (or rather, they release smaller fragments upon hydrogenation). (2) The sites most active and also surviving the longest are on all metals the steps and kinks (lowest coordination sites). (3) The fragments of different molecules are removed with different rates; e g , from Pt the fragments of neopentane are removed faster than those of hexane, the consequence being that under steady-state conditions the structure of the “carbon” layer is different with different HC molecules. (4) Heating of the “carbon” layer leads to its reconstruction and even recrystallization into a graphitic-like layer, and this ordering and recrystallization may lead to changes in activity (and selectivity) of the catalysts (229).

In view of this complexity and the evident interrelation between reaction and self-poisoning, it is actually less surprising that the literature does not offer a clear picture of the particle size effects in the HC reactions. Hydrogenolysis of ethane by Pt and Ir reveals a decrease in TON by a factor of 10-15, when the dispersion D is increased (antipathic relation) from z O . 1 to ~ 0 . (231). 8 An early paper on ethane hydrogenolysis by Ni reports a strong and a later paper reports a less pronounced variation of activity with D. However, in contrast to Pt and Ir, this variation is always a sympathetic one (232, 233). The same holds true for Rh (234).

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VLADlMlR PONEC

With higher hydrocarbons the structure sensitivity is usually much less pronounced: whereas Ir shows a clear sensitivity with ethane, with higher hydrocarbons the sensitivity is much less (235). Cyclopentane shows a pronounced antipathic correlation with D on Pt but an independence of D with Ir and Pd (231).With Rh the correlation of activity with D is antipathic for cyclopentane (236) but sympathetic for n-pentane. Thus, both a metal and the structure of the molecule are likely essential for the structure sensitivity to occur. Davis and Somorjai collected and evaluated kinetic data from more than 150 papers on the H C reactions (181) and their survey shows more of such differences (see cyclopentane versus pentane on Rh) which are not easy to rationalize. However, when one considers what has been said above [effects (1)-(4) of “carbon” layers], the following hypothesis emerges : The differences among the data of different authors are due to the differences in the structure of the “carbon” layer on their metal surface. This idea is supported by the following information. Lankhorst et al. (204) prepared a homogeneous series of Pt catalysts (almost all of them with the same metal loading) whereby the variations in the average particle size were achieved by using varying reductions (flow and H 2 0 content, temperature and duration of reduction). It appears (204) that within a factor of 3, all catalysts show the same TON in skeletal reactions of hexane at low temperature. Also, the selectivities did not vary strongly. However, this picture changed strongly when the catalysts were subjected to a brief (4hr) self-poisoning by hexane/H, at elevated temperatures (450°C) and then reinvestigated at low temperatures. The small particles were affected least by this poisoning; the big particles most. After poisoning, the same conversion was measured at a temperature higher by AT This AT appeared to be clearly correlated with the particle size, as can be seen in Fig. 12. The existence of such a correlation can be expected on the basis of the work on monocrystals (flat and stepped surfaces) (181, 229, 230). It is interesting to note that in compliance with earlier data, selectivity also varies with self-poisoning (204). Self-poisoning increases the selectivity of dehydrocyclization and decreases that of isomerization (238), whereby the effect of self-poisoning is most pronounced with big and least pronounced with small particles. Vogelzang et al. (235) made analogous experiments with two Ir catalysts of two extreme dispersions-very small and very big particles. It appears that (1) it is much more difficult to lay down a standing deposit on Ir than on Pt ; (2) the sensitivity of Ir for self-poisoning is much less than that of Pt ; and (3) the sensitivity of the selectivity for the particle size effects is much less than that of Pt. This all suggests that self-poisoning being structure sensitive causes (or strengthens) the structure sensitivity of a given reaction.

185

CATALYSIS BY ALLOYS IN HYDROCARBON REACTIONS

I

I

z

I

I

I

I

I

I

3

II

5

6

7

8

P A R T I C L E S I Z E “MI

FIG. 12. Effect of self-poisoning (ihr at 450°C. by the reaction mixture) on hexane/H, reactions. AT is the temperature increase necessary to achieve the same overall conversion after poisoning over that before poisoning. AT is plotted as a function of the average particle size of various Pt/SiO, catalysts. From V . Ponec et al., in “Catalyst Deactivation,” p. 93, Elsevier, Amsterdam (1980).

Several examples supporting this idea are also presented in the review by Tetenyi (222). Gault (157) reviewed the data obtained in his laboratory. This review will not be reproduced here, but it is essential to mention several points: (1) Particle size influences considerably the contributions to the overall isomerization on Pt/Al,O: of the 3C and 5C mechanisms; the 5C mechanism prevails on smaller particles, but it prevails in a rather broad range of particle sizes. (2) Particle size influences the ratios hexane/2MP and hexane/3MP upon hydrogenolysis of MCP. On very small particles or diluted alloys of Pt, the products are of random distribution (Gault calls it a nonselective fission) on massive particles, which favors the 3C mechanism-hexane is missing selectively. According to Gault (157), this is because it is not formed; according to other authors (188), it is because the intermediate leading to hexane is decomposed in an adsorbed state by multiple reactions and the products of this fission do not desorb as easily as hexane isomers do.

I86

VLADIMIR PONEC

(3) Ir shows a strongly prevailing 5C isomerization and selective MCP splitting, without any particle size effect. No definitive explanation of the above data is available. Several hypotheses have been formulated to explain the data (157) and there is certainly no urgent need to add a new one to the existing list. However, one important factor has not yet been considered among the possible explanations already suggested ; namely, that of the structure-sensitive formation of the carbon layer, which in its turn would determine the selectivities. Yet it is easy to imagine that this layer controls not only the overall selectivities (as in 204) but also the contributions of various (3C and 5C) mechanisms, as well as the destiny of the MCP intermediates on the surface. Let us consider the following. In principle, both mechanisms may have a “two”-site and a “one”-site alternative, the former leading to a faster reaction (see Figs. 6-8). It is conceivable that as long as a metal can split the C-C bonds of the adsorbed intermediates, the intermediate which would lead to hexane (an attack starts on the most reactive terminal C) might be the first victim of this activity (see 239). Then the following hypothesis can be made. The largeparticle Pt is usually investigated at lower temperature, where extensive cracking can still be observed (see Fig. 10). With these catalysts the 3C isomerization and the selective MCP fission are observed, since both employ “two”-site ensembles, which are still present on the surface, the latter being self-poisoned at low temperature only. The small-particle Pt is usually a low-loading Pt and such a catalyst is usually investigated at higher temperature, when isomerization already prevails (“two”-site ensembles are reduced in number). On Ir, large- or small-particle catalysts, there are always sufficiently large ensembles to (a) convert all 3C complexes into cracking products and (b) let 5C complexes be formed in the “two”-site mode. These might be ideas worth considering.

V.

Hydrocarbon Reactions on Alloys

A. CLASSIFICATION OF THE REACTIONS ON METALS AND GENERAL DESCRIPTION OF ALLOYING EFFECTS One of the most studied alloys is undoubtedly the Ni-Cu system. When the studies started, many problems remained open with regard to the phase and surface composition of these alloys and their electronic structure. Today, it seems that these problems have basically been solved; admittedly not in all details, but to a sufficient extent to allow discussion of the results. At temperatures 450-500 K, Ni-Cu alloys form only one phase. Since

CATALYSIS BY ALLOYS IN HYDROCARBON REACTIONS

187

the catalysts are usually prepared and very often tested above this temperature, there should not be many problems with the phase composition. The surface composition of monocrystals, polycrystalline films, or alloy powders is now relatively well established: Addition of small amounts of Cu (e.g., 10% Cu) to the bulk leads to a high surface enrichment in Cu and the surface Ni concentration drops to about 10 & 5% Ni, staying within these limits up to about 80% bulk Cu (for Ni-Cu alloys on carrier, see below). In the whole range of Cu concentrations, magnetic measurements show (242-244) that Ni is ferro- or paramagnetic, and easily forms clusters. Also there is no electron transfer from Cu to Ni as demonstrated by magnetic measurements or by XPS/UPS (no shift in either the edge of the Cu d-band or the position of any of the d-bands) (10,13) and soft X-ray emission (16) (for review of the electronic structure of Ni-Cu, see 5). As mentioned above, thermal programmed desorption reveals that the states characterized by the position of the desorption peaks are almost the same for pure Ni and Ni-Cu alloys. With this in mind, we must conclude that when changes in activity and selectivity occur with alloying, they should be primarily ascribed to the changes in surface concentration of Ni. This statement holds true most probably for other Group VIJI-IB metal alloys as well. Presently, data on about 40 reactions are known for Ni-Cu alloys. If the activity (defined as the relative rate with regard to Ni, measured at standard reaction conditions for both metals and all alloys) is plotted as a function of the Cu bulk concentration, all available data for various reactions are split into two groups, characterized as follows (245) (see Fig. 13): Group I : Alloying leads to an increase in activity or a decrease which is substantially no higher than the decrease in Ni surface concentration. These reactions are hydrogenation/dehydrogenation on C-C or C-0 bonds (most probably also on C-N bonds), HC/D, exchange reactions, and possibly other reactions analogous to those mentioned. Group I I : The activity drops more than the Ni surface concentration (Fig. 13), i.e., at least about 20 times. However, for several reactions this drop is two or more orders of magnitude. The reactions included in this group are methanation and Fischer-Tropsch synthesis, isomerization, dehydrocyclization or hydrogenolysis of alkanes, ether formation from alcohols, metathesis of alkylamines, and possibly other reactions.

It was speculated (245) that the first group of reactions could proceed also on isolated active sites, whereas the reactions of the second group require an ensemble of the size of several atoms. The increase in activity with alloying found with some reactions of group I is most probably caused by the fact that these reactions are accompanied by an often invisible side reaction of group 11, e.g., C-C bond breaking leading to the deposition of “carbon”

188

VLADIMIR PONEC Activity

-I

standard.Ni

lo-’

10-3

10-54

NI

cu

FIG. 13. Summary of the literature data on various reactions (x40) studied on Ni/Cu alloys. Relative activity as a function of the bulk alloy composition. All data can be subdivided into two groups of reactions: sensitive (11) or insensitive (I) to alloying. The surface Ni concentration follows approximately the lower limit of the band for reactions of group I . From V. Ponec, Int. J . Quantum Chem. 12, Suppl. 2, I - 12 (1977).

(246) and C-N bond breaking leading to C and N deposition (247). Such side reactions are suppressed by alloying more than the group I reactions under study (246). This classification reminds one of the classification of structure-insensitive (I) and structure-sensitive (11) reactions by Boudart (223). It is probably not true that all reactions of group I1 are structure-sensitive and vice versa, but it is to be expected that the possible candidates for structure sensitivity are indeed in group I1 rather than in group I of the above classification. Recent data on other alloys confirm the overall classification presented above, but at the same time lead to some refinement of the picture. For example, the most diluted Pt-Au alloys revealed isomerization, identified as running via 3C intermediates. This evidence was obtained (248) by establishing the fact that pentane isomerizes on most diluted Pt-Au alloys with 100% selectivity, whereas this molecule can only isomerize via the 3C complexes. This conclusion has been confirmed by the isotopic labeling method (269). It is therefore reasonable to assume that this isomerization can also proceed on isolated Pt sites, as can a part of dehydrocyclization and the dehydrogenation. We must conclude on the basis of this information that on metals like Pt, the fast multisite and the slow one-site mechanisms of hydrocarbon reactions may operate in parallel with each other. The overall effects of alloying Ni with Cu can now be summarized as follows : Skeletal reactions are strongly suppressed, hydrogenolysis more than isomerization, but a part (one type) of isomerization running via the 3C complexes is affected less by alloying; there is no indication of isomerization on Ni via the 5C complexes.

CATALYSIS BY ALLOYS IN HYDROCARBON REACTIONS

189

A study of neohexane reactions revealed (267)that Ni has a strong preference for hydrogenolytic cracking of the terminal type, via the 2C complex formation. Deposition of carbon [it is known (249)that this carbon occupies preferentially the valley positions] as well as alloying decrease the activity of the catalysts so that at higher temperatures the 3Ccry complexes are also being formed and reactive. On alloys, these 3C complexes undergo along with fission (on alloys the contribution of internal splitting is higher) also isomerization. Examples of the behavior are given in Figs. 14 and 15 (267). Alloying of an active with an inactive metal selectively suppresses the reactions requiring the biggest ensembles. Interesting results can also in principle be obtained with alloys of two active metals when these metals behave sufficiently different in catalytic reactions. A quantitative analysis (like, e.g., determination of the exact size of ensembles required) of the alloying effect is impossible at this time. The surface composition, in particular that of small particles, under a running catalytic reaction is generally not known. Further, it is not known how metals and alloys differ in extent and composition of the carbonaceous deposits and in their influence on the reaction under study. Therefore, no reliable quantitative conclusions can be made on the number of metal atoms involved in the formation of individual complexes. For the sake of simplicity

80 Conc %

60

LO

20

= isobut

0

L

T

IKI

FIG.14. Product patterns as a function oftemperature in neohexane/H, reactions: (a) Pure Ni measured at low temperatures; (b) pure Ni diluted by SiO, and self-poisoned by the running reactions (notice that methane 2 neopentane, i.e., multiple reactions are also running at the lowest conversions); (c) alloy of Ni/Cu in the ratio 65:35 (increase in methane < decrease in neopentane; this indicates that molecules other than methane are formed, i.e., the role of a? is larger here). From V. Ponec et a/., Faraduy discus.^. No. 72, p. 33.

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VLADIMIR PONEC

FIG.15. Selectivity for nondestructive reactions in hexane (HEX) and neohexane (NEOHex) reactions (in H2)in the temperature range 173-603 K . From V. Ponec e t a / . , Faraday Discuss. No. 72, p. 33.

one speaks of “one”-, or “two”-, or “mu1ti”-site complexes or reactions, but actually one should always use the terms “big” or “small” ensembles. Sometimes, the effect of adding an inactive to an active metal was surprisingly small; usually it was with metals on carrier. We suspect that in these cases a small (X-ray diffraction-) amorphous part of the active metal was present unalloyed. Another point is that an inactive metal might be actually active, at least moderately, in particular in cooperation with an active Group VIII metal. This is probably the case with some Cu alloys (168). In some other cases the opposite was true. The effect of inactive additives was surprisingly high and it would imply a requirement for an unreasonably big ensemble. Several reasons could be responsible for such effect : (1) Inactive metal atoms preferentially occupy the most active sites, like those on corners, edges, etc. (2) Above a certain temperature, alloys get more poisoned by the reaction mixture than a pure active metal, whereas for the low temperature the opposite might be true. (3) The surface composition is different from that expected and the surface concentration of a Group VIII metal is actually much lower than assumed. This is probably the case met in various studies on Pd-Ag alloys (44). Having all these possible complications in mind, we can start a cautious discussion on the ensemble size requirements of various reactions and their

CATALYSIS BY ALLOYS IN HYDROCARBON REACTIONS

191

intermediates. Modest progress has been made in recent years in this very field of the use of alloys in catalysis.

B. SOMEPARTICULAR ALLOYSYSTEMS 1.

N i Alloys

Ni-Cu. Several aspects of the results with these alloys have been already discussed, so it will suffice here to make only some additional remarks. The first surprising point is that the Ni surface concentration and the overall activities vary only marginally on alloys with 10-807; Cu (in bulk) concentration (see Fig. 13), whereas the selectivity varies in a much more pronounced way (see Figs. 14 and IS). The reason for this is not exactly known, but one can speculate along the following lines. Ni forms clusters the size of which depends on Ni concentration; variation of the size and concentration of clusters is responsible for the changes in the ferromagnetic saturation moment (242-244). At only marginally varying Ni concentration in the surface, Ni available in the surface may be a part of ensembles varying in size. When these variations are around the critical value of the ensemble size, they may have a very pronounced effect on the selectivity. The summary of data in Fig. 13 concerns the activity; Figs. 14 and 15 concern the selectivity. An inspection of the data in Figs. 14 and 15 (as well as an analysis of other pieces of information on the same subject) reveals that of all complexes, the 2C complexes of hydrogenolysis are the most easily formed and the most affected by alloying; in other words, the 2C hydrogenolytic complexes require the biggest ensembles. Another interesting point is that with hexane a much higher selectivity can be achieved than with neohexane (see Fig. 15). Most likely, the selectivity is not only regulated by alloying, but also by carbonaceous deposits which are more extensively, or differently, formed from hexane than from neohexane or lower hydrocarbons. Ni-Sn (250,251).In many respects this system is very similar to the Ni-Cu system. Ni-Pd. Moss et nl. (252) reported that 60% Pd (in bulk) catalysts (i.e., those which have almost lOOo/, Pd in the surface) had almost the same activity in ethane hydrogenolysis as pure Ni, although pure Pd itself is not very active. This might be an indication that for this reaction mixed ensembles of Pd-Ni can operate. In this respect it is interesting that Driessen recently found that in contrast to this, a 75'; Pd (bulk) catalyst [the exchange reaction detected (253)the presence of some Ni in the surface of a catalyst of this composition] showed no activity in methanation, compared to Ni.

192

VLADIMIR PONEC

Evidently, methanation is a reaction where the mixed ensembles cannot operate. 2. Pi and Pd Alloys Several papers (24, 75, 254-256) confirmed the earlier results (248, 257) that alloying of Pt with metals that are virtually inactive for the reactions discussed (Ag, Au) suppressed all group 11 reactions very strongly. However, as already mentioned, hydrogenolysis is always suppressed most. Least affected is the high-temperature dehydrogenation (group I reactions) of alkanes [e.g., propane into propene (255)] and reactions induced by this dehydrogenation such as aromatization. There are indications that also a part of the dehydrocyclization into 5-ring products running at high temperatures is little affected by a diminished (by alloying) ensemble size (254). Diminution of the ensemble size, leading to the effects just described, can be achieved by various elements: Au, Ag, or Sn (167, 168, 254-257). A study on neohexane reactions revealed (167, 168) that the “inactive” additives suppress hydrogenolysis of neohexane in a way similar to that which suppresses hydrogenolysis of hexane. It can be further seen (see Fig. 16) that these additives do not substantially alter the regularly observed preference of Pt for the reactions induced by 3Cay complexes, as opposed to those which are induced by 2 C a j complexes. Alloying with Au or Ag only strengthens this preference. Rather surprisingly, the addition of Cu has quite different consequences (see below). Whereas pure massive Pt catalysts prefer the 3Cay to the 5C mechanism, catalysts with very small pure Pt particles prefer the 5C mechanism (157).

:1

100

,

80-

60-

FIG.16. Selectivity in the formation of various adsorbed 3C and 2C complexes upon reactions of neohexane with hydrogen for Pt/SiO,, Pt-Ag/SiO,, and Pt-Au/SiO, with compositions and temperatures of reactions as indicated (H-hydrogenolysis).

CATALYSIS BY ALLOYS IN HYDROCARBON REACTIONS

193

Moderately diluted Pt alloys probably do the same, but the most diluted Pt (in Au) alloys again reveal (199) a pure 3Ccry-type isomerization. It is suspected that this is a 3C isomerization by a one-site pseudo-carbenium mechanism (248). Interesting results were obtained with Pt-Pd and Pt-Rh films (258-260) i.e., with the alloys of two active metals. With neopentane (and also with neohexane) (167, 168) both metals (Pt, Pd) can form 3Cuy complexes, but Pd has a higher hydrogenolytic selectivity than Pt. The alloys reveal a continuous shift from isomerization to hydrogenolysis with an increasing Pd concentration in Pt. It is interesting to note that when Rh is added to Pt (261),the selectivity for the “internal” hydrogenolysis (Pt-like) decreases and that for “terminal” and “multiple” hydrogenolysis (Rh-like) continuously increases. Palladium alloys and their behavior in hexane reactions were already mentioned above. Figure 17 shows that the behavior of neohexane is similar to that of hexane. The system of Pt and Cu behaves substantially differently from other Pt/IB alloys or other analogous bimetallics (262). Platinum itself has a rather low hydrogenolytic activity and selectivity, but the addition of Cu while decreasing (see Fig. 18) the overall activity increases (at a given temperature) the hydrogenolytic selectivity ! This holds true for hexane, pentane, and neohexane (168). Experiments with neohexane (168) revealed that the preference of Pt for the formation of 3Cmy over 2Ccr/J is preserved on all alloys. 3C complexes lead the reaction into isomerization only on Pt and on Pt-rich alloys, and into hydrogenolysis on Cu-rich alloys. With hexane reactions only a marginal, almost negligible increase in the 2Ccr/J terminal splitting of hexane due to alloying was observed (262).Here too, the increase in hydrogenolytic selectivity is mostly because the 3Ccry complexes induce

Pd/Si02,fresh, 610K Pd/Si02+

20

-Pd

Cs, 610 K

75%.Ag25% ISiO, 613 K

FIG. 17. Selectivity in the formation of various adsorbed 3C and 2C complexes upon reactions of neohexane with hydrogen catalysts: Pd/SiO,, a fresh catalyst: Pd/SiO, , after selfpoisoning by the reaction mixture (carbon deposition); Pd-Ag/SiO, with composition as indicates (H-hydrogenolysis).

194

VLADIMIR PONEC

70

60 ACT

I%) 50

LO

30

20

10

0

i0

6‘0

% Pt

100

FIG. 18. Activity of Pt-Cu/SiO, catalysts in the hexdne and pentane skeletal reactions. ACT = a:F/ W , where a: is the overall conversion (;a). F is the feed, and W is the weight or the catalyst. T = 573 K . From H. C. de Jongste and V . Ponec, Proc. 7rh In/.Congr. Culul. Tokyo, 1 9 8 0 ~ 186. .

splitting rather than isomerization when Pt is diluted in Cu. Thus, the situation is very similar to that with neohexane. In the last-mentioned results, there is an indication that Cu is somehow involved in the formation of 3Cay complexes on the surface. Figure 19 schematically shows two possible positions of 3C complexes around the Pt atoms; in both cases there is a possibility for Cu to play some role in binding the 3Cay complex: either directly, in binding carbon atoms, or indirectly, in breaking the C-H bonds, releasing H,, or something similar. Perhaps, the “on top” complex (Fig. 19b) deserves our preference as the candidate with the correct structure, since one can then more easily explain the fact that deposition of carbon or sulfur or alloying with Ni/Cu increases in the same way the propensity of the catalyst to form 3Coly complexes and suppress formation of 2CaB complexes. It is known that carbon and sulfur occupy the valley positions on the surfaces of metals (249, 263-265). On those metals where CO occupies the same position on pure surfaces, carbon and sulfur cause CO to be pushed

CATALYSIS BY ALLOYS IN HYDROCARBON REACTIONS

195

FIG. 19. Pt/Cu alloys in interaction with 3C complexes. Speculation on various positions in which the 3C complexes interact with both alloy components, Cu being involved either in binding (a, b) or hydrogen-carbon bond dissociation.

from its “natural” valley position to the “summit” position (265-268). One can expect that (a) the same happens also for metal-carbon bonds upon adsorption of hydrocarbons and (b) if a metal prefers the formation of summit-like metal-carbon bonds, this is further strengthened by C or S deposition (or alloying). If the above conclusion is correct, i.e., if the 3Ccry complexes are, indeed, preferentially formed on the summit of atoms, we can deduce from the selectivity effects of alloying and of C and S deposition that the formation of the 2CaP complexes requires one or, even more likely, two valley positions. We shall turn to this point later. As far as the 5C complexes are concerned, the following can be said. These complexes have been experimentally proved to exist on Pt and Ir (157) and on alloys of these metals; up to now they were not observed with Ni or Ni/Cu catalysts. This might indicate that they prefer a certain metal atom. It is also interesting that 5C complexes also “survive” the alloying of Pt with Cu or Au. In addition, they resist hydrogenolysis better than the 3C complexes do and they cope better with increasing isolation of Pt atoms upon alloying. In other words, the 5C complexes seem to be, when necessary, better accommodated on small ensembles (single atoms?). This is actually not surprising since adsorbed cyclopropanes or metallocyclobutane intermediates may be expected to be less stable than the metallocyclohexanes (see, e.g., 283). On the other hand, it is quite reasonable and acceptable to think that the 3C complexes are more easily formed and are better stabilized

196

VLADIMIR PONEC

wherever they can interact simultaneously with more than one metal atom. In other words, they prefer larger ensembles where they can possibly be formed with less strain and where they are more activated than the pseudometallocarbenium ions bound to only one site. An assumption that Cu (but not Au or Ag) atoms somehow participate in the complex formation is a plausible explanation, consistently supported by the data known to date, but it is not the only possible explanation. We saw in Figs. 10 and 1 I that hydrogenolysis selectivity decay and isomerization selectivity growth are almost mirror images. This behavior might be related to the “carbon” deposited at higher temperatures, this causing “carbon” part of the 3C hydrogenolysis to become 3C isomerization. Alloying of Pt with Cu causes the S-cracking and S-isomerization curves to shift toward higher temperatures. The result can then be interpreted as follows: a higher temperature is needed on Pt/Cu than is needed on pure Pt in order to achieve the same shift hydrogenolysis + isomerization, because a higher temperature is necessary to create a comparable “carbon” layer. Evidently, more experiments are necessary in this direction, some of them currently being performed in the author’s laboratory. Results on Pt/Au alloys were also interesting with regard to benzene hydrogenation. This is a reaction which on Ni appeared to be insensitive for alloying, but on Pt, where much lower surface Pt concentrations could be experimentally tested, the benzene hydrogenation appeared to require E 12 Ac

10

8

6

L1 98

1L inactive L

1

kcallmol

~1

- 12 -8 -L

II

I

98

L

%Pt

FIG.20. Activity (-) in benzene (left) and cyclohexane (right) hydrogenation by Pt/Au alloys on S O , . The content of Pf (in 7;) is indicated. Activation energy of cyclohexene hydrogenation (----) does not show any dramatic change when going from Pt-rich to Pt-lean alloys, whereas activity in the hydrogenation of benzene does. From V. Ponec, Proc. hi.Congr. Catal. London, 61h. 197611977 p. 851.

CATALYSIS BY ALLOYS IN HYDROCARBON REACTIONS

197

definitely larger ensembles than required for cyclohexene hydrogenation (315) (Fig. 20). This is in accordance with the fact that no data exist which would reliably (a presence of colloidal metals is always the problem) demonstrate that hydrogenation of benzene is possible with mononuclear homogeneous complexes which can easily hydrogenate olefins. What is the reason for the insensitivity with Ni alloys? Two suggestions can be made: Either Ni is never really atomically diluted in Ni/Cu alloys, but Pt is in Pt/Au alloys, or Cu can play the role of an additional active center in the mixed ensembles hydrogenating benzene.

3. Pt- Re and Pt-lr Alloys

These are probably the industrially most important alloys. While their superiority to pure Pt catalysts has been well demonstrated (6, 271), the exact role of Re or Ir is not known. However, there are already several suggestions in the literature, each of them probably expressing correctly one particular aspect of the role of Re or Ir. Let us summarize briefly the ideas and also some experimental data relevant to this problem. From the experiments at low temperatures ( 5 6 0 0 K), we know which modifications may occur in the metallic function of these alloy/carrier catalysts (272-275). First, there is obviously a maximum when the hydrogenolytic activity in pentane, neopentane, or cyclopentane reactions is plotted against the Pt/Re alloy composition, which maximum is at 60-80% of Re in bulk. The addition of Re also changes the behavior of the surface complex : Isomerization changes into hydrogenolysis, and the internal splitting typical for Pt turns into terminal and multiple splitting. With Pt/Ir alloys, the situation seems to be a bit more complicated. Rasser et al. (276) found no pronounced variation with composition in the selectivity of Pt/Ir alloys, whereas Leclercq et al. (272) found a hydrogenolytic activity of some of the Pt/Ir alloys which was higher than that of the pure metals and otherwise a behavior similar to that of Pt/Re alloys. The reason for this difference is not yet clear. In the literature there is also information on the behavior of Pt/Re and Pt/Ir alloys at temperatures 2800 K, where also the acidity of the carrier plays an important role. A group of Indian scientists (277) summarized their findings as follows: The admixture of Ir lowers formation of the firmly bond unreactive coke precursor, but enhances the hydrogenolytic activity also with respect to the precursors. These two phenomena are probably related, as was pointed out by the Exxon group (271,278).According to the authors (277), the higher hydrogenolytic activity can be neutralized by modifying the catalyst with sulfur. Very similar thoughts have been formulated for Pt/Re catalysts by Menon and Prasad (279). They also mentioned

198

VLADIMIR PONEC

that although pure Pt catalysts need some sulfur in the feed in order to maintain a proper degree of modification, once administered sulfur is well kept by Pt-Re catalysts (Re has a high affinity to sulfur) and a further continued supply of sulfur is then undesirable. Biloen et al. (280) suggested an even more detailed picture explaining the behavior of the Pt/Re S catalysts: Sulfur, firmly bound to Re, blocks the cracking power of Re and dramatically diminishes the ensemble size of Pt ensembles. This leads to a suppression of all reactions except those running as and through dehydrogenation ; thus, Pt/Re S alloys behave in a way similar to, e.g., Pt/Au alloys (“ReS” = Au). At high temperatures ( 2 700 K), an alumina carrier participates in the overall catalytic reforming. It is then not surprising that the function of Re (or Re ions) is seen by some authors (281) as modifying the carrier. In earlier papers on this subject, it was even in doubt whether a Re oxide could indeed be reduced on Al,O, and form any alloy. Recent papers (277, 279) favor a picture according to which a substantial part of the Re is present in a reduced form (Re”) and alloyed with Pt, thereby modifying the properties of Pt. Of course, the presence of small amounts of Re ions modifying the properties of the carrier is always possible. Russian authors (282) favor the idea that Re ions are (a) anchoring and thus stabilizing the ultra-small Pt particles (notice that it is more difficult to poison very small particles by carbonaceous deposits than a massive metal) and (b) modifying very small Pt particles electronically or in their shapes. As already mentioned above, it is also possible that when added to Pt/AI,O,, “Re” plays all of these roles simultaneously.

+

+

C. OPENPROBLEMS IN CATALYSIS BY ALLOYS

It has been shown above that a self-consistent explanation of the selectivity effects of alloying can be suggested, this being based on, essentially, the following points: (1) Since various reactions evidently differ in their requirements with regard to ensemble size, the size (in alloys of an active metal with an inactive metal) and composition (in alloys of two active metals) of ensembles of active sites are responsible for the changes of selectivity due to alloying. (2) A pure metal and a bimetallic ensemble may differ in the way in which they bind hydrocarbon, hydrogen, or the deposited “carbon.” (3) All other effects are minor in their importance. The problems are mainly with point (3). Some authors point to the fact that in some cases the activity of alloys is higher than that of pure metals, and they conclude that such “synergism” cannot be explained without

CATALYSIS BY ALLOYS IN HYDROCARBON REACTIONS

199

speculating on the operation of the ligand (or, in general, electronic structure) effects of alloying (284-292). A possibility that this is true should always be considered, but it is important to realize that the maximum on the activity versus alloy composition curve is not evidence of the ligand effect. The observed activity of a metal is dependent on its intrinsic activity (per site) and on the number of “working” sites, i.e., those which are not selfpoisoned by the reaction running. The sites not covered by inactive species bind the reactants and/or serve as portholes for adsorption of molecules which may react in a later stage elsewhere. Alloying suppresses self-poisoning by “carbon,” oxygen, or hydrogen dissolved (Pd alloys) (293). Also, the already mentioned mixed (bimetallic) ensembles must always be considered as a possible way in which alloying may influence the activity and/or selectivity. These mixed ensembles might be more active than the individual pure components. Thus, there are several effects which can potentially lead to a “synergism” and the synergistic effects are by no means unambiguous evidence of ligand effects. Synergistic behavior is sometimes observed with Ni/Cu but also with Pt/Au, Pd/Au, and Pd/Ag. Since in the first two cases the number of d-holes does not vary upon alloying and in the other two cases it does, a varying occupation of the d-band cannot be responsible for the synergism (if observed). The same conclusion holds with regard to the selectivity. With both Pd/Au (298) and Ni/Cu alloys (see Section V,A) the selectivity in the nondestructive reactions increases by alloying the Group VIII metal with the Group IB metal in a very similar way. Thus, also these selectivity changes cannot be ascribed to the varying number of d-holes. The possible role of mixed ensembles has been insufficiently investigated up to now. The main question-Do they operate or not?-is still open. There are some arguments in favor of this idea (Pt/Cu and Pd/Ni, as mentioned above), but there are also papers in which the results suggest a negative answer (as, e.g., in Ref. 294 on the Rh/Ir catalysts). The uncertainties surrounding the possible role of mixed ensembles are close related to those concerning the role of hydrogen. Although it has been pointed out already, in the early stages of the research on alloys, that hydrogen coverage might be an important factor (295) in the overall effects caused by alloying, very little is known in this respect. Bimetallic clusters may have quite a different composition in their surfaces exposed to the gas phase than is the case for the bigger metal particles. This is usually admitted in the literature, but at the same time it is overlooked that not only the composition but also the distribution of metal components (mutual dispersion) may be different in small and big particles or on different carriers. Strong indications for the last carrier effect have been recently presented (296).

200

VLADIMIR PONEC

With small crystals on carriers, alloying might change the shape and size of metal (alloy) particles. If certain sites (corners, edges, vacancies) are the only active sites or are dominating the activity, and if these sites decrease or increase in number with particle size variations, this can be an additional important effect of alloying. A recent paper presents a strong indication in favor of the existence of such an effect (300). Up to now our discussions has concerned selectivity in systems with parallel-running reactions, with all reactions starting from the same initial molecule. However, there are also important reactions of hydrocarbons which are either parallel reactions of two different molecules in the same reaction mixture (hydrogenation in a mixture of substituted benzenes) or parallel-running consecutive reactions (hydrogenation of acetylene or of unsaturated aldehydes). However, in these cases the selectivity for a certain product is very much determined by the ratios of the heats of adsorption (162, 167, 297) and since we know that alloying can cause only marginal changes in the latter, a rather modest effect of alloying (298) on the selectivity of some of these reactions is less surprising. On the other hand, one can easily understand why in this situation, e.g., a gas admixture may cause a more substantial change in selectivity than alloying: The admixture displaces the desired intermediary product from the surface and dictates in this way the selectivity in consecutive reactions (299). With alloys, one also confronts problems which are in principle trivial, but nevertheless are very difficult to solve: a well-defined preparation of supported alloys, the characterization of the distribution of alloy components on the carrier, and the control of alloying. We have already mentioned one paper in this respect (296),but there are more examples available on problems along this line. For example, it was long a source of puzzlement why some groups found that alloying of Pt with Au causes a higher selectivity of isomerization of n-hexane (248, 256), whereas other groups found an increased selectivity of dehydrocyclization (270, 301) with the nominally same alloys. The only difference was the way in which alloy was prepared and carrier was used. These two factors also appeared to be the key to the solution of the discrepancy. When well-defined Pt/Au alloys (238) are mounted on a SiOz support, an oxidation/reduction cycle repeated with the same catalyst decreases the activity slightly (by decreasing most likely the surface) but leaves the selectivity unchanged. However, when the same alloys are on A1,0,, then the oxidation/reduction cycle as applied by some authors (270, 301) leads to the separation of a small amount of small Pt particles, which, under the given conditions, indeed favors dehydrocyclization (316). It is obvious that the surface segregation in alloys is suppressed by increasing the dispersion D of alloys : Small particles might not have “enough” Group IB metals to achieve a pronounced segregation and, moreover, the

CATALYSIS BY ALLOYS IN HYDROCARBON REACTIONS

20 1

very small particles have negligible "bulk," so that any segregation is impossible because of that. However, some puzzling cases are known. For example, Ni/Cu alloys with rather large particles do not apparently show any segregation, although the Cu segregation is thermodynamically very much favored (302, 303). If one excludes from consideration trivial reasons like separation of a small number of pure Ni particles on the support surface, the most likely alternative explanation is an assumption that with small particles (up to a quite large size) even the weak adsorption of hydrogen is strong enough to cause gas-induced Ni segregation to the surface to occur up to the bulk Ni concentration. With CO, such segregation certainly exists (67, 249). Evidently, the problem is still open. Although with well-defined films, monocrystals, or carrier-free alloy powders, alloying of Group VIII metal with Group IB metal always decreases hydrogenolysis more than other skeletal and hydrogenation/dehydrogenation reactions, there are several, rather surprising reports that in some cases hydrogenolysis was promoted by such alloying. Examples are Pt/Au (304, Ni/Cu (305, 307), Co/Cu (306), and Pt/Ag (308) alloys. A recent paper on Pt/Sn alloys (309), where higher adsorption of hydrogen has been reported for alloys than for pure Pt, might indicate in which direction the solution should be sought. All Group VIII metal/support catalysts contain a certain amount of unreducible ions of the Group VIII elements (see, e.g., 310-312) which can be extracted by a complexing compound (e.g., acetylacetone) or displaced by another ion (313,314),e.g., Sn2+and Sn4+.The not completely simultaneous precipitation of both components (Ni/Cu, Pt/Sn, etc.) from a solution, or the addition of Sn or other salts to the already reduced Pt or Ni catalyst, may cause such secondary Ni2+ or Pt2+ displacement. This displacement facilitates the subsequent reduction of Pt2+ by the reaction mixture and formation of very small Pt" or Ni" particles, which are very difficult to self-poison, even at high temperatures (204).However, it is clear that more experimental work is needed on this particular subject.

D. A SPECULATIVE MODELOF HYDROCARBON REACTIONS ON METALS AND ALLOYS In general, when speaking about the mechanism of chemical reactions, not very much can be considered to be well understood. Of course, this holds true also for the particular case of HC reactions on metals. However, the studies performed in the 1960s and 1970s have accumulated so much information that one must at least try to form a generalized picture, despite the fact that the result of such an effort is highly speculative and certainly of restricted (in time) validity.

202

VLADIMIR PONEC

TABLE VI Bond involved 1

2 3 4

5

Type of reaction

C-H C-H

c-c c-c

Rupture Rupture Rupture Rearrangement

C-C/ C-H

Multiple rupture

Chemical conversion Exchange Hydrogenation Hydrogenolysis Isomerization dehydrocyclization Aromatization. multiple hydrogenolysis

Let us start the discussion by repeating several essential points. The first one concerns the ease of performing various reactions. The spectrum of HC reactions grows with increasing temperature as presented in Table V1. The second important aspect is the ensemble size requirement. The size of necessary ensembles has the following order : hydrogenolysis > fast isomerization and dehydrocyclization > slow isomerization and dehydrocyclization 2 hydrogenation/dehydrogenation, including a part of aromatization. While the two statements above are almost phenomenological, one inevitably enters the field of speculation upon attempting to find an answer for the following questions: Where does the reaction or binding to the surface take place-on the summits of the surface atoms or in the valley positions? Do metals and various molecules differ in this respect? Metals which with adsorbed CO prefer to form metal-carbon bonds on the summits are Pt and Ir (Cu?); metals which promote binding in the valley are Pd > Ni > Rh, Re. Metals promoting multiple metal-carbon bonds (with hydrocarbons) are Ni, Ru, Rh; Pt and Pd are much worse in this respect. Let us extrapolate and assume that what holds for C O also holds for hydrocarbon molecules, and that the characterization of the multiplebond formation propensity is valid also at higher temperatures than were established experimentally by exchange reactions. Then we can attempt to rationalize the available information on the formation and the role of various hydrocarbon complexes. We know already that metals differ in their ability to form 2C and 3C complexes. In general, under the conditions when solely the exchange reactions are running, formation of 2CaB complexes is always easier than formation of 3Cay complexes. However, the results obtained with alloys (active + inactive metals) show that we have to discern two different types of 2C complexes :

(i) One type of 2Ccomplex is not strongly affected by alloying (with alloys

CATALYSIS BY ALLOYS IN HYDROCARBON REACTIONS

203

like Ni/Cu, Pt/Au, Pd/Ag, etc.) (245) and is an intermediate of exchange reactions (ii) Another type of 2C complex, the formation of which is strongly suppressed by alloying, is the intermediate of hydrogenolysis Several suggestions have been made in the literature regarding these two groups : Group (i) R

R\

I

/R

CHI I

H/ C ’ f C L H

CHI I

Ni-Cu-NI-Cu-Cu-

N!

CH3

,R

c-C\

I

CH-R

/R

H I’ -NI-CU-I

I

-N~-~-NI-

H

Group (iil

H2C - CH2

/

NI

\

NI

HC - C H

/\

A

NI NI NI NI

C-

/I\

NI NI NI

C

/I\

NI NI NI

Most likely, the two groups differ in the sense suggested by Tetenyi (222), namely, in their degree of dehydrogenation and in the number of bonds formed between the individual carbon atoms and the metal surface atoms. It seems to be a reasonable assumption that not only the 2C complexes of group (i) but also those of group (ii) are formed more easily than the 3C complexes of exchange and skeletal reactions. (Note that the 3C complexes of skeletal reactions also require a dehydrogenation of the adsorbing molecule.) However, on some metals (see, e.g., the results on the mechanism of reactions on Pt and Pd) the products of 2C hydrogenolytic complexes do not appear in the gas phase before (i.e., at lower temperature) the 3C complexes. Thus, the assumption, when accepted, implies that when the 2C hydrogenolytic complexes [group (ii)] are formed more easily on all metals than are the 3C complexes, on some metals (Pt, Pd) they do not desorb readily and stick to the surface. Results on neohexane reactions (167) show that hydrogenolytic splitting can also occur with the 3C complexes (see, e.g., Pd). The question remaining is: Can the two types of hydrogenolytic splitting (2C and 3C) be related to two different ways of hydrogenolysis? There are indications that the answer should be positive. On Ni, for example, the terminal splitting of n-hexane is very rapid and occurs at such low temperature that the neohexane assay shows almost exclusively the formation of 2CaD complexes. In other words, the terminal splitting can likely be associated with the 2CaP splitting. On the other hand, Pt shows hydrogenolysis at higher temperatures than Ni does, and at low temperatures (i.e., relatively low; low for Pt) this splitting is of highly internal character. At slightly elevated temperatures this splitting

204

VLADlMlR PONEC

goes from this type to increasingly terminal splitting and to isomerization (3C). The internal fission of hexane is promoted by the same factors which promote formation of 3C complexes (see Fig. 15) from neohexane, namely “carbon” deposition and alloying like that of Ni with Cu and Pt with Au. Both factors are known (see above) to invalidate the valley position of metals which prefer to bind carbon atoms there, or one may assume that they strengthen the intrinsic preference of other metals for the summit positions. All these observations and tentative conclusions can be combined to form the following self-consistent picture : (1) The terminal splitting is a reaction via the 2C complexes which, when belonging to group (ii), are preferentially bound to the valleys. (2) The internal splitting is mainly a reaction initiated by the 3C complexes. This is a reaction which seems to prefer the summit position for the carbon atoms of the reacting hydrocarbon molecule. When conditions allow, this complex induces isomerization instead of hydrogenolysis. (3) The terminal 2C splitting complex formation always occurs more easily than the 3Cuy fission, but products of the former splittings stick to the surface of some metals too firmly and do not desorb under the temperature at which the 3C-induced hydrogenolysis or isomerization occurs. This seems to be the case for Pt and Pd and to some extent also for Ir. (4) “Carbon” deposition blocks the valleys rather than the summits, which relatively enhances formation of 3C complexes and relatively suppresses formation of 2C complex. Moreover, “carbon”-covered surface has a lower concentration of hydrogen and this relatively promotes isomerization and, with higher hydrocarbons-dehydrocyclization.

The ratio of rates of formation and removal (by H,) of firmly bound species (“carbon”) is different with different metals. Evidently, Pt and Pd keep more “carbon” on their surfaces than do the good methanation catalysts such as Ni, Ru, or Rh. The surface of, say, Pt is better blocked and thus protected against hydrogenolysis than are surfaces of other metals. The often-found particle size sensitivity of hydrocarbon reactions on Pt (less on other metals) might be related to this. The steady-state concentration of carbon is also a function of the hydrocarbon molecular structure: Higher hydrocarbons are more efficient in modifying the metal surfaces than smaller molecules are. Two extremes emerge from comparison of the Group VIII metals: Ni, Rh, Co, and Ru (the left corner of the Group VIII metal block of the periodic table) prefer terminal splitting, already show multiple splitting at rather low temperatures, are the best catalysts (with 0 s ) in hydrogenolysis of ethane (only 2C complexes possible), and catalyze well the reaction of carbon atoms to methane. Pt is the other extreme in all of these respects, with Pd and Ir

205

CATALYSIS BY ALLOYS IN HYDROCARBON REACTIONS

TABLE VII Complexes Operating upon Reforming of Hydrocarbons on Metals"

Function

Hydrogenolysis type

Hydrogenolysis

Terminal

2 or more

3Cuy

Isomerization or hydrogenolysis

lnternal

5c

Isomerization dehydrocyclization

Fast isomer: 2 or more Slow isomer: 1 Hydrogenolysis: 2 or more Slow dehydrocyclization: 1 Fast dehydrocyclization : 2 or more

Complex 2Cup

-

Required sites

Likely location At least IC in the valley ; most

likely- both "On top" position

"On top" position

T = 500-600 K.

somewhere in between. Based on these facts and on speculation, Table VII summarizes the features of various complexes and reactions in the skeletal reactions of hydrocarbons on the Group VIII metals. The metal not discussed yet is iron. It appeared to be a rather inactive metal. The possible reason for this is that iron is, under a running skeletal reaction or under conditions when more difficult dehydrogenation/hydrogenation can occur, covered by carbon to such an extent that one can rather speak of Fe carbides being the catalyst here. Most likely, the same holds for Group 111-VI transition metals. However, carbides (with an imperfect structure) of these metals are, in contrast to Fe, active in skeletal reactions.

VI.

Conclusions

Considerable progress has been made in accumulating information on the electronic structure of metals and alloys, on some aspects of the structure of hydrocarbon adsorption complexes, etc. Also, information on the relative importance of the electronic structure effects of alloying-as contrasted to the geometric, ensemble size effects-has grown appreciably. With solution alloys the effect of alloying on the electronic structure is surprisingly small, and also with intermetallic compounds these effects are not very pronounced. The effect of alloying on catalytic reactions depends

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VLADIMIR PONEC

very much on the mechanism of the catalytic reaction and on the type of intermediates operating upon reaction. In parallel-running, simultaneous reactions, the alloying effect is very pronounced if in order for a reaction to occur, large ensembles of certain metal atoms are required. However, the fact that certain metal atoms are required is itself evidence of the importance for catalysis of the electronic structure of metal atoms. The selectivity in consecutive reactions (hydrogenation of multiple unsaturated molecules) sometimes depends on the (relative) heat of adsorption of the starting molecules and intermediate products (e.g., acetylene/ethylene), and since heats of adsorption are usually only marginally affected by alloying, alloying would not change this kind of selective behavior of metals. Various reactions or reaction systems (parallel or consecutive reactions) are influenced by alloying to a quite different degree. This should be kept in mind when attempting to find new or better alloy catalysts for a given reaction.

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21 1

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21 3

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ADVANCES IN CATALYSIS, VOLUME 32

Modified Raney Nickel (MRNi) Catalyst: Heterogeneous Enantio- Differentiating (Asymmetric) Catalyst YOSHIHARU IZUMI Institute for Protein Research Osaka University Osaka, Japan

1. What Is MRNi? . . . . . . . . . . . History of Discovery and Development of MRNi. . Profileof MRNiin Hydrogenation. . . . . . A. Hydrogenation Activity. . . . . . . . B. Kinetics . . . . . . . . . . . . IV. Profile of MRNi in Stereo-Differentiation . . . . A. Enantioface-Differentiating Ability . . . . B. Other Stereo-Differentiating Abilities . . . . V. Other Profiles . . . . . . . . . . . . VI. Surface Conditions . . . . . . . . . . A. Amount of Adsorbed Modifying Reagent . . B. Adsorption Mode of Modifying Reagent . . . VII. Mechanism of Enantio-Differentiation . . . . . VIII. Characterization of Catalyst by Modifying Technique . . . . . . . . . . IX. TA-NaBr-MRNi X. Other Investigations . . . . . . . . . . References . . . . . . . . . . . . . 11. 111.

1.

. . . . . .

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

.

. . . . .

. . . . .

. . . . .

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

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

. 215 . 218 , , , , , , , , ,

224 224 225 229 229 245 248 249 249 . 250 . 254 ,262 .264 ,267 ,269

What Is MRNI?'

Metal catalysts can be endowed with various new properties by a simple chemical treatment. This catalyst with the new property is called a modified We shall use the following abbreviations throughout: AA for acetylacetone; DNi for nickel catalyst prepared by thermal decomposition of nickel formate; EDA for enantio(face)-differentiating ability (see footnote 2); GA for glycolic acid; HNi for nickel catalyst prepared from NiO by reduction; HNi-1 for HNi prepared from light-green NiO; HNi-2 for HNi prepared from dark-green NiO ; MAA for methyl acetoacetate; MHB for methyl 3-hydroxybutyrate; MRNi for modified Raney nickel catalyst (see p. 216); X-MRNi for Raney nickel catalyst modified with reagent X (see p. 216 and Scheme I ) ; RNiA for RNi pretreated with 2-hydroxy acid; OY for optical yield; and TA for tartaric acid (optically active). 215 Copyrighl 0 1983 by Academic Press. Inc All rights of reproducuon in any form reserved ISBN 0-12-007832-5

216

YOSHIHARU IZUMI sp2-prochiral center

I CH2COOCHj

CHzCOOCH3 I

HO-C -H I

CH3

f

<

(S,SI-TA-MRNi si-face

(SI -MHB

+2H

+2H

(R,RI-TI-MRNi

Enontioface MAP.

H-C-OH CH3 I

re-face

(RJ-MHB

FIG. 1. Enantioface-differentiating (asymmetric) hydrogenation of MAA to MHB. TAMRNi: RNi catalyst modified with tartaric acid.

catalyst. Among the modified catalysts, Raney nickel (RNi) modified with optically active compounds has the longest history of investigation and has been the most systematically studied. By modification with optically active compounds, the RNi is endowed with the new function of enantiofacedifferentiation2 (asymmetric) in addition to the function of hydrogenation. Thus, the RNi modified with an optically active compound can catalyze an enantioface-differentiating (asymmetric) hydrogenation.’ For instance, RNi modified with an optically active amino acid or hydroxy acid hydrogenates methyl acetoacetate (MAA) to produce optically active methyl 3-hydroxybutyrate (MHB) as shown in Fig. 1. Raney nickel modified with an amino acid or hydroxy acid can be prepared by a very easy and simple method. Scheme 1 shows the standard procedure for the preparation of modified RNi (MRNi). Raney nickel is prepared from 1.5 g of alloy (Ni/Al = 42/58) by the digestion with 20 ml of 20‘x aqueous sodium hydroxide at 100°C for 1 hr followed by 1 5 successive washings with 30-ml portions of water. The RNi thus obtained is modified by soaking for 1 hr with occasional shaking in 100 ml of aqueous modifying ”Enantioface-differentiating hydrogenation” is a new terminology based on the new concept “stereo-differentiation’’ (63) described in Section VII (see footnote 3). However, readers will be able to understand most of this review, except for particular parts, if they replace “enantio(face)-differentiating” or “enantio(face)-differentiating ability” by “asymmetric” or “asymmetric activity,” respectively. An “enantioface-differentiating reaction” is a reaction in which one enantiomer is produced more than the other from the sp2-prochiral compound represented in a general form

as M A A , as shown in Fig. I . Both sides of the molecular plane around the sp*-prochirdl center are called enantiofaces. The optically active compound is produced when the catalyst o r reagent differentiates one of the enantiofaces prior to the addition reaction. Thus, we call this type of reaction an “enantioface-differentiating reaction.”

MODIFIED RANEY NICKEL CATALYST

21 7

Raney-Ni alloy

1

DIGESTION (1) with NaOH (2) washed with water

I

Raney-Ni MODIFICATION

( I ) with optically active compound - - - - - - - - - - - - - - Modifying reagent at specific pH _ _ - - _ - -- - - - - _ _ _ _ _ _ _ _ Modifying pH and temperature - - - - - - - - - - - - - - - - - - - - - - - - - - - Modifying temperature ( 2 ) washed with water and methanol MRNi HYDROGENATION 1

______

__

CH,-CO-CH,-COOCH,~

n

CH,-CH-CH,-COOCH, AH

MAA

[alpof (R)-MHB =

MHB -22.95 (neat)

[a]? of (S)-MHB = +22.95 (neat)

SCHEME 1. Preparation of MRNi

solution, which has been adjusted to the specified pH and temperature. After decantation of the modifying solution the catalyst is washed with water, methanol, and reaction solvent. Since the pH and temperature of the modifying solution affect the enantiodifferentiating ability (asymmetric activity) (EDA) of the catalyst, they are very important factors and are called modifying pH and modifying temperature, respectively. Methyl acetoacetate is used as the standard substrate for the determination of the EDA of MRNi, and the standard conditions for hydrogenation are 60°C and 100 kg/cm2 of hydrogen pressure. The EDA (%) of MRNi is defined as optical yield (OY) (%). The OY of the hydrogenation is based on the value of [a]? = 22.95 (neat) for optically pure MHB. Since MRNi is prepared from commercially available cheap materials by the simple method described above, MRNi is one of the most economical catalysts to be devised for the practical enantio-differentiating reaction. Most of the studies on MRNi have been carried out by our research group with the support of Japanese physicochemists since 1962 (I-52e). Thus, MRNi is one of the very few catalysts which has been studied deeply and systematically with respect to its nature and catalytic properties on the same experimental base. Furthermore, MRNi has a much longer history than enantio-differentiating organometallic catalysts. In the present review the profile of MRNi will be introduced from several viewpoints.

218

YOSHIHARU IZUMl

II.

History of Discovery and Development of MRNi

The historical origin of MRNi stems from our discovery of silk-palladium (silk-Pd) catalyst in 1956. This was the first of the enantioface-differentiating heterogeneous catalysts. The silk-Pd catalyst was discovered from a very primitive concept for the reaction mechanism of leucineaminopeptidase. That is, we expected simply that if the reaction was performed in an optically active chiral environment, an optically active product should be produced from the influence of the optically active environment, like baking a waffle. Since silk fibroin contains a crystal structure as shown in Fig. 2 and has no sulfur amino acids, we used silk fibroin as a “waffle iron.” Palladium metal was introduced between silk-fibroin molecules by the hydrogenation of silk fibroin-PdCl, (silk-PdCl,) complex. The silk-PdCl, complex was prepared by boiling silk fibroin with an aqueous solution of PdCl, . For several months we obtained successful results with the resulting silk-Pd catalyst (53). Later we found, unfortunately, that the EDA (asymmetric activity) depends greatly on the nature of the silk fibroin, and some very poor reproducibility in enantio-differentiation was observed with silk from different sources. In order to solve this problem, the effect of silk fibroin on the property of metal catalyst was investigated with silk-Pd (54-56), silk-Pt (57, SS), and silk-Rh (59). All these silk catalysts were found to have very poor hydrogenation activity for the hydrogenation of =C=O to =CHOH, though they had rather stronger activities than the corresponding metal catalysts in the hydrogenations of -NO,, =C=N--, and =C=C=.

FIG.2. Schematic model of silk fibroin: (-)

fibroin molecule; (----) hydrogen bond.

219

MODIFIED RANEY NICKEL CATALYST

To understand the above finding, the idea of competitive inhibition in biochemistry was introduced. That is, we were concerned that the -C=O group of the peptide bond in protein molecules might inhibit the active site for the hydrogenation of =C=O in MAA, since the =CO group in protein has a much stronger affinity for the active site for =CO hydrogenation and is more resistant to hydrogenation than is the =C=O in MAA. It was thus expected that a variety of hydrogenation catalysts should lose their activity for =C=O hydrogenation by the treatment of these catalysts with

TABLE I Hydrogenations of Diethyl2-Oxoglutarate (1) and 2- Methyl-4-benzaloxazoline ( 3 ) with RNi or RNis Treated with (S)-AminoAcids EtOOC-CH,-CH,-CO-COOEt

EtOOC-CH,-CH,-CH-COOEt

I

OH 1

2

H &

Ph-CH=C-CO

I

N

I

\ /

HO

Ph-CH,-CH-CO

O

I

I

N

O

\ /

Ph-CH,-CH-COOH

I

NH,

C

C

4

3

5

Reaction conditions Reaction Catalyst

RNi RNi treated with (S)-tyrosine in 2 N HCI RNi treated with (S)-glutamic acidin2NHCl

Substrate Product

1 1 3

1

Pressure Temp. (atm) ("C)

2 2 4

2

70 30

56

[a]? of 5. OY was calculated from [a], of 5 obtained.

25 85 75 60 60 70

80

Solvent

[a],

Benzene AcOEt AcOEt AcOEt MeOH MeOH

+0.35 -0.15 +0.15 +0.45 - 1.0 -0.3"

MeOH

-0.25

OY (%) Ref.

::;1 3.3 9.8 :b

6

]

60a

)

606

220

YOSHIHARU IZUMI

aqueous solutions of protein. This working hypothesis was proven by treating RNi with protein solutions ( I ) . Further experiments were carried out with RNi treated with various chelating reagents with simple functional groups as well as carboxyl groups, such as amino acids or hydroxy acids. In all experiments mentioned above, the treated catalysts lost their activity for =C=O hydrogenation at 10°C,but a small activity remained at above 60nC ( I ) . When the hydrogenation was carried out at above 60°C with RNi treated with an optically active reagent, an enantio-differentiating reaction occurred. (S)-Glutamic acid and MAA were chosen as an optically active modifying reagent and a substrate, respectively, and with them we found enantio-differentiating hydrogenation (2). We named this catalyst “modified catalyst” instead of poisoned catalyst, because the effect of treatment with the chelate reagents on the activity of the catalyst depended on the nature of the substrate. Independently, Isoda et al. .in 1958 reported strange results from hydrogenations in the presence of RNi (60a) and RNis treated with optically active amino acids (606). Table I shows examples of some of the results of their experiments. TABLE 11

Efeecr

of

Suhstituent on C(2),N,or 0 of Modvying Reagent on the EDA

oJ M R N i “

Product Modifying reagent

Modifying pH

[aJioofMHB

OY (%)

Ref.

15.3

3

(S)-HOOC-CH,-CH2-CH-COOH

5.1

-3.51

NH2 (S)-HOOC-CH,-CH,-CH-COOH

5.55

- 0.61

2.92

3

5.01

- 0.15

3.21

3

5.0

- 5.30

5.2

-2.15

6 .I

-

I

I

NHCOC,H,

(S)-HOOC-CH,-CH,-CH-COOH

I

N(CH,), (R)-HOOC-CH-CH-COOH

I

I

23.1

4

OH OH (R,R)- HOOC-CH-CH-COOH

I

I

9.31

4

8.37

6

OH OCOC,H, CH,

I I

(i)-HOOC-CH,-CH,-CC-COOH

1.92

NH, Modifying temperature: 0°C. Reaction conditions: MAA (neat), 6 0 ’ C . 80 kg/cm2

MODIFIED RANEY NICKEL CATALYST

22 1

In 1968, the following experimental rules were established with respect to the correlation between the structure of the modifying reagent and the EDA of MRNi (24) during the enantio-differentiating hydrogenation of MAA at 60°C : (1) R-CHX-COOH (X = NH2 or OH) is the preferred structure of the modifying reagent. The presence of any substitutent on C-2 and N or 0 decreases the EDA. The examples are shown in Table 11. (2) The direction of enantio-differentiation (the predominant enantiomer R or S , to be produced) is decided by two factors. One factor is the configuration of the chiral structure, that is, if the catalyst modified with (S)glutamic acid [(S)-Glu-MRNi] produces (R)-MHB from MAA, then (R)Glu-MRNi produces (S)-MHB (2). The other factor is the nature of X. That is, when the amino acid or hydroxy acid with the same configuration is used as the modifying reagent, the configurations of the predominant products are enantiomers of each other in most cases. For example, (S)aspartic acid-MRNi produces (R)-MHB and (S)-malic acid-MRNi produces (S)-MHB (19).

I

I

R = H-C-H

A

H-C-H I

CH3

CH3-C-H I

CH,-C-CH, I

CH3

C"3

FIG. 3. Effect of substituent of modifying reagent, R-CH-COOH.

on EDA: ( A )

I X X = NH, (modified at pH 6 . O'C); (0) X = OH (modified at pH 5.0, 0 C). Reaction conditions: M A A (neat). 60 C, 80 kgjcm'.

Y OSH 113A K U I Z U M I

112

( 3 ) The degree of EDA is governed by the nature of the substituent (R). The effect of R is observed often in opposite directions between amino acid MRNi and hydroxy acid-MRNi. For example, the increase in bulkiness of R increases the EDA of amino acid-MRNi but decreases that of hydroxy acid -MRNi as shown in Fig. 3 (11, 12). An increase (decrease) in electron density at the chirdl center of the modifying reagent increases (decreases) the EDA of amino acid-MRNi and decreases (increases) the EDA of hydroxy acid-MRNi as shown in Table 111 (52a). (4)In all cases, when an acidic modifying reagent is used, the modification at or near pH 5.0 gives MRNi with the highest EDA (2, 3, 10). ( 5 ) If the modifying reagent has two chiral centers, the configuration of the second chiral center greatly affects the EDA of MRNi (10, 21, 22). Examples are shown in Table IV.

On the basis of the above findings, optically active tartaric acid (TA) and its derivatives were determined to be the best modifying reagents of all.

Modifying conditions Modifying reagents

Product

pH

Temp.( C)

[ci]i5 of MHB

2.9

0

+0.14

0.61

+ 0.25

I .09

Reaction conditions: MAA (neat), 6 0 C , 80 kg/cm2

OY

('lo)

223

MODIFIED RANEY NICKEL CATALYST

TABLE IV Effect of Configuralion of Second Chiral Center of Modifying Reagent on EDA of MRNi" Modifying reagent, (R-CH-COOH)

I

X Modifying conditions

Configuration

R

X

C(2) tS

-CH

-CH

/ \ /

CH 2-

COOH

CH 3 COOH

\ OH

OH

(" R

Product

[a]? of Temp. ("C) MHB" OY (%)

C(3)

pH

s

5.94

R

5.94

-2.13

S

2.9

+0.68

2.96

R

2.9

- 0.22

0.96

S

4.0

0

R

4.0

0

R

5.0

0

s

5.0

0

0

- 1.95

8.50

-5.0

21.8

0

0

Ref.

>

Substrate: MAA (neat). Reaction conditions: 6 0 T , 80-100 kg/cm2. Neat.

Especially from the economic viewpoint, tartaric acid is the best. Therefore, research on MRNi has been focused mainly on RNi modified with optically active tartaric acid (TA-MRNi). Among the factors to affect the EDA of MRNi in the reaction system are trace amounts of organic acids, amines, and hydrogen acceptor compounds (23-25, 28). Among all, the positive effect of acids was one of the most important findings for the effective performance of enantio-differentiating hydrogenations with MRNi. There had been two major problems to overcome for the development of MRNi with high EDA. One was the possibility of the presence of two kinds of differentiating sites on the surface of MRNi. That is, even though the same modifying reagent was used, MRNi often produced an opposite direction of EDA when modifying conditions were varied. The other problem was the possibility of the presence of unmodified surface area on the catalyst. The former problem was overcome by finding conditions for the treatment of RNi with hydroxy acid under acidic condition. Uniform differentiating sites were prepared by this treatment (47).

224

YOSHIHARU IZUMI

The latter problem was overcome by the discovery of a second modifier. Since the discovery of MRNi, whenever we got an unusual result, we explored factors such as modification and reaction conditions to determine if they might increase the EDA of MRNi. We accidentally found that anions in our water supply remarkably increased the EDA of TA-MRNi during the dry summer of 1978 when we were supplied very dirty water in Osaka. Because of this result, sodium halides were examined for their abilities as the second modifier, and NaBr was found to be the best second modifier. This led to our discovery of the practical MRNi, TA-NaBr-MRNi (37,47). The durability of TA-NaBr-MRNi was greatly improved by embedding it in silicon rubber (48). During the development of MRNi, in 1977 (61~7,6Zh) we proposed an hypothesis about the mechanism of hydrogenation on the surface of metal catalysts ( 6 1 q 616). In 1971-1974 we proposed the name “stereo-differentiation,” which is the basic principle for the so-called asymmetric reactions (24, 32, 34, 38. 62, 63). These have been the working hypotheses for the development of MRNi.

111.

Profile of MRNi in Hydrogenation

A. HYDROGENATION ACTIVITY Before the discovery of MRNi, we believed the conventional theory of hydrogenation. That is, the adsorption of substrate and hydrogen on the surfaces of catalysts were essential conditions to promote hydrogenation, and other substances adsorbed on the surface behaved as poisons. The results obtained in the studies of MRNi, however, often turned out contrary to these expectations as shown in Tables V-VII ( I ) . The effect of the modification on the hydrogenation activity of RNi greatly depended on the sorts of substrates and modifying reagents. Without exception, RNi lost substantially all its activity for the hydrogenation of =C=O. However, in the hydrogenation of =C=C= or -NO,, the activity of RNi was not inhibited, and in some cases it was enhanced by the modification. In the case of substrates which are corrosive to Ni, when the hydrogenation was carried out under high pressure, TA-MRNi exhibited the highest reaction rate, whereas RNi and Glu-MRNi exhibited the same rate as shown in Fig. 4 (30). Hydrogenations of these substrates under atmospheric pressure did not proceed in liquid phase over unmodified RNi because of the corrosion of the catalyst by the substrate.

225

MODIFIED RANEY NICKEL CATALYST

TABLE V Effect of Modu.ving Reagent on Hydrogenation Activity of RNi"

Hydrogenation activity Modifying conditions

MEK~ Concentration Reagent None Glutamic acid (Glu) Monosodium glutamate Aspartic acid (Asp) Leucine (Leu) Glycine (Gly) Threonine (Thr) Hydroxyproline (Hypro Phenylalanine (Phe) Arginine (Arg) Glycylglycine (Gly Gly) Gelatin EDTA ' 2 N a Succinic acid Sodium acetate Dimethylglyoxime Ethylenediamine hydrochloride

(%I

PH

1 .o 1.2 1 .o

4

0

4 7 7 I

0 0 0 0 0 0

I .o I .o I .o

1.o

2

I

I

I .o

I I1

0.5 1.85 I .3 0.5 0.8 I .o

6 4 3

1.o 1.o

10°C

5

5

I

0 0 0 0.5 0.5 0 2 0.5 0

60°C 20 I .5 2 2

3.5 3 3 3 0.5-1 3.5 2 1.5 4 3-3.5 6

5.5 3

All' I 0-'C

34 36 30 35 34 35 35 38 36 22 45 38 31

25 (50) (52)

a Catalyst: RNi. Modifying condition: room temperature. Substrate: ethyl methyl ketone (MEK) (neat), allylalcohol (All) (neat). Reaction condition: 95 kg/cm2. Activity is expressed by percentage of conversion in 30 min. ' Activity is expressed by percentage of conversion in 5 min.

B. KINETICS Figure 5 shows Arrhenius plots of reaction rates and OYs in hydrogenations of MAA with several MRNis under atmospheric pressure (34). Arrehenius plots for all of the catalysts gave parallel straight lines with an apparent activation energy of 10.5 f 0.5 kcal/mol, regardless of the values of OY. Arrhenius plots for catalysts modified with homologs lay on the same line. Since the hydrogenation of MAA with unmodified RNi did not proceed as mentioned in the previous section, the kinetic parameters of the liquidphase reaction with MRNi under atmospheric pressure could not be compared with those of RNi. However, it can be expected that the modification does not change the nature of hydrogenation with RNi since the activation energies of MRNis were exactly the same as each other and independent of the sort of modifying reagent. This expectation was confirmed by the results

226

YOSHIHARU IZUMl TABLE V1 Hydrogenation Activity of RNi Modified with Glutamic Acid (Glu-MRNI)" Hydrogenation activityh Reaction temp. : SubstrateiCatalyst Ketone Acetone Me Et ketone Acetophenone Cyclohexanone C=C double bond Ally1 alcohol Cinnamic acid Maleic acid Ethyl acrylate Diethyl maleate Cinnamic aldehyde a

10°C RNi

60°C

Glu-MRNi

0 0

5.7

2 8 1.8

0

0

RNi

Glu-MRNi

55 20 120 24.3'

8 2 22

I

34

34 16 41 100 100 12.5

16

63 3 mind

4 mind 14

0.5

-

Reaction condition: 70-90 kg/cm2. Hydrogenation activities for \ ,C=O

are expressed, respec-

and

tively by the percentage of conversions in initial 30 and 5 min. ' Hydrogenation was carried out at 80°C. Time for completion of hydrogenation.

of the gas-phase hydrogenation obtained by Yasumori (64). He found that the activation energy over the nickel catalyst prepared from nickel formate (DNi) was mostly the same as that over DNi modified with tartaric acid (TA-MDNi). Kinetic parameters for the hydrogenation of MAA on DNi, MDNi, and MRNi are summarized in Table VIII. TABLE VII Hydrogenation Activity of MRNis for Nitrobenzene" Catalyst Reaction temperature ("C) 10

40

RNi

Glu-MRNi

9 16

2 8

DimethylglyoximeMRNi 32 45

Hydrogenation activity is expressed by the percent of conversion in an initial 10 min.

227

MODIFIED RANEY NICKEL CATALYST

Time Ihr)

FIG.4. Rates of hydrogenations of AA (a) and MAA (b) with MRNis: (0) RNi; (0) TA-MRNi (modified at p H 5.0,O"C);(A) Glu-MRNi (modified at pH 5.0,O 'C). Reaction conditions: AA or MAA (10 ml, neat), MRNi (prepared from 1.0 g of alloy), 65°C. 100 kg/cm2.

TABLE Vlll Kinetic Parameters for the Hydrogenation of M A A on RNi and TA-MRNi -~

Reaction conditions:

Gas phase (100-300 Torr) NiD"

Catalyst: Activation energy (kcal/mol) Reaction order MAA H2

From Ref. 64. From Ref. 34.

Modified with alcoholic s o h . of T A Unmodified 10.6 k 0.6

1.OkO.l 0.0*0.1

10.5 & 0.5

1.1kO.1 O.O+O.l

~~~

Liquid phase (atm. pressure) RNib

Modified with aqueous s o h . of T A 10.5 f 0.5

10.3 f 0.5

0.8k0.1

0.2-0.3

O.O*O.l

1.0*0.1

10.5

+ 0.5

0.2-0.3 0

Modifying Reagent

Optical Yield

(%I 25.6 2-Hydroxy-3methylbulyric acid A N,N-Dimethylalonins 0 N-Methylvoline

4

0.7 0.1 3.0

0 Alanine A Butyrine

1.1 4.1 13.2 4.2

Voline 3

8.9 6.7

7.3

-

I

5

A

2

1

0 2.0

2.9

3.0

3.1 ( VT x

3.2

3.3

3.4

lo3)

FIG.5. Arrhenius plots and optical yields of hydrogenations of MAA with MRNis. Catalyst: RNi (digested at 20 f 2°C and kept at 75-78'C for 45 min). Modifying conditions: isoelectric point, 0 'C. Reaction conditions: MAA (neat), atmospheric pressure. TABLE IX Corretution between the Rate, Optical YieU and Absorbed Amounf of Modifying Reagent on the Catalyst"

Modifying reagent (R,R)-Tartaric acid L-2-Hydroxy-3-methylbutyricacid L-N,N-dimethy lalanine L-N-methy haline L-Alanine L-Butyrine L-Valine L-Leucine L-Glutamic acid L-Omithine L-Lysine ~

~~

Rate of hydrogenation (mmol/hr)

18.1 14.8 13.2 13.2 10.6 10.3 10.7 10.3 7.2 3.5 3.9

Optical yield (%)

25.6 0.7 0.I 3.8 1.1 4.I 13.2 4.2 8.9 6.7 7.3

Adsorbed amount of modifying reagent (mmol/g cat.)

0.I33

0.185 0.216 0.273

~~

RNi: digested at 75-78°C for 45 min. Modifying conditions: at isoelectric point, 0°C. Reaction conditions: MAA (neat), 70°C atmospheric pressure.

229

MODIFIED RANEY NICKEL CATALYST

TABLE X Effect of Bulkiness of Ester Group ( R ) of Acetoacetate on the Hydrogenation Rate and ihe Optical Yield (0Y )”

R

Concentration neat (mol/liter) at 25°C

Rate of hydrogenation (mmolihr)

Methyl Ethyl n-Propyl n-Butyl i-Propyl t-Butyl

9.22 7.87 6.67 6.18 6.84 6.1 I

10.7 9.7 8.7 7.0 6.2 3.8

OY

(%) 11.7 16.4 14.9 11.1 14.3 ~

a Preparation of RNi and modifying and reaction conditions are the same as that stated in Table IX.

There exists a good correlation between the hydrogenation rate of MAA with MRNi and the amount of adsorbed modifying reagent as shown in Table IX (34). This finding indicates that the hydrogenation rate of MAA greatly depends on the surface area of the catalyst which is not occupied by the modifying reagent. In other words, the reaction rate depends on the surface density of the substrate. This finding was also supported by the following phenomena. That is, the hydrogenation rates of various esters of acetoacetic acid over RNi modified with valine (Val-MRNi) were found to have a good correlation with the molecular volume of the ester as shown in Table X (34). IV.

Profile of MRNi in Stereo-Differentiation3

A. ENANTIOFACE-DIFFERENTIATING ABILITIES Enantioface-differentiating ability is the most widely studied character of MRNi because MRNi has been studied with the goals of establishing the fundamental concept of stereo-differentiation and developing enantiofacedifferentiating catalysts for practical use. The EDA is the important parameter indicating the ability of MRNi for the production of optically active The classification of stereo-differentiation (63) (see Section VII) is as follows : enantiomerdifferentiation includes enantioface-differentiation, enantiotopos-differentiation,and enantiomer-differentiation ; diastereo-differentiation includes diastereoface-differentiation, diastereotopos-differentiation, and diastereomer-differentiation.

230

YOSHIHARU IZUMl

compounds. Furtheremore, the EDA gives information about the reaction mechanism and the surface conditions of the catalyst. This information is quite different from that obtained by conventional methods from kinetic or physicochemical studies. Moreover, the surface study of catalysts by means of modification can be carried out under practical reaction conditions and special conditions as the use of high vacuum are not required. The focus of the present section will be not only on the stereochemistry of the reaction with MRNi, but also on the nature of the surface area of RNi. 1. Eflect of’ Modifying Conditions Most of the properties of MRNi were determined by varying the modifying conditions. Modifying conditions not only affect the degree of EDA of MRNi but often affect the direction of its EDA. a. Modifying Temperature. The temperature of the modifying solution (modifying temperature) affects the EDA of MRNi (2-7, 10-12, 15, 19,21, 30,47), as shown in Fig. 6, in a way which depends also on the structure of the modifying reagent. For example, the modifying temperature does not

-

-

m

-I

I

N 0

m I

I

-3.’

L

0

O

s o

a

T

Y

; = clO.0 5 Y

.0 a

t

-2.

:

12.0

V

c

0

.-cC0 L

-e

C

* 8.0

fj c

-e

-1.

c6.0 ;. .a

0

..n

0

0

4.0

+ 2.0 0

20

40

60

80

100

Modifying temperature (‘C)

FIG.6 . EfTect of modifying temperature on EDA with the following modifying reagent and conditions: (0) ( +)-erythro-2-methyltartaric acid, pH 5.0-5.2, 0°C; ( 0 )6,s)-tartaric acid, pH 5.0-5.2, 0°C; ( A ) (+)-2-methyl glutamic acid, pH 5.0, 0°C; ( 0 )(S)-valine, isoelectric point, 0 C; ( 0 )(S)-glutamic acid, pH 5.2, 0°C. Reaction conditions: MAA (neat), 60”C, 80-100 kg/cmz.

23 1

MODIFIED RANEY NICKEL CATALYST

affect so much the EDA of MRNi in the cases of modifications with 3-alkylamino acid and 3-alkylhydroxy acid. In some special cases such as modifications at different temperatures with (S)-glutamic acid [(S)-Glu], two kinds of MRNi were prepared. The (S)-Glu-MRNi modified below 80°Cproduced (R)-MHB preferentially, whereas modification above 80°C produced (S)MHB (2).This phenomenon indicates the possibility that there are two kinds of differentiating sites functioning in opposite directions of enantio-differentiation on the surface of the catalyst. If the proportion of these two sites on the catalyst surface may change depending on modifying temperature, then the overall OY and direction of enantio-differentiation may significantly change from one direction to the other. The ratio of the two differentiating sites must be one of the important factors which decide the degree of EDA of MRNi. b. Modifying p H . The EDA of MRNi also depends on the modifying pH (2-12, 14, 19, 12,29,30,33,47) (Fig. 7). There.exists an optimum modifying pH near pH 5.0 for obtaining an effective MRNi when modified with an optically active acid. One hydroxy acid containing a phenyl group gave two kinds of MRNi with different directions of EDA by modifying below and above pH 10.5. This phenomenon also indicates the possibility that the EDA of MRNi does

I 2

4

6

8

10

12

Modifying pH

FIG. 7. Effect of modifying pH on EDA of MRNi: ( 0 )(S)-manderic acid MRNi (33);

(0) (S)-2-hydroxy-3-phenyl-propionic acid-MRNi (33); ( 0 )(S)-aspartic acid-MRNi (IY); ( A ) (R,R),tartaric acid-MRNi (25). Modifying conditions: 0°C. Reaction conditions : MAA (neat), 60'C, 80-100 kg/cm2.

232

YOSHIHARU IZUMl

not only depend on the density of differentiating sites, but also on the ratio of the two kinds of differentiating sites, as in the case of the effect of the modifying temperature. c. Pretreatment with Hydrogen Acceptor. When MRNi was treated with hydrogen acceptors prior to the reaction, its EDA was increased (28) (Table XI). This phenomenon is difficult to explain by the conventional theory, because such mild reagents should change neither the structure of the modifying reagents nor the surface structure of the catalyst. d. Pretreatment with Hydroxy Acids. MRNi reveals a complicated relationship between the effects of modifying pH and temperature on the EDA of MRNi as mentioned in Sections IV,A,l,a and b. However, when RNi is pretreated with hydroxy acid prior to modification, the complicated relationship between the effects of modifying temperature and pH disappears and the EDA of modified catalyst becomes stable to the changes in modifying conditions. Figure 8 ( 4 7 ) shows the effect of modifying temperature on the EDA of MRNi prepared from RNi pretreated with 1% aqueous glycolic acid at 100°C for 1 hr [RNiA (GA)]. Results obtained with MRNi without pretreatment also are shown in Fig. 8 for comparison. Only a small effect of modifying temperature on EDA was observed with MRNiA and the greater EDA was obtained with RNiA. Especially in the case of modification with TABLE XI Eflecr of Various Hydrogen Acceptors on the EDA of MRNi"

oy (%J Reagent (ml) N o treatment Acetone (1 8) Methyl ethyl ketone ( I 8) Cyclohexanone (18) Methyl acetoacetate (lot 70) (18) Cyclohexene ( I 8) Diethyl maleate (18) Diethyl fumarate (18) Acrylonitrile (18) Nitromethane ( I 8)

Time for treatment

(S)-ValineMRNi

(S)-AlanineMRNi

90 90 90

10.8 13.3 15.8 10.7 10.8

0.17 0.13 0.78

90

18.1

90 90 90 90 90

16.3 18.0 Unhydrogenated

I .39 1.31 2.31 2.44 Unhydrogenated 0.83 0.00

-

Modifying conditions: isoelectric point, 0°C. Reaction conditions: MAA (neat), 60°C. 80 kg/cm2.

233

MODIFIED RANEY NICKEL CATALYST

FIG.8 . Effect of modifying temperature on EDA of MRNi and MRNiA. Catalyst: RNi (standard), RNiA (GA) (pretreated with glycolic acid at IOO'C for I hr). Modifying conditions: pH 5.0, 0 C. Reaction conditions: MAA (neat), 60°C. 110-130 kg/cm2.

+4

c

I

2

4

6 8 Modifying pH

10

12

FIG.9. Effect of pH on EDAs of HNi and RNi modified with (S)-mandelic acid: ( A ) HNi,

(0) RNi, ( 0 )HNiA, (0) RNiA. Modifying condition: 0 C. Reaction conditions: MAA (neat), 60 C, 100 kg/cm2.

234

YOSHIHARU IZUMI

glutamic acid, the change in EDA with change in modifying temperature was no longer observed with RNiA. A very similar phenomenon was observed in the effect of modifying pH on the EDA of MRNi. Figure 9 (52a) shows the effect of modifying pH on the EDAs of (S)-mandelic acid-MRNi and (S)-mandelic acid-MRNiA. In the case of modification of RNiA, the EDA of MRNiA did not change its direction with the change of modifying pH. The above findings indicate that the surface condition of RNi is simplified by the pretreatment with acid, and only one kind of differentiating site seems to be formed on RNiA. Since RNi contains a large amount of aluminum and 2-hydroxy acid is a strong chelating reagent, one difference between RNi and RNiA could be ascribed to their difference in aluminum contents. Table XI1 (49) shows the correlation between the aluminum content and the EDA of those catalysts modified with tartaric acid. The aluminum content of RNi was decreased by pretreatment with hydroxy acid. Moreover, reduced nickel prepared from NiO (HNi-1) gives an effective modified catalyst and its pretreatment with hydroxyacid does not affect its EDA. However, this consideration could not become a conclusive one because another complicating phenomenon was observed, as shown in Fig. 9 (52a). In the modification of HNi with (S)-mandelic acid, MHNi-1s producing (S)- and (R)-MHB were obtained by the modifications at pH 2.4 and pH 5.0, respectively. However, HNi pretreated with glycolic acid [HNiA- 1 (GA)] gave only one kind of MHNi-I which produced (R)-MHB regardless of the modifying pH. TABLE XI1 Enantio-Differentiating Abilities of Various Catalysts Modified with Tartaric Acid" A1 content OY No.

Preparation of catalyst

I

Raney alloy was leached with 20% aq. NaOH at low temperature (20°C) Raney alloy was leached with 20% aq. NaOH at high temperature (80°C) NiO (light green) was reduced with H, at 350°C RNI (H) wdS treated with a 1% solution of TA at 100°C RNi (H) was treated with a 1% solution of glycolic acid at I00"C HNi was treated with a 1% solution of TA at 100°C

2

3

4 5 6

Abbreviation

(%)

(%)

RNi(L)

6

35

RNi(H)

5

40

HNi-I(GA) RNi-A(TA)

0 2

76 62

RNi-A(GA)

3

72

HNi-A(TA)

0

76

' Modifying conditions: tartaric acid, pH 5.0, 0°C. Reaction conditions: MAA (11.5 ml), methyl propionate (23 ml), HOAc (0.2 ml), catalyst (0.8 g), I O O T , 90 kg/cm2.

MODIFIED RANEY NICKEL CATALYST

235

Furthermore, the EDA of MHNi strongly depends on the nature of NiO. Light green NiO gave an excellent MHNi (MHNi-I) without pretreatment with hydroxy acid. In contrast, reduced nickel (HNi-2) prepared from dark green NiO gave MHNi with poor EDA (MHNi-2). However, even HNi-2 gave MHNiA with high EDA when pretreated with TA (Table XIII). Since modification at pH 7.3 is not expected to have as much effect on the surface structure of the catalyst as modification at pH 3.2, the EDA of the catalyst modified at pH 7.3 can be expected to carry the information of the original surface structure of the catalyst. The EDAs in Table XI11 suggest that HNi-I has a much more uniform surface structure than HNi-2 and the pretreatment with hydroxy acid prepares a uniform surface structure on the catalyst (51). Since the catalyst is considerably corroded during pretreatment with acid, the particle size of the catalyst is expected to change during pretreatment. Two studies were carried out to examine the effect of crystallite size of the catalyst on the EDA of the modified catalyst. One was reported by Nitta (65a) in which the larger-crystallite catalysts gave the higher EDA. Very recently they detailed the relation between EDAs of various supported nickel catalysts modified with TA and the crystallite size of nickel (6%). That is, the EDA of a modified catalyst primarily depends on the crystallite size of nickel. The catalysts with larger-sized crystallites had higher EDA and lower hydrogenation activities than those with small-sized crystallite. Thus, the EDA of modified catalysts greatly depends on the crystallite size distribution (CSD). The catalysts containing around 10-nm crystallites with narrow CSD gave best results. To obtain reproducible results, careful attention should be paid to the precipitation process, the reduction temperature, and TABLE XI11 Enantio-Differentiating Abilities of M H N i and MHNiA"

Modifying pH Catalyst

3.2

7.3

TA-MHNi-I TA-MHNiA- 1 TA-MHNi-2 TA-MHNiA-2

71 77 60 77

71 81 10 59

~~

a Catalysts: HNi-I and HNi-2 were prepared from NiOs with light green and dark green, respectively by the reduction at 350°C for 1 hr. Modifying and reaction conditions were the same as those stated in Table XII.

a

HNi-2

Recovered TA-MHNi-2 ( Mod. at pH 7.3, 0

OC

1

Recovered TA-MHNi-2 ( Mod. at DH 3 . 2 , 100

OC

1

FIG.10. (A) Scanning electron micrographs of HNi-I and TA-MHNi-1 recovered from the reaction system. (B) Scanning electron micrographs of HNi-2 and TA-MHNI-2 recovered from the reaction system.

238

YOSHIHARU IZUMl

the time periods in the preparation of the catalyst. An adequate preparation of Ni AI,O, gave as high an EDA as Ni-SiO, . Ni-SiO, ( 1 : I ) prepared by reduction at 400'C for 3 hr was suggested as the best for hydrogenation activity and EDA. In contrast, we could not find a significant relation between the EDA of MHNi and the size of the catalyst (51). The above findings indicate that two kinds of surface structures exist cvcn on the surface of HNi and one kind can be removed by pretreatment with hydroxy acid. On the surface of RNi, aluminum does not directly participate in the formation of the differentiating site, but must participate in the formation of the surface structure of nickel which is removed by the pretreatment with hydroxy acid. The remarkable physical difference between HNi-1 and HNi-2 has not been discovered. Their electron microscope pictures are shown in Fig. 10 (51) for reference. e. S r w n d Modifying Reagent. Several sodium salts were found to be effective as second modifying reagents as shown in Tables XIV and XV ( 3 7 , 4 7 ) .Among them, sodium bromide (NaBr) was the best. The second modifying reagent causes a strong enhancement of the EDA of MRNi, even though it has no enantio-differentiating ability itself. The second modifying reagent is used as the subcomponent of the modifying solution. Figure 1 1 shows the correlation between the concentration of NaBr and the EDA of MRNiA as well as the correlation between the amounts of TA and NaBr adsorbed on TA-NaBr-MRNiA (47).The absorbed amount of TA decreased in the presence of a trace amount of NaBr, and no further decrease of absorbed TA was observed. The EDA of the catalyst increased TABLE XIV EfJrerr of Second Modifyitzg Reuyent on oJ TA N u X - M R N i (11)"

the EDA

Second modifying reagent "ax) (g)

OY

None NaH,PO4.2H,O ( I ) Na2S04 (10) NaNO, (0.I ) NaCl (1 0)

39.2 35.1 56.4 53.0 72.I

(%)

a Modifying solution: Ta(1g) and NaX in 100 ml of water. Modifying conditions: pH 3.2, 100°C. Reaction conditions were the same as those stated in Table XII.

MODIFIED RANEY NICKEL CATALYST

239

TABLE XV Efjtct of'Secnnd Modifying Reagent on the E D A of 7'A-NuX-MRNi (I)" Second modifying reagent (8)

N a F (3) NaCl ( 10) NaBr (10) NaBr NaBr (10)' NaI ( 5 x NiBr, (1)

OY

(%) 60.8 72. I 83. I 86. I 88.6 51.2 62.6

Modifying and reaction conditions were the same as those stated in Table XIV. Modification was performed twice. ' Modification was performed three times.

NaBr concentration (mol/liter)

FIG. 1 I . Effect of NaBr concentration of modifying solution o n amount of adsorbed T A T A ; ( 0 )OY; o r NaBr and enantioface-differentiating ability of TA-NaBr-MRNiA: (0) (0) Br. Catalyst: RNiA (TA) (RNi pretreated with 1% T A at p H 3.2 and 100 C for I hr). Modifying condition: T A (1%) + NaBr, p H 5.0, 0°C. Reaction conditions: MAA (11.5 ml), methyl propionate (23 ml), AcOH (0.2 ml), IOO'C, 110- 130 kg/crn2.

240

YOSHIHARU IZUMI

Comparison

TABLE XVI TA-NaBr- M R N i in Enantio- Differentiating Hydrogenation.F if Various Ketones

of' EDA hetween T A - M R N i and

(R.R)-TAMRNi

(R.R)-TA NaBr- MRNi ~

OY

OY

(73

(23 Substrate

CH,-CO-CH,--COOCH,~ CH,-CO(CH,),-COOCH,~ CH,-CO(CH,),-CH,'

(I)

39 3.4 40.3

([I)

(Conf.)"

Ref.

(R)!' (R)' (S)d

47 12

83' 38'

52d

65.6'

(Conf.)"

Ref.

11!l

(R) (R) (S)

47 52c 52d

2.1 11.1 1.52

" Conf. denotes the configuration of the preferential product. Reaction conditions: substrate (1 I .5 ml), methyl propionate (23 ml). AcOH (0.2 ml). 100 C, 85 kg:cm2.

' Reactionconditions:substrate(lOml),THF(20ml), pivalicacid(l8ml).

'

100 C. 100kg:cm'.

Modifying conditions: pH 3.2, 100 C. Modifying conditions: standard (see Table IX). Modifying conditions: pH 6 . 5 , 100 C.

TABLE XVll Comparison o / E D A hetween T A - M R N i and T A - - N a B r - M R N i in the ~ n a n t i o - n i ~ e r e n t i a t i ~ i g Hydrogenations of Ketones

(R,R)-TAMRNi

(R,R)-TANaBr-MRNi

OY

OY

(22 Substrate CH3-CO(CH~),-OCH,h CH,-CO(CH,),-OH~ CH,-CO-CH,-COOCH,' CH,-CO-CH,-CO-CH,d

(I)

(Conf.)"

33' 27.3" 39' 35'

(R) (R) (R) (R)

Ref.

(%) (11)

5 2 ~ 68#

42 47 46

59.8O 83' 74'

(Conf.)

Ref.

ll/I

(R) (R) (R) (R)

52r

2.1 2.2 2.1 2.1

42 47 46

C o d . denotes the configuration of the preferential product. Reaction conditions: substrate (10 g), T H F (20 ml), AcOH (0.2 ml), 85'C, 90 kg/cm*. ' Reaction conditions: substrate ( I I .5 ml), methyl propionate (23 ml), AcOH (0.2 ml), IOO"C, 85 kg/cm2. !' Reaction conditions: substrate ( I 10 g), T H F (320 ml), AcOH (2 ml), IOO'C,95 kgjcm'. ' Modifying conditions: pH 4.0, 100°C. Modifying conditions: pH 3.2, IOO'C. ' Modifying conditions: standard (see Table IX). a

'

MODIFIED RANEY NICKEL CATALYST

24 1

at approximately the same rate as the amount of NaBr absorbed on the catalyst. The rate of hydrogenation of acetone over TA-MRNi was higher than over TA-NaBr-MRNi (47). Thus, the NaBr inhibits partially the hydrogenation activity ofTA-MRNi, and the increase in EDA ofTA-MRNi with NaBr can be ascribed to the inhibition of hydrogenation at the unmodified surface of TA-MRNi. In other words, most of the hydrogenation with TA-NaBr-MRNi must be performed at the enantio-differentiating site. As shown in Table XVI, TA-NaBr-MRNi catalyzed the enantio-differentiating hydrogenation of ketones with a much higher EDA than TAMRNi. The increase of EDA of TA-MRNi with NaBr for each substrate can be correlated with the hydrogenation rates of each substrate at the modified surface and the unmodified surface. Table XVII shows the comparison of results of enantio-differentiating hydrogenations of ketones which have a general structure of R-COCHz-X-0over TA-NaBr-MRNi and TA-MRNi. The EDA over TA-NaBr-MRNi was twice as much as that over TA-MRNi without except ion. 2 . Effect of Reaction Conditions

Reaction conditions affect strongly the OY4 in the enantio-differentiating hydrogenation of MAA with MRNi and the stereochemical reaction mechanism. a. Temperature. Figure 12 shows the effect of reaction temperature on the OY of the enantio-differentiating hydrogenation of MAA with MRNi under atmospheric pressure (34, 36). The reaction temperature not only changed the OY, but also often changed the configuration of the predominant product. These findings indicate the possibility that two enantio-differentiations, producing (R)- and (S)-products, were carried out side by side in the reaction system. The change of configuration of the predominant product could be ascribed to the change in nature of the force contributing to the interaction between the modifying reagent and the substrate. b. Pressure ofHydrogen. The effect of pressure of hydrogen on the OY of the enantio-differentiating hydrogenation of MAA was carefully studied with a modified catalyst of silica supported nickel by Nitta e t a / . (66). They found that the OY decreases with increase of pressure of hydrogen up to 10 kg/cmz as shown in Fig. 13. This phenomenon is difficult to explain by the conventional theory based on the principle that the hydrogenation Since i t has not been decided whether the reaction conditions affect the E D A ofthe catalyst or the efficiency of EDA of the catalyst, optical yield (OY) is used instead of EDA in this section.

242

YOSHIHAKU IZUMI

0 (R,R)- Tartaric acid-MRNi 0 (S)-Glutamic acid-MRNi 0 (S)-Valine-MRNi (S)-Butyrine-MRNi 0 (S)-Alanine-MRNi 0 (S)-Hydraxy-msthyl butyric acid-MRNi

30

40

50 60 10 Temperature ("C)

80

Fiti. 12. Effect of hydrogenation temperature on optical yield. Reaction conditions: M A A (neat), atmospheric pressure. RNi and modifying conditions as slated as Table IX.

takes place after the substrate molecules absorb on the catalyst. If that is the case, the proportions of si- and re-faces of the substrate which are facing the catalyst in their adsorption state (see Fig. 1) should be independent of the pressure of hydrogen and the OY should be unaffected by the pressure of hydrogen. 60

I

I

I

5

10

20

I

50

H p pressure ( k g crnb2)

FIG 13. Effect of hydrogen pressure on the optical yield. Catalyst: Ni/SiO, = 1 : I . Modifying conditions: pH 5.1, 83 'C. Reaction conditions: M A A (10 ml). ethyl acetate (10 ml), 60 C .

243

MODIbIEI) KANEY NICKEL CAIALYSI

Also, Klabunovskii reported pressure dependences of the OYs in enantiodifferentiating hydrogenations of ethyl acetoacetate (EAA) with ruthenium (67). Raney cobalt (68), and RNi catalysts (69) modified with TA. c. Additires. Additives which are added to the reaction system oftcn exert a remarkable effect on the OY of the enantio-differentiating hydrogenation of MAA (23-25). Water is one such additive. For example, in most hydrogenations with amino acid-MRNis, the direction of differentiation was reversed by the addition of small amounts of water as shown in Fig. 14 (23, 25). Another additive, fatty acid, reveals a desirable effect on the OYs in the enantio-differentiating hydrogenations of ketones (24, 50, 52). The amount of fatty acid which gives an optimum OY greatly depends on the structures of the substrate and the acid. In the hydrogenation of MAA with TA-NaBrMRNi, acetic acid was not very effective (Fig. 15) (524. However, a drastic effect of acetic acid and pivalic acid was observed in the enantio-differentiating hydrogenation of 2-octanone with TA-NaBr-MRNi (Fig. 16) (50-52d). Acetic acid has a stronger effect than pivalic acid when a small amount is added, but the latter gives a higher optical yield than the former at the optimum amount. The greater effect of the structure of the carboxylic acid additives appears

0

2

4

6

8

10

H20 (mll FIG. 14. Effect of water on optical yield in the enantiofdce-differentiating hydrogenation wlth M R N I : (0) (S)-GIU-MRNI;( A ) (S)-Val-MRNi; (0) (S)-Ala-MRNi. Modifying condi-

tions: isoelectric point, 0 C. Reaction conditions: MAA (17.5 ml), water. 60 C, 80 kg/cm*.

244

YOSHIHARU IZUMI

3

.-

2

70.

0 .c

n

60

FIG. 15. Relation between the optical yield and amount of acetic acid in the enantiofacedifferentiating hydrogenation of MAA with TA-NaBr-MRNi. Catalyst: standard (see Table IX). Reaction conditions: MAA ( I 1.5 i d ) , T H F (23 ml), AcOH, 100 C, 100 kgicm’.

60

-ae

-

4c

0 ._ 2.

0

.c

a

0

2c

0

20

10

30

Acid ( m l l

FIG. 16. Relation between the optical yield and the amount of piviilic acid and acetic acid in the enantioface-dilt’erenti~ting hydrogenation of 2-octanonc with TA-NaBr- M R N i : (0) pivalic acid; ( A )acetic acid. CatalysI : standard (see Table IX). Reaction conditions: 2-octnnone (10 ml), THF: (20 ml). acid. 100 C. 90 kglcni’.

24 5

MODIFIED RANEY NICKEL CATALYST

in the hydrogenation of monoketones rather than in the hydrogenation of MAA (Table XVIII) (50, 52d).

B. OTHERSTEREO-DIFFERENTIATING ABILITIES In this section the diastereoface-differentiating5 ability and the enantiomer-differentiating‘ ability of MRNi will be introduced as examples. By modification with an optically active compound, RNi can acquire both enantiomer-differentiating ability and diastereoface-differentiating ability in addition to the enantioface-differentiating ability. The diastereoface- and enantiomer-differentiating abilities of MRNi can be observed when a substrate containing both chiral and sp2-prochiral centers is used, because such a compound has a diastereoface and a chirality. 4-Hydroxy-2-pentanone is one of the substrates with a chiral and sp2-prochiral center, as shown in Fig. 17. Diastereoface- and enantiomer-differentiating hydrogenations of 4-hydroxy-2-pentanone (6) with TA-MRNi and TA-NaBr-MRNi (46) will be introduced as an example. As found in Table XIX, (R,R)-TA-MRNi and (R,R)-TA-NaBr-MRNi produced (R,R)-2,3-pentandiol (7) from (R)-6. RNi and (S,S)-TA-NaBrMRNi produced (R,S)-7 predominantly in the hydrogenation of (R)-6. Those results indicate that RNi, which originally had a diastereoface-differentiating ability (DDA),’ producing (R,S)-7 in excess, gained a new DDA, producing (R,R)-7 by the modification with (R,R)-TA. The DDA of RNi was enhanced A diastereofacc-differentiatingreaction is a reaction in which one diastereomer is produced more than the other from the substrate containing both chirality and sp*-prochirality, a s shown in Fig. 17. Both sides of the molecular plane of such a molecule are called diastereofaces. One of the diastereomers could be produccd more than the other when the catalyst or reagent differentiates one of the diastereofaces and performs an addition reaction. Thus, we call this type of reaction a “diastereoface-differentiatingreaction.” An enantiomer-differentiating reaction is a reaction which resolutes one enantiomer from the chiral substrate as racemic compound. One enantiomer will be derived to the other compound when the catalyst or reagent differentiates one of the enantiomers and carries out the reaction. Optical resolution of acylamino acid with acylase is a typical one. (R):(S)-R-CH-COOH

I

NHAc

-+

(K)-R-CH-COOH

I

NHAc

+ (S)-R-CH-COOH I

NH,

Thus, an enantiomer-differentiating reaction is a kinetical resolution. not a synthetic reaction. ’ DDA is the parameter indicating the ability of the catalyst in the diastereo-differentiation. DDA is estimated by the difference ( y g ) of diastereomers in the product. D D A is a parameter comparable to EDA in the enantio-differentiation.

TABLE XVIII Effect of Additives on the Optical Yield in the Enantio-Differentiating Hydrogenations of 2-Octanone and MAA with TA-NaBr-MRNi 2-Octanone” Substrate additives None Acetic acid Propionic acid lsobutyric acid Pivalic acid Caproic acid Lauric acid Stearic acid a,a-Dimethyl caproic acid Diphenyl acetic acid 4-Cyclohexyl butyric acid I -Adamantancarboxylic acid Cyclohexanecarboxy lic acid 1 -Methyl- 1-cyclohexanecarboxylic acid

Amount

OY (%)

20.8 g 9.0 g 23.0 g

39.9 29.4 58.0

10.0 g 2.0g

41.9 14.7

18.8 g

48.8

13.5 g

42.1

14.9 g

52.5

Reaction conditions: (R,R)-TA-NaBr-MRNi

MAAh Ref.

Amount (ml)

OY (%)

Ref.

+

52d

(prepared from 3.8 g of alloy), 2-octanone

(10 ml), THF (20 ml), additive, IOO”C, 100 kglcm’.

’Reaction conditions: (R,R)-TA-NaBr-MRNi

(prepared from 1.9 g of alloy), MAA (1 1.5

ml),THF (23 ml), additive, 100”C, 100 kg/cm2.

FIG.17. Diastereoface-differentiatinghydrogenation of 4-hydroxy-2-pentanone.

247

MODIFIED RANEY NICKEL CATALYST

TABLE XlX Slereo-Differenrialing Hydrogenation of (R)-l-Hydroxy-2-penlanone(6) to 2,4-Pentandiol(7) will1 TA-NaBr-MRNi' CH,-CH-CH,-CH--CH,,-

I

CH,-CH-CH,-CH-CH,

II

I

0

OH

I

OH

Subst rate

OH

Product Product (%)

Catalyst

(2R,4R)-2

(2R,4S)-2

DE (%)"

RN i (R,R)-TA MRNi (R,R)-TA-NaBr-MRNi (S,S)-TA-NaBr -MRNi

49 64 90 45

51 36

- 2' 28 80

~

10

55

-10'

a Reaction conditions: catalyst (1.2 g), (R)-6 (1 1.5 ml), THF (23 ml), AcOH (O.? ml), 100°C. 100 kg/cm2. DE denotes diastereomer excess, which can be estimated as (2R,4R)-7 P0)-(2R,4S)-7 (%). ' The sign ( - ) indicates that (2R,4S)-7 was produced in excess

by modification with (S,S)-TA. The DDA with (R,R)-TA-NaBr-MRNi was much higher for (R)-6 than with other catalysts. The hydrogenation of substrate 6, which is a chiral compound, with an optically active catalyst (R,R)- or (S,S)-TA-NaBr-MRNi, causes not only diastereoface-differentiation but also enantiomer-differentiation of the hydrogenated product. Since the enantiomer-differentiation is one kind of kinetic resolution, the enantiomer-differentiating ability of a catalyst can be observed when the reaction is stopped at the 50% hydrogenation of racemicd. Table XX shows the results of the halfway hydrogenation of racemic-6 with (R,R)-or (S,S)-TA-NaBr-MRNi. When (R,R)-catalyst was used and hydrogenation was stopped at 55% conversion, (S)-6 with 30% of optical purity was recovered from the reaction mixture and (R,R)-7 was obtained in excess. When (S,S)-catalyst was used, (R)-6 with 29% of optical purity was recovered and (S,S)-7 was produced preferentially. The above facts indicate that (R,R)- and (S,S)-catalysts hydrogenate (R)-6 and (S)-6 at much higher rates than (S)-6 and (R)-6, respectively. That is, a high enantiomer-differentiating ability can be given to RNi by modification with optically active TA and NaBr.

248

YOSHIHAKU IZUMI

TABLE XX Entmliomer- DIfl>rentia/ing Hvdroyenation o/ Racemil, 4- Hydros~-2-i,c.ntanone( 6 ) 1vi1h T A NuBr ~ ~ MRNi"

Catalyst : Conversion of hydrogenation Recovered 6 Conliguration Optical purity Diastereomer ratio in hydrogenation product

(R.R)-TA--NaBr-MRNi

(S.S)-TA NaBr- MRNi

55

43

(x)

R

Sh 30"

(R.R)-7/(R,S)-7

=

80h/20

29' (S3S)-7/(R,S)-7= 77'/23

" Reaction conditions were the same as those stated in Table XIX. This result indicates that (R)-6 was hydrogenated more readily than (S)-6, and (S)-6 remained in excess in the reaction system (see the scheme in Table XIX). ' This result indicates that (S)-6 was hydrogcnated more than (R)-6.

V.

Other Profiles

RNi acquires durability toward corrosion from acetylacetone (AA) (CH,-CO-CH2-CO-CH,) by modification (30). Table XXI shows the amount of corroded nickel in the reaction system from the catalyst when the hydrogenation of AA (10 ml) with RNi or MRNi (400 mg) was carried out at 65°C under 100 kg/cm2 of initial hydrogen pressure and was stopped at the point where 1 mol of hydrogen had been consumed. The durability of TABLE XXI

Ejj+ct

01' Modifii,ation on Amount

?/Corroded N i 1vi111 AA ,from ~ l r cCatalyst"

Modifying conditions Catalyst (mg) 400 400 400 400 400 400 400 400 400 400

Amount of AA (ml) 10 10

10 10

10 10 10 10

10 10

Modifying reagent __ D-Tartaric acid D-Tartaric acid D-Tartaric acid D-Tartaric acid D-Tartaric acid L-Glutamic acid L-Glutamic acid L-Histidine -

pH

2.0 5.0 10.0 5.0 10.0

Temp. (' C)

0 0

0 I00 1 00

5.0

0

5.0

I on 0

7.9

Corroded Ni (mg)

121 35 29 91 64 97 9 28 10 303

Reaction conditions: MRNi (0.4 6). AA (10 ml). 65°C. 100 kgjcm2. Shaken lor the same number of hours as in Expt. No. I without hydrogen at 65 ' C

(';A) 30 9

7 23 16 24 2 7 3 76"

MODIFIED RANEY NICKEL CATALYST

249

MRNi was found to correlate with the chelating ability of the modifying reagent. However, as mentioned in Section III,A, the hydrogenation activity of the catalyst was not affected, but rather enhanced, by the modification. The phenomenon which has a close connection with the above finding was observed id the hydrogenation of MAA mentioned in Section II1,B. The hydrogenation of MAA with RNi could not be carried out at atmospheric pressure due to corrosion of the catalyst by MAA (32,34). The above findings suggest that both MAA and AA have difficulty in chelating the nickel surface of the MRNi catalyst. That is, MAA and AA have difficulty in adsorbing on the surface of the catalyst due to hindrance by the modifying reagent.

VI.

Surface Conditions

When compared with other heterogeneous catalysts, studies of surface conditions of these modified catalysts are quite difficult because the amounts of modifying reagent adsorbed on the catalyst are very small and the catalyst consists mostly of metal. Especially, the physical study of the adsorption mode of the modifying reagent is difficult because it is adsorbed as a monolayer or close to it. In the next section, the surface conditions of MRNi will be discussed in connection with the adsorbed modifying reagent. OF ADSORBED MODIFYING REAGENT A. AMOUNT

The amount of adsorbed modifying reagent on the catalyst is greatly influenced by the modifying conditions. Figure 18 shows the effect of modifying pH on the adsorbed amount of 2-hydroxy-3-phenylpropionic acid as an example (33).The EDA of MRNi does not completely correlate with the amount of adsorbed modifying reagent in most cases, because there often exists two kinds of differentiating sites as mentioned in Section 1V.A. The density of absorbed TA on TA-MRNiA was estimated as 1.7 x mol/m2. The estimate was based on the amount of absorbed TA on the MRNiA modified at pH 5 and 0°C and on the s u r f x e area of RNiA measured by means of BET (52b). The reason for making the comparison at pH 5 was because the change of surface conditions of RNiA by the modification is expected to be not so drastic at pH 5 as it is at a more acidic pH. The TA adsorbed on the nickel catalyst (DNi) prepared from nickel formate had been studied by chemical and physicochemical methods by Yasumori (64), and by electrochemical methods by Fish and Ollis (70), respectively. The number of nickel atoms occupied by TA on the surface of the catalyst was estimated to be 30% by both authors.

250

YOSHIHARU IZUMI

Modifying pH (adjusted with 1 M NaOH)

FIG. 18. ERicct of modifying pH on the adsorption amount of modifying reagent and EDA of 2-hydroxy-3-phenylpropionicacid-MRNi. (0) amount of modifying reagent; (0) EDA of MKNi. Modifying condition: 0 C. Reaction conditions: MAA (neat). 60'C. RO kg/cm2.

MODEOF MODIFYING REAGENT B. ADSORPTION The absorption modes of (S)-3-phenyl-2-hydroxypropionicacid, (S)-3phenyl-2-aminopropionic acid, and (S)-alanine adsorbed on a nickel plate or RNi were studied by Suetaka's group (71, 72). From the measurement of infrared (IR) dichroism in the reflection spectrum, the molecular orientation of the modifying reagent was deduced. Figures 19-21 show molecular orientations of (S)-2-hydroxy-3-phenylpropionicacid on a nickel plate and (R)-alanines on RNis modified at 5" and lOO"C,respectively.

Ni FIG.

19. Molecular orientation of thin (S)-phenylalanine film formed on nickel metal surface.

MODIFIED KANEY NICKEL CATALYST

25 I

Ni

FIG. 20. Orientational model in thin (R)-alanine crystal formed on nickel metal surfacc a1 5

c.

/

\

/

0

\

0

FIG.21. Orientational model in thin (R)-alanine crystal formed on "Raney nickel" surface at 100 C.

0

H -c

H-C

I -0' I -0.. I

,*'

',

'0-

,-0-C

-;Ni:' I

C --H

I -H I

.

'0,'

FIG.22.

Schematic projection of binuclear Ni tartarate anions.

252

YOSHIHARU IZUMI

Sachtler's group (73) and Yasumori (64) studied the 1R spectra of silicasupported Ni modified with amino acid and 2-hydroxy acid and the XPS of TA-MRNi. Both authors deduced almost the same model as proposed by Suetaka. Recently Sachtler's group proposed other models as shown in Fig. 22 from results obtained in enantio-differentiating hydrogenations of MAA with nickel catalysts modified with nickel and copper tartrates (74). The nickel tartrate adsorbs at the vacant coordination site of nickel in this model. To elucidate the role of one of the two carboxyl groups of TA in the adsorbed state on the catalyst, a study was conducted on the effect of the cation which was used for the pH adjustment of the modifying solution on the EDA of MRNi (29). As shown in Fig. 23, the EDA of TA-MRNi was strongly affected by the kind of cation used, and sodium was found to be the most favorable one, although the EDA of (S)-2-hydroxyisovaleric acidMRNi was not affected, as shown in Table XXII. From this finding it can be deduced that one of the carboxyl groups of T A participates in the adsorption, while the other must exist as a carboxyl anion and that the counter cation must be present near the carboxyl ion. Uyeda's group investigated by electron microscopy and diffraction (75) the epitaxial growth of amino acids on the (001) plane of nickel film or on the one modified with amino acid at a specific pH and temperature. The most preferable positions for the mutual orientation of both crystals, nickel

-10.0

-

0

01a

U

u

c U 0 3

2n c 0

-2 -

.-

-5.0

0

U 0

.-

n

0

0 Ionic radius ( % I

FIG.23. Effect ofcation used for iheadjustment ofmodifyingpH on EDAs of MRNis: (0) modified with (K,K)-TA, pH 5.0. 0 C ; ( 0 )modified with (R,R)-TA, pH 5.0. 100 C ; ( 0 ) modified with (R)-malic acid, pH 5.0.0 C. Reaction conditions: MAA (neat), 60 C , 90 kg/cm*.

MODIFIED RANEY NICKEL CATALYST

253

TABLE XXll Effecr oj'(btion Wscdjiw tlir Adjustmcw o/'Mod(/jing p H on EDA q/ (S)-2-HJ~dro.n.i.so1~uleric Acid-MRNi" Cation Li Ma K+ NH,' +

+

Optical purity of MHB (%)

I .39 1.35 I .35 1.35

'I Modifying conditions: modifying solution was adjusted to pH 5.0 with aqueous solution of metal hydroxide. RNi was modilied :it 0 C. Reaction conditions: MAA (neat), 60' C, 90 kg/cm2.

4 3 . 5 2

FIG.24. Lattice fitting for (001) nickel surface and overgrown (S)-glutamic acid: A and C. specific for low-temperature type; B. common to both types.

254

YOSHIHARU IZUMl

and amino acid, were determined by statistical treatment of the angular distribution of the appearance frequency. Three types (A, B, and C) of orientation of amino acid were observed. Figure 24 shows the schematic drawing of orientation of (S)-glutamic acid. A and C types of orientations were found specifically on the surface of samples modified at pH 5 and 0°C. B type existed as the predominant one in samples modified at pH 5 and at both 0” and 100°C. VII.

Mechanism of Enantio-Differentiation

Before our proposal of the new concept “stereo-differentiation,” it was believed that the “asymmetric reaction” proceeds by the difference of activation energy between reactions producing R- and S-enantiomers, and the difference of activation energy was simply ascribed to the difference of energy at the transition state. However the kinetic study of the hydrogenation of MAA with MRNi gave a quite different result from the one expected from the conventional theory as already mentioned in Ill-A. That is as shown in Fig. 5, all of apparent activation energies of hydrogenations of MAA with MRNi’s had been the same regardless of the reaction rate and of the optical yield. On the basis of the above findings, we assumed that activation energies of hydrogenations of si- and re-enantiofaces of substrate (see Fig. I ) must be the same as each other, and OY must be governed by the molecular ratio of substrates adsorbed with si- and re-enantiofaces with MRNi. In other words, the OY of the reaction is ruled by the difference of energies of adsorptions of si- and re-enantiofaces, and not by the difference of activation energy of hydrogenations of si- and re-enantioface reactions. For further elucidation of the reaction mechanism of MRNi, a stereochemical study of the enantio-differentiating hydrogenation of methyl CH,CO-CH-COOCH,

I

1 CH,-CH-CH-COOCH, I

I

OH CH,

CH, 8

9

Susceptible for racemization

Stable for racemization

-< -<

(W-8

(2s)-8

(2R,3R)-9 (2R,3S)-9 (2S,3S)-9 (2S,3R)-9

SCHEME 2. Enantio-differentiating hydrogenation of methyl 2-methyl-3-oxobutyrate.

255

MODIFIED RANEY NICKEL CATALYST H

CH3

I

CH,CO-C-COOCH,

I

H SCHEME3.

p

I I

CH,CO-C-COOCH, C”3

Racemization of methyl 2-methyl-3-oxobutyrate

2-methyl-3-oxobutyrate (8) (Schemes 2 and 3) was studied, because C(2) of 8 is highly susceptible to racemization, and C(2) and C(3) of the produced methyl 3-hydroxy-2-methylbutyrate (9) are very stable toward the racemization of Schemes 2 and 3 (39). The experiment was carried out under the following working hypotheses : (1) If the substrate racemic-8 interacts with the modifying reagent on the surface of the catalyst prior to the hydrogenation, the enantiomer distribution of 8 would deviate from O8 by the interaction of the modifying reagent. And the degree of the interaction between the modifying reagent and the substrate would reflect on the extent of deviation of the enantiomer distribution of 8. The enantiomer distribution of 8 would be kept in the chirality distribution’ of C(2) of the hydrogenation product, methyl 3-hydroxy-2-methylbutyrate (9). (2) If the differentiation was completed prior to the hydrogenation, the close correlation between the chirality distributions of C(2) and the newly produced C(3) of the product might be found in the results of the experiment.

Table XXIII shows the result of enantio-differentiating hydrogenations of 8 with various MRNis. A good correlation was observed between degrees of deviations of chirality distributions between C(2) and C(3). That is, the product 9 with higher deviation of chirality distribution of C(2) had also a higher deviation of C(3). Thus, the degree of chirality distribution of the newly produced chiral center is decided prior to the hydrogenation, as shown in Fig. 25. Based on the above findings, a new concept and a new term, namely “stereo-differentiation” (62, 63) is proposed. This concept consists of the The term “enantiomer distribution” expresses the difference (%) of the numbers of respectiveenantiomers [e.g., S (%) - R (%)I existing in the system. Since C(2) of 8 is highly susceptible to racemization, 8 exists as a racemate in natural. The enantiomer distribution of racemate should be 0. because a racemate consists of equal amounts of (R)- and (S)-isomers. The term “chirality distribution” indicates the difference (%) of numbers of specific chiral centers with (R)- and (S)-configurations in the system regardless of the sort of diastereomer. For example, the chirality distribution of C(2) in the reaction product can be calculated as can be calculated as follows. The contents (%) of four 2 s (%) - 2R (%). 2 s (%) and 2R (2,) stereoisomers (see Scheme 2) can be calculated from the diastereomer ratio [threo (%)/erythro (%)I and the optical purity of each diastereomer. The contents (%) of 2R can be estimated as the sum of the contents (%) of (2R,3R)-9 and (2R,3S)-9. In the same way, the contents (%) of 2s in the product can be obtained as the sum of the contents (%) of (2S,3S)-9 and (2S,3R)-9.

256

YOSHIHARU IZUMI

TABLE XXIll Enanrio-DflYrrmriatiny Hydrogenation of Merliyl2- Methyl-3-oxohulyrate ( 8 )wirli Various ModiJied Nickel Caral.v.sts"

Product 9 Chirality distribution

Optical purity

Ca talysth

Substrate

(R,R)-TA-MRNi (R,R)-TA - MDNi (R,R)-TA- M HNi (S)-Val MDNi (S)-Glu- MDNi DNi

a-MeMAA

Diastereomer ratio : erythrolthreo

(23

~

~

Erythro

Thrco

24.0 55.8 56.7 5.5 0.6

17.3 41.2 64.4 0.5 0

65.1/39.4 11.9122. I 18.4121.6 62.3137.7 61.6132.4 62.6137.4

~

c-2 12S-2RI

c-3 13R-3SI

8.3 34.4 30.4 3.2 0.4 0

22.0 52.6 58.4 3.6 0.4 0

' Reaction conditions: catalyst (8 g), 8 (100 g), T H F (300 ml). AcOH (1.5 g). 12O'C. I10 kgicm'. RNi: Raney nickel; DNi: catalyst prepared by the thermal decomposition of nickel formate; HNi: powder prepared by the hydrogenolysis of nickel oxide.

following basic principles : The differentiation was performed on the substrate by the topological recognition of the catalyst or the reagent, and the result of differentiation was decided not only at the transition state but also prior to the reaction. Differentiation is the factor which rules the prior step of the reaction as shown in Fig. 2 5 . Stereo-differentiation is divided into enantio- and diastereo-differentiations with their recognition modes. Both differentiations are subdivided into face-, topos-, and isomer-differentiation, respectively, depending on the location where the recognition is taking place.' Substrate

Product

aq

Modifier Modifier

\\\\\\\\\\\\\\\\<\\t\\\\\\\\\\ Differentiotinq Step

Hydropenation Step

FIG.25. Schcmatized model of enantio-differentiating hydrogenation on MRNi.

MODIFIED RANEY NICKEL CATALYST

257

On the basis of enantio-differentiating hydrogenation of methyl 2-methyl3-oxobutyrate (8) with TA-NaBr-MRNi, the model of enantioface-differentiation of MAA with TA is proposed in Fig. 26 (39). The stereochemical mechanism of enantioface-differentiation of MAA with TA was studied in more detail by Tai et al. (52c). Table XXIV shows the results of enantiodifferentiating hydrogenations of MAA with RNis modified with NaBr and various analogs of TA. All of the four functional groups of TA appear to be essential to perform the effective enantio-differentiation. The EDA of TA-NaBr-MRNi became half when one of the four functional groups of TA was derivatized, and when two of them were derivatized, the EDA of the catalyst was almost lost. As already mentioned, properties of MRNis in the hydrogenation are different in several points from the ordinary catalysts, and the mechanism of hydrogenation, based on findings described in Sections III,A, IV,A, I ,c. IV,A,2,b, and V, is difficult to explain by conventional theories. Thus, a new hypothesis is suggested (61a,b). This new hypothesis is constructed from the following situations. (1) Hydrogen molecules pile up on the surface of the catalyst and form a multimolecular layer in such a way that the bottom molecules are chemisorbed, whereas upper molecules are only physically adsorbed. (2) The polarization induced by the chemisorbed molecules can be transmitted to the hydrogen molecules in the upper part of the layer, and polarizable substances other than hydrogen can also serve as mediators of polarization transmission as shown in Fig. 27.

____-Hydrogen Bonding

Ni FIG.26. Schematic drawing of interaction between (R.R)-TA and MAA

TABLE XXIV.

Modifying reagenl

No. 1

Enuntio-Df~erenriuringAbilily ( E D A ) of T A or 11s Anulog

EDA of Configuration MRNi ofthe product (Configuration)

(x)

Structure

TA

COOH

I H-C-OH I

(2R,3R)

83

R(-)

(2R,3R)

74

W-1

(2R.3R)

65

R(-)

(2R,3R)

68

R(-)

OH-C-H

I

COOH 2

2-Methyl-TA

3

0-Benzoyl-TA

COOH

I CH,-C-OH I HO-C-H I

COOH COOH

I

H-C-OC-Ph

I I

HO-C-H

I1

0

COOH 4

0-Methyl-TA

5

Malic acid

COOH

I H-C-OCH, I HO-C-H I

COOH COOH

I I H-C-OH I

CH,

COOH

6

2.3-Dihydroxy butyric acid

I .2

S(+)

(2R,3R)

8

R(-)

(2RJR)

0.2

N-)

CH,

I

HO-C-H H-C-OH

I

COOH 7

0.0’-DibenzoylTA

COOH PhCO-C-

I1

0

I

H

I H-C-0-C-Ph I II

COOH 0

8

0.0’-DimethylTA

COOH

I MeO-C-H I H-C-OMe I

COOH

MODIFIED RANEY NICKEL CATALYST

H

H

359

H

FIG.27. Schematized model of activation of hydrogen o n the metal catalyst

(3) The hydrogenation reaction can take place at any place in the layer where favorably polarized hydrogen molecules are available. That is, the “hard and soft acids and bases” (HSAB) principle must be applied to hydrogenations on the surfaces of heterogeneous catalysts.

On the basis of the new hypothesis, it can be stated the hydrogenation must take place at an area distant from the surface of the catalyst where the chiral center may be present. A high enantio-differentiation can be carried out when areas where hydrogenation takes place overlap with areas where the differentiation is performed. The hydrogenation of a carbonyl group could take place at the rather vicinal place from the surface of catalyst where the highly polarized hydrogen (hard hydrogen ion) is present, because the carbonyl group is a rather highly polarizable one.’O In 1967, Klabunovskii’s group (76, 77) proposed a mechanism based on the hypothesis that the enantio-differentiation of ethyl acetoacetate (EAA) l o The hydrogenation of C=C must take place at a distant place from the surface with a weakly polaized hydrogen, because polaization of C=C is very weak. Thus, the hydrogenation activity of RNi for C=C is not affected by modification. Direct adsorption is not an essential factor for the hydrogenation of AA. The hydrogenation of AA on MRNi could be performed without corrosion and a fall of rate. The change of hydrogen pressure changes the density of the hydrogen layer o n the catalyst and results in change of the degree of fitness between the hydrogenation area and the differentiating area. And the efficiency of differentiation by the modifier could be influenced. In the same sense, the pretreatment of M R N i with a hydrogen accepter prior to the hydrogenation could bring the same results with change of the hydrogen pressure.

260

YOSHIHARU IZUMI A

B

FIG.2X. Stereochemical models of (A) intcrtnediate complex and (B) heteroligand complcx.

or AA with MRNi would be performed through the hydrogenation of a heteroligand complex which was the intermediate produced with the substrate and the modifying reagent adsorbed on the catalyst as shown in Fig. 28. In connection with the above mechanism, they reported in 1980 (78) the correlation between the conformation of the carbonyl ligand (EAA or AA) in a metal complex (Cu, Co, or Ni) which was heteroliganded with EAA or AA and modifying reagents, and the OY of enantio-differentiating hydrogenation of EAA or AA with corresponding MRNi. The conformational distortion of EAA or AA in the complex was studied by means of the CD spectrum of the complex prepared. There was a good agreement between the cdrbonyl ligand comformation and the results of enantio-differentiating hydrogenation in the case of AA. In the same paper he dealt with the mechanism of enantio-differentiating hydrogenation of AA with TA-modified catalyst on the basis of results in the IR spectrum of adsorbed tartaric acid

FIG.29. Stereochemical model of intermediate complex AA-Me-TA on the metal catalyst.

26 1

MODIFIED RANEY NICKEL CATALYST

r q 1p:^l o,

preferred coadsorption complex

. , ‘Ni’

IS) - omino acid modifier

%Ni*‘

( S ) - hydroxy acid modifier

Preferred orrongemenls of the coodsorption complexes

FIG.30. Stereochemical model of enantio-differentiation of MAA with MRNi

on silica supported Cu-Ni and proposed another stereochemical model in which hydrogenation was performed at the step of metal surface as shown in Fig. 29. Sachtler’s group (79) in 1976 studied the structure of the adsorption complexes of relevant compounds on silica supported nickel by means of I R spectra. They proposed stereochemical models as shown in Fig. 30 for the enantio-differentiating hydrogenations of MAA with amino acidand hydroxy acid-MRNis, and rejected the mechanism proposed by Klabunovskii in 1967 because of the difficulty in having tetracoordinated complexes on the surface of the catalyst. According to this mechanism, the hydrogen addition was performed on the side of the substrate facing the modifying reagent. However, the mechanism for the transfer of hydrogen from the surface of the catalyst to the substrate was not mentioned in this paper. In 1979, Sachtler’s group proposed a new mechanism (74)and speculated that TA forms a Ni-tartrate complex, in which each nickel atom was

FIG.31. Stereochemical model of enantio-differentiation of M A A with TA on the catalyst.

162

YOSHIHARU IZUMI

SUbSlrdle

CH,-CO-CH,-COOCH, CH,-CO-C(CH,),-COOCH,

Ref. 80

Ref. 63

38" 30"

3jh -. 1-4.6"

" Modifying conditions: pH 5.0, 0 C. Reaction conditions: substrate (neat), 60 C, 80 kg/cm2. Modifying conditions: pH 5.0, 100°C. Reaction conditions: substrate (neat), atmospheric pressure.

tetracoordinated as shown in Fig. 22. One of the remaining two coordination sites of nickel participates in the hydrogenation and the structure of the nickel complex acts as a template. The hydrogen used for the reaction is supplied from the metallic nickel to the hydrogenation site of the complex by a spillover transport via the tartaric ligand. Yasumori's group (64) proposed the stereo model of enantio-differentiating hydrogenation of MAA with MRNi as shown in Fig. 31 based on the observations that the hydrogens on C(2) of MAA had been replaced by deuterium during the deuteration of MAA with MRNi in the gas-phase reaction. According to his proposal, hydrogenation is carried out on the enolate of MAA and the enantioface of enolate of MAA is differentiated with the modifying reagent. However, this proposal cannot apply to the liquid-phase reaction, because methyl 2,2-dimethyl-3-oxobutyrate, which has no enolate form, was hydrogenated with TA-MRNi in an almost similar OY to that obtained in the enantioface-differentiating hydrogenation of MAA, as shown in Table XXV (63, 80). That is, we do not consider the enolation to be an essential factor in carrying out the enantioface-differentiating hydrogenation in the liquid-phase reaction.

VIII.

Characterization of Catalyst by Modifying Technique

As the surface condition of catalyst reflects the EDA of MRNi as already mentioned, the EDA is a very sensitive parameter from which to get information on the surface character of the catalyst. For example, when MAA (%) change of the surface condition is is used for the substrate, a 1 x observable, because the optical rotation of optically pure MHB is 22.95"

263

MODIFIED RANEY NICKEL CATALYST

in neat and the minimum observable value of optical rotation by a conventional instrument is 0.002". In this section, the characterization of surface conditions of nickel catalysts by means of modifications with NaBr and (R,R)-TA will be introduced as an example (49). In the modification with TA and NaBr, NaBr adsorbs on the surface areas where TA does not adsorb and unstably adsorb as already mentioned in Section lV,A,l ,e. The correlation diagram between the fractional saturating ratio of NaBr (f)on the surface of the catalyst and the EDA of the modified catalyst as shown in Fig. 32 can be deduced from the diagram of correlation between the EDA of the modified catalyst, the amounts of NaBr and TA adsorbed, and the concentration of NaBr in the modifying solution as shown in Fig. 1 1 . When the rates of the enantio-differentiating hydrogenation ( E ) and nonenantio-differentiating hydrogenation ( N ) are postuated to be equal, and the EDA at the enantio-differentiating site is defined as l o o x , the following equation is derived: N E

P-IIa P-I

- _ _ -

P-IIb + -(l P-I

-

1')

In the equation P-I and P-11 are the parts on the surface area where enantiodifferentiation and non-enantio-differentiation proceed, respectively. P-IIa is a portion of P-11 where NaBr cannot block the hydrogenation, and P-IIb is another part of P-I1 where NaBr can inhibit the hydrogenation.

RNi-A

0

.

4

4

I 0.2 0.4 0.6 0.8

O0.0

Degree of NoErsoturotion (1)

FIG.32. Dependence of (IOOjOY) - I on degree of NaBr saturation on the catalyst ( / ) .

264

YOSHIHARU IZUMI

RNi

40%

RNi-A

HNi

62%

76%

FIG.33. Evaluation of surface characters of various catalysts with respect to the modifying method: (I) enantio-differentiating hydrogenation; (IIa) non-enantio-differentiating hydrogenation part which cannot be blocked by NaBr ; (Ilb) non-enantio-differentiating hydrogenation part which can be blocked by NaBr.

Based on Eq. (l), P-IIb/P-I and P-IIa/P-I are estimated from the slope of the line and the N / E value at f = 1 in Fig. 32, respectively. From these values, proportions of P-I, P-IIa, and P-IIb are readily calculated. Figure 33 shows the result of the characterizations of RNi, RNiA, and HNi-1. IX.

TA-NaBr-MRNi

Since TA-NaBr-MRNi is a nearly completed enantio-differentiating catalyst for practical use, the EDA of this catalyst will be summarized in this section. The standard method for preparation of TA-NaBr-MRNi is followed. That is, RNi prepared from 1.9 g of RNi alloy is modified with 100 ml of an aqueous solution of (R,R)-TA (1.0 g) and NaBr (10.0 g) at TABLE XXVl Enantio-Differentiating Hydrogenation of AIkyl Methyl Ketones" Substrate (CH,COR) R C*H, C,H,

OYb [a]?

(%)

Configuration

6.79 7.62 6.40

48.9 65.9

S S

65.6

S

Reaction conditions: (R,R)-TA-NaBr-MRNi (prepared from 3.8 g of alloy), alkyl methyl ketone (10 ml), THF (20 ml), pivalic acid (1 8 ml), I OOT, 100 kg/cm2. Optical yields are calculated from the specific rotation of pure enantiomers: (S)-2-butanol, [a];' + 13.87 (neat); (9-2hexanol, [a];' + 11.57 (neat).

265

MODIFIED RANEY NICKEL CATALYST TABLE XXVII Enantio-Differentiating Hydrogenations of 8-Diketones with (R',R')-TA- NaBr- MRNi' Proportion of diastereomers (%) in the hydrogenation product

Substrate (R-CO-CH,-CO-R) R

(R',R')-diol

CH3 C2Hs n-C,H, n-C,H I 3 Ph

Isolation of (R,R)-diol from the hydrogenation product

(R*,S')-diol

Yield (%)

13 20 15 20 23

65 30

87 80 85 80 11

[a]:

of diol - 54.5 -39.1 -21.4 - 10.8 -72.7

II 21 20

Reaction conditions: (R,R)-TA-NaBr-MRNi (prepared from 38 g of alloy), substrate (1 mol), THF (220 ml), HOAc (2 ml), 100°C. 95 kg/cm2.

TABLE XXVIIl Enantio- Differentiating Hydrogenution of' R C O C H , C O O C H , with TA-NaBr-MRNi" RCOCH,COOCH,

+

RCHCH,COOCH,

I

OH Product Substrate R

Configuration of TA

Configuration

OY (%)

R S R R R S R S R S R S

85 85 86 87 83 88 87 85 86 84 87 84

" Catalyst: standard. Reaction conditions: substrate (10 g), methyl propionate (30 ml), HOAc (0.1 ml), IOOC, 100 kg/cm2.

266

YOSHIHARU IZUMl

pH 3.2 and 100°C for 1 hr with occasional shaking. After the removal of modifying solution by decantation, the modified catalyst is washed with a 10-ml portion of water, two 50-ml portions of methanol, and a 10-ml portion of a reaction medium successfully (37). TA-NaBr-MRNi has been found to be an effective catalyst for enantiodifferentiating hydrogenations of ketones which have a general structure of as shown in Table XVII (52c) and methyl ketones R-CO-CH2-X-Oas shown in Table XXVI (524. Among all, P-diketones and P-ketoesters are the most favorable substrate for this catalyst. Specific rotations [a]? of (R*, R*)-diols produced from P-diketones by hydrogenation with this catalyst are summarized in Table XXVII (44). In the hydrogenation of 6-ketoesters, more than 80% of the enantiodifferentiation was performed regardless of the type of alkyl residue in the acyl or ester moieties as shown in Tables XXVIJI and XXIX, respectively (52a). When the hydrogenation of AA with (R,R)-TA-NaBr-MRNi had been stopped at the point of 70% conversion and the enantiomer-differentiation (see Section IV,B) was controlled, (R,R)-2,4-pentanediol 7 with 98% optical purity was obtained by a single distillation of the product (46). Although TA-NaBr-MRNi is rather more unstable than TA-MRNi in the persistency of EDA, TA-NaBr-MRNi was extremely stabilized not only with EDA but also with hydrogenation activity by embedding it in a silicone polymer as shown in Fig. 34 (48). The embedded catalyst can be stored very stably for a long period without special precautions. The embedded catalyst with high EDA is expected to be obtained in the near future. -0

loo(HA)

\-TA-NaBr-MRNi

80

!ia

--+--*-*.-*---*4--+4

-

60-

TA-NoBr- MRNi

c)

w

40

-

20

-

- 20 -TA-NaBr-MRNi

(EDA)

* 0.'

*

.

a

'

'

. . 10 . '

I

'

'

'

'

.

'

. .20 '

'

' '

' .

'

'

'

30 .

'

'

. G

Repetition numbers

FIG. 34. Durability of TA-NaBr-NRNi embedded in silicone polymer (TA-NaBrMRNi/SR) with respect to EDA and HA.

MODIFIED RANEY NICKEL CATALYST

267

TABLE XXIX E n u n r i o / u ~ i ~ - I ~ i ~ e r m r i aHydroyenution riny o/ C H , C O C H , C O O R w f i i ( R , R ) - T A NuBr M R N I "

CH,COCH ,COOR

+

CH ,CHCH ,COOR

I

OH Product Substrate R

Configuration

OY (%)

Catalyst: standard. Reaction condition: substrate ( I 1.5 ml), THF (23 ml), HOAc (0.1 ml). at lOO"C, 100 kg/cm2.

X.

Other Investigations

Gross and Rys (81) investigated correlations between optical purity of product and the degree (%) of conversion in the enantio-differentiating hydrogenation of MAA with TA-MRNis prepared from various kinds of RNi. The degree (%) of conversion at which the highest OY was obtained was found to depend on the sort of RNi as shown in Fig. 35. Smith and Musoiu (80) reported results of an energetic research on MRNi with respect to the effect of the sort of RNi, effects of conditions for the preparation of RNi and MRNi, and effects of reaction conditions on the EDA of MRNi. Nickel, cobalt, copper, ruthenium, and copper-ruthenium catalysts modified with optically active amino acid or hydroxy acid have been extensively investigated by Klabunovskii's group since 1964 (82). However, those catalysts have been reported to have lower EDA than that of MRNi. Orito's group reported that OY = 90.6%'' had been obtained in the I ' The EDAs of those catalysts must greatly depend on the impurities contained in the kieselguhr used (47) and the conditions for the preparation of the catalyst (656). They calculated the optical purity of MHB based on the value of [a]:' = 20.9 (neat). However, [a]:' for optically pure MHB equals 22.95 (neat). The OY reported by them should be recalculated based on the correct value. For example, the highest OY of 99.4% should be reduced to 90.6% (37,47).

268

YOSHIHARU IZUMI

w-1

w-0 w-1

w-2

A

W-6 20

40

60

80

100

Conversion (XI

FIG. 35. Relation between the degree of conversion ( Y o ) and the optical yield C 0 )in the enantio-differentiating hydrogenations of MAA with TA-MRNis prepared from various type RNis ( W scale). Modifying conditions: pH 4.9, 100 C. Reaction conditions: 60 C, 90 kg/cm*.

enantio-differentiating hydrogenations of M A A with nickel-platinumkieselguhr and nickel-palladium-kieselguhr catalysts modified with TA (83-85). They also found a platinum-carbon catalyst modified with cinchonidine (Q-MPt-C) which has a quite high EDA (81.9%) in the enantiodifferentiating hydrogenation of methyl benzoylformate (86). Since TAMRNis perform very poor enantio-differentiating hydrogenation of a-ketoesters, Q-MPt-C must have a quite different property from TAMRNi in the enantio-differentiation. ACKNOWLEDGMENTS The study of silk-Pd began when Dr. S. Akabori led our research group. The establishment of the experimental rule with MRNi was achieved through the contributions of Mr. M. Imaida, Dr. S. Tatsumi, Dr. T. Tanabe. Dr. T. Ninomiya, and Dr. K. Okubo. The new concept of stereo-differentiation was ratlonalized by the efforts of Drs. A. Tai and H. Ozaki. The development of TA-NaBr-MRNi mostly depended on the efforts of Dr. T. Harada.

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The author wishes to express his sincere thanks to Dr. S. Akabori for his valuable advice and encouragement throughout this work, and his co-workers for their great contribution to this work. Special thanks are due to professor Gerard V. Smith for editing the original English manuscript of this article. REFERENCES 1 . Fukawa, H . , Izumi, Y., Komatsu, S., and Akabori, S., Bull. Chem. Soc. Jpn. 35, 1703 (1 962). 2. Izumi, Y., Imaida, M., Fukawa, H., and Akabori, S., Bull. Chem. Soc. Jpn. 36,21 (1963). 3. Izumi, Y., Imaida, M., Fukawa, H., and Akabori, S., Bull. Chem. Soc. Jpn. 36, 155 (1963). 4. Tatsumi, S., Imaida, M., Fukuda, Y., Izumi, Y., and Akabori, S., Bull. Cliem. Soc. Jpn. 37, 846 (1964). 5. Izumi, Y., Akabori, S., Fukawa, H., Tatsumi, S., Imaida, M., Fukuda, T., and Komatsu, S., Proc. Int. Congr. Caral., 3rd, Amsterdam, p. 1364 (1964). 6 . Izumi, Y., Tatsumi, S.,Imaida, M., Fukuda, Y., and Akabori, S., Bull. Chem. Soc. Jpn. 38, 1206 ( I 965). 7. Izumi, Y., Tatsumi, S., Imaida, M., Fukuda, Y., and Akabori, S., Bull. Chem. Soc. Jpn. 39, 361 (1966). 8. Izumi, Y., Tatsumi, S., and Imaida, M., Bull. Chem. Soc. Jpn. 39, 1087 (1966). 9. Izumi, Y.. Tatsumi, S., and Imaida, M., Bull. Chem. Soc. Jpn. 39, 2223 (1966). 10. Tatsumi, S . , Bull. Chem. Soc. Jpn. 41,408 (1968). I f , Izumi, Y., Tanabe, T., Yajima, S., and Imaida, M.,Bull. Chem. Soc. Jpn. 41,941 (1968). 12. Izumi, Y.. Matsunaga, K., Tatsumi, S., and Imaida, M.,Bull. Chem. Soc. Jpn. 41, 2515 (1968). 13. Izumi, Y., Imaida, M., Harada. T., Tanabe, T., Yajima, S., and Ninomiya, T., Bull. Chem. Sac. Jpn. 42, 241 (1969). 14. Izumi, Y.. Tatsumi, S., and Imaida, M., Bull. Chem. Soc. Jpn. 42, 2373 (1969). I S . Izumi, Y., Tatsumi, S., Imaida, M., and Okubo, K.. Bull. Chem. Soc. Jpn. 43,566 (1970). 16. Izumi, Y and Ninomiya, T., Bull. Chem. Soc. Jpn. 43, 579 (1 970). 17. Izumi, Y., Takizawa, H., Nakagawa, K., Imamura, R., M., Imaida, M., Ninomiya, T., and Yajima, S., Bull. Chem. Soc. Jpn. 43, 1792 (1970). 18. Tanabe, T., Ninomiya, T., and Izumi, Y., Bull. Chem. Soc. Jpn. 43,2276 (1970). 19. Izumi, Y., Yajima, S., Okubo, K., and Babievsky, K. K., Bull. Chem. Soc. Jpn. 44, 1416 (1971). 20. Izumi, Y., Harada, T., Tanabe, T., and Okuda, K., Bull. Chem. Soc. Jpn. 44,1418 (1971). 21. Izumi, Y., and Ohkubo, K., Bull. Chem. Soc. Jpn. 44, 1330(1971). 22. Harada, T., Imaida, M., and Izumi, Y.. Bull. Chem. Soc. Jpn. 44, 1419 (1971). 23. Higashi, F., Ninomiya, T., and Izumi, Y., Bull. Chem. Soc. Jpn. 44, 1333 (1971). 24. Izumi, Y., Angew. Chem. Inr. Ed. Engl. 10,871 (1971). 25. Ninomiya, T., Bull. Chem. SOC.Jpn. 45,2545 (1972). 26. Ninomiya, T., Bull. Chem. Soc. Jpn. 45,2548 (1972). 27. Ninomiya, T., Bull. Chem. Soc. Jpn. 45. 2551 (1972). 28. Ninomiya, T.. Bull. Chem. Soc. Jpn. 45,2555 (1972). 29. Tanabe, T., Okuda. K.. and Izumi, Y., Bull. Chem. Sac. Jpn. 46, 514 (1973). 30. Tanabe, T., Bull. Chem. Soc. Jpn. 46, 1482 (1973). 31. Tanabe, T.. and Izumi. Y.. BUN. Chem. Soc. Jpn. 46, 1550 (1973). 32. Ozaki, H., Tai. A., and Izumi. Y., Cliem. Lett. p. 935 (1974). 22. Harada, T.. Bull. C h m . Soc. Jpn. 48,3236 (1975). 34. Harada, T., Hiraki, Y., Izumi, Y., Muraoka, J., Ozaki, H., and Tai, A,, Proc. In/. Congr. Cuiul., 6111, London p. 1024 (1976). 35. Harada, T., Onaka. S.. Tai, A.. and Izumi, Y., Cliem. Lerr. p. I131 (1977). .1

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526. Harada, T., unpublished. 52c. Tai, A., Harada, T., Hiraki, Y.,and Murakami, S., Bull. Chem. Soc. Jpn. 56, 1414 (1983). 52d. Osawa, T., Harada, T., Ozaki, H., Tai, A,, and Izumi, Y., Prep. Discuss. Cuiul. 50th 4G204, Niigata (1982). 52e. Murakami, S., Harada, T., Tai, A., and Izumi, Y.,Prep. Discuss. Caral. 44th A-4L03, Fukuoka (1979). 53. Akabori, S., Sakurai, S., Izumi, Y.,and Fujii, Y . , Nature (London) 178,323 (1956). 54. Izumi, Y., Bull. Chem. Soc. Jpn. 32, 932 (1959). 55. Izumi, Y.,Bull. Chem. SOC. Jpn. 32,936 (1959). 56. Izumi, Y ., Bull. Chem. Soc. Jpn. 32, 942 (1 959). 57. Akamatsu, A., Izumi, Y., and Akabori, S., Bull. Chem. Soc. Jpn. 34, 1067 (1961). 58. Akamatsu, A., Izumi, Y., and Akabori, S., Bull. Chem. SOC.Jpn. 34, 1302 (1961). 59. Akamatsu, A., Izumi, Y.,and Akabori, S., Bull. Chem. Soc. Jpn. 35, 1706 (1962). 60a. Isoda,T., Ichikawa, A., and Shimamoto, T., J . Sci. Res. Inst. (Riken hokoku)34,32(1958). 606. Isoda, T., Ichikawa, A., and Shimamoto, T., J . Sci. Res. Ins(. (Riken hokoku) 34, 143 (1958). 61a. Izumi, Y., Shokubai (Catalyst) 12, 201 (1970). 616. Izumi, Y . , Proc. Jpn. Acad. 53,38 (1977). 62. Izumi, Y . ,and Tai, A., “Rittai-Kubetu Hanno” (in Japanese). Kodansha Tokyo, 1975. 63. Izumi, Y., and Tai, A., “Stereo-Differentiating Reactions.” Kodansha Tokyo and Academic Press, New York, 1977. 64. Yasumori, I., Pure Appl. Chem. 50,971 (1978). 65a. Nitta, Y.,Sekine, F., Imanaka, T., and Teranishi, S., Bull. Chem. Soc. Jpn. 54,980 (1981). 656. Nitta, Y . , Sekine, F., Imanaka, T., and Teranishi, S., J. Catal. 74, 382 (1982). 66. Nitta, Y., Sekine, F., Imanaka, T., and Teranishi, S., Chem. Len.p. 541 (1981). 67. Klabunovskii, E. I . , Vedenyapin, A. A., Talanov, Yu.M., and Sokolova, N. P., Kinet. Catal. 16, 595 (1975). 68. Neypokoev, V. I., Petrov, Yu. I., and Klabounovskii, E.I., Izu. Akad. Nauk SSSR Ser. Khim. p. 113 (1976). 69. Lipgart, E. N., Petrov, Yu.I., and Klabunovskii, E. I., Kinet. Catal. 12, 1326 (1971). 70. Fish, M. J., and Ollis, D. F., J . Catal. 50, 353 (1977). 71. Hatta, A., and Suetaka, W., Bull. Chem. SOC.Jpn. 48,2428 (1975).

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72. Hatta, A., Moriya, Y., and Suetaka, W., Bull. Chem. Soc. Jpn. 48, 3441 (1975). 73. Groenewegen, J . A,, and Sachtler, W. M . H.. J. C a r d 38, 501 (1975). 74. Hoek, A,, and Sachtler, W. M. H., J . Card. 58, 276 (1979); Hoek, A., Woerde, H. M., and Sachtler, W. M. H., Proc. Int. Congr. Catal., 7th,Tokyo p. 376 (1980). 75. Tanabe, R., and Uyeda, N., Bull. Inst. Chem. Res. Kyoto Univ. 52,616 (1974). 76. Klabunovskii, E. I., and Petrov, Yu. I., Dokl. Akad. Nauk SSSR 173, I125 (1967). 77. Petrov., Yu. I., and Klabunovskii, E. I., Kinet. Catal. 8, 814 (1967). 78. Klabunovskii, E. I., Vedenyapin, A. A,, Karpeiskaya, E. I., Pavlov, V. A,, and Zelinskii, N . D., Proc. Int. Congr. Catal., 7th,Tokyo p. 390 (1980). 79. Groenewegen, J. A,, and Sachtler, W. M. H., Proc. Inr. Congr. Catal., 6th, London p. 1014 (1976). 80. Smith, G . V., and Musoiu, M., J. Catal. 60, 193 (1979). 81. Gross, L. H., and Rys, P., J. Ory. Chem. 39,2429 (1974). 82. Klabunovskii, E. I., and Bedeyapin, A. A,, "Asymmetric Catalysis-Hydrogenation Metals" (in Russian). Nauk, Moscow, 1980. 83. Orito, Y., Niwa, S., and Imai, S., J. Syn. Org. Chem. Jpn. 34, 236 (1976). 84. Orito, Y . , Niwa, S., and Imai, S., J. Syn. Org. Chem. Jpn. 34,672 (1976). 85. Orito, Y., Niwa, S., and Imai, S., J . Syn. Org. Chem. Jpn. 35, 753 (1977). 86. Orito, Y . , Imai, S., Niwa, S., and Nguyen-Cia-Hung, J. Syn. Org. Chem. Jpn. 37, 173 (1979).

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ADVANCES IN CATALYSIS, VOLUME 32

Analysis of the Possible Mechanisms for a Catalytic Reaction System JOHN HAPPEL Department of Chemical Engineering and Applied Chemistry Columbia University New York, New York AND

PETER H. SELLERS The Rockefeller University New York, New York 1. Introduction . . . . . . . . . . . . . . 11. The Structure of a Chemical System . . . . . . . A. The S-Dimensional Space of All Mechanisms and the Q-Dimensional Space of All Reactions . . B. The &‘-Dimensional Subspace of All Steady-State Mechanisms and the &Dimensional Subspace of All Overall Reactions . . . . . . . . . C. Direct Mechanisms and Simple Reactions . . . . 111. General Formulas for Mechanisms and Reactions . . . A. A Change of Basis for the Mechanism Space. . . . B. Basic Overall Reactions, Steady-State Mechanisms, and Cycles . . . . . . . . . . . . . C. Algebraic Formulas. . . . . . . . . . . D. Multiple Overall Reactions. . . . . . . . . IV. A Procedure for Finding Every Direct Mechanism . . . A. The Cycle-Free Subsystem . . . . . . . . . B. The Direct Mechanisms . . . . . . . . . V. Systems with a Simple Overall Reaction . . . . . . Example I . Sulfur Dioxide Oxidation (No Cycles) . . . Example 2. The Hydrogen Electrode Reaction ( I Cycle) . Example 3. Ammonia Synthesis (2 Cycles) . . . . . Example 4. Dehydrogenation of I-Butene to 1,3-Butadiene (3 Cycles) . . . . . . . Example 5. Hydrogenation of Isooctenes (4 Cycles). . . VI. Overall Reactions with a Multiplicity Greater Than One. . Example 6. Ethylene Oxide Synthesis (NoCycles) . . . Example 7. Ethylene Oxide Synthesis ( I Cycle) . . . . Example 8. Isomerization of Butenes (1 Cycle) . . . .

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Example9. n-Butane Dehydrogenation (3 Cycles) . Example 10. Methanation of Synthesis Gas (3 Cycles) VII. Discussion. . . . . . . . . . . . . A. Thermodynamics . . . . . . . . . B. Kinetics . . . . . . . . . . . . VIII. List of Symbols . . . . . . . . . . . References . . . . . . . . . . . . .

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Introduction

Complex chemical reactions proceed through a network of intermediates that are connected by elementary reaction steps. A reaction mechanism is defined as a combination of specific elementary steps which can be taken in appropriate proportions to produce the degrees of change of terminal species in a reacting system. Usually more elementary steps may be considered possible than are required to produce any single mechanism accounting for the observed overall reaction. It is thus necessary to consider how steps may be combined, given an appropriate initial choice of species and steps. The purpose of this article is to review methods for accomplishing this and to present detailed information on a new method with this objective developed by Happel and Sellers (I). For our purpose elementary steps can be chosen to include any reaction that cannot be further broken down so as to involve reactions in which the specified intermediates are produced or consumed. Ideally, elementary steps should consist of irreducible molecular events, usually with a molecularity no greater than two. Such steps are amenable to treatment by fundamental chemical principles such as collision and transition state theories. Often such a choice is not feasible because of lack of knowledge of the detailed chemistry involved. Each of these elementary reactions, even when carefully chosen, may itself have a definite mechanism, but theory may be unable to elucidate this finer detail [Moore (2)]. Regardless of how the possible intermediates and elementary steps are selected, the procedure given in this article presents a method for the unambiguous enumeration of all possible minimal reaction mechanisms that will generate the observed overall consumption and production of terminal species under given conditions of temperature, pressure and concentration. In developing this procedure two basic assumptions are made. The first is concerned with the fact that possible mathematical networks of reaction steps can be decomposed into two sets of steps, namely those that can be combined to give the observed overall production and consumption of chemical species and those that form cycles resulting in no net change in

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terminal species. Since the latter would correspond to no change in free energy, steps in such paths would be at equilibrium. According to the principle of microscopic reversibility (2),such cyclic paths would not occur physically. Their mathematical properties are important, as discussed in Section 111. The second assumption employed in this article is that all species designated as intermediates-those that do not enter into a given system as either terminal reactants or products--will be present at constant concentrations. This includes stationary systenis that can be described by a unique steady state rather than those which exhibit transient or oscillatory behavior. The general subject of enumerating reaction mechanisms has been considered previously in reviews that have appeared in this series over the past 30 years [see the reviews by Christiansen ( 3 ) ,Horiuti and Nakamura ( 4 ) , and Temkin ( 5 ) ] . Each of these successive contributions presented developments based on concepts developed earlier, as does the present treatment. Studies conducted by those authors are of special interest here since the subject matter is largely in the field of heterogeneous catalysis. Christiansen’s kinetic treatment of sequences forms the basis of much of a chapter in Boudart’s (6) excellent elementary treatment of the kinetics of chemical processes. A key concept is the recognition that there are two distinct types of sequences leading from reactants to products through active centers. One of these is an open sequence, in which an active center or site is not reproduced in any other step of the sequence. (One should not confuse this terminology with open systems referring to the continuous passage of reactants over a catalyst.) The second type is a closed sequence, in which an active center is reproduced so that a cyclic reaction pattern repeats itself and a large number of molecules of products can be made from only one active center. As Boudart notes, a closed sequence constitutes in some sense a good definition of catalysis. Although all reactions showing a closed sequence could be considered to be catalytic, there is a difference between those in which the entity of the active site is preserved by a catalyst and those in which it survives for only a limited number of cycles. In the first category are the truly catalytic reactions, whereas the second comprises the chain reactions. Both types can be considered by means of the steady-state approximation, as in Christiansen’s treatment. This important development dates to 1919 (6) when Christiansen as well as Herzfeld and Polanyi independently proposed an explanation of the kinetics of the reaction between hydrogen and bromine reported earlier by Bodenstein and Lind. In treating catalytic sequences of elementary steps, Christiansen adopted the simplification that each elementary step is first order in both directions with respect to concentration of a single active species. The resulting rates

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of production of terminal species could then be expressed quite generally by formulas involving a matrix whose elements are products of reaction velocity constants and concentrations of species. Such a treatment is especially useful for considering simple reactions such as isomerizations. Christiansen also noticed that some closed sequences would not yield an overall reaction and appropriately called such sequences cyclic. He was among the first to advance the viewpoint that the only possible stationary value for flow in such a sequence is zero and identified this with the principle of microscopic reversibility. Horiuti and his school, although not referring directly to the research of Christiansen, were no doubt familiar with it since Horiuti studied with Polanyi. Horiuti’s studies are summarized in a recent monograph devoted to the use of tracers in heterogeneous catalysis (7). He made an important advance in considering the velocities of mechanistic steps directly instead of relating them to reaction velocity constants as Christiansen had done. He showed that a chemical reaction mechanism can be described by specifying the number of occurrences of the elementary steps for each single occurrence of a given simple overall reaction. In Horiuti’s terminology these occurrences are the stoichiometric numbers associated with the specified reaction mechanism. Thus, the choice of possible reaction mechanisms reduces to the selection of appropriate stoichiometric numbers. Horiuti accomplished this selection by employing a step-by-species matrix in which the elements are the stoichiometric numbers and obtained the linearly independent sets. He described these sets as reaction routes. This method was generalized by Horiuti to systems in which more than one simple overall reaction could occur. Its implications are discussed further in Section II,C. The mathematical concept of linear independence introduced by Horiuti, although correct, will allow mechanisms to be combined in ways resulting in the cancellation of segments of the elementary reaction steps. Horiuti’s procedure in the case of simple overall reactions places a lower bound on the number of possible reaction mechanisms but does not furnish a complete listing. This limitation was recognized by Milner ( B ) , who introduced the concept of direct paths, each of which is unique in the sense that it cannot be considered to result from the superposition of any other member of the set of elementary reactions. Milner applied this idea to the enumeration of mechanisms for a number of simple overall reactions involving electrochemistry. He arrived at the rule that for such a reaction the number of nonzero stoichiometric numbers specifying a direct path can be no more than one greater than the number of intermediates. By a trial-and-error procedure he was able to count all mechanisms consistent with a given choice of possible unit steps. Sellers (9) developed a theory of chemical reaction networks for the

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generation of all possible direct mechanisms involving a simple overall reaction. In the present article this theory is developed more completely than in our earlier paper (1). Temkin (5, 10, 11)presented additional studies extending the original ideas of Horiuti to establish the number of routes or mathematically independent mechanisms consistent with a given initial choice of elementary steps. He showed that the algebra of reaction routes was consistent with the specification of the dimension of the space of such routes and that such a basis could include empty routes for which no net reaction occurs. However, instead of using such empty routes or cycles to generate the complete list of direct mechanisms as discussed in Section III,A, he assumed that such cycles could be disregarded in their effect on reaction mechanisms but not on kinetics. This is inconsistent with the treatment in this article since we assume that such cycles would not occur. A recent development aimed at classifying types of chemical mechanisms has been presented by Sinanoglu and co-workers (12, 13). It is an enormous classification problem, which they simplify by using a graph theoretic model. We have not adopted any such simplifying assumptions. Instead we require that a particular chemical system be given, and then we introduce procedures for analyzing that system, however complicated it may be from a combinatorial viewpoint, rather than undertaking to classify all chemical mechanisms in a general sense. The procedure presented here places no limit on the number of reactants in an elementary reaction or the number of elementary reactions which have a reactant in common. We use an incidence matrix to define the combinatorial structure of a system, and we analyze it by the methods of linear algebra. The graph theoretic model corresponds to a special type of incidence matrix (where each row has only two nonzero entries). Our procedure is applicable not only to this type, but to any matrix of integers. It is an algorithm, which could be carried out by computer, for finding all chemical mechanisms in a fixed context. In the following sections of this article we first define the terms necessary to identify a chemical system. After this, the use of an algebraic technique is developed for the expression of general reaction mechanisms and is compared with the previous treatments just mentioned. Next, a combinatorial method is used to determine all physically acceptable reaction mechanisms. This theoretical treatment is followed by a series of examples of increasing complexity. These examples have been chosen to illustrate the technique and for comparison with previous studies. They do not constitute a survey of all the most significant studies concerned with the mechanisms illustrated. Finally, a discussion is presented of the relationship of the present treatment to studies concerned with thermodynamics, and of the relationship between kinetics and mechanisms.

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II. The Structure of a Chemical System Let us view a chemical system as a network of elementary chemical reactions linked to one another by common reactants. The broad question of what kinds of network configurations are possible for chemical systems can be answered mathematically if it is stated in the following way: Given a hypothetical finite list of elementary chemical reactions, determine all the ways the reactions (or a subset of them) can be combined to form a specified overall reaction, or, more generally, to form any member of a specified family of overall reactions. The first stage in a mathematical solution of this problem is to define a chemical system formally. The mathematics we shall need is confined to the properties of vector spaces in which the scalar values are real numbers. From a mathematical viewpoint the whole discussion will take place in the context of two vector spaces, an S-dimensional space of chemical mechanisms and a Q-dimensional space of chemical reactions, which are related to each other by the fact that each mechanism m is associated with a unique reaction R(m) which it produces. The function R is a transformation of mechanisms to reactions which is linear by virtue of the fact that reactions are additive in a chemical system and that the reaction associated with combined mechanisms m, + m2 is R(m,) + R(m,). All mechanisms are combinations of a simplest kind of mechanism, called a step, which ideally consists of a one-step molecular interaction. Each step produces one of the elementary reactions which form a basis for the space of all reactions. For instance, let step s, be a collision process between a , and a, to form a3, and let step s2 be an isomerization of a3 to a4. Then R(s,) is a vector a4, and R(s, + s2)is a vector which - a , - a 2 + a3, R(s,) is a vector -a3 equals the sum -a, - a, + a4 of the first two vectors. This illustrates the linearity of R, which may be expressed in general by the linearity equation R(s, + s2) = R(s,) + R(s,). In particular, ifs is repeated cr times, the linearity equation becomes R(crs) = crR(s). This principle can be extended to all real values of cr, where cr is regarded as the rate of reaction, including the possibility of a negative value for cr to express the possibility of a reverse reaction. For simplicity we speak of a mechanism or a reaction, rather than a mechanism vector or reaction vector. The distinction lies in the fact that a reaction r (or mechanism) is essentially the same whether its rate of advancement is p or u, whereas pr and crr are different vectors (for p # 0 ) .Therefore, a reaction could properly be defined as a one-dimensional vector space which contains all the scalar multiples of a single reaction vector, but the mathematical development is simpler if a reaction is defined as a vector. This leaves open the question of when two reactions, or two mechanisms, are “essentially different” from a chemical viewpoint, which will be taken up

+

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279

separately. It has not been universally recognized that, even though a onedimensional space contains essentially one reaction or mechanism, an N-dimensional space (for N > 1) generally contains more than N essentially different reactions or mechanisms, where the number varies from case to case and is not a function of N alone. SPACEOF ALL MECHANISMS A. THES-DIMENSIONAL AND THE Q-DIMENSIONAL SPACEOF ALLREACTIONS Let us begin here with a formalization of some of the ideas which have already been introduced. A chemical system contains species which will be denoted by a , , a,, . . ., aA and elementary reactions among these species which will be denoted by the S vectors in Eqs. ( l ) , rl = u l l a l r, = a Z l a l

+ a12a2+ . . . + ulAaA + a2,a2 + . . . + u2AaA

rs

+ us2a2 + . . + C(SAaA

=

aSlal

(1)

'

in which the a's are stoichiometric coefficients. Ordinarily, each elementary reaction will have one or two positive coefficients, one or two negative coefficients, and the remainder equal zero, but more nonzero coefficients are possible. Let us assume, however, that there is at least one positive coefficient and at least one negative coefficient. Any reaction in Eqs. (1) may be written as a conventional chemical equation by setting it equal to zero and transposing the negative terms to the other side of the equation. This notation has been discussed by Aris (14). Chemical equality, denoted by the symbol e, has been shown by Sellers (15) to be a group equivalence, thus satisfying ordinary rules of mathematical equality. Except when specific reservations are stated, every reaction is assumed to be reversible, that is, to be capable of any real rate of advancement, positive or negative. The elementary reactions in Eqs. (1) are not necessarily linearly independent, and, accordingly, let Q denote the maximum number of them in a linearly independent subset. This means that the set of all linear combinations of them defines a Q-dimensional vector space, called the reaction space. In matrix language Q is the rank of the S x A matrix ( 2 ) of stoichiometric coefficients which appear in the elementary reactions ( 1):

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JOHN HAPPEL AND PETER H . SELLERS

Next, let us define the space of all chemical mechanisms in the chemical system under consideration. Let step si denote the molecular interaction which produces the reaction denoted by ri or R(si). Let a mechanism m be any linear combination of steps of the following form: m

=

alsl

+ a2sz +

*

+ asss

(3)

where each coefficient oi is a real number, describing the rate of occurrence of si. The set of all such mechanisms constitutes an S-dimensional vector space, called the mechanism space. The reaction r produced by m is found by applying the linear transformation R to Eq. (3), which gives the following: r

=

a,rl

+ a2r2+ . . . + asrs

(4)

Since we have explicit expressions (1) for each f i rthey can be substituted into expression (4),so as to express the reaction r as the following explicit linear combination :

thus, obtaining a general expression for any reaction r in the system. It is usual in algebra to express vectors by linear combinations, but it is conventional also, particularly in working examples, to use matrix notation, where only the scalar coefficients are written. Thus, m would be expressed by (al* . . as)and Eq. ( 5 ) by the following matrix equation :

B. THEP-DIMENSIONAL SUBSPACE OF ALLSTEADY-STATE MECHANISMS AND THE R-DIMENSIONAL SUBSPACE OF ALLOVERALL REACTIONS To define a system in a steady state it is necessary to distinguish two kinds of species, the intermediates a , , . ,,a, and the terminal species a,, ,. . . , a , + T , where I + T = A. In such a system a steady-state mechanism is one whose reaction only involves terminal species. The net rate of production of each intermediate in such a mechanism is zero, which is equivalent to saying that the first I coefficients are zero in the general expression (5) for a reaction. This gives us the characterization, introduced by Horiuti ( 4 , 7 ) ,for a steady-

.

,

ANALYSIS OF MECHANISMS FOR REACTION SYSTEMS

28 1

state mechanism as one whose coefficients p l , . . ., ps satisfy the I linear equations expressed by the following matrix equation: (PI . ’ ’ P S I

i“‘ us1

... @;I) ..*

= (O...O)

(7)

us1

If H denotes the rank of the S x I matrix in Eq. (7), then the dimension P of the space of all steady-state mechanisms equals S - H , and the dimension R of the space of all reactions which they produce equals Q - H . Let us describe the reactions in this R-dimensional space as the overall reactions, their essential property being that they involve terminal species only. Horiuti calls H the “number of independent intermediates.” Temkin (10) describes the equation P = S - H as Horiuti’s rule, and the equation R = Q - H as expressing the “number of basic overall equations.” To avoid confusion, let us confine the term basis and the concept of linear independence to sets of vectors, and let numbers such as H,P, Q, R, S be understood as dimensions of vector spaces. This makes it simple to determine their values and the relations among them, as will be done in Section 111. The dimension of a space equals the number of elements in a basis, which is defined as a set of elements such that every element in the space is equal to a unique linear combination of them. Therefore, P steady-state mechanisms can be chosen in terms of which all others can be uniquely expressed. This gives us a unique way to symbolize each steady-state mechanism and its overall reaction, but it does not provide a classification system for them which is valid from a chemical viewpoint, because the choice of a basis is arbitrary and is not dictated, in general, by any consideration of chemistry. A classification system for mechanisms is our next topic. C. DIRECT MECHANISMS AND SIMPLE REACTIONS

In a chemical system there is a unique collection of mechanisms, called the direct mechanisms of the system, which will be shown to be the fundamental constituents of any mechanism. Milner (8) called them “direct paths” and Sellers (9)-“cycle-free mechanisms.” Let m be any mechanism, and let r be the reaction which it produces; then m is defined as direct if it is minimal in the sense that, if one step is omitted, then there is no mechanism for r which can be formed from any linear combination of the remaining steps. Similarly, we can define r as a simple reaction if it is minimal in the sense that, if one of its reactants is omitted, then no reaction in the system involves only the remaining reactants. Let us use the word multiple as an antithesis to direct or simple. As we shall

282

JOHN HAPPEL AND PETER H. SELLERS

see in the examples of Sections V and VI, direct mechanisms for both simple and multiple reactions are of major interest. The set of all direct mechanisms in a system contains within it a basis for the vector space of all mechanisms. In general, there are more direct mechanisms than basis elements, which means that there can exist linear dependence relations among direct mechanisms but, even so, they differ chemically. That is, a direct mechanism with a given step omitted cannot be considered to result from a combination of two other mechanisms in which that step is assumed to occur. In the latter case the net velocity of zero for that step would result from a cancellation of equal and opposite net velocities rather than from the complete absence of the step. The set of all direct mechanisms (unlike a basis) is a uniquely defined attribute of a chemical system. In fact what we have called a direct mechanism is what is usually called a mechanism in chemical literature, even though the definition may be implicit. There are special cases where the direct mechanisms are linearly independent and constitute a basis. If all the direct mechanisms for a particular reaction r are disjoint, in the sense of containing no steps in common, then they are obviously linearly independent, or if there is only one direct mechanism for r, it is independent. This last case suggests a way of finding all the direct mechanisms in a chemical system. If we can find a subsystem which contains at most one mechanism m for any reaction r, then m is direct. In other words, m is a direct mechanism if S = Q in the chemical system, consisting just of the steps in m. Accordingly, to find all the direct mechanisms in a system where Q < S, we may consider each of the (i)subsystems, where

Hence in the entire system there are at most (i)direct mechanisms for r, but usually there are many less than this, not only because of the excluded subsystems, but also because different subsystems can contain the same direct mechanism for a particular r. This approach to finding direct mechanisms is implemented in Section IV, where an efficient procedure is given for making a complete list of all the direct steady-state mechanisms for a given reaction, simple or multiple. All mechanisms can be classified in terms of direct mechanisms. In this article we consider the problem of listing all direct mechanisms in a given system, but we do not undertake to list all combinations which consist of two or more direct mechanisms, advancing simultaneously at independent rates. Each direct mechanism contains a minimal number of elementary steps. Combinations of direct mechanisms in which additional steps must

ANALYSIS OF MECHANISMS FOR REACTION SYSTEMS

283

appear may also be termed mechanisms over the allowable range of such combinations without cycle formation. Such combinations are unique if they are composed of only two direct mechanisms. Combinations involving more than two direct mechanisms are not necessarily unique, in the sense that a given rate of reaction can no longer be represented by combinations of only those direct mechanisms. The combinations of increasing numbers of direct mechanisms will finally include all steps and thus constitute the most general mechanism consistent with the initial choice of elementary steps. The extent to which any given direct mechanisms may be combined without cycle formation can be determined by noting whether such combinations contain irreducible cycles. The latter are the cycles with a minimal number of steps which characterize a given system. They can be determined by a procedure that is analogous to that for finding direct mechanisms [Sellers (9a)I.For a multiple overall reaction, the relative degrees of advancement for each of the simple overall reactions chosen as a basis introduce additional restrictions on the allowable cycle free combinations) [Sellers (9b)I. Except for modeling isomerization systems involving multiple overall reactions, it is generally assumed that a single predominant direct mechanism is sufficient to characterize a given system. Usually the further simplifying assumption is made that there is a single rate-controlling step, other steps in a mechanism being taken at quasi-equilibrium. Another simplification is the assumption of unidirectional steps for reactions that are far from equilibrium.

111.

General Formulas for Mechanisms and Reactions

Equation (7) characterizes a steady-state mechanism algebraically, but it does not provide an explicit formula for any such mechanism. Therefore, using only the matrix (2) of elementary reaction coefficients and the knowledge of which columns in the matrix correspond to terminal species, let us derive a general formula for any steady-state mechanism. A. A

CHANGE OF

BASISFOR

THE

MECHANISM SPACE

The basis s l , . . ., ss for the mechanism space will be changed to one which contains H non-steady-state mechanisms and P steady-state mechanisms. The latter will be what Temkin (16) calls a “basis for all routes.” A route is what we are calling a steady-state mechanism, and a basis is a maximal linearly independent set of them. The basis is “stoichiometric” in Temkin’s

284

JOHN HAPPEL AND PETER H. SELLERS

terminology when it is selected as follows: R of the P basis elements are mechanisms whose reactions constitute a basis for the space of overall reactions, and the remaining C basis elements are what Temkin (10) calls “empty routes,” each of which has a reaction equal to zero. Let us use the mathematical term cycle to describe any element m of the mechanism space such that R(m) = 0, instead of “empty route.” The C elements mentioned are a basis for the space of all cycles. In algebra this space is known as the “kernel of R.” To construct the new basis, described above, we will change the basis s l , . . ., ss of the S-dimensional space of all mechanisms into a basis (9), which separates into 3 parts, ml,...,mH;

mH+,,...,mQ;

mQ+,,...,mS

(9)

where each mi is a linear combination of the original basis elements, such that (mQ+ , . . ., m,) is a basis for the subspace all cycles; (mH+ . . ., mQ) is a set of steady-state mechanisms no linear combination of which is a cycle, and (ml , . . ., mH) is a set of mechanisms no linear combination of which is a steady-state mechanism. To get basis (9), start with following matrix :

,

ss \us,

.’ ’

as1

%,I+,

’

.’

%,I

+TJ

which is the same as matrix ( 2 )except that we now require the first I columns to correspond to intermediates and the remaining columns to correspond to terminal species. Furthermore, the rows are labeled by the steps which correspond to them [i.e., aij is the stoichiometric coefficient of species a j in R(s,)]. Next, perform elementary row operations on matrix (lo), so as to put it in the diagonal form shown in Fig. 1. This will require interchanging some pairs of columns (but a column corresponding to an intermediate is never interchanged with one corresponding to a terminal species). Every time a row operation is performed it is also applied to the column of basis elements at the left of the matrix, with the result that the entries in this column become linear combinations of steps which will achieve form (9) when the diagonalization is complete. For instance, if the first row operation is to replace row j by row j minus row i, then the column entry sj is replaced by ( s j - si).There is no need to describe the diagonalization procedure in detail, except to say that it must be performed in such a way as to insure that the combinations m, , . . ., m, are linearly independent. This will be achieved if the elementary row operations are confined to changing a row by adding to

ANALYSIS OF MECHANISMS FOR REACTION SYSTEMS

285

FIG. I . This diagonalized matrix ( Pij) is formed from matrix ( a i j )by elementary row operations and column permutations. It has the same rank as (aij).

or subtracting from it one of the rows above. The only row permutation needed is moving a row of zeros to the bottom of the matrix. B.

BASICOVERALL REACTIONS, STEADY-STATE MECHANISMS, AND CYCLES

An explicit basis for the space of overall reactions is characterized by rows H + 1 through Q of the diagonalized matrix of Fig. 1. If the matrix is expressed by (Pij), then the desired basis is R(mH+I) = P H + l , l + l a l + l + ." + P H + l . A a A R ( m ~ + 2=) B H + ~ , I + ~ ~ I *+" I -I PH+2,AaA R(mQ)

=

DQ,l+laI+l

+

* * '

+

( 1 1)

PQAaA

A basis for the space of all steady-state mechanisms is (mH+1 , . . ., ms). Since mi is a linear combination of steps, this basis has the following form: mH+l

=

YH+1,lSl

+

" '

+ YH+l,SSS

286

JOHN HAPPEL A N D PETER H. SELLERS

The rows in (12) from m?+ through ms are a basis for the space of all cycles, and the coefficients in these rows form a C x S matrix (yij), which will be needed in Section IV for the construction of the unique set of all direct steady-state mechanisms.

C. ALGEBRAIC FORMULAS

Every element of a space is a unique linear combination of its basis elements. Therefore, a general expression for any steady-state mechanism m, including cycles, has the following form: (13) =pH+lmH+l + C1H+2mH+2 + " ' + h m S where the coefficients are any real numbers. A general expression for any overall reaction r is obtained by applying the function R to Eq. (13), which gives the following: = pH+

lR(mH+1)

+

' * *

+ PQR(mQ)

(14)

These expressions for m and r are made explicit by substituting into them the values for mi and R(mi)stated in bases (1 1) and (12).

D. MULTIPLE OVERALL REACTIONS

If R = 1 in a chemical system, it means that all steady-state mechanisms [i.e., all m which can be obtained by assigning particular numerical values to p l , . . ., ps in Eq. (13)] will have the same overall reaction r or a multiple of it, because then Eq. (14) reduces to r = pH+lR(mH+]). In this case the system is said to have a simple overall reaction, and, when we come to list all the direct mechanisms for r, there is no loss of generality if we take the multiple pH+ to be unity. If, however, R > 1, then the general formula (14) for an overall reaction r involves two or more independent parameters and is said to be a multiple overall reaction. In such a case each direct mechanism for r must also involve these parameters, unless we are prepared to choose particular overall reactions and determine a list of direct mechanisms for each. However, if we take a basic set of overall reactions and combine the direct mechansims of all of them, the process of combining them will lead to nondirect mechanisms. Accordingly, to generate all the direct mechanisms in a system, we must

287

ANALYSIS OF MECHANISMS FOR REACTION SYSTEMS

find them for the general overall reaction (14). This is contrary to the approach suggested by Lee and Sinanoglu (13).

IV.

A Procedure for Finding Every Direct Mechanism

A procedure, which was introduced by Happel and Sellers (I) for finding all direct mechanisms for a given reaction will be demonstrated here from the standpoint of how to apply it in practice. We demonstrate it by applying it to an arbitrary S-step chemical system, as defined in Section 11, to find all the direct mechanisms for the general overall reaction (14) derived in Section 111. A. THECYCLE-FREE SUBSYSTEM In a chemical system with S steps and a maximum of Q linearly independent elementary reactions every set of Q steps whose reactions are linearly independent constitutes a cycle+ee subsystem. It is apparent that, if R(sl),. . ., R(s,) are linearly independent, then no cycle can be formed with the steps s l , . . ., sQ(unless all the coefficients are zero). Accordingly, a laborious way ways in to find all the cycle-free subsystems would be to consider all the which Q rows can be chosen from the S x A matrix (2) and select those which are row independent. Since each row corresponds to a step, each selection of Q independent rows gives a cycle-free subsystem. The same result can be achieved by considering columns C at a time in the following smaller C x S matrix: . . . YQ+ 1.S ?Q+ 1 . 1 ?Q+ 1 . 2

(z)

?Q+2.1

YQ+2.2

Ys2

...

".

YQ + 2,s

Yss

called the cycle matrix, whose entries are defined by the basic cycles constructed in Section III,B and given explicitly in the last C rows of basis (12). Each column of this matrix corresponds to a step in the system, and each linearly independent set of C columns corresponds to a set of steps which, if removed from the system, would leave a Q-step cycle-free subsystem. This procedure for finding the Q-step cycle-free subsystems was introduced by Happel and Sellers ( I ) , and is equivalent mathematically to the procedure described in the last paragraph, but it takes advantage of the fact that the

288

JOHN HAPPEL AND PETER H. SELLERS

basic cycles for the system have already been determined by diagonalizing matrix (2).

B. THEDIRECTMECHANISMS Let us find every direct mechanism for a given overall reaction r. Assume r to be of the general form given in Eq. (14) and of multiplicity R ( R = Q - H), which means that an expression for it contains R parameters. Any mechanism for r is of the general form given in Eq. (13) and depends not only on the R parameters in its reaction, but on C additional parameters, where C is the dimension of the space of all cycles ( R C = S - H). Therefore, to determine a unique mechanism for r, we need to determine C parameters, and they can be chosen to make it a direct mechanism by the following reasoning: (i) Every direct mechanism belongs to a Q-step cycle-free subsystem and (ii) every Q-step cycle-free subsystem is obtained, as shown in Section IV,A, removing C appropriate steps from the system as a whole (Q = S - C ) . Taking (i) and (ii) together, we obtain a direct mechanism for r by rewriting the general mechanism (1 3) as a linear combination of steps, then setting C coefficients equal to zero, where they are the coefficients of C steps whose removal defines a cycle-free system. Let us clarify this procedure by applying it to four cases of increasing complexity: (1) C = 0; (2) C = 1, R = I ; (3) C>l,R=l;(4)C>l,R>l. Case I . If C = 0, there are no cycles in the system. For any reaction r in the system there is one mechanism, and it is direct. This applies to Example 1 in Section V and Example 6 in Section VI. Case 2. If C = 1 and R = 1, then the general mechanism, as given by To simplify the notation, Eq. (14), takes the form pH+lm,+l + pH+2mH+2. let this be written as pm + 4n, where m is a mechanism, n is a cycle, and the coefficients p and 4 are unrestricted. The overall reaction depends only on p and is a simple reaction. Ordinarily, with simple reactions the scalar coefficient is omitted, since r and pr are essentially the same reaction. Therefore, in the present case the general mechanism is taken to be of the form m + 4n. It will be recalled that m and n are fixed linear combinations of steps, which result from the diagonalization procedure described in Section III,A. Accordingly, m + 4 n can be expanded to a linear combination of steps, whose coefficients depend on 4 or else are constant. This becomes a direct mechanism if any single nonconstant coefficient is set equal to zero. In the first place this removes one step, which means that the mechanism belongs to a cycle-free subsystem and thus is direct. Second, setting a nonconstant coefficient equal to zero allows us to solve for 4, which means that all other coefficients are known, and the direct mechanism is completely

+

ANALYSIS OF MECHANISMS FOR REACTION SYSTEMS

289

determined. Furthermore, we get every possible direct mechanism if we go through the above procedure for every nonconstant coefficient. The above procedure applies to Example 2 of Section V, where it is carried out in full detail. Case3. If C > 1 and R = 1, then the general mechanism, as given by Eq. (1 4), takes the form p H+ mH+ + . . . + psms, in which mH+ is a mechanism and the other m’s are cycles. Rewrite it as pm + 41nl + . . . + &n,, where m is a mechanism, n, through n, are cycles and the coefficients are unrestricted, which means, as in Case 2, that we may set p equal to unity without loss of generality. To find each direct mechanism, expand m + 4 ,n, + . . . + &nc to a linear combination of steps, and set C coefficients equal to zero, where the choice of coefficients is made as follows: The cycles are of the following form:

,

n,

=

vllsl

+ . . . + vlsss

n,

=

vclsl

+ + vcsss

(16)

* * *

The C steps whose coefficients are set equal to zero and must correspond to C columns in (16) whose C x C matrix of coefficients is nonsingular. This choice will guarantee that the resulting mechanism is direct. Furthermore, each coefficient which we set equal to zero gives rise to a linear equation in the variables 41,. . ., 4,. We have C such equations, which are solvable because of the nonsingularity of the C x C matrix of coefficients. Accordingly, for each nonsingular C x C matrix of coefficients in (1 6), there is a complete solution for 41,.. ., &-,which means that the direct mechanism is completely determined. Furthermore, we get every possible direct mechanism, if we go through this procedure for every nonsingular C x C matrix of coefficients in (16). The above procedure applies to Examples 3, 4, and 5 of Section V, and it is carried out in full detail in Example 3. Case 4. If C > 1 and R > 1, then we have the general situation described in Section II1,B. Using Eqs. (12) and (13) from that section, we arrive at an explicit expression for the general mechanism as the sum of all of the following expressions : pH+1YH+1,ls1

+

~H+RYH+R.ISI

+ ... + P H + R Y H + R . S S S

PSYSlSl

” *

+ pH+lYH+l,SSS

+ . ’ . CCSYSSSS

The first R rows add up to a mechanism for the general overall reaction, and the remaining C rows are cycles. As in Case 3, our object is to choose coefficients for the cycles such that C of the columns in (1 7) add up to zero. The

290

JOHN HAPPEL A N D PETER H . SELLERS

C columns we choose must have the property that matrix ( y i j ) from these columns and from the last C rows of (17) form a nonsingular C x C matrix M. For simplicity of exposition let us suppose that the first C columns are the ones chosen, then M = (yij) where H + R + 1 5 i IS and 1 Ij IH . Our object now is to have the first C columns of (17) add up to zero. dSss,which is a Denote the sum of the first R rows of (17) by glsl * * mechanism for the overall reaction. Then the statement that the first C columns of (23) add up to zero is equivalent to the following matrix equation:

+

( O l * * ' O R )f (p(H+R+I"'pS)M

=

+

(18)

whose solution is (pH+R+I

" ' p S ) = (-Ol

* "

- CTR)M-

(19)

In other words, if the C coefficients P,,+~+ . . . p s are given the values determined by Eq. (19), then the total of the expressions in (17) will be a direct mechanism. Furthermore, if we go through this procedure for every selection of C columns in (1 7) such that the C x C matrix M is nonsingular, then we get every direct mechanism for the overall reaction (14). Altogether there are R + C undetermined coefficients pH+ 1 , . . ., ps in (17), the last C of which are determined for each direct mechanism. The remaining R parameters pH+ . ., P,,+~ are in the expression (14) for the overall reaction, which is of multiplicity R. Similarly each direct mechanism must be a function of these R parameters. The above procedure is carried out in the treatment of Example 9 in Section VI to obtain an initial direct mechanism. In all of the cases treated above the set of direct steady-state mechanisms which has been generated is exhaustive. However, it is possible for repetitions to occur among the mechanisms, but we can eliminate the possibility of repetitions in the following manner. Each time C values for pH + R + . . ., ps are determined, substitute them in the general mechanism, express it as the following linear combination of steps : ZilSl

+

..*

+

ZiSSS

(20)

and put its coefficients in the last row of Table I, which is a display of all direct mechanisms found, so far. To get the next line, choose a set of C columns from (17) which has not yet been considered. There are ( 5 ) ways of choosing columns which must be considered, but the computation (19) does not have to be carried out if either of the following conditions holds: (i) There is a row in Table I which already has zeros in the C columns we are considering. (ii) The matrix M determined by these columns is singular. These precautions will eliminate repetitions among the rows of Table I

ANALYSIS OF MECHANISMS FOR REACTION SYSTEMS

29 1

TABLE I

more simply than if they are thrown out after they are found. This saves unnecessary matrix inversions. Each of the (): column selections must be tested. Any systematic way of ordering these selections so that none are omitted is acceptable. This additional procedure to avoid repetitions is carried out in the last part of Example 9 in Section VI. V.

Systems with a Simple Overall Reaction

Most systems treated in the literature exhibit a simple overall reaction, which can be uniquely represented by a conventional chemical equation. In addition, the elementary reactions are usually selected so that all of them must be combined to form the overall reaction, which means that the system is cycle free and that there is no mathematical distinction between an elementary reaction and the step which produces it. Often the combination of steps giving the overall reaction is such that each intermediate is produced by exactly one step and consumed by exactly one step. The following example illustrates such a system. 1 . SULFURDIOXIDE OXIDATION (No CYCLES) EXAMPLE

The important commercial process of sulfur dioxide oxidation has been studied by a number of investigators. A set of steps that has been proposed for both platinum and vanadium oxide-based catalysis by Horiuti (7) for the overall reaction 2 S 0 2 + O2 + 2 S 0 3 is as follows: sl:

0,+ 2 1 s 2 0 1

s,:

so, + 1 ==S 0 , I

sj: S 0 , I s4:

+ 01=

S0,l

+1

so,/= so, + I

This is the form in which steps will be listed in all our examples-a

symbol

29 2

JOHN HAPPEL AND PETER H. SELLERS TABLE I1 OI

S0,I

2 0

0

SI s2

s, s4

I

-I

-I 0

0

so,/ 0 0 1 -1

0,

I

-2

so, 0

-1

-1 1

I

0

0 0 0 I

so,

-I

0 0 0

so,

0

TABLE I11 01

S0,I

SO&

I

0,

0 I 0

0 0 2

-2

-1

0

0

SI

+ 2s, + 2s,

2 0 0

SI

+ 2s, + 2s, + 2s,

0

SI s2

-I

-2

0 0 0

-1

-2

2

0

-1

-2 0

0

so,

-1

for the step, followed by a chemical equation for the elementary reaction produced by that step. In this example, but not in the subsequent ones, let us rewrite the elementary reactions in the following more formal vector notation: R(s,) = - 0 2 - 21 201

+ R(s2) = -SO2 - 1 + S 0 2 f R(s3) = - S 0 2 1 - 01 + S03f + 1 R(s4)= - S 0 3 1 + SO3 + 1

(22)

This displays the convention, tacitly assumed later, that the positive direction of a step corresponds to the advancement from left to right of the stated chemical equation. The matrix of stoichiometric coefficients for these reactions is shown in Table 11. The diagonalization of the matrix in Table I1 gives the matrix in Table 111, from which the steady-state mechanism is s1 + 2s, + 2s3 + 2s4. In Horiuti’s terminology the “stoichiometric numbers’’ are 1 for s1 and 2 for s 2 , s 3 ,and s4. Often, even in the case of simple reactions, it is possible to encounter systems with cycles. The following is an illustration of this situation.

EXAMPLE 2. THEHYDROGEN ELECTRODE REACTION ( 1 CYCLE) This system has received considerable attention. Milner (8) includes it in a study of several electrode reactions, and Horiuti (7) uses it as the basis of a general discussion.

293

ANALYSIS OF MECHANISMS FOR REACTION SYSTEMS

TABLE IV H

H,

Ht

*

The overall reaction of the system is given as 2H+ + 2eH,, and 3 steps are postulated whose elementary reactions are as follows : s,: H t + H + e - = H 2 s,:

+ e- + H +HeH,

H+

s3: H

Since H + and e- are always together, let us regard H+ + e - as a single component and write it simply as H + . The matrix of stoichiometric coefficients is given in Table IV, the diagonalization of which gives the matrix in Table V. From this we conclude that the general steady-state mechanism is as follows :

+ ~ 2 +) 4 ( ~ -1 sz

( ~ 1

-

(1

~ 3= )

+

+ (1

4)~1

-

4 1 ~ 2- 4

~

3

(24)

where the coefficient 4 is unrestricted. Following the method of Case 2 in Section IV,B, we find each direct mechanism by setting one of the coefficients equal to zero in the right-hand side of Eq. (24). This gives three possible values - 1, 1,0 for 4. Putting them in Eq. (24), we get all the possible direct mechanisms as follows : m,

=

2s,

+ s3

m,

=

2s,

-

m3 = s1

s3

(25)

+ s2

This result may be tabulated as shown in Table VI. In subsequent examples this sort of table will be the principal way of listing direct mechanisms. There are three elementary steps and one independent intermediate, so TABLE V

s2

H

H,

1

1 0

H+ -I

294

JOHN HAPPEL A N D PETER H . SELLERS T A B L E VI

m, m2

m,

0 2 I

2 0 1

I

-I 0

that there are 3 - I = 2 reaction routes according to Horiuti’s rule. Miyahara ( 1 7 ) and Horiuti (7) noted that any two of the three direct mechanisms could be combined algebraically to obtain the third. However, they are distinct from each other chemically since any two of them contain a step that is not involved at all in the third.

EXAMPLE 3. AMMONIA SYNTHESIS (2 CYCLES)

*

The reaction N, + 3H2 2NH, has been studied extensively from a mechanistic viewpoint. Horiuti (7) and Temkin (11) have proposed entirely different mechanisms for this reaction. Recognizing all steps in both mechanisms as possibilities, we find that there are in all 6 direct mechanisms, including the proposed ones, all of which produce the same overall reaction. In the following system for ammonia synthesis steps s l , s2,s 3 ,and s, were proposed by Temkin and s4, s5, s6, s8, and sg by Horiuti :

+ I* N,I N,I + H, * N,H,I N,H,I + I * 2NHI N, + 2/* 2NI NI + HI*NHI + I NHI + HI* NH,I + I NHI + H, * NH, + I H , + 21* 2HI NH21 + H I + NH, + 21

s , : N, s2:

s,: s4:

s5: s6: s,:

sg: sg:

The symbol I in system (26) refers to an active surface site on the catalyst. Every species with I in it is an intermediate and the rest are terminal species. For the purpose of our analysis we could omit I wherever it appears alone as a reactant. Notice that by including it as an intermediate, we get a case of a system where H < I , or in Horiuti’s terminology, the intermediates are not all independent. By diagonalizing the matrix of stoichiometric coefficients, we obtain the matrix given in Table VII. The seventh row shows that there is a simple

295

ANALYSIS OF MECHANISMS FOR REACTION SYSTEMS

TABLE VII N21 N,H21 NHI N I SI s2

+ s1

s3

s2

+ + s1

s4

+ sq - s3 - s2 - s, - 2s, - s4 + 2s3 + 2s, + 2s1 2% + s3 + s2 + s , sg + 2s, + s4 - s3 - s2 - S I + - s, + S 6 2s, 2s,

sg

sg

HI

NH21

I

N2

H2

-1 -I

-I

I 0

0 1

0

0

0

0

-

I

0

0

0

0

-

I

0 0 0 0

0 0 0 0

2 0 0 0

0 0 0 2 0 0 0 - 2 0 0 0 2

0

0

0

0

0

0 0

0

0 0

0 0

0 0

NH3

0

2 2 2 -2

-I -I

-I

-2

0

0

0

-I

-3

2

0 0

0

0 0

0 0

-I 0

0

0

1

0 ~

0

0

0 0

overall reaction, and the last two rows show that the space of all cycles is of dimension 2. The general mechanism is given' by the following equation: 2S7

+ + s2 + + s3

s1

&s8

+ 2 S 5 + s4 -

s3

4)(s1

+ $(s9 + s8 - s7 + s6) +

- s2 - s1)

+ s2 + s3) + &s4 + 2s5) + $ ( s 6 + ( 2 - $ b 7 f (6+ 9 b 8

= (I -

s9)

(27)

and $ are unrestricted. Notice that the combinations of steps (sg + sg) may be treated as single steps. The cycle matrix is given in Table VIII. There are two singular 2 x 2 submatrices in Table VIII, consisting of columns 1 and 2 and columns 3 and 4. The remaining eight 2 x 2 submatrices are nonsingular. This means, according to the Case 3 in Section IV,B, that each direct mechanism is obtainable by setting a pair of coefficients equal to zero in the second line of Eq. (27) and solving for 4 and $. This leads to six distinct solutions, as shown in Table IX, which considers every pair of coefficients. Cases 1 and 8, which correspond to singular 2 x 2 submatrices of Table VIII do not have solutions, and cases 7 and 9 are repetitions of previous solutions. Accordingly, we are left with six pairs of values for 4 and $, which may be substituted in Eq. (27) to get the direct mechanisms given in the following list: where

(sl

CI#

+ s2 + s3), (s4 + 2s,), and

+ 2s5) + 2s7 + s8 m2 = (s4 + 2s5) + 2(s6 + s9) + 3s8 (Horiuti) m3 = (s4 + 2s5) + s9) + 3s7 m4 = ( s , + s2 + s3) + 2s7 (Temkin) m5 = (sl + s2 + s3) + 2(s6 + s9) + 2.5, m6 = 3(s, + s2 + sj) - 2(s4 + 2 4 + 2(s6 + s9)

m, = (s4

- (sg

and tabulated in Table X.

0 0 0

296

JOHN HAPPEL AND PETER H. SELLERS

4 9

I 0

-I 0

0 I

0

1 1

-I

TABLE IX Case

Pairs of coefficients

1-4.4 I -4.G 1-4,2-9 1 -l#J,4+9 4, 4*2- 9 4.4 t G 9.2 - 9 9.4 + 9 2 - 9.4 t J,

I 2 3

*

4

5 6

I 8 9 10

Solutions none fj=I,+=O

4=1.+=2 +=I,$= -1 fj = 0, I j = 0 4=0,$=2

+=o,g=o 4 4

= =

none 0,9 = 0

-2.9

=

2

TABLE X SI

m1 m2 m3

m4

m, m6

+ s2 + sj 0 0 0 I I 3

s4

+ 2s,

S6

+ sg

s,

sg

0 2

2 0 3 2 0 0

1 3 0 0 2 0

I 1

I 0

0 -2

-I 0 2 2

m, and m4 are the mechanisms proposed by Horiuti and Temkin, respectively. m3 and m6 might be omitted on the grounds that some of their steps proceed in the wrong direction, but m, and m5 remain for consideration. Rate equations for simple reversible reactions are often developed from mechanistic models on the assumption that the kinetics of elementary steps can be described in terms of rate constants and surface concentrations of intermediates. An application of the Langmuir adsorption theory for such development was described in the classic text by Hougen and Watson (18), and was used for constructing rate equations for a number of heterogeneous catalytic reactions. In their treatment it was assumed that one step would be rate-controlling for a unique mechanism with the other steps at equilibrium.

297

ANALYSIS OF MECHANISMS FOR REACTION SYSTEMS

Such rate expressions are often termed Langmuir-Hinshelwood-HougenWatson (LHHW) equations and are widely used in chemical engineering [see Froment and Bischoff (19)].The usual procedure is to postulate plausible mechanisms without considering cycles, as in Example 1. In such cases it may be desirable to develop the complete list of possible direct mechanisms even if further considerations can rule out some as being unlikely. The following example illustrates a typical case. EXAMPLE 4. DEHYDROGENATION OF 1-BUTENE TO 1,3-BUTADIENE (3 CYCLES) Froment and Bischoff (19) report a study of the dehydrogenation of 1-butene to butadiene on a chromia-alumina catalyst. Neglecting isomerization of 1-butene, the following steps are postulated:

+ I * C,H,I + I*H21 H,I + I = 2HI H, + 2/* 2HI C,H,I + I * C,H,I + HI C,H,I + I* C,H,I + HI C,H,I * C4H6 + I C4H,I + I* C,H,I + H,I C,H,I + 21* C,H6/ + 2HI

s, : C,H, s,:

s3: s,: s5 :

s6: s, : s,:

s9:

H,

The overall reaction is

TABLE XI

1 0 0 0 0

'6 s5

s3 s2

s2

s, s, s*

+ s, + s5 - s3 - s2 + s, -

sg - s,

- S6 -

s9 - S6 -

s5 s5

+ s3

-

1

I 0 0 0

0

0

0 0 0

0 0 0

1

1 2

-

0 0

0

0 0 0 - 1 1 0 1 0 0 1

0 0 0 0

- 1 - 1 - 1 - 1 - 1 - 1

0 0 0 0

0 0 0

0 0 0 0

0 - 1

0 0 0

0 0 0

0 0

0

0 0 0 1 0

I

I

0 0 0

0 0 0

0 0 -

298

JOHN HAPPEL AND PETER H. SELLERS

The diagonalization of the matrix of stoichiometric coefficients is simplified in this case ifthe rows are not ordered as in steps (35).The result of diagonalizing is given in Table XI. Then, using the methods of Section IV,B, we find all the direct mechanisms of Table XI1 for the overall reaction

+ C,H,

C4H,

+ H,

Examination of the matrix in Table XI1 shows that 6 mechanisms are possible on the basis of the steps proposed by Bischoff and Froment. They identified m , , m2, m4, m5, and m6, and developed 15 rate equations corresponding to various choices of rate-controlling steps. After a set of experiments involving sequential testing and model discrimination, they retained mechanism m4 with step s1 as the rate-controlling step. According to the present procedure m, might be an additional mechanism to consider. A scheme which was considered by Hougen and Watson (18)and is slightly more complicated is the hydrogenation of isooctene codimer. This illustrates a reaction with a large number of cycles compared with the number of intermediates.

EXAMPLE 5. HYDROGENATION OF ISOOCTENES (4 CYCLES) A supported nickel catalyst was used to study the reaction in which mixed isooctenes, commercially known as codimer, are hydrogenated in the vapor phase to the corresponding isooctanes. Neglecting isomerization, the following steps are assumed to occur:

+ I*

s , : C,H,,

s,:

H,

+ I*

s3: C,H,,/

C,H,,I

H,I

+HJe

s4: C , H , , I ~ C , H , , s5:

H,

+I

+ 2 / * 2HI + 2 H I e C,H,,/ + 21

C,H,,I s,: C,H,, s,:

C,H,,I +I

s,:

C,H,,

sg:

H,

+ H,/* C,H,,I + 2 H I e C,H,,/ + /

+ C , H , , I ~C,H,,/

The overall reaction is It is not necessary to know in advance what the overall reaction is (Hougen and Watson assumed that it might also occur as an uncatalyzed reaction in the gas phase). For our purposes it is enough to know what the terminal

299

ANALYSIS OF MECHANISMS FOR REACTION SYSTEMS

TABLE XI1 s1

m1

+ s,

s,

I I I

m2 m 3 .

1

-1 -1 -1

m6

I

ss

s4

-I -1 -I 0 0 0

0 0 1 0

0 0 0

m4 m5

s3

-I -1

+

s*

sg

0

0

1

1

0 1 I

0 0 0

0 0

1 0

S6

0 0 0 0

species are, since the overall reaction among them is furnished by the diagonalization (see Table XIII) of the matrix of stoichiometric coefficients in steps 30. Using the methods of Section IV,B, we find all the direct mechanisms (see Table XIV) in this system for the overall reaction H2

+ CSH,, * CSH,,

The matrix given in Table XIV shows that there are nine direct mechanisms. Five of these, namely, m2, m5, m7, m,, and m,, were identified by Hougen and Watson. Seventeen different mechanisms with single ratecontrolling steps were modeled and tested for agreement with observed kinetic data. The model corresponding to m7 with s, as the rate-controlling step was chosen as the recommended rate equation. The mechanisms obtained by our procedure in addition to those developed by Hougen and Watson are m, , m3, m4, and m6. The large number of these mechanisms is related to the fact that four cycles appear in the diagonalized matrix. These models have unusual features that would probably not be noticed unless a procedure like this were employed. For example, in mechanism m, neither isooctane nor hydrogen adsorbs directly on the

TABLE XI11

-1

s3 S2

SS SS

+ sg + ss s1 + s, + -s2 + s5 + s2 - ss + s, S) + sg s4

s3

sg -

S)

S8

-s2

-

S6

s*

0 0 0 0 0 0 0 0

-1 1

1

0

0

0

0 0 0 0 0 0 0

0 0 0 0 0 0

1

-2

2

1 -1

I -2

-I

0 0 -1 0

0 0 0 0

-I

-1

1 0 0 0 0

0 -1 0

~

0

0

0 0 0 0

0 0 0 0

0 0 0 0

0

0 0 0

300

JOHN HAPPEL AND PETER H . SELLERS

TABLE XIV

m, m2 m , m 4 m5

m, m, mR m,

'I

'2

'3

0 0

0 0 0 0 I I 0 0 I

0 0 -1 -1 0 1 0 0 1

O 0 0 0 I I I

' 5

'6

1 1 1 1 1

0 1 0

-I

I

0

1 1 1

0 1 0

'4

'7

0 1

1 1 0

0 0 0

'8

'9

1 0 1

I

I

I

0 . 0

0

0

1

0 0 1 0

0

0 1

0 0

0 1 0 0

0

0

0

-I

0

catalyst, but instead reacts with adsorbed species. In this mechanism HI is not produced from the initial reactants and is recycled. If it is assumed that the reaction will be characterized by a single direct mechanism as well as a single rate-controlling step, one possible model is exhibited for every nonzero entry in the matrix of Table XIV. Tracer techniques with high-speed computers are useful in relaxing the requirement of a single rate-controlling step. Direct mechanisms can also be combined, if care is taken to avoid the possible occurrence of cycles. In Example 5 all nine direct mechanisms can be combined without cycle formation, though this is not always the case [Sellers (9a)I. VI.

Overall Reactions with a Multiplicity Greater Than One

Each system considered in this section has a space of overall reactions whose dimension exceeds one. In many industrial reactions involving organic substances a major product is formed, but a side reaction contributes to loss in selectivity or yield of the desired product. Such cases may be said to exhibit a multiple overall reaction, unless the ratio of desired product to by-product remains constant over a range of operating conditions, so that a simple chemical equation might be employed to express the stoichiometry. It is important to note that in these cases one cannot add up the separate direct mechanisms for all the simple reactions which add up to the overall reaction and expect, in general, to get direct mechanisms for the overall reaction, unless there are no common steps in the mechanisms of the simple reactions that form a basis for the system. The direct mechanisms for the reaction systems are, however, unique and any observed rates of appearance or disappearance of terminal species can

ANALYSIS OF MECHANISMS FOR REACTION SYSTEMS

30 1

be expressed in terms of these mechanisms. If one such mechanism predominates, it will be possible to express the rates of changes of terminal species in terms of step velocities for that mechanism. A typical process is the oxidation of ethylene over silver catalyst with a side reaction to produce undesirable carbon dioxide. Miyahara (17) was among the first to appreciate the problem of assigning unique mechanisms to this system and demonstrated that an appropriate set of steps could be chosen that corresponds to a single mechanism. In this mechanism, discussed by Happel and Sellers (Z), there are seven elementary steps and five independent intermediates, and following Horiuti’s rule there are 7 - 5 = 2 independent “routes.” This is also equal to the number of independent reactions and there are no cycles. Miyahara and Yokohama (20)considered this reaction further, employing a different mechanism with four steps and two intermediates and again arrived at a single unique mechanism for the system. Both these mechanisms followed the original view that dissociative adsorption of oxygen occurred and that the epoxide was formed as a result of interaction of ethylene with one adsorbed oxygen atom. However, later results [see Patterson (21)]suggest that diatomic oxygen is involved in the formation of the epoxide. The following two examples employ such mechanisms for the purpose of illustration, although a recent survey by Sachtler et al. (22)indicates that both views are still tenable.

EXAMPLE 6. ETHYLENE OXIDESYNTHESIS (No CYCLES) The following steps are postulated for the oxidation of ethylene:

sg:

+ 0,I + C,H, * C,H,O + OI C,H4 + 601 * 2C0, + 2H,O + 61

s4:

201*

s , : 0, I*O,I s,:

0,

(31)

+ 21

where all species involving the symbol I are intermediates and the rest are terminal species. From the diagonalization of the matrix of stoichiometric coefficients given in Table XV, the general mechanism (32) and the general overall reaction (33) can be found:

+

+ + ~ ( 2 +~ 21 ~ 2+

~ ( 6 ~ 61 ~ 2 sj) P(-7CzH4

-

~ 4 )

(32)

6 0 2 + 6 C 2 H 4 0+ 2 C 0 2 + 2 H 2 0 )

+ a(-2C2H4

-

0,

+ 2C2H40)

(33)

Since there are no cycles in the system, the general mechanism (32) is a direct mechanism for a multiple overall reaction (33), where p and a are unrestricted

302

JOHN HAPPEL AND PETER H. SELLERS

TABLE X V

6s, 2s,

+ 6s, + s3 + 2s, + s4

0,I

01

0 0

0 0

I

0 0

H,O

0,

6

2

-6

2

0

-I

CO,

C,H4

C,H40

2 0

-1 -2

values in both expressions. The form in which the multiple reaction (33) is written suggests that it is made up of two specific reactions operating independently, but this separation is not inherent in the chemical system. Any two basis elements for the reaction space could be used to construct an expression identical to (33). What is inherent in the system is the fact that there are four simple reactions in the space of all overall reactions. Their equations are

+ 0, * 2C,H40 + 4H,O * 2C,H40 + 5 0 , + 2H,O * C,H4 + 3 0 ,

2C,H4 4C0,

2C0,

2C0,

(34)

+ 2H,O + 5C,H4 * 6C,H40

having been determined by a procedure which is analogous to the procedure for finding direct mechanisms. Any two of them constitute a basis for the space of overall reactions and could have been used to express the general overall reaction. Notice that the first reaction, used in (331, is not simple. Let us collect terms and rewrite the general mechanism (32) and its overall reaction (33) in a more conventional way:

+ 2 0 ) ~ i+ (6p + 2 0 ) ~ 2+ + (35) ( 7 p + 20)C2H4 + (6p + 0)Oz * (6p + 20)C2H40 + 2pC02 + 2 p H 2 0 (6p

ps3

0 ~ 4

If the ratio p/rr remains constant, the last equation can be expressed with the ratio as a selectivity so that only a single free parameter remains, as recently suggested by Temkin (23). Examination of the matrix given in Table XV brings up an item of special interest. If the combination s4 of atomic oxygen were assumed not to occur, we would still be able to produce ethylene oxide by a combination of the first three steps. This scheme could place a lower limit on the selectivity at 6:7 or 85.7%, corresponding to a simple overall reaction rather than a multiple overall reaction. This serves to illustrate that we get fewer overall reactions than would be predicted by considering only the atom-by-species matrix, as a result of a more restricted choice of possible steps.

303

ANALYSIS OF MECHANISMS FOR REACTION SYSTEMS

EXAMPLE 7. ETHYLENE OXIDE SYNTHFSIS ( 1 CYCLE) Let us consider a more complicated case, similar to Temkin’s proposed system (5) for ethylene oxide formation, but without any prior assumption about the direction of any step in it. The overall reaction space remains the same as in Example 6, but there are additional intermediates. In particular, acetaldehyde (CH3CHO)is an intermediate which is not bound to the catalyst. Its role still requires clarification, as indicated in recent studies by Wachs and Chersick (24, but, whether or not Temkin’s scheme proves to be correct, it illustrates our method. The steps are as follows: sl:

0,+ I* 0,I

*0,I + 1 + C,H, * 01 + CH,CHO

s, : 201

s,:

0,I

s,:

+ I + C,H,O/ + C,H, * C,H,O + 01 + CH,CHO * 5 0 1 + 2C0, + 2H,O C2H,01 * 01 + C,H,

(36)

C,H,O

s s : O,/ s6: SO,/

s7:

Diagonalization of the matrix of stoichiometric coefficients gives Table XVI, from which we read off the general steady-state mechanism (37) and its overall reaction (38), where p , a, and C#J are unrestricted:

+ 3 4 s , + ( p + 30 + + d s 3 + + (2p + + 4(s4 + s,) p ( - 0, - 2C2H4 + 2C2H40)+ a( - 3 0 2 - C2H4+ 2 C 0 2 + 2 H 2 0 )

(p

$ 0 2

(37)

4)Sg

S6)

(38)

Following the method given in Section IV,B for determining those values of which make (37) into a direct mechanism, we arrive at the three direct mechanisms for the overall reaction (38), which are given by the matrix in Table XVII. Temkin (5)gave mechanism m3, but m, and m, would be equally valid choices, if all steps can occur in both directions. In interpreting his

4

TABLE XVI 0,I SI

-

s2

SI

+ s1 + s2

2s, sA

SI

+ s* + 2s,

3SI

+ 3s, + s, + S6

S,+S,+S,+S,

1 0

0 0

01 CH,CHO

C,H,O/

0

0

0 0

0 2 0

0 0 0 I

-2

1

C,H,O

-I

0 0 0

2 0 -1

-I

0

0

0

0

0 0

0 0

0 0

2 0

0

0

0

0

0

0

C,H,

0, C O , H,O

0 0 -2 0

-I

-2

-I -3

-1

0

I

-I 0

0

0 0 0 0

0

0 2

0 2

0

0

0 0 0

304

JOHN HAPPEL A N D PETER H . SELLERS

TABLE XVll SI

m, m2

p p p

m3

s2

+ 3a + 30 + 3a

’3

0

+ 3a + 3a

-p p

+ ‘h

SS

U

p-3a

U

0

U

2P

s4

+Sl

-p-3a

- 2P 0

results, Temkin concluded that step s7 should be included in a rate expression although it does not occur in mechanism m3. From the viewpoint developed in this article, we would conclude that if s, is to be employed, it must occur in the negative direction to accommodate the only possible other direct mechanisms m, and m 2 . Combinations of these three direct mechanisms which are cycle-free are discussed by Sellers (9b). There would be no essential change in the above results if we had diagonalized the matrix differently. If line 6 in the matrix of Table XVI were replaced by twice line 6 minus line 5, the matrix would still be in the appropriate diagonal form, but the general mechanism (37) would appear as (37’) and the overall reaction (38) would appear as (38’):

+ 5 0 ’ ) s , + ( p ’ + 50‘ + + 20’(s, + Sg) + 4”s4 + s7) + (2p’ - 20’ + 4 ’ ) S S p’( -02 - 2C2H4 + 2C,H40) + - 2C2H4O + 4 c o 2 + 4H2O)

(p’

4’)SZ

(37‘) (38’)

a’(-502

The matrix given in Table XVII would take the form given in Table XVIII. Each of the three mechanisms listed in Tables XVII and XVIII corresponds to elimination of the same step. The mechanisms in Table XVIII correspond to 0 = 20’ and p = p‘ - of, so we have simply changed the way of writing the two arbitrary advancement parameters without altering the mechanisms. One other item is worth noting in this example. Since s4 and s7 appear only in the cycle, to omit either one from the choice of possible steps would reduce the system to a unique direct mechanism with a multiplicity of two. But it would not be possible to eliminate further steps and still obtain a reaction among all the terminal species as we were able to do in Example 5. TABLE XVIII SI

m, m2 m3

p’ p’ p’

+ 5a’ + 5a’

+ 5a‘

s2

0 -p‘ p’

+ 7a’ +5d

s.3

+ Sh

s5

s4

+ s7

2a’

p, - 7a’

- p ( - 5ar

2a’

0 2p’ - 20’

2a’ - 2p‘ 0

2a’

ANALYSIS OF MECHANISMS FOR REACTION SYSTEMS

305

Another type of reaction that has received considerable study is that of hydrocarbon isomerizations [see Pines (25)l. These and similar multicomponent systems have been the subject of an elegant kinetic analysis by Wei and Prater (26).In this method pseudomonomolecular reactions following mass action laws are assumed. With these simplifications they treat the general mechanism which includes the occurrence of all possible steps without cycle formation. Data are obtained for concentration changes of reacting components versus time and individual rate constants are calculated by a novel method of integration of the differential equations that model the systems. The Wei and Prater method has been applied to n-butene isomerizations, as well as to several other systems [see Christoffel(27)l.The following example illustrates a different way of considering such systems. In this example models are first generated in which all possible elementary step velocities may not occur.

EXAMPLE8.

ISOMERIZATION OF

BUTENES (1

CYCLE)

Isomerizations (39) among the species 1-butene, trans-2-butene, and cis-2butene are postulated: s, : C4H8-1

+ I*C4H8-II

s*: C4H8-II= C4H8-2CI

s, : C4H8-lI* C4H8-2TI s4 : s5 : S6

:

C,H8-2CI= C4H8-2TI

(39)

* C4H8-2C + I C4H8-2TI * C4H8-2T + I C4H8-2CI

Let the isomers be denoted by 1,2C, and 2T. Then diagonalizing the matrix of stoichiometric coefficients gives matrix of Table XIX. From the matrix of Table XIX we obtain the general steady-state mechanism (40) with p and a unrestricted: P(S,

+ s5) +

+ S 6 ) + ( P + 4)sz + (a - 41% + 4 s 4

(40)

whose overall reaction is as follows : p( -(C4H8 - I)

+ (C4H8- 2C)) + a( -(C4H8 - 1) + (C4H8- 2T))

(41)

By the method of Section IV,B we arrive at three direct mechanisms tabulated in the matrix of Table XX. All six elementary steps assumed possible for any mechanism are shown in Fig. 2,

306

JOHN HAPPEL AND PETER H . SELLERS

TABLE XIX 11 S1

SI

SI

+ s2 + s3

I

2C

2T

I

-I -I -I

0

0

0 0

0 0

-I -I

I

0 1

0

0

0 0 I

0 0

0 0

0 0

I 0

0 I

0

0

0

0

0

S]

+ s2 + ss + s3 + s,

s2

-

+ s4

0

s3

2Tl

0

0 0

S]

2CI

-I

-1

-I 0

TABLE XX

Wei and Prater, in treating this system, assumed in effect that the surface species would be in equilibrium with butenes in the gas phase so that only reactions s2, s 3 , and s4 were employed, corresponding to six first-order rate constants. We have used the more general form shown in Fig. 2. The three direct mechanisms in the matrix of Table XX can be diagrammed as shown in Fig. 3. The arrows in Fig. 3 show the net direction the reaction velocity steps would take for the case where a mixture of reactants is employed such that 1-butene would produce both of the 2-butenes. The steps are not assumed to

C4Hg - 1

1lS1 C4Hg-2CP

54

C4Hg-2TP

551t C4Hg- 2C FIG.

C4Hg - 2 T

2. Possible elementary steps for isomerization of butenes. This diagram corresponds

to listing (39).

ANALYSIS OF MECHANISMS FOR REACTION SYSTEMS

307

R

FIG.3. Direct mechanisms for isomerization of butenes.

be unidirectional. The directions of these two independent reactions can be calculated from thermodynamic data. If experimental data cannot be satisfactorily modeled by one of the three direct mechanisms, a combination of m, and m, will accommodate any possible values for step s4, except when s4 does not occur in either direction, but that case would have already been ruled out since sq is not contained in the direct mechanism m3. If adsorption steps s l , s 5 , and s6 were considered to be at equilibrium, as assumed by Wei and Prater, the situation would be considerably simplified. Since the 2-butenes are connected by a direct reaction, it would be possible from thermodynamics to calculate the direction of the reaction connecting them. Thus, it would be necessary at the outset to model only two direct mechanisms, namely either m, or m2, and m3. If neither of these direct mechanisms were capable of modeling the data, a combination of the two would be sufficient to evaluate all six reaction velocity constants. Such modeling would, of course, be strengthened by supplementing the usual overall reaction rate experiments by tracer data. Recent studies of the kinetics and mechanism of n-butene isomerization over lanthanum oxide by Rosynek et al. (28) indicate that for this catalyst interconversion of the two 2-butene isomers (s4 in Example 8) is very slow and in that case the system could be described by mechanism m3. Studies by Goldwasser and Hall (29) indicate that as temperature is increased, there is appreciable direct conversion via s4 so that one or both of the other two direct mechanisms may be involved. These authors suggest that further studies with all three isomers, at several temperatures and with tracers, would be desirable. The monomolecular conversion of three components has also been considered in some detail by Kallo (30). Rate equations based on Langmuir adsorption were developed assuming a number of different mechanistic schemes including steps in which surface adsorption was not at equilibrium. Since the rate equations developed became complicated, practical application was devoted to cases in which only initial reaction rates were observed, so

308

JOHN HAPPEL AND PETER H. SELLERS

that the final steps involving product formation could be considered unidirectional. The scheme shown in Fig. 2 is one of a number of alternatives considered by Kallo (his scheme VII), and that corresponding to s4 = 0 in Fig. 3 was also identified by him, but not the alternatives with s2 = 0 and s3 = 0. A number of additional alternates could be developed following our procedure, consistent with an appropriate choice of possible elementary steps. Dehydrogenation, hydrogenation, and aromatization of hydrocarbons have also been widely studied [see Pines (25)]and applied industrially. Since olefinic compounds produced by such reactions can simultaneously react to form isomers, it is of interest to explore effects of such combined reactions on the mechanisms involved. Model studies of hydrogenation-dehydrogenation of the n-butenex-butene:hydrogensystem by Happel et al. (31)and of the isobutane:isobutene:hydrogensystem by Happel et al. (32) showed that they are governed by different kinetics. This seems to be due to the occurrence of isomerization reactions in the former case. Studies by Hnatow (33) indicated that in the case of n-butane dehydrogenation 1-butene is the primary product. However, the 2-butenes hydrogenate more rapidly than 1-butene. The following example illustrates how mechanisms can be developed to reflect these observations.

EXAMPLE 9.

BUTANE DEHYDROGENATION (3 CYCLES)

Following a variation of the well-known Horiuti-Polanyi mechanism, we consider the following steps as possible for the system n-butane-n-buteneshydrogen over chromia-alumina catalyst : C4Hl0

+ I*

C4Hl0I

+ I*H,I H21 + I* 2HI C4Hl0I + I* C4H9-II + HI C4HloI + I* C4H9-2/+ HI C4H9-11 + I*C4H,-II + HI C4H9-21 + I = C4H,-2CI + HI C4H9-21+ I * C4H8-2TI + HI H,

C4H8-II* C4H,-2CI C,H,-II*

C4H8-2TI

C,H,-ZTI*

C4H8-2T

C4H8-2CI

* C4H,-2C

C4H,-11* C4H8-2CI

C4H8-I

+I +I

+I

= C,H,-ZTI

TABLE XXI C,H8-2Tl

C4H,-2CI

C,H,-I1

C4H,-21 -1 -1

se

+ s5 + S8 + S I 1 + s5 + + sl, + s4 + Sb + SI3 - s5 + - S) + s9

S)

-

S8

- s,

'7

-

'8

+ '14

s1 - s, - s3 s1 - sz - s3 S6

S)

-- 1

1

0

+ SIO

0 0 0

0 0

0

0 0

0

0

0

0

0 0

0 0

0 0 0

0 0

0

0 0

0 0 0 0 0 s1 - s1 - s3

C4H9-II

0

0

1

H,I 0 0 0 0 0

C4HlOI HI 0 0 0 -1 -1

I

C4H8-2T

1 1

-1 -1

1

-I

1 1

-1 -1

-1

C4H,-2C

C4H,, 0 0 0 0 0

C4H,-I 0 0 0 0 0 0 0 0

H, 0 0

0 0 0 -1

0

1

0

0

0

0

0

I 0

0 0 2

0 0 0

0 0 0

0 0 0

0 0 0

0 0 0

1

0

-1

0

1

0 0

1

-1 -1

0 1

1 1

0 0 0

0 0 0

0 0 0

0 0 0

0 0 0

0 0

0 0 0

0 0 0

0

-1 -2

-1

0

0

0

0 0 0

0 0 0

0 -I

310

JOHN HAPPEL A N D PETER H . SELLERS

Diagonalization of the matrix of stoichiometric coefficients in (42) gives the matrix of Table XXI, from which we read off the steady-state mechanism (43) and its overall reaction (44), which has a multiplicity of three:

+ + s g + + a(s1 - s2 - + + + + T(s1 - s2 + + + + 4(s4 - + - + + s9 + + - f =(P + + T)(S1 - S2 psi1 + as12 + T S 1 3 ( P f a - 4 ) S g + (T 4 x S 4 + (a - 4 -t X +

p ( s , - s2 - s3

- s3

x(s7

s3

s11)

sg

sg

$(s7

slO)

s8 -

s13)

s6

s8

s5

s7

s12)

s5

s6

s7

sg)

s14)

S3)

Sg)

+(P - x

-

Il/ls8

$Is7

+ (4 - d S 9 + X s 1 0

(43)

f $s14

+ a(-C,Hio + (C4H8 - 2C) + + ~ ( - C 4 H l o+ (C4H8 - 1) + H2)

P ( - C ~ H , O+ (C4Hg - 2T) +

H2)

H2)

(44)

Now let us determine all the direct mechanisms for reaction (44) in detail to illustrate the method introduced in Case 4 of Section IV,B. Each direct mechanism for reaction (44) must depend on the parameters p, a, and T , because they appear in the reaction (44), but 4, X, and $ must be evaluated, which is done by means of formula (19) in Section IV,B. Start with the general mechanism (43) and extract from it the cycle the matrix of Table XXII and one mechanism ( 4 9 , written as a row vector: TABLE XXll

4 x

*

0

0 0 p+(T+T

0 0 0 /3

0 0 0

0 0 0

-1

T

p+a

0 0

1

-I

't

0

I

0 0

-I

1 - 1

a

p

1

-1 0

0

0

0

I

0

0

1

0

0

(45) The cycle matrix of Table XXll is a tabulation of mechanism (43) with y = 0, a = 0, and T = 0, and the row vector (51) consists of the coeEients in (43) with 4 = 0, x = 0, and Il/ = 0. Any three independent cycles could have been chosen to generate Table XXII and any mechanism for the overall reaction could have been chosen to establish the row vector (45).The choices we made are arbitrary and depend on the diagonalization procedure used to find the matrix of Table XXI, which is far from unique. The important point is that the list of direct mechanisms we are looking for is unique and independent of how the above choices are made. Starting from the left side of the matrix of Table XXII, choose the first three columns which constitute a nonsingular 3 x 3 matrix, which will be

31 1

ANALYSIS OF MECHANISMS FOR REACTION SYSTEMS

those headed by ss, s7, and s,. In these three columns of Table XXII and row vector ( 4 9 , we find matrix (46) and row vector (47):

I/ = (

p

+ 0,0,0)

(47)

Apply formula (19) to these to get an evaluation for (4,x, $) as follows:

\

0

-1

1/

+ 6,p + 0,-0) (48) Put these values, that is, 4 = p + 0,x = p + 0,and $ = -6, in the general =(

p

mechanism (43), which gives the first direct mechanism. It appears as the first row of the matrix in Table XXIII. The subsequent rows are determined in the same way with an additional procedure to avoid repetitions, which can be explained by first looking at the row m3 in Table XXIII, which is the direct mechanism corresponding to the columns s 5 , s,, s 1 4 .Since s I 4 is the last column, the next three columns to consider are s s , s8, s,. Before testing the 3 x 3 submatrix M of the matrix of Table XXII with those columns for nonsingularity, look for any mechanisms already listed in Table XXIII which have zeros in those columns. If any are found, dismiss that choice of columns. Mechanism m, has zeros in columns ss, s8, s9, wfiich means that this choice of columns would lead to the same mechanism as m , , unless M was singular, in which case we would also want to dismiss this choice. Thus, we avoid the test for nonsingularity in a case such as this. The next two choices would be s 5 , s 8 ,s l 0 and s S r sg, s14,which would be dismissed for the same reason. The next choice is ss, sg, s l 0 ,for which matrix M is singular. Finally, the next choice s 5 , s,, s14is accepted, and it determines the direct mechanism m4. A continuation of this procedure gives the complete set of direct mechanisms through m18. Mechanism m18 corresponds to the case where the isomerization steps s9, s l 0 , and s14are assumed to be very slow compared with steps of hydrogenation and dehydrogenation. In that case, if p and r~ were taken as negative and 5 as positive, we would model a situation in which n-butane was dehydrogenating to produce l-butene and at the same time the 2-butenes were being hydrogenated to produce n-butane. This qualitatively follows the observations of Hnatow (33). This mechanism is of interest because it calls attention to the limitation of a rule of thumb sometimes employed in catalytic research. It has at times

TABLE XXIII (si

-

'2

-

'3)

'11

'12

'13

'5.

(s4

+ '6)

s7

'8

s9

'10

0

P + U

m1 m 2

U

m4

0

m 5

0 -'I

m 8

U

0

m9 m10 m11

-p

m12 ml 3

m14 m15 m16 m17 m18

0

-U

-T

0 - T -T

- 'I

0

-T

0 U

0 0 0

-U

0

P + T

0

-T

0 0 0

P + U

0

m 7

P

P

P + U

m6

-U

0

P + O

m 3

' 1 4

P -'I

0 0 0 0 P

0

P

0 0 0 -U

0 P

0 0

T

ANALYSIS OF MECHANISMS FOR REACTION SYSTEMS

313

been suggested that a useful approach to improve catalyst development may be to study the formation of reactants from products instead of the desired forward reaction [see Bond (34)].This example shows how this idea may need to be modified for mechanisms involving isomer formation. Another point illustrated by Table XXIII is the need to carefully consider the effect of lumping isomers for convenience, when mechanistic models are generated. Thus, in Example 4, isomerization of 1-butene is neglected in selecting the elementary steps for butadiene production. In effect, it is assumed that all intermediates are indistinguishable whether 1-butene or a mixture of n-butenes reacts. If that scheme were used in the present case, we could consider a model in which only sl, s2,s3, s4, s6, and s I 3were retained, which would correspond to only a single direct mechanism. However, if instead we chose to retain all the elementary steps as possibilities except s l , and s12,we would obtain five direct mechanisms for a system producing only 1-butene (in which p = (T = 0). Reactions involving the catalytic hydrogenation of carbon monoxide to produce hydrocarbons and oxygenated products are important for chemical and fuel production from coal. The following example, in which the methanation of synthesis gas is simulated, illustrates a typical system.

EXAMPLE 10. METHANATION OF SYNTHESIS GAS(3

CYCLES)

A 16-step system for methanation over a nickel catalyst is selected that includes many features discussed in the literature :

+ I*CI + 01 + HI* CHI + I CHI + HI* CH,I + I CH,I + HI * CH,I + I CH,I + HI* CH, + 21 OH/ + HI* H,O + 21 co, + I* COJ co + I*COI COI CI

+ 21* 2HI C0,I + HI* CHOOI + I CHOOI + HI* CHOI + OH1 01 + HI* OH1 + I COI + OI* C0,I + I CHOOI + I * OH1 + COI CHOI + HI* CHI + OH1 COI + HI*CHO/+ I H,

(49)

314

JOHN HAPPEL A N D PETER H. SELLERS

It is by no means exhaustive but does exhibit the main possibilities for reaction mechanisms. The idea of a CHO,,, intermediate has been advocated by Pichler and Schultz (35),although other intermediates such as CHOH,,, could have been equally well postulated to take into account the hypothesis that C O dissociation might be assisted by hydrogen. The water gas shift reaction is considered to occur via the CHOO,,, intermediate according to studies of Oki et al. (36).The hydrogenation of carbidic species by atomic rather than molecular hydrogen follows the findings of Happel et a/. (37). By diagonalizing the matrix of stoichiometric coefficients in (49), we get the matrix of Table XXIV, from which we can read off the overall reaction

+ 0(-2CO

-

2Hz

+ CH4 + C 0 2 )

( 50)

which has a multiplicity of two. Then we can apply the methods of Section IV,B to determine all the direct mechanisms for r (shown in Table XXV) from which are omitted the following steps with constant coefficients:

+ 0)(s3 + s4 + +

+ ( p + 20)sS + (3p + 20)s9 (51) The maximum number of steps is obtained ( H + R = 13) for m,, m I 2 , (p

sg)

ps6

- cs7

m I 3 , m14, and m15.If the CHOI is thought to be unlikely [see Yates and Cavanagh (38)],we are left with only m a , m,,, and m, for consideration, so it can be seen that establishment of the presence or absence of such an intermediate is important. Mechanism ma is probably the simplest since it does not require the CHOOl intermediate that appears in the water gas shift reaction. However, if it were assumed that over certain catalysts the reaction of O/ with COI ( ~ 1 3 )could not occur, then mechanism m,, should be considered as a possibility. In this example we only listed the following two overall simple chemical reactions to represent the system: r,:

3H,

r,:

2H,

+ CO * CH, + H,O + 2CO * CH, + CO,

Altogether there are five simple overall reactions in the system, the others being as follows :

+ H,O * CO, + H, (water gas shift) CO, + 4H, * CH, + 2H,O (Sabatier) 4CO + 2H,O + CH, + 3C0, (Kolbel-Engelhardt)

r 3 : CO

r4: rs :

(53)

Any 2 of these 5 equations could be employed to represent the overall reaction with a multiplicity of 2 and the same 15 direct mechanisms would be obtained. In this example we expressed the general overall reaction as

I

l

l

oooooooo--m I

0 0 0 0 0 0 0 - 0 0 0

I

I

1

0 0 0 0 0 0 - 0 0 N N

m w I I I

I

- N

0 -

--

0 0 0

0 0 0

0 0 0

0 0 0

0 0 0

0 0 0 0 - 0 0 0 0 0 0

- 0

0 0 0

I

3 0

I 0 0 0

I 3 0

0 0 0

I

3 0

0 0 0

I

3 0

0 0 0

I

I

0 - - 0 0 0 0 0 0 0 0

I

0 0 - - 0 0 0 0 0 0 0

I

0 0 0 - - 0 0 0 0 0 0

I

- 0 0 0 0 0 0 - 0 0 0

I

oooooooooom

3 0

0 0 0

- - - - N N - - N N m

O O O O O O O O O N O

D O

0 0 0

I

3 0

0 0 0

I

3 0

0 0 0

I

0 - - - - - 0 0 N 0 0

3 0

0 0 0

I

0 0 0 0 0 0 - 0 0 0 0

3 0

0 0 0

I

0 0 0 0 0 - 0 0 0 0 0

3 0

I

- - 0 0 0 0 0 0 0 0 0

m

I

m

n

0 0 0

N - 0 1-

-

3 0

- 0 0 0 0 0 0 0 0 0 0

m

N

c

m

N

mm

' D P

m v I

t +

v)N

m m

::

'1

m

N

m-

m

m

m

d v)

n m N

m

II

m

I

p

s

b c

+

o+

m-

m-

m N

N I m-

lo+ v)-

I

+ " Z

0 1

m-

- + I f + my + + d, + + + 7 A: + - I y + 4

mo" vm)w N

mm

++,

++

t l + -

316

JOHN HAPPEL AND PETER H. SELLERS

TABLE XXV 6 1

+ s2) 0 0 0 0 0 0

m1 m2

%I

0 0

p+a

-a -a -a P + U

0

-a p + a P+a

0 0

a P + O

P

0 0

P + U

p + a

P+

--d

P fP

s12

P

P

tP

m13

tP +a

mi4

/J + O

-0

-a

ml5

p+2a

-a

-a

'1.3

0 0 0 -p

- 2a

0 P

0 0 P+a

0 0 P+a p+2a

'14

s1s

0

P + U

a

-a -a

0

0 0 0

a

m12

810

p

a

- p - a

0 0 0

-a

+ 20 a a a

P + U

0

-U

P+a fP +

0 0

0 - 2a 0 0 0 -P P

-- P

=

0 0 0 0

p+a p+a P + O

p + a P + a

P

0 P

0 0 0 PI2 0 -a

'16

P + O

0 p + a p 0 2a

+

0 P

0 0 0 0 -P

0 a

0

pr, + or,. If we had separately calculated direct mechanisms for r l and r2 instead of considering the combined reaction system, i.e., by leaving out the terminal species CO, for r1 and H,O for r,, we would have obtained 7 direct mechanisms for rl and 10 direct mechanisms for r,. Therefore, if we took m, as 1 of the 7 direct mechanisms for rl and m, as 1 of the 10 direct mechanisms for I-,, then there would be 70 combinations of the form pm, am2. However, if we proceeded to examine the result obtained in this manner, we would find that only 15 of them are direct mechanisms and the other 55 are combinations of 2 of the 15 without any canceled steps. Similarly, the numbers of direct mechanisms for r3, r4, and r5 are 7, 10, and 15, respectively. Obtaining the direct mechanisms for the multiple reaction at the outset is clearly a more effective procedure than combining results for separate individual simple reactions. In fact, without the complete list of possible steps for all reactions, it would not be possible to know whether the complete list of direct mechanisms for each had been derived. The reverse reaction, steam cracking of methane, involves the same elementary steps as the methanation reaction. The kinetics for that reaction have been developed for a single direct mechanism by Snagovskii and Ostrovskii (39). A recent comprehensive review of a set of elementary reactions that can be chosen to represent the Fischer-Tropsch synthesis has been presented by Rofer-De Poorter (40). Such sets of elementary reactions form the starting point for our treatment as discussed in Section 11.

+

ANALYSIS OF MECHANISMS FOR REACTION SYSTEMS

VII.

317

Discussion

This survey has been concerned with the enumeration of all possible mechanisms for a complex chemical reaction system based on the assumption of given elementary reaction steps and species. The procedure presented for such identification has been directly applied to a number of examples in the field of heterogeneous catalysis. Application to other areas is clearly indicated. These would include complex homogeneous reaction systems, many of which are characterized by the presence of intermediates acting as catalysts or free radicals. Enzyme catalysis should also be amenable to this approach. The subject of reaction mechanism also has a bearing on other fundamental problems of physical chemistry. In the following two sections the relationship of the material presented here to thermodynamics and chemical kinetics is considered. A. THERMODYNAMICS In carrying out the procedure for determining mechanisms that is presented here, one obtains a set of independent chemical reactions among the terminal species in addition to the set of reaction mechanisms. This set of reactions furnishes a fundamental basis for determination of the components to be employed in Gibbs’ phase rule, which forms the foundation of thermodynamic equilibrium theory. This is possible because the specification of possible elementary steps to be employed in a system presents a unique a priori resolution of the number of components in the Gibbs sense. The number of comporienfs as defined for application in the phase rule is equal to the number of terminal species minus the number of independent chemical reactions and minus the number of any restrictive conditions, such as material balance or charge neutrality. The number of independent reactions includes only those that actually occur under the conditions in question. For example, Gibbs (41), in citing the case of equilibrium in a vessel containing water and free hydrogen and oxygen, states that we should be obliged to recognize three components in the gaseous part since the reaction H, + +02 H,O is assumed not to occur (i.e.,3 - 0). If, however, a suitable catalyst were present, or if the temperature were high enough for reaction (2),the system would reduce to 3 - 1 = 2 components. If we imposed the condition that all the H, and O2 came from dissociation of H 2 0 , we would have a system of only one component. Thus, Gibbs clearly recognized that it was the reactions that would actually occur that should be employed in thermodynamic calculations. A method for calculating the number of independent reactions discussed

318

JOHN HAPPEL A N D PETER H. SELLERS

by Gibbs has been called Gibbs’ rule of stoichiometry by Christiansen (3). Aris and Mah (42) have discussed this rule at length and stated that it gives an upper bound to the number of independent reactions. This method is based on determination of the number of independent reactions by means of an atom by species matrix, as also developed by Amundson (43).Such a method will give the maximum number of independent reactions, assuming no isomerizations, rather than those required by the phase rule itself. Aris and Mah developed an ingenious scheme whereby experimental observations of rates of change of species consumed and produced can be used to determine an observational matrix which will show how many independent reactions are actually accounting for the observed composition changes. They applied this procedure to an example given by Beek (44) in which ethylene was converted to ethylene oxide as well as to CO, and HzO, so that an atom-species matrix would correspond to at most two independent overall reactions, similar to Examples 6 and 7 in Section VI of this article. It was shown that the data indeed were consistent with two independent overall reactions, but this procedure may be difficult to apply practically given the usual uncertainties in kinetic data. Bjornbom (45)has discussed the relation between reaction mechanism and the stoichiometric behavior of chemical reactions. He pointed out that the set of linearly independent reactions which is obtained from the atom-by species matrix may be larger than the number of independent reactions required to describe an actual physical system because the latter must be related to its reaction mechanism. He gave several examples of complex homogeneous reaction systems including a trial-and-error procedure for hydroperoxide decomposition, based on data obtained by Hiatt and coworkers (46).The system considered was the metal catalyzed decomposition of secondary butyl hydroperoxide in n-pentene solution to give secondary ethyl butyl alcohol, methyl butyl ketone, water and oxygen. The atom-byspecies matrix analysis showed that the maximum number of independent reactions was two, but by forming matrices of the intermediates he showed that the number of independent reactions was actually equal to one. Then by trying linear combinations of the elementary reaction such that the intermediates cancel, he found the single reaction consistent with Hiatt’s experiments: 3C4H,00H

* 2C4H,0H + CH,COC,H, + 0, + H,O

(54)

The procedure described here is consistent with his approach but is simpler to use in more complicated cases. Often thermodynamic calculations for complex systems are made assuming that all chemical changes can take place that are allowed within the framework of the atomic material balances. This approximation may be appropriate at high temperatures but is often not true for catalytic systems.

ANALYSIS OF MECHANISMS FOR REACTION SYSTEMS

319

Smith (47)and Bjornbom (48)have discussed the introduction of restrictions into equilibrium calculations in addition to the elemental abundance constraints. Often only a restricted set of terminal species is chosen, but it would seem logical in choosing such additional restrictions to revert to Gibbs’ original idea of specifying possible reactions as well as possible species. The choice of elementary reactions, rather than of overall reactions with complicated stoichiometry, would be simplified by modern developments in theoretical chemistry and surface science. B. KINETICS Our object has been to enumerate all sets of steps corresponding to possible direct mechanisms. Insight into how to choose the elementary steps themselves can often be obtained from physicochemical principles and experimental surface examination as well as from rate data. This information will also throw light on the most likely mechanisms from among those generated. The magnitudes of concentrations of intermediates and of step velocities appearing in these mechanisms are the parameters in kinetic models that form the next step for further discrimination. A detailed treatment of model building for this purpose is beyond the scope of this article. The subject is briefly discussed here in the context of the methods presented. At the outset it may also be advantageous to consider problems of structural identifiability and distinguishability. A model is not identifiable if in principle it is impossible to determine the desired parameters on the basis of proposed data to be obtained even if there is no experimental uncertainty or inadequacy in computer programming. Even when it is theoretically possible to determine the parameters for a given model, it may also be possible to compute those for a competing model from the same data and the models will then be indistinguishable. Resolution of these problems is often not simple. The subject is discussed in an advanced monograph by Walter (4Y) as well as in papers by Park and Himmelblau (49a)and Walter et al. (4%). Most standard chemical engineering tests on kinetics [see those of Carberry (50), Smith (52), Froment and Bischoff (19), and Hill (52)],omitting such considerations, proceed directly to comprehensive treatment of the subject of parameter estimation in heterogeneous catalysis in terms of rate equations based on LHHW models for simple overall reactions, as discussed earlier. The data used consist of overall reaction velocities obtained under varying conditions of temperature, pressure, and concentrations of reacting species. There seems to be no presentation of a systematic method for initial consideration of the possible mechanisms to be modeled. Details of the methodology for discrimination and parameter estimation among models chosen have been discussed by Bart (53)from a mathematical standpoint.

3 20

JOHN HAPPEL AND PETER H. SELLERS

Information on the steps in a reaction mechanism can be extended significantly by isotopic tracer measurements, especially by transient tracing [see Happel et al. (54,55)].Studies by Temkin and Horiuti previously referenced here have been confined to steady-state isotopic transfer techniques. Modeling with transient isotope data is often more useful since it enables direct determination of concentrations of intermediates as well as elementary step velocities. When kinetic rate equations alone are used for modeling, determination of these parameters is more indirect. The use of tracers in this manner has also been considered by Le Cardinal et al. (56), with special reference to homogeneous systems, and discussed by Happel (57) and Le Cardinal (58).Such an approach parallels the viewpoint of Aris and Mah (42) in which they distinguished between the kinematics and kinetics of overall reactions. Rates of change of species are considered without reference to their correlation in terms of rate equations related to particular physical conditions. To summarize, the type of information that has been presented in this review should be useful in furnishing a logical first step in comprehensive understanding of complex chemical reaction systems. Consideration of a chemical system in terms of unique direct reaction mechanisms required to produce observable rates of change of terminal species has distinct advantages, especially when multiple overall reactions are involved. The required necessary assumptions regarding possible elementary reaction steps are becoming increasingly accessible through modern tools for surface spectroscopy and fundamental theories of chemical kinetics of elementary reaction steps. A number of examples worked out in detail illustrate that the procedure can be rather readily followed even if one does not wish to go into mathematical details. In many cases insights are obtained that are not immediately obvious from more superficial considerations.

VIII. A

(4 C=S-Q

H I I M m m, N n,

P=S- H

List of Symbols

Number of species in chemical system. ith species in chemical system. Dimension of cycle space. Rank of matrix of stoichiometric coefficients of intermediates only. Number of intermediates in chemical system. Active site on a catalyst. C x C submatrix of matrix (oij). Mechanism. ith element in a basis for mechanism space. Dimension of unspecified vector space. ith cycle. Dimension of steady-state mechanism space.

ANALYSIS OF MECHANISMS FOR REACTION SYSTEMS

Q R = Q - H

R R(m) ri = R(si) S S Si

T = A - I V aij Bij

Yij

Pi v..

P U bi

T T..

i’ 4i x

*

32 1

Dimension of reaction space. Dimension of overall reaction space. Linear transformation of mechanism space to reaction space. Reaction produced by mechanism m. ith elementary reaction in chemical system. Dimension of mechanism space; also the number of steps (types of molecular interaction) in chemical system. Step in a mechanism. ith step (type of molecular interaction) in chemical system. Number of terminal spacies in chemical system. Row vector for a steady-state mechanism. Stoichiometric coefficient of species a j in elementary reaction T i . Stoichiometric coefficient of species a j in overall reaction R(mi). Net rate of advancement of step sj in steady-state mechanism mi. Coefficient of mi in a mechanism m. Net rate of advancement of step sj in cycle ni. Rate of advancement of a mechanism. Rate of advancement of a mechanism. Coefficient of step si in a mechanism m. Rate of advancement of a mechanism. Net rate of advancement of step sj in the ith direct mechanism. Rate of advancement of a cycle. Coefficient of cycle ni in a cycle. Rate of advancement of a cycle. Rate of advancement of a cycle. REFERENCEB

1. Happel, J., and Sellers, P. H., Ind. Eng. Ckem. Fundam. 21,67 (1982). 2. Moore, W. J., “Physical Chemistry,” 4th ed., p. 332. Prentice-Hall, Englewood Cliffs, New Jersey, 1972. 3. Christiansen, J. A,, Ado. Catal. 5, 31 1 (1953). 4. Horiuti, J., and Nakamura, T., Adu. Caral. 17, I (1967). 5. Temkin, M . I . , Adu. Catal. 28, 173 ( I 979). 6. Boudart, M . , “Kinetics of Chemical Processes,” Chap. 3. Prentice-Hall, Englewood Cliffs, New Jersey, 1968. 7. Horiuti, J., Ann. N.Y. Acad. Sci. 213, 5 (1973). 8. Milner, P. C., J. Electrochem. SOC.111,228 (1964); Milner, P. C., J. Elec/roclrem. Sor. 111, 1437 (1964). 9. Sellers, P. H., Arch. Ration. Mech. Anal. 44, 23 (1971); Sellers, P. H., Arch. Ration. Mecli. Anal. 44, 376 (1972). 9a. Sellers, P. H.. SIAM J. Appl. Math. (1983) in press. 9h. Sellers, P. H., in “Proceedings of the Symposium on Chemical Applications of Topology and Graph Theory” (R. B. King, ed.). Elsevier, Amsterdam, 1983. 10. Temkin, M. I., In!. Chem. Eng. 11,709 (1971). 11. Temkin, M. I., Ann. N.Y. Acad. Sci. 213, 79 (1973). 12. Sinanoglu, O., J. Am. Cliem. SOC.97,2309 (1975). 13. Lee, L..and Sinanoglu, O., Z. Phys. Chem. 125, 129 (1981). 14. Aris, R., “Elementary Chemical Reactor Analysis,’’ p. 8. Prentice Hall, Englewood Cliffs, New Jersey, 1969. 15. Sellers, P. H., SIAM J. Appl. Math. 15, 637 (1967).

322

JOHN HAPPEL AND PETER H. SELLERS

16. Temkin, M. I., I n / . Chrm. Eng. 16, 264 (1976). 17. Miyahara, K . , J. Res. Inst. Catal. Hokkaido Uniu. 17, 219 (1969). 18. Hougen, 0. A,, and Watson, K . M., “Chemical Process Principles,” Vol. 3. Wiley, New

York, 1947. 19. Froment, G . F.. and Bischoff, K. B., “Chemical Reactor Analysis and Design.” Wiley,

New York, 1979. 20. Miyahara, K.. and Yokohama, S., J . Res. Inst. Catal. Hokkaido Uniu. 19, 127 (1972). 21. Patterson, W. R., in “Catalysis and Chemical Processes” (R.Pearce and W. R. Patterson, 22. 23. 24. 25.

26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43.

44. 45. 46. 47. 48. 49. 49a. 496.

50.

eds.), p. 253. Blackie, London, 1981. Sachtler, W. M. H., Backx,C.,and Van Santen, R. A,, Catul. Rev. Sci. Eny. 23,127(1981). Temkin, M. I . , Kinet. C a d . Engl. Trans. 22,409 (1981). Wachs, I . E., and Chersick, C. C., J. Card. 72, 160 (1981). Pines, H., “The Chemistry of Catalytic Hydrocarbon Conversions.” Academic Press, New York, 1981. Wei, J., and Prater, C. D., Adu. Catal. 13, 206 (1962). Christoffel. E. G . . Catal. Reu. Sci. Eng. 24, 159 (1982). Rosynek, M. P., Fox, J. S., and Jensen, J. L., J . Catal. 71.64 (1981). Goldwasser, J., and Hall, W. K., J . Catal. 71, 53 (1981). Kallo, D., J. Catal. 66, 1 (1 980). Happel, J . , Hnatow, M. A., and Mezaki, R., Adu. Chem. Ser. (97). 92 (1970). Happel, J . , Kamholz, K., Walsh, D., and Strangio, V., I&EC Fundam. 12, 263 (1973). Hnatow, M. A., Ph.D. thesis, New York University, 1970. Bond, G . C., “Heterogeneous Catalysis-Principles and Applications,” p. I I I . Clarendon, Oxford, 1974. Pichler, H., and Schultz, H., Chrm. l n g . Tech. 42, I162 (1970). Oki, S., Happel, J., Hnatow, M. A., and Kaneko, Y., Proc. In!. Congr. Caral., 5th. p. 173 (1972). Happel, J., Fthenakis, V., Suzuki, I . , Yoshida, T., and Ozawa, S . , Proc. Inr. Congr. C u td ,, 7th, Tokyo p. 542 (1981). Yates, J. T., Jr., and Cavanagh. R.R., J . Caral. 74,97 (1982). Snagovskii. Y. S., and Ostrovskii, G . M., “Modeling the Kinetics of Heterogeneous Catalytic Processes” (in Russian). p. 232. Khimia, Moscow, 1976. Rofer-De Poorter, C. K., Chem. Rev. 81, 447 (1981). Gibbs, J. W., “The Scientific Papers of J. Willard Gibbs,” Vol. 1, Thermodynamics, pp, 63. 70, 138. Dover, New York, 1961. Aris. R.,and Mah, R. H. S . , Ind. Eng. Fundam. 2,90 (1963). Amundson, N.R.. “Mathematical Methods in Chemical Engineering,” p. SO. PrenticeHall, Englewood Cliffs, New Jersey, 1966. Beek, J . , A h ) . Chem. Eng. 3,204 (1962). Bjornbom, P. H., AIChEJ. 20,285 (1977). Hiatt, R., Irwin, K. C., and Gould, C. W., J . Org. Chem. 33, 1430 (1968). Smith, W. R.. Ind. Eng. Chem. Fundam. 19, l(1980). Bjornbom, P.. Ind. Eny. Chem. Fundam. 20, 161 (1981). Walter, E.. “Identifiability of State Space Models.” Lecture notes in “Biomathematics.” Vol. 46. Springer-Verlag, Berlin and New York, 1982. Park, S. W., and Himmelblau, D, M., Chem. Eng. J . 25, 163 (1982). Walter, E., Le Courtier, Y., and Happel, J., 1EEE Trans. Autom. Conrrol, in press (1983). Carberry, J. J., “Chemical and Catalytic Reaction Engineering.” McGraw-Hill, New York. 1976.

ANALYSIS OF MECHANISMS FOR REACTION SYSTEMS

323

51. Smith, J. M., “Chemical Engineering Kinetics,” 3rd ed. McGraw-Hill, New York, 1981. 52. Hill, C. G., Jr., “An Introduction to Chemical Engineering Kinetics and Reactor Design.” Wiley, New York, 1977. 53. Bard, G., “Nonlinear Parameter Estimation.” Academic Press, New York, 1974. 54. Happel, J., Suzuki, I., Kokayeff, P., and Fthenakis, V.,J. C u d . 65, 59 (1980). 55. Happel, J., Cheh, H. Y., Otarod, M., Ozawa, S., Severdia, A. J., Yoshida, T., and Fthenakis, V.,J. Curd. 75, 314 (1982). 56. Le Cardinal, G., Walter, E., Bertrand, P., Zoulalian, A,, and Gelas, M., Chem. Eng. Sci. 32, 733 (1977). 57. Happel, J., Chem. Eng. Sci. 33, 1567 (1978). 58. Le Cardinal, G., Chem. Eng. Sci. 33, 1568 (1978).

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ADVANCES I N CATALYSIS. VOLUME 32

Homogeneous Catalytic Hydrogenation of Carbon Monoxide : Ethylene Glycol and Ethanol from Synthesis Gas B . D . DOMBEK Union Carbide Corporation South Charleston. West Virginia 1. Introduction . . . . . . . . . I1. Cobalt Catalysts . . . . . . . . A . Background . . . . . . . . B. Catalytic Activity and Selectivity . . C . Solvents . . . . . . . . . D . Catalyst Stability . . . . . . . E . Mechanism . . . . . . . . 111. Rhodium Catalysts . . . . . . . A . Background . . . . . . . . B. Catalytic Activity and Selectivity . . C . Solvents and Promoters . . . . . D . Catalyst Stability . . . . . . . E . Mechanism . . . . . . . . 1V . Unpromoted and Carboxylic Acid-Promoted Ruthenium Catalysts . . . . . . . A . Background . . . . . . . . B. Catalytic Activity and Selectivity . . C . Solvents . . . . . . . . . D. Catalyst Stability . . . . . . E . Mechanism . . . . . . . . V . Lewis Base-Promoted Ruthenium Catalysts A . Background . . . . . . . . B . Catalytic Activity and Selectivity . . C . Solventsand Promoters . . . . . D. Catalyst Stability . . . . . . . E . Mechanism . . . . . . . . VI . Other Catalysts . . . . . . . . . VII . Conclusions . . . . . . . . . References . . . . . . . . . .

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325 Copyright Q 1983 by Academic Press. Inc . All rights of reproduction in any form reserved . ISBN 0-12-007832-5

326

B. D. DOMBEK

1.

Introduction

Hydrogenation of carbon monoxide by heterogeneous catalysts has been extensively researched and reviewed ( I - - ] @ ,but homogeneous catalysts for this reaction have received less attention (7-11). Hydrocarbon synthesis by heterogeneous catalysts, often broadly referred to as the Fischer-Tropsch synthesis, has been studied in depth as a route to liquid fuels from synthesis gas (H2/CO), which is obtainable from fossil and renewable raw materials. Products of this catalytic process are usually paraffinic and olefinic hydrocarbons, although some heterogeneous catalysts can produce oxygenates such as alcohols, aldehydes, acids, esters, and ketones. Oxygenates are very desirable as chemical intermediates and products, apart from their possible use as fuels or fuel additives. For chemical purposes, however, high selectivity with respect to both chemical functionality and molecular weight distribution must be achieved, a result rarely found in heterogeneously catalyzed synthesis gas conversion. An exception is methanol synthesis, which is conducted commercially with high selectivities over heterogeneous catalysts (9, 12-14). Certain two-carbon oxygenates can also be formed with good selectivities by supported rhodium catalysts (15- 19). Polyhydric alcohols such as ethylene glycol and glycerine, however, are very rarely found as products of heterogeneously catalyzed reactions. Indeed, Storch noted in 1948 that “the synthesis of polyalcohols . * . [from] hydrogen and carbon monoxide remains as one of the prizes of further research” ( 1 ) . Although a 1951 patent (20) claimed the production of hydroxylic compounds including ethylene glycol and glycerol by the use of solid Mn-Cr catalysts under at least 2000 atm of H2/C0, this work does not appear to have been further developed or replicated since. The only other reports known to describe the production of diols by heterogeneous catalysis involve the use of supported palladium catalysts (14, 21). Traces of ethylene glycol and other diols were obtained along with the major products methanol and methyl formate. Thus, heterogeneous catalysts, with these exceptions, are not known to yield polyalcohols from HJCO, whereas homogeneous catalysts, with few exceptions (22-26), produce nearly exclusively monoand polyalcohols. Indeed, when nonoxygenated hydrocarbons are observed as products in supposedly homogeneous systems, the possibility of a heterogeneous component must be considered. In view of this background, the present review is limited to reactions employing homogeneous catalysts for the direct conversion of synthesis gas to oxygenates, and is especially directed toward the synthesis of ethylene glycol. Ethanol is sometimes observed as a major project in such catalytic systems, and these reactions are also considered. Other two-carbon oxy-

ETHYLENE GLYCOL AND ETHANOL FROM H2 AND CO

321

genates, acetaldehyde and acetic acid or its esters, are sometimes observed in these same catalytic systems as minor products; their formation will not be addressed except incidentally. The thermodynamics of CO hydrogenation has been discussed in several publications (3, 4, 8, 27). Reactions forming hydrocarbons from H, and CO are thermodynamically favorable over a wide range of temperatures and pressures; the equilibrium constants decrease with increasing temperature. Methane formation has the most negative free energy change per carbon atom. Alcohols are produced with smaller negative free energy changes than corresponding hydrocarbons; this effect is particularly pronounced for the formation of methanol and polyalcohols, reactions in which water is not produced as a by-product. Lower operating temperatures and higher pressures are beneficial for formation of these oxygenated products. It is evident that for reactions converting CO and H, to methanol and polyhydric alcohols, which are not the most thermodynamically favored products, kinetic control is important in determining product selectivity. Direct conversion of synthesis gas to chemicals is not presently commercialized in any homogeneously catalyzed process. The major impetus behind the consideration of synthesis gas-based processes is the anticipated widening in the price differential between coal and petroleum or natural gas. Synthesis gas from coal is expected to be a very competitive feedstock for the production of chemicals having relatively simple molecular structures, including ethylene glycol (28, 29). The actual timing in the replacement of petroleum-based technology by coal-based technology will be dependent on numerous factors, but most directly on hydrocarbon feedstock prices and capital costs for the coal-based route being considered. Although chemicals can be made from H2/C0 in ways which involve several processing steps, direct routes are ideally the most economical. Selectivities are potentially higher, whereas capital costs and energy consumption may be lower because of the fewer reaction steps required. Technological improvements may be required before a homogeneous synthesis gas conversion process is commercialized, but the impact of such improvements could be very great. For these reasons, the literature reviewed here is approached from a chemical and mechanistic point of view, in terms of what it may signify for future research, rather than entirely from a practical applications standpoint. For the sake of consistency and convenience, rates to products quoted throughout this review are given in terms of turnover frequency, defined here as the number of moles of product formed per gram-atom of metal per hour. This provides units of reciprocal hours, rather than the more commonly accepted reciprocal seconds. For the reactions described here, however, use of the former units affords numbers of a magnitude convenient for ready comprehension and comparison.

328

B. D. DOMBEK

II. Cobalt Catalysts

A.

BACKGROUND

Reactions of carbon monoxide and hydrogen appear to be almost universally catalyzed by cobalt complexes. Indeed, HCo(CO), has been termed “the quintessential catalyst” by Orchin (30),because of its participation in hydroformylation, olefin isomerization, hydrogenation, and carbonylation reactions. It is therefore not surprising that the first known references to the conversion of H, and CO to ethylene glycol involve the use of cobalt catalysts. Several patents issued to DuPont in the early 1950s which claim the use of cobaltous fluoride (31) and other cobalt complexes (32,33) as catalyst precursors for an apparently homogeneous reaction under high H ,/CO pressures. Solvents employed included acetic acid, water, toluene, benzene, heptane, and cyclohexane; pressures and temperatures reported in examples ranged from 1400 to 3000 atm and from 180 to 290”C, respectively. Products listed were ethylene glycol diacetate and glycerol triacetate, with smaller amounts of methyl acetate, ethyl acetate, and n- and i-propyl acetates when acetic acid was employed as the solvent. For reactions conducted in other solvents, the only products specifically reported were ethylene glycol and its mono- and diformate esters, although lower-boiling fractions are mentioned. During this same time period, ethylene glycol was reported as a product in cobalt-catalyzed methanol homologation reactions; methanol, ethanol, and glycol ethers were also found in similar reactions carried out to homologate n-propanol and n-butanol(34). Reaction conditions were 800-1000 atm of HJCO at 225°C. These reports appear to have attracted little attention at the time of their publication, perhaps because the high pressures and low productivities made any potential application or thorough study of such reactions seem difficult or impossible. However, a renewed interest in the use of H,/CO as a feedstock stimulated further investigation (or rediscovery) of this chemistry by several groups. Indeed, in the absence of the more recent investigations, the reliability and interpretations of the earlier work might be open to question.

B. CATALYTIC ACTIVITY AND SELECTIVITY An initial report by Rathke and Feder of Argonne National Laboratory on cobalt-catalyzed hydrogenation of CO (35)has been followed by extensive study of this chemistry by the same workers (36-38). Reaction conditions adopted for these investigations (pressures below 375 atm and temperatures below 230°C) are much less severe than those employed in the earlier work

ETHYLENE GLYCOL AND ETHANOL FROM H2 AND CO

329

noted above, so a systematic study of the catalytic chemistry was more easily undertaken. Products observed in these studies (conversions up to 1 M total products) are alcohols from methanol to pentanol and their formate esters, acetaldehyde and propionaldehyde, ethylene glycol and its formate esters, are carbon dioxide and water. A first-order dependence of total activity on the concentration of HCo(CO), was noted (35, 38). Under conditions of higher pressure (2000 atm), Keim and co-workers (39,40,40u)have observed ethylene glycol, glycerol, and other polyols, consistent with the earlier findings of Gresham (32). Under some conditions, substantial amounts of methyl acetate were also observed (Table I). Individual rates are not given, but the amount of H2/C0 consumed by the reaction in toluene solvent was about five times that in N-methylpyrrolidone, CH3NC(0)CH2CH2CH2 (NMP). The distribution of products in these reactions can change very substantially with reaction time, as illustrated in Fig. 1. The rate of ethylene glycol formation remains quite constant, but rates to other products change markedly as the reaction proceeds, indicating that secondary reactions are taking place. Some of the plausible secondary reactions in this system have been listed by Feder et ul. (37): I

I

(a) alcohol homologation, the conversion of an alcohol to the next higher alcohol through metal alkyl, metal acyl, and aldehyde intermediates (41); TABLE I Products from CO Hydrogenation by Cobalt CataIysisa.b Solvent Product (wt. %)

Toluene

Methanol Methyl formate Methyl acetate Ethanol n-Propanol Ethylene glycol Ethylene glycol monoformate I ,2-Propyleneglycol 1.3-Propyleneglycol Glycerol

31.4 34.1 0. I 0.4 0. I 25.1 2.1 0.6 0.6 2.1

Data from Ref. 39. Reaction conditions: 2000 atm, HJCO Co, 1 hr. N-Methylpyrrolidone.

=

NMP' 14.4 6.4 28.9 5.9 ~

-

I , 230°C 0.2 M

330

B. D. DOMBEK

(b) transesterification, the conversion of various alcohols to their formate esters by reaction with methyl formate; (c) methanolysis, the interception of an intermediate metal acyl by methanol to produce methyl esters such as methyl acetate; (d) alkane formation by hydrogenolysis of a metal alkyl; (e) aldolization, reactions of intermediate aldehydes ; and (f) C 0 2 formation, by hydrolysis of formate esters to give formic acid which decomposes. It appears that methanol, methyl formate, and ethylene glycol are primary reaction products. Upward curvature in the amounts of higher alcohols (Fig. 1) indicates that their formation rates are dependent on the concentration of the next lower alcohol. Indeed, methanol homologation under these conditions was shown to be first-order in methanol concentration (38).Thus, during CO hydrogenation the methanol concentration can reach a steadystate level where its rate of production is balanced by the rate at which it is converted to higher products. Fahey has reported results which show ethanol as the major reaction product (42, 4 3 , with a molar selectivity on the order of 50%. These results are obtained at 31 3 atm and 200% conditions under which the overall rate of CO hydrogenation is slow and secondary reactions such as homologation may be more significant. Similar reactions under higher pressures were found to give a lower proportion of ethanol (42). However, decomposition of the CH3(0CH2CH,),0CH, (tetraglyme) sol-

0

10

20

30

40

YIIO~. M set

FIG.I . Product distribution as a function of reaction time in cobalt-catalyzed CO hydrogenation. (Reprinted from Ref. 38. by courtesy of Marcel Dekker. Inc.) Reaction conditions: 26.5 atm H , , 340 atm CO, 182 C, 1.4-dioxane solvent. Y = fo[HCo(CO),]dr; cobalt concentration changes throughout reaction because of sampling. Average HCo(CO), concentration is 0.05 I M .

ETHYLENE GLYCOL AND ETHANOL FROM H2 AND CO

33 I

vent to the observed products could be significant in these experiments (vide infra). Formate esters of the various alcohols formed are observed as major products in these cobalt-catalyzed reactions, and the mole ratio of formates to alcohols remains constant throughout a reaction. This observation would be consistent with the occurrence of a rapid carbonylation equilibrium process, ROH

+ CO * R0,CH

(1)

a reaction known to be catalyzed by bases (44). Further work by Rathke and Feder, however, indicates that addition of alkyl formate to a catalytic reaction does not lead to a rapid formate/alcohol equilibrium (38). Instead, it appears that methyl formate is produced as a primary product, and transesterification with other alcohols present in the system, as in reaction (2), leads to the various formates observed: CH,O,CH

+ ROH 5 CH,OH + RO,CH

(2)

The equilibrium constant for R = CH2CH, was observed to be about 0.8 at 200°C. It also seems probable that some of these formate esters are produced when intermediate aldehydes, such as acetaldehyde, are hydrogenated : C H , C H O B CH,CH,OH

+ CH,CH,O,CH

(3)

These results are presented here to emphasize the fact that selectivity and rates to various products can be subject to great variation as a result of secondary reactions. Any attempt to determine the fundamental responses of a catalytic system to changes in reaction variables must recognize the potential complications of such secondary reactions. Rathke and Feder have carried out calculations to determine the amounts of primary products actually produced by the cobalt system, assuming that these products are methanol, methyl formate, and ethylene glycol (38). The amounts of these primary products were estimated by the following relationships : [CH,OH], = [CH,OH] -

+ [higher alcohols] + [aldehydes]

[HOCH2CH202CH] - 2[C2H,(O2CH),]

[CH302CH], = [CH,O,CH]

(4)

+ [higher formate esters]

+ [HOCH2CH202CH] + 2[C2H4(02CH)J

(5)

+

[HOCH2CH20H], = [HOCH2CH20H] [HOCH2CH202CH]

+ [C2H4(02CH)2]

(6)

According to Eq. (4), the amount of primary product methanol is related to the amount of methanol observed plus the number of moles of alcohols

B. D. DOMBEK

332

and aldehydes produced from methanol via homologation; concentrations of glycol formates are subtracted because they are assumed to be equivalent to the amount of methanol produced from methyl formate via transesterification: CH,O,CH

+ HOCH,CH,OH

-+

CH,OH

+ HOCH,CH,O,CH

(7)

(Higher alcohol formate esters do not appear here because an equivalent amount of methanol is consumed in the homologation process which produced them.) In reactions using trifluoroethanol as solvent, the observed

0.10

0.00

z! 2

0 t

a

0.06

2

w

u

z 0

u

0.04

0.02

0

0.04

0.00

0.12

0.16

TP. M FIG.2. Calculated amounts of primary reaction products formed relative to total products in cobalt-catalyzed CO hydrogenation. (Reprinted from Ref. 38, by courtesy of Marcel Dekker. Inc.) Reaction is the same as that of Fig. I .

333

ETHYLENE GLYCOL AND ETHANOL FROM H 2 AND CO

amount of trifluoroethyl formate (a secondary product formed by transesterification with methyl formate) is also subtracted. Similar reasoning leads to Eqs. ( 5 ) and (6) for the other primary products. The success of this approach is illustrated by Fig. 2, which displays results for the reaction of Fig. 1. This plot depicts the linear increase in the amounts of primary products (including their derivatives) as a function of total product concentration, which is also linear in Y , the abscissa of Fig. 1. Experimental data treated in this manner thus indicate the selectivity to primary products formed by a reaction, regardless of the extent of secondary reactions, and meaningful interpretation of the effects of reaction variables becomes possible. Under some conditions the secondary reactions can be suppressed, and the concentrations of primary products then increase linearly as the reaction proceeds. For example, Fig. 3 gives the results of a reaction in which the homologation process has been inhibited by added tri-n-butylphosphine. A similar inhibition of the homologation reaction was found in experiments carried out in 2,2,2-trifluoroethanoI solvent (36). This alcohol is itself resistant to homologation under these conditions, but does undergo transesterification with methyl formate. Data listed in Table I1 are those obtained after derivation of the actual rates of primary product formation, using Eqs. (4)-(6). These data are

? i

0.08

50: 2 t

0

4

12

8 YIIO?

16

20

M soc

FIG.3. Product distribution in cobalt-catalyzed CO hydrogenation with added tri-n-butylphosphine. (Reprinted from Ref. 38, by courtesy of Marcel Dekker, Inc.) Reaction conditions: 149 atm H,, 149 atm CO, 182’C, 1,4-dioxane solvent, 3(n-C4H,),P/Co, average HCo(CO), concentration is 0.03 M .

TABLE I1 Rates to Primary Products in Cobalt-Catalyzed CO Hydrogenation ai 182°C"

Rateb (hr-' x 10') Expt. 1 2 3 4 5 6 7 8 9 10 I1 12

Notes C C

C C C

C

c, d

c, e

f f f

f,9

[HCo(CO),l Av. ( M ) 0.12 0.10 0.078 0.089 0.051 0.1 1 0.03 0.084 0.081 0.034 0.052 0.0025

PH(atm)

96.5 I92 30.2 100

26.5 102(D2) 149

Pc0 ( a m )

115 121 113 123 340 111 149

115

115

154 97.1

179 170 108 108

161

161

Data from Ref. 38, by courtesy of Marcel Dekker, Inc.

* Turnover frequency, moles of product per mole of catalyst per hour. 1,4-Dioxane solvent. = 3. ' 8.5 M H,O. 2,2,2-Tri!luoroethano1 solvent. 214°C.

'(n-C,H,),P/Co

107k (atm-' sec-')

2.0 1.7 1.8 2.0 1.9 2.7 0.69 2.4 2.7 2.0 2.0 46

CH,OH 3.48 5.15

1.47 3.46 1.18 4.26 2.66 5.17 6.75 3.11 7.89 239

CH,O,CH

I .74 2.93 0.45 I .37 0.51 1.98 0.25 0 3.90 1.68 1.39 26.6

HOCH,CH,OH 1.75 3.86 0.05 2.38 0.14 3.47 0.81 4.17 4.35 1.54 2.32 0

ETHYLENE GLYCOL AND ETHANOL FROM H2 AND CO

335

consistent with the rate equation d[P]/dt = k[H c O ( c o ) , ] P ~

(8) where [PI represents the molar concentration of total products, and PH is the partial pressure of hydrogen in atmospheres. The temperature dependence of the reaction rate constant has been determined (37) to fit the equation log,& (atm-' sec-') = A - 41,000/2.303RT

(9)

where A is 12.40 in 1,Cdioxane and 13.02 in trifluoroethanol. Thus, the free energy of activation is approximately 41 kcal/mol in both solvents. The indicated first-order rate dependence on [HCo(CO),] is observed in 1,Cdioxane solvent, as shown by the constant value of k over a range of HCo(CO), concentrations. A slightly higher concentration dependence may be observed in 2,2,2-trifluoroethanol (e.g., Expt. 9). It is proposed that such behavior, if significant, could be the result of greater ionic dissociation of HCo(CO), at the lower concentrations in this more polar solvent. A catalyst concentration effect on product selectivity was reported by Keim et al. (39), but rate effects are not reported and possible secondary reactions are not taken into account. Also listed in Table I1 are turnover frequencies to the primary products, in units of moles of product per mole of HCo(CO), per hour, which allow a comparison of the relative activity of the catalyst to the primary products under various reaction conditions. It is evident that the activity of this system is quite low under the conditions recorded. The maximum turnover frequency to the glycol product is below 0.05 hr-', and the highest rate to methanol is slightly above 2 hr-'. When the primary products of CO hydrogenation are estimated according to Eqs. (4)-(6), Rathke and Feder have suggested (37,38)that the following relationships hold :

The ratio of formates to alcohols [relation (lo)] was observed to remain constant throughout a catalytic reaction, as had earlier been reported in the hydrogenation of an aldehyde by HCo(CO), (45).This observation provides further evidence that methyl formate is actually a primary product of the reaction. The formate/alcohol ratio was found to be independent of H, partial pressure, but to increase substantially with CO pressure (nearly

336

B. D. DOMBEK

second-order in P , in 2,2,2-trifluoroethanoI solvent, and somewhat less dependent on CO in 1,Cdioxane). This finding is taken as evidence that C H 3 0 H and HOCHzCHzOH are formed from a single intermediate and CH30,CH is formed from a different intermediate. An increase in temperature was also noted to cause a slight decrease in the formate/alcohol ratio. Increasing temperature appears to have an adverse effect on the ethylene glycol/methanol ratio (39), although little information is available. This ratio is found to increase rapidly with increasing pressure (37, 39). Fahey has reported similar results concerning ethylene glycol/methanol selectivity (43).Experiments over a wide range of pressures (1 26- 1973 atm) are reported to show a first-order dependence of the total product formation rate on overall pressure, in agreement with the results of Rathke and Feder which were observed in a much smaller pressure range. (A first-order rate of gas uptake was also reported, but this result appears fortuitous. As noted above, the product distribution can change markedly with time because of secondary reactions, and the number of moles of gas consumed need not be simply related to the number of moles of organic products formed.) The mole ratio of ethylene glycol-derived products to methanol-derived products was found to change substantially with pressure, as shown in Table 111. (Alkyl formates are apparently regarded as methanol-derived products in this analysis.) Because of the overall first-order dependence of reaction rate on pressure (specifically, on hydrogen partial pressure) in combination with the rather complex selectivity relationships among primary products, it is regarded as quite probable that all of the primary products (and the separate interTABLE 111 Product Selectivity and Rate as a Funcrion of' Pressure for Cobalt-Catalyzed C O Hydrogenation"*h Pressure (atm)

Ethylene glycol/methanol'

Rate (hr-')d

313' 1361 1973

0.10 0.19

6.8 22.3 42.1

0.60

"Data from Ref. 43. (Copyright 1981 American Chemical Society. Adapted with permission.) Reaction conditions: 100 ml tetraglyme solvent, 2 mmol Co, HJCO = I , 230°C. Mole ratio of ethylene glycol-derived products t o methanolderived products. Rate of total product formation. 200°C.

ETHYLENE GLYCOL AND ETHANOL FROM H2 AND CO

337

mediates leading to them) derive from a single common intermediate whose rate of formation is first-order in (hydrogen) pressure. Implications of this assumption will be addressed further in Section II,E. A reaction related to CO hydrogenation to alcohols has recently been reported (46,46a,466,46c);the products are apparently siloxanes. Reactions were carried out in the presence of high concentrations of hydrosilanes, as follows : CO

+ H,

Catalyst R,SiH

.XOCH,CH,OX

+ CH,OX

(12) where R = n - C6HI3,C,H,; X is not specified, but presumably is mainly R,Si. Catalysts reported for this reaction include Co2(CO), as well as complexes of rhodium and ruthenium, and reported conditions range from 272 to 544 atm of H 2 / C 0 at 270°C. Turnover frequencies are reported to be as high as 7 hr-’, with up to 45% molar selectivity (among reported products) to the glycol derivative. This represents “a remarkable acceleration by hydrosilanes of the transition-metal-catalyzed reduction of carbon monoxide to compounds containing methoxy and 1,2-ethanedioxy groups.” The acceleration noted appears to result from carrying out the reactions at much higher temperatures than those commonly used for comparable CO hydrogenation experiments. (The maximum temperature is usually dictated by the stability of the catalyst.) The hydrosilane thus seems to provide a remarkable degree of stabilization to the cobalt catalyst. The reaction rate, however, is reported to decline with time, perhaps suggesting catalyst instability since silane conversions appear to be low. The most significant result of this work is the observation of relatively high selectivity to the two-carbon product at the high temperature employed. It is suggested that this effect may be a result of the hydrosilane functioning as a trap for a reversibly formed intermediate which may be a precursor of the two-carbon product.

C. SOLVENTS The solvent is a sine qua non of a homogeneous catalyst system. Solvent properties are indeed very important in determining the activity, selectivity, and stability of a catalyst. Solvent stability is also essential, if the catalytic system as a whole is to be stable. As described above, several solvents have been employed in studies of cobalt-catalyzed CO reduction. Keim et al. (39) noted a substantial difference in activity and selectivity between catalyst solutions in toluene and N-methylpyrrolidone (Table I). Most of the information in this area again comes from the work of Feder and Rathke (36). Listed in Table IV are their results showing changes in the activity of the cobalt catalyst corresponding to changes in solvent polarity. The rates

338

B. D. DOMBEK

TABLE IV Rates of'Cobalt-Catalyzed CO Hydrogenation in Dij'erent Solvents at 200°C"

Solvent

0)

Pressure (atmy

Rate (hr-' x 10')

Heptane Benzene 1 ,4-Dioxaned 2,2,2-TrifluoroethanoI

I .9 2.3 2.2 26.7

290 290 296 306

zl.8 > 7.9 > 13.3 46.4

" Data from Ref. 36. Room temperature dielectric constant. HJCO = 1. I96 'C.

are seen to increase by a factor of about 20 from the least polar solvent, heptane, to the most polar, trifluoroethanol. In view of the relatively large span of dielectric constants covered by these solvents, an ionic mechanism is concluded to be unlikely. Instead, the more polar solvents are proposed to better stabilize a polar transition state or intermediate. These solvents are all relatively nonbasic compared to N-methylpyrrolidone (NMP), a highly polar solvent [t = 32 (47),pK2" = -0.9 (48)] which Keim er cil. found to give relatively poor activity compared to toluene (39). The low activity in this solvent is perhaps caused by appreciable deprotonation of the relatively strongly acidic HCo(CO), [pK, < 2 (49)], to give the inactive Co(C0)i anion. Changes in solvent were also found to have significant effects on product selectivity, mainly relating to secondary product formation. As mentioned above, the use of trifluoroethanol solvent was found to suppress alcohol homologation. and only primary products were observed (36). This inhibition of the homologation process was attributed to strong hydrogen bonding of the relatively acidic fluoroalcohol to the product alcohol oxygen atoms. Such an interaction would make the alcohol less available for protonation by HCo(CO), , a step believed to be involved in the homologation sequence (41). Addition of a Lewis base, tri-n-butylphosphine, was also found to inhibit alcohol homologation (38).Such behavior could be due to both the competition of the more strongly basic phosphine with alcohol for the HCo(CO), proton, and to the conversion of HCo(CO), to a more weakly acidic phosphine-substituted complex. High-pressure infrared studies have indicated that the complex HCo(CO),P(C,H,), is the predominant equilibrium species when HCo(CO), and tri-n-butylphosphine are heated at 190°C under 80 atm of HJCO (50). The addition of 16% water to dioxane solvent was found to increase the

ETHYLENE GLYCOL AND ETHANOL FROM H2 AND CO

339

rate of reaction slightly (Table 11, Expt. 8), which could be the result of increasing the solvent dielectric constant. The addition of water also has significant effects on product selectivity. Formate esters are apparently hydrolyzed to alcohol and formic acid, which decomposes to CO, and H, . Selectivity to ethylene glycol was also substantially increased by the addition of water. Since hydrolysis of the dioxane solvent in the presence of acidic HCo(CO), could lead to ethylene glycol, similar experiments were carried out with methanesulfonic acid instead of the cobalt hydride. Slow hydrolysis of dioxane was indeed observed, but the major product was diethylene glycol. Examination of cobalt-catalyzed experiments did not reveal this product, so it was concluded that little if any of the observed ethylene glycol was the result of solvent decomposition (36). Similar rates to ethylene glycol have been observed under comparable conditions in a solvent, trifluoroethanol, which cannot produce ethylene glycol upon decomposition. Other studies of cobalt-catalyzed CO reduction must be viewed critically from the standpoint that solvent decomposition could contribute significantly to the observed products. Cobalt carbonyl in (CH,OCH,CH,),O (diglyme), CH30CH2CH20CH3(glyme), and CH,OCH,CH,OH solvents has been reported to catalyze rapid CO hydrogenation at H,/CO pressures below 300 atm and temperatures ~ 2 0 0 ° C(51, 52). High selectivities to ethanol and other two-carbon oxygenates are claimed, and novel mechanisms are presented to account for this activity and selectivity. Although it was noted that solvent decomposition could be partially responsible for the observed results, recent work by others suggests that only a minor fraction of the products originates from CO hydrogenation (53).This study confirmed that high apparent selectivities to ethanol could be realized when diglyme, tetraglyme, or 2-methoxyethanol were used as solvents (Table V). No ethanol was detected, however, after similar experiments in sulfolane, tetrahydrofuran, or dioxane solvents. (Relatively short reaction times are reported, which could limit secondary reactions.) Use of 2-ethoxyethyl ether as solvent gave substantial yields of n-propanol, suggesting that the terminal alkyl group in the ether solvents is being cleaved and homologated. Because of the complexity of the solvent mixture, the source of alcohol products in the experiments was investigated by use of 13C0 in the H,/CO reaction mixture. An experiment in diglyme under the conditions of Table V gave ethanol product of which only 5% was derived totally from CO hydrogenation. [The instability of related solvents in cobalt-catalyzed methanol homologation has also been reported (M).] These results also raise doubts about the source of ethanol in the similar experiments of Fahey described above (e.g., Table 111), which were performed in tetraglyme solvent. Indeed, it is noted that “at very high pressures, the tetraglyme solvent reacted and yielded both higher and lower molecular weight materials” (42). It is not

B. D. DOMBEK

340

TABLE V Product Distribution in Cobalt-Catalyzed C O Hydrogenation in Various Solvents",b Selectivity (mol %) Solvent (MeOCH,CH,),O (EtOCH2CH,),0

CH,

C,H,

CH,OH

CH,CH,OH

21.0 30.0

-

2.6

67.8 28.0

5.2 39.9

68.0 72.1

22.7

-

-

-

__

26.0 54.5

-

5.8

52.6

-

I

OCH,CH,CH,CH~ I ,4-Dioxane Sulfolane' MeOCH,CH,OH" MeOCH,CH,OMe a

9.8

~

CH,CH,CH,OH

~

CH,CHO 2.2

Total products (mmol)

-

40 16

-

-

10

-

-

4

-

11.4

20

-

-

-

Data from Ref. 53. Reaction conditions: 0.1 M Co, 200 atm, HJCO = I , 220°C. 4 hr. I

I I

' S0,CH,CH,CH,CH2, trace conversion noted. Ethylene glycol also detected.

clear how solvent decomposition was established in these experiments or excluded in others.

D. CATALYST STABILITY Relatively little reference has been made to cobalt catalyst stability in the studies cited above. Fahey has reported that reactions in tetraglyme at 200°Cunder 136 atm of 1 : 1 HJCO gave precipitates of cobalt metal ( 4 2 ) . Some experiments which produced elemental cobalt were found to give "nonliquid products," presumably methane and other light hydrocarbons. Experiments under higher pressures showed no evidence of cobalt precipitation. Heterogeneous cobalt catalysts are known to be effective for the Fischer-Tropsch process; supported cobalt metal is a catalyst for the synthesis of methane and other hydrocarbons at 240°C under I atm of HJCO (55). The relatively high fraction of methane (and the presence of ethane) reported in Table V may thus be the result of partial cobalt precipitation, although no comment is made regarding catalyst instability. Investigations of cobalt stability as a function of catalyst concentration, temperature, and CO partial pressure have been carried out in connection with cobalt-catalyzed hydroformylation (56-58). The stability of Co,(CO), in heptane is shown by Fig. 4, which relates to the equilibrium

q,, + 8CO,,I

* CO,(~O),,,,,,,

(13)

ETHYLENE GLYCOL AND ETHANOL FROM H2 AND CO

34 1

r' YYJ

f 2m YI

-K 100 x m

:.

E M LO

20 lemperoture

-

0

FIG.4. Stability of cobalt carbonyl catalyst [Co,(CO), and HCo(CO),] as a function of CO partial pressure and reaction temperature (57,58). (Reproduced with permission of Ernest Benn Ltd. and Springer-Verlag.)

The partial pressure of CO necessary to maintain Co2(CO), in solution rises rapidly with temperature. The decomposition of C o 2 ( C 0 ) , may, however, be kinetically slow in the absence of compounds which could catalyze this conversion (56), and the decomposition process is reported to be autocatalytic (59). Thus, a catalytic reaction was possible in an unstable temperature-pressure region for some time before cobalt metal precipitation became noticeable. Although operation with a metastable catalyst may be possible in short batch experiments, it would be undesirable in a continuous reaction where stability over extended periods is essential. It should be recognized that the stability of cobalt complexes under carbon monoxide can be enhanced by the addition of ligands, as is the case for phosphine-modified cobalt hydroformylation catalysts (57, 58). The stability will also probably depend on properties of the solvent employed. Nevertheless, the plot shown in Fig. 4 appears to be quite useful for assessing long-term cobalt stability under HJCO in the absence of strongly coordinating solvents or ligands. Inspection of the reaction conditions adopted for the experiments reported in Tables I-IV suggests that, in general, they are within the region of cobalt stability. An obvious exception is Expt. 12 of Table 11. Indeed, significant quantities of precipitated cobalt were reported for this experiment (36). Experiment 8 also gave precipitated cobalt, which suggests that the presence of water may lower cobalt stability. Both of these reactions produced, in addition to the normal products, significant amounts of methane and a distribution of straight-chain alkanes. The reaction conditions of Table V also appear to lie in the unstable region where cobalt precipitation might be expected. Indeed, heterogeneous catalysis by precipitated cobalt metal

342

B. D. DOMBEK

may be the cause of the relatively high fraction of methane product observed in these experiments.

E. MECHANISM Much of the information bearing on a catalytic mechanism comes from kinetic observations made on the catalytic process. Any proposed mechanism must then be consistent with the cyclic conversion of the starting catalyst species to the transition state of the rate-determining step. The identity of the stable catalyst species under reaction conditions must therefor be known in order to interpret kinetic results. Although a variety of cobalt complexes may be used as catalyst precursors in the reactions described above, most of these complexes are found to be converted largely to HCo(CO), under reaction conditions. King has found that Co,(CO), , (C,H,),C,Co,(CO), ,CH,CCo,(CO), , and (C,H,P),Co,(CO),, are transformed into HCo(CO), in n-tetradecane solution under 200 atm of 1 : I H,/CO at 110-190°C (60). The same precursors were found to provide active catalytic systems when studied in preparative reactions (dioxane solvent, 173-200"C, 200 atm of I : 1 H,/CO). Two other cobalt complexes, (CH,),SnCo(CO), and [CH3N(PF2),Co2(C0),, were found not to be converted to HCo(CO), ,and did not catalyze CO hydrogenation. Other studies by high-pressure infrared spectroscopy under conditions similar to those used for catalytic CO hydrogenation have been reported by Whyman (50). At 150°C and 290 atm of 1 : 1 H,/CO in heptane solution the equilibrium 2HCo(CO),

* H, + CO,(CO),

(14)

was found to lie well toward the hydride complex, although small absorptions due to the carbonyl dimer could be detected. Rathke and Feder have employed Co2(CO),as the catalyst precursor in their studies. Samples withdrawn from reactions under pressure were analyzed for both total cobalt and for HCo(CO), (35);conversion to HCo(CO), was observed to the extent of 50-90%, varying according to (14) with temperature and hydrogen pressure. Experiments with different levels of catalyst showed that the overall rate of CO reduction was first-order in the HCo(CO), concentration, as determined by titration of reaction samples. Thus, there is substantial evidence that the catalyst in this system (or more precisely, the species present in the transition state of the rate-determining catalytic step) is a mononuclear cobalt complex. The observed kinetic dependences [Eq.

ETHYLENE GLYCOL A N D ETHANOL FROM H2 A N D CO

343

(8)] are useful in consideration of how HCo(CO), is transformed on the pathway to the transition state. Fachinetti el nl. have proposed that a trinuclear hydroxymethylidyne cobalt cluster could possibly be an intermediate in CO hydrogenation (6163). Evidence for the equilibrium

has been presented, including isolation of the amine adduct of the cluster from a reaction between HCo(CO), , Co,(CO), , and triethylamine (62). This trinuclear cluster is very thermally labile, decomposing to HCo(CO), and Co,(CO),. Model studies were carried out with a related derivative, Co,(CO),CO-X (X = CH3), which was found to react with H2/C0 (1 15 atm, 120°C) in the presence of Co,(CO),, yielding the organic products X-OCH,, X-OCH2CH20H, and X-OCH2CH202CH (61).This is presented as evidence that the trinuclear hydroxymethylidyne cluster could be a precursor of methanol and two-carbon alcohols in CO hydrogenation. It is difficult, however, to reconcile the observed kinetic dependences of the cobalt-catalyzed process with a mechanism which involves conversion of HCo(CO), to the trinuclear cluster. If the rate-determining step is proposed to occur after cluster formation, a dependence on HCo(CO), concentration of greater than first-order would be expected, in disagreement with observed results. On the other hand, plausible rate-determining steps before or during cluster formation also do not lead to the observed dependences. For example, the forward step in (14) has been shown to have an inverse kinetic dependence on CO partial pressure and a second-order dependence on HCo(CO), concentration (64, 65). Involvement of (14) as a preequilibrium step in cluster formation would contribute an inverse hydrogen partial pressure dependence as well as a higher cobalt concentration dependence. For these reasons, it appears probable that under conditions employed for the research described here, cobalt cluster formation is not involved in the major catalytic pathway. The HCo(CO), complex is therefore inferred to be involved in initial hydrogen transfer to carbon monoxide. This step was initially proposed to comprise rate-determining hydrogen atom transfer from HCo(CO), to free CO, affording a formyl radical, HCO; subsequent reaction with further HCo(CO), would lead to the observed products (35). However, kinetic observations (the zero-order dependence on CO partial pressure) were later made which are inconsistent with such a process (36). All experimental observations are consistent with the first step in the CO hydrogenation process being a rapid, reversible equilibrium in which the

344

B. D. DOMBEK

hydrogen atom in HCo(CO), migrates to a carbonyl ligand :

In the subsequent, rate-determining step

hydrogen is added to the coordinatively unsaturated formyl complex, and the formyl ligand is converted to coordinated formaldehyde. The dihydride species may be a transient intermediate or may be representative of the transition state in the rate-determining step. A mechanism involving rapid, equilibrium formation of H,CO followed by rate-determining addition to HCo(CO), would also be consistent with the observed kinetics. This pathway is regarded as less probable, however, because added formaldehyde is not rapidly converted back to H2 and CO as would be expected if such an equilibrium were important ( 3 6 ) . The conversion of a metal hydride into a metal formyl by hydride migration, as in (16), has been regarded for some time as a very difficult or improbable reaction. Certainly, there is evidence to suggest that for a number of metal carbonyl complexes alkyl migration to a carbonyl ligand, as follows: R

I

L,,M-CO

0

II

+ L’ * L,L’M-CR

is more easily accomplished and observed than analogous hydride migration (66-69). Nevertheless, there are recent ceports which show that hydride-toformyl conversion is an observable process in certain stoichiometric systems ( 7 0 7 2 ) .This step is therefore plausible in catalytic systems, although it is probably an endothermic process with a very small equilibrium constant. Participation of the hydride-formyl equilibrium in (16) is also plausible in light of an apparent inverse kinetic deuterium isotope effect for the catalytic process. Use of deuterium gas instead of hydrogen (cf. Expts. 6 and 4 in Table 11) causes an increased rate, with k,/k, = 0.73 (37).The existence of an isotope effect implies that hydrogen atom transfer occurs before or during the rate-determining step, and an inverse kinetic isotope effect may be possible in the case of a highly endothermic, product-like transition state (73). On the other hand, Bell has concluded that inverse kinetic isotope

ETHYLENE GLYCOL A N D ETHANOL FROM H2 A N D CO

345

effects are not observed for single-stage hydrogen transfer reactions (74). The observation of such an inverse rate effect can instead imply that the observed process includes an endothermic equilibrium which possesses an inverse equilibrium isotope effect. An equilibrium will exhibit such inverse behavior ( K H / K D< 1) if the hydrogen vibrational frequencies are higher on the product side than on the reactant side. This may be seen to be the case for (16), where (Co-H) z 1830 cm-' and (C-H) z 2900 cm-'. If any normal equilibrium or kinetic isotope effects in other steps before or during the rate-determining step are not large enough to negate this inverse effect, an apparent inverse kinetic isotope effect will result. This appears to be a plausible situation for the combination of (16) and (17). Fahey presents the products of (1 7) as uncomplexed formaldehyde and HCo(CO), rather than a bound-formaldehyde species (43). Free formaldehyde is a thermodynamically unfavorable product from H2 and CO (a),and significant stabilization may be expected as the result of coordination in a metal complex. However, thermodynamic calculations are presented which indicate that small equilibrium concentrations of formaldehyde could be present under the conditions of these cobalt-catalyzed reactions (43). Although small amounts of uncoordinated formaldehyde are indeed expected as a result of the following endothermic ( 3 6 , 3 7 ) equilibrium: H I 0 (CO)&o- II C H

/ \

H

I 0 * (CO),CO + II C H

H

/ \

H

the significance of this equilibrium is minor if it is the complexed formaldehyde which is consumed in subsequent steps. The concept of a (bound) formaldehyde intermediate in CO hydrogenation is supported by the work of Feder and Rathke (36) and Fahey (43). Experiments under H,/CO pressure at 182-220°C showed that paraformaldehyde and trioxane (which depolymerize to formaldehyde at reaction temperatures) are converted by the cobalt catalyst to the same products as those formed from H,/CO alone. The rate of product formation is faster than in comparable H,/CO-only experiments, and product distributions are different, apparently because secondary reactions are now less competitive. However, Rathke and Feder note that the formate/alcohol ratio is similar to that found in H2/CO-only reactions (36). Roth and Orchin have reported that monomeric formaldehyde reacts with HCo(CO), under 1 atm of CO at 0°C to form glycolaldehyde, an ethylene glycol precursor (75). The postulated steps in this process are shown in (19)-(21), in which complexes not observed but

346

B. D. DOMBEK

presumed as intermediates are shown in brackets : HCo(CO),

+ HCHO

4

[(CO),Co-CHZOH]

(19)

0 [(CO),Co-CH,OH]

+ CO

II

4

0

(20)

0

I1

[(C0)4C~-CCH,0H] + HCo(CO),

[(CO),Co-CCH,OH]

II

4

HCCHzOH

+ Co,(CO),

(21)

Other work has shown that cobalt complexes under HJCO pressure and at higher temperatures can catalytically convert formaldehyde to glycolaldehyde (76) or ethylene glycol (77-79); methanol is observed as a product as well. It has therefore been well demonstrated that formaldehyde can be converted by cobalt catalysts to the same products observed from CO reduct ion. A very simplified possible scheme for subsequent reactions of a formaldehyde intermediate, modeled largely after one presented by Rathke and Feder (38),is the following: 0

0

Hydrogenolyses leading to products or their organic precursors are shown as reactions with [HI, which may be either H2 or HCo(CO), . Recent studies by Pino and co-workers (80) suggest that in the related cobalt-catalyzed hydroformylation process, it is H, which is largely responsible for this hydrogenolysis. It has also been shown that H2 can undergo analogous oxidative addition to (CO),CoH, forming H,Co(CO), at low temperatures (81). The essentially irreversible hydrogenolysis reactions are presumed to occur for the coordinatively unsaturated (tricarbonyl) intermediates shown.

ETHYLENE GLYCOL AND ETHANOL FROM H2 AND CO

347

The "CO insertion" reactions shown are likely to involve the coordinatively saturated (tetracarbonyl) complexes, and are expected to be reversible. The coordinatively unsaturated tricarbonyl and saturated tetracarbonyl complexes are presumed to be in equilibrium, the position lying predominantly toward the coordinatively saturated species. Glycolaldehyde is postulated as the ethylene glycol presursor, as will be further described below. This scheme appears to be consistent with the selectivity relationships (10) and ( 1 I ) reported by Feder er a/. (37). Changes in H, or CO partial pressure are not expected to significantly affect the first branching. Increases in CO partial pressure are anticipated to favor CO insertion reactions (22) and (24) by increasing the concentration of coordinatively saturated, tetracarbonyl, reactants. This will lead to the observed increases in selectivity to glycolaldehyde (ethylene glycol) and methyl formate. The effects of increased H, partial pressure are less easily rationalized. Increased hydrogen pressure could perhaps increase the fraction of glycolaldehyde relative to methanol, consistent with ( 1 I), by a concerted H, addition-alkyl migration process involving the tetracarbonyl hydroxymethyl complex. Coordinating solvents have been shown to be involved in alkyl migration processes (82), and it appears possible that H2 could behave similarly. Rathke and Feder make the assumption that the pathway to methanol of (25) is minor relative to (23), based on the observation of a relatively constant formate/alcohol ratio at different hydrogen partial pressures, (10). However, this does not seem to be a necessary conclusion. Cobalt-catalyzed hydroformylation of olefins proceeds, under similar reaction conditions (59), through a cobalt alkyl complex analogous to the hydroxymethyl complex shown in (22) and (23). In the hydroformy lation process, little hydrocarbon, a hydrogenolysis product analogous to the methanol produced by (23), is normally produced. Indeed, acyl complexes of the formula 0

II

(CO),CoCR

are observed as a major component of the hydroformylation system under catalytic conditions (50). This indicates that alkyl migration (22) competes very effectively with hydrogenolysis (23) in hydroformylation, and a similar result should probably be expected in CO hydrogenation. All of the observed effects appear to be consistent with a mechanism as shown in (22)-(25) in which substantial amounts of methanol are formed via (25), if the first branching is indeed reversible and if the CO insertion process in (24) is slower than that in (22)-a reasonable assumption (67,83).However, in the absence of additional data relevant to (22)-(25), further rationalizations of product selectivities do not appear to be warranted. The fate of a glycolaldehyde intermediate in these reactions may be somewhat analogous to that of formaldehyde, as outlined by Fahey (43)

348

B. D. DOMBEK

Interaction of glycolaldehyde with the catalyst can thus lead to glyceraldehyde (a glycerol precursor), ethylene glycol, or ethylene glycol monoformate. Pathway (26) represents an aldehyde hydroformylation, or chain-growth route, and provides a plausible pathway to the glycerol which is observed as a product at higher pressures (53).Higher polyols may also be expected as products by continuation of this chain-growth process through a sequence of aldehyde intermediates. However, the selectivity of branching for higher aldehydes will presumably differ from that in the formaldehyde case because of steric and electronic effects. As a result of these influences, the first branching in (26)-(29) should lead more selectively to the metal-oxygen bonded intermediate than was the case for formaldehyde. This will cause a rapid diminution in chain growth beyond the two-carbon polyol stage. An increased proportion of glycerol and possibly other polyols at higher pressures (32) can be rationalized, since increases in both CO and H, pressure are expected to enhance the proportion of CO insertion (26) relative to hydrogenolysis (27). Semiempirical molecular orbital calculations based on modified extended Hiickel theory have been used to model the geometries and energies of the transition-state species of (16), (17), and (22)-(25) (37). Within the uncertainty limits, these calculations are supportive of the proposed mechanism. A formaldehyde complex of the formula (CO),CoH(CH,O) was found to have the cobalt atom closer to the formaldehyde carbon atom (2.34 A) than the oxygen atom (2.85 A). This agrees well with data reported for a formaldehyde complex of osmium, [(C,H,),P],(CO),Os(CH,O) (84).A slightly less

ETHYLENE GLYCOL A N D ETHANOL FROM H2 A N D CO

349

stable structure was found with the oxygen atom closer to the cobalt center. This, it is suggested, may indicate that hydrogen atom transfer in the next reaction step can proceed either to the carbon or the oxygen atom. A hydroxycarbene isomer, (CO),CoH(=CHOH), was found to be significantly less stable. The possible intermediates (CO),Co(CH,OH) and (CO),Co(OCH,), formed by hydrogen atom transfer in (CO),CoH(CH,O), were also briefly examined ; it was concluded that highly accurate calculations would be required to gain information about factors possibly responsible for the branching to thsese two species. Further calculations have been done which suggest that a hydroxycarbyne complex, (CO),CoECOH, may be more stable than the (CO),CoCHO formyl complex. These results have led Nicholas to propose that this species is a possible intermediate in the CO reduction process (84a).Comparison of the relative stabilities of the above two complexes is not altogether valid, however, since the hydroxycarbyne species is coordinatively saturated whereas the formyl is coordinatively unsaturated. In order for the former complex to react with H z , as proposed (84a), loss of a carbonyl ligand to give a higher-energy species would presumably be required (which could also lead to kinetic dependences at variance with those observed). Reaction with hydrogen and CO might then be expected to involve transient formation of the hydroxycarbene intermediate (CO),CoH(=CHOH); as noted above, this is calculated to be a relatively unstable complex in comparison with the formaldehyde species derived from a formyl intermediate. The intermediacy of a hydroxycarbyne species therefore does not appear to be energetically feasible for CO reduction processes. Selectivity observations described here for cobalt catalysts and in Section IV for ruthenium catalysts are also not explained by invoking a hydroxycarbyne intermediate.

111.

Rhodium Catalysts

A. BACKGROUND Knowledge of patents claiming cobalt catalysts for the conversion of HJCO mixtures to ethylene glycol (31-33) appears to have led to initial investigation of rhodium catalysts for this reaction at Union Carbide (27, 85-87). Early experiments by Pruett and Walker at pressures of about 3000 atm indicated that the activity of rhodium was notably greater than that found for cobalt. Several other potential catalyst precursors, including compounds of Sn, Ru, Pd, Pt, Cu, Cr, Mn, Ir, and Pb, were screened for activity under pressures of about 1500 atm and found not to produce detectable

350

B. D. DOMBEK

amounts of polyhydric alcohols. Solvents initially examined with the rhodium catalyst included tetrahydrofuran, water and alcohols, NMP, dioxane, o-dimethoxybenzene, tetraglyme, and toluene. Experiments with added methanol or in methanol solvent indicated that this alcohol is not an intermediate in glycol formation (27). Various organic “ligands” were found to promote the glycol-forming reaction; among those reported were pyrocatechol, bipyridyl, piperazine, and various substituted pyridines, especially 2-hydroxypyridine (85). With the realization that anionic rhodium complexes are present in catalyst solutions, cationic counterions such as alkali and alkaline-earth metals were found beneficial in the process (86). Experiments first reported in the patent literature were carried out largely at pressures x3000 atm, although some at pressures as low as 400 atm were also described. After these initial findings, a considerable amount of progress has been made in improving the activity and stability of this catalytic system, and increasing our understanding of it.

B. CATALYTIC ACTIVITY AND SELECTIVITY Products formed by the rhodium catalyst are generally the same as those produced by the cobalt system. Table VI (Expt. A) presents a summary of products reported by Fahey (43) from a rhodium-catalyzed reaction at 1973 atm. Ethylene glycol and methanol are the two major products, comprising more than 60% of the total product mixture. The overall rate of product formation in this example (moles of products per gram-atom of metal per hour) is 473 hr-’, and the rate to ethylene glycol is 222 hr-’. Some of the compounds found to be secondary products in the cobalt system are also observed in this reaction ; ethanol and higher linear alcohols (and their formate esters) are presumed likely to be secondary products in this reaction also. 1,3-Dioxolane and 2-(hydroxymethyl)-l,3-dioxolaneare the ethylene glycol acetals of formaldehyde and glycolaldehyde, respectively. Their observation is evidence that these aldehydes may be intermediates in the CO reduction process (43). Examples from the patent literature, as shown in Expts. B and C of Table VI, are generally found to contain possible secondary products in much lower proportions. These examples illustrate selectivities to the two major primary products, methanol and ethylene glycol, of greater than 90%. The higher proportion of probable secondary products in Expt. A would appear to be a result of carrying out this reaction to very high product concentrations ; the volume of liquid products formed during the experiment is approximately equal to the initial volume of solvent. Results of experi-

ETHYLENE GLYCOL AND ETHANOL FROM H2 AND CO

35 1

TABLE VI Products from Rhodium-Catalyzed CO Hydrogenation

Amt. (mmol) Product

Expt,: A0.b

Methanol Methyl formate Ethanol Ethyl formate 1 -Propano1 I-Propyl formate 1-Butanol Ethylene glycol Ethylene glycol monoformate Propylene glycol Glycerol 1,3-Dioxolane 2 4 Hydroxymethy1)- 1.3-dioxolane

284 52 191 20 121 29 < 31 1000 118

118 120 2 20

B

d

Ce.I

281 25 6.3

52 2.5

329 14

69 0.9

22

0.5

1.0

Data from Ref. 43. (Copyright 1981 American Chemical Society. Adapted with permission.) Reaction conditions: 100 ml tetraglyme solvent, I mmol Rh, 5 mmol 2-hydroxypyridine, 1973 atm, HJCO = I , 230°C. 4.5 hr. Data from Ref. 86. Reaction conditions: 76 ml tetraglyme solvent, 3 mmol Rh, 12 mmol2-hydroxypyridine, I333 atm, H J C O = 1.5, 220°C 4 hr. ‘ Data from Ref. 88. Reaction conditions: 75 ml tetraglyme solvent, 3 mmol Rh, I0 mmol 2-hydroxypyridine, 0.5 mmol cesium formate, 544 atm, HJCO = I , 220°C. 4 hr.

ments run to more moderate conversions, such as Expts. B and C of Table VI, are therefore believed to be representative of largely primary reactions. Although secondary reactions which increase with product concentrations in these rhodium-catalyzed reactions may not complicate rate interpretations as severely as in the cobalt systems, other complexities can arise as product amounts increase. Results presented in a patent (89) indicate that increased concentrations of ethylene glycol in the reaction medium have an adverse effect on the ethylene glycol/methanol ratio produced. High concentrations of ethylene glycol cause a decrease in the rate of its formation and may enhance the rate of methanol formation. In another patent, apparently referring to this effect, it is reported that ethylene glycol is “destroyed” during the catalytic process (90).Other studies in which I4C-labeled ethylene

352

B. D. DOMBEK

glycol was added to experiments showed that the label was recovered essentially quantitatively as ethylene glycol, indicating that there was no destruction or transformation of this product under the particular conditions employed (91). The buildup of glycerol is also reported to have an adverse effect on the rate of ethylene glycol formation (90), which suggests that the inhibitory effect noted for ethylene glycol is common to polyalcohols. Addition of methanol at similar levels does not inhibit the glycol-forming process (91). Ethylene glycol concentrations of less than 5 M (about 30 wt. %) are claimed to be desirable in order to minimize its inhibitory effects (89).Since most of the experiments reported here do not involve such high product concentrations, major changes in selectivity during these reactions are not expected. Nevertheless, the existence of this effect must be recognized. The reaction rates in this system are presumably first-order in catalyst concentration, as implied by the scaling of product formation rates proportionately to rhodium concentration (90, 92, 93). Responses to several other reaction variables may be found in both the open and patent literature. Fahey has reported studies of catalyst activity at several pressures in tetraglyme solvent with 2-hydroxypyridine promoter at 230°C (43). He finds that the rate to total products is proportional to the pressure taken to the 3.3 power. A large pressure dependence is also evident in the results shown in Table VII. Analysis of these results indicates that the rate of ethylene glycol formation is greater than third-order in pressure (exponents of 3.23 . 9 , and that for methanol formation somewhat less (exponents of 2.3-2.8). The pressure dependence of the total product formation rate is close to third-order. A possible complicating factor in the above comparisons is the increased loss of soluble rhodium species in the lower-pressure experiments, as seen in Table VTI. Experiments similar to those of Fahey have also been

eflecr nJ' Pressure on

TABLE VII Rhodium-Catalyzed CO Hydrogenurion",h

Solvent

Pressure (atm)

Ethylene glycol rate (hr-')

Methanol rate (hr-')

Rhodium recovery rAy

Sulfolane Sulfolane SITd

544 1020 544 I020

9.5 72.5 10.25 92.5

13.5 71.5 15.5 67.5

81 I 07 67 91

SITd

Data from Ref. 94. Reaction conditions: 75 ml solvent, 3 mmol Rh, 7 mmol ethylenedimorpholine, 260°C. ' Amount of rhodium soluble or suspended in solution after reaction, based on amount charged; determined by atomic absorption spectroscopy. 1 : 1 volume ratio of sulfolane and tetraglyme.

ETHYLENE GLYCOL AND ETHANOL FROM H, AND CO

353

noted to give considerably less than quantitative recovery of soluble rhodium at the lower pressure (94, 95). Such catalyst instability effects could lead to abnormally high apparent pressure dependences. Another study which provides information on the pressure dependence is shown in Fig. 5. This series of experiments, which gave rhodium recoveries within the range of 78-930/, (96),exhibits somewhat lower pressure dependences. The total rate of product formation is proportional to the pressure taken to the 2.3 power, and the reactions which produce methanol and ethylene glycol have very similar pressure dependences in this study. The ratio of formates to alcohols, however, was seen to increase with pressure. Rate dependences on CO and H, partial pressures have not been determined, but the experiment in Fig. 5 at a different HJCO ratio (0.67 instead of 1) suggests that the dependence on hydrogen pressure is similar to, or perhaps slightly greater than, the dependence on CO pressure. Results obtained by other workers (97, 98) lead to the same conclusion.

Pressure, atm

FIG.5. Plot of log(rates) vs. log(pressure) for rhodium-catalyzed CO hydrogenation. Reaction conditions: 75 ml sulfolane, 3 mmol Rh, 1.25 mmol pyridine, HJCO = I , 240 C, 4 hr (96). Total rate includes rates to methanol, methyl formate, ethanol, ethylene glycol monomethanol;).( ethylene glycol. Open figures are formate, and propylene glycol: (A)total;).( for an experiment with HJCO = 0.67.

354

B. D. DOMBEK

The effect of increasing the reaction temperature under otherwise constant conditions may be seen in Fig. 6 . Changes in temperature have very little effect on the ethylene glycol/methanol ratio. The routes to both products have Arrhenius activation energies of approximately I8 kcal/mol, as estimated from Fig. 6 . A possible deviation from this dependence at the highest temperature in this series (25OOC) may suggest partial catalyst instability, but rhodium recoveries were not reported. The effect of increased temperature on catalyst stability (based on analyses of rhodium in solution at the end ofcatalytic experiments) is shown in Table VIII ; rhodium recovery is significantly lower for the higher-temperature experiments. Before proceeding to a more detailed description of the effects of various solvents and promoters on catalyst activity and stability, it should be noted that the responses described above are possibly, or even probably, influenced by solvents and promoters. The responses shown, however, appear to be generally characteristic of these rhodium-containing systems. It is apparent that the rate of product formation is significantly accelerated by increases in reaction temperature. Higher temperatures, however, can bring about catalyst instability unless the pressure is simultaneously increased. Higher Temp., "C

10.0

I

2

8.0

6.0

0)

2

40

2.0

i.01 . 1.90

I 2.00

T - ' , K-' x

2.10

lo3

FIG. 6. Effect of temperature on rhodium-catalyzed CO hydrogenation: (u)methanol; ( 0 )ethylene glycol. Reaction conditions: 75 ml y-butyrolactone solvent, 3 mmol Rh, 10.5 mmol 2-hydroxypyridine, 0.5 mmol cesium 2-pyridinolate, 544 atm, H,/CO = I , 4 hr (88).

ETHYLENE GLYCOL AND ETHANOL FROM H2 AND CO

355

TABLE V l l l Acriuirv und Rhodium Recowry in Tetraglyme Soluent at 544 arm and 220-240 C" ~~

Temp. ( C) 220' 240h 220h

240' 220' 240'

Ethylene glycol rate ( h r - ' )

Methanol rate ( h r - ' )

Rh recovered

Salt Cs-2HP Cs-2HF PPN-OAcd PPN-OAcd BTB' BTB'

5.51 6.59 4.97 7.12 5.13 6.90

4.43 9.64 3.39 7.55 3.98 6.66

81 64 81 61 83 46

r<;)

' Reaction conditions: 75 ml tetraglyme solvent, 3 mmol Rh, 544 atm, HJCO I. Data from Ref. 95. 10 mmol 2-hydroxypyridine also added. " Cesium 2-pyridinolate. amount unspecified. Bis( tripheny1phosphine)iminium acetate, amount unspecified. ' Data from Ref. 99. 3.3-Bisdimethylamino-N,N,N',N'-tetramethylacrylamidiniumbenzoate, 0.65 mmol. =

'

pressures also are beneficial for improving the reaction rate, but are not desirable from a practical or economic standpoint. A major emphasis in the work reported in the patent literature has therefore been the attainment of higher product formation rates without resorting to increased pressure. This has been possible through the use of solvent/promoter combinations which give increased inherent catalyst activity, and by the observation that certain solvents and promoters provide enhanced catalyst stability, thus permitting the use of higher reaction temperatures. Selected examples which illustrate rates and selectivities obtained at various reaction conditions are shown in Table IX. The ethylene glycol/methanol ratio produced by these reactions ranges from about equimolar to as high as 1.75. This selectivity, as well as the selectivity to other products, can be manipulated to some extent by the use of certain promoters. For example, the selectivity to n-propanol may be enhanced by addition of vanadium compounds (loo), and the productivity of propylene glycol is increased by the use of aluminum compounds (101). Combinations of rhodium with iron ( 1 0 1 ~ )and rhenium (1016) complexes in the presence of basic promoters have recently been reported to provide catalytic activity for ethylene glycol formation, but rates and selectivities do not appear to be greatly affected by the second metal.

356

B. D . DOMBEK TABLE IX Selected Examples o j Rhodium-Catalyzed CO Hydrogenation Solvent (75 ml)

Notes a

Tet raglyme Tetraglyme Sulfolane 18-Crown-6 18-Crown-6 Tet raglyme

.f

CIS

9

18-Crown-6 18-Crown-6

h

6 c

d e

i

Pressure (atm)

Temp.

("(3

Ethylene glycol rate (hr-')

544 544 544 544 544 850 850 1020 1020

220 240 240 270 270 270 275 270 280

6.05 9.4 9.5 20.6 25.8 I I4 I38 175 I88

Methanol rate ( h r - ' ) 4.42 9.5 8.9 18.8 17.5 101

125 I00 120

Rh recovered 93 79 83 78 72 72 92 96 82

3 mmol Rh, 0.90 mmol bis(tripheny1phosphine)iminiurn acetate, 2.5 mmol pyridine (102). 3 mmol Rh. 0.75 mmol potassium acetate, 4 mmol 222-crypt (103). 3 mmol Rh, 0.80 mmol ammonium acetate (96). 1.5 mmol Rh, 0.85 mmol cesium acetate, 7 mmol N M M , 100 mmol trimethylphosphine oxide (90). ' 30 ml 18-Crown-6, 300 mmol trimethylphosphine oxide, I .5 mmol Rh, 0.85 mmol cesium acetate. 7 mmol N M M (90). 1.5 mmol Rh, 0.45 mmol potassium acetate. 16 mmol 222-crypt (103). Equal volumes of 18-crown-6 and sulfolane, 1.5 mmol Rh. 0.375 mmol cesium benzoate, 4 mmol ethylenedimorpholine (92). 1.5 mmol Rh, 0.375 mmol cesium benzoate, 4 mmol N M M (92). 1.5 mmol Rh, 0.375 mmol cesium benzoate, 7 mmol NMM. 100 mmol triphenylphosphine oxide (YO).

C. SOLVENTS AND PROMOTERS 1.

Amine Promoters

I t was recognized in initial patents that amines were beneficial for the activity and glycol selectivity of rhodium-catalyzed CO hydrogenation reactions (85).A number of amines were investigated as promoters, including a variety of substituted pyridines; 2-hydroxypyridine appeared to give somewhat better results than other amines (86). Trialkanolamine borates were also shown to be effective promoters (104). Amines and other Lewis bases may be functioning as ligands, as described in early patents (85, 87). However, the observation of anionic rhodium complexes in reaction mixtures (86) indicates that an important role of these additives is that of a proton base, thus promoting the formation of charged species. Solutions of Rh(CO),(acetylacetonate) in tetraglyme at 210°C under 714 atm of HJCO were monitored by high-pressure infrared spectroscopy (105) and found not

ETHYLENE GLYCOL AND ETHANOL FROM Hz AND CO

357

to display absorptions characteristic of anionic species ; an absorption band possibly due to Rh,(CO),, was observed. Under these conditions such solutions are not effective for the production of ethylene glycol. Identical reaction mixtures containing 2-hydroxypyridine or piperidine were found to exhibit absorptions characteristic of an anionic rhodium species tentatively identified by Chini and Martinengo (106) as [Rh,2(C0),,]2-. (This species was later isolated and shown to be [Rh,(CO), ,I- (107).)Solutions exhibiting these absorptions were found to be capable of producing ethylene glycol at significant rates (86). Catalyst solutions in which an infrared band assignable to [Rh(CO),]- is dominant tend to produce a relatively large proportion of methanol (102). Highly sterically hindered amines such as 2,4,6-trimethylpyridine and 1,8-bis(dimethylarnino)naphthalene are effective promoters in this system (108). These compounds cannot coordinate to rhodium and must therefore be serving as promoters only by virtue of their proton basicity. The degree of basicity of an amine might then be expected to have an influence on its effectiveness as a promoter. That this is the case is shown by Table X, which compares the optimum observed aminelrhodium ratio with the pK,,+ of the amine (108).Those amines which are classified as “strong bases,” having a pK > 5 , generally provide an optimum rate to glycol at a relatively low amine/rhodium ratio, on the order of 0.1-0.5. For amines with a pK < 5 , a higher amine/rhodium ratio is required to achieve the highest ethylene TABLE X Amounts of Amines Required for Optimum Promoter Eflect” Amine

pK (H,O, 25°C)

1,8-Bis(dimethylamino)naphthalene Sparteine Dibutylamine Triethylamine N-Methylpiperidine Piperazine Ammonia I ,4-Diazabicyclo[2.2.2]octane 2,4,6-Trimethylpyridine N-Methylmorpholine Pyridine 1,IO-Phenanthroline Aniline

12.3 12.0 11.3 10.7 10.4 9.7 9.3 8.8 7.4 7.4 5.2 4.8 4.6

Amine/rhodiumb -0.1

0.2-0.3 0.3-0.5 0.3 0.3-0.5 0.3-0.5 0.3 0.2-0.3 0.2-0.3 0.3-0.4 -0.1 1.6 2.3

’ Data from Ref. 109. Minimum ratio of moles of amine to moles of Rh which provides the maximum rate to glycol.

358

13. I). DOMBEK

glycol productivity; bascs progressively weaker are required in greater amounts. Shown in Fig. 7 are the results of adding increasing amounts of two “strong bases,” 1,4-diaza[2.2.2]bicyclooctane (dabco) and N-methylmorpholinc (NMM), to rhodium catalyst solutions. The optimum ethylene glycol productivity is achieved in both cases at amine/rhodium ratios of less than 0.5. Differences in behavior are observed as further amounts of these amines are added. Higher concentrations of dabco cause diminished productivity, while increased amounts of N M M have little effect (109). Rates to methanol are affected little by either amine. Since the basicities of these amines are similar, other factors appear to be important in determining the degree of ethylene glycol rate decrease with excess amine. Kaplan has proposed that ion pairing between rhodium complex anions and the positively charged counterions has an adverse effect on catalytic activity for ethylene glycol formation (96, 109, 110).The following scheme: Aminc

+

[Amine H f &-I&!= Amine

[amine H t B-1

(30)

[amine H]++ &--catalysis

(31)

+ (alcohols. Rh species, etc.) % [amine HIf

r

P

(32)

Dabco

2 N- Methylrnorpholine

I

0.5

I

I

1.0 1.5 2.0 Arnine/Rh, Mole Ratio

1 2.5

FIG.7. Effect of amine/Rh ratio on product rates: (m) methanol; ( 0 )ethylene glycol. Reaction conditions: 75 ml sulfolane, 3 mmol Rh, 544 atm, HJCO = 1, 240’C,4 hr (109). Upper graph is for I ,4-diazabicyclo[2.2.2]octane(dabco); lower graph is for N-methylmorpholine.

ETHYLENE GLYCOL AND ETHANOL FROM H2 AND CO

359

is based on this assumption, and is consistent with the observation that amines act as promoters but can inhibit the reaction of interest at higher levels. In the scheme, &is a generalized rhodium complex, and &-is the catalyst or its immediate precursor. The amine and hydrogen act to reduce the rhodium species in (30), thus forming an anionic rhodium species as the ammonium salt. Less weakly basic amines may be required in larger amounts to force equilibrium (30) to the right, but strong. bases can apparently accomplish this step quantitatively. (Note the nearly constant amine/ rhodium ratio in Table X for bases with a pK > 5.) Increases in the concentration of added amine will also provide higher concentrations of [amine HI', by deprotonation of other rhodium species and hydroxylic products in solution as in (32); the more strongly basic the amine, the greater the fraction of it which will be converted to its conjugate acid. This increased concentration of [amine H]+ can be expected to shift equilibrium (31), an ion pairing process, toward an inactive (or less active) anion-cation complex. Thus, two factors determine the inhibitory ability of an amine: its basicity (K32)and the ion pairing ability of its conjugate acid ( K 3 , )(109). A weakly basic amine will produce little [amine H I + , and an excess of amine will therefore not greatly affect equilibrium (31). On the other hand, a strongly basic amine may be an excellent promoter even if it produces a high concentration of [amine HI', so long as this cation is poor at forming an ion pair in equilibrium (31). This would seem to be the optimum situation, since even that [amine HI+ formed by the (promoting) process of (30) will not inhibit the catalytic reaction. In general, it appears that amines which are found to cause little inhibition at higher levels [such as NMM, 2-hydroxypyridine, and 1,8-bis(dimethyIamino)naphthalene] have structures which allow, in the protonated form, delocalization of the positive charge over more than one atom. The freedom to use an amine at levels higher than optimum without causing a large glycol rate inhibition presents several benefits. The criticality of precisely controlling the amine/rhodium ratio is reduced, and higher levels of amine have been noted to improve the catalyst stability (109). Changes in reaction temperature and solvent dielectric constant are expected to affect the equilibria (30)-(32), and such effects are indeed observed (108).The addition of salts as promoters can also alter the optimum amounts of amine promoters to be used. 2. Salt Promoters Counterions for the anionic rhodium complexes present in catalyst solutions may also be provided by the addition of salts. A salt may be used as the sole promoter, but it appears that under many conditions a combination of salt and amine provides the best results. Table XI indicates that

360

B. D. DOMBEK

TABLE XI Efccr of Salts as Promorers in Rhodiutn-Carulyzcd CO Hydrogenation".' ~

Cation None Li+ Na+ Kt Rbt cs Mg' Sr2 Bat' PPN +

+

f

r.d

~~

~

Ethylene glycol rate ( h r - ' )

Methanol rate ( h r - I )

1.34 2.22 3.45 2.89 3.01 5.24 0.94 I .01 0.87 4.91

5.49 4.47 3.49 2.32 9.35 4.1 I 3.46 3.07 I .82 3.39

" Data from Ref. 88.

' Reaction conditions: 75 ml tetraglyme solvent, 3 mmol Rh, 10 mmol 2-hydroxypyridine, 0.45-0.50 mmol of acetate anion with cation specified, 544 atm. HJCO = I , 2 2 0 C , 4 hr. ' Data from Ref. 102. Bis(triphenylphosphine)iminium.

TABLE XI1 Effecr ?/'Cesium Salr.~on Rhodium-Catalyzed CO Hydrogenationanh

Anion

Ethylene glycol rate ( h r - l )

Methanol rate (hr- ')

Fluoride Chloride Bromide Iodide Formate Acetate 2-Pyridinolate Sulfate

5.51 4.77 3.49 2.42 5.71 5.24 5.85 I .94

3.85 4.97 4.40 4.53 4.32 4.1 I 4.43 4.04

Data from Ref. 88. Reaction conditions: 75 ml tetraglyme solvent, 3 mmol Rh, 10 mmol 2-hydroxypyridine, 0.45-0.50 mmol ofcesium cation with anions specified, 544 atm. HJCO = 1, 220°C. 4 hr. a

ETHYLENE GLYCOL A N D ETHANOL FROM H2 A N D C O

36 1

addition of various acetate salts to a catalyst solution already containing an amine promoter can cause substantial improvements in rate and selectivity to ethylene glycol. Under the conditions of these experiments (in tetraglyme solvent), cesium and bis(tripheny1phosphine)iminium ([(C,H,),P],N+ ; PPN') salts provide the best rates to the glycol product. In Table XI1 are shown the results of a series of experiments with a variety of cesium salts; formate and pyridinolate anions are found to be the most effective under these conditions. Halides were found not to be as effective in these experiments, although the use of higher levels of iodides is reported in a patent as being useful ( I ION). The amount of cesium salt added to these reactions was found to have a dramatic effect on the activity and selectivity of the system, as illustrated by Fig. 8. Maximum rate and selectivity to ethylene glycol are observed at a Rh/Cs' ratio of 6. At higher levels of salt the overall activity increases very substantially, but the amount of the glycol product diminishes rapidly. A similar series of experiments using the bulky PPN' cation was found to give somewhat different results (102), as shown in Fig. 9. The ethylene glycol rate maximum is observed at a rhodium/cation ratio of 4, and changes in glycol/methanol selectivity are not as pronounced as was found for the cesium cation. The bulky PPN' cation therefore has the advantage of providing a system in which the ethylene glycol rate and selectivity are not

Cs*/Rh. Mole Ratio

FIG. 8. Plot of rates as a function of C s + / R h ratio: ( 0 )methanol;).( ethylene glycol Reaction conditions: 75 ml tetraglyme solvent, 3 mmol Rh, 10 mmol 2-hydroxypyridine, 544 atm, H,/CO = I . 220 C, cesium formate promoter as indicated. 4 hr (88). Methanol and ethylene glycol rates at C s + / R h = 0 are 5.21 and 1.34 hr- I , respectively.

362

B. D. DOMBEK

as sensitive to increased cation/rhodium ratios. Similar results have been observed for quaternary phosphonium (If I) and ammonium cations (1f2), and the 3,3-bisdimethylamino-N,N,N"'-tetramethylacrylamidinium cation ( 9 9 ) .An advantage of being able to operate with higher salt concentrations is the possible increase in catalyst stability in the presence of higher promoter concentrations ( I f2, 113). Ion pairing interactions appear to be an important factor in rate inhibition for the glycol product as indicated above for amine promoters, and such effects apply in the case of salt promoters as well. The PPN' ion, for example, is large and has delocalized charge; it is therefore expected to interact only weakly with anions in solution. Alkali metal cations, however, may interact with rhodium complex anions as well as with the solvent and other anions in competitive processes. Cations with varying degrees of solvation will exhibit differences in ion pairing ability. A number of possible equilibria must then be considered if the effects of salt promoters are to be rationalized. Consistent with experimental observations, anionic rhodium complexes essentially free of ion pairing interactions are proposed (I 13) to provide the best catalyst. The best salt promoter would then be one in which the cation ion pairs the least with the rhodium catalyst. The anion of the salt is believed to act as a promoter by forming or transforming the anionic rhodium complexes involved in catalysis. Possible equilibria representing processes involved in inhibition by the cation are the following:

+ Rh--catalysis [ M + X - ]& M + + X-

[M+Rh-]%

M+

(33) (34)

x - + ROHK".XH + RO-

(35)

+ RO-

(36)

[MtRO-]&Mt

where Rh- is the active rhodium species formed by interaction of the salt promoter, MX, with the rhodium precursor. Any effects which reduce the interaction between M and jth- are expected to lead to increased rates of product (ethylene glycol) formation. It follows that the use of cation complexing agents o r high-dielectric-constant solvents, as described below, may enhance catalytic activity by increasing the amount of free &-. On the other hand, increased amounts of free M + will diminish the rate of catalysis by a mass law effect on equilibrium (33), decreasing the amount of free Rh-. The anion X- of the salt promoter can also have an effect on the amount of free M + (inhibitor) through the equilibria (34)-(36). A more +

ETHYLENE GLYCOI.. AND ETHANOL FROM H, A N D CO

-

0.083

0.167

0.250 0.333

363

0.5000.667

PPN + / R h , Mole Ratio

FIG.9. Plot of rates a s a function of P P N + / R h ratio: (m) methanol; ( 0 )ethylene glycol. PPN acetate was used as promoter ( / 0 2 ) .Reaction conditions are the same as those of Fig. 8.

basic anion will decrease the concentration of free M f both through a smaller equilibrium constant K , , and a larger equilibrium constant K , , . Because an excess of salt promoter has, under some conditions, been found to improve catalyst stability, it is regarded as desirable to employ as much salt as possible without substantially reducing the rate to ethylene glycol (113). By use of cesium carboxylates with a range of basicities, it was shown that, consistent with (33)-(36), the salts with the most basic anions gave the least rate inhibition when added at increasing levels (113). The ion pairing ability of cations can also be reduced by addition of complexing agents. For example, the use of alkali metal salts in the presence of “cryptands” ( 114, 1140) such as 4,7,13,16,2 I ,24-hexaoxa-1,1 O-diazabicyclopentatriacontane (222-crypt) :

is claimed to give increased catalyst stability and higher rates to ethylene glycol (103). The alkali metal cation can be enveloped in the interior of this and related molecules, thus preventing it from direct interaction with anions in solution. The size and geometry of the cryptand may be varied to provide an optimum fit for different metal cations. Similar results have been reported for “spherand” compounds, which also complex alkali or alkaline-earth metal cations (1146).

364

B. D. DOMBEK

3. Solvents The solvents used in these rhodium-catalyzed reactions may also act as complexing agents for counterions of the anionic rhodium complexes. For example, tetraglyme is known to coordinate alkali metal cations. Such solvation decreases the possibility of the cation interacting with the anionic rhodium catalyst and lowering its activity or solubility. The crown ethers, such as [ 181-crown-6

cone> to ,4

(38)

u

and [15]-crown-5, comprise a class of compounds which also can complex cations very effectively (114); these cyclic polyethers have been found to be excellent solvents for rhodium-catalyzed CO hydrogenation (92). The cyclic structures are multidentate ligands for the metal cations, and the effectiveness of the complexation is a function of the degree to which the ether can conform to the size of the metal cati0.n. An effective combination for the catalytic process has been found to be [18]-crown-6 with cesium salts as promoters (92). Although the cesium cation is too large to fit within this polyether, effective complexation is apparently achieved by the formation of a 2 : 1 crown ether-cesium “sandwich” complex, as indicated by crystal structure determinations (115, 116). Since the crown ethers are very effective complexing agents, the amount of free M + in solution, as in (33)-(36), is expected to be small; the crown ether competes very well with &-and X - for M’. Indeed, it is found that the addition of excess salt causes a much lower degree of rate inhibition in [18]-crown-6 as compared to some other solvents. For example, Fig. 10 illustrates the differences between [ 18]-crown-6 and tetraglyme as the level of salt promoter is increased. The capability of using an excess of salt reduces the criticality of precisely controlling the salt concentration and is beneficial for the stability of the catalyst (92). Another method of reducing ion pairing is to use a solvent having a high dielectric constant, such as sulfolane: n

o*

s) %O

(39)

This material has a dielectric constant of 43.3 at 30°C; it has very low proton basicity (pK,,,+ = - 12.9) and is a weak Lewis base (117). Indeed, sulfolane is an excellent solvent for the rhodium catalytic system, giving good rates

ETHYLENE GLYCOL AND ETHANOL FROM H2 AND CO

I

0.167

0.217

365

I 0.267

CS+/ R h , Mole Ratio

FIG. 10. Effect of cesium concentration on ethylene glycol rates in 18-crown-6 ( 0 )and tetraglyme (B)(92). Reaction conditions: 75 ml solvent, 3 mmol Rh, cesium benzoate, 544 atm, HJCO = I , 220'C, 4 hr.

and high rhodium recoveries (96). The potential instability of this material at high temperatures and resultant adverse effects on catalyst activity have been noted (94). Another patent shows that copper and its salts are useful in negating adverse effects of sulfur compounds which may be present as a result of the decomposition of sulfur-containing solvents (118). Another class of solvents having high dielectric constants is the lactones, such as y-butyrolactone :

and 6-valerolactone (95). The dielectric constant of butyrolactone is 39 at 20°C (47), and this solvent appears to give good rates and improved catalyst recoveries. These lactones, however, will polymerize to some extent during the reaction ( 9 3 , and may also react with hydroxylic products in a saponification process (93). Substituted butyrolactones are more stable toward these ring-opening reactions, and 2,2-dimethyl-y-butyrolactone, I

I

C(CH,),CH,CH,OC(O), has been shown to be superior to the unsubstituted analog (93). N-Methylpyrrolidone was noted by Keim et ul. (39) to be a good solvent for CO hydrogenation by rhodium catalysts in the absence of added promoters. The basicity of the compound [pKBH+ = -0.9 (48)]probably allows it to serve the same function as weakly basic amine promoters. The high

366

B. D. DOMBEK

dielectric constant of this solvent [ 3 2 at 25°C (47)] also indicates that it should be effective at separating ion pairs. Compounds related to this are the cyclic ureas, such as 1,3-dimethy1-2-imidazolidinone

which has been shown to be a very effective solvent ( 1 19). Some of the above solvents, such as tetragiyme and crown ethers, are effective because of their ability to complex cations, whereas others, such as sulfolane and butyrolactone, are useful by virtue of their high dielectric constants. Mixtures of these two types of solvents can lead to improved results, better than those obtainable in the single solvents. For example, Fig. 1 I shows that mixtures of sulfolane (high dielectric constant) and tetraglyme (complexing) solvents give improved rates and selectivities to the glycol product. The catalyst stability is substantially better in sulfolane than in tetraglyme under the conditions of these experiments, but it may be seen that a large fraction of the sulfolane can be replaced by tetraglyme before an adverse effect on stability is observed. However, as catalytic conditions become more severe (higher temperature or lower pressure), a higher sulfolane/tetraglyme ratio must be used to maintain the stability of the catalyst (94). The use of mixed complexing and high-dielectric-constant solvents is also illustrated for cyclic ureas (119), crown ethers (Y2), and phosphine oxides (YO), in the appropriate combinations.

0

20

40

60

80

100

Sulfolone, Volume Percent

FIG. I I . Effects on rate and catalyst stability of using sulfolane-tetraglyme mixtures as solvent: ( 0 )methanol; (m) ethylene glycol; ( A ) rhodium recovery. Reaction conditions: 75 mi solvent, 3 mmol Rh, 0.65 mmol cesium benzoate, 544 atm, HJCO = I . 240°C 4 hr (Y4).

ETHYLENE GLYCOL AND ETHANOL FROM H2 AND CO

361

Organic phosphine oxides are reported to be useful solvents for several reasons (90). Phosphine oxides are strong Lewis bases and can complex the positively charged counterions, thus increasing the amount of non-ionpaired rhodium complexes in solution. These compounds also possess relatively high dielectric constants, which decreases the attractive forces between anions and cations in solution. Additionally, the strong hydrogen bond acceptor capability of phosphine oxides allows them to complex with ethylene glycol and glycerol, which are bidentate hydrogen bond donors. Increasing concentrations of these polyalcohols, as noted above, can cause a decreased rate of ethylene glycol production and may also lower the catalyst stability. Apparently, phosphine oxides can counteract these harmful effects by the hydrogen-bonding interaction.

D. CATALYST STABILITY Many of the reactions described above are seen to give less than quantitative recovery of the rhodium catalyst component. The amount of rhodium remaining in a catalyst solution was determined by atomic absorption spectroscopy, and is reported as the percent of the rhodium charged which remains soluble or suspended in the reaction mixture at the end of the reaction (95). After some experiments a wash procedure was employed to dissolve rhodium complexes possibly left in the reactor; heating a charge of pure solvent in the reactor under H 2 / C 0 pressure sometimes dissolved substantial amounts of rhodium species (94-96, 104, 108, 109). High recoveries of rhodium are essential in a practical process because of the scarcity and high price of this metal (120, 121). The form of the unrecovered rhodium in experiments shown above is not certain. Although metallic rhodium has been found to be taken into solution to provide an active homogeneous catalyst under some conditions (85, 91), it appears unlikely that metallic rhodium is being dissolved in the reactor washes described above. In reaction media similar to those employed for catalytic reactions (tetraglyme or [ 181-crown-6 solvents, amine and salt promoters), rhodium clusters of decreased solubility may be produced from more soluble precursors. These clusters include [Rh,4(C0)2,]4- (122, 123), [Rhi 5(cO)27l3- (123), [Rh22(CO)35Hx+nI'5-')- (115, 124), clusters which are possibly [Rh,3(CO)24H,]'5-"'- (x = 0, 1) (122), and [Rh,,(C0)3,]4(116). These are relatively large, highly charged metal complexes whose growth may be reversed by the application of carbon monoxide pressure (115, 125, 126). It therefore appears possible that at least some of the precipitated rhodium noted in catalytic experiments could be in the form of similar highly reduced clusters, perhaps even higher in nuclearity. Under some conditions, reactions of ionic clusters under hydrogen ( 1 atm, 25°C)

368

B. D. DOMBEK

have been reported to give a precipitate of rhodium metal (126). Effects of the counterion on the growth and decomposition of anionic rhodium clusters have been reported (126a). The enhanced stability of the rhodium catalyst system under carbon monoxide is the subject of several patents. Separation of the alcohol products from a catalyst solution, by distillation for example, might involve the use of conditions under which the catalyst becomes insoluble. Contacting the catalyst-product mixture with carbon monoxide while distilling the product is claimed to minimize catalyst instability (127). Examples show the use of CO gas to strip products from heated catalyst solutions. Comparisons with the use of N, gas demonstrate that CO does indeed stabilize the catalyst. Another patent describes the use of a continuous reactor with catalyst solution recycle in which such a CO stripping column is used to remove products from the reaction mixture (128). The rhodium catalyst was found to be continuously lost from solution at a low rate during extended operation of the continuous unit. The rhodium level, however, could be increased nearly to the initial value by lowering the H2/C0 ratio in the reactor vessel, i.e., increasing the CO partial pressure while decreasing the H2 partial pressure (128). This apparently resolubilizes rhodium species which have been lost from solution in the reactor vessel. Another patent shows that periodically lowering the temperature of the reactor also has the effect of raising the rhodium concentration, apparently by causing the solubilization of precipitated rhodium species (129). Other patents illustrate the use of a solvent extraction process to separate the alcohol products from the catalyst (130, 131). When a catalyst solution containing alcohol products is mixed with water and a water-immiscible solvent, the alcohol products are extracted into the aqueous phase and the rhodium species enter the water-immiscible solvent. The effectiveness of the extraction and the stability of the rhodium catalyst can be greatly increased by carrying out the process under CO pressure (131). The general behavior of rhodium catalysts with respect to stability thus appears to be similar to that seen for cobalt catalysts; an inverse relationship between carbon monoxide partial pressure and reaction temperature is apparent. Stability decreases rapidly with increasing temperature, and raising the pressure tends to improve catalyst stability. It is not certain whether the adverse effects of increasing the H2/C0 ratio are merely the result of a decreased CO partial pressure, or whether increased hydrogen partial pressure induces catalyst instability. Catalyst stability in this system is substantially influenced by the characteristics of solvents and promoters. Indeed, the properties of solvents and promoters which improve the catalytic activity for ethylene glycol production (increased dielectric constant, greater cation complexing ability, or

ETHYLENE GLYCOL AND ETHANOL FROM H 2 A N D C O

369

lower ion-pairing ability) also appear, in general, to improve catalyst stability.

E. MECHANISM The characteristics of the rhodium catalytic system described above suggest that this is a very complex system. No simple concentration dependences are evident, and subtle ion-pairing effects can have a large influence on activity and selectivity. Studies of the rhodium chemistry also indicate a high degree of complexity in this system. Although many rhodium complexes may be used as catalyst precursors (86),the most commonly used precursor is Rh(CO),(acac) (acac = acetylacetonate). It is known that reduction of this and similar mononuclear rhodium species by bases under carbon monoxide affords a rhodium cluster anion, [Rh 2(CO)3,]2- (106, 132-135), whose structure is shown in Fig. 12. This complex is useful as a catalyst precursor, and various mixed-metal clusters of the same general structure containing cobalt, rhodium, and iridium in the cluster framework have been prepared (136-138). Although these compounds are reported to be useful as catalyst precursors, no catalytic results are given. The [Rh,,(CO),,12- cluster is in equilibrium under CO with another anionic cluster, initially identified as [Rh, 2(CO),,]2- (106). Later studies, which involved the low-temperature isolation of this very labile complex, showed it to be [Rh,(CO),,]- (107),whose solution structure (139) is shown in Fig. 13. Studies by infrared (126, 140) and NMR (139) spectroscopy have shown that [Rh, 2(CO)3,]2- is essentially completely converted to [Rh,(CO),,]- under relatively low CO pressures (5 atm at 25°C) as follows:

a

3[Rh, r ( C O ) 3 0 1 Z ~ 6[Rh5(C0),s1-

b F c . 12.

+ RhJCO),,

d

Molecular structure of [Rh,,(CO),o]Z- (135).

(42)

B. D. DOMBEK

370

0

FIG. 13. Solution structure of [Rh,(CO),,]- (139). x represents a bridging carbonyl ligand.

Catalytic reaction solutions prepared from Rh(CO),(acac) in the presence of amines and/or carboxylate salts show the presence of [Rh,(CO),,]- and a mononuclear species, [Rh(CO),] - , when observed by high-pressure infrared spectroscopy (86, 140). The spectral features of these mixtures remain unchanged as the temperature is increased up to 180°C (at 500 atm of HJCO) of 210°C (at 1000 atm). At temperatures above these values significant broadening and shifting of the absorption bands occur, and the species present cannot be identified with certainty (140). (It is interesting to note in this regard that most of the reported catalytic experiments have been carried out at temperatures above these values.) The changes reported at higher temperatures are reversible, and bands assignable to [Rh5(CO)l,I- and [Rh(CO),]- reappear upon cooling the solutions below 180-210°C under pressure. Although the identity of rhodium complexes present at temperatures and pressures generally employed for catalytic experiments cannot be determined with certainty by infrared spectroscopy, rhodium clusters of the formula [Rh, 3(CO),,H,]'5-"'- (x = 2, 3) are possibly present under such conditions. These are known complexes (141), stable under an atmosphere of carbon monoxideat room temperature, and obtainable from [Rh,,(C0),,]2by reaction with H, ( W C , 1 atm). The structure of [Rhl,(CO),,H,]2- is shown in Fig. 14. It has also been found that [Rh,(CO),,]- can be converted to these clusters by the following reaction with HJCO, the temperature required being determined by the H,/CO pressure (140): [Rh,(CO),,]-

-+

[Rh,3(C0)24H,]'5-X)-(x

=

2 , 3)

+ higher clusters

(43)

ETHYLtNE GLYCOL A N D ETHANOL FROM H 2 AND CO

37 I

d FIG.14. Molecular structure of [Rh,,(CO),,

1’-

(13

The temperature was 80, 190, and 240 C, respectively, at HJCO pressures of 1, 600, and 1000 atm. [The higher clusters presumably include [Rh,,(CO)z5]4- and [RH I 5(CO)27]3-(142).] Such clusters therefore seem likely to be present in reaction solutions during catalysis. The shifting of equilibria to higher clusters with an increasing number of metal-metal bonds upon raising the temperature appears to be a general phenomenon (126,143); it has been observed for other rhodium clusters at lower temperatures and pressures (135). The 13-Rh-atom clusters of (43) are interconvertible by protonation and deprotonation; amines such as N M M serve to deprotonate the trihydride. These clusters can be fragmented by carbon monoxide as follows :

These interconversions illustrate the general finding that CO pressure causes cluster fragmentation, whereas replacing CO by H, allows cluster growth to occur. An important feature of the fragmentation process shown is the removal ofmononuclear [Rh(CO),]- or HRh(CO), units from larger clusters by reaction with CO. Evidence for further fragmentation of [Rh,(CO),,]to [Rh(CO),]- and Rh,(CO), is reported based on high-pressure infrared spectroscopy (126), but such a transformation is not observed in highpressure NMR experiments (139, 144).

372

B. D. DOMBEK

One function of amine and other basic promoters may be to facilitate cluster transformation by allowing facile protonation/deprotonation processes to occur. The [Rh,3(C0)24H3]2-cluster is a sufficiently strong acid to be deprotonated by NMM, as seen in (44).The HRh(CO), species has been found to be fully deprotonated by N M M and N,N-dimethylaniline (145). Protonation and deprotonation in reaction systems containing hydrogen are linked to oxidation and reduction processes. For example, addition of hydrogen to a metal cluster followed by deprotonation leads to a net reduction of the metal species; conversely, protonation and elimination of H 2 causes oxidation of the metal species (123, 125). The basicity and amount of promoter employed can therefore determine the metal oxidation state of the system. The preferred oxidation state of rhodium in the catalytic system for ethylene glycol formation appears to be between the extremes of 0 and - 1. Catalytic systems containing a high proportion of [Rh(CO),](oxidation state - 1) produce largely methanol and little ethylene glycol (102), whereas systems containing neutral rhodium complexes such as Rh,(CO)], (oxidation state zero) have low activity without a basic.promoter (reducing agent). Another possible function of certain promoters may be to facilitate the transfer of “Rh(C0);” fragments between clusters (123, 126), as follows: [Rhl,(C0)z,]3- t 2L

* [Rh14(CO)z5]4-+ [Rh(CO),L,]+

(45)

The ligand which removes the “Rh(C0);” fragment may be a halide ion (146), an amine, or a solvent molecule (123). Reactions according to (45) were observed for N M M , bis(N,N-dimethyl)-ethylenediamine,and 1,lOphenanthroline. In reactions of the first two amines, infrared absorptions were detected which could possibly arise from the [Rh(CO),(amine),]+ species; such complexes have been studied previously (147). The [Rh,, .(CO)2,]3- cluster is observed to react also with certain solvents according to (45). Although this cluster is stable at ambient conditions in acetone and tetraglyme (relatively low-polarity solvents), it reacts in sulfolane and [ 181-crown-6 to form [Rh,,(C0)2,]4- (123). It is suggested that a decrease in ion pairing in the latter two solvents may have facilitated release of the “Rh(C0):” fragment. Carboxylate promoters may also be able to coordinate to an “Rh(C0):” fragment, and therefore facilitate a process such as that shown in (45). Reactions of [Rh, s(CO)2,]3- with H2 in the presence of cesium carboxylates are reported (123) to be consistent with the formation of small equilibrium amounts of Rh(CO),(O,CR) by reaction (45). Carboxylates could therefore be involved in cluster growth or transformation during catalysis. The possibility that a cluster framework could be an important feature in determining the activity of a catalyst has led to investigations of less labile

ETHYLENE GLYCOL AND ETHANOL FROM H2 AND CO

373

rhodium clusters as catalyst precursors. A number of rhodium clusters containing encapsulated main-group elements have been prepared, including [Rh, 7S2(CO)32]3- (148-15O), [Rh,C(CO), 532- (151), [Rh,P(CO), (152- 154), [Rh ,Sb(CO), J 3 - ( 1 5 9 , [Rh oP(CO),,]3- (156), and [Rh ,As (C0),,l3 - (157). The encapsulated atoms stabilize the cluster framework, and several of these complexes have been observed to remain intact under high pressures of H,/CO in the presence of promoters, conditions which normally lead to CO hydrogenation (4). Table XI11 shows that the activity of such systems is substantially lower than that of a normal rhodium catalytic system, so the possibility that small amounts of the clusters are fragmented (perhaps reversibly) to form catalytically active species cannot be excluded. The role of metal clusters in the rhodium-catalyzed hydrogenation of CO remains uncertain. It is evident that various cluster species are present during catalytic operation, but it is also clear that labile fragmentation and rearrangement processes are possible. Indeed, these processes are facilitated by the species observed to promote catalytic activity. Criteria set forth by Laine for identifying cluster-catalyzed reactions (158) are not definitive for this process. The pressure dependence of the reaction has been suggested to be attributable to a shifting of equilibria between clusters in solution at varying H,/CO pressures (43); however, the identity or characteristics of the active species are not apparent. The general outline of steps leading to the primary oxygenated products presented above for cobalt catalysts (a chain growth process which proceeds through aldehyde intermediates) may also apply to the rhodium system. Certainly, the same array of products is observed in both systems, although secondary reactions are evidently less predominant in most of the rhodium TABLE Xi11 Catalytic Activity of Systems Based on Stabilized Clusters"~h

Complex

Ethylene glycol rate (hr- I )

Methanol rate (hr- I )

Data from Ref. 140. (Adapted with permission. Copyright 1980 American Chemical Society.) Reaction conditions: 75 ml sulfolane, 3 mmol Rh, 5 mmol NMM, 1000 atm, H,/CO = 1,260"C. 0.375 mmol cesium benzoate added.

374

B. D. DOMBEK

reactions. The pathways leading to 1,2-propyleneglycol (101)and n-propanol (100) in certain promoter-modified rhodium reactions are not certain. Although the products are possibly formed entirely by secondary reactions, definitive experiments are not reported. The possible intermediacy of formaldehyde in CO hydrogenation has been addressed above with regard to the cobalt catalytic system. Fahey has observed a small amount of 1,3-dioxolane (the ethylene glycol acetal of formaldehyde) as a product of the rhodium system (43). Thus, there is evidence that formaldehyde or a complexed form of this molecule could be an intermediate in the CO reduction process by this system. Rhodium catalysts are indeed found to be useful for the hydroformylation of formaldehyde to glycolaldehyde (159-261); methanol is a by-product in these reactions. An experiment in which I4CH2Owas added to a rhodium-catalyzed CO reduction system showed that the label was incorporated into all of the expected products, including ethylene glycol, methanol (and their formate esters), ethanol, and the ethylene glycol acetal of glycolaldehyde (91). The label was not found in CO or C 0 2 .These results support the general mechanism described above in which (coordinated) formaldehyde is a precursor of methanol and glycolaldehyde, which is itself a precursor of ethylene glycol and higher polyalcohols. An interesting result of this experiment is that the ethylene glycol/methanol mole ratio from I4CH2O (3.5) is substantially higher than that for the overall reaction (1 3.This may indicate an alternative route to glycol via hydrodimerization of formaldehyde, which would result in a higher concentration of label in the glycol product. This finding may also imply that there are slight differences in selectivity for the hydroformylation of free (labeled) formaldehyde and coordinated (unlabeled) CHzO-an entirely plausible possibility. The absence of I4CO at the end of this experiment may indicate that formation of formaldehyde by this system is an essentially irreversible process. This cannot be definitely concluded, however, in light of the probable reactivity differences between coordinated and uncoordinated formaldehyde. Several differences between the cobalt- and rhodium-catalyzed processes are noteworthy with regard to mechanism. Although there is a strong dependence in the cobalt system of the ethylene glycol/methanol ratio on temperature, CO partial pressure, and Hzpartial pressure, these dependences are much lower for the rhodium catalyst. Details of the product-forming steps are therefore perhaps quite different in the two systems. It is postulated for the cobalt system that the same catalyst produces all of the primary products, but there seems to be no indication of such behavior for the rhodium system. Indeed, the multiplicity of rhodium species possibly present during catalysis and the complex dependence on promoters make it

ETHYLENE GLYCOL AND ETHANOL FROM H2 AND CO

315

appear possible that several species may be catalytically active, each having its own product-forming selectivity. Any conclusions about the mechanism of the rhodium-catalyzed process, however, must await more detailed catalytic and chemical studies. IV.

Unpromoted and Carboxylic Acid-Promoted Ruthenium Catalysts

A. BACKGROUND Homogeneous ruthenium catalysts have been reported to convert H 2 / C 0 to methane (22) and a distribution of linear alkanes (10, 162). The argument was presented that a metal cluster, such as Ru,(CO),~,was an essential catalyst component in order to form the alkane product, and it was claimed that the mononuclear Ru(CO), was inactive (10). Further studies have shown, however, that methane and alkanes are formed by rutheniumcontaining catalytic systems only when metallic ruthenium (formed by decomposition of the homogeneous catalyst) is present (163-167). Strictly homogeneous solutions of ruthenium complexes are found not to produce alkanes, but instead usually yield methanol as the major product. The use of certain additives or solvents has been found to improve the activity or selectivity to the methanol product (164-168). It has further been found that carboxylic acids can cause this system to form ethylene glycol as its carboxylate ester in addition to the methyl ester (166, 167, 169-172). The similarity in many details of the chemistry in the above homogeneous systems suggests that they are modifications of the same basic ruthenium catalyst system. The most notable common feature of these reactions, to be discussed in this section, is the presence of predominantly Ru(CO), as the stable ruthenium species during catalysis. Addition of ionic promoters, particularly halide salts, to ruthenium-containing solutions has been found to provide catalytic systems with very different characteristics. Such systems contain ruthenium complexes other than Ru(CO), during catalysis, and will be described in Section V.

B. CATALYTIC ACTIVITY AND SELECTIVITY Catalyst solutions generated by the reaction of Ru(acac), or Ru,(CO),, with H 2 / C 0have been reported by Bradley to produce methanol and methyl formate as the major products (264, 165). Methyl formate is produced at a constant rate, suggesting that it is a primary product and not derived from

376

B . D. DOMBEK

initially formed methanol. Reactions at lower pressure were found to give much smaller relative yields of methyl formate (166), consistent with the effects of pressure on the formate/alcohol ratio observed for the cobalt and rhodium catalytic systems. Reactions in a variety of solvents give a range of activities, as seen in Table XIV. Tetrahydrofuran (THF) and sulfolane provide lower activities than those observed in certain other solvents, such as ethanol and ethyl acetate. Carboxylic acids provide methanol (ester) rates comparable to those found in the latter solvents, but are notable in that ethylene glycol esters are also observed. Ethylene glycol is not normally observed as a product in the absence of carboxylic acids. Small amounts of ethylene glycol have been reported as products after ruthenium-catalyzed reactions in NMP and toluene solvents at 2000 atm (39). However, observations of minor amounts of this product must be viewed with caution unless great care is taken in the experimental procedure. For example, it was earlier reported that a catalyst derived from Ru,(CO),, TABLE XIV Ruthenium-Catalyzed CO Hydrogenation ~

Expt.

Solvent

I 2 3 4 5 6 7 8 9

TH F Ethanol Ethanol THF Sulfolane Ethyl acetate Acetic acid Acetic acid Propionic acid THF THF THF

10

II 12

~~

Pressure Temp. -OCH2CHLONotes (atm)" ("C) rate ( h r - l ) b C C C C

d e e

.f g

h i

1300 340 340 340 340 340 340 340 340 1200 1200 1200

268 260 230 230 230 230 230 260 230 275 275 290

-

0.29 0.34 0.22

HJCO = I unless specified otherwise. HJCO = 1.5 (164). 'Methyl formate levels very low (166). Includes methyl acetate formed by transesteritication (166). 'Methanol and ethylene glycol detected as acetate esters (166). Methanol and ethylene glycol detected as propionate esters (166). From Ref. 165. Includes added triphenylphosphine, P/Ru = 3 (165). H,/CO = I . 5 . Methanol rate includes methyl formate (164). j Not detected.

~

CH30rate (hr-I) 27.5 23.2 11.8 4.12 3.19 7.04

Methyl formate rate (hr-l) 3.06 n.d.1 n.d. n.d. n.d. n.d.

11.1

29.6 13.0 35 51 I I6

-

18

3.7

ETHYLENE GLYCOL AND ETHANOL FROM Hz AND CO

377

and 2-hydroxypyridine in tetraglyme solvent under pressures of at least 1700 atm gave substantial amounts of ethylene glycol (97, 98). Later studies by the same researchers (173, 174) led them to conclude that the ethylene glycol in these experiments was actually produced by rhodium species leached from the walls of the vessel during reaction, and originating from earlier rhodium-containing experiments. A patent also claims the production of ethylene glycol by a ruthenium catalyst in the presence of 2-hydroxypyridine in n-propanol at 1700 atm (175); this result appears suspect in light of the conclusions cited above. Traces of ethylene glycol have been detected in catalytic solutions derived from RU,(CO),~in T H F solvent, after reaction at pressures of 1000-1500 atm (176); a blank run containing no RU,(CO),~immediately preceding these experiments produced no detectable glycol. The major products of these ruthenium-catalyzed experiments were found to be methanol and methyl formate. Other products have also been reported from ruthenium-catalyzed CO reduction experiments. Ethylene glycol and its ethers are reported as products from ruthenium-containing solutions in the presence of polyhydric phenols and, optionally, mineral acids (177). The fact that the only solvent employed in the examples cited is the dimethyl ether of tetraethylene glycol (tetraglyme) suggests that solvent decomposition could lead to these products, particularly in the presence of the acidic “promoters.” The decomposition of glyme solvents in catalytic solutions containing acidic HCo(CO), has already been discussed (53). Methanol, methyl formate, dimethyl ether, and acetone are reported as products from ruthenium-containing solutions in 2-methoxyethanol(144). Once again, solvent decomposition is implicated, and the source of these products remains uncertain. Disregarding ambiguous results perhaps caused by metal precipitation, catalyst contamination by rhodium and possibly other metals, and catalyzed solvent decomposition, it appears probable that homogeneous ruthenium catalysts in the absence of ionic promoters can produce essentially only the one-carbon products methanol and methyl formate. [Under much higher pressures (3000-3600 atm) it is reported that higher linear alcohols may also be obtained (174).] Ethylene glycol is at most a trace product, except in the presence of carboxylic acids which cause the formation of ethylene glycol esters. [Siloxane derivatives of ethylene glycol are also obtainable by carrying out ruthenium-catalyzed reactions in the presence of reactive hydrosilanes (46, 46b, 46c).] The amount of ethylene glycol product formed in acetic acid solvent is usually minor relative to the methanol product. Table XIV, for example, shows examples in which the C , / C , product ratio is within the range of about 35-90. Esters of the three-carbon polyalcohol, glycerol, have also been

378

B. D. DOMBEK

detected in these product mixtures. The CJC, product ratio is even smaller, generally within the range of 200-300 (179). Other minor products in these reaction solutions include esters which result from the hydrogenation of carboxylic acid to alcohol (166). A first-order dependence of the CO reduction rate on metal concentration has been observed in these systems (164-167). Dependences on hydrogen and carbon monoxide partial pressures are less simple. A study of these effects on the rate of ethylene glycol ester formation in acetic acid solvent (164) showed that the dependence on hydrogen partial pressure was constant, with an order of about 1.3 (Fig. 15). The dependence on CO partial pressure is more complex, exhibiting a high dependence at low partial pressures (100 atm at 23OoC),but showing zero-order dependence at higher CO partial pressures (200 atm). (The effect of these partial pressures on the rate of methyl ester formation was parallel ; only minor changes in selectivity were noted over the range of pressures investigated.) Bradley has reported that under yet higher partial pressures of CO (ca. 500 atm), increases in CO pressure cause decreased rates of CO hydrogenation (164). Increases in CO partial pressure have also been reported to enhance the

0.30

r

170 atrn

co

170 atrn H2

-

0.05

75

100 150 200 250 Pressure of CO or Hz, atrn

FIG. 15. Dependences of ethylene glycol diacetate yield (formation rate) on CO and H, partial pressures. (Adapted from Ref. 166 with permission. Copyright 1980 American Chemical Society.) Partial pressure of reagent not being varied is 170 atm. Reaction conditions: 50 ml acetic acid, 2.35 mmol Ru, 2 3 0 C , 2 hr.

ETHYLENE GLYCOL A N D ETHANOL FROM H, A N D CO

379

formate/alcohol ratio. It was further found that addition of triphenylphosphine had a negligible effect on the overall rate of product formation, but caused a substantial decrease in the formate/alcohol ratio [cf. Expts. 10 and 1 1 in Table XIV (1641. The temperature dependence of the CO reduction process has also been studied. Over the range 250-290°C under 1200 atm of H,/CO, an Arrhenius activation energy of 32 kcal/mol was reported (164). The activity of this ruthenium system is comparable to, or somewhat greater than, that of cobalt catalysts under the same conditions of temperature and pressure. Rhodium catalysts provide substantially higher activity than either of these systems. As will be seen later, however, addition of ionic promoters can greatly increase the activity of ruthenium-based catalysts. C. SOLVENTS As described above and shown in Table XIV, the identity of the solvent may have significant effects on the rate of CO reduction. Alcohols, esters, and carboxylic acids appear to provide the highest rates, whereas T H F and sulfolane are somewhat less effective. Heptane solvent has been reported to afford poor rates of CO reduction by this system (163). Differences in rates among these solvents appear small enough to be attributable to an effect such as the enhanced stabilization of a polar transition state by the more polar solvents. The presence of certain additives, such as boric acid and aluminum alkoxides, has also been found to increase the rate of CO reduction, perhaps for similar reasons (168). Carboxylic acids appear to have a role in this system more complex than that of other solvents, since they are incorporated into the products and alter the selectivity by promoting formation of the two-carbon glycol product. Noncarboxylic acids have not been found to possess this ability to induce glycol formation; Br~nstedacids with a range of acidities have been investigated but found not to be effective (166). For example, pentachlorophenol, which has a pK, similar to that of acetic acid, does not promote the formation of significant amounts of a glycol product when used as solvent (167). Addition of other acids, such as H,PO,, to carboxylic acid solvents is not observed to enhance the rate or selectivity of two-carbon product formation. However, a variety of carboxylic acids promote formation of the glycol ester products (166, 167, 171). Thus, carboxylic acids are quite specific promoters for glycol formation, and acidity alone is not the source of this promoter effect. The influence of carboxylic acid concentration on the rate of two-carbon product formation has been investigated. Dilution with other solvents causes

380

B. D. DOMBEK

.-

0.30

0.20 0.15

-

0.10

-

rn U

3

2 0.08-

0 3

P

5 0.06 -

.0

- 0.04

F 0.030

(3

t

5 0.02-

5

L 4

6

8 10

15

20

Acetic Acid, M

FIG.16. Log-log of ethylene glycol diacetate yield vs acetic acid concentration when diluted with varying amounts of methyl acetate and water. (Adapted from Ref. 166 with permission. Copyright 1980 American Chemical Society.) Reaction conditions: 50-75 ml solvent, 2.35 mmol Ru, 340 atm, HJCO = 1, 230 C, 2 hr.

a rapid decline in the rate of glycol formation (166, 169). The rate of glycol production is approximately proportional to the second power of carboxylic acid concentration, as shown in Fig. 16. In contrast, the rate of methanol or methyl ester formation is changed little upon altering the acid concentration. Studies of ruthenium-catalyzed reactions in carboxylic acid solvents have been reported by Knifton (171, 172), but most of these experiments contain added salt promoters which greatly modify the catalytic behavior. These experiments will be considered in Section V, along with other Lewis basepromoted ruthenium systems.

D. CATALYST STABILITY The stability of soluble ruthenium carbonyl species toward decomposition to metal is a function of both carbon monoxide partial pressure and reaction temperature, similar to the situation described earlier for cobalt complexes and shown in Fig. 4. However, a quantitative study of these variables on ruthenium stability has not yet been reported.

ETHYLENE GLYCOL AND ETHANOL FROM H2 AND CO

38 1

Solutions of ruthenium carbonyl complexes in acetic acid solvent under 340 atm of 1 : 1 H,/CO are stable at temperatures up to about 265°C (166). Reactions at higher temperatures can lead to the precipitation of ruthenium metal and the formation of hydrocarbon products. Bradley has found that soluble ruthenium carbonyl complexes are unstable toward metallization at 271°C under 272 atm of 3 : 2 H,/CO [I09 atm CO partial pressure (165)]. Solutions under these conditions form both methanol and alkanes, products of homogeneous and heterogeneous catalysis, respectively. Reactions followed with time exhibited an increasing rate of alkane formation corresponding to the decreasing concentration of soluble ruthenium and methanol formation rate. Nevertheless, solutions at temperatures as high as 290°C appear to be stable under 1300 atm of 3 :2 H,/CO. Careful studies by Doyle et ul. (163) have also shown that soluble ruthenium species are inactive for hydrocarbon formation. A soluble system could be maintained in heptane solvent at 250°C under 100 atm of 1 : 1 H,/CO for many hours by taking precautions to avoid the possible introduction of impurities into the system and by slowly raising the temperature. No hydrocarbon formation was observed in this reaction. Only upon heating to about 260°C was the disappearance of soluble ruthenium complexes noted, along with the formation of linear alkanes. These results may suggest that metastable homogeneous ruthenium solutions can be formed, as has been reported for cobalt complexes (56); precipitation of the metal may be an autocatalytic process.

E. MECHANISM Information from several sources is relevant to the identity of ruthenium species present in these catalytic solutions. Reactions of ruthenium complexes under 200 atm of 1 :2 H,/CO at 180°C (180) or 80 atm of CO at 150°C (181) have been reported to produce mainly Ru(CO), . The mononuclear species is formed as an equilibrium product from Ru,(CO),, under CO pressure, the position of the equilibrium +RU,(CO),~+ CO + Ru(CO),

(46)

depending on the temperature and CO pressure. Studies of this equilibrium have been carried out at temperatures of 75-125°C and CO pressures of 10-60 atm (182). At 100°C under 60 atm of CO, an equilibrium solution approximately 3 x M in ruthenium contains Ru(CO), to the extent of about 99%. Under 10 atm of CO, slightly more than 50% of the ruthenium is in the form of Ru(CO),. Higher temperatures favor the equilibrium formation of the Ru,(CO),, cluster.

High-pressure infrared studies of ruthenium carbonyl solutions under HJCO at temperatures employed for CO reduction have also been reported. I n Ir-tetradecane solution at 180 C under 1 : 1 H 2 / C 0 , mainly Ru(CO), is detected (60).In acetic acid solvent at 200 C, only Ru(CO), is detected under 400 atrn of 1 : 1 H,/CO; at HJCO pressures of 200 atm, Rii3(CO)12is also observed (166). Reaction solutions have also been studied by sampling under reaction conditions, rapidly cooling the samples to low temperatures, and analyzing them by infrared spectroscopy; after reaction at 265 atm of 1 : 1 HJCO at 180 C , only Ru(CO), could be detected (164). At higher temperatures and lower pressures (100 atm of 1 : 1 HJCO and 250°C). evidence was seen for the clusters Ru,(CO),, and H,Ru,(CO),, as well as Ru(CO), ( 163). The presence of mainly the mononuclear Ru(CO), species under catalytic conditions, in combination with the observation of first-order rate dependences on ruthenium concentration, indicates that a mononuclear catalyst is involved in this CO reduction process. Reactions before or during the rate-determining step which involved metal cluster formation, or other processes requiring more than one metal species, would cause rate dependences of higher order in metal concentration. The observation of equilibrium (46) can also explain the changing rate dependences on CO partial pressure (Fig. 15). Under high CO partial pressures (e.g., > 200 atm under the conditions of Fig. 15) zero-order or negative rate dependences are observed; under these conditions equilibrium (46) is shifted essentially completely to the mononuclear species. Under lower partial pressures of CO (e.g., 100 atm under conditions of Fig. 15) cluster formation is observed to occur, thus lowering the concentration of effective (mononuclear) catalyst present. Therefore, the initial rate enhancement observed upon increasing a low CO partial pressure is the result of increased cluster fragmentation and the generation of active catalyst species. At the same time, however, CO may be an inhibitor of the actual hydrogenation process. Once the cluster fragmentation equilibrium has been shifted far toward the mononuclear species, only the inhibition effect is observed. The effect on CO reduction rates of H, partial pressure has been found to be somewhat greater than first order (ca. 1.3) (166). The existence of a nonintegral dependence on hydrogen pressure suggests the participation of an equilibrium involving hydrogen addition prior to the rate-determining step. It is known that Ru(CO), reacts with H z under pressure to form H,Ru(CO), (181), and this reaction is a plausible equilibrium process under catalytic conditions. A scheme consistent with the observed behavior of the system can be constructed if a second molecule of H z reacts with a catalytic intermediate before or during the rate-determining step, as follows: RU(CO),+

H,Ru(CO),&

product

(47)

ETHYLENE GLYCOL AND ETHANOL FROM H2 AND CO

383

Assuming steady-state behavior, the rate law for this process is as follows:

d [ product] - k k , [Ru(C0),]Pi dt k-IP,, + k2PH This equation predicts a rate dependence on H, partial pressure of between first- and second-order and a CO dependence of between zero- and negative first-order, as well as first-order dependence on Ru(CO), concentration. Based on this scheme, it may be suggested that H,Ru(CO), is the active catalyst species in this system. The steps by which this metal hydride forms the observed organic products are perhaps similar to those already discussed for cobalt catalysts. Steps which may be involved are intramolecular hydride migration to produce a formyl ligand : (CO),RuH,

* (CO),HRu-CHO

(49)

followed by rate-determining H, addition and production of a formaldehyde intermediate :

H

7\ H

(As discussed previously, thermodynamics indicate that free formaldehyde

will not be a major product of this reaction, although a small equilibrium concentration may be formed.) 0

0

II II ( C O ) ~ H R U - C C H Z O H-HCCHzOH

cY \

cv

(51)

(CO), HRu-CHZOH

0 (c0)3H1zRu

-Id. H' \ H

(52)

CH3OH

0 II

k

-

Y

0 11

(CO~HRU-COCHJ-HCOCH~

(53)

CH30H

(54)

(CO). HRu OCHS

384

B . D. DOMBEK

The glycolaldehyde shown in (51) results from a “CO insertion” reaction followed by reductive elimination, and is presumed to be a precursor of ethylene glycol. Since ethylene glycol is, however, at most a trace product of this catalytic system, step (51) appears to be essentially inoperative. Methyl formate, a major primary product of this system under some conditions, is also presumed to be formed by a CO insertion process, (53). Methanol may be formed by a reductive elimination (hydrogenolysis) of either a hydroxymethyl ligand, (52), or of a methoxy ligand, (54). The scheme in (51)-(54) is useful in considering possible reasons that ethylene glycol is not produced by this catalytic system. One possibility is that the alkyl migration, or “CO insertion” process of (51) is particularly unfavorable for the hydroxymethyl ligand. However, Roth and Orchin have demonstrated a reaction which apparently involves alkyl migration of a cobalt hydroxymethyl complex, (20), at quite low temperatures (75). Also, ethers and esters of the hydroxymethyl ligand (which are expected to be very similar electronically to the parent hydroxymethyl) have been shown to undergo CO insertion in manganese complexes under mild conditions (68). These studies suggest that differences in alkyl migration behavior between hydroxymethyl complexes and other simple alkyl complexes are small and of a quantitative rather than qualitative nature. Another possible reason that ethylene glycol is not produced by this system could be that the hydroxymethyl complex of (51) and (52) may undergo preferential reductive elimination to methanol, (52), rather than CO insertion, (51). However, CO insertion appears to take place in the formation of methyl formate, (53), where a similar insertion-reductive elimination branch appears to be involved. Insertion of CO should be much more favorable for the hydroxymethyl complex than for the methoxy complex (67, 83). Further, ruthenium carbonyl complexes are known to hydroformylate olefins under conditions similar to those used in these CO hydrogenation reactions (183, 184). Based on the studies of equilibrium (46) previously described, a mononuclear catalyst and ruthenium hydride alkyl intermediate analogous to the hydroxymethyl complex of (5 1) seem probable. In such reactions, hydroformylation is achieved by CO insertion, and olefin hydrogenation is the result of competitive reductive elimination. The results reported for these reactions show that olefin hydroformylation predominates over hydrogenation, indicating that the CO insertion process of (51) should be quite competitive with the reductive elimination reaction of (52). The evidence then suggests that the reason ethylene glycol is not formed by this system is that its hydroxymethyl precursor is not efficiently produced in the first step of (51). It follows that most of the methanol produced by this catalytic system must be formed by pathway (54), through a methoxide

ETHYLENE GLYCOL AND ETHANOL FROM Hz AND CO

385

intermediate. The very low selectivity of this system for the glycol product then appears to be determined by the preferred conversion of coordinated formaldehyde into a methoxy ligand. [Added paraformaldehyde has also been observed to be converted quite effectively to methanol by this system (179), presumably via (54).] Factors influencing the direction of formaldehyde insertion into a metalhydrogen bond, and thus the product selectivity in a scheme such as (51)(54), are expected to include the acidity of the hydride ligand. A highly acidic hydrogen atom may be more selectively transferred to the formaldehyde oxygen atom, producing the hydroxymethyl ligand as in (55) :

This appears to be the case for cobalt catalysts, which can hydroformylate formaldehyde to glycolaldehyde with high selectivity, apparently through the hydroxymethyl intermediate (75). The HCo(CO), hydride is known to be a strong acid, having a pK, < 2 (49).In contrast, reactions of HMn(CO), , a much weaker acid [pKa = 7 (49)],with formaldehyde under H 2 / C 0 have been found to produce instead the hydrogenated product methanol (1 79). The hydroxymethyl intermediate appears not to be formed, since under the identical conditions its ether and ester derivatives are converted in high yield (via CO insertion and hydrogenation) to ethylene glycol-containing products according to the following reaction (68, 179) : (co),M~--cH,oRJ!+

HOCH,CH,OR

(57)

It may be concluded that formaldehyde inserts into the less acidic manganese-hydrogen bond to form a methoxide ligand as shown in (56). As already mentioned, ruthenium catalysts also convert added formaldehyde into methanol rather than hydroformylated products. The H,Ru(CO), complex is not highly acidic (185) and is expected to have a pKa somewhat greater than that of H,Fe(CO), [pK, = 6.8 (186)].In this respect, H,Ru(CO), is anticipated to resemble HMn(CO), more than HCo(CO), . Reactions of added formaldehyde may differ somewhat from those of a possible coordinated formaldehyde intermediate generated by CO hydrogenation. It may be unnecessary for an added formaldehyde molecule to be coordinated to the metal before reacting with its hydride ligand. Such an addition could take place by an ionic or even a radical process. However, the trends in selectivity appear to be consistent in those systems which both

386

B. D. DOMBEK

reduce carbon monoxide to organic products and convert added formaldehyde; the cobalt and rhodium catalytic systems produced ethylene glycol (or glycolaldehyde) both from H,/CO and HCHO, and the ruthenium system produces only methanol from both. Slight quantitative variances in product selectivity from the two reactants have already been noted for the rhodium system (91), suggesting that there may indeed be small differences in the conversion mechanisms of bound and added formaldehyde. It is interesting to consider the function of carboxylic acids in promoting ethylene glycol ester formation by this system. Knifton has presented a thermodynamic argument, suggesting that the more favorable free energy of ethylene glycol ester formation, relative to that of free ethylene glycol, may be responsible for the formation of this product (171). However, the reactants and products in this system are not at equilibrium, and thermodynamics past the transition state of the product-determining step seem unlikely to be applicable to product selectivity. (As described in the next section, other ruthenium catalysts can produce ethylene glycol under similar conditions of temperature and pressure without the need for carboxylic acids.) Spectroscopic and chemical studies indicate that the ruthenium species present during catalysis in carboxylic acid solvents is Ru(CO), , as in other solvents (166).The chemical behavior, including the overall rate of CO reduction and the responses to reaction variables, is very similar to that observed in other solvents. The effect of carboxylic acids on product selectivity therefore appears to occur at a reaction stage after the rate-determining step of CO reduction. Remembering that the promoter function of a carboxylic acid is not a result only of its acidity, the scheme shown in (51)-(54) may be examined for intermediates which could interact with a carboxylic acid. One such intermediate is the hydroxymethyl complex of (51). If this intermediate were reversibly formed in small concentrations, it could be converted to the carboxylate ester by reaction with a carboxylic acid, as follows :

H

/-\

H

in a process similar to that of simple alcohol esterification. Hydroxymethyl complexes are normally quite unstable, apparently decomposing via a P-hydride shift from the oxygen atom (187),i.e., the reverse of hydroxymethyl formation in (58). However, carboxylate esters of the hydroxymethyl ligand are stable and not readily converted back to formaldehyde (68).Such an acyloxymethyl ligand would then be capable of undergoing CO insertion and hydrogenation to glycolaldehyde (ester) or ethylene glycol (ester) products, analogous to the reaction of (51). The carboxylic acid may also react

ETHYLENE GLYCOL AND ETHANOL FROM H2 AND CO

387

directly with the coordinated formaldehyde species, acylating it to form the ester without proceeding through a hydroxymethyl intermediate. Both of these reactions forming the acyloxymethyl ligand could be expected to exhibit the observed second-order dependence on acid concentration (166, 167). The direct acylation of a coordinated formaldehyde species has recently been observed (188). Reaction of the vanadium formaldehyde complex Cp2V(CH20)(Cp = qs-CsHj) with benzoyl chloride yields the 0

1I

Cp2(CI)V-CH20CC,H,

analog of the ruthenium ester shown in ( 5 8 ) . Formaldehyde added to ruthenium catalytic solutions in acetic acid is found to be converted, as expected, to ethylene glycol and methyl esters (179). Somewhat different selectivity is observed, however, from that found in standard H 2 / C 0 runs. Under conditions which give a C,/C2 ratio of 38 from CO hydrogenation, a reaction with added formaldehyde gave a C,/C2 ratio of 21 (179). As described above, slight differences in the reaction mechanisms involving bound and added formaldehyde could be responsible. Additionally, formaldehyde is known to react with carboxylic acids to form acetals and hemiacetals (189). These species could also react with ruthenium hydrides to form acyloxymethyl complexes without ever proceeding through hydroxymethyl or coordinated formaldehyde intermediates. The observation of glycerol triacetate as a trace product of CO hydrogenation by this ruthenium system in acetic acid solvent ( I 79) suggests that glycolaldehyde (ester) can undergo further chain growth by the process outlined in (26) for the cobalt system. As with formaldehyde, however, a carboxylic acid is apparently necessary to promote formation of the metalcarbon bonded intermediate which can produce the longer-chain product. Except for the modification of (58) in reactions (51) and (52), the scheme of (49)-(54) appears to apply to catalytic systems containing Ru(CO), in carboxylic acids and a variety of other polar and nonpolar solvents. As described in the next section, introduction of ionic promoters brings about significant changes in the catalyst chemistry. V.

Lewis Base-Promoted Ruthenium Catalysts

A.

BACKGROUND

The addition of certain ionic promoters to ruthenium catalytic solutions has been found to dramatically affect the rate and selectivity of CO hydrogenation. Whereas ruthenium solutions do not otherwise produce ethylene glycol as a significant product (except as its derivatives in in reactive solvents),

388

B. D. DOMBEK

ionic promoters can cause the formation of large amounts of this product in a variety of solvents. The species employed as a promoter need not be added in an ionic form, but it appears that the capability of forming ionic species under reaction conditions is essential. Halides are particularly effective as ionic promoters. Although the ruthenium species observed during CO reduction in the absence of promoters is Ru(CO),, its concentration can be reduced to unobservable levels by promoters which cause the formation of ionic ruthenium complexes. Because this system differs from unpromoted ruthenium catalysts in as many respects-rates, selectivities, catalytic species observed, and mechanism-it is addressed separately in this section. B. CATALYTIC ACTIVITY AND SELECTIVITY

Ruthenium complexes in nonreactive solvents such as sulfolane and NMP in the presence of halide promoters are found to possess high activity for the nium catalysts in many respects-rates, selectivities, catalytic species observed, and mechanism-it is addressed separately in this section. TABLE XV Hydrogenation o f C 0 by Halide-Promoted Ruthenium Catalysts

Expt.

Solvent

Notes”

1 2 3 4 5 6 7 8

Sulfolane NMP 18-Crown-6 Sulfolane 18-Crown-6 NMP Acetic acid Bu,PBr

b c

d e

f g

h i

Pressure (atm)

Temp. (“C)

Ethylene glycol rate (hr-’)

408 544 544 850 850 1020 430 430

200 250 250 1no 200 240 220 220

0.65 4.68 4.10 2.05 8.60 48.4 0.74 I .63

Methanol rate (hr-I)

Ethanol rate (hr-’)

2.86 36.8 141 1.71 30.4 384 6.46 19.8

0.29 4.7 1 I 8.4 0.35 1.51 24.5 0.70 5.63

HJCO = I, ruthenium source is Ru,(CO),,, unless otherwise specified. 75 ml solvent, 30 mmol Ru, 180 mmol KI, 2 hr (190). 75 ml solvent, 6 mmol Ru, 18 mmol KI. 0.77 hr (190). 75 ml solvent, 3 mmol Ru, 60 mmol KI, 0.63 hr, H,/CO = I .5 (190). 75 ml solvent, 30 mmol Ru, 180 mmol KI, 1.68 hr (190). 75 ml solvent, I5 mmol Ru. 60 mmol KI, 0.47 hr (190). 75 mol solvent, 6 mmol Ru, 120 mmol KI,0.13 hr (190). 50 g solvent, 3.75 mmol RuCI,. H,O, 37.5 mmol heptyl(triphenyl)phosphonium acetate, 18 hr (171). Products are acetate esters. 15 g tetrabutylphosphonium bromide “solvent,” 4 mmol RuO,. H,O, 2 hr (199).

ETHYLENE GLYCOL AND ETHANOL FROM H2 AND CO

389

glycerol, the ethylene glycol acetals of acetaldehyde, glycolaldehyde and formaldehyde, and small amounts of methane. Free acetaldehyde is also sometimes observed. Methanol and ethylene glycol are the major primary products; ethanol is largely a secondary product derived from methanol via homologation, through acetaldehyde (191). Reactions which are allowed to proceed for extended periods are thus found to produce higher relative yields of ethanol and other related secondary products. Increased product levels, particularly ethylene glycol, can reduce the rate of glycol production (192). Under such conditions, higher relative yields of glycol derivatives such as acetals and ethers may be observed. For these reasons, experiments designed to study chemical responses of this catalytic system were carried out using relatively short reaction times to avoid large contributions of secondary reactions and product interactions with the catalyst (191). Iodide-promoted reactions in phosphine oxide solvents have been observed under some conditions to produce ethanol from H2/C0 with good rates and high selectivities (193-195) (Table XVI, Expts. 1-3). Experimental evidence suggests that the ethanol is a secondary product, although its selectivity is high even after very short reaction times (193). An acid component is believed to be involved in alcohol homologation by this system, which will be described in more detail below. Related work has been reported in amide solvents with halide or hydrohalic acid promoters (196). Ethanol and acetaldehyde as well as methanol are observed. Enhanced yields of acetaldehyde appear to be obtainable by operating such a system at reduced temperatures, although overall rates of CO reduction suffer. Reactions of ruthenium catalyst precursors in carboxylic acid solvents with various salt promoters have also been described (170-172, 197) (Table XV, Expt. 7). For example, in acetic acid solvent containing acetate salts of quaternary phosphonium or cesium cations, ruthenium catalysts are reported to produce methyl acetate and smaller quantities of ethyl acetate and glycol acetates (170-172). Most of these reactions also include halide ions; the ruthenium catalyst precursor is almost invariably RuCl, . H 2 0 . The carboxylic acid is not a necessary component in these salt-promoted reactions : as shown above, nonreactive solvents containing salt promoters also allow production of ethylene glycol with similar or better rates and selectivities. The addition of a rhodium cocatalyst to salt-promoted ruthenium catalyst solutions in carboxylic acid solvents has been reported to increase the selectivity to the ethylene glycol product (198). Very similar reactions using a ruthenium catalyst, carboxylic acid solvent, and a slightly different promoter system have been reported (197) to give increased amounts of ethyl ester product (Table XVI, Expts. 4 and 5). Most examples show the use of RuO, * H,O as the catalyst precursor in a carboxylic

390

13. D. DOMBBK

acid containing a quaternary phosphonium bromide salt, under 430 atm of H,/CO at 220°C. The long reaction times reported (1 8 hr) and the observation of higher alcohol ester products suggest that secondary, alcohol homologation, processes are involved in the formation of the ethyl ester. Amounts of methane (calculated in approximate fashion based on reported typical levels in the vented gas, the free reactor volume, and the reaction temperature and pressure) found in these reactions appear to be quite high, TABLE XVI Eilianol Production Jrom HJCO by Ruthenium Catcilys/s"

Expt.

Solvent Pr PO Pr PO Pr,PO Propionic acid Propionic acid Bu,PBr Bu,PBr Bu,PBr

"

HJCO

=

Pressure Temp. (am) ( C)

408 408' 850 430 430 272 430 272

210 240 230 220 220 220 220 220

Promoter or cocatalyst

Ethanol rate (hr- I )

CH,(C,H,),PBr C,H I ,(C,H5),PBr TiO,(acac) Zr(acac), Co(acac),

6.6 17.4 14 2.4" 2.6' 2.6 2.6 2.3

12

4

I2

Ethanol C efficiency

("6)"

Notes

59 51 59 31 34' 31 31' 20

d e

./' 11

.I

k 1?1

n

I unless specified otherwise.

* Carbon efficiency to ethanol, defined as moles of CO converted

to ethanol divided by total moles of CO converted to organic products. ' H,/CO = 2. Other products and rates (hr- ')are methanol (4.0),n-propanol(O.25). n-butanol(O.1). ethylene glycol ( O . l ) , and methane (3.6)(194). ' Methanol ( l O . l ) , n-propanol(O.38), ethylene glycol (0.07).and methane (20)(194). Methanol ( 5 . 5 ) , n-propanol (0.87), n-butanol (0.30), ethylene glycol ( I .8), and methane (6.9) (193). Product is ethyl propionate. Methyl propionate (2.58),n-propyl propionate (0.43),n-butyl propionate (0.04).and methane (6.84)(197). ' Assuming reported "typical" methane levels are formed in this experiment ; ethanol carbon efficiency among liquid products is 63%. Methyl propionate (1.3), n-propyl propionate (0.5).n-butyl propionate (0.04). and ethylene glycol dipropionate (0.04)(197). Methanol ( I .47),n-propanol (0.21).n-butanol (0.17).methyl acetate (0.14).ethyl acetate plus methyl propionate (0.26).propyl acetate plus ethyl propionate (0.13), and methane (6.67)(204). ' Assuming reported "typical" methane levels are formed in this experiment; ethanol carbon efficiency among liquid products is 52%. Methanol ( I .07,n-propanol (0.30).n-butanol (0.22),methyl acetate (0.08), ethyl acetate plus methyl propionate (0.36).and n-propyl acetate plus ethyl propionate (0.08) (204). " Methanol ( I .17).n-propanol(O.83), n-butanol (0.10), methyl acetate (0.29).ethyl acetate (0.83). n-propyl acetate (0.35).and methane (8.22)(203).

ETHYLENE GLYCOL AND ETHANOL FROM H 2 AND CO

39 1

and selectivities to the ethyl ester are <40%. The methane observed in these and similar reactions appears to be derived from methanol as a by-product of the homologation process. Ruthenium-catalyzed CO hydrogenation has also been reported in a reaction medium consisting of molten quaternary phosphonium salts, generally halides (199,200) (Table XV, Expt. 8). Major products are rnethanol, ethanol, ethylene glycol, and glycol ethers. Although studies of the effects of a number of reaction variables (pressure, H,/CO ratio, and Ru concentration) are reported (199), these results must be interpreted with caution. The reaction times are long (6- 18 hr), and product concentrations become high during the process. This not only leads to extensive secondary reactions, but can also change the catalytic activity throughout the experiment. One-point rates determined by final product concentrations after a fixed reaction time in such runs are unlikely to reflect accurately the chemical responses to changes in reaction variables. The addition of certain cocatalysts to ruthenium catalytic systems is reported to improve the selectivities for certain products. Rhenium and manganese additives are claimed to improve the selectivity to methanol (201), and cobalt (202), titanium, or zirconium additives (203) to enhance the ethanol selectivity when added to quaternary phosphonium salt systems. These additives appear to act by inhibiting or enhancing secondary conversions during the long reaction times employed. Table XVI lists some of the reactions (Expts. 6-8) which are reported to give increased yields of ethanol. It is evident that selectivities to this product remain quite low, methane productivities are high, and numerous other products are formed by these catalyst systems. The addition of rhodium complexes to ruthenium catalytic systems has been found to provide increased selectivity to the ethylene glycol product (198, 204, 205, 2051). Rhodium complexes themselves, of course, can produce ethylene glycol under these conditions. Combination of the two metals, however, affords higher activity to this product than would be expected based on the sum of the individual activities, thus indicating synergistic action (205). Although mechanistic details have not been presented, it has been suggested that the ruthenium component is involved in an early stage of CO reduction, such as metal formyl formation, and the rhodium component is important in later product-forming steps (205). Reactions with iron (20%) and rhenium (205c) additives have also been described. The effects of several reaction variables on the activity and selectivity of the iodide-promoted catalyst in nonreactive solvents have been studied (190-191). These reactions have been generally performed with relatively low conversion under conditions where constant rates to the primary products are observed; secondary reactions are usually minor. Increases in

39 2

B. D. DOMBEK

reaction temperature are found to enhance the rates to both methanol and ethylene glycol, the two major primary products (Fig. 17). The observed Arrhenius activation energy for methanol formation (18 kcal/mol) is substantially higher than that for the production of ethylene glycol (9 kcal/mol). Thus, selectivity to the two-carbon product may be greatly increased by operating at lower temperatures, but this enhanced selectivity is achieved at the expense of catalyst activity. As the temperature continues to be increased, a point is reached beyond which the catalytic activity does not increase, or actually declines. The temperature at which this complex behavior is initially observed may be in the range 220-270°C and above, and is a function of the reaction pressure. Increased reaction temperatures are permitted by the use of higher pressures. Temperatures slightly above the onset of this behavior do not appear to cause irreversible changes in the catalytic system, although such changes (to be described below) are observed at sufficiently high temperatures. Reaction pressure has a dramatic effect on the rate of product formation in the halide-promoted ruthenium system. As shown in Fig. 18, the dependence of the glycol-forming reaction on total HJCO pressure is approximately fourth-order, whereas that for the methanol-producing reaction is

Y 0,

0"

Methanol

20

C 0

u

10

Ethylene Glycol

0

I-

2 ( 2 6 0 ' ) (230')

(2OOOC)

I

1.90

2.00 2.10 T - ' x lo3

2.20

FIG.17. Effect of reaction temperature on methanol ( E , = 18 kcal/mol) and ethylene glycol (E. = 9 kcal/mol) formation rates by an iodide-promoted ruthenium catalyst (191). Reaction conditions: 75 ml 18-crown-6 solvent, 15 mmol Ru, 60 mmol KI, 850 atm, HJCO = 1.

ETHYLENE GLYCOL A N D ETHANOL FROM H 2 A N D CO

393

Pressure, MPa

FIG. 18. ERect of pressure on methanol and ethylene glycol formation rates by an iodidepromoted ruthenium catalyst (191). Reaction conditions: 75 ml N-methylpyrrolidone solvent, 15 mmol Ru, 45 mmol NaI, H,/CO = I , 230 C. I MPa = 9.87 atm.

somewhat above third-order. This is markedly different behavior from the first-order or slightly higher dependence observed for the unpromoted ruthenium system. Individual dependences of product formation rates on H2 and CO partial pressures are not precisely known, but the evidence suggests that the dependences are approximately equal (176). Reactions pressured further with inert nitrogen gas also show increases in rates to both methanol and ethylene glycol (191). This effect may be attributable to a negative volume of activation for the CO reduction process, and may be responsible for some of the observed dependence on synthesis gas pressure. Because of this effect and the fact that the activities of CO and H2 in solution are not known under the various reaction conditions studied, the pressure dependences observed on changing H2 and CO partial pressures can indicate only very approximately the kinetic orders in these reagents. An unusual effect of the catalyst concentration on product rates has been observed. Increasing the concentrations of both the iodide promoter and the ruthenium catalyst precursor while holding their ratio constant leads to

394

B. D. DOMBEK

Metha no1

0

I-

Glycol

0.02

0.040.060.10

0.20

0.4(

CRuI, M o l a l

FIG. 19. Effect of catalyst concentration on rates to methanol and ethylene glycol by an iodide-promoted ruthenium catalyst (191). Reaction conditions: 75 ml of N-methylpyrrolidone (m) or sulfolane (0) solvent, [KI] = ~ [ R u ]850 , atm. 230°C. H,/CO = 1.

increased activity on a metal atom basis for the ethylene glycol product (191). The normalized activity for methanol either remains constant or declines, depending on the solvent employed, as shown in Fig. 19. Similar observations have been made in acetic acid solvent with a RuC1,-Bu,POAc catalyst (171). For a process first-order in the catalyst species, the activity per metal atom would remain constant with changing catalyst concentrations. The practical consequence of this result is that higher concentrations of the catalyst form the glycol product with substantially higher selectivity and activity than lower concentrations. When compared to the rhodium catalytic system, it can be seen that under identical conditions of temperature and pressure the iodide-promoted ruthenium system produces ethylene glycol at a comparable or somewhat lower rate. However, the rate of methanol formation is substantially higher than for the rhodium system. Thus, the overall activity of this ruthenium system is higher than that of the rhodium-based system, but the selectivity to the two-carbon product is lower.

C. SOLVENTS AND PROMOTERS Addition of various promoters to ruthenium-containing solutions can increase the overall rate of CO reduction, but the most remarkable effect is the change in selectivity to ethylene glycol. The effects of several potassium salts are illustrated in Table XVII. The acetate, phosphate, and fluoride

ETHYLENE GLYCOL AND ETHANOL FROM H2 AND CO

395

TABLE XVll Ruthenium-Calalyzed CO Hvdrogenarion wiih Porassiuwi Sulr Promoters","

Anion

Ethylene glycol rate (hr- I )

Methanol rate (hr- ' )

None Acetate Phosphate Fluoride Chloride Bromide Iodide

n.d.' 0.25 0.32 0.17 1.1 2.9 7.2

2.31 9.3 10.0

7.5 6.6 13.6 33.7

Data from Ref. 190. (Copyright 1981 American Chemical Society. Adapted with permission.) Reaction conditions: 75 ml sulfolane solvent, 6 mmol Ru (added as Ru,(CO),,). 18 mmol potassium salt, 850 atm, HJCO = I , 230 C. Not detected.

'

salts are seen to increase the rate of methanol production and promote the formation of small amounts of ethylene glycol. Chloride, bromide, and iodide salts are even more effective, particularly for the glycol product. Experiments were also carried out to determine the effect of the promoter cation on catalytic rates and selectivities. As shown in Table XVIII, all of the alkali metal iodides investigated give similar results in sulfolane solvent. TABLE X V l l l Ruthenium-Catalyzed CO Hydrogenation with Jodide Saltsash

Cation

Li Na+ +

K+ cs

+

PPN H + d.e

+

Ethylene glycol rate (hr- I ) 8.27 8.77 7.17 7.00 8.77 <0.3

Methanol rate (hr-I) 48.7 39.7 50.7 40.0 47.7 <0.3

Data from Ref. 191. Reaction conditions: 75 ml sulfolane solvent, 3 mmol Ru [added as Ru,(CO),,], 18 mmol iodide salt, 850 atm, HJCO = 1,230"C. Bis(tripheny1phosphine)iminium cation. 6 mmol HI (added as 12), 6 mmol Ru. Data from Ref. 193. a

B. D. DOMBEK

396

Even the large, delocalized PPN’ cation affords similar results. However, HI or species which can generate it under reaction conditions are not effective promoters in this solvent. The addition of increasing amounts of iodide salts to ruthenium-containing solutions provides an interesting effect, as shown in Fig. 20. Small amounts of ionic iodide cause the ethylene glycol rate to increase with a dependence somewhat greater than first-order. At an I-/Ru ratio of approximately 0.5, however, this dependence changes to about 0.45 order, which is maintained for salt concentrations up to the solubility limit at reaction conditions. The methanol rate dependence on iodide concentration appears to remain quite constant, at about 0.6 order. Thus, increased rates to ethylene glycol can be obtained by increasing the iodide concentration, but the selectivity is adversely affected at higher salt concentrations. The increased rates caused by higher levels of iodide promoter do not appear to be the result of a general salt effect or cation effect, since addition to iodidecontaining systems of a chloride salt causes only the smaller rate increases expected based on its lower individual promoter activity (191). The iodide promoter effects seen in Fig. 20, and some of the catalyst behavior to be described below, can be partially understood in terms of the ruthenium chemistry involved. Iodide salts have been found (191) to react

“““F 400

200

1

P

/Methanol Slope = 0.60

P Ethylene Glycol Slope = 0 45

NaI ,mmol

FIG. 20. Effect of increasing iodide promoter concentration on rates to methanol and ethylene glycol by a ruthenium catalyst. (Adapted from Ref. 190 with permission. Copyright 1981 American Chemical Society.) Reaction conditions: 75 ml sulfolane solvent, 6 mmol Ru, 850 atm. H,/CO = I 230 C.

ETHYLENE GLYCOL AND ETHANOL FROM H 2 AND CO

397

with Ru,(CO),, or Ru(CO), under H2 or H,/CO to yield two rutheniumcontaining products :

+ 31- + H, -+2[HRu,(CO),,]- + [Ru(CO),I,]- + IOCO

(59) This redox process involves conversion of a Ru(0) species into both oxidized and reduced products. These ruthenium complexes are observed in reaction solutions after catalysis, and high-pressure infrared spectroscopic studies to be described later also indicate that they are present under reaction conditions. The amount of I - necessary to convert the ruthenium catalyst precursor into these two complexes according to (59) is given by the I-/Ru ratio of 0.43. Thus, the break in the ethylene glycol rate plot of Fig. 20 at an I-/Ru ratio of about 0.5 may be explained as being the point at which sufficient I - has been added to convert all of the ruthenium catalyst precursor into the species present in active catalyst solutions. Reaction solutions with less iodide contain unconverted Ru(CO), , which does not produce ethylene glycol. However, methanol is produced by the Ru(CO), catalyst precursor, so the methanol rate plot in Fig. 20 does not drop as sharply as that for ethylene glycol below this I-/Ru = 0.43 point. The fact that excess I - beyond this stoichiometric point enhances the formation of both methanol and ethylene glycol indicates that it serves a promoter role beyond the reaction shown in (59). Its further function will be considered later. The two ruthenium complexes produced by (59) have been studied for their individual catalytic activity (190, 191). The [Ru(CO),I,]- species has no catalytic activity for CO hydrogenation either with or without iodide salts. This complex is observed unchanged in such solutions after reaction. The [HRu,(CO),,]- complex has a low activity in the absence of iodide salts which increases upon the addition of iodide. (This complex also remains unchanged after such reactions.) However, a combination of the two ruthenium complexes is significantly more active, particularly when present = 2. Ratios different from in the ratio of [HRu,(CO),,]-/[Ru(CO),I,]this give catalytic solutions of lower activity, as shown in Fig. 21. Although this figure shows only the response of the rate to ethylene glycol, a similar response is seen in the methanol rate. The activity of this system can then be related to the average ruthenium oxidation state present during catalysis (193). A highly oxidized ruthenium species is not active (nor is a highly reduced complex) in the absence of reagents which promote reduction (or oxidation). The ineffectiveness of hydrogen iodide as a catalyst promoter can now be rationalized, since HI reacts with RU,(CO)~, or Ru(CO), to produce Ru(CO),I, or [Ru(CO),I,]-, Ru(I1) halide complexes which are stable under H 2 / C 0 and inactive as catalysts (191). These oxidized halide complexes can 7Ru(CO),

398

HRu,(CO),,-,

mmol

FIG. 21. Effect on ethylene glycol rate of changing the ratio of [HRu,(CO),,]- to [Ru(CO)31,]-. (Adapted from Ref. IYO with permission. Copyright 1981 American Chemical Society.) Reaction conditions: 75 ml sulfolane, 0.86 mmol PPN[Ru(CO),I,], PPN[HRu,(CO),,l as indicated, 36 mmol Nal, 850 atm, HJCO = I , 230 C.

be converted into an active catalyst, however, by reaction with H, (or H,/CO) in the presence of a base or hydrogen halide acceptor. The presence of these additives allows reduction of the oxidized species to [HRu,(CO), and generates ionic iodide. For example, addition of appropriate amounts of NaOH to solutions of [Ru(CO),I,]- can convert this inactive complex into an active catalytic system. The solvent, if it is sufficiently basic, can also perform this function. Thus, it is reported that Ru(l1) halide complexes such as [Ru(CO),CI,],, or mixtures of Ru,(CO),, and HCl, are precursors of active CO reduction catalysts in NMP solvent (196).Such catalyst precursors are inactive in less basic solvents, but NMP may be sufficiently basic [pK,, + = - 0.9 (48)] to permit the formation of reduced ruthenium species apparently essential for CO hydrogenation activity. Phosphine oxides are also solvents whose basic properties may be involved in catalytic reactions. Addition of HI or I, to Ru,(CO),, in phosphine oxide solvents gives catalyst solutions active for CO hydrogenation, but with remarkable selectivity and activity for ethanol production (193). Tri-npropylphosphine oxide [pKBH+% -0.5 (48)] is a sufficiently strong base to promote the reduction of Ru3(CO),, under H,/CO according to the following reaction : Ru,(CO),,

+ R,PO + H , * [ R , P O H ] + [ H R U ~ ( C O ) ~ , ] -

(60)

The addition of HI oxidizes some of the [HRu,(CO),,]~to [Ru(CO),I,]-, the other species necessary for optimum activity. The resulting increase in catalytic activity is shown in Fig. 22. The addition of further HI can cause reduced catalytic activity as increasing amounts of [Ru(CO),I,]- are formed at the expense of [HRu,(CO), ,I-. These catalyst solutions derived from

ETHYLENE GLYCOL AND ETHANOL FROM H2 AND CO

399

FIG.22. Etfect on catalytic activity to methanol (0) and ethanol ( A ) of adding HI to a Ru,(CO, catalyst in tri-n-propylphosphine oxide solvent (193). Reaction conditions: 75 ml solvent, 6 mmol Ru, 850 atm, H,/CO = I , 230°C.

phosphine oxides, ruthenium complexes, and HI may owe their effectiveness for ethanol production to the acidity of the [R,POH]+ counterion generated in solution. This cation appears to possess the appropriate acidity to catalyze the homologation of methanol to ethanol, while still being capable of coexisting in solution with reduced ruthenium species apparently involved in the CO reduction process (193). When, instead of HI or 1 2 , alkali metal iodides or other stable iodide salts are added to ruthenium carbonyl solutions in phosphine oxide solvents, catalyst solutions are generated which are comparable in rate and selectivity to such systems in sulfolane or NMP solvents. In these catalytic mixtures the major counterion is the alkali metal cation or other cation provided to the system, and not an acidic species which can participate in secondary reactions. Activities of KI-promoted catalyst systems in a number of solvents are shown in Table XIX. Sulfolane, NMP, [18]-crown-6, and tri-n-propylphosphine oxide solvents provide similar rates to ethylene glycol, but differences in the rates of methanol formation appear to be significant. (This is also apparent in Fig. 19.) These are all polar solvents which can dissolve the ionic promoters and ruthenium species present during catalysis. Other potential solvents give considerably inferior results. Tetraglyme, for example, is a much less effective solvent than [18]-crown-6. This may be attributable to the lower solubility of KI in the open-chain polyether. Hydroxylic solvents are also inferior solvents, although they are capable of dissolving the promoter and catalyst species. This may be the result of a

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B. D. DOMBEK

TABLE XIX Ruthenium-Catalyzed CO Hydrogenation with K l Promoter in Diflerent Solvents",h Ethylene glycol rate (hr-')

Solvent Sulfolane NMP Pr,POd 18-Crown-6

Tetraglyme n-Butanol Water

7.3 8.0 8.3 7.5 0.87

Trace Trace

Methanol rate (hr-') 45 66 73 63 12 2.7 3.8

" Data from Ref. 191. Reaction conditions: 75 ml solvent, 6 mmol Ru [added as Ru,(CO),,], 18 mmol KI, 850 atm, HJCO = 1,230'C. N-Methylpyrrolidone. Tri-n-propylphosphine oxide.

chemical reaction involving the hydroxyl group, a subject which will be addressed below. As described previously, carboxylic acids may be used as solvents for halide-or Lewis base-promoted CO reduction. However, these solvents have the disadvantage of forming the usually less desirable carboxylate esters rather than free alcohol products. Rate or selectivity advantages are not apparent in these solvents. Reactions conducted in molten quaternary phosphonium salts require no other solvent (199). This material serves as both promoter and reaction medium. Care must be exercised in choosing the salt in such a reaction, since any decomposition could lead to products such as trialkylphosphines and alkyl halides which are expected to be deleterious to catalyst performance. Tetrabutylphosphonium bromide is reported to provide a stable catalyst medium which can be recycled (199, 200), but other related salts show evidence of thermal decomposition during catalytic reactions. Experiments in tetrabutylphosphonium acetate, for example, are found to produce large amounts of methyl and ethylene glycol acetate esters (199).

D. CATALYST STABILITY Unpromoted ruthenium catalysts have been shown to become unstable toward metallization with increasing temperature and/or decreasing CO pressure. lonic promoters, especially halides, are found to provide a large stabilizing effect toward precipitation of metal, and significantly higher

ETHYLENE GLYCOL A N D ETHANOL FROM H2 A N D CO

40 1

temperatures can be attained in such systems before any metal formation is noted (191). As noted previously, however, catalytic activity may not continue to increase with temperature, even though the solution remains homogeneous. Rate maxima can be observed at temperatures well below those which give any evidence of metal formation. Temperatures slightly above such rate maxima do not appear to cause irreversible changes in the catalyst system. However, at sufficiently high temperatures (usually 260-290°C, depending on the pressure and catalyst concentration), catalyst transformations occur which are essentially irreversible but do not involve metal precipitation. The major ruthenium species observed in such deactivated solutions is a carbido cluster, [Ru,C(CO),,]~-, which is very stable toward metal formation. This cluster is essentially inactive as a catalyst for CO hydrogenation ( 1 9 4 , and is only slowly reconvertible to an active catalyst system under the proper conditions of temperature and pressure. Thus, initial catalyst deactivation in this system does not involve any loss of catalyst components from solution, as has been observed for the other catalyst systems previously described. There is no evidence of the formation of the inactive carbido cluster under normal catalytic conditions.

E. MECHANISM Since knowledge of the catalyst species present during reaction is basic to interpreting kinetic observations, several studies have been devoted to obtaining such information in this system ( I 91). Iodide-promoted catalyst sohtions, after being returned to ambient conditions, contain [HRu,(CO), ,]and [Ru(CO),I,]- in approximately a 2: 1 molar ratio, apparently formed according to (59). Both complexes are detectable by infrared spectroscopy, and the hydride cluster has been observed by 'H NMR. The [Ru(CO),I,]species has been detected in catalyst solutions by 99Ru NMR (191, 206). Additionally, both complexes have been isolated from solutions after being subjected to catalytic conditions. Studies of I-/Ru stoichiometry previously discussed and shown in Fig. 20 suggest that these two complexes, or at least a catalyst composition of the same stoichiometry, are present during catalysis. Studies of active solutions during catalysis by high-pressure infrared spectroscopy have also confirmed the presence of these complexes (191). Under 544 atm of HJCO at 230°C in sulfolane solvent, the infrared absorptions for the carbonyl ligands of both complexes are observed clearly. No other carbonyl absorptions are evident. Samples have also been withdrawn from catalytic reactions and cooled immediately to low temperature before analysis by infrared spectroscopy; these solutions also are found to contain only [HRu3(CO),,]and [Ru(CO),I,]-.

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B. D. DOMBEK

The observed ratio of [HRu,(CO),,]- to [Ru(CO),I,]- in solutions after catalysis is sometimes found to vary from the 2 : 1 ratio shown by (59). This may be expected if acids or bases (e.g., a basic solvent) are involved in oxidation or reduction processes, which can interconvert the two; such equilibria can change with pressure (19.3). Nevertheless, these two species are normally observed to be stable under catalytic conditions, and a combination of the two is found to provide the optimum catalytic rates (e.g., see Fig. 21). Catalyst solutions derived from nonhalide salt promoters are presumed to contain [HRu3(CO),1]- and an oxidized ruthenium species analogous to [Ru(CO),I,]-, although no detailed studies of such systems have been reported. The rates to methanol and ethylene glycol have qualitatively similar dependences on pressure (Fig. 18) and promoter concentration (Fig. 20), but somewhat different dependences on catalyst concentration (Fig. 19). The latter study shows that the ethylene glycol-producing reaction is more highly dependent than the methanol-forming reaction on processes intermolecular in catalyst components. There appears to be no evidence that all of the methanol and ethylene glycol are formed from a common precursor. Indeed, since methanol is known to be produced by unpromoted ruthenium catalysts, this product could be formed by several independent pathways in the promoted system. Any mechanism proposed for the iodide-promoted (or, more generally, Lewis base-promoted) catalytic system must account for the requirement of the two ruthenium complexes for optimum activity, the relatively high dependence on HJCO pressure, and the fractional dependence on salt promoter concentration. Since both [HRu3(CO),,I- and [Ru(CO),I,]- are involved in catalysis by the iodide-promoted system, it appears probable that they, or complexes in equilibrium with them, interact in some manner related to catalysis. A possible mechanistic scheme involving both ruthenium catalyst components could involve hydride transfer from an anionic ruthenium hydride complex to a carbonyl ligand on an oxidized ruthenium species. The [HRu3(CO), anion has been shown to behave as a hydride donor under some circumstances (207). The higher-oxidation-state species [Ru(CO),I,]- possesses a negative charge and therefore is not expected to be easily attacked by a hydride reagent, but this complex may be capable of undergoing the following iodide displacement reaction under high CO pressure : [Ru(CO),I,]-

+ CO * Ru(CO),I, + I -

(61)

The equilibrium must lie far toward the anionic complex, which is observed under catalytic conditions. The Ru(CO),I, complex does indeed react rapidly with 1- according to the reverse of (61). The carbonyl ligands in this

E T H Y L E N E G L Y C O L AND ETHANOL. FROM H2 AN11 ('0

403

neutral complex are expected to be quite susceptible to attack by nucleophiles, including hydride. Nevertheless, a mechanistic scheme involving reaction of this species with [HRu,(CO), ,I- does not appear to yield kinetic behavior consistent with that observed (1Y1). A scheme more consistent with the observed results is generated if [HRu,(CO), 1 ] - is also postulated to be involved in the following equilibrium process : [ H R u , ( C O ) , , ] ~+ 3CO S [IHRu(CO),]-

+ 2Ru(CO),

(63)

This equilibrium must lie toward the cluster, according to high-pressure infrared observations. A model for this reaction is the fragmentation of the analogous [HFe,(CO), ,I- cluster, which is observed to proceed to completion under CO pressure (208, 210). Since Ru(CO), formed by the equilibrium of (62) is expected to react rapidly under catalytic conditions with H2 and 1- according to (59), a net stoichiometry for fragmentation of during catalysis is as follows: [HRu,(CO), [HRu,(CO),,]-

+ :HI + :CO + 21-

+;[HRu(CO)J

+ :[Ru(CO),IJ

(63)

Combination in a catalytic scheme of this reaction, which generates [HRu(CO),]-, with reaction (61), which generates Ru(CO),I,, gives the cycle shown in Fig. 23. Transformation of Ru3(CO),, according to (59) provides [HRu,(CO), 1 ] - and [Ru(CO),I,]-, the stable species during catalysis. The outer circle represents the transformation of the reduced

f 517 I -

FIG. 23. Scheme showing possible interconversions of ruthenium complexes in an iodidepromoted system during catalysis (191).

404

B. 1). DOMBEK

species, whereas the inner circle concerns the oxidized ruthenium complex. Interaction of [HRu(CO),]- with Ru(CO),I, is shown to be involved in product formation, and this interaction will be discussed in more detail below. Assuming that the rate-determining step in this scheme occurs at or shortly after the interaction of these two species, the scheme provides kinetic dependences in qualitative agreement with those observed : a relatively high dependence on H 2 / C 0 pressure, a fractional dependence on iodide concentration, and a dependence on catalyst concentration of greater than first order. Other observations are also consistent with the scheme of Fig. 23. Neither [HRu,(CO), ,I- nor [Ru(CO),I,]- alone (in the absence of promoter) possess catalytic activity for glycol formation, as expected based on the scheme. However, [HRu,(CO), J- in the presence of I - has some glycolproducing activity. This may be possible because the cluster can be fragmented in the presence of iodide to produce both [HRu(CO),]- and [Ru(CO),I,]-, a precursor of Ru(CO),I,, as shown in (63) and Fig. 23. This scheme also suggests a reason for the poor performance of hydroxylic compounds as solvents for this system. Water and alcohols are known to react with [HRu(CO),]-, producing H2 and [HRu,(CO), ,I- (209). Thus, one of the important catalytic intermediates in this system could be destroyed by reaction with a high concentration of hydroxylic compounds before it can carry out productive catalysis. Reactions of [HRu(CO),]- and Ru(CO),I, have been investigated at room temperature and below (191, 210). The instability of initial products generated, however, prevented a detailed study of the process. The use of [CpRe(CO),(NO)]+ (Cp = $-CsH5) as a model for Ru(CO),12 has provided some useful results. This rhenium complex, like Ru(CO),I,, contains electrophilic carbonyl ligands which should be susceptible to attack by nucleophiles. Comparison of the v( CO) frequencies in the two complexes suggests that these ligands should possess comparable reactivities, so the rhenium complex appears to be a reasonably accurate model for Ru(CO),12. Reaction of [CpRe(CO),(NO)] with borohydride reagents has been previously shown to produce a relatively stable formyl complex, CpRe(C0) .(NO)(CHO) (211). Indeed, [HRu(CO),]- is found to react similarly with [CpRe(CO),(NO)]+ at room temperature to generate CpRe(CO)(NO)(CHO) in a yield of 20-30%, apparently via a hydride transfer reaction (210). Also observed is CpRe(CO)(NO)H, presumably formed by a parallel reaction pathway not involving a formyl intermediate. The observed ruthenium product is [HRu,(CO), which appears to be formed according to the stoichiometry of the following reaction : +

+ [CpRe(CO),(NO)]+ [HRu,(CO), ,]- + 2CpRe(CO)(NO)(CHO) + CO

3[HRu(CO),]-

4

(64)

ETHYLENE GLYCOL AND ETHANOL FROM H2 AND CO

405

The [HRu,(CO), complex reacts with [CpRe(CO),(NO)]+ only relatively slowly, and by a pathway which does not involve nucleophilic hydride attack (210). Reaction (64) demonstrates the production of a metal formyl complex by intermolecular hydride transfer from a metal hydride which is expected to be regenerable from H2 under catalytic conditions. Further, it provides a plausible model for the interaction of [HRu(CO),]- with Ru(CO),12 during catalysis, and suggests a possible role for the second equivalent of [HRu(CO),]- which the kinetics indicate to be involved in the process (see Fig. 23). Since the Ru(CO), fragment which would remain after hydride transfer (perhaps reversible) from [HRu(CO),]- is eventually converted to [HRu,(CO), ,]- [as in (64)] by reaction with further [HRu(CO),]-, the second [HRu(CO),]- ion may be involved in a kinetically significant trapping reaction. A mechanism possibly involving intermolecular hydride transfer in this promoted ruthenium system is thus very different from the reaction pathways presented for the cobalt and unpromoted ruthenium catalysts, where the evidence supports an intramolecular hydrogen atom transfer in the formyl-producing step. Nevertheless, reactions following this step could be similar in all of these systems, since the observed products are essentially the same. Thus, a chain growth process through aldehyde intermediates, as outlined earlier, may apply to this ruthenium system also. The mechanism of ethanol formation by HI-promoted ruthenium catalysts in phosphine oxide solvents has been studied in some detail (193). Experiments with added 13CH30Hconfirmed that methanol is converted to ethanol, as well as n-propanol, rz-butanol, and methane by this system. ( N o evidence was seen for conversion of the methanol to ethylene glycol.) Kinetic experiments which could allow an assessment of the proportion of ethanol formed through intermediate methanol were not possible. A study of the product distribution as a function of reaction time (Fig. 24) showed that the amount of methanol in the system reached a plateau very early in the reaction, suggesting that secondary reactions which consume it are very significant. (Although upward curvature is expected in the plots for secondary products such as ethanol, the methanol concentration stabilizes so early in the process that essentially linear plots are obtained.) Comparison of the initial rate of methanol formation in these experiments with the sum of the rates to possible secondary products after the methanol concentration stabilizes confirms that these products could indeed be largely or entirely derived from initially formed methanol. The relatively high selectivity of this ystem for ethanol formation, even with very short reaction times and low Lonversions, thus appears to be due to a very active homologation system acting in combination with methanol-producing catalysts. Ruthenium systems with iodide-containing promoters have been previously shown to

406

B. D. DOMBEK

250 0 200

1

E

E 0

a1150

E

LL

100 3 U

2

a

50

0

I

0

01

0 2

0.3

04

05

0

Reaction Time, hr

FIG.24. Products of an HI-promoted ruthenium catalyst in tri-n-propylphosphine oxide, as a function of reaction time (193):( A ) ethanol; ( 0 )methane; (0) methanol; (0) ethylene glycol; (A)n-propanol. Reaction conditions: 75 ml solvent, 15 mmol Ru, 15 mmol HI, 850 atm, H,/CO = I , 230 C.

convert methanol to ethanol under H,/CO (212-215). Under conditions of these studies, however, it is improbable that a significant amount of CO reduction to methanol would have occurred, since the concentration of reduced ruthenium species appears to be low. VI.

Other Catalysts

The preceding discussion has dealt with synthesis gas conversion by homogeneous catalysts based on cobalt, rhodium, and ruthenium. Complexes of many other metals have also been investigated as potential catalysts. It is clear that no known homogeneous catalyst based on other metals is as active as the Co, Rh, or Ru catalysts described. A significant problem when testing metal complexes of potential low activity is the possibility that traces of these active metals remaining in the reaction vessel from earlier experiments may be responsible for any activity observed. The activity of trace rhodium levels has been previously mentioned (173, 174). Thus, catalytic reaction vessels must be scrupulously clean to properly assess catalytic activities. Other possible problems are presented by solvent reactivity or decomposition, and catalyst heterogenization. Complexes of a variety of metals, including Pd, Pt, Cr, Mn, and Ir, were screened under about 1500 atm of HJCO and are reported not to produce

ETHYLENE GLYCOL AND ETHANOL FROM H2 AND CO

407

detectable amounts of polyhydric alcohols (27, 85). Possible activities for methanol or ethanol are not mentioned. Under about 2500 atm of H,/CO, carbonyl complexes of V, Cr, Mn, Mo, Re, and Ir are reported to produce very small amounts of methanol; Os,(CO),, was not active (43).The possible low activity of Mn,(CO),, for methanol formation has also been noted in another work (35). Complexes of Fe, Ni, Pd, Os, and Pt are reported to produce low yields of methanol and traces of ethanol under 2000 atm of H 2 / C 0 (39).Traces of ethylene glycol, in addition to methanol and ethanol, are noted for iridium catalysts. A claim of relatively high activity for an iridium catalyst (216) appears to be inconsistent with other results described here, and is perhaps complicated by rhodium contamination. Attempts to reproduce this result under identical conditions have not been successful (217). Keim et al. have found, however, that under certain conditions in pentane or toluene solvent, small amounts of ethylene glycol could be reproducibly formed (217). Reactions were generally performed at 230250°C and 2000 atm, and major products were methanol and methyl formate. However, the total rate of product formation was observed to be less than 1 hr-', and ethylene glycol was found to comprise less than 5 wt. % of the total liquid products. Under very high pressures (3300-3600 atm), catalysts based on Os, Ir, Fe, Pd, and Pt are reported to have low activities for HJCO conversion, largely to methanol (174). No ethylene glycol was detected, and only for Pt was ethanol observed. A description of catalytic activity by Fe and 0 s complexes (178) is likely to involve solvent decomposition. Reactions of Os,(CO),, with H 2 / C 0 in acetic acid are claimed to produce unspecified amounts of methyl acetate and ethylene glycol diacetate (172). Solutions containing copper compounds have been reported to form oxygenates from H,/CO, including ethylene glycol (218). However, amounts of ethylene glycol are small (much less than the amount corresponding to one turnover) and there is some doubt about the homogeneity of the system. The activities of the catalyst systems cited here appear to be very low. Under the circumstances, it is not certain that all of the catalysts are homogeneous systems based on the metals charged ; catalyst contamination can present a severe problem. Additionally, the predominant product in the reactions described, methanol, can be formed from H 2 / C 0by heterogeneous systems, and the metal reactor walls may participate in such reactions. Although it is possible that some of the catalyst systems mentioned here possess authentic homogeneous activity for methanol production under high H 2 / C 0 pressures, their abilities to produce ethanol and ethylene glycol are even less certain. The present evidence thus supports catalytic activity for hydrogenation of CO by homogeneous systems based on cobalt, rhodium, ruthenium, and possibly iridium. In the absence of more detailed studies, it can only be concluded that presently known homogeneous catalysts derived

408

B. D. DOMBEK

from other metals have perhaps minimal, but questionable activities for conversion of synthesis gas to oxygenates. Several metal carbonyl complexes have been investigated as potential CO hydrogenation catalysts in reactive Lewis acid solvents (23-25). In apparently homogeneous systems, Ir4(CO)lzin molten AICI,. NaCl(2: 1) and OS,(CO),~in BX3 (X = C1, Br) produced methane and higher saturated hydrocarbons under 1-2 atm of HJCO. Activities are low and oxygenates are not observed; they would react rapidly with the solvent. (Water byproduct formed in these systems will also decompose the solvents.) Halogenated hydrocarbons are indeed detected in some reactions. The possibility exists that these reactions are heterogeneous, although solids were not detected in the catalytic solutions. Other metal carbonyls also showed activity, but gave precipitates during the process. Interaction of the Lewis acid with the oxygen atom of a coordinated carbonyl ligand is postulated to be an important factor in determining the catalytic activity of these systems. Although these results are not directly applicable to the production of oxygenated products, the Lewis acid enhancement of CO activation may be useful in designing more active catalytic systems which could form oxygenates.

VII.

Conclusions

Because of the variety of catalyst responses to pressure, temperature, promoter levels, catalyst concentration, and other factors, a direct comparison of activities among different catalysts is not entirely meaningful. In general terms, however, it can be concluded that rhodium-based catalysts have shown the highest activity and selectivity to ethylene glycol. Halidepromoted ruthenium catalysts are found to possess comparable or even greater overall activities for synthesis gas conversion under similar conditions, but with rates and selectivities to the ethylene glycol product generally lower than for rhodium catalysts. Unpromoted ruthenium catalysts show relatively low activities, and very low selectivities to ethylene glycol. Cobalt catalysts appear to possess an inherent activity for synthesis gas conversion similar to that of unpromoted ruthenium catalysts under comparable conditions, but this activity is somewhat limited by the lower temperature stability limit of the cobalt catalyst. The selectivity to ethylene glycol found for cobalt can be substantial, especially at higher pressures. Selectivities to ethanol are relatively high for certain cobalt and ruthenium catalyst systems. In both metal systems, most of the ethanol observed is

ETHYLENE GLYCOL A N D ETHANOL FROM H z A N D CO

409

probably a secondary product formed by homologation of methanol. Homologation of ethanol to higher alcohols also occurs, but at a lower rate than the conversion of methanol; selectivity to ethanol can thus be quite high. The highest rates to ethanol have been found in halide-promoted ruthenium systems. The requirement of relatively high pressures to attain substantial catalytic activity (and in some cases, stability) in these systems is a disadvantage, but does not in itself disqualify this technology from commercial application. A number of processes, such as hydroformylation (58) and methanol carbonylation (219), have been successfully operated under conditions similar to those employed in the studies described here. Much of the pressure effect in these reactions appears to be kinetic rather than thermodynamic in origin ;thus, advances in catalyst performance at lower pressures are possible. The catalytic selectivity to ethylene glycol or ethanol in some of these catalytic systems is also an area for improvement. However, the major by-product, methanol, is a large-volume commodity chemical whose demand is expected to grow substantially in coming years. Experimental results from the catalytic systems studied can be interpreted as supporting a general conversion mechanism to the observed primary products, which are mainly methanol, ethylene glycol, and their formate esters. This reaction sequence may involve a formaldehyde intermediate, probably coordinated to a metal species, whose reactions determine the selectivity of the process. Higher aldehydes also appear to be precursors of the corresponding alcohols and longer-chain products. Details of the catalytic mechanism are certain to be different for the various catalytic systems discussed. As an example, metal formyl production, a probable early step in the general catalytic sequence for homogeneous CO reduction, is apparently accomplished by an intramolecular hydride migration process in the cobalt and unpromoted ruthenium systems, but may involve an intermolecular hydride transfer in halide-promoted ruthenium systems. Thus, there are limits in the generality of any single mechanism presented for homogeneous synthesis gas conversion and few guidelines of broad generality in the search to discover novel catalytic systems. Despite numerous screening studies, the literature contains little evidence that homogeneous catalyst systems based on metals other than Co, Rh, or Ru have significant activity for catalytic CO reduction. As seen for the known active catalytic systems, however, properties of solvents and additives or promoters can have enormous effects on catalytic activities. Solvents and additives can serve many roles in these catalytic systems. One important function of promoters in the Rh and Ru systems appears to be that of stabilizing metal oxidation states involved in catalytic chemistry. Other

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ADVANCES IN CATALYSIS. VOLUME 32

Cyclodextrins and Cyclophanes as Enzyme Models IWAO TABUSHI

AND

YASUHISA KURODA

Department oj’syntlietic Chemistry Kyoto University K j w t o . Jripan

I. Introduction.

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

. . . . . . . . .

,

.

.

.

A. Cyclodextrin . . . . . . . . . . . . . B. Cyclophane . . . . . . . . . . . . . 11. Basic Principles of Molecular Recognition . . . . . . A. X-Ray Analyses of Inclusion Hosts and Their Complexes B. Driving Force for Inclusion by Cyclodextrin . . . . 111. Enhancement of Binding and Catalysis by Host Design . . A. Multifunctionalization of Cyclodextrin . . . . . . B. Approach to Enzyme Model via Host Design . . . . 1V. Enhancement of Binding and Catalysis by Guest Design . . V. Conclusion . . . . . . . . . . . . . . . References and Notes . . . . . . . . . . . .

I.

. . . .

. . . .

. . . . . .

417 418 420 420 422 428 436 436 445 456 462 462

Introduction

Among many interesting and important fields of chemistry, biomimetic chemistry is now undoubtedly one of the most promising and productive fields. The increasing number of interesting reports clearly shows that biomimetic chemistry (bioorganic and bioinorganic chemistry) took off rapidly during the past several years and some successful applications of biomimetic chemistry to the synthetic, analytical, pharmaceutical, and industrial chemistries have been made. Since the biomimetic chemistry was born in the interface between organic (and inorganic) chemistry and biochemistry, it was quite natural that the first and major target of this area would be the construction of “enzyme models.” Enzyme has several unique characteristics-specific catalysis, regulation, biosynthesis, biodegradation, transport, etc. However, the “first generation” of enzyme models (or artificial enzymes) mostly deal with the simplest 417 Copyright & 1981 by Academic Press Inc All rights of reproduction #ndny lorm reserved ISBN 0-1?-007832-5

418

IWAO TABUSHI AND YASUHISA KURODA

catalytic activity. Thus, simple intramolecular catalysts, micelle catalysts, and polymer catalysts were used to afford a special local environment around a certain catalytic group, to enhance its reactivity. After many successes in the preparation of artificial enzymes having satisfactory reactivity ( I ) , chemists tired of continuing this rather routine trial to prepare artificial enzymes of the first generation and started to go further. Thus, more sophisticated enzyme characteristics have been investigated, such as substrate specificity or termolecular or multimolecular reaction. In the following enzymatic reactions: E

+S

E . S + Following catalytic reaction

(1)

where E denotes the enzyme and S denotes the substrate, formation of an E S complex practically determines the substrate specificity. Formation of the E . S complex is controlled by the combination of various types of intermolecular interactions between the enzyme and the substrates, as will be discussed in Section 11. This process becomes a kind of “trigger” for the following processes, since the substrate molecule and the catalytic functional group(s) of the enzyme are forced to take the most favorable position for the further reaction(s) in the E . S complex. Thus, for the construction of excellent enzyme models, it is essential to provide the molecule with a binding site of known or reasonably expected “shape” and a catalytic site consisting of an appropriate combination of the functional groups. From these purposes, inclusion compounds are chosen as the enzyme model. The present review will deal with two types of the most typical inclusion host molecules: cyclodextrins and cyclophanes. A. CYCLODEXTRIN

Cyclodextrins are doughnut-shaped, macrocyclic oligosaccharides constructed of glucose units linked by a tl(1 + 4) bond. Hexamer, heptamer, and octamer are the most common, often called the tl-, b-, and y-cyclodextrins, respectively (Fig. 1). Although cyclodextrin has been known as one of the oligoglucoses produced by Bacillus maceruns since 1891 (2), its spectacular behavior as the possible enzyme model was first observed by Cramer and his co-worker (3). Cyclodextrin has the following apparent merits as an enzyme model :

(1) Its chemical and physical properties are well known. (2) Its crystal structures (freeand complexed)are satisfactorilyestablished. (3) The stoichiometry of the inclusion processes is easily defined. (4) A new strategy was found recently by the authors for the specific introduction of desired catalytic functional group(s) on cyclodextrins.

419

CYCLODEXTRINS AND CYCLOPHANES AS ENZYME MODELS OH

\ -

:1I

0

0

03‘ ‘OH

‘0H HO

FIG.1. Chemical structure of cyclodextrins and numbering of the atoms.

Extensive studies on cyclodextrin as the enzyme model began in the mid-1960s. The “first generation” of cyclodextrin chemistry, where the groups of Cramer and Bender played the most significant roles, shows that cyclodextrin behaves as an enzyme-like catalyst in a mechanistic sense in a limited number of reactions such as ester hydrolyses. The kinetics follows the Michaelis-Menten-type equation (4) and very often covalently bound E . S complexes such as acylcyclodextrins are involved as key intermediates. However, some problems still remained that the actual catalytic activities (k,,, and/or l/K,,,) of cyclodextrins are very often much lower than those of native enzymes and generally the substrate specificity is low. In order to enhance catalytic activity and substrate specificity, additional functional groups should be introduced, as shown in pioneering works on enzyme models by the use of functionalized cyclodextrins carried out by Cramer (5) and Breslow (6).Imidazole and Ni(I1) oximate, respectively, are introduced to C D as catalytically active groupings for the hydrolysis of p-nitrophenyl acetate. In spite of these successful attempts for preparation of enzyme models of “second generation,” there still remains a serious barrier between artificial enzymes and native enzymes. This barrier may be overcome by further sophistication of artificial enzymes and this sophistication requires new modification techniques for cyclodextrin. In the past several years, however, there have been some remarkable developments in cyclodextrin chemistry. This review deals with these recent developments, which lead us to the “third generation” of cyclodextrin chemistry. And since a good review

420

IWAO TABUSHI AND YASUHISA KURODA

of this field which collected the reports up to 1977 already exists (7), the present article mostly describes later developments. B. CYCLOPHANE The term “cyclophane” is the general name and means a macrocyclic compound containing aromatic moieties within its cyclic skeleton. Chemical and physical properties of cyclophanes, especially those of the relatively small ring sizes such as [2,2]-paracyclophane, had been attracting considerable attention from organic and physical chemists since the 1950s. In spite of the relatively long history of cyclophane chemistry, enzyme model studies by use of cyclophane have only started recently in the author’s group. The properties of cyclophanes should be as follows: (1) Cyclophanes must have the appropriate binding sites at least 3-4 A shorter in diameter. (2) Cyclophanes must be water soluble, although this condition is sometimes accompanied by difficulties in synthesis. (3) Cyclophanes should have appropriate functional groups for catalysis. This is also accompanied by some synthetic difficulties. Cyclophanes has several great advantages in use as enzyme models. First, the design of the preparation of substituted cyclophanes is well established. Second, they are very stable, even more by stable comparison with corresponding cyclodextrins. Moreover, a cyclophane of certain cavity size is readily available. Because of these advantages, cyclophanes attract increasing attention from chemists. II.

Basic Principles of Molecular Recognition

Native enzymes show remarkably sharp substrate specificities. Actually, an enzyme can sometimes distinguish a slight difference of “shapes” between its specific and nonspecific guests, even if the electronic states of both are almost the same. The term “molecular recognition” is used to describe such ability of the molecule (enzyme). Inclusion compounds are supersimplified, but very appropriate systems for elucidation of the nature of the molecular recognition, since interactions between the inclusion host and a substrate are not more complicated than can be measured or even calculated. In this section, we will survey the recent results on the crystal structures of the inclusion compounds, and based on the information, the probable driving force for the inclusion is discussed in detail.

TABLE I

X-Ray Crystallography of Cyclodexrrin Complexes Cyclodextrin a a

B Y a a a a a a U

a a a U

a a a a a a a a a

B B B P P Y

Guest

Chemical composition”

(H,o)h (Hzo)’’ (H,o)h (H,Qh Kr Kr 1, (1)” (1)” MeOH CH,CO,K n-PrOH n-PrS0,Na DMSO. MeOH DMF 2-Pyrrolidone PhS0,Na p-IPhNH, m-NO,PhNH, p-N0,PhOH m-N0,PhOH p-1PhOH P-HOPhCO,H Methyl orange EtOH n-PrOH p-IPhOH p-NO,PhNHAc 2,S-(~O,),PhCO,H n-PrOH

a-CD. (H,O), a-CD.(H,O),,,,‘ B-CD. (HZO), Z C Y-CD (H,O) 1 7 ‘ a-CD ’ Kr0.,6 ‘ (H 2O)S .2SC a-CD.KrO.,,.(H,O),,,,‘ a-CD. I, .(H20), (a-CD), ‘ L ; 1 3 ~ I z ~ ( H z 0 8 (a-CD), ’ Cd,., . I , . (HzO)27 a-CD. CH,O, (H,O),‘ a-CD.C,H,O,K.(H,O),,, a-CD. C,H,O.(H,O),,B a-CD. C,H,SO,Na .(H,O), a-CD.C,H,SO~CH,O(H,O), a-CD. C,H,ON (H20), a-CD. C,H,ON .(H20), a-CD~C,H,O,SNa~(H,O),, a-CD.C,H,NI .(H,O), a-CD.C,H,N,O, .(H20), a-CD.C,H,NO,. (H,O), a-CD.(C,H,NO,),.(H,O), a-CD.C,H,IO~(H,O), a-CD C,H,O, (H,O), (a-CD), .C,,H,,0,N3SNa. (Hzo)19.5 ~-CD.C,H,O~(H,O), (P-CD)2‘(C,H,O), .(H20),4 (p-CD); .(C,HiOI), .(H,O),, B-CD. C,H,N,O, ’ (HzO), P-CD, C,H,0,N2, (H20),”d -d

Cyclophane (CP) 1,6,20.25-Tetraaza[6,1,6, I](CP) .(C, ,H ,). 4( HCI . H,O) paracyclophane N,N.N‘,N’”-Tetramethyl(CP). CHCI, I . 10,19.28-tetraaza[3.3.3.3]paracyclophane N, N,N’,N’,N”,N”,N”’,N”’-Octa- (CP) .( H ,O).. (BF,-), methyl-I, 10,19,28-tetraaza[3,3.3,3]paracyclophane

Space group

Ref. Y i0

i1 12

13 13 14 15

15 16 8

17 18 19 20 20 21 22 23 24 25 26 24 27 28 29 P1 PI

2Y

P4

30 31 32

P21

33

c2

34

P2,IC

34

c2

Chemical compositions of a-cyclodextrin (a-CD) = C36H6003O18-CD = C,,H,,O,,, y-CD = C ~ ~ H , ~ O ~ ~ . Uncomplexed- hydrated cyclodextrins. Guest molecules are largely disordered. Guest molecules are not traced.

and

422

IWAO TABUSHI A N D YASUHISA KURODA

A. X-RAYANALYSES OF INCLUSION HOSTS AND THEIR COMPLEXES 1.

Unsubstituted Cyclodextrins

Since the first X-ray analysis of the complex of a-cyclodextrin with potassium acetate was reported by Hyble et al. (8),the various types ofcyclodextrin complexes have been studied by X-ray crystallography as shown in Table I. In contrast with cyclodextrin, only two examples of X-ray analyses of cyclophanes as inclusion hosts are known, which are also shown in Table I. These X-ray studies reveal that both a- and P-cyclodextrins have a round and slightly conical structure with a narrow opening at the primary O(6)H hydroxyl side as shown in Fig. 2 and Table 11. The structures of a-cyclodextrin in “empty” (not complexed with a guest but hydrated) states are illustrated in Fig. 3. In Table I11 are listed typical parameters characterizing empty cyclodextrins. As shown in Fig. 3, in the “empty” a-cyclodextrin one (glucose 5 in Table 111) of the glucose units tilts to form a O(6) * . H20(in the cavity) hydrogen bond in the cavity. Based on the assumption of the strained macrocycle, the strain relief with a concomitant expulsion of so-called “activated water” from the cavity was taken as the major driving force of inclusion of organic molecules (35).Recently, another crystal form was found for a-cyclodextrin (form 111) ( l o ) , where the cavity maintains an almost ideal “ r o u n d shape without significant deformation. Form I11 binds 2.57 H20molecules in its cavity. Considering that manyfold crystal forms have been observed to result from slight changes in the conditions of crystal formation, there may still be more crystal forms possible. This polymorphism

- d2

F

d

3

--I

FIG.2. Schematic representation of the conical structure of cyclodextrin.

TABLE I1 Typical Dimensions of Cyclodexrrins"

a

B Y

5.6 6.8 8.0

4.2 5.6 6.8

8.8 10.8 12.0

7.8 7.8 7.8

Measured with the aid of CPK models. Notations are given in Fig. 2; d , , d 2 , and d3 are diameters of cavities measured at C(6). H(5). and O(2) positions, respectively.

FIG.3. The crystallographic structure of empty a-cyclodextrin

424

IWAO TABUSHI A N D YASUHISA KURODA

TABLE 111 Interplanar Angles and Diameters Characterizing Empry Cyclodextrin

Cyclodextrin a (form 1)

Glucose (n)

1 2 3 4 5 6 7 8 R,,, R,i,

0"

lp

I)"

IyJ

71 81 83 78 54 82

59 63 59 47 79 52

78 79 85 86 65 89

-

(A)d

Ref

P-

a (form 111)

Y

I)"

*h

I)"

*h

59 59 64 56 63 61

71 78 83 77 77 79 84

52 49 59 49 48 53 52

-

-

71 88 72 84 77 79 82 70

50 45 46 45 41 46 45 45

-

6.7 2.0

6.7 3.8

7.3 4.4

9.0 5.7

9

10

I1

I5

The angle between the mean plane [C(2), C(3). C(5), O(5) of each glucose unit and that of all O(4) atoms. The angle between the normals of two adjacent glucose planes. Values estimated from the figure in Ref. 11. Estimated maximum and minimum diameters at C(6) positions.

may suggest continuous and smooth conformation change of cyclodextrins in the aqueous solution (36). Empty /3- (11) and y-cyclodextrins (12) also take normal torus shapes, as revealed by X-ray crystallography, and no significant deformations are observed for these two cyclodextrins when they bind guest molecules. It is noteworthy that all empty cyclodextrins include water molecules in their cavities as shown in Table IV and Fig. 4. Since no water molecules were observed in inclusion complexes of cyclodextrins with organic guest molecules, it is evident that the expulsion of these water molecules in the cyclodextrin cavities is one of the important factors for formation of the inclusion complexes. The recent X-ray analyses of cyclodextrin inclusion complexes showed

CYCLODEXTRINS AND CYCLOPHANES AS ENZYME MODELS

425

TABLE IV Water Molecules in Cavilies of Empty Cyclodextrins

Cyclodextrin

Site of water

a (Form I)" a (Form 11)' a (Form Ill)'

B'

2 1 4 8

Yd

12

Occupancy factor

Average number of water

1 I

2 1

0.64

2.57 6.5

0.81' 1

12

* Ref. 12.

Ref. 9. Ref. 10. ' Ref. 11.

a

Average value.

additional interesting aspects of the inclusion phenomena. As shown in Fig. 5, the a-cyclodextrin-polyiodide complex has a typical channel-type crystalline structure and polyiodide molecules included in the channel have the structure I, * 1; (15, 37), although a "channel"-type structure cannot be formed in a membrane due to strong hydrophilicity of the parent cyclodext rin. Based on the X-ray analyses reported, the following generalizations may be made with regard to the cyclodextrin inclusion: (1) In the inclusion, cyclodextrins take a more symmetrical conformation than that in the empty state, i.e., 6 and Ic/ values (Table 111) lie in the ranges 70-90" and (360/n & 5)O, respectively, in the usual inclusion complexes. (2) The guest molecules having a benzene ring are included by cr-cyclodextrin at the very similar position (depth) of the cavity, independent of the different inclusion mode for para- and meta-substituted benzene, e.g., as seen in Fig. 6. (3) b-Cyclodextrin includes a benzene ring, often in a form that tilts toward the apparent C, axis (see 0 in Table V), most probably in order to gain better van der Waals contact.

(a)

(bJ

FIG. 4. Schematic representation of water molecules in cyclodextrin cavities: (a) a-cyclodextrin (Form I); (b) ,fl-cyclodextrin.

FIG. 5 . The crystallographic structure of the inclusion complex of a-cyclodextrin with iodine (IS).

FIG,6 . The inclusion of a benzene ring by u-cyclodextrin: (a) para-substituted benzene; (b) meta-substituted benzene.

TABLE V Typical Inclusion Modes of Benzene Derivatives b.v Cyclodextrins

Cyclodextrin U

a

j3

B

e Guest

(deg)

Ref.

p-HOdCO,H p-NO,+OH p-N024NHAc P-WH

6 9 30 36, 30"

24 24 30 29

' Values for two different inclusion modes in the single crystal.

CYCLODEXTRINS AND CYCLOPHANES AS ENZYME MODELS

427

2. Substituted Cyclodextrins In order to prepare certain “excellent” and sophisticated enzyme models, preparation of cyclodextrins having one or more functional groups is essential. At the same time, firm experimental evidence for their structures should be provided. Recently, X-ray analysis of b-cyclodextrin having a t-butyl-thio moiety at the C(6) position was reported as the first example of mono-substituted cyclodextrin (38).Interestingly, a single molecule of this cyclodextrin behaves as the host at its cavity, on the one hand, and also as the guest at the t-butyl moiety, on the other hand, as shown in Fig. 7. This interesting result strongly suggests that even for the substituted cyclodextrins the capacity of inclusion formation is much the same as for the parent cyclodextrin. Therefore, we may extend the basic concept of the structure of cyclodextrin inclusion to “molecular design” for the preparation of artificial enzymes having satisfactory substrate specificities and catalytic activities.

3. Cyclophanes Because of their relatively young history, there are only a few examples of X-ray analyses of water-soluble cyclophanes. The macrocyclophanes are

FIG.7 . The crystallographic structure of mono-6-f-butyl-thio-6-deoxy-~-cyclodextrin.

428

IWAO TABUSHI AND YASUHISA KURODA

known to take statistically a face-to-face conformation in the solution, and in this conformation, they are expected to have hydrophobic cavities of appropriate sizes. They have satisfactory freedom of conformational motion at room temperature. The rapid conformation change (AG 5 3.8 kcal mol-’) is also very favorable for the rapid incorporation of a guest molecule. Among the various types of macrocyclophanes investigated as artificial inclusion hosts by the authors (39, 40), 1,10,19,28-tetraaza-[3,3,3,3]paracyclophane systems and [2,2,2,2]paracyclophane having two or three ammonium substituents are interesting, since these systems have been successfully shown to behave as excellent enzyme models-to bind organic substrate with unique specificity in the aqueous solution (40,41).As shown in Fig. 8,the octamethyl derivative shows the parallelogram structure, where two benzene rings completely tilt into the hydrophobic cavity of this cyclophane. This watersoluble cyclophane of face-to-face conformation has a hydrophobic cavity of 3 x 7 W in an aqueous solution.

B. DRIVING FORCEFOR INCLUSION BY CYCLODEXTRIN

A variety of mechanisms are presented to interpret how the inclusion complexes are formed, which may be summarized as follows: (1)

The “hydrophobic” interaction is a kind of “combined” interaction comprising the following elemental interactions: (a) The van der Waals interaction between a guest molecule and the cyclodextrin cavity. (b) The entropy gain due to the destruction of water assembly around the guest molecule. (c) The entropy loss due to “freezing” motional freedom of the guest molecule in the cyclodextrin cavity.

( 2 ) The expulsion of “high-energy” waters which were found in the “empty” cyclodextrins (46,35). (3) Release (change, in general) of the strain energy of cyclodextrin (Y, 4 2 ) . (4) Hydrogen bonds between guest and cyclodextrin molecules (43). (5) The polar interaction between guest and cyclodextrin molecules ( 4 4 ) .

The theory of hydrophobic interaction is based on the large entropy loss and small enthalpy gain observed for a hydrophobic molecule to dissolve in water, which is ascribed to the growth of water assembly around the hydrophobic molecules ( 4 5 ) . In order to recover this entropy loss, molecules having large hydrophobic surfaces tend to associate by themselves in order to expose minimum total surface to the water (minimum water assembly),

CYCLODEXTRINS AND CYCLOPHANES AS ENZYME MODELS

429

FIG.8. The crystallographic structure of N,N,N’,N’,N”,N”,N”’,N”’-octamethyl-I, 10,19,28tetraaza[3,3,3,3]paracyclophane.

hopefully with maximum van der Waals contacts as shown in Fig. 9. The significance of the van der Waals interaction operating in inclusion complexes has been suggested by many examples based on X-ray analyses. The direct experimental evidence supporting the mechanism of hydrophobic interaction was presented by the flexibly (46)and rigidly (47) capped

L

+

Bulk

FIG. 9. Schematic representation of hydrophobic association: diagonal lines-water assembly; stippling-van der Waals interaction; cross-hatched lines-water assembly destroyed by the guest association.

FIG. 10. Schematic representations of the hypothetical thermodynamic process of inclusion.

CYCLODEXTRINS AND CYCLOPHANES AS ENZYME MODELS

43 1

cyclodextrins, which showed remarkably enhanced binding for the hydrophobic guest molecules. The first theoretical calculation to evaluate the energies for the inclusion was carried out by Harata ( 2 4 ,but it is much too simplified. The more comprehensive and quantitative, semiempirical evaluation of the thermodynamics of the inclusion process was presented in 1978 by Tabushi et a!. (48). The evaluation starts from the drawing of the hypothetical thermodynamic picture of the inclusion process based on the established crystal structure of the initial and final states, since thermodynamics is based only on these states (Fig. 10). The necessary thermodynamic quantities are estimated as follows: (1) Release of two water molecules from the cavity of cr-cyclodextrin (form I) (19) is accompanied not only by the loss of van der Waals interaction ( F a d w ) and hydrogen bonding ( - 2 AHH-bond), but also by the gain of motional freedoms of two water molecules as to translation (2Gans) and At the same time, a change in conforthree-dimensional rotation (2T0,,,-,,). mational energy of a-cyclodextrin is involved which is estimated by the use of Allinger’s method (49). (2) The transformation of the extruded gaseous water molecules into the liquid state results in the enthalpy change ( - A V a , ) and entropy change (ASras-liq 1.

FIG.11. Assumed change in “loose water assembly.” According to our preliminary estimation of the thermodynamic quantities involved the interaction in the formation of the “assembly” should be much weaker than hydrogen bonding.

432

IWAO TABUSHI AND YASUHISA KURODA

(3) An apolar solute (guest) is transferred from water (surrounded by water molecules) to the ideal gaseous state, leaving a structured cavity behind. The cavity then collapses with redistribution of the water molecules. Counting the number of water molecules using a space-filling model gives the free energy change of this step (50). Usually the water assembly is only partly destroyed during the inclusion process, depending on the shape and size of a guest molecule (Fig. 11). Such a partial stripping of water during the inclusion is also suggested from the comparison of free energy change of benzoate transferred from water to dioxane with that from water to inclusion (51). (4) Binding of the guest molecule to the cavity is accompanied by van der Waals stabilization (Htdw),which is calculated by using Hill's potential (52).Freedom of one-dimensional rotation is assumed to be allowed for the inclusion complex (S:o,,l-D,). Based on this somewhat simplified picture, overall enthalpy (AHinclusion) and entropy changes (ASinclusion) relevant to the inclusion are calculated by using Eqs. (2) and (3), respectively, where subscripts c, w, and g refer to inclusion complex, water molecules, and guest molecule, respectively : AHinclusion

"inclusion

+ (H:onf

= ( H l d w - Hydw)

=

AHap

(';ol(l-D)

-

-

Hr",nf)

-

AHElus~er

(2)

2HH-bond

':01(3-D)

+ 2 ASEs-1i-q

-

-

-

':am)

SEluster

-

2(s",(3-D)

- sEans) (3)

Calculated values thus obtained are listed in Tables VI, VII, and VIII, which are in good agreement with observed values. The results reveal many important aspects of the inclusion process of a-cyclodextrin. (1) The entropy loss due to the "freezing" of motional freedoms of the guest molecule ( - 50 to - 60 cal deg-' mol-') is to a large extent compensated by the entropy gain of other origin, especially from loss of the water cluster around the guest molecule in the bulk medium. The significant role of the water structure was also suggested by the interesting observation that the formation of the 8-cyclodextrin guest inclusion complexes in DMSO does take place but much more weakly than in H,O (K,",2,"/KEyso= 80- 180) (53). (2) Water molecules in cyclodextrin's cavity (H';b, + 2HH-bond = - 16 kcal mol - ' ) comprise "high-energy'' water, as suggested by Hingerty and Saenger (9),compared with those in the liquid phase (2 AH;aV,,= -20.92 kcal mol-I).

433

CYCLODEXTRINS AND CYCLOPHANES AS ENZYME MODELS

TABLE VI Calculated Values of Enrhalpy Change"

H L - H=)dUr H&r

Guest Benzene P-Iodoaniline Methyl orange

-

H L

- 3.15 - 10.09

+4.38 +4.38'

-8.35

+ 3.83'

AH^^,,,,,,

-2

+4.10

+7.08 + 6.7 1

- 2Hwbond

- 20.92 - 20.92

+ 12.2 + 12.2

- 20.92

+ 12.2

The units of enthalpy change are kcal mol-I, 25°C.

* The conformation of a-cyclodextrin was taken from Refs. 22 and 27. TABLE V11 Calculared Values of Entropy Change"

Benzene P-Iodoaniline Methyl orange

- 52.0 -62.9

+20.1 +34.8

+ 33.6 +33.6

+1.7 +5.5

- 61.6

f32.9

+ 33.6

- 1.1

The units of entropy change are cal deg- I mol-

', 25°C.

TABLE Vlll Free- Energy Change in Inclusion Complex Formarion" AGincIwion Guest Benzene p-lodoaniline Methyl orange

AHinslusion - 3.99 -1.35 -6.53

- TASinelusion Calculated

-0.51 - 1.64 +0.33

-4.50 - 8.99

- 6.20

Observed ~

- 5.9 -5.1

The units of free energy change are kcal mol-I, 25°C.

Strain relief seems to be ruled out as a major significant driving force for the following reasons: (a) No remarkable change of binding was observed (rather slightly strengthened binding was observed) on 6-0-methylation of cyclodextrins, which apparently prevents the tilting of glucose units as shown in Table IX (54). (b) Strong (often stronger) binding is observed for fl- and y-cyclodextrins (11, 12), which have no remarkably tilted glucose unit in

434

IWAO TABUSHI AND YASUHISA KURODA

TABLE 1X Binding of p-Nitrophenorate by 0-Methylated Cyclodextrin”

AH

Kdiss

Cyclodextrin

x

5.0 I .27

a-

Dodekakis-2.6o-meth yl-a-

15.9 12.4

8Tetradekakis-2.6o-methyl-8Heptakis-3methyl-8a

lo4 at 25°C

(kcal mol-’) -9.06 - 10.06

AS (e.u.)

+ 0.08

0.06

-15.7 - 16.8

* 0.2 0.3

-3.79 f 0.06 -3.30 f 0.08

+O.l +2.3

0.2 0.3

-

1 6b

-

From Ref. 54a. From Ref. 54b.

their “empty” state. (c) According to the thermodynamic calculation discussed above, strain relief is not important at all. However, the conformational change during the binding process may affect the binding rate (55). Hydrogen bonding between guest and host molecules usually does not seem important for the following reasons: (a) No drastic decrease in the substrate binding was observed for the cyclodextrin incapable of hydrogen bonding (54, 56) (see Table IX). (b) Guests incapable of hydrogen bonding to cyclodextrins, such as xenon, krypton, methane, ethane, and propane, are included by cyclodextrins relatively strongly (57). Polar interaction was recently suggested as the most important driving force for inclusion by cyclodextrins. This claim is based on the relatively high compensation temperature found in the AH-AS plotting and the linear relationships between 3C chemical shifts of C, carbon on cyclodextrin and AH of the inclusion (44). However, this is not true in most cases for several TABLE X Compensation Temperatures in AH-AS Relationships

Cyclodextrin a a a

8 a, B

8

Compensation temp. (K)

Ref.

405 265 422 284 319 330

44 61 62 63 64 65

CYCLODEXTRINS AND CYCLOPHANES AS ENZYME MODELS

435

reasons. (a) Some rare gases are strongly bound (57). (b) By statistical treatment of reported AH and AS (58), the compensation temperature becomes much lower than proposed, being satisfactory for the hydrophobic binding (59,60) (Table X). (c>The total free energy change including small additional factors is represented by Eq. (4): AG(tota1) = AG(vdw) + AG(G, water assembly)

+ AG(motiona1 freedom) + AC(water state) + AG(conformation) + AG(hydrogen bonding) + AG(po1ar)

(4)

If two substrates similar in structure but not polarity are compared, then AAG

=

AG(tota1 for substrate i) - AG(tota1,j)

=

AAG(po1ar)

(5)

If, for example, a series of benzene derivatives having a small or medium substituent are studied, a linear relationship with a polar parameter will be easily observed [Eq. (5)] as was reported by Gelb et a/. ( 4 4 , but obviously, it is not the essential discussion about the nature of the inclusion. It also should be noted that every elemental process involves entropy and enthalpy changes which are very large in magnitude, but opposite in sign, leaving only a small difference. Therefore, the calculation strongly demonstrates that the sign of the final (observed) values of AH and AS have little significance since it is simply the remainder after taking one large quantity from another. Thus, the hydrophobic interaction is concluded to be the most important driving force for inclusion, although some additional factors may contribute. ,Coulombic

interaction

7

recognition

Coordinate interact ion Multiple recognition

Simple

inclusion

FIG.12. Recognition by inclusion compounds.

436

IWAO TABUSHI AND YASUHISA KURODA

Hi669

CH2

FIG. 13. Multiple recognition by carboxypeptidase A : (----) hydrophobic recognition; Coulombic interaction; (......) hydrogen bond; (11111) coordination interaction.

(0000)

More sophisticated molecular recognition requires many of these additional interactions to operate simultaneously in order to distinguish delicate differences in the shapes of guests. The term “multiple recognition” may be introduced for such recognition of a guest molecule by the inclusion host through these multiple interactions. The multiple recognition is, of course, widely observed for native enzymes. As a typical example, the binding of a substrate by carboxypeptidase A is shown in Fig. 13 (66). The comparison of Fig. 12 with Fig. 13 clearly shows how one can approach artificial enzymes by the use of inclusion complexes. For the purpose of the preparation of artificial enzymes showing high selectivity and activity as native enzymes, it is absolutely necessary to design the host structure (or guest structure) according to this “multiple” recognition principle.

111.

A.

Enhancement of Binding and Catalysis by Host Design

MULTIFUNCTIONALIZATION OF CYCLODEXTRIN’

The most difficult problem in the design of the enzyme model clearly lies in the difficulty of specific introduction of the functional groups into the host skeleton. For cxample, preparation of an enzyme model by use of /bcyclodextrin often requires the introduction of two or more functional groups at certain positions among 7 primary (C,) and 14 secondary (C, and C,) reactive positions. Unless one expects an accidental success by the use of any nonspecific functionalization, it is inevitably necessary for the host design to solve these problems.

’ For review of the monofunctionaii;ration of cyclodextrin. see Ref. 7.

CYCLODEXTRINS AND CYCLOPHANES AS ENZYME MODELS

437

SCHEME I . First capped cyclodextrin.

In the recent progress of cyclodextrin chemistry, the first solution for bifunctionalization was established by the series of investigations on “rigidly capped cyclodextrins.” The first successful preparation of this type of capped cyclodextrin was achieved by a diphenylmethane-p,p’-disulfonatecap (Scheme 1) (47). The great advantage of the capping procedure lies in the excellent availability for the preparation of the variety of disubstituted cyclodextrins. Since I is activated at two sulfonate carbons toward the nucleophilic displacement, various types of functional groups can be easily introduced via SN reactions as shown in Scheme 2 (67). Similar double nucleophilic displacement can also be applied to a-cyclodextrin (67). Usually the capping reaction proceeds under mild conditions in good yield and separation and/or purification is easy. In the case of such a weak nucleophile as imidazole, however, the direct reaction of capped cyclodextrin with a nucleophile requires rather severe conditions (68).This problem was successfully solved by the use of a new intermediate, diiodocyclodextrin (2), as shown in Scheme 3 (69). Thus, it is concluded that the diiodocyclodextrin route is very convenient for the preparation of various types of difunctionalized cyclodextrin. /3-Cyclodextrin has 7 primary hydroxyl groups and the complex problem remains unsolved in the regiochemistry of bifunctionalization, i.e., the existence of A,B-, A,C-, and A,D-isomers (Fig. 14). The problem, first suggested by Breslow, is that capped /?-cyclodextrin ( I ) is the mixture of A,Cand A,D-isomers (68).

1

+

x:-B X = SPh

, N3 , NH2 , NHEt

SC(NH2):

NH2

, SH

SCH2CH2NH2

S C H ~ 2.M ~Double nucleophilic displacement of capped cyclodextrin.

438

IWAO TABUSHI AND YASUHISA KURODA 85

I 1

-

9O‘C

,

.

96hr

N ~ N H

W

+ 2

SCHEME 3. Diiodo route for bifunctionalized cyclodextrin (Q.Y.,quantitative yield).

A catalytic site of a native enzyme usually consists of two or more functional groups with a strict “spatial” arrangement. In order to mimic this unique situation, regiospecific functionalization of cyclodextrins should be achieved. The first success is the regioselective A,C-capping reaction of /3-cyclodextrin with benzophenone-m,m’-disulfonyl chloride to give a mixture of 78% A,C-isomer and 22% others. From the mixture, the A,C-regioisomer is readily isolated as a pure regioisomer by repeated reprecipitation. The purified isomer (3) was converted to the corresponding dideoxy derivative via the diiodide (Scheme 4)(70). The structure determination of regioisomers is discussed later. This regiospecific or highly regioselective capping can be understood as a result of the so-called “looper’s walk” of the capping reagent (70), i.e., when one reaction site of a bifunctional capping reagent is fixed by the OH group of an A-ring, the other reaction site searches for an OH group at the juxtaposition determined by its rigidity, length, and angle as shown in Fig. 15. Through looper’s walk capping, various types of capped cyclodextrins are successfully prepared as shown in Table XI. In an active site of a native enzyme, two (or more) functional groups are generally different; acid and base, reductant and oxidant, metal ligand and hydrogen bonding site, etc. Therefore, in order to mimick enzyme activity, unsymmetric di- (or poly-) substitution of the cyclodextrin cavity is necessary and important. Although

A , B- isomer

FIG.

A.C-isomer

A.D-isomer

14. Three possible isomers of capped cyclodextrins.

CYCLODEXTRINS AND CYCLOPHANES AS ENZYME MODELS

B+&

439

0

0g2

0 0 SOZCl

SO2Cl

3

SCHEME 4. Conversion of benzophenone-capped cyclodextrin to dideoxy derivative.

it is possible, in principle, to prepare unsymmetrically disubstituted cyclodextrins through classical modification techniques or through the ordinary capping technique described above, classical modification leads to a complex mixture of regioisomers of mono-, di-, tri-, tetra-, penta-, hexa-, and heptasubstitution products (75) and even from capped cyclodextrin, a random mixture of isomers should still be obtained, i.e., CDX,, CDXY, and CDY, in 25,50, and 25%, respectively. For the combination-specific synthesis of CDXY, a special new technique is necessary. For this purpose, the

\

\slow \

&!?-

i

polymer

\

7 2 " \

FIG. 15. Looper's walk mechanism of the capping reaction.

440

IWAO TABUSHI AND YASUHISA KURODA

TABLE XI Capped Cyclodextrin Family”

Ratio of AB. AC, and A D capping Capping reagenth

X

AB

Go@

AC

AD

Ref.

62

38

47

78

22

70, 71

0

I00

72

58

42

72

89

I1

72

50

50

73

2

98

73

-

-

74

X

X

Exclusive

&x The ratios of three isomers are determined by us. x = S0,CI.

“flamingo-type” capping was newly developed (76). As shown in Fig. 16, the flamingo cap (5), obtained by the oxidation of N-benzyl-N-methy ladinep,p’-disulfonate-capped cyclodextrin (4), has the two S, reaction sites of quite different activity toward a nucleophile, where the sulfonate carbon of the N-oxide side ( k , ) is 13 times more reactive than the methylene side

CYCLODEXTRINS AND CYCLOPHANES AS ENZYME MODELS

441

.?J7

U

U

very rapid strong nucleophile)

FIG. 16. The reaction of “flamingo” capped cyclodextrin.

( k 2 ) (kl/k2 x 13) toward the N; substitution. Thus, the first treatment of 5 with a relatively weak nucleophile (X)followed by treatment with a strong nucleophile (Y) gave the unsymmetrically substituted cyclodextrin (CDXY) in reasonably high selectivity (85%). Specific (or selective) polyfunctionalization of cyclodextrin is, in principle, possible based upon stepwise regiospecific flamingo-type capping as is shown schematically in Scheme 5. Simple tetrasubstitution (or hexasubstitution) is more easily attained through double (or triple) capping. A typical example is the successful preparation of tetrasubstituted P-cyclodextrin (8), prepared from 7 by the authors as a channel-forming compound for metal ion transport (77).

SCHEME 5 . Stepwise looper’s walk modification of cyclodextrin.

442

IWAO TABUSHI AND YASUHISA KURODA

Thus, in the present stage, we can freely design an enzyme model by the use of cyclodextrin, although introduction of intermediate numbers of functional groups on cyclodextrin is not very easy. Generally, separation of regioisomers of capped cyclodextrins is difficult. Only in limited ideal examples is each isomer easily separated through the column and purified by recrystallization or reprecipitation without any difficulty. The phenanthrenedisulfonate cap is a typical example of this kind (72). But usually column separation (even a very efficient HPLC column) is not very helpful. Regioselective capping is very important in these sense and from a mixture of regioisomers, the most abundant isomer is usually purified by careful and repeated reprecipitation. Structure determination of regioisomers is also not easy in a practical sense, although the basic principle is clearly depicted as follows : (a) A,D capping-single cap, (b) A,C capping-single and double caps, (c) A,B capping-single, double, and triple caps, therefore, if doubly capped cyclodextrin is formed in a reasonable yield, only two possibilities remain-A,B or A,C. If the triple cap is formed in a reasonable yield, a single possibility remains-A,B. Based on the above concept of a regiospecific cap (or highly regioselective cap), the structure determination is usually easy, although less selective capping leads to a very difficult situation as shown in the following hypothetical example: A,B; C,D A,B; D,E double cap

A,B; C,E A,B; D,F A,B; C,F

single cap

P-CD-

A,B; C,E

A,C; D,F

LA,.

doublecap

c

B,c A,B; C,F

Once a disubstituted cyclodextrin is satisfactorily purified, 3C-NMR is helpful in determination of the A,B isomer. It is reported that a C,-substituent

443

CYCLODEXTRINS AND CYCLOPHANES AS ENZYME MODELS

6

7

FIG.17. Tetrasubstituted cyclodextrin as a channel-forming compound.

on ring B affects the 13C chemical shift of ring A carbon appreciably (78). However, the remote (de)shielding effect of a C,-substituent on ring C or D upon the 13Cchemical shift of ring A carbon is much smaller. This criterion was successfully applied to the structure determination of our capped P-cyclodextrins (70, 71).Another criterion for structure determination is the isomerization of the cap, as is exemplified by the cis- and trans-stilbenedisulfonate cap (see Fig. 18) (72). The cis cap consisted of two isomers P, and P,, neither of which has the A,B regiochemistry based on 13C-NMR. One of the isomers (P,) was identical with the photoproduct obtained from the pure regioisomeric trans-stilbenedisulfonate cap. And cis cap (P1 P,) gave the corresponding double cap upon further treatment with the cis reagent, but no double cap was prepared from the trans cap. Based on these observations, it was concluded that P, was a A,C- and P, was a A,D-isomer.

+

TABLE XI1 Cycbdextrins Modified on All 0 ( 2 ) ,0 ( 3 ) ,and/or O(6)Positions Cyclodextrin

a-

P-

Functional group

Position"

-CH, -CH,CH=CH,

Ref.

80 O(6) 0(2),0( 6 )

81 81

SO2CI

Q?+

SOpCl

€3

FIG.18. Photoreactions of stilbenecapped cyclodextrin.

CYCLODEXTRINS AND CYCLOPHANES AS ENZYME MODELS

445

Tr

FIG.19. Trisubstituted a-cyclodextrin.

Only one example is known for random multifunctionalization (see Fig. 19). Symmetrical (A,C,E) tritrityl-or-cyclodextrin (9) was isolated (78). Although the reaction of tritylchloride with or-cyclodextrin is rather random, the yield of 9 is relatively high (23%), probably due to the bulkiness of the trityl group. After careful column chromatography, the pure A,C,Eregioisomer was obtained. Much easier multimodification is, of course, that all hydroxyl groups on C, ,C, ,and/or C, positions are modified. The typical examples are shown in Table XII. Among these functional groups, tosylate and mesitylsulfonate are used as intermediates for further conversion. Benzoate and allyloxyl groups are used as the protecting groups during the modification. TO ENZYME MODELVIA HOSTDESIGN B. APPROACH

1. Cyclodextrin

We now can prepare, in principle, enzyme models by use of the concept of “host design,” where artificial enzymes are so designed as “multiple recognition” hosts schematically shown in Fig. 20. Although unsubstituted cyclodextrins are well known to catalyze some organic reactions such as ester hydrolysis, their catalytic activities are relatively small. Recent progress in cyclodextrin chemistry has shown that it is possible to enhance the catalytic Molecular recognition

Catalytic r e a c t ion

Hydrophobic interact ion

u FIG.20.

Artificial enzyme design based on the concept of multiple recognition

446

IWAO TABUSHI AND YASUHISA KURODA

activity remarkably by the use of appropriately designed cyclodextrin derivatives. For example, capped cyclodextrin 1 shows a remarkably enhanced association constant for hydrophobic guests as shown in Table XI11 due to its extended hydrophobic recognition (4 7). This remarkable enhancement of the hydrophobic binding affects the catalytic effect of cyclodextrin upon the ester hydrolysis. The capped cyclodextrin 1 shows far larger catalytic constants for the hydrolysis of substituted phenyl acetates than does the parent P-cyclodextrin. As shown in Table XIV, the larger catalytic effect ( k , . K,,,) of 1 is clearly due to the large association constants ( K a s s of ) 1 (82). Furthermore, 1 shows the para selectivity which is mainly determined by the term K,,,although parent fl-cyclodextrin, as is well known, exhibits the meta selectivity determined by the term k, (4b).Thus, even this rather simplified example of host design (the expansion of the hydrophobic binding area) demonstrates how one can use the concept of host design effectively for the preparation of an enzyme model. Preparation and catalysis of disubstituted cyclodextrin as an excellent enzyme model is demonstrated by the RNAase model reported by Breslow et al. (68, 83). The enzyme models 10 and 11, derived from I , show a bellshaped pH versus rate profile for the hydrolysis of the cyclic phosphate of 4-tert-butylcatechol, indicating the cooperative catalysis by two imidazole groups (Fig. 21). The reactions catalyzed by 10 and 11 give exclusively 12 and 13, respectively. This interesting specificity indicates that the geometry of the P-0 bond cleavage is quite different from each other. Another interesting "enzyme-like" kinetic behavior that these hosts exhibited is successful demonstration of the so-called "bell-shaped pH profile.

TABLE XI11 Association Constants of B-Cyclodextrin and 1" Guest

8-Cyclodextrin

58

At 2S"C, pH 6.86, Ref. 47.

1

1,300

CYCLODEXTRINS AND CYCLOPHANES AS ENZYME MODELS

441

TABLE XIV Catalyiic Rate Consianis ( k , ) and Association Constants .for Hydrolysis of Phenyl Acetate"

O2N-O-o~c ~

O

A

C

2.71

5.37

3.00

33.6

0.31

0.45

833 90.9

2.08

2260

11.2

1.63

272

54.7

8.3

0.21

3.4

0.7

OaN CH~-@OAC

P O A c

0.48

0.97

188

35.7

4.41 2.85

17.1 58.4

1.9 2.7

CH 3

* 25"C, pH 10.60.

Another interesting enzyme model obtained by use of difunctionalized cyclodextrin having two imidazole groupings also afforded carbonic anhydrase models as reported by the authors (69). Carbonic anhydrase has Zn2+ surrounded by three imidazoles in the active site and C O , is bound to the active site is close proximity to Zn2+ with the assistance of hydrophobic 00

0

t

10

12

+ t

qoH OPOi

13

FIG.21

RNAase model by the bisimidazole derivative of p-cyclodextrin

448

IWAO TABUSHI AND YASUHISA KURODA

SCHEME6. The zinc complex of bishistaminocyclodextrin(14).

interactions. As a simplest model, zinc complex of bishistaminocyclodextrin (14) was prepared and its C 0 2 hydration rates were investigated (see Scheme 6). From the observed rate constants for a series of catalysts, a conclusion is drawn that three factors-the hydrophobic environment, the imidazoleZn2+ complex, and an additional base or its conjugate acid-operate to enhance the rates of catalysis. In the course of our host design, a more sophisticated synthetic recognition system is prepared by the use of capped cyclodextrin. Triple recognition should be the minimal condition necessary for modeling a ligase-type activity (to show specificity toward both S , and S , as well as toward the functional group(s) F, and F, in Fig. 22). Duplex cyclodextrin (15) satisfies these minimum requirements (84). This specially designed host shows interesting binding characteristics toward guest molecules having two hydrophobic recognition sites. One typical example of this multiple recognition is the binding of 6-p-toluidinylnaphthalene-2-sulfonate (TNS), where its fluorescence maximum largely shifts (demonstrating the hydrophobic environment) according to its binding mode. As shown in Table XV,the A,, of TNS included by 15 is equal to that of the 1:2 TNS-8cyclodextrin complex even at the low host concentration where parent 8-cyclodextrin (and corresponding 8-cyclodextrin tetramine) forms only 1: 1 complex. It demonstrates that both of the hydrophobic moieties of the guest

n

15

FIG.22. Triple recognition. Duplex cyclodextrin as a ligase model.

CYCLODEXTRINS AND CYCLOPHANES AS ENZYME MODELS

449

TABLE XV Fluorescence Maximum of TNS" Host None B-CD 8-CD B-CD(NzC2H6)z 15 a

Binding mode (guest/host)

A,, (nm)

-

480 451

1:1

1:2 1:l

444

1:1

444

452

From Ref. 84.

are bound to two hydrophobic binding sites of the host. Methyl orange is also bound much more strongly by 15 (k,,, = 3160 M-')than by p-cyclodextrin tetramine (k,,, = 520 M-').These examples demonstrate the multiple recognition mechanism is operating for binding by 15 as shown in Fig. 22. Another beautiful example of multiple recognition by the use of multifunctionalized cyclodextrin is symmetrical triamino-0-permethyl-oc-cyclodextrin (16), which is designed for the specific recognition for the phosphate group (Fig. 23) (85).As is expected, 16 shows a higher affinity toward benzyl phosphate (K,,, = 3.2 x lo4) than does the corresponding monoammonium derivative (K,,, = 3.3 x 10'). Introduction of a single catalytic group in cyclodextrin generally affords enzyme models as shown in many examples listed in Table XVI. Thus, reasonable acceleration and substrate specificity were observed in these models. However, monosubstituted cyclodextrins seem to have limitations and introduction of two or more functional groups is usually necessary for multiple recognition and for a sophisticated enzyme model. Enzyme models 18 and 19, which catalyze the decarboxylation, are typical examples of double recognition (87). These metalloenzyme models recognize a-ketoacid as the specific substrate by the hydrophobic interaction and coordination interaction as shown in Fig. 24 and Table XVII.Thus, the presence of the second recognition site, triamino-Zn", results in an increase

FIG.23. Triammonium-a-cyclodextrin.

86

L6

96

56

P6 F6

26

16

06

68

88

88

L8

98

CYCLODEXTRINS AND CYCLOPHANES AS ENZYME MODELS

45 1

\N and/or interact ion

coordination

Hydrophobic interaction

FIG.24. Double recognition by a metalloenzyme model.

by a factor of 300 in the association constant for the specific substrate, 2-adamantanone- 1-carboxylic acid. If the concept of double (or multiple) recognition is generally applicable to the host design, one can prepare the multiple recognition model simply by connecting several elemental recognition sites via appropriate chemical bonds. Intramolecular hydrophobic recognition is sometimes very important in biological systems. Artificial rhodopsin, as a typical example of intramolecular recognition, was investigated by use of the cyclodextrin recognition hosts 25 and 26 (92,93).The rhodopsin model 25 successfully mimics the large red shift of the retinal Schiffs base in native rhodopsin as shown in Table XVIII. It was concluded that the remarkable red shift observed for 25 in the aqueous solution was due to the electronic destabilization of the ground state caused by the ammonium-ammonium unfavorable interaction in the hydrophobic microenvironment. The chromophore is forced to take this unfavorable orientation because of the tight binding of double recognition. Twisting of the conjugated olefin chain of retinal due to the tight binding may also contribute to some minor extent. The enzyme model having the pyridoxamine moiety 24 presented by Breslow's group (92) is one of the most important enzyme models. The TABLE XVII Guest Binding of B-Cyclodextrin and Double Recognition Model"

2-AdCO. CO,H l-Ad.CO,H Cyc-Hex .CO,H a-NaphCOCH,CO,H p-Nitrophenol ~~~~

~

From Ref. 87.

830 230 I40 710 470

270,000 5,300 1,900 4,200 560

452

IWAO TABUSHI AND YASUHISA KURODA

TABLE XVllI Absorption Maxima of Retinal Pigment Models

Comp.

Aq."

Org.b

Aq. CD inclusion

25 26

497 445

476 460

-

n-BuNH=Retinal

450

450

430

47 1

470

444

498

-

-

Bovine rhodopsin 4

-

0.1 N HCI.

In ethylene glycol.

artificial enzyme 24 specifically reacts with keto acids having aromatic moieties such as indolepyruvic acid and phenylpyruvic acid to give corresponding amino acids. Interestingly, the reaction shows significant optical induction (e.g., 52% enanfiomeric excess of L-isomer for phenylpyruvic acid). Although this model does not show any turnover, it seems to be possible to provide turnover activity by further sophistication. These results clearly suggest that cyclodextrin derivatives appropriately designed will show a wide variety of artificial enzyme activity. Recognition capacity of the cyclodextrin cavity may be used for the preparation of various artificial functions other than artificial enzyme function. One example is the substrate-specific energy transfer, as shown in the following example. As shown in Fig. 25, bemophenone-capped cyclodextrin (34)shows highly effective triplet-triplet energy transfer even at the low concentrations of host and guest molecules, where the open-chain benzophenone derivatives show none of such energy transfer (99). The remarkable significance of the guest binding by the host cavity to the effective transfer is also shown by the absence of the energy transfer from 34 to trisodium naphthalene-1,3,6-trisulfonate,which is a very hydrophilic and inappropriately shaped guest.2

2. Cyclophane In contrast to cyclodextrins, there is no parent cyclophane structure available in nature. Therefore, the design of the host molecule starts from the Another type of photoactive capped cyclodextrin, azobenzene-cappedcyclodextrin, is also reported, but regioisomer distribution is not mentioned and their treatment does not seem quite satisfactory. See Yoshimura, H., Saka, H., and Osa, T., J. Am. Chem. SOC.101,2779 (1979); Chem. Lett. 841,1007 (1979); Chem. Lett. p. 29 (1980).

CYCLODEXTRINS AND CYCLOPHANES AS ENZYME MODELS

453

Phosphorescence Of

guest

34

FIG.25. Specific triplet-triplet energy transfer between host and guest.

construction of the basic structure of the desired host. In this case, artificial enzyme models based on the cyclophane structure and modified with appropriate functional groups are more conveniently prepared by the classical strategy of the “synton” combination than by functionalization after the skeleton synthesis. The earliest example of cyclophane host synthesis is 2”-paracyclophanes investigated by the authors (41). Since the space-filling model suggested that 2’- and 26-paracyclophanes had suitable hydrophobic cavities (6 and 8 A, respectively) inside the molecule, the introduction of quaternary ammonium groups was attempted in order to solubilize these too hydrophobic cyclophanes into the aqueous solution (Scheme 7). These water-soluble cyclophanes behaved as artificial enzymes of simple function-rate acceleration, specific binding of guests, etc. As other examples, a series of heterocyclophanes containing nitrogen or sulfur atoms were investigated by the authors. Tetrathiacyclophane tetrasulfonium (35) was prepared by high-dilution condensation followed by permethylation as shown in Fig. 26 (100). Cyclophanes take several stable and metastable conformations, such as face-toface (a) or lateral (b) (101). In the face-to-face conformation, the cavity size of 35 is about 7 A and is expected to be capable of including a naphthyl moiety. Tetraazacyclophane (36) has a similar hydrophobic cavity surrounded by four benzene rings, but its hole size (5.5 A) is smaller than that of 35 (102). As shown in Table XIX,both 35 and 36 can bind l-anilino-8naphthalenesulfonate (1,8-ANS) to form 1 :1 inclusion complexes and their association constants are significantly larger than that of fl-cyclodextrin.

n=

5

and

0

rn = 4 (average1

SCHEME7. Water-soluble2”-paracyclophane.

454

IWAO TABUSHI AND YASUHISA KURODA

FIG.26. Tetrathia- and tetraazacyclophane: (a) face to face; (b) lateral.

TABLE XIX Inclusion of 1,8-ANS by Artificial Hosts"

PH

Ls(M-')

7.0 4.2 2.0 7.0

I600 380 550 57

M e - S w - M e

7.0

< 50

M e 2 H N+&

4'2

<4

Host

35 36,(H+), 36.(Ht)4

8-CD

4h

NHMe2

From Refs. 100 and 102.

CYCLODEXTRINS AND CYCLOPHANES AS ENZYME MODELS

455

4BF;

37 FIG.27. Artificial cyclophane catalyst

Thus, tetraammoniumcyclophane (37) was investigated as the "inclusionelectrostatic" catalyst for ester hydrolysis. As shown in Table XX, the watersoluble heterocyclophane 37 showed a unique substrate specificity in the hydrolysis of chloroacetate (ClCH,CO,R) (40, 103). The rate acceleration observed (the rate constant ratio k,/k,) strongly indicates that 37 is more effective and much more discriminating than any of the CTAB miscelles or open-chain analogs. Very interestingly, however, a unique inhibition by 37 is observed for a-chloro-P-naphthylchloroacetate.These observations strongly suggest that the quaternary ammonium residue of 37 in the inclusion complex markedly favors the stabilization of the negatively charged quasi-tetrahedral transition state for the specific substrate such as P-naphthyl ester. The deceleration observed for a-chloro-S-naphthyl ester is attributed TABLE XX Catalytic Hydrolysis of Aromatic Esters by 37 Chloroacetate

k , x lo3 (sec-'y

k , x lo3 (sec-I)

K,,, x lo3

a-Naphthyl" /I-Naphthyl" p-Ni trophenyl" a-CI-Pnaphthyl'

0.82 0.77 5.54

4.9 19.2 14.6

5.5 I .9 2.0

2.60

0.22

5.0

0.085

(CH3)3Y + 2BP4-

P-Naphthyl"

0.82

1.32

0.38

1.6

CTAB

a-Naphthylb P-Naphthyr

0.81 0.87

4.5 5.9

Catalyst

37

.

a

pH 8.10. pH 8.25. Spontaneous rate of hydrolysis.

(M-')

42 33

kzlko 6.0 25 2.6

5.6 6.8

456

IWAO TABUSHI AND YASUHISA KURODA O*c’R

(a 1

(b)

(C)

FIG.28. Three possible modes of catalysis: (a) correct f i t ; (b) reverse binding; (c) induced disfit.

to the inhibition of the N+-oxyanion interaction due to either “reverse binding” of a substrate or the “induced disfit” as schematically shown in Fig. 28. I t is also reported that various types of [20]- and [10,10]-paracyclophane derivatives having oxime grouping accelerate the deacylation of p-nitrophenyl esters, although structures of these cyclophanes seem to be much too flexible to expect any excellent substrate specificity as enzyme models (104).Thus, it is quite likely that water-soluble cyclophane is a very promising host family as the enzyme model and it may be reasonably expected that their substrate specificity, catalytic activity, and mechanism of the catalysis are quite different from those of cyclodextrin. IV.

Enhancement of Binding and Catalysis by Guest Design

By “guest design” is meant preparation of that framework of the guest molecule most appropriate to mimic a native enzyme reaction for a given reaction and a given host. In this section, we will discuss the scope of guest design by the use of unmodified cyclodextrins. Cyclodextrins, as is well known, catalyze the hydrolysis of various types of ester and their kinetic behaviors are clarified by many examples (f05). However, the magnitude of acceleration for the acylation of the unmodified cyclodextrin is usually I02-fold, which is significantly smaller than that by native enzymes (usually 10’- to 107-fold).Recently, Breslow et al. showed that 102-fold acceleration was not the upper limit even for unmodified cyclodextrins, if one used very carefully designed substrates (106). The largest acceleration ever achieved was for ferrocene derivatives and was so designed as to retain as much binding as possible on proceeding from bound substrate to bound tetrahedral intermediate to derive the conversion. As shown in Table XXI, lo6-fold acceleration (kEomplex/kun) was observed for the

457

CYCLODEXTRINS AND CYCLOPHANES AS ENZYME MODELS

TABLE XXI Binding and Rate Constants for Deacylation of Various p-Nitrophenol Esters"

3900 3900 26.1

Ad-propiolateb

3-t-Bu- Ad-propiolateb Fer-acrylate' Fer-exocyclic ester'

2.25 2.25

38s

38b Fer-endocyclic ester' 39a 39b a

15.3 15.3

84 590 96

3300 560 110

2,200 15,000 360,000

72.3 3.56

260 220

3,200,000 160,OOO

10.1 1.56

260 220

66,000 10,000

In 60% (v/v) DMSO, at 30°C. pH 6.8;Ad, adamantane.

pH 10.0; Fer, ferrocen; %a,

; 38b, optical isomer of 380;

CQ2PNP

; 39b,optical isomer of 39a.

substrate 38. This reaction, catalyzed by fl-cyclodextrin, also shows relatively large chiral selectivity for 38 and 39. The results can be rationalized by the term "geometry optimization" of the substrate, i.e., best stabilization of a transition state as shown in Fig. 29. In Table XXIl are shown reactions catalyzed by cyclodextrins. It is shown that some condensation reactions are also remarkably catalyzed by the use Optimal Substrate

Good Subst rate

Poor Substrate

FIG.29. "Geometry optimization" of substrate.

458

IWAO TABUSHI AND YASUHISA KURODA

TABLE XXII Typical Reactions Catalyzed by Parent Cyclodextrin Cyclodextrin

P

Reaction R-COY

P

Ref.

+ H,O

4

+

+ R-C/o

H20

+ CO, + H +

R-H

107

108

'NHFHCO~H

109

PhCH2SCH2Ph + I

OH- PhCHSCH3

+

@fH3

&H2Ph

CH3

CHSCH3 I Ph

I10

111

P a

PH

17

(Me01 2P-CHCC13

+ HOCl

&H3

;1

+ ( M e O ) 2POCH=CC12

112

__t

113

H H

P

ArCOCF3

+

0

or NaEH4

-

ArCH (OH)CF3

114

Pr

8 CN

+

B X

HCN

d

I15

X

6 .b R - 4 R

117

(continued)

CYCLODEXTRINS AND CYCLOPHANES AS ENZYME MODELS

459

TABLE XXII (continued) Cyclodextrin

Reaction

Ref.

Z

z HON

P

P

Y

‘OCH3

CN

I I8

hv_

&OCH

3 +

6” 6 0

+

IIy

I20

P

2[CoCl2en2I+

+

H2N(CH2ll2NH2

+

B-CD

I22

of simple cyclodextrin based on the suitable guest design. Thus, electrophilic allylations of hydroquinone derivatives (116) are generally applicable for various types of quinones, leading to bioactive quinones such as vitamines K , and K, (1164, tocopherolquinone, and ubiquinone (1164. The mechanistic study on this reaction reveals that p-cyclodextrin acts as the inclusion catalyst resembling “ligase and oxidase” as summarized below (116b): (1) lowering the pK, value of the substrate, hydroquinone, by hydrogen bonding;

460

IWAO TABUSHI AND YASUHISA KURODA

(2) enhancement of the nucleophilic reactivity of the delocalized carbanion in the hydrophobic cavity (ligase-typeactivity); (3) stabilization of the one-electron oxidation state of the bound substrate (oxidase-type activity); (4) protection of the product from undesirable oxidative cleavage by hydrogen peroxide formed during the reaction.

The detailed mechanism of this reaction is depicted in Fig. 30. Another successful example of such guest design is the Diels-Alder reaction markedly accelerated by the cyclodextrin inclusion. As shown in Table XXIV, P-cyclodextrin accelerates the addition of a small dienophile to cyclopentadiene, but inhibits that of N-ethylmaleimide to anthracene-9carbinol (117). Thus, the “guest design” is a really helpful concept for the remarkable catalysis. However, there seems to be some limitation in the choice of reactions, if cyclodextrins have no special functional group for the TABLE XXIll One-Step Preparation of Bioactive Quinone Analog“ Yield

R I L r B r( X-B r )

Hydroquinone

R p R ,

q

R,

=

R, = R,

R,

=

CH,,R, = H = R, = H R, = H,R, = CH,

H

3

R,

=

=

R , = R, CH 3

=

=

I

H

R, = CH,,R, R,

(XJh

Product

CH,. R,

=

86 (19)

4KH3

77 (22) 76 (47) 78 (40)

81 (35)

H CH3

See Refs. 116(1-c. is based on the consumed material. The yields in the absence of /I-cyclodextrin are shown in parentheses. “

* The yield

46 1

CYCLODEXTRINS AND CYCLOPHANES AS ENZYME MODELS

t I-H'

t I++

FIG.30. Reaction scheme of allylation and oxidation of hydroquinonederivatives catalyzed by cyclodextrin.

effective catalysis. It is also important to realize that the "guest design" cannot be adopted by simple examination of the molecular model. For the excellent guest design, it is necessary to examine a geometry of included ground and transition states, hydrophobicity and hydrophilicity of a substrate, some specific (hydrogen bonding, polar, charge transfer, etc.) interactions involved, change in solvation, and change in molecular and intramolecular motions. TABLE XXIV Rate Constantsfor Diels- Alder Reactions" ~

Diene

~~

Dienophile

CH 3

N

Jb C

e

Cyclodextrin None 8-CD a-CD None 8-CD a-CD

k, x (M-' sec-*) 4,400 10,900 2,610 59.3 531 47.9

None

22,600

8-CD

13,800

0

~~

From Ref. 117.

462

IWAO TABUSHI AND YASUHISA KURODA

V.

Conclusion

Inclusion catalysts as enzyme models are now undoubtedly one of the most promising fields in biomimetic or bioorganic/bioinorganic chemistry. As described in this article, the recent successes in this field clearly suggest that the molecular design of any type of enzyme model is possible, in principle, by use of the concepts of host design. Further detailed information about the transition states may be obtained by careful guest design as well as host design. Since practical modification techniques for host molecules such as capping are established, it now is possible to prepare enzyme models specifically catalyzing certain reactions remarkably. Several years ago, Breslow wrote as follows in his first report on the guest design (106a): “Some pessimists have concluded that cyclodextrins can give selective reactions, but with only modest rate accelerations over the control. However, it seemed to us that the optimal systems had not yet been examined.” This statement shows that we should be very optimistic. It also seems to us that much more sophisticated and also complicated recognition of a guest by a native enzyme can be generally understood and mimicked by the use of a completely artificial host appropriately designed, being much simpler than the corresponding native enzyme from a structual viewpoint, but having the same recognition capacity based on exactly the same recognition principle from mechanistic viewpoints. We do hope this rather optimistic dream will come true in a variety of important reactions presently catalyzed by native enzymes. This chemistry may be called “synzyme catalysis.” REFERENCES AND NOTES lo. Jencks, W. P., “Catalysis in Chemistry and Enzymology.” McGraw-Hill, New York, 1969. I h . Bender, M. L., “Mechanism of Homogeneous Catalysis from Proton to Protein.” Wiley, New York, 1971. Ic. Dugas, H., and Penney, C., “Bioorganic Chemistry.” Springer-Verlag. Berlin and New York, 1981. 2. Villiers, A., C.R. Acad. Sci. Paris 112, 536 (1891). 3a. Cramer, F., “Einschlussverbindungen.” Spring-Verlag, Berlin and New York, 1954. 36. Cramer, F.. and Hettler, H., NaturMfissenschafen54, 625 (1967). 4a. Hennrich, N . , and Cramer, F., J . Am. Chem. Soc. 87, 1 121 (1965). 4b. Van Etten, R. L., Clowes, G. A., Sebastian, J. F., and Bender, M. L., J . Am. Chem. Soc. 89,3253 (1967). 5. Cramer. F., and Mackensen, G., Anyrw. Chem. 78,641 (1966). 6. Breslow, R., and Overman, L. E., J . Am. Chem. Soc. 92, 1075 (1970). 7. Bender, M . L., and Komiyama, M . , “Cyclodextrin Chemistry.” Springer-Verlag, Berlin and New York, 1978. 8. Hyble, A., Rundle, R. E., and Williams, D. E., J . Am. Chrm. SOC.87, 2779 (1965).

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463

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107. Cramer, F., and Kampe, W.. J. Am. Chem. SOC.87, 1115 (1965);Stranb, T . S.,and Bender, M. L., J . Am. Chem. SOC.94, 8875 (1972);Motozato, Y., Furuya, Y., Matsumoto, T., and Nishihara, T., Bull. Chem. SOC.Jpn. 53,2578 (1980). 108. Daffe, V., and Fastrez, F., J . Am. Chem. SOC.102,3601 (1980). 109. Griffiths, D. W., and Bender, M. L., J . Am. Chem. SOC.95, 1679 (1973). 110. Mitani. M.,Tsuchida, T., and Komiyama, K., Chem. Comrnun. p. 869 (1974).

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Author Index Numbers in parentheses are reference numbers and indicate that an author’s work is referred to although his name is not cited in the text. Numbers in italics show the page on which the complete reference is listed.

A

Amsson, A., 160(125), 161(125),209 Amundson, N. R., 318,322 Andersen, T., 82(328), 139, 144 Anderson, D. L., 81(320), 143 Anderson, J. R., 163(152, 155), 164, 168(155), 169(155), 171(155), 172(194, 195), 173 (155), 174(195), 200(301, 304). 210, 211,

Aaenger, W., 422(35), 428(35), 463 Abdo, S., 28(105b), 29(105b), 74, 77, 126 (105b). 129(105b), 137 Abe, M., 421(28), 463 Abet, E. W., 78(306), 143 Abou-Kais, A., 22(89), 25(89), 63(266), 65 (266), 67(266), 68(266), 69(266), 75, 91 (266), 126(89,266,289), 129(89), 137,142,

214

Anderson, R. B., 326(3), 327(3), 410 Andreev, N. S., 40(177), 106(177), 139 Andrews, L., 31(133a, 133b. 133c), 86, 97, 98 (386), 131(133a, 133b, 133c, 458), 138,

143

Abraham, F. F., 153(31, 32), 207 Abrahams, S. C., 8(34), 135 Adamic, K., 24(94b), 129(94b), 137 Adams, H.-N., 343(63), 412 Adamson, A. W., 6, 135 Adrian, F. J., 129(450), 147 Aguilo, A., 409(219), 416 Aika, K., 104(402), 146 Akabori, S., 217(2, 3, 4, 5, 6, 7), 218(53, 57, 58, 59). 220(2, 3, 4, 6), 221(2), 222(2, 3), (230(2, 3, 4, 5, 6, 7), 231(2, 3, 4. 5, 6, 7), 248(6), 269, 270 Akamatsu, A,, 218(57, 58, 59), 270 Akimoto, M., 53, 102, 103(399), 121,141,146, 147

Albano, V. G . , 369(135), 370(141), 371(135, 142). 414 Aldridge, C. L., 411 Alfeev, V. S., 191(250), 212 Ali, A. H., 151(7), 206 Allan, G., 160(96), 161(96), 208 Allinger, N. L., 431,464 Allison, E. G., 199(290), 213 Allpress, J. G., 159(85), 208 Al-Mashta, F., 5, 32, 54, 131(196), 134 Amir-Ebrahimi, V., 172(196, 198), 173(198), 174(196, 198), 176(196), 211

144,145,147

Andrews, M. A., 404(21 I), 416 Andrich, G. P., 416 Anker, W. M., 370(141), 371(142), 314 Anpo, M., 59(256), 79(418), 106(256), 107, 125(256), 142, 146 Anstock, M., 407(217), 416 Anufrienko, V. F., 5(20b), 6,12(20b), 31(20b), 40(181), 41, 47(20b), 124(20b, 22f). 127 (20b), 131(20b), 133(20b), 134, 135, 140 Aoyama, S., 17(65c), 136 Aoyama, Y.,465 Apai, G., 160(107, 135), 161(135), 209 Aptekar, E. L.,49(213,216), 53(215,216), 140, 141 Arieti, A., 28(110), 29(110), 37(110), 38(110), 39(110), 55(110), 81(110), 89(110), 91 (110). 125(110), 137 Aris, R., 279, 318, 320, 321, 322 Arnett, E. M., 338(48), 365(48), 398(48), 411 Arpe, H.-J., 326(18, 19), 331(44), 410 Atanasova, V. D., 73(284), 81(284), 143 Atkins, P. W., 20(79), 82(323), 83(332), 84 (332), 95(373, 375), 137. 143, 144, 145 Au, C. T., 109(422), 146 Auk, B. S., 86(341, 344), 144 Aykan, K., 120(435). 147

467

468

AUTHOR INDEX

B Baba, N., 458(114), 466 Babievsky, K. K., 217(19), 221(19), 230(19), 231(19), 269 Backx, C., 131(462), 147, 301(22), 322 Baer, Y., 152(12), 206 Baerends, E. J., 159,208 Baetzold, R. C., 160(100, 128, 130), 161(128, 130). 209 Bagherzadah, E., 377(173, 174),406( 173,174), 407(174), 415 Baiker, A., 188(247), 212 Baird, T., 193(260), 213 Bajaj, V. B., 363(114a), 413 Baker, B. G . . 163(152), 210 Baker, R. T. K., 160(118), 209 Balaz, P., 201(305, 306). 214 Balistreri, S., 141 Balocchi, L., 343(62), 412 Banks, R. L., 51(221), 141 Banky-Elod, E., 458(112), 466 Baptista, 1. L., 139 Barbaux. Y., 120(437b), 147 Barbier, J., 183(231), 184(231), 212 Bard, G., 319, 323 Barreto. C. L., I52( 17). 206 Barron. Y ., 163(154), 210 Bartel, K.,421(29), 426(29), 463 Barthomeuf, D., 160(115, 116), 209 Bartlett, N., 8(27), 135 Bartley, W. J., 326(15), 410 Bartram, R. H.. I I(43a). 136 Barzaghi, M., 80(250), 126(250), 142 Basolo, F., 15(46b), 24(98), 28(46b), 32(134f), 33(46b), 34(148), 78(46b, 134f, 148, 307, 310). 79(98, 148, 313, 315), 81(320), 94 (98). 131(148.315). 136,137,138, 139, 143 Bastein, A. G . T. M., 158(81, 82). 162(143), 208, 210 Bates, J. B., 31(132), 86(132), 131(132), 138 Batta, I., 160(109), 209 Bauer. R . S., 32(135a), 13X Beck, P. A., 187(242), 191(242), 212 Bedeyapin, A. A., 267, 271 Beek, J., 318, 322 Behm, R. J., 157(72, 73). 208 Beier, B. F., 326(22). 375(22), 411 Beindorf, W. H., 197(276), 213 Beisner. R. W., 368( 127. 128. 129). 413

Bell, R. P., 345(74), 412 Bellemans, A., 153(29), 207 Bellstedt, F., 384(183), 415 Belyi, A. S., 191(250), 212 Bender, M.L., 423(7), 436(7), 450(94,95), 458 ( 109). 462,465 Benndorf, G., 156(68), 208 Bennemann, K. H., 154(39), 207 Bennett, A., 8(32), 9(32), 135 Bennett, J. E., 123(446), 147 Ben Taarit, Y.,17(63), 20(76), 21(84), 22(63, 81. 84), 23(84), 25(63), 35, 38, 50(63), 51 (81, 84), 52(84), 61(258, 261), 69(81), 73 (152), 80(319b), 81(152), 96(379), 100 (76). 105, 125(81), 126(63, 127(84), 128 (84). 129(63, 76, 81, 84, 152), 136, 137, 139, 142, 143, 145, 146 Benzon, S. W., 8(31b), 135 Berdnikov, V. M., 28(119), 138 Berger. M., 329(39, 40), 335(39), 336(39), 337 (39), 338(39), 365(39), 376(39), 411 Bergeron, R., 432(51). 433(54b), 434(54a, 54b). 443(81). 464, 465 Bergman, R. G . , 337(46c), 347(82), 377(46c). 411,412 Berke, H.,347(83), 384(83), 412 Bertizeau, C., 197(272), 213 Bertrand, P., 320(56), 323 Berty, J., 340(56), 341(56), 381(56), 411 Besoukhahova, C., 160(116), 209 Beyer, H. K., 76(291), 143 Bhasin, M. M., 326(15),4/0 Bickley, R. I., 47, 105, 106(206, 405). 107(206, 405). 108. 140, 146 Bielanski, A,. 2(2), 54, 55. 56, 134, 141 Bill. H., 144 Biloen, P., 131(462), 147. 153(24), 192(24, 255). 198. 207,213 Binder, K., 157(74), 208 Binenboym, J.. 131(456), 147 Bischoff. K . B., 297, 319, 322 Bishop. A. R., 155(52), 207 Bishop, R. J., 63(264), 64(272), 65. 67, 68. 71 (272), 128(272), 142 Bjerre. N., 3(12b). 134 Bjornbom, P. H., 318, 319,322 Blackborow, J. R.. 339(51, 52). 407(178), 411, 415 Blakeley, D. W., 183(230), 184(230), 212 Block, J., 160(108), 20Y

469

AUTHOR INDEX

Blue, E. M., 150(6), 197(6), 206 Blunt, F. J., 5(21), 6(21), 31(21), 34(21), 35 (21), 114(21), 131(21), 135 Blyholder, G.,40(173), 106(173), 139, 158(76), 208 Boccuzzi, F., 38(164b, 165), 139 Boger, J., 443(78, 80). 445(78), 449(85), 465 Bolivar, C., 197(272), 213 Bond, G . C., 160(114), 164, 166(162), 167 (162), 168(162), 171(162), 182, 199(290), 200(162), 209,210, 212,213, 313,322 Bonelle, C., 160(103), 209 Bonnelle, J . P., 120(437b), 147 Bonneviot, L., 73(283), 129(283), 142 Boonstra, A. H., 47(207), 106(207), 140 Bor, G . , 341(59), 347(59), 412 Boreham, C. J., 79(312), 143 Borello, E., 131(465), 147 Boronin, V. S . , 159(88), 208 Bory, P.. 440(73), 464 Boss. H. J., 102(394), 145 Botman, M. J . P., l64( 167, 168). 165(167, 168), 169(167, 168), 172(167, 168), 189(167), I90( 168). I92( 167, I68), 193(167, 168), 200( 167). 203( 167). 210 Bouchy, A,, 167(85), Boudart, M., 80(319b), 89(354), 112(354), 143, 160(111. 112), 169(187), 182, 188, 199 (287), 209, 211, 212, 213, 275,321 Boudeville, Y., 52(231a), 141, 160(136). 161 (1 36), 209 Bourgonje, A. F., 156(67), 201(67), 208 Bovy, P., 446(83), 465 Bowman, F., 81(320), 143 Bozon-Verduraz, F.. 73(282), 142 Braca, G., 416 Bradley, J. S . , 375. 376(164, 165), 378(164, 165), 379(164). 381, 384(184), 414,415 Bradshaw, A. M., 160(124, 131). 161(124, 131), 209 Brandenberger, S . G., 177(112), 211 Branscomb, L. M., 9(38), 88(38), 135 Brazdil, J . F., 120, 147 Breakspere, R. J . , 106(407), 146 Breck, D. W., 59(257), 62(257), 142 Brenner, P. G . , 443(78), 445(78), 465 Breslow, R., 419, 429(46), 432(53), 437(68), 440(73), 446, 450(88, 91, 98), 451(91), 458(113, 117), 460(117), 461(117), 462 (106a), 462,464,465,466

Breysse, M., 17(62), 25(62), 27, 43(62, 189, 190), 124(189), 126(62), 129(62), 136,140 Bricker, J . C., 402(207), 415 Brivati, J. A., 82(323), 143 Brooker, M. H., 31(132), 86(132), 131(132), 138 Brooks, R. E., 346(77), 412 Brown, E. S., 351(88), 353(95), 355(95), 360 (88), 362(112), 365(95), 367(95), 369(132, 133, 134, 136, 137, 138). 412,413,414 Brown, L. F., 199(294), 214 Brown, T. G., 24(98), 79(98), 94(98), 137 Brown, T. L., 343(65), 412 Brummer, J . G . , 6, 135 Brundle, C. R., 109(423), 146 Bryant, D. R., 351(88), 360(88), 362(112), 369 (132), 412,413 Buchler, E., 30(120), 55(120), 125(120), 138 Biissemeier, B., 326(6), 410 Bulko, J . B., 326(13), 410 Bunger, W . B., 365(47), 366(47), 411 Bunker, R. J . . 212 Burch, D. S., 9(38), 88(38). 135 Burch, R., 201(309), 214 Burness, J . H . , 33(135b), 138 Burrington, J . D., 118(431), 120(431), 147 Bursian, N . R., 213, 214 Burton, J . J., 153(34), 159(89), 207, 208 Burwell, R . L., 81(320), 143 Burwell, R. L., Jr., 164, 166(160. 178, 179). 210,211 BuSCd, G., 4, 5(19b), 32(19b), 54(19b), 131 (19b), 132(18b), 133(18b), 134, 134 Busetto, C.. 143 Bushurova, V. S . , 28( I 18), 30( I 18). 48( I 18), I38 Buttet, J., 160(102), 209 Byberg, J . R . , 82(328), 93, 144, 145 Bystrikov, A. V., 59(251), 122(251), 142

C Calderazzo, F., 344(67), 347(67), 384(67), 412, 415 Callender, W. L., 177(212), 211 Campadelli, F., 80(250), 126(250), 142 Campbell, 1. D., 39(167), 40(167), 124(167), 139 Campbell, P., 458(113), 466 Candy, F. P., 159(92), 208

470

AUTHOR INDEX

Car, R.,160(102), 209 Carberry, J. J., 319,322 Cardelino, B.,428(44), 434(44), 435(44), 464 Cariati, F., 142, 143 Carniti, P., 80(250), 126(250), 142 Carrington, A,, 83(337), 144 Carter, J. L., 151(9), 155(9), 183(232), 197 (271), 206,212, 213 Casey, C . P., 404(21 I), 416 Casey, E. J., 90(357, 358), 91(357, 358). 144 Castner, T. G . , 35(153), 139 Cavanagh. R. R., 314,322 Cawse, J. N., 356(102, 103), 360(102), 361 (102). 362(11 I), 363(102, 103). 372(102), 413 C a m , B.,434(56), 464 Celotta, R.J., 8(32), 9(32), 135 Ceriotti, A,, 370(141), 371(142), 414 Cerruti, L., 131(465), 147 Cesaiotti, E., 358(110), 413 Chako. K. K., 421(10), 422(10), 424(10), 425 (lo), 463 Chan, Min-Nan, 163(153), 210 Channing. M. A., 432(51), 464 Chantry, G. W., 145 Charcosset, H., 197(272), 213 Che, M., 2, 3, 4(1), 13(45), 16(58, 59), 20(45, 70, 72, 75, 76), 21(58, 82, 84, 93). 22(58, 82, 84, 85. 87. 90, 93). 24(85, 96, 97, 99), 25(58,59,96), 26(87), 27(59), 28(108), 29 (108), 30(90, 120). 31(75, 108). 33(141), 34(141, 145b), 35(1), 36(96), 37(70, 72), 40(174c, 184), 42(188), 43(45, 188, 191), 44(45, 191), 45(75, 141, 174c, 195, 196), 46, 48(82, 212a), 50(87), 51(82, 84, 85, 191, 228), 52, 55(120), 56(243), 57(1), 58, 59(255), 61(258, 261), 68(76), 73(282, 283). 82(96), 84(1), 90(361), 92(99), 93 (99), 94(1), 95(1),96,99, 100, 104, 105(1), 106(1, 108), 110(1), 111(1), 112(1), 116 (I), 117(1), 118(1), 121(1), 123(70. 72), 124(45, 75, 90, 174~.184, 191). 125(85, 108, 120, 191), 126(87), 127(58, 84, 93), 128(45, 70, 72, 75, 82, 84, 85, 212a). 129 (58, 76, 84, 87, 90, 93, 255, 283). 130 (453b). 131(1), 132(453b), 134, 136, 137, 138, 139,140. 141, 142, 144, 145 Chebab, F., 156(65), 208 Cheh, H.Y ., 320(55), 323

Chen, M. J., 328(37). 329(37), 335(37), 336 (37), 344(37), 345(37), 347(37), 348(37). 41 I Cherkashin, A. E., 108(419), 146 Chersick, C. C., 303.322 Chiesi-Villa, A,, 387(188), 415 Chini, P., 357, 369(106, 107, 135), 370(141), 371(135, 142), 372(146), 413,414 Choi, H. W., 326(24, 25, 26), 348(26), 408(24, 25), 411 Chono, M., 96(381), lOO(381). 145 Christe, K. 0.. 33(136), 138 Christensen, J. J., 363(114), 364(114), 413 Christiansen, J. A,, 275, 318,321 Christmann, K., 157(72, 73), 208 Christoffel, E. G . , 305, 322 Chuvylkin, N. D., 16, 22(86), 25(86), 47(202), 50(86), 92(366, 367). 93(367), 99(367), 125(86, 447), 126(86, 447), 129(86, 447), 136, 137, 140, 145, 147 Ciani, G., 370(141), 371(142), 372(146), 414 Ciapetta, F. G., 150(6), 162(145), 197(6), 206, 210 Cibeily, G., 433(54b), 434(54b), 464 Cini, M., 160(101), 208 Cirillo. A. C., 75(286, 287), 143 Claasen, H. H., 131(456), 147 Clark, G. R.,348(84), 412 Clarke, J. K. A,, 150(1), 151(1), 153(23), 155 (I), 163(156), 164(156), 169(156), 171 (156), 173(156), 192(254), 193(260, 261), 197(275),206, 207, 210, 213 Clarkson, R. B.,4, 16(56, 57), 28(56), 29(56), 75, 77(299), 126(56, 299), 134, 136. 143 Claudel, B.,17(62), 25(62), 27(62), 43(62, 189, 190), 124(189), 126(62), 129(62), 136,140 Claus, H., 187(242), 191(242). 212 Clowes, G. A,, 419(4b), 428(4b), 446(4b), 462 Codell, M., 33( 137), 40, 44( 137), 45( 137). 124 (137), 138, 139 Coenen, J. W. E., 160(98),209 Coetzee, J. F., 364(117), 413 Cohen, M. H., I I, 15, 123(42), 136 Cole, T., 95(377), 145 Collin, R. L., 8(34), 135 Collins, M. A,, 144 Collman, J. P., 344(66), 412 Coluccia, S . , 20(70,72), 31(129,130), 32(134e), 37(70, 72, 129). 38(129, 164a, 164b. 165).

AUTHOR INDEX

39(134e), 56(134e), 80(134e), 123(70), 72, 129), 128(70, 72), 131(134e, 465). 133 (134e), 136,138, 139,144,147 Conway, D. C., 96(384), 145 Cope, J. O., 39(167), 40(167), 24(167), 139 Corcoran, R. C., 443(80), 465 Corden, B. B., 78(304), 79(304), 143 Cordischi, D., 28(110), 29(1 lo), 37, 38(110), 39(110), 55, 57, 87, 89(110, 158),91(110, I S ) , 123(158,160), 125(110), 137, 139, 141 Cornaz, P. F., 33(138, 140), 44(138), 45(138), I38 Cornet, D.. l63( 154). 210 Cornils, B., 326(6), 340(58), 341(58), 409(58), 410,412 Corro, G., 199(284,285), 213 Coryell, C. D., 55(238), 141 Cosby, L. A., 356(105), 373(148, 149, ISO), 413,414 Cosgrove, M. M., 144 Cossee, P.,48(209), 140 Cotton, F. A., 352(93), 365(93), 412 Courbon, H., 107(413), 146 Cramer, F., 418(3a, 3b), 419, 428(43a), 434 (57). 435(57), 439(75), 443(75), 458(107), 462,463.464,465 Cramor, F., 458( 1 15), 466 Creighton, J. A , , 131(455), 147 Criado, J., 199(287), 213 Cropley, J. B., 351(89), 352(89), 412 Crossley, A,, 158(78),208 Crumbliss, A. L., 32(134f), 78(134f), 138 Csiscery, S. M., 177(21I ) , 211 Cullis, C. F., 118(432), 147 Curtiss, L. A., 328(37), 329(37), 335(37), 336 (37). 344(37), 345(37), 347(37), 348(37), 411 Cusumano, J. A,, 183(232), 212 Cyrot-Lackmann, F., I55(53), 160(97. 136). 161(97, 136. 137). 207,208,2UY Cyrot, M., 155(53). 207 Czarniecki, M. F., 462(106a), 465 Czycholl, G., 155(54),207

D Daffe, V., 458(108), 465 Daiguji, M., 434(64), 458(1 I I), 464,466

47 1

Dalla Betta, R. A., 160(11I , I12), 209 Dalmai, G., 59(254), 125(254), 142 Dalmai-Imelik, G., 63(266), 65(266), 67(266), 68(266), 69(266), 91(266), 126(266), 142 Dalmon, J. A., 201(302), 214 Daroda, R. J., 339(51, 52), 407(178), 411,415 Dartiques, J. M., 193(258), 213 Date, Y., 464 Dauben, C. H., 8(29), 135 Dautzenberg, J. M.,153(24), 192(24),207,212 Dautzenberg, R. M.,192(255), 198(280), 213 Davignon, L.. 1 lO(4251). 146 Davis, M. S., 169(181), 180, 182, 183(181), 184, 210 Davis,R. H., 164(163, 164,165,166, 177),210, 21 I Davydov, A. A., 5, 12(20b), 31, 32, 35, 41 (187), 47(20b), 50,54, 124(20b), 127(20b), 131(20a, 20b), 133(20a, 20b, 149a, 469). 134, 138, 139,140,141,147 Davydova, 2. A,, 214 Deane, A. M.,31(130), I38 Deb, S. K., 31(126), 90(126), 138 de Boer, N. H., 120(434), 147 Defay. R.. 153(29), 207 de Groot, P. P. M., 131(462), 147 de Jongste, H. C., 152(20), 176(204), 177(204), 180(204). 184(204, 238), 185(188), 186 (204), 188(269), 193(262), 194, 200(238), 201(204), 207,211,212,213 Del Angel, G., 199(285),213 Delbouille, A., 89(354), 112(354), 114 Delmon, J . A., 195(268), 213 Delrue, P., 156(66), 208 Deluzarche, A., 353(97, 98), 377(97, 98, 173, 174). 406(173, 174), 407(174), 412, 413, 415 Dembinski, J . W., 176(206), 211 Demitras, G. C.,326(23, 26), 348(26), 408 (23), 411 de Montgolfer, Ph., 52(231a), 141 de Montgolfier, C., 17(63), 22(63), 25(63), 38 (63), 50(63), 126(63), 129(63), 136 Dempsey, E., 65(273), 68(275), 142 Denise, B., 372(147), 414 Derouvane, E. G., 37, 89(354), 112(354), 123 (157), 114, 139 Desjongueres, M. C., 160(97), 161(97), 208 Desquesnes, W., 110(425f), 146

472

AUTHOR INDEX

Dessing, R. P., 188(248), 192(248, 257), 193 248), 200(248), 212,213 DeVries, K. L., 71(280). 73(280), 96(280), 97 (280), 126(280), 142 Diaz, G., 199(284), 213 Diegruber, H., 80(316), 143 Diemente, D. L., 15(46b), 28(46b), 33(46b), 78(46b, 307). 136, 143 Dietler, U.K., 341(59), 347(59), 381(182), 412, 415 Dietsche, W., 458( 1 I5), 464,466 Dillard, J. G . , 33(135b), 138 Dimmey, L. J., 25(101), 129(101), 137 Disdier, J., 107(414,415), 146 Djega-Mariadassou, G. J., 110(4251), 146 Djeghri, N., 107(417), 108(417), 146 Dmuchovsky, B., 7, 135 Dobashi, K., 465 Doherty, J. B., 437(68), 446(68), 464 Doi, Y.,48(211), 50, 96, 125(218), 140, 141 Domansky, R., 201(305, 306), 214 Dombek, B. D., 344(68), 375(166, 167, 168, 169). 376(166), 377(176), 378(166, 167, 179), 379(166. 167, 168), 380(166, 169), 381(166), 382(166), 383(166), 384(68), 385(68, 179), 386(68, 166). 387(166, 167, 179), 388(176, 190, 191), 389(191, 192, 193). 390(193), 391(190, 191, 205). 392 (191), 393(176), 394(191), 395(190, 191, 193). 396(190, 191), 397(190, 191, 193), 398(190, 193), 399(193), 400(191), 401 (191), 402(193), 403(191, 210), 404(191, 210), 405(193, 210), 406(193), 412, 414, 415 Dominguez, J. M., 200(300), 214 Don, J. A., 200(299), 214 D’Orazio, L. A., 9(36). 135 Dori, Z., 15(49), 27(49, 102b), 28(49, 102b). 79(49), 130(49, 102b), 136, 137 Dougherty, S. J., 368(130), 413 Doyle, M. J., 375(163), 379(163), 381, 382 (163), 414 Drago, R. S., 14(48), 15(48), 24(48), 28(48), 78(48, 304, 308), 79(48, 304). 133, 136, 143 Driessen, J . M., 201(312), 214 Duck, M. J., 39(168), 139 Dufaux, M., 13(45), 20(45), 43(45, 191), 44(45), 51, 124(45. 191), 125(191), 128 (45), 160(113), 136, 140, 141,209

Dufour, G., 160(125), 161(125), 209 Dugas, H., 462 Duke, J . M., 367(120), 413 Duplyakin, V. K., 191(250), 212 Durham, P. J., 155(51), 207 Dutel, J. F., 35(152), 73(152), 81(152), 129 (152). 139 Dykstra, R. W., 401(206), 415 Dyrek, K., 38(166), 54, 55, 56, 57, 125(242), 130(453b), 132(453b), 139, 141

E Eachus, R. S., 82(324, 325), 144 Eastland, G. W., 16(60), 28(60), 136 Eastman, D. E., 161(140), 210 Eatorigh, D. J., 363(114), 364(114), 413 Eberhardt, W., 5, 135 Echigoya, E., 53, 102, 103(399), 121, 141, 146, 147 Eckle, E., 421(29), 426(29), 463 Edwards, P. R.,82(324), 144 Edwards, A. J., 131(457), 147 Egelhoff, W. F., Jr., 160(132), 161(132), 209 Eguchi, T., 369(139), 370(139), 371(139, 144), 377( 144), 414 Ehrenreich, H.,155(49), 207 Eiki, T., 450(89), 465 Eischens, R. P., 158(77), 198(281), 208, 213 Eisenbeis, A., 329(40), 411 Eisenberg, R., 326(1 I), 410 Elek, L. F., 326(14), 410 Eley, D. D., 28(111), 29(111), 58(111), 125 (1 1I), 137, 157(71), 199(289), 208,213 Ellgen, P. C., 326(15), 410 Emert. J., 429(46), 464, 465 Emmett, P. H., 326(4), 327(4), 373(4), 410 Engels, A., 201(308), 214 Englehardt, P. A., 193(258), 213 Erley, W., 157(70), 208 Erskine, R. W., 32(134c), 98(134c), 133(134c), 138 E d , G., 154(41), 157(72, 73), 207,208 Evans, J. C . , 35(149b), 131(149b), 139 Everett, D. H., 153(29), 207 Ewing, G. E., 5, 131(22b), 135 Eyal, A., 44,124(193), 140 Eyring, E. M., 71(280), 73(280), 96(280), 97 (280), 105(404b), 126(280), 142, 146, 434 (55), 464

473

AUTHOR INDEX

F Fabian, D. J., 152(14),206 Fachinetti, G., 343.412 Fadley, C. S., 161(138),209 Fagan, P. J., 344(70), 412 Fahey, D. R., 330(42, 43), 336(43), 339(42), 345, 347, 350, 351, 352, 373(43), 374,407 (43), 41 1 Falbe, J., 340(57), 341(57), 412 Falconer, W. E., 131(457), 147 Falikov, L. M., 155(56), 160(95), 161(95),207, 208 Fargues, D., 160(104), 209 Fastrez, F., 458( 108).465 Faulkner, J. S . , 155(50,57), 207 Feder, H. M., 328, 329(35, 37, 38), 330(38), 331(38), 332(38), 333(36, 38). 334(38), 335, 336(37), 337(36), 338(36, 38). 339 (36), 341(36), 342, 343(35, 36). 344(36, 37), 345(37), 347, 348(37), 407(35), 411 Fee, J . A., 2(5), 134 Fehsenfeld, F. C . , 9(40), 136 Feighan, J . A., 164(163), 177(163), 210 Ferguson, E. E., 9(40), 136 Fessenden, R. W . , 129(451,452), 147 Fiato, R. A., 371(143), 373(148, 149, 150),414 Field,B.O.,32(134~),98(134c), 133(134c),138 Figueras, F., 51(228), 141, 184(236), 199(284), 212,213 Filimonov, V. N., 6(24d, 24e), 34(24d), 56 (24d), 131(24d, 24e), 133(24d, 24e), 135 Finn, B. P., 326( l3), 410 Fischer, C. B.,194(263), 213 Fischer, F., 31(123), 138 Fischer, T. E . , 194(264), 213 Fish, M. J., 249,270 Fisher, G. B., 131(460), 147 Floriani, C., 387(188), 415 Forster, H., 131(466), I47 Foger, K., 172(194, 195), 174(195), 201(304), 211,214 Fokina, E. A., 53(233), 125(233), 141 Fonseca, R., 353(97,98), 377(97,98), 412,413 Ford, P.C . , 385(186), 404(209), 415 Forissier, M.,51(228). 110(425g), 141, 146 Formenti, M., 105, 106(406), 107(406, 413, 416), 108, 146 Fouilloux, P.,159(92),208 Fournier. J. T., I1(43a), 136

Fournier, M., 24(97), 137 Foyos, J., 421(14, 17), 463 Fox, J. S., 307(28), 322 Fraissard, J., 97(385), 145 Francis, S. A,, 158(77), 208 Frennett, A., 179(219), 212 Frety, R., 197(272), 213 Fricke, R., 28(107), 29(107), 48, 103, 121, 124 (107), 137 Friebele,E. J., 22(91), 59,127(91), 128(91),137 Frimer, A,, 6(24c), 135 Freed, J. H.,13(43b), 17(66), 19(43b), 20(66), 21(66), 23(66), 24(66), 27(66), 45(43b), 46(66), 48(66), 106(66), 124(66), 128(66), 136 Freerks, M. C., 7(26c), 135 Frohning, C. D., 326(6,9), 410 Frolkina, I. T., 156(63), 208 Frolov, A. M., 49(216), 53(216), 141 Froment, C. F., 160(119), 209 Froment, G. F., 297, 319,322 Fruend, T., 90(362), 144 Fthenakis, V., 314(37), 320(54, 55). 322,323 Fuentes, S., 184(236), 212 Fuggle, J. C . , 152(14). 206 Fuhrman, H. S., 428(44), 434(44), 435(44), 464 Fuijiwara, T., 191(251), 212,421(28), 463 Fujii, Y . , 218(53), 270 Fujita, K., 427(38), 429(47), 437(47, 67). 440 (47), 446(47, 82), 452(99), 459(116a, 116c),460(116a, 116c), 463,464,465,466 Fukawa, H., 217(1, 2, 3, 4, 5), 220(1,2, 3,4), 221(2), 222(2, 3), 224(1), 230(2, 3, 4, 5). 231(2, 3, 4, 5), 269 Fukuda, T., 217(5), 230(5), 231(5), 269 Fukuda, Y.,217(6, 7), 220(6), 230(6, 7). 231 (6, 7), 248(6), 269 Fukuzawa, S., 33(139), 40(171), 45(139), 124 (171), 125(171), 138, 139 Fukuzumi, S., 20(71), 37(71), 78(303), 123 (71). 128(71), 136, 143 Fumagalli, A., 357(107), 369(107), 413 Fung,S. C., 160(117, 118, 121),209

G Gaebler, W., 160(134), 161(134), 209 Galizzioli, D., 143 Gallezot, P., 160(112), 209

474

AUTHOR INDEX

Gallivan, J . B., 31(126), 90(126), 138 Gambarotta. S., 387(188), 415 Gamble, F. T., 82(330), 144 Garbowski, E., 73(281), 126(281), 142 Gardner. C. L., 90(357,358),91(357,358), 144 Garin, F., 170(189), 172(196, 197, 198), 173 ( I 98). 174(196. 197, 198), I76( 198). 211 Garrone, E., 38, I12(427), 139. 147 Garten, R.L., 160(1 17), 209 Garuyuk, L. N., 40(176), 106(176), 139 Gaspard, J. P.,154(40), 207 Gates, B. C . ,99(388), 118(428), 120(428), 145, 147

Gault, F. G . , 163(154, 157), 164, 166(157), 167, 169(157), 170. 171(192), 172(157, 196, 198, 199). 173(157, 198). 174(157, 196, 198), 176, 185. 186(157). 188(269), 192(157), 193(199, 258). 195(157), 200 (270), 210,211,213 Gavezotti. A., 167(182),211 Geddes, A., 421(31), 463 Geenen, F.V., 102, 145 Gelas, M., 320(56), 323 Gelb, R. I., 428(44), 434(44), 435, 464 Gel’bstein, A. I., 156(63), 208 Geltham, S., 9(38), 88(38), 135 Gerensen, L. J., 160(129), 161(129), 209 Germain, J. E., 110(425b, 425c, 425d, 425e, 425h), 146, 163(151), 176(151), 210 Getz, D., 15(49), 27, 28(49), 79(49). 130(49), I36 Gezalov, A. A.. 40(175), 57, 106(175, 248), 125(248), 139. 142 Ghiotti, G . , 38(164b, 165). I3Y Giamello, E., 52, 128(231b), 141 Gibbens, H. R., 191(252),212 Gibbs, J. W., 317, 322 Gideoni, M.,20(73), 43(192), 45(73), 124(73, 192). 128(73), 136, 140 Giguere, P.A., 86(343), 144 Gillet, M., 159(90. 91). 208 Gisser, H., 40(174b), 139 Gladysz, J. A,, 344(69). 412 Gland, J. L., 131(460), 147 Goddard, W. A., 16(55), 59(55), 136 Goetschel, C. T . , 176(305), 211 Goetz, R.W., 374(159), 414 Goldberg, 1. B., 3(9), 33(136), 106(9), 134,138 Goldwasser, I., 307, 322

Golumbic, M.S . , 326(3), 327(3), 410 Goman, A. A,, 6(24c), 135 Gomez, R.,199(284,285),2 / 3 Gonzalez-Elipe, A. R., 7(26h), 22(87, 90), 26 (87), 30(90), 47(205), 50(87), 90(205), 91 (205). 94(205), 106(205, 408), 124(90), 126(87). 129(87,90), 135, 137, 140, 146 Gopel, W., 32,40(180), 124(180), 138,140 Gordy, W., 25(101), 129(101), 137 Gorokhovatskii, Y.B., 40(176), 106(176), 139 Gosbee, J., 28(105b), 29(105b), 74(105b). 77 (105b). 126(105b), 129(105b), 137 Could, C. W., 318(46), 322 Graham, G. M., 3(10, I I ) , 134 Graham, W. A. G . ,386(187), 415 Grange, P.,51(222), 141 Grant, B., 77(295), 143 Grasselli, R. K., I18(429, 430, 431). 120(431, 436,438), 147 Gravelle, P.C . , 45( 196). 47(208), 75(289), 106 (208). 126(289), 140, 143, 156(61), 208 Gray, H. B., 32(134b), 138 Gray, P.,8(35). 135 Greaves, E. O . , 130(467), 147 Gregorio. G., 416 Gresham, W . F., 326(20), 328(31,32,33), 329, 346(77), 348(32), 349(31, 32, 33), 359 (31). 384(20), 410,411, 412 Gressmann, K. N., 156(68), 208 Grieger, R.A., 435(60), 464 Griffiths, D. W., 458(109), 465 Griffiths, D. W. L.,4, 32,54, 131(19a), 134 Griffiths, J. E., 131(457), 147 Griffiths, T. R.,34(145a), 139 Griscom, D. L., 22(91), 59(91), 127(91), 128 (91), 137 Griva, A. P.,91(364), 92(364), 94(364), 96,97, 99(364), 145 Groenewegen, J. A., 177(283), 189(249), 194 (249), 195(283), 201(249), 212, 213, 252 (73), 261(79), 271 Gross, L. H., 267,271 Grundig, H., 31(123), 138 Grunze, M..160(131), 161(131), 162,209,210 Gryder, J. W., 145 Grzybowska, B., 120(437b), 147 Guastini. C., 387(188), 415 Guczi, L., 166(177), 171(177), 179, 199(286, 295), 211,213,214

AUTHOR INDEX Guggenheim, E. A., 153(36), 207 Guidot, J., 160(116), 209 Guilliot, G., 437(68), 446(68), 464 Guillory, J. P.,7, 135 Gundrizer, T. A,, 6(22f), 41(22f), 124(22f), 133(469), 135,147 Gustafson, B. L., 22(83b), 73(83b), 81, 126 (83b). 129(83b), 137 Gyorffy, B. L., 155(51),207

H Haber, J., 2(2), 106(409), 109, 121(439), 134, I36 Habgood, H. W., 61(239), 64, 65(239), 73 (239), I41 Haensel, V., 150(6), 162(144), 197(6), 206,210 Hagan, A. P.,56, 141 Hagen, D. I., 192(256), 200(256), 213 Haining, I. H. B., 197(274), 213 Hale, J. W., 88(349), 90(349), 144 Halko, D. J., 7(26i), 135 Hall, J. L., 8(32), 9(32), 135 Hall, W. K., 64(270), 307, 142, 322 Hallam, H.E., 4(19a), 32(19a), 54(19a), 131 (19a), I34 Haller, G. L., 199(294,296), 200(296), 214 Hamaguchi, H . H., 465 Hamilton, J. A,, 421(31), 463 Hamilton, J. F., 160(107, 128, 130), 161(128, I30), 209 Hammaker, R. M., 158(77), 208 Hammond, M., 450(91), 451(91), 465 Haneman, D., 4, 7, 14, 28(47, 114), 29(47, 114). 30(47, 114), 91(363), 126(47, I14), 134, 136, 138. 145 Hansen, L. D., 434(61), 464 Hanson, L. K., 79(314), 143 Hansson, G., 32(135a), 138 Happel,J.,274,277(1),287,301,308,314,319, 320,321,322,323 Harada, T., 21 7( 13, 20, 22, 33, 34, 35, 37, 39, 40, 41, 42, 43, 44,45, 46, 47, 48, 49, 50, 51, 52a. 52b, 52c, 52d), 222(22, 52a), 223 (22,47), 224(34, 37,47), 225(34, 50), 229 (34). 230(47), 231(33, 47). 232(47), 234 (49, 52a). 235(51), 238(37, 41, 51), 240 (42,46,47), 241(34,47), 243(50, 51, 52a, 52b, 52c, 52d), 245(46, 50, 52d), 246(50,

475

52d). 249(33, 34, 52b), 255(39), 256, 263 (49), 266(37, 46, 48, 52a, 52c, 52d), 267 (37,47), 269,270 Harata, K., 421(18, 19, 20, 21, 22, 23, 24, 25, 26, 27), 426(24), 431, 433(22, 27), 434 (65), 463, 464 Harberts, J. C. M.,156(67), 201(67), 208 Hardeveld, van, R., 159(84), 208 Harding, M. M . , 421(30), 426(30), 463 Hardy, W. A., 199(292), 213 Hariharan, N., 82(326), 144 Harmsworth, B. J., 88(349), 90(349), 144 Harrison, A. M., 401(206), 403(210), 404(210), 405(210), 415 Harrison, P.G., 131(464), 133(464), 147 Hart, P. W., 366(119), 391(205), 413,415 Hartmann, W., 47(203), 124(203), 140 Hartsuck, J. A,, 436(66), 464 Harvey, J. S . M.,3(10, I t ) , 134 Hasegawa, A., 59(253), 125(253), 141, 142 Hashiba, K., 154(46), 207 Hassan, L. A. R.,106(407), 146 Hatta, A,, 250(71,72), 270,271 Hattori, K., 86(347, 348), 144 Hattori, T., 121(444), 147 Hatzenbuhler, D. A,, 31(133b), 131(133b),138 Haul, R., 47(203), 90(359), 91(359), 124(203), 140, 144 Hauser, C., 33( 140), 46, 138, 140 Hausmann, A,, 39(169), 139 Hawthorne, W. P.,61(259), 142 Hayes, J. C . , 150(6), 197(6), 206 Headford, C. E.L., 348(84), 412 Heaton, B. T., 357(107), 369(107, 139), 370 (139), 371(139, 144). 377(144), 413,414 Heden, P.H., 152(12),206 Hedman, J., 152(12), 206 Heine, V., 160(94), 161(94), 208 Heinemann, H., 162(146), 210 Held, A. M., 7(26i), 135 Helle, J. N., 153(24), 192(24), 198(280), 207, 21 3 Hemidy, J. F., 31(129, 130), 37(129). 38(129), 123(129), 138 Hendra, P.J., 5(21), 6(21), 31(21), 34(21), 35 (21). 114(21), 131(21), 135 Hendricksen, D. E., 326(1I), 410 Henglein, F. M.,434(57), 435(57), 464 Hennrich, N., 419(4a), 462

476

AUTHOR INDEX

Herman, K., 86(343), 144 Herman, R. G . . 326(13), 410 Herrmann, J. M., 107(414,415), 146 Herschbach, D. R., 13(44), 28(44), 127(44), 128(44), 136 Hersch, C. L., 440(73), 446(83), 464, 465 Herzberg, G., 3(6), 6(24f), 8(28), 9(6,28), 131 (24f, 454). 134, 135, 147 Herzberg. L., 6(24f), 131(24f), 135 Hettler, H., 418(3b), 462 Hiatt, R..318, 322 Nigashi, F., 217(23), 223(23). 243(23), Higuchi, T., 427(38), 463 Hill, C. G., Jr., 310, 323 Hill, T. L., 432,464 Hilsch, R., 31(123), 138 Himmelblau, D. M., 319,322 Himpsel, F. J., 161(140), 210 Hindermann, J. P.,201(312), 214 Hingerty, B., 421(9, 16), 422(35), 423(9), 424 (9), 425(9), 428(9, 35). 432,463 Hiraki, Y., 217(34,45. 52c), 224(34), 225(34), 229(34), 240(52c), 241(34), 243(52c), 249 (34). 256, 266(52c), 269, 270 Hirata, F., 434(64). 464 Hirayama, F., 458(11 I), 466 Hirota, K., 96(381), lOO(381). 145 Hirotsu, K., 427(38), 463 Hirsch, J. A., 431(49), 464 Hnatow, M. A., 308, 31 I . 314(36), 322 Hodges, R. J., l66(176), 211 Hoefs, E.. 121(443). 147 Hoek, A., 252(74), 261(74), 271 Hoffman, B. M., 15(46b), 24(98), 28(46b), 33 (46b), 78(46b, 307), 79(98,310,313,314), 94(99), 136, 137, 143 Hoffman, G. A., 371(144), 377(144), 414 Hoffman, R., 347(83), 384(83), 412 Holocek, D. R.,339(53), 340(53), 348(53), 377 (53). 41 I Holroyd, P.,18, 19(68), 20(68), 30(68), 36(68), 37(68), 101(68), 123(68), 128(68), 136 Honrath, W., 31(124), 138 Hori. Y., 17(65c), 136 Horiuti, J., 275,276(7), 280,291,292,294,321 Horsfeld, A., 95(378), 145 Horsley, J. A., 160(118), 209 Hougen, 0. A., 296,298,322

Howard, J. A., 24(95), 130(453a), 137, 147 Howe, R. E., 50,51(219), 53,96,125(219), 141 Howe, R. F., 16(61), 20(61), 25(61), 28(105b, 115, 116), 29(105b, 115, 116). 51, 53(61, 234). 74(105b), 77(105b), 79(115, 116). 80, 125(61,226), 126(105b, 115, 116). 129 (61. 105b, 116), 131(463), 136, 137, 138, 141, 147 Hucknall, D. J., 51(220), 118(432), 141, 147 Hiifner, S., 152(13, 19), 187(13), 206, 207 Hugues, F., 76(292), 143 Hunter, W. G., 435(60), 464 Hurst, J. K., 7(26i). 135 Hutta, P. J., 82(321), 143 Hwang, J. T., 31(133c), 97(133c), 131(133c), 138 Hyble, A., 421(8), 422,423(8), 462

I Ibach, H., 157(70), 208 Ibarbia, P. A., 326(14), 410 Ichikawa, A , , 219(60a, 60b), 220(60a, 60b), 2 70 Ichikawa, K., 152(18), 207 Ichikawa, M., 326(16, 17), 410 Iida, H., 459(118), 466 litaka, Y.,421(33), 463 Iizuka, T., 37(161, 162), 139 Ikeda, B., 434(62, 63). 464 Ikezawa, M., 31(125), 138 Imachi, Y.,217(48, 51), 235(51), 238(51), 243 (51). 266(48), 270 Imai, S., 268(83. 84, 85, 86). 271 Imai, T., 61(239), 64,65(239), 73(239), 141 Imaida, M., 217(2,3,4,5,6,7,8,9, 10, 11, 12, 13, 14, 15, 17, 22,47), 220(2, 3,4,6), 221 (2, Il), 222(2, 3, 10, 12, 22), 223(10, 22, 47), 224(47), 230(2,3,4,5,6,7, 10, I I , 12, 47), 231( 2, 3,4, 5, 6, 7, 8, 10, 11, 12, 14, 15, 47), 238(47), 240(47), 241(47), 248 (6). 256(8), 267(47), 269, 270 Imamura, R., 217(17), 269 Imanaka, T., 235(65a, 65b). 241(66), 267(65b), 2 70 Imelik, B., 40(174c, 184), 45(174c, 195). 59 (254), 106(174c), 124(174c, 184), 125

477

AUTHOR INDEX

(254), 139, 140, 142, 160(113), 195(268), 209,213 Imoto, T., 427(38), 446(82), 463, 465 Inami, S . H., 199(288),213 Indovina, V., 28(110), 29(1 lo), 37, 38(110), 39(110), 55(110), 81(110), 87(158), 89 (110, 158, 354), 91(110, 158), 112(354), 114, 123(157, 158, 160), 125(110), 132, I39 Ingold, K. U., 24(94b), 129(94b), 137 Ingrarn, D. J. E., 123(446), 147 Inomata, Y.,1 lO(425i). 146 Inoue. Y.,109, 146 Inouye, Y ., 459( 120), 466 Irwin, K. C . , 318(46), 322 Ishida, S . , 48(21 I ) , 100(392), 121(392), 140, I45 Ishii, Y.,51(227), 141 Ismailov, E. G., 40(181), 41(187), 55,65(240), 73(240), 78, 81(240), 140, 141 Isoda, T., 219(60a, 60b). 220, 270 Itai, A., 421(33), 463 Ito, K., 217(39, 44, 45, 46). 240(46), 245(46), 255(39), 266(46), 270 Ito, T., 97(385), 145 Iwakura, Y.,450(94), 465 Iwamoto, M.,39(170), 40(170), 41(170), 46, 48, 57, 90, 91(355), 101, 102, 103(393), 105(403). 106, 107(403), 117(393), 120, 124(170), 127(170), 139, 144, 145, 146 Iwasawa, Y.,78(305), 143 lyengar, R. D., 33(137), 34(142, 143), 40(172, 174a. 174b). 44(137), 45(137, 143). 124 (137, 143, 172, 174a), 138, 139 Izatt, R. M.,363(114), 364(114), 413 Izumi, Y.,216(63), 217(1, 2, 3, 4. 5, 6,7, 8, 9, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 29, 31, 32, 34, 35, 37. 38, 39, 47, 48, 49, 51, 52a, 52e), 218(53, 54, 55, 56, 57. 58, 59). 220(1, 2, 3, 4, 6), 221(2, 19, 24). 222(2, 3, 11, 12, 21, 22, 52a), 223 (21, 22, 23, 24. 47), 224( 1, 24, 32, 34, 37, 38,47,61a, 61b, 62,63), 225(34), 229(34, 63), 230(2, 3,4, 5, 6, 7, 11, 12, 15, 19, 21, 47),231(2,3,4,5,6.7,8,9,11,12,14,19, 47). 232(47), 234(49, 52a), 235(51), 238 (37, 47. 51). 238(37, 47, 51). 240(37, 47, 52d, 52e, 54), 241(34,47). 243(23, 24,29,

51, 52a), 246(52d), 248(6), 249(32, 34), 252(29), 255(39, 62, 63), 256(8), 257(61a, 61b). 262(63), 263(49), 266(37, 48, 52a). 267(37,47), 269,270

J Jacobi, K., 160(126, 127, 133, 134), 161(126, 127, 133, 134), 209 Jacobs, P.A., 76(291), 143 Jacox, M.E., 86,90(342), 97, 144 Janakiraman, R., 142 Jarjoui, M.,75(289), 126(289), 143 Jaunay, D., 197(272),213 Jayne, J. P., 36(155), 37(155), 39(155), 40 (159, 101(155), 123(155), 124(155), 139 Jencks, W.P.,462 Jenner, G., 353(97,98), 377(97,98, 173, 174). 406(173, 174). 407(174), 412,413,415 Jensen, J. L., 307(28), 322 Jerschkewitz, H. G., 28(107), 29(107), 48 (107), 103(107), 121(107), 124(107), 137 Jiru, P.,120(437a), 147 Jogun, K. H., 421(29), 424(36), 426(29), 453 (36). 463 Johnson, M. F. L., 201(310), 214 Johnson, R. F., 428(44), 434(44), 435(44), 464 Johnston, H. S . , 8(31a), 135 Jonas, J., 369(139), 370(139), 371(139, 144). 377( 144), 414 Jonassen, H. B.,411 Jones, R. D., 34(148), 78(148), 79(148, 310). 131(148), 139,413 Juillet, F., 47(208), 106(208), 107(416), 140, 146 Julg, A,, 95(374), 145

K Kacmarek, A. J., 87(346, 347, 348), 144 Kadelka, J., 329(40), 411 Kadushin, A. A., 55(241), lOO(391). 125(241), 141, 145 Kallne, K., 152(14), 206 Kalechits, I. V . , 176(209), 211 Kallo, D., 307, 322

478

AUTHOR INDEX

Kamei, Y ., 434(62), 464 Kamholz, K., 308(32), 322 Kampe, W., 458(107), 465 Kane, A. F., 192(254), 213 Kaneko, Y . , 314(36), 322 Kanzaki, N., 28(105a), 29(105a), 64(267), 65 (105a. 267), 70, 72, 74(267), 91(105a), 126(267), 137, 142 Kanzig, W . , I I , 15, 18(69b), 20(69b), 35(153). 123(42,69b), 128(69b), 136, 139 Kapicak, L. A,, 364(116), 367(116), 413 Kaplan, L., 337(46, 46a. 46b, 46c), 351(90), 352(90, 92, 93, 94), 353(94, 96). 355(99, 100, 101). 356(90,92, 104). 357(108, 109). 358, 359(108, 109). 362(99, 113). 363 ( 1 13). 365(92, 93, 94, 96, 118). 366(90, 92,94), 367(90,94,96, 104,108,109), 374 (100, 101). 377(46, 46b, 4 6 ~ ) .382(46), 411,412,413 Karpinski, Z . , 153(23), 193(259), 207, 213 Karra, J. S . , 33(137), 44(137), 45(137), 124 (137). 138 Kasai, P. H., 31(127b), 63(264), 64(127b,272), 65,67.68,71(272), 126(127b), 127(127b), 128(127b. 272). 138. 142 Kashiwabara, 17(65c), 136 Kashiwazaki. T., 48(212b), 101(212b), 124 (212b), 140 Katzer, J. R., 78,99(388), 118(428), 120(428), 126(302), 143. 145, 147, 183,212 Kaufherr, N., 20(73), 45(73), 90(356), 106 (356), 124(73), 128(73). 136, 144 Kawakubo, H., 459(116a, 116b, 116c), 460 (116a. 116b. 116c),466 Kawazu, M., 443(79). 465 Kaye. G . W. C., 136 Kazanskii. B. A,, 163(153). 210 Kazansky. V. A,, 176(207), 211 Kazansky, V. B., 7(26e). 16(54), 22(86), 25 (86). 28(106, 109, 117). 29(106, 109). 30 (109). 45(197), 46, 47(202), 48(106, 117, 210), 49(198), 50(86, 217). 51(117, 198), 52(109), 59(252), 73(284), 78(301), 81 (284, 301, 318, 319a), 91, 92(364, 365, 366, 367), 93(198, 367. 369). 94(364, 365, 369, 370, 371, 372). 96(383), 97 (383). 99(364. 367), I12(106), 124(106, 197). 125(86, 109, 198, 252, 447). 126(86,

447). 129(86, 447), 135, 136, 137, 138, 140, 141, 142, 143, I45 Kazusaka, A,, 6, 16. 20(61). 25, 53, 125(61), 129(61). 135, 136 Kearns, D. R.,6, 135 Keen, N., 82(323), 143 Keii, T., 31(121), 68(121), 71(279), 97(279). 125(121), 126(121,279), 138, 142 Keier, N. P., 32( 134a), 35( 149a), 50( 149a). 133 (149a). 138, 139 Keim, W.. 329(39,40.40a), 335, 336(39), 337, 338,365, 376(39), 407,411, 416 Keleman, S . R., 194(264), 213 Keller. G . E., 368(127), 413 Kellerman, R.,34( 142, 143). 45( 143). 82, 124 (143). 138, 143 Kelley, M., 154(47), 207 Kelley, M. J.. 153(35), 207 KembalLC., 164, 165, 166, 168(158), 171(158, 159). 178(159). 181( 159). 197(274), 199 (294). 210.213,214 Kemeny, E., 199(286). 213 Kerker, G., 154(39), 207 Kermarec, M.. 56(243), 73(282), 130(453b), 132(453b), 141, 142 Kerr, G . T., 63(265). 142 Kessler, J., 156(68), 208 Keulen, van, B . . 153(22), 207 Keulks, G. W . , 121, 147 Kevan, L., 17, 31(69b), 136, 142 Keys, L. K., 5(22c, 22d). 135 Khalif, V. A.. 49, 53, 140. 141 Khan, A. U., 6, 7, 7(23b), 135 Khanna, S. N., 160(136), 161(136), 209 Kibblewhite, J . F. J.. 13(45), 20(45), 31(128), 35(151), 43(45), 44(45), 87(353), 88, 96 (382), 99(382). 124(45), 128(45), 136,138, 139, 144, 145 Kida, M., 465 Kiefte, H., 3(10), 134 Kinnemann, A., 353(97,98). 377(97,98), 412, 413 Kikuchi, J., 465 Kikuchi, R., 96(380), 145 Kilty, P. A,, 76, 77, 102, 143 Kimura. Y.. 428(40), 453(102), 454(102), 455 (40, 103), 463,465 King, A. D., 342(60), 382(60). 412

AUTHOR INDEX

King, D. A., 158(78), 208 King, R. B.. 342(60), 382(60), 412 King, W. J., 31(122), 138 Kinnemann, A., 377(173, 174), 406(173, 174), 407(174), 415 Kirino, Y., 40(171), 124(171), 139 Kirkpatrick, S . , 155(49), 207 Kirstein, W., 156(65. 68). 208 Kitaguchi, H., 440(76), 465 Kitajima, N., 78(303), 143 Kitaura, Y., 450(95), 465 Kitko. D. J., 14(48). 15(48), 24(48), 28(48), 78 (48). 79(48), 136 Kitson, M., 77(296), 143 Kittler, R.C . , 155(56). 207 Kiyosuke, Y., 431(48), 432(64), 464 Klabounovski, E. I . , 243(67,68,69), 259,267, 270, 271 Klar, B., 422(35), 463 Klasson. M., 152(12), 206 Kleiman, G. G . , 152(17), 206 Klein, M. J.. 86(348), 144 Kleppa, 0. J., 199(293), 214 Klier, K., 82(321), 143, 326(13), 410 Kljushnikov, 0. L., 152(12). 206 Kluksdahl, H. E.. 150(6), 197(6), 206 Knifton, J. F., 375(170,171, 172). 377(177), 379(171), 380, 386, 388(171, 191), 389 (170, 171, 172, 197), 390(197, 203, 204). 391(199, 200, 201, 202, 203, 204), 394 (171). 400(199,200), 407(172), 414,415 Knowls, J. R.,443(78), 445(78), 449(85), 465 Kobatake, S., 217(38), 224(38). 270 Koby1inski.T. P., 326(13), 377(175), 407(216), 410, 415, 416 Kodama, S., 79(418), 107, 146 Koermer, G. S . , 339(54), 411 Koetzel, T. F., 357(107), 369(107), 413 Koga, K., 421(34), 463 Kogan, S. B., 214 Kokayeff, P., 320(54), 323 Kokes, R.J., 4(17), 39(17), 134, 145 Kolb,D. M.,160(126. 127). 161(126,127),209 Kolosov, A. K., 28(109), 29(109), 30(109), 52 (109), 125( 109). 137 Komarova, M . P., 5(20b), 12(20b), 31(20b), 47(20b), 124(20b), 127(20b), 131(20b), 133(20b), 134

479

Komatsu, S., 217(1, 5),220(1), 224(1), 230(5), 231(5), 269 Komiyama, K., 458(110), 465 Komiyama, M., 423(7), 436(7), 462 Kon, H., 3(12a), 134 Kon, M. Y . ,92(365), 94(365), 145 Kondo, T., 164(161), 166(161), 210 Kooser, R . G . , 16(57), 75, 136 Koscielski, T., 193(259), 213 Kosinski, K., 106(409), 109, 146 Kostikov, S. V . , 44(194), 124(194), I40 Kotov, A. G . , 102(398), 146 Kouno, I., 459(121), 466 Kouwenhoven, A. P., 375(163), 379(163), 381 (163). 382(163), 414 Kozuka, M., 133(468), 147 Krebs, J. J., 28(113), 29(113), 30(113), 126 (113), 137 Krenzke, L. D., 121(442), 147 Kresge, C. T . , 385(186), 415 Krug, R. R.,435(60), 464 Krylov, 0. V . , 7(26b), 49(213, 214,215,216). 51, 53(213, 215, 216, 233), 55(241), 100 (391), 102, 124(213, 214), 125(213, 233, 241), 135, 140,141, 145 Krzyanowski, S., 27(102a), 74(285), 79(102a), 126(102a, 285), 129(102a), 137, 143 Ku, R . C . , 153(30), 207 Kubokawa,Y., 59(256), 79,106, 107, 108, 125 (256), 142, 146 Kiihl, H., 155(54), 207 Kiippers, J., 154(41), 207 Kugler, B. L., 145 Kuijers, F. C . , 184(238), 200(238), 212 Kuijers, F. J., 154(43, 44, 45), 190(44), 193 (262). 207,213 Kumamoto, M.,459(121), 466 Kung, H. H., 182(226), 212 Kupferschlager, K., 329(40a), 411 Kurita, H.,443(79), 465 Kuroda, Y., 428(39b, 41), 435(58), 437(69), 438(70), 440(70, 71, 74), 441(77), 443(70, 71), 447(69), 448(84), 449(84), 450(92, 96), 451(92), 453(41, 100, 101, 102), 454 (100, 102), 463,464,465 Kuznicki, S. M.,71, 73(280), 96(280), 97, 105 (404b), 126(280), 142, 146 Kuzubor, B. N., 191(250), 212

480

AUTHOR INDEX

Kwan, T.. 33(139), 40, 44,45(139), 124(171, 182). 125(171, 182). 138, 139,140 Kwiram. A . L., 13(44), 28(44), 127(44), 128 (44). 136

L

Lewis, P. H., 160(122), 209 Libermann, A. L., 163(153). 210 Libre, J. M., 120(437b), 147 Liddy, J. P., 131(463), 147 Liengme, B. V., 64(270), 142 Lin, J.-J., 391(202), 415 Lin, M. J., 22(83b), 73(83b), 81(83b), 126 (83b). 129(83b), 137 Linder, K., 421(11, 32),424(11),425(11),433 (1 I), 463 Lindsay, D. M., 13(44), 28(44), 127(44), 128 W), 136 Ling, D. T., 156(64), 157(69), 208 Linnett, J. W., 199(292),213 Lipatkina, N. I., 7, 22(86), 25(86), 50(86), 92, 93, 99(367), 125(86, 447), 126(86). 129 (86,447), 135, 137. 145, 147 Lipgart, E. N., 243(69), 270 Lippincott, E. R.,131(455), 147 Lipscomb, W. N., 8(34), 135,436(66), 464 Lipsett, F. R.,31(122), 138 Lipsey, C., 437(68). 446(68), 464 Lischke, G., 28(107), 29(107), 48(107), 103 (107), 121(107), 124(107), 137 Lock, C. J. L., 130(467), 147 Lofthouse, M. G., 31(131), 56(244c), 87(131), 138, 141 Loginov, A. Y., 44, 124(194), 140 Lohmann, D. H., 8(27), I35 Long, C. A., 5, 131(22b), 135 Longoni, G., 369(135). 371(135), 414 Lorenzelli, V., S(19b). 32(19b), 54(19b), 131 (i9b), 134 Losee, D.8.. 28(112), 29(112), 58, 125(112),

Laby, T. H., 136 3(9), 106(9), I34 Laeger, H. 0.. Laine, R. M., 373, 414 Lam, Y. L., 199(287). 213 Lambert, R. M., 77(295, 296.297). 143 Lambin, Ph., I54(40), 207 Landau, D. P., 157(74).208 Laner, M., 450(91), 451(91), Langwitz, A,, 201(308), 214 Lankhorst, P. P., 176(203,204), 177(204), 180 (203, 204). 184, 186(204),201(204), 211 Latour. J. M.. 79(31 I , 312). 143 Laufer, D. A,, 428(44), 434(44), 435(44), 464 Launay, J . P., 24(97), 137 Law, D. S., 109(424), 146 Lawson, T., 21(80), 40(80), 83, 86, 87(353), 88, 106(338), 124(80), 128(80), 137, 144 Lazzaroni, R., 343(61), 412 Leal, 0.. 81(320). 143 Lebedev, V. M., 156(63),208 Le Cardinal, G., 320, 323 Leclercq, C., 184(236), 212 Leclercq, G., 171(190). 180, 197,211,212,213 Leclercq. L., 171(190),211 Le Courtier, Y., 319(49b), 322 Ledoux, M. J., 167(183, 184, 185). 211 Lee, E. P. F., 109(424), 146 137 Lee, L., 277(13), 287,321 Lott, K. A. K., 34(145a), 139 Lee, P. A., 153(28), 207 Louis, C., 21(84), 22(84, 87). 23(84), 26(87), Lee, S. T.. 160(129, 135). 161(129, 135). 209 W87). 51(84), S2(84), 126(87). 127(84). 34(145b), 139 Legendre, 0.. 128(84), 129(84, 87). 137 Lehn, J. M., 443(80), 465 Lowry, R. P.,409(219), 416 Lehwald, S., 157(70), 208 Loza, G. V., 163(153), 210 Leith, 1. R., 51, 125(226), 141 Lubitz, W., 80(316), 143 Lemke, B. P., 4, 7, 91(363), 134, 145 Ludlum, K. H., 198(281), 213 Leonard, A . L., 197(277). 198(277), 213 Luk’kanov, A. E., 28(118), 30(118), 48(118), L’Eplattenier, F., 415 138 Leupold, E. I., 326(18, 19), 410 Lumpov, A. 1.. 47(202), 81, 140, 143 Lever, A. B. P., 32(134b), 138 Lumry, R., 435(59), 464 Levine, J., 8(32), 9(32), 135 Lunsford. J. H.. 2(3), 12(3), 15(51), 21(81), 22 Levine. J . D., 112(426), 146 (51, 81, 83b). 28(51. 103, 104, 115, 116), Lewis, E. A,, 434(61), 464 29(51, 103, 104, 115, 116). 36(155). 37

AUTHOR INDEX

(155). 39(155), 40(155), 51(81), 58(104), 62, 64, 65(263), 67, 68, 69(81, 103, 104), 70,71(103), 73(83b), 79(81, 115, 116),80, 81(83b), 83(339), 84(334, 335). 85(334, 335), 86, 88(334), 89(334), 90, 91(334, 355), 92(334), 96(3, 379). 101. 102, 103 (393), 104, 105(403), 106, 107(403), 117 (393), 120, 123(155), 124(115), 125(81), 126(51, 83b, 103, 104, 115, 116), 128(81, 334), 129(51,81,83b, 116), 134,136,137, 138,139, 142,144, 145, 146 Luyten, L. J. M., 201(303), 214 Luz, Z., 83(340), 128(448), 144 Lyashanko, L. V., 40(176), 106(176), I39 Lytle. F. W., 153(27), 207 Lyubimova, 0.I., 102(398), 146

M McAlister, D. R., 404(21 I), 416 McAteer, J. C., 28(108), 29(108), 31(108), 51 (228), 52(108,229), 58(108), 106(108), 125 (108). 137, 141 McCain, D. C., 16, 136 McClellan, S., 77(299), 126(299), 143 McDonough, J . M., 86(347), I44 McGill, T. C . , 16(55), 59(55), 136 MacGvern, K. A,, 432(51), 464 Machlin, E. S . , 153(34), 207 Mclntosh, D., 31(127a), 77(127a), 97, 131 (127a), 138 McKee, D. W., 199(291), 213 Mackensen, G.. 419(5), 462 Mackenson, G., 439(75), 443(75), 464 Mackenzie, J . R., 5(21), 6(21), 31(21), 34(21), 35(21), 114(21), 131(21), 135 McLachlan, A. D., 82(331), 83(337), 87(331), I44 Maclennan, J. M., 421(12, 30), 424(12), 425 (12). 426(30), 433(12), 463 McMullan, R. K., 421(14, 17). 463 McVicker, G . B., 197(271,273), 213 Madix, R. J., 131(461), 147, 156(66), 208 Mah, R. H. S., 318, 320,322 Maine, G., 193(258), 223 Maire, G., 163(154), 170(189), 210, 211 Maitlis, P. M., 130(467), 147 Maksic, Z., 212 Maksimov, N. G., 5(20b), 6(22f), 12(20b), 31 (20b), 40(181), 41(22f, 187), 47(20b), 124

48 1

(20b, 22f). 127(20b), 131(20b), 133(20b), 134,135, 140 Maksimovskaya, R. I., 51(224), 125(224), 141 Malkova, A. I., 142 Manninger, J., 193(260), 213 Manonque, W. H., 183(228), 212 Manor, P. C . , 422(35), 428(35), 463 Marachi, C., 16(59), 25(69), 27(59), 136 Marchon, J. C., 79(311, 312), 143 Maricot, P., 183(231), 184(231), 212 Mariot, J. M., 160(125), 161(125), 209 Mark, P., 112(426), 146 Marko, L., 340(56,57a), 341(56), 343(64), 381 (56), 411,412 Marks, T. J., 344(70), 412 Marques, A. R., 110(4251), 146 Mars, P., 120, 147 Marsden, K., 348(84), 412 Martin, G. A., 183(233), 195(268), 212, 213 Martin, S. T., 356(105), 413 Martinengo, S., 357, 369(106, 107), 370(141), 371(142), 372(146), 413,414 Martins, J. L., 160(102), 209 Masai, M., 191(251), 212 Mashchenko, A. I., 45(197), 124(197), 140 Masini, J. J., 167(183, 184), 211 Mason, M. G., 160(129, 130, 135). 161(129, 130, 135), 209 Massardier, J., 22(89), 25(89), 63(266), 65 (266), 67(266), 68(266), 69(266), 91(266), 126(89,266), 137,142 Massey, N. S. W., 88(351, 352). 144 Masters, C., 326(10), 375(10, 162), 410, 414 Mathieu, M. V., 195(267), 213 Matsuda, T., 403(208), 415 Matsuda, Y . , 465 Matsui, Y., 450(86, 97). 464, 465 Matsumoto, K., 96(380), 145 Matsumura, T., 78(303), 143 Matsumura, Y ., 458( 1 14), 466 Matsunaga, K., 217(12), 222(12), 230(12), 231 (12), 269 Matsushita, K., 428(39b), 463 Matsuura, I., 51(227), 141 Matsuzaki, T., 48(2 12b), 100(392), I0 I(212b). 121(392), 124(212b), 140,145 Matusek, K., 199(286), 213 Maubert, A,, 199(284), 213 Maurel, R., 171(190), 180, 197(272), 211,212, 213

482

AUTHOR INDEX

Maxwell, 1. E.. 61(262), 142 May, C. J., 386(187), 415 Meeley, M. P.,443(81). 465 Meeloy, M. P., 434(54a). 464 Mehta, M., 161(138). 20Y Meistrich, M. L., 13(46a), 136 Melamud, E., 15(49), 20(78), 21(92), 22(92), 27(49, 102b). 28(49,102b), 79(49), 129(92, 449), 130(49, 102b). 136, 137, 147 Melander, L., 344(73), 412 Mendez, H., 197(272), 213 Mennon, P. G . , l60( 119). 209 Menon, P. G.. 197. 198(279), 213 Mercati, G.. 80, 126(250), 142 Meriaudeau, P., 17(62), 20(74, 75), 22(88), 25(62, 88), 27(62), 31(75), 40(74), 41, 43 (62, 190). 45(75, 196), 46(75), 47(88,208), 52(230, 231a). 90(88), 91(88). 94(88), 96 (75). 106(88, 208). 124(74, 75, 88). 126 (62, 88). 128(74, 75), 129(62, 88). 136, 137. 140

Merkulov, A. A,, 28(119), 138 Merta, R.,178(216), 181(216), 212 Messmer, P.P., 160(99), 209 Mester, Z. C. , 373(151), 414 Metcalf, L. P., 109(423), 146 Metcalfe, A,, 131(459, 463, 464), 147 Meyens, C. G., 162(147), 210 Mezaki. R.,308(31), 322 Mihelcic, D., 90(359), 91(359), 144 Mikheikin, 1. D., 45(197), 47(202), 81(318, 319a). 92(366), 124(197), 140, 143, 145 Millar, M. A., 431(49), 464 Miller, D. J., 14, 28(47, 114). 29(47, 114), 30 (47, I14), 126(47, 114), 136, 138 Miller, T. A,, 6, I35 Milligan, D. E., 86, 90(342), 97, 144 Millikan, T. H:, 162(146). 210 Mills, G . A., 162(142), 210 Milner. P. C . , 276, 281, 292, 321 Minachev, Kh. M., 55(240). 65(240), 73(240), 78(240), 81(240), 141 Mink, J., 199(286), 213 Mirra, C., 38(164b), 139 Mischenko, Y.A,, 156(63), 208 Mitani, M., 458(1 lo), 465 Mitchell, E. J., 338(48), 365(48), 398(48), 411 Miura, M., 59(253), 125(253). 141, 142 Miyahara, K., 106(412), 146, 294, 301,322 Miyamoto, A,, llO(425i. 425j). 146

Miyoshi, I., 156(60), 208 Mizokawa, Y.,40(183), 41, 124(183), 140 Mizutani, T., 440(71), 443(71), 464 Mizutani, Y.,435(58). 464 Mochida, K., 450(86), 464,465 Mochizuki, A., 437(69), 447(69), 464 Moloy, K. G . , 344(70), 412 Moni, K., 191(251), 212 Monnier, J. R.,121(443), 147 Montfoort, V., 159(84), 208 Mookerjee, A. J., 155(52), 207 Moore, P. B.,157(71), 208 Moore, W. J., 274, 275(2), 288(2), 321 Mootz, D., 421(14), 463 Mooz, D., 421(17), 463 Moran-Lopez, J. L., 154(39), 207 Moraweck, B.. I IO(425f). 146, 159(93), 208 Morazzoni, F., 80(250), 126(250), 142. 143 Mori, K., 48(212b), 101(212b), 1 lO(425i). 124 (212b), 140, 146 Morigaki, K., I55(58), 208 Moriya, Y.,250(72), 271 Moro, G., 17(66), 20(66), 21(66), 23(66), 24 (66). 27(66), 46(66), 48(66), 106(66). 124 (66), 128(66), 136 Morterra. C., 38(165), 13Y Morton, J. R., 20(77), 24(94b), 68(77), 95 (378). 129(94b), 137, 145 Moseler, R., 80(316), 143 Moskovskaya, T. E., 28(118), 30(118), 48 (118), 138

Moss, R. L., 191, 212 Mowat, W., 165(170), 210 Moyer, C. E., 368(127), 413 Mrowca, J. J., 165(169), 210 Miiller, J. M., 163(154), 171(192), 210,211 Muetterties, E. L., 326(8. 22, 23, 24, 25, 26), 327(8). 345(8), 348(26), 375(22), 408(23, 24, 25), 410, 411 Mulay, L. N., 5(22c, 22d). 135 Munch, R. H., 7(26c), 135 Munns, G. W.. Jr., 162(147), 210 Munuera, G., 7, 47(204, 205). 96(205), 91 (205), 94(205), 106(204, 205. 408). 135, 140, 146

Murakami, S., 217(42, 52c). 240(42,52c, 52e), 243(52c), 256,266(52c). 370 Murakami, Y.,IlO(425i. 425j), 121(444), 146, 147,465

Muramoto. H., 191(251), 212

48 3

AUTHOR INDEX

Muraoka, J., 217(34), 224(34), 225(34), 229 (34). 241(34), 249(34), 269 Murphy, J., 35(154), 139 Murty, T., 338(48), 365(48), 398(48), 411 Musoiu, M. J., 262(80), 267, 271 Mustafin, A. I . , 166(175), 211 Mutsaers, C. A. H. A,, 47(207), 106(207), 140 Myahara, K., lOO(389). 145

N Nabeshima, T., 440(76), 465 Naccache, C., 13(45). 17(63), 20(45. 74, 75, 76), 22(63), 24(99), 2363). 31(75), 33 (141). 34(141), 35(152), 38(63), 40(74, 174~.184). 41(74), 43(45), 44(45, 141, 191) 45(75), 46, 50(63), 51(191, 223), 52(231b), 68(76), 73(152), 80(319b). 81 (152). 83(339), 86(339). 90(360), 91(360), 92(99), 93(99), 96, 100(76), 104, 106 (174c), 124(45, 74, 75, 174c, 184, 191), 125(191), 126(63). 128(45, 74, 75, 231b), 129(63, 76). 136, 137, 138, 139. I40, 141. 143, 144, 160(113), 2OY Nagel, C. C.. 402(207), 415 Najbar, M..54. 55. 56. 141 Nakagawa. K.. 217(17), 26Y Nakajima. M.. 79(31 I ) , 143 Nakamoto. K., 77. 79(315). 131(315). 133 (468). 143, I47 Nakamura, I., 459(120), 466 Nakamura, S., 40(183), 41, 124(183), 140 Nakamura, T., 275, 280(4), 321 Nakano, A., 465 Naldrett, A. J., 367(120), 413 Narayana, M., 142 Nason, D., 153(33), 207 Naurse, H., 464 Navio, A., 7(26g, 26h), 135 Nazymek, D., 201(307). 214 Neiva, S., 268(83, 84, 85, 86), 271 Nelson, R. L., 18, 39(168), 57(246), 88, 89 (69). 90(348), 91(67), 136, 139, 141, 144 Nemethy, G., 428(45), 464 Nemmonov, S., 152(12), 206 Nepijko, A. S., 160(106), 209 Nesbitt, L. E., 96(364). 145 Neypokoev, V. I., 243(68), 270 Nguyen-Cia-Hung, 268(86), 271

Nicholas, K. M., 349(84a), 412 Nikisha, V. V., 40(175), 46, 57(248), 91(364), 92(364, 365). 93(369), 94(364, 365, 369, 370, 371, 372), 96(383), 97(383), 99, 106 (175,248), 125(248), 139,140, 142, 145 Nilson, R.,152(12), 206 Nilsson, P. O., 152(15). 206 Ninomiya, T., 217(13, 16, 17, 18, 23, 25, 26, 27, 28, 52a). 222(52a), 223(23, 25, 28). 232(68), 234(52a), 243(23, 25, 52a). 266 (52a), 269, 270 Nislick, A. S., 327(29), 41 I Nitta, Y.,235(65a, 65b). 241, 267(65b), 270 Nogaito, T., 96(380), 145 Nogueira, L., 210 Noltemeyer, N., 421(13,15),422(35),424(15). 425(15), 426(15), 428(35,42), 463 Nordling, C., 152(12), 206 Novakova, J., 98. 145

0 Oblad, A. G . , 162(146), 210 Occhiuzzi, M., 28(110), 29(110), 37(110, 158, 160), 38(110), 39(110), 55(110), 81(110), 87(158), 89(110, 158), 91(110, 158), 123 (158, 160), 125(110), 137, 139 O’Cinneide, A,, 200(270), 213 O’Connor, G. L.,391(205), 415 Oda, J., 459(120), 466 Oda, L., 428(39a, 39b), 463 Odashima, K., 421(33), 463 Ogasawara, S., 78(305), 143 Ogino, H., 459( 122). 466 Ogino, Y., 4(14), 134 Ohara, M., 459(119), 466 Ohkubo, K., 217(21), 222(21), 223(21), 230 (21). 269 Ohlman, G . Z., 28(107), 29(107), 48(107), 103 (107), 121(107), 124(107), 137 Ohriaki, S., 191(251), 212 Okamoto, S., 217(41), 270 Oki, S., 314,322 Okimoto, A,, 450(97), 465 Okubo, K., 217(15, 19.20, 52a), 221(19), 222 (52a), 230(15, 19), 231(19), 234(52a), 243 (52a), 266(52a), 269, 270 Okuda, K., 217(29), 231(29), 252(29), 269

484

AUTHOR INDEX

Oliver, D., 21(84), 22(84), 23(84), 51(84), 52 (84). 127(84), 128(84), 129(84), 137 Olivier, D., 16(59). 25(59), 27(59), 73(282, 283), 129(283), 136, 142 Ollis, D. F., 249, 270 Olsen, K.J., 82(328), 144 Olson, D. H., 64(271), 142 Okay, E., 340(56), 341(56), 381(56), 411 Onaka, S . , 217(35, 47). 223(47), 224(47), 230 (47). 231(47), 232(47), 238(47), 240(47), 241(47), 267(47), 269, 270 Onda, Y ., 416 Ono, Y.,20(71). 31(121), 37(71), 68, 71(279), 78, 97, 123(71), 125(21), 126(121, 279), 128(71), 136, 138, 142.143 Onoda, T., 346(78), 412 Orchin. M.. 328, 345(75), 384, 385(75), 4 1 1 , 4 I2 Orito. Y.. 268(83. 84. 85. 86). 271 Orlova. G. N.. I56(63), 208 Osawa, T., 217(50. 52d). 225(50), 240(52d), 243(50. 52d). 245(50. 52d). 246(5O, 52d), 266(52d). 270 Ostermaier, J . J.. 183(228). 212 Ostrovskii. G. M., 316,322 Otagiri. M.. 434(62. 63). 464 Otarod. M..320(55), 323 Otero Arean. C.. 56(244b), 141 Overman, L. E., 419(6). 450(98), 462,465 Ozaki, H., 217(32. 34. 36,38,47.49). 223(47), 224(32. 34, 38.47). 225(34). 229(34), 230 (47). 231(47). 232(47). 234(49). 238(47), 240(47. 52d). 246(52d), 249(32. 34), 263 (49). 267(47). 260. 270 OZdWa, s., 4(14), 134. 314(37), 320(55). 322, 323 Ozias, Y., 95(374), 145 Ozin, G. A., 31(127a), 32(134b), 76(292), 77 (127a). 78(300), 97, 131(127a), 138, 143

P Paal, Z., 172(193), 175(201), 177(209), 211 Palke. W. E., 16, 136 Pannetier, G., 372(147), 414 Pariisky, G. B.,40(175), 49(213, 214). 51(213, 225). 53(213), 57(248), 59(252), 106(175,

248). 124(213, 214). 125(213, 248, 252). 139, 140, 141, 142 Park, S. W., 319,322 Parker, D. G . , 352(91), 367(91), 374(91), 386 (91). 412 Parkes, D., 104(400), I46 Parshall, G. W . , 165(169). 210 Paton, R. M.,421(30), 426(30), 463 Patterson, W. R., 301. 322 Paukert, T. T . , 8(31a), 135 Pauling, L., 55(238), 141 Paxson, T. E., 339(53), 340(53), 348(53), 377 (53), 411 Pearce, R., 352(91), 367(91), 374(91), 386(91), 412 Pearson, R. G., 6, 136, 385(186), 415 Pellet, R.J., 182(226), 212 Pen, J. B.. 166(178), 211 Pendry, J. B., 153(28), 207 Penney, C., 426 Perrier, J. P., 187(243), 191(243), 212 Perrin, M., 51(228), 141 Petrov, Yu. I., 243(68, 69), 259(76. 77). 260 (78). 270, 271 Peyrot, M., 193(258), 213 Pham, T. P., I10(425g), 146 Phichitkul, C., I lO(425h). 146 Pichat, P., 107(413, 414, 415). 146 Pichler, H., 314,322, 326(2, 5). 410 Pick, F. M., 25(100), 90(100), 129(100), 137 Pickert. P. E., 68, 142 Pierce, R., 344(72), 412 Pierron, E. D., 7(26c), 135 Pillor, D. M., 433(54b). 434(54b), 464 Pindor, A. J., 155(51),207 Pines, H., 162(150), 163(150), 175(150). 176 (1 50, 205, 206). 210, 21 I , 305, 308, 322 Pino, P., 341(59), 346, 347(59), 381(182), 412, 415 Pinter, A,, 450(88), 465 Pippel, E., 160(106), 209 Plank, C. J.. 61(259), 142 Platz, G. M., 86(348), 144 Pluijm, F. J., 33(138), 44(138), 45(138), 138 Plummer, E. W., 5(22a). 135 Plunkett, T. J., 193(261), 213 Poe, A., 341(59), 347(59), 412 Poels, J. K.,201(312), 214 Politzer, E. L., 150(6), 197(6), 206

485

AUTHOR INDEX Poltorak, 0. M.. 159(86, 87, 88), 208 Ponec,V., 150(4, 5 ) . 151(4, 5 , 8, I I ) , 152(4, 5 , 20,21), 153(5,22), 154(43,44,45,47), 155 (4, 5 , 8), 156(62, 67), 158(11, 79, 80, 81), 160(123), 161(123), 162(142, 143), 164 (167), 165(167), 167(180), 169(167, 180). l72( l67), 176(204), 177(204), 178(216), 179(219). 180(5, 204). 181(5), 184(204, 216, 235, 2381, 185(188), 186(204, 239), 187(5, 245), 188(246, 248, 269), 189(167, 249). 190(44), 191(253), 192(167, 248, 257), 193(167, 248, 262), 194(249), 196, 197(315), 199(298), 200(167, 238, 248, 297, 298), 201(67, 204, 249, 312), 203 (167, 24% 206. 207, 208, 209, 210, 211, 212,213,214 Poonia, N. S., 363(114a), 413 Poort, G. J., 162(142), 210 Pope, P., 191(252), 212 Portefaix, J. L., 51(228), 141 Pott, G . T., 102(394), 145 Potts, A. W., 109(424), 146 Pound, G. M.,153(31, 32). 207 Poutsma, M.L., 326(14, 21). 410 Povich, M.J., 4(18a), 134 Power, W. J., 78(300), 143 Praliaud, H., 51(228), 52(230), 141 Prasad, J., 197, 198(279), 213 Prater, C. D., 162(148), 210, 305, 322 Pratt, J. M., 78(306), 143 Prayre, F., 172(198), 173(198), 174(198), 176 (198). 211 Prest, D. W., 352(91), 367(91), 374(91), 386 (91). 412 Prettre, M.,45(195), 140 Priggogine, I., 153(29), 207 Primet, M.,195(267, 268), 213 Prinsloo, J. J., 156(61), 208 Prochaska, E. S., 86(341), 144 Prudnikov, 1. M.,37(156), 40(177, 178, 179), 106(177, 178, 179). 139 Pruett, R. L., 327(27). 349(27,85,86. 87). 350 (27, 85, 86), 351(86), 356(85, 86). 365 (118), 367(85), 369(86, 133, 134), 370 (86). 371(143), 373(153), 407(22,85). 411, 412,413,414 Przheral’skaya, L. K., 7(26e), 50(217), 135, 141 Pshezhetskii, S. Ya., 102(398), 146

Pshezhetsky, S. Ya., 142 Ptak, L., 169(187), 211 Pucci, S., 343(61), 412 Puddu, S., 197(315), 214 Pyerimhoff, S. D., 212

R Rabo, J . A,, 68(275), 142, 326(14,21), 410 Radtsig, V. A., 59(251), 125(251), 142 Ragaini, V.,80(250), 126(250), 142 Rajinder, S., 435(59), 464 Ramaswamy, A. V., 197(277), 198(277), 213 Randic, M.,212 Rasser, J. C., 197, 213 Rathke, J. W., 328, 329,330(38), 331(38), 332 (38), 333(36, 38), 334(38), 335, 336(37), 337(36), 338(36, 38), 339(36), 341(36), 342,343(35,36), 344(36,37), 345(37), 347 (37), 348(37), 407(35), 411 Ratnasamy, P., 197(277), 198(277), 213 Rauke, W., 160(134), 161(134), 209 Redondo, A., 16(55), 59(55), 136 Reggiani, M.,434(56), 464 Reilly, C. A., 339(53), 340(53), 348(53), 377 (53), 411 Reinalda,D., 151(11), 158(11, 79), 206, 208 Reman, W. G., 151(7), 206 Renou, A,, 159(91), 208 Renouprez, A. J., 159(92, 93). 200(300), 208, 214 Resasco, D. E., 199(296), 200(296), 214 Reuveni, A,, 128(448), 147 Rewick, R. T., 194(265), 195(265), 213 Richard, M.,73(282,283), 129(283), 142 Richarz, W., 188(247), 212 Riddick, J. A., 365(47), 366(47), 411 Rideut, D. C., 458(117), 460(117), 461(117), 466 Rinz, J. E., 404(21 I), 416 Risch, A. P., 326(14. 21). 410 Rives-Arnau, V., 7(26g), 47(204), 106(204), 135, 140 Robbins, C. G., 187(242), 191(242), 212 Robert, W., 433(54b), 434(54b), 464 Roberti, A,, 186(239), 212 Roberts, M.W., 77(298), 109(421, 422). 143, 146 Robinson, W. T., 15(50), 136

486

AUTHOII INDEX

Rodgers. M . A . J.. 6(24c), lL5 Rodionova, T. A,. O(24c). 56(24c), 131(24e), I33(24e). 135 Rodley. G . A,. I S ( 5 0 ) . 136 Rodriguez. L. J.. 434(55). 464 Riiper. M., 407(217). 416 Rofer-De Poorter, C. K., 316,322,326(7). 410 Rogers, J . D., 152(17), 206 Rohrbach, R. P., 434(55). 464 Rolfe, J.. 31(122. 125). I38 Rol, N . C., 76(193), 77(293), 102(293). 143 Rooney. J . K.. 163(156), 164, 169(156), 171 ( 156). I73( 156). 210 Roper, W . R.. 348(X4), 412 Rosinski, E. J., 61(259), 142 Rosynek. M. P., 307.322 Roth. J. A., 345(75). 384,385(75). 412 Rouco, A. J., 199(296). 200(296), 214 Roulet, N., l60(125). l61(125), 20Y Rousseau-Violet. J.. 160(136), 161(136),20Y Roussy, G., 167(183, 1x4, 1x5). 211 Rozentuller, B. V.. 49(215,216). 5X215.216). 140, 141 Rudakov, E. S., 166(175). 211 Rundle. R . E., 421(8). 422(X). 423(8), 462 Rusiecka. M., IOh(409). 109. 146 Russell. W. W., ISh(59). 208 Rys. P.. 167. 271

S Sabesdn, M . N., 421(31), 463 Sachtler, W. M. H.. 76(293). 77(293), 102 (293). 120(434), 143. 147. 150(3). 151(3, 8). 153(24. 37), 155(3, 8). l56(3, 62), 157, 177(283), 186(239), 192(24, 75, 255). 195 (266, 267. 283). 206. 207, 208, 211, 213, 252,261,271,301,322 Saenger, W., 421(9, 10, 1 I . 13, IS, 16, 17, 28, 32). 422(10). 423(9), 424(9, 10, 11, 15), 425(9, 10, I I , 15), 426(15), 428(9, 42), 432,433(1 I ) , 463 Saito, S., 164(161), 166(161), 210 Sakaguchi, M., 156(60), 208 Sakatd, Y.,450(96), 465 Sakurai, S., 218(53). 270 Samuel, D., 25(100), 90(100), 129(100), 137 Sancier, K. M., 33(139),45(139).90(362), 121, 138, 144, 147 Sanchez-Delgado, R. A., 384(184), 415

Sanders, J. V.. 159(85). -711s Sanz, J., IOh(408). 146 Sarichev. M . E.. 48(210). 140 Sasaki, H., 4 5 3 100). 454( 100). 465 Saten. van. R. A., 153(37). 207 Sayers. D. E.. 153(27). 207 Sbrana, G., 416 Schaap, C. A,, 373163). 37Y( 163). 381(163). 382( 163), 414 Schastnev. P. V.. 6(221'), 2X( 118). 30( I IX), 41 ( 2 2 0 , 48( I 18). I24(22f), IJS, 138 Scherdga, H. A,, 428(45), 464 Schindler, R. N., 90(359), 91(359). 144 Schlick, S., 17. 21(92), 22(92), 31(69b), 82 (322). 84(322), 92(322). 129(92), 136.137, 143 Schlosser, E. G., 182(225), 212 Schlupp, J., 329(39,40,40a), 335(39), 336(39), 337(39), 338(39), 36339). 376(39), 407 (217), 411. 416 Schmeisser, D.. 160(126. 127. 133), 161(126, 127, 133). 209 Schmeltekopl', A. L., 9(40), 136 Schmidt, H.-J.. 326(18. 19). 4111 Schoenberg, A,, X5(340), 144 Schoening, R. C., 364( 116). 367( 116,122, 123, 125, 126). 368(126, 126a). 369(126), 371 (126. 143). 372(125, 126). 373(153, 156), 413.414 Scholten. J. J . F.. 197(276), 200(299). 213,214 Schomaker, V., 68(275), 142 Schonland, D., 123(446). 147 Schoonheydt. R.. 64(270), 142 Schuit, G. A,, 151(7), 206 Schuit, G. C. A,, 33(138), 44(138), 45(138), 78 (302). 99(388), I18(428), 120(428), 126 (302). 138, 143. 145. 147 Schuldt, M., 131(466), 147 Schuler, R. H., 129(452), 147 Schultz, H., 314, 322 Schultze, D., 160(108), 209 Schulz, H., 326(5), 410 Schulz, H. F.. 384(183), 415 Schunn, R. A., 326(26), 338(49), 348(26), 385 (49). 41 I Schuster, L.,201(308), 214 Schwab. G. M., 160,209 Schwartz, L. M., 428(44), 434(44), 435(44), 464 Schwerdt, S., 329(40a), 411

AUTHOR INDEX

Schweitzer, C. E., 328(31), 349(31), 359(31), 41 I Sebastian, J. F., 419(4b), 428(4b), 446(4b),462 Secco, F., 343(62). 412 Segall, R. L., 144 Seib, D. H., 151(10), 152(10), 187(10). 206 Seiyama, T., 39(170), 40(170), 41(170), 46 (1 70), 48( 170), 57( 170), 124(170). 127 (170). 139 Sekine, F., 235(65a, 65b), 241(66), 267(65b), 2 70 Sellers. P. H., 274, 276,277(1), 279, 281,283, 287.300, 301,304,321 Sermon, P. A,, 160(114), 209 Setaka, M., 40, 44, 124(171, 182), 125(171, 182). 13Y, 140 Severdia, A. J., 320(55), 323 Sexton, B. A,, 131(460. 461), 147 Seymour, R. C . , 4, 134 Shamir, J., 131(456), 147 Shamsuddin, M., 199(293), 214 Shaw, R.,8(31b), 135 Shchekochikhim, Yu. M., 32(134a), 54(235), 138, 141 Shchekochikhin, Yu. M. 35(149a), 50(149a), 133(149a), 139 Sheik Krush, N., 44(194), 124(194). 140 Shelef, M., 34, 139 Shelimov, B. N., 24(99), 46(201), 48(212a). 81(318), 91(364),92,93(369),94(364,365, 369,370,371,372), 96(382,383), 97(383), 99(364, 382). 100(212a), 128(212a), 137, 140, 143. 145 Sheppard, N., 5(19b),32(19b, 134d), 51( I34d). 54(19b, 131d). 131(19b, 134d), 133(134d), 134, 138 Shevets, V . A.. 7(26e), 22(86), 25(86), 28(106, 109, 117). 29(106, 109), 30(109), 46. 48, 49(198), 50, 51(117, 198), 52(109), 73 (284). 78(301), 81(284, 301), 91, 92(364, 365, 367), 93(198, 367). 94(364, 365), 99 (364, 367). 112(106), 124(106, 198), 125 (86, 109, 198,447), 126(86,447), 129(86, 447). 137. 138, 140, 141, 143, 145, 147 Shiblom, C. M., 7, 135 Shield, L. S . , 156(59), 208 Shigehiro, K., 459( 118). 466 Shilov, A. E.. 166(173, 174, 175). 210,211 Sbimamoto, T., 219(60a, 60b), 220(60a, 60b), 270

487

Shimizu, N., 75, 126(288), 143, 429(47), 437 (47), 440(47), 446(47), 449(87), 450(87), 451(87), 464, 465 Shimokawa, K., 429(47), 437(47, 67), 440(47, 71), 443(71), 446(47), 448(84), 449(84), 450(92,93), 451(92, 93), 464,465 Shimokoshi, K., 75(288), 126(288), 143 Shinoda, A,, 427(38), 463 Shinoda, T., 446(82), 465 Shiotani, M., 13(43b), 19(43b), 20(66), 21(66), 23(66), 24(66), 27(66), 45(43b), 46, 48, 106(66), 124(66), 128(66), 136 Shirakata, H., 429(47), 437(47). 440(47), 446 (47), 464 Shore, S. G., 402(207), 415 Shortland, A., 165(170). 210 Shrage, K., 166(179), 167,211 Shteinman, A. A., 166(174, 175). 211 Shubin, V. E . , 78,81(301), 125(447), 126(447), 129(447), 143, 147 Siegbahan, K., 152(12), 206 Siegel, B., 432(53), 450(88,90), 464, 465 Siegel, M . W., 8(32), 9(32), 135 Silsbee, R. H., 24(94a), 137 Silver, B. L., 15(49). 20(78), 21(92), 22(92), 27 (49,102b). 28(49,102b), 79(49), 128(448), 129(92,449), 130(49, 102b). 136,137,147 Silverman, F. M., 156(66), 208 Simmonetta, M., 167(182), 211 Simmons, G. W., 326(13), 410 Simoneau, R.,3(1 I), 134 Sinanoglu, 0.. 277( 12, 13). 287,321 Sinfelt, J. H., 150(2), 151(2,9), 153, 155(2,9). 178(213, 214). 183(232), 197(271, 278). 206,212,213 Sironi, A,, 372(146), 414 Sivasankar, S., 197(277), 198(277), 213 Slawson, V., 6, 135 Slecbta, J., I55(55), 207 Slinkard, W. E., 339(54), 411 Slocum, D. W.. 329(41), 338(41), 411 Smardzewski, R.R.,77(294), 98(386), 143,145 Smith, D. W., 389(196), 398(196), 415 Smith, G. V., 168(186), 211, 262(80), 271 Smith, I. G . , 18(69a), 136 Smith, J. M., 319, 323 Smith, S. J., 9(38), 88(38), 135 Smith, W. R., 319,322 Snagovskii, Y. S . , 316,322 Sobhanadri, J., 82(326), 144

488

AUTHOR lNDEX

Sojka, Z., 56, 141 Sokolova, N . P., 243(67), 270 Sokolovskii, V. D., 40(181). 41(187), 140 Solomon, 1. J.. 87(346, 347, 348), 144 Solonitsyn, 37( 156). 139 Solymosi, F., 160(109, 110). 209 Soma-Noto, Y., 195(266),213 Somorjai, G. A., 157. 182, 183, 184, 192(75, 256). 200(256), 208, 211, 213 Soria, J., 7(26h), 47(205), 90(205), 91(205), 94 (205), 106(205,408), 135, 140, 146 Sorokin. Yu.A,, 142 Sorokina, M. F., 152(12). 206 Sotak, I . , 201(306), 214 Soven, P., 155(48),207 Spencer, A.. 374(160, 161), 414 Spencer, N . D., 77(297), 143 Spicer, W. E., 151, 152(10), 156(64), 157(69), I87( 10). 206, 208 Spirodonov, K. N., 40(175), 49(213, 214, 215, 216). 51(213, 225), 53, 55(241), 57(248), lOO(391). 106(175, 248), 124(213, 214), 125(213, 233, 241, 248). 139, 140, 141, 142,145

Spitz, P. N., 327(28), 411 Spoto. G., 32(134e), 39(134e), 56(134e), 80 (134e). 131(134e). 133(134e), 138 Stamires, D. N., 71(278), 142 Stapelbrock, M., 22(91), 59(91), 127(91), 128 (91), 137 Stauss, G. H., 28(113), 29(113), 30(113), 126 ( I l3), 137 Stein, J., 326(8), 327(8), 345(8), 410 Stein, R. A.. 6(26a), 135 Steinberg, M., 20(73), 43( 192). 44,45(73), 124 (73, 192, 193). 128(73), 136, 140 Steinemann. S., 152(16), 187(16), 206 Steiner, P., 152(19), 207 Steinrauf, L. K., 421(31), 463 Stephan, J. J., 156(62, 67). 201(67), 208 Stern, E. A., 153(27), 207 Stezorvski, J . J., 421(12,29), 424(12, 36). 425 (12). 426(29), 433( I2), 453(36), 463 Stocks, G. M., I55(50, 57), 207 Stohr. J., 160(107), 209 Stone, F. S., 31(131), 38(163). 56(244b, 2 4 4 ~ ) . 87(131), 112(247), 138, 139, 141, 147 Stoop, F., I58(80), 162(142),208, 210 Storch, H. H., 326(1, 3). 327(3), 410 Strlhle, J., 343(63). 412

Strandberg, M. W., 3(7,8a, 9a), 134 Strangio, V.. 308(32), 322 Strona, L., 369(139), 370(139), 371(139), 414 Subba Rao, V. V., 40(172), 124(172), 139 Subramanian, S., 82(324), 144 Suetaka, W., 250, 270,271 Sugimoto, T., 431(48), 458(114), 459(120), 464,466

Summerville, D.A., 34(148), 78(148), 79( 148. 310), 131(148), 139, 143 Sundarajan, P. R., 464 Sundaram, V. S., I52( 17), I53(38), 206. 207 Sunder, W. A., 131(457), 147 Suresh, D. D., 120(436,438), 147 Surin, S. A,, 81(318), 94(371, 372), 143, 145 Suryanarayana, D., 17(65d, 65e). 136 Suss, J. T., 85(340), 144 Sutton, L. E., 8(30, 33a), 135 Suzuki. l., 314(37), 320(54), 322,323 Suzuki, K., 71(279), 97(279), 126(279), 142 Svejda, P., 47(203), 90(359),91(359). 124(203), 140. 144

Swartzfager, D. G., 153(35), 207 Sweany, R. L., 346(81), 412 Swegler, E. W., 162(149). 211) Swenberg, C. E., 1 l(43a). 136 Swoap, J . R.. 168(186), 211 Sychev, 0. F., 28( 119). I3X Syrnons. M . C. R., 16(60). 18(69a).20(79). 28 (60). 34(145a). 35, 82(323,324,325, 331), 84(332), 87, 88. 95(373. 375). 105(404a). 136, 137, 139, 143, 144. 14s. 146

Szabo, Z. G., 160(109), 209 Szapiro. S., 85(340), 144 Szeitli, J., 458(112). 466 Szyrnanski, T., 24(98), 79(98). 94(98), 137

T Tabasaranskaya, T. E.,55, 125(241), 141 Tabushi. I ., 427(38), 428(39a, 39b. 40.41). 429 (47), 431, 432, 435(58), 437(47, 67, 69). 438(70), 440(47, 70, 71, 72, 74, 76). 441 (77). 442(72), 443(70, 71, 72), 446(47), 447(69), 448(84), 449(84. 87), 450(84, 92, 93,96), 451(87,92,93), 453(41. 100, 101, 102), 454(100, 102). 455(100, 102). 459 ( I l6a, I16b. 1 I ~ c )460( , I 16a, I 16b, 1 1 6 ~ ) . 463,464,465.466

AUTHOR INDEX Tagaki, W., 450(89), 465 Tagawa, T., 121, 147 Tagaya, K., 96(380), 145 Tagaya, R., 82(327). 144 Tagiev, D. B., 55(240), 65(240), 73(240), 78 (240), 81(240). 141 Tai, A., 216(63), 217(32, 34, 35, 38, 39,40,41, 42, 44, 45, 46, 47. 48, 49, 51. 5 2 ~ ) 223 . (47). 224(32, 34, 38, 47. 62, 63). 225(34), 229(34, 63). 230(47), 231(47), 232(47), 234(49), 235(51), 238(47. 51), 240(42, 46, 47, 52c-e), 241(34,47), 243(51, 52c), 245 (46), 246(52d), 249(32, 34), 255(39, 62, 63). 256, 262(63), 263(49), 266(46, 48, 52c), 267(47), 269,270 Tait, J. C., 130(453a). 147 Takabatake, 156(60), 208 Takaishi, T., 4, 134 Tskagiwa, H., 20(71), 37(71), 123(71), 128 (71). 136 Takasu, Y., 160(131), 161(131), 209 Takeshi, N., 82(327), 144 Takeshita, H., 459(121), 466 Takeuchi, T., 156(60), 208 Takita, Y., 104. 105, 107, 146 Takizawa, H., 217(17), 269 Takusagawa, F.. 357(107), 369(107), 413 Talanov, Yu. M., 243(67), 270 Tamaru, K., 164, 166(161), 210 Tanabe, K., 37(162), 139 Tanabe, R., 252(75), 271 Tanabe,T., 217(11, 13, 18,20,29,30,31),222 (11),224(30),230(11,30),231(11,29,30), 249(30), 252(29), 269 Tanaka, J., 421(25), 463 Tanaka, K., 6, 100, 106, 107(411), 135, 145, 146, 342(60), 382(60), 412 Tanaka, K. I., 40(173), 106(173), 139 Tanaka, S . , 75(290), 102(290), 126(290), 143, 434(62), 464 Tarama. K., 48(211, 212b), lOO(392). 101 (212b). 121(392), 124(212b), 140,145 Tarasova, D. V., 6(22f), 41(22f), 124(22f), 135 Tatibouet, J. M.. 110, 146 Tatsumi, K., 13(43b), 19(43b), 45(43b), 136 Tatsumi, S., 217(4, 5, 6, 7, 8, 9, 12. 14, 15), 220(4, 6), 222(10, 12), 230(4, 5, 6, 7, 12, 15). 231(4, 5, 6, 7, 8, 9, 12, 14), 248(6), 256(8), 269 Tauster, S . J., 160(117, 118, 121), 209

Taylor, J. F., 197(275).213 Taylor, L. T., 33( I35b), 138 Teichner, S. J., 45(196), 47(208), 105, 106 (208,406), 107, 108. 140, 146 Temkim. M. I., 275, 277, 280(5), 281, 283, 284,294, 302, 305(5), 321, 322 Templeton, D. H., 8(29), 135 Tench, A. J., 16(58), 20(45), 21(58,80,82,93), 22(58, 82, 85, 93), 23(82, 85), 24(85). 25 ( 5 8 ) . 28(108), 29(108), 30(68), 31(75, 108, 128, 129, 130). 35(1, 151). 36(68), 37(68, 70, 129), 38(164a), 40(75, 80), 41(74), 43 (45). 44(45), 45(75), 48(82), 51(82,85). 52 (82, 85, 108, 219). 57(246), 57(1, 246), 68 (76). 83, 84(1, 333), 85, 86, 87(333, 353), 88, 89(67, 333). 90(333. 356), 91(67,333). 94(1), 95(1), 96(382), 99(382), 100, 101 (168), 104(333), 105(1,404a), 106(1, 108, 338,356), 104(333), 110(1), 1 1 1 ( 1 ) , 112(1), 116(1), 117(1), 118(1), 121(1), 123(68, 70, 129). 124(45, 80). 125(85, 108). 127(58, 93), 128(45, 68, 70, 74, 75, 82, 85), 129 (58,76,93), 130(453b), 131(1), 132(453b), 134,136, 137,138,139, 141 Tenkham, M.,3(7, 8a, Bb), 134 Teranishi, S . , 235(65a, 65b), 241(66), 267 (65b), 270 TersolT, J., 160(95), 161(95). 208 Tesche, B., 160(131), 161(131), 209 Tetenyi, P., 172(193), 177(209), 181. 185, 203, 211,212 Tevault, D. E., 77(294), 143 Theobald, F., 52(231b), 110(425g), 128(231b), 141

Thieme, F., 156(65, 68), 208 Thomas, M. G . , 326(22), 375(22), 411 Thomas, W. J., 4(19a), 32(19a), 54(19a). 131 (19a), 134 Thompson, A., 160(107), 209 Thornton, E. W., 131(464), 133(464), 147 Tibbets, G. G . , 160(132), 161(132), 209 Tichy, J., 120(437a), 147 Tieman, B. M., 154(45), 207 Timmer, W., 53(234), 141 Tissler, B., 187(243), 191(243), 212 Toda, F., 450(94), 465 Tokunaga, H., 3l(l21),68(121), 125(121), 126 (121), 138 Tokuoka, R.,421(28), 463 Tomita, K., 421(28), 463

490

AUTHOR INDEX

Tomita, S., 346(78), 412 Tomizawa, T., 459(118). 466 Toolenaar, F. J . C. M., 151(11), 158(11, 80, 81, 82), 162(142. 143). 2W,208,210 Topchieva, K.V., 44(194), 124(194), 140 Tournayan, L., 197(272), 213 Tournier, R., 187(243), 191(243), 212 Tovrog, B. S . , 14, 15, 24(48), 28, 78. 79(48), 136, 143 Townsend, M. G . , 82(331), 87(331), 144 Trainer, G . L., 465 Tran, C . D., 465 Trapeznikov, V. A,. 152(12), 206 Trevalion, P. A., 82(323), 143 Trevethan, M. A,, 56(244c), 141 Trindle, C., 31(133c), 97(133c), 131(133c), 138 Troup,J.M.,364(115, 116),367(115,116),373 (155), 413 Tsau, N. H.,153(31, 32), 207 Tsuchida, T., 458(110), 465 Tsujihara, K., 443(79), 465 Tsyganenko, A. A., 6. 34(24d), 56, 131(24d, 24e), 133(24d, 24e), 135 Tuck, D. G., 9, I35 Tupikov, V. I., 142 Turkevich, J., 4, 30(120), 33(137), 44(137), 45 (137). 55(120), 124(137), 125(120), 134, 138 Turner, K., 367(121), 413 Tyabin, M. B., 166(174), 210 Tyminski, I. J., 431(49), 464

U Ude Shankar, S., 131(459,464), 147 Uedaira, H., 421(22), 433(22), 463 Uekama, K., 434(62, 63, 64), 458( 11l), 464, 466 Ugo, R.,358( 1 lo), 413 Ui, T., 1 lO(425j). 146 Ujzaszi, P.L., 166(177), 171(177), 211 Ungvary, F., 343(64), 412 Uno. K., 450(94), 465 Unwin, R., 160(124, 131). 161(124, 131), 209 Urban, M. W., 77(294),79(215), 131(315), 143 Ustimov, 1. A., 40(176), 106(176), 139 Uyeda, N., 252,271 Uytterhoeven, J. B., 64, 142

V Valenti. G . ,416 Valentine, J. S . , 2(5), 32(134c), 98(134c), 133 (134~).134,138 Valentini, G., 416 van Barneveld, W. A. A., 188(246), 212 van Catledge, F. A., 431(49), 464 Vanderspurt, T. H., 30(120), 55, 125(120), 138 Van der Veen, J. F., 161(140), 210 van Dijk, W. L., 189(249), 194(249),201(249), 212 van Doorn, J. A,, 375(162), 414 van Eck, M., 201(303), 214 Van Etten, R. L., 419(4b), 428(4b), 446(4b), 462 van Grondelle, J., 201(303). 214 Van Helden, J. F., 40(185), 124(185), 140 Van HOOKJ. H. C., 33(138), 40(185, 186). 44 (138), 45, 78(302), 124(185, 186). 126 (302). 201(303), 138,140, 143,214 van Hove, M. A., 157(73), 167(182), 208,211 van Keulen, K., 171(191), 177(191), 211 van Krevelen, D. W., 120, 147 Vannerberg, N. G . , 8(33b), 34(33b), I35 Vannice, M. A,, 160(120), 209,340(55),411 van Oort, B.. 375(163), 379(163), 381(163), 382(163), 414 Van Reijen, L. L., 48(209), 140 Vansant, E. F., 15(51), 22(51), 28(51), 29(51), 79(51), 126(51), 129(51), 136 Van Santen, R. A., 301(22), 322 van Schaik, J. R. H., 188(248), 192(248), 193 (248), 200(248), 212 Vasile, M. J., 131(457), 147 Vaska, L., 2(4), 8(4), 78(4), 79(4), 134 Vazquez, A. S . , 200(300), 214 Vebeek, H., 198(280), 213 Vedaira, H., 421(25), 463 Vedenyapin, A. A., 243(67), 260(78), 270,271 Vedrine, J. C., 17(63), 22(63, 88, 89). 24(83a), 25(63, 88, 89), 27, 35(152), 38(63), 47 (88). 50(63), 52(230, 231b). 59(254), 63 (266), 65(266), 66(83a), 67, 68, 69(83a, 266). 73(152, 281), 74, 81(152), 90(88), 91(88. 266), 94(88), 106(88), 124(88), 125 (83a. 254). 126(63,88,89, 102a, 266,281, 289), 128(231b), 129(63,83a, 88,89,102a, 152). 136, 137, 139, 141, 142, 143, 160 ( 1 13), 209

AUTHOR INDEX

Velicky, B.. I55(49). 207 Venlurini, M.. 343(62). 412 Vergand, F., 160(103, 104, 105). 209 Veron, J.. 43( 189). 124( 189). 140 Vidal. J. L., 356( 103). 363( 103). 364( I 15. 116). 367(115. 116, 122. 123, 124, 125, 126). 368(126. 126a). 369( 126, 140), 370(140), 371(126. 143). 372(125, 126, 145). 373 (140. 148, 149, 150, 151. 152. 153, 154. 155, 156). 413, 414 Villiers. A,. 418(2), 462 Visser, C.. 199(298), 200(298). 214 Vlasveld. J. L., 191(253), 213 Volter, J.. 201(31 I ) , 214 Voevodsky. V . V., 59(252), 125(252), 142 Vogelzang, M. W., 164(167), 165(167), 169 ( I 67). I72( 167). 184, I89( I67), I92( 167). 193(167), 200(167), 203(167). 210, 212 Vogt, E., 187(244), 191(244), 212 Volodin, A. M., 108, 146 Volta, J. C . , I10(425f, 425g), 146 Vorotinzev, V. M., 28(106), 29(106). 48(106), I12(106), 124(106), 137 Vorotyntsev,V. M.,28(1 17),48(1 l7),5l(ll7), 138

W Wachs, 1. E., 303,322 Wada. F., 403(208). 415 Waenger, W., 421(14), 463 Wagner, G. R., 35(154), 139 Wakamatsu, H., 346(76), 412 Walker, H. W . , 385(185, 186), 403(210), 404 (210). 405(210), 415 Walker, J. F., 387(189), 415 Walker, W. E., 349(85, 86, 87), 350(85, 86), 351(86, 88, 89). 352(89, 94), 353(94, 95), 355(95), 356(85, 86, 87, 105), 360(88), 362(112), 365(94, 99, 366(94), 367(85, 94, 95, 126). 368(126, 127). 369(86, 126, 132, 133, 140), 370(86, 140), 371(126, 143), 372(126, 145), 373(140, 151, 152, 153, 154, 156), 407(85), 412, 413, 414 Wall, R . G . , 346(79), 412 Wallace, D. N., 150(6), 197(6), 206 Walsch, A. D., 211 Walsh, A. D., 83, 144 Walsh, D., 308(32), 322 Walter, E., 319, 320(56), 322, 323

49 1

Walters, A. B., 89(354). I12(354). 114 Wandelt, K., 109(420), 146 Wang, H.-K., 326(24), 408(24), 411 Wang, K. M., 28(103, 104). 29(103, 104), 58 (104). 62, 64, 65(263), 67, 68. 69( 103, 104). 70, 71(103), 126(103. 104). 137, 142 Ward, J. W.. 64, 142 Warren, B. K . , 389(193, 194, 195), 390(193. 194). 395(193), 397(193), 398(193), 399 (193). 402( 193), 405( 193). 406( 193), 415 Watanabe. H., 217(38,40), 224(38), 270 Watanabe, K.,9(37), 154(46), 135, 207, 459 ( I 19). 466 Watson, K . M., 296, 298,322 Watson, L. M . , 152(14), 206 Wax, M. J., 347(82), 412 Wayland, B. B., 344(71. 72), 412 Weber, R., 160(112), 209 Webster, D. E., 166(176), 211 Weeks, R. A., 22(91), 59(91), 127(91), 128(91). 137 Wegman, R. W., 343(65), 412 Wei, J., 305, 322 Weinberg, W. H., 157(73), 208 Weisang, F., 172(196), 174(196), 193(258), 211,213 Weisberg, J., 40(174b), 139 Weissermel, E., 331(44), 411 Weissman, W., 197(271), 213 Weisz, P. B., 61(260), 142, 162(148, 149), 210 Wells, P. B . , 166(176), 211 Wells, R. C . , 368(130. 131). 413 Wender, I., 326(12), 410 Wenger, A., 152(16), 187(16), 206 Wentrcek, P. R., 121(440), 147 Wernick, J. H., 152(13), 187(13), 206 Wertheim, G. K . , 152(13), 161(139), 187(13), 206,209 Weschler, C. J., 79(313), 143 Whan, D., 197(274), 213 Whelan, R., 78(306), 143 Whiffen, D. H., 95(378), 145 Whyman, R., 338(50), 342,347(50), 381(181). 382(181), 389(198), 391(198), 411,415 Wichers, W. R., 153(22), 207 Wichterlova, B., 120(437a), 147 Wicker, G., 27(102a), 74(102a), 126(102a), 129(102a), 137 Wilde, M., 201(308), 214 Wilkinson, F., 6, 135

492

AUTHOR INDEX

Wilkinson, G., 165(170, 171). 210, 339(51, 52). 384(184). 407(178), 411,415 Williams, D. E., 421(8), 422(8), 423(8), 462 Williams, F. L., 153(33). 207 Williams, R. W., 155(50). 207 Williamson, R. C.. 377(175), 407(216), 415, 416 Williamson. W. B.. 83, 86(339), 144 Wilson, R. D., 33( 136). 138 Wilson, T. P., 326( I5), 410 Winans, S. C., 368(128, 129). 413 Wins.com, C. J., 80, 143 Winter, S. R., 344(66), 412 Wise, N., 121(440), 147, 154(42), 194(265), 195(265), 199(288). 207, 213 Witko. M., 121(439), 147 Woltersdorf, J., 160(106), 209 Wong, N. B., 84(334, 335). 85(334, 335), 86, 88(334), 89(334), 91(334), 92(334), 104 (334). 128(334), 144 Wong, T. C., 199(294), 214 Wood, B. J., 154(42), 207 Wood, J. C., 4, 134 Wood, R . H.. 9(36), 135 Woods, B. A., 344(71, 72). 412 Wreesman, C. T. T.. 176(202). 180(202), 211 Wsgner. H., I57(70), 208 Wunder, F., 326(18, 19), 410 Wynblatt, P.,153(30, 38), 207

Y Yacaman, M. J., 200(300), 214 Yagupsky, G., 165(170), 210 Yajima, S . , 217(11. 13, 17, 19). 221(19), 222 (11),230(11. l9),231(11, 19).269 Yamada, H., 428(39a, 39b), 453(101),463,465 Yarnada. K.. 459(118), 466 Yamada, M., 440(74), 464 Yamada. Y., 59(253), 125(253), 141, 142 Yamagishi, M., 78(305). 143 Yamamoto, M.. 217(47,49), 223(47), 224(47), 230(47), 231(47), 232(47), 234(49), 238 (47). 240(47), 241 (47). 263(49), 267(47), 270 Yamamura, I., 141 Yamamura. K., 428(40), 431(48), 432(50), 440(76), 449(87), 450(87). 451(87), 455 (40,103). 459( I 16b), 460(116b), 463,464, 465,466

Yamashina. T., 75(290), 102(290), 126(290), 143, 154(46), 207 Yamazoe, N., 39(170), 40(170), 41(170), 46 ( I 70), 48( 170). 57( 170). I24( 170). I27 (170). 139 Yao, H. C., 34, 139 Yasumori, I., 28(105a), 29(105a), 64(267), 65 (105a. 267). 70,72,74(267), 75,91(105a), 109(425a), 126(267, 288), 137, 142, 143, 146,226,249,252, 262,270 Yates, D. C., 151(9), 155(9), 206 Yates, J. T., Jr., 314, 322 Yermakov, Yu. I., 191(250), 198(282). 212, 213 Y oda, Y .. 39( 170). 40( I70), 41 (170). 46( 170). 48(170), 57(170), 124(170), 127(170), 139 Yokohama, S., 301,322 Yokoi, T., 450(86). 465 Yokota, K., 438(70), 440(70,71), 441(77), 443 (70, 71), 464,465 Yonezawa, F., 155(58). 208 Yong, L. K., 16(61), 20(61), 25(61), 53(61), 125(61). 129(61), 136 Yoshida, S., 48, 100, 101(212b), 121, 124 (212b), 140, 145 Yoshida, T., 314(37), 320(55), 322, 323 Yoshida, Z., 428(39a, 39b). 463 Yu, K. Y ., I56(64), 157, 208 Yuan, L. C., 438(70), 440(70,71,72), 442(72), 443(70, 7 I , 72), 452(99), 464, 465 Yukawa, T., 346(76), 412 Yun, C., 59(256). 79(418). 106(256), 107, 125(256), 142, 146 Yusa, A., 4(14), 134

Z Zanaraa, 0.. I70( 189). 21 I Zahraa, O., 193(258), 213 Zakharenko, V. S., 108(419), 146 Zammitt, M. A,, 28(111), 29(111), 58(111), 125(111), 137 Zecchina, A., 20(70, 72). 31(131), 32, 37(70, 72), 38(163), 39, 56, 80, 87, 112(427), 123 (70, 72), 125470, 72), I3 I( I34e, 465), 133 (134e). 136, 138, 139,147 Zeif, A. P., 32(134a), 138 Zeller, H.R.,18(69b), 20(69b), 123(69b), 128 (69b). 136

AUTHOR INDEX Zettlemoyer, A. C., 40(172), 124(172), 139 Zhabrava, G . M., 40(175), 57(248), 106(175, 248), 125(248), 139, 142 Zhidomirov, G . M., 16(54), 28(117), 47(202), 48(117), 51(117), 81(319a). 92(366), 96 (383), 136, 138, 140, 143, 145 Zhu, A. R., 109(422), 146

493

Zienty, F. B., 7(26c), 135 Ziesecke, K.-H., 328(34), 411 Zuidwijk, J. G. P., 199(298), 200(298), 214 Zul’fugarov, Z. G . ,40(181), 55(240), 65(240), 73(240), 78(240), 81(240), 140,141 Zoulalian, A,, 320(56), 323 Zyryanov, V. G., 152(12), 206

Subject Index A AB, radicals. 83 84 Absorption hand. 356-359, 370 Abstraction reaction. I 19- I2 I Acetaldehyde. ethylene oxide synthesis. 303305 Acetic acid. MRNi hydrogenation. 243-245 Acetoacetate. ester group, effect on hydrogenation. 229 Acetylacetone. 248- 249 hydrogenation. 259 262 Activated water expulsion in cyclodextrin, 422 426.432 Active sites. S I X and composition. I73 174, 19% 201 Activity alloys. 187.191 carboxylic acid-promoted ruthenium catalysis. 375- 379 catalytic, 149- 150. 155- I56 CO hydrogenation. 407-410 cobalt catalyst solutions, 337-340 Lewis base-promoted ruthenium catalysis, 388-394 metals. 178- 182 Pt-Au alloys, 196-197 Pt-Cu alloys, 194-197 rhodium catalysts, 350-356, 361, 373-375 unpromoted ruthenium catalysis, 375-379 Additives catalysis, 391, 409-410 reaction conditions, 243-246 Adsorbed oxygen species. 1 18- 122, 133- I34 Adsorption steps in butene isomerization, 306-308 Alcohol cobalt catalysis, 337-340 oxidation, 108 rhodium catalysis, 368 ruthenium catalysis. 379-380 494

Alcohol homologation, 329 -330. 338. 409 410 ruthenium catalysis. 390,405, 409 -41 0 Aldolization, 330 Algebraic formulas, 276-279. 286 Alkali and alkali-earth zeolites, 68-74 Alkali halides, 31-32 Alkali metal cations rhodium catalysis. 363-364 ruthenium catalysis, 395-396. 399-400 Alkaline-earth oxide, 36 39 Alkane formation, 330. 341-342 isomerization reactions, 182 oxidation. 107-108. I18 ruthenium catalysis. 375 -376 Alkene, 107 - 108 Alkyl migration, 346-347 Alkyl radical, 104-105 Alloys chemisorption bond strength. 156- I59 electronic structure, I51 - I53 hydrocarbon reaction model, 201 -205 metals, 186- 19 I texture and surface composition. 153-1 54 theories, 155-156 Ally1 radicals, 101-102, 118-121 Allylations of hydroquinone derivatives, 45946 1 Alumina, 57-58 Aluminosilicates. see specific compounds Aluminum content of RNI. 234, 238 Amide solvents, 384 Amine promoters, 356-359. 371-373 Amine-rhodium ratio, 357-359 Amino acid growth on Ni lattice, 252-254 modifying reagent, 221 -224 Ammonia synthesis, 294-297 Anionic rhodium complexes, 356-364 Anionic ruthenium complexes, 402-406 Anions, 224

SUBJECT INDEX

1,8-ANS inclusions. 453-454 Apolar solute, 432 Archetype molecules, 164-165, 171 Aromatic oxidation, 118-121 Aromatization hexanes, 176- I82 hydrocarbons, 176- 182, 308 Artificial enzymes, 41 9-420 Association constants, 446-447 Atom-by-species matrix, 302-303, 318-319 Auger electron spectroscopy, I54 Azobenzene-capped cyclodextrin. 452

B Basicity, amines, 357-359 Basis elements, 281-283 mechanism space, 283-287 Binary oxides, 119 Binding energy, 160- 162 Biomimetic chemistry, 417-418 Bis(tripheny1phosphine)iminium salt, 361363 Bond frequency, 1 I4 Bond shift mechanism, 170- 171 1,3-Butadiene production, 297-298. 313 n-Butane dehydrogenation, 308-31 3 Butene, isomerization, 305-308, 31 1-313 I-Butene. 297-298

C C(2). 254-255 C(3). 254-255 Cadmium oxide, 39-40 Carbido cluster, 401 Carbon, on metal surfaces, 194-197 Carbon complexes n complex, 167-168, 175-176 2C complex, 202-205 3C complex, 168-169, 173-174, 187-191, 195- 197,203-205 3Cny complex, 173- 174, 186 5C complex, 195-197 hydrocarbons, 167- 168 Carbon layer structure. 184-186 Carbonic anhydrase, 447-448 Carboxylate ions, I I 7- I I8

495

Carboxylic acid Lewis base-promoted ruthenium catalysis. 389,400 promoted ruthenium catalysis, 375. 379380,386-387 Catalysis cyclodextrins. 457-459 cyclophanes, 455-456 Catalyst enzyme models, 417-418 modified, 220 Catalytic rate constants, 446-448 Catalytic reaction, I 18- I22 Cation adsorption sites, 28-31, 61 inhibition in rhodium catalysis, 362-363, 366-367 modifying pH, 252-254 C basis elements, 284-285,287-291 C-C bond fission, 163-164, 175-176, 178, I86 13Cchemical shift in cyclodextrin, 442-443 C=C hydrogenation, 259 Cesium*/Rh ratio, 361 Cesium salt, 360-365 Chemical system, 278-283 Chemisorption behavior, 162 Chemisorption bond strength, 156- 159 Chemisorption complexes 2C, 167-168 3C,168-171 5c, 170-171 metal-carbon bonds, 165-167 metal differentiation, 171- 172 Chiral structure, 221-224 Chirality distributions, 255-256 ~4~~~0,374-375 Chromium oxide, 50-51 L3C-labeledmolecules, 164-165, 170-171 Closed sequence in reaction mechanisms, 275276 Cluster formation, 343-344 -C=O and RNi hydrogenation, 219-220, 224

co

alloys, 194- 195 chemisorption bond strength, 156-159, 162 hydrogenation, 313-316 Coal, gas synthesis, 327 Cobalt additives, ruthenium catalysis, 391

496

SUBJECT INDEX

Cobalt catalysts. CO hydrogenation, 3?8--349, Cyclic mechanism, 170-171 408-4 I0 Cyclic ureas, 366-367 Cobalt dioxygen adducts, 14-16, 28-31, 33, Cyclodextrin, 418-420 78 -82 activated water expulsion, 422-426. 431 Cobalt dioxygen carriers. 28-31. 33 binding, 433-435.456-457 Cobalt ions, 38-39 capped, 437- 445.452 Cobalt oxide, 55-56 capped isomers, 438-440.442 Cobalt precipitation. 340-342 catalysis, 457 -459 Coefficients in reactions, 284-286, 288- 289, empty, 422-426.428.433-434 see also Diagonalized matrix flamingo capped, 440-441 CO, formation, 330 host design, 445-452 Coherent potential approximation theory, modifications, 442-445 155-156 monofunctionalized, 450-45 I C O insertion multifunctionalization, 436-445 cobalt catalysis, 347 multiple recognition, 436, 448 ruthenium catalysis. 384 stilbene capped. 443-444 Compensation temperatures in AH-AS relastructure, 419, 422-427, 442-443 tionships, 434-435 substituted, 427 Complex homogeneous reaction systems, unsubstituted, 422-426 317-3 I9 water molecules, 422-426, 428-432 Complexes a-Cyclodextrin, 422 -426. 43 1-432 n, 167-168, 175-176 triammonium, 449 aa, 166-168, 171-172 trisubstituted. 445 UP. 166- 168 /I-Cyclodextrin, 422-426. 432-433 ay, 168-172 association constants, 446-447 Components in Gibb phase rule, 317-319 capping, 436-445 C parameters and direct mechanisms. 288-291 guest binding, 449-451 Copper, 365-366 inclusion catalyst, 459-461 Covalent model, 0; ions. 14- I8 tetrasubstituted, 441 -443 Crown ethers y-Cyclodextrin, 422-426. 433 rhodium catalysis, 364, 366. 372 Cyclophanes, 420,427-428 ruthenium catalysis, 399-400 binding, 455-456 Cryptands, rhodium catalysis, 363 host models. 452-456 Crystallography cyclodextrin, 421 -426 cyclophane, 427-429 D Cycle ammonia synthesis, 294-297 d band n-butane dehydrogenation, 308-31 3 alloy electronic structure, I51 I53 butene isomerization. 305-308 particle size, 160- 162 dehydrogenation of I-butene to 1,3-buta- Dehydrocyclization. carbon complexes, 179 diene, 297-298 182 direct mechanisms, 283 Dehydrogenation ethylene oxide synthesis, 303-305 1 -butene to I ,3-butadiene, 297-298 hydrogen electrode reaction, 292-294 n-butane. 308-313 hydrogenation of isooctenes, 298-300 hydrocarbon, 308 mechanism space. 284--287 Desorption of oxygen, 106 methanation of systhesis gas, 313-316 Deuterium, CO hydrogenation. 344-345 reaction mechanisms. 274-277 Diagonalized matrix. 284-286. 288 Cycle-free subsystem, 287-288 ammonia synthesis, 294-297 Cycle matrix, 287-291 n-butane dehydrogenation, 309-3 I3 -

497

SUBJECT INDEX

I-butene to 1,3-butadienedehydrogenation,

297-298 butenes isomerization, 305-308 ethylene oxide synthesis, 303-305 hydrogenation of isooctenes, 299-300 methanation of synthesis gas, 314--316 sulfur dioxide oxidation, 292 Diastereoface-differentiating hydrogenation of MRNi, 245-248,256-262 Diels-Alder reactions in cyclodextrin inclusion, 460-461 Differentiating sites, 223-224 Diiodocyclodextrin. 437-438 Dimerization, oxygen, 5 Dioxygen adducts, 78-82 Dioxygen species. 8-10,18-35,see ulso specific species aluminosilicates, 57-74 characteristics, 36,112-1 I5 dioxygen adducts, 78-82 EPR spectroscopy, 113-115 g,, values, I 13-I I5 ionic oxides, 36-44 0; ions, 10-13 0: ions, 33-34 0 ; - ions, 34-35 0 ; - ions, 35 support metals, 74-78 transition metal oxides. 44-57 Direct mechanism chemical systems, 276,281-283 overall reactions, 287-291,293-294 procedure, 287-291 Direct paths, see Direct mechanisms Double nucleophilic displacement, capped cyclodextrin, 437 Double recognition models, 451-452

E Electron density in chiral center, 222-223 Electron paramagnetic response spectroscopy adsorbed oxygen species, 121-122 alumina, 57-58 chromium oxide, 50-51 cobalt oxygen adducts, 38-39,78-82 C 0 , - 0 , ions, 25-26 dioxygen species, I 13-1 I5 g tensors, 1 1-1 8 inequivalent oxygen nuclei, 23

iron group oxides, 54-57 isotopic exchange of molecular ions, 99-100 molybdenum ions, 28-31 mono-oxygen species, 1 I2 0; ions, 10-11, 18-25,31-32,101-102 reaction, 101-102 0: ions, 33-34 0; ions, 85-91 reaction, 104 0, ions, 96-98 0:-ions, 34-35 photo-induced reactivity, 108-109 rare earth oxides, 44-45 silica, 59 support metal, 75-78 thorium oxide, 43 tin oxide, 41-43 titanium oxide, 44-48 tri/polynuclear complexes, 91-95 tungsten oxide. 53-54 vanadium oxide, 48-50 zeolites, 62-74 zinc oxides, 39-40 Enzyme models, cyclodextrins, 417-420 Enzymes and catalytic activity, 417-418 Enzyme, substrate complex, 418-420 Epoxide formation, 301 Equilibrium cobalt catalysis, 343-347 hydride-formyl, 344-349 ruthenium catalysis, 381-382,403-406 Ethylacetoacetate, EDA, 259-262 Ethylene glycol/methanol ratio cobalt catalysis, 336-337,374 rhodium catalysis, 351-356,374-375 Ethylene oxide synthesis, 301-305 Exchange broadening, see Electron paramagnetic response spectroscopy Exchange reactions hydrocarbon-hydrogen, 164-165 molecular ion, 98-100 Extended X-ray absorption fine structure,

153-154

F Fatty acid, MRNi hydrogenation, 243-245 Formaldehyde cobalt catalysis, 345-349,409

498

SUBJECT INDEX

rhodium catalysis, 374, 409 ruthenium catalysis, 385-387.409 Formate/alcohol ratio cobalt catalysis, 335-337, 345-347 rhodium catalysis, 353-354 ruthenium catalysis, 376 Formate esters, 331-333, 339 Fragmentation of rhodium clusters, 371-375 Free-energy change in inclusion complex, 433, 435

G y tensor, 11-18, 28-31

alumina, 57-58 chromium oxide, 50-51 covalent or spin-pairing model, 14-18 dioxygen adducts, 78-82 ionic model, I I - 14 iron group oxides, 56-57 molybdenum oxide, 51 -53 mono-oxygen species, 112 motion, 16-18 0: ions, 33-34 0;ions, 94-98 ozonide O ; , 82-91 rdre-earth oxides, 43-44 silica, 59 so-called 0;complex, 94 supported metals, 75-78 titanium oxide, 47-48 tri/polynuclear complexes, 92-95 vanadium oxides, 49-50 zeolites, 62-74 zinc oxides, 39-40 g,, values dioxygen species, I 13- 1 15 0;.123-128 Gas phase values of dioxygen species, 8-10 Gibb phase rule, 317-319 Gibb rule of stoichiometry, 318-319 (S)-Glutamic acid, 220-221,224-229 modifying temperature, 230-23 I nickel lattice orientation, 253-254 Glyceraldehyde, 348 Glycerol, 352 Glycoaldehyde, 345-349 Glycol formation in ruthenium catalysis, 380 Graph theoretic model, 277

Guest design in cyclodextrin, 456-461 Guest molecules in cyclodextrin, 424-426, 428-430,432

H Halide promoters in ruthenium catalysis, 388394, 397-400,408-410 Hard and soft acids and bases principle, 259 HJCO ratios carboxylic acid-promoted ruthenium catalysis, 375-376, 387 CO hydrogenation, 407-410 Lewis base-promoted ruthenium catalysis, 390, 392-393.398-406 rhodium catalysis, 368, 370-375 unpromoted ruthenium catalysis, 375-376, 387 Heptane CO hydrogenation, 338-340 reactions with hydrogenation, 18 1 - I82 Heterocyclophane, water-soluble, 453-456 Heterogeneous catalysis, 188- 12 I cobalt catalysis, 326-327, 341-342 Heteroligand complex, 260-262 Hexane reactions with hydrogen, 181-182 High-energy waters, 422,428-429.432 HNi, 233-238 Homogeneous catalysis CO hydrogenation, 407-410 ruthenium catalysts, 375-377, 381 Host design concept cyclodextrin, 4 4 - 452 cyclophane, 452-456 Hydride-formyl equilibrium, 344-349 Hydrides, ruthenium catalysis, 383-385.401 402,405-406 Hydrocarbon chemisorption complex, 158- 159, 165- 172 feedstock, 327 hydrogen reactions chemisorption complex, 165-172 exchange reaction, 164- 165 particle size, 182- 186 Pt catalysts, 162-1 65 skeletal reactions, 172- 182 isomerization, 305 molecule bonding metal-carbon multiple bond, 165- I67 metal-carbon single bond, 165

499

SUBJECT lNDEX reaction alloy catalysis. 198-201 alloy systems. I9 I I98 classification, 186- 191 reaction model, 201 -205 Hydroformylation process cobalt catalysis, 346-349, 409 ruthenium catalysis, 384-385, 409 Hydrogen chemisorption bond strength, 156- I59 molecules, MRNi hydrogenation. 257-259 0-ion. 89-91 0;ion, 101-102 simple reaction of oxygen species. 1 17- I 18 Hydrogen acceptors, EDA of MRNi, 232 Hydrogen bonding alcohol homologation, 338 between guest and cyclodextrin molecules, 428-43 1,434 Hydrogen electrode reaction, 292-294 Hydrogen iodide in ruthenium catalysis, 397400,405-406 Hydrogen pressure, reaction conditions, 241 243 Hydrogenation butenes, 31 1-313 carbidic species, 341-316 hydrocarbons, 308 isooctenes, 298-300 MRNi, 224-229 Rni, 219-220 surface location, 259-262 Hydrogenolysis alloys, 201 carbon complex, 168-169, 172, 196-197 cobalt catalysis, 346-348 methylcyclopentane, 170-171. 185-186 Pt and Pd alloys, 192-197 Pt-Re and Pt-Ir alloys, 197-198 skeletal reactions, 176, 178- I82 Hydrolysis of dioxane, 339-340 Hydrophobic interaction cyclodextrin, 424-425, 428-430. 435-436, 446,448-449 cyclophane, 453 Hydrosilanes, 337 Hydroxy acid modifying pH, 231-234 modifying reagent, 221-224 pretreatment of Rni, 232-235, 238 -

Hydroxycarbyne intermediate, 349 Hydroxylation and catalysis, 109- I10 Hydroxymethyl intermediates in ruthenium catalysis, 384-386 Hyperfine parameters for "0.128-130 Hypertine tensor constants, 20-21 dioxygen species, 18-25 equivalent oxygen nuclei, 18-21 ionic oxides, 40 mono-oxygen species, 1 I2 motion, 25-28 nonequivalent oxygen nuclei, 21 -25

I Identitiability of structure, 319-320 Imidazole and catalysis rates, 448 Inactive additives and alloys, 190-191 Inclusion compounds, 418,420-426 cyclodextrin, 428-436,462 unsubstituted compounds, 422-426 Independent reactions, 317-319 Infrared band spectroscopy, 4-6, 8, 31 -32 adsorption complex, 261 -262 characterization of oxygen species, 121123, 130-134 chemisorption bond strength, 158-1 59 chromium oxide, 50-5 1 dioxygen adducts, 79-82 dioxygen species, 1 13- 1 16 0;ions, 33-34 0;ions, 101-104 0:-ions, 34-35 0;ions, 105 0; ions, 97-98 particle size, 162 supported metals, 78 surface conditions of modifying reagents, 251 -254 zeolites, 64-65 Intermediate species, 280-281 Intermediates methanation of synthesis gas, 314-316 multiple overall reactions, 301-305 reaction mechanisms, 274-277,319-320 lntramodular hydrophobic recognition, 45 1 Iodide promoters in ruthenium catalysis, 389406 Ionic models for g tensor, 11-14

500

SUBJECT INDEX

Ionic oxides, see specific oxides Ionic promoters in ruthenium catalysis, 387406 Ion-pairing in rhodium catalysis, 359, 362367. 372 lridium and CO hydrogenation, 407-410 Iron and hydrocarbon reaction, 205 Iron group oxides, 54-57 Iron oxides, 54-55 I-/Ru ratio, 397-398,401 lsomerization alloys, 188-191 n-butane dehydrogenation, 31 1-313 butenes, 305-308, 311-313 carbon complexes, 169- 170, 178- 182 3C complexes, 173- 174, 185- 186, 188-191, 196-197 5C complexes, 185- I86 chemisorption complexes. 170- 172 hydrocarbon reaction models, 202-205 Pt-Re and Pt-Ir alloys, 197-198 5-ring dehydrocyclization, 172- 176 Isooctenes, 298 - 300 Isotope effect on 0;ions, 24 Isotopic exchange of molecular ions, 99-100 Isotopic tracer measurements, 320

K Kernel of R, 284-285 Ketones EDA hydrogenation, 264-267 EDA of MRNi. 240-241 Kinetic dependencies, cobalt catalysis, 342349 Kinetic isotope effect, 344-345 Kinetics MRNi hydrogenation, 225-229, 254 reaction mechanisms, 319-320

Ligand field strength, 15-16 Ligands cobalt catalyst stability, 341-342 ruthenium catalysis, 383-384,401-404 Linear dependence among direct mechanisms, 281 -283 Looper’s walk capping, 438-445

M Magnesium oxide 0;ion reactions, 104-105 simple reactions of oxygen species, I 17- I 18 (S)-Mandelic acid, 233-234 Manganese additives in ruthenium catalysis, 391,406-410 Manganese oxide, 56-57,84-91 Mars-van Krevelen reaction, 120- I 2 I Matrix, see also Diagonalized matrix atom-by-atom species, 302-303, 318-319 cycle, 287-288 stoichiometric reaction coefficients, 279280,283-287 M ( C x C ) matrix, 290-291, 31 1-313 Mechanism cobalt catalysis, 342-349 rhodium catalysis, 369-375 ruthenium catalysis, 381 -387 space, 280 Metal-carbon bonds, 165- 167,202-203 Metal catalysts, EDA hydrogenation, 267268 Metal formyl complex, 404-406, 409-410 Metal oxides, 199-121 Metal particles and electronic structure, 159161 Metals catalysis and CO hydrogenation, 406-41 0 classification of hydrocarbon reaction, 186191 aa complexes, 17I 172 hydrocarbons, 198-205 selectivity and skeletal reactions, 172- 176, 178-182 supported, 74-78 Methanation of synthesis gas, 313-316 Methane and cobalt catalysis, 330, 341-342 Methanol cobalt catalysis, 346-347, 409-410 -

L Lactones in rhodium catalysis, 365-366 Lattice oxygen, 118-121 Lewis acid-base reactions, 16 Lewis base-promoted ruthenium catalysis, 387-406,408 Ligand effect, 156, 158

50 I

SUBJECT INDEX

rhodium catalysis, 351-356, 374-375 ruthenium catalysis, 375-377,384,409-410 Methanol oxidation, 118-121 Methanolysis, 330, 409 Methyl acetoacetate hydrogenation -C=O, 219-220 corrosion, 248-249 EDA with TA, 257-259 MRNi, 225-229 RNi, 224-229,254-262 TA-MRNi, 267-268 Methylcyclopentane hydrogenolysis, 170171, 178-182, 185-186 Methyl formate, 377, 384 Mixed ensembles in alloy catalysis. 198-201 Models alloy surface composition, 153- I54 mechanisms, 313, 319-320 Modified Raney nickel catalyst (MRNi) defined, 215-217 hydrogenation, 224-229 Modifying technique of catalysts, 262-264 Molecular design in cyclodextrin, 427 Molecular recognition, 420-436 Molecular structures in rhodium catalysis, 369-371 Molten quaternary phosphonium salts, 391400 Molybdenum ions, 24-25 0;ion reaction, 103-104 oxide, 51 -53 Monomolecular conversion, butene isomerization, 307-308 Mononuclear cobalt complex, 342-343 rhodium species, 369-375 ruthenium complex carboxylic acid-promoted catalysis, 375, 381-382 Lewis base-promoted catalysis. 397-398 unpromoted catalysis, 375, 38 I -382 surface oxygen species, electron spectroscopic and catalysis studies, 109-1 10 Mono-oxygen species, characterization, I 1 I 112 ma Multiple bonds, 166 aaa Multiple bonds, 166 Multiple over reactions, 300-316

Multiple reactions, 282-283, 286-287 Multiple recognition hosts, 445-446, 448449,451 -452 Multiple recognition of cyclodextrin, 438,448

N n-type semiconductors, I19 Native enzymes, 419-420 Neohexane hydrogenolysis, 192- I97 reactions, 189-191,203-205 NH,Y, 66-74 Nickel alloys, 186- 192 catalyst, methanation, 313-316 complexes, 261 -262 Nickel formate as nickel catalyst, 226-229 Nickel lattice orientation, 252-254 Nickel oxide, 56-57, 235 Nickel zeolites, 72-73 Ni-Pd, 191-192 Ni-Sn alloys, 191 Nitrobenzene, MRNi hydrogenation, 226 Nitrogen in rhodium catalysis, 368 NMP solvent, ruthenium catalysis, 388-389, 398-399

0 0-ion abstraction reaction, 119-121 electron spectroscopic and catalysis studies, 110 mono-oxygen species, 1I 1 - I 12, I I 5- 1 16 oxygen reactions, 122 simple reactions, 1 16- I I8 0’ion, 54-57 catalytic reactions, 119-120 mono-oxygen species, I 1 1-1 12 oxygen reactions, 122 0;ion, 33-34 dioxygen species, I 12- 1 I5 0;ion alumina, 57-58 bonding interactions, 14-15 catalytic reactions, I19 chromium oxide, 50-51 covalent or spin pairing models, 14-18

502

SUBJECT INDEX

dioxygen species, I 13- I I5 y tensor. 10-18 gvalues, 12-14, 17-18, 123-128 hyperfine tensor, 18-28 ionic model, 1 I - I4 ionic oxides, 36-37, 40 iron group oxides. 54-57 molybdenum oxide, 5 1-53 "0 hyperfine parameters, 128-130 optical properties. 3 1-32 oxidation reaction, 100-104 oxygen reactions, 122 photoelectron spectroscopy, 32-33 photo-oxidation reactions, I 18 rare earth oxides, 44 silica, 59 simple reactions, 1 16- 1 18 spectroscopic constants adsorbed on TiO,, 46 superhyperfine tensor. 28 -31 supported metals, 75-78 tin oxides, 40-43 titanium oxide, 45-48 tungsten oxide, 53-54 vanadium oxide, 48-50 zeolites alkali and alkaline-earth, 68-74 decationated, 66-68 experimental conditions, 62-71 general features, 61-62 transition metal ions, 71-74 0:-ion, 34-35 dioxygen species, 112-1 I5 oxygen reactions, 122 0:-ion, 35 dioxygen species, 112, 114- 115 0;ion exchange reactions. 100 formation and stability, 88-91 magnetic parameters. 84-85 normal ozonide ion, 82-91 other species, 95 oxidation reaction, 104-105 oxygen reactions, 122 polyoxygen species, 1 15- I 16 related tri/polynuclear complexes, 91 -95 simple reactions, 116-1 18 so-called 0;ion, 91-95 zeolites, 60-66 0: ion and dioxygen species, I 12- I 15 ~

0; ion, 95-98 exchange reactions, 99-100, 105 "022, 85-88 I

lo

exchange reactions, 99- 100 hyperfineconstants, 17-I8,20-28,128-130 labeling, 7 dioxygen adducts. 78-82 iron group oxides. 56-57 oxygen species, I21 - I22 silica, 59 0, ion,96-98 ozonide ion, 83-88 tri/polynuclear complex, 94-95 tungsten oxide, 53-54 zeolites, 68-74 180.22 Ih02, 76-77, 79 dioxygen adducts, 79-82 110,

ionic oxides, 36-37,40 molybdenum oxide, 52-53 rare earth oxides, 44 titanium oxide, 45-48 "0,and supported metals, 76-77 OH, ions, 38-39 Olefin chemisorption, carbon atom complexes, 167-168, 175-176 Olefin hydroformylation. 384-385 Olefinic compounds, 308 160-1 8 0

exchange reactions, 99 mixed isotopes, 5-6 Open sequence in reaction mechanisms, 275 Optical yields adsorption energies, 254 EAA or AA hydrogenation, 260 EDA hydrogenation of MAA, 267-268 modifying reagent, 227-229, 231 reaction conditions, 241-246 surface conditions, 263-264 Optimum promoter effect of amine/rhodium ratio, 357-359 Orbitals dioxygen bonding, 132-134 0;ion, 33-34 spin pairing, 14- I5 IT# Orbital occupancy and energies of neutral oxygen, 3,9-10 Overall reactions, 281,285-287, see also Mul-

503

SUBJECT INDEX

tiple overall reactions; Simple overall reactions Oxidation reactions, 100-105 Oxides alloy catalysis, 177-178 catalysis, 6-7 Oxygen singlet, 5-7, 112-1 13 triplet, 3-6, 112-1 13 Oxygen bonds, 132- I34 Oxygen nuclei equivalent, 18-21 unequivalent, 21-25 Oxygen species catalytic reactions, I I 1-123 characterization, 11 1-1 16 IR characterization, 130-134 Oxygenates, 326 Ozonide characterization, 82-88 formation and stability, 88-91 0;ion, 82-91

P Palladium, 73-74 alloys, 192-197 2"-Paracyclophanes, 453 Paraffin oxidation, I 18- I21 Parameter estimation, 288-291, 319-320 Particle size catalysis of HC reactions, 182-1 86 chemisorption behavior, 162 electronic structure, 159-161 MRNi catalyst, 235 P-dimensional subspace, 280-28 I n-Pentane reactions with hydrogen, 180 Peroxy radical, 34-35 Peroxy species, 16-18, 24-25 PH modifying, 216-217, 227,231-235 surface conditions, 249-250,252-254 profile in enzyme models, 446 Phosphine oxides, 366-367 Photoadsorption of oxygen, 106 Photoelectron spectroscopy, 32-33 Photoemission alloy valence bands, I51 - 153 oxygen, 5 transition metals, 160-161

Photo-induced reactions, 105-1 09 Photo-oxidation reactions, 118 Pivalic acid and MRNi hydrogenation, 243245 Place exchange of molecular ions, 99-100 Polar interaction between guest and cyclodextrin molecules, 428-432, 434-435 Polyoxygen species, I 15 Potassium salts in ruthenium catalysis, 394395,399-400 PPN+ cation and rhodium catalysis, 361-363 Pressure cobalt catalysis, 334-337, 340-342 rhodium catalysis, 352-356, 368, 370-371 ruthenium catalysis, 378-384,386,392-394 Primary reaction products in cobalt catalysis, 330-333.336-337 Product distribution cobalt catalysis, 328-335, 340 rhodium catalysis, 350-356 ruthenium catalysis, 405-406 Product rates cobalt catalysis, 333-337 rhodium catalysis, 358-359 Promoters, see also specific promoters rhodium catalysis, 354-367.409-410 ruthenium catalysis, 388-400,409-410 Protein and -C=O hydrogenation, 220 Pseudo-monomolecular reactions, 305 p-type semiconducting oxides, 119

Q Q bands, 92-93 Qdimensional space of all reactions, 278-280, 287-288 Qualitative studies of simple reactions, I16 Quinone, bioactive, 459-461

R Racemization of substrates, 255 Raney nickel catalyst adsorption modes, 250-254 corrosion durability, 248-249 optically active modification, 216 Rare-earth-exchanged zeolites, 74 Rare-earth oxides, 43-44

504

SUBJECT INDEX

Rate constants cyclodextrin, 446-447,456-457 cyclophane, 453-457 R-dimensional space, 280-28 1 Reaction conditions cobalt catalysis, 328-337, 341-342 EDA of MRNi, 241-245 Reaction mechanism defined, 274-277 equations, 278-280.284-287 thermodynamics, 3 17-319 Reaction rates cobalt catalysis, 334-340 rhodium catalysis, 352-356, 361-364, 366 ruthenium catalysis, 382-387, 391-394 Reaction space, 279 Reactivity of surface oxygen species, 116- 121 catalytic reactions, 118-121 photo-oxidation reactions, 11 8 simple reactions, 1 16- 1 18 Reagents, modifying correlations on catalyst, 228 EDA of MRNi, 220-224 modifying temperature, 230-23 I RNi hydrogenation, 224-229 second modifying reagent, 238-241 surface conditions, 249-254 Redox process in ruthenium catalysis, 398400 Re-enantiofaces, 254 Regiospecific functionalization of cyclodextrin, 438-442 Retinal pigment models, 451-452 R groups, 16 Rhenium additives, ruthenium catalysis, 391, 404 Rhodium additives in ruthenium catalysis, 391 Rhodium catalysts, CO hydrogenation, 349375,408-410 Rhodium clusters, 367-375 Rhodium ions, 73-74 Rhodium recovery, 354-356,367-369 Rigid band theory of solids, 150-151, 155 RNAase model of cyclodextrin, 446-447 ROO '0hyperfine parameters, 128- 130 R parameters and direct mechanisms, 28829 1 Ruthenium catalysts carboxylic acid-promoted, 375-387

Lewis base-promoted, 387-406,408-410 unpromoted, 375-387,408-410 Ruthenium complexes, 397-406 Ruthenium ions, 73-74

S Salt promoters rhodium catalysis, 359-363 ruthenium catalysis, 389-406 Scandium oxide, 43-44 Schwab effect, metal particles, 160 S-dimensional space of all mechanisms, 278280,284, 287-288 Secondary reaction cobalt catalysis, 331-333, 338-340 rhodium catalysis, 373-374 ruthenium catalysis, 391-394 Segregation in alloys, 200-201 Selective oxidation reactions, 119-121 Selectivity alloys, 150-151, 155-156, 198-201 carboxylic acid-promoted ruthenium catalysis, 375-379 cobalt catalysis, 328-340 homogeneous catalysts, 408-410 hydrocarbon reactions, 200-201 Lewis base-promoted catalysis, 388-394 metals, 172-182 neohexane reactions, 192-193 rhodium catalysts, 350-356,361-364 unpromoted catalysis, 375-379 Self-poisoning alloys, 199-201 hydrocarbon reactions, 183-184 Sheland compounds, 363 Si-enantiofaces, 254 Silica, 59 Silk fibroin, 218 Silk-palladium catalyst, 218-219 Siloxanes, 337 Silver, 0;ion reaction, 102 Simple overall reactions, 291 -300 Simple reactions, 276,281-283,286-287 qualitative studies, 116-1 17 stoichiometric studies, 117-1 18 Skeletal reactions of hydrocarbon-hydrogen, 172-1 82 Sodium bromide

505

SUBJECT INDEX modifying reagent, 224,238-241 surface conditions, 263-264 Sodium halides as modifying reagents, 224, 238-24 1 Solid acid catalysis, 163-165 Solution models, 153- 154 Solution structures in rhodium catalysis, 370 Solvents, see also specific solvents cobalt catalysis, 337-340 rhodium catalysis, 354-356, 364-367 ruthenium catalysis, 376-380, 388-400 Species in chemical systems, 279-281, see also Intermediate species; Terminal species S-3-phenyl-2-hydroxypropionicacid, 250-25 1 Spin delocalization, 28-31 Spin densities, 19-22. 27, 79-82 Spin pairing, 14- I8 Stability carboxylic acid-promoted ruthenium catalysis, 380-38 1 cobalt catalysis, 340-342 Lewis base-promoted ruthenium catalysis, 400-401 rhodium catalysis, 362, 366-369 unpromoted ruthenium catalysis, 380-381 Steady-state mechanisms, 280-281.283-287 ethylene oxide synthesis, 303-305 sulfur dioxide oxidation, 292 Step velocities, 306-308, 319-320 Stereochemical models EDA of MAA, 261 -262 hydrogenation, 260-262 Stereo-differentiating hydrogenation, 247248,256-262 Stereo-differentiation, 224, see alro specific type of stereo-differentiation EDA, 229-230 modifying conditions, 230-241 reaction conditions, 241-245 Stoichiometric studies of simple reactions, 117-118 Stoichiometry. 276, 279-280, 283-285, see also Diagonalized matrix Gibb rule, 318-319 Strain energy of cyclodextrin, 428 Strain relief in cyclodextrin, 433-434 Strong metal support interaction, 160-161 Structure insensitive hydrocarbon reactions, 182-189 Structure sensitivity, 110. 182-189

Substituents, effect on modifying reagents, 220-224 Substituted cyclodextrin. 427 Substrate energy transfer, 452-453 Substrates enzymes, 4 I8 methyl 2-methyl-3-oxobutyrate, 254-255 methyl 3-hydroxy-2-methylbutyrate, 254255 reaction conditions, 242-245 RNi hydrogenation, 224 surface density, 229 Sulfolane carboxylic acid-promoted ruthenium catalysis, 376-377.379-380 Lewis base-promoted ruthenium catalysis. 395-396.399-401 rhodium catalysis, 364-366, 372 unpromoted ruthenium catalysis, 376-377. 379-380 Sulfonic acid, 7 Sulfur dioxide oxidation, 291-292 Sulfur on metal surfaces. 194-197 Superhyperfine interactions and y,, values, 127-128 Superhyperfine tensor, 28-31,48-50 Surface conditions adsorbed modifying reagent, 249-254 characterization, 263-264 Surface enrichment of alloys, 153-1 54 Synergism of alloys, 198-201 Synthesis gas, 313-316 Synton combination in cyclophanes. 453 Synzyme catalysis, 462

T Tartaric acid AA hydrogenation, 260-262 EDA, 234-241.257-259 modifying reagent, 222-224, 227-229 reaction conditions. 246-247 surface conditions, 249,252,263-264 Temperature butenes isomerization, 307-308 carboxylic acid-promoted ruthenium catalysis, 378-384, 386 catalytic reactions, 120-1 22 cobalt catalysis, 332-337, 340-342 hydrocarbon reactions, 202-205

506

SUBJECT INDEX

Lewis base-promoted ruthenium catalysis, 392-394,404 modifying, 216-217,220,227,230-234 neohexane reactions, 189-190 reaction conditions, 241-242 rhodium catalysis, 354-355, 368, 370-371 unpromoted ruthenium catalysis. 378-384, 386 Terminal species, 280-281, 300-30 1 ethylene oxide synthesis, 301 -305 thermodynamics, 3 17-319 Tetraammoniumcyclophane, 455 Tetraazacyclophane, 453-454 Tetraglyme solvent cobalt catalysis. 339-340 rhodium catalysis, 355-356. 361, 364, 366, 372 ruthenium catalysis, 399-400 Tetrahydrofuran, 376-377,379-380 Tetrathiacyclophane tetrasulfonium, 453-454 Thermal programmed desorption spectra, 156-159 Thermodynamics inclusion process, 430-433 reaction mechanism, 317-319 Thorium oxide, 43 Tin oxides, 40-43 Titanium, 103-104 additives, 391 oxide, 7, 33,4448.71-73 photo-induced reactivity, 106- I08 photo-oxidation reactions, 118 TNS, fluoresence maximum, 448-449 Transesterification, 330-331 Transition metal ions, 71-74, 131-134 Transition metal oxides, 44-57 alkaline-earth oxides, 36-37 chromium oxide, 50-51 dioxygen adducts, 81-82 iron group oxide, 54-57 molybdenum oxide, 51-53 titanium oxide, 44-48 tungsten oxide, 53-54 vanadium oxide, 48-50 Transition metals, photoemission, 160-161 Transition-state species, 347-349 Triammonium a cyclodextrin, 449 Tritluorethanol. 338-340 Trifluorethyl formate, 333-337

Trinucledr hydroxymethylidyne cobalt cluster. 343-344 Tri/polynuclear complexes, 91 -95 Tungsten oxide, 53-54 Turnover frequencies, 327, 335-337 Turnover members, 182- I86

U Ultraviolet photoelectron spectroscopy alloy electronic structure, 15I 153, I55 dioxygen species, 122 particle size, I62 surface oxygen species, 109- I I0 Ultraviolet radiation miscellaneous reactions, 108- I09 ozonide ions, 90-91 photoadsorption and desorption, 106 photo-induced exchange reaction, 106-1 07 titanium oxide, 47-48 zeolites, 62-74 -

V Valine, hydrogenation modifier, 229 Vanadium oxide, 48-50 van der Waals interaction, cyclodextrin, 428432 V centers, 66-68. 85, 90-95 Vector spaces, and reaction mechanism, 27928 I Velocities. 306-308. 319-320

W Water cobalt stability, 339-342 MRNi hydrogenation, 243-245

x X-ray analyses cyclodextrin complexes, 425-426 cyclophanes, 427-428 X-ray photoelectron spectroscopy alloy electronic structure, 151-153, 155 dioxygen species, I22 particle size, 160-162 surface oxygen species, 109- I 10

SUBJECT INDEX

Y Yttrium oxide. 43- 44

Z Zeolites alkali and alkali-earth. 68-74 cation exchanged. 70 decationated, 66-68 experimental conditions. 62-66 g z z values, I27 128 0; species, 61 -74 0;ions, 97-98 ~

507

pretrcatmenr conditions. 62 64 rare-edrth exchanged. 74 starting materials. 65 66 structure. 59-61 transition metal ions, 71- 74 ultrastable, 63-64 UV radiation, 62-74 x-type, 60-62 y-type, 60-62 Zinc complex of bis-histaminocyclodextrin, 448 Zinc oxides, 39-40 Zirconium additives and ruthenium catalysis, 39 I

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Contents of Previous Volumes About the Mechanism of Contact Catalysis GEORG-MARIA SCHWAB

Volume 1

The Heterogeneity of Catalyst Surfaces for Chemisorption HUGHS. TAYLOR Alkylation of lsoparaffins V . N. IPATIEFFAND LOUISSCHMERLING Surface Area Measurements. A New Tool for Studying Contact Catalysts P. H. EMMETT The Geometrical Factor in Catalysis R. H. GRIFFITH The Fischer-Tropsch and Related Processes for Synthesis of Hydrocarbons by Hydrogenation of Carbon Monoxide H. H. STORCH The Catalytic Activation of Hydrogen D. D. ELEY Isomerization of Alkanes HERMAN PINES The Application of X-Ray Diffraction to the Study of Solid Catalysts M . H. JELLINEK A N D I. FANKUCHEN

Volume 3

Balandin's Contribution to Heterogeneous Catalysis B. H. W. TRAPNELL Magnetism and the Structure of Catalytically Active Solids P. W. SELWOOD Catalytic Oxidation of Acetylene in Air for Oxygen Manufacture J. HENRYRUSHTONA N D K. A. KRIEGER The Poisoning of Metallic Catalysts E. B. MAXTED Catalytic Cracking of Pure Hydrocarbons VLADIMIR HAENSEL Chemical Characteristics and Structure of Cracking Catalysts A. G. OBLAD,T. H. MILLIKEN,JR., AND G. A. MILLS Reaction Rates and Selectivity in Catalyst Pores AHLBORNWHEELER Nickel "Ifide Catalysts WILLIAM J. KIRKPATRICK

Volume 2

The Fundamental Principles of Catalytic Activity FREDERICK SEITZ The Mechanism of the Polymerization of A1kenes LOUISSCHMERLING A N D V. N. IPATIEFF Early Studies of Multicomponent Catalysts ALWINMITTASCH Catalytic Phenomena Related to Photographic Development T. H. JAMES Catalysis and the Adsorption of Hydrogen o n Metal Catalysts OTTOBEECK Hydrogen Fluoride Catalysis J . H. SIMONS Entropy of Adsorption CHARLES KEMBALL

Volume 4

Chemical Concepts of Catalytic Cracking R. C. HANSFORD Decomposition of Hydrogen Peroxide by Catalysts in Homogeneous Aqueous Solution J. H. BAXENDALE Structure and Sintering Properties of Cracking Catalysts and Related Materials HERMAN E. RIES,JR. Acid-Base Catalysis and Molecukdr Structure R. P. BELL Theory of Physical Adsorption TERRELL L. HILL 509

510

CONTENTS OF PREVIOUS VOLUMES

The Role of Surface Heterogeneity in Adsorption GEORGED. HALSEY Twenty-Five Years of Synthesis of Gasoline by Catalytic Conversion of Carbon Monoxide and Hydrogen HELMUT PICHLER The Free Radical Mechanism in the Reactions of Hydrogen Peroxide JOSEPH WEISS The Specific Reactions of Iron in Some Hemoproteins PHILIPGEORGE

Volume 5 Latest Developments in Ammonia Synthesis ANDERSNIELSEN Surface Studies with the Vacuum Microbalance: Instrumentation and Low-Temperature Applications T. N . RHODIN,JR. Surface Studies with the Vacuum Microbalance: High-Temperature Reactions EARLA. GULBRANSEN The Heterogeneous Oxidation of Carbon Monoxide MORRISKATZ Contributions of Russian Scientists to Catalysis G . S. JOHN,A N D E. FIELD J. G . TOLPIN. The Elucidation of Reaction Mechanisms by the Method of Intermediates in QuasiStationary Concentrations J. A. CHRISTIANSEN Iron Nitrides as Fischer-Tropsch Catalysts ROBERTB. ANDERSON Hydrogenation of Organic Compounds with Synthesis Gas MILTON ORCHIN The Uses of Raney Nickel EUGENE LIEBERA N D FREDL. MORRITZ

Some General Aspects of Cheniisorptron and Catalysis TAKAO KWAN Nobel Metal Synthetic Polymer Catalysts and Studies on the Mechanism of Their Action P. DUNWORTH A N D F. F. NORI) WILLIAM Interpretation of Measurement in Experimental Catalysis P. B. WEISZA N D C. D. PRATER Commercial lsomerization B. L. EVERING Acidic and Basic Catalysis MARTINKILPATRICK Industrial Catalytic Cracking RODNEYV. SHANKLAND ~

Volume 7 The Electronic Factor in Heterogeneous Catalysis M . McD. BAKERA N D G. 1. JENKINS Chemisorption and Catalysis on Oxide Semiconductors G . PARRAVANO A N D M. BOUDART The Compensation Effect in Heterogeneous Catalysis E. CREMER Field Emission Microscopy and Some Applications to Catalysis and Chemisorption ROBERTGOMER Adsorption o n Metal Surhces and Its Bearing on Catalysis JOSEPHA. BECKER The Application of the Theory of Semiconductors t o Problems of Heterogeneous Catalysis K. HAUFFE Surface Barrier Effects in Adsorption, Illustrated by Zinc Oxide S. ROY MORRISON Electronic Interaction between Metallic Catalysts and Chemisorbed Molecules R. SUHRMANN

Volume 6

Volume 8

Catalysis and Reaction Kinetics at Liquid Interfaces J. T. DAVIES

Current Problems of Heterogeneous Catalysis J. ARVIDHEDVALL

CONTENTS OF PREVIOUS VOLUMES

Adsorption Phenomena J. H. DE BOER Activation of Molecular Hydrogen by Homogeneous Catalysts S. W. WELLERAND G. A. MILLS Catalytic Syntheses of Ketones V. 1. KOMAREWSKY AND J. R. COLEY Polymerization of Olefins from Cracked Gases EDWIN K. JONES Coal-Hydrogenation Vapor-Phase Catalysts E. E. DONATH The Kinetics of the Cracking of Cumene by Silica-Alumina Catalysts CHARLESD. PRATERA N D RUDOLPHM. LAGO Volume 9

Proceedings of the International Congress on Catalysis, Philadelphia, Pennsylvania, I956 Volume 10

The Infrared Spectra of Adsorbed Molecules R. P. ElSCHENS A N D w . A. PLISKlN The Influence of Crystal Face in Catalysis ALLANT. GWATHMEY AND ROBERTE. CUNNINGHAM The Nature of Active Centers and the Kinetics of Catalytic Dehydrogenation A. A. BALANDIN The Structure of the Active Surface of Cholinesterases and the Mechanism of Their Catalytic Action in Ester Hydrolysis F. BERGMANN Commercial Alkylation of Paraffins and Aromatics EDWINK. JONES The Reactivity of Oxide Surfaces E. R. S. WINTER The Structure and Activity of Metal-on-Silica Catalysts G. C. SCHUITA N D L. L. VAN REIJEN Volume 11

The Kinetics of the Stereospecific Polymerization of a-Olefins G . NATTAAND 1. PASQUON

51 1

Surface Potentials and Adsorption Process on Metals R. V. CULVER AND F. C. TOMPKINS Gas Reactions of Carbon P. L. WALKER, JR., FRANK RUSINKO, JR., A N D L. G. AUSTIN The Catalytic Exchange of Hydrocarbons with Deuterium C. KEMBALL lmmersional Heats and the Nature of Solid Surfaces J. J. CHESSICK AND A. c . ZETTLEMOYER The Catalytic Activation of Hydrogen in Homogeneous, Heterogeneous, and Biological Systems J. HALPERN Volume 12

The Wave Mechanics of the Surface Bond in Chemisorption T. B. GRIMLEY Magnetic Resonance Techniques in Catalytic Research D. E. OREILLY Bare-Catalyzed Reactions of Hydrocarbons HERMAN PINESA N D LUKEA. SCHAAP The Use of X-Ray and K-Absorption Edges in the Study of Catalytically Active Solids ROBERT A. VANNORDSTRAND The Electron Theory of Catalysis on Semiconductors TH. WOLKENSTEIN Molecular Specificity in Physical Adsorption D. J. C. YATES Volume 13

Chemisorption and Catalysis on Metallic Oxides F. S. STONE Radiation Catalysis R. COEKELBERGS, A. CRUCQ,AND A. FRENNET Polyfunctional Heterogeneous Catalysis PAULB. WEISZ A New Electron Diffraction Technique, Potentially Applicable to Research in Catalysis L. H. GERMER

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CONTENTS OF PREVIOUS VOLUMES

The Structure and Analysis of Complex Reaction Systems JAMES WE1 A N D CHARLES D. PRATER Catalytic Effect in Isocyanate Reactions A N D G . A. MILLS A. FARKAS

Volume 14 Quantum Conversion in Chloroplasts MELVIN CALVIN The Catalytic Decomposition of Formic Acid P. MARS,J. J. F. SCHOLTEN, AND P. ZWEITERINC Application of Spectrophotometry to the Study of Catalytic Systems JR. H. P. LEFTINA N D M. C. HOBSON. Hydrogenation of Pyridines and Quinolines MORRIS FREIFELDER Modern Methods in Surface Kinetics: Flash. Desorption, Field Emission Microscopy, and Ultrahigh Vacuum Techniques GERTEHRLICH Catalytic Oxidation of Hydrocarbons L. YA. MARGOLIS

Volume 15 The Atomization of Diatomic Molecules by Metals D. BRENNAN The Clean Single-Crystal-Surface Approach to Surface Reactions N . E. FARNSWORTH Adsorption Measurements during Surface Catalysis KENZITAMARU The Mechanism of the Hydrogenation of Unsaturated Hydrocarbon on Transition Metal Catalysts G . C. BONDA N D P. B. WELLS Electronic Spectroscopy of Absorbed Gas Molecules A. TERENIN The Catalysis of Isotopic Exchange in Molecular Oxygen G . K. BORESKOV

Volume 16 The Homogeneous Catalytic Isomerization of Olefins by Transition Metal Complexes MILTONORCHIN The Mechanism of Dehydration of Alcohols over Alumina Catalysts HERMAN PINESAND JOOSTMANASSEN n Complex Adsorption in Hydrogen Exchange on Group Vlll Transition Metal Catalysts J. L. GARNETTA N D W. A. SOLLICHBAUMGARTNER Stereochemistry and the Mechanism of Hydrogenation of Unsaturated Hydrocarbons SAMUEL SIEGEI. Chemical Identification of Surface Groups H. P. BOEHM

Volume 17 On the Theory of Heterogeneous Catalysis JUROHORIUTIA N D TAKASHI NAKAMURA Linear Correlations of Substrate Reactivity in Heterogeneous Catalytic Reactions M. KRAUS Application of a Temperature-Programmed Desorption Technique to Catalyst Studies R. J. CVETANOVIC A N D Y.AMENOMIYA Catalytic Oxidation of Olefins HERVEY H.VOCE AND CHARLES R. ADAMS The Physical-Chemical Properties of Chromia-Alumina Catalysts CHARLES P. POOLE, JR. A N D D. s. MAClVER Catalytic Activity and Acidic Property of Solid Metal Sulfates Kozo TANABE AND TSUNEICHI TAKESHITA Electrocatal ysis S. SRINIVASEN, H. WROBLOWA, A N D J. O’M. BOCKRIS Volume 18 Stereochemistry and Mechanism of Hydrogenation of Napthalenes in Transition Metal Catalysts and Conformational Analysis of the Products A. W. WEITKAMP

CONTENTS OF PREVIOUS VOLUMES The Effects of Ionizing Radiation on Solid Catalysts H. TAYLOR ELLISON Organic Catalysis over Crystalline Aluminosilicates P. B. VENUTOA N D P. S. LANDIS On the Transition Metal-Catalyzed Reactions of Norbornadiene and the Concept of II: Complex Multicenter Processes G. N. SCHRAUZER Volume 19 Modern State of the Multiplet Theory of Heterogeneous Catalysis A. A. BALANDIN The Polymerization of Olefins by Ziegler Catalysts M. N. BERGER,G . BOOCOCK,A N D R. N. HAWARR Dynamic Methods for Characterization of Adsorptive Properties of Solid Catalysts L. POLINSKI A N D L. NAPHTALI Enhanced Reactivity at Dislocations in Solids J. M. THOMAS

513

Molecular Orbital Symmetry Conservation in Transition Metal Catalysis FRANKD. MANGO Catalysis by Electron Donor-Acceptor Complexes KENZITAMARU Catalysis and Inhibition in Solutions of Synthetic Polymers and in Micellar Solutions H. MORAWETZ Catalytic Activities of Thermal Polyanhydroa-Amino Acids DUANE L. ROHLFING A N D SIDNEY W. Fox

Volume 21

Kinetics of Adsorption and Desorption and the Elovich Equation C. AHARONI AND F. C. TOMPKINS Carbon Monoxide Adsorption on the Transition Metals R. R. FORD Discovery of Surface Phases by Low Energy Electron Diffraction (LEED) JOHNW. M A Y Sorption, Diffusion, and Catalytic Reaction in Zeolites Volume 20 L. RlEKERT Chemisorptive and Catalytic Behavior of Adsorbed Atomic Species as Intermediates Chromia in Heterogeneous Catalysis L. BURWELL, JR., GARY L. HALLER, CARL ROBERT WAGNER A N D JOHNF. READ KATHLEEN C. TAYLOR, Correlation among Methods of Preparation of Solid Catalysts, Their Structures, and Volume 22 Catalytic Activity TAKAYASU SHIRASAKI, Hydrogenation and Isomerization over Zinc KIYOSHI MORIKAWA. Oxide A N D MASAHIDE OKADA R. J. KOKESA N D A. L. DENT Catalytic Research o n Zeolites Chemisorption Complexes and Their Role in A N D Y. ONO J. TURKEVICH Catalytic Reactions on Transition Metals Catalysis by Supported Metals Z. KNOR M . BOUDART Influence of Metal Particle Size in NickelCarbon Monoxide Oxidation and Related on-Aerosil Catalysts on Surface Site DisReactions on a Highly Divided Nickel tribution, Catalytic Activity, and SelecOxide tivity A N D S. J. TEICHNER P. C. GRAVELLE R. VAN HARDEVELD A N D F. HARTOG Acid-Catalyzed lsomerization of Bicyclic Adsorption and Catalysis on Evaporated Olefins Alloy Films JEANEUGENE GERMAIN AND R. L. Moss A N D L. WHALLEY MICHELBLANCHARD

514

CONTENTS OF PREVIOUS VOLUMES

Heat-Flow Microcalorimetry and Its Applications to Heterogeneous Catalysis P. C. GRAVELLE Electron Spin Resonance in Catalysis JACKH. LUNSFORD Volume 23

Metal Catalyzed Skeletal Reactions of Hydrocarbons J. R. ANDERSON Specificity in Catalytic Hydrogenolysis by Metals J. H. SINFELT The Chemisorption of Benzene P. B. MOYESAND P. B. WELLS The Electronic Theory of Photocatalytic Reactions on Semiconductors TH. WOLKENSTEIN Cycloamyloses as Catalysts DAVIDw . GRIFFITHS AND MYRONL. BENDER Pi and Sigma Transition Metal Carbon Compounds as Catalysts for the Polymerization of Vinyl Monomers and Olefins D. G. H. BALLARD Volume 24

Kinetics of Coupled Heterogeneous Catalytic Reactions L. BERANEK Catalysis for Motor Vehicle Emissions JAMESWE1 The Metathesis of Unsaturated Hydrocarbons Catalyzed by Transition Metal Compounds J. C. MOLA N D J. A. MOULIJN One-Component Catalysts for Polymerization of Olefins YU. YERMAKOV A N D v. ZAKHAROV The Economics of Catalytic Processes J. DEWING AND D. S. DAVIE Catalytic Reactivity of Hydrogen on Palladium and Nickel Hydride Phases W. PALCZEWSKA Laser Raman Spectroscopy and Its Application to the Study of Adsorbed Species R. P. COONEY, G . CURTHOYS, AND NGUYEN THETAM Analysis of Thermal Desorption Data for Adsorption Studies

MILOS SMUTEK, SLAVOJ FRANTISEK BUZEK

CERN+,

AND

Volume 25

Application of Molecular Orbital Theory to Catalysis ROGERC. BAETZOLD The Stereochemistry of Hydrogenation of a,B-Unsaturated Ketones ROBERT L. AUGUSTINE Asymmetric Homogeneous Hydrogenation J. D. MORRISON, w . F. MASLER.A N D M. K. NEUBERG Stereochemical Approaches to Mechanisms of Hydrocarbon Reactions on Metal Catalysts J. K. A. CLARKE A N D J. J. ROONEY Specific Poisoning and Characterization of Catalytically Active Oxide Surfaces HELMUT KNOZINGER Metal-Catalyzed Oxidations of Organic Compounds in the Liquid Phase: A Mechanistic Approach ROGERA. SHELDON AND JAYK. KOCHI Volume 26

Active Sites in Heterogeneous Catalysis G . A. SOMORJAI Surface Composition and Selectivity of Alloy Catalysts AND R. A. V A N SANTEN W. M. H.SACHTLER Mossbauer Spectroscopy Applications to Heterogeneous Catalysis JAMFS A. DUMESIC AND HENRIK TOPS@E Compensation Effect in Heterogeneous Edt a Iy si s A. K. GALWEY Transition Metal-Catalyzed Reactions of Organic Halides with CO, Olefins. and Acetylenes R. F. HECK Manual of Symbols and Terminology for Physicochemical Quantities and UnitsAppendix I1 Part 11 : Heterogeneous Catalysis Volume 27

Electronics of Supported Catalysts GEORG-MARIA SCHWAB

CONTENTS OF PREVIOUS VOLUMES

515

The Effect of a Magnetic Field on the Cata- Site Density and Entropy Criteria in Identifying Rate-Determining Steps in Solid-Catlyzed Nondissociative Parnhydrogen Conversion Rate alyzed Reactions P. W. SELWOOD RUSSELLW. MAATMAN Hysteresis and Periodic Activity Behavior in Organic Substituent Effects as Probes for the Catalytic Chemical Reaction Systems Mechanism of Surface Catalysis VLADlMfR HLAVACEK A N D JAROSLAV M. KRAUS VOTRUBA Enzyme-like Synthetic Catalysts (Synzymes) G . P. ROVER Surface Acidity of Solid Catalysts A N D B. H. C. WINQUIST Hydrogenolytic Behaviors of Asymmetric H. A. BENESI Selective Oxidation of Propylene Diarylmethanes A N D TADASHI KAWAI W. KEULKS,L. DAVIDKRENZKE, YASUOYAMAZAKI GEORGE AND THOMAS N. NOTERMANN Metal-Catalyzed Cyclization Reactions of u-II Rearrangements and Their Role in Hydrocarbons ZOULTAN PAAL Catalysis AND MINORU Tsu~sui BARRY GOREWIT Characterization of Molybdena Catalysts Volume 30 F. E. MASSOTH Mechanisms of Skeletal Isomerization of HyPoisoning of Automotive Catalysts drocarbons on Metals M. SHELEF, K. OTTO,AND N. C. OTTO F. G. GAULT Tin- Antimony Oxide Catalysts Volume 28 FRANK J. BERRY Selective Oxidation and Ammoxidation of Elementary Steps in the Catalytic Oxidation Propylene by Heterogeneous Catalysis of Carbon Monoxide on Platinum Metals AND ROBERT K. GRASSELLI T. ENGELAND G . ERTL JAMES D. BURRINGTON The Binding and Activation of Carbon MonMechanism of Hydrocarbon Synthesis over oxide, Carbon Dioxide, and Nitric Oxide Fischer-Tropsch Catalysts and Their Homogeneously Catalyzed ReA N D W. M. H. SACHTLER P. BILOEN actions Surface Reactions and Selectivity in ElectroA N D DAN E. HENRICHARDEISENBERG catalysis DRIKSEN P. SAKELLAROPOULOS GEORGE The Kinetics of Some Industrial HeterogeneSolvent and Structure Effects in Hydrogenaous Catalytic Reactions tion of Unsaturated Substances on Solid M. 1. TEMKIN Catalysts Metal-Catalyzed Dehydrocyclization of AlN ~ VLASTIMIL RO~I~KA LIBORC E R V EAND ky laromatics SIGMUND M. CSICSERY Metalloenzyme Catalysis Volume 31 AND JOSEPHJ. VILLAFRANCA Nonacid Catalysis with Zeolites FRANKM. RAUSHEL I. E. MAXWELL Characterization and Reactivity of MonoVolume 29 nuclear Oxygen Species on Oxide Surfaces M. CHEA N D A. J. TENCH Reaction Kinetics and Mechanism on Metal Sulfur Poisoning of Metals Single Crystal Surfaces C. H. BARTHOLOMEW, P. K. AGRAWAL, AND J. MADIX ROBERT J. R. KATZER Photoelectron Spectroscopy and Surface Methanol Synthesis Chemistry K. KLER M. W. ROBERTS

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