Advances in Catalysis, Volume 29

ADVANCES IN CATALYSIS VOLUME 29

Advisory Board

G. K. BORESKOV Novosibirsk, U . S . S . R .

M. BOUDART Stanford, Calijorn iu

P. H. EMMETT

A. OZAKI

Portland, Oregon

Tokyo, Japan

G. A. SOMORJAI Berkeley, California

M. CALVIN Berkeley, California

G.- M. SCHWAB Munich. Germany

R. UGO Milan, Iiaiy

ADVANCES IN CATALYSIS VOLUME 29

Edited by

D. D. ELEY The University Nottingham, England

HERMAN PINES Northwestern University Evanston, Illinois

PAULB. WEISZ Mobil Research and Development Corporation Princeton, New Jersey

1980 ACADEMIC PRESS A Subsidiary of Harcourt Brace Jovanovich, Publishers

New York

London Toronto Sydney San Francisco

COPYRIGHT @ 1980, BY ACADEMIC PRESS,INC. ALL RIGHTS RESERVED. NO PART OF THIS PUBLICATION MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM OR BY ANY MEANS, ELECTRONIC OR MECHANICAL, INCLUDING PHOTOCOPY, RECORDING, OR ANY INFORMATION STORAGE AND RETRIEVAL SYSTEM, WITHOUT PERMISSION IN WRITING FROM THE PUBLISHER.

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LIBRARY OF

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ISBN 0-12-007829-5 PRINTED IN THE UNITED STATES OF AMERICA 80 81 82 83

9 8 7 6 5 4 3 2 1

Contents CONTRIBUTORS ............................................................... PREFACE .................................................................... GlULlONATTA(1903-1979) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ix xi

...

Xlll

Reaction Kinetics and Mechanism on Metal Single Crystal Surfaces ROBERT J. MADIX I. 11.

111.

IV. V.

Introduction .............................. . . . . . . . . . . . . . . . . . . The Tools of Surface Reactivity.. . . . . The Reactions of Carboxylic Acids and .................................. The Reactions of Alcohols Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 3 21 36 49 50

Photoelectron Spectroscopy and Surface Chemistry I.I.

11. 11.

111. 111.

1v. 1v.

V. V. VI. VI.

VII. VII.

VIII. VIII. IX. IX.

M. W. W. ROBERTS ROBERTS M. . . . . . . . . . . . . . . . Introduction Introduction . . . . . . . . . . . . . . . . . . . . . . .......................... . . . . . . . . . . . . . . . . . .. .. .. .... . . .................... X-Ray and UV Photoelectron Spectroscopy ............................. ron Intensity Data. Data. .. .. .. Calculation of Surface Concentrations from Photoelectron Intensity Experimental Strategy Strategy .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..................... Experimental .......... Chemisorption of Diatomic Molec ....... Chemisorption of More Complex Molecules . . . . . . . . . Metaloxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alloys and Surface Segregation . . . . . . .................... Conclusion .. .. ................. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion References ......... ..................................

55 55 556 6 59 59 62 65 80 80 88 88 91 91 92 93 93

Site Density and Entropy Criteria in Identifying Rate- Determining Steps in Solid-Catalyzed Reactions RUSSELL W. MAATMAN I. 11. 111.

97 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Theory . . . . . . . . . . . . . . ...... . . . . . . . . . . . . . . . . . . . . 99 121 Analysis of Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . 147 Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 References . . . . . . . . . . . . . . . . . . . . . . . . V

vi

CONTENTS

Organic Substituent Effects as Probes for the Mechanism of Surface Catalysis M. KRAUS I. 11. 111. IV. V. VI . VII.

........................ Introduction . . . . . . . . . . sis ..................... Structure Effects on Rate .......................... Quantitative Treatment ... Heterogeneous Acid-Base Catalysis . . . . . . . . . . . Heterogeneous Redox Catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . dsorptivity . . . . . . . . . . . . ...

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

References . . . . . . . . . . . .

151 153 156 163 172 189 191 192

Enzyme-like Synthetic Catalysts (Synzymes) G. P. ROYER

I.

Introduction . . . . . . . . . .

11.

111. IV. V. VI. VII. VIII.

......................... 205 Linear Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalyst Based on Polyethyleneimine: A Branched Synthetic Polymer . . . . . . . 215 Immobilized Catalysts . . ........... Semisynthetic Enzymes . ........... .............................................. 223 References . . . . . . . . . . . .

Hydrogenolytic Behaviors of Asymmetric Diarylmethanes YASUOYAMAZAKI AND TADASHI KAWAI I. 11.

111. IV. V. VI. VII.

Introduction ................................ ... Preparation of Asymmetric Diarylmethanes ............................. Catalyst for Hydrogenolysis of Diarylmethanes. . ............... Kinetics of Catalytic Hydrogenolysis of Diphenylmethane . . . . . . . . . . . . . . . . . Catalytic Hydrogenolysis of Asymmetric Diarylmethanes . . . . . . . . . . . . . . . . . . Active Species of MoO,-AI,O, Catalyst for Hydrogenolysis ........................... of Diarylmethanes . . . . . Conclusions. . . . . . . . . . . ........................... ... References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

229 232 239 241 243 258 269 270

Metal-Catalyzed Cyclization Reactions of Hydrocarbons ZOLTANP A L I. 11. 111. IV. V.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . “Simple” Cyclization Reactions Cyclization with Skeletal Rearrangement . . . . . . . Cyclization over Dual Function Catalysts and Oxides ..................... Interpretation of Metal Activity in Catalytic Cyclization . . . . . . . . . . . . . . . . . . . References ................................. ...

273 31 1 317 329

CONTENTS

AUTHORINDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SUBJECT INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CONTENTS OF PREVIOUS VOLUMES ...............................

vii . . . . . . . . 335 . . . . . . . . 353 . . . . . . . . 361

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Cont r ibut0rs Numbers in parentheses indicate the pages on which the authors’ contributions begin.

TADASHI KAWAI,Department oj Industrial Chemistry, Faculty oj Technology, Tokyo Metropolitan University, Tokyo, Japan (229) M. KRAUS,Institute of Chemical Process Fundamentals, Czechoslovak Academy ojsciences, 165 02 Prague 6-Suchdol, Czechoslovakia (15 1) RUSSELLW. MAATMAN, Department of Chemistry, Dordt College, Sioux Center, Iowa 51250 (97) ROBERT J. MADIX,Department of Chemical Engineering, Stanford University, Stanjord, California 94305 ( 1 ) ZOLTANPAAL,Institute of Isotopes of the Hungarian Academy of Sciences, Budapest, Hungary (273) M. W. ROBERTS,Department of Physical Chemistry, University College, Cardiff CF1 I X L , United Kingdom (55) G. P. ROYER,Department of Biochemistry, Ohio State University, Columbus, Ohio 43210 (197) YASUOYAMAZAKI, Department of Industrial Chemistry, Faculty of Technology, Tokyo Metropolitan University, Tokyo, Japan (229)

ix

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Preface The last two decades have witnessed great strides in the contribution of catalysis to the advancement of petrochemistry and related fields. New catalytic systems and catalysts added much to the discovery and improvement of catalytic processes as a result of a better understanding of catalytic reactions and the availability of new analytical tools. Introduction of zeolites into catalytic cracking improved the quality of the product and the efficiency of the process. It was estimated that this modification in catalyst composition in the United States alone saved over 200 million barrels of crude oil in 1977. The use of bimetallic catalysts in reforming of naphthas, a basic process for the production of high-octane gasoline and petrochemicals, resulted in great improvement in the catalytic performance of the process, and in considerable extension of catalyst life. New catalytic approaches to the development of synthetic fuels are being unveiled. In homogeneous catalytic systems we witnessed a new process for the production of acetic acid from methanol and carbon monoxide using a transition metal complex, thus displacing the earlier process employing ethylene as the starting material. The use of immobilized enzymes makes possible the commercial conversion of glucose into fructose. The present volume continues to provide an entire spectrum of interdisciplinary exposures to catalysis. As stated in the introduction to the first chapter by R. J. Madix, heterogeneous catalysis is a complex phenomenon to understand at the molecular level, and the key to understanding such processes lies in the ability to dissect the catalytic event into its separate components. This chapter describes physical and spectroscopic approaches to make the explanation of a variety of catalytic reactions on clean metal surface possible. M. W. Roberts reviews the contribution of photoelectron spectroscopy to provide chemical information at the molecular level to the catalytic reactions on surfaces. The use of organic probes to study the rate-determining steps and mechanisms of catalytic reactions is reviewed by R. W. Maatman and M. Kraus, respectively. Attempts to make enzyme-like catalysts, synzymes, from nonbiological systems is described by G. P. Royer. The final two chapters by Y. Yamazaki and T. Kawai, and Z. Paal deal with catalytic hydrocarbon conversions using acids and metals, respectively, as catalysts. HERMAN PINES xi

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Giulio Natta 1903-1 979 Giulio Natta, Nobel prize winner for chemistry in 1963, died in Bergamo, Italy on May 2, 1979. His scientific interest was originally centered on the use of x rays for the determination of the crystalline structure of organic and inorganic materials. While visiting the University of Fribourg, Fribourg, Germany, in 1923, he became interested in exploring new techniques concerning the interference of electrons. At this time he also knew and appreciated the work of Staudinger in the field of macromolecular chemistry. As a result of this visit he initiated the investigation of the crystalline structure of high polymers. While engaging in this investigation he developed an interest in heterogeneous catalysis. In this field he made some landmark contributions concerning methanol and higher alcohol synthesis from syn gas. He was a Professor of Industrial Chemistry, School of Engineering, Polytechnic Institute of Milan, Milan, Italy since 1937. He became involved with applied research, which led to the production of synthetic rubber in Italy, at the Institute in 1938. He was also interested in the synthesis of petrochemicals such as butadiene and, later, 0x0 alcohols. At the same time he made important contributions to the understanding of the kinetics of some catalytic processes in both the heterogeneous (methanol synthesis) and homogeneous (oxosynthesis) phase. In 1950, as a result of his interest in petrochemistry, he initiated the research on the use of simple olefins for the synthesis of high polymers. This work led to the discovery, in 1954, of stereospecific polymerization. In this type of polymerization nonsymmetric monomers ( e g , propylene, 1-butene, etc.) produce linear high polymers with a stereoregular structure . Initially Professor Natta used catalysts for ethylene polymerization that had been already proposed by Ziegler. However, he subsequently improved the catalytic system in such a way as to synthetize polymers with a very high stereoregularity. This discovery has been the origin of new classes of macromolecular materials with excellent mechanical and thermal properties. These materials are particularly suitable for the production of plastics, films, and fibers from low-cost raw materials. The investigations of Professor Natta and his co-workers in the field of olefin polymerization were not limited to research on new catalysts, but were enlarged to include study of the mechanisms of catalysis, definitions of the structural characteristics of the many stereopolymers produced, study of the kinetics of polymerization, and study of the organometallic chemistry of catalytic systems. In 1963 Professor Natta, ...

XI11

xiv

GIULIO NATTA

together with Professor Ziegler, became Nobel laureate for chemistry as a result of contributions to polymerization. Professor Natta was also a honorary member of many academies (including the New York Academy of Sciences, the Academy of Sciences of URSS, and the Academie des Sciences de 1’Institut de France) and chemical societies (including the Belgian Chemical Society, the Swiss Chemical Society, and the French Chemical Society). He also received the laurea honoris causa from the University of Louvain, Louvain, Belgium, the University of Turin, Turin, Italy, and the University of Genoa, Genoa, Italy. Among his numerous medals and awards were the first Medal International in Synthetic Rubber (1961), the Lavoisier Medals (1963), the STATS Medal (1962), the Lamonsor Medal (1969), and the Perkin Medal of Dyers and Colourists (1963). ITALOPOSQUON

Department of Industrial Chemistry Polytechnic Institute of Milan Milan, Italy

ADVANCES IN CATALYSIS VOLUME 29

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ADVANCES IN CATALYSIS, VOLUME 29

Reaction Kinetics and Mechanism on Metal Single Crystal Surfaces ROBERT J. MADIX Department of Chemical Engineering Stanford University Stanford, California

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 11. The Tools of Surface Reactivity . . . . . . . . . . . . . . . . A.LEED.. . . . . . . . . . . . . . . . . . . . . . . . B,AES . . . . . . . . . . . . . . . . . . . . . . . . . . . C. UPSandXP S . . . . . . . . . . . . . . . . . . . . . . D. FDSorTPD . . . . . . . . . . . . . . . . . . . . . . E.TPRS . . . . . . . . . . . . . , . . . . . . . . . . . 111. The Reactions of Carboxvlic Acids and Related Reactions . . . . A. Historical . . . . . . . . . . . . , . . . . . . . . . . . B. Results for Formic Acid Decomposition on Clean Metals . . . C. Discussion of Formic Acid Decomposition . . . . . . . . . D. Reactions for Formic Acid Decomposition on Metal-Adlayer Surfaces . . . . . . . . . . . . . . . E. The Decomposition of Acetic Acid . . . . . . . . . . . . . IV. The Reactions of Alcohols . . . . . . . . . . . . . . . . . . A. Adsorption . . . . . . . . . . . , . , . . . . . , . . . B. Reaction on Clean Surfaces . . . . . . . . . . . . . . . . C. The Oxidation of Methanol and Ethanol on Copper and Silver . D. Other Oxidation Reactions on Ag(ll0) . . . , . . . . . . . V.Summary . . . . . . . . . . . . , , . . . . . . . . . . . References . . . . . , . . . . . . . . . . . . . . . . . . .

. . . . . . .

. . . . . . . .

I 3

4 7 10 15

. . . . . .

18 21 21 21 28

. . . , . . . . . . . . . . . . .

32 35 36 36 37 38 48 49 50

.

1. Introduction Heterogeneous catalysis is clearly a complex phenomenon to understand at the molecular level. Any catalytic transformation occurs through a sequence of elementary steps, any one of which may be rate controlling under different conditions of gas phase composition, pressure, or temperature. Furthermore, these elementary processes occur catalytically on surfaces that are usually poorly understood, particularly for mixed oxide catalysts. Even on metallic catalysts the reaction environment may produce surface compounds such as carbides, oxides, or sulfides which greatly modify 1

Copyright 0 1980 by Academic Press, Inc. All rights of reproduction in any form reserved. ISBN 0-12-007829-5

2

ROBERT J. MADIX

the intrinsic reactivity of the metal. Coupling all of these complications with the fact that in any given reaction system stable, adsorbed surface intermediates can strongly affect the chemical behavior of the surface, one quickly concludes that catalytic phenomena may be very difficult to understand in a fundamental manner that has real practical significance. The key to understanding such processes lies in our ability to dissect the catalytic event into its separate components. Numerous ingenious experiments have been performed by workers in the field of catalysis for many years, and it is not the intent of this article to review these contributions. It is important to note that such studies have advanced the field of catalysis to a refined science and that a number of general observations have been developed which serve as guidelines for the development and improvement of catalytic materials. Insofar as surface science and the study of reactions on macroscopic single crystal surfaces is related to catalysis, its purpose should therefore be to contribute a more exact and, thereby, a more general understanding of the basic phenomena involved. The combined use of the modern tools of surface science should allow one to understand many fundamental questions in catalysis, at least for metals. These tools afford the experimentalist with an abundance of information on surface structure, surface composition, surface electronic structure, reaction mechanism, and reaction rate parameters for elementary steps. In combination they yield direct information on the effects of surface structure and composition on heterogeneous reactivity or, more accurately, surface reactivity.* Consequently, the origin of well-known effects in catalysis such as structure sensitivity, selective poisoning, ligand and ensemble effects in alloy catalysis, catalytic promotion, chemical specificity,“volcano” effects, to name just a few, should be subject to study via surface science. In addition, mechanistic and kinetic studies can yield information helpful in unraveling results obtained in flow reactors under greatly different operating conditions. There are, of course, limits to the studies possible within the framework of surface reactivity. These limits are imposed largely by the sensitivities of the techniques employed, though in some cases the limitations do arise from a lack of surface definition. It is impossible to prepare a surface totally free of contaminants or undesired defects such as step edges or kinks. Therefore, since one of the major objectives of studies with single crystals is to associate reactivity with specific structural and compositional features of the surface, reaction events that occur in numbers close to the limiting defect or impurity concentrations must be viewed with suspicion and very care-

* This term shall be used to describe studies of heterogeneous reactivity done using the methods of surface science.

METAL SINGLE CRYSTAL SURFACES

3

fully scrutinized. Typically, reactions which involve less than 10’ sites/cm2 are in this category. Aside from the question of the surface sensitivity of the analytical tools, this factor is the most restrictive condition for studies of surface reactivity. It applies as well to studies done on overlayer structures (e.g., sulfur on platinum single crystals), since perfect order cannot be expected in adsorbate layers. Thus, though a square unit of sulfur atoms on a Pt(100) surface may represent 95-98% of the surface, the remaining 2-5% disorder may contribute selectively and significantly to the observed chemistry. It can easily be seen that such considerations direct the course of study in surface reactivity to reactions that occur with high probability over the surface. The second most apparent limitation on studies of surface reactivity, at least as they relate to catalysis, is the pressure range in which such studies are conducted. The lo-” to Torr pressure region commonly used is imposed by the need to prevent the adsorption of undesired molecules onto the surface and by the techniques employed to determine surface structure and composition, which require relatively long mean free paths for electrons in the vacuum. For reasons that are detailed later, however, this so-called “pressure gap” may not be as severe a problem as it first appears. There are many reaction systems for which the surface concentration of reactants and intermediates found on catalysts can be duplicated in surface reactivity studies by adjusting the reaction temperature. For such reactions the mechanism can be quite pressure insensitive,and surface reactivity studies will prove very useful for greater understanding of the catalytic process. II. The Tools of Surface Reactivity

The experimental techniques most commonly used to characterize surfaces in studies of surface reactivity are as follows : 1. Low energy electron diffraction (LEED) (1, 2) 2. Auger electron spectroscopy (AES) ( 3 , 4 ) 3. Ultraviolet photoelectron spectroscopy (UPS) (5, 6) and x-ray photoelectron spectroscopy (XPS) (7, 8) 4. Flash desorption (FDS) or temperature programmed desorption (TPD) (9,101 5 . Temperature programmed reaction spectroscopy (TPRS) These techniques have been reviewed extensively (Il-Z4), and the interested reader should consult the references for details. For convenience a brief outline of the features of each technique and the most important results for studies of surface reactivity will be presented.

4

ROBERT J. MADIX

A. LEED Low energy electron diffraction is the most commonly used method for determining surface structures. A collimated beam of electrons of energies of the order of 100 eV is directed onto the surface, and the elastically backscattered electrons are accelerated through a 5-kV potential and observed on a phosphorescent screen, providing a visual display of the diffraction pattern (2).This display allows the investigator to observe the general symmetry features of the diffraction pattern quickly and to monitor changes in the structure as adsorption or desorption of adatoms proceeds. A series of LEED patterns formed by the adsorption of sulfur on Ni(100) is shown in Fig. 1 (15). Usually the unit cell of the overlayer structure is referenced to that of the underlying metal surface. The structure shown in Fig. l b is designated p(2 x 2) to indicate that the sulfur atoms occupy a square array with a unit cell distance equal to twice that of the underlying surface. Other structural notations are used as well (16, 17). The most general notation utilizes the two-fold matrix which transforms the clean surface unit cell vectors into the adlayer unit cell vectors. This notation is exemplified in Fig. 2 for the W( loo)(: - 7)C surface carbide on tungsten (18).Also included in Fig. 2 are several known surface tungsten carbide structures. In order to determine the position of surface atoms with LEED the variation of the intensity of selected diffraction spots with beam voltage must be accurately measured and interpreted. For this purpose a rotatable electron “catcher’s mitt” (Faraday cup) is employed (29).These I-V plots (intensityvoltage) are then compared to theory, and the surface structure is determined (20).A comparison of experiment and theory is shown in Fig. 3 for Ni(100)p(2 x 2)s ( 2 0 ~ )i.e., ; sulfur adsorbed on the Ni(100) surface in the p(2 x 2) structure. The general conclusion drawn from such results is that adatoms such as sulfur, carbon, and oxygen adsorb in high coordination sites between the metal atoms on the surface as shown in Fig. 1 (21).In addition, they often distribute themselves so as not to occupy nearest neighbor sites, forming either p(2 x 2) or c(2 x 2) structures (Fig. 1) at one-quarter and one-half monolayer coverages, respectively (22, 23). It is clear that these two structures offer different binding sites to adsorbing species. For Ni(100)c(2 x 2)S, for example, the vacant fourfold hollows (see Fig. 1) are ineffective for H2S decomposition. It is reasonable to expect that, generally, the p(2 x 2)s and c(2 x 2)s structures would show different surface reactivities on (100) surfaces of all face-centered cubic metals (Ag, Cu, Au, Ni, Pd, Pt, Rh, Ir), leading to selective poisoning by sulfur in submonolayer quantities. Some metal surfaces reconstruct either in the clean state or in the presence of adsorbed gases. Platinum, iridium, and gold (100) surfaces, which have square symmetry, all reconstruct to hexagonal close-packed (1 11) surfaces

5

METAL SINGLE CRYSTAL SURFACES

REAL SPACE 0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

LEED NiIlOOl ( l x l l

(a 1

REAL SPACE

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

0

0 . 0 . 0 . 0 . ‘ 3

0

0 . 0 . 0 . 0 . 0

0

0 . 0 . 0 . 0 . 0

0

LEED Ni 11001p(2X2)‘4 S

.... .... REAL SPACE

0 . 0 . 0 . 0 . 0

0

0

0

0

0

0

0

0

.... 0

0

0

0

0

0

0

0

LEED Ni[1001c12x21’/2 5

(b) (C 1 FIG. I . Real space and LEED structures for Ni(100) surfaces with ordered sulfur overlayers.

6

ROBERT J. MADIX

0 OOGO 0000 0000 0 0000

0 0

LEED PATTERN (a)

0

0

0

LEED PATTERN

REAL SPACE

(c)

(1x1)

0

oxxxxxo X X X X X

0

LEED PATTERN

0

x x x x

REAL SPACE

X

0

0

X X X X

oxxxxxts REAL SPACE

LEED PATTERN

(b) (32x2)

(d)

REAL SPACE

(6x1)

oxxxxxo X X X X X

X X X X X

oxxxxxo LEED PATTERN

(e 1

x o x x x x

REAL SPACE

( 5 x I)

x o x x

x x x x x

x x x x x x x x x x o x x o x

LEED PATTERN (f

REAL SPACE

W(110)-(15~3)R14"

FIG.2. Real space and LEED structures for surface carbides of W(100).

in the top layer (24). The adsorption of hydrogen, CO, or oxygen on these surfaces may cause reversion to the square surface structure (25,26)or the formation of more complex structures, particularly in the formation of surface oxides (27).Surface reconstruction can also be produced by carbon, particularly if the metal carbide is very stable. Both Mo and W surfaces show complex LEED sequences as the surface carbon coverage is increased (28),

METAL SINGLE CRYSTAL SURFACES

7

FIG.3. Comparison of experimental and theoretical I-Vplots for Ni(100)c(2 x 2)s (200).

indicative of the formation of surface carbides of differing stoichiometry (Fig. 2). The results of LEED studies related to the results described in the next section are given in Table I. More complete tabulations are published elsewhere (22).

B. AES Auger electron spectroscopy is the primary method employed for the determination of surface composition. Standard LEED optics can easily be used for AES, so they are normally employed. An electron gun producing electrons of approximately 2 kV is directed at the surface at an angle of about 15". These incident electrons cause ionization of metallic atoms, and the neutralization of these ions by electrons from higher lying orbitals by the Auger process (29) produces energetic secondary electrons which exit the solid into the vacuum. These electrons have an energy distribution characteristic of the elemental composition of the surface. The Auger spectrum is normally displayed as a derivative spectrum (3, 4 ) to enhance the peaks. Other types of energy analyzers are also employed (30). An Auger spectrum for Fe(100) with C, S, and 0 impurities is shown in Fig. 4 (30u) as an example. Each peak is clearly labeled on the figure. One of the primary advantages of AES is that it is a surface-sensitive technique. Auger electrons with energies between 50-100 V have mean free

8

ROBERT J. MADIX

TABLE I Selected Overlayer Structures for Adsorbates on Metal Surfaces Ni(ll0)

Clean

(1 x 1) (2 x 1)O

0

(3

x

1)O

( 5 x 1)O

NiO(100) c(2 x 2)s (2 x 1)C (4 x 5)C

S C Cu( 100)

Clean

(1 (1 (2 P(2 c(2 P(2 (2

0

S Cu(ll0)

Clean

x

210

x 2)O x 2)s x 1)s

(1 x 1) (2 x 1)O

0

A d 1 10)

x 1) x 1)O x 1)O

Clean

(1 x (2 x (3 x (4 x

0

1)

1)O 1)O 1)O

(5 x 1)O (6 x 1)0 (7 x 1)O

Pt(100)

W(100)

Clean 0 S Clean C

(2J2

( 5 x 20) x 2<2)R45"-0

P(2 x 2)s c(2 x 2)s (1 x 1) c(2 x 2)

(6 x l)C (5 x 1)C

paths in the metal between one and two bulk lattice constants. The Auger electrons thus emerge primarily from the top two layers of atoms, though the contribution of deeper layers cannot be totally ignored. For adsorption systems in which other evidence indicates that penetration of the adsorbate into the lattice does not occur, the relative peak heights of the adsorbate

9

METAL SINGLE CRYSTAL SURFACES

AES OF CONTAMHATED

Fd1001 SURFACE

Fo

C

1

200

400 ELECTRON ENERGY

600

800

(rv)

AES of chon FdIOO)

1

0

1

200

1

400 ELECTRON

600 ENERGY

gob

(rV)

FIG.4. Auger electron spectra of clean and contaminated Fe(100) surfaces. The presence of C, 0, S,and N atoms is clearly visible ( 3 0 ~ ) .

10

ROBERT J. MADIX

and a chosen metal peak can serve as an accurate relative measure of adsorbate concentration. To obtain an absolute coverage from AES a calibration must be made. This is most conveniently done using LEED structures, particularly if there is more than one LEED pattern with changing coverage. Thus, for example, the ratio of Auger peak heights for sulfur in the p(2 x 2) and c(2 x 2) structures on Ni(100) should be 1 :2 (when properly corrected to standard operating conditions by referencing the sulfur peak to a nickel peak). Recent measurements with x-ray photoelectron spectroscopy lend strong support to the use of such structural fiducial points (31). The state of bonding of carbon on the surface can easily be detected by AES. The carbon Auger peak shows a different fine structure for carbidic and graphitic carbon, as shown in Fig. 5a and b, respectively (32). Carbidic carbon characteristically shows three sharp peaks near 270 V ; the graphitic form shows a more rounded spectrum. It is therefore possible to differentiate the effects of carbidic and graphitic carbon on surface reactivity.

While LEED and AES provide information about the physical state of the surface with and without adsorbates, photoelectron spectroscopy can proENERGY (eV) 240 260 280

ENERGY (eV) 220

300

320

-

I

V

z

Y

-

r

-6 -8

€

I

-

-

30 -

-

20 10 0 - , 220

ENERGY (eV

I

, , , , , , 240 260 280 ENERGY (eV)

I

, -

300

FIG.5. Auger fine structure for (a) carbidic and (b) graphitic carbon on Ni(ll0) (32).

320

11

METAL SINGLE CRYSTAL SURFACES

vide chemical knowledge. Photoelectrons are ejected from electronic states of both the surface and the adsorbate when photons strike the surface. With ultraviolet radiation valence electrons are probed (UPS), whereas X radiation causes ionization of core electrons (XPS). Both methods are sensitive to the bonding of the adsorbate (33). Physically, the absorption of the photons causes the electrons to be ejected into the vacuum with kinetic energies characteristic of the difference between the initial and final state of the atom and the resulting positive ion. Typically, UPS spectra for adsorbates are compared to the gas phase spectrum of the same molecule (34, 35). Orbital assignments are made on the basis of this comparison. Surface bonding orbitals are often inferred from the relative shift of peaks in the spectrum (36).As a definitive tool for understanding surface bonding, UPS must be used with extreme caution. On the other hand, intermediates, once identified by other techniques, appear to have characteristic spectra which do not change appreciably from surface to surface (37).Furthermore, the formation of new surface species from adsorbed molecules is usually quite evident through changes in the UPS spectra. An example of such be1

1

1

1

I I I I

I

I

I

I

~

I

I

I

-

-w

W

za

Y

Z

J 0

I 5

10

15 BINDING ENERGY (eV)

FIG.6. He II(40.8 eV) ultraviolet photoelectron spectrum of CH,OH adsorbed on Cu(l10). (Ia) Clean Cu(ll0); (Ib) Cu(ll0) with CH,OH adsorbed at 140 K; (Ic) Cu(ll0) with CH,OH adsorbed at 140 K and heated to 300 K. Part I1 shows the difference spectra and the spectrum for gaseous methanol (38). Reprinted with permission of North-Holland Publishing Company, Amsterdam (in press).

12

ROBERT J . MADIX

havior is given in Fig. 6 . This figure shows the UPS for methanol adsorbed on Cu(ll0). The spectrum at 140 K is typical of gas phase CH,OH, modified by a shift in the orbitals due to bonding via the lone pair electrons on the oxygen (38-40). Heating the adsorbate complex to 270 K produces a dramatic change in the spectrum; the number of peaks collapses from five to three. The significance of this change will be discussed later. The main point to be made here is that such spectra can reveal substantial changes in surface bonding and the temperature range in which they occur, even though a detailed, orbital-bonding understanding of the peak positions may be difficult. Both UPS and XPS may be used to detect the conditions under which complete dissociation of molecules takes place. The UPS spectra for CO adsorbed on Fe(100) for various thermal treatments are shown in Fig. 7 (41).

I"'

I

" '

16

'

I

'

CO I on ' IFe(100) '

14 I2 10 8 6 4 2 E. ELECTRON BINDING ENERGY W )

OaEp

FIG.7. UPS spectra for CO chemisorbed on Fe(100) as a function of temperature. (a) Clean surface; (b) CO saturation at 123 K ; (c) (b) - (a); heated to (d) 300 K ; (e) 373 K ; and (f) 773 K (41). Reprinted with permission from Solid State Communications 23, 275 (1977). Copyright (1977), Pergamon Press, Ltd.

13

METAL SINGLE CRYSTAL SURFACES

1

1

1

f

1

1

1

1

1

1

1

1

1

1

FIG.8. XPS C(ls) core level spectra for CO adsorbed on polycrystalline iron and Fe(100). (a) Clean surface; (b) saturation CO coverage at 20°C; (c) warmed to 100°C. Lines I and I1 indicate the C(1s) positions for atomic carbon and carbon in molecular CO, respectively (43).

TABLE I1 C(1s) Core Level Shifts for Surface Bonded Iniermediaies Intermediate

-q P

A Surface carbide

0.464

B -CH3

0.236

AE (from carbide)

1.2 k 0.3

k 0.1

E -O--CH,

-0.103

3.1

F 4-CH,-CH,

-0.143

3.2 f 0.2

G

-0.182

3.5

k 0.3

-0.364

4.5

k 0.3

B p CH

-0.624

6.0

+ 0.2

BC-CH3

-0.663

6.5

+_

t

-O-CH(CH3),

t

H --O=CH2

-0 J

-0 -0

K

.y -0

0.3

14

ROBERT J. MADIX

In the undissociated state the 40, In, and 50 molecular orbitals are observed, the 1n and 50 orbitals being degenerate. Near room temperature these orbitals disappear, and the spectrum shows peaks characteristic of atomic oxygen and carbon (42). Similar results are obtained by XPS (43), since the C(1s) and O(1s) ionization energies are very sensitive to the electronic environment of the atom. As shown in Fig. 8, the C(1s) peak for molecular CO is easily distinguished from that for atomic carbon. A careful examination of both the C(1s) and O(1s) lines shows evidence for more than one bonding state of molecular CO. Similar results have been reported for the different binding states of CO on tungsten (44). The correlation of electron binding energy with Pauling charge on the atom observed with gas phase molecules (33) is also observed for some adsorbed species (45). Table I1 lists the C(1s) peak energies relative to carbidic carbon for a number of intermediates identified on Fe(100) (46,47). Figure 9 shows this correlation to be roughly linear. The large shifts occur due to the increasing number of oxygen atoms bound to the carbon. Clearly large chemical differences typically seen by XPS in the gas phase are not obscured by adsorption, though smaller shifts in core level positions may be caused by more subtle physical effects, and care must be taken in interpreting 1-2 eV core level shifts (44,48). BINDING ENERGY CORRELATION

WITH PAULHG CHARGE

FIG.9. Electron binding energy correlation for the XPS C(1s) core level with Pauling charge for adsorbed species (45).

METAL SINGLE CRYSTAL SURFACES

D. FDS

OR

15

TPD

Temperature programmed desorption directly measures the kinetics of desorption of simple molecules. In addition, the existence of different binding states may be inferred from structure in the desorption spectrum. The technique was first applied to adsorption of gases on polycrystalline filaments (49),and its use in studies on single crystals has been extensive (10, 50-52). The surface to be studied is cleaned and then exposed to a prescribed dose of the desired gas at a temperature preselected to be well below the desorption temperature of the adsorbed species. After the dosing gas is pumped away, heating of the sample produces a partial pressure burst as the gas desorbs. This burst is easily detected with a mass spectrometer. If the characteristic pumping speed of the system is high compared to the desorption rate, the desorption pulse yields directly the rate of desorption as a function of temperature. A series of TPD curves is shown in Fig. 10a and b for increasing initial coverages of CO desorbing from Ni(ll0) (53). Provided all curves are taken with identical heating rates, the relative area under the curves yields the relative amount desorbed. Careful calibration of the system pumping speed and mass spectrometer sensitivity allows absolute coverages to be calculated. There are several rules of thumb which can be employed in interpreting such spectra. First, provided proper experimental precautions are taken (54), the symmetry of the peak suggests the reaction order. This is true particularly for clean surfaces; surface adlayers may skew desorption curves appreciably (55). For first-order desorption the curve is asymmetric ; the ratio of area of the curve to the right of the desorption peak to that of the total area is equal to l/e. A second-order curve is nearly symmetric about the peak maximum. Furthermore, a first-order process exhibits desorption peaks that simply grow in magnitude with increasing dosage of reactant; the peak position remains constant. Peaks for a second-order process shift to the left with increasing coverage, as shown for H, desorption from Cu(ll0) in Fig. 1Oc (56). Note the different peak behavior for CO and H, with increasing coverage due to the different kinetic orders. Kinetic parameters can be extracted from TPD curves by a variety of methods. A rough estimate of the activation energy in kcal/mol can be obtained by dividing the peak temperature by 16. Reaction orders can be determined by plotting data for a single curve (57), or by cross-plotting data obtained by varying the initial coverage (58, 59). Preexponential factors v and activation energies E can be extracted by the same methods or by varying the heating rate at a constant initial coverage (53). The latter method does not require determination of the reaction order to evaluate v and E.

16

14 12 110

a

z % F j N 00

$ 6

a

5 4 2

250

275

300

350 375 400 425 TEMPERATURE ( O K 1

325

450

47:

_I

a z 9 r n

N

1

300 350 TEMPERATURE

400

(K1

FIG. 10. TPD spectra of (a) and (b) CO/CO (300 K) from Ni(ll0) (53); (c) HJH, from Cu(l10) (56). Reprinted with permission of North-Holland Publishing Company, Amsterdam, 1979

17

METAL SINGLE CRYSTAL SURFACES

Temperature programmed desorption studies provide information vital to the study of surface reactivity. The desorption kinetics of any species, A, following the adsorption of A at temperature T is designated as A/A(T). Since multiple adsorption states are often observed, the notation A(ol)/A(T) is employed to designate which desorption state, designated by a Greek letter, is referred to. The COjCO(l50 K) spectrum from Fe(100) is shown in Fig. 11. The CO(a)/CO(lSO K) peaks are due to the desorption of molecularly bound CO, whereas the CO(p) peak is due to the recombination of carbon and oxygen atoms resulting from the dissociation of CO on the surface (43, 60). Spectra of this type are typical of COjCO from transition metal surfaces (61);molecular CO desorbs near 425 K, and recombination of adsorbed carbon and oxygen atoms occurs near 800 K. The existence of multiple peaks for molecular desorption has been attributed to lateral interactions among adsorbed species (62-64). As discussed previously, adsorption onto the surface lattice may occur preferentially in next nearest neighbor sites to form p(2 x 2) structures. Even at low coverages, attractioe forces may cause adatoms to occupy next nearest neighbor positions, so that clusters of adsorbate form which have local twofold periodicity (65) with respect to the surface. Such effects are entirely consistent with the perturbations of the surface electronic wave functions due to adsorption (66-68) which show that these binding sites represent the

I

CO/C0(150 K) COVERAGE VARIATION

\ n\

I

I

I

200

1

I

6

I

400

TEMPERATWE

I

600

I

I

1

800

(K)

FIG.1 1 . The temperature programmed desorption spectra for CO from Fe(100) following adsorption at 180 K. Curves (a)-(e) are arranged in order of increasing initial coverage by

co (45).

18

ROBERT J. MADIX

minimum of energy for the adsorbate-surface system. As these sites become filled, the energetically less favorable nearest neighbor sites become occupied, and the apparent binding energy of the species decreases. This dependence of the binding energy on adsorbate coverage produces a multiplepeaked desorption spectra. Calculations of desorption spectra based on lattice statistics (69) qualitatively fit the results obtained (62). These calculations are based on the assumption that the adsorption site does not change as the adsorbate coverage increases. This assumption, though necessary to a solution of the model, does not appear to be accurate for many adsorption systems. Adsorbed species may change binding sites as the adsorbate coverage changes. Direct evidence for bonding changes have been obtained with vibrational spectroscopy of adsorbed species (70- 72). Thus, for example, CO adsorbs at low coverages in both bridged and linear sites on Ni(100) at 295 K, whereas the linear form predominates at 0.5 ML (monolayer) coverage (73). It is evident that the surface must be viewed as a lattice with distinct binding sites and that significant interactions among adsorbed species occur. The question of whether different binding sites are simultaneously occupied or whether site binding changes uniformly with surface coverage may depend upon the specific metal-adsorbate system. E. TPRS Temperature programmed reaction spectroscopy makes use of the principles of TPD for the study of the kinetics and mechanism of more complex reaction systems. An apparatus typical of such studies is shown in Fig. 12. Surface definition is accomplished with LEED and AES, as discussed earlier. Reactant gases are normally dosed to the surface from a gas manifold through a stainless steel needle. Condensable species are maintained at fixed pressures above constant temperature baths for controlled dosing. Typically a desired amount of reactant is dosed onto the surface at a predetermined temperature, and the surface is heated linearly. Reaction products then leave the surface at temperatures roughly characteristic of their activation energies, and they are detected mass spectrometrically by a quadrupole mass spectrometer. The products do not undergo a significant number of secondary collisions with the surface due to the high pumping speed of the system. The rate-temperature or rate-time data for the formation of each product can be monitored individually by locking the mass spectrometer onto a given mass during the flash. Sequential flashes yield the total product spectrum as a function of temperature. Such a spectrum is shown in Fig. 13 for the products of CH,OD oxidation on Cu(ll0) with preadsorbed "0 (74). There are four major steps in the interpretation of TPRS. First, the peak

19

METAL SINGLE CRYSTAL SURFACES

FIG.12. Schematic of the apparatus used for TPRS, LEED, AES, UPS, and XPS (30a). #

200

I

,

250

,

,

300

I

,

350

l

400

1

1

450

l

1

500

TEMPERATURE ( K )

FIG.13. TPRS product spectrum for CH,OD adsorbed on Cu(l10) following a predose with 'SO*.

temperature of each product peak is indicative of its rate of formation at the temperature of the peak. Second, it then follows that if a peak temperature for a given product lies above the characteristic desorption temperature for that species (as determined by TPD), a process different from desorption must be rate limiting for its formation. Such observations are clear evidence for surface reaction-limited processes. Third, species with

20

ROBERT J. MADIX

the same peak temperature originate from the same rate-limiting step. Peak temperatures can be resolved to within k2 K, which gives an energetic resolution of approximately lo-, kcal/mol. To be precise then, products which exhibit the same peak temperature to within experimental error show the same activation energy to within lo-, kcal/mol. This is strong evidence that these products are formed simultaneously in a single rate-limiting step. Reference to Fig. 13 shows that there are three groups of products. Each group corresponds to a different surface reaction, as discussed later. The correspondence of the peak positions in each group indicates three ratelimiting molecular rearrangements are observed. As a more stringent test of rule three above, the rates of two products may be normalized and plotted together as a function of temperature. Coincidence of these spectra through the entire peak clearly indicates a common rate-limiting step. The evolution of H, and CO, from HCOOH decomposition on Cu(ll0) is shown in Fig. 14 as an example (75). Fourth, a quantitative determination of the relative amounts of products desorbed at a common peak temperature provides direct information on the atomic composition of the rate-determining active intermediate. For the reaction observed in Fig. 14, H, and CO, were evolved in 1 :2 ratio (75). It was easily inferred from this that the rate-determining intermediate was HCOO(a). In combination with the quantitative methods of TPD discussed previously, studies by TPRS provide a direct identification of the atomic composition of reaction intermediates and their rates of reaction. In most cases this intermediate can be identified by analogy

373

I

398

423

1

I

TEMPERATURE (K) 4 4 8 473 498

523

548

I

I

1

I

I

,

I

1

1

z w

0.6 0

w 9 0.4

~

a

I

8 0.2 z 0-

I

I

100

125

I

150

175 200 225 250 TEMPERATURE ("C)

I

275

FIG.14. TPRS spectra for H, and CO, from HCOOH on Cu(l10). The exact superposition of the product peaks indicates a common rate-determining step for their formation.

METAL SINGLE CRYSTAL SURFACES

21

to known organometallic complexes or stable compounds. With this method the effects of surface composition and structure on reaction kinetics and mechanism can then be examined.

111. The Reactions of Carboxylic Acids and Related Reactions

A. HISTORICAL The decomposition of formic acid is one of the most extensively studied catalytic reactions. Several excellent review articles have been written on the subject (76-78). From a large number of studies on supported metal catalysts the following, and sometimes contradictory, observations were made : 1. IR data show evidence for both surface formates and anhydrides following the adsorption of HCOOH on metal surfaces (79-82). 2. The decomposition of the formate is the rate-limiting step on supported nickel catalysts (79). 3. Conversely, the presence of surface formate species retards the decomposition reaction on supported nickel catalysts (82). 4. Different activation energies were observed for the decomposition on single crystals of both Cu and Ag with different crystallographic orientation (83-87). 5. A strongly coupled surface complex between a formate ion and adsorbed formic acid is the decomposition intermediate on silver (88). 6. Heating a nickel wire to elevated temperatures in vacuum increases the catalytic activity of the wire for the decomposition by several orders of magnitude (89). Thus, though the participation of a formate ion in the decomposition is generally agreed upon, the mechanistic details are generally not understood. In the discussion to follow some of these points will be addressed.

B. RESULTS FOR FORMIC ACIDDECOMPOSITION ON CLEAN METALS 1. Copper

The decomposition of formic acid was studied on clean Cu(ll0) by TPRS (75).Formic acid adsorbed at either 190 K or 300 K with an initial sticking probability of unity. The adsorption roughly followed Langmuirian behavior. Multilayers were not adsorbed down to 190 K; condensed HCOOH was reported to desorb from copper films below 190 K (90). Chemisorbed

22

ROBERT J. MADIX

-100

I

-50 I

0 I

TE M PERAT U RE ("C 1 50 100 150 I

I

I

200

250

I

I

TEMPERATURE (K)

FIG.15. TPRS spectra from HCOO on Cu(l10). The two sets of peaks at 273 and 473 K show two different reaction events (75).

formic acid desorbed intact at 175 K ; its binding energy was approximately 10 kcal/mol. Upon heating, carbon dioxide and hydrogen were the sole products observed, as shown in Fig. 15. The low temperature peak originated from the acidic hydrogen, and the coincident H, and CO, peaks at 475 K (see Fig. 14) originated from the adsorbed formate (75). Adsorption at room temperature led to dissociative adsorption with immediate desorption of H, and formation of the formate ion; the decomposition kinetics of the HCOO(a) was unchanged. Since this copper surface desorbed H, well below 475 K (92) and did not adsorb CO, at all (at least above cryogenic temperatures), the simultaneous evolution of CO, and H, at 475 K was clear evidence of a reaction-limited process. The decomposition of the surface formate was determined to be first order from (a)a plot of log R vs. log OHCOO at constant temperature; and (b) the invariance of the peak temperature with initial coverage. The activation energy and preexponential factor were 3 1.9 kcal/mol and 9.4 x 10' sec- respectively. Surface formate was also produced by reacting preadsorbed oxygen with formaldehyde (92) according to

',

The observed reaction product spectrum showed that exchange of oxygen

METAL SINGLE CRYSTAL SURFACES

23

isotopes occurred between H 2 C 0 and adsorbed oxygen. This is strong evidence for the formation of the H\ /H O/c\*

intermediate. Close examination of the H2 peak in step (2) showed it to be desorption limited. The UPS spectra for the adsorbed formate are shown in Fig. 16 (92). These spectra were very similar to those previously observed on copper films (90).The spectrum obtained from coadsorbing H,CO and 0 at 140 K and heating to 400 K was identical to that of the formate, as expected from the TPRS results. Electron loss vibrational spectra (ELS) (70) for HCOOH adsorbed on Cu( 100) at 350 K also show the vibrational frequencies expected for HCOO(a) (93). The strong signal obtained by ELS indicated that the HCOO(a) was adsorbed nearly perpendicular to the surface. The same conclusion was reached from JR measurements (94). XPS results have been used to show that both oxygens are equivalently bonded on the surface (90, 92), and work function changes suggest that the oxygens are bound to the surface (92).

DCOOH/Cu (110)

’I FIG.16. UPS spectra for DCOOH adsorbed on Cu(l10) (92).(a) Clean surface; (b) DCOOH adsorbed at 140 K ; (c) heated to 400 K to form the formate. Reprinted with permission of North-Holland Publishing Company, Amsterdam (in press).

24

ROBERT J. MADIX

2. Iron

Formic acid adsorbed with near unit sticking probability on clean Fe(100). The reaction product spectrum for HCOOH on Fe(100) is shown in Fig. 17 (95).As with Cu( 110) the reaction proceeded via two steps, one which evolved H, at 350 K and the other which formed H,, CO,, and CO at 490 K. A small amount of CO was evolved at 800 K due to the reaction of residual carbon and oxygen atoms on the surface. From these results it was concluded that CO, H,, and CO, were formed by a common rate-limiting step at 490 K. Since the H,/H, TPD peak appears at 400 K and below, this step was determined to be a surface reaction. No water was formed. Evidently the reaction proceeded by the pathways HCOO(a) + H(a)

+ CO,(g)

HCOO(a) + H(a)

+ CO(g) + O(a)

and The difference in enthalpy for these reaction pathways is equal to the difference between the heat of formation of CO, and the heats of formation of CO and FeO, which is less than 5 kcal/mol. These two steps would therefore be expected to compete, as observed. The XPS results tabulated in Table I11 clearly illustrate the presence of the formate and its decomposition (95). Prior to adsorption a small amount of residual carbon was present on the surface in carbidic form shown by the C(1s) peak at 282.3 eV binding energy. Allowing for this residual carbon, the XPS-determined coverages clearly show the expected 2: 1 OjC ratio. The O(1s) peak position observed following heating to 360 K, which was F o r k A c h Decdrnpositibn on 6e1100j

I

I

200

I

I

400

I

I

600 TEMPERATURE ( K )

I

I

I

800

FIG. 17. TPRS spectra for the products evolved from Fe(100) subsequent to HCOOH adsorption at 180 K (95).

25

METAL SINGLE CRYSTAL SURFACES

TABLE I11 C(Is)and O(ls) X P S Energies for HCOOH Adsorpiion and Reaction on Fe(100) Binding energies (eV) C(1S)

O(1s)

(a) HCOOH adsorbed at 160 K

282.3 288.3

(b) Heated to 360 K (c) Heated to 500 K (d) Heated to 900 K (e) Before adsorption

Coverages (monolayers) ec

00

531.2 532.5

0.36

0.62

282.3 288.3

531.7

0.35

0.60

282.3

530.0

0.12

0.20

__

530.0

-

0.07

282.3

-

0.06

-

sufficient to desorb the acidic hydrogen, was 531.7 eV-identical to the O(1s) position for the formate on Cu(l10) (92). The high C(1s) binding energy at 288.3 eV clearly revealed the preservation of the 0 - C 4 bonds in the intermediate at 160 and 360 K. Heating to 500 K decomposed the formate (Fig. 17) and produced adsorbed atomic oxygen with a O(1s) binding energy of 530.0 eV. The ratio of CO to CO, produced was about 1:1. The rate constant for the first-order decomposition of the formate was 7 x 1013sec-2exp{-31.1 kcal/mol/RT}. 3. Tungsten Tungsten reacted strongly with formic acid. The molecule was dissociatively adsorbed at 300 K with a sticking probability near one on W(100). For exposure less than 2 x lOI4 molecules/cm2 HCOOH was completely decomposed to adsorb carbon, oxygen, and hydrogen atoms, as evidenced by the AES spectra shown in Fig. 18. The only gaseous product evolved was H,. The surface remained covered by carbon and oxygen in a 1 :2 ratio. Further heating to 1500 K desorbed CO and left the excess oxygen on the surface. Heating to 2500 K desorbed the oxygen as tungsten oxide (96). The clean tungsten surface was the most reactive surface observed; it totally decomposed formic acid.

4. Nickel The decomposition of formic acid on nickel single crystals showed unusual features not observed on Cu(llO), Fe(100), Ag(l10), or W(100) surfaces. Adsorption of isotopically labeled formic acid, HCCOD, or Ni(ll0)

26

ROBERT J. MADlX

AES for HCOOH Decomposition on W(100)

100

2 00

300

V

E L E C T R O N ENERGY

400

500

1

(eV)

FIG. 18. AES for the HCOOH decomposition on W(100) (96).(a) Clean surface; (b) clean surface exposed to HCOOH at 300 K and heated to 800 K; (c) heated further to 1500 K to desorb carbon and oxygen atoms. The carbon and oxygen peaks in (b) indicate dissociation of the HCOOH.

at 220 K and subsequent heating showed that, predominantly, the acid hydrogen formed water by an intermolecular condensation reaction 2HCOOD +D,O + [intermediates] (97).The stoichiometry of the remaining intermediates was H2C20,. Obviously the adsorbed species were some combination of HCO and HCOO. The fact that only a single H, TPRS peak was observed upon heating indicated the presence of a single intermediate. This intermediate was concluded to be formic acid anhydride. Similar results were obtained on the Ni(100) surface (98). The decomposition of this intermediate on both the Ni(ll0) and Ni(100) surfaces occurred by an autocatalytic mechanism (99) for adsorbate coverages above about one-tenth of a monolayer. In fact, the decomposition rate was observed to accelerate isothermally as the reaction proceeded on both the Ni(ll0) and Ni(100) surfaces (98,99);the rate of acceleration was more pronounced on the (1 10) surfaces. Furthermore, the intermediates were observed to form islands, as if a two-dimensional phase condensation occurred at about one-tenth monolayer coverage. The formation of this 2D condensed phase was clear indication of attractive interactions among the adsorbed species. These attractive interactions were responsible for the autocatalytic be-

METAL SINGLE CRYSTAL SURFACES

27

havior. From absolute rate theory the decomposition rate can be expressed as

h

A,,

exp(-E,/RT)N,

If the Bragg-Williams approximation (69)is employed to describe the attractive interactions in the adsorbed layer, and the transition state is taken to be free of such interactions, the rate can be described by R = v exp[ -(EA

+ oO,,,)/RT]N,

where v is the lumped preexponential factor, Oloc is the local fraction of sites occupied by reactive intermediates, and o is the local attractive interaction potential; Oloc varies from one to zero during heating. Effectively, the activation energy decreases as Oloc decreases, and the rate increases. The rate conwOloc) kcal/mol/RT] sec-'; for stant was k = 6 x lOI5 exp[-(25.5 Ni(llO), o = 2.7 kcal/mol; for Ni(100), o = 1.4 kcal/mol. The origin of the attractive interaction and its difference on Ni(ll0) and Ni(l00) is somewhat uncertain, but it is reasonable to speculate that it arises from dipole-dipole attractions in the adlayer. Quantitative TPRS (99) and results on (110) oriented Cu/Ni alloy surfaces (100) showed that the anhydride required four nickel atoms for stabilization on the (1 10)

+

FORMIC ANHYDRIDE ISLAND STRUCTURE ON N i ( l l 0 )

NICKEL ATOM

t FORMIC ANHYDRIDE (a)

FORMIC ANHYDRIDE ISLAND STRUCTURE ON Ni(100)

NICKEL ATOM FORMIC ANHYDRIDE (b)

FIG. 19. Schematic drawing of the arrangement of surface dipoles for formic anhydride adsorbed on Ni(l10) and Ni(100) (98). Reprinted with permission of North-Holland Publishing Company, Amsterdam, 1979.

28

ROBERT J. MADIX

surface. Due to the structural anisotropy of the (1 10) surface, head-to-tail alignment of dipoles is preferred, whereas on the (100) surface a more random arrangement is possible. This effect reduces the total strength of the interaction on Ni(100) relative to Ni(1 lo), yielding a reduction in the net value of o in the rate expression. These structural features are shown schematically in Fig. 19. 5 . Ruthenium

The decomposition behavior of formic acid on the close-packed Ru( 1070) surface parallels the reaction on nickel, except that the autocatalytic process was not observed (101). Water was desorbed at 183 K by apparent secondorder kinetics following adsorption of HCOOH at 100 K. Subsequent desorption of H,, CO,, and CO suggested the formation of the surface anhydride. The rate constant for decomposition was 2.6 x 1015 sec-' exp( - 26.9 kcal/mol/RT}. 6. Silver Formic acid did not adsorb on clean A g ( l 1 0 ) above 180 K. In order to obtain the adsorbed species, the surface was predosed with oxygen to produce adsorbed oxygen atoms. Surface species could then be stabilized by reacting formic acid with surface oxygen to produce water and the formate (102). Subsequent heating of the surface produced decomposition near 425 K. Only CO, and H, were observed as products from the HCOO intermediate. Additionally, some back reaction to reform HCOOH occurred between HCOO(a) and the H(a) liberated by the decomposition. The rate constant for the decomposition was k

=

8 x 1015sec-l exp{-30.0 kcal/mol/RT).

The results of these studies are tabulated in Table IV. This table lists the surfaces studied, the intermediates observed, the products, the adsorption the initial sticking probability of HCOOH (So),the peak temperature temperature for product evolution (Tp), and the activation energy (Eact) and the preexponential factor (v) determined by methods discussed earlier (see Section 1,D). Data for HCOOD is given where available in order to distinguish the two hydrogens in the acid.

(zds),

c. DISCUSSION OF FORMIC ACIDDECOMPOSITION The relative decomposition temperatures for the formate, Tp, on the clean metals (M) can be related to the surface metal-oxygen and metal-hydrogen bond strengths listed in Table V. With the exception of silver, T, decreased

TABLE IV Kinetic Parameters for the Decomposition of HCOOD or HCOOH on Single Crystal Surfaces T.ds

Surface

Intermediate

Product (K)

So ~

Cu(ll0) (HCOOD)

HCOO

Fe( 100) (HCOOH)

HCOO

Ni(ll0) (HCOOD)

Anhydride

Ni(100) (HCOOD)

Anhydride

Ru( 1010) (HCOOH)

Anhydride

A d 1 10) (HCOOD)

HCOO

Ni(110)(2 x l)C (DCOOH)

HCOO

Ni(100)p(2 x 2)C (HCOOH)

HCOO

W(100)(5 x l)C (HCOOH)

HCOO

Ni(110)(2 x 1 ) 0 Anhydride (HCOOD)

Pt(ll0) (DCOOH)

Undetermined

175

200

180

220

100

300 300

225

200

Eact

V

(kcal/mol)

(sec-I)

~~

-1

-1

-1

-1

-

300 <10-4 200

T, (K)

-1

-1 -1

-1

-1

~

475

31.9 31.9

-

9 x 1013 9 x 1013

350 490 490 490 810

31.1 31.1 31.1 -

325 350 350 445

26.6 26.6 32.0

1015 1015

200 370 370 440

25.5 25.5 27.8

6 x lOI5 6 x lOI5 6 x IOl5

180-225 400 400 500

26.9 25.8 35.2

2 x 1015 5 x 1014 5 x 1015

425 42 5

30.0 30.0

8 x lOI5 8 x lOI5

300 450 450

25.5 25.5

440 440

28.4

350 540 540 540 540 250 423 423 433 260 -

260

-

7 x 1013 7 x 1013 7 x 1013

1015

1012.5 1012.5

-

-

-

36.5

2 x 1014

-

-

~

21 21 -

-

1012 10'2

30

ROBERT J. MADIX

TABLE V Metal-Oxygen and Metal- Hydrogen Surface Bond Strengths Compared to TPRS Peak Temperature for Formic Acid Decomposition”

Metal

M-0 (kcal/mol) ~

A d 1 10) Pt(l1 I) Cu(l10) Ni(ll0) Fe(100)

80.3 84 97 11s 124

M-H (kcal/ mol)

T&CO,/HCOOH) (K)

~

< 5s 57

57 63 62

420 265 475 350 (anhydride) 490

a Bond strengths were calculated from published values for heats of adsorption or heats of formation of the metal oxide.

with decreasing M-0 bond strength. Thus the stability of the formate toward decomposition was governed primarily by the M-0 bond strength. As shown in Table IV, the apparent relationship between oxygen bond strength and reactivity was not simply related to the activation energy. Silver, copper, and iron surfaces gave E,,, = 31 k 1 kcal/mol, yet q spanned 65 K. Additionally, weaker M-0 bonds in the formate must lead to weaker perturbations of the C-0 bonds in the adsorbed formate. For Cu, Fe, and Pt the binding energy for adsorbed hydrogen is about 57, 65, and 57-62 kcal/mol, respectively (64, and no trend of Tp with hydrogen bond strength is evident. It appears likely that the low-frequency wagging motion of the adsorbed formate which brings the hydrogen atom close enough to the surface to promote reaction is an important reaction coordinate. The amplitude of this vibration would be expected to increase as the M-0 bond weakens, producing a lower T p with reduced oxygen bond strength, provided the affinity of the surface metal atoms for hydrogen is approximately the same from surface to surface. In the case of silver, however, the M-H bond is sufficiently weak that the driving force for reaction is low, and the surface formate is more stable than expected from the M - 0 bond strength alone. The unexpected stability of the formate on Ag and the low probability for dissociative adsorption combine to produce a low overall activity for silver. This low activity has been well documented (76, 79), and the low adsorption probability of HCOOH on silver and extra stability of the formate are clearly the origin of the rapid decrease in activity of Ag and Au observed on the so-called “volcano” curve shown in Fig. 20 (102~). The low probability for dissociative adsorption of HCOOH on Ag is clear indication that adsorption was activated. For such a process, one would expect the dissociative sticking probability to depend on the structure of the surface, since alignment of the 0 - H bond with the metal surface

METAL SINGLE CRYSTAL SURFACES

iI

31

0 HIGH PRESSURE, .Pt(llOl

SUPPORTED METAL .LOW PRESSURE, SINGLE CRYSTALS

300

b

ITP

400

FIG.20. The “volcano plot” for HCOOH decomposition on high surface area catalysts ( 0 ) and single crystals ( 0)(1024. Reprinted with permission of North-Holland Publishing Company, Amsterdam, 1979.

atoms becomes more critical. The observed structural sensitivities for this reaction (103, 104) are thus not surprising. [It must be cautioned, however, that trace amounts of oxygen can increase the reaction rate dramatically (102).] This reaction system is one which shows a significant pressure effect. At several torr pressure formic acid decomposes on silver crystals with zero-order kinetics without significant formation of water, which eliminates the oxygen-activated route as a possibility. This very fact indicates that the formation of HCOO(a) from HCOOH(a) was not observed on clean silver at low pressures owing to the low probability of dissociative adsorption. It has also been observed that the presence of surface oxygen or formate can increase the heat of nondissociative adsorption of species containing oxygen, such as water and alcohols (105); this effect may be the origin of the bimolecular complex proposed for HCOOH decomposition on silver. The autocatalytic process observed on Ni(ll0) was very similar to results reported previously on nickel powder (82). In that work the rate of decomposition was measured as a function of the amount of formic acid adsorbed and the formic acid pressure. At 60°C and pressures from 0.4 to 1.4 Torr the rate of decomposition decreased as the formate coverage increased. Such behavior can be accounted for if the attractive interactions between the adsorbed species observed for the formic acid decomposition on Ni(ll0) or

32

ROBERT J. MADIX

Ni( 100) stabilized the adlayer and only isolated formate ions decomposed easily at this temperature. Thus, inhibition of the reaction by adsorbed formate ions is possible, if they form islands.

D. REACTIONS FOR FORMIC ACIDDECOMPOSITION ON METAL-ADLAYER SURFACES 1. Surface Carbides and Oxides

Surface adlayers altered surface reactivity both with respect to rate and selectivity. On Ni(l10) the (2 x l)C adlayer is believed to consist of C, fragments bridging the space between the close-packed rows (106), whereas on Ni( 100) the carbon atoms presumably occupy fourfold hollow positions in a square array of twice the metal-metal atoms spacing. On both of these surfaces (107, 108) the autocatalytic reaction was suppressed, though the total amount of formic acid that adsorbed and reacted was not significantly reduced. The clean surface formed equal amounts of CO, CO, , and H2; CO, and H, were the primary products on Ni(ll0) (2 x l)C, with little formation of H,O observed. The formate was the dominant intermediate. The small amount of CO and H,O formed reached a saturation value at low exposures to formic acid, indicating that only a few sites were active for their formation. These sites may have been due to the existence of a small number of uncarbonized nickel atoms (107). The rate of decomposition to form CO, was nearly two orders of magnitude slower on the carbide surfaces than on the clean surfaces. It should be noted that the acid was more easily decomposed on all the nickel surfaces, including the carbides, than on copper. For the carbide surfaces this was probably due to a weakened Ni-0 bond, since the Ni-H bond strength on Ni(ll0) (2 x l)C is roughly equal to that on copper (109). Similar results were found for the decomposition on surface carbides of tungsten. The W(100) surface reconstructs to form a W(100) (5 x l)C structure, as shown in Fig. 2. This structure has been interpreted to be a hexagonal layer of W,C on top of the (100) surface (110)with the carbon atoms in interstitial positions under the first layer of tungsten atoms (111). The presence of the carbon atoms strongly modified the reactivity of the surface. Rather than dissociating formic acid completely like W(loo), the carbide surface selectively formed the formate. The decomposition proceeded at 540 K with

k

= 2 x 1014 sec-1 exp(

- 36.5 kcal/mol RT

METAL SINGLE CRYSTAL SURFACES

33

to form CO, and H,. A slight amount of surface oxidation occurred. This oxygen was reduced to form H,O by the hydrogen atoms liberated in the decomposition. The adsorption of oxygen on the Ni(ll0) surface produced an effect on the decomposition that was strikingly different from carbon (112). The clean surface was exposed to oxygen in varying amounts up to exposures equivalent to 10 monolayers. For oxygen exposures up to 10 L the autocatalytic mechanism was preserved, and a second mechanism, apparently characteristic of the fully developed oxide, gradually appeared. As the surface oxygen coverage was increased, the autocatalytic peak shifted to higher temperatures, and its magnitude decreased. The autocatalytic mechanism was still dominant on the Ni( 110) (2 x 1)0 surface with half-monolayer oxygen coverage. Preservation of the autocatalytic mechanism with a change in the rate constant gave clear indication that ligand effects on the metallic behavior due to adsorbed oxygen exist. 2. Copper-Nickel Alloys The large differences observed in the reaction of formic acid on Ni(ll0) and Cu( 110) afforded a unique opportunity for the study of ligand and ensemble effects on the decomposition using Cu/Ni(l 10) single crystals. With AES the surface composition of the alloy can be determined with reasonable accuracy. It has been shown that copper atoms segregate preferentially to the (1 10) surface (113-116). This segregation provides a convenient way to vary the surface composition. Following argon ion bombardment of the surface, which creates a pure nickel surface, annealing to various temperatures produced a range of surface copper concentrations between zero and 63% copper (114). The surface composition was unchanged by formic acid adsorption and decomposition, as monitored by AES before and after the TPRS experiment. On the alloy surface the reaction proceeded both via the anhydride and formate intermediates (117).As the copper concentration was increased, the formate species dominated the reaction, until at 63% copper the CO/CO, ratio was less than 0.1. This change was due to the decrease in the amount of anhydride formed with increasing copper and the corresponding increase in formate. Since only the anhydride decomposition produced CO, the relative amount of anhydride formed could be determined as a function of surface composition. This relationship is shown in Fig. 21 ; the anhydride concentration fell as the fourth power of the nickel concentration, suggesting the requirement of four nickel atoms for its stabilization. This value agreed with the earlier determination for the saturation density of anhydride intermediates on Ni(ll0) (99).

34

ROBERT J. MADIX

"c'4

3.5 30

CO/HCOOH

25 2.0 0,

I

1.5 1005 0-

-10

-08

-06

-04

-02

0

AIen.X,,

FIG.21. Plot showing the falloff in CO produced from formic anhydride on a Cu/Ni alloy with changing surface composition (100). Reprinted with permission from Journal of Inorganic Chemistry 17, 1978. Copyright 1978, American Chemical Society.

The reaction system showed only weak ligand effects. The decomposition of the anhydride 0 0

II II

DCOCD(a) + 2D(a)

+ (1

- x)CO(a)

+ xCO(g) + CO,(g)

produced both a desorption-limited CO peak near 420 K and a reactionlimited peak near 350 K. These peaks were well separated, and the effect of surface composition on the rates could be studied by observing the peak shift. The results are shown in Fig. 22. The reaction-limited peak shifted to higher temperature with increasing copper concentration, while the desorption-limited peak shifted to lower temperatures. This result indicates that the activation energy for anhydride decomposition on the nickel clusters increased with increasing copper concentration, while the CO desorption energy decreased. Both of these effects correspond qualitatively to the admixture of the relative rate behavior for HCOOH decomposition and CO desorption from clean nickel and copper, respectively. It should be noted that the maximum changes in activation energy with surface composition for both decomposition and desorption were small (about 2 kcal/mol). It should also be noted that no reaction path characteristic of clean copper was observed. The decomposition of the formate occurred well below that

35

METAL SINGLE CRYSTAL SURFACES

298

323

348

I

I

I

TEMPERATURE ( K ) 3-73 398 423 448 I

I

473

498

I

5 TEMPERATURE ("C)

FIG.22. TPRS spectra for CO/HCOOH from Cu/Ni alloys of varying surface composition (100). (a) 37% Ni; (b) 46% Ni; (c) 54% Ni; (d) 61% Ni; (e) 68% Ni. The peak shifts indicate weak ligand effects (see text). Reprinted with permission from Journal of Inorganic Chemistry 17, 1978. Copyright 1978, American Chemical Society.

expected for clean Cu(ll0) even on the 65% copper surface. In this sense the surface more resembled a modified nickel surface than a copper surface. Apparently, adsorbed formate intermediates formed at high copper concentrations preferred Cu-Ni bridged sites to Cu-Cu clusters.

E. THEDECOMPOSITION OF ACETIC ACID The decomposition of acetic acid was studied by TRPS in order to determine the similarities of its surface reactivity to formic acid. Reactions on Fe(100) ( 9 9 , Ni(ll0) (118), and Cu/Ni(llO) (100) alloys were studied. On Ni(ll0) acetic acid adsorbed at 300 K with a sticking probability near unity to form the anhydride intermediate and release H,O. The decomposition of this intermediate proceeded by the two-dimensional autocatalytic process observed for formic acid to yield CO, , H,, CO, and surface carbon. The rate of reaction was well described by the same equation describing the decomposition of the formic anhydride. The rate constant at low coverages was

kanhy= 6.4 x 1014sec-' enp{

- 28.2 kcal/mol

RT

On the Cu/Ni(llO) alloy containing 35% surface nickel and 90% bulk nickel the acetate was the predominant surface intermediate observed. The

36

ROBERT J. MADIX

autocatalytic decomposition was completely suppressed. The rate constant for decomposition of the acetate at low coverages was

At higher surface coverages interactions among the adsorbed intermediates were evident, as the TRPS peaks shifted to higher temperatures, indicating attractive lateral interactions. At saturation coverage the activation energy for decomposition increased by 14 kcal/mol. In fact the kinetics became more complex, and the reaction could not be described exactly by simple first-order decomposition. For both Ni( 110) and Cu/Ni(llO) the activation energies were higher than those for the decomposition of formic acid, and the preexponential factors were lower. Qualitatively, the mechanistic behavior was similar to HCOOH. It is interesting to note that the ratio of preexponential factors for the decomposition of 0 0

II II

HCOCH,

0 0

II II

0 0

II II

DCOCD, and CH,COCCH,

produced from formic acid and acetic acid on Ni(1 lo), respectively, scaled closely according to the square root of the masses of H, D, and CH, . This result suggests that the normal mode of vibrations which represents the reaction coordinate involves extension of the R-C bond (118). IV. The Reactions of Alcohols A.

ADSORPTION

The adsorption of methanol on polycrystalline palladium (119), Ru(O0 1) (120),Ni(ll1) (121), and Cu(ll0) (122)surfaces has been studied extensively by UPS by several investigators. It was generally observed that at temperatures low enough to prevent dissociation the orbital energies of the nonbonding, lone pair valence electrons on the oxygen were shifted to higher electron binding energies (B.E.) relative to the 0- or 7c-bonding electrons. This shift was indicative of a bonding interaction of the alcohol with the surface via these electron pairs. On palladium only the highest lying of the two nonbonding orbitals shifted, whereas on the other surfaces both lone pair orbitals moved to higher B.E. This different behavior indicated a difference in the bonding mode for CH,OH on the polycrystalline palladium. The nondissociative adsorption of methanol also decreased the work function, indicating that the molecule adsorbed with the negative end of

37

METAL SINGLE CRYSTAL SURFACES

TABLE VI Work Function Changes Following Methanol Adsorption on Clean Metals (Monolayer Coverages) Metal

A 4 (eV)

T,"

Pd (polycrystalline) Ru(l10) Ni(ll1) Cu( 1 10)

-1.4(120K) - 1.7 (80 K) - 1.6 (80 K) - 1.9 (140 K)

300 175 160 300

a T, is the temperature at which the UPS spectra indicate that a new surface species formed from adsorbed CH,OH.

its dipole moment nearest the surface. The work function changes at saturation coverage are listed in Table VI. These results indicated that the molecule adsorbed with the oxygen down, in agreement with the UPS results. Furthermore, on Cu(ll0) the presence of a monolayer of CH,OH did not impede the dissociative adsorption of oxygen at 140 K, which indicated that the molecular methanol occupied sites different from those producing dissociation (122). Since oxygen adsorbs in the troughs on the (1 10) surface (124, the methanol probably sits on top of the ridges, i.e., on the outermost copper atoms on the surface. The binding energies of nondissociatively adsorbed alcohols and aldehydes, as well as H,O and HCOOH, are similar on all surfaces studied. The binding energies range from 12 to 16 kcal/mol, and the range observed due to multiple desorption peaks on a given surface is larger than the difference in binding energies for the different molecules (see Table VII). Evidently, the bond strength due to the surface-lone pair interaction is not significantly affected by the molecular geometry.

B. REACTIONON CLEANSURFACES Heating the surface with nondissociatively adsorbed methanol produced different reaction behavior on nickel and copper versus palladium and ruthenium, respectively. As discussed later, surface methoxy species can be produced by reacting methanol with preadsorbed surface oxygen on Cu(ll0) or Ag(l10) (74, 124). The UPS spectra for this surface methoxide differed significantly from that of methanol, allowing ready identification of CH,O(a). The UPS spectra observed following heating of adsorbed methanol on clean Ni(ll1) or Cu(ll0) indicated formation of the surface methoxide. On palladium and ruthenium, hydrogen and CO formed, and the methoxide was not observed. Heating of CH,O(a) on Ni(ll1) produced hydrogen and CO as the only gas phase products, whereas on Cu(l10) hydrogen atoms

38

ROBERT J. MADIX

TABLE VII Binding Energies and Peak Temperatures f o r Species Desorbed from Cu(ll0) and A g ( l l 0 ) Surfacef Cu( 110)

State

co/co co/co, HzO(GO/HzO HzO(B)l HzO C,H,/C*H, H2CO/H,C0 D,/D atoms CH,OH(a,)/CH,OH CH,OH(a,)/CH,OH CH,OH(a,)/CH,OH CH,CHzOD(a,)CH,CH,OD CH,CH,OD(a,)/CH,CH,OD CH,CHO(~,)/CH,CH,OD

T,(K)

E*(kcal/ mol)

223 223 235 285 224 225 336 200 5 245 5 275 220 282 220

13.5 13.5 14.2 17.3 13.5 13.5

228 228 235 210 249 f 5 265

13.3 13.3 13.7 12.2 14.0 15.5

-

**

12.1 14.8 16.7 12.8 16.6 12.8

Molecule Dz /D,,.,, H,CO/H,CO HCOOCH, /HCOOCH, CH,CH,OD(a,) /CH,CH,OD CH,CH20D(a,)/CH,CH,0D CH,CH,OD(a,)/CH,CH,OD

E* was calculated assuming first-order desorption with v

=

lo1, sec-'.

recombined with CH,O to reform CH,OH. This reaction occurred instead of the recombination of H(a) to form H, due to the unusually low desorption rate of hydrogen from this surface (124). The stable surface methoxy species was also formed from methanol on Fe(100) (99, W(100) ( 5 x l)C (225), Ni(100)c(2 x 2)s (126), and Ni(100)c(2 x 2)C (127); it was not observed to form on Ag(ll0) in the absence of surface oxygen (see later). The formation of ethoxy intermediates was also observed on Cu(ll0) (128), Ag(ll0) (128), and Fe( 100) (95) following ethanol adsorption. C. THEOXIDATION OF METHANOL AND ETHANOL ON COPPER AND SILVER 1. Methanol The catalytic oxidation of methanol to formaldehyde was discovered

METAL SINGLE CRYSTAL SURFACES

39

more than a century ago, but the mechanism of oxidation and the role of oxygen are still not completely understood (128). The classical catalytic process employs either copper or silver catalysts in the form of gauze or pellets operated at 600 to 725°C. An oxygen-lean mixture is employed. The major side product formed is CO,; minor amounts of CO, HCOOH, CH,, and methylal are also formed. Various mechanisms have been proposed for the reaction (129-133). Most recently investigators have concluded that oxygen must be present in copper for it to be active, however the relative roles of dehydrogenation and direct oxidation of the alcohol were not clearly established (133). It is evident from results presented earlier that CH30H dissociatively adsorbed on Cu(l10) at room temperature. The fact that little H,CO was then formed from the CH30(a) was due to the preferential recombination of the adsorbed hydrogen atoms with the methoxy species. Since surface oxygen on copper is easily reduced by hydrogen atoms (124), its presence could easily provide a low energy pathway for removal of the hydrogen atoms via water formation, leaving the methoxide to decompose to H,CO and H, . Additionally, adsorbed oxygen atoms could facilitate the dissociative adsorption via direct interaction with the hydroxyl hydrogen. These two mechanisms are discussed later. It is most instructive to consider the alcohol oxidation on silver first. Adsorption of oxygen on silver has been shown previously to be a complex phenomenon (134). It was shown that oxygen dissociatively adsorbed with a low probability on all low index planes of silver; it was most reactive with the (1 10) surface, on which it adsorbed with a probability near (134136). When the oxygen coverage was progressively increased at room temperature, LEED patterns exhibited (7 x l), (6 x l), (5 x l), (4 x l), (3 x l), and (2 x 1) symmetry; the adatoms occupied the troughs on the (1 10) surface and spaced themselves evenly (134). These results indicate that, in effect, the adatoms repelled one another along the troughs and attracted one another in the direction perpendicular to the troughs. They thus appear to form many one-dimensional chains which are equally spaced along the closepacked direction. The desorption behavior with heating was also complicated. Molecular oxygen was evolved by atom recombination at about 575 K in a TPD peak with near first-order symmetry (134-136). Isotope exchange experiments showed the recombination of atoms to be random (136). At temperatures near the desorption temperature the adatoms must gain appreciably mobility. The adatom binding energy was 99.7 kcal/mol (134, 136) at low coverages. Only a small amount of methanol adsorbed on the Ag(ll0) surface at 180 K in the absence of preadsorbed oxygen. In the presence of oxygen adatoms, however, methanol adsorbed both dissociatively and nondissociatively. The TPRS product spectrum obtained for a low exposure of

40

ROBERT J. MADIX I

I

I

I

I

PRODUCT/ CH,OD 60L'%* -I

a z 12 v)

I-

u

3

n 0

CK

a (b)xl I

I

200

250

I

I

300 350 TEMPERATURE ( K )

I

400

FIG.23. TPRS product distribution for CH,OD oxidation on Ag(ll0) following predosage of the surface of 1 8 0 2 (137). (a) CH,OD; (b) D,180; (c) H,CO; (d) CH,OH; (e) HCOOCH,. Reprinted with permission of North-Holland Publishing Company, Amsterdam, 1978.

Ag(ll0) with 0.20 monolayer of l80adatoms to CH,OD is shown in Fig. 23 (137). This figure clearly shows that CH,OD interacted with the "O(a) to produce D, " 0 near 225 K. Since dissociative adsorption of CH,OD did not occur on the clean surface, and since no D, was observed as a reaction product, this reactive interaction must have occurred directly with the adsorbed oxygen, leading first to l80D and then to D, " 0 via interaction with a second molecule of CH,OD. As the exposure of this predosed surface to CH,OD was increased, the D, '80/CH,0D (180) peak shifted to lower temperatures and finally disappeared altogether at saturation exposures of CH,OD, suggesting that D, l80was displaced from the surface as the reaction of CH,OD with "O(a) proceeded. This displacement was indeed confirmed by monitoring the desorption of D, l80mass spectrometrically during dosing. At saturation exposure to CH,OD all of the predosed "O(a) was reacted to form D, "0, which was displaced from the surface by the adsorbed intermediates. These intermediates also stabilized the adsorption of molecular CH,OD, which was not observed on the clean surface. Several reaction products appeared at higher temperatures. Simultaneous evolution of H,CO, H, , and CH30H in a first-order, reaction-limited step was observed near 350 K. These products clearly resulted from the decomposition of CH,O(a) which was formed following initial reaction of CH,OD and "O(a). The CH,OH product was formed by the reaction of the hydrogen atoms released by the reaction CH,O(a) +H(a) + H,CO(a) with the CH,O(a) on the surface. The predominant mechanism was

METAL SINGLE CRYSTAL SURFACES

41

CH30D(g) +CH,OD(a)

-+ I80(a) CH,O(a) + I80D(a) + 180D(a) --+CH,O(a) + D,lsO(a)

CH,OD(a) CH,OD(a)

-+

DZ180(a)-+ D,I80(g) CH,O(a) 2H(a) H(a)

+ CH,O(a)

-+

+

-+

H(a)

+ H,CO(a)

%(g) CH,OH(g)

H2CO(a) + H,CO(g)

Methyl formate and C'60180 were also detected as products which formed at higher oxygen coverages and, consequently, higher coverages of the adsorbed species, as shown in Fig. 24. Coadsorption of D,CO and CH,O(a) produced DCOOCH3 and D, simultaneously at 275 K by firstorder kinetics. The mechanism was suggested to be (137) D,CO(a)

+ CH,O(a)

-+

D,

D I ,OCH, C

I

3

D(a)

+ DCOOCH,(g)

0

The CO, was produced via the formate intermediate, but the mechanism of its formation is not entirely clear. It was observed to form (138) from H,CO according to the reaction H,CO(a)

+ O(a) + H,CO,(a)

HCOO(a)

'

--t

H(a)

+ H(a)

+ HCOO(a)

+ CO,(g)

a

J

v)

cn

a

5

EXPOSURE OF '?IN I2 LANGMUIRS FIG. 24. Plot of the dependence of the products of CH,OD oxidation on Ag(l10) on the prior exposure of the surface to ' * 0 2H,CO . and CH,OH form at low oxygen exposures, whereas CO, and HCOOCH, grow in only at higher exposures (137).Reprinted with permission of North-Holland Publishing Company, Amsterdam, 1978.

42

ROBERT .I. MADIX

In this reaction H,CO, was observed as a stable reactive intermediate. With increasing coverages of reactive intermediates, evidence for repulsive interactions between intermediates was observed. Typically the TPRS spectra showed the emergence of new low temperature product peaks similar to that shown for H,CO/CH,OD in Fig. 25. The low temperature peaks grew in strongly after the high temperature p2 peaks were nearly saturated. As discussed earlier in Section II,D, this type of behavior is indicative of repulsive lateral interactions among the adsorbed species. The magnitude of the repulsive interactions was about 3 kcal/mol. This interaction, in effect, increases the rate of reaction due to a destabilization of the adsorbed state. The D,COOCH,(a) intermediate may actually be the result of attractive forces between CH,O(a) and DzCO(a). Both of these species have weak dipole moments with a net negative charge on the oxygen and a net positive charge on the carbon. A head-to-tail alignment of these species would produce

which is the correct configuration for the formation of methyl formate. The oxidation of CH,OD on Cu(ll0) was qualitatively similar to that on Ag(l10). H,CO, Hz, and CH,OH were formed simultaneously at 360 K due to the decomposition of the methoxide. No D, was observed at saturation exposure to CH,OD, so apparently the alcohol intereacted solely with the

TEMPERATURE

(K)

FIG.25. TPRS spectra for H , C 0 / ' 8 0 , , CH,OD showing the evolution of the two peaks with increasing coverage of intermediates on the surface (137). Reprinted with permission of North-Holland Publishing Company, Amsterdam, 1978.

43

METAL SINGLE CRYSTAL SURFACES

surface oxygen to produce D, l8O as with Ag(ll0). The presence of the methoxy species enhanced the nondissociative adsorption of CH30D as it did on Ag(ll0). The decomposition of CH,O(a) occurred at a higher temperature on Cu(ll0) than on Ag(ll0). The activation energy for H,CO(B,)/ CH,OD the primary product peak, was 4-5 kcal/mol higher on the Cu(ll0) surface. No evidence for lateral interactions was observed in the product peaks on Cu(ll0). Carbon dioxide was formed via the formate decomposition, but no methyl formate was formed on copper. The reaction HZCO + 180(a) -+ HC0I80(a)

+ H(a)

was also observed to occur on Cu(ll0) (56). It should be noted that the formate is the most stable surface intermediate formed in this reaction sequence and steady-state operation below its decomposition temperature would lead to a buildup of HCOO(a), producing an inactive surface. The desorption parameters and the rate constants for the preceding reactions are given in Tables VIII and IX. The peak temperature is the temperature at which the surface lifetime of the reaction intermediate is approximately 1 second. The values of v and E can be used to extrapolate this characteristic time to higher or lower temperatures. The methoxy intermediates produced on Cu(1lo), Ag(llO), Ni(1 ll), and Fe(100) (95) showed a wide range of stability. On Ni(ll1) and Fe(100) the methoxy decomposed primarily to CO and H,, whereas on Ag(ll0) and TABLE VIII Reaction Rate Constants for Reactions of C H 3 0 H on Cu(ll0)" Product/ Reactant

Tp

E(kcal/ mol)

330 f 5 365 390 365 392 325 5 370 390 470 470 238 290 320 470

-

-

-

-

+

v (sec-')

E*

-

-

22.1 k 0.1 19.3 f 0.4

5.2 f 1.6 x 10" 1.5 k 0.7 x 10'"

-

22.0

-

3.6 x 10"

-

-

30.9 k 0.2 30.9 0.2

+

8.0 f 2.0 x 1013 8.0 2.0 x 1013

-

-

+

-

-

-

-

30.9 f 0.2

8.0

+ 2.0 x

1013

20.1 22.4 23.9 22.4 24.0 19.8 22.6 23.9 29.0 29.0 14.3 17.6 19.5 29.0

a E and Y were determined by methods discussed in the test. E* was calculated by assuming first-order kinetics and v = 1013sec-1.

44

ROBERT J. MADIX

TABLE IX Reaction Rate Constunts f o r Reactions of CH,OD on Ag(ll0)" State

T,(K)

E(kcal/mol)

250 280 3 252 280 f 3 300 340 250 300 340 I): 10 250 312 350 402 402 273

13.1 k 0.6 13.3 f 0.4

v (sec-')

4.5 2.5

k 3.5 k 1.5 -

~

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

22.2 & 0.5 14.0 0.5

x 10" x 10''

-

1.1 2.4

k 0.7 k 2.0

x 10l2 x 10"

E*(kcal/mol) 14.6 16.3 14.7 16.3 17.6 20.0 14.6 17.6 20.0 14.6 18.3 20.6 23.8 23.8 16.0

E and v were determined by methods discussed in the text. E* was calculated by assuming first-order kinetics and v = 10'3sec-1.

Cu(ll0) significant amounts of H,CO were formed. Accurate values of the decomposition temperature are available only for the Ag, Cu, and Fe surfaces. For these surfaces T,(Fe) > T,(Cu) > T,(Ag) for the methoxide decomposition ; i.e., the characteristic rate increased with the decreasing strength of the metal-oxygen and metal-hydrogen bond. This result is identical to that observed for the decomposition of HCOO(a). The rocking vibrational mode of vibration of the adsorbed CH30 to facilitate hydrogen transfer is probably the most important reaction coordinate; a lower oxygen-metal bond strength apparently produces a greater amplitude of vibration at a given temperature, facilitating reaction. 2. Ethanol

The TPRS spectrum produced following adsorption of EtOD to near saturation at 180 K on a preoxidized Ag(ll0) showed two distinct temperature regimes for product formation, as shown in Fig. 26 (139).Upon adsorption D, '*O was displaced from the surface. At 200-230 K evolution of CH3CH0, EtOD, D 2 I6 0 , C,H,, and H2 occurred. Near 275 K EtOH, H,, and CH3CH0 were desorbed. This high temperature branch corresponded to aldehyde formation via dehydrogenation of the ethoxide (139) in a fashion similar to CH,O decomposition. The low temperature reaction branch involved another more complex mechanism.

45

METAL SINGLE CRYSTAL SURFACES

FIG.26. TPRS product spectra for saturation coverages of ethanol on Ag(ll0) predosed with "O2 to form 0.20 monolayers of oxygen adatoms (139). Reprinted with permission of North-Holland Publishing Company, Amsterdam, 1978.

Ethylene and D 2 0 were formed only above a threshold exposure of the surface to ethanol. Figure 27 shows that acetaldehyde was first formed; this was the result of the reactions 2CH3CH,0D(a)

+ l8O(a)

CH,CH,O(a)

-+

2CH,CH,O(a)

-+ CH,CHO(g)

+ D,180(g) + H(a)

at 275 K. The presence of the ethoxide stabilized further adsorption of ~

d

zm

15

CH3CHO

0

a

w Iw

Ba

I-

8 u) u)

a

I

FIG.27. Evolution of products from ethanol oxidation on Ag( 110) as a function of the amount of EtOD dosed onto the preoxygenated surface (139). Reprinted with permission of NorthHolland Publishing Company, Amsterdam, 1978.

46

ROBERT J. MADIX

molecular ethanol. Apparently at the higher ethanol concentrations the hydrogen transfer reaction 2CH3CH,0D(a) + CH3CH,0(a)

+ CH3CH20DZ(a)

occurred. This was followed by CH,CH,O(a) +CH,CHO(g) CH3CH20DZ(a)-+ CH,CH,(a)

+ H(a)

+ DZO(g)

CH3CHz(a) + C2H4(g) + H(a)

and 2H(a)

--+

H&)

The C2H, peak positions did not change with increasing reactant coverage ; i.e., the rate-limiting step was at least pseudo-first-order. Owing to the presence of lateral interactions in the adsorbed layer, it is not possible to say what step was rate limiting. The reactions of ethanol on Cu(ll0) were qualitatively similar to the results on Ag(ll0). Prior adsorption of oxygen was not needed to produce a product distribution very similar to that in Fig. 26 (139).This difference in reactivity, also observed for methanol dissociation, reflects the relatively higher binding energies of copper for oxygen and hydrogen which favors the reaction ROH(g) RO(a) + H(a) on Cu, but not on Ag. The relative stabilities of the ethoxides on Cu(ll0) and Ag(ll0) were less than the methoxides. On copper this decreased stability produced decomposition of the ethoxide rather than reconstitution of EtO(a) and D(a) to form EtOD, whereas the methoxide recombined with D(a) to reform CH,OD(g). This difference in stability of these surface species agrees with the order of stabilities for zirconium alkoxides, Zr(OCH,), > Zr(OCH,CH,), > Zr(OCH(CH,),), (140). It also correlates with the relative bond strengths of the u-hydrogen C-H bonds in CH,OH (- 92 kcal/mol) and CH,CH,OH (88 kcal/mol). This result strongly suggests that the reaction coordinate involves transfer of the hydrogen on the carbon adjacent to the oxygen to the surface. The interactive differences observed between Cu and Ag for CH,OD and EtOD are small, but they produce dramatic differences in chemical behavior. The kinetic parameters for the reactions of both methanol and ethanol listed in Tables VIII-XI show some interesting features. First, the frequency factors for the decomposition of the alkoxide intermediates to form the aldehydes were observed to be within an order of magnitude of 10l3 sec-' as is expected from simple transition state theory. The activation energy for the transfer of the hydrogen atoms from the alkoxide to the surface was -+

47

METAL SINGLE CRYSTAL SURFACES

TABLE X Reaction Rate Constants for C H 3 C H 2 0 D Reactions on Ag(100)a

State

T,(K)

C,H,/CH,CH,OD D 20/C H,C H 20D CH,CHO(B, )/CH,CH20D CH,CHO(~,)/CH,CH,OD CH,CHO(B,)/CH,CH,OD CH,CHO(/I,/CH,CH,OD H~(BI)/CH,CH~OD H2(B3)/CH3CHZ0D H2(B4)/CH3CH20D CH,CH20H(P,)/CH3CH,0D CH,CH,0H(B,)/CH3CH20D

220 3 220 3 215 240 5 273 320 k 5 230 283 320 5 276 320 & 5

E*(kcal/mol)

+

12.8 12.8 12.5 14.0 16.0 18.8 13.4 16.6 18.8 16.1 18.8

*

+

E* was calculated by assuming first-order kinetics and Y = 10'3secC1.

TABLE XI Rate Constants for Reactions of CH,CH,OD on Cu(ll0)"

State

T,(K)

C,H,/CH,CH,OD DZO/CH,CH,OD CH,CHO(~,)/CH,CH,OD CH,CHO(B,)/CH,CH,OD CH,CH,OH/CH,CH,OD D, + H D + H,/CH,CH20D

225 228 316 350 316 340

E(kcal/mol)

v (sec-')

E*(kcal/mol)

-

__

13.1 13.3 18.7 20.7 18.7

__

20.4

k 1.0

5.0 f 4.0 x

-

__

-

-

-

-

-

a E and v were determined by methods described in the text. E* was calculated by assuming first-order kinetics and v = 10'3sec-'.

lower for the ethoxide, in agreement with the lower bond strength of the a-hydrogen in the ethoxide. Thus, in effect, the barrier height for transfer of the hydrogen is reduced. The higher frequency factor for the ethoxide has an interesting origin. First, the value of the frequency factor is given approximately by (kT/h)Cf /A). A higher frequency factor signifies that j#/A is higher, or, in other words, that the entropy difference between the transition state and the adsorbed state is larger. We picture the transformations as occurring according to

I

cu-cu-cu

cu-cu-cu

48

ROBERT J. MADIX

and

cu-bu-cu

cu-cu-cu

There will be entropy changes associated with the formation of the cyclic transition states, but these values should be approximately the same for both species. The additional entropy of the methyl group for the ethoxy intermediate cancels in the adsorbed and transition states. The difference in frequency factors cannot be found in differences in transition states. On the other hand, there is one pronounced difference in the adsorbed states. The UPS results for CH,OD adsorption on Cu(ll0) indicate that the methoxy sits upright on the surface, bonded through the oxygen to the copper (141). In this configuration there will be no barrier to rotation of the methyl group (142), whereas the bent configuration of the ethyl group relative to the 0-Cu bond will produce a barrier to internal rotation. In other words, the formation of the cyclic transition state eliminates free rotation of the methyl group in CH,OD, whereas the lowering of the entropy of the ethyl group is not as large, since the ethyl group’s rotation about the C-0 bond is hindered. If the barrier height for rotation of the ethyl group was taken to be 3 kcal/mol (143), the calculated difference in frequency factors due to the additional loss of the entropy of the rotation of the CH, group in CH,O was nearly an order of magnitude. This result agrees with the observed difference. D. OTHEROXIDATION REACTIONS ON Ag(ll0) A variety of reactions have been observed to be produced by oxygen preadsorbed on Ag(l10) (137, 144). In general these reactions do not proceed on clean silver under the conditions normally utilized in studies of surface reactivity. These reactions are listed below : RCOOD RCOOD ROD ROD

+ O(a) -+ RCOO(a) + OD(a)

+ OD(a)

-+

RCOO(a)

+ D20(g)

+ OD(a) -+ RO(a) + OD(a) + OD(a) -+ RO(a) + D,O(g)

(4) (5)

(6) (7)

0

II

HCOCH,

+ I80(a)

C02 + O(a) CO

-+

HC160180(a) + CH,O(a)

-+

CO,(a)

+ O(a) -+ C02

(8) (9) (10)

METAL SINGLE CRYSTAL SURFACES

+ O(a) -+ RHCO,(a) + (n - l)RHCO(a) nH,O + O(a) -+20H(a) + (n - l)H,O(a) C,H2 4- O(a) -+ C,(a) + H 2 0

nRHCO

49 (1 1)

(12) (13)

These reactions clearly illustrate that the adsorbed oxygen atom is a very strong base. It can abstract hydrogen atoms from molecules of low acidity (as measured in aqueous solution) and can attack carbon nucleophilically, as shown in reactions (8), (9), (lo), and (11). In addition to this striking property, the presence of atomic oxygen induces nondissociative adsorption as exemplified in reactions (11) and (12). In these reactions adsorption of molecular species up to ten times the number of oxygen adatoms is induced by the intermediate formed in the primary reaction with the oxygen. These effects are most striking on silver since it is, itself, a very unreactive surface. There is every reason to expect, however, that oxygen will behave similarly on other metals. More complex reaction behavior will, of course, be observed as the intrinsic reactivity of the metal increases. Oxygen adsorbed on platinum should show similar properties. In fact the formation of surface OH groups from H,O and O(a) was recently reported (145). The ability of platinum itself to break C-H and C-C bonds complicates oxidation mechanisms, but future work should provide a greater understanding of the relative role of surface oxygen in oxidation catalysis. V. Summary

The combined use of temperature programmed reaction spectroscopy and physical methods of surface characterization has made possible the study of the effect of surface composition and structure on surface reactions. Both mechanistic and kinetic studies have been performed on model reaction systems. The results show strong effects due to changes in surface composition. These effects include the partial passivation of tungsten by surface carburization and the pronounced selective activation of silver by adsorbed oxygen. For copper-nickel alloys both ligand and cluster effects were observed for the formic acid decomposition. The ligand effect was minor. Kinetic perturbations on the behavior of nickel due to adsorbed oxygen have also been observed. The autocatalytic decomposition of formic acid is believed to originate in attractive interactions among adsorbed intermediates. The kinetics of methanol oxidation on silver also shows evidence for lateral interactions between adsorbed methoxy groups. It is evident that a much clearer picture of surface reactivity is emerging from these model studies.

50

ROBERT J. MADlX

ACKNOWLEDGMENTS The author gratefully acknowledges the support of the National Science Foundation, the American Chemical Society/Petroleum Research Found, and the Center for Materials Research at Stanford University.

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ADVANCES IN CATALYSIS. VOLUME 29

Photoelectron Spectroscopy and Surface Chemistry M. W. ROBERTS Department of Chemistry University College Cardiff, South Wales, United Kingdom

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

55 56

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

59 62 65 65 68 73 75 80 80 80 82 85 88 91 92 93

I. Introduction . . . . . . . . . . . . . . . . 11. X-Ray and UV Photoelectron Spectroscopy .

111. Calculation of Surface Concentrations from Photoelectron Intensity Data . . . . . . IV. Experimental Strategy . . . . . . , . . V. Chemisorption of Diatomic Molecules . . A. CarbonMonoxide . . . . . . . . . B. NitricOxide . . . . . . . . . . . . CNitrogen . . . . . . . . . . . . . D.Oxygen.. . . . . , . . . . . . . VI. Chemisorption of More Complex Molecules A. Ammonia and Hydrazine . . . . . . B.Water.. . . . . . . . . . . . . . C. Formic Acid . . . . . . . . . . . . D . Hydrocarbons . . . . . . . . . . . VII. Metaloxides . . , . . . . . . . . , . VIII. Alloys and Surface Segregation . . . . . IX. Conclusion . . . . . . . . . . . . . . References . . . . . . . , . . . . . .

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1. Introduction

The scientificinterest in solid surfaces whether in the context of heterogeneous catalysis, solid state electronics, or corrosion is a direct consequence of the fact that their properties are unique with relation to the corresponding properties of the bulk solid. This is not difficult to appreciate since at the surface there is a breakdown of translational symmetry, extreme gradients of chemical composition are feasible, and perturbation of both bulk structure and charge are possible. There are, therefore, formidable problems to overcome if we are to arrive at a situation where surface structure, electronic 55 Copyright 0 1980 by Academic Press, Inc. All rights of reproduction in any form reserved. ISBN 0-12-007829-5

56

M. W. ROBERTS

structure, and atomic composition are all known factors in any discussion of the solids’ inherent chemical reactivity. A mere cursory examination of the surface chemistry literature over the last fifty years will establish that these three central problems have been recurrent themes; Langmuir, Otto Beeck, J. K. Roberts, E. K. Rideal, H. S. Taylor, and Balandin being active in such discussions over forty years ago. Descriptive surface chemistry in the early 1960s very much depended on the various experimental approaches adopted by such people. They initiated the use of metal films (as exemplifying atomically clean substrates), the use of metal filaments, the application of isotopic exchange studies, the determination of heats of chemisorption, the development of kinetic models, and so on; and today the subject of surface chemistry is still largely based on the ideas and experimental data generated prior to 1950. The important difference between contemporary studies and those undertaken prior to, say, 1970 is that experimental methods have emerged over the last decade which enable problems in interfacial science to be probed at the atomic level. Ideas that have been prevalent for a generation or more become tractable experimental projects, and not surprisingly with the wealth of detail that becomes available from these experimental techniques new ideas emerge. In this article the object will be to examine the impact that electron spectroscopy has had on our understanding of molecular events occurring at solid surfaces. To date the greatest effort has been made with metals, but we will also consider some aspects relating to metal oxides. The chemisorption of diatomic molecules, their mode of adsorption, the interplay between associative and dissociative states, the relation between chemisorption and catalysis, the modification of surfaces (e.g., oxides) during chemisorption, surface segregation, catalytic reactivity and electronic structure, surface charge, and the problems inherent in the quantification of the concentration of surface species will be considered. We will, therefore, be emphasizing the more recent experimental data since they undoubtedly will be the basis for the confirmation of old ideas, the development of new ones, and the possible rejection of hitherto established views. II. X-Ray and UV Photoelectron Spectroscopy

When a photon of energy hv impinges on a solid, the ejected photoelectrons have a kinetic energy distribution made up of a series of discrete bands reflecting the sample’s electronic structure. The experimental determination of the kinetic energy EK of the photoelectrons enables, through the relationship EK = hv - EB

PHOTOELECTRON SPECTROSCOPY AND SURFACE CHEMISTRY

57

the binding energy (B.E.) of the photoelectrons to be determined. The photoelectron spectrum is therefore obtained by scanning the kinetic energy EK of all the photoelectrons, peaks being observed at discrete values of EK corresponding to particular values of EB. It is the binding energy EB that enables the origin of the photoelectron to be characterized, and therefore provides the information concerning the species being ionized. Strictly speaking, photoelectron spectra should be interpreted in terms of the manyelectron states of the final ionized state (say M’) rather than the occupied one-electron states of the neutral species M. We will not, in general, concern ourselves with problems associated with “final-state’’ effects, but merely consider how values of EB enable diagnostic information to be deduced concerning solid surfaces and the nature of the bonding of molecules to surfaces. Although there was no information available in 1970 on the application of x-ray photoelectron spectroscopy (XPS) to solid surfaces, there were many examples of studies of gases reported, largely by Siegbahn ( I ) and his colleagues, where “chemical shifts” (i.e., different B.E. values for a given ionization process) had been observed for a particular atom in different chemical environments. Siegbahn had shown the existence of a strong correlation between “charge” associated with the species and the binding energy observed. It was this particular parameter that made the application of XPS to problems in surface chemistry so potentially attractive, since it was well known that adsorbed species exhibited distinct charges as reflected by surface potential data. The key question to be answered was whether or not XPS would be sufficiently surface sensitive. There were two important clues which engendered optimism: (a) the XPS data of Siegbahn et al. indicated that iodostearic acid, a comparatively large molecule (- 25 A) could be “observed,” and (b) the chemical shifts reported in the electron energy distribution results for the nickel oxygen system (2) suggested that the escape depth of photoelectrons was no more than about 20 A. Both of these provided sufficient impetus for us to develop an ultrahigh vacuum compatible electron spectrometer suitable for the study of chemisorption on well-defined clean metal surfaces. The prototype spectrometer (incorporating both x-ray and UV radiation sources) would, if successful, be suitable for studies in the broad area of interfacial science. The successful application of XPS to the study of adsorption on metal surfaces was reported in 1972, and the data indicated (3,3u)that “in general less than 10%of a monolayer is detectable.” In particularly favorable cases, such as mercury adsorption, less than 1% of a monolayer could be observed by XPS. The surface sensitivity of XPS was therefore established (Fig. 1). What information might accrue regarding the details of surface bonding, and so on, however remained to be explored. There were also important questions to be answered regard-

+

58

M. W. ROBERTS

O(ld monolayer exposure

hc

-

*

-. $ ! b 0

531

A

537 543 519 binding energy/eV

0 erporure (torr

L

x

Au

B a1

100 binding enerpy/eV

FIG. 1 . (A) O(ls) spectra for gold exposed to carbon dioxide at - 8 5 K : (a) clean Au; (b), (c). and (d) after exposure to CO,. (B) Au(4f) and Hg(4f) spectra after exposure of gold foil to mercury vapor at 290 K. The mercury and gold peaks are well separated and in contrast to Auger spectra which overlap.

ing the determination of absolute surface concentrations from photoelectron intensity data, the estimate of the photoelectron escape depth, and whether it was energy dependent. There was also a dearth of information regarding relative photoionization cross sections for different electron shells. Ultraviolet photoelectron spectroscopy (UPS) as a tool for the study of gases was well established in the early 1960s largely due to the efforts of Turner (4) and Price (5). Bordass and Linnett (6) had shown in 1969 that there were grounds for optimism in the application of UPS to surface studies (they reported data for CH,OH adsorption on tungsten), and in 1970 Eastman and Cashion (7) reported the first UPS study of carbon monoxide chemisorbed on nickel (Fig. 2). The combined UPS and XPS facilities available in the Vacuum Generators ESCA-3 Spectrometer (the name by which the prototype became known) provided a powerful means ( 3 , 3 a ) of exploring solids, their surfaces, and in turn the reasons for their inherent chemical reactivities. We shall see that XPS and UPS enable both qualitative and quantitative characterization of the solid surface at the atomic level, the chemical environment of a particular

PHOTOELECTRON SPECTROSCOPY AND SURFACE CHEMISTRY

-12

-10

-8

-6

-L

-2

59

0 EF

ELECTRON ENERGY L t V t L S (eV1

FIG.2. Emission spectra (hv temperature.

= 40.8 eV) for

nickel and nickel exposed to 0.6 L of CO at room

surface atom, whether there is any significant bonding present between two atoms in a diatomic molecule in the gas phase still present after adsorption, and the electronic structure of the solid surface as reflected in a “density of states” curve. 111. Calculation of Surface Concentrations from Photoelectron Intensity Data

In order to estimate the photoelectron yield from a chemisorbed layer we follow the approach of Henke (a), applied by Madey et al. (9),and modified by Carley and Roberts (10). A monoenergetic beam of x rays strike a surface at an angle 8 to the surface normal; the photoelectrons, assumed to be generated in a layer of depth X and thickness dx, escape from the sample to a detector positioned at an angle 4 with respect to the surface normal. The differential probability dP of absorption of x rays at depth X within the thickness dx is dx dP=crp-exp cos e where CI is the mass x-ray absorption coefficient and p the sample density. The probability P , that a photoelectron generated at X below the surface

60

M. W. ROBERTS

will escape and be detected within the angle undergoing any inelastic collisions is given by

P , = exp

4 by the detector without

(lio:4) ~

where l is the mean free path of the photoelectrons. The total no-energy loss photoelectron yield Y, from the solids is therefore x= m

Ys = FK

i;.

pedp

(3)

where F is the x-ray flux and K an instrumental constant involving the solid angle subtended by the detector slit. Combining Eqs. (l),(2), and (3) and since a p l << 1, integrating Eq. (3) leads to Y, = FKa,plZ-

cos 4 cos 8

(4)

For a monolayer containing a atoms cm-2 the probability of x-ray absorption by the monolayer is given by

P, = amMrna N cos 8 ~

where a, is the absorption coefficient, M, the molecular weight, and N Avogadro’s number. Assuming no electron attenuation by the monolayer (i.e., P , N 1.0) the total photoyield from the monolayer Y, is given by

Y, =

FKa,M,a N cos 8

If we divide Eq. (6) by Eq. (4), then the photoelectron yield from the monolayer (Y,) relative to the total yield from the substrate (Y,) is given by

We can write pN/M for a so that

Equation (8) therefore allows us to use directly tabulated subshell photoionization cross sections (p)instead of mass absorption coefficients (a). Y, is the integrated photoelectron signal from an appropriate subshell of the monolayer adatom; Y, the integrated signal from the relevant subshell of the substrate which is not simply the area of the core-level peak; ,urnand ,us

PHOTOELECTRON SPECTROSCOPY AND SURFACE CHEMISTRY

61

the photoionization cross sections for the particular subshell of the adatom and substrate; p the density of the substrate; 1 the electron escape depth in the substrate for the particular photoelectron; N Avogadro’s number; and 4 the angle of collection of the photoelectrons with respect to the sample normal. The surface concentration G can therefore be calculated. We must also take into account two further factors. First, the fact that the transmission efficiency of the analyzer is a function of the kinetic energy (K.E.) of the photoelectrons; in the ESCA-3 Vacuum Generators instrument the transmission is inversely proportional to the K.E. of the electrons (3a). Second, photoelectron yields must refer to total yield from a particular ionization process and this need not, for example, be just the area of the relevant peak. Account must be taken of all processes that divert electrons from the primary peak, e.g., shake-up, shake-off, and plasmon peaks. In some cases, e.g., emission from the Cu 2P,,, level, the contribution of “additional processes” is small; but in others, and emission from the Al(2p) shell is an example, the “no-loss peak” is substantially less than the “true” Al(2p) emission. Both the calculated photoelectron ionization and escape depth data of Scofield (11) and Penn (12) are invaluable in estimating surface concentrations from Eq. (8). More recently, experimental cross section data have been reported by Thomas and his group (13);the reported data are relative to the F(1s) peak taken as unity. There are clearly examples where Scofield’s calculated cross section values are at variance with the experimentally determined ones ; the variation is particularly noticeable when we consider outer levels, e.g., for K(2p) there are serious discrepancies, whereas the K(2s) data are acceptable. Although the present article will be mainly concerned with molecular events occurring at “clean” metal or oxide surfaces, if for some reason we are dealing with a substrate heavily contaminated by carbon, then the observed photoelectron peaks from the underlying substrate will be influenced by the passage of the electrons through the carbon overlayer. This is particularly serious if we are concerned with estimating a ratio of two species, say tungsten and oxygen, and we are monitoring W(4f) and O(1s) photoelectrons since their respective escape depths are dependent on kinetic energy through the relation I -f (K.E.)’’*. We will not concern ourselves here with problems associated with line broadening, overlapping peaks, and background subtraction. There are, however, examples discussed later where both deconvolution and curve fitting procedures are shown to be essential in unraveling the contributions of differently bonded species of the same molecule to the total photoelectron yield. Carley and Joyner (14) have discussed recently deconvolution procedures for photoelectron spectra.

62

M. W. ROBERTS

Prior to the advent of electron spectroscopy there were no experimental facilities available that could provide, even at the most elementary level, a qualitative analysis of the chemical composition of a solid surface. Surface chemists therefore find acceptable estimates of absolute concentrations which might only be accurate to no better than 20%. There is, however, substantial evidence to suggest that photoelectron spectroscopy can provide data that are at least within these limits; and where relative concentrations are being considered, the accuracy is somewhat better. It should be emphasized that the photoelectron signal is not generated entirely by the surface atoms. The precise definition of /I (the “escape depth”) is the depth from which a fraction lie of the electrons escape without losing energy throclgh inelastic collisions. This follows from Y, = Ym(l - e-d’A)

(9)

where Y, is the “peak” intensity from a solid layer thickness d and Y, that from an “infinitely thick” solid for photoelectrons whose escape depth is 2. Using Eq. (9) we can show that approximately 80% of the total signal is generated from a depth of 23L or 25 A (a typical value of 3L N 12 A) and from the surface layer alone the contribution to the total signal would be about 30%. One advantage of the dependence of 3L on the kinetic energy of the photoelectrons is that it is possible to “analyze” at different depths by choosing two peaks characteristic of a given element but differing widely in kinetic energy.

IV. Experimental Strategy

The strategy adopted in the application of electron spectroscopy to elucidate problems in surface chemistry and catalysis is illustrated by the three instruments used in our studies. The ultrahigh vacuum compatible electron spectrometer (ESCA-3) combined x-ray with UV facilities (3) enabling exploration of core levels with photons of energy 1486 eV (AIKu radiation) and valence levels with photons of energy 21.2 and 40.8 eV [He(I) and He(I1) radiation, respectively]. Our second instrument was designed ( I S ) specifically for studies with single crystals; it was, in contrast to ESCA-3, a single-chamber instrument, but in addition to x-ray and UV sources there was the provision made for investigating surface structure by means of low energy electron diffraction (LEED). Both these instruments suffered the disadvantage that the maximum gas pressure at which spectra could be obtained was about lo-’ Torr. Above this pressure electron-molecule interactions become troublesome. Although this “cutoff pressure” is not a serious disadvantage for studies involving strongly chemisorbed species, it does

PHOTOELECTRON SPECTROSCOPY AND SURFACE CHEMISTRY

63

restrict the study of weakly adsorbed species. This point is best illustrated by reference to the “residence time” 9of a molecule at a surface. 9is related to the surface temperature T and activation energy of desorption through Eq. (10) where f,is a constant. .Y = Yo exp(Ed/RT)

(10)

Using the concept of partition functions and making certain assumptions concerning the adsorbed molecule and the activated complex, it can be shown that f ,N hv/kT which at 300 K is 1.6 x sec. If the experimental strategy is such that adsorbed species are to be observed only after the removal of the gas phase molecules, then obviously desorption from the surface must be negligible on the time scale of the experiment and we would require 9to be at least lo3 or lo4 sec. Similarly, if we carry out the experiment in the presence of gas, lower values o f f may be tolerated. The surface coverage is related to f through (r = Sv,where S is the sticking probability and v the rate of impingement of molecules from the gas phase. Suppose we wish to detect by electron spectroscopy 5 x 10l2 species cm-2 (say 1% of a monolayer), then we would require the following condition to hold f > 5 x lO”/Sv. Since v for, say, N, at 295 K is 3.8 x lo2’ P (Torr) molecules cm-2 sec-’ and if we assume S = 0.1 and P = Torr, then adsorption would only be detectable if 9-> 0.1 sec; whereas if P = 1 Torr. 9could be as small as lo-’ sec. Obviously we can also consider the problem in terms of the heat of adsorption (or Ed), since this is related to F through Eq. (10).

G A S IN

G A S IN

G A S IN

FIG.3. Schematic drawing of the high pressure electron spectrometer. A, Argon ion gun: D, differentially pumped region; EL,electron lens; G , gas cell; HSEA, hemispherical electron analyzer; LO, two-grid LEED optics; LV, leak valve; M, long travel rotatable manipulator; P, pirani gauge; S, sample; TSP titanium sublimation pump; W, window; X, twin anode x-ray source.

64

M. W. ROBERTS

The “high pressure” spectrometer should, therefore, enable an XPS study of “weakly adsorbed” species at room temperature or above by increasing their surface coverage through the dependence of coverage on pressure. The spectrometer was designed with a view to being able to obtain a spectrum of the solid surface in the presence of gas at a pressure of 1 Torr. Obviously the path of the electrons through the gas should be as short as possible, and to achieve this an electron lens system was constructed (16) between the gas cell and the retarding element of the electron energy analyzer. This contrasts with the analyzer used in the ESCA-3 spectrometer and enabled differential pumping to be used, which was essential to decrease the gas pressure from 1 Torr at the sample surface to no greater than 5 x Torr in the analyzer. A diagramatic representation of the instrument is shown in Fig. 3, a feature being the tandem design with a common analyzer. This allows two distinct research projects to be underway simultaneously, one with single crystals since LEED facilities were incoporated into one side. In addition to A1 and Mg x-ray anodes, UPS facilities using He radiation were also available.

0.2-

0.2

0.L

0.6

0.8

1.0

1.2

1.6

1.L

1.8

2.0 2.2

LN/forr

@

FIG.4. Ag(3d5,,) signal intensity as a function of argon pressure; (0) experimental points and (-) calculated.

525.0

529.0

533.0

53 7.0

561.0

BINDING ENERGY ( C V )

FIG. 5. O(1s) spectra for oxidized silver in the presence of oxygen at 0.5 Torr pressure. A is the “oxide” peak, B is assigned to the 0,-species, and C is the gas phase spectrum.

PHOTOELECTRON SPECTROSCOPY AND SURFACE CHEMISTRY

65

Two conditions had to be fulfilled if the spectrometer was to achieve the design objective: First, it should be possible to obtain simultaneously gas phase and surface spectra with little, or no overlap; and second, the loss in intensity of the signal from the surface should be small in the presence of gas at a pressure of 1 Torr. Figure 4 shows the attenuation of the Ag(3d5,,) signal as a function of increasing pressure of argon. Clearly, loss of signal from the silver surface is not a serious problem. Furthermore, there is little overlap between the O(1s) signals from O,(g) and O(a) (Fig. 5 ) (16, 17). V. Chemisorption of Diatomic Molecules

Prior to 1970 our understanding of the bonding of diatomic molecules to surfaces, and in many cases the type of adsorption (i.e., molecular or dissociative) was almost entirely dependent on indirect experimental evidence. By this we mean that deductions were made on the basis of data obtained from monitoring the gas phase whether in the context of kinetic studies based on gas uptake or flash desorption, mass spectrometry, or isotopic exchange. The exception was the important information that had accrued from infrared studies of mainly adsorbed carbon monoxide, a molecule that lent itself very well to this approach owing to its comparatively large extinction coefficient. The advantages of electron spectroscopy for the study of adsorbed diatomic molecules are illustrated by reference to the adsorption of carbon monoxide, nitrogen, nitric oxide, and oxygen on different metal surfaces. A. CARBON MONOXIDE This was the first adsorbed molecule to be investigated by electron spectroscopy, initially through the use of helium radiation and subsequently with x-ray radiation. There were, however, inherent advantages in having in situ capabilities for exploring both valence and core electrons simultaneously, a possibility conceived in the design of the ESCA-3 electron spectrometer. The first studies of chemisorbed CO were on nickel surfaces by Eastman and Cashion (7) who used helium radiation. The assignment of the peaks observed at about 7 and 11 eV to the ( 5 0 and ln) and 40 orbitals in CO(g) is now agreed upon. Although there was considerable debate concerning orbital assignment, the suggestions of Lloyd (18) and Mason and his colleagues (19) for the above assignment were confirmed by the synchrotron radiation studies of Gustafsson et al. (20). It appears that the outer 5a orbital is centered primarily on the C atom and looks like a lone pair in that it extends out well beyond the carbon atom; the 1n level, ac-

66

M. W. ROBERTS

commodating four electrons, is centered on the oxygen; whereas the 40 is like an oxygen lone pair extending beyond the 0 atom. Evidence for both molecular adsorption and dissociation of carbon monoxide on metal surfaces emerged from studying O(ls), C(ls), and the valence electrons. With such metals as tungsten, molybdenum, and titanium (21-23, dissociation was facile at room temperature; with nickel, ruthenium, and copper (24-26), adsorption was molecular; whereas with iron (27), adsorption was only molecular at 80 K, slow dissociation occurring at 295 K. The role of a surface impurity in the interplay between associative and dissociative states of adsorption is exemplified by preadsorbed sulfur on iron (27). An iron surface with approximately 30% of a monolayer of chemisorbed sulfur was shown to chemisorb CO molecularly at 295 K with no evidence for dissociation. The experimental evidence for these conclusions was based first on the O(1s) binding energy, there being strong grounds for associating a dissociated molecule with an O(1s) value of 530 eV. Chemisorbed oxygen adatoms on (most) metals was also characterized by an o(1s) value of 530 eV. Second, the lack of orbital structure in the helium-induced spectra at the characteristic energies of molecularly adsorbed CO [i.e., the electrons associated with the degenerate (5a and 171) and 40 orbitals] suggested that there was little bonding between the carbon and oxygen atoms in the adsorbed state. These conclusions can be rationalized in terms of the heat of adsorption where enhanced back bonding leads to an increased electron density on the oxygen, weakening of the carbon-oxygen bond, and ultimately dissociation. A correlation was shown to exist between experimentally observed O(1s) values for adsorbed CO, heats of CO adsorption, and the presence or absence of peaks in the helium-induced spectra (Fig. 6) (28). This suggests that for heats of adsorption greater than about 300 kJ the state of adsorbed CO is dissociative.

-

-

Oils1 5LOeV

He PEAKS AT 7eV AND lOeV

He PEAKS ABSENT

535eV

530 eV 50

100

300kJ(AH)

FIG.6. Correlation between O(1s) binding energy for adsorbed CO on different metals, the heat of adsorption (AH), and the presence or absence of peaks in the He spectra characteristic of molecular CO.

PHOTOELECTRON SPECTROSCOPY AND SURFACE CHEMISTRY

67

Although the early kinetic work had suggested that carbon monoxide was the archetype of molecular chemisorption (29), more recent kinetic studies have favored dissociation (30). The absence of infrared active bands for CO adsorbed on such metals as tungsten and iron (except at very high exposure, i.e., for CO adsorbed molecularly on a carbidic-oxide surface generated at low exposure as a result of dissociation) and the occurrence of isotopic exchange studies can now be understood in terms of the interplay between associative and dissociative states at low temperatures (295 K and less). For example, isotopic exchange was reported with iron by Webb and Eischens (31) in 1955 at about 170 K, whereas recent electron spectroscopic results indicate that dissociation is reasonably facile at 290 K so that incipient dissociation leading to exchange through a “four-centered’’ mechanism is probable at significantly lower temperature. The fact that surface structure, in particular steps and coordinatively unsaturated sites, has an influence on the state and reactivity of carbon monoxide is entirely in keeping with the empirical correlation (Fig. 6 ) between heat of adsorption, electron binding energies, and molecular state. Elegant studies by Mason, Somorjai, and their colleagues (32, 33) have established that with Pt(ll1) surfaces, dissociation occurs at the step sites only, and once these are filled carbon monoxide is adsorbed molecularly (Fig. 7). The implications of the facile dissociation of carbon monoxide by such metals as iron, molybdenum, and tungsten for the conversion of carbon monoxide into hydrocarbons (the Fischer-Tropsch process) have been emphasized and discussed by a number of people (32,34). CO adsorption on metals has, therefore, been extensively studied by electron spectroscopy, many of the results have been compared with theoretical carbidic carbon

282

286

I

282

286

BINDING ENERGY

BINDING ENERGY

la)

(b)

FIG. 7. Dissociative and molecular adsorption of CO on (a) stepped Pt(ll1) surface at 290 K, (b) same as (a) but with preadsorbed oxygen present. C(1s) spectra with increasing exposure to CO(g), spectra (i)-(v).

68

M. W. ROBERTS Dissociation

I

Molecular

FIG.8. CO reactivity pattern at 290 K ; dissociative chemisorption occurs to the left of the heavy line. With iron, dissociation is comparatively slow at this temperature, and so is a borderline case.

calculations, and a broad pattern of reactivity has emerged (Fig. 8). Many good reviews have been published (35-39), and we see clearly the influence of the metal, surface crystallography and the role of preadsorbed impurities such as sulfur and oxygen delineated. B. NITRICOXIDE

Interest in nitric oxide was stimulated by the conclusions reached regarding the molecular state and bonding involved in the adsorption of carbon monoxide on metal surfaces. The bond energy of NO (600 kJ) is appreciably smaller than that of CO (1000 kJ), the extra electron in NO going into the antibonding n* orbital. In view of the model proposed for CO bonding and implicit in the correlation shown in Fig. 8, analogous molecular events were anticipated for nitric oxide chemisorption. Data were first reported with polycrystalline iron surfaces (40), and even at 80 K both dissociative and molecular chemisorption were observed. Nitrogen adatoms formed at 80 K were shown to be characterized by an N(1s) binding energy value close to 397 eV. The molecular state of adsorbed nitric oxide had associated with it an N(1s) value of about 400 eV, but the peak was very broad which suggested contributions from other surface species involving nitric oxide. The presence of molecular nitric oxide at 80 K was confirmed by valence level studies using helium radiation, peaks being observed in the spectrum which could be assigned to the molecular orbitals of NO(g). At 295 K dissociation of the molecularly adsorbed species occurred [as also shown by Kishi and Ikeda (41) at 5 Torr pressure of NO], and the He(I1) (pseudodensity of state) spectrum was featureless and typical of an iron oxide-nitride surface. On exposure to further nitric oxide at 295 K evidence from both XPS and UPS indicated that the now heavily oxidized surface adsorbed molecular NO (40).

I

,i\

O(ls1

&lqL 53 0

290K

535

5LO (eV1

Nils) (bl

290K 170K

lLOK llOK

]*OK .. . 397

LO5

LO1

LO9

lev)

FIG.9. (a) O(1s) spectra during exposure of Cu(100) surface to NO(g). A, Clean surface at 80 K ; B, after exposure (6 L) to NO at 80 K; C, after exposure (24 L) to further NO; D, after warming adlayer to 100 K ; E, after warming adlayer to 290 K. Peak assignments are shown. (b) N( Is) spectra for NO saturated adlayer on Cu(100) surface at 80 K and during various stages of warming to 290 K. The dotted line shows the spectrum for a Cu(l11) surface exposed to NO at 80 K.

70

M . W. ROBERTS

Clearly the molecular events with iron were complex even at 80 K and low NO pressure, and in order to unravel details we chose to study NO adsorption on copper (42),a metal known to be considerably less reactive in chemisorption than iron. It was anticipated, by analogy with carbon monoxide, that nitric oxide would be molecularly adsorbed on copper at 80 K. This, however, was shown to be incorrect (43), and by contrast it was established that the molecule not only dissociated at 80 K, but NzO was generated catalytically within the adlayer. On warming the adlayer formed at 80 K to 295 K, the surface consisted entirely of chemisorbed oxygen with no evidence for nitrogen adatoms. It was the absence of nitrsgen adatoms [with their characteristic N(1s) value] at both 80 and 295 K that misled us (43) initially to suggest that adsorption was entirely molecular at 80 K. These studies were with polycrystalline copper, but more detailed results (Fig. 9) were obtained subsequently (44) with Cu(lOO), Cu(l1 l), and Cu(ll0) surfaces; XPS, UPS, LEED, and mass spectrometric data being combined to provide a self-consistent model. The surface species formed with their associated N(1s) and O( 1s) binding energies (eV) on exposing copper surfaces to NO(g) are listed in Table I. The presence of N,O at 80 K was confirmed in four ways; first by determining a “difference spectrum” between 80 and 120 K (when it desorbed). Two N(1s) peaks (Fig. 10) (one at 402 eV and the other at 406 eV) and one O(1s) peak (at 531 eV) were lost on warming; the intensities of the N(1s) peaks were identical. Second, the difference spectrum was shown to be identical with N,O molecularly adsorbed at 80 K on a Cu(ll1) surface. Third, a mass spectrum analysis of the gas phase on warming from 80 K showed the presence of N,O ; and last, helium-induced valence-level spectra at 80 K were consistent with a NO-N,O mixed adlayer (44,45). There were small but experimentally insignificant differences between binding energies observed with polycrystalline and different crystal planes of copper. We have assigned the two quite distinct N(1s) peaks at 399.5 and 401 eV to “bent” [NO(b)] and “linearly” [NO(/)] bonded molecular TABLE I Surface Species Formed and Their Associated N ( l s ) and O ( l s ) Binding Energies“ on Exposing Copper to NO( 9 )

In electron volts. Only one peak observed, but the full width half-maximum (FWHM) suggests presence of two distinct species. This is confirmed by two distinctly resolvable peaks in N(1s) spectra.

PHOTOELECTRON SPECTROSCOPY AND SURFACE CHEMISTRY 402eV

395

Loo

71

406&

605 (d)

530

535

5LO

FIG.10. (A) N(1s) and O(1s) difference spectra for NO adlayers on Cu(100) at 80 K and 110 K. (B) N(ls) and O(1s) spectra for N,O adsorption on Cu(ll1) surface at 80 K .

nitric oxide. We consider first the arguments that led us to conclude that there were present two distinct molecularly adsorbed NO species. 1. The O(1s) peak at -531 eV observed at 80 K had an unacceptably large FWHM value (- 3 eV) for a single species. 2. At low NO exposure at 80 K the first N(1s) peak appeared at 399.5 eV. 3. With increased exposure a second N(1s) peak appeared at -401 eV binding energy. 4. With time the 399.5 eV diminished in intensity; this was accompanied by an increase in the O( Is) intensity at 530 eV. 5 . Increasing the temperature of the surface (after exposure to NO at 80 K) to 140 K resulted in a single peak at -401 eV remaining [contributions to the N( 1s) spectral features from N,O(a) having been removed due to N,O desorption]; this proved that the N(1s) peak at 401 eV was not entirely to be associated with one of the nitrogens of N,O(a). 6. A single N( 1s) peak at 402 eV is observed on preoxidized nickel [with clean nickel at 80 K the N(1s) peak is at 399.5 eV]. The intensity of the 402-eV peak is both temperature and pressure dependent, increasing in intensity with decreasing temperature and increasing pressure (46).

-

-

The spectroscopic evidence for the existence of two molecular NO surface species is, therefore, unambiguous. The dynamic behavior suggests that the

72

M. W. ROBERTS

more strongly adsorbed species of lower N( 1 s) binding energy dissociates and the one of higher binding energy is more weakly adsorbed. This is entirely in keeping with the general observation in other studies of adsorbed species (CO, NH,, H20, 0 2 ,N2): The smaller the heat of adsorption, the larger the binding energy. Furthermore, the more extensively dissociated the species (e.g., NH, +NH2 -+NH +N), the smaller the binding energy (47-47c); the N(1s) values for the adsorbed species being about 400.5, 399, 398, and 397 eV, respectively. There is insufficient information available as to how exactly the charge in the NO(a) redistributes itself between the “nitrogen” and the “oxygen” atoms. With CO Fuggle (48) has brought up to date the O(1s) versus AH correlation (Fig. 6 ) and extended it to include the corresponding C(1s) data. Over the systems reported the values of the O(1s) binding energies decrease from 534 to 530 eV (except for one value at 537 eV), while the corresponding C(1s) values decrease from 287 to 283 eV. We speculate that the species of lower binding energy is the more strongly chemisorbed, bent form; i.e., the analog of the metal-nitrosyl complexes with metal-ligand angles of 109”and 120” for sp3 and sp2 hybridized states, respectively. Clearly we cannot distinguish on the basis of electron spectroscopy between bent and “bridge-bonded’’ NO. The species characterized by the higher N(1s) value is likely to have an electronic configuration closest to the free nitric oxide molecule, and we assign this to a bonding that is essentially unspecific (e.g., it occurs on preoxidized surfaces) and probably of a linear configuration with respect to the surface. Although it is recognized that surface coverage calculations based on XPS intensity data can only be accurate to & 15% of a monolayer, the calculation of total coverage based on each species present (0, N, and NO) nevertheless suggests that the two NO species must each be bonded to a single copper substrate atom (44). It would be difficult to accommodate bridged NO and still maintain 8 5 1.0 at 80 K. The experimental data with copper are reasonably clear-cut compared, for example, with our observations with both aluminum and iron. In the case of aluminum (lo), overlapping O(1s) peaks assigned to molecular NO, surface oxide, and N 2 0 made individual assignments impossible. However, desorption of the N20(a) by exposing the adlayer to H20(g) at 80 K, thus removing the two N( 1s) peaks characteristic of N20(a), established the presence of a molecular NO species in addition to nitrogen adatoms. With the hindsight of our results with copper and nickel (Figs. 9 and 11) it would be profitable to reexamine the interaction of nitric oxide with iron (40). Although there are no published data (which can be compared directly with the results obtained with nickel, iron, and aluminum surfaces), several studies with NO have been reported recently with platinum, ruthenium, and iridium (49,50). In the main, these have relied on electron energy loss spectroscopy N

PHOTOELECTRON SPECTROSCOPY AND SURFACE CHEMISTRY

73

n

N O (b)

395 400 405 410 r e v 1 FIG.11. Comparison of N( Is) spectra observed after NO adsorption: 1, clean nickel exposed to NO at 290 K ; 2, nickel preoxidized at 290 K, NO adsorbed at 80 K ; 3, nickel with chemisorbed oxygen layer present (formed at 80 K), NO also adsorbed at 80 K ; 4, clean nickel exposed to NO at 80 K.

(EELS) and LEED. In two of these studies, NO adsorbed on Pt(100) and Ru(OO1) surfaces, two states of adsorption have been detected. In the latter system, Thomas and Weinberg proposed a reaction sequence very similar to that proposed for copper, viz., the complete dissociation of a bridged form at 316 K while the linear form remains on the surface until 500 K. In other words, it is the species formed first which dissociates, the activation energy for dissociation of the linear species being appreciably higher. With Pt(100) Pirug et al. interpret the single species they observe with the unreconstructed Pt(100) 1 x 1 surface as a molecularly adsorbed species in the bent configuration. With the reconstructed 5 x 20 surface a second species is observed which the authors suggest may be linearly bonded NO. The involvement of NO present in a pairlike formation (as distinct from dimers) is also considered. C. NITROGEN The adsorption of nitrogen on tungsten (51) was the first “nitrogen” system to be studied (Fig. 12) by XPS; subsequently nitrogen interaction with iron was studied (52, 53), and two distinct N(1s) peaks were observed at 80 K, one at about 405 eV and the other at 400 eV. At room temperature with both single crystals and polycrystalline iron surfaces only a single peak is

74

M. W. ROBERTS

rn t-

Z 3 0 0

I

;0

0L5

040

e;5

,

860

ELECTRON K I N E T I C ENERGY I eVl

Torr, FIG. 12. N(1s) spectra for N, adsorbed on tungsten: (a) after exposure (5 x 60 sec) to N, at 100 K; (b) after N, adsorption at 100 K the sample was heated to 300 K in uucuo and recooled to 100 K ; (c) after exposure to N, at 300 K (5 x lo-’ Torr, 60 sec); (d) clean tungsten.

present, and this is at a binding energy of about 397 eV; there is no doubt that this reflects the presence of nitrogen adatoms. The assignment of the two higher binding energy peaks is more problematical, but one model that has been proposed (53)is that they reflect two molecularly adsorbed nitrogen species which are essential for the formation of chemisorbed nitrogen adatoms. The intensity of both peaks is observed to decrease with simultaneous enhancement of intensity in the 397-eV spectral region, which strongly suggests a precursor model. Furthermore, if N adatoms can only form via these molecular states, then since their heats of adsorption are no more than about 20 kJ/mol we can compare the residence times of the molecular species at 300 and 80 K. At 300 K the value is about lo8 times smaller, which may account for the very low sticking probability ( lo-’) value reported by various groups for nitrogen chemisorption on iron at room temperature. Relevant to the synthesis of ammonia over iron catalysts is the observation of Ertl et al. (54) that potassium preadsorbed by an iron catalyst (oK 2: 0.1) increased the rate of synthesis at 430 K by a factor of about 300. This effect the authors attributed to an enhancement of the heat of adsorption of molecular nitrogen due to transfer of electronic charge from potassium to the surface of the iron catalyst. This would be entirely in keeping with the precursor model proposed for nitrogen chemisorption (53). Fuggle and Menzel (55) have studied nitrogen adsorption on W(110). N

PHOTOELECTRON SPECTROSCOPY AND SURFACE CHEMISTRY

75

They, in contrast to Kishi and Roberts (52) and Johnson and Roberts (53), interpret the N(1s) peak at 405 eV as a satellite of the “400-eV peak;” moreover Fuggle and Menzel observe two components to the 400-eV peak, one at 399.1 eV and the other at 400.4 eV. These they attribute to the two nonequivalent nitrogen atoms in a vertically adsorbed N, molecule and draw the analogy with the splittings observed in dinitrogen complexes of transition metals. The question that is immediately raised is the interpretation of the relatively high intensity of the satellite peak, some 60% of the main peak. Fuggle et al. (56) have considered this point.

D. OXYGEN The chemisorption of oxygen on metals was attractive to study by electron spectroscopy since, in principle, XPS offered the possibility of distinguishing oxygens in different chemical environments : oxygen adatoms, incorporated oxygen, oxide, molecular oxygen species, etc. Earlier data from work function and photoemission studies and in particular electron energy distribution results (2) were gounds for optimism. In general, the initial X P S results were somewhat disappointing for two reasons : first, the apparent insensitivity of the O(1s) binding energy to the oxygen environment; and second, the insensitivity of the metal core electrons to oxygen interaction. Oxygen adatoms were found to be usually characterized by O(1s) values of about 530 eV, whereas more weakly, possibly molecularly adsorbed species, had substantially higher binding energies. This certainly appeared to be the case of oxygen interaction with Cu(100) surfaces at 80 K (15). The important question for heterogeneous catalysis, particularly in the context of the selective oxidation of hydrocarbons, was whether molecularly adsorbed oxygen species could be observed under conditions where catalysis occurred. This raised experimental problems that could only be overcome by a new design of electron spectrometer (see Fig. 3) because it is vital that spectra be obtained in the presence of oxygen at, say, 1 Torr, i.e., some lo4 times higher than is possible in conventional electron spectrometers (e.g., ESCA-3). By monitoring the surface at relatively high oxygen pressure both weakly and strongly adsorbed species could be observed spectroscopically. The first system investigated was the Ag + 0,; and the advantages of the “high pressure” spectrometer became immediately obvious (16) since by studying the dynamics of oxygen interaction at about 500 K three distinct stages of oxygen interaction were delineated : oxygen chemisorption, followed by oxygen incorporation leading to oxidation, and finally molecular adsorption. The molecular oxygen was characterized by an O(1s) value of -532.5 eV, i.e., well away from the chemisorbed oxygen value at 528.3 eV (see Fig. 5). By studying the “molecular species” at two different electron take-off angles, and

76

M. W. ROBERTS

also using UV photoelectron spectroscopy, it was established that the 532.5eV peak was due to a surface species probably of the peroxidic kind. The concentration of the molecular species is estimated to be about 1 x 1014 cm-’ at an oxygen pressure of 0.5 Torr. The conclusions from this electron spectroscopic study are very similar to those of Kilty et al. (57) who drew on kinetic, uptake, and infrared data. An interesting observation with bismuth (58) has revealed the generation of a second surface oxygen species (Fig. 13) formed in situ from chemisorbed oxygen at 145 K [O(ls) N 529.6 eV] under the influence of the x-ray photons. This is the only example where the x-ray flux has been reported to have a direct influence on the nature of chemisorbed species on metals, in this case the generation of a second species at the expense of the first. The second species [O(ls) i~ 532.8 eV] is unstable in that it can be thermally activated back into its precursor, the initial chemisorbed state

I

I

I

I

I

526

528

530

532

53L

eV

FIG.13. O(1s) spectra after exposure of a clean bismuth surface at 145 K to oxygen at 0.1 Torr pressure as a function of time of exposure. to x-ray flux. The spectra in ascending order correspond to (a) the clean surface; (b) after 10’ L exposure to oxygen; (c), (d), (e), and (f) after increasing times of exposure to the x-ray flux; (g), (h), and (i) during warming of the adlayer to 290 K.

PHOTOELECTRON SPECTROSCOPY AND SURFACE CHEMISTRY

77

(Fig. 13), the total O(1s) intensity during formation and decay remaining constant. The process can only be observed at low temperature presumably because the surface lifetime of the x-ray generated chemisorbed state decreases with increasing temperature. The most likely assignment of the . analogy second species with an O(1s) binding energy of 532.8 eV is 0 2 -The with our observations with silver is obvious. The bismuth results highlight the interplay between more than one chemisorbed oxygen species which in heterogeneous catalysis will be influenced by surface considerations alone. But the direct observation of controlled interconversion between two species (albeit using photons) is relevant to a better understanding of selective oxidation reactions. We shall not discuss here oxygen interaction with nickel since, although it is one of the most extensivelystudied systems (59),there is still considerable ambiguity and confusion regarding possible models. Two systems that, however, have lent themselves to investigation by XPS are the oxidation of lead (60, 604 and aluminum surfaces (20). In both cases substantial shifts were observed in the metal core levels, which as emphasized previously is more the exception than the rule.

F

E

D

C B

70

75 BINDING ENERGY / P V

BINDINGENERGVI C V

-

FIG. 14. (a) AI(2p) during exposure of aluminum to oxygen at 290 K : A, clean surface; B, 12 L; C, 24 L; D, 30 L; E, 78 L; F, saturation at oxygen pressure of Torr. (b) Deconvolution of AI(2p) spectrum F, removing instrumental and x-ray broadeningfrom the raw data.

78

M. W. ROBERTS

With aluminum the 2p level is shown to exhibit a shifted component at a binding energy of -74.5 eV after oxygen interaction at 80 K. A curve fitting and deconvolution analysis of the shifted peak indicate (Fig. 14) that it is made up of two components, one (designated 01) at a binding energy of 74.0 eV and the other (p) at 75.3 eV. The LY component develops preferentially at 80 K, whereas B which we believe reflects the formation of A1,0, dominates at 290 K. Comparison of the development of the O( 1s) intensity at 80 and 290 K indicates that c1 is a kind of precursor state, the rate of oxygen uptake at 80 K being about four times faster than at 290 K. Confirmation of the existence of the two components (LY and p) to the Al(2p) “oxide peak” is available from the high resolution variable excitation energy electron spectroscopy data of Flodstrom et al. (61). Data obtained at room temperature indicated at low oxygen exposures an Al(2p) peak shifted by about 1.3 eV from the metal peak, whereas at higher exposures (200 L) a second peak shifted by 2.4 eV dominates. The presence and quenching by oxygen of plasmon loss features associated with the Al(2s) peak facilitated (10) the interpretation of the AI(2p) core-level spectra. The O(1s) data were comparatively noninformative regarding the details of the chemisorption, incorporation, and oxidation processes due to overlapping peaks; they did, however, provide information on the stoichiometry of the oxide formed and support for conclusions based on the Al(2p) data. With polycrystalline lead, Pb(100), and Pb(ll0) surfaces, the Pb(4f7,,) core-level peak became asymmetric during oxygen exposure (60, 60a), the asymmetry appearing at a binding energy about 0.9 eV higher than the Pb(4f,,,) peak for the clean metal. This was in keeping with the initial “as obtained” surface (i.e., before argon ion bombardment), where the Pb(4f) doublet showed two distinct peaks. The more intense peak is initially at the higher binding energy (owing to the presence of oxide), but with progressive ion bombardment it becomes the minor component and finally disappears when the surface is clean. An interesting feature of the progressive oxidation of lead is the development of a single O(1s) peak with a binding energy of 529.5 eV which is invariant. The full width half-maximum (FWHM) value of the O( 1s) peak is only 1.4 eV, which also does not change during oxidation. A value of 1.4 eV is unusually small suggesting that only one kind of “oxygen” is present during oxide growth. The LEED evidence (60,604 for both Pb(100) and Pb(ll0) was interpreted as the growth of orthorhombic PbO virtually from the lowest exposures studied (< 100 L). This is in keeping with the known facile incorporation of oxygen by lead indicated by work function data. The PbO structure does not change during oxide growth, and the LEED pattern retains its high intensity. This is unusual for low temperature metal oxidation and is indicative of a highly ordered structure being

PHOTOELECTRON SPECTROSCOPY AND SURFACE CHEMISTRY

79

retained up to about 9 A. The oxide appears to grow as four domains with dimensions very close to that of bulk PbOo,,ho,the essential difference being that the domains are not precisely rectangular (0 = 89"). The interaction of oxygen with Cu( 100) contrasts (15) very obviously with the behavior observed with lead (60,604 and aluminum (10). At 80 K the O(1s) peak after 20 L exposure to oxygen is centered at 530.5 eV, the FWHM value is exceptionally large (-4.5 eV), and there is only weak ordering within the 2 x 2 adlayer as reflected by LEED. After 300 L exposure the diffraction pattern exhibits ( $ x $)R 45" symmetry, and the O( 1s) peak is now centered at 531.7 eV. At 290 K, the clean surface transforms through a "4-spot" pattern through the ($2 x f l R 45" pattern to give at high exposure ( > 2000 L) two domains of a ( p x 2 f l ) R 45" structure. The O(1s) value is between 530.5 eV initially and 531.1 eV at the highest oxygen exposure with a comparatively small FWHM value of -2.4 eV (cf. 4.5 eV at 80 K). The ($ x p ) R 45" structure is interpreted as a chemisorbed layer of oxygen adatoms bonded in positions of maximum (fourfold) coordination and sitting "on top" of the Cu(100) unreconstructed surface. In other words, there is a distinct chemisorbed adlayer formed. The ( f lx 2 4 2 ) R 45" mesh (which forms next) is considered to reflect reconstruction of the surface. Using the satellite structure (15, 62) associated with the Cu(2p3,,) peak as diagnostic evidence for the presence of Cu(I1) or Cu(1) species, it was possible to conclude that the ($z x 2 G ) R 45" structure was due to the formation of Cu,O (no shake-up satellitespresent), but for more extensive oxidation ( 5 Torr at 290 K) evidence for the formation of CuO (shake-up satellites observed) was obtained. The LEED pattern showed no evidence for an ordered structure although heating to 520 K in uacm regenerated the ( P x 2 P ) R 45" structure and the disappearance of the satellites, i.e., the thermally induced conversion of CuO to Cu,O had occurred. LEED has also provided some interesting ideas on the mechanism of oxygen chemisorption on a more open plane of Cu, the (210) surface (63). By combining LEED with Auger electron spectroscopy and using optical methods for the simulation of the LEED patterns, the way in which the highly defective adlayer is formed at room temperature has been investigated. The LEED pattern after exposure to oxygen (< 2 L) at 290 K showed streaks running parallel to the clean surface (120) direction, but with some concentration of intensity in the (h + 3,k) positions. The only model that satisfactorily simulated the diffraction pattern was one where the number of dissociation sites was restricted to a few ( w 1%) special sites. Furthermore the diffusion of the dissociated oxygen adatoms had to occur over comparatively large distances ( > 100 A) before becoming chemisorbed.

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M. W. ROBERTS

VI. Chemisorption of More Complex Molecules

A. AMMONIA AND HYDRAZINE N(1s) spectra have provided very clear evidence for the nature of adsorbed ammonia and hydrazine (47-47c, 52,64): whether adsorption is molecular, or if dissociated, the extent of the dissociation, i.e., whether the surface species are NH,, NH, or N. Although no high resolution studies have been reported for ammonia interaction with iron surfaces, two main states of adsorption were recognized. At 80 K adsorption is entirely molecular with a characteristic N( 1s) binding energy of 400 eV, but on warming the adlayer to 290 K the N( 1s) intensity is mainly at 397 eV, typical of chemisorbed nitrogen adatoms with only a small contribution at 400 eV. Studies of hydrazine adsorption on iron were particularly rewarding in that a variety of states were observed: N,H,, NH,, NH, and N. These surface species had characteristic N(1s) values of 400.5, 399, 398, and 397 eV, respectively. Furthermore, it was possible to delineate temperature regimes over which first N-H cleavage took place (85-233 K) and then N-N cleavage occurred (> 233 K). There were strong analogies between the results with . was, howiron (47, 47b) and those observed with aluminum ( 4 7 ~ )Copper ever, very much less active in that mainly the molecular states of hydrazine and ammonia formed. However, preexposing the copper surface to oxygen (42) led to enhanced reactivity toward ammonia: The O(1s) intensity decreased to zero during exposure to ammonia and an N(1s) peak developed at 398 eV. From the O(1s) intensity lost and the N(1s) intensity gained it could be shown that one oxygen atom was replaced by a nitrogen atom indicating that the following reaction occurred :

--

NHdg) + o(a) + NWa) + H 2 0 W

Clearly, strong 0-H interaction occurs leading to dissociation of ammonia, formation of OH groups, dehydroxylation, and surface imide formation. The NH(a) species has a characteristic N(1s) value of 398 eV, i.e., 1 eV greater than N(a) and 1 eV less than NH,(a). B. WATER Three distinct types of behavior with clean metal surfaces have been observed : Type 1. Examples of this category are gold (3, 3 4 22, 65), copper (66), and lead ( 6 6 4 . Adsorption is only observed at low temperature (80 K) and

PHOTOELECTRON SPECTROSCOPY AND SURFACE CHEMISTRY

81

is molecular in character. The O(1s) binding energy is -533 eV. There is little or no evidence for any interaction at room temperature. Hydrogen sulfide is known to behave in an analogous manner with lead. Confirmation of the molecular nature of water was obtained from helium spectra (23,65). Type 2. An example of this is molybdenum (22), where interaction is clearly dissociative [O(ls) value of 530.3 eV] at 295 K. Further exposure to water vapor results in molecular adsorption [O(ls) value -533 eV]. Type 3. In this category dissociation of the water molecule is only partial and surface OH species dominate. Cobalt is an example (67). Iron appears (68) to form a mixed oxyhydroxide adlayer analogous to FeO.OH. This renders the surface unreactive to further attack by O,(g), even though it is estimated to be no more than a single monolayer. Oxidation of iron by O,(g) is well known to be multilayer at 295 K. Similar data have also been reported by Gimzewski et al. (69)with iron. In the case of Cu(100) and (Cu(ll1) surfaces (66) XPS showed that an adlayer formed at 80 K [O(ls) 2: 533.5 eV] desorbs rapidly at about 150 K indicating a heat of adsorption of about 34 kJ/mol. There is evidence for some hydroxyl species present at 290 K characterized by an O(1s) value of

B.E./eV ____)

FIG. 15. Curve-fitted O(1s) spectra: A, Cu(ll1) surface after exposure to 300 L oxygen at 290 K ; B. after exposure of A to water vapor (1.5 L at 80 K); C, D, E, and F during warming to 290 K .

82

M. W. ROBERTS

531.5 eV. The concentration of OH species at 295 K is estimated to be about 0.14 x lOI5 cm-, compared with the atom density of the Cu(ll1) plane of 1.76 x lOl5 cm-’. If the Cu( 111) surface is pre-exposed to oxygen at 290 K and subsequently to water vapor at 80 K, two O(1s) peaks are present, one at 530 eV (due to chemisorbed oxygen) and the other at 533.3 eV (due to molecularly adsorbed water). After first carrying out deconvolution removing the background intensity and instrumental broadening, the curve-fitted spectra obtained by a least squares procedure are shown in Fig. 15. As the molecular adlayer is warmed to 290 K the profiles of the spectra change with a third component [O(ls) 2: 531.5 eV] emerging at 173 K, apparently at the expense of the 533-eV peak. Data are shown for 153, 173, 220, and 290 K. Estimates of the concentration of the three species O(a), H,O(a), and OH(a) as a function of temperature indicate that at 173 K about half of the chemisorbed oxygen species have been “hydroxylated,” but above this temperature dehydroxylation occurs with the concomitant replenishment of the chemisorbed oxygen. At 290 K the surface is composed of 0.35 x oxygen adatoms cm-2, and 0.1 x lot5 hydroxyl species cm-’. Activation of the 0-H bonds was also effected by exposing H,O(a) on clean Cu(ll1) to NO(g) (66). The influence of chemisorbed oxygen on the subsequent interaction with water vapor is significant for heterogeneous catalysis and is analogous to the interaction of hydrogen sulfide with oxidized lead (60). In the absence of chemisorbed oxygen Pb( 100) surfaces are inactive in the chemisorption of H,S; however, a surface sulfide is formed readily in the presence of chemisorbed oxygen, the oxygen being desorbed as H20(g). These are examples of surface processes being induced by strong 0-H interaction leading to the activation of bonds (H-0 in H,O and H-S in H2S) which otherwise would remain unreactive. Similar phenomena have been observed with ammonia adsorption on preoxidized copper (42); in this case activation of N-H bonds occurred leading to the formation of a copper “immide” surface. In the absence of chemisorbed oxygen there was little evidence of ammonia interaction with copper at room temperature. C. FORMIC ACID Interest in studying formic acid adsorption on metals by XPS and UPS was stimulated largely by its use as a “probe molecule” for investigating the role of the electronic factor in heterogeneous catalysis as in the work of Schwab (70), Dowden and Reynolds (711, Eley and Leutic (72), and Fahrenfort et al. (73). The advantages of XPS and UPS are fourfold.

PHOTOELECTRON SPECTROSCOPY AND SURFACE CHEMISTRY

83

1 . The density of electron states close to the Fermi level can be monitored directly. 2. The surface species present including their concentration can be observed through their core-level spectra. 3. The molecular nature of the surface species can be studied by ultraviolet photoelectron spectroscopy. 4. Assignments of experimentally observed orbital energies for HCOOH(g) were available.

By choosing the three metals, copper, nickel, and gold, two of them (Cu and Au) with low density of states close to the Fermi level (EF) and one (Ni) with a high electron density of states close to EF, the significance of the “density of states concept” could be explored (74). At low temperature (80 K) formic acid condensed forming a hydrogenbonded molecular solid; but on increasing the temperature to 120 K, the hydrogen bonds break; and at about 160 K a proton splits off to give a formate ion. With gold the formate ion is stable over only a limited temperature range, decomposing below 200 K to give gaseous products; in the case of nickel the formate decomposed at -300 K ; and with copper the formate was stable up to -400 K. Activation energies estimated for the decomposition of the three surface formates (Au, Ni, and Cu) were 50-60, 90, and 1 15- 130 kJ/mol, respectively. These values agree with published data for the catalytic reactions (55,85, and 110 kJ/mol) based on monitoring the gas phase reaction: HCOOH(g)

-

CO&)

+ HZW

No evidence was obtained for any correlation between activity and the density of states at EF (74). When we turn our attention to the catalytic decomposition of formic acid over lead and copper “oxides” we see (75) analogies with our observations for water, ammonia, and hydrogen sulfide, i.e., activation of bonds arising from strong 0 - H interactions. With PbO (Fig. 16) at 190 K and above the surface after exposure to HCOOH(g) at 80 K has on the basis of XPS data the structure PbOH.HCO0 (a hydroxy formate), but on further exposure to formic acid at 295 K regeneration of the “clean” Pb(100) surface occurs [Eqs. (1 1) and (12)]. These conclusions are supported by the observation that the clean Pb( 100) surface does not chemisorb formic acid at 295 K, which is comparable with the behavior of gold. It is significant that with oxidized Pb( 100) surfaces comparatively large shifts (- 1.7 eV) were observed in the Pb 4f7,, peak resulting from the formation of the surface hydroxy formate (Fig. 16). These shifts were reversible in going from metal to oxide to formate

Oils1

COOH (a)

1

1

PblLfl Ib)

I

I

526.0

5300

53L.O

137.0

538.0

BINDING ENERGY lev

A

I

I

I

1L1.0

I

1L5.0

BINDING ENERGY lev PblLfl

Olls) AT 295K FCR PbO + HCOOHlgl

/

2 X L -

‘Pb’~ 7 L

1300L

6

526.0

530.0

53L.O

BINDING ENERGY l e v

I

I

,

137.0

1L1.0

1L5.0

BINDING ENERGY I cV

FIG. 16. (a) O(1s) spectra for oxidized lead (1) and on exposure to formic acid vapor at 80 K followed by warming adlayer to 295 K; (b) Pb(4f) spectra corresponding to (a); (c) O(ls) spectra for PbO and during exposure to HCOOH(g) at 295 K ; (d) Pb(4f) spectra corresponding to (c).

PHOTOELECTRON SPECTROSCOPY AND SURFACE CHEMISTRY

85

and back to metal (Fig. 16). With oxidized copper, a CuOH.HCO0 surface forms, but this is stable at 290 K and only after heating to -500 K is the clean copper surface regenerated. XPS and LEED confirmed the regeneration of the clean surface. PbO

+ HCOOH(g)

--+

PbOH.COOH

PbOH.COOH

+ HCOOH(g)

--+

Pb

+ H,(g) + CO,(g) + H,O(g)

(1 1) (12)

Temperature programmed desorption studies of formic acid decomposition by metals was reviewed recently by Madix (76); the significance of formate formation is paramount to the discussion. This is also apparent in the recent electron energy loss spectra of formic acid adsorption on Cu(lO0) reported by Sexton (77). D. HYDROCARBONS The first electron spectroscopic study of adsorbed hydrocarbons was that reported by Eastman and Demuth (78)who used He radiation to probe the valence electrons of benzene, acetylene, and ethylene. Figure 17 shows the difference spectrum of C2H, adsorbed on Ni(lI1) at 100 and 230 K compared with the results of Clarke et al. (79)for ethylene adsorption on Pt(100) at 290 K, propylene adsorption on Pt(100), and ethylene adsorption on Pt( 1 11). At 100 K the spectrum with Ni( 111) could be interpreted in terms of a

FIG. 17. Photoelectron spectra of metal-alkene surfaces: (a) Ni(lll)-C,H, at 100 K difference (21.2 eV) spectrum; (b) Ni(1 11)-C,H, difference (21.2 eV) at 230 K ; (c) Pt(100)-C,H4 difference (40.8 eV) spectrum at 290 K ; (d) Pt(lOO)-C,H, at 290 K, 40.8 eV radiation; (e) Pt(1 1 1)-C,H, difference spectrum, 40.8 eV radiation.

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M. W. ROBERTS

“shifted C2H, spectrum,” whereas on warming dehydrogenation occurs with the spectrum at 230 K having strong similarities to that of a Ni( 111)C,H2 surface. In the case of platinum there was little evidence for dehydrogenation, the surface species being mainly ethylene and propene. On the other hand, with W(110) at room temperature Plummer el al. (80) suggest that dehydrogenation of ethylene leading to “acetylenic” residues occurs readily. These are the species implicit in the ideas of Jenkins and Rideal(81) and Trapnell(82) based on kinetic studies with nickel and tungsten, respectively. Lewhald and Ibach (83) have used EEL spectroscopy to study hydrocarbon reactions at stepped and flat Ni( 111) surfaces, and again we see the general conclusions of Somorjai (84) regarding the high and unique reactivity of stepped sites being sustained. Mason and his colleages have obtained detailed information on the adsorption of fluorinated alkenes on platinum single crystals using XPS combined with LEED (39). The results on Pt(100) and Pt(ll1) can be summarized as follows : The vinyl halides are dissociated extensively; the difluoroalkenes either are adsorbed molecularly or dissociate according to the isomer; and the dichloroalkenes are molecularly adsorbed. Apparently dissociation is more extensive on the close-packed platinum( 111) surface. The implications of these and other experiments, particularly with relation to possible surface elimination reactions, of whether a Hinshelwood-Langmuir or Eley-Rideal reaction is involved are discussed by Mason and Textor (39). Using the same experimental approach that was used in the studies of benzene adsorption, etc. (Fig. 17), Rubloff et al. (85) studied the dehydrogenation of cyclohexane on clean Pd surfaces. With both polycrystalline and Pd( 111) surfaces at low temperature ( 5120 K) cyclohexane was adsorbed molecularly, but at 300 K it dehydrogenated to leave chemisorbed benzene. Benzene itself adsorbed without decomposition. Recent electron energy loss spectra reported by Bertolini and Rousseau (86) for benzene adsorption on Ni(ll1) and Ni( 100) surfaces at room temperature have been interpreted in terms of a molecular species .n bonded and parallel to the plane of the surface. The adsorption and decomposition of benzene and pyridine has also been investigated by XPS (86a). With oxide surfaces the Krakow group (87) have reported interesting information on propionic acid interaction with oxides of molybdenum (MOO,, MOO,, and CoMoO,). There are two distinct C(1s) peaks, one at 285.8 eV (due to the C2H, group) and the other at 289.8 eV (due to the carbon of the COOH group); the intensity of the former remains almost unchanged on heating, whereas the latter decreases to zero. These observations are explained by the suggestion that heating simultaneously effects two pro-

PHOTOELECTRON SPECTROSCOPY AND SURFACE CHEMISTRY

87

cesses : desorption of the “acid molecules” and their decarboxylation, that is, CH,CH,COOH(a) -+ CO,(g)

+ 2H(a) + C,H,(a)

leaving at the surface C,H, species. A further conclusion from the XPS data, including the Mo(3d) peaks, is that reduction of the surface favors decarboxylation. One of the most interesting questions relevant to the catalysis of hydrocarbons over the last decade has been the question of the participation of allylic species. Such species may result from an alkene by hydrogen abstraction. What evidence do we have from electron spectroscopy for the activation of a carbon-hydrogen bond in the adsorbed state? We consider the influence of a “surface oxide” on the reactivity of a metal surface: A number of very striking examples now exist where preexposure to oxygen enhances the reactivity of the surface through the activation of the adsorbate. We recall the evidence already discussed (42,60,60a,66,664 75) for NH,(a), H,S(a), H,O(s), and HCOOH(a). The interaction of ammonia with atomically clean cu( 1 1 1) and Cu( 100) surfaces is extremely weak (88) (desorption occurs above 150 K) with little evidence for any significant interaction at 295 K. Preadsorbing oxygen, however, results through hydrogen abstraction in the desorption of water and the formation of immide (NH) species [see also Matloob and Roberts (42)]. Hydrogen sulfide behaves in an analogous fashion with Pb(100). It does not interact with the clean surface at 295 K, whereas oxidized lead (PbO) forms PbS with H,S at the same temperature. Clearly the S-H bond is activated through strong 0 - H interaction. Water adsorption on Cu( 111) is molecular at 80 K and desorption is virtually completely at 150 K. However, with preoxidized copper there is evidence for hydroxylation at 150 K, again an indication that strong 0-H interaction leads to weakening of the hydroxyl bond in the molecularly adsorbed water. Lastly, formic acid interacts only very weakly with Pb(100) surfaces, but PbO formed by oxidizing a Pb(100) surface adsorbs HCOOH readily, and the surface after warming from 80 K to 295 K is considered to be PbOH.COOH, a lead hydroxy formate [(Eq. ll)]. The Pb(4f) spectrum shows a very distinct “new peak” shifted by about 1.8 eV from the clean metal peak and at a binding energy nearly 1 eV greater than what is observed after the oxidation of lead (Fig. 16). It is clear that “PbO” activates the carbonhydrogen bond in formic acid with the formation of the stable hydroxy formate. The interaction of this surface with excess formic acid vapor at 290 K generates the clean Pb(100) surface, obvious from both LEED and XPS data, presumably because Pb(I1) formate is unstable. The relative stabilities of the formates of gold, copper, and nickel have been discussed

-

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M . W. ROBERTS

recently (74). What is common to all these studies (H,S, NH3, H,O, and HCOOH) is the influence of chemisorbed oxygen on the reactivity of the metal surface. Recently, analogous phenomena have been reported (74a) for acetic acid adsorption and discussed in terms of acid-base reactions. The relative inactivity of the clean surface (compared with the “oxidized” surface) suggests that a precursor adsorbed state involving hydrogen bonding, leading to enhanced surface life time of the molecular species and eventually leading to dissociation may well be the common and unifying feature. Evidence for the activation of the carbon-hydrogen bond in ethylene is apparent (88) from XPS studies of the interaction of C,H, with Cu(ll1) and preoxidized Cu( 11 1) surfaces. Figure 18 shows the curve fitted O( 1s) spectra for an oxidized Cu( 11 1) surface, after exposure to C,H,. There is a clear indication of the development of a strong O( 1s) component with a binding energy of 531.5 eV after exposure to C,H, at 373 K. That this peak is due to OH-like species is now established (88). There is associated with this O(1s)

0’

0

cu

cu

I

I

peak a C(ls) peak at 285.5 eV so that we may have a surface configuration akin to which is stable at 373 K. We have, therefore, experimental evidence for the occurrence of the hydrogen abstraction reaction C,H,(g) + O(a) --t C,H,(a) + OH(a). Whether the process proceeds to C,H, through the type of interaction envisaged above can only be deduced from more detailed studies. These results also raise a number of interesting general questions related to the stability of surface species when more than one kind of species is present. For example, “OH” groups on copper are unstable above 200 K, whereas CuOH. COOH is stable above room temperature ; hydroxylation of lead by H,O(g) is not feasible, whereas PbOH+COOHis stable at the same temperature in uacuo. The analogy between the surface hydroxy formate (Fig. 16) and ethylene interaction with “CuO” is striking. VII. Metal Oxides

Tungsten oxide offered a number of interesting challenges to electron spectroscopy : the possibility of observing various valence states of tungsten and relating these to conductivity, electrochromic effects, and catalytic properties, for example. Studies of amorphous tungsten oxides by XPS have been reported by Hollinger et al. (89) and Haber et al. (90,91); whereas

PHOTOELECTRON SPECTROSCOPY AND SURFACE CHEMISTRY

lb)

I

I

1

i\ /

\

"

ld I

la)

I 1 525

89

1

530

1

,

535

FIG. 18. O(1s) spectra for (a) Cu(l11) surface exposed to oxygen at 295 K ; (b) after exposing (a) to C,H, at 295 K ; (c) on warming (b) to 373 K; and (d) after further exposure to C,H,(g) at 313 K.

in the work of Salje et al. (92) grown single crystals of WO, and also W,Mo,-,O, were used. For pure WO, the fine structure of the W(4f) peak shows considerable changes after reduction of the crystal. With the nonreduced stoichiometric crystal only two levels 4f7,, and 4f5,, are observed, but after reduction the superposition of two doublets induces broadened features. Shifts due to the transformation of W6+ to W5+ are revealed by a curve fitting procedure; however, more drastic reduction can only be effected by hydrogen and argon ion-bombardment when W4+ and Wo states appear (Fig. 19). Studes of three different oxides of niclel (93) prepared by preheating at 700, 1100, and 1450°C with known defect concentrations, surface morphology, and purity showed characteristic surface charging when examined by XPS. The magnitude of the experimentally observed charge could be related to the defect nature of the particular oxide, being greatest (-4 eV) for the most perfect oxide (Ni01450)and least (- 1 eV) for the most defective oxide (Ni0700).Charge removal could be effected by increasing the sample temperature, for example, with Ni01450 the surface charge is +4.3 eV at 20"C, but only +1.5 eV at 125°C. The charge observed with

90

M. W. ROBERTS INT

C

B

A

31

37

3L

LO

$C.V)

FIG.19. W(4f) spectra from,a WO, sample A, as prepared; B, after argon ion bombardment at 300 K for 30 min; C, same as for B, but taken with a reduced take-off angle; D, after further extensive bombardment (1 50 min).

Ni01450appears to be independent of temperature below room temperature and above 100°C with a linear region of log (charge) versus 1/T between 30 and 100°C. The process of charge removal is clearly thermally activated and intimately related to the electronic structure of the nickel oxide. An interesting consequence of the above relationship between surface charge and the defect nature of the oxide was the observation that the chemisorption of such molecules as carbon monoxide or nitric oxide could also influence the surface charge. Nitric oxide was shown to act as an electron acceptor, whereas carbon monoxide behaved as an electron donor, these characteristics being reflected in removal and enhancement, respectively, of the surface charge (Fig. 20). Again a correlation was shown between defect nature and charge redistribution after chemisorption. We have, therefore, a direct method of monitoring changes in “surface charge” resulting from adsorption on semiconducting oxides with, at the same time, complete characterization of the surface in question. The thermally induced interconversion of two oxides, e.g., Co,O, and COO, and Cu,O and CuO, has been followed by electron spectroscopy (15, 67). In both cases the “shake-up” satellites associated with the C 0 ( 2 p ~ , ~ )

PHOTOELECTRON SPECTROSCOPY AND SURFACE CHEMISTRY

960

KINETIC ENERGY lev)

91

950

FIG.20. O(ls) spectra for NiO,,,, and NiO,,,, before (broken line spectra) after exposure (solid line spectra) to NO@) at 290 K. The respective O(1s) shifts are shown.

peaks are very different. With Co,O, the satellite intensity is only about 20% of the main Co(2p) line, whereas with COOit is between 70 and 80%. Such differences enabled a better understanding of the oxidation of cobalt, when for low oxygen exposures at 290 K COOformed and at high oxygen exposures at the same temperature C o 3 0 , developed. The latter could be converted to COOby heating in uucuo. Chuang et al. (94) have also reported XPS and UPS data for cobalt oxides, and Haber and Ungier (95) have studied both their Auger and photoelectron spectra. The latter authors pay particular attention to the role of the relaxation energy in determining Co(2p) peak positions.

VIII. Alloys and Surface Segregation

Current views on the surface enrichment of one component over another in alloy systems are, surprisingly, more a consequence of “gas titration” and Auger electron spectroscopy than XPS and UPS. There is little doubt, however, that looking to the future XPS will provide important clues regarding the mechanism of bimetallic catalysts, the significance of promoters,

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M . W. ROBERTS

and the onset of surface segregation induced by the catalytic reactions in question. We have seen (54) how potassium present at a concentration of about 1.5 x 1014 atoms cm-2 increases the rate of nitrogen chemisorption on an Fe( 100) surface by a factor of 300 at 430 K. Also the surface segregation of potassium, present at a bulk concentration of less than 1 in lo6 in nickel, has been shown (94) to occur readily during the formation of a 25 A thick oxide film. At about 250°C the surface concentration of potassium is estimated to be about 2 x 1014 cm-2. Again in studies (96) involving indium-doped zinc oxide, most of the indium segregated to the surface rather than being uniformly distributed throughout the bulk. The experimental and theoretical problems associated with bulk versus surface compositions have been reviewed by Spicer et al. (38). IX. Conclusion

The significance of the development of photoelectron spectroscopy over the last decade for a better understanding of solid surfaces, adsorption, surface reactivity, and heterogeneous catalysis has been discussed. The review is illustrative rather than exhaustive, but nevertheless it is clear that during this period XPS and UPS have matured into well-accepted experimental methods capable of providing chemical information at the molecular level down to 10% or less of a monolayer. The information in its most rudimentary state provides a qualitative model of the surface; at a more sophisticated level quantitative estimates are possible of the concentration of surface species by making use of escape depth and photoionization crosssection data obtained either empirically or by calculation. The spectroscopic results have shown the subtle interplay between molecular and dissociated states of adsorption, and the different surface bonding configurations for a given molecule; enabled dynamic studies to be made of simple catalytic processes ; explored the role of surrface impurities either as promoters or poisons; and shown the range of valence states present at the surface of a reduced metal oxide. The accomplishments are impressive when cognizance is taken of the fact that no suitable electron spectrometer capable of surface studies by XPS and UPS was available in 1970. One fact already established is that considerable ambiguity can be removed in developing models for surface reactions by the application of electron spectroscopy, and this is central to any advance in our understanding of heterogeneous catalysis. It is, however, disappointing that reports of studies of real catalytic systems have, to date, not been very prevalent. There is no question that a firm basis now exists for such work and the near future may well see an upsurge of activity on the “real catalyst” front.

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REFERENCES 1 . Siegbahn, K . , Philos. Trans. R . Soc. Landon, Ser. A 268,33 (1970). 2. Wells, B. R., and Roberts, M. W., Discuss. Faraday Soc. 41, 162 (1966); Quinn, C. M., and Roberts, M. W., Trans. Faraday Sac. 61, 1775 (1965). 3 . Brundle, C. R., and Roberts, M. W., Proc. R. Soc. London, Ser. A 331,383 (1972). 3a. Brundle, C. R., Roberts, M. W., Latham, D., and Yates, K., J . Electron Spectrosc. Relat. Phenom. 3,241 (1974). 4 . Turner, D . W., and Al-Joboury, M. I., J . Chem. Phys. 37,3007 (1962). 5 . Price, W. C., in “Molecular Spectroscopy” (P. Hepple, ed.), p. 221. Inst. Petroleum, London, 1968. 6 . Bordass, W. T., and Linnett, J. W., Nature (London) 222,660 (1969). 7. Eastman, D. E., and Cashion, J. K., Phys. Rec. Lett. 24, 310 (1970). 8. Henke, B. L., Phys. Rei:. A 6, 94 (1972). 9 . Madey, T. E., Yates, J. T., and Erickson, N. E., Chem. Phys. Lett. 19,487 (1973). 10. Carley, A. F., and Roberts, M. W., Proc. R. Soc. London, Ser. A 363,403 (1978). 1 1 . Scofield, J. H., J . Electron Spectrosc. Relat. Phenom. 8, 129 (1976). 12. Penn, D. R., J . Electron Spectrosc. Relat. Phenom. 9, 29 (1976). 13. Evans, S., Pritchard, R. G., and Thomas, J. M., J . Phys. C 10, 2483 (1977). 14. Carley, A. F . , and Joyner, R. W., J . Electron Spectrosc. Phenom. 13,411 (1978). 15. Braithwaite, M. J., Joyner, R. W., and Roberts, M. W., Faraday Discuss. Chem. Soc. 60, 89 (1975). 16. Joyner, R. W., Roberts, M. W., and Yates, K., Surfi Sci. 87, 501 (1979). 17. Joyner, R. W., and Roberts, M. W., Chem. Phys. Lett. 60,459 (1979). 18. Lloyd, D. R., Faraday Discuss. Chem. Sor. 58, 136 (1974). 19. Clarke, T. A,, Gay, I. D., Law, B., and Mason, R., Chem. Phys. Lett. 31,29 (1975). 20. Gustafsson, T . ,Plummer, E. W., Eastman, D. E., and Freeouf, J. L., Solid State Commun. 17,391 (1975). 21. Atkinson, S . J., Brundle, C. R., and Roberts, M. W., J . Electron Spectrosc. Relai. Phenom. 2, 105 (1973). Chem. Phys. Lett. 24, 175 (1974). 22. Atkinson, S . J., Brundle, C. R., and Roberts, M. W., Faraday Discuss. Chem. Soc. 58, 62 ( 1974). 23. Eastman, D. E., Solid State Commun. 10, 933 (1972). 24. Joyner, R. W., and Roberts, M. W., J . Chem. Sac., Faraday. Trans. I 70, 181 (1974). 25. Fuggle, J. C., Madey, T. E., Steinkilberg, M., and Menzel, D., Phys. Lett. A 51,163 (1975). 26. Isa, S. A,, Joyner, R. W., and Roberts, M. W., J . Chem. Soc., Faraday Trans. 1 7 4 , 546 (1978); Kuppers, J., Nitscke, F., Wandelt, K., Ertl, G., and Brundle, C. R., J . Chem. Sac., Faraday Trans. I 74,984 (1979). 27. Kishi, K., and Roberts, M. W., J . Chem. Soc., Faraday Trans. 171, 1715 (1975). 28. Joyner, R. W., and Roberts, M. W., Chem. Phys. Lett. 29,447 (1974). 29. Ford, R. R., A h . Catal. 21, 51 (1970). 30. Goymour, C. G., and King, D. A,, J . Chem. Sac., Faraday Trans. I 69,736 (1973); Bickley, R. I., Roberts, M. W., and Storey, W., J . Chem. Soc. p. 2774 (1971). 31. Webb, A. N., and Eischens, R. P., J . Am. Chem. Soc. 77,4710 (1955). 32. Iwasawa, Y., Mason, R., Textor, M., and Somorjai, G., Chem. Phys. Lett. 44,468 (1976). 33. Somorjai, G. A,, and Blakely, D. W., Nature (London) 258, 580 (1975). 34. Joyner, R. W., J . Catal. 50, 176 (1976). 35. Mrnzel, D., J . Vac. Sci. Technol. 12,313 (1975). 36. Fuggle, J. C., in “Handbook of X-ray and Ultra-violet Photoelectron Spectroscopy” (D. Briggs, ed.), p. 273. Heyden, London, 1977.

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37. Bradshaw, A . M., Cederbaum, L. S., and Domcke, W., Struct. Bonding (Berlin) 24, 134 (1975). 38. Spicer, W. E., Yu, K. Y., Lindau, I., Pianetta, P., and Collins, D. M., Surf. Defect Prop. Solids 5, 103 (1976). 39. Mason, R., and Textor, R., Surf. Defect Prop. Solids 5, 189 (1976). 40. Kishi, K., and Roberts, M. W., Proc. R. Soc. London, Ser. A 352, 289 (1976). 41. Kishi, K., and Ikeda, S . , Bull. Chem. Soc. Jpn. 47,2532 (1974). 42. Matloob, M. H., and Roberts, M. W., J . Chem. Soc., Faraday Trans 173, 1393 (1977). 43. Johnson, D . W., Matloob, M. H., and Roberts, M. W., J . Chem. Soc., Chem. Commun. p. 41 (1978). 44. Johnson, D. W., Matloob, M. H., and Roberts, M. W., J . Chem. Soc., Faraday Trans. I 75,2143 (1979). 45. Carley, A. F., and Roberts, M. W., unpublished results. 46. Carley, A. F., Rasias, S., Roberts, M. W., and Wang, T. H., Surf. Sci. 84, L227 (1979); J . Catal. 60, 385 (1979). 47. Matloob, M. H., and Roberts, M. W., J . Chem. Res. p. 336 (1977). 470. Johnson, D . W., and Roberts, M. W., J . Electron. Spectrosc. Relat. Phenom. 19,185 (1980). 47b. Grunze, M., Surf. Sci. 81, 603 (1979). 47c. Kishi, K . , and Roberts, M. W., Surf. Sci. 62,252 (1977). 48. Fuggle, J. C., in “Handbook of X-ray and Ultra-violet Photoelectron Spectroscopy” (D. Briggs, ed.), p. 273. Heyden, London, 1977. 49. Pirug, G., Bonzel, H. P., Hopster, H., and Ibach, H., J . Chem. Phys. 71 (2), 593 (1979). 50. Thomas, G. E., and Weinberg, W. H., Phys. Rev. Lett. 41, 1181 (1978); Zhdan, P. A., Boreskov, G. K., Boronin, A. I., Schepelin, A. P., Egelhoff, W. F., and Weinberg, W. H., Appl. Surf. Sci. 1, 1 (1977). 51. Madey, T., Yates, J. T., and Erickson, N. E., Surf: Sci. 43, 526 (1974). 52. Kishi, K., and Roberts, M. W., Surf. Sci. 62, 252 (1977). 53. Johnson, D. W., and Roberts, M. W., Surf. Sci. 87, L255 (1979). 54. Ertl, G., Weiss, M., and Lee, S. B., Chem. Phys. Lett. 60, No. 3, 391 (1979). 55. Fuggle, J. C., and Menzel, D., Surf: Sci. 79, I (1977). 56. Fuggle, J. C., Umbacb, E., Menzel, D., Wandelt, K., and Brundle, C. R., Solid State Commun. 27,65 (1978). 57. Kilty, P. A., Rol, N. C., and Sachtler, W. M. H., Proc. Int. Congr. Catal., 5th, 1972 p. 929 (1973). 58. Singh-Boparai. S., Joyner, R. W., and Roberts, M. W. (unpublished results). 59. Brundle, C. R., and Carley, A. F., Chem. Phys. Lett. 31, No. 3, 423 (1975); Evans, S., Pielaszek, J., and Thomas, J. M., Surf. Sci. 55, 644 (1976); Norton, P. R.,and Tapping, R.L., Discuss. Faraday Soc. 60,71 (1975). 60. Kishi, K., and Roberts, M. W., J . Chem. Soc., Faraday Trans. 171, 1721 (1975). 60a. Joyner, R. W., Kishi, K.,andRoberts, M. W., Proc. R. Soc. London, Ser. A358,223 (1977). 61. Flodstrom, S. A., Bachrach, R. Z., Bauer, R. S., and Hagstrom, S . B., Phys. Rev. 36, 151 (1976). 62. Evans, S . , Evans, E. L., Parry, D. E., Tricker, M. J., Walters, M. J., and Thomas, J. M., Discuss. Faraday Soc. 58,97 (1974). 63. McKee, C. S . , Renny, L. V., and Roberts, M. W., Surf. Sci. 75,92 (1978). 64. Gay, I. D., Textor, M., Mason, R., and Ywasawa, Y., Proc. R. Soc. London, Ser. A 356, 25 (1977). 65. Brundle, C. R.,and Roberts, M. W., Surf. Sci. 38,234 (1973). 66. Au, C. T., Breza, J., and Roberts, M. W., Chem. Phys. Leii. 66, No. 2, 340.

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Kishi, K., and Roberts, M. W. (unpublished results). Moyes, R. B., and Roberts, M. W., J . Catal. 49,216 (1977). Roberts, M. W., and Wood, P. R., J . Electron. Spectrosc. Relat. Phenom. 11,431 (1977). Gimzewski, J. K., Padalia, B. D., Affrossman, S., Watson, L. M., and Fabian, D. J., Surf. Sci. 61,468 (1976). 70. Schwab, G. M., Discuss. Paraday Soc. 8, 166 (1950). 71. Dowden, D. A,, and Reynolds, P. W., Discuss. Faraday SOC.8, 184 (1950). 72. Eley, D. D., and Leutic, P., Trans. Faraday SOC.53, 1483 (1957). 73. Fahrenfort, J., Van Reyen, L. L., and Sachtler, W. M. H., in “Mechanism of Heterogeneous Catalysis” (J. H . deBoer, ed.) . Elsevier, Amsterdam. 74. Joyner, R. W., and Roberts, M. W., Proc. R. Soc. London, Ser. A 350, 107 (1976). 7 4 ~ Kishi, . K., and Ikeda, S., Appl. Surj. Sci. 5, 7 (1980). 75. Roberts, M. W., Chem. Soc. ReE. 6 (4), 373 (1977); Isa, S., Joyner, R. W., Matloob, M., and Roberts. M. W.. Appl. Surf: Sci. (in press). 76. Madix, R. J., Surj. Sci. 89, 540 (1979). 77. Sexton, B. A., Surf: Sci. 88,319 (1979). 78. Eastman, D . E., and Demuth, J. E., Jpn. J . Appl. Phys., Suppl. 2, 827 (1974); Phys. Rev. Lett. 32, 1123 (1974). 79. Clarke, T. A., Gay, 1. D., Law, B., and Mason, R., Discuss. Faraday SOC.60, 119 (1975). 80. Plummer, E . W., Waclawski, B. J., and Vorburger, T. V., Chem. Phys. Lett. 28, No. 4, 510 (1974). 81. Jenkins, G. I., and Rideal, E. K., J. Chem. SOC.p. 2490 (1955). 82. Trapnell, B. M. W., Trans. Faraday SOC.48, 160 (1952). 83. Lewhald, S., and Ibach, S., Surf. Sci. 89, 425 (1979). 84. Somorjai, G. A,, Ado. Catal. 26, l(1977). 85. Rubloff, G. W., Liith, H., Demuth, J. E., and Grobman, W. D., J . Catal. 53,423 (1978). 86. Bertolini, J. C., and Rousseau, J., SurJ Sci. 89,467 (1979). 86a. Kishi, K., Chinomi, K., Inoue, Y., and Ikeda, S . , J . Catal. (to be published). 87. Haber, J., Marczewski, W., Stoch, I., and Ungier, L., Proc. In?. Congr. Catal. 6th, 1976 p. 287. 88. Au, C . T . , and Roberts, M. W., Chem. Phys. Lett. (in press). 89. Hollinger, G., Duc, T. M., and Deneuville, A,, Phys. Reo. Lett. 37, 1564 (1976). 90. Haber, J., Stoch, J., and Ungier, L., J . Solid State Chem. 19, 113 (1976). 91. Haber, J., J . Less-Common Metals 54, 243 (1977). 92. Salje, E., Carley, A. F., and Roberts, M. W., J . Solid State Chem. 29, 237 (1979). 93. Roberts, M . W., and Smart, R. St. C., Chem. Phys. Lett. 69, 234 (1980). 94. Chuang, T. J., Brundle, C. R., and Rice, D. W., Surf. Sci. 59,413 (1976). 95. Haber, J., and Ungier, L., J . Electron Soectrosc. Relat. Phenom. 12, 305 (1977). 96. Lauks, I. R., and Green, M., Surf. Sci. 71, 735 (1978). 66a. 67. 68. 69.

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ADVANCES IN CATALYSIS, VOLUME 29

Site Density and Entropy Criteria in Identifying Rate- Determining Steps in Solid-Catalyzed Reactions RUSSELL W. MAATMAN Department of’ Chemistry Dordt College Sioux Center, Iowa

I . Introduction . . . . . . . . . . . . 11. Theory . . . . . . . . . . . . . . A. The Use of Transition State Theory B. The Calculation of Site Densities . . C. The Entropyof Activation . . . . 111. Analysis of Reactions . . . . . . . . A. One-Reactant, Inorganic Examples . B. One-Reactant, Organic Examples. . C. Two-Reactant, Inorganic Examples . D. Two-Reactant, Organic Examples . Symbols . . . . . . . . . . . . . . References . . . . . . . . . . . . .

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I. Introduction

Study of the kinetics of a reaction frequently makes it possible to determine the mechanism of the reaction. In a kinetic study one can attempt to reduce a reaction that might be complicated to a series of steps, each of which may be relatively easy to understand. What can we say a priori about the rates of individual steps? We find immediately that there is very little relation between the rates for the various steps of a reaction. In fact, normally the rates of the different steps of a reaction vary over a wide range. This situation enables us to make an important conclusion. The rate of one step of a reaction is often so much slower than the rates of the other steps that this one step is the rate-determining step. As a consequence, one can frequently design an experiment so that the observed rate is actually the rate of that one step. 97

Copyright 0 1980 by Academic Press, Inc. All rights of reproduction in any form reserved.

ISBN 0-12-007829-5

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Although there is very little physical constraint on the range of rates that might exist for the various steps of a reaction, there is considerable physical constraint on the range of rates that we might find for a specific step. Therefore, to the extent that our rate data apply to a rate-determining step, we may be able to make deductions about the nature of that step. For example, an adsorption step cannot proceed more rapidly than the time it takes for a molecule to diffuse from the gas phase to the surface. We can also use rate data to calculate the magnitude of the energy barrier, the activation energy. Then, if the step being considered is a bond-breaking step, the activation energy must be at least as great as the bond energy. Also, we usually cannot have a large activation energy for gas adsorption. In this work we analyze the rate data for over 100 solid-catalyzed gas reactions and ask questions about one other physical constraint, namely, a constraint concerning the concentration of active sites on the surface of the catalyst, L. If the observed rate is assumed to be the rate of a certain step, a postulated rate-determining step, and calculation reveals that L is greater than is physically possible, then that step cannot be rate determining. Or, if the calculated value of L is so small that the turnover frequency (the number of molecules reacting per site per unit time) is calculated to be larger than the number of molecules that strike the site in unit time, then this value of L and this step must also be rejected. In our earlier work on this subject (1-3) we concluded that because of (a) the possibility of finding small L values, near the lower physical limit, and (b) approximations inherent in the method of calculation, that a step could not be rejected because of the calculated value of L if the value were in the large range between lo5 and l O I 7 sites cm-2. Even though this range is large, most calculated L values fall outside the range. Evidently the L criterion, if it is valid, enables one to narrow down the choice for a rate-determining step. Using the L criterion to reject a postulated rate-determining step is more complicated than maintaining, for example, that adsorption cannot be more rapid than that allowed by known diffusion rates. The principal problem lies in the assumptions that one must make in developing the equations needed to make the L calculation. We discuss this question in Section II,A and attempt to show that the number of assumptions is not as great as is sometimes assumed. In Section II,B we give the details of how we calculate L, omitting, however, the details of those cases described in the standard literature. We can calculate either L or some limit on L for 9 different postulated rate-determining steps if there is one reactant and 16 different postulated rate-determining steps if there are two reactants. Others have used the calculated value of L as a criterion (4). There has, however, been more interest in comparing the observed entropy of activation with what is physically possible for a postulated rate-determining step.

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99

We show in Section II,C the relation between the L criterion and the entropy criterion and how the two criteria can be used to complement each other. Section I11 is a summary and discussion of the L calculations we have made on over 100 published systems, taken largely from the literature appearing in 1975-1978.

II. Theory

A. THEUSE OF TRANSITION STATE THEORY In a solid-catalyzed gas reaction any one of several steps may be a ratedetermining step. Consider the possibilities when only one molecule reacts. The rate-determining step could be adsorption, reaction on the surface after adsorption, or desorption. We can conceive of some special cases of these possible rate-determining steps. Thus, dissociation of the molecule might or might not necessarily accompany adsorption. A surface reaction might be one of several kinds. The adsorbed molecule could isomerize or decompose on a fixed site, and either kind of step could be rate determining. In some cases reaction occurs only when two adsorbed molecules react with each other. Regardless of whether the surface reaction is monomolecular or bimolecular, the reacting molecules could possess surface mobility and not be attached to fixed sites; here the “site” is the unit area of surface, as described in Section II,B,3. Molecules could have surface mobility before reacting on fixed sites, or they could react without ever attaching themselves to fixed sites. A desorption rate-determining step could be simple desorption from a site upon which reaction has occurred. Surface mobility could also be a factor in desorption. For example, in some cases the adsorbed molecule must move from the site of reaction to a site from which it can desorb. For a solid-catalyzed reaction between two different molecules all the possibilities for the rate-determining step of the one-molecule reaction still exist. Thus, adsorption of either molecule can be rate determining; a surface reaction involving only one of the molecules can be the rate-determining step; and so forth. The two-molecule case introduces some new possibilities for the rate-determining step. In both the one-molecule and the two-molecule case the rate-determining step can be a surface bimolecular reaction. With the surface reaction in the two-molecule case, however, the reaction depends upon the relative abilities of the molecules to adsorb on the active sites. There are several possible cases. Both can adsorb weakly; one can adsorb moderately well as the other adsorbs weakly; they can both adsorb moderately well, competing effectively with each other for sites; and one can adsorb very well as the other adsorbs weakly. Another problem arises when the two

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RUSSELL W. MAATMAN

different molecules adsorb on two different kinds of sites and therefore do not compete with each other. Also, for both the one-molecule and the two-molecule cases the reaction might be catalyzed simultaneously by two different kinds of sites. Still another complexity arises when two kinds of surface sites are required for different steps of the reaction sequence. Such a dual-site mechanism can involve diffusion between solid phases. In this article we use transition state theory (TST) to analyze rate data. But TST is by no means universally accepted as valid for the purpose of answering the questions we ask about catalytic systems. For example, Simonyi and Mayer (5) criticize TST mainly because the usual derivation depends upon applying the Boltzmann distribution law where they think it should not be applied, and because thermodynamic concepts are used improperly. Sometimes general doubts that TST can be used reliably are expressed (6). But TST has also been used with considerable success. Horiuti, Miyahara, and Toyoshima (7) successfully used theory almost the same as TST in 66 sets of reported kinetic data for metal-catalyzed reactions. The site densities they calculated were usually what was expected. (Their method is discussed further in Section 11,B,7.) To examine TST, let us fix our attention on a system of reactant gas molecules and the surface of a catalytic solid at a certain temperature and pressure. The free energy of this system, G,, is defined. Suppose that there is spontaneous reaction. For reaction to occur spontaneously the free energy of the system after reaction, G,, must, according to the first and second laws of thermodynamics, be less than G, . In fact, such a system will always change “immediately” t o a position of lower free energy, provided that (a) there is no intermediate free energy barrier and (b) “immediately” allows, however, for diffusion time and time for atoms to take new positions, perhaps by vibration. Notice that Condition (a) removes the requirement that only equilibrium states be considered. The two conditions together mean that, if there is no free energy barrier, we can say something about the rate of spontaneous change from an initial state to any final state. Normal diffusion rates and the time elapsed during atomic movements are usually such that reaction would be very fast if there were no intermediate free energy barrier. But in most cases rates are not that fast. We conclude that normally there is a free energy barrier. That is, Gi-G, > O

where i refers to the system when it is at the top of the free energy barrier. The extension to nonspontaneous reactions is obvious. We can now write, as we can for any free energy change, Gi - GI = AGf = AH’ - T A S f

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(Throughout this article standard-state thermodynamic quantities will be assumed, and therefore there is no need to use the standard-state superscript.) We shall return to this equation later. We next turn to the empirical Arrhenius equation k

= Ae-EIRT

(3)

where A and E are constants. Unless otherwise specified, our use of Eq. (3) is limited to a single step of a reaction sequence. The validity of Eq. (3) has been amply demonstrated in both homogeneous and heterogeneous systems. What d o we know about E, a positive constant that has the dimensions of energy? Kineticists have concluded that E is the energy that a system must have in order that there be reaction. This conclusion seems reasonable when we realize that in an assemblage of molecules e-E’RTis the fraction of those molecules that possess an energy at least as great as E . There is general agreement that E is a threshold energy. But E is not the height of the free energy barrier; we can see this when we consider how E is determined. Thus, with nonreacting gas molecules is the fraction possessing kinetic energy E or greater; when two molecules collide, the complex formed possesses potential energy equal to the kinetic energy they had before collision, where the kinetic energy is calculated using their relative velocities. Since E evidently is not related to the entropy of the reactants, E cannot be the height of the free energy barrier. Let us consider in more detail the concept of a free energy barrier. Transition state theory also uses the idea that there is such a barrier in the reaction path. What is special about TST is that it ascribes certain properties to the species at the top of the barrier, the activated complex. According to TST for a unimolecular reaction, k = KVe-AGftRT (4) where v is the frequency at which the activated complex reacts and K, the transmission coefficient, is the probability that the reaction is a forward reaction. We need not concern ourselves with v and K now. What is important is that Eq. (4) contains the free energy barrier, which, as we have concluded, must exist if the reaction does not occur immediately. Since this free energy is not the potential energy of the complex, the exponential factors in Eqs. (3) and (4) are not identical. We might, however, intuitively expect E to be close to AH$,the molar enthalpy of reaction, regardless of the reacting system. If one does assume the validity of TST, then it can be shown that E = AH^

+ RT

(5)

We shall assume Eq. ( 5 ) to be valid. This assumption does not introduce a large error into the development if TST is not valid, since RT is usually small

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RUSSELL W. MAATMAN

with respect to E or AH*, and so Eq. (5) does little more than change the symbols used in the equations. We now remove AG* from Eq. (4)by the use of Eqs. (2) and (5). We then have k = eKVeASt/Re-E/RT (6) Equation (6) is important for the following reason. If we assume the validity of the empirical Arrhenius equation, Eq. (3), then any concern we have about TST as it is expressed in Eq. (6) should be confined to the quantity e l c v for, from the argument given, it seems that the eAstiRand e-EiRT factors should arise in any case, once we concede that a free energy barrier exists. What can be said about e w ? Normally one expects IC to be not far from unity, although very low values are not unknown, and so e K is of the order of unity. Evidently questions about TST ought to focus on v, taken as k T / h in TST. Or, a negative attitude toward the use of TST in catalytic systems seems not to be justified if it does not focus on the frequency factor question. One further point about E of Eq. (6) must be made. The quantity E can be determined experimentally using the Arrhenius equation, and, since experimental results may involve more than one step, E can therefore involve more than the activation energy of a single step. Situations in which E is not the activation energy of a single step are discussed in Section II,B. But just because such complexities arise, we might be confused by another situation that can arise, namely, instances in which the endothermic heat of reaction is greater than the observed activation energy. Best and Wojciechowski (8)pointed out that the activation energy for cumene dealkylation over silica-alumina is about the same as (and could perhaps be less than) the heat of reaction. They propose a model in which the reactant in the ratedetermining step, the step for which the observed activation energy applies, is at a much higher energy level than the reactant was initially. Thus, the observed activation energy can be smaller than the heat of reaction.

B. THECALCULATION OF SITEDENSITIES 1. Adsorption

To calculate the site density, L, for a given reaction step we use Eq. (6). [Equation (6) is for a unimolecular reaction; the appropriate change is made when the reaction is not unimolecular.] Suppose that the reaction step is the adsorption of a gas (B) on a catalytically active site (D):

-

+ D k,

(7) Then if adsorption is first order, where the number of bare sites per square B

BD(ads)

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centimeter is approximated by L, u

=

k,PL

where P is the partial pressure of B. To calculate L, we combine Eqs. (6) and (8). We also assume the validity of TST; that is, we take v = kT/h and IC x 1. Then u = PL(kT/h)eeASt’Re-EiRT

(9)

But the entropy of the reacting species can be calculated using statistical mechanics if the partition functions are known. Making the appropriate substitution and rearranging Eq. (9) we have

where = (FFD/FhDcg)

(11)

F is the partition function of the gas. (The symmetry factors that appear in the partition functions and the other parts of the calculation usually cancel and in any case do not contribute a large factor, and so they are not included in this article.) FD and FhDboth approximate unity. For the gas F

=

FtrFrotFvibFel

(12)

We neglect Fviband F,, because they approximate unity. For example, for N, at 1 atm and 25”C, the contributions to its entropy calculated from the four partition functions (where S is essentially proportional to In F) are, in entropyunits(e.u.),S,, = 35.92, S,,, = 9.82, Svib= 2.89 x 10-4,andS,, = 0. We also have F,,

=

(2~rnkT)~”/h~

(13)

For linear molecules, Fro,= 8nZIkT/h2

For nonlinear molecules,

Fro,= 8712(8n31,Z,I,)”2(kT)3’2/h3

(15)

In certain cases Eq. (15) is only approximate. Using the Arrhenius activation energy and u at a specified P and T , we can calculate L for adsorption from Eq. (10) when C is determined from Eqs. (11)-(15). We pointed out above that any one of many different steps may be the rate-determining step. Usually L can be either estimated or given a lower limit by using Eq. (lo), provided that C is known. In fact, C is the only

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RUSSELL W. MAATMAN

TABLE I Calculation of L

Step No.”

Description of rate-determining stepb

1

Adsorption of one reactant

2

Adsorption of one reactant with dissociation

3

Mobile activated complex for one reactant

4

Single reactant; bimolecular surface reaction‘’

5

Surface saturated with one reactant

6

First-order surface reactionf

7

Bimolecular reaction on a sparsely covered surface

8

Bimolecular surface reaction; mobile reactantsh

9

Bimolecular surface reaction; mobile reactants; rotating activated complexh

10

Bimolecular surface reaction; one reactant appreciably adsorbed‘

11

Bimolecular surface reaction; one reactant very strongly adsorbed’

12

Bimolecular surface reaction; both reactants appreciably a bsorbedk

C in Eq. (10)’

Relevant equation in textd

a Some of the steps which are also given in Reference (3) are assigned different numbers in this work. Except where otherwise noted, fixed sites are assumed. (I) For some steps the apparent activation energy is to be used in Eq. (lo), and in others, the true activation energy. See text. (2) Where relevant, it is assumed that the symmetry number approximates unity; it is also assumed that (2/s) 5 0.5, where s is the number of sites adjacent to a given site in a surface bimolecular reaction. (3) Both cgr gas concentration in molecules ~ m - and ~ , P, gas pressure in atmospheres are used in this work. For an ideal gas, cg = 7.34 x loz1PlT. (4) Except where otherwise noted, FJD5 FD 5 1. ( 5 ) An adsorption reaction is a “Rideal-Eley” reaction; a surface reaction is a “Langmuir-Hinshelwood” reaction.

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105

quantity in Eq. (10) that depends upon which step is being considered. We give C for 12 steps in Table I. The expressions for C in Steps 2, 5, 7, 10, and 11 given in Table I are developed in the standard literature, and so their development is not repeated here (9). The value of C for Step 1 is given by Eq. (11). The other steps are now taken up. 2. First-Order Unimolecular Surface Reaction For the reaction B

k, k +D+ BD(ads)

products

k,

(16)

the steady-state rate can easily be shown to be given by P

v = k,L

+ (k2 + k3)/kl

(For conversion between cg and P see footnote c, Table 1.) If k , << k , , a situation that frequently arises because surface reactions tend to be more complex than the adsorption reaction, then Eq. (17) becomes

v

=

k,L

P

+ (l/Kads)

where Kads is ( k , / k 2 ) and is the equilibrium constant for adsorption. If ( l l K a d s ) >> p , 21

=

Kadsk,LP

(19)

and the rate is first order in B: that is, the amount of B adsorbed corresponds to a point on the linear part of the B adsorption isotherm. In Eq. (19) L is the sum of the occupied and the unoccupied sites. We now remove k , and Kadsfrom Eq. (19). For k , , we note from Eq. (17)

Where an equation number is not given, the appropriate equation including C is given in Reference (9). The L value calculated is a minimum for the site density. See text. If the surface reaction is first order and the L value calculated is too large, the adsorbed species may retain translational and/or rotational degrees of freedom. See text for reason for relating Eqs. (1 1) and (25). In the absence of specific information, we assume that cB, = cB2 = l O I 4 molecules cm-’. The calculation provides a value no greater than L/4; see text. j Molecule 1 is strongly adsorbed. The F / c , ratio to be used is the larger of the two. See text for further details.

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RUSSELL W. MAATMAN

that v = k,L when the surface is saturated, that is, when P >> (k, + k3)/kl. In such acase C = 1 (Step 5, Table l), and Eq. (10) becomes on rearrangement

v

=

k,L

=

L(kT/h)e-E’RT

k,

=

(kT/h)e-EiRT

(20)

whence Equation (21) shows that k , is the rate of reaction per site, and therefore it can be used regardless of the degree of surface saturation. For K,,,, we know that AG,,, = - R T l n Kads= AH,,, - T AS,,,

(22)

Using Eqs. (21) and (22) in Eq. (19), we have v

=

LP(kT/h) exp(AS,,,/R) exp[ -(E

+ AH,,,)/RT]

(23)

Equation (23) shows that the measured or “apparent” activation energy, Eapp,is Eapp= E

+ AH,,,

(24)

Combining Eqs. (23) and (24) and rearranging,

Suppose that we replace the second bracketed factor in Eq. (25) by using partition functions. Then, since the adsorbed activated complex and the adsorbed molecule (assuming that each is on a fixed site) differ insignificantly, we have, except for the matter of the activation energy, essentially Eq. (lo), with C given by Eq. (Il), where adsorption is the rate-determining step. (Thus, we assume that AS,,, x AS*.) If the observed activation energy is the true activation energy and adsorption is the rate-determining step, then Eqs. (10) and (11) give L correctly. On the other hand, when the observed activation energy is the apparent activation energy as defined by Eq. (24) and the slow step is a first-order surface reaction, then Eq. (25) (or its equivalent using partition functions) gives L correctly. Notice that, for fixed-site surface species, the calculated L value is the same in the two cases, using Eqs. (10) and (ll), or using Eq. (25), for the same experimental conditions and results. Later we shall encounter examples of unimolecular reactions that are first order but give, when the observed activation energy is used, L values that are impossibly large when Eqs. (10) and (11) or Eq. (25) is used. Why

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107

does this occur? We can analyze this situation by considering Steps 1 and 5 of Table I. Taking L(Step 1) and L(Step 5) to be the site densities calculated for those steps and using Table I and Eq. (25), we have

while L(Step 1) is given by Eq. (25). If in a first-order unimolecular reaction we were to calculate L(Step 5), we would thereby be assuming incorrectly that the reaction is zero order. Adsorption is bond formation, and therefore AH,,, is a negative quantity, or, according to Eq. (24), in the first-order case Eappis less than E. Thus L(Step 5) would be too small to apply to the first-order case. The adsorption step is an ordering process, and therefore ASadsis negative; the calculated value of L(Step 1) [using Eq. (25)] is larger than the calculated value of L(Step 5) [using Eq. (26)], provided that the same rate data are used in the two calculations. We shall see that there are usually several orders of magnitude difference between L(Step 1) and L(Step 5). If the reaction is first order, but L(Step 1) [Eq. (25)] is larger than is physically possible, then the absolute value of AS,,, is too large. That is, there is not as great an entropy change upon adsorption as was initially assumed. We know quite accurately the entropy of the reactant gas. Therefore, if the entropy change of the surface site is unimportant, then the entropy of the adsorbed molecule is greater than we had thought. Suppose, for example, we decide that it is physically impossible for L to be greater than 10’’ sites cm-2, and yet L(Step 1) is calculated from Eq. (25) to be lo’* sites cm-2. We shall discuss the entropy-site density question in more detail in Section 11,C; we can, however, give a simple conclusion now. If L is too large by a factor of lo3, AS,,, is too negative by 13.8 e.u. The molar entropy of a gas might by 40-50 e.u. If L(Step 1) were determined assuming that the adsorbed molecule is immobile and does not rotate, then our entropy calculation indicates that the adsorbed molecule actually retains at least 25-30% of the entropy it possesses in the gas phase. (The percentage could be greater than 25-30% since the actual L value could be less than lo1 sites cm-2.) A calculation of this kind could show that the adsorbed molecule retains some rotational-translational freedom. Such a conclusion was made by Eley and Russell for the equilibration of 28N2and 30N2over Re at 1373 K (10). The reaction was first order in nitrogen, and Step 1 was rate determining. By using a TST argument equivalent to the one just given, they showed that the activated complex possessed some surface rotational and translational freedom.

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RUSSELL W . MAATMAN

Although the surface first-order case is formally the same as the first-order adsorption case, they are separated in Table I for the sake of clarity..and are Steps 6 and 1, respectively. 3. Mobile Activated Complex Suppose that there are no fixed sites; the entire surface is active. Then upon adsorption the gas molecule loses only one-third of its translational degrees of freedom as the activated complex forms. Thus, we replace F,r, used when the gas loses all of its translational degrees of freedom, with F:!3. Equation (11) becomes

We approximate using the reasons given by Miyamota and Ogino (11). If FD % 1, as before, Eq. (10)becomes for the mobile activated complex case

But if there are no localized sites and the entire surface is active, there is only one “site” per unit area, that is, L = 1. Thus, when the mobile activated site L calculation is made, the physically possible value is of the order of unity, not lo1’ or some such value. The mobile activated complex case is Step 3 in Table I.

4. Bimolecular Surface Reaction with a Single Reactant When the C expression for Step 7 of Table I is used in Eq. (lo),the standard TST equation for a bimolecular surface reaction is produced. If the two molecules are the same, we have after rearrangement

Then c5, =

LvF~rF~ot F,eEiRT cgZ(kT/h)Ft

Some B molecules are adsorbed, and the adsorption isotherm of B can be

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109

given in terms of partition functions (9): cs

=

cDc,FBD exP(AHa,s/RT) FFD

(32)

But L

= CB

+

(33) We combine Eqs. (32) and (33) to eliminate cB and combine the result with Eq. (31). Assuming F* = FBD, as before, then CD

+

where the observed activation energy is E AH,,,. Note that the left side of Eq. (34) is at its maximum when the surface is 50% covered, that is, when cD/L = 0.50. Then the left side is equal to 0.25L. If the surface is 10 or 90% covered, the left side is 0.09L. Thus, if the surface is between 10 and 90% covered, the right side of Eq. (34) should be multiplied by a number between 4 and 11 to give L. In any case, the right side of Eq. (34) gives a lower limit for L for this step. Notice that the right side of Eq. (34) has the same form as the L equation for Step 1, and thus we will not be able to distinguish between a measurement of L for Step 1 and a lower limit measurement for the present case. The single-reactant, bimolecular case is Step 4 in Table I. If much less than 10% of the surface is covered, then the conventional Langmuir-Hinshelwood TST equation, given by Eq. (10)and the C expression for Step 7 of Table I, applies for both one- and two-reactant cases. Step 7 is second order; in the intermediate coverage range, where Eq. (34) can best be used, the reaction is first order; and at high coverage the reaction is zero order even when bimolecular. For a zero-order reaction, Step 5 of Table I applies. (The assumption that the entropy of activation is zero for Step 5 is not necessarily correct when the surface reaction is bimolecular.) We studied the metathesis of propylene over W0,-Si02, an example of a single-reactant, bimolecular surface reaction. We calculated L for various conceivable rate-determining steps, including the step for which Eq. (34) applies (12). 5 . Reactants which Migrate over the Surface to Fixed Sites

In earlier work we pointed out that very rarely is Step 7 of Table I the rate-determining step: the value calculated for L is almost always too large (3).This is an expected result, since on a sparsely covered surface two mole-

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RUSSELL W. MAATMAN

cules are seldom close enough to react with each other, with the consequence that v would be very low, too low to measure. But L is proportional to v, and so the correct L for this step is much lower than that calculated using measurable rates. The reactants in Step 7 are gas molecules; Aldag, Lin, and Clark (13) pointed out, however, that the calculated value of L is entirely different if the reactants are taken to be mobile surface species that subsequently react on fixed sites. The calculation one makes for such a model has particular relevance for the sparsely covered case, but it applies also to cases in which the coverage is larger. A much different value of L will be obtained if it is assumed that the reactant is mobile on the surface and not a gas. After all, mobile species have much more of an opportunity to react, and therefore the number of sites needed to account for the observed reaction is much less. The value of L obtained from Eq. (10) is much smaller in the surface-mobile case because the partition function of the surface species is smaller than that of the gas species. Two cases are discussed. First, suppose that two mobile species react on a fixed site to form an activated complex that has neither translational nor rotational degrees of freedom. If the reactants have the usual rotational degrees of freedom, but only two-thirds of the translational degrees of freedom, then

Since the activated complex has no rotational degrees of freedom, F:ot = 1. Then, rearranging,

We can compare Eq. (36) with the L equation for Step 7, which is

Then from Eqs. (36) and (37),

We use again cg = 7.34 x 1OZ1(P/T)molecules ~ r n - ~ Using . Eq. (13) for F,,, Eq. (38) becomes L(Step 7)/L(Step 8) =

27tk(m1? & ) 1 / 2 C ~ 1 C ~ 2T3 (7.34 x 102')2P,P , h2

(39)

CRITERIA IN SOLID-CATALYZED REACTIONS

111

To complete the calculation we must assume values for cB1and cB2, the total number of molecules migrating over the surface, not related to the number of active sites. For our calculation we assume cB1= cB2= 1014 molecules cmP2. Using these values and converting masses to molecular weights, Eq. (39) becomes L(Step 7)/L(Step 8) = [6.10 x 10-3(MlM2)1/2T3]/(PlP2) (40) Thus, L calculated for Step 8 of Table I can be related to the site density calculated for Step 7. The other case arises when the activated complex, although on a fixed site, retains the rotational motion of the gas molecule. Then

Equation (35) then becomes after substitution and rearrangement

Aldag, Lin, and Clark used an equation similar to Eq. (42) in their analysis of the metathesis of ethylene over rhenium oxide-alumina; see Example 18, Table VI (13).Equation (42) is for Step 9 of Table I. Summarizing the bimolecular, sparsely covered case, we have the following situation. When the reactants are gas molecules and the activated complex is fixed and nonrotating, the entropy of activation is large, and L(Step 7) is very large. When the reactants are mobile, adsorbed molecules and the activated complex is still fixed and nonrotating, the entropy of activation is smaller, and therefore L(Step 8) is smaller. When the reactants are mobile, adsorbed molecules and the activated complex is fixed but rotating, the entropy of activation is still smaller, and L(Step 9) is the least of the three calculated values. Since it is probably not correct to insist that the activated complex possesses either all or none of the rotational degrees of freedom of the reactants, the correct site density could well be between L(Step8) and L(Step 9).

6. Bimolecular Surface Reaction: Both Reactants Appreciably Adsorbed Steps 10 and 11 of Table I give the appropriate expressions for C to be used in Eq. (10) when the adsorption of only one reactant is significant. If in a bimolecular surface reaction a significant amount of each reactant is adsorbed, then the number of unoccupied sites is consequently small. Therefore, L

%

csl

+ cB2

(43)

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RUSSELL W. MAATMAN

For each reactant there is an adsorption isotherm of the form given in Eq. (32). Then Eq. (43) becomes L=

‘DCgl FBID

exp(AHads,,IRT) Fl F D

+

cDcgZFB2D

exp(AHads,2/RT)

(44)

F2FD

Since neither the sites nor the activated complexes are mobile, FslDx FDx FszDFZ 1

(45)

Also, if there is not a large difference between the two gas concentrations, we can assume that the reason that the two reactants compete effectively with each other for adsorption sites is that AHads31

AHads,2

(46)

Equation (46) holds, of course, only if the entropies of adsorption are similar. Equation (44) then becomes, when rearranged,

The general equation for a bimolecular surface reaction is, neglecting the small factor that takes into account the number of adjacent active sites (9),

vF, FZeEiRT - cg,cg2(kT/h)

* L -

(‘Dl

Equating the right sides of Eqs. (47) and (48) and rearranging, we have

We assumed earlier that both the concentrations and the entropies of adsorption are similar for the two gases. Then the partition functions are about the same and Eq. (49) can be simplified:

Since only approximate calculations can be made, (2F/cg) will not easily be distinguished from (Flc,), and so, when we look at actual cases, we will not attempt to differentiate between Step 12, Table I, for which Eq. (50) applies, and Step 1. Note that Eq. (50) yields the value of cD,not L, and since in this case cD<< L, the method gives only a lower limit to L. For purposes of approximating the lower limit to L when both reactants adsorb appreciably, Eq. (50) can be used even when one (c,/F) term is much larger than the other; the form of Eq. (50) is still produced.

CRITERIA IN SOLID-CATALYZED REACTIONS

113

7. The Approach of Horiuti, Miyahara, and Toyoshima We have already mentioned that Horiuti, Miyahara, and Toyoshima (7) determined the site densities of metal catalysts by using a theory similar to TST wherever the cases they consider are the same as the cases considered in classical TST development. We can now show this similarity in the light of Eq. (10) and the definition of C in Table I. Using our symbols where applicable, the equation Horiuti, Miyahara, and Toyoshima use is log,, L

=

log,, u

+ (E/2.3RT) - log,,(ekT/h) + v1 log,o(Fl/c,l)

where v, and v, are the orders with respect to species 1 and 2, respectively; where there is only one reactant, v2 = 0. We differentiate Eqs. (13) and (14) with respect to T, use the resulting equations in Eq. (51), and rearrange to get for linear molecules

For nonlinear molecules [for which Eq. (15) must be differentiated] the coefficient (5/2) is replaced by 3. Equation (52) differs from TST as we have outlined it in previous sections in the following ways: 1. The coefficients v1 and v2 can be assigned whatever are the observed orders; there is no limitation to zero, integers, and one-half, unlike TST. Thus, in Step 1 of Table I, v, = 1 and v2 = 0; in Step 11, v, = - 1 and v2 = 1, and so forth. 2. The Horiuti group treats the temperature coefficient of the rate differently from the way it is usually treated in TST. They clearly identify E as the experimentally observed activation energy, but according to TST [cf. Eq. (5)] the ( E - R T ) quantity of Eq. (52) is the enthalpy of activation. The R T term in Eq. (5) arises because the assumption that the Arrhenius plot is linear is equivalent to the assumption that the preexponential factor A of the Arrhenius equation is constant, whereas, according to TST, A always contains the factor (kT/h). In addition, the partition function factors of Table I are also part of A, and most of them are functions of T. Since the Horiuti group takes this temperature dependency of the preexponential factor into account, the factor exp[(5/2)(v1 + v,)] (where 5/2 is replaced by 3 for nonlinear molecules) arises.

+

Concerning the first point, there are obvious arguments both for and against the idea of allowing v, and v2 to take on any value. In discussing a

114

RUSSELL W. MAATMAN

specific catalytic system in Section 111, we prefer to begin with a model for which C is given in Table I and then modify the model only when necessary. Concerning the second point, the different approach concerning the temperature coefficient of rate introduces (since L values can usually be known only to within an order of magnitude) a relatively small difference in L in almost all cases. Experimental error tends to mask any curvature in a plot of In v versus (l/T). Miyahara and Kazusaka (14) used an equation like Eq. (51), except that it was modified by adding a term that was a function of surface coverage. 8. Determining Site Density from High-Conversion Data

To calculate L values using Table I we need forward reaction rates, the number of molecules converted per unit area per second. These reaction rates are most easily obtained at low conversions. But frequently-often for practical purposes-high conversions are reported. We have developed a method for the determination of site densities in a certain class of systems even though the conversion and back reaction are both large. Also, the method can be used under certain conditions even if product isomerization is involved. We shall describe the theory here and in Section II1,B apply it to the isomerization of 1-butene to cis- and trans-Zbutene over silicaalumina. Although in this article we do not use this method to analyze any other systems, we present it in some detail because it may have potential for further use. For a solid-catalyzed reaction in which a single gaseous reactant is transformed to either of two products via a common surface complex, the reaction scheme is

Obviously, the interconversion of two isomers, such as between B and N alone, is much more common than the three-isomer case. We shall develop the three-isomer case, and our results will hold for the two-isomer case if we make k , = k6 = 0. Our problem has four parts. First, we must determine the rate law for an infinitesimal length of catalyst bed. Second, the resulting equation must be integrated over the length of the bed. Third, the values of certain of the system parameters must be determined using the integrated equation and experimental conversions. Finally, the site density is determined from the values of these parameters.

CRITERIA IN SOLID-CATALYZED REACTIONS

d[BD]/dt

=

0 = ki[B][D]

+

+ k&"J][DI + k6[QI[DI

+ k5"DI

(56) [BD], we can combine Eqs. (54)-(56) and obtain for the -

Since L = [D] two rates

(kz + k 3

115

The rate law for the conversion of B is the sum of the right sides of Eqs. (57) and (58). When conversions are not small, the change in concentration of reactant and product over the length of the catalyst bed must be taken into account. It is therefore convenient to substitute for the concentrations in Eqs. (57) PB(l - fN - &); [N] PBfN; [Q] --t P B f Q . Also, and (58) SO that [B] in a flow system the number of moles of reactant converted in an element of catalyst bed is equal to the reaction rate per gram times the mass of the catalyst in that element of the bed. That is, --+

--+

U df

=

RtotdW

(59)

where f = fN +fa. The flow rate is constant and, substituting for f and Rtot their component parts, we can integrate over the catalyst bed to get U =

s

+

RN dfN RQ +

s

+

R, dfQ R,

We shall confine our interest to the saturated surface case. Under such conditions we might expect a simple relationship between fN and fa ;that is, fN =

afQl

where a is a constant. Combining Eqs. (57), (58), (61), and the definitions given, we have

(61)

116

RUSSELL W. MAATMAN

Then, using Eqs. (60) and (62), we have [y

+ b/PJ

s

+ (Z - Y)

d f / ( u - fx)

+

s

f d f / ( u - fx)

+

+

-

W / u = 0 (63)

+

+

where y = k , / L ; u = kl(k3 k,); x = kl(k3 k , ) kz[(k4a k6)/(l a)]; z = (1/L)[(k4a + k6)/(l a)]; b = ( k , + k3 + k,)/L. We integrate Eq. (63) and remove the constant of integration by using the boundary condition that f = 0 when W = 0. Then

+

W/u

+ (y/X + b / P B X + ZU/Xz

- yU/X’)

h(1 - fX/U) f

f(Z

- y)/X = 0

(64)

where the absolute value of (1 - fx/u) is to be taken. We now determine the system parameters by evaluating Eq. (64).First, although it is not necessary to limit our considerations to the saturatedsurface, zero-order case, we do so to simplify the analysis of high-conversion systems. [We earlier assumed in connection with Eq. (61) that the surface is saturated and that a is constant.] Equation (62) indicates that the total rate is zero order when ( k , k3 + k , ) is small in comparison to the rest of the denominator. Thus, since b = ( k , + k3 k,)/L, b = 0 in the zeroorder case, and the (b/PBx)term can be removed from Eq. (64). In the resulting equation the experimentally determined quantities are W/U and f, and for our purpose we take the unknown quantities to be (x/u), (zlx), and (y/x). We can then obtain three equations of the form of Eq. (64) [with the (b/P,x) term omitted], one equation for each of the three different pairs of ( W / U )and f values. With the subscripts 1, 2, and 3 used for the three values of both ( W / U )and f , these three equations are solved simultaneously to give

+

+

In the 1-butene isomerization example which we discuss in Section III,B we solved Eq. (65) for (x/u) by Newton’s method on a high-speed computer to facilitate this otherwise laborious approximation method. The solutions to Eqs. (66) and (67) are then straightforward. From the definitions of u, x, y,

CRITERIA IN SOLID-CATALYZED REACTIONS

117

and z already given, we can note the following equations:

Taking the reciprocal of the product of each side of Eqs. (68) and (70), we have

Equation (65) can have many roots. For each of these values of (x/u) there exists a value of (z/x)and a value of (y/x),as determined by Eqs. (66) and (67). But since Eqs. (68)-(70) contain only positive constants, these equations provide criteria that the roots must satisfy in order that they apply to the model. Thus, according to Eq. (68), (x/u)> 1 ; according to Eqs. (69) and (70), both (z/x)and (y/x)must be positive. Since (x/u),(zlx),and (y/x) are linked, rejection of one root means that the two associated roots must also be rejected. In the 1-butene case we were thus able to remove the multiple-root problem from Eq. (65), although we have not proved that such removal can always be accomplished by using these two criteria. To estimate a value of L, we assume that the sum of k3 and k , , both rate constants for the reaction of adsorbed BD, can be approximated by a single surface rate constant. This approximation is justified if we do indeed observe an appreciable amount of N and Q in the product. Using zero-order TST (that is, using Eq. (10)for Step 5 of Table I), we then obtain from Eq. (71) (u/y)= L(k3

+ k , ) = Lk' = L(kT/h)e-E/RT

(72)

For the zero-order case, (u/y)= v, and Eq. (72) can be handled just as is Step 5 of Table I. We can determine (x/u)and (y/x)and therefore (u/y)at different temperatures, and so E can be determined from an Arrhenius plot, enabling us to obtain a value for L. The method cannot be used if there is not a common surface complex for interconversion among the three isomers, according to the mechanism given in reaction (53). C. THEENTROPY OF ACTIVATION

Sometimes workers analyzing the kinetics of reactions have focused atten: tion on the entropy of activation, or, in certain cases, on the entropy of reac-

118

RUSSELL W. MAATMAN

tion (15). Thus, a postulated step can be shown to be improbable when the entropy of activation calculated for that step is improbable. Using the calculated entropy of activation in determining the feasibility of a postulated step is similar to using the calculated value of L and deciding whether the value is physically possible. There is a clear functional relationship between L and AS%-indeed, Eq. (9) demonstrates this relationship for adsorption. Therefore, there is no question of claiming that either the site density method or the entropy of activation method is “right” and the other “wrong.” Rather, it is a matter of convenience. We choose to emphasize the site density method in this article chiefly because it enables us to examine easily many postulated steps for a given reaction. But there are also good reasons for using the entropy of activation in deciding which postulated steps are possible. Thus, one can sometimes compare the observed and the calculated (by statistical mechanics) values of the entropy of activation. Also, the entropy change has an obvious relation to other thermodynamic quantities. Although these two methods of deciding upon the feasibility of postulated steps are formally equivalent, in practice they complement each other. The purpose of this section is to show the relation between the two methods for the 12 steps listed in Table I. The general equation for L is Eq. (lo), and the expressions to be used for C in that equation are listed in Table I in terms of partition functions. But the entropies of both the activated complex and the reactants, and therefore AS*, can also be expressed in terms of partition functions. Therefore, C can be expressed in terms of the entropies of the activated complex and the reactants. As we shall see, it is possible to eliminate partition functions entirely. Also, in all but one case, Step 11, the entropy factor in C can be determined if one knows only the entropy of activation; in those cases the entropy of a reactant or the activated complex is not needed. The expressions for C in terms of entropy instead of partition functions are given in Table 11. The method of obtaining these expressions is illustrated in the following discussion by developing the appropriate expression for Step 1. For Step 1, the adsorption of a gas to form an activated complex, the entropy of activation is (73) ASt = Sactivatedcomplex - Sreactant - 0 - Sreactant = -(S’tr + ’rot) As indicated earlier, the very small vibrational and electronic contributions can be neglected. From statistical mechanics we have

+

S,,, = R In Fro, gR

(75)

119

CRITERIA IN SOLID-CATALYZED REACTIONS

TABLE I1 Entropy of Activation

Step"

AS$ definition

Cb

e- 712e-A91R,

e-4e-A.StlR

e- 7 1 4 e - A S t 1 2 R , c,

e-2e

e - 712e-AStlR.

c

H1 82

-9.2 -4.6

e

-4e - A W R

-4.6

1

- 4.6

- 4e -AS?/R

- 4.6

e - 7 e - A % l R . e - Be(c

-4.6

-A 4 l 2 R

- 213 - 3 . 1 4 e - A Q l R

e - 712e-ASt/R.

A(AS$)IA log,, L (e.u.)

g l g2

AalR

-4.6

)213e-5.33e-ASt/R

-4.6

10

- 4.6 - 4.6

1I d

-4.6

12'

- 4.6

9'

For description of each step, see Table I. The C of Table I is here given in terms of entropies of activation. Where two values are given, the first is for a linear molecule and the second for a nonlinear. ' See Footnote h, Table I, concerning cB values. The first C expression is to be used when both molecules are linear or both nonlinear; the second, when Molecule 1 is linear and Molecule 2 is nonlinear; the third, when Molecule 1 is nonlinear and Molecule 2 is linear. S is for the molecule for which F/c, is larger. a

where g = 1 for linear molecules and g = 3/2 for nonlinear molecules. Combining Eqs. (73), (74), and (75) for a linear molecule, (AS*/R)= -1n(Ft,Fro,/cg)- 7/2

(76) But (Table I) C = (F/c,) = (F,,F,,,)/c,, and after substitution and rearrangement Eq. (76) becomes

c = e-7/2e-ASt/R For adsorption Eq. (10) is then

(77)

120

RUSSELL W. MAATMAN

Thus, for given u, E, and T values, an order of magnitude decrease in L corresponds to an increase in ASt of 4.6 e.u.; that is, A(AS$)/A(logL) = -4.6 e.u. Values of A(ASt)/A(logL) for all steps are given in Column 4 of Table 11. Since these values depend upon what the reactants and activated complexes are assumed to be, ASt is defined in Column 3. An example illustrates the usefulness of Table 11. Suppose a certain adsorption reaction is 0.5 order, and it is concluded that dissociation accompanies adsorption; that is, Step 2 applies. Suppose also that L has been found by a nonkinetic method to be l O I 4 sites cm-*, and that according to TST L is calculated to be 10l6 sites cm-2. To decrease the calculated value of L by a factor of 100 means that ASt (a negative quantity) as calculated from the model is 18.4 e.u. (that is, 2 x 9.2e.u.) too low. Thus, in this example the gas did not lose as much entropy upon adsorption as had been supposed. Such a result could indicate that the dissociated fragments are mobile, not limited to fixed sites.

TABLE 111 Some Moments of Inertia

Molecules Linear molecules H2 D2 N2

co

Nonlinear molecules H2O NH, D*0 H2S CH4 C2H4 Methanol NO* C2H6 Formic acid

so2 Cyclopropane Propene

Moment of inertia”

Molecules

0.459 0.918 13.99 14.48

2.41 5.93 6.35 7.04 12.33 73.9 89.0 123.9 136.5 267 277.4 383.7 410.7

Moment of inertia“

16.41 19.36 66.9 71.9

Ethanol Acetone Propane cis-2-butene Isobutene Acetic acid trans-2-butene I-Butene Isopropanol Isobutane Benzene sec-butanol Cyclohexane

492 551.3 566.7 602.9 621.3 1017 1175 1257 1347 1506 2536 2812 3552

Where I is the moment of inertia, the quantity given for linear molecules is I x lo4’ g cm2; for nonlinear, (IxIyzz)1’2x lo6’ g3I2cm3.

121

CRITERIA IN SOLID-CATALYZED REACTIONS

TABLE IV S for Typical Molecules and Conditions M (g mole-')

T ("K)

2 2 2 20 40 40 40 60 60 80 80

300 300 500 500 500 500 500 500 500 500 500

Moment of inertia" 0.5 0.5 0.5 5.0 75 125

400 500 1250 2000 4000

P (at4

S

(e.u.

0.1

37.26 32.69 36.24 47.68 55.13 56.14 58.45 60.11 61.93 63.72 65.09

1.o 1.o 1 .o 1.o 1 .o 1 .o 1.o 1.o 1.o 1.o

~~~~~~

The moments of inertia for linear and nonlinear molecules are given in Table 111 and defined in Footnote a of that table. The S values are for linear molecules; for nonlinear molecules, add (- 1.54 + 0.99 In T ) e.u. The symmetry factor is omitted. For all molecules, add 4.58 e.u. for a tenfold decrease in P and subtract 1.55 e.u. for a 20% decrease in T. a

To facilitate the use of Table 11, we provide equations for the entropies of gases. For a linear molecules, (SIR) = - 3.86

+ (3/2) In M + (7/2) In T + In(l x

lo4') - In P

(79)

For a nonlinear molecule, (SIR) = -4.64

+ (3/2) In M + 41n T + ln[(ZxZ,,~z)l~z x lo6']

-

In P (80)

In Table I11 we list typical moments of inertia. With these values, others can be estimated well enough for the approximations needed to use Eqs. (79) and (80) in connection with Table 11. In some cases we need only approximate values of S. To aid such calculation we present in Table IV S for typical values of P , T, M , and the moment of inertia. Thus, in connection with the discussion of specific systems in Section 111, the reader is given the opportunity, as he or she uses Tables II-IV and Eqs. (79) and (go), to relate ASt to the L calculations that we report.

111. Analysis of Reactions

In this section we present our calculations of L for many conceivable rate-determining steps for catalyzed reactions reported in the recent liter-

TABLE V Log L Values: One Reactant, Inorganic Log L, Step T

L N

Example

Catalyst

Reaction"

(OK)

molecules

mole-')

cm-, sec-')

1,4,6

2

3

9

-4

7

8

9

Ref.

4 24

10

9

16

6 22 11

10

17

8

18

3 13 35 27 23

19

5

1) + 4 0 (ads) + 2 H 2 0

373

13.2

4.1 x lo9

14

343

5.9

5.7 x IOl4

14

733

14.2

5.2 x 10''

11

7

Ru/Fe

w3, 1) 0-H, D, (0.1,0.9) + 2H (ads) 2HD 14N2(0.16,0.5) + 2 5N(ads) 2 14N''N

690

52

8.0 x lo9

24

19

Ir/A1,03

2N (ads) + N2

645

26.8

2.5 x l o i 3

-

-

-

MnO,

0, (1, 1) + 2 0 (ads)

418

30

6.6 x 10"

25

20

4

CUO

0,(1.3 x

373

15.3

3.8 x 10''

20

13 - 1

7 33 18 14

16

513

9.9

7.4 x 10"

15

10 -6

4 26

16 11

22

3.8 x 10''

22

15

9 35 22 16

23

CUO

H, (1.3 x

Tab.,,

p-H, (2.6 x

BP

+

+

+

-1

+

2 0 (ads)

0, (0.04,0.5) + reduced surface +oxidized Th surface molybdate

9

E (kcal

cu molybdate

0.7) + I6O (ads) + " 0 (ads)

"0, (2.6 x " 0 l60

+

774

40

10 -4

-8

0

3 20

11

9 - - 16 34 27 23

20 21

10

V205I K2S04

0, (0.2, 1) + reduced surface surface

11

Ni/Al,O,

2CO (1,O) + 2C (ads)

12

NiO

CO (0.3, -)

13

e

N W

Pt/A120,

-+

-+

oxidized

+ 0,

CO (ads)

+ 40, N2O + 40,

NO (5 x lo-,, -) +4N2

14

BaO

2 N 0 (1 x lo-,, -)

15

NiO

CO, (ads) -+ CO,

16

ZnO

+

17

BaO

18

Pt

19

Fe

+ 40, N,O (1 x lo-’, 1) N, + 40, 2NH, (1.4 x lo-’, 1) N, + 3H2 2NH,(1.3 x 1 0 - 2 , 0 ) + N 2 + 3H2

20

MnO,

NH3 (1, 1) + surface.

21

Pt

N,O (7.9 x lo-’, 1) 4 N, +

+

2NH, (2.6 x

-+

-)

N H (ads), H,O

+

N,

+ 3H2

593

29.3

9.7 x 1017

26

21

5

573

32.8

2.8 x 10’’

22

17

1 12 31 24

373

13.8

1.3 x 1017

22

17

1

-

-

1073

17.1

9.1 x 1014

-

16 37 28 23

24

19

25

19

26

5 - -

-

27

28

12 31 23

1023

16.8

8.0 x 10l6

-

-

7 - -

-

330

6.8

2.0 x 1017

-

-

-

9 - -

-

26

3.5 x 1013

22

15

-1

9 34 23

17

29

6 33 20

14

28

833 1023

34 15.4

1.0 x 10l6

20

13 -3

800

21

6.5 x

23

17

1 10 35 23 17

30

890

49.6

1.0 x 10l6

28

22

7

31

418

17

700

0.6

lOI7

1.2 x lo’,

18

14 -3

1.8 x 1017

-

-

-

15 42

28 22

9 27 21

15

21

4 - -

-

32

The first number in the parenthesis following the gas reactant is its pressure in atmospheres; the second is the order with respect to that reactant. No order is recorded where the data were reported so that they can apply to only one step, or where none was given.

TABLE VI Log L Values: One Reactant, Organic

Log L, Step Example

-

Catalyst

T E (kcal (OK) mole-')

Reaction"

(molecules cm-2sec-') D

1,4,6

2

3

413

19

1.8

109

-

-

-

n-C3H,OH (-, 0)+ C3He H2O

468

22.7

1.6 x lo',

-

-

-

Si/Al/Mg

t-C,H,OH (0.4, 0)--+ i-C,He + H2O

393

22

1.5 x lo',

26

19

4

ZnMoO,

i-C3H,OH (1,O) + C& H,O

573

23.5

2.2 x lo',

23

5

ZnMoO,

I'-C,H,OH (1, 1) 4 H, + CH,COCH,

573

24.4

1.3 x

6

SrO

i-C,H,OH (0.03, 0) + H2 CH3COCH3

593

32.5

Cu/AI,O,

sec-C,H,OH (0.05, 0) + H, CH3COC2H5

486

8

Th molybdate

C2H,0H (0.01, 1) + oxidized surface + reduced surface

9

SO, iNaX

ZnO

1

TiO,

C2HSOH (-0.1,O) H2O CzH4

2

BPO,

3

10

+

--+

+

8

9

Ref.

-

-

-

33

1 0 -

-

-

34

7

5

13

39

30

20

35

17 -1

10

36

28

18

36

24

18

0

11

37

29

19

36

5.5 x 10"

25

18

1

11

40

29

18

37

12.5

1.2 x lOI3

20

13 -4

6

35

24

14

38

513

9.3

3.5 x 1013

19

12 -5

4

33

21

12

22

cis-C4H8(0.17,O) -+ trans-C,H,

298

7.8

9.9 x lo9

15

-8

3

27

19

10

39

cis-C,H, (0.13, 0)

313

11.2

6.9 x 10"

18

12 -5

6

31

22

13

40

13

41

42

+

+

+

-+

I-C,H,

IOl5

9

2

5

11

La203

l-C,H, (0.13, 0) + cis-C4H8

294

4.6

1.1 x 10l5

18

12 - 5

6

31

22

12

cuix

l-C,H, (0.03, 1) + cis- + trans-C,H,

313

12.6

2.6 x lo9

19

12 -1

5

32

22

13

13

Pd,iAl,O,

l-C,H, (1, 0)

473

28.8

1.9 x lo',

27

21

15

40

32

22

43

MgO

l-C,H, (1, 1) cis- + trans-C,H,

299

4.7

4.0 x 10"

15

9

3

27

20

10

44

14

+

-+

c~s-C~H,

3 -8

15

ZnO

cis-C,H, (0.1, -0) + trans-C,H, + I-C4H,

593

30.5

16

CI/A120,

cis-C,H, (-, 0) + I-C4H,

453

27

17

MnO

HCOOH (5.3 x 10- z, 0) H 2 0 + CO

573

32

-+

9.0 x 10"

-

10"

24 -

17 -

0

-

_

2.9 x 10"

25

18

2.4 x 1OI2

16 21

10

2

11

37 -

27 -

18 -

45 46

12

38

28

19

47

11 -6

5

26

19

12

13

14 -2

7

35

23

14

48

-

10

18

2CzH3D (1, -) C2HzD2 + CzH4

348

9.5

19

C,H6(1.0 x

-)+

741

35.5

6.0 x 109

0) +

773

42.7

5.0 x 10"

C3H, (4.0 x lo-', 1) +

633

30.5

3.5 x 1013

25

18

1

11

39

28

19

50

0

10

39

29

18

51

-+

C2H4 f H 2

20

CrZ03

21

Pt

C2H, (3.9 x CZH4 + H2 C3H6

-

_

-

-

-

49

+ H2

22

1%Pt/Al,O,

c ~ c / o - C ~(0.07, H ~ 0) ~ + CsH.5 f 3Hz

463

17.1

5.7 x 1014

24

17

23

0.2% Pt/

c ~ c / o - C ~(0.07, H ~ 0) ~ + CsH, 3H2

463

14.3

7.4 x 1014

23

16 - 1

9

30

21

15

52

523

16.6

1.2 x 1OI6

24

17 - 1

10

37

29

18

53

521

19

6.0 x l o L 2

23

15

-1

8

38

27

16

54

18

11

-7

4

32

24

12

55

A1203

+

24

Pt/AIZ03

c ~ c ~ o - C ~ (0.9,O) H,, C6H.5 f 3H2

25

Pt

cyclo-C6H12(1.2 x C6H6 f 3H2

26

M n 0 2I MOO,

cyclo-C,H,, (1, -) + oxidized surface -+C,H,, H,O

726

11.5

4.8

C,,H, (0.01, 0) + oxidized surface products

573

13

1.5 x 1OI6

623

33.5

27

v205

I

K2S04

28

Fe203

-+

2CH3COOH (1, CH,COCH,

See Footnote a, Table V.

-+

-)+

-+

1) +

C02

+ H2O

-

1013

10'2

-

-24

-18

-0

-11

-

-

- 37 -29 - 19

24 56

126

RUSSELL W. MAATMAN

TABLE VII

Example

Reactiona

Catalyst

+ D,

1

MgA1204

H, (3.2 x lo-’, 1)

2

Pd/A120

3H2 (0.75, 1.03) + CO (0.25,0.03)

3

Fe/Si02

3H2 (0.75, -)

+ CO (0.25, -)

4

Ni/Al2O3

3H2 (0.75,0.8)

+ CO (0.25, -0.3)

5

Ni/Zr02

3H2 (0.6, -)

+ CO (0.2, -)

6

Ni/SiO,/

3H2 (0.8, -)

+ CO (0.2, -)

(3.2 x

1)

+

+

-+

CH4

-+

CH4

667

+ H20

548

+ HZO

CH4

+

2HD

+

+ H2O

548 548

CH4

+ H2O

523

CH4

+ H2O

573

CH4

+ H2O

548

CH4

+ H2O

548

A1203

7

Ni/A120,

3H2 (7.5, -)

+ CO (2.5, -)

8

Ni/ misc hmetal

3H2 (7.5, -)

+ CO (2.5, -)

9

Fe

3H2(4.5, -)

+ CO(1.5, 0)

10

Rh

3H2 (0.7, -)

+ CO (0.2, -)

11

Fe

4H2 (4.5, -)

+ CO2 (1.5, 0)

12

Rh

4H2 (0.7, -)

+ C02 (0.2, -)

13

Ni/Cr,03

4H2 (-, -)

+ CO, (3.7 x

14

Ru/Fe

3H2 (0.6, -)

+ N2 (0.2, -)

15

CeRu,

3H2 (37.5, -)

+ N2 (12.5, -)

+

2NH3

723

16

Mo/SiO,

3H2 (37.5, -)

+ N2 (12.5, -)

+

2NH3

723

17

Pt/SiO,

2H2 (4.5 x

0)

+ 0,(4.5 x

1) + 2H20

273

18

Pt/Si02

2H2 (9.0 x

1)

+ O2 (9.0 x

0)

2H20

273

+

-+

-+

CH4

-+

+

+ H20

663

CH4

+ H2O

573

CH4

+ 2H20

573

--+

CH4

+ 2H2O

0.5) -+ CH4

+ 2H20

2NH3

573 448 690

-+

-+

127

CRITERIA IN SOLID-CATALYZED REACTIONS

Log L Values: Two Reactants, Inorganic Log L, Stepb E (kcal

t' (molecules

mole-')

cm-*sec-')

7.2 19.7 6.9 25.1

1,6

2

5

3

7,lO

8

9

11

12

Ref.

4.6 x 10"

11 12

7 7

-7 -7

2

21

9

7

2 1

12

57

1.2 x

14 18

11

-4 -3

8

25

17

14

11 4

18

58

13

9 13

6 8

-9

2

19

11

8

6 -2

13

59

17 21

14 16

-1

11

27

20

16

14 7

21

60

19 22

15 17

0 1

12

29

21

18

15 8

22

61

0 1

11

28

21

17

15 7

22

62

10

25

19

16

14 6

20

63

14

28

23

20

17 10

23

63

11

27

20

17

14 7

21

64

1013

5.2 x 10" 3.7 x

1013

-8 0

28

2.3 x 10i3

27.7

8.3 x

27.6

1.8 x 10"

16 20

13 15

32.4

8.6 x

lOI3

19 23

17 18

23

3.3 x

1OI6

17 21

14 16

-1

lOI3

-

_

-3 -I 1 2

0

24

1.3 x 1014

17 21

14 15

-1 0

10

27

19

16

14 6

21

65

17

1.0 x 10l6

15 20

12

-3 -2

9

26

19

16

14 4

20

64

15

14 19

11 13

-4 -3

8

26

18

14

12 3

19

65

_ -1

8

16

3.6 x 1014

17.3

1.4 x

13

10.3 25 3.4 1.8

_

_

-

-

-

-

-

66

20

14

10 13

6

-9 -8

2

20

12

9

6 -2

13

19

8

10

7 9

-9 -7

5

19

14

11

8 1

14

67

14 2.0 x 1O'O

10 14

7 9

-9 -8

5

19

14

11

8 1

14

68

1.3 x 1015

12 16

8 11

-6 -4

5

23

13

10

9 0

16

69

7.2 x

11 13

7

-7 -7

4

21

12

9

6 1

13

69

8

3.1 x 10" 6.0 x l O I 4

1014

2

(continued)

TABLE VII ~

Example

Reaction‘

Catalyst

19

NiO

2H, (4.4 x

-)

+ 0, (2.2 x

20

HfC

2H, (5.5 x lo-,, 1)

+ 0, (0.2, 0 )

21

ZnO

2CO (4.4 x lo-,, 0.9)

22

NiO

2 c o (0.1 1, 1)

23

NiO

2CO (2.6 x lo-,, - )

24

CU/Y

2 c o (5.9 x 10-3, 1)

+ 0 , (0.2,o) .-+ 2 c 0 2

623

25

Ag

2CO (6.2 x lo-’, 1 )

+ O 2 (4.6

326

26

Cr(III)/Y

0, (5 x lo-,, 0.3)

27

Pt/AI,O,

0, (0.33, 0 ) + 2S0, (0.14, 1 )

28

CUO

30, (0.9,O) + 4NH3 (0.1, 1 )

29

PNQ‘

0, (0.2, 0 )

30

Ag

31

-)

+ 0, (0.22, 0) +0

2

-+

2H,O

623

lo-’, 0.1) + 2 0 ,

413 523

2c0,

(1.4 x lo-,, --)

-+

2C02

x lo-,, 0) + 2C0,

+ 2 N 0 (3 x -+

578

2H,O

-+

+ 0,(2.2 x

+

0.7)

-+

2N0,

513

513 683

250,

+ 6HZO

513 328

U oxide

+ 2H2S (1, -) 2s + 2HzO CO (1.6 x lo-,, 1) + 2 N 0 (1.6 x lo-,, -) N2O + C02 CO (0.1, 1) + NO (0.2, 0.4) N,, N,O, CO,

32

CUO

c o ( 7 x 10-3, -1 NO(^ x

33

Mo oxide/ SiO,

CO (7.8 x lo-’, 0) + N,O (3.8 x lo-,, 1)

34

Mo oxide/ SiO,

CO (9.3 x lo-’, 0)

35

Sn(IV) oxide

CO (0.4, 0.2)

36

Na2S04/

CO(5.3 x lo-,, 1)

-+

2N2

--+

380

-+

583

+

+ N,O

+ N,O

(0.2, 0)

+

(0.1, 0.5) + CO,

+ N,O(5.3

638

-)+N,,N,o,co,

CO,

-+

CO,

+ N,

+ N2

333

+ N,

x lo-’, 0) +CO,

333

463

+ N,

713

v205

See Footnote D of Table V for each reactant, For Steps 1, 2, 3, 6, and 11 the first (upper) log L value is for the first reactant; the second (lower) value is for the second reactant. Polynaphthoquinone. 128

(Continued)

Log L, Stepb E (kcal mole-')

(molecules crn-'sec-') l'

1,6

2

3

15 18

10 11

-4 -3

5

28

14

11

8 1

18

70

7,lO

5

8

9

11

12

Ref.

12.3

1.1

18

1.0 x lo'*

13 16

9 11

-5 -5

5

24

15

11

8 2

16

71

21

2.0 x 1 O ' O

18 18

12 13

-3 -3

7

29

19

14

7 6

18

72

22.5

1.5 x 1013

20 20

15 15

-1

10

30

21

17

9 9

20

73

-1

- lo9

12 16.4 9.3

1013

-13 -13

2.2 x lo6 1.1 x 1013

-

11.5

109

-7 - - 8 -7 - - 8

-2

11 10

4

-10 -11

17 17

12 12

-4

-12 -14

5

-4

-6 --9 -7 - - 7

-1 6 -1

-24 - 13

-9

-2 -1

-13

74

22

11

6

-3 0

11

75

27

17

13

6 6

17

76

- 25 - 13 -8 - -2-2

-14

77

17.5

6.7 x 1013

17 20

12 13

-4 -4

6

30

21

14

9 3

20

78

20

1.6 x 1013

19 19

14 14

-3 -2

9

29

21

16

9 8

19

79

13

2.9 x 10"

-

-

14.4

2.2

1013

20 20

14 14

-1 -1

9

30

20

16

9 8

20

76

15

2.7 x 1014

18 18

12 12

-3

7

28

19

14

6 7

18

81

-4

-

-

_

7

__

-

-

80

14

1.1

1014

18 18

12 12

-3 -3

6

30

18

13

6 5

18

82

10.8

2.5

109

14 15

9 9

-7 -6

4

25

15

10

5

15

83

2

14.9

1.4 x 109

16 17

11 11

-5 -5

6

26

18

13

6 5

17

83

24.4

1.4 x 10"

20 21

15 15

-1 -1

10

31

22

17

11 8

21

84

32

2.1 x 1013

21 22

16 16

10

33

23

17

11

22

85

0 0

9

130

RUSSELL W. MAATMAN

TABLE VIII

Example

Catalyst

Reaction"

1'

cu

C2H4(2.6 x lo-,, -)

2'

cu

H, (2.6 x lo-,, -)

3'

cu

C2H4(ads)

-+

+ H (ads) (ads) + H (ads)

4'

cu

C,H,

5'

Pt

C,H4 (2.6 x lo-,, -)

6'

Pt

C2H4 (ads)

7'

Pt

H, (2.6 x lo-,, -) 2H (ads) + H,

-+

-+

370

2H (ads)

+

+

C2H5 (ads)

370

C2H6

370

C2H4(ads)

357

-+

357

C,H4 -+

2H (ads)

357

8'

Pt

9'

Pt

CzH4(ads)

+ H (ads)

10'

Pt

C2H5(ads)

+ H (ads)

11

Pt/SiO,

Hz (0.91, -)

12

Pt/SiO,

H2 (0.9, -)

13

Pt /SiO

14

Pd/A1,03

H, (0.1, 0.5) + cyclo-C6H,, (9.9 x cyclo-C6H12

15

Pt

H2 (0.9, -)

16

Pt

3H2 (0.9, -)

17

Ni/A1,0,

3H2 (1.3, 1.25) + C,H6 (0.13, 0.3)

18

Pt/Y

3H2 (0.9, -)

19

Pt-PA66*

H, (0.99, 0)

20

Pd

H, (0.7,

21

Pt/SiO,

,

370

C2H4(ads)

357 +

+

C2H5 (ads)

357

C2H6

357

+ CyCio-C3H6 (0.07,O)

+ cyclo-CH,

-+

C,Hs

- C,H, (0.07, 0) + i-C4Hlo

H, (0.1, 0.8) + cyclo-C,H,, (1.7 x lo-*, 0) c~c~o-C 1 2~ H

-1

0)

+ c ~ c ~ o - C ~(0.06, H ~ O-) + CsH,j (0.06, -)

+ C6H6 (0.09,O) + styrene (1.3 x

cyclo-C6H12

273 273 338 338

cycio-C,Hl,

398

C JX ~ O - C~ H , ,

298

-+

lo-,, 1)

+ acetone (0.3, 0.4)

-+

273

c~c~o-C~H,,

-+

-+

-+

-+

273

-+

-+

C6H5C2H5

i-C3H,0H

374 448 216

131

CRITERIA IN SOLID-CATALYZED REACTIONS

Log L Values: Two Reuctants, One of which Is Organic

Log L, Stepb E (kcal mole-’)

1.5

(molecules crn-’secC’) L’

1,6

2

3

5

7,lO

8

9

11

12

Ref.

6.8 x lOI4

15

9

-7

3

-

86

11.3

4.5

1013

15

11

-3

7

-

86

19

1.9

1015

-

86

4

-

86

-1

9

-

87

-1

9

-

87

-2

9

-

87

-

87

6

3.0 x lo”

10.1

1.0 x 1OI6

10.4

3.1 x l O I 5

8.9

7.6 x lOI5

13.7

14

1.1

21

16

15

12

1015

0

11

9.7

6.8 x 10l6

8.2

5.7

9.6

1.6 x I O l 4

15 21

12 15

-3 -1

9

26

9.5

1.4

1014

14 22

12 15

-3 -1

9

27

20

8.0

9.4

10i4

15 23

12 16

-3 -1

9

29

9.3

9.7 x 10i4

16 24

13 17

-2 0

10

11.7

3.3 x 1OI6

17 25

14 18

-1 1

11.9

2.6 x

15 23

12 16

-3

10

8

1015

87

~

-

-

87

16 2

21

88

15

16 1

22

88

20

14

16 1

23

89

30

21

15

17 1

24

90

11

31

23

17

19 3

25

91

9

29

21

15

17

23

91

1

-1

10.1

4.2

1013

12 20

9 13

-6 -4

6

26

18

12

14 -2

20

92

11

2.4 x

1013

14 22

11 15

-4 -2

9

27

20

15

16 1

22

93

- 21 - I5 -- 17 -24

94

17

95

24

88

13 8.1 10.4

- 10’4 1.6 x 7.5 x

IOl3

1013

-- 2414 --1116 --- 1- 4 11 17

7 11

17 24

14 18

-8

-1

4

-8

-29 23

16

10

10

-3

-7 -1 1

12

28

21

17

18 5

(continued)

132

RUSSELL W. MAATMAN

TABLE VIII

Example

Catalyst

22

Pt/A1,03

23

CoO/MoO,/ A1203

Reaction’

573 H, (0.8, I) + benzothiophene (4.3 x C ~ H ~ C Z H ,HzS

+

+ C,H,

1) -+

623

1) +

423

24

Ag

302 (1.1 x 0) 2C0, + 2H,O

25

Ag

30, (2.4 x lo-’, 0.5) 2C0, + 2H,O

26

Bi,FeMozO1,

40, (0.16, 0)

27

FeZ(MoO4)3/ 0, (0.2, -) MOO,

28

Pt

29

CrzO,

D2 (0.15, -0.1)

30

NiO/AI,O3

NO (0.07,

31

Rh/Cr,O,

723

32

Ru/A1,03

713

33

Rh/A120,

673

(1.3 x

+ C,H,

(1.3 x

+ 1 - C4Hs (0.16, 1)

+ 2CH3OH (0.01, 0)

-+

3

1) +

C4H6 + HZO

2HCHO

+ 2H,O

413 703 553 328

-

1)

+ CH, (1.5 x + C3H6(0.6, 0)

lo-’, 1) -+

+

CH,D, etc.

C,H,N, etc.

646 683

See Footnote u of Table V for each reactant.

* See Footnote b, Table VII.

Examples 1-4 and 5-10 are each groups of steps constituting a single reaction, and it is the overall reaction, not all of the individual steps, that falls into the classification given by the title of the table. Pt polyamide 66.

ature. The experimental conditions and results used to make the calculations and the results of those calculations are given in Tables V-VIII (13, 26- 105).

We have already given some reasons that the calculation of L is only approximate. In addition, Pritchard and Bacon (106) and Cvetanovic and

133

CRITERIA IN SOLID-CATALYZED REACTIONS

(Continued)

Log L, Step" E (kcal mole-')

(molecules cm-'sec-')

23

1.5 x 1013

4.9

I'

5.2 x 10"

1,6

5

7,lO

8

9

11

21

1.5

2

3

15 23

12 16

-3 -1

9

8 18

5

-10 -7

1

25

15

8

10

9

40

22

30

12

16

Ref.

23

96

11 -9

18

97

16

11 6

26

98

1

21

1.1 x 10"

23 26

16 17

11

2.0 x 10"

20 20

12 12

-1 -2

3

36

16

10

4 3

20

98

17.9

2.8 x 1014

18 21

12 14

-3 -3

7

32

22

15

10

21

99

19 22

14 16

-2 0

9

33

22

16

11 6

22

100

19 26

15

0 2

12

33

24

18

19 4

26

54

15 18

10

-5 -3

6

27

17

12

10

18 I01

12

19.5

1.7

19

6.0 x 10"

23

1014

4.3 x 10"

19

2 3

3

2

21

5.9 x 10"

17 18

11 12

-5 -6

5

29

20

13

7 3

27.5

4.8 x

1013

19 24

14 16

-2 -1

9

34

25

16

14 3

24

103

42.5

3.3 x 1013

24 29

19 21

3 4

13

39

29

21

18 8

29

104

32

1.3 x

20 25

15 18

-1 0

10

35

26

18

15 5

25

105

1013

18 102

Singleton (107) have shown that there are statistical questions concerning the usual kinetic model analysis. Consequently, there is often some uncertainty in the reported kinetic constants, particularly E . For these reasons we give only the order of magnitude of L ; that is, in Tables V-VIII log L is rounded off to an integral value. Several points must be made concerning Tables V-VIII and the discussion of those tables.

1. In some articles cited the data are presented so that they can apply to only one step. In such cases our calculations and discussion focus on that step.

134

RUSSELL W. MAATMAN

2. The lower and upper limits of the true site density are taken to be about lo7 and 1015 sites cm-2, taking into account (at the lower end) the number of molecules which strike the site per unit time and the number that react, and (at the upper end) the minimum area per site. For Step 3, however, L = 1 for reasons already given. In addition, the error introduced by approximations must be taken into account, and so the practical range in logL is from about 5 to 17. 3. Some L calculations are omitted because they are for steps for which the reported data cannot apply. 4. The observed order of the reaction is extremely important and is reported in many cases. Where the order is omitted, it is either not reported or not relevant because the reported data apply to only one step. In some cases we report an order on the basis of our deduction from information given in the article. 5. In most of the articles surveyed several catalytic systems are described. Usually, however, we selected only one of those systems, since in most cases we could choose a system that was representative of all those reported on.

A. ONE-REACTANT, INORGANICEXAMPLES These examples are described in Table V. In Examples 1-3 reasonable log L values for Steps 1,4, and 6 are found. The reactions are close to first order, as required for these steps. Step 4 seems unlikely, but the information presented here does not rule it out. For all three examples one could calculate reasonable log L values for a model incorporating a slow step intermediate between Step 7 (large entropy change as the gas adsorbs on fixed sites) and Step 8 (smaller entropy change as the mobile adsorbed molecules move to fixed sites). But Step 7 is second order, and one expects Step 8 (as well as Step 9) to be greater than first order, perhaps nearly second order in most cases. In all three cases Step 2 should be ruled out because the reactions are not one-half order. For Step 3 to be tenable, log L must be near zero, and Step 5 is ruled out because the reactions are not zero order. If we then choose Step 1,4, or 6, we conclude that there is a rather low site density (log L = 11)in Example 3. We cannot increase this site density estimate by changing the entropy of activation, since for Steps 1, 4, and 6 the only change possible is such that the log L value calculated becomes even smaller. Analyses similar to those just given can be made for many examples in the remainder of the discussion. We shall attempt to minimize repetition. Examples 4 and 8 are both one-half order, but the log L calculations indicate that the explanations of the results in these two cases might not be the same. With Example 8, the rate-determining step could be Step 2, dissoci-

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ative adsorption, with a rather low site density. Since a zero-order reaction is a saturated surface reaction and a first-order reaction can be a surface reaction on a sparsely covered surface, an order between zero and one, such as one-half, can indicate that the reaction occurs on a partially filled surface. Example 8 could be such a reaction. (Obviously, the order would then be a function of reactant partial pressure, even though the order might change so slowly with reactant partial pressure that the dependence could not be observed.) But it does not seem that either this possibility or Step 2 can be used to explain Example 4. For Example 4, logL = 19 for Step 2. This is about the same value that we would obtain were the data for the middle of the nitrogen adsorption isotherm, that is, a value midway between 13 (Step 5, zero order) and 24 (Step 1, first order). Suppose that we attempt to devise a model so that log L = 15, an acceptable value, for Example 4. Let mobile atoms be the adsorbed species. The value of 19 for Step 2 must be decreased by four units. According to Table 11, the gas must lose 36.8 e.u. (that is, 4 x 9.2 e.u.) more than is postulated for Step 2. But S for N, at 690K and 0.16atm [Eq. (79)] is only 56.6e.u. A loss of only 19.8 e.u. (that is, 56.6 e.u. - 36.8 e x ) upon adsorption seems impossible, since the rotational loss alone (which must be included since the model calls for dissociation into atoms) is 12.9 e.u. The difficulty with Example 4 is that an activation energy of 52 kcal mole- is extremely large. We cannot choose a possible rate-determining step from the data. The calculations made for Example 7 indicate that it too may be a reaction in which the rate-determining step is dissociative adsorption ;the log L value of 13 for Step 2 is reasonable. However, the order of the reaction was not reported. Examples 5 and 15 are for desorption reactions, and therefore Steps 5 8 , and 9 could be relevant. The values for Steps 8 and 9 could not be calculated; the partition functions for the mobile species were not available. If Step 5 is to be chosen for Examples 5 and 15, the site densities are low. Such a low site density situation in desorption is not unknown. Hayward, Herley, and Tompkins (208)found log L 8 for hydrogen desorption from Ni, and they suggested that, even when surface coverage is large, desorption might take place from only a very few favored sites. Is it possible that low site densities are obtained in some desorption reactions because an incorrect assumption is made about the entropy of activation? For Step 5 we have assumed that ASx = 0. Were we to modify this step to obtain a larger log L , we would have to postulate that the adsorbed molecule loses more entropy as the activated complex forms (4.6 e.u. per unit change in logL) than it does in Step 5 as we have described it. Such a sequence of events is not impossible for a surface reaction. But if the adsorbed molecule is immobile, it is difficult to imagine such a species losing

-

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RUSSELL W. MAATMAN

entropy as the activated complex forms; if in desorption AS$ # 0, one would expect AS* to be positive, not negative. Examples 6, 10, 16, 17, 18, and 20 are similar to Examples 1-3 in that they are first order, but they differ in that the log L values calculated for Steps 1, 4, and 6 are large. For Example 20, Il’chenko and Golodets (21) assumed log L 15 and calculated AS* to be significant. They obtained similar results for the same reaction over CuO, Fe,O,, and V,O,. Thus, surface mobility and/or rotation may be indicated. Il’chenko and Golodets suggested the rotational possibility and also the idea that there may be significant vibration in the surface species not found in the gas phase. Is the explanation used for Example 20 tenable for Examples 6, 10, 16, 17, and 18? The percentages of entropy that the reactant gas molecule would have to retain (instead of losing loo%, or retaining upon adsorption on a fixed site) for the log L values of Steps 1,4, and 6 to be reduced to 15 are 91, 90, 50, 33, and 57%, respectively; the corresponding value for Example 20 is 27%. If the adsorbed species is mobile, it could retain as much as two-thirds of its translational entropy and conceivably all of its rotational entropy. Thus, this explanation seems reasonable for Examples 16-18, but not for Examples 6 and 10. Butt and Kenney (24) do comment for Example 10 that their kinetic equations can be used to correlate the data but that they do not account for the mechanism. Also, even though we have suggested a model for Example 17, one should be cautious in assuming that the TST assumptions, particularly those concerning the transmission coefficient and the transmission frequency, hold at very high temperatures, in this case 1023 K. For Example 9 the order is 0.7, suggesting a model for which the log L value is between that for Step 2 (0.5 order, log L = 15) and Step 1 (1.0 order, log L = 22). Thus, the rate-determining step may be a reaction on a partially filled surface. Since the log L value calculated in this way for 0.7 order is rather large, some surface mobility and/or rotation is indicated. The zeroorder reactions of Examples 11 and 19 are clearly surface reactions for which expected site densities are obtained. For Example 11 Tottrup (25)suggested that the rate-determining step is C - 0 scission in adsorbed CO. For Example 12 the order was not given, but from what has already been discussed it is not difficult to devise a model. Concerning the reaction in Example 21, Pignet and Schmidt (32) cautioned against using their results for the purpose of constructing a model; the extremely small activation energy may confirm their warning. In any case, their data that we use can apply only to a surface reaction. The data reported for the reactions in Examples 13and 14are for saturatedsurface reactions, and therefore only Step 5 can be considered. The calculated site densities are extremely low, and, as before, one must take care in using TST at these high temperatures.

-

Ox,

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137

B. ONE-REACTANT, ORGANIC EXAMPLES This section is a discussion of the examples listed in Table VI. As before, the examples will be taken up in order as much as possible. For Examples 3, 6, 13, 16, and 17 the reaction is zero order and the calculated value of log L is between 11 and 15 for Step 5, as expected. For Examples 2, 4, 15, 20, and 27 the reaction is zero order, and the calculated value of log L for Step 5 is between 8 and 10. Thus, if the surface reaction is the rate-determining step, the site density is very low. This is possible; we suggested earlier that by various means some site densities can be shown to be that low ( I ) . A comparison of Examples 19 and 20 presents an interesting confirmation of a point made earlier. In Example 20 Konig and Tetenyi (49)reported a true activation energy of 42.7 kcal mole-' and for Step 5 log L = 10. Konig and Tetenyi (48) also reported the apparent activation energy, that is, the difference between the true value and the heat of ethane adsorption. The calculations for this case are given in Example 19, and, as expected, log L is significantly smaller. In Example 27 the site density might not be low: the calculations are made on the same system described in Example 10, Table V, and, as noted earlier, the authors warned against using their data to elucidate a mechanism (24). Examples 22-25, all for cyclohexane dehydrogenation over Pt, are similar to the examples just discussed in that the data are all for surface reactions. Also with each of them the log L value calculated for Step 5 is between 8 and 10. We reported similar results and log L calculations (109)and suggested, using independent evidence of Boudart (110),that the site density is not as low as our TST calculations indicate. Thus, the log L calculation can be used to demonstrate the existence of a complexity that might not otherwise be detected, in this case, the possibility that cyclohexene is an intermediate. For Examples 1, 7,9, 10, and 11 the reaction is zero order, but there are probably significant differences between these examples and those just discussed: the values calculated for log L for Step 5 are from 3 to 6 . Log L = 6 is near the lower limit (depending upon P, T , and the area of a single site) allowable, since the site turnover frequency cannot be greater than the number of molecules striking the site. Evidently there are three possibilities for these examples. 1. The site density is actually this low, provided that the criterion concerning the turnover frequency is not violated. It is not impossible that a site density be very low; we expect a priori to find solids ranging from those having a large number of active sites all the way to those having no active sites. 2. The entropy change between reactant and activated complex might not be zero. Thus, if the reactant had some rotational and/or translational

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freedom, AS' could be negative. According to Table 11, for each 4.6e.u. decrease in entropy of activation, log L increases by one unit. Log L could therefore be several units larger. 3. The reaction might be more complicated than indicated. The existence of an intermediate, as postulated in cyclohexane dehydrogenation, is sometimes a possibility. We noted earlier that often in alcohol decomposition reactions the calculated value of logL is low (3). Examples 1 and 7 are for alcohol decomposition; the low Step 5 log L values could perhaps be accounted for by the second possibility, that is, the explanation involving the entropy of activation. Such an explanation has often been given; thus, for Example 1 Carrizosa and Munuera (33) interpret their data by assuming AS' = 30 e.u. For Examples 2, 4, and 6, zero-order alcohol decompositions already discussed, the Step 5 log L values are also low. Examples 9-11 are three of the butene isomerization examples. For Example 9 Otsuka and Morikawa (39)postulated a complicated mechanism, the third possibility listed above for cases in which log L is very low. Lombardo et al. (40) suggested for Example 10 that their results uncovered the existence of a complication. They stated that the problem might lie with the TST frequency factor. (Their results are similar to those given in Example 15, discussed above, where the log L values are only slightly higher.) The frequency factor conclusion can also be made for Example 11, where the log L values are like those of Example 10 (even though the kinetic parameters are significantly different) and for Example 26, for the reaction of cyclohexane with an oxidized surface. For Example 14, for first-order isomerization of 1-butene, either Step 1, or 4,or 6, with a log L value of 15, is apparently the rate-determining step. Example 18 is a reaction between two adsorbed molecules of C,H,D. Aldag, Lin, and Clark (13) postulated that the reactant molecules are mobile and that both reactants and activated complex are free to rotate. That is, Step 9 is the rate-determining step. For that step log L = 12, a reasonable value. For Examples 5, 8, 12, 21, and 28, all first order, the logL values for Steps 1, 4, and 6 are too large. With Examples 8 and 12 one can obtain reasonable log L values by postulating (as was done in connection with similar examples listed in Table V) that the gas molecule does not lose all of its entropy upon adsorption. For Example 28 Kuriacose and Jewur (56) postulated a bimolecular surface mechanism, that is, Step 4. The L value for that step is very large; the authors claimed that an intermediate is ferric acetate. If this is correct, one would indeed expect the L calculation to indicate that the reaction is more complex than any of our models. For

139

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Examples 5 and 21 we cannot choose rate-determining steps consistent with the orders and the log L values calculated. For Example 21 Biloen, Dautzenberg, and Sachtler (50) postulated a rate-determining surface step that is, since the reaction is first order in propane, Step 6. They reported the reaction is - 1.1 order in PH2. In Section II,B,8 we discussed the question of determining site densities using high-conversion data. We developed a method applicable i n the interconversion of three isomers when there is a common surface complex for the three possible reactions. We have tested this method using the conversion of 1-butene to cis- and trans-2-butene over silica-alumina, a system that, according to Hightower and Hall, proceeds through a common surface complex (111).Their conclusion has been confirmed experimentally (112) and by semiempirical quantum-chemical calculations (113). We carried out the reaction in a flow system under conditions such that the conversion level was high but well below equilibrium conversion. We used C.P. 1-butene from Matheson and passed it over 100-200 mesh Mobil silica-alumina catalyst [10% Al,O,; surface area, 393 m2 g- (BET)]; the batch was heated 1 hr at 450°C in an air stream and kept in a closed container. Gas chromatographic analysis was used ; neither reactant impurity nor a thermal rate was found to be a complicating factor. The reaction was carried out at 120, 135, 150, and 165°C at several partial pressures, using N, as diluent, up to 0.95 atm. The reactant flow rate was always 1.56 x lop3 mole min-'. A steady state was achieved in about 20 min, and the activity for a run was taken to be the average of three determinations made between 35 and 50 min. Conversions were determined at initial 1-butene partial pressures between 0.58 and 0.95 atm and using several weights of catalyst between 0.50 and 1.00g. We determined rounded-off conversions for three ( W / U ) values, enabling us to use Eqs. (65)-(67). However, two prior questions need to be settled.

'

1. Equations (65)-(67) may be used only if a in Eq. (61) is truly a constant. We concluded (see Table IX) that this is an adequate approximation. 2. Also, to make our calculations we must assume that the reaction is zero order; that is, in Eq. (64), b = 0. Using 1.00 g catalyst, the temperature and the total conversion at 0.58,0.76, and 0.95 atm were, respectively: 120°C, 0.101, 0.111, 0.129; 135"C, 0.190, 0.192, 0.201; 15OoC, 0.278, 0.281, 0.290; 165"C, 0.358, 0.364, 0.375. Essentially the same results, but with more scatter, were obtained using 0.75 and 0.50 g catalyst. We conclude that the reaction is near zero order at 0.95 atm. Table X presents the conversions for the three different values of W , the weight of catalyst, at the four temperatures. These conversions and W values

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TABLE IX cis Fraction of 1-Butene Conversion”

0.50 0.62 0.75 0.88 1.oo @

P

=

393

408

423

438

0.86 0.83 0.84 0.82 0.85

0.89 0.83

0.76 0.65 0.70 0.61 0.58

0.53 0.55 0.60 0.49 0.53

-

0.80 0.69

0.95 atm.

TABLE X I-Butene Conversion”

(OK)

393 408 423 438

Treatment of Eqs. (65)-(67)b

Conversions

-

I

A

x

x

0.081 0.151 0.217 0.347

0.096 0.172 0.244 0.364

0.129 0.201 0.290 0.375

XIU

8.54 5.22 3.64 2.69

5.27 2.31 3.78 1.68

x

lo4

x lo4 x x

UlY

YIX

ZIX

lo4 lo4

1.45 1.13 8.27 4.02

x

lo4

x lo4 x x

lo3 lo3

8.08 1.70 3.32 9.25

x x x x

P = 0.95 atm;J , A , a n d x are for 0.50, 0.75, and 1.00 g catalyst, respectively. The dimensions of the ratios are such that ( u / y ) is given in moles of I-butene converted per second per gram of catalyst.

were used in Eqs. (65)-(67); the roots of those equations are also given in the table. (All other sets of roots violated one of the two criteria given in Section II,B,8, and therefore the sets in Table X are unique.) The quantity (u/y) was determined using Eq. (71). In an Arrhenius plot (not shown) of (u/y) the scatter was significant, but it could be determined that E x 18 kcal mole- Using Eq. (72) we obtained log L z 9. This calculation applies to Step 5. Some of the Step 5 log L values for the other zero-order butene isomerization reactions given in Table VI are greater than 9 and some are smaller. The reasons for these variations have been discussed, and they can be used considering the present case also. It seems, however, that conversion data better than ours can be obtained, and that therefore fairly reliable log L values can be derived from the high-conversion method described.

’.

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C. TWO-REACTANT, INORGANIC EXAMPLES We now turn to the examples given in Table VII. For Example 1, reaction of H, with D,, Vickerman (57)reported that the reaction is first order in total pressure. Since the molecules are almost identical, the reaction is presumably first order in each reactant. From log L calculations we deduce that Steps 1 and 6 for each reactant and Step 12 are all possibilities. The Step 12 calculation is made, however, assuming that the species reacting on the surface are the same as those in the gas phase, in this case H, and D2 molecules. Probably the existence of a surface ratedetermining step would require that the reacting species be adsorbed atoms, not molecules. Examples 2-10 are for the hydrogenation of C O to produce CH,. For Example 2, the order and calculated log L values suggest that Step 1 or 6 for hydrogen is the rate-determining step. If it is Step 1, the rate-determining step is the adsorption of H, on a CO-saturated surface. If it is Step 6, it is a surface reaction between hydrogen and CO, where the surface is saturated with CO but the amount of hydrogen adsorbed corresponds to the linear part of the adsorption isotherm. We do not have enough information on reaction order in Examples 3 and 7-10. The log L calculations indicate that the explanations for these examples could be the same as that given for Example 2. Probably one would have to modify the reaction model in some cases according to the method using the entropy of activation. As with many of the examples already discussed, the log L values given suggest other possibilities. For example, there could be, depending upon which example is considered, rate-determining reaction of C O with the surface (Steps 1 and 6, CO), surface reaction between adsorbed species (Step 12), or zero-order reaction of a surface complex (Step 5). For Example 4 the order is 0.8 in H, and -0.3 in CO. If the order were - 1 in C O and + 1 in H,, Step 11 for strong adsorption of CO with a very low site density (log L = 7) would be a possibility. Thus, some modification of Step 11 might well be a good description of the rate-determining step. Concerning Example 5, the authors obtained reaction orders-second order in hydrogen, - 1 order in CO-under conditions different from those used to obtain the other data. If the orders are relevant, another modification of Step 11 for CO adsorption may be indicated. Dalla Betta and Shelef suggested in another article that the rate-determining step is scission of the C - 0 bond in adsorbed CO (114). For Example 6, the experimental results are for a reaction between two adsorbed species, and it is the surface rate constant that is given. Therefore, since the surface rate constant, not the surface rate, is used, Step 5 is probably the applicable step. The log L value of

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11 for Step 5 is not unreasonable. Huang and Richardson (62) claimed for this system that the rate-determining step is reaction between C O molecules and H atoms, each adsorbed on nickel atoms. For Examples 11-13, for the hydrogenation of CO, to produce CH, over metal catalysts, the log L values calculated are about the same but the orders are not. For Examples 11 and 12 the rate-determining step may be H, adsorption on a carbonaceous surface, that is, Step 1 for H,. For Example 13 P,, was not given, but H, was in excess. The order in CO, and the log L value for Step 2 for CO, are consistent. For Example 14-16, all for NH, synthesis, the authors implied that a surface reaction is the rate-determining step. For given steps in these three cases the logL values are similar. Either Step 8 or Step 12 could be rate determining; but the reacting surface species are probably not N, and H,, and therefore these steps can probably be ruled out. Almost certainly none of the steps in Table VII are rate determining in NH, synthesis. Examples 17-20 are for the production of water from hydrogen and oxygen. For Example 17, where an excess of H, is used with Pt catalyst, either Step 1 or Step 6 , both for oxygen, could be rate determining. In the same system but with an excess of oxygen (Example 18) the orders are reversed. Hanson and Boudart (69) suggested that their results indicate for Example 18 that hydrogen attacks adsorbed oxygen not from the gas phase but from a small number of sites. Such an explanation is consistent with Step 6 for hydrogen. Example 20, with HfC catalyst, where there was also an excess of oxygen, is somewhat similar to Example 18, where either Step 1 or Step 6 , both for hydrogen, is indicated to be the rate-determining step. For Example 19 Jamieson, Klissurski, and Ross (70) reported the order to be 0.7 in total pressure and gave the mechanism to be one consisting of H, attacking an oxidized site to produce water and a reduced site, followed by oxygen reaction with the reduced site. Combining the information on order and the log L values, we conclude that the rate-determining step is a surface reaction (either of the two steps just described) such that the surface is somewhere in the middle of the adsorption isotherm of whichever species is attacking it. The appropriate logL value would then be between 15 or 18 (Step 6 for H, or 0,, respectively) and 5 (Step 5). Examples 21-25 are all for CO oxidation. A total order of one was reported for Example 23; the other four are first order in C O and zero order in oxygen. Evidently Step 1 or 6 for CO is a possibility in all cases, provided that the modifications discussed earlier are made where the log L values are too large. Kobal, Senegacnik, and Kobal (73) reported for Example 22 that their activation energy was anomalously large, since other values reported in the literature are in the 5-17 kcal mole-' range. If we arbitrarily change

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their activation energy from 22.5 to 15.0 kcal mole-', log L for Step 1 or Step 6 (either reactant) becomes 17, a more reasonable value. Arai, Tominaga, and Tsuchiya (77) postulated for Example 26 that NO adsorbs on previously adsorbed oxygen. The log L values given in Table VII for this example cannot be applied directly to their reaction because they report fractional orders. But a combination of the information on orders and logL values suggests that the site density is low, that the surface is unsaturated with respect to both reactants, and that a surface reaction is rate determining. Example 27 is for the oxidation of SO, in the presence of He diluent. As with some other cases discussed, Step 1 (for SO,) could be chosen, provided that adsorbed SO, retains considerable freedom of rotational and/or translational motion. But the question is complicated because using Ar diluent gives different results; log L values are then about two units larger. Example 28 is for the oxidation of NH,. Here the large value of log L for the steps indicated by the orders reported (log L = 19 for Steps 1 and 6, both for NH,) suggests that the activated complex on the surface may possess some freedom of motion. The calculation of log L in Example 29 is based on the surface'rate constant, and so Step 5 should apply, although the log L value of 7 is rather low. Example 30 is especially interesting. By changing the temperature (results not shown in the table) Cant and Fredrickson (76) were able to show that the NO heat of adsorption on active sites is - 5.2 kcal mole-' and that the NO adsorbs more strongly than CO. We therefore expect that CO adsorption is less exothermic. It can then be shown that the true activation energy, that is, the activation energy for the surface step, is between 9.2 and 14.4 kcal mole-', the precise value depending upon the exact value of the CO heat of adsorption on active sites. Consequently, log L for Step 5 is between 6 (for E = 9.2 kcal mole-') and 9 (for E = 14.4 kcal mole-' as given in Table VII). Thus, it seems at first that a low site density is indicated. But the surface reaction is between two surface molecules, and it is possible that a significant amount of entropy is lost as the activated complex forms. If so, the correct value of log L would be larger. Nozaki, Matsukawa, and Mano (81) suggested for Example 31 that the rate-determining step is CO reaction with a surface that has been oxidized by NO; thus, Step 1 or 6 for CO can provide a lower limit (since the reaction is 0.4 order in NO) for log L. But log L = 18 is rather large for a lower limit, and other possibilities may have to be considered. Dissociative adsorption of NO (Step 2 for NO) could be the rate-determining step. Example 32 is for the same reaction, but using a different catalyst and the log L calculations are similar.

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RUSSELL W. MAATMAN

+

Examples 33-36 are for reactions of CO with N,O to produce CO, N,. For Examples 33 and 34, for the same experimental system but at different partial pressures, we estimate that the catalyst used (2.1% Mo oxide/SiO,) had a surface area of 300 m2 g- '. If this estimate is too large by a factor of 10, the log L values calculated would be one unit larger, and so forth. For Example 34, zero order in both components, Step 5 should apply, and even if the surface area were overestimated by a factor of 100, log L would be 8, a very low value. At significantly lower PNZ0 (Example 33) the reaction is first order in N,O and Step 1 or 6 for N,O should apply. But then, regardless of the surface area, the site density in Step 1 or 6 for N 2 0 in Example 33 is nine orders of magnitude larger than it is in Step 5 of Example 34. Since the only difference between the two is reactant partial pressure, it may well be that the explanations just presented are not correct. Kazusaka and Lunsford (83)suggested for these examples that the reaction involves complexes of N 2 0 and CO on metal ion clusters. For Example 35 the calculation of log L values and the orders reported suggest that Step 2 for dissociative adsorption of N,O is the rate-determining step. This is consistent with the proposal of Fuller and Warwick (84) that the rate-determining step is N,O reoxidation of the catalyst after the catalyst has been reduced by CO. For Example 36 Krupay and Ross (85) suggested a mechanism of several steps involving two different oxidized sites. The complexity of the reaction and the large value of log L for Steps 1 and 6 for CO, the only steps in the table that seem to be relevant, suggest that the reaction is not easily handled.

D. TWO-REACTANT, ORGANIC EXAMPLES The examples discussed in this section are listed in Table VIII. Examples 1-4 are for the various steps in the hydrogenation of C2H4 over Cu. Sat0 and Miyahara (86) reported E and u for each step. They postulated that adsorbed C2H4 reacts with adsorbed H in two steps to form C,H,. Examples 5-10 are for the same reaction scheme with Pt catalyst; here CzH4 and H desorption steps are included (87). Most of the irrelevant log L values are not given. For Examples 1 and 2 the rate-determining step can be nondissociative adsorption; Example 3 is evidently a surface step in which, as with a unimolecular zero order reaction, A S f = 0. But it is not easy to see why log L is so small for Step 5 in Example 4. If this is a correct calculation, it implies that the reactants (adsorbed C,H, and H) have much larger freedom of motion than the activated complex. The reactions described in Examples 5-10 are somewhat more difficult to analyze. It seems unrealistic to postulate that C2H4 adsorbs dissociatively (Step 2 of Example 5); the alternative is Step 1 with the adsorbed species having considerable

CRITERIA IN SOLID-CATALYZED REACTIONS

145

freedom of motion. Desorption (the reaction of Example 6 ) could occur, as discussed earlier, using only a few surface points, and thus the log L value of 9 for Step 5 can be rationalized. For Example 7, hydrogen adsorption can be associative (Step 1) or dissociative (Step 2); hydrogen desorption, given in Example 8, could be such that Step 5 is the rate-determining step. For Examples 9 and 10, the log L values are low but still not as low as that of Example 4. Using our procedures we cannot calculate the log L values for Steps 8 and 9 for Examples 3, 4, 9, and 10 because the reacting surface species in those steps are not the same as the gas phase species. But if we make the calculations for those steps assuming that the surface species are the same as the gas phase species, we obtain the following log L values for Steps 8 and 9, respectively-Example 3: 23, 19; Example 4: 14, 10; Example 9: 20, 15; Example 10: 18, 13. Thus, surface motion may be a factor in these examples. Examples 11-23 are hydrogenation reactions. Although not all the orders have been reported, the available orders and the log L values indicate for Examples 11-18, 21, and 22 that the rate-determining step may be found among Steps 1, 2, and 6 for hydrogen. Of these ten examples, it is possible in all but Example 17 that the surface is covered with hydrocarbon before hydrogen reacts. For Example 17 the 0.3 order in benzene indicates incomplete benzene coverage. The 1.25 order for hydrogen may, of course, indicate for this example that none of the steps listed is a rate-determining step. Leclerq, Leclerq, and Maurel (96) reported on the hydrogenation of isopentane (Example 22) and several other saturated hydrocarbons (results not given in the table). They found fractional orders in several cases, including - 1.8 order in H, in one instance. The log L values for a given step (given in the table only for isopentane) vary widely from hydrocarbon to hydrocarbon. Evidently the situation is complicated. Example 23 is of interest for a special reason. The log L value for Step 8, suggested by the orders, seems physically possible. But one would hardly expect benzothiophene to have surface mobility. Bartsch and Tanielian (97) concluded for another reason that their activation energy was not correct, and they suggested that the kinetics are diffusion limited. Thus, our inability to find a suitable log L value might have been expected and, in fact, our calculations may perhaps be used to indicate that there is a problem with the data. Examples 24-27 are for oxidation reactions, three of hydrocarbons and one of CH,OH. Examples 24 and 25 are for the same reaction, with Example 24 for high oxygen pressure and coverage and Example 25 at low oxygen pressure and coverage. For the low-pressure case, Korchak and Tretyakov (98) postulated for their system that the surface-active oxygen is atomic;

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RUSSELL W. MAATMAN

then the first surface reaction is the rearrangement of C,H,O-, attached to a site to give CH,CHO--, also attached to a site. For Example 26, Steps 1 and 6 for C,H,, on a surface previously saturated with oxygen, are both possible rate-determining steps if C2H4 retains some surface motion. Linn and Sleight (99) found for their system a much different Arrhenius slope at low temperature, to give an activation energy of 50 kcal mole-'. The logL values calculated for the low temperature range (not shown in the table) are impossibly large. The authors suggested that the large activation energy is the consequence of product inhibition. If so, this is another case for which the log L calculation indicates a kinetic difficulty. For Example 27 the order with respect to oxygen was not reported, but the oxygen pressure was evidently large enough for the reaction to be zero order in oxygen. Step 5 is therefore relevant. Either the site density is very low or the entropy of activation is not zero. In a reaction of this type the latter reason is probably the correct one. Examples 28 and 29 are for deuterium-hydrogen exchange reactions. The reaction in Example 28 was carried out at a temperature low enough so that the only reaction after adsorption was that of C,H, (ads) with D (ads), followed by desorption. Sarkany, Guczi, and Tetenyi (54) suggested for their reaction that the rate-determining step was cyclohexane adsorption. The log L values indicate that dissociative adsorption of cyclohexane (Step 2, cyclohexane), for which log L = 19, is possible. But some surface freedom of cyclohexane is required. Dissociative adsorption of D,, for which log L = 15, seems more likely. Kalman and Guczi (101) considered for Example 29 that breaking the C-H bond is the rate-determining step. Thus, since the reaction is first order in CH,, Step 6 for CH,, for which log L = 18, is indicated; once again some surface freedom of the hydrocarbon is called for. But the order in D, is -0.1, and the authors reported for the same temperature, 646 K, D, orders of -0.3 and -0.8 for exchange with C,H, and C,H,, respectively. Thus, there is indication that D, and hydrocarbon compete for sites, with D, more strongly adsorbed, presenting the possibility that the rate-determining step is Step 11 for D,, for which log L is 10. Since the order in D, is -0.1 and not - 1 as required for Step 11, we therefore expect Step 11 to be only an approximation. For Example 30 Zidan et al. (102) postulated that NO oxidizes a surface site and produces atomic N; propylene then reduces the site, and in a subsequent fast reaction the resulting adsorbed hydrocarbon fragment reacts with the adsorbed atomic N to produce acrylonitrile. According to the orders given, either Step 1 or 6 for NO, that is, NO oxidation of the surface, should be the rate-determining step. Since several questions are involved- which surface species there are, what the entropy of surface atomic N would be,

CRITERIA IN SOLID-CATALYZED REACTIONS

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and so on-the rather large value of 17 for logL does not rule out some modification of Step 1 or 6 for NO. Examples 31-33 are for the steam dealkylation of toluene. The reaction is complicated, and there are several possibilities for a rate-determining step. For Example 31 Kochloefl (103) found that water adsorbs better than toluene, suggesting the possibility of Step 11 for H,O, for which log L = 14. Grenoble (104) reported several transition-metal catalyzed dealkylations of toluene; one of his systems is described in Example 32. The orders in the various reactions reported vary; in several the order in toluene is negative. But log L = 8 for Step 11 for toluene in Example 32 is rather low. It is just possible that in Examples 31-33 that a modification of Step 5 should be considered. The modification would take into account the fact that H,O and toluene compete for sites and the entropy of reaction is not zero, contrary to the usual entropy assumption made for Step 5.

Symbols A a

B C

% cB, cD C B l 1 CB2

D E F

f G H h

I I,, I, > I* Kads k L M m N

P

Q R R N , RQ

Rt, S

preexponential factor in Arrhenius equation proportionality factor in Eq. (61) gas species partition function factor of Eq. (10) and Table I gas concentration, molecules cmsurface concentration of B and unoccupied site, respectively, cm-' surface concentration of molecules 1 and 2, respectively, cm-' unoccupied active site activation energy, cal or kcal mole-' partition function fraction converted Gibbs free energy, cal or kcal mole-' enthalpy, cal or kcal mole-' Planck's constant moment of inertia of a linear molecule, g cm2 moments of inertia of a monhnear molecule, g cm2 adsorption equilibrium constant rate constant or Boltzmann constant; see context site density, cm-' molecular weight, g mole-' mass of a molecule, g product molecule in reaction (53) pressure, atm product molecule in reaction (53) ideal gas constant rate of appearance of N and Q in reaction (53), respectively R?i + R, entropy, cal deg-' mole-', or entropy units (e.u.) per mole

,

148 T U v

W

RUSSELL W. MAATMAN

temperature, K flow rate reaction rate, molecules cm-’ sec-’ mass of catalyst SUBSCRIPTS

tr, rot, vib, el translational, rotational, vibrational, electronic app apparent BID, B2D designating B, and B, molecules adsorbed on site D, respectively SUPERSCRIPT

r

denotes activated complex ACKNOWLEDGMENTS

We are indebted to Mr. Paul Wiersma, who carried out the experimental and much of the theoretical work on 1-butene isomerization. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. REFERENCES Maatman, R. W., J. Catal. 19,64 (1970). Maatman, R. W., Catal. Rev. 8, 1 (1973). Maatman, R. W., J. Catal. 43, l(1976). Baetzold, R. C., and Somorjai, G. A., J . Catal. 45,94 (1976). 5 . Simonyi, M., and Mayer, I., Acta Chim. Acad. Sci. Hung. 87, 15 (1975). 6. Galwey, A. K., Adv. Catal. 26, 247 (1977). 7. Horiuti, J., Miyahara, K., and Toyoshima, I., J. Res. Inst. Caial., Hokkuido Uniu. 11, 59 (1966). 8. Best, D. A., and Wojciechowski, B. W., J . Catal. 47, 343 (1977). 9. Glasstone, S., Laidler, K., and Eyring, H., “The Theory of Rate Processes.” McGraw-Hill, New York, 1941. 10. Eley, D. D., and Russell, S . H., Proc. R . SOC.London, Ser. A 341,31 (1974). 11. Miyamoto, A., and Ogino, Y., J. Catal. 27, 31 1 (1972). 12. Maatman, R. W., and Friesema, C., Proc. Iowa Acad. Sci. 86,26 (1979). 13. Aldag, A. W., Lin, C. J., and Clark, A,, J. Catal. 51,278 (1978). 14. Miyahara, K., and Kazusaka, A,, J. Res. Inst. Catal., Hokkaido Univ. 24, 65 (1976). IS. Boudart, M., Mears, D. E., and Vannice, M. A., Ind. Chim. Belge 32,281 (1967). 16. Ostrovskii, V. E., and Dyatlov, A. A,, Kinei. Katal. 17,405 (1976). 17. Poleski, M., Frackiewicz, A., and Palczewska, W., React. Kinet. Catal. Lett. 4, 199 (1976). 18. Moffat, J. B., and Scott, L. G., J. Catal. 45, 310 (1976). 19. Urabe, K., and Ozaki, A., J . Catal. 52, 542 (1978). 20. Falconer, J. L., and Wise, H., J. Cuiul. 43,220 (1976). 21. Il’chenko, N. I., and Golodets, G. I., J. Catal. 39, 57 (1975). 22. Srihari, V., and Viswaneth, D. S., J . Catal. 43, 43 (1976). 23. Klissurski, D. G., Ross, R. A., and Griffith, T. J., Can. J . Chem. 52, 3847 (1974). 24. Butt, P. V., and Kenney, C. N., Proc. Int. Congr. Catal., 61h, 1976 Vol. 2, p. 779 (1977). 1. 2. 3. 4.

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Tottrup, P. B., J . Cutal. 42, 29 (1976). Ueno, A., Hochmuth, J. K., and Bennett, C. O . , J . Catal. 49, 225 (1977). Amirnazmi, A., and Boudart, M., J . Curd. 39, 383 (1975). Meubus, P., J . Electrochem. Soc. 124,49 (1977). Pepe, F . , Schiavello, M., and Ferraris, G., Z . Phys. Chem. (Frankfurt am Main) [N.S.]96, 297 (1975). 30. Loffler, D. G., and Schmidt, L. D., J . Cural. 41,440 (1976). 31. Loffler, D. G . , and Schmidt, L. D., J . Catal. 44,244 (1976). 32. Pignet, T . , and Schmidt, L.D., J . Cural. 40, 212 (1975). 33. Carrizosa, I., and Munuera, G., J . Cural. 49, 174 (1977). 34. Moffat, J. B., and Riggs, A. S . , J . Catul. 42,388 (1976). 35. Balikova, M., and Beranek, L., Collect. Czech. Chem. Commun. 42,2352 (1977). 36. Varadarajan, T. K., Viswanathan, B., and Sastri, M. V. C . , Indian J . Chem., Sect. A 14, 851 (1976). 37. Szabo, Z . G . , Jover, B., and Ohmacht, R., J . Cutul. 39,225 (1975). 38. Echevin, B., and Teichner, S. J., Bull. Soc. Chim. Fr. p. 1495 (1975). 39. Otsuka, K., and Morikawa, A., J . Cutul. 46, 71 (1977). 40. Lombardo, E. A,, Comer, W. C., Jr., Madon, R. J., Hall, W. K., Kharlamov, V. V., and Minachev, K. M., J . Cutul. 53, 135 (1978). 4 1 . Rosynek, M. P., and Fox, J. S . , J . C a r d 49,285 (1977). 42. Dimitrov, C., and Leach, H. F., J . Curd. 14,336 (1969). 43. Carra, S., and Ragaini, V., J . Cutul. 10, 230 (1968). 44. Baird, M. J., and Lunsford, J. H., J . Curul. 26,440 (1972). 45. Uematsu, T., Inamura, K., Hirai, K., and Hashimoto, H., J . Catul. 45, 68 (1976). 46. Basset, J., Figueras, F., Mathieu, M. V., and Prettre, M., J . C u r d 16, 53 (1970). 47. Krupay, B. W., and Ross, R. A,, J . Catul. 39,369 (1975). 48. Konig, P., and Tetenyi, P., Acfa Chim. Acad. Sci. Hung. 89, 123 (1976). 49. Konig, P., and Tetenyi, P., Acfa Chim. Acad. Sci. Hung. 89, 137 (1976). 50. Biloen, P., Dautzenberg, F. M., and Sachtler, W. M. H., J . Card. 50,77 (1977). 51. Haro, J., Gomez, R., and Ferreira, J. M., J . Cutul. 45, 326 (1976). 52. Ruiz-Vizcaya, M. E., Novaro, O., Ferreira, J. M., and Gomez, R., J . Catul. 51, 108 (1978). 53. Ramaswamy, A. V., Ratnasamy, P., Sivasanker, S., and Leonard, A. J., Proc. Znr. Congr. Cutul., 6th, 1976 Vol. 2, p. 855 (1977). 54. Sarkany, A., Guczi, L., and Tetenyi, P., J . Cural. 39, 181 (1975). 55. Abd El-Salaam, K. M., 2. Phys. Chem. (Frankfurt am Main) [N.S.]95,147 (1975). 56. Kuriacose, J. C., and Jewur, S . S., J . Cutul. 50, 330 (1977). 57. Vickerman, J. C . , J . Card. 44,404 (1976). 58. Vannice, M. A,, J . Curul. 31,449 (1975). 59. Vannice, M. A,, J . C u r d 50, 228 (1977). 60. Vannice, M . A,, J . Card. 44, 152 (1976). 61. Dalla Betta, R. A,, Piken, A. G., and Shelef, M., J . Cutul. 40, 173 (1975). 62. Huang, C. P., and Richardson, J. T., J . Cutul. 51, 1 (1978). 63. Atkinson, G. B., and Nicks, L. J., J. Cutul. 46,417 (1977). 64. Dwyer, D. J., and Somorjai, G. A,, J . Cural. 52, 291 (1978). 65. Sexton, B. A., and Somorjai, G . A,, J . Caral. 46, 167 (1977). 66. Muller, J., Pour, V., and Regner, A,, J . Cutul. 11, 326 (1968). 67. Takeshita, T., Wallace, W. E., and Craig, R. S . , J . Card. 44, 236 (1976). 68. Kuznetsov, B. N . , Kuznetsov, V. L., and Ermakov, Y . I., Kinet. Kutul. 16, 915 (1975). 69. Hanson, F. V., and Boudart, M., J . Curul. 53,56 (1978). 70. Jamieson, D. M., Klissurski, D. G., and Ross, R. A., Z . Anorg. Allg. Chem. 409,106 (1974). 25. 26. 27. 28. 29.

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Chebotareva, N. P., Il’chenko, N. I., and Golodets, G. I., Teor. Eksp. Khim. 12,202 (1976). Murphy, W. R., Veerkamp, T. F., and Leland, T. W., J . Caial. 43,304 (1976). Kobal, I., Senegacnik, M., and Kobal, H., J . Catal. 49, 1 (1977). Zielinski, S., and Wachowski, L., Rocz. Chem. 50, 1023 (1976). Beyer, H., Jacobs, P. A,, Uytterhoeven, J. B., and Vandamme, L. J., Proc. Int. Congr. Caial., 6rh, 1976 Vol. 1, p. 273 (1977). 76. Cant, N. W., and Fredrickson, P. W., J . Caial. 37,531 (1975). 77. Arai, H., Tominaga, H., and Tsuchiya, J., Proc. Ini. Congr. Catal., 6ih, 1976 Vol. 2, p. 997 (1977). 78. Silva, A. E. M., Hudgins, R. R.,and Silveston, P. L., J . Caial. 49, 376 (1977). 79. Il’chenko, N. I., Avilova, I. M., and Golodets, G. I., Kinet. Katal. 16,679 (1975). 80. Iwasawa, Y., and Ogasawara, S., J . Caial. 46, 132 (1977). 81. Nozaki, F., Matsukawa, F., and Mano, Y., BUN. Chem. SOC.Jpn. 48,2764 (1975). 82. Alkhazov, T. G., Gasan-Zade, G. Z., Osmanov, M. O., and Sultanov, M. Y., Kinet. Katal. 16, 1230 (1975). 83. Kazusaka, A., and Lunsford, J. H., J . Caial. 45, 25 (1976). 84. Fuller, M. J., and Warwick, M. E., J . Caial. 39,412 (1975). 85. Krupay, B. W., and Ross, R. A,, J . Catal. 50,220 (1977). 86. Sato, S., and Miyahara, K., J . Res. Insr. Catal., Hokkaido Unio. 22,51 (1974). 87. Sato, S., and Miyahara, K., J . Res. Insi. Catal., Hokkaido Uniii. 23, l(l976). 88. Otero-Schipper, P. H., Wachter, W. A., Butt, J. B., Burwell, R. L., Jr., and Cohen, J. B., J . Catal. 50,494 (1977). 89. Segal, E., Madon, R. J., and Boudart, M., J . Catal. 52,45 (1978). 90. Gonzo, E. E., and Boudart, M., J . Caial. 52,462 (1978). 91. Puddu, S., and Ponec, V., R e d . Trav. Chim. Pays-Bas 95,255 (1976). 92. Sica, A. M., Valles, E. M., and Gigola, C. E., J . Catal. 51, 115 (1978). 93. Gallezot, P., Datka, J., Massardier, J., Primet, M., and Imelik, B., Proc. Ini. Congr. Caial., 6th, 1976 Vol. 2, p. 696 (1977). 94. Bernard, J. R., Hoang-Van, C., and Teichner, S. J., 1. Chim. Phys. 73,988 (1976). 95. Nakamura, M., and Wise, H., Proc. Int. Congr. Caral., 6ih, 1976 Vol. 2, p. 881 (1977). 96. Leclercq, G., Leclercq, L., and Maurel, R., J . Caial. 44, 68 (1976). 97. Bartsch, R., and Tanielian, C., J . Caial. 35, 353 (1974). 98. Korchak, V. N., and Tret’yakov, I. I., Kinei. Kaial. 18, 171 (1977). 99. Linn, W. J., and Sleight, A. W., J . Catal. 41, 134 (1976). 100. Edwards, J., Nicolaidas, J., Cutlip, M. B., and Bennett, C. O., J . Catal. 50, 24 (1977). 101. Kalman, J., and Guczi, L., J . Caial. 47, 371 (1977). 102. Zidan, F., Pajonk, G., Germain, J. E., and Teichner, S. J., J . Catal. 52, 133 (1978). 103. Kochloefl, K., Proc. Int. Congr. Caial., 6th, 1976 Vol. 2, p. 1122 (1977). 104. Grenoble, D. C., J . Catal. 51, 203 (1978). 105. Kim, C. J., J . Caial. 52, 169 (1978). 106. Pritchard, D. J., and Bacon, D. W., Chem. Eng. Sci. 30, 567 (1975). 107. Cvetanovic, R. J., and Singleton, D. L., Int. J . Chem. Kinet. 9,481 (1977). 108. Hayward, D. O., Herley, P. J., and Tompkins, F. C., Surface Sci. 2, 156 (1964). 109. Maatman, R. W., Mahaffy, P., Hoekstra, P., and Addink, C., J . Caial. 23, 105 (1971). 110. Boudart, M., Adti. Catal. Relai. Subj. 20, 153 (1969). 111. Hightower, J. W., and Hall, W. K., Chem. Eng. Prog., Symp. Ser. 63, No. 73, 122 (1967). 112. Ballivet, D., Barthomeuf, D., and Trambouze, Y., J . Caiul. 34,423 (1974). 113. Chuvylkin, N. D., Zhidomirov, G. M., and Kazansky, V. B., J . Catal. 38, 214 (1975). 114. Dalla Betta, R. A., and Shelef, M., J . Caral. 49, 383 (1977). 71. 72. 73. 74. 75.

ADVANCES IN CATALYSIS. VOLUME 29

Organic Substituent Effects as Probes for the Mechanism of Surface Catalysis M . KRAUS Institute of Chemical Process Fundamentals Czechoslovak Academy of Sciences Prague. Czechoslovakia

I . Introduction . . . . . . . . . . . . . . . . . . . I1 . Structure Effects on Rates and Equilibria in Catalysis A . Electronic and Steric Effects of Substituents . . . B. Kinetic Isotope Effects . . . . . . . . . . . . . C . Stereochemical Effects . . . . . . . . . . . . . I11 . Quantitative Treatment of Structure Effects . . . . A . Types of Correlations . . . . . . . . . . . . . B. LFERs in Heterogeneous Catalysis . . . . . . C.Data . . . . . . . . . . . . . . . . . . . . D . Interpretation of Slopes of Linear Correlations . E . Catalyst Characterization by the Slopes of LFERs IV . Heterogeneous Acid-Base Catalysis . . . . . . . . A . Elimination Reactions . . . . . . . . . . . . . B . Substitution Reactions . . . . . . . . . . . . . V . Heterogeneous Redox Catalysis . . . . . . . . . . A . Reactions on Metals . . . . . . . . . . . . . . B. Reactions on Metal Oxides and Sulfides . . . . VI . Structure Influence on Adsorptivity . . . . . . . . VII . Conclusions . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . .

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

151 153 153 155 155 156 156 158 159 160 161 163 163 170 172 172 186 189 191 192

.

I Introduction

In studies of reaction mechanisms. the necessary experimental information is. in general. obtained from kinetic measurements and from nonkinetic exploration of the reaction course. intermediates. and products . With noncatalytic homogeneous reactions. the kinetic evidence usually plays an important role. and the nonkinetic results often serve only for support. independent confirmation. and elucidation of finer points . However. the 151

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kinetics of catalytic reactions, especially those on solid catalysts, are very complicated owing to the multistep cyclic reaction schemes. Therefore, rate equations of acceptable complexity can be obtained only at the cost of a number of assumptions. These sometimes quite drastic simplifications make the form of the rate equation practically useless for mechanistic considerations as the precise physical meaning of the constants is difficult, if not impossible, to ascertain. In such situations, the attention of researchers into the mechanism of heterogeneous catalysis has turned to the nonkinetic methods for the identification of surface complexes. The recent literature usually classifies these methods according to the physical probes applied to the exploration of the structure of surface species. The main problem in the interpretation of this indirect knowledge is whether the observed complexes are true intermediates in the reaction of interest. Progress toward the study of surface phenomena by means of various probes under dynamic conditions, that is, during a catalytic reaction, is quite slow. Nevertheless, the kinetic approach to heterogeneous catalysis can be rewarding if relative data for two or more structurally related reactants or catalysts are acquired and interpreted. Instead of applying several assumptions that simplify the reaction scheme and the model of the surface, which are necessary for absolute kinetic description, it is accepted that, under certain conditions, the same reaction scheme holds for all members of the series of reactants or catalysts and that all of the unknown but identical simplifications in the relative data cancel out. However, it is much safer to select a series of reactants in which the structural change from one member to another will be small enough to uphold the basic features of the mechanism than to assume the same for a set of catalysts that are not minor variations of a basic preparation. The mechanistic evidence from relative kinetic data can be greatly enhanced when correlations with other independent quantities are constructed, and thus links between the catalytic processes and other phenomena are found. Boudart (1)was first to point out the possibilities of such correlations. When a relationship of a catalytic reaction to a noncatalytic chemical transformation is established in this way, the catalytic mechanism can be elucidated on the basis of analogy. Moreover, if the relationships are linear, the interpretation of their slopes yields additional information. The approach outlined is based on measurements of how small perturbations of the structure of a reactant or catalyst affect the rate, and thus utilizes the structural change as a probe. In combination with nonkinetic methods, deep insight into the mechanism can be obtained in this way. In recent years, progress and encouraging results have been achieved in this field.

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II. Structure Effects on Rates and Equilibria in Catalysis

The structure of a reacting molecule can be used as the chemical probe for the reaction mechanism in several ways. Ample experience is available with these methods from the research of noncatalytic homogeneous reactions, and their possibilities and limitations are well known. However, the solid catalyst restricts the scope to some extent on the one hand, but opens new applications on the other. For this reason, the methods of physical organic and inorganic chemistry developed for noncatalytic reactions cannot simply be transferred into the field of heterogeneous catalysis. The following remarks should identify some of the problems. A. ELECTRONIC AND

STERIC

EFFECTS OF

SUBSTITUENTS

The analysis of the influence of substituents in organic molecules upon rates and equilibria has led to the recognition that they operate in two different ways, either by changing the electronic density, in comparison with a reference substituent, at the reaction center of the molecule or by blocking the access to the reaction center. The same is true for heterogeneous catalytic reactions. However, the interaction of a molecule with a surface can disturb the “normal” effect of a substituent. Consider a molecule X.Y.Z, where X is the substituent, Y the constant part of the molecule (the link between X and Z ) , and Z the reaction center. Now we wish to compare its reactivity with the molecule H . Y . Z bearing hydrogen as the reference substituent. The electronic influence of the substituent X may depend on whether it comes into contact with the surface or not. In terms of adsorption this means whether the molecule will be attached to the surface horizontally or perpendicularly : X

Z

Y

Y

In Case A, the proximity of the group to the surface or to a greater extent its bonding to the surface necessarily causes a shift in the original electron density distribution within the group X and, in consequence, also in the whole

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molecule X.Y.Z. Then the influence of the substituent X on the reaction center Z is different than that in reactions where such secondary interaction does not occur. Beside the normal Case B we also can anticipate Case C, where the molecule is bonded to the active center through the nonreactive group X; this results in the competition of two adsorption modes, B and C, for the active centers. The kinetic consequence of Cases A and C is the blocking of a fraction of active centers and a decrease in reaction rate only because there are fewer sites available for the reaction. Again, the observed influence of the substituent will be other than expected. Published data showing the unusual effect of some substituents, whereas other behaved “normally” (i.e., as in noncatalytic transformations) can be explained in this way. An example of such behavior has been found with the addition of organic acids R.CH,COOH on acetylene, yielding vinyl esters, and catalyzed by zinc salts on active carbon (2). The rate decreased in the series R = H, CH,, CzH5,that is, with the increasing electron-donating ability of the groups. However, the methoxy group CH,O, contrary to expectation based on its electronegativity, showed low reactivity.This can be attributed to competitive adsorption of the acid by means of the free electron pairs of the oxygen atom in the substituent on active centers of the catalyst (Zn2+ ions). The reported erratic results (3-7) on the influence of ring substituents on the hydrogenation rate of nitrobenzenes may have a similar cause. The steric effects may be more pronounced in heterogeneous catalysts than in homogeneous reactions in solution. The rigid, solid surface restricts the approach of the reactants to the active centers and interaction between the reactants. The steric requirements are quite stringent when a two-point adsorption is necessary and when, in consequence, the internal motion of the adsorbed molecules is limited. In this way, the stereoselectivity of some heterogeneous catalytic reactions, for example, the hydrogenation of alkenes on metals (8)or the dehydration of alcohols on alumina and thoria (9),have been explained. In spite of these problems, the study of electronic and steric effects in reactions of organic compounds over solid catalysts can be successful, especially when quantitative correlations are attempted (see Section 111). The observation of unusual behavior sometimes can be more informative than the standard (expected) influence as it indicates some peculiarities of the mechanism. An implicit conviction forms the background of the application of electronic and steric effects as the probes for the mechanism that the reactivity of molecules is governed by the same general rules in all their transformations, notwithstanding the way in which the molecule is activated, whether thermally, by radiation, by acid or bases in solution, by metal complexes, or by solid catalysts. Then the observed electronic and steric effects of sub-

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155

stituents in heterogeneous catalysis can be explained on the basis of parallel findings for homogeneous noncatalytic reactions, which, owing to their relative kinetic simplicity, are easier to explore. The problem of interpretation therefore shifts partly to the necessity of finding suitable noncatalytic reactions in which the mechanisms are sufficiently known. It should be noted that this is much more possible with reactions on solid catalysts with acidic or basic surfaces than with metallic catalysts. Practically all the knowledge of the rules of organic reactivity is based on results concerning homogeneous reactions that are ionic in nature (cf., e.g., 10, 12). The current deep insight into mechanistic features of catalytic eliminations, additions, esterifications, etc. on solid acidic or basic catalysts (cf. 12, 13) has been achieved with extensive support from analogies to ionic reactions in solution. On the contrary, our progress toward the understanding of metal-catalyzed reactions is hampered by a lack of necessary analogies. They may be sought in reactions of metal complexes, which, however, have not been sufficiently generalized until now.

B. KINETIC ISOTOPEEFFECTS The measurement of kinetic isotope effects is used (14, 15) for finding out which bond is split or formed in the slow reaction step. The structure of the reacting molecule, changed by substituting an atom by its isotope, again serves here as a probe. In comparison to substituent effects, the perturbation of the original molecule by isotopic substitution is quite small. With heterogeneous catalytic reactions in which the reproducibility is relatively low, only primary kinetic isotope effects of deuterium are distinguishable from experimental errors. Therefore, the applicability of this method is limited to cases where it can answer specific questions about timing of the rearrangement of the various bonds of hydrogen. However, in this respect it is unsubstitutable.

C.

STEREQCHEMICALEFFECTS

In typical stereochemical experiments, the reactivity of two or more compounds of the same structure but of different configuration is compared either in separate or competitive experiments. The method has been reviewed several times for heterogeneous catalytic reactions, mostly with respect to reactions of hydrocarbons on metals (16-Zd). The results concerning eliminations on acidic catalysts have been summarized in an article dealing with the mechanism of this type of reactions (22). Clarke and Rooney (17) have broadened the notion of the stereochemical approach to heterogeneous catalysis when they included into it all work in which mechanistic conclu-

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sions are made from an analysis of the structure of the products formed from a certain, structurally complex and usually rigid, starting compound. The usually tedious work of synthetizing the stereochemical probes is rewarded in most cases by information that is not obtainable by other means. However, the mechanistic interpretation of the results may not be simple as the data are sometimes obscured by side reactions, interconversion of isomers, and the multistep character of catalytic reactions. Stereochemical approaches to heterogeneous catalysis are directed toward only qualitative information in most cases, for example, which stereoisomer reacts more slowly or which is formed preferentially. However, if these questions are answered in a more quantitative way, that is, on the basis of kinetic data, the information obtained is more fundamental.

111. Quantitative Treatment of Structure Effects

A. TYPESOF CORRELATIONS The preferred way of quantitatively expressing the influence of molecular structure on rates and equilibria of organic reactions is by means of linear correlations with quantities describing related processes. In theory, these empirical correlations are explained on the basis of free energy change as an additive function and the well-known relations between the free energy change and the equilibrium constant or the activation free energy change and the rate constant, respectively, (1)

A,G" = -RTlnK A,GS

=

- RTln (kT/h)

+ RTln k

(2)

where the subscript r denotes the change due to reaction. The term linear free energy relationships (LFER) has been coined for them; their detailed derivation, based on the original idea of Hammett (19), has been published in refined form many times (20-22) and need not be repeated here. For the purpose of this review, it is important to stress some features that present limitations to their applicability. A linear correlation of structure effects is a special case, in general, of the complex relationships between two processes and can be obtained only under certain simplified conditions. The principal approximation is the inclusion of only first order interaction terms in the sum of free energy contributions to the overall free energy change. Higher order interactions among various parts of the molecule are neglected, and this requires that a relatively narrow set of structural changes be used within the correlated

SUBSTITUENT EFFECTS AS PROBES

157

series of compounds. In other words, in order to obtain a good linear correlation, the perturbation of the structure must be kept small enough to allow the approximation of linear response. The second consequence of the simple linear model is the necessity of having a single type of interaction between the rest of the molecule and the reaction center (or more types of interactions, but operating in constant proportions in both compared processes) if one wishes to use a relationship of the form

(3) where K is a rate or equilibrium constant, subscript 0 is the reference compound of the series, cz is the proportionality constant, and p is the term Iog(ic/K0),, for a reference process p . In terms of the free energy changes, we may write log Krel = lOg(K/Ko) = up

log K , , ~ z As A,G*

(or As A , G )

(4)

where the subscripts denotes change due to reactant structure. For more than one interaction mechanism between the rest of the molecule and the reaction center, an expression of the type log Krel

= CIJ1

+ a& + . . .

(5)

is necessary. Three types of the LFERs may be distinguished. The first group (A) includes expressions valid for a specific type of compound and certain processes. They are based on well-defined reference processes, which serve for the determination of the values of the constants p. These LFERs usually are named after the authors who have introduced them (Hammett, Taft, Brown, etc.), and special symbols for the parameters CI and p are used (see Table I). The advantage of these established LFERs is the availability of TABLE I Some Special Substituent Constants Symbol 0 (a,,

c+

up 1

(d, c;)

l 7

U*

E,

4 , EP

fl

in Eq. (3)

Name

Application

Hammett

Polar and resonance effects in conjugated systems (for meta and para substituents) Polar and resonance effects in conjugated systems when a positive charge, permanent or transient, is developed on the reaction center The same but with a negative charge Polar effects in aliphatic systems Steric effects in aliphatic and ortho-substituted aromatic systems The same but corrected to hyperconjugation

Hammett

Hammett Taft polar Taft steric

158

M. KRAUS

sets of proven values of the /? constants for a large number of organic groups that act as substituents. Their disadvantage is that they require a clear division of the reacting molecule into the parts substituent-link-reaction center, which, for some reactants, is impossible to achieve. In the second type of correlation (B) a search is made for relationships between two processes, one of them being under study and the other one being well known from the viewpoint of mechanism. The advantage of this approach lies in the clear kinship of the two processes when they show parallel structure effects on rate or equilibrium manifested by a good correlation. However, one must use the same derivatives in both series of compounds, and this may cause experimental problems. In the third type of correlation (C), experimental rate or equilibrium data are compared with reactivity indices calculated by some (usually) semiempirical method of theoretical chemistry. The main problem here is in the design of a suitable molecular model as the basis for calculation.

B. LFERs IN HETEROGENEOUS CATALYSIS The broad applicability of LFERs for heterogeneous catalytic reactions has been demonstrated independently by Kraus (23) and Yoneda (24-27). The first author concentrated mostly on the established relationships such as the Hammett and Taft equations, whereas Yoneda has concentrated particularly on correlations with reactivity indices and other quantities. Since then, LFERs have been widely applied to heterogeneous catalytic reactions, and experience has been gained as to the suitability of each different type. An important step has been made toward an interpretation of the slopes of linear correlations (parameter ct in Eq. 3) as the quantities that are closely connected with reaction mechanisms. Among Type A relationships, the Hammett and Taft equations are most frequently employed for noncatalytic reactions. When utilizing them for catalytic reactions we must consider the reliability of the substituent parameters and their suitability for the given structural type of reactants. The Hammett equation

(6) for inductive and mesomeric substituent effects in conjugated systems and its various modifications, such as the Yukawa-Tsuno (28) equation, log Krel =

log KreI

= pa

+

- a)

(7) can be used quite safely, and inexplicable deviations are rare with noncatalytic reactions. When, with a catalytic reaction, some points do not fit an otherwise good correlation, or if, instead of a line, a scatter diagram is obtained, we must look for some factor that has been neglected. Y(Q+

SUBSTITUENT EFFECTS AS PROBES

159

The Taft equation log K,,,

= p*cr*

(8)

for polar substituent effects in aliphatic compounds is less reliable, and correct values of CT* are discussed (for a summary of the problems see, e.g., 29, 30). Deviations of some points are observed even with noncatalytic reactions, and their explanation is difficult. This uncertainty is, of course, transferred to heterogeneous catalytic reactions, where the problems increase. Taft has also introduced the steric substituent constant E,, which is used separately in an expression of the type shown in Eq. (3) or together with polar CT* constants in a four-parameter equation, log qe1 = p*a*

+ sE,

(9)

Their values also are a matter of discussion (see, e.g., 30). For the alkyl groups, the values of cr* and E, are strongly interrelated (32)with the exception of the t-butyl and other bulky groups. For this reason, when only alkyl groups are used as substituents and when they are not properly selected, both sets of constants may yield a good fit. Special attention has been paid to this problem in the field of heterogeneous catalysis (32-35), where, in order to avoid the competitive adsorption through a substituent containing a heteroatom, alkyl groups have been often used as the only substituents. In the case of Type B linear correlations of two presumably related processes, the main problem is to find a suitable partner to a heterogeneous catalytic reaction ; the requirements include a good knowledge of its mechanism, easy measurement of structure effects, and the possibility of using the same reactants in both series. It already has been mentioned that this task may be more easily fulfilled with heterogeneous acid-base reactions but may be impossible with reactions on metals or some oxides. Type C correlations of catalytic data with theoretical quantities, such as change of charge on atoms and change of bond strength or length, are seldom used because of the problems in devising a suitable model of the catalysts. Although modem computers allow relatively extensive sets of atoms as models for a reactant interacting with the active center of the catalyst, special questions arise when the catalyst is composed of heavier atoms. The progress in this promising type of correlation therefore is relatively slow.

C. DATA All correlations require reliable kinetic data, which may be obtained either from individual or competitive measurements. In the case of experimentation with individual compounds separately, great care should be taken

160

M. KRAUS

to obtain consistent sets of data for all members of the series of compounds. The reproducibility of catalytic kinetic measurements is usually low, and, moreover, small amounts of poisons in some compounds of the series may alter the activity of the catalyst and the apparent reactivity of these compounds. Frequent tests of catalyst activity with a reference compound are therefore strongly recommended. The individual rate data may be used directly for a correlation, with a clear knowledge that they reflect structure influence on both adsorptivity and reactivity, or the data may be worked up into kinetic constants of a suitable rate equation, which then are treated in correlations. According to the author's experience, the form of the rate equation is of less importance, provided that it fits the data satisfactorily. The probable reason is that only relative constants are required for linear correlations; their sensitivity to the way in which they have been computed from primary data is low. Therefore, data from pulse flow experiments also give a satisfactory basis for linear correlations in spite of their unsuitability for kinetic analysis. The competitive data, obtained with pairs of simultaneously reacting members of the series, are less subject to experimental error and to changes in catalyst activity, especially when cross checking is achieved by altering the pairs and extent of conversion. However, relative data measured in this way constitute in most cases a product of the rate constant and adsorption coefficient divided by the same product for the reference compound. Some authors (cf., e.g., 36, 37) were able to separate these products into relative rate constants and relative adsorption coefficients by conducting separate individual kinetic measurements with at least one member of the series.

D. INTERPRETATION OF SLOPES OF LINEARCORRELATIONS A successful correlation of catalytic data for a series of related compounds is of little value for obtaining insight into the mechanism if its slope is not interpreted with respect to the sign and value. The slope a of Eq. (3) is proportional to the free energy change caused by the difference in mechanisms of the two processes being compared, the one under study and the reference one : a = lOg(Krel/Krcl,p)

At As

ArG'

(or

At

As Arco)

(10)

where the subscript t denotes change due to processes. Various situations arise according to this type of correlation (cf. 38). For Type B correlations the proportionality between the structure influence on a catalytic and (usually) noncatalytic process is strong evidence of mechanistic kinship. The only condition for the value of the slope is its significant difference from zero; the absolute value is of no consequence.

SUBSTITUENT EFFECTS AS PROBES

161

Positive values are most often found, and then the interpretation is straightforward. Concrete examples will be presented in the following sections. Practically the same is true for Type C correlations. However, when the theoretical model is oversimplified, the linear correlation does not need to be obtained, and only parallel trends in value are observed. In order to gain an insight into the mechanism on the basis of the slope of a Type A correlation requires a more complicated procedure. Consider the Hammett equation. The usual statement that electrophilic reactions exhibit negative slopes and nucleophilic ones positive slopes may not be true, especially when the values of the slopes are low. The correct interpretation has to take the reference process into account, for example, the dissociation equilibrium of substituted benzoic acids at 25°C in water for which the slope was taken, by definition, as unity @ = 1). The precise characterization of the process under study is therefore that it is more or less nucleophilic than the reference process. However, one also must consider the possible influence of temperature on the value of the slope when the catalytic reaction has been studied under elevated temperatures; there is disagreement in the literature over the extent of this influence (cf. 20,39). The sign and value of the slope also depend on the solvent. The situation is similar or a little more complex with the Taft equation, in which the separation of the molecule into the substituent, link, and reaction center may be arbitrary and may strongly influence the values of the slopes obtained. This problem has been discussed by Criado (35)with respect to catalytic reactions. However, there are enough examples of slope values that can be interpreted safely as will be shown in the following sections. These are the catalytic reactions for which the slopes of the Hammett and Taft equations have been found to be significantly different from zero, being well in the positive or negative range. Otherwise, careful comparison with the values for related processes has to be made, as Dunn (38) has pointed out. The interpretation of slopes also requires meaningful rate data. When the reaction consists of a series of elementary steps (and this is always so with heterogeneous catalytic reactions), the rate coefficients obtained from a superficial treatment of a limited set of measurements may be composites of several rate and equilibrium constants for individual steps, in favorable cases constituting a product. As every step may be influenced by the substituents, the resulting effect can be easily attributed to a false elementary step. E. CATALYST CHARACTERIZATION BY THE SLOPES OF LFERs

Linear correlations of structure effects on rate and equilibria may be obtained for a single reaction proceeding on different catalysts. In this way,

162

M. KRAUS

a unique opportunity is obtained for a relative characterization of catalysts on the basis of slopes the values of which are initimately connected with the reaction mechanisms. The relative slopes reflect the differences in the energy of interaction between the active centers of the catalyst and the reaction center of the reactant. The application of logarithms of rate constants in a LFER leads to the separation of the extensive (concentration of active sites per unit surface area, L ) and intensive (rate constant per one active center, k ) components of the experimental rate constant kexp= kL, viz. log k,,

= log L

+ log k

(1 1) When L is the same for all members of the reactant series (an assumption fulfilled when the size of the reactants is similar and when no secondary interactions of the reactants with the surface are possible), the term L is canceled out and the slope found expresses only the change in the energy of interaction due to the change of structure : The subscript i denotes the catalyst. In the relative values of the slope, arel= tx2/al, this sensitivity to structural changes is canceled out, and the value of arelreflects the difference in the strength of the active center. In the terms of the free energy change, we may write the proportionality

arel!z A, A, As A,Gt where the subscripts r, s, and t correspond to the changes in activation free energy due to reaction, substituent, and type of reaction, respectively, and c denotes the change caused by the transition from one to another catalyst. The requirement of small structural differences within the series of reactants for obtaining a LFER has its parallel in series of catalysts. Meaningful values of arelresult only when the catalysts operate principally in the same way, that is, when the reaction mechanism is basically the same. This is most likely to occur when the catalysts differ only by minor modifications in the method of preparation or when their composition is only slightly modified by the addition of promoters. With chemically different catalysts the similarity is achieved when the active centers have as their decisive component a common species, for example, protons on solid acidic catalysts. Catalyst characterization by the relative value of slopes, are,, is most useful when parallel trends in the properties of the catalysts, measured by other probes, chemical or physical, are discovered. Examples are the estimation of acid strength of the surface sites or the estimation of energy of interaction between surface atoms on the basis of shifts in spectra. All of the quantities used for comparison must be intensive, that is, they must express some form of energy or be proportional to energy.

SUBSTITUENT EFFECTS AS PROBES

163

IV. Heterogeneous Acid-Base Catalysis

As has been mentioned previously, one is most likely to find analogies to catalytic reactions on solids with acidic and/or basic sites in noncatalytic homogeneous reactions, and therefore the application of established LFERs is safest in this field. Also the interpretation of slopes is without great difficulty and more fruitful than with other types of catalysts. The structure effects on rate have been measured most frequently on elimination reactions, that is, on dehydration of alcohols, dehydrohalogenation of alkyl halides, deamination of amines, cracking of the C - C bond, etc. Less attention has been paid to substitution, addition, and other reactions.

A. ELIMINATION REACTIONS Studies of structure effects on rate have helped substantially to bring researchers to the present deep understanding (12, 13) of the mechanism of elimination reactions. Beside stereochemical evidence, successful linear correlations have yielded the desired information. The published series of reactants and correlations are summarized in Table 11. The fit of straight lines to experimental data is usually good or very good, and only a few points deviate significantly. Details of the correlations may be found in the original literature; here we will concentrate on the values of the slopes. An inspection of Table I1 (26,40-64) shows that, in most cases, the slopes have negative values, some being highly negative. There may be two reasons for these unusual magnitudes. Elimination reactions bring the problem of the division of the reacting molecule into the reaction center and the substituent. When, for example, the dehydration of aliphatic alcohols is studied R 2 . CH2 * CH2. OH

R 2 . CH=CH2

+ H20

the reaction center may be defined alternatively as - O H , - C H 2 * O H and , - C H 2 . C H 2 . 0 H , respectively, and, in consequence, the substituent as R2.CH2-CH,--, R'.CH,--, and R--, respectively. The correct selection depends on the contribution of the fission of the C,-H bond to the observed rate, that is, it depends on the details of the mechanism. When this step is rapid in comparison to the splitting of the C - O H bond, which is then rate determining, the use of the Taft substituent constants o* for R is quite appropriate. However, on using o* constants for R2 or R' instead for R, good correlations can be obtained because the same methylene chain is inserted between the substituent and the reaction center in all members of the reactant series. This link, of course, diminishes the sensitivity of the reaction center to the inductive effect and, in consequence, the value of the slope. An example of this possible manipulation of the data is presented in

164

M. KRAUS

TABLE I1 LFERs for Elimination Reactions

Series

Catalyst

Temperature (“C)

Number of points

Reactants

p”

RefSlope erence

Dehydration of alcohols to alkenes 1 2 3 4 5 6 7 8 9

10 11 12 13 14 15 16

A1203 A1203

SiO, Ti02 ZrO, AI,O, + NaOH A120,-Si02 NaOH Hydroxyapatite Nonstechiometric hydroxyapatite

+

TiO, A1203

SiO, TiO, ZrO, A1,0, + NaOH Acid clay

380 220 300 300 300 300 250

ROH RCH(OH)CH(CH,), R.CH(OH)CH(CH,), RCH(OH).CH(CH,), RCH(OH)CH(CH,), RCH(OH)CH(CH,), ROH

395 230 282 350 200 200 220 220 220 220 350

R’R2R3COH R’R2R3COH

5 4 4 4 4 4 6

a* - 16 U* - 2.0 -2.8 U* a* -0.8 U* 0.3 U* 1.2 - 13.3 U*

40b 41 41 41 41 41 42

-2.3 -5.1 -4.5 U* a* - 3.9 U* - 10.7 -2.6 U+ U+ -2.4 a+ -2.2 U+ -2.1 U+ - 2.2 C

43 43 43 43 44 45 46 46 46 46 47

U*

a*

ROH RC6H4CH(OH)CH3 RC6H4CH(OH)CH3 RC6H4CH(OH)CH3 RC6H4.CH(OH)CH, R.C,H,CH(OH)CH, ROH

Deamination of alkylamines 17

A1203

350

RNH,

5

U*

- 13.4

48b

5 5

U*

- 39

U*

- 34

U*

- 40 -

49b 50 50 26

Dehydrochlorination of chloroalkanes 18 19

KCI BaSO,

20

SrO

325 220 280 300

RCI RCI C2H6-.Cl,(n

=

2, 3,4)

5

C

Dealkylation by cracking 21 22 23 24 25 26 21 28

A120,-Si0, A1,03-Si02 Al,O,-SiO, AI,O,-SiO, A1BF4-A1,03 AI,O,-SiO, A1,03-Si0, AI,O,-SiO,

550 500 400 400 400 490 400 400

U*

a* U*

B U U U U

-9.1 - 23 - 5.0 -

-9.5 -2.0 -1.0 - 3.2

51b 52” 53 53,54 55 56b 57b 54

(continued)

165

SUBSTITUENT EFFECTS AS PROBES

TABLE I1 (Continued)

Series 29 30 31 32 33 34 35

Catalyst AI,O,-SiO, + NaOH Zeolite Kaolin AI20,-Si0,

Temperature ("C)

Reactants

Number of points

Slope

Reference

400

RC,H4CH(CH3),

3

a

-4.2

54

414 500 400

RC6H4.CH(CH3), (RC6H4),CHCH, (RC6H4)2CHCH,

4 6 4

-5.0 -2.8 -2.4

400 400

p-R.CsH4OH o-RC~H~OH

3 5

u 0 u C u a

58' 5P 60 60 6Ib 61'

5 4

B B

3

-3.3 -6.0 -10.1 U* a* -4.3 +4.3 u*

~

-22 -19

Dehydrosulfidation 36 37

A1,03-Si02 Al,O,-SiO,

250 250

RSH RSR

-

62 62

Decomposition of esters 38

39a 39b

BaSO,

AI,0,-Si02

370 412 470 230

CH,COOR

u*

o*

CH3COOCHR'R2 RCH,COOCH(CH,),

9 5 ~

63 63 63 64 64 ~

' Parameter of Eq. (3) applied to linear correlation. For symbols see Table I. B and C denote Type B and Type C correlations. Data only.

Fig. 1; it is based on actual data (Case 1 in Table 11) for the dehydration of primary alcohols on alumina. (Note that the fit deteriorates from left to right, but this may be connected with the still not quite certain o* values for higher alkyl groups.) A similar example may be found in the paper by Kibby and Hall (43). The other cause for the unusually high values of the slopes may be the absence of a solvent as all the data on catalytic eliminations have been obtained in gas-phase experiments. With highly polar transition states, the solvent compensates for the influence of the separation of charges. It should be noted that the correlation of the data for the pyrolysis of alkyl halides similarly gave very high negative values of the slopes (65). Before analyzing the slopes in the light of experience with noncatalytic elimination reactions, which have been measured mostly in the range of 20"-100°C, it is necessary to consider the possible influence of temperature on the values of o*, as the data for Table I1 have been obtained in the range of 200"-500°C. There is considerable disagreement in the literature as to

166

M. KRAUS

0I

-0.16

-a%

I

-0.12

I

-0.x) 6*

I

I

I

-0.15 -O.x)-O.O5

0

a,

-0.2

I

I

0

0.2

0.4

06 s"'

FIG.1. Correlation of the rates of primary alcohol dehydration (40) (series 1 from Table 11) in the coordinates of the Taft equation (8) for different separations of the reactants molecules into the parts substituent-link-reaction center.

the extent of this influence (cf. 20,39). Table I1 adds three other controversial cases to the scarce information from the literature : series 9 shows a decrease of p* with increasing temperature, whereas series 19 and 39 indicate an opposite trend. It is difficult to determine what causes the changes as they express the temperature influence on the electron densities in both the reactants and the catalysts; in the liquid-phase reactions the temperature influence on solvation also contributes to the change. However, it is not probable that the sign of the slope may be altered to a positive one by decreasing the temperature under 100°Cin the cases summarized in Table 11. Thus, taking the negative sign of the slopes as an identification of the slower step in the elimination of the product HX from the grouping -CH-CX--, we may say that most catalytic eliminations are electrophilic from the viewpoint of the reactants (other evidence, mostly stereochemical, is available). This means an attack of a positively charged active center of the catalyst on an electronegative part of the reactant molecule. With the exception of the cracking of alkanes we can easily recognize the atom or group X as this negatively charged part of the reaction center. Consequently, its partner on the surface must be a metal ion or a surface hydrogen atom (most probably from a surface hydroxyl group). This attribution leaves the role of the positively charged part of the reaction center to the H atom from the C,-H bond and the role of its partner on the surface to the anionic atoms of the lattice. This picture of a two-point interaction of the reactant with the surface is consistent with other observations (cf. 12, 13). Its generalization led

SUBSTITUENT EFFECTS AS PROBES

167

to the following description by Mochida, Anju, Kato, and Seiyama (66) of eliminations on solid surfaces consisting of acidic and basic centers. The adsorption complex is similar for all reactants and all catalysts:

-c-cI

x

I

y

The timing of the fission of the C-X and C-H bonds depends on the nature of the catalyst, the substituents on both carbon atoms, and probably also the temperature (Fig. 2). The different timing is described in terms of E l , E2, and ElcB mechanisms borrowed from noncatalytic elimination reactions (67) and slightly modified. With the increasing polarity of the C-X bond, with increasing acid strength of the catalyst, and with increasing temperature the mechanism is shifted toward the E l operation mode, that is, to the case in which C-X splitting distinctly precedes C-H splitting. The E2 mechanism corresponds to simultaneous fission of both bonds and the ElcB mechanism to the case of elimination starting at the C-H bond. Two intermediate modes, E2cA and E2cB, have been inserted (66) between the standard three mechanisms as additional fixed points in an otherwise continuous spectrum of possible timings. The series 3-6 in Table II constitute a strong support for this approach to elimination mechanisms. The slopes of the Taft relationships (p,*) change with the nature of the catalysts and could be correlated with their other intensive properties, which were determined independently (Fig. 3). The

-

Cp-H

bond strength

Ca-X

-

FIG. 2. Schematic representation of the influence of reactant structure, catalyst acid-base properties, and temperature on the selection of the elimination mechanism. For an explanation of symbols, see text. [Reprinted with permission from Berhnek and Kraus (13, p. 276). Courtesy Elsevier Scientific Publishing Company.]

168

M. KRAUS

1

9: 0-

-1

-

-2

-

-3

-

(C2H5)20

~

4

0 I 0

I

12

0

Od

0.2

FIG.3. Correlation of the slopes p: for the dehydration of secondary alcohols on various catalysts (series 3-6) with independently measured heats of adsorption of water and diethyl ether, sensitivity to pyridine poisoning ( 4 4 , and deuterium kinetic isotope effects (68). [Reprinted with permission from Beranek and Kraus (13, p. 294). Courtesy Elsevier Scientific Company.]

quantities used for comparison have been the sensitivity of catalyst poisoning to dehydration by pyridine (y), the heats of adsorption of water and diethyl ether (AH), and the slopes of the Taft relationships for adsorption of a series of dialkyl ethers (pa*) on these catalysts as measured chromatographically. Later, these correlations were supplemented by Kochloefl and Knozinger (68) with values of deuterium kinetic isotope effects of the dehydration of deuterated isopropanols CH,.CHOD.CH, and CD,.CDOH.CD,, which also depended on the nature of the catalysts. It has been suggested (41) that the change of pr* reflects the transition from an almost El mechanism on S O , to an almost ElcB mechanism on alkalized A1203, all other data and correlations with the acidity of the catalysts being in agreement with it. The dehydration of alcohols on stoichiometric and nonstoichiometric (calcium-deficient) hydroxyapatite (series 8 and 9 in Table 11) gave results consistent with the above findings. Although there is a difference in the reaction temperature, it is evident that with the nonstoichiometric catalyst, which must be more acidic, the slope found is more negative than that with the stoichiometric calcium phosphate. The influence of reactant structure on the mechanism is evident from the comparison of series 12-15 with series 3-6, both sets of which have been measured on the same set of catalysts. In series 12-15, the reactants were ring substituted 1-phenyl ethanols in which a positive charge developing on

SUBSTITUENT EFFECTS AS PROBES

169

the a-carbon atom in the transition state came into conjugation with the aromatic ring. This structural feature of the reactants overcame all the differences in the acidity of the catalysts and led to an El-type mechanism in all cases, as the equal negative slopes show. This interpretation is supported further by the fact that better correlations of the rate data have been obtained with the ' 0 substituent constants than with the standard 0 constants. The structure effects on rate in the catalytic dehydration of alcohols on acidic catalysts also have been elucidated by quantum-chemical modeling of the adsorption complex in a series of alcohols R.CH(OH).CH,, using a proton as a simple model of the catalyst (69). It has been found that the protonation of the hydroxyl group causes an increasing weakening of the C - 0 bond in the order R = CH,, C2H,, i-C3H7,t-C,H,. This corresponds well to the negative slopes of the Taft correlations on acidic catalysts. Also other Type B and C series from Table I1 are consistent with the above elimination mechanisms. The dehydration rate of the alcohols ROH on an acid clay (series 16) increased with the calculated inductive effect of the group R. For the dehydrochlorination of polychloroethanes on basic catalysts (series 20), the rate could be correlated with a quantum-chemical reactivity index, namely the delocalizability of the hydrogen atoms by a nucleophilic attack; similar indices for a radical or electrophilic attack on the chlorine atoms did not fit the data. The rates of alkylbenzene cracking on silicaalumina catalysts have been correlated with the enthalpies of formation of the corresponding alkylcarbonium ions (series 24). Similar correlations have been obtained for the dehydrosulfidation of alkanethiols and dialkyl sulfides on silica-alumina (series 36 and 37); in these cases, correlation by the Taft equation is also possible. The rate of cracking of 1,l -diarylethanes increased with the increasing basicity of the reactants (series 33). Dautzenberg and Knozinger (70) have shown that the Taft equation also can be successfully used for the correlation of structure effects on the stereochemical course of an elimination. With the dehydration of secondary alcohols RCH,.CH(OH)*CH, on an alumina catalyst, they observed the influence of the group R on the ratio of the 2- and 1-alkenes formed and on the ratio of cis-2- and trans-2-alkenes. The positional selectivity (S2J conformed to the o* constants (series 40a, pf = 2.2), whereas for the conformational selectivity (SCJthe steric E, constants gave a better fit (series 40b, s = 0.3). The interpretation states that the direction of the elimination is governed by the relative bond strength of C-H in the CH, and CH, groups, which is influenced by the inductive effect of the substituent R, and that the conformation of the adsorbed molecule is strongly affected by the size of R, which must interact with the CH, group when the cis isomer is formed. The prevailing negative slopes in Table I1 indicate a strong tendency toward the El-type elimination mechanism, which begins by the splitting of

170

M. KRAUS

the C-X bond, and is followed by the rupture of the C-H bond. The difference in the timing of these two steps should correspond to the degree of development of the positive charge on C,, a carbonium ion being the limiting case (neat El mechanism). As a highly polar transition state should manifest itself by a large slope of a LFER, we can examine the data for different reactions in Table I1 in light of this requirement and the hypothesis by Mochida et al. (66) on the continuous spectrum of possible mechanisms. However, the differing approaches to the division of the reactant molecule into substituent, link, and reaction center and an inadequate description of the catalysts makes this task rather difficult. It seems that the dehydration of alcohols requires a less polar transition state than the dehydrochlorination of alkyl chlorides, in agreement with conclusions from stereochemical studies (12). The same is true for the decomposition of esters. Noller et al. (63) interpreted the increase in the value of the slope for decomposition of esters on BaSO, (series 38) with the temperature as a transition from a more or less concerted (E2-type) mechanism toward an El-type; this view has been criticized by Criado (35),who pointed out that the change may be caused by the change of activation energy within the series of reactants examined. Data are available (71) on the influence of ring size upon the rate of dehydration of C,-C, cycloalkanols on alumina; they will be discussed in Section V,A, 1, where they are summarized, together with other data concerning the reactivity of cyclic compounds in various reactions, in Table V. Here let it suffice to state that the reactivity pattern in dehydration fit the general rules known from noncatalytic chemistry well. B. SUBSTITUTION REACTIONS Under the general term of substitution, we will deal with several transformations in which two molecules of reactants form the product and in which a new C - C or C - 0 bond or bonds are formed by replacing a C-H bond or another C - 0 bond. Aldol condensation, esterification, or transesterification and the formation of ethers from alcohols fall into this broad category. We also will include in this section addition to multiple C - C bonds. The published LFERs are summarized in Table I11 (2, 72-76). Furthermore, Venuto (77, 78) has applied a special type of LFER, the Brown selectivity relationship (79), to the deuteration (series 50) and ethylation (series 51) of toluene and benzene catalyzed by zeolites. The Brown equation

171

SUBSTITUENT EFFECTS AS PROBES

TABLE I l l LFERs f o r Substitution and Addiiion Reactions

Series

Catalyst

Temperature (“C)

Reactants

Number of points

8”

Slope

Reference

3 5

B d

1.1

72 73

Aldolization

41 42

MgO Anion exchanger

300 35

+

CH2O CH3.R C,H,CHO RC,H,COCH3

+

Esterification and transesterification 43 44 45

46

Cation exchanger Cation exchanger Al2O3-SiO2 NaOH A1203-Si02 NaOH

+ +

120

CH3C02C2H, + ROH

4

E,

1.4

74

120

4

E,

0.6

74

250

RCH2C02C2H, C,H,OH CH3C02C2H, + ROH

8

u*

0

42

250

RC02H

7

E,

0.1

42

4 7

u*

E,

3.5 0.6

75 76

3

u*

0.5

2

+

+ C2H,0H

Dehydration of alcohols to ethers 47 48

Ni/Si02

183 160

ROH ROH

+ ROH + ROH

Vinyl ester formation 49

RC0,Zn

210

RC02H + C2H2

‘See Table 11.

relates the influence of the methyl substituent on the rate of electrophilic aromatic substitution and the ratio of the rates in the para and meta positions of the nucleus @/m selectivity). The proportionality constant, b, has been determined by means of about 40 noncatalytic homogeneous reactions, for example, nitration and halogenation. The fit of the points for the heterogeneously catalyzed substitutions to this general relationship is very strong evidence for a mechanistic relationship between homogeneous and heterogeneous transformations. It means that the zeolite catalyst acts as a strong acid, transforming, for example, ethylene into a positively charged species, probably into the ethyl carbonium ion, C,Hl, which then attacks the aromatic nucleus in the manner well known from homogeneous substitution. The data in Table I11 require a few comments. All slopes are positive or

172

M. KRAUS

zero, in agreement with what could be expected from experience with homogeneous reactions, catalyzed by soluble acids or bases. The steric constants E, often gave a better fit than the polar (T* constants; however, in the case of ether formation on Ni/SiO, (series 48) their application by Simonik and Pines (76) has been criticized by Criado (35). The positive slope for ether formation found on alumina (series 47), which contrasts with the negative one for alkene formation, has been interpreted by Knozinger as evidence of different mechanisms for these two, often in parallel proceeding transformations of alcohols. It has been suggested that the first step of the dehydration to an ether is the formation of a surface alkoxide, which is then attacked by a weakly bonded alcohol molecule. The zero slope found for transesterification (series 45) can be explained in accordance with the general view on acid-catalyzed reactions of organic acids and esters. The first step is the protonation of the acid or ester, which is followed by interaction with the alcohol (or water in ester hydrolysis). The absence of any observable influence of the alcohol structure on rate indicates that the rate-determining step must be the protonation of the ester. This is in contrast to the homogeneous reaction, in which this step is usually very rapid. The parallel dehydration of the alcohols exhibited a large structure effect on rate (Case 7 from Table II), confirming the independence of the two reaction routes.

V. Heterogeneous Redox Catalysis

A. REACTIONS ON METALS 1. Hydrogenation of the C=C Bond

The old and lasting problem of heterogeneous catalysis, the mechanism of alkene hydrogenation, has also been approached from the viewpoint of structure effects on rate. In 1925, Lebedev and co-workers (80) had already noted that the velocity of the hydrogenation of the C=C bond decreases with the number of substituents on both carbon atoms. The same conclusion can be drawn from the narrower series of alkenes studied by Schuster (81) (series 52 in Table IV). Recently authors have tried to analyze this influence of substituents in a more detailed way, in order to find out whether the change in rate is caused by polar or steric effects and whether the substituents affect mostly the adsorptivity of the unsaturated compounds or the reactivity of the adsorbed species. Linear relationships have been used for quantitative treatment.

173

SUBSTITUENT EFFECTS AS PROBES

TABLE IV Structure Influence on the Rate of Hydrogenation of Unsaturated Noncyclic Compounds

Series

Catalyst

Temperature ("C)

Reactants

Number of points

52 54

Ni Pt/SiO, Pt/C

0 20

R1R2=CHz R'R2C=CR3R4

53

PdjC, Pd/SiO, PtjC, Pt/SiO,

20

RCH=CH,

55 56

Ru/C Pt/SiO,

3 20

R'RZC=CR3R4 R1R2C=CR3R4

11 11

51

Pt

20

R1RZC=CHR3

5

58 59 60 61 62

Rh Pd/C PdjC Pd/SiO, PdiC

22 20 20 120 30

5 15

I

CH,=CH.X CH,CH=CH.X (CH3),CH=CH.X (CH,),CH=CH.X X.C,H,C.CH,

7 4 4 4 18

Observed influence

Reference

Decrease of rate with increasing number of substituents Decrease of rate with increasing R Decrease of rate with increasing number of substituents Correlation with K(Ag + 1 No effect of X No effect of X No effect of X No effect of X No effect of X

4 87 87 88 89

No effect of X

89

81 84

83

85 82

87

II C(CH3)2 63

Pd/C

30

X

q

A CH3 CH3

The works of Maurel and Tellier (82), RPliiEka and CervenL (83, 84, Litvin, Freidlin, and Tilyaev (85) and Brown and Ahuja (86),who have used extensive series of alkenes, confirmed the Lebedev's rule. With 1 -alkenes (C6-C, 7) on palladium, platinum, and rhodium catalysts, the initial reaction rate decreased with the length of the chain, and with Pd and Pt a linear dependence on the number of carbon atoms was obtained (83) (series 53). An example of the influence of the number of substituents on the carbon atoms of the double bond is shown in Fig. 4.It is evident that the mere presence of the substituent is more important than its nature. However, this secondary factor has been accounted for by using the sums of the Taft polar (T*or steric E, constants for all substituents on C=C. Cerveny and RfiiiEka (84) have found excellent linear relationships between the initial hydrogenation rate of 15 alkenes on 3 different Pt catalysts and C E, (series 54), and

174

M. KRAUS I

I

I

I

I

I

FIG.4. Dependence of the hydrogenation rate of alkenes on the number n of substituents on C=C in R1R2C=CR3R4. [Data by Maurel and Tellier (82), series 56.1 No.

R'

R2

R3

R4

No.

R'

R2

R3

R4

1 2

C4Hy C6H13 i-C4Hy CH3 CH3 C2H5

H H H C,H, i-C,H, C4Hg

H H H H H H

H H H H H H

7 8 9

CH3 CH3 CH, C2Hs CH3

H CZH, CH, C2H5 CH3

i-C,H, CH3 C2H5 CH3 CH3

H H H H CH3

3 4 5 6

10

11

slightly worse ones on using I: o*.Later, they extended the analysis (33) of these sets of data to alternative models of the surface reaction, which included hydrogen radicals, protons, or hydride ions as attacking species and simultaneous or stepwise addition, in the latter case the first or second step determining the rate. Good correlations yielded only the models for the simultaneous addition of two hydrogen radicals or for addition of H' or H + to the adsorbed alkene. Litvin et al. (85) have obtained a good correlation using polar g* constants (series 55). Maurel and Tellier (82) have made an important contribution when they separated the reaction rates found into the rate constants and adsorption coefficients. Whereas the rate constants varied only slightly and irregularly, probably in the range of experimental error, large differences in adsorption coefficients were observed. The adsorption coefficients gave a good correlation with the sum of the constants I; o* (series 56 and 155 from Table IX in Section VI). The important influence of adsorptivity also has been confirmed by Jardine and McQuillin (87), who correlated the rate of pentene and methylpentene hydrogenation on platinum with the equilibrium constant of the mcomplexing of these alkenes and Ag' ions measured chromatographically (series 57).

SUBSTITUENT EFFECTS AS PROBES

175

All of these studies have used alkyl groups as the substituents on the C=C bond, which, however, differ only slightly in their polar effects. In order to find out the extent of electronic contribution to the overall reactivity, a broader range of substituents is necessary. The literature yields earlier data of this type (3) for the hydrogenation of unsaturated compounds CH,=CH .X (where X = -CH,NH,, -CH,COOH, -CH,CN, -CH,OH, -CH20COCH3, -CH,OCH,, and -CH,CHO) on Rh. In spite of large differences in the polarity of X, the rates cover only a narrow range and give a scatter diagram in the coordinates of the Taft equation (series 58). The same is true for the data by Jardine and McQuillin (87)on the hydrogenation of the compounds CH,.CH=CH.X and (CH,),C=CH.X on palladium, where the second methyl group affected the rate more than the different X substituents (series 59 and 60). A similar system, (CH,),C=CH.X, was studied by Endrysova and Kraus (88) in the gas phase in order to eliminate the possible leveling influence of a solvent. The rate data were separated in the contribution of the rate constant and of the adsorption coefficient, but both parameters showed no influence of the X substituents (series 61). A definitive answer to the problem has been published by Kieboom and van Bekum (89),who measured the hydrogenation rate of substituted 2-phenyl-3-methyl-2-butenesand substituted 3,4-dihydro-1,2-dimethylnaphtalenes on palladium in basic, neutral, and acidic media (series 62 and 63). These compounds enabled them to correlate the rate data by means of the Hammett equation and thus eliminate the troublesome steric effects. Using a series of substituents with large differences in polarity, they found relatively small electronic effects on both the rate constant and adsorption coefficient. The conclusions on the mechanism of the double bond hydrogenation on metallic catalysts can be summarized as follows: (1) with respect to structure effects on rate, all transition metals behave similarly; (2) the reactivity of the unsaturated compounds is governed mostly by the number and size of the substituents on the carbon atoms of the double bond through their influence on adsorptivity ; (3) the electronic nature of the substituents plays a minor if any role. Point (3) has been interpreted by Kieboom and van Bekkum (89) as evidence of the similarity in the electronic character of the initial and transition states. However, an alternative explanation would be that the ratedetermining step does not involve the unsaturated compound but only the activation of hydrogen; the overall rate then will be determined by the equilibrium adsorption of the unsaturated compound, the extent of which is sensitive to steric effects. Structure effects on hydrogenation rate also have been studied in series of cycloalkenes. The influence of substituents on C=C is similar to that in aliphatic series (e.g., 82, 87, 90), but the point of interest is the observed

176

M. KRAUS

TABLE V Effect of Ring Size (in Number of C Atoms) on the Relative Rates of Catalytic Reactions Ring size Series

Catalyst

Temperature (“C)

5

6

7

8

10

12

Reference

Hydrogenation of cycloalkenes 64 65 66 67

PdjC Pt/AI,O, Pt Pd

20 25 50 50

68

Cu/SiO,

200

0.75 1.07 -

-

1.0 1.0 1.0 1.0

0.90 0.69 -

0.15 0.09 0.26 0.13

-

-

-

-

-

0.52

-

-

87 90 91 91

4.9

7.6

92

Dehydrogenation of cycloalkanols 1.2

1.0

3.4

3.8

Dehydration of cycloalkanols 69

A1203

200

1.9

1.0

2.3

8.6

-

-

71

effect of ring size upon the rate. Table V (71,87,90-92) summarizes the data on a few series of cycloalkenes, together with two observations concerning dehydrogenation and dehydration of cyclohexanols. With the exception of the point for C, in the series 65, all results conform to the general rule, derived from homogeneous reactions of cyclic compounds, that the reactivity shows a maximum at the C6 compound when the reaction causes a change from sp2 to sp3 hybridization at the carbon atom (series 64-67) and a minimum for the opposite process (series 68 and 69). This rule usually is interpreted in terms of changes in ring strain ( E strain) due to changes in the bonding, and Table V proves that it is also valid for heterogeneous catalytic reactions. 2. Hydrogenation of the Aromatic Nucleus A large number of papers has been devoted to the influence of substituents upon the reactivity of benzene nucleus. Extensive studies concerning various benzene derivatives and catalysts from the platinum group metals have been published by H. A. Smith and his co-workers (for a summary see 36). The most consistent sets of data on alkylbenzenes are available from him and other groups of authors. Table VI summarizes the influence of the structure of a single alkyl group; Table VII (94, 95, 97-103) summarizes the influence of the number and position of the methyl groups. Both series show very similar behavior on all metal catalysts, a decrease in rate with the size

177

SUBSTITUENT EFFECTS AS PROBES

TABLE VI Relative Rates of Hydroyenation of Alkylbenzenes Catalyst and temperature ("C)

R in R.C6H,

Pt 30

Ni/AI,03 150

Ni 170

100 62

100

100" (100)b

45 41

50 43 45

77 (29) 46 (41)

33 37

44

56

(18)

Ru 30

I 004 (1 O O ) ~ 15 (300) 9 (500) 5 (750) 3 (610)

29

Series Reference a

23 26

_

41 42

41 40

38 39

_

70 93

71 94

40 -

72 95

73 96

Rate constants. Adsorption coefficients

and with the number of alkyl substituents. A more detailed examination of the data reveals the finer effects of the branching of the alkyl groups or of the arrangement of the methyl groups on the ring. The decrease in hydrogenation rate with increasing size of the alkyl group (Table VI) (93-96) can be correlated by the Taft equation. However, the correlation of the data by Smith and Pennekamp (93) (series 70) using the polar o* constants published by Kraus (23) has been criticized by Mochida and Yoneda (32), who have shown that a somewhat better fit could be obtained when a four-parameter Taft equation is applied that also includes steric E, constants. Similarly, Kieboom (34) has discussed Yoshida's correlation of series 73 based on o* constants, but his main conclusion has been that the data do not allow a clear distinction between steric and polar effects. It seems that both operate in the same direction. Series 72 and 73, in which the rate data have been separated into the rate constants and adsorption coefficients, show opposite trends with the latter parameter. A similar problem has been encountered by Volter, Hermann, and Heise (ZOO) and by Najemnik and Zdraiil (103) in the series of methylbenzenes (Table VII) and is discussed in this connection.

TABLE VII Influence of Methyl Substitution on the Relative Rates of Hydrogenation of Benzene Nucleus Number of position of methyl groups

Catalyst and temperature ("C)

Pt 30

None 100"(100)b 62 (55) CH3 1,2-Me, 32 (30) 1,3-Me, 49 (18) 1+Me, 65 (10) 1,2,3-Me3 14 (10) 1,2,4-Me, 29 (6) 1,3,5-Me3 58 (3) 1,2,3,4-Me4 10 (3) 1,2,3,5-Me4 11 (2) 1,2,4,5-Me4 18 (1) 3,5 (0,6) Me5 Me, 0, 6 (0,2) Series Reference

74 97

Rate constants. Adsorption coefficients.

Rh/AI,O, 30 100"(100)b 43 (65) 25 (34) 16 (23) 22 (12) 8 (18) 7 (9) 15 (2) 3 (10) 5 (2) 6 (2) 1, 1 (4) Small 75 98

Ni 170

Ni/MgO 90

Co/MgO 90

Rh/MgO 90

MoS, 420

WS, 420

100"(100)b 77 (29) 16 (43) 21 (21) 26 (21)

100 87 -

100 45

100

100 230

-

-

-

-

54

30

30

100 99 108

-

-

-

_

-

-

-

-

-

25 -

5

24

111

_ -

-

-

-

430 -

-

-

-

-

-

-

630 150

Ni/AI,O, 150 100 50 24 23 31 -

10 -

4 0,5 Small

- _ _ __

-

_ - _ -

-

76

77

94

95

52

-

-

-

-

-

-

-

-

92

-

-

-

78 99

79

80

81

100

100

101

-

230 -

82 102

Co0-Mo0,/A1,03 50 100" (100)b

98 21 34 25 10

(164) (318) (264) (264)

(609)

5 (527) 5 (464)

_ - _ _ -

-

- _

83 103

SUBSTITUENT EFFECTS AS PROBES

179

The decrease of the hydrogenation rate with methyl substitution has been expressed by Lozovoi and Diakova (94) by a simple relation between the rate v and the number of methyl groups n : v = 2-"Vb,

(15)

where vb denotes the rate for benzene. Volter and co-workers (99, ZOO) have pointed out that this order of reactivities resembles (in opposite direction) the relative stabilities of n complexes of methylbenzenes with picric acid. They were able to draw a satisfactory linear correlation between the relative rates and stability constants, with a single line for three different catalysts of their own (series 78-80) and the rhodium catalyst of Rader and Smith (series 75). However, the finer data by Smith and co-workers (series 74 and 75) show that the position of the groups also plays a role. The crowding of the groups (1,2-, 1,2,3-, and 1,2,3,4-derivatives)decreases the rate, and the symmetrical compounds (1,4-, 1,3,5-, and 1,2,4,5-derivatives) react most rapidly within their group of substances with an equal number of methyl groups. No satisfactory explanation has been suggested for this influence ; however, similar differences between isomers are observed in the basicities of methylbenzenes, especially in 0 basicities (cf. 104). Several authors have separated the rates into rate constants and adsorption coefficients (series 72 and 73 in Table VI and series 74, 75, and 77 in Table VII). A decrease in the adsorption coefficient with increasing methyl substitution and in one case with the increasing size of the alkyl groups has been observed. On the other hand, Yoshida (series 73) found an opposite trend, and Volter, Hermann, and Heise (ZOO), who measured the adsorptivity independently, found it to increase with the number of the methyl groups. Two explanations for this discrepancy are possible. First, the treatment of the data in calculating the rate constant and adsorption coefficient from competitive experiments is based on biased assumptions and the values of the adsorption coefficients are artifacts. The second has been suggested by Volter, Hermann, and Heise (ZOO), who considered a three-step hydrogenation mechanism consisting of adsorption of the hydrocarbon in form of a n complex, which is transformed into a c complex, and then, by the successive action of the adsorbed hydrogen atoms, to the saturated hydrocarbon. The first two steps are assumed to be in equilibria, the positions of which depends on the catalyst and reaction conditions (including the solvents) and are differently influenced by the structure. In independent adsorption measurements the n complexing is reflected, whereas from kinetic data the c complexing is determined. However, it is not clear why the stabilities of the n and 0 complexes should show opposite trends with methyl substitution; both the n and 0 basicities increase with the number of methyl groups (104).

180

M. KRAUS

Nieuwstad, Klapwijk, and van Bekkum (105) have added to the knowledge of aromatic hydrogenation by their study of the influence of alkyl substituents in the 1 and 2 positions of naphthalene on the rate. Tetrahydronaphthalenes were the products of hydrogenation over palladium at 80°C. The selectivity of the reaction was also followed and expressed as the ratio of the rate constants for the saturation of the unsubstituted and substituted rings, respectively. Steric effects play an important role, and, beside steric hindrance by the bulky substituents, steric acceleration also has been observed, the latter being caused by a release of the strain between the 1-alkyl group and hydrogen in position 8.

3. Hydrogenation of the C=O Bond Only two series of data on structure effects on the hydrogenation rate of aldehydes are available in the literature (106,107). Both could be correlated with the Taft equation, using the (r* constants for the substituents R in RCH,CHO. The slopes have negative values [series 84,data by Oldenburg and Rase (100, three aldehydes on Ni/Si02 at 170"C, slope -0.6 (23); series 85, data and correlation by Sporka and RPliiEka (107),six aldehydes on Cu/SiO, at 190°C, slope - 1.31. The latter series was composed in such a way as to allow for the distinction between the polar and steric influence; the o* constants gave a much better fit than the E, constants. The hydrogenation of ketones has been studied more frequently and in a more detailed way (Table VIII) (108-115). With dialkylketones, all measureTABLE VIII Correlation of Structure Effects on the Rate of Hydrogenation of Ketones

Series

Catalyst

Temperature ("C)

86 87 88 89 90 91 92 93 94 95

Cu/SiO, Ni Ni Ni Ni Cu/SiO, Pt/SiO, Rh/SiO, Ni PdIC

150 125 25 30 10 150 150 150 36 25

a

Reactants RCOCH, RCOCH, R'COR' RCOCH, RCOCH, RCOCH, RCOCH, RCOCHj XC6H4COCH, XC6H,COCH,

LFER Taft Taft -

Taft Taft Taft Taft Taft Hammett YukawaTsuno

Number of points 3

4 10 4 4 5 5 5 8 22

Slope

Reference

-3.3 19 6 3.2 3.3 2.9 2.5 Scatter

I08

0.7b

115

109 110 111 112

113 113 113 114

Positive slopes in partial correlations of small series for which the Taft equation is suitable. For a resonance parameter r = 0.8.

SUBSTITUENT EFFECTS AS PROBES

181

ments have shown a decrease of rate with an increase in size of the alkyl groups; only a single exception has been recorded (series 86). Several series gave very good correlations versus cr*, E, being unsuitable. The work of van Bekkum, Kieboom, and van de Putte (115) has contributed to the understanding of the mechanism of ketone hydrogenation. Beside the series 95, included in Table VIII, they worked with about 20 other ketones of different structures on the same palladium catalyst. Series 95 shows that the hydrogenation is accelerated by electron-withdrawing and retarded by electron-donating substituents. However, the reaction rate also is negatively influenced by steric effects of substituents in the ortho position and by bulky groups in the meta and para positions. Several adsorption coefficients have been determined from competitive experiments, and it has been found that all substituents, irrespective of their position on the ring, decrease the adsorptivity to about one-third of the value for acetophenone. However, the adsorption coefficient of phenylacetone was one order of magnitude smaller and that of acetone two orders of magnitude smaller. This observation and the correlation by the Yukawa-Tsuno equation of the series of meta- and para-substituted acetophenones indicate that the adsorbed state involves both the carbonyl group and the aromatic ring, which are in conjugation, and that the greater part of this conjugation is lost in the transition state. The structure effects on the hydrogenation rate of ketones also have been used for comparisons of catalysts. Simonikova, Ralkova, and Kochloefl (113) have pointed out that the slopes of the Taft relationships for series 91-93 for copper, platinum, and rhodium catalysts, together with the similar results of Iwamoto, Yoshida, and Anouma (112) for a nickel catalyst (series 90), exhibit an opposite trend from the d character of the metals. The findings of Tanaka, Takagi, Nomura, and Kobayashi (116), based on competitive hydrogenations of cyclohexanone and 2-, 3-, and 4-methylcyclohexanoneson eight transition metal catalysts and on quantum-chemical calculations of the reactants, revealed that the effect of the substituents consists mostly of steric hindrance to adsorption. The plots of the relative rates versus the atomic radii of the metals gave smooth curves for the group Ru, Os, Ir, Pt, and the pairs Rh, Pd and Ni, Co showing separate but parallel trends. The deuteration experiments have distinguished similarly among members of the tetrad Ru, Os, Ir, and Pt, which operate by a simple addition mechanism to the C=O bond, and members of the diad Rh and Pd, which tend to form diadsorbed species involving the C , and C , or C , and C6atoms in the ring. It might be of interest to compare the observed structure effects on the hydrogenation rate with the parallel results concerning the noncatalytic reduction of ketones by some chemical reagent. Data on the reduction of

182

M. KRAUS

ketones by lithium aluminum hydride are available (117); the results could not be simply interpreted, as the observed trends indicate that, beside steric and electronic effects, other factors also may intervene. However, the general decrease of the rate with increase in size of the alkyl groups agrees with that recorded in Table VIII in most cases. Because the reduction of the C=O bond by a hydride involves a nucleophilic attack (by H - ) on the carbon atom we may hypothesize that, similarly, the rate-determining step in the hydrogenation of ketones is the addition of the adsorbed hydride ion to the carbon atom or a simultaneous addition of a polarized hydrogen molecule (Ha+-Hd -) to the C=O double bond.

4. Hydrogenolyses of the C-C and C-X Bonds The efforts (118-122) to obtain insight into the mechanism of the hydrogenolysis of the C-C bond by means of structure effects on rate have not led to conclusive results. The relative reactivities of various bonds in a hydrocarbon molecule depend very much on the metal. Whereas the splitting of the carbon chain on nickel starts from the less branched end of the molecule and continues stepwise (120), the opposite is true for platinum (123). Large differences in the reactivity patterns among various metals also have been observed with cyclopropyl derivatives (124). Both electronic and steric effects seem to influence the reaction rates (118,122, 124,125) and selectivities. As the splitting of the C-C bond very probably requires diadsorbed hydrocarbon species, the atomic distances in the metal catalysts, or more precisely, the distances between two surface atoms capable to act as active sites, must play an important role, and the reaction pattern must depend very much on the nature of the metal. Leclerq, Leclerq, and Maurel (122) concluded that, beside 1,2-diadsorbed molecules, 1,3-, and I$-, and 1,5-diadsorbed molecules also may be involved on platinum. This, of course, complicates the rules governing the hydrogenolytic reactivity of hydrocarbons. That such rules do exist is confirmed by the regularities observed by several authors. The rates of the hydrogenolysis of the alkyl chain in alkylbenzenes (118) and in alkylnaphthalenes (119) on the nickel catalyst gave a good correlation (119) with a slope 2.5, reflecting the difference in the electronic influence of the benzene and naphthalene nucleus. Leclerq, Leclerq, and Maurel (122) were able to calculate standard bond reactivities in saturated hydrocarbons on platinum from measurements with a large series of individual compounds. In contrast, considerable understanding of the hydrogenolyses of the C - 0 and C-halogen bonds has been gained by means of structure effects on rate. Hydrogenolytic fission of the C-0 bond in alcohols and their derivatives

SUBSTITUENT EFFECTS AS PROBES

183

is particularly easy when the carbon atom is attached to a phenyl group. Kieboom, de Kreuk, and van Bekkum (126) have studied this type of compounds on a palladium catalyst at 30°C. Their series of reactants of the general structure R' I X . C,H4-C--O-RZ

I

R3

consisted of the following combinations of groups: series 96, R' = R2 = R3 = H, nine different X groups in the meta and para positions; series 97, R' = CH,, R2 = H, R3 = i-C3H7,nine X groups; series 98; X = R2 = H, 12 combinations of R' and R2 groups; series 99, X = R' = R3 = H, nine R2 groups, including hydrogen. The series 96 and 97 could be correlated by the Yukawa-Tsuno equation [Eq. (7)] with slopes -0.37 and - 1.43, respectively, and corresponding resonance parameters 0.71 and 0.64, respectively. All series have been interpreted in the following way. The reaction mechanism is cyclic; the electron deficient carbon atom in the C - 0 group, which is in conjugation with the aromatic ring, is attacked by adsorbed hydride (H-). The second hydrogen atom simultaneously forms a bond with the leaving group R2 or goes into solution as a proton. With primary alcohols and their derivatives (R' = R3 = H), the displacement occurs in a concerted fashion (S,2), with tertiary alcohols the breaking of the C - 0 bond precedes somewhat the formation of the C-H bond, that is, the timing corresponds more to an S,1 substitution. Zdraiil and Kraus (127) studied the related hydrogenolysis of 24 esters R'COOR', where R' and R2 were alkyl groups (series 100). The catalyst was rhodium, and the temperature was 300°C. The relative reactivities of the esters could be correlated by a modified Taft equation log Rre, = p*o*

+ h An,

(16)

where An = 6 - Z n, n being the number of hydrogen atoms on &carbon atoms. The second term in Eq. (16) is usually interpreted as an adjustment to hyperconjugation (20-22). As the signs of the proportionality constants (p* = -25.7, h = -2.28) show, both effects operate in the same direction, indicating that increasing electron density on the ester group facilitates the reaction. However, the relative reactivities also could be correlated with the boiling points of the esters and with their heats of evaporation. The mechanistic interpretation of the results was similar to that by Kieboom et al. (126), namely, that the reaction mechanism is cyclic, the ester is adsorbed on an electrophilic surface center, and the surface reaction consists

184

M. KRAUS

of a nucleophilic attack on the C-0 bond by negatively polarized adsorbed hydrogen atom H-. Both groups of authors came to the conclusion that in the hydrogenolysis the hydrogen atoms do not act as equal species with lone electrons (freeradical type) but as hydride and proton species. The same interpretation was applied by Kraus and Baiant (123) to the hydrogenolysis of chlorobenzenes on palladium at 200°C. The influence of ring substituents could be correlated by the Hammett equation (series 101, six points, slope 4.0) and with the data on the dehalogenation of substituted chlorobenzenes by a metal hydride. Together with the results on deuterolysis, which yielded benzene derivatives with a single deuterium atom in the place of the chloro atom, the correlations led to the suggestion of the following mechanism. Chlorobenzene is adsorbed on the surface through its chloro atom on an electrophilic (electron deficient) center and is attacked by a hydride species. The leaving C1- atom is accommodated by the adsorbed H'. 5 . Hydrogenolysis of the N - 0

Bond

A number of authors measured the influence of ring substituents on the rate of catalytic reduction of aromatic nitro compounds by hydrogen (3-7, 224, 228). The series have been composed in such a way as to allow the Hammett correlations, but, with a single exception, scatter diagrams resulted. The successful case by REitka and Santrochova (128) (series 102, 12 points, slopes for three different platinum catalysts 0.24, 0.34, and 0.92, respectively) differs from the others in the use of platinum catalysts, whereas the other authors worked with rhodium (4,5), palladium (5,214), ruthenium (6),axid nickel (7). RPliiEka and Santrochovh also (128) failed to correlate the data for a palladium catalyst. The complexity of the kinetics may be the cause of these unsuccessful attempts. However, Finkelsthein and Kuzmina (7) were able to correlate their data for a nickel catalyst with the solvatochromic effect (shift in electronic spectra due to a solvent). 6. Dehydrogenation of Alcohols

Some information about structure effects on the rate of dehydrogenation of alcohols to aldehydes and ketones on metals is found in the older literature (129-132) from which it follows that secondary alcohols react more easily than the primary alcohols (229) and that the reactivity decreases with the length of the carbon chain (131). Some series can be correlated by the Taft equation using o* constants (Ref. 131, series 103, Cu-Cr,O, catalyst, 350"C,four points, slope 18; Ref. 132, series 104, Cu catalyst, four points, slope 22). Linear relationships have been used in a systematic way by

SUBSTITUENT EFFECTS AS PROBES

185

Hajek, Duchet, and Kochloefl(133,134), who studied the dehydrogenation of secondary alcohols of the type R.CH(OH)CH, on Cu (133) and Pt, Pd, and Rh (134) catalysts. With copper, the data were correlated using the CT* constants; however, the separation of the points into two groups has been observed (series 105). The first line consisted of R = CH,, C2H,, C5Hll, and C,H,; the other one included the branched alkyl groups i-C3H7, i-C4H9, and t-C4Hg; both lines had the same slope of 0.95. However, it is doubtful whether the alcohol with the phenyl group, which can achieve direct resonance with the reaction center in the transition state, belongs in this series. When it is omitted, the series can be correlated by the modified Taft equation (16), yielding a single line. With platinum, palladium, and rhodium catalysts, the observed influence of the alkyl groups (series 106, 107, and 108, six points each) could be put into a dependence on the steric E, constants with slopes 0.86, 0.48, and 0.61, respectively. Hajek, Duchet, and Kochloefl (134) also tried to correlate the adsorption coefficients calculated from the rate data, but the relationships obtained have not been very convincing. In spite of great effort, the analysis of structure effects on the dehydrogenation of alcohols on metals has not helped much toward an understanding of the mechanism. A broader range of substituents would be necessary in order to distinguish between the electronic and steric influences. 7. Miscellaneous Reactions on Metals Decarbonylation of aldehydes is particularly easy when the group - C H O is bonded to a conjugated system. Hoffman and Puthenpurackal (135) measured the rate of the decomposition of cc-alkylcinnamaldehydes C6H,. CH=CR.CHO (series 109, Pd, 191"C, five points). A correlation of these data with the CT*constants (slope 1.7) has been published ( 2 3 ,but the steric E, constants give, as has been found now, a slightly better fit (slope 0.7). The absence of electronic influence on the rate was the result of the extensive work by Smolik and Kraus (136), who worked with substituted benzaldehydes X-C,H4CH0 on palladium, rhodium, platinum, and ruthenium at 180°C. Five to six substituents in the meta and para positions were used, but the rates differed only slightly and without any visible trend. It seems that, in spite of the spread of catalyst activities over two orders of magnitude, the mechanism is similar for all metals and is characterized by a small difference in the electronic structure of the initial and transition states. Deuterium exchange in alkylbenzenes, catalyzed heterogeneously by platinum and homogeneously by platinum complexes, has been found by Hodges and Garnett (137) to have similar patterns and has been interpreted as an evidence of the similarity in mechanism. Shopov, Andreev, Petrov,

186

M. KRAUS

and Gudkov (138) found a correlation between the exchange rate in cyclohexane and methylcyclohexanes on a nickel catalyst at 177°C and the delocalizability of hydrogens, calculated by a quantum-chemical method. Kubelka and Kraus (139) measured the kinetics of deuterium exchange in benzene, toluene, anisol, and methyl benzoate on palladium at 135°C. The observed reactivities and especially the selectivities of the substitution in the meta and para positions indicated a nucleophile type of reaction, Dbeing the attacking species. The oxidation of alkenes by nitrous oxide on silver at 350°C has been studied from the viewpoint of structure effects on rate by Belousov, Mulik, and Rubanik (140), and very good correlations of Type B have been found with ionization potentials and with the rate of oxidation by atomic oxygen (series 110 and 111). The oxidative dehydrogenation of secondary alcohols to ketones on iridium at 130°C has been measured by Le Nhu Thanh and Kraus (141), and the rates have been correlated by the Taft equation [series 112, four reactants of the structure R.CH(OH)CH,, slope 4.71. The synthesis of phenylbromosilanes from bromobenzenes and silicon, catalyzed by copper (142) at 411"C, could be correlated by the Hammett equation (series 113, seven points, slope 0.63). €3. REACTIONS ON METAL OXIDES AND SULFIDES

1. Hydrogenation of Alkylbenzenes The published data have been already summarized in Table VII. With the cobalt-molybdenum oxide catalyst (series 83), the decrease of reactivity with increasing number of methyl groups found by Najemnik and Zdraiil (103) corresponds to that for metal catalysts. The relative adsorption coefficients, given in parentheses, have been measured independently, using the gas chromatographic technique. They increase with increasing molecular weight as also shown by the data from Volter, Hermann, and Heise (100) for cobalt and rhodium catalysts. The results of Lozovoi (101, 102) on MoS, (series 81) indicate no influence of structure on rate, and on WS2 (series 82) an increase of reactivity with an increasing number of methyl groups. However, the arrangement of the experiments does not exclude the intervention of transport processes. 2. Dehydrogenation of Hydrocarbons and Alcohols

Hishida, Uchijima, and Yoneda (25) have measured the rates of the dehydrogenation of cyclohexane, mono-, di-, and trimethylcyclohexanes to

SUBSTITUENT EFFECTS AS PROBES

187

aromatic hydrocarbons on Cr,O,-AI,O, and MOO,-AI,O, at 350"-5OO0C and correlated the data with a quantum-chemical reactivity index (delocalizability) calculated for the abstraction of one hydrogen atom in the radical form (series 114 and 115). Balandin and co-workers (143) have shown that the apparent activation energies for the dehydrogenation of ethylbenzenes to styrenes, as well as their similar previous data (144), can be correlated by the Hammett equation (series 116 and 117, three reactants in each, probably an Fe,O, catalyst, negative slopes). Structure effects on the dehydrogenation rate of secondary alcohols to ketones have been studied by Nondek and co-workers (145-147) as a means for the elucidation of the mechanism and for the characterization of different Cr,O, catalysts, and by Kibby and Hall (43)asa side reaction to the dehydration on hydroxyapatite catalysts. The latter authors found a satisfactory correlation between the rate constants and the g* constants, with a positive slope of 1.5 (series 118, 395"C, five reactants), in contrast to the dehydration on the same catalyst giving a negative slope (series 8, Table 11). Nondek and SedlaEek (145) have obtained a good correlation between the rate constants for the dehydrogenation of five 2-alkanols on chromia and the change of charge on Ha caused by the deprotonation of the hydroxyl group calculated by the CNDO/2 method (series 119). Later, SedlaEek, Avdeyev, and Zakharov (148) repeated this quantum-chemical modeling with CH, * CH(OCr).R as a representation of the surface species and again found a good correlation with the rate constants (series 120). Nondek and Kraus (146), using a broader series of 2-alkanols, found a linear relationship between the rate constants and the g* constants (series 121, eight reactants, 350"C, slope 2.9). On the basis of these observations and other experimental evidence the authors suggested a mechanism consisting of a rapid loss of the hydrogen atom from the hydroxyl group under formation of surface alcoholate, followed by a slow loss of the hydrogen atom from the C, atom. For the characterization of the six different chromia catalysts, the rate constants for five alcohols on one of these catalysts have been chosen as standards, and the data for each of the other catalysts were plotted against these reference values (146) (series 122-127). The slopes of the obtained linear dependences of Type B served as a relative measure of the energy differences among the active sites of individual catalysts. An excellent correlation (147) has been found between these slopes and B,, , an empirical parameter calculated from the positions of two bands in the electronic reflectance spectra of the catalysts ; the parameter B3, reflects the energy of interaction among the surface chromium atoms. This correlation shows that the preparation variables of the catalyst affect both the state of surface chromium atoms and their energy of inter-

188

M. KRAUS

action with the reactants. The same procedure has been applied to a series of chromium oxide catalysts containing various alkali metals by Kraus, Andreev, Mihajlova, and Nondek (149). Again, the suitability of the Taft equation for series of 2-alkanols on individual catalysts has been confirmed (series 128-133, 350°C, four reactants, slopes from 1.1 to 3.2), but for the correlation with the B,, parameter the relative slopes, based on the catalyst without any alkali metal, have been used; a good linear correlation has been obtained (series 134). 3. Oxidation of Hydrocarbons and Sulfides Structure effects on the rate of selective or total oxidation of saturated and unsaturated hydrocarbons and their correlations have been used successfully in the exploration of the reaction mechanisms. Adams (150) has shown that the oxidation of alkenes to aldehydes or alkadienes on a Bi,O,-Moo, catalyst exhibits the same influence of alkene structure on rate as the attack by methyl radicals; an excellent Type B correlation has been gained between the rate of these two processes for various alkenes (series 135, five reactants, positive slope). It was concluded on this basis that the rate-determining step of the oxidation is the abstraction of the allylic hydrogen. Similarly, Uchijima, Ishida, Uemitsu, and Yoneda (151) correlated the rate of the total oxidation of alkenes on NiO with the quantum-chemical index of delocalizability of allylic hydrogens (series 136, five reactants). Bobianko and Gorokhovatskii (152) and Roiter, Golodets, and Pyatnitskii (153) found correlations between the rates of the total oxidation of alkenes and alkanes on copper catalysts and the strength of the C+ bond, which is ruptured in the rate-determining step (series 137, six reactants; series 138, seven reactants). Trimm and Irshad (154) have used the influence of the substituents upon the rate of the oxidation of toluene and its derivatives to corresponding aldehydes on a molybdenum oxide catalyst at 460°C for obtaining insight into the mechanism. The rate constants could be correlated by the Hammett equation (series 139, five reactants, 450°C, slope - 1). Mashkina, Markoveev, and Zeif (155) measured the oxidation of an extensive series of dialkyl sulfides and alkylaryl sulfides to corresponding sulfoxides and sulfones on VzO, and tested a number of Type A, Type B, and Type C correlations. Good or excellent linear dependences have been obtained with the four-parameter Taft equation (9) (series 140, (r* = -0.9, s = OS), with the corrected rate constants (log k - sE,) against the enthalpy of formation of the sulfide -AlBr, complexes (series 141), and with (log k sE,) against several quantum-chemical indices: the charge on the S atom (series 142), delocalizability (series 143), and energy of highest occupied

SUBSTITUENT EFFECTS AS PROBES

189

orbital (series 144). The authors concluded that the reactivity depends both on steric hindrance and on electron density in sulfur, which is attacked by oxygen atoms.

Vi. Structure influence on Adsorptivity

Three different sources of data in the form of adsorption coefficients or quantities proportional to them may be encountered in correlations of adsorptivity. The first one, usually yielding quite reliable series, is based on independent adsorption measurements; as static methods are difficult to apply with higher-boiling adsorbates (100), the gas chromatographic (dynamic) technique is preferred by some authors (156, 157). The second sets are products of kinetic analysis of a given reaction, based on the Langmuir-Hinshelwood model of surface reactions. The corresponding rate equations contain the adsorption coefficients as adjustable parameters, which, in dependence on the form of the equation and on the propagation of experimental error into individual constants, are sometimes determined with considerable uncertainty (larger than that for the rate constants). This problem has been overcome by estimation of the relative adsorption coefficients from competitive experiments with pairs of compared reactants. The relative rates thus obtained are (e.g., 36), in the case of validity of the Langmuir-Hinshelwood kinetics and of the assumption about the surface reaction as the rate-determining step, a product of rate constants and adsorption coefficients: relative rate = k , K , / k , K , .

(17) From this expression, the relative adsorption coefficients of the starting compounds 1 and 2 Krel= K , / K , can be calculated when the values of the rate constants k , and kz are known from individual kinetic measurements. However, the success of this procedure depends very much on the reliability of the estimation of the rate constants. Sometimes simple measurements are conducted under the assumption that the reaction is zero order with respect to the concentration of the organic compound. When this assumption has not been adequately tested, this third source of data must be judged with care. Tables VI and VIII contain in parentheses several sets of adsorption coefficients of aromatic hydrocarbons that have been estimated from competitive experiments or adsorption measurements. The problems with the interpretation have been mentioned in Section V,A,2. Other series that have been correlated with Type A and Type B expressions are summarized in Table IX (48,52, 74,82, 96, 100, 103, 156-159). The series showing parallel

TABLE IX Correlations of Adsorpticity

Series

Source of data"

Adsorbate

s.a.

Methylbenzenes

GC GC

Ethylbenzenes Methylbenzenes

GC

Alkanes

i.k. i.k. i.k. i.k. i.k. c.k.

Alkylbenzenes Alcohols Amines Alcohols Alkylbenzenes Alkenes

147a 147b 148 83a 83b 83c 149a 149b 150 151 152 153 154 155

Sorbent (catalyst)

Pd AI,O,-SiO, AI,O,-SiOz A1203 Ion exchanger Ru Pt-SiO,

Correlation of log K, with

Number of points 4 4 4 10 10 10 9 9 5

3 4 5 5 7

s.a., static adsorption; GC, gas chromatographic; i.k., individual kinetic; c.k., competitive kinetic measurements -, negative slope. ' PA, picric acid. TCNE, tetracyanoethylene. Bond strength alkyl-CH,. Data only.

* + , positive;

Slope*

+ + + + + + + + + + -

Reference 100 100 158 103,156 103, 156 103, 156 157 157 52 159 48 74 96 82

SUBSTITUENT EFFECTS AS PROBES

191

trends with stability constants of 71 complexes are strong evidence for the existence of this form of surface species. The point is, however, whether they participate in the catalytic transformation as intermediates. Zdraiil’s (103, 156) successful combination of competitive kinetic data with gas chromatographically determined adsorptivities for alkylbenzenes and alkylthiophenes on a CoO-MoO,/Al,O, catalyst is encouraging: the relative rates from competitive experiments showing unusual order have yielded meaningful sets of rate constants (see Table VIII) when treated with adsorption coefficients reflecting the complexing.

VII. Conclusions

A physical organic approach to the problems of heterogeneous catalysis, which is the basis of the methods and results reviewed, has brought attention to some new or neglected aspects and hitherto dormant ideas. Frequent close parallelism in the behavior of organic compounds over solid catalysts and in noncatalytic reactions (when this counterpart could be found) supports the view that the mechanism of heterogeneous catalysis may be solved within the framework of a general theory of chemical reactivity. Although this fact has never been explicitly contradicted, many experimental findings have been interpreted as if the heterogeneous catalysis would be governed by quite special rules. There is, of course, no doubt that the solid catalyst introduces some specific factors, but they only modify the behavior determined by general laws; the geometrical arrangement of the surface and its relative immobility seem to be the most important factors. This viewpoint is in accord with the recently growing conviction (e.g., 160) that the chemical properties of the reaction sites are more closely represented by those of single atoms than by the bulk physical properties of a metal or oxide. The study of heterogeneous catalysis with the emphasis on the effects of reactant structure stimulates consideration of the reacting system in terms of mutual interactions. Modification of the catalyst surface by the action of reactants is a part of these interactions. This idea is not new, but hitherto little evidence supported it; now it is an inherent component of the accepted mechanism of elimination reactions. In general, the working surface may be quite different from the initial surface. Even the solvent may participate in the mechanism, as the results of the Delft school (125, 161, 162) indicate, by temporally accommodating hydrogen species formed in a reaction step from the reactants or hydrogen molecules on the surface. The study of structure influence on rate, which is the specific topic of this review, has contributed to the understanding of the different types of catalytic

192

M. KRAUS

reactions in varying degrees. As has been already mentioned, important information has been gained in the field of acid-base reactions on solids. With metallic catalysts, new problems have been opened by this approach. One of the challenging findings is the observation that the activation of hydrogen molecules on metals need not consist of homolytic loosening of the H-H bond into equal species (free-radical type), but that heterolysis into H - and H+ may be preferred. Similarly, in the dehydrogenation of alcohols the two hydrogen atoms leave the organic molecule very probably as a proton and a hydride. Various metals, in spite of their large differences in activity for individual reactions, behave so similarly with respect to structure effects on rate in many reactions that the question arises whether the observed activities are determined mostly by differences in the concentration of active sites on their surface and less by the electronic properties of the active sites. The analysis of structure effects has stressed the importance of a sound kinetic analysis as a basis for the design of the reaction mechanism, the necessity of a clear distinction between the contribution of various steps, and between the influence on adsorptivity and on reactivity, which may even act in opposite directions. Linear correlations as the means for expressing the influence of reactant structure and for catalyst characterization on the basis of their slopes have the advantage of operating with relative quantities. The extensive factors (the concentration of active sites) are eliminated by using relative rate and equilibrium data, and therefore only logarithms of intensive factors proportional to free energy changes are interpreted. This procedure has special merits in the case of Type C correlations, where the reference series is obtained on the basis of theoretical calculations that cannot model extensive parameters. Moreover, the simplifying assumptions in some quantumchemical methods, which make them not very safe for absolute calculations, very probably play a minor role in relative data for series of structurally related compounds. The experimental techniques for obtaining data suitable for a LFER are relatively simple, but the number of necessary measurements is large, in comparison with most physical probes for which the opposite is usually true. However, the otherwise inaccessible information gained makes the task worthwhile ; this may stimulate the development of methods for rapid or automated accumulation of data.

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4. Hernandez, L., and Nord, F. F., J. Colioid Sci. 3, 363 (1948). 5. Hsien-Cheng Yao, and Emmett, P. H., J . Am. Chem. Soc. 81,4125 (1959). 6 . Taya, K., Sci. Pap. Inst. Phys. Chem. Res. (Jpn.)56, 285 (1962). 7 . Finkelsthein, A. V., and Kuzmina, Z. M., Zh. Fiz. Khim. 40, 166 (1966). 8. Kieboom, A. P. G., and van Rantwijk, F., “Hydrogenation and Hydrogenolysis in Synthetic Organic Chemistry,’’ p. 36. Delft Univ. Press, Delft, The Netherlands, 1977. 9 . Sedlacek, J., J . Catal. 57, 208 (1979). 10. Ingold, C. K., “Structure and Mechanism in Organic Chemistry,” 2nd ed. Cornell Univ. Press, Ithaca, New York, 1969. 11. Hughes, E. D., ed., “Reaction Mechanisms in Organic Chemistry,’’ Vols. I-IV. Elsevier, Amsterdam, 1963-1966. 12. Noller, H., and Kladnig, W., Catal. Rev. 13, 149 (1976). 13. Beranek, L., and Kraus, M. Compr. Chem. Kinet. 20,263 (1978). 14. Melander, L., “Isotope Effects on Reactionn Rates.” Ronald Press, New York, 1961. 15. Weissberger, A., ed., “Technique of Organic Chemistry,” Vol. VIII. Wiley (Interscience), New York, 1961. 16. Burwell, R. L., Jr., Ace. Chem. Res. 2, 289 (1969). 17. Clarke, J . K. A., and Rooney, J. J., Adv. Catal. 25, 125 (1976). 18. Kieboom, A. P. G., and van Rantwijk, “Hydrogenation and Hydrogenolysis in Synthetic Organic Chemistry,” pp. 36, 55, 71, 80, 105, 121, 133, 138, and 143. Delft Univ. Press, Delft, The Netherlands, 1977. 19. Hammett, L. P., J . Am. Chem. Soc. 59,96 (1937). 20. Leffler, J . E., and Grunwald, E., “Rates and Equilibria of Organic Reactions.” Wiley, New York, 1963. 21. Wells, P. R., “Linear Free Energy Relationships.” Academic Press, New York, 1968. 22. Hine, J., “Structural Effects on Equilibria in Organic Chemistry,” p. 55. Wiley, New York, 1975. 23. Kraus, M., A&. Catal. 17, 75 (1967). 24. Mochida, I., and Yoneda, Y., J . Cutal. 7 , 393 (1967). 25. Hishida, T . , Uchijima, T., and Yoneda, Y., J . Catal. 11,71 (1968). 26. Mochida, I., Take, J., Saito, Y., and Yoneda, Y., J. Org. Chem. 32, 3894 (1967). 27. Yoneda, Y., Proc. Int. Congr. Catal., 4th, 1968 Vol. 2, p. 449 (1971). 28. Yukawa, Y., and Tsuno, Y., Bull. Chem. Soc. Jpn. 32,971 (1959). 29. Hine, J . , “Structural Effects on Equilibria in Organic Chemistry,” p. 92. Wiley, New York, 1975. 30. Shorter, J., in “Advances in Linear Free Energy Relationships” (N. B. Chapman and J. Shorter, eds.), p. 71. Plenum, New York, 1972. 31. Koppel, I. A,, Reakts. Sposobn. Org. Soedin. 2, No. 2, 26 (1965). 32. Mochida, I., and Yoneda, Y., J . Cural. 11, 183 (1968). 33. RuiiEka, V., Cerveny, L., and Pachta, J., Collect. Czech. Chem. Commun. 34,2074 (1969). 34. Kieboom, A. P. G., BUN. Chem. Soc. Jpn. 49, 331 (1976). 35. Criado, J. M., Iberoam. Symp. Catal., 4th 1974, Preprint. 36. Smith, H. A,, Ann. N . Y . Acad. Sci. 145, 72 (1967). 37. Zanderighi, L., Setinek, K., and Beranek, L., Collect. Czech. Chem. Commun. 35, 2367 (1970). 38. Dunn, I. J., J. Catal. 12, 335 (1968). 39. Exner, O., in “Advances in Linear Free Energy Relationships” (N. B. Chapman and J. Shorter, eds.), p. 20. Plenum, New York, 1972. 40. Stauffer, J. E., and Kranich, W. L., Ind. Eng. Chem., Fundam. I, 107 (1962). 41. Kochloefl, K., Kraus, M., and Baiant, V., Proc. Int. Congr. Catal., 4th, 1968 Vol. 2, p. 490 (1971).

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Mochida, I., Anju, Y., Kato, A,, and Seiyama, T., Bull. Chem. Soc. Jpn. 44,2326 (1971). Kibby, C. L., and Hall, W. K., J. Curul. 29, 144 (1973). Carrioza, I., and Munuera, G., J . Cutul. 49, 189 (1977). Kraus, M., and Kochloefl, K., Collect. Czech. Chem. Commun. 32,2320 (1967). Kraus, M., Collect. Czech. Chem. Commun. 34,699 (1972). Liu Ta-Chuang, Wang Fu-An, Yang Wen-Hsiieh, Lee Yun-Zing, Lee Ben-Kuo, and Lu Tai-Chung, Actu Chim. Sin. 32,89 (1966). 48. Catry, J. P., and Jungers, J. C., Bull. Soc. Chim. Fr. p. 2317 (1964). 49. Noller, H., and Ostermeier, K., Z. Elektrochem. 60,921 (1956). 50. Lopez, F. J., Andreu, P., Blassini, O., Paez, M., and Noller, H., J . Catul. 18, 233 (1970). 51. Franklin, J. L., and Nicholson, D. E., J . Phys. Chem. 60,59 (1956). 52. Rase, H. F., and Kirk, R. S., Chem. Eng. Progr. 50,35 (1954). 53. Mochida, I., and Yoneda, Y., J. Cutal. 7,386 (1967). 54. Mochida, I., and Yoneda, Y., Shokubui 6,281 (1964). 55. Strnad, P., and Kraus, M., Collect. Czech. Chem. Commun. 30, 1136 (1965). 56. Georgiev, C. D., and Kazanskii, B. A., Izv. Akud. Nuuk SSSR, Otd. Khim. Nuuk pp. 491, 499 (1959). 57. Roberts, R. M., and Good, G. M., J . Am. Chem. Soc. 73, 1320 (1951). 58. Schwab, G. M., and Mandre, G., Z. Phys. Chem. (Frankfurt am Main) 47,22 (1965). 59. May, D. R., Saunders, K. W., Kropa, E. L., and Dixon, J. K., Discuss. Furaduy Soc. 8, 290 (1950). 60. Fukui, Y., Takaoka, H., Ishii, J., Hirai, K., and Takahashi, T., Kogyo Kuguku Zasshi 69, 82 (1962). 61. Schneider, P., Kraus, M., and Baiant, V., Collect. Czech. Chem. Commun. 26, 1636 (1961); 27,9 (1962). 62. Sugioka, M., and Aomura, K., Bull. Jpn. Pet. Inst. 17, 51 (1975). 63. Andreu, P., Linero, M. A,, and Noller, H., J . Cutal. 21,349 (1971). 64. Ali, D., Kripylo, P., and Prinzler, H., J. Prukt. Chem. 315,47 (1973). 65. Kraus, M., Chem. Ind. (London) p. 1263 (1966). 66. Mochida, I., Anju, Y., Kato, A,, and Seiyama, T., J . Org. Chem. 39,3785 (1974). 67. Banthorpe, D. V., “Elimination Reactions.” Elsevier, Amsterdam, 1963. 68. Kochloefl, K., and Knozinger, H., Proc. In?. Congr. Cutul., 5th, 1972 p. 1171 (1973). 69. SedlaEek, J., and Kraus, M., Collect. Czech. Chem. Commun. 41,248 (1976). 70. Dautzenberg, D., and Knozinger, H., J . Cutul. 33, 142 (1974). 71. Kochloefl, K., Kraus, M., Chou Chin-Shen, Beranek, L., and Baiant, V., Collect. Czech. Chem. Commun. 27, 1199 (1962). 7 2 . .Malinowski, S . , Basinski, S., Szepanska, S., and Kiewlicz, W., Proc. In?. Congr. Cutal., 3rd, 1964 p. 441 (1965). 73. Cziiros, Z . , Deak, G., Haraszthy-Papp, M., and Prihradny, L., Acta Chim. Acud. Sci. Hung. 5 5 4 1 1 (1968). 74. Beranek, L., Setinek, K., and Kraus, M., Collect. Czech. Chem. Commun. 37,2265 (1972). 75. Knozinger, H., Angew. Chem. 80,778 (1968). 76. Simonik, J., and Pines, H., J . Cutul. 24, 31 1 (1972). 77. Venuto, P. B., J. Org. Chem. 32, 1272 (1967). 78. Venuto, P. B., and Landis, P. S.,Adv. Cutal. 18,259 (1968). 79. Stock, L. M., and Brown, H. C., Adv. Phys. Org. Chem. 1, 35 (1963). 80. Lebedev, S. V., Kobliansky, G. G., and Yakubchik, A. O., J. Chem. Soc. 127,417 (1925). 81. Schuster, C., 2.Elektrochem. 38,614 (1932). 82. Maurel, T., and Tellier, J., Bull. Soc. Chim. Fr. p. 4650 (1968). 83. RuiiEka, V., and Cerveny, L., J. Prukt. Chem. 311, 135 (1969). 42. 43. 44. 45. 46. 47.

SUBSTITUENT EFFECTS AS PROBES

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84. Cerveny, L., and RBiiEka, V., Collect. Czech. Chem. Commun.34, 1570 (1969). 85. Litvin, J. F., Freidlin, L. C., and Tilyaev, S. K., lzv. Akad. Nauk SSSR, Ser. Khim. p. 1220

(1968). Brown, C. A., and Ahuja, V. K., J . Org. Chem. 38,2226 (1973). Jardine, I., and McQuillin, F. J., J . Chem. SOC.C p. 458 (1966). Endrysova, J., and Kraus, M., Collect. Czech. Chem. Commun. 35,62 (1970). Kieboom, A. P. G., and van Bekkum, H., J. Catal. 25,342 (1972). Hussey, A. S., Keulks, G. W., Nowack, G. P., and Baker, R. H., J . Org. Chem. 33, 610 (1968). 91. Balenkova, J. C., Alekseeva, V. I., Khromova, G. I., and Khromov, S . I., Nefrekhimiya 9, 184 (1969). 92. Davidova, H., and Kraus, M., Collect. Czech. Chem. Commun. 43,725 (1978). 93. Smith, H. A., and Pennekamp, E. F. H., J . Am. Chem. Soc. 67,276 (1945). 94. Lozovoi, A. V., and Diakova, M. K., Zh. Obshch. Khim. 9,895 (1939). 95. Waquier, J. P., and Jungers, J. C . , Bull. Sor. Chim. Fr. p. 1280 (1957). 96. Yoshida, T., Bull. Chem. SOC.Jpn. 47,2061 (1974). 97. Smith, H. A., and Pennekamp, E. F. H., J . Am. Chem. Soc. 67,279 (1945). 98. Rader, C. P., and Smith, H. A,, J . Am. Chem. SOC.84, 1443 (1962). 99. Volter, J., Lange, B., and Kuhn, W., Z . Anorg. Allg. Chem. 340,253 (1965). 100. Volter, J., Hermann, M., and Heise, K., J. Catal. 12,307 (1968). 101. Lozovoi, A. V., and Seniavin, S . A., Sh. Statei Obshch. Khim. 1, 254 (1953). 102. Lozovoi, A, V., and Seniavin, S. A., Sh. Statei Ohshch. Khim. 2, 1035 (1953). 103. Najemnik, J., and Zdraiil, M., Collect. Czech. Chem. Commun. 41, 2895 (1976). 104. Perkampus, H. H., Adv. Phys. Org. Chem. 4, 195 (1966). 10.5. Nieuwstad, T. J., Klapwijk, P., and van Bekkum, H., J . Catal. 29,404 (1973). 106. Oldenburg, C. C., and Rase, H. F., AlChE J . 3,462 (1957). 107. Sporka, K., and RuiiEka, V., Collect. Czech, Chem. Commun. 33, 1247 (1968). 108. Sporka, K., RBiiEka, V., and RoEnakova, M., Sb. Vys. Sk. Chem.-Technol., Praze, Org. Chem. Technol. C11,33 (1967). 109. van Mechelen, C., and Jungers, J. C . , Bull. SOC.Chim. Belg. 59,597 (1950). 110. Seljakh, I. V., Dolgov, B. N., Zh. Prikl. Khim. 38,2034 (1965). 111. Kishida, S., Murakami, Y., Imanaka, T., and Teranishi, S . , J . Catal. 12,97 (1968). 112. Iwamoto, I., Yoshida, T., and Aonuma, T., Nippon Kagaku Zasshi 92,504 (1971). 113. Simonikova, J., Ralkova, A., and Kochloefl, K., J . Catal. 29,412 (1973). 114. Petro, J., Kalman, V., Lengyel, A., and Mathe, T., Period. Polytech., Chem. Eng. 15, 99 (1971). 115. van Bekkum, H., Kieboom, A. P. G., and van de Putte, K. J. G . , Reel. Trav. Chim. PaysBas 88,52 (1969). 116. Tanaka, K., Takagi, Y., Nomura, O., and Kobayashi, I., J . Catal. 35,24 (1974). 117. Geneste, P., Larnaty, G., and Vidal, B., Bull. Soc. Chim. Fr. p. 2027 (1969). 118. Berinek, L., and Kraus, M., Collect. Czech. Chem. Commun. 31,566 (1966). 119. MachaEek, H., Kochloefl, K., and Kraus, M., Collect. Czech. Chem. Commun. 31, 576 (1966). 120. Kochloefl, K., and Baiant, V., J . Catal. 10, 140 (1968). 121. Poulter, S . R., and Heathcock, C . H., Tetrahedron Lett. pp. 5539,5543 (1968). 122. Leclercq, G., Leclercq, L., and Maurel, R., J . Catal. 50, 87 (1977). 123. Kraus, M., and Baiant, V., Proc. lnt. Congr. Catal., Sith, 1972 Vol. 11, p. 1073 (1973). 124. Smejkal, J., and FarkaS, J., Collect. Czech. Chem. Commun. 28, 1557 (1963). 125. Irwin, W. J., McQuillin, F. J., Tetrahedron Lett. p. 2195 (1968). 126. Kieboom, A. P. G., de Kreuk, J. F., and van Bekkum, H., J . Catal. 20, 58 (1971). 86. 87. 88. 89. 90.

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Zdraiil, M., and Kraus, M., Collect. Czech. Chem. Commun. 39,3515 (1974). RBiiEka, V . ,and Santrochova, H., Collect. Czech. Chem. Commun. 34,2999 (1969). Bork, A. C., Acta Physicochem. URSS 9,697 (1938). Brihta, I., and Luetic, P., Croat. Chem. Acta 31,75 (1959). Sun Cheng-E., and Wang Hsiu-Shan, Acta Chim. Sin. 31, 11 (1965). Thonon, C., and Jungers, J. C., Bull. Soc. Chim. Belg. 59,604 (1950). Hajek, M., and KochloeR, K., Collect. Czech. Chem. Commun. 34, 2739 (1969). Hajek, M., Duchet, J. C., and Kochloefl, K., Collect. Czech. Chem. Commun. 35, 2258 (1970). 135. Hoffman, N. E., and Puthenpurackal, T., J. Org. Chem. 30,420 (1965). 136. Smolik, J., and Kraus, M., Collect. Czech. Chem. Commun. 31, 3042 (1972). 137. Hodges, R. J., and Garnett, J. K., J. Catal. 13, 83 (1969). 138. Shopov, D., Andrew, A,, Petrov, L., and Gudkov, B., Dokl. Bolg. Akad. Nauk 22, 1273 (1969). 139. Kubelka, V., and Kraus, M., Collect. Czech. Chem. Commun. 34,2895 (1969). 140. Belousov, V. M., Mulik, I. Ya., and Rubanik, N. Ya., Kinet. Katal. 10, 841 (1969). 141. Le Nhu Thanh, and Kraus, M., Collect. Czech. Chem. Commun. 38,2931 (1973). 142. Krosnar, T., Rathousky, J., and Baiant, V., Collect. Czech. Chem. Commun. 34, 1286 ( 1969). 143. Artamov, A. A., Balandin, A. A,, and Marukjan, G. M., Dokl. Akad. Nauk SSSR 169, 132 (1966). 144. Bogdanova, 0. K., Balandin, A. A,, and Belomestnykh, I. P., Izv. Akad. Nauk SSSR, Old. Khim. Nauk p. 61 1 (1963). 145. Nondek, L., and SedlaEek, J., J . Catal. 40,34 (1975). 146. Nondek, L., and Kraus, M., J. Catal. 40,40 (1975). 147. Nondek, L., Mihajlova, D., Andrew, A,, Palazov, A,, Kraus, M., and Shopov, D., J. Catal. 40,46 (1975). 148. SedlaEek, J., Avdeyev, V. I., and Zakharov, I. I., Collect. Czech. Chem. Commun. 40,3469 (1975). 149. Kraus, M., Andeev, A,, Mihajlova, D., and Nondek, L., Collect. Czech. Chem. Commun. 40,3856 (1975). 150. Adams, C. R., Proc. Int. Congr. Catal., 3rd, 1964 p. 240 (1965). 151. Uchijima, T., Ishida, Y., Uemitsu, N., and Yoneda, Y., J . Catal. 29,60 (1973). 152. Bobianko, I. I., and Gorokhovatskii, Ya. B., Katal. Katal. 2,29 (1966). 153. Roiter, V . A., Golodets, G. I., Pyatnitzkii, Yu. I., Proc. Int. Congr. Catal., 4th, 1968Vol. I, p. 466 (1971). 154. Trimm, D. L., and Irshad, M., J. Catal. 18, 142 (1970). 155. Mashkina, A. V . , Makoveev, P. S., and Zeif, A. P., Kinet. Katal. 10,823 (1969). 156. Zdraiil, M., Collect. Czech. Chem. Commun. 42, 1484 (1977). 157. Moro-oka, Y., Kitamura, T., and Ozaki, A., J. Catal. 13, 53 (1969). 158. Kraus, M., and Stmad, P. J. Catal. 3, 560 (1964). 159. Heath, C. E., M.S.Thesis, University of Wisconsin, Madison, 1956. 160. Derouane, E. G., Ind. Chim. Beige 36,359 (1971). 161. van Rantwijk, F., van Vliet, A,, and van Bekkum, H., J. Chem. SOC.,Chem. Commun. p. 234 (1973). 162. van Rantwijk, F., Kieboom, A. P. G., and van Bekkum, H., J . Mol. Catal. 1,27 (19751976).

127. 128. 129. 130. 131. 132. 133. 134.

ADVANCES IN CATALYSIS. VOLUME 29

Enzyme-like Synthetic Catalysts (Synzymes) G . P. ROYER Department of Biochemistry Ohio State University Columbus, Ohio

. . . . Structure and Properties . .

. . . . . . . . . . . . . . . . . . . . . . . . . . A. . . . . . . . . . . . . . B. Catalysis by Cycloamyloses and Derivatives Thereof. . . 111. Other Macrocycles . . . . . . . . . . . . . . . . . . . A.Amines.. . . . . . . . . . . . . . . . . . . . . B. Paracyclophanes . . . . . . . . . . . . . . . . . . C. Cyclic Peptides . . . . . . . . . . . . . . . . . . . IV. Linear Polymers . . . . . . . . . . . . . . . . . . . . A. Polypeptides. . . . . . . . . . . . . . . . . . . . B. VinylPolymers. . . . . . . . . . . . . . . . . . . I. Introduction

11. Cycloamyloses

.. . . .

. . . . . .

V. Catalysts Based on Polyethyleneimine: A Branched A. Structure . . . . . . . . . . . . . . . . B. Binding of Small Molecules . . . . . . . . C. Catalysis . . . . . . . . . . . . . . . . IV. Immobilized Catalysts . . . . . . . . . . . . VII. Semisynthetic Enzymes . . . . . . . . . . . . VIII. Conclusions. . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . .

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

. .

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

. . . . . . . . . . . . . . Synthetic Polymer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . .

..

197 199 199 200 205 205 206 208 208 208 210 215 215 216 218 220 223 223 224

I. Introduction

There are two basic reasons for attempting to make enzyme-like catalysts from nonbiological materials. First, the preparation of inexpensive, stable catalysts with high efficiency would be of obvious practical benefit. Second, the modeling of enzymes has provided new concepts and valuable confirmation of concepts arrived at by enzymologists. Additional supportive work and new ideas are expected in the future. Enzymes occur in every living cell and are the basic elements in the execution and control of metabolic processes. They are very sophisticated catalysts. In addition to bringing about spectacular rate enhancements, enzymes 197 Copyright 0 1980 by Academic Press, Inc. All rights of reproduction in any form reserved. ISBN 0-12-007829-5

198

G . P. ROYER

exhibit a very high degree of specificity in terms of reaction and substrate structure. Perhaps the most remarkable property of enzymes is their sensitivity to control. Cooperativity in allosteric enzyme systems and covalent modulation of enzymes permit rapid and effective responses to metabolic demands that result from changes in the environment. From the practical viewpoint, enzyme-like synthetic catalysts, or synzymes, need not be specific for a given reactant structure. In nature enzymes distinguish among closely related molecules and transform only the substrate for which it is specific. Mixtures of molecules may not be involved in the industrial reaction to be catalyzed. Reaction specificity is, of course, a requirement. A synthetic hydrolase should not catalyze other reactions such as decarboxylation. Enzymes bring about rate enhancements of lo6- 1014. A synzyme could be of great practical importance with far less efficiency than the natural enzyme if it is cheap and stable. In other words, a near miss in an attempt to mimic enzymes could be a fabulous success. Enzymes are polymers of L-a-amino acids with molecular weights in the range of 10,000-500,000. The active center, which is made up of a binding area and a catalytic area, represents only a small part of the surface area of the enzyme molecule. The following question arises : Is the rest of the molecule necessary or simply excess baggage that is a consequence of the limited building blocks which nature and evolution had at hand? The destruction (denaturation) of the three-dimensional structure of an enzyme results in inactivation in the cases where the active site is perturbed. However, there are many examples in which large pieces of protein can be removed under mild conditions (enzymatic cleavage) without loss in activity. Substrate- and ligand-induced conformational changes occur, but these changes are generally involved in specificity or control. The preparation of rigid active center models with the correct geometry should produce a catalyst with some enzyme-like characteristics such as strong binding of substrate and large rate enhancements. The synthesis of active centers is not a small problem. The enzyme carboxypeptidase A is a pancreatic exopeptidase that catalyzes the sequential release of amino acids from the C terminus of polypeptide chains as shown in reaction (1). Much work has been done on the structure (31).Although the

ENZYME-LIKE SYNTHETIC CATALYSTS

199

mechanism has not been firmly established, the following groups appear to play important roles at the active center: the guanidinium group of Arg-145, the phenolic group of Tyr-248, the carboxylate ion of Glu-270, and Zn2+. The synthesis of a model that contains these functional groups in the correct three-dimensional arrangement, along with a binding site, would not be easy. The arrangement of catalytic groups in synthetic catalysts to yield concerted, multifunctional mechanisms has been accomplished as we shall see. Also, synthetic polymers with very powerful binding capabilities have been prepared and characterized. In other words, it is not a short-range project to fabricate an enzyme active center, but some progress has been made in producing catalysts with enzyme-like characteristics. The scope of this article is limited to models that involve a substrate binding step prior to the catalytic step, shown in reaction (2). Many models Cat+S+Cat.Sk..’cat

+ P

(2)

demonstrating intramolecular reactions analogous to enzyme reactions have been made (4, 15,36, 46). These studies have been valuable in defining the nature and relative importance of factors that contribute to rate enhancement in enzyme catalysts. However, a successful synzyme for practical use should be a good catalyst at low substrate concentration for complete conversion of substrate to product. This feature implies a high affinity of catalyst for substrate. The binding of substrate by enzymes is important in the imposition of strain on the substrate and provision of a microenvironment that favors the transition state. It seems appropriate, therefore, to concentrate on models that work, as shown in reaction (2). II. Cycloamyloses

A. STRUCTURE AND PROPERTIES Cycloamyloses are cyclic oligosaccharides made up of D-glucopyranose rings in a cr-1,4 linkage (Fig. 1). Cycloamyloses with six, seven, and eight glucose units have been extensively studied (for review, see 6, 18, 26, 27).

---o

HO

l

FIG. 1. The backbone structure of cycloamyloses D-glucopyranose units in w1,4 linkage.

200

G . P. ROYER

HT FIG.2. Space-filling models of CL (6 units), j3 (7 units, and y (8 units) cyclodextrins. The cavity diameters range from 4.5 to 8.5 A.

These compounds are also called cyclodextrins: c1, six units; p, seven units; y , eight units. These molecules are toroidal in shape with cavity diameters that range from 4.5 to 8.5 A (Fig. 2). The lining of the cavity is composed of C-H groups along with glycosidic oxygens and is, therefore, apolar. The primary hydroxyl groups contributed by C6 of the glucose units are arranged on one end while the secondary hydroxyls of C2 and C3 are on the other. Most of the interest in cycloamyloses stems from the fact that these structures bind small molecules by inclusion in the apolar cavity. Since many enzymes bind substrates via apolar bonds, the use of cycloamyloses as enzyme models is justified. Moreover, on the basis of x-ray and neutron diffraction studies, an “induced-fit” mechanism has been proposed, which means that the binding of “substrate” is accompanied by a conformational change (90). Bender’s group and Cramer’s group have done thermodynamic and kinetic studies on the binding of small molecules by cycloamyloses (21, 99). In most cases 1 : 1 complexes are formed. Size dependence and spectrophotometric data strongly indicate that these are, in fact, inclusion complexes. The free-energy changes associated with ligand binding are at the low end of the binding free-energy range characteristic of enzymes (4-10 kcal mole- l). B. CATALYSIS BY CYCLOAMYLOSES AND DERIVATIVES THEREOF Cycloamyloses have been studied extensively as models of a-chymotrypsin and other serine proteases. Chymotypsin works via a double displacement pathway in which the hydroxyl group of serine-195 acts as a nucleophile. This is shown in Scheme I. In nature, the substrate is an amide. Synthetic esters are also used in model studies and routine assays. Nitrophenyl and

20 1

ENZYME-LIKE SYNTHETIC CATALYSTS

-

,CH,(Ser-l95) HN,pN:

t coo-

H-0

HN\+N---H--*

coo-

I

l

Asp-102

Asp-102

CH,(Ser-195) I

y ' FO'

HN / N . COO-

I

0

-

!

c \ H z ( S e r - 1 9 5 ) N+N , -H

0 I

r

COOH I Asp- 102

R

Asp-102

i"

X-C-R I! 0

+Hx

\+

deacylation

SCHEME I

other substituted phenyl esters have been used in studies of cycloamyloses as enzyme models. Van Etten et al. (99) showed that the esterolysis rates deviated strongly from the expected Hammett relationship when cycloamyloses were present. Rate accelerations brought about by the cycloamyloses were always larger for the meta-substituted esters than for the corresponding para-substituted compounds. This selectivity varies as the size of the cavity changes. Rates for release of phenol (acyl transfer) showed hyperbolic dependence on catalyst concentration, indicating complex formation. The reaction of phenyl esters with cycloamyloses occurs in two steps (98). In the first step, a secondary hydroxyl group is acylated, and phenol is released. In the second step, which is relatively slow, the acyl moiety is transferred to water. In a study of the hydrolysis of acyl benzoates the rates of benzoate appearance were the same for three different substrates, rn-nitrophenyl benzoate, rn-chlorophenyl benzoate, and rn-t-butylphenyl benzoate, which indicates the presence of a common benzoyl intermediate (98). Kaiser's group has demonstrated existence of a covalent intermediate and enantiomeric specificity in the hydrolysis of the spin label (1) (25). For cyclohexaamylose catalysis the binding constants for deacylation are similar for the enantiomers of (l),but the rate constants for acylationdiffer bya factor of eight. It is interesting that the reactions with cycloheptaamylose show no

202

G . P. ROYER

(11

enantiomeric specificity. Kurono et al. (51) prepared and isolated cinnamate and acetate derivatives of cycloheptaamylose. The deacylation of these compounds was accelerated by noncovalently complexed 6-nitrobenzimidazole. The reaction showed saturation by nitrobenzimidazole at neutral pH values. Czarnieki and Breslow (22) have studied the rate of acyl transfer from a substrate that is bound by the acyl part rather than by the leaving group. Having shown that ferrocene binds strongly to P-cyclodextrin, Czarniecki and Breslow employed the p-nitrophenyl ester of ferrocinnamic acid in kinetic studies using DMSO-buffer mixtures. A rate acceleration of 51,000 times background was observed for acylation of P-cyclodextrin. Unsubstituted cycloamyloses have been used to catalyze a number of reactions in addition to acyl group transfer. Brass and Bender (8)showed that cycloamyloses promoted phenol release from diphenyl and bis(p-nitrophenyl) carbonates and from diphenyl and bis(m-nitropheny1)methyl phosphonates. Breslow and Campbell ( 1 0 , I I ) showed that the reaction of anisole with HOCL in aqueous solution is catalyzed by cyclohexaamylose and cycloheptaamylose. Anisole is bound by the cyclodextrins and is chlorinated exclusively in the para position while bound. Cycloheptaamylose has been used to promote regiospecific alkylation followed by the highly selective oxidation shown in reaction (3) (95). In addition cycloheptaamylose effec-

m

c

H

3

aq RR'C=CHCH,Br O H - , cycloheptaamylose

CH,CH=C,

OH

0

(3)

/R R'

tively catalyzes a variety of specific allylation-oxidation reactions of the type shown in reaction (4) (96).

" R2

'

0 O

R

OH

'

XYC=CHCH2Br

RIO R2

CH,CH=CXY 0

+

R

2

R2

0

(4

H 0

Much interesting work has been done on catalysis by cycloamyloses in cases where no covalent intermediate is formed. Catalysis in these cases is

203

ENZYME-LIKE SYNTHETIC CATALYSTS

similar to enzymatic catalysis in that the microenvironment of the cycloamylose provides the catalytic driving force. This microenvironmental effect is manifested in two ways. First, the apolar character of the cycloamylose cavity may favor formation of the transition state. Second, the inclusion of the substrate into the binding cavity may bring about strain, distortion, or limitation of rotational modes, all of which may result in promotion of the reaction. Excellent discussions of these effects with numerous examples have appeared (5,fi). Unmodified cycloamyloses are not generally effective catalysts for acyl transfer and other reactions at pH values near neutrality. Also, the covalent adducts in many cases turnover very slowly. As a result of these drawbacks of unmodified compounds, many derivatives have been made. Since histidine is at the active site of many enzymes, it is only logical that the imidazolyl group was among the first to be chemically attached to cycloamyloses. Cramer and Mackensen (19,20) attached this group to the primary hydroxyl side of the cycloamyloses. A cycloheptaamylose containing about two imidazoyl groups per molecule produced a threefold rate enhancement over that produced by imidazole alone for the hydrolysis of p-nitrophenyl acetate at pH 7.5. The Bender group subsequently produced a cyclodextrin with the imidazoyl function attached at the secondary hydroxyl groups rather than at the primary hydroxyls of C6 (35). This derivative produced a rate 6.3 times greater than that of the control at pH 8.37. An interesting bisimidazole adduct has been reported by Breslow et al. (12). The imidazole derivative (3) was prepared from the capped disulfonate (2) originally made by Tabushi et al. (97). The cycloamylose (3) was used as

t

(3)

a ribonuclease model in studies of the hydrolysis of a cyclic phosphate (4). Studies with the monoimidazole analog of (3) and the pH-rate profile indicate a bifunctional mechanism similar to that proposed for ribonuclease as

G . P. ROYER

204 0 II

OP0,Z-

0-P-0-

OH

@ @"' L + shown in reaction (7). The hydrolysis of (4) can give either the 1-phosphate

(3 or the 2-phosphate (6). Alkaline hydrolysis gives a 6 : 4 mixture of

(5)

and (6). The cycloamylose-catalyzedreactions yield (6) almost quantitatively.

CH,-C

I

-CH3

Cycloheptaamylose cavity

The small amount of (5) produced was ascribed to the background reaction. In addition to the regioselective and bifunctional nature of the reaction, saturation with catalyst was also possible. Hence, even though the rate enhancement brought about by (3) is far below that of the enzyme ribonuclease, this catalyst is a good example of an enzyme model. Catalytic groups other than imidazole have been covalently linked to cycloamyloses. Breslow and Overman (14) bound a pyridine dicarboxylic acid group to a secondary hydroxyl by means of the m-nitrophenyl ester. When this group is chelated to pyridine carboxaldoxime through Ni2+,an

ENZYME-LIKE SYNTHETIC CATALYSTS

205

active catalyst is formed (7). Moderate rate enhancements were found at pH 5.2 for hydrolysis of nitrophenyl and dinitrophenyl esters. In contrast to unsubstituted cycloamyloses, the rate accelerations are greater for p-nitrophenyl acetate than for m-nitrophenyl acetate. This finding is consistent with the spatial relationships derived from model building studies (9). Gruhn and Bender (29)attached a hydroxamate group (8) to a secondary hydroxyl in an attempt to attain rapid turnover of catalyst in phenyl ester hydrolysis. The catalytic rate of the cycloamylose-hydroxamate adduct was compared to the rates brought about by (9) free in solution. Relative to the O \\

'r:

HzC,

OH

/"

/CH,

0

F H 3

C-N\

-N

1

1

HzC,

OH

0 H J

(8 )

(9)

derivatives discussed up to now, the rate enhancements brought about by (9) are large-as high as 70 for a nitrophenyl sulfone. The hydroxamate also exhibits preference for p-nitrophenyl acetate over m-nitrophenyl acetate. Even more effective catalysts resulted when additional functional groups were attached to cyclohexamylose in conjunction with the hydroxamate. Kitaura and Bender (43) reported the preparation of cyclodextrins (10) and (11). Compound (10) exhibited optical selectivity in the hydrolysis O f D- and L-acetylphenylalaninep(m)-nitrophenyl esters. Compound (10) proved to be a true catalyst at neutral pH in that it deacylates very fast.

111. Other Macrocycles

A. AMINB The properties of natural macrocycles, the cyclodextrins, has stimulated interest in the preparation of synthetic macrocycles. Three basic types have been made : macrocyclic amines, cyclophanes, and cyclic peptides. Hershfield and Bender (33) prepared a bicyclic amine with hydroxamate sub-

206

G . P. ROYER

0 HONCCH2N 11 /(cH*)lz\

I

CH,

\(CHd12/

0 NCH,bNOH CH, I

(12)

stituents (12). The cavity of compound of (12) is about 5.6 A in diameter and will presumably bind apolar substrates in a manner analogous to that for cycloamyloses. Impressive rate enhancements are brought about by (12) in the hydrolysis of p-nitrophenyl carboxylates at pH 6.8. The catalytic rate produced by (12) was compared to that of compound (13). For

the hydrolysis of p-nitrophenyl dodecanoate a rate enhancement of 7600 was observed. A correlation between rate enhancement and apolar character of the substrate was found. In analogy to enzymes, “burst” kinetics, competitive inhibition, and saturation were demonstrated. It was shown that Cu2+ accelerates the reaction of (12) with p-nitrophenyl carboxylates, but has no effect on the reaction catalyzed by (13).

B. PARACYCLOPHANES Murakami et al. (71) have prepared macrocyclic oximes, (14) and (15). At high pH values, 10-hydroxyl-1l-hydroximino[20]paracyclophane(14) reacts rapidly with p-nitrophenyl laurate and decanoate but not with p-nitrophenyl acetate or hexanoates. The inner diameter of the cavity of (14) is about 6.5 A. The association constants for the two long-chain nitrophenyl esters are about lo5 AK’, which is in the range for enzyme-substrate interaction. The negative effects of the presence of organic solvent and urea suggest, as expected, that the driving force for association is apolar bonding. The catalytic power of (14) is eliminated by Cu2+ presumably because the metal coordinates to the active catalytic form. It is interesting that the (14)-Cu2+

207

ENZYME-LIKE SYNTHETIC CATALYSTS

complex reduces the rate ofp-nitrophenyl laurate hydrolysis below the background hydrolysis rate, which indicates that the bound substrate is not accessible to OH- in the solvent. Further studies by Murakami et al. (72) have confirmed the initial observations on substrate specificity and binding. Relatively small rate enhancements were observed ( < loo), and even at high pH values the deacylation was slow. In an attempt to accelerate the deacylation step, Murakami et al. (69) attached an imidazole ring to the macrocycle. In a recent report the same group reported evidence for bifunctional catalysis by a paracyclophane, (16), in which Cu2+ polarized the

’0-

‘NO,

carbonyl oxygen to enable nucleophilic attack at the carbonyl carbon of an ester by the protonated oxime group (68).The authors point out that this is a good model of the proposed mechanism of carboxypeptidase A, which involves the Zn2+-assisted nucleophilic attack of glutamic acid 270 (58). This model, (16), embodies a binding cavity (K,,,,, = lo5 M - ’ ), a nucleophile (-C=N--OH), and a metal ion acting as an electrophile (Cu2+). Acylation occurs at pH 8.2, and saturation kinetics were observed. The “catalyst” does not turnover, however. Sunamoto et al. (94) have studied the reaction of (14) with 2,4-dinitrophenyl sulfate in aqueous organic solvent with 0.1 N sodium hydroxide. The rate of esterolysis by (14) was greater than the rate of the reaction catalyzed by p-cyclodextrin. Better binding by the paracyclophane cavity and the greater nucleophilicity of the oxime group as compared to the secondary hydroxyl group of /I-cycdextrin were cited to explain the difference in catalytic efficiency.

208

G. P. ROYER

C. CYCLIC PEPTIDES In a number of laboratories macrocyclic peptides have been studied as enzyme models (47,63,70, 72a, 91). A bicyclic peptide (17) with considerable apolar character was made by Murakami et al. (70) in which Hex is 6-aminohexanoyl ; Und is o-aminoundecanoyl. The esterolysis of p-nitrophenyl carboxylates by (17) was studied over the pH range 10-12. The rate depends on the ionization of one group (pK, 12.3), which is apparently the anionic imidazole group.

J

(!ily-Fys-Gly-His-Hex-Und

ply -Cy s-Gly -His-Hex-Und

1

IV. Linear Polymers

A. POLYPEPTIDES In the late 1950s it was shown that imidazole catalyzes the hydrolyses of p-nitrophenyl acetate (7, 16) and that histidine was at the active site of a-chymotrypsin (2). These findings led Katchalski et al. (39) to synthesize a number of histidine-containing polymers for evaluation as catalysts. Second-order rate constants were calculated on the basis of the concentration of neutral imidazole, that is, k, = (kobs- k,)/a[IM], where k, is the rate constant in the absence of catalyst and a is the fraction ionized. Some of these rate constants appear in Table I. All of the polymers possess less than TABLE I EfSect of Various Histidine Peptides on the Rate of Hydrolysis of p-Nitrophenyl Acetate at p H 7.73"

Catalyst Copoly(L-His, L-Ser) Poly-L-His (low MW) Copoly(L-His, L - A s ~ ) Poly-L-His (high MW) Copoly(L-His, L-ASP-L-SU) Imidazole Chymotrypsin a

Data of Katchalski et al. (39).

Histidine content (%)

38.9 100

45.4 100 6.2 -

0.67

a

k2

(estimated)

(liters/mole- '/min-')

0.95 0.99 0.84 0.98 0.91 0.77

9.7 9.5 1.4 5.5 3.4 35

-

104

ENZYME-LIKE SYNTHETIC CATALYSTS

209

one-thousandth of the activity of chymotrypsin as a catalyst for the hydrolysis of p-nitrophenyl acetate. Although the results were negative, the work was important in that it was the first demonstration that a simple combination of amino acids in a linear peptide was not adequate to produce an effective catalyst. Polypeptides containing tyrosine and glutamic acid have been studied as catalysts (74, 101). The interaction of the phenolic hydroxyl of tyrosine with the carboxyl group of glutamic acid was implicated in catalysis of the hydrolysis of p-nitrophenyl acetate. Michaelis-Menten kinetics were observed; the pH-rate profile was bell shaped, analogous to many enzymes. In the case of poly(L-Tyr, L-G~u,L-Ala) the K, at pH 5.7 was reported as 2.2 x M ( l o ] ) ,which means that the substrate is complexed fairly well by the polymer. However, the catalytic rate constants for these systems are far below the range for natural enzymes. Photki and Sakarellou-Daitsiotou (85) synthesized the series of histidinecontaining peptides shown below: Gly- His- Gly

Phe- Gly-His-Gly Gly- His- Gly- Gly- His- Gly

Ser- Gly- Gly- His- Gly- Gly- His- Gly Asp- Ser- Gly- Gly- His- Gly- Gly- His- Gly- OEt

Ser- Gly- Gly- His- Gly- Gly- His- Gly- Asp Gly- Asp- Ser- Gly- Gly- His- Gly- Gly- His- Gly- OEt

For the hydrolysis of p-nitrophenyl acetate at pH 7.7 the most effective catalyst was Gly-His-Gly-Gly-His-Gly. However, this peptide had only 50% of the catalytic activity of imidazole. For the seven peptides the range of catalytic effectiveness was found to be 30-50% that of imidazole. In one case, a small peptide with enzyme-like capability has been claimed. On the basis of model building and conformation studies, the peptide Glu-Phe-Ala-Ala-Glu-Glu-Phe-Ala-Ser-Phe was synthesized in the hope that the carboxyl groups in the center of the model would act like the carboxyl groups in lysozyme (17). The kinetic data in this article come from assays of cell wall lysis of M . lysodeikticus, chitin hydrolysis, and dextran hydrolysis. All of these assays are turbidimetric. Although details of the assay procedures were not given, the final equilibrium positions are apparently different for the reaction catalyzed by lysozyme and the reaction catalyzed by the decapeptide. Similar peptide models for proteases were made on the basis of empirical rules for predicting polypeptide conformations. These materials had no amidase activity and esterase activity only slightly better than that of histidine (59, 60). Heller and Klotz (32) prepared a series of peptides that contained histidine

210

G . P . ROYER

and cysteine in a variety of arrangements. For the hydrolysis of nitrophenyl esters, no multifunctional catalysis was observed. However, studies with thiol directed reagents revealed a rapid, reversible transacetylation reaction that involved cysteine and histidine. The authors suggested that thiol proteases have evolved in such a way that back-attack of cysteine on acylhistidine is inhibited.

B. VINYLPOLYMERS Morawetz and co-workers did pioneering work on reactivity and conformation of vinyl polymers in solution. Their initial goal was to use reactive groups on the polymer backbone to probe conformation. In one early study Morawetz and Gaetjens (65) reported on the preparation of a copolymer of methacrylic acid and p-nitrophenyl methacrylate (1-2%). Hydrolysis of the ester on the polymer involves the neighboring carboxyl group. The Morawetz group also reported on reactions of polymers that exhibited cooperative effects of the type illustrated in (18) (52,53).The groups A and

r'-----N

A' on the polymer may be chemically identical, or they may represent different ionization states. Studies on catalysis by polyions were another important topic addressed by Morawetz (64, and references therein). Polyions inhibit reactions of species of opposite charge. Reaction (8) would A-

+ B+-+C

(8)

be decelerated by a polyanion since B+ would be concentrated in the vicinity of polymer but A- would be repelled. Morawetz and Shafer (66,677 demonstrated this effect for the base-catalyzed hydrolysis of a positively charged ester. Acceleration would be expected if both reactants carried the charge opposite to that of the polymer. For example, Morawetz and Vogel (67a) demonstrated a spectacular rate enhancement (176,000-fold) with low levels of poly(vinylsu1fonate) in reaction (9). Co(NH3),Cl2+

+ H g 2 + + H 2 0 -+

Co(NH3),H203++ HgCl+

(9)

Letsinger and Savereide (55, 56) demonstrated catalysis by poly(4-vinylpryridine) in the solvolysis of 3-nitro-4-acetoxybenzenesulfonatein 50%

21 1

ENZYME-LIKE SYNTHETIC CATALYSTS

ethanol. The catalysis results from electrostatic binding of the negatively charged substrate to the positively charged polymer. The maximum of a kobsversus a plot (Fig. 3) occurs at a = 0.6, which corresponds to pH 3.6. The fully deprotonated polymer (a = 1) shows no unusual catalytic ability. Also, poly(4-vinylpyridine) shows less catalytic effect than the monomer with neutral nitrophenyl esters. These studies were extended to include polymeric substrates and catalysis by poly(N-vinylimidazole) (54). Kirsh et al. (42) prepared apolar derivatives of poly(4-vinylpyridine) by benzylation. With nitrophenyl acetate as the substrate the benzylated catalyst is 100 times more effective than 4-ethylpyridine. A double-displacement mechanism was observed. The rate constants for deacylation of the acylpoly(viny1pyridine) derivatives were about 4 x lOP4/sec. The comparable value for a-chymotrypsin is 8 x 10-3/sec. The factor of 20 seems small, but it should be kept in mind that deacetylation of a-chymotrypsin is very slow compared with the deacylation reactions involving the natural substrates of the enzyme.

0.08

7 0.06

L

.-C

E

:: Y

0.04

0.02

0

0.2

0.4

0.8

1 .o

a FIG.3. A plot of the observed rate constant versus c( for the hydrolysis of 3-nitro-4-acetoxybenzene sulfonate in the presence of (1) 0.016 M 4-methylpyridine (control) and (2) poly(4-vinylpyridine) with 0.01 M pyridine units. Line (3) is a calculated line projected from the pH dependence of the hydrolysis of a neutral substrate, dinitrophenyl acetate. From Letsinger and Savereide (55).

212

G . P. ROYER

Imidazole, substituted at the 4(5) position, was first incorporated into a vinyl polymer by Overberger and Vorchheimer (83). The synthesis of 4(5)-vinylimidazole was accomplished by decarboxylation of urocanic acid in vacua at 200°C. A variety of copolymers have been made (80,83). Three types of cooperative effects were observed : (1) imidazole acting as a nucleophile with imidazole anion acting as a general base (78);(2) imidazole acting as a nucleophile and a neutral imidazole acting as a general base (79);(3) attraction of an anionic substrate by imidazole cation with a nearby neutral imidazole acting as a nucleophile in analogy to Letsinger's model with poly(viny1pyridine) (78). Overberger et al. (81) reported significant rate enhancements with copoly[4(5)-vinylimidazole,p-vinylphenol]. In the case of p-nitrophenyl acetate, a large increase in rate was observed in the pH range in which the phenol groups were partially ionized (Table 11). Poly[4(5)-vinylimidazole]was considerably less effective in the high pH region. This cooperativity was observed with negatively charged substrates as well. Table I11 illustrates that, notwithstanding repulsion of the substrates by the polymer, a significant rate enhancement results from cooperativity between imidazole and the phenolate anion at high pH. As expected, a large acceleration of rate was observed with a positively charged substrate (3-acetoxy-N-trimethylanilinium iodide, ANTI) in the presence of the imidazole-phenol copolymer. At the pH value at which 10%of the phenol residues in the polymer were ionized a rate constant of 151 M-' min-' was found compared to 2.3 M-' min-' for imidazole. The copolymer of 4(5)-vinylimidazole and p-methoxystyrene did not bring about a rate enhancement. A probable mechanism for participation of the phenolate ion is shown in (10). A copolymer of 4(5)-vinylimidazole and acrylic acid was studied by Overberger and Maki (76). A TABLE I1 First-Order Observed Rate Constants f o r 1 : 1.95 Imidazole- Phenol Copolymer. Poly-4(5)-vinylimidazole,and Imidazole-Catalyzed Soluofyses of PNPA /cobs( x lo4 min-')

PH

1 : 1.95 Copolymer of 4(5)-vinylimidazole and p-vinylphenol

Poly-4(5)-vinylimidazole

Imidazole

7.4 8.2 9.1

3.0 5.1 28.6

2.1 3.0 3.2

2.6 2.4 2.1

' In 80% methanol-water at an ionic strength of 0.02. From Overberger et al. (81).

213

ENZYME-LIKE SYNTHETIC CATALYSTS

TABLE 111 First-Order Observed Rate Constants for I : I .95Imidazole-Phenol Copolymer, Poly-4(5)-vinylimidazole, and Imidazole-Catalyzed Solvolyses of NABSa.b /cobs( x lo3 min-')

PH

1 : 1.95 Copolymer of 4(5)-vinylimidazole and p-vinylphenol

Poly-4(5)-vinylimidazole

Imidazole

3.3 5.1 6.1 7.4 8.2 9.1

0.4 6.2 8.3 4.1 9.5 17.5

0.7 8.9 10.3 6.9 3.8 1.3

0.0 -

0.5 1.2 2.5

In 80% methanol-water at an ionic strength of 0.02.

* NABS is 3-nitro-4 acetoxylbenzenesulfonate. From Overberger et al. (81).

sequence of carboxylate-imidazole-carboxylate was the most effective arrangement for catalysis. With ANTI as a substrate this polymer showed significant catalytic activity.

Apolar binding of substrates has been demonstrated with polymers of vinylimidazole. Overberger et al. (77) studied the hydrolysis ofp-nitrophenyl acetate and p-nitrophenylheptanoate by poly[4(5)-vinylimidazole]in ethanol water mixtures. As one might expect the rate of p-nitrophenyl heptanoate hydrolysis increased at low ethanol concentrations as a result of apolar binding. The rate of p-nitrophenyl acetate hydrolysis also increased markedly at low ethanol concentration. This finding was explained by a conformational effect on the polymer, that is, lower ethanol concentration brings about a shrinkage of the polymer, which increases concerted interactions of the imidazole residues. The hydrolysis of 3-nitro-4-dodecanoyloxybenzoatewas found to be 1700 times faster in the presence of poly[4(5)-vinylimidazole]compared to free imidazole (77).A double-displacement mechanism was demonstrated for this system (75).

214

G. P. ROYER

Polymers of 4(5)-vinylimidazole and copolymers containing this monomer are usually studied with ethanol-buffer mixtures as solvent because of their insolubility in water. Overberger and Smith (82) found that poly( 1-Me-5vinylimidazole) was soluble in water. Negatively charged substrates with long apolar side chains were bound very strongly to this polymer. A rate enhancement of 106 over the monomeric analog, 1,5-dimethylirnidazole, was observed. On the basis of work on enzyme models of low molecular weight, Kunitake and his associates have prepared a variety of vinyl polymers containing the hydroxamate group. G r u h and Bender (28, 30) investigated compound CH,C-N, II

0

,Cff,OH

CH, I

NCH, I

CH, (19)

(19). Kunitake et af. (50) showed that deacylation of the hydroxamate of compound (20) was accelerated by a factor of 13 due to the presence of the

$=0

(20)

imidazole group. A solvent deuterium isotope effect of 2 indicates that the imidazole group functions as a general base. The enhanced deacylation rate was observed for acyl derivatives of polymer (21) (48, 49). Although the hydroxamate group does not occur in enzymes, the analogy of the hydroxylimidazole arrangement in (21) with the serine proteases is obvious. The acrylamide residues of (21) were added to provide solubility. The deacylation

ENZYME-LIKE SYNTHETIC CATALYSTS

215

rate of the hydroxamate polymer is two orders of magnitude greater than the deacylation rates of the hydroxamate or imidazole units alone. Manecke and his collaborators have synthesized polymers of vinylimidazole hydroxamic acid (73) (22, 23,24). Hydrolysis of p-nitrophenyl acetate

in the presence of the polymers shown above exhibits "burst" kinetics. It was found that the acyl group was transferred from the substrate to the hydroxamate of the catalyst. The monomeric unit was a better catalyst than the imidazole or carbohydroxamate. The monomeric unit also catalyzed the hydrolysis of p-nitrophenyl acetate faster than did the polymers. The activity decrease of the polymer-bound hydroxamate was ascribed to the steric hindrance of the polymer backbone. The oxamate group has been incorporated into a vinyl polymer by Kirsh and Kabanov (41). These workers prepared a copolymer of 4-vinyl-N(phenacy1oxime)pyridinium bromide and vinylpyridine. For the hydrolysis of p-nitrophenyl acetate the oxamate polymer produced a significant rate enhancement over the monomeric analogs. V. Catalysts Based on Polyethyleneimine : A Branched Synthetic Polymer

A. STRUCTURE Enzymes are compact, globular polymers. Vinyl polymers are extended structures with high intrinsic viscosities. For example, polyvinylpyrrolidone has an intrinsic viscosity of 22 ml g-' whereas comparable values for pro-

216

G . P. ROYER

teins are < 5 ml g- l . For water-soluble polymers of the linear type, the binding of small molecules is not strong relative to the strength of substrate complexation by enzymes. Since binding is the first step of any enzyme reaction, Klotz sought a globular synthetic polymer with the binding characteristics of proteins. Polyethyleneimine (PEI) is an inexpensive branched polymer made by the acid-catalyzed polymerization of aziridine (reaction

7 H

-+

HZN(CH~CH~NH)X(CH,CH~N)Y(CH~CH~NHZ) I CHz

(1 1)

I I

CHz N-

I

11) (23). The branching of the polymer is shown schematically in (25). The distribution of amino groups in 25% primary, 50% secondary, and 25% tertiary. The primary amines occur on the outside of the molecule and are easily substituted by acylation and alkylation.

B. BINDINGOF SMALLMOLECULES Klotz et al. (45) showed that PEI with pendant apolar groups possessed remarkable ability to bind small molecules. Serum albumin is a frequently used model for protein binding studies. Although it is not an enzyme, the binding sites of serum albumin resemble enzyme active sites both in terms of strength and nature of binding. Figure 4 shows the very powerful binding of the dye methyl orange to apolar (but watersoluble) derivatives of PEL For methyl orange the binding ability of lauroyl-PEI far exceeds that of serum albumin. The AGO for methyl orange binding by serum albumin under comparable conditions is about - 6 kcal mole- I . The apolar derivatives of PEI are therefore at the upper end of the range for strength of small molecule-protein interactions. The next question to be asked was whether or not this binding ability could be translated into correspondingly large acceleration of reaction rates. Royer and Klotz (89) investigated the rates of aminolysis of p-nitrophenyl esters using PEI with pendant apolar groups. The results of this study appear in Table IV. The rate constant for the reaction of lauroyl-PEI with p-nitro-

217

ENZYME-LIKE SYNTHETIC CATALYSTS I

1

300

a

%

3 0 a

m

2 200 \

w

> a a

z 3

0

_. 100

cn W

d5 - 6.0

- 5.0

-5.5

-4.5

- 4.0

LOG (FREE METHYL ORANGE)

FIG.4. Extent of binding of methyl orange at pH 7.0 and 25°C as a function of free (nonbound) dye concentrations: (1) polyethylenimine with 8.4% of residues acylated by lauroyl groups; (2) polyethylenimine with 11.5% of residues acylated by hexanoyl groups; (3) polyethylenimine with 10% of residues acylated by butyrl (0) or isobutyrl (m) groups; (4) Polyethylenimine, PEI-600; (5) bovine serum albumin.

TABLE IV First-Order Rate Constants for Amine Acylation by p-Nitrophenyl Esters".b

k ( x 10' min)"

Amine

p-Nitrophynel acetate

p-Nitrophenyl caproate

p-Nitrophenyl laurate

Propyl PEI-6d PEI-I 8 d PEI-600d L( lO%)-PEI-6'

0.98 3.60 4.38 4.60 15.2

0.51 1.41 1.57 1.80 68.1

0.053 0.1 1 0.11 0.17 698

Measurements made at pH 9.0 in 0.02 M tris(hydroxymethy1)aminomethane buffer, 25°C. Stock solutions of substrate were made in acetonitrile; hence, the final buffer also contained 6.7% acetonitrile. * From Royer and Klotz (89). ' k = k,, where k, is the measured rate constant in the presence of amine and k , is that for the hydrolysis in tris buffer alone, k , is 0.94 x min-' for the acetyl ester, 0.61 x lo-' min-' for the caproyl ester, and 0.023 x lo-' min-' for the lauroyl ester. The numeral following "PEI" multiplied by 100 is the molecular weight of the polymer sample. This sample of PEI-6 has 10% of its nitrogens acylated with lauroyl groups.

218

G. P. ROYER

phenyl laurate is 700 times greater than the rate constant for the reaction of propylamine and p-nitrophenyl acetate. Presumably a large part of this rate enhancement is due to complexation of the apolar ester by the alkyl groups on the polymer. The direct comparison of the rates of aminolysis of p-nitrophenyl laurate by lauroyl-PEI and propylamine reveals a rate enhancement of lo4. However, the p-nitrophenyl laurate is associated in solution even at the low concentration employed in the study. In either case the effect of introducing long-chain alkyl groups into the polymer is large. C. CATALYSIS

The next step in the elaboration of a synthetic catalyst based on the PEI matrix was the introduction of catalytic groups along with the binding groups. Reaction of PEI with chloromethyl imidazole and dodecyl iodide produced (26), a potent catalyst for the hydrolysis ofp-nitrophenylcaproate.

R = dodecyl

The derivative, containing 10% of its residues alkylated with dodecyl groups and 15% with methyleneimidazolegroups, brought about a rate enhancement of nearly 300 over imidazole for the hydrolysis of p-nitrophenylcaproate at pH 7.3 and 25" (44). Kinetic data indicated catalyst turnover and a two-step pathway. In chymotrypsin and other serine proteases the imidazole moiety of histidine acts as a general base not as a nucleophile as is probably the case in the catalysis of activated phenyl ester hydrolysis by (26). With this idea in mind, Kiefer et al. (40) studied the hydrolysis of 4-nitrocatechol sulfate in the presence of (26) since aryl sulfatase, the corresponding enzyme, has imidazole at the active center. Dramatic results were obtained. The substrate, nitrocatechol sulfate, is very stable in water at room temperature. Even the presence of 2M imidazole does not produce detectable hydrolysis. In contrast (26) cleaves the substrate at 20°C. Michaelis-Menten kinetics were obtained; the second-order rate constant for catalysis by (26) is 1OI2 times

ENZYME-LIKE SYNTHETIC CATALYSTS

219

greater than that for imidazole catalysis. Indeed, the enzyme arylsulfatase is 100-fold less effective than (26), in the hydrolysis of nitrocatechol sulfate. In addition to strong binding with approximation of substrate and catalytic groups, a microenvironmental effect may be important in this system. Batts (3) has reported a huge solvent effect on the rate of hydrolysis of alkylhydrogen sulfates. Moist dioxane was better than water by a factor of 10’. The Klotz group has also found rate enhancements of decarboxylation reactions with PEI derivatives. Catalysis of decarboxylation of P-keto acids by small amines goes via a Schiff base intermediate. Hine’s group has shown that unmodified PEI catalyzes dedeuteration effectively and that the reactions involve Schiff base intermediates (34, and references therein). DodecylPEI containing free amino groups and quaternized nitrogens, dodecylPEI-Q-NH,, was found to be an effective catalyst for the decomposition of oxaloacetate (reaction 12) (92). At pH 4.5 the polymer is lo5 times as effective as ethylamine. K, was found to be 3.5 x M at pH 4.5. HO,CCCH,CO~

II

+ dodecyl-PEI-Q-NH,

-+

0 CH,CCO,H

II

+ CO, + dodecyl-PEI-Q-NH,

(12)

0

Suh et al. (93)reported that dodecyl-PEI with fully quaternized nitrogens (no free amines) was an effective catalyst for the decarboxylation of nitrobenzisoxazolecarboxylic acid (reaction 13). Strong binding and stabiliza-

tion of the charge-delocalized transition state were cited to account for a rate enhancement of lo3 over background. Weatherhead et al. (100) have shown that PEI, benzylated to the extent of 10% of the amines, effectively cleaves Elman’s reagent, as shown in reaction (14). Evidence for a binding step and a rate enhancement of lo6 were re-

+ Bid-PE1,NH-S ‘fD

0

QNoz

coo-

”f&% coo-

(14)

220

G . P . ROYER

ported. The rate enhancement was calculated with an ideal simple amine as a reference. Benzylated PEI has an apparent pK of 7.49. The rate constant for the reference amine (pK, 7.49) was taken from a Br~nstedplot ( I ) .

VI. Immobilized Catalysts

There are at least three reasons for attempting to prepare solid-phase catalysts that resemble enzymes. Synthetic procedures would generally be simplified. Catalytic groups are fixed on the support so that they cannot interact with one another, for example, thiols cannot deactivate by forming disulfides and metal ions cannot deactivate by forming binuclear structures. Finally, if the successful catalyst is eventually made, it will almost certainly be used in heterogeneous systems. The first question that we faced in our work was the choice of the solid support. Porous glass seemed appropriate since it is rigid and has a very high surface area. In a study with an imidazole-thiol combination on glass we found that the silanized surface of the glass was not stable over extended periods in slightly alkaline solution. We then decided to investigate other supports, which lead to the development of polymer “ghosts” (Fig. 5). These structures result from a three-step process (61).First, the polymer is absorbed to porous alumina beads. The second step is crosslinking. In the final step the alumina core is dissolved away to yield a hollow “ghost.” The amount of structure in the wall and the thickness of the wall can be controlled by the geometry of the inorganic bead, the amount of polymer adsorbed, and the degree of crosslinking. For a catalyst support we used PEI “ghosts” since PEI is readily derivitized. It was crosslinked with glutaraldehyde to give a rigid structure that is compatible with many solvents including water, and is stable both chemically and mechanically. Poly(viny1amine) can be used in essentially the same way. PEI “ghosts” containing histidine residues and lauroyl groups were prepared from the respective active esters. The catalyst was first tested against p-nitrophenyl caproate. The assay required the removal of solution at timed intervals using a syringe fitted with a nylon net. The absorbance was then determined at 410 nm. The results with p-nitrophenyl caproate hydrolysis at pH 7.2 appear in Fig. 6 (62). The lauroyl-histidyl “ghost” show a rapid initial reaction rate. The reaction levels off, however, at a point less than onethird of the way to completion. At this point the catalyzed reaction rate actually falls below the background rate. Very strong binding and substrate inhibition may explain these findings. The rate enhancement in the initial stage of the reaction is about ten fold (catalyst over imidazole). Since imidazole is a known catalyst for hydrolysis of p-nitrophenyltrifluoroacetanilide

22 1

ENZYME-LIKE SYNTHETIC CATALYSTS

a. + (-CH,CH,-NH,-),

---)

-

alumina bead b.

+ glutaraldehyde

-

X

X

C.

Hollow

“Ghost”

FIG. 5. Preparation of polymer (PEI) ghosts in a three-step process: (a) adsorption; (b) stabilization; (c) removal of core. These structures were substituted with lauroyl groups and histidyl groups to yield the solid-phase analog of (26).

FIG.6. Hydrolysis of p-nitrophenyl caproate at pH 7.2,25”C:(0) lauroyl, histidyl “ghosts”; ( 0 )lauroyl ghosts; imidazole; ( 0 )background.

(m)

222

G . P. ROYER

(86) and since the apolar product is not negatively charged, we felt that our catalyst with strong binding sites would be effective with this substrate. The hydrolysis was followed to 75% of completion, and a rate enhancement of 230-fold over the imidazole catalysed rate was found at pH 8.2 (Fig. 7). Constant activity after repeated treatments with excesses of substrate showed that the catalyst was regenerated. Breslow et al. (1 3) have prepared an insolubilized cyclodextrin resin by crosslinking with epichlorohydrin. The resin was used for the chlorination of anisole via a three-step process. The column was loaded with anisole, that is, the available cycloclextrin cavities were filled with anisole. Aqueous HOCl was passed through and the product, 99% p-chloroanisole, was eluted with tetrahydrofuran. Problems such as diffusional limitations and the analysis of catalyst composition occur with solid-phase catalysts. Much work has been done on diffusion in bound enzymes (for reviews, see 24 and 88). In our work we used ninhydrin, which is a reagent ideal for surface analysis; amino acid analysis is used wherever possible. Amine depletion as followed by ninhydrin is not exact, but some quantitative guides are obtained. Certainly synthetic catalysts must be made with bonds other than amide bonds and components other than those compounds that are detectable on the amino acid analyzer.

Minutes

FIG.7. Kinetics of the hydrolysis of p-nitrophenyltrifluoroacetanilide: ( 0 )lauroyl, histidyl“ghosts”; (m) imidazole; ( 0 )background. A indicates substrate depletion from solution.

ENZYME-LIKE SYNTHETIC CATALYSTS

223

One alternative would be to use isotopically labeled intermediates to trace incorporation of groups in to the catalyst. In spite of the drawbacks I feel that the solid-phase approach will be used successfully in the synthesis of catalysts with enzyme-like properties.

VII. Semisynthetic Enzymes

Means and Bender ( H a ) observed that a specific binding site on serum albumin reacts very rapidly with p-nitrophenyl acetate. Acetylation occurs at the phenolic hydroxyl group of a tyrosine residue. The prospect of producing a catalyst from albumin by introducing catalytic groups at this reactive site appears interesting. In our laboratory we have done limited experiments on trying to make enzymes from antibodies. Antigens and haptens are bound to antibodies very tightly and specifically. Using the specificity of the antibody, we designed an affinity label that would label the binding site with nucleophilic groups. Antidinitrophenyl antibodies were reacted with bocdinitrophenylhistidine-p-nitrophenylester. However, insignificant rate enhancements were observed with the hydrolysis ofp-nitrophenyl acetate probably because of misorientation of the bound substrate. In another approach the specificityof an existing enzyme has been changed. Levine and Kaiser (57) have transformed a protease, papain, into a redox enzyme by alkylation of the active site thiol with (27),a derivative of xanthine.

(27)

The enzyme termed flavopapain, reacts with dihydronicotinamides. The catalyst exhibits saturation kinetics and modest rate enhancements over appropriate model compounds. VIII. Conclusions

Many of the basic elements of enzyme catalysis have been illustrated here, including binding of substrate, multifunctional catalysis, microenvironmental effects, covalent catalysis, and strain effects. The most remarkable rate enhancements reported to date are those brought about by apolar derivatives of PEI, a polycation. These rate enhancements are very en-

224

G . P. ROYER

couraging in that they are most likely produced by strong binding and microenvironmental effects. The structure of apolar PEI derivatives has been probed using fluorescence spectroscopy and 19F-NMR (37, 38, 87). The results of these studies indicate that the apolar groups of PEI cluster to form a hydrophobic pool in the interior of the molecule. This area is accessible to water, however. One could picture substrates being bound in the pool, and migrating until a catalytic group is encountered. If this model is correct, the very large rate enhancements brought about by PEI are probably not a result of highly precise alignment of catalytic groups with the substrate molecule. Strain effects would also seem unlikely. Proximity and medium effects would appear to be the most important factors. The conclusion is, therefore, that a rigid catalytic center using a specific two- or three-site attachment of substrate would be even more effective than the catalysts described here. As stated earlier, to make such a catalytic center is not a simple job. However, it can be said with confidence that, in view of the successes with some of the structures described here, the goal appears a great deal more attainable now than it was two decades ago when Morawetz, Bender, Bruice, and others began looking at enzyme models. REFERENCES 1. Al-Rawi, H., Stacey, K. A,, Weatherhead, R. H., and Williams, A., J . Chem. Soc., Perkin

Trans. 2 p. 663 (1978). 2. Barnard, E. A., and Stein, W. D., Adv. Enzymol. 20,51 (1958). 3. Batts, B. D., J . Chem. Soc. B p. 547 (1966). 4 . Bender, M. L., “Mechanisms of Homogeneous Catalysis from Protons to Proteins,” p. 281. Wiley, New York, 1971. 5. Bender, M. L., and Komiyama, M., Bioorg. Chem. I, 31 (1977). 6. Bender, M. L., and Komiyama, M., “Cyclodextrin Chemistry.” Springer-Verlag, Berlin and New York, 1978. 7. Bender, M. L., and Turnquest, B. W., J . Am. Chem. Soc. 79, 1652 (1957). 8. Brass, H. J., and Bender, M. L., J . Am. Chem. SOC.95, 5391 (1973). 9. Breslow, R., Ada. Chem. Ser. 100, 21 (1971). 10. Breslow, R., and Campbell, P., J . Am. Chem. Soc. 91,3085 (1969). 11. Breslow, R., and Campbell, P., Bioory. Chem. 1, 140 (1971). 12. Breslow, R., Doherty, J. B., Guillot, G . , and Lipsey, C., J . Am. Chem. Soc. 100, 3227 (1978). 13. Breslow, R., Kohn, H., and Siegel, B., Tetrahedron Lett. p. 1645 (1976). 14. Breslow, R., and Overman, L. E., J . Am. Chem. Soc. 92, 1075 (1970). 15. Bruice, T. C., and Benovic, S . J., Bioorg. Mech. 1, 1199 (1966). 16. Bruice, T. C., and Schmir, G. L., J . Am. Chem. Soc. 79,1663 (1957). 17. Chakravarty, P. K., Mathur, K. B., and Dhar, M. M., Indian J . Chem. 12,464 (1974). 18. Cramer, F., and Hettler, H., Naturwissenschaften 54, 625 (1967). 19. Cramer, F., and Mackensen, G., Angew. Chem., Inr. Ed. Engl. 5,601 (1966).

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20. 21. 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. 50. 51. 52. 53. 54. 55.

56. 57. 58. 59. 60.

61.

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Cramer, F., and Mackensen, G., Chem. Ber. 103,2138 (1970). Cramer, F., Saenger, W., and Spatz, H.-C., J . Am. Chem. Soc. 89, 14 (1967). Czarnieki, M. F., and Breslow, R., J . Am. Chem. Soc. 100,7771 (1978). Davis, L. E., in “Water-Soluble Resins” (R. L. Davidson and M. Sitlig, eds.), p. 216. Van Nostrand-Reinhold, Princeton, New Jersey, 1968. Engasser, J.-M., and Horvath, C., Appl. Biochem. Bioeng. 1, 128 (1976). Flohr, K., Paton, R. M., and Kaiser, E. T., Chem. Commun. p. 1621 (1971). French, D., Ado. Carbohydr. Chem. 12, 189 (1957). Griffiths, D. W., and Bender, M. L., Ado. Catal. 23,209 (1973). Gruhn, W. B., and Bender, M. L., J . Am. Chem. Soc. 91, 5883 (1969). Gruhn, W. B., and Bender, M. L., Bioorg. Chem. 3,324 (1974). Gruhn, W. B., and Bender, M. L., Bioorg. Chem. 4,219 (1975). Hartsuck, J. A,, and Lipscomb, W. N., in “The Enzymes” (P. D. Boyer, ed.), 3rd ed., Vol. 3, p. 1 . Academic Press, New York, 1971. Heller, M. J., and Klotz, I. M., J . Am. Chem. Soc. 99, 2780 (1977). Hershfield, R., and Bender, M. L., J . Am. Chem. Soc. 94, 1376 (1972). Hine, J., Ace. Chem. Res. 11, 1 (1977). Iwakura, Y., Uno, K., Toda, F., Onozuka, S., Hattori, K., and Bender, M. L., J . Am. Chem. Soc. 97,4432 (1975). Jencks, W. P., “Catalysis in Chemistry and Enzymology,” p. 8. McGraw-Hill, New York, 1969. Johnson, T. W., and Klotz, I. M., J . Phys. Chem. 75,4061 (1971). Johnson, T. W., and Klotz, I. M., Macromolecules 7,618 (1974). Katchalski, E., Fasman, G. D., Simons, E., Blout, E. R., Curd, F. R. N., and Koltun, W. L., Arch. Biochem. Biophys. 88,361 (1960). Kiefer, H. C., Congdon, W. I., Scarpa, I. S., and Klotz, I . M., Proc. Narl. Acad. Sci. U.S.A.69,2155 (1972). Kirsh, Y. E., and Kabanov, V. A., Dokl. Akad. Nauk SSSR 193,889 (1975). Kirsh, Y. E., Kabanov, V. A,, and Kargin, V. A,, Dokl. Acad. Nauk SSSR 177,112 (1967). Kitaura, Y., and Bender, M. L., Bioorg. Chem. 4,237 (1975). Klotz, I. M., Royer, G. P., and Scarpa, I. S., Proc. Natl. Acad. Sci. U.S.A.68,263 (1971). Klotz, I. M., Royer, G. P., and Sloniewsky, A. R., Biochemistry 8,4752 (1969). Komiyama, M., Bender, M. L., Utaka, M., andTakeda, A,, Proc. Natl. Acad. Sci. U.S.A. 74,2634-2638 (1977). Kopple, K. D., and Nitecki, D. E., J . Am. Chem. Soc. 84,4457 (1962). Kunitake, T., and Okahata, Y., Chem. Letr. p. 1057 (1974). Kunitake, T., and Okahata, Y., Macromolecules 9, 15 (1976). Kunitake, T., Okahata, Y., and Tahara, T., Bioorg. Chem. 5, 155 (1976). Kurono, Y., Stamoudis, V., and Bender, M. L., Bioorg. Chem. 5,393 (1976). Ladenheim, H., Loebl, E. M., and Morawetz, H., J . Am. Chem. Soc. 81,20 (1959). Ladenheim, H., and Morawetz, H., J . Am. Chem. Soc. 81,4860 (1959). Letsinger, R. L., and Klaus, I. S., J . Am. Chem. Soc. 87,3380 (1965). Letsinger, R. L., and Savereide, T. J., J . Am. Chem. Soc. 84, 114 (1962). Letsinger, R. L., and Savereide, T. J., J . Am. Chem. Soc. 84, 3122 (1962). Levine, H. L., and Kaiser, E. T., J . Am. Chem. Soc. 100,7670 (1978). Makinen, M. W., Yamamura, K., and Kaiser, E. T., Proc. Nail. Acad. Sci. U.S.A.73, 3882 (1976). Mehta, R. V., Mathur, K. B., and Dhar, M. M., Indian J . Chem., Sect. B 15B, 458 (1977). Mehta, R. V., Mathur, K. B., and Dhar, M. M., Indian J . Chem., Sect. B 16B, 118 (1978). Meyers, W. E., and Royer, G. P., J . Am. Chem. Soc. 99,6141 (1977).

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Meyers, W. E., and Royer, G. P., unpublished (1977). Mitchell, A. R., Gupta, S. K., and Roeski, R. W., J . Org. Chem. 35, 2877 (1970). Morawetz, H., Acc. Chem. Res. 3, 354 (1970). Morawetz, H., and Gaetjens, E., J . Polym. Sci. 32, 526 (1958). Morawetz, H., and Shafer, J., J . Phys. Chem. 67, 1293 (1963). Morawetz, H., and Shafer, J., Biopolymers I, 71 (1963). Morawetz, H.. and Vogel, B., J . Am. Chem. SOC.91, 563 (1969). Murakami, Y., Aoyama, Y., Kida, M., and Kirkuchi, J., J . Chem. SOC.,Chem. Commun. p. 494 (1978). 69. Murakami, Y., Aoyama, Y., Kida, M., and Nakano, A,, Bull. Chem. SOC.Jpn. 50, 3365 (1977). 70. Murakami, Y., Nakano, A., Matsumoto, K., and Iwamoto, K., Bull. Chem. SOC.Jpn. 51,2690 (1978). 71. Murakami, Y., Sunamoto, J., and Kano, K., Bull. Chem. SOC.Jpn. 47,1238 (1974). 72. Murakami, Y., Sunamoto, J., Okamoto, H., and Kawanami, K., Bull. Chem. SOC.Jpn. 48, 1537 (1974). 73. Nishide, H., Storck, W., and Manecke, G., J . Mol. C a r d 6, 23 (1979). 74. Noguchi, J., and Yamamoto, H., J . Biochem. (Tokyo) 64,703 (1969). 75. Overberger, C. G., and Glowaky, R. C., J . Am. Chem. SOC.95,6014 (1973). 76. Overberger, C. G., and Maki, H., Macromolecules 3,214 (1970). 77. Overberger, C. G., Morimoto, M., Cho, I., and Salamone, J. C., J. Am. Chem. SOC.93 3228 (1971). 78. Overberger, C. G., St. Pierre, T., Vorchheimer, N., Lee, J., and Yaroslavsky, S . , J . Am. Chem. SOC.87,296 (1965). 79. Overberger, C. G., St. Pierre, T., Yaroslavsky, C., and Yaroslavsky, S . , J . Am. Chem. SOC. 88, 1 184 (1 966). 80. Overberger, C. G., and Salamone, J. C., Acc. Chem. Res. 2,217 (1969). 81. Overberger, C. G., Salamone, J. C., and Yaroslavsky, S . , J . Am. Chem. SOC.89, 6231 (1967). 82. Overberger, C. G., and Smith, T. W., Macromolecules 8,416 (1975). 83. Overberger, C. G., and Vorchheimer, N., J . Am. Chem. SOC.85,951 (1963). 84. Paton, R. M., and Kaiser, E. T., J . Am. Chem. SOC.92,4723 (1970). 85. Photaki, I., and Sakarellou-Daitsiotou, M., J . G e m . SOC.Perkin Trans. 1 p. 589 (1976). 86. Pollack, R. N., and Dumsha, T. C., J . Am. Chem. SOC.97,377 (1975). 87. Prank, R. A., and Klotz, I. M., Biopolymers 16, 299 (1977). 88. Royer, G. P., in “Immobilized Enzymes, Antigens, Antibodies, and Peptides” (H. H. Weetall, ed.), p. 49. Dekker, New York, 1975. 89. Royer, G. P., and Klotz, I. M., J. Am. Chem. SOC.91,5885 (1969). 90. Saenger, W., Naltemeyer, M., Manor, P. C . , Hingerty, B., and Klan, Bioorg. Chem. 5 , 187 (1976). 91. Sheehan, J. C., Bennett, G. B., and Schneider, J. A,, J . Am. Chem. SOC.88,3455 (1966). 92. Spetnagel, W. J., and Klotz, I. M., J . Am. Chem. SOC.98, 8199 (1976). 93. Suh, J., Scarpa, I. S., and Klotz, I. M., J . Am. Chem. SOC.98, 7060 (1976). 94. Sunamoto, J., Kondo, H., Okamoto, H., and Taira, K., Bioorg. Chem. 6, 95 (1977). 95. Tabushi, I., Fujita, K., and Kawakubo, H., J . Am. Chem. SOC.99, 6456 (1977). 96. Tabushi, I., Kuroda, Y.,Fujita, K., and Kawakubo, H., Tetrahedron Lett. p. 2083 (1978). 97. Tabushi, K., Shimokawa, N., Shirakata, H., and Fujita, K., J . Am. Chem. SOC.98, 7855 (1976). 98. Van Etten, R. L., Clowes, G. A., Sebastian, J. F., and Bender, M. L., J . Am. Chem. SOC. 89,3253 (1967).

62. 63. 64. 65. 66. 67. 67a. 68.

ENZYME-LIKE SYNTHETIC CATALYSTS

227

99. Van Etten, R. L., Sebastian, J. F., Clowes, G. A., and Bender, M. L., J . Am. Chem. SOC.

89,3242 (1967). 100. Weatherhead, R. H., Stacey, K. A., and Williams, A., J . Chem. SOC.Perkin Trans. 2 p. 800 (1978). 101. Yamamoto, H., and Noguchi, J., J . Biochem. (Tokyo) 67, 103 (1970).

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ADVANCES IN CATALYSIS, VOLUME 29

Hydrogenolytic Behaviors of Asymmetric D iarylmethanes YASUO YAMAZAKI

AND

TADASHI KAWAI

Department of Industrial Chemistry Faculty of Technology Tokyo Metropolitan University Tokyo, Japan

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . Preparation of Asymmetric Diarylmethanes . . . . . . . . . . . Catalyst for Hydrogenolysis of Diarylmethanes . . . . . . . . . . Kinetics of Catalytic Hydrogenolysis of Diphenylmethane . . . . . Catalytic Hydrogenolysis of Asymmetric Diarylmethanes . . . . . . A. Experimental . . . . . . . . . . . . . . . . . . . . . . . B. Hydrogenolytic Behavior of Phenylarylmethanes . . . . . . . . C. Hydrogenolytic Behavior of Asymmetric Diarylmethanes . . . . D. The Kinetics and the Scheme of the Catalytic Hydrogenolysis of Asymmetric Diarylmethane . . . . . . . . . . . . . . . . VI. Active Species of Moo,-Al,O, Catalyst for Hydrogenolysis of Diarylmethanes . . . . . . . . . . . . . . . . . . . . A. Experimental . . . . . . . . . . . . . . . . . . . . . . . B. Effects of Pretreatment of Catalyst . . . . . . . . . . . . . . C. Active Species of Catalyst . . . . . . . . . . . . . . . . . D. Mechanism of Interaction between Active Species and Substrates. VII. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . .

I. 11. 111. IV. V.

.*

. . 229 . . 232 . . 239 . . 241 . . 243 . . 244 . . 244 . . 246 .

.

252

. . . . . . .

. . . . . . .

258 259 259 262 265 269 270

I. Introduction

1,2,4,S-Benzenetetracarboxylicdianhydride (pyromellitic dianhydride) is a typical bifunctional acid anhydride, and it is a useful raw material for preparing many useful chemicals. Polyimides and polyimidazopyrrolons prepared from this dianhydride have excellent heat and chemical resistance, as well as excellent mechanical and electrical properties. Pyromellitic dianhydride is produced by the oxidation of 1,2,4,5-tetraalkylbenzenessuch as 1,2,4,5-tetrarnethylbenzene(commonly known as durene) and 4,6-diisopropyl-l,3-dimethylbenzene.Durene, in particular, is a fundamental raw material for the production of the dianhydride (Z-8). 229 Copyright 0 1980 by Academic Press, Inc. All rights of reproduction in any form reserved. ISBN 0-12-007829-5

230

YASUO YAMAZAKI AND TADASHI KAWAI

The Clo fraction in reformates has been considered a likely petroleum source for obtaining durene (9). However, there are 28 kinds of isomeric arenes in the fraction, and the durene content is only 5-9%. Therefore, the isolation of durene from such a complicated mixture is inefficient, and some complicated techniques are required for it. In contrast, 1,2,Ctrimethylbenzene (pseudocumene) can be easily isolated simply by the distillation of the C, fraction of the reformates, and a high-purity pseudocumene has been produced on an industrial scale. The following routes have been published for converting pseudocumene to durene: 1. Transmethylation (ZO-16). 2. Methylation (17-26). 3. Condensation with formaldehyde and catalytic hydrogenolysis of the condensation products (27-35). It has been reported that the durene content in the tetramethylbenzenes obtained by the method of transmethylation or methylation is only 40-50%. However, 90% durene can be obtained by the last method. The authors’ studies have focused on the last method, and detailed investigations have clarified the following points (36-39) : 1. Formic acid is the most suitable catalyst for the condensation reaction of pseudocumene with formaldehyde, and the isomeric mixture of hexamethyldiphenylmethanes (HexMeDPM) with the following composition is obtained in good yields:

2 , 4 , 5, 2 ‘ , 4 ‘ , 5’-HexMeDPM,

84%

2, 3, 6 , 2 ’ , 4 ’ , 5’-HexMeDPM,

15%

2, 3, 5, 2‘, 4’, 5’-HexMeDPM,

0.4%

H3c&+

CH3 CH,

H,C‘

H3C

H,C’

HYDROGENOLYTIC BEHAVIORS OF ASYMMETRIC DIARYLMETHANES

H,C@cH;+

23 1

3

2, 3, 6, 2’, 3’, 6’-HexMeDPM,

CH3

0.6%

H,C

2. The hydrogenolysis of the condensates over a Co0-MoO3-A1,0, catalyst takes place quantitatively under mild reaction conditions, and a tetramethylbenzene mixture with the following composition is obtained with 92-95% selectivity to C,, arenes: CH3

CH,

CH3

H3C

CH, CH3 Prehnitene (3.7%)

CH3 Durene (94.3%)

H3C Isodurene (2.073

The ratio of the rates of hydrogenolysis of the aryl-methylene bonds (a’ and b’) in A I - ~ C H , ~ will A ~determine ’ the product selectivity. If we suppose that the rates of hydrogenolysis of bonds, a’ and b’, are equal, then the durene content from the mixture of HexMeDPM should be 91.7% and that of prehnitene, 8.1%; however, the durene content in the C,, arenes is 94.3% due to the hydrogenolysis of the condensates. The only asymmetric HexMeDPM in sufficient quantity to affect the amount of durene is 2,3,6,2',4,5'-HexMeDPM ; the amount of durene formed suggests that the hydrogenolysis on the 2,3,6-trimethylphenyl side will be about four times faster than that on the 2,4,5-trimethylphenyl side, thus favoring durene production. In fact, it was found that this pure compound hydrogenolyzes 80% on the 2,3,6-trimethylphenyl side. Such hydrogenolysis indicates that the following two parallel reactions occur and that their ratio will vary substantially with degree and location of methyl group in the catalytic hydrogenolysis of asymmetric diarylmethanes (Ar-CH, -Ar’) : /Ar-H

AI-CH,-Ar’

+ H, LAr-CH,

+ Ar‘-cH3 + Ar’-H

It is interesting to consider the factors affecting the product selectivity. Our experimental studies have examined precisely such phenomena. In this article we review the hydrogenolysis of asymmetric diarylmethanes based on our investigations.

232

YASUO YAMAZAKI A N D TADASHI KAWAI

Initial research centered on the hydrogenolytic behavior of asymmetrically methyl-substituted diarylmethanes (hereafter abbreviated as asym DAMs) on Co0-Mo0,-A1,03 catalyst. Subsequently, the hydrogenolytic behavior of 2,5,3'-trimethyldiphenylmethane (2,5,3'-TrMeDPM) was investigated as a function of the pretreatment of MOO,-Al,O, catalyst. The main points determined in this work were as follows: 1. The relation between the structures of asym DAMs and (a) their hydrogenolytic behaviors and (b) their overall rates of hydrogenolysis. 2. The kinetics and the scheme of the hydrogenolysis of asym DAM. 3. The relationship between the hydrogenolytic behavior of an asym DAM and the variations of the catalytic properties. 4. The active species of the catalyst and the reaction mechanism. II. Preparation of Asymmetric Diarylmethanes

Thirty-five different asym DAMs were synthesized by the condensation reaction of benzyl alcohols and arenes in the presence of an acid catalyst. The mechanism of the condensation reaction of pseudocumene and formaldehyde showed that trimethylbenzyl alcohols were formed as intermediates. However, these alcohols were not detected in the reaction products because they are more reactive than formaldehyde and condense immediately with pseudocumene to produce the corresponding HexMeDPM (39). Ordinarily, p-toluenesulfonic acid has been used as the catalyst. Although p-toluenesulfonic acid has high activity, its use results in the formation of by-products such as methylbenzyl-p-toluenesulfonates, high molecular weight condensates, and esters. We found that formic acid is a better catalyst for this reaction and gives almost quantitatively the mixture of HexMeDPM in a pure state without any contamination due to undesirable by-products. Formic acid also serves as a solvent. From this study, it was found that benzyl alcohols react easily with arenes to form asym DAMs, and formic acid is the most favorable catalyst for the benzylation reaction. Five kinds of methylbenzyl alcohols were prepared through the following routes :

OcH3+&""' p

FHzCl

3

SOCI, (400)

CH,COOK (41) CH,COOH

HYDROGENOLYTIC BEHAVIORS OF ASYMMETRIC DIARYLMETHANES

&-&

LIAIH, (43)

CH3 bp 8Oo-81"C/2 Torr (42)

bp 107"-108"C/8 Torr (42)

bH3 bp 16lo-163"C/54 Torr (42)

233

234

YASUO YAMAZAKI AND TADASHI KAWAI

Benzyl alcohols, including commercially available benzyl alcohol and 4-methylbenzyl alcohol, were purified through rectification, followed by recrystallization when the alcohols were solids. A series of asym DAMs were synthesized by the condensation reaction of arenes with benzyl alcohols. Benzene, p-xylene, mesitylene, durene, and isodurene were used as arenes because they give only one condensation product. For example, pure 2,5,3‘-TrMeDPM is easily prepared by a distillation of the crude product obtained by the following reaction:

-

H3C H02H & @

+ H3c@

H 3 c b C H y *

CH3

CH3

In most cases, the reaction was carried out with formic acid. However, p-toluenesulfonic acid was used for the condensation of 4-methylbenzyl alcohol with benzene, benzyl alcohol with isodurene, and 2-methylbenzyl alcohol with p-xylene because formic acid was too weak an acid in these cases. The condensation products were purified by rectification and, when possible, by subsequent recrystallization. Through gas chromatographic analysis the purity of asym DAMs was found to be 99% +. The structures of these compounds have been identified by elemental analysis, infrared, nuclear magnetic resonance, and mass spectra (50, 51). The properties of the asym DAMs which we prepared are listed in Table I (42). TABLE I Properties of Asymmetric Diarylmethanes

No.

Compound

bp (“C Torr- ’)

mp (“C)

ni5

1.5721

1 S659

1.5691

(continued)

235

HYDROGENOLYTIC BEHAVIORS OF ASYMMETRIC DIARYLMETHANES

TABLE I (Continued)

4

5

6

7

bp ("C Torr-')

Compound

No.

D

C

*

@4$ 0

-

C

-

b

9

-

C

-

P

10

d

11

c

5

-

l37i6.0

-

1.5677

104

-

107/1.0

__

1S648

125

-

126i2.0

134

0

4

I33

0-C-k-

8

mp ("C)

-

38.4

136i3.5

39.6

1.5555

-

144

-

148i3.5

58.4

-

59.3

158

-

164i4.5

58.0

-

60.0

129

- 130i3.5

I35

- 13813.5

-

1S669

*

&-@

61.2

- 62.2

-

(continued)

236

YASUO YAMAZAKI A N D TADASHI KAWAI

TABLE I (Continued) No.

Compound

bp ("C Torr-')

-

mp ("C)

ni5

12

155

15713.5

95.0

-

96.2

13

154- 156/3.0

84.5

-

86.2

14

132

-

134/3.0

-

1.5655

15

138

-

139/3.5

-

1.5650

16

146

-

149/2.0

74.0

-

15.2

17

145

-

147/1.5

50.0

-

51.7

18

142

-

14416.0

19

141

-

14213.5

54.6

~

-

~

-

-

1.5620

-

-

55.6

(contimed)

HYDROGENOLYTIC BEHAVIORS OF ASYMMETRIC DIARYLMETHANES

237

TABLE I (Continued) No.

Compound

bp ("C Torr-')

mp ("C)

20

166

-

16713.5

105.1

-

107.2

21

172

-

173/4.0

71.7

-

72.5

22

156

-

151/7.0

103.5

-

105.0

23

149

-

151/2.0

119.0

-

120.3

24

182

-

184/9.0

116.0

-

117.2

25

122

-

124/1.0

26

161

-

163/3.0

12.6

-

73.1

27

139

-

142/2.0

68.8

-

69.7

ni5

-

(continued)

238

YASUO YAMAZAKI AND TADASHI KAWAI

TABLE I (Continued) No.

Compound

bp (“C Torr-’)

mp (“C)

np

28

148

- ISO/l.O

80.2

- 81.5

29

149

-

I5l/l.5

76.4

- 77.5

30

172

- 175/1.5

150.2

-

152.1

-

31

157

-

160/2.0

132.2

-

133.7

-

32

160

-

162/2.0

104.5

-

106.1

-

33

195

-

197/8.0

103.0

-

105.0

-

34

150

-

152/1.5

102.2

-

103.5

-

100.0

- 101.5

35

-

-

-

-

HYDROGENOLYTIC BEHAVIORS OF ASYMMETRIC DIARYLMETHANES

239

111. Catalyst for Hydrogenolysis of Diarylmethanes

The possible reaction routes in the catalytic hydrogenolysis of 2,5,3’TrMeDPM are shown in Fig. 1. In Fig. 1, dearylation 1 refers to the dearylation reaction of 2,5,3’-TrMeDPM; the hydrolysis occurs between the two aryl groups and the methylene group. Demethylation 1 shows the demethylation reaction of 2,5,3’-TrMeDPM. Demethylation 2, the demethylation reaction of the various methylbenzenes formed, was negligible under our reaction conditions. Dearylation 2 indicates the dearylation of the demethylated compounds of 2,5,3’-TrMeDPM. High selectivity for dearylation 1 is desirable in this approach. If other reactions such as demethylation 1 and dearylation 2 do not take place, the molar ratio of toluene and pseudocumene (T/TrMeB) will be unity. However, if side reactions occur, then the toluene produced will increase, whereas pseudocumene decreases. This means the ratio of T/TrMeB deviates from 1 and increases with the extent of the side reactions. Therefore, the T/TrMeB ratio provides an evaluation of the selectivity of dearylation 1 as well as side reactions: a = T/TrMeB

Another index of demethylation will be defined as follows :

’

(MeDPM + DMeDPM) formed x 100 (%) = TrMeDPM converted

where MeDPM, DMeDPM, and TrMeDPM mean methyldiphenylmethanes, dimethyldiphenylmethanes, and 2,5,3’-TrMeDPM, respectively. The

Dearylatlon 1

c

-0 Demethybtlon 2

FIG.1 . Competitive and consecutive hydrogenolysis of 2,5,3’-trimethyldiphenylmethane.

240

YASUO YAMAZAKI AND TADASHI KAWAI

greater the value of p, the greater is the extent of the demethylation; with a catalyst completely selective for dearylation 1, M would be unity and p, zero. In addition, the ratio of hydrogenolysis on paths b and a in dearylation 1 (product selectivity) was measured by

_b -- m-X a

+

p-X 2 x TrMeB

where m-X and p-X mean m-xylene and p-xylene, respectively. Many catalysts were screened in order to find the most selective for the dearylation 1. The results are shown in Table I1 (52).The 10 wt% MOO,A1,0, catalyst appears to be the most selective catalyst tested. The 10 wt % MOO,-A1,0, catalyst has high activity, an M value close to 1, and the smallest p value. However, the loss in activity during a run is relatively large for the 10 wt % MOO,-Al,O, catalyst; the activity after a 6-hr run drops to one-half of that of the fresh catalyst. The addition of COO to MOO,-Al,O, as a third component lowers the activity of the fresh catalyst. However, the Co0-Mo0,-A1,0, catalyst decreases deactivation and it also provides good selectivity for dearylation 1. The most suitable composition of the catalyst is 1 3 wt 7; COO-10 COO-12 20 wt % MOO,-Al,O, (52). A commercially available 4 wt wt MoO,-Al,O, catalyst was employed both for the investigations of

-

-

TABLE I1 Activities and Catalytic Behaviors of Various Metallic Oxides"

Catalysts A1203

20% NiO-AI,O, 10% NiO-MgO 10% V2OS-AIZOj 10% CoO-AI,O, 25% AI,O,-SiO, 10% W0,-AI,O, MOO, 10% MoO,-MgO 10% Moo,-SiO, 10% MoO3-A1,0,-Si0, 10% M0O3-AI2O3

Calcination condition ("C hr-')

Conversion'

55012 55012

550/2 60012 55012 55012 65013 650/1 65013 650/3 650/3 55014

WIF

(%I

1.9 10.5 10.5 1.9 10.5 10.5 7.9 1.9 1.9 7.9

5 3 1

I 9 31

1.9 10.5 ~

U

a

2.15 1.10

23 19

~

45 1

1.32 2.35 2.90 2.13 2.34

1 21 87 61

1.35 1.83 1.01

-

~~

Reaction temperature, 350"C, H2/(2,5,3'-TrMeDPM + benzene); molar ratio, 2.0. Pretreated with hydrogen at 450°C for 2 hr. ' Determined after the feed had passed over the catalyst for 60 min.

-

10

29 22 15 12 20 10 12 5

HYDROGENOLYTIC BEHAVIORS OF ASYMMETRIC DIARYLMETHANES

241

the kinetics of the hydrogenolysis of diphenylmethane and the hydrogenolytic behaviors and the reactivities of asym DAMS. MOO,-Al'O, catalysts were used in studies of the relations between hydrogenolytic behavior and catalytic properties. IV. Kinetics of Catalytic Hydrogenolysis of Diphenylmethane

Diphenylmethane (DPM), the basic hydrocarbon of the diarylmethane series, is the most appropriate compound for studying the kinetics of the catalytic hydrogenolysis of diarylmethane. This study involved a CoOMOO,-Al,O, catalyst (4 wt % COO,12 wt % MOO,) (53). The reaction was carried out in a flow system using a fixed-bed reactor made of Pyrex (25 mm4 x 400 mm) provided with an internal thermocouple well. The catalyst was packed in the reactor and was pretreated in situ with hydrogen at 535°C for 2 hr. The mesh size used was 10-40. The reaction conditions were as follows: temperature, 490-530°C; hydrogen partial pressure (pH),0.27-0.91 atm; DPM partial pressure (P,), 0.050.20 atm; and W/F, 8.8 g catal hr mole-' (here, W is the weight of the catalyst used and F is the moles of DPM per hour). Reaction rates under these conditions were not affected by the mass transfer of internal and external diffusion of the reactants. The observed values are shown in Table 111. Under the reaction conditions (temperature, 515-535°C; W/F, 4.1-10.5 g catal hr mole-' ; and HJhydrocarbons molar ratio, 5), the apparent initial rate ( r o ) was represented as r,, = k,,,PDPi.5 (1) where P , and P , are the partial pressures of DPM and hydrogen. Equation (1) suggests that the rate-determining step of the catalytic hydrogenolysis of DPM is the reaction between DPM and the dissociated hydrogen on the catalyst. As shown in Table 111, the conversion of DPM increases with the increase in hydrogen partial pressure, and it decreases with the increase in DPM partial pressure. These tendencies suggest that DPM and hydrogen absorb on the same kind of active sites of the catalyst. Furthermore, linear relationships are found between (PD/ro)'/' and P,, and between (P&,)'/' and (pH)'/'from the kinetic data shown in Table 111. The following elementary reactions are proposed to reflect the above relationships:

+2ue2H H, + D*(DH), P H I , + H, =$ B, + To H,

(2) (3) (4)

242

YASUO YAMAZAKI AND TADASHI KAWAI

B,*B T,=T

+u +u

(5)

(6)

where H and H, are unadsorbed and adsorbed hydrogen, respectively; D and (DH), are unadsorbed DPM and adsorbed DPM on hydrogenated active site (H,), respectively; B and B, are unadsorbed and adsorbed benzene, respectively ;T and T, are unadsorbed and adsorbed toluene, respectively; and (r is the vacant active site of the catalyst. TABLE 111 Kinetic Data on Catalytic Hydrogenolysis of Diphenylmethane Temperature ("C)

Conversion

(%I

r,(exp.) x lo3

P"

PD

490

0.273 0.455 0.636 0.909 0.800 0.800 0.800 0.800

0.091 0.091 0.091 0.091 0.050 0.100 0.150 0.200

2.6 3.2 3.4 3.7 4.2 3.5 3.0 2.6

0.28 0.34 0.36 0.40 0.24 0.41 0.53 0.61

505

0.273 0.455 0.636 0.909 0.800 0.800 0.800 0.800

0.091 0.091 0.091 0.091 0.050 0.100 0.150 0.200

3.5 4.5 5.2 5.9 5.7 5.1 4.4 3.9

0.36 0.47 0.54 0.63 0.33 0.59 0.76 0.90

520

0.273 0.455 0.636 0.909 0.800 0.800 0.800 0.800

0.091 0.091 0.091 0.091 0.050 0.100 0.150 0.200

5.1 6.5 7.4 8.1 8.3 7.5 6.8 6.2

0.52 0.67 0.76 0.83 0.47 0.85 1.15 1.40

535

0.273 0.455 0.636 0.909 0.800 0.800 0.800 0.800

0.091 0.091 0.091 0.091 0.050 0.100 0.150 0.200

7.5 8.8 9.8

0.76 0.89 0.99 1.16 0.58 1.06 1.46 1.77

11.5 10.5 9.6 8.8 8.0

HYDROGENOLYTIC BEHAVIORS OF ASYMMETRIC DIARYLMETHANES

243

Assuming that the rate determining step is the surface reaction on the catalyst [Eq. (4)], the following rate equation is derived:

where k and k‘ are the velocity constants of the forward and reverse reactions in Eq. (4), respectively; KH, K D H , KB and KT are the adsorption coefficients of hydrogen, DPM, benzene, and toluene, respectively; and PH,P,, P,, and P, are the partial pressures of hydrogen, DPM, benzene, and toluene, respectively. When the partial pressures of the reaction products are negligibly small, Eq. (7) is simplified as follows:

If P, or P , is constant, Eq. (8) can be arranged as a linear function of P, or PD,such as

(9)

The experimental data satisfy Eqs. (9) and (10) as previously mentioned. In the same way, all of the possible rate-determining steps of the surface reactions, which are based on both Langmuir-Hinshelwood and RidealEley mechanisms, were formulated and were compared with the experimental values. The equation for the surface reaction between adsorbed DPM and adsorbed hydrogen was fairly good agreement with slight experimental scatter. However, it was found that the surface reaction between adsorbed DPM and dissociatively adsorbed hydrogen agrees best with the experimental data.

V. Catalytic Hydrogenolysis of Asymmetric Diarylmethanes

In the catalytic hydrogenolysis of asym DAMS (ArCH’Ar’) two competitive reactions are possible : Ar-CH,-Ar’

+ H, / Ar-H + Ar’-cH3 Ar-CH,

+ Art-H

244

YASUO YAMAZAKI AND TADASHI KAWAI

The product selectivity was assumed to be markedly different as explained in Section I. The relations between the structures of asym DAMs and their hydrogenolytic behaviors, as well as their reactivities, are the next point for discussion. In our studies, a fixed catalyst was used in order to maintain the same effect on the hydrogenolysis of asym DAMs (53).

A. EXPERIMENTAL A Pyrex tube (25 mmb x 400 mm) was used as a fixed-bed flow reactor for the catalytic hydrogenolysis of asym DAMs. A commercially available Co0-Mo0,-A1,0, catalyst, composed of 4 wt % COOand 12 wt % MOO,, was used for the experiments; about 4 g of 10-20 mesh catalyst were used. The asym DAM was fed into the reactor as a 5 wt % benzene solution after the catalyst had been pretreated with hydrogen at the reaction temperature for 2 hr. The reaction conditions were as follows : temperature, 400°C ; W/F, 8 g catal hr mole-' (LHSV = 3-4 hr-'); H,/hydrocarbons molar ratio; 2; total pressure, 1 atm. The activity of the catalyst declined with time at the beginning of the reaction and became constant after 30 min of the feed, so the sample of the reaction products was taken for 15 min, using a cooling bath after 30 min of the feed. Depending on the structure of the asym DAM, the conversion of asym DAM was between 30 and 100%. Side reactions such as demthylation, isomerization, and disproportionation of the asym DAMs and the polymethylbenzenes produced were negligible under these reaction conditions. The reaction products were analyzed with gas chromatography. An Apiezone Grease L column [3 m. 200-240°C, H,, thermal conductivity detector (TCD)] and a PEG 6000 column (4 m, 130-140°C, H,, TCD) were used for asym DAMs. An Apiezone Grease L column (3 m, 130-1WC, H,, TCD) was mainly used for polymethylbenzenes; also a Hitachi Gorey column (R-90, 85"C, N,, FID) was used when the resolution of the effluent was incomplete using the Apiezone Grease L column.

B.

HYDROGENOLYTIC BEHAVIOR OF PHENYLARYLMETHANES

Now for an explanation of the hydrogenolytic behavior of phenylarylmethane, in which only one benzene ring has methyl groups. In the reaction of 4-MeDPM, the molar ratio of toluene and p-xylene was found to be 3.56: 1 from the distribution of the reaction products. From this ratio it

HYDROGENOLYTIC BEHAVIORS OF ASYMMETRIC DIARYLMETHANES

245

can be calculated that the following two reactions are 64 and 36%, respectively :

The above result can be written as follows:

The hydrogenolytic behaviors of a series of phenylarylmethanes are summarized in Table IV. Several conclusions may be drawn from Table IV: 1. The predominant hydrogenolysis occurs between the methylene group and methyl-substituted phenyl group. 2. The selectivity toward the hydrogenolysis between the methylene group and the methyl-substituted phenyl group increases as the number of methyl groups increases. 3. The influence of the position of the methyl group on the hydrogenolytic behavior is as follows: (a) the hydrogenolysis between the methylene group and the tolyl group is favorable in the order of o-tolyl >> p-tolyl 2 m-tolyl; (b) when two methyl groups occupy two ortho positions in the same benzene ring (as in Compounds No. 6,8,9),the hydrogenolysis on the side of the aryl group increases considerably and becomes highly selective. 4. The experimental data are reflected quite accurately in the following empirical equation : a ' : b' = 1 : [1

+ 0.8m + (4.9 x 5"-'

-

l)n],

(11)

where m is the total number of m- and/or p-methyl substituents in an aryl group and n is the number of o-methyl substituents in an aryl group. The values within parenthesis in Table IV are calculated values from Eq. (11).

HYDROGENOLYTIC BEHAVIORS OF ASYMMETRIC DIARYLMETHANES

247

TABLE V Hydrogenolytic Behaviors of Asymmetric Diarylmethanes

Product selectivity Diarylmethane No.

Structure

Found

Calculated values from Table IV

Calculated values from Eq. (12)

i7:n3

-

17233

36 :64

-

36 :64

37 :63

36:64

12:nn

30:70

2:98

1832

(continued)

248

YASUO YAMAZAKI AND TADASHI KAWAI

TABLE V (Continued) Product selectivity Diarylmethane

No.

Structure

10

Found

42:58

Calculated values from Table IV

40 :60

11

14

15

46:54

9:91

12

13

Calculated values from Eq. (12)

d-C-f&-

bc@ @&-@

9:91

10:90

9:91

7:93

10:90

9:91

17:83

19:81

24:16

3:97

3:91

4:96

(Continued)

HYDROGENOLYTIC BEHAVIORS OF ASYMMETRIC DIARYLMETHANES

249

TABLE V (Continued) Product selectivity Diarylmethane Structure

No.

\

\

Found

18

19

Calculated values from Eq. (12)

/

16

17

Calculated values from Table IV

**

26:74

3 :97

4:96

3:97

4:96

20 :80

24:76

4:96

4:96

20

21

22

(Continued 1

250

YASUO YAMAZAKI AND TADASHI KAWAI

TABLE V (Continued) Product selectivity Diarylmethane No.

Structure

Found

Calculated values from Table IV

Calculated values from Eq. (12)

23

9:91

13:87

10:90

24

6 :94

13:87

10:90

78:22

76 :24

69:31

26

31 :69

51:43

47:53

27

4:96

5:95

5:95

25

kc4

28

2 :98

5 :95

5:95

29

2:98

5:95

5:95

(Continued)

HYDROGENOLYTIC BEHAVIORS OF ASYMMETRIC DIARYLMETHANES

25 1

TABLE V (Continued) Product selectivity Diarylmethane No.

Structure

Found

Calculated values from Table IV

Calculated values from Eq. (12)

30

15:85

9:91

12:88

31

11:89

9:91

12:88

32

46 :54

50:50

5050

33

34:66

50:50

50 :50

34

84: 16

91 :9

88:12

35

19:81

-

12:88

252

YASUO YAMAZAKI AND TADASHI KAWAI

An estimate of the product selectivity for the hydrogenolysis of asym DAM (Ar-CH,-Ar') can be obtained from the experimental values for the hydrogenolysis of the two phenylarylmethanes (Ph-CH,-Ar and Ph--CH,-Ar'). If the rate of hydrogenolysis on the phenyl side is regarded as 1, values of y and 6 for the aryl side can be calculated from the experimental data: Ph

CH,-Ar 1

:1

--+

Y

Ph

;

CH,-Ar'

1

A-CH,-Ar' Y

I

b

b

These y and 6 will be the effect of the aryl group on the hydrogenolytic behavior. For example, 2,5,2'-TrMeDPM will be constituted from 2-MeDPM and 2,5-DMeDPM. When the value for the phenyl side is taken as 1, the value of the tolyl side becomes 4.88 (y), using the value obtained experimentally. In the same manner, that of a xylyl group becomes 7.33 (6). The product selectivity for 2,5,2'-TrMeDPM becomes 4.88 :7.33 or 40 :60 if these values are converted into percentages. This estimated value is very close to the experimental value (42:58). Applying this method to 25 types of asym DAMs, produced the results shown in Table V. The estimated values correspond well with the experimental data. This shows that the hydrogenolytic behavior of asym DAM can be estimated from two kinds of phenylarylmethanes. Also, the hydrogenolytic behavior of asym DAM is prescribed by the structure of the aryl group: the position and the number of the methyl substituents. This suggests that the methylene group insulates the interaction between the two aryl groups. Furthermore, the following Eq. (12) derived from the modification of Eq. (11) is adaptable for asym DAMs: a' :b' = [1

+ 0.8m + (4.9 x

5"-

-

l)n] :[l

+ 0.8~1'+ (4.9 x

5"'-

- l)n']

(12) The values calculated from Eq. (12) are shown in Table V and, with a few exceptions, they correspond well with experimental data. This fact means that the equation could be applied to an estimate of the product selectivity for asym DAMs, which was not investigated. AND THE SCHEME OF THE CATALYTIC D. THE KINETICS HYDROGENOLYSIS OF ASYMMETRIC DIARYLMETHANE

The hydrogenolytic behavior of asym DAM can be estimated fairly well from two corresponding phenylarylmethanes. This suggests that the hydro-

HYDROGENOLYTIC BEHAVIORS OF ASYMMETRIC DIARYLMETHANES

253

genolytic behavior of asym DAM will be determined by the structures of the two aryl groups, and, furthermore, the interaction between two aryl groups will be insulated by the methylene group. As indicated previously, the selectivity in hydrogenolysis of asym DAM increases as the number of methyl groups increases. And of course, the increase in the number of methyl groups means an increase in the basicity of the aryl group. In the catalytic hydrogenolysis of 2,5,3’-TrMeDPM, the main reaction was demethylation when a basic catalyst (NiO-MgO) was used (see Table 11). However, in the catalytic hydrogenolysis of this compound over a Co0-Mo0,-A1,0, catalyst, the demethylation was negligible and the predominant reaction was the hydrogenolysis between the methylene group and p-xylyl group. The Co0-Mo0,-A1,0, catalyst used in this study is an acidic catalyst. These facts indicate that the two aryl groups are adsorbed on the acid sites of the catalyst before the hydrogenolysis between the methyl2 .o

1.5

A

--

1.0

h

LI

CI

V

2

0.5

a c)

Y 3 0

K

Y

O

-

Y 0

- 0.5 I ,

-1.0

:

@-C-Ar

I

-&: &c-Ar I

I

-0.6

-0.4

I

-0.2

I

1

1

0

0.2

0.4

log (rolrtlvo brslcltr)

FIG.2. Relation of the product selectivity between methylene and aryl groups and relative basicity (n complex) of two corresponding hydrocarbons.

254

YASUO YAMAZAKI AND TADASHI KAWAI

ene group and the two aryl groups occurs. Moreover, we can assume that the product selectivity corresponds with the ratio of strength of the interaction between aryl group and acid sites on the catalyst. Consequently, the relationship between the hydrogenolytic behavior and relative basicity of the two aryl groups was examined. There are no prior data on the basicity of the aryl group, so we assumed that the relative basicity of the two aryl groups was equal to that of two separate corresponding alkylbenzenes. Thus, for the asym DAM (Ar-CH,-Ar’), the two corresponding alkylbenzenes are ArCH, and Ar’CH, .The basicity used was that from the n complex between hydrogen chloride and the various methylbenzenes. Figure 2 shows the relation between the product selectivity of asym DAM and the relative basicity of the two corresponding methylbenzenes. There were linear relations between them when a particular methylbenzene of a pair was fixed. The linear relationship suggests that the stronger the basicity of the aryl group, the more the interactions between the aryl group and acid sites occur. Also, the ratio of these interactions would seem to determine the product selectivity. The kinetic data indicate that the rate equation of DPM could be expressed as a surface reaction between adsorbed DPM and dissociatively adsorbed hydrogen as mentioned above. Applying this result to the asym DAM, and taking the above discussions into account, the reaction scheme of asym DAM could be drawn as in Fig. 3. The reaction scheme of asym DAM can be described as follows. Both aryl

FIG.3. Schematic model of the catalytic hydrogenolysis of diarylmethane.

HYDROGENOLYTIC BEHAVIORS OF ASYMMETRIC DIARYLMETHANES

255

groups are adsorbed on acid sites, and form complexes in a certain ratio (perhaps, in a ratio according to their basicities as it was suggested above). Then the reactions between an adsorbed individual aryl group and dissociatively adsorbed hydrogen will occur. Finally, the reaction products will be produced according to the reaction routes. Table VI shows the relative reactivities of various asym DAMs. An equimolecular mixture of two kinds of asym DAMs was fed as a 5% benzene solution and hydrogenolyzed in order to check the effect of the methyl group on the reactivity. Two kinds of asym DAMs having similar reactivities were selected as a combination. The reaction conditions were temperature, 400°C; HJhydrocarbons molar ratio, 2. The contact time was changed since the reactivities of asym DAMs differed considerably according to their structures; this made it possible to evaluate the different reactivities. Side reactions such as demethylation, isomerization, and disproportionation were negligible under these reaction conditions. The relative values for the reactivities of the asym DAMs shown in Table VI are determined when the value of 2,5-DMeDPM as a standard material is fixed at 100. Table VI shows the following points for the reactivities of asym DAMs: 1. The reactivity of DAMs increases as the number of methyl groups in the aryl group increases. 2. When the total number of methyl groups in the DAMs is equal, their reactivities increase as the number of methyl groups on one of the aryl groups increases. For example,

>

> \

404

3. The order of the reactivity of asym DAMs is as follows (provided that the other aryl group is kept constant in the series): o-tolyl < rn-tolyl < p-tolyl < 3,5-xylyl < 2,5-xylyl < 2,4,5-trimethylphenyl < 2,4,6-trimethylphenyl < 2,4,5,6-tetramethylphenyl < 2,3,4,6-tetramethylphenyl.

The rate equation of catalytic hydrogenolysis of DPM can be expressed as the surface reaction between adsorbed DPM and dissociatively adsorbed hydrogen. In the reaction of asym DAM, both aryl groups are adsorbed in a certain ratio, and the reactions between an adsorbed individual aryl group and dissociatively adsorbed hydrogen will occur according to the scheme presented. It was thought that the methylene group would insulate the interaction between two aryl groups, and the unadsorbed aryl group

TABLE V1

70

bca

I 40

83

I44

87

205

93

43 1

100

219

43 I

559

242

470

585

446

560

590

510

62 I

65 I

Relative Reactivity of Diarylmethanes

475

570

516

625

I

440

568

640

I

408

560

588

I

+* 470

560

585

590

683

683

L

qc* 713

251

65 1

258

YASUO YAMAZAKI AND TADASHI KAWAI

would not affect the reaction between the adsorbed aryl group and adsorbed hydrogen. These findings suggest that the rate equation of asym DAM can be expressed by the sum of the reaction rates between the adsorbed individual aryl group and dissociatively adsorbed hydrogen as follows:

where k and K refer to the rate constant and adsorption equilibrium constant, respectively. The subscripts of Ar, Ar’, and H refer to both aryl groups and the hydrogen, and P, and P, to the partial pressure of hydrogen and asym DAM, respectively. The first term and the second term in Eq. (13) are the initial rates of an individual aryl group. The ratio of the individual KAr,.This ratio should rate equation of both aryl groups becomes kArKAr/kAr, be the product selectivity of asym DAM, that is, b‘la‘ = kArKAr/kArrKArTr suggesting that the product selectivity is proportional to the ratio of the adsorption equilibrium constants. There is a relationship between the product selectivity and relative basicity of both aryl groups, as shown in Fig. 2. This fact verifies the scheme of the two aryl groups adsorbing on the acid sites of the catalyst according to the basicity of both aryl groups, making a n complex and further reacting with dissociatively adsorbed hydrogen. Equation (13) shows that if one of the two aryl groups is fixed, the rate of hydrogenolysis of thz asym DAM should depend on the rate of the other unfixed aryl group, since the rate of the fixed aryl group remains constant. Therefore, the order of the reactivities of phenylarylmethanes should be the same as the order of asym DAMs, even if the phenyl group is substituted with another aryl group. Table VI shows that the reactivities of asym DAMs depend on the rate of the second aryl group if one of the two aryl groups is fixed. This fact also supports both the scheme given for the hydrogenolysis of asym DAM, as well as the rate equation of asym DAM.

VI. Active Species of MoO,-AI,O, Catalyst for Hydrogenolysis of Diarylmethanes

The relations between the structures of asym DAMs and their hydrogenolytic behaviors were examined in order to consider the factors affecting

HYDROGENOLYTIC BEHAVIORS OF ASYMMETRIC DIARYLMETHANES

259

the product selectivity in Section V. In these experiments, a fixed CoOMOO,-AI,O, catalyst was used in order to maintain the same effect on the asym DAMS. Another approach for understanding the factors affecting the hydrogenolytic behavior is to examine the changes in the hydrogenolytic behavior of a fixed asym DAM using catalysts with different properties. In other words, the changes of the hydrogenolytic behavior by using different catalytic properties directly reflect the changes in the catalyst, assuming that a fixed asym DAM keeps the same effect on the catalyst. A catalyst with different properties can be obtained by changing the treatment conditions of the catalyst. The relationships between the hydrogenolytic behavior and the treatment conditions of a catalyst are discussed here in order to clarify the interaction between an asym DAM and the catalyst. A MOO,-AI,O, catalyst was selected because it is the most selective catalyst for the dearylation of asym DAM as shown in Table I1 (54).

A. EXPERIMENTAL 2,5,3'-TrMeDPM was used as the standard asym DAM and it was fed into a flow type fixed-bed reactor as a 10% benzene solution. A MOO,-Al,O, catalyst containing 4-40 wt % MOO, was prepared by the impregnation technique. Calculated amounts of y-A1,03 (Tohkai Kohnetsu Co.) were impregnated in an aqueous solution containing calculated amounts of (NH,),Mo,O,, * 4H,O. After standing overnight, it was evaporated to dryness, further dried at 110"-120°C and finally calcined at a temperature between 500" and 800°C in air for 3 hr. The particle size of the catalyst used was 10-40 mesh. Reaction conditions were temperature, 350°C; W/F, 7.9 g catal hr mole-'; and H,/hydrocarbon molar ratio, 2. The sample of the reaction product after a 1-hr run was collected in a cold trap and analyzed. Possible reaction routes in the hydrogenolysis of 2,5,3'TrMeDPM are shown in Fig. 1.

B. EFFECTS OF PRETREATMENT OF CATALYST 1. Calcination Temperature The effects of calcination temperature on the hydrogenolytic behaviors are shown in Fig. 4. The activity increases as the calcination temperature is increased up to 600°C. Then, the conversion is constant until the temperature reaches 750°C, whereupon it drops rapidly. The values of b/a indicate almost the same tendency as the conversion. However, the degree of demethylation is almost constant for all calcination temperatures and ol(T/TrMeB)= 1.02, = 2.8-4.7%. The increase of conversion by calcination at temperatures

260

O i F u

YASUO YAMAZAKI AND TADASHI KAWAI b/a

200P

E

8

c \

40 v)

0

20

500 600 700 800

7

500 600 700 800

500 600 700 800

C a i c i n a t ion Temperature ( *C 1 FIG. 4. Effect of calcination temperature on conversion and hydrogenolytic behaviors

catalyst: 10 wt % MOO,-AI,O, reduced at 450°C for 2 hr after calcination; reaction temperature, 350°C; W/F, 7.9 g catal hr mole-', H2/(2,5,3'TrMeDPM + benzene) molar ratio, 2.0; total pressure, 1 atm.

up to 600°C suggests that weaker acid sites are formed, because it is expected that b/a increases as weak acid sites increase (see Section V,D). The decrease in activity above 800°C is caused by the decrease of MOO, content due to sublimation. For example, MOO, content after calcination of 10 wt % MoO,-AI,O, catalyst at 650" and 850°C were 9.7 and 4.7 wt %, respectively. Figure 4 also shows there is no correlation between surface area and the activity change. 2. MOO, Content

The effects of the MOO, content in the catalyst on the hydrogenolysis are shown in Fig. 5. Both the activity and bla increased with an increase in MOO, content up to approximately 20 wt %, and after that they remained constant with higher loading. On the contrary, the ratio of T/TrMeB and the index of demethylation decreased with an increase in MOO, content up to 10 wt %, and then remained constant with higher loading; their values were a = 1.05 and /I= 2.0%, respectively (52). The surface areas of the unreduced MOO,-A120, catalysts decreased significantly with an increase in the MOO, content, but the surface areas of the reduced catalysts increased. For example, although the surface areas of unreduced 17.1 and 36.8 wt % MoO,-AI20, catalysts were 135 and 95 m2 g- they increased to 145 and 125 m2 g- after reduction at 550°C with hydrogen for 1.5 hr, respectively. Infrared spectra of the catalysts with loading of greater than 25 wt % MOO, showed bands between 990 and 440 cm-', and their intensities increased with MOO, content. Some of these bands are identical with those of

',

HYDROGENOLYTIC BEHAVIORS OF ASYMMETRIC DIARYLMETHANES

a

3

30

2

20

1

10

26 1

B (%I

80

9 60

-.

8

A

8

bla

40

c

7

0

0

20

6

n 0

10

Moot

20

30

40

Content, w t %

FIG. 5. Effect of MOO, content on conversion and hydrogenolytic behaviors. Catalyst, MoO,-A1,0, calcined at 650°C for 3 hr and reduced at 450°C for 2 hr; reaction temperature, 350°C; W/F, 7.9 g catal hr mole-’, H2/(2,5,3’-TrMeDPM benzene) molar ratio, 2.0; total pressure, 1 atm.

+

Al,(MoO,), and free MOO, (55). The hydrogenolytic behaviors were checked using a pure A1,(Mo04), prepared by Giordano’s method (55). Although the AI,(MoO,)~ (reduced at 550°C with hydrogen for 1.5 hr) had a high activity (9973, the value of b/u was rather small (7.7), and the T/TrMeB value was relatively high (1.51) compared with a MoO3-A1,0, catalyst. Free MOO, and A1,(Mo04), are found in high MOO, content in MOO,Al,O, catalysts (e.g., 55-57), and they are reduced more easily than MOO,A1,0, catalysts (e.g., 57,58). Thus, one of the reasons for keeping the activity and the values of b/u, a, and p constant above 20 wt % MOO, content is that the calculated MOO, value to make a monolayer on y-Al,03 by MOO, is 21.8 wt % (57),which means that MOO, above 21 wt % exists mainly as free MOO, and is inactive when it is reduced. In fact, the activity of a pure MOO, is only 1%, as shown in Table 11. 3 Hydrogen Treatment

Figure 6 shows the effect of hydrogen treatment of the catalyst on the hydrogenolysis. The activity of the catalyst treated at 350°C increases with

262

YASUO YAMAZAKI AND TADASHI KAWAI

-

1.2

1.1

-0.1

0 R e d u c t i on

120 Time

240

-L

0

120

24(

(mln.

FIG.6. Effect of reduction condition on conversion and hydrogenolytic behaviors. Catalyst, 14.8 wt % MOO,-AI,O, calcined at 650°C for 3 hr; reduction temperature, ( 0 )550°C (0) 350°C; reaction temperature, 350°C; W/F, 7.9 g catal hr mole-', H2/(2,5,3'-TrMeDPM benzene) molar ratio, 2.0; total pressure, 1 atm.

+

the treatment time up to about 2 hr, and thereafter the activity is constant. The effect of reduction temperature on the maximum activity is minimal. The ratio of b/a increases as the reduction time increases, and reduction temperature significantly affects the ratio. However, the demethylation reaction (T/TrMeB ratio) decreases as the degree of reduction increases. These results indicate that the catalyst treated with hydrogen at 550°C has more selective sites for the dearylation 1 in Fig. 1, suggesting that the active sites for the selective dearylation are the coordinatively unsaturated molybdenum sites generated during reduction. The increase in b/a and the decrease in demethylation with an increasing extent of reduction suggest that the weak acid sites are formed. Active species for hydrogenolysis of diarylmethanes (DAM) will be further discussed in Sections VI,C and D.

C. ACTIVE SPECIES OF CATALYST The most active and selective catalyst for the hydrogenolysis of DAM is 15-20 wt % Mo03-A1203 calcined at 6OO0-700"C and then treated with hydrogen at 550°C. Giordano et al. (55) have studied the solid state properties of the MOO,A1203 catalyst, which was prepared by impregnation of y-alumina with an aqueous solution of ammonium molybdate, using various chemical and physical techniques. According to this study, there are tetrahedral Mo(V1) and octahedral Mo(V1)species in Mo03-A1,03. Tetrahedral Mo(V1)species are found as a main component at lower calcination temperatures, and tetrahedral Mo(V1) species are changed progressively to octahedral Mo(V1) species as the temperature is increased up to 500°C. Furthermore, the octahedral Mo(V1) species are mainly found between 500" and 700°C, but

HYDROGENOLYTIC BEHAVIORS OF ASYMMETRIC DIARYLMETHANES

263

a sharp reversal of the tendency occurs at 700°C. They also have reported that Mo(V1) on a catalyst calcined at 500°C is initially tetrahedrally coordinated, and it evolves toward a high octahedral/tetrahedral ratio with increasing MOO, content in the system. The octahedral Mo(V1) species are mainly found at the higher MOO, content. If we suppose that tetrahedral Mo(V1) species have low activity and give a low value of b/a, whereas octahedral Mo(V1) species have high activity and give a high value of b/a, our data (Figs. 4 and 5 ) appear to be entirely consistent with the results of Giordano et al. (55).That is, the changes in the hydrogenolytic behavior caused by the calcination temperature and also the amount of MOO, in MOO,-Al,O, catalyst can be explained by the structures of Mo(V1) species. However, our data do not exactly compare with their results, because a MOO,-AI,O, catalyst treated with hydrogen was used in our studies. These Mo(V1) species are regarded as precursors, which should be changed into the species generated from the reduction of octahedral and tetrahedral Mo(V1) species. Active species for the hydrogenolysis of asym DAM will be discussed further in Section VI,D. The hydrogenolytic behavior can be compared to the number of acid sites in the MOO,-AI,O, catalyst. Kabe et al. (59)have reported that weak acid sites of the MOO,-AI,O, catalyst increase corresponding with MOO, content, and the amounts of the weak acid sites become constant above 12 wt % MOO, content. They also have reported that the amounts of acid sites increase by the hydrogen treatment of the MOO,-AI,O, catalyst. Their results also correspond to the changes of the hydrogenolytic behavior shown in Figs. 5 and 6. This favorable connection supports the scheme presented above. Depending on their basicities, the more selective adsorption of the two aryl groups will occur, and this causes b/a to increase when the nature of the acid sites is weak. However, if strong acid sites exist, the nonselective adsorption of both aryl groups will occur and cause b/a to be small. The activity increases as the amount of acid sites increases; this, too, is predictable from the scheme. Weak acid sites are favorable for the selective dearylation reaction of asym DAM. The 14.8 wt % Moo,-Al,O, catalyst was treated with water or aqueous ammonia in order to obtain a correlation between the chemical state of the molybdenum oxides and the hydrogenolytic behavior. The catalyst was impregnated in ion-exchanged water or 5 wt % aqueous ammonia; the weight of the solution was 20 times that of the catalyst. The catalyst remained in the solution for 2 days, following which the aqueous layer was removed and the catalyst was washed with ion-exchanged water. The catalyst was dried at 120"C,and finally calcined at 650°C for 1 hr. Predictably, the activity will change after the treatment with water or ammonia because the extraction of MOO, by these treatments causes the

264

YASUO YAMAZAKI AND TADASHI KAWAI

30 40[ 0

/ 60

120

-

'6 t t 0

Hydrogen

60

Treated

12

Tlme

0

60

121

(rnin.)

FIG.7. Effect of water and ammonia treatment of MoO,-A1,0, catalyst on conversion and hydrogenolytic behaviors. ( 0 )Catalyst treated with water (MOO, content, 11.1 wt %); (0) catalyst treated with ammonia (MOO, content, 4.4 wt %); (0) catalyst calcined at 650°C for 3 hr (MOO, content, 14.8 wt %); reduction temperature, 550°C; reaction temperature, 350"C, H2/(2,S,3'-TrMeDPM + benzene) molar ratio, 2.0; total pressure, 1 atm; W/F, 7.9 g catal hr mole I . ~

MOO, content in the catalyst to change. Figure 7 shows the results using the catalyst treated with water and ammonia. The difference between the original 14.8 wt % catalyst and 11.1 wt % catalyst obtained after the water treatment should indicate the nature of the MOO,, which is soluble in water. The MOO, that is soluble in water differs from free MOO, because the activity of free MOO, is only 1% (see Table 11). The MOO, in the original 14.8 wt % catalyst covers the surface as a monolayer (57) and the existence of free MOO, is negligible or small. Then it seems that the MOO, that is soluble in water has a weak chemical interaction with Al,O, as reported by Hashimoto.et al. (56).The 11.1 wt % catalyst had somewhat lower values for b/a and slightly higher values for T/TrMeB, although the catalytic activity decreased due to the decrease of MOO, content, in comparison with the original 14.8 wt % catalyst. These changes in the catalytic activity and the hydrogenolytic behavior indicate that the MOO,, which is soluble in water, is an active and selective species for the hydrogenolysis of asym DAM. The differences in the hydrogenolytic behavior between the 11.1 wt % catalyst and ammonia-treated catalyst (4.4 wt % catalyst) 'will indicate the nature of the MOO, that is soluble in ammonia. Whereas the values in b/a decreased substantially from 9 to 7, the index of demethylation (T/TrMeB) increased significantly in comparison with the 11.1 wt % catalyst, when the ammoniatreated catalyst was used. The decrease in b/a and the increase in the demethylation reaction indicate that the ammonia-treated catalyst is not favorable for the selective hydrogenolysis of asym DAM. Thus, the most

HYDROGENOLYTIC BEHAVIORS OF ASYMMETRIC DIARYLMETHANES

265

suitable molybdenum oxide species for the selective hydrogenolysis of asym DAM are molybdenum oxides, which are soluble in water and also in ammonia. Hashimoto et al. (56) have reported on the relation between the MOO, content (up to 16.7 wt % MOO,) in MOO,-Al,O, catalysts and the amounts of three kinds of molybdenum oxides-soluble in water, insoluble in water but soluble in ammonia, and insoluble in ammonia. According to their data, the MOO, that is soluble in aqueous ammonia increases almost linearly with the increase in MOO, content until 16.7 wt %. This relation corresponds with the effect of MOO, content on the hydrogenolysis of asym DAM, suggesting that the MOO, that is soluble in ammonia is the suitable precursor species as discussed above. Octahedral Mo(V1) species are extracted preferentially by the ammonia treatment of a Moo,-Al,O, catalyst (55). This finding also supports our results, since the octahedral Mo(V1) species, which are precursors of the reduced species, are thought to be main species for the selective dearylation of asym DAM as mentioned above.

D. MECHANISM OF INTERACTION BETWEEN ACTIVESPECIES AND SUBSTRATES The activity and the selectivity for the hydrogenolysis of asym DAM are impressively improved by hydrogen treatment of the MOO,-Al,O, catalyst as mentioned above. The oxidation states, the active species, and their structures have been extensively studied for reduced MOO,-Al,O, catalysts. The oxidation states of the molybdenum in reduced MOO,-Al,O, catalysts range from Mo(V1) to Mo(O), depending upon the temperature used in reduction, the reduction time and the MOO, content (57, 58, 60-62). The formation of the lower valence states is enhanced by increasing the MOO, content, time of reduction, and temperature. Figure 8 shows the relationship between the hydrogenolytic behaviors and reduction time (52).The Mo(V) in the reduced catalyst is related neither to the catalytic activity nor to the hydrogenolytic behaviors. The electron spin resonance signal reaches a maximum within a very short reduction period, then drops and reaches a constant with continued reduction. This variation of Mo(V) concentration is compatible with the data obtained by Seshadri and Petrakis (61) and Massoth (58).The changes in the b/a ratio and the catalytic activity with the time of reduction agree with the amount of Mo(1V) species reported by Massoth (58), as quoted in Fig. 8. Hall et al. (63)found that the active species in the hydrogenolysis of cyclopropane are Mo(1V) in reduced MOO,-Al,O, catalysts. Also, Burwell and Bowman found that the hydrogenolysis of cyclopropane at 100°C (64)and also propane at 300°C (65) occurs over Mo(IV), Mo(II), and Mo(0) catalysts, which were prepared from Mo(CO), on Al,O, . The average valence state

266

YASUO YAMAZAKI AND TADASHI KAWAI

R e d u c t i o n T i m e , min

FIG. 8. Effect of reduction time on conversion and hydrogenolytic behaviors. Catalyst, 10 wt % MoO,/Al,O, calcined at 650°C for 3 hr and reduced at 450°C; reaction temperature, + benzene) 350°C; W/F, 7.9 g catal hr/mole; total pressure, 1 atm, H2/(2,5,3'-TrMeDPM molar ratio, 2.0. [Data from Massoth (%).I

of the molybdenum in our catalysts is about 4 based on our reduction conditions (45O-55O0C,14.8 wt %) (57, 58, 60, 61). These results suggest that Mo(1V) is the active species for the hydrogenolysis of asym DAM. Several configurations for Mo(1V) species have been reported. However, the active species for the hydrogenolysis of asym DAM should have some relationship with the Mo(1V) species originated from reduction of the octahedral oxomolybdenum species as mentioned previously. One of the most probable Mo(1V) species is illustrated below (58, 62, 66):

HYDROGENOLYTIC BEHAVIORS OF ASYMMETRIC DIARYLMETHANES

267

where 0represents an anion vacancy created by reduction of Mo(V1). The anion vacancy sites have acid properties. As stated above, the catalytic activity and the hydrogenolytic behavior correlated with the acid properties of the catalyst, as well as the extent of reduction. Therefore, the adsorption of an aryl group will occur on the coordinatively unsaturated molybdenum sites generated during reduction. According to the reaction scheme for the hydrogenation of ethylene over a reduced MOO,-AI,O, catalyst, ethylene becomes n-bonded at a second vacant ligand position of a coordinatively unsaturated Mo4+ species and inserts to form the a-bonded alkyl (66). The reaction mechanism for the catalytic hydrogenolysis of asym DAMS is shown in Fig. 9 for DPM. From the mechanism proposed in Fig. 9, the

FIG.9. Reaction mechanism of catalytic hydrogenolysis of diphenylrnethane.

268

YASUO YAMAZAKI AND TADASHI KAWAI

following relationships are obtained:

c,, = c4, c6H

=

c4H

+ cD

where C4, and C,, are the numbers of vacant site of o4 and 0 6 , respectively; and c 4 H , c 6 , , and C, are the numbers of 04, 06,and oD, respectively. Therefore, assuming that the rate-determining step is Eq. (iii) in Fig. 9, the following initial rate equation is derived:

If KZPD is negligible compared with 1, Eq. (15) agrees with Eq. (8) derived from the kinetic study on the reaction of DPM. The other mechanisms to be considered are the direct nucleophilic displacement of an alkylated benzyl cation by protonic hydrogen [Eq. (16)] and homolytic displacement of an alkylated benzyl radical by atomic hydrogen [Eq. (17)]. However, it is recognized that reactions in Eqs. (16)

‘u

and (17) rarely participate in the reaction over MoO3-Al2O, catalysts. As indicated in Table 11, one piece of evidence is found from the hydrogenolysis of asym DAM over a Si02-A1,0, catalyst, which is a typical protonic catalyst. When the reaction is run over this catalyst, there are considerable amounts of demethylation products. Other evidence is obtained from a thermal hydrogenolysis of asym DAMs. The asym DAMs are

HYDROGENOLYTIC BEHAVIORS OF ASYMMETRIC DIARYLMETHANES

269

FIG.10. Thermal hydrocracking behavior of diarylmethanes.

thermally hydrogenolyzed above 600°C; Eq. (17) is the key elementary reaction for free radical chain reactions (67). However, in the thermal hydrogenolysis the hydrodemethylation reaction occurs predominantly as shown in Fig. 10. Therefore, Eqs. (16) and (17) are not probable in the catalytic hydrogenolysis of asym DAMs over MoO3-Al2O, catalysts.

VII. Conclusions

The study of the hydrogenolytic behaviors of asym DAMs, in connection with their structures and the properties of the catalyst, leads to several fruitful conclusions. The position and the number of methyl substituents significantly affect the product selectivity between a methylene group and two aryl groups. The product selectivity (y:6) can be estimated with high accuracy by using the relative values of the two phenylarylmethanes (Ph“CH2LAr, PhLCH2LAr’) from which an asym DAM (ArWH24Ar’) can be constituted. Furthermore, the empirical equation for the hydrogenolytic behavior of asym DAM (ArflH, FAr’) including phenylarylmethanes is a’:b’ = [I

+ 0.8m + (4.9 x

5”-’ - l)n]:[I

+ 0.8m’ + (4.9 x

5’”’

-

1)n’l

where m and m‘ are the number of meta- and/or para-methyl substituents in the Ar and Ar’ groups. Also, n and n’ are the numbers of ortho-methyl substituents in the Ar and Ar’ groups. The experimental data coincides accurately with the empirical equation for the product selectivity. These results suggest that the methylene group insulates the interaction between

270

YASUO YAMAZAKI AND TADASHI KAWAI

two aryl groups; then the chemical properties of each individual aryl group are essentially independent of the other aryl group. In fact, the product selectivity of asym DAMs is closely related to the relative basicities of the two aryl groups as estimated from the corresponding methylbenzenes. The structure of asym DAM also affects the reactivity. The kinetic studies of the hydrogenolysis of DPM indicate that both the DPM and hydrogen are adsorbed on the same kind of active sites on the catalyst. Also, the rate-determining step of the hydrogenolysis is a surface reaction between adsorbed DPM and dissociatively adsorbed hydrogen. When the rate equation for DPM is applied to asym DAMs, their reactivities can be satisfactorily explained, and it is suggested that the product selectivity is proportional to the ratio of the adsorption equilibrium constants of the two aryl groups. A MOO,-A1,0, catalyst is the most selective for the hydrogenolysis between a methylene group and two aryl groups; side reactions such as demethylation. disproportionation, and isomerization are minimized by the use of the MOO,-Al,O, catalyst. Variables, including calcination temperature, MOO, content, and reduction conditions, affect the catalytic activity and the hydrogenolytic behaviors. The changes of activity and the hydrogenolytic behaviors correspond accordingly to the changes of the acidic properties of the catalyst and the structures of molybdenum oxides. The weaker the nature of acid sites, the more selective is the interaction between two aryl groups and acid sites of the catalyst and the less is the demethylation. That is, the ratio of hydrogenolysis becomes selective. The molybdenum oxides that are soluble both in water and in ammonia are the most suitable species for the selective dearylation reaction of asym DAM. Finally, the active sites for the hydrogenolysis of asym DAM are Mo(1V) species that originated from the reduction of the octahedral Mo(V1) species. The adsorption of the aryl group occurs on the coordinatively unsaturated molybdenum sites, which have acidic properties; this fact, in turn, leads to the reaction mechanism of the interaction between the active species and the substrates.

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27 1

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ADVANCES IN CATALYSIS, VOLUME 29

Meta I- Cat a Iyzed Cyc Iizat ion Reactions of Hydrocarbons ZOLTAN PAAL Institute of Isotopes of the Hungarian Academy of Sciences Budapest, Hungary

. . . . . A. Some General Problems. . . . . . . . . B. C, Dehydrocyclization . . . . . . . . . C. C, Cyclization. . . . . . . . . . . . . 111. Cyclization with Skeletel Rearrangement . . .

. . . . . . . . . . . . . . . . . . . . . . . . A. Metal-Catalyzed Skeletal Isomerization Processes. .

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

B. Cyclization of Open Chain Hydrocarbons with Skeletal Rearrangement. . . . . . . . . . . C. Interconversion between Ring Systems . . . . . . IV. Cyclization over Dual Function Catalysts and Oxides. . A. Ring Closure over Bifunctional Catalysts . . . . . B. Cyclization over Oxide Catalysts . . . . . . . . . V. Interpretation of Metal Activity in Catalytic Cyclization A. Structure and Catalytic Activity of Metal Surfaces . B. Astoichiometric Components and Surface Activity . C. Concluding Remarks . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . .

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I. Introduction

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11. “Simple” Cyclization Reactions . . .

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273 275 275 279 292 297 297

. . . 298 . . . 303 . . . 311 . . . 311 . . . 316 . . . 317 . . . 318 . . . 322 . . . 328 . . . 329

1. Introduction

Five- and six-membered rings are quite common in organic compounds because of the tetrahedral geometry of the carbon atom. Hydrocarbons are reluctant to form new C-C bonds. Even so, five- and six-membered hydrocarbon rings can be created naturally, as proved by the composition of petroleum. This reaction was first achieved in research in 1936 by means of heterogeneous catalysts (1-4). Since that time, catalytic reforming has become a large-scale commercial 273 Copyright 0 1980 by Academic Press, Inc. All rights of reproduction in any form reserved. ISBN 0-12-007829-5

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process producing, among others, aromatics and C 5cyclics from open-chain hydrocarbons. Despite continual technological development, the elementary processes of catalytic reforming are still not fully understood. This article will deal with metal-catalyzed cyclization reactions, with reference to oxide and dual-function catalysts. Product cycles may contain five or six carbon atoms. The respective prefixes C5and C, will point to the resulting structure (5). The term “dehydrocyclization” will be applied to reactions that end up with aromatic products; the formation of saturated (cycloalkane) rings will henceforth be called “cyclization.” The last exhaustive review dealing with cyclization proper was published in 1958 (6). Since then, several aspects of the various cyclization steps have been further discussed, together with other hydrocarbon processes (7, 8 ) . The review papers by Kazansky provide excellent summaries, mainly of the Soviet research done in this field (9-11). The work presented here is based to a great extent on the author’s own results and attempts to avoid repeating the details of previous studies. A common feature of any cyclization reaction is that a new intramolecular C-C bond is produced that would not have been formed in the absence of the catalyst. Those reactions in which one ring closure step is sufficient to explain the formation of a given cyclic product will be called “simple” cyclization processes, although their mechanism is, as a rule, complex. We shall distinguish those cases in which any additional skeletal rearrangement step(s) is (are) required to explain the process. Some specific varieties of hydrocarbon ring closure processes are not included. A recent excellent review deals with the formation of a second ring in an alkyl-substituted aromatic compound (12). Dehydrocyclodimerization reactions have also to be omitted-all the more since it is doubtful whether a metallic function itself is able to catalyze this process (13). As few as six different kinds of adsorption have been proposed as being responsible for a great variety of hydrocarbon transformations over metal catalysts (14). We fully accept this approach-that the character of primary adsorption determines the structure of the product. One of the main points that will be stressed is that very different reactions may often be concealed behind the expression “cyclization.” An attempt will be made to correlate primary adsorption (consequently the reactions expected) with two main factors: the nature of the metal and the amount of hydrogen available during the catalytic process. The latter may be of paramount importance: the amount of surface hydrogen may govern which type of chemisorbed species is formed and, by doing so, determine catalytic selectivity.

METAL-CATALYZED CYCLIZATION REACTIONS OF HYDROCARBONS

275

II. “Simple” Cyclization Reactions

A. SOMEGENERAL PROBLEMS 1. Elementary Acts in Diflerent Forms of Cyclization

As far as the “simple” cyclization step (involving theoretically the formation of a new intramolecular C-C bond) is concerned, the approaches published so far belong to two fundamentally different groups. a. The majority of authors regards the formation of C, and c6 cyclic products as two variants of a common “cyclization” process (7, 8). At least one carbon atom participating in new C-C bond formation is assumed to be in the sp2 hybrid state. Ring closure with the participation of a terminal olefin bond (“alkene-alkyl insertion”) (15, 154 can be traced back to the Twigg mechanism of aromatization (16). Another, “carbene-alkyl insertion,” (17) has been suggested for hydrocarbons with quaternary carbon atoms where alkene formation is impossible. The theories give no exact predictions as far as the structure of the ring produced is concerned. Eiectronic factors [such as partial carbonization of the surface (8) or the difference between the partial charge of the primary and that of the secondary carbon atom (18)]have been proposed to explain the predominance of C, or c6 cyclic production, respectively. b. The Soviet catalytic school has always insisted that C, and c6 cyclizations should be differentiated (5, 10). At the same time, explanations in terms of mechanistic details have sometimes been incomplete. This school interpreted both types of ring closure by the formation of a new C-C bond between two sp3 carbon atoms. The importance of hydrogen has been pointed out in the common surface complex suggested for both C,-ring opening and closure (19). We also regard C, cyclization and c6 dehydrocyclization as being two different processes, each having different surface intermediates and different ring closure steps. In the literature, the following pathways have been mentioned so far for various types of cyclization : 1. “Stepwise” c6 dehydrocyclization (aromatization) involving the gradual loss of hydrogen atoms from an alkane followed by a triene +cyclohexadiene ring closure step (20, 21). This can be : (a) Catalytic (21): this requires an “all-cis”-triene conformation on the surface. We think that this should predominate over most metals at not too

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high temperatures. Sufficient hydrogen is necessary so that geometric isomerization via half-hydrogenated surface species be rapid enough to produce this precursor. (b) Thermal : rapid spontaneous cyclization of cis-triene after having desorbed from the surface. This may occur over oxides (and perhaps metals) at high (2500°C) temperatures (22, 23). The dehydrogenation of the cyclohexadiene species into aromatics should be catalytic. 2. “Direct” C , cyclization of alkanes into cyclohexane-type rings. The occurrence of this process is claimed under the effect of unreduced surface platinum complexes which have no metallic properties (24). 3. Direct (“hydrogenative”) C , cyclization of alkanes to give saturated C , cyclic products. This is a typical metal-catalyzed reaction occurring in a hydrogen-rich atmosphere over a narrow group of metals (25). 4. “Dehydrogenative” C, cyclization (25, 26). Its probable pathway is an alkene-alkyl insertion (8). A carbene-alkyl insertion mechanism may eventually also be possible. “Hydrogen sensitivity” of individual product formation (i.e., yields as a function of hydrogen pressure) helps to select from among the possible pathways (27). Figure 1 depicts yields of benzene and methylcyclopentane from n-hexane as a function of hydrogen pressure (27~).Reactions favored by low hydrogen pressures (e.g., benzene formation) should involve more dissociated surface intermediates than those promoted by higher amounts

FIG. 1. Yields of benzene and methylcyclopentane from n-hexane (mole % in the effluent) as a function of the hydrogen percentage in the carrier gas (the other component being He). Pulse system, catalyst: 0.4 g Pt black, T = 360°C ( 2 7 ~ ) .

METAL-CATALYZED CYCLIZATION REACTIONS OF HYDROCARBONS

277

of hydrogen (such as the production of methylcyclopentane). Similar hydrogen dependence is observed for palladium and iridium, too. Thus, in the first rough approximation, stepwise C, dehydrocyclization and “hydrogenative” C, cyclization can be regarded as the most important processes over metals. All these reactions will be discussed later in detail. 2. Thermodynamic Considerations A necessary precondition of cyclization is that the reaction must be thermodynamically feasible. C,-alkane isomers have approximately the same stability, and they are much more stable near room temperature than are cycloalkanes or aromatics (28). The formation of methylcyclopentane becomes favorable at above 323°C (600 K). At this temperature, that of benzene is even more so. Cyclohexane is thermodynamically unstable from 223°C (500 K) upward. The entropy factor should also be considered since cyclization results in a more ordered structure. The C, cyclization of n-hexane involves an entropy decrease of about 15-17 entropy units (e.u.). The corresponding values for cyclohexane and benzene formation are about 25 and 38-45 e.u., respectively. These values are comparable with calculated values of adsorption entropy (29). Thus, adsorption of a molecule to be cyclized may supply a considerable part of the entropy change; in other words, adsorption should take place in a geometry favorable for cyclization. This is one of the main roles of the catalyst. The increase in hydrogen pressure should suppress both benzene and methylcyclopentaneformation. Equilibrium composition for the five hexane isomers, methylcyclopentane, and benzene in sixfold hydrogen excess consists of nearly 100% of benzene at about 400°C (673 K) at 3 atm and at about 600°C (873 K) at 20 atm. Cyclohexane and unsaturated products should be present in concentrations between and lo-’ mole %. In fact, less cyclohexane and more unsaturated products are observed (30). The yields of both benzene and methylcyclopentane show maxima as a function of the hydrogen pressure. Whereas thermodynamics permit a very broad maximum in methylcyclopentaneconcentration, the yields of benzene should increase monotonically. Over platinum black, n-hexane gives 2- and 3-methylpentanes, methylcyclopentane, and benzene. Actual concentrations are compared in Fig. 2 with equilibrium ones as a function of hydrogen pressure. Unreacted n-hexane is ignored since it would not be able to equilibrate with all its products. Realistic values are obtained if methylcyclopentane plus isomers are compared with the amount of benzene. These, however, correspond to much higher “effective” hydrogen concentrations than measured in the gas phase (31).

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1-0

1.5

21)

9( (finol),alm

FIG.2. The selectivity of saturated C, products (2MP + 3MP + MCP) and benzene produced from n-hexane (total C, conversion = 100%) as a function of the final hydrogen pressure. Thick full lines represent calculated equilibrium concentrations. Dashed lines denote experimental data with respect to benzene ( x ) and saturated C, products (0). Pulse system, catalyst: 1.0 g platinum black, T = 327 3°C (- 600 K) (31).

Two types of surface intermediates should be assumed here: one gives benzene; the other gives methylcyclopentane and isomers. Their interconversion must be strongly hindered: that is, C, and c6 cyclization represent thermodynamically separated systems. That is why observed methylcyclopentane to benzene ratios are much higher than the thermodynamics would permit under any conditions (at a given temperature).

3. Catalysts for Cyclization The first metallic catalyst used for dehydrocyclization of alkanes ( I ) was platinum on carbon (10-40 w/w% metal). It is typically used around atmospheric pressure and temperatures not exceeding 300°C. Such catalysts are inadequate for practical purposes. This is the reason for commercial “dual-function’’ catalysts-typically platinum on silica-alumina-having been developed (32). Platinum is still the best and most thoroughly studied dehydrocyclization catalyst. Several other metals also show aromatizing activity. Group VIIIB metals (except for Fe and 0 s ) (33, 34), Re (39, Cu, and Co (36) have been reported to catalyze C6 dehydrocyclization. Aromatization is not uncommon, and generally speaking, may occur over almost every good dehydrogenating contact, although the extent of aromatization may vary very widely. Systematic investigations to discover all possible aromatizing metals have not been carried out, maybe because of the outstanding importance of platinum. The significant activity of several oxides in c6 dehydrocyclization should also be pointed out (37). C, cyclization is much more specific: in addition to platinum (38, 38a), palladium (39, 40) and iridium (41, 4 1 4 have been reported to catalyze it.

METAL-CATALYZED CYCLIZATION REACTIONS OF HYDROCARBONS

279

Rhodium can be added as the fourth member of this group-particularly since skeletal isomerization over rhodium also involves C5 cyclic intermediates (42). Its alloying with copper results in the appearance of C, cyclic products (43). The actual selectivity depends on the nature of the catalyst. For example, the following data were reported for n-hexane transformed over platinum and palladium supported on the same alumina (44) (pulse system, hydrogen carrier gas, T = 520°C): Selectivity (total conversion = 1.OO) Catalyst

Pt/A1,0, Pd/AI,O,

C , cyclic

C, cyclic (benzene)

0.21 0.07

0.22 0.19

The conclusions to be drawn are mainly based on experiments carried out with platinum catalysts. The very complex phenomena of bi- and multimetallic catalyst are far beyond the scope of this review. Excellent recent reviews can be consulted for further data (45, 46). Only a few relevant results will be mentioned.

B.

c 6

DEHYDROCYCLIZATION

1. Stepwise Mechanism of C6 Dehydrocyclization The first mechanistic concepts of aromatization (16) originate from pregas-chromatography times. A direct alkane cycloalkane reaction was proposed by Kazansky and co-workers (47). Several authors have interpreted the formation of six-membered rings over metal catalysts in terms of alkene-alkyl insertion (i.e., analogous to the Twigg mechanism) (7, 8, 14). Stepwise cyclohexane dehydrogenation revealed the possible importance of unsaturated intermediates in benzene formation (48). Pines and Csicsery reported on the formation of diolefins in chromia catalyzed dehydrocyclization of c5-c6 hydrocarbons (49). The kinetic behavior of heptadienes and heptatrienes in chromia and molybdena catalyzed aromatization of unsaturated n-C7 hydrocarbons (22, 49a) indicated that they were intermediates of the reaction. That diolefins play a role in benzene formation has also been shown over over a nickel-on-alumina catalyst. Product composition from 1-heptene as a function of the catalyst amount is shown in Fig. 3. This points also to diene intermediates (50). The same was found with carrier-free nickel and platinum (51). --+

280

ZOLTAN

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0

0

li!!!Ll 5

0

0.1

0.2

0.3

0.4

0.5 g C.tOly.I

FIG.3. Yields of heptadienes and aromatics from 1-heptene, as a function of the amount of Ni/A1,0, catalyst. Pulse system, carrier gas: He; ( x ) aromatics; (0) heptadienes. (a) T = 355°C; (b) T = 370°C; (c) T = 390°C (50).

Radiotracer experiments gave final proof of the reaction pathway. The mixture of I4C-labeled n-hexane with inactive 1-hexene was reacted over platinum catalyst. The same was done with the mixture of labeled n-hexane and inactive cyclohexane (52-54). The three components involved in the mixtures all give benzene. A fraction of benzene should be radioactive, and its specific activity will reflect how much of this product was formed from the radioactive and how much from the inactive component of the starting mixture. The components of the starting mixture are in rapid adsorption-desorption interaction with the surface. For example, a part of adsorbed n-hexane desorbs as n-hexane; another part reacts to give benzene. If benzene formation involves an n-hexene surface intermediate, this hexene-the concentration of which may be eventually so small that it does not appear in the gas phase-interacts with the inactive hexene in the starting material and increases its specific radioactivity.

METAL-CATALYZED CYCLIZATION REACTIONS OF HYDROCARBONS

28 1

If cyclohexane is added as a second component to n-hexane, a similar increase of its radioactivity should be observed if it is really produced from n-hexane. The appearance of radioactivity in the assumed intermediate can be observed even if its concentration in the gas phase does not correspond to sorption equilibration. Comparative results are shown in Table I. A considerable increase in n-hexene radioactivity is observed, whereas no radioactivity appeared in cyclohexane. These results indicate the formation of 1-hexene from n-hexane in both helium and hydrogen. The absence of cyclohexane is due to the lack of its formation and not to its rapid further reaction to benzene. The rate of hexene aromatization is more rapid than that of hexane (52,54). Similar experiments showed that neither cyclohexane nor cyclohexene is formed from labeled 1-hexene. However, the formation of 1,3$hexatriene TABLE I Radiotracer Studies with the Mixtures of I4C Labeled n-Hexane with Nonradioactive I-Hexene and Cyclohexane (54) Specific radioactivity”

No. of Run

n-Hexane

1-Hexene

Cyclohexane

Benzene

Starting mixture I b 111 112

1.46 1.15 1.18

0.066 0.39 0.42

Absent

-

Absent 0.41 0.43

Absent

0.013 0.012 0.014

Absent 0.74 0.69

0.21‘ 0.63 0.57

Absent

Absent 0.63 0.40

Starting mixture 11‘ 1111 II/2 Starting mixture IIId IIIjl 11112

1.60 1.49 1.60 2.69 1.35 1.30

-

-

‘ Expressed as the ratio of percentage of radioactivity in the given component to its w/w percentage. T = 390°C, carrier gas, helium; 3 p1 pulses of 65% n-hexane plus 35% 1-hexene on to 0.76 g Pt black. ‘ T = 390°C, carrier gas, helium; 3 pl pulses of 62% n-hexane plus 38% cyclohexane on to 0.76 g Pt black. T = 480”C, carrier gas, hydrogen; 3 p1 pulses of 31% n-hexane plus 69% 1-hexene on to 0.16 g Pt black. The radioactivity of hexene fraction was due to incomplete separation from n-hexane (“tailing”).

TABLE I1 Tracer Studies on the Possibility of Formation of Hexatriene and Various Six- Membered Rings During Dehydrocyclization of 1 - H e x e d (20, 53)

Specific radioactivityb

Hexenes

Cyclohexane

Cyclohexene

Hexadienes

1,3-Cyclohexadiene

Trans-],3,5-

Hexane

hexatriene

Benzene

Starting mixture I'

None

2.28

0.013

Absent

Absent

Absent

Absent

Absent

I/ 1 112

2.16 2.44

2.38 2.50

0.008 0.011

1.24

Absent Absent

No. of Run

Starting mixture 11'' IIj2 IIj2 Starting mixture 111' IIIjl IIIj2 Starting mixture IVf IVjl IVj2

-

1.28 1.26 -

4.75 3.55 3.55

-

3.88

4.27 4.1 1

-

0.07 0.07 0.07

Absent

Absent

-

-

-

1.60 1.49

0.82 0.86

Absent 1.13 0.99

Absent

Absent

-

-

Absent 0.72 0.72

0.0038 0.012 0.022

Absent 0.04 0.05

Absent

Absent 0.030 0.030

Absent 0.12 0.25

~

Absent

6.85

Absent

-

-

-

-

0.75

-

0.013 0.074 0.051

Catalyst, 0.76 g platinum black; T = 360°C; 3 p1 hydrocarbon pulses; carrier gas, 55 ml min-' helium. See Footnote a to Table I. ' 1.4% n-Hexane plus 43.6% [14C]-l-hexeneplus 55.0% cyclohexane. 79.4% ['4C]-l-Hexene plus 20.6% cyclohexene. 21% [14C]-l-Hexene plus 79% 1,3,5-hexatriene (cis + trans). 24.2% [14C]-l-Hexene plus 1.O% cyclohexene plus 74.8% cyclohexadiene. a

-

METAL-CATALYZED CYCLIZATION REACTIONS OF HYDROCARBONS

283

and 1,3-cyclohexadiene could be shown (Table 11). The absolute effects are rather small, but one cannot expect more considering the extremely great differences between the reactivities of the components in the starting mixture (20, 53). The stepwise mechanism was also shown later to be valid over supported platinum ( 5 9 , and palladium (56, 56a) catalysts, as well as over aluminasupported rhodium (57). Calculations according to the kinetic isotope method for the mixture of inactive n-hexane and [14C]-l-hexene showed that 92-97% of the total amount of benzene formed over Pt/C at 300°C formed uia hexene (58). 2. The Cyclization Step

The importance of the above radiotracer experiments is not restricted to the demonstration of the stepwise aromatization mechanism. Even more important is the evidence against the formation of any cyclohexane or cyclohexene during aromatization (53,55, 58). Product concentrations as a function of the contact time suggested the following ring closure pathway of heptadiene over chromia (22):

/ heptadiene

cis-cis-heptatriene

-+

methylcyclohexadiene

i

trans-cis- and trans-transheptatrienes

Obviously, once the hexatriene stage is reached, its cis isomer very rapidly gives 1,3-~yclohexadiene(59). Radiotracer studies have confirmed this cyclization step (20).However, trans- 1,3,5-hexatrienemay also be produced with almost the same probability (Scheme I). We do not agree with Rozen1,J-Hcx a d i e n e

3

2

1

P

4

MS-He xa t r icn c cis

trans

C>=p=YL 5 6 7

?=f==4=f 0 9 10

SCHEME I

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gart et al. (22) that geometrical isomerization might occur in the cis +trans direction only. Our view is supported by the ease of aromatization of 1,4hexadiene, where trans- 1,3,5-hexatriene is a mandatory intermediate (21). Even thermal trans-cis isomerization (followed by a very rapid cyclization) is noticeable above 45OoC, as indicated by the product composition obtained in an empty reactor (21) : T (“C)

25

cis-Triene &) trans-Triene (%) 1,3-Cyclohexadiene (%)

29.7 70.3 -

240 17.0 68.0 15.0

300

360

420

450

-

-

-

-

69.1 30.9

67.2 32.8

38.6 61.4

9.0 91.0

The process is much more rapid over platinum. Dautzenberg and Platteeuw (23) assumed the formation and thermal cyclization of hexatriene [similarly to the earlier suggestion with respect to oxides (22)]. However, it is not likely that such an extremely unstable intermediate would leave the catalyst surface just in order to cyclize and then rapidly readsorb to complete aromatization. Still, thermal cyclization cannot be a priori excluded at high temperatures where the equilibrium concentration of triene is higher and its adsorptivity lower, but its appearance may be rather exceptional. We suggest, instead, a surface cyclization step of cis-l,3,5-hexatriene. There is a very significant difference between the rate of aromatization of trans- and cis-hexatriene (Table 111), which shows that geometrical isomerization prior to cyclization may be rate limiting. Since this occurs via halfhydrogenated species (60), it is promoted by the presence of hydrogen, and so is benzene formation. It should be noted that cyclohexane and cyclohexene are produced from cis-triene. The hydrogenation of cyclohexadiene may explain their formation here and in other cases of stepwise C6 dehydrocyclization. With no sufficient hydrogen present, the molecules “get stuck” on the surface. Owing to purely statistical reasons (Scheme I), this is more probable in an “elongated” position. Such molecules may combine with each other to give high molecular weight polymers (“coke”). Metal-catalyzed polymerization has actually been observed with lower molecular weight hydrocarbons (61). Such reactions are responsible for more rapid deactivation of the catalyst by trans isomers (Table 111). The application of helium permits the “freezing” of the reaction in such stages that the otherwise very reactive intermediates become detectable. Hydrogen thus influences the relative rates of elementary steps but not the overall mechanism which is shown in Fig. 4a (21, 62).

TABLE 111 Catalytic Reactions of cis- and trans-1,3,5-Hexatrienen (21) Composition (mass %) 1,3,5-Hexatriene No. of pulse


Hexane

Hexenesb

A. Starting hydrocarbon: trans-1,3,5-hexatriene 1 2.2 0.1 1.7 2 5 ~

B.

Starting hydrocarbon: cis-l,3,5-hexatriene 6.2 0.7 0.4 1.2 2.5 5 0.2 0.1

1 2

Hexadienes

1.3 0.6 0.4

Cyclohexane

Cyclohexene

1,3-Cyclohexadiene'

trans

cis

Benzene

-

-

62.6 65.9 69.2

-

-

18.8 30.6 29.5

13.4 3.0 0.8

-

-

~

-

-

-

-

2.3 3.5

0.4 1.8

2.5 2.6

12.1

55.8

1.o 3.7

Catalyst, 0.4 g Pt; T = 360°C; carrier gas; 60 ml min-' He; 5 p1 hydrocarbon pulses each. No regeneration between pulses. With 1,5-hexadiene. ' With cis-trans- and cis-cis-2,4-hexadiene.

-

-

0.4

92.7 78.3 32.0

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ZOLTAN PAL

FIG.4 (a) Stepwise dehydrocyclization of n-hexane (21, 62). (b) Temperature programmed desorption of benzene originating from various adsorbates over Pt-AI,O,. Temperature ofadsorption: 25°C. Rate of heating: 23°C per minute. Detector: monopolar mass spectrometer, the ordinate corresponds to the I intensity of mass number 78, in arbitrary units. For clarity, the thermodesorption curves for other compounds (starting hydrocarbon) hexene from hexadienes and hydrogen have not been shown (62c).

3. TPR Studies of Aromatization Temperature programmed reaction (TPR) studies involve the adsorption of a substance on the catalyst at relatively low temperatures. After evacuating the vessel, the catalyst is gradually heated and the appearance of gas phase products is monitored. Apart from the desorption of the starting substance, its reaction products also appear; thus the method gives information on the

METAL-CATALYZED CYCLIZATION REACTIONS OF HYDROCARBONS

287

surface process(es). The temperature of the TPR (or desorption) peak (T,,,) is characteristic of the rate determining (surface reaction and/or desorption) step(s) (62a). If various feeds give the same TPR spectrum for their end product, a common rate determining step can be assumed. This was the situation when TPR spectra of benzene formed over Pt-Al,O, from adsorbed n-hexane, 1-hexene, and 1,Shexadiene were studied. This re-confirms the hexanehexene-hexadiene stepwise mechanism since cyclohexane, cyclohexene, and cyclohexadiene gave another type of TPR spectrum (62b). Whereas a single TPR peak of benzene (T,,, = 230°C) was reported for n-hexane (62b), a peculiar triple benzene peak system (with T,, values of 210", 230", and 255"C, respectively) was observed with 1,3-, 1,4- and 2,4hexadienes as adsorbates (62c) (Fig. 4b). The middle peak (the only one for n-hexane) was absent during thermodesorption of benzene, therefore this can be tentatively assigned to the ring closure step (as discussed in Section II,B,2). Essentially the same triple peak system also appeared with Pt-black, but the entire spectrum was shifted toward lower temperatures (T,,, = 155", 175", and 195"C, respectively). Truns-hexatriene gave the same spectrum, but with cis-triene a fourth, low-temperature peak appeared at 140°C. This should correspond to the facile cyclization of cis-triene. The fact that a peak of similar T,,, appears when benzene is desorbed from the catalyst is another strong argument against the purely thermal (noncatalytic) character of cyclization of cis-l,3,5-hexatriene. With TPR of n-heptane, the number of peaks depended on the nature of the catalyst. While some peaks may be attributed to "direct" cyclization (Section II,B,4) or dual function aromatization (Section IV,A), the above triple peak system can be recognized with some catalysts (62d).If deuterated heptane was adsorbed, only the third peak was accompanied by appearance of deuterium, indicating that dehydrogenation may belong to the rate determining steps in this case. The various TPR peaks may correspond to different active sites. One hypothesis assumed cyclization over metallic and "complex" (Section II,B,4) platinum sites (62e); the participation of various crystallographic sites (Section V,A) cannot be excluded either. Alternatively, the peaks may represent three different rate determining steps of stepwise aromatization such as cyclization, dehydrogenation, and trans-cis isomerization. If the corresponding peak also appears in the thermodesorption spectrum of benzene, it may be assumed that the slow step is the addition of hydrogen to one or more type of deeply dissociated surface species which may equally be formed from adsorbed benzene itself (62f)or during aromatization of various n-C, hydrocarbons. Figure 11 in Section V,A shows the character of such a species of hydrocarbon.

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ZOLTAN PAL

With n-heptane feed, a high-temperature benzene peak appeared apart from the low-temperature toluene peak. This also appeared with toluene, but was absent with benzene feed. Benzene must have been formed, therefore, after ring closure via toluene bonded to the surface through its methyl group. This could be removed only by demethylation (62e). The benzene peak gradually decreased with more and more sintered Pt/Al,O, catalysts (62d). Further research in this field may be very promising. This must include studies of hydrogen effects as pointed out already in (62d).The importance of hydrogen will be discussed in detail in Section V,B.

4. “Direct” C , Cyclization In addition to the stepwise mechanism, Dautzenberg and Platteeuw proposed another “platinum-catalyzed cyclization mechanism” (2.3). This might correspond simply to a “disguised” stepwise aromatization where the further reaction of unsaturated intermediates is very rapid compared with their desorption. Thus, hydrogen pressure would govern the probability of desorption versus further reaction. Since the cyclization of triene is irreversible, a very low steady-state surface triene concentration must be sufficient to ensure a measurable reaction rate. McHenry et al. (63) suggested that a special “soluble platinum” [which is a not fully identified complex containing Pt(1V) chloride and hydroxide complexed with alumina] may be active in aromatization. Its presence was denied in a reducing atmosphere (64). Electron spin resonance (ESR) and optical spectroscopy gave evidence, however, that a part of it still exists even after reduction (65,66). Bursian et al. (66a) suggested metallic platinum sites for dehydrogenation and Pt4+ sites for ring closure. They studied the effect of several elements added to platinum-on-silica catalyst on the aromatizing activity of n-hexane. Benzene yield increased parallel to the amount of “soluble platinum” (66b); at the same time, the crystallinity of platinum decreased in the presence of additives promoting aromatization. These are elements (e.g., Ce, Sc, Zr) which do not form an intermetallic compound with platinum (66c). Volter (66) reported that a part of Pt(1V) (which is present as a nonstoichiometric oxychlorinated complex over his alumina support after calcination) is reduced reversibly below 550°C to a “Pt complex” related to “soluble platinum.” Above this temperature, reduction to the metallic state is complete. He attributed “direct” alkane +cycloalkane cyclization to this “platinum complex” (24, 66). If this pathway does, in fact, exist, it may not be a true “metal-catalyzed’’ cyclization, but a rather special case of acid catalysis. Further research is necessary to elucidate this question.

METAL-CATALYZED CYCLIZATION REACTIONS OF HYDROCARBONS

289

5 . Aromatization of Hydrocarbons with More Than Six Carbon Atoms

Aromatization of hydrocarbons with more than six carbon atoms is thermodynamically even more favorable than the reaction of ri-hexane (28). These hydrocarbons offer more than one way of cyclization, for example, n-octane may give either o-xylene or ethylbenzene. n-Nonane can even give bicyclic products (indane system) (12). If a “stepwise” cyclization mechanism is assumed-for example, for n-C,-two octatriene intermediates may be formed, viz. 1,3,5octatriene would lead to ethylbenzene, and 2,4,6-octatriene to o-xylene (Scheme 11). The dehydrogenation of the latter would give octatetraene, which, in turn, gives styrene via vinylcyclohexadiene. Dehydrogenation and cyclization of octatriene were reported to compete over chromia and molybdena catalysts (67); with less hydrogen present (e.g., in a pulse system with in helium carrier gas), styrene formation is enhanced. Levitsky et al. (68, 68a) reported that the o-xylenelethylbenzene ratio from n-octane increased at higher hydrogen pressures with platinum on alumina, too. They suggested that metal becomes more electron accepting when it adsorbs a great deal of hydrogen; therefore, n-octane would adsorb on its secondary carbon atom carrying minimum negative charge via proton elimination. This would give 2,4,6-octatriene by stepwise dehydrogenation. The adsorption would take place on the primary carbon atom via hydride ion elimination with less hydrogen present; this, in turn, would lead to 1,3,5-0ctatriene and ethylbenzene. The prediction of the site of the primary adsorption is in agreement with the literature (8), but one may wonder whether double bond isomerization-which is not negligible over a platinum catalyst (2Z)-can be disregarded. Davis (69) found no considerable variation in the o-xylene versus ethylbenzene ratio as a function of hydrogen pressure. He also observed that the relative amount of o-xylene from n-octane increased (a) with decreasing Pt loading of the catalyst (70); (b) with increasing tin addition (69, 7 1 ) ; (c) with the poisoning of the catalyst with thiophene (71); and ( d )if octenes or octynes

SCHEME 11

290

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PAAL

were used as starting hydrocarbons (71). The author explained aromatization by “direct” C, ring closure. He offered an explanation in terms of the electronic factor : when the catalyst is electron deficient, the probability of the adsorption of primary and secondary carbon atoms would be almost equal (with o-xylene versus ethylbenzene ratio near to unity). Over catalysts that are not very electron deficient, the adsorption of secondary carbon atoms (and hence, o-xylene formation) would predominate (71) because of the lower bond energy of the secondary C-H bond. All the effects under (a)-@) are claimed to be in agreement with this concept but they predict an opposite selectivity compared with the above hypothesis (6th). As far as Point (a) (see p. 289) is concerned, very small metal crystallites (with low Pt loading) are supposed as being more electron deficient than large ones (72). Yet, the formation of more o-xylene over low platinum content catalysts is reported by Davis et al. (71). The formation ofp-xylene from 3-methylheptane was doubled over low platinum content catalysts (71). Here stereochemical reasons must be important. Platinum loading effects are, therefore, not unambiguous, all the more because hydrogen spillover effects are much more marked with smaller metal particles (73). Hydrogen spillover, and the use of un%aturatedreactants, as well as thiophene poisoning, can all influence surface hydrogen concentration of the catalyst, so that an interpretation in these terms can also be considered. Some additives (e.g., tin) decrease the hydrogen adsorptivity of platinum catalysts (74) : Pt-A1,03 : Pt-Sn-Al,O,:

(0.5%) 54 ml H,/g Pt (0.5-0.5%) 20 ml H,/g Pt

The admixture of lead to platinum has a similar effect (Fig. 5). At the same time, the aromatizing activity increases up to about 1 : 1 Pt :Pt atomic ratio (24). With even more lead it scatters around somewhat lower values (66). Electron donation from lead to platinum has been proved by infrared spectroscopy, so one may wonder whether lead is present as “metal” in the catalyst (75). The additive effect can also be interpreted by its creating hydrogen-deficient surface sites favorable for aromatization. When more lead is present than platinum (i.e., where no more continuous platinum surface is probable), the inverse correlation between hydrogen adsorptivity and activity ceases to exist. Recent experiments confirmed the role of tin added to platinum on alumina (present mainly as Sn2+)as a regulator for surface hydrogen (75a). A very specific spillover mechanism is assumed between platinum and tin atoms. The hydrogen accessibility of Pt increases proportionally to the amount of added Sn. To this end, it is sufficient that 3-6% of the tin atoms

METAL-CATALYZED CYCLIZATIONREACTIONS OF HYDROCARBONS

Hack

KDHC

29 1

[s] 0.9

400

0.7

3.5 300 0.3

1.1

200 1

05 10 15 20 Fiti. 5. Hydrogen adsorptivity (left-hand ordinate, in arbitrary units) and rate o f n-heptane aromatization (right-hand ordinate) as a function of relative lead content o f a supported platinum catalyst (24).

0

participate in hydrogen transfer. This is claimed to be one of the reasons for the very high selectivity and stability of Pt-Sn catalysts. Szebenyi et al. (76) reacted n-nonane over alumina-supported Pt and Pt-Re catalysts. With increasing temperature (Le., less surface hydrogen present) the o-ethyltoluene/n-propylbenzeneratio increased :

o-Ethyltoluene n-Propylbenzene

T ( C)

475 f 5

500

513 f 2

(Pt) (Pt-Re)

I .7 I .6

2.0

2.3 2.1

-

We believe that although the above interpretations all have some merit they cannot explain all the very complex experimental facts. For a completely adequate explanation, other factors should also be considered. One of them is the carbonization of the catalyst, which, in fact, could influence considerably the aromatic composition (69-71); another is the unknown ratio of

292

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various cyclization mechanisms (stepwise, “direct” and dual function pathways-see also Section IV,A) over catalysts of various preparation.

C. C, CYCLIZATION C, Cyclization of various alkanes (38, 38a) over platinum on carbon was first observed in 1954. Barron et al. (15a) postulated the formation of a “surface C, cyclic intermediate,” which may desorb as a cyclopentanic hydrocarbon or may produce skeletal isomers without desorption. Scheme I11 shows Liberman’s “associative” ring closure mechanism (19). The participation of surface hydrogen atoms (26) in the cyclization-ringopening complex is noteworthy. The other atoms of the C5ring are claimed as lying on the metal linked to it by physisorption forces.

2%

P 2 ,

H2t\ ,W2 H HzC-CH2 H

I

.

M--d-h--M

I

1

H2C\

FH2

H-.H2C--CH2.-v , , 0 , M.-M..---M...M .

I

I

I

ZZ H2C\ H-H2C . ,

,CH2 CH2-’;l

t&$f-

I

,

M-M

b

SCHEME 111

M

(B)

(A)

SCHEME IV

Dissociative ring closure (15, 15a, 17) would involve unsaturated surface intermediates (Scheme IV). 1. Hydrogenative C , Cyclization

The formation of methylcyclopentane from hexanes proceeds in the presence of hydrogen only (27, 27a). A singly dissociated surface intermediate is suggested by the hydrogen order of about - 1 on the right-hand side of the bell-shaped curves over platinum black between 300” and 360°C (Fig. 6 ) (77). A comparison of the cyclization rates of alkanes and alkenes may help to distinguish between associative and dissociative ring closure mechanisms, just as in the case of C6 dehydrocyclization of hexane and hexenes.

METAL-CATALYZED CYCLIZATION REACTIONS OF HYDROCARBONS

293

lg w+10

I

25

i

I 1.4 1.6 1.8 20 2.2 2.L 2.6 [g pHz

1.2

FIG.6. Rates of methylcyclopentane formation from 3-methylpentane, as a function of hydrogen pressure. Static-circulation system; catalyst, 0.16 g platinum black; hydrocarbon pressure, 5 Torr (77).

None of the 3-methylpentene isomers formed more methylcyclopentane than did 3-methylpentane, With the alkene-alkyl insertion mechanism the reverse should be expected, especially for 3-methyl-1-pentene. The following statements can be made as far as the actual surface intermediates are concerned (25): (a) hydrogenation and cyclization of 3-methylpentene isomers require a common surface intermediate (since cyclization yields are proportional to the hydrogenation rate) ; (b) this intermediate seems to be identical for 3-methylpentenes and 3-methylpentane (since their cyclization rates are close to each other); and (c) the surface species in question does not keep the original geometry of the starting 3-methylpentane (because cis and trans isomers cyclized with identical rates). The most satisfactory species fulfilling these requirements is a halfhydrogenated radical. This is linked to the surface preferably through the tertiary carbon atom (which has the lowest C-H bond energy). The loss of a second hydrogen atom from its vicinity may lead to 3-methyl-2-pentene

294

ZOLTAN PAAL

-2H

*2H

SCHEME V

(Scheme V). The surface species-owing to the sp3 geometry of the adsorbed carbon atom-lies “parallel” to the surface. This “dual-site’’ mechanism (25) involves a “binding” and a “catalytic” site. Ring closure may take place between associatively adsorbed sp3 carbon atoms, although possibly fewer metal atoms participate in it than proposed by Liberman (29).The necessity of hydrogen for this ring closure step-where hydrogen is eliminated from the molecule-supports this suggestion. The surface complex shown in Scheme V may be identical with the common “C” intermediate of C , cyclization and C5 cyclic isomerization (152). Its degree of dissociation may not be too high (32), and the bond between it and the metal may have some sort of “fluxional” character. The geometry of chemisorbed half-hydrogenated species must be favorable for cyclization. The repulsion between the side methyl group and the hydrogen atom in the fl-position bends methylpentane molecules into an appropriate “horseshoe-like’’ conformation (Scheme VI). This so-called

SCHEME VI

“l,3 effect” also contributes to the higher methylcyclopentane yields from methylpentanes as compared with n-hexane (36, 78) (data obtained in a pulse system, at 360°C with 1 : 1 H,-He mixture as the carrier gas):

METAL-CATALYZED C Y C L I Z A T I O ~REACTIONS ~ OF HYDROCARBON

295

Starting hydrocarbon Methylcyclopentane yield (%)

n-Hexane

2-Methylpentane

3-Methylpentane

On Pt black: On Pd black:

0.84 1.3

3.4 1.9

3.3 2.9

Kazansky et al. reported (over platinum on carbon under identical conditions) about 5% C , cyclic yield from n-alkanes (38),12% from 3-ethylpentane (79), and 35% from 2,2,4-trimethylpentane (38~2,80). The C , cyclization of 2,5-dimethylhexane did not take place, presumably because the end methyl group removes carbon atom No. 2 from the surface (81) (Scheme VII). The

SCHEME VII

latter fact also supports the necessity for the “lying down” of the molecule on the catalyst surface for C , ring closure. Further checking of this hypothesis with other hydrocarbon isomers would be desirable. Single platinum atoms in the surface complex state have been suggested to catalyze “direct” C , ring closure according to Scheme IVB (65). 2. Dehydrogenatiue C, Cyclization Garin and Gault (82) reported a hydrogen order of - 3.4 for I3C position isomerization of n-pentane over alumina-supported platinum between 240” and 300°C. They concluded therefore that a surface species that lost 3-4 hydrogen atoms would cyclize. It is not clear to what extent this discrepancy (cf. 77 and 82) may be attributed to different experimental conditions, to different catalysts, or actually to different mechanisms : that is, C5 cyclization with participation of unsaturated intermediates. Such a mechanism is a reality. Methylpentenes are able to form an unsaturated C5cyclic compound 1-methylcyclopentene, even under conditions when 3-methylpentane does not cyclize. Its amount is highest from 3-methyl1-pentene (having a terminal olefin bond) (25). This points to another, dehydrogenative route. The experiments of Bragin et al. (26) support the existence of two cyclization mechanisms. They introduced alternative 2-methylpentane and

296

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2-methylpentene feed on to carbon- or alumina-supported platinum. At about 300°C, unsaturated hydrocarbons rapidly deactivated both types of catalyst. At higher temperatures, however, olefins cyclized faster than 2-methylpentane (26). Apparently, cyclization involving unsaturated intermediates predominates above 400°C. (This reaction proceeds faster with olefin feed.) Radiotracer studies also confirmed the existence of two C, cyclization pathways (83). A similar mechanism may produce 1-methylcyclopentene from 1-hexene (51), various hexadienes (84), and hexatriene (21, 56a). The enhanced reactivity of 1,Shexadiene (84) points to the importance of a terminal double bond in this reaction. This reaction also proceeds in the presence of hydrogen only; thus, it may never predominate since under these conditions more rapid hydrogenation of the double bond competes very efficiently with it. Due to rapid double bond isomerization the product contains predominantly the most stable isomer : 1-methylcyclopentene. The dependence of the selectivity of hydrogenative versus dehydrogenative cyclization on the structure of the starting hydrocarbon (Fig. 7) shows that methylcyclopentene (MCPe) is not the product of secondary dehydrogenation (85). The “dehydrogenative” route is probably identical with the alkene-alkyl insertion mechanism (I54 (Scheme IVA) rather than with the “dicarbyne” cyclization (8%). The latter was based on the unreactivity of n-hexane in C 5 cyclic reactions over iridium ( 4 1 ~ ) .

50 HI,%

x)o

FIG.7. Percent selectivity of “hydrogenative” C, ring closure as a function of the hydrogen content of the carrier gas. Pulse system; catalyst, 0.4 g platinum black; T = 360°C. Starting 3-methyl-1-pentene; (V)tmns-3-methyl-2-pentene; hydrocarbons: ( 0 )3-methylpentane; ( A ) cis-2-methyl-2-pentene. Selectivity is expressed as methylcyclopentane (MCP) % in the total C, cyclic product (MCP + MCPe) (85).

(m)

METAL-CATALYZED CYCLIZATION REACTIONS OF HYDROCARBONS

297

111. Cyclization with Skeletal Rearrangement

A. METAL-CATALYZED SKELETAL ISOMERIZATION PROCESSES Two main pathways of metal-catalyzed skeletal rearrangement have been distinguished : “bond shift” mechanism and C, cyclic isomerization (7, 8). 1. Bond Shift

Three explanations have been suggested for bond shift. The AndersonAvery mechanism (Scheme VIIIA) assumes surface hydrocarbon species multiply bonded to the metal (7, 86).

C

F\/\I

-A\. C

[cJ7c\c]

c c c

(C)

SCHEMEVIII

Muller and Gault proposed (87) an adsorbed cyclopropane-like intermediate (Scheme VIIIB). The McKervey-Rooney-Samman mechanism involves a complex like adsorption of two carbon atoms (88) (Scheme VIIIC). Little essential differences can be found between the intermediate structures suggested by these hypotheses. The dispute concerns only the extent to which it can be regarded as a cyclopropane species. We can agree with

298

ZOLTAN

PAL

Anderson that at this level the mechanistic details are a matter of opinion (7). There is, however, a difference as far as the number of surface atoms participating in the reaction is concerned. Mechanism A requires more than one; Mechanism C, however, requires only one metal atom. Van Schaik et al. (89) reported skeletal isomerization according to Mechanism A over platinum-rich platinum-gold alloys, whereas over gold-rich catalysts, isolated platinum atoms could promote Mechanism C only. Garin and Gault (82) assumed the formation of a C4 cyclic intermediate with the insertion of a platinum atom as the fourth member of the ring. This concept of Mechanism B would also involve one metal atom. Pines and Csicsery (90,90a) proposed three and/or four-membered cyclic intermediates in the isomerization of various branched alkanes over “nonacidic” chromia-alumina. A similar, 1,3-rnethyl shift has recently been reported with an oxygenated reactant (tetramethyloxetane) over supported Pt, Pd, and Rh (906).Future experiments are necessary to elucidate whether hydrocarbons, too, can form C4 cyclic intermediates over metal catalysts. Some products assumedly formed via ethyl shift could be interpreted by C4 cyclic isomerization. 2. C , Cyclic Isomerization This pathway is restricted to four metals : platinum, palladium, iridium, and rhodium (Section 11,A,3). In addition to cyclization, it should also involve the opening of the C, cycle (see Section III,C,l). Isomerization-wherever the structure of the reactant permits the C, cyclic pathway-is accelerated by hydrogen (27, 27a). The parallelism between the isomer and C, cyclic yields (27a, 62), and also the composition of isomers (91-92) indicate a prevailing C, cyclic pathway of isomerization in the presence of hydrogen. Isomer formation from dimethylbutanes is much lower, and hardly any acceleration from increasing hydrogen pressure is observed (78). Small amounts of 2,2-dimethylbutane produced over platinum black from 3-methylpentane indicate a slow bond shift isomerization at any hydrogen pressures (25).

B. CYCLIZATION OF OPENCHAINHYDROCARBONS WITH SKELETAL REARRANGEMENT 1. Aromatization of Isohexanes over Metal Blacks

If methylpentanes and dimethylbutanes are able to give C, or C6 cyclic products, this must involve some sort of skeletal rearrangement. Aromatiza-

TABLE IV Selectioity of Formation of Various Products from Hydrocarbons of Different Structure" [ P t : recalculated after (91); Pd: Ref. ( 9 / a ) ] ~~

Carrier gasb: Helium

Starting hydrocarbon n-Hexane 2-Methylpentane 3-Methylpentane 2,2-Dimethylbutane 2,3-Dimethylbutane Methylcyclopentane

Catalyst'

Hydrogen

Conversion,

(%I

SH

Pt Pd

28.8 31.8

53 19

-

Pt Pd

23.9 36.7

Pt Pd

SAC

So,

2

45 79

-

65 21

2 2

33 76

-

25.6 26.1

68 24

2 2

30 73

-

Pt Pd

27.3 8.9

93 53

-

Pt Pd

20.7

84 85

-

20.2

Pt Pd

15.8 40.5

36 12

-

Si+c.

-

1

Conversion, (%)

SH

61.O 22.1

sol

s i

sC,

sAr

73 73

5 10

4 7

18 10

36.8 12.5

70 46

14 16

12 19

4 9

10

1

38.8 13.3

73 40

10 16

13 20

4 7

16

7 43

4

19.1 3.8

95 78

4 16

11 10

5 5

43.4 20.2

92 90

59 86

5 1

59.6 6.8'

50 26

2

1

2.5

0.5 -

1.5

2

-

44

-

36

-

~

-

-

0.5 4

-

2 0.5

2 7.5

5 29

2

1 9

a The subscripts denote as follows: H, hydrogenolysis; i, isomerization; C,, C, cyclization; Ar, aromatization; 01, olefin formation. Ring opening products from methylcyclopentane are given under Si. Carrier gas, 60 ml min-': T = 420°C, except for experiments with palladium in helium where T = 390°C; pulses, 2 pl (Pt) and 3 pl (Pd) of hydrocarbons. Catalysts: 0.51 g Pt black (91), and 0.31 g Pd black ( 9 1 ~ ) . T = 390°C.

300

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tion may proceed via C, ring closure. A subsequent C, + C6 ring expansion or ring opening and reclosure may lead to benzene. Alternatively, skeletal isomerization should precede cyclization. In helium, all hexanes give benzene (Table IV) (91, 91a). Methylcyclopentzne and methylpentanes give similar benzene yields. No saturated isomers are detected, and the ring opening of methylcyclopentane is negligible. Benzene formation from all isohexanes had a similar energy of activation value. With platinum this was nearly twice as high as that of n-hexane aromatization (62) ; with palladium black, however, nearly the same values were found for n-hexane and isohexanes (91a). This indicates a common rate-determining step for aromatization with skeletal rearrangement. This is not the formation and/or transformation of the C , ring. We attribute benzene formation to bond shift type isomerization preceding aromatization. It requires one step for methylpentanes and two steps for dimethylbutanes; this is why the latter react with a lower rate, but with the same energy of activation. A monotonic decrease of benzene yield from methylpentanes is observed as a function of the hydrogen pressure over both metals (27a, 91a). The intermediates of bond shift type dehydroisomerization are likely to be unsaturated. This points to the McKervey-Rooney-Samman mechanism (88). This pathway obviously has a higher energy barrier over platinum than over palladium as compared with the aromatization of n-hexane. This is reflected also by the similar aromatization selectivity (,SAT) values of n-hexane and methylpentanes over palladium (Table IV). The rearrangement and formation of 2,2-dimethylbutane-with a quaternary carbon atom-is only possible via the above mechanism. Over platinum, 2,3-dimethylbutane (78) gave more benzene; over palladium, 2,2-dimethylbutane (91a) gave more benzene. This is the opposite selectivity, as reported by Muller and Gault, for ring expansion of 1,1,3-trimethylcyclopentane (87). This may be more evidence that at least two different types of bond shift mechanism can occur. The appearance of very small amounts of methylcyclopentane from dimethylbutanes (78) indicates subsequent C5cyclization of skeletal rearrangement products like those reported by Muller and Gault (93). This reaction has not been observed over palladium. 2. Aromatization of Hydrocarbons with Quaternary Carbon Atoms The Kazansky-Libzrman mechanism of “direct” alkane cyclization was postulated on the basis of experiments with 3,3-dimethylhexane (47). There was some hint even in this early publication that 2,2-dimethylhexane behaved in a different way. Of the two alkanes, 3,3-dimethylhexane can form

METAL-CATALYZED CYCLIZATION REACTIONS OF HYDROCARBONS

30 1

1S-diene, which may cyclize similarly to triene. The resulting unsaturated C , ring aromatizes with demethylation. 2,2-Dimethylhexane, however, cannot form 1,5 diolefin. It should isomerize prior to cyclization. It is also possible that the hindered aromatization of hydrocarbons with quaternary carbon atoms may involve some sort of one metal atom catalyzed cyclization. Here the intrinsic activity of the catalyst in forming a new C-C bond becomes important. Platinum is a better catalyst in this respect than palladium. It may be for this reason that 2,2,4,4-tetramethylpentanegives a C , ring over palladium more slowly than over platinum (17). A similar mechanism may be responsible for the reverse reaction: C5 ring opening of gemdialkylcyclopentanes. The stepwise dehydrocyclization of hydrocarbons with quaternary carbon atoms over chromia was interpreted by Pines (94).Here a skeletal isomerization step prior to cyclization was assumed. This is not of a cationic type reaction, and the results were explained by a free radical mechanism accompanied by vinyl migration (Scheme IXA). Attention is drawn to the fact that

tH3

Cn3

SCHEME IXA

ZOLTAN PAL

302

Step 10 is a thermal cyclization reaction not requiring a catalyst. The participation of a radical intermediate in the rearrangement reaction was verified by radiotracer studies (942). Davis (946)aromatized several C8 and C9 hydrocarbons with a quaternary carbon atom over chromia- and platinum-on-alumina catalysts. Here the reactions of 1,l -dimethylcyclohexane, and 2,2- and 3,3-dimethylhexanes will be compared (Table V). 1,l-Dimethylhexane suffered demethylation predominantly over chromia and alkaline platinum ; however, with less alkaline platinum, isomerization to xylenes occurred. The product composition obtained from dimethylhexanes over chromia does not agree with that of 1,l-dimethylcyclohexane,indicating that the latter is probably not an intermediate of aromatization. Vinyl shift according to Scheme IXA would give nz-xylene from 2,2-dimethylhexane and o-xylene from 3,3-dimethylhexane (the data in Table V are in agreement with this prediction). Other isomers may be produced via methyl shift. These data are in favor of stepwise dehydrocyclization over a chromia catalyst, although the author did not express such an opinion. On the other hand, the strong dependence of aromatic distribution on the alkaline character of the catalyst points to dual function effects with platinum on alumina (see Section IV,A). Thus, the above guesses about the reactivity differences of 2,2- and 3,3-dimethylhexanes could not be confirmed-all the more since no data have been given for the overall aromatic yield from the two isomers. TABLE V Aromatic Selectivities in Dehydrocyclization of Various Hydrocarbons with Quaternary Carbon Atoms." After (94b) Aromatic selectivity (%) Xylenes Ethylbenzene

Starting hydrocarbon

o

m

p

Toluene

1 2.5 6

88 80 20 20 59 14 4 14

~

1,I-Dimethylcyclohexane

2,2-Dimethylhexane 3,3-Dimethylhexane

Cr Pt' Ptd Cr PtC Cr Ptd Ptd

-

9

1.5 4 3

10

2 6

60 35 16 42 33 23

35 19 18 37 36

1

9 9 8

10

7

4 17 17 19

T = 500"C, flow system, no hydrogen added, analysis at about 30 min after the start of the runs. Catalysts: Cr, "nonacidic" Cr,O3-Al2O3 (13 wt % chromia). Pt: Pt-AI20,-K (0.6 wt % Pt, 1 wt % K). Pt Pt-AI20,-K (0.6 wt % Pt, 0.4 wt % K).

METAL-CATALYZED CYCLIZATION REACTIONS OF HYDROCARBONS

303

C. INTERCONVERSION BETWEEN RINGSYSTEMS These are reactions in which one cyclic system is formed catalytically from another. Very large numbers of compounds and reactions could be treated here (see, e.g., 9-11). Of these, some related to our main problems have been selected. Ring interconversion is often of minor importance in total reactivity ; therefore, some related reactions should also be tackled. Stereochemical aspects may be particularly important here. Small rings are planar and have high Baeyer strain owing to the distortion of the valence angle) (95). Strain energy is at a minimum with five- and six-membered rings that are not quite planar. With larger rings, the Baeyer strain is less significant. Here Pitzer strain (owing to the hindrance of free rotation) and the van der Waals repulsion between “crowded” hydrogen atoms (on neighboring carbon atoms as well as between those in the transannular position) are more important. As a result, the overall strain energy of the molecule increases again. Cyclooctane and cyclononane have the highest strain. In cyclododecane only transannular interactions manifest themselves, whereas cyclopentadecane is as free from strain as n-pentadecane (11). 1. Small Cycles The opening of small cycles, that is, the reverse reaction of ring closure, is their main catalytic reactions. For platinum, palladium, iridium, and rhodium the opening of the fivemembered rings has common features that are different from those of alkane hydrogenolysis. The reaction (a) operates already at much lower temperatures, (b) has a lower energy of activation than alkane hydrogenolysis [Pt : 167 vs. 117 kJ mole-’ (96); Rh: 117 vs. 54 kJ mole-’ (96a)], and (c) is promoted by higher hydrogen pressures. The increase of hydrogen pressure causes a sharp monotonic increase in the yield of alkanes from both methylcyclopentane (91,914 and ethylcyclobutane (97,97a). In spite of the different strain and the different overall reactivities of these cyclic systems, their ring opening should be essentially identical process. It represents a peculiar sort of C--C bond rupture. It must also involve surface intermediate different from that of alkane hydrogenolysis : as “flatly” adsorbed ring, (cf. Scheme V). If a denotes C-C bond rupture in the vicinity of the substituent and b that in the next position, b/a values are usually higher than unity for the four metals mentioned. This is the situation, for example, with C, ring opening over carbon-supported (98) and unsupported (62) platinum, as well as rhodium, iridium, and palladium (42). Ring opening ov&r nickel is also selective (42,99); much less (or even the opposite) selectivity has been found with other metals (42). A methyl substituent exerts a stronger directing effect than an ethyl group.

ZOLTAN PAAL

304

This is also true for C4 ring opening where the selectivity is less marked (97, 974 100). The following b/a values were obtained with platinum black at 360°C (pulses introduced into hydrogen) (85):

Hydrocarbon: b/a :

Methylcyclobutane 5.5

Ethylcyclobutane 1.6

Methylcyclopentane 13.5

Ethylcyclopentane 3.5

Probably, with ethyl substituent, a n-allylic adsorption [such as suggested by Paal and Dobrovolszky (97)]competes with the dual site mechanism. Much has been published about crystallite size effects on selectivity [for a review, see Clarke and Rooney (S)].Two (or possible three) ring opening pathways were postulated by Maire et al. (101). Later work of this group has been confined to two mechanisms: one nonselective, the other selective. One may wonder if everything can be sufficiently explained by the combination of these two mechanisms or whether there is a third, “partly selective” mechanism that has been neglected up until now. Crystallite size is, undoubtedly, the most important factor determining ring-opening selectivities over platinum catalysts on various supports (92).With increasing hydrogen pressures, monotonically increasing b/a values are observed for all catalysts : apparently the chances for the dual-site mechanism increase as opposed to any other less selective ring opening pathway. Ethylcyclobutane also gives benzene and methylcyclopentane over platinum black. The apparent energy of activation of benzene formation is close to that obtained for n-hexane (about 40 W mole-’) (52). It is assumed to have formed by its secondary C, cyclization. Methylcyclopentane may be formed via ring expansion or by consecutive C, cyclization of ring opening products. Its yield has a maximum at low hydrogen pressures (Fig. 8a). This curve is very similar to that obtained for benzene formation from methylcyclopentane (Fig. 8b). This latter has a higher (about 70 kJ mole-’) activation energy. The C , -P C, and the C, -P C6 ring enlargement reactions are analogous and represent a bond shift type rearrangement. This is easier over palladium than over platinum (Table IV). This is confirmed by recent results of Bragin et al. ( 1 0 1 ~obtained ) over Pt-C and Rh-C catalysts. Isopropenyl- and isopropylidene-cyclobutanes in hydrogen more readily underwent C4 C5 ring expansion than saturated isopropyl-cyclobutane. The C,-cyclic products were predominantly of unsaturated character. In helium, ring enlargement became predominant. Although some effect of acidic centers of the carbon support may not be exluded, the hydrogen sensitivity (which agrees to that shown in Fig. 8) points to a true bond shift mechanism with unsaturated type intermediates. --+

METAL-CATALYZED CYCLIZATION REACTIONS OF HYDROCARBONS

5

305

-I 0

50

'1. H,

100

FIG.8. Yields of ring enlargement products (percentage of effluent) as a function of the hydrogen content of the carrier gas. Pulse system; catalyst, 0.4 g platinum black; T = 360°C. (a) C,cyclics (methylcyclopentane plus methylcyclopentene) from ethylcyclobutane (974. (b) C, cyclics (mainly benzene) from methylcyclopentane (92,9Ia).

There are no such conditions that would ensure significant aromatization of C5cyclic systems over pure metals. Bond shift type ring enlargement is rather slow. c6 reclosure of ring opening products is also not likely; under low hydrogen pressures ring opening is hindered ; higher hydrogen pressures do not favor c6 dehydrocyclization. The ring enlargement of five-membered rings containing a quaternary carbon atom occurs predominantly with its participation (5). This can be judged from the isomer composition of aromatics from 1,1,3-trimethylcyclopentane (Scheme IXB) (m-xylene :p-xylene :toluene = 62 :34 :4). With palladium, the tertiary C atom is more reactive (87).

SCHEME IXB

ZOLTAN PAL

306 2. Medium and Large Cycles

Cyclohexane dehydrogenates rather rapidly to benzene. Its rearrangement has not been reported over pure metals until now. C6 Ring opening is negligible over platinum and palladium (48, 53); slight hexane formation was reported over carbon supported rhodium, iridium, and, especially, osmium and ruthenium (102), as well as over nickel on alumina (99). Cycloheptane is about the largest ring in which the participation of the whole molecule can be expected in catalystic reactions. This molecule exhibits practically all the reactions described for cyclic hydrocarbons. The ring opening of methylcycloheptane is also selective, but total reactivity is much less than that of methylcyclopentane (103) : Bond : Relative reactivity:

U

None

b 1.o

C

0.8

d 0.8

The flat adsorption is obviously much more hindered with the greater C7 ring than with C5or C6 ones. Cycloheptane exhibits a high ring contraction tendency (via methyl shift) to give aromatic products or even 1 ,Zdimethylcyclopentane (104, 1044. The outstandingly favorable elimination of one CH, group during ring contraction increases benzene yield (Table VI). With even larger rings, only appropriate parts of the cyclic hydrocarbon react catalytically, according to a few elementary reaction types. With Cs-CIs rings, the extent of hydrogenative ring opening over platinum on carbon at 310°C increased from 1 to 2% up to almost 100%selectivity under identical conditions (11) (Fig. 9). The relative importance of ring rearrangement reactions, in turn, decreased. Nickel always led to more ring opening and more ring contraction than platinum. With cyclooctane, transannular cyclization (an intramolecular form of C, cyclization) is especially favored (105). It gives bicyclo(3,3,0)octane TABLE VI Formation of Aromatics,from Various C , Hydrocarbons" (104)

Aromatic selectivity (%) Feed

Aromatic Yield (%I

Benzene

Toluene

n-Heptane Ethylcyclopentane Methylcyclohexane Cycloheptane

1.85 1.35 15.5 42.1

19 19 1 41

81 81 91 59

Catalyst, 0.4 g platinum black; T = 360°C; carrier gas, 60 ml min-' helium; 3 pI pulses.

,

O f L;

METAL-CATALYZED CYCLIZATION REACTIONS OF HYDROCARBONS

20

8

10

12

307

1L n

FIG.9. Selectivity of ring opening (percentage of the total conversion) under identical conditions as a function of the number of carbon atoms in the ring (n) (11).

(cis-pentalane). Transannular C, cyclization and ring opening (of cyclooctane and of one or both of the fine-membered rings of pentalane) was significant in hydrogen only, as in the case of alkanes and cyclopentanes. Aromatization of cyclooctane is slower than that of cycloheptane and is promoted by the absence of hydrogen (Table VII). The detection of products with cycloheptane ring point to the stepwise character of ring contraction, involving two subsequent bond shift type steps (85). The selectivity of ring opening of one or both five-membered rings of cis-pentalane may proceed similarly to that observed with simple C5cycles. Positions b , and b, are apparently equivalent, that is, the other tertiary TABLE VII Selectivity of Cyclooctane Transformations in Helium and Hydrogen" (85) Selectivity for carrier gas (%)b

~

Product

He

C,-C, Alkanes C6-C, Cyclopentanes C6-C, Aromatics Cycloheptane n-Octane iso-Octanes Cs Alkylcyclopentanes C, Aromatics Cyclooctene &-Pen talane'

20.7 0.3 10.6 2.7 2.4

15.4 40.1 7.8

5.2 1.1 1.6 -

28.7 11.6 7.4 6.6 1.6 37.1

~

Catalyst, 0.4 g platinum black; T = 360°C; carrier gas, 60 ml min-'; 3 pl pulses. Total conversion (%) for He is 6.7 and H, is 58.8. ' Together with methylcycloheptane and methylcycloheptene. In hydrogen, these consist of about 40% of the peak, in helium, probably more. a

ZOLTAN PAAL

308

carbon atom-which is supposedly not attached to the surface-does not appear to screen bz (Scheme X).

&-. h

A

SCHEME X

TABLE VIII Relative Reactivities of Cyclic Hydrocarbons with 4-7 Membered Rings" (78) Total conversion

(%I

Relative reactivityb

A. Carrier gas: helium Ethylcyclobutane

7.67

1.oo

Methylcyclopentane

3.17

0.49

Eth ylcyclopentane

3.38

0.44

Methylcyclohexane

15.54

9.85

Cycloheptane

44.80

5.84

61.12

1 .oo

Methylcyclopentane

36.08

0.59

Ethylcyclopentane

32.7

0.54

Methylcyclohexane

81.06

1.33

Cycloheptane

18.2

0.30

Feed

B. Carrier gas: hydrogen Ethylcyclobutane

Main reactions and their approximate relative rates'

RH > HC > A > DH 5: 4: 2: 0.2 HC > D H % RH % A 2: I: I: 1 HC > D H % A > DM 7: 1: I : 0.5: DH= A >DM 14: 1 A > HC > D M > RH 5: 3: 2: 0.3

RH 0.1

RH> I >HC> A 8: I: 1: 0.1 RH>HC>DH> A 9: 1: 0.2: 0.05 RH>HC>DM> A 8: 1: 0.2: 0.1 D H = A >>DM 160: 1 A > RH > HC 6: I: 0.5

0.4 g platinum black, T = 360°C; carrier gas, 60 ml/min, 3 pl pulses. Related to ethylcyclobutane. Abbreviations: RH, hydrogenative ring opening; HC, hydrocrack to fragments; A, aromatization; DH, dehydrogenation; DM, demethylation; I, skeletal isomerization (both in the ring and ring opening products).

METAL-CATALYZED CYCLIZATION REACTIONS OF HYDROCARBONS

309

Whenever six-membered rings are formed, they dehydrogenate rapidly to an aromatic one : hydrindane formed via transannular cyclization of cyclononane very rapidly gives indane. The flatly adsorbed benzene ring exerts a significant screening to the “a” bonds of the five-membered rings : the b/a ring opening selectivity of the latter was around 11 (1052) (Scheme X). A similar directing effect is observed in ring closure (12). Hydrogen effects in reactions of various cyclic molecules can be interpreted in terms of hydrogen sensitivities of the elementary steps (dehydrogenation, ring opening/closure, “bond shift” type isomerization) involved in them (Table VIII) (78). Molecular structure is more important here than with alkanes and may eventually have a decisive role (e.g., bond shift type ring rearrangement of ethylcyclopentane is not too significant but that of cycloheptane is rapid). 3. Ring Systems

The series of studies carried out by Kazansky et al. with various higher cyclic compounds provide a mine of information for up-to-date studies. Data published for three spirocyclic hydrocarbons have been selected (106-106b). Their recalculation also confirms the validity of the abovementioned principles for polycyclic compounds (Table IX). The following points must be stressed : a. The quaternary carbon atom is involved in the formation of practically all products. The only exception is spiro(4,5)decanewhere the opening of the five-membered ring can occur in positions b or c, too (which would be predominant with gem-dialkylcyclopentanes). Hydrogen enhances ring opening in positions b and c. b. The main product is thermodynamically stable aromatic hydrocarbon in each case, but the type of reaction leading to it depends on the structure of the starting hydrocarbon (Scheme XI).

SCHEME XI

ZOLTAN PAAL

310

TABLE IX Selectivity of Various Transformations of Spirocyclic Hydrocarbons" [Calculated after (106-106b)l Percent selectivity if the feed is Spiro(4,5)decane Reaction

Alone

WithH,

Spiro(5,5)undecane Alone

WithH,

Spiro(5,6)dodecane alone

Degradation*

tr.

1

11

5.5

0.2

Ring opening In position Q In other positions

52 5

63 12

70

69 -

17 -

43

24

19

22.5

82.5

-

-

8

5.5

6

43

24

4.5

-

-

6.5*

Quaternary-tertiary bond shift' Keeping original ring structured With ring enlargemente With ring contraction

-

12 5f

10 669

' Catalyst: 15% platinum on carbon; T = 320°C; flow system, space velocity, 0.2 hr-' (for hydrocarbon, no space velocity is given for hydrogen). Analysis data are related to liquid products. Products: aromatics (alkylbenzenes or naphthalene) with less carbon atom than the parent hydrocarbon. ' Products: conjugated bicyclic hydrocarbons with the same number of carbon atoms as the parent hydrocarbon. Conjugated ring systems with the same carbon atoms in each ring as the parent hydrocarbon, i.e., 1-methylnaphthalene (6 6) and benzcycloheptane (6 7 formed with the loss of one carbon atom), respectively. Naphthalene, benzcycloheptane, and benzbicyclo(3,3,O)octane, respectively. 1-Ethylindane- and 2-methylnaphthalene (the latter being the product of secondary ring contraction of benzcycloheptane). 65% Diphenyl and 1.5% bicyclohexyl.

+

+

Ring opening is preferential with spirodecane and spiroundecane, whereas the seven-membered ring in spirododecane favors aromatization with C , +C6 ring contraction likewise in cycloheptane. Naphthalene is one of the main products from spirodecane. As opposed to the authors (106), we attribute its formation to bond shift with the transformation of the quaternary carbon atom to a tertiary one rather than secondary ring closure of butylbenzene. If this reclosure were significant,the overall yield of methylnaphthalenes from spiroundecane should have been much higher (l06a).

METAL-CATALYZED CYCLIZATION REACTIONS OF HYDROCARBONS

3 11

This type of bond shift is also suppressed by hydrogen. No naphthalene derivative was identified from spirododecane (106b). IV. Cyclization over Dual Function Catalysts and Oxides

A. RINGCLOSURE OVER BIFUNCTIONAL CATALYSTS Dual function catalysts contain metallic and acidic active sites. No distinction will be made here between mono- and multimetallic catalysts-all the more since our knowledge on the latter type of catalysts is far from perfect. This study has collected evidence on the following metal catalyzed elementary processes. Numbers 1 and 2 occur with open chain hydrocarbons and Numbers 3 and 4 occur with cycles. 1. C, Dehydrocyclization. Arguments have been put forward that primary ring closure produces six-membered rings over three important catalyst types : oxides, supported platinum, and bimetallic catalysts (107). The postulation of metal catalyzed C, ring closure does not involve any definite suggestion whether its mechanism is “direct” or “stepwise.” 2. C, Cyclization. This is typical for lower temperatures and higher hydrogen pressures. The parallel occurrence of these processes has been pointed out. 3 . Ring opening. This occurs selectively (far from the substituent) with C, (and smaller) cycles, under hydrogen-rich conditions. Over four metals (Pt, Pd, Ir, and Rh) it may be much more rapid than cracking. The C6 cycle is not liable to open over metals.

4. Ring Enlargement. The ring enlargement of five-membered cycles to six-membered ones is possible over pure metallic sites. This reaction is, however, generally slower than any of the cyclization processes. We believe that it proceeds by some kind of radical type bond shift mechanism (see Section II1,A) rather than via ionic intermediates owing to some kind of acidic properties of platinum assumed by Lester (108). The presence of acidic centers in the catalyst promotes acid-catalyzed processes in addition to the above reactions. Of course, their rates may be much higher than those of metal catalyzed ones. Acid-catalyzed reactions involve carbonium-ion intermediates. Their different product spectra can be used for detection of the probable reaction mechanism.

ZOLTAN PAAL

312

The following main processes can be mentioned : a. Curbonium Ion Type Cyclizution (109). This alone can produce either five- or six-membered rings. Here the structure of the carbonium atom precursor is important. Secondary carbonium ions are more stable; therefore, n-hexane gives mainly C5-, n-octane c6 cyclic products (18). This is not true for metal-catalyzed reactions: the rates of C6 cyclization of n-hexane and n-octane have been reported to be nearly identical over platinum on carbon (110). b. Carbonium Ion Type Ring Opening. This is preferential adjacent to the substituent. Its selectivity is thus just the opposite of any reaction over metallic sites (112). Ring opening under the effect of an acidic alumina support proceeds about ten times more rapidly than in the platinumcatalyzed reaction (112). At least two mechanisms seem probable (12 2 ~ ) . c. Curbonium Zon Type Zsomerization. This produces mainly tertiary and quaternary carbon atoms. The abundance of such products indicates acidcatalyzed isomerization. d. Curbonium Ion Type Ring Enlargement (113,114). Aqueous acid treatment of an otherwise nonacidic silica support for platinum catalyst accelerates C, --t C, ring enlargement by up to 25 times (115). Over platinum on alumina support the C, c6 ring interconversion can proceed in both directions (116). Dehydrogenation to benzene represents a strong competition to ring contraction of cyclohexane (114). Methylcyclohexane gave also some 1,l-dimethylcyclopentanewith a quaternary carbon atom (116). c6 Ring contraction (likewise its ring opening) is practically absent over monofunctional metals.

*

The main feature of bifunctional catalysts is not only the appearance of novel catalytic functions but also its mode of operation: various functions may (and sometimes should) participate alternately in the transformation of one single molecule. Consequently, various multistep pathways may lead to the same end product. In this situation it can be extremely difficult to distinguish between the role of individual catalytic functions. Three experimental approaches have been offered : 1. The use of mechanical mixtures of metals on nonacidic supports and acidic catalysts showed that dual function effect appears without an intimate contact between the two functions (114,117,118). 2. Selective poisoning of one and/or another function by added poisons (118-119) or by self-deactivation ( 1 1 2 ~as ) well as the following fact: 3. Changing the concentration of the assumed active component in the catalysts (119, 220) may reveal the main pathways leading to one or another class of products.

METAL-CATALYZED CYCLIZATION REACTIONS OF HYDROCARBONS

3 13

A simplified “two-dimensional chart” was proposed by Mills et al. (121) as early as in 1953 to visualize the variety of reactions in bifunctional catalysis. The scheme restricts metallic functions to dehydrogenation reactions. These sites would produce alkenes from alkanes, which subsequently would form C, cycles over acidic sites. Acidic centers would be active in C, C, ring enlargement. Benzene formation would be the result of metal catalyzed dehydrogenation of these c6 cyclic intermediates. The reality of these reactions cannot be denied. The original paper (121) presents very good experimental evidence on C5+C6 ring enlargement over acidic sites via olefin intermediates. There is an interesting contradiction in the literature as far as this “twodimensional scheme” is concerned. On the one hand, it gives a good qualitative description of reforming reactions. This may be the reason for its having been adopted by so many handbooks and papers (e.g., 111, 118-119). On the other hand, it has one serious drawback: it restricts ring formation to acid catalyzed C5 cyclization of olefins. Examples quoted in this review may offer sufficient evidence that the attribution of mere hydrogenationdehydrogenation activity to the metal and the denying of its cyclization activity is an oversimplification. The inclusion of a metal catalyzed cyclization step in this two-dimensional chart, that is, its adoption with proper criticism, would be essential. Silvestri et af. (118a) found that a platinum-on-carbon catalyst promoted the formation of both five- and six-membered rings. Thiophene deactivated both cyclizing functions leaving the dehydrogenation activity almost unchanged. The mechanical mixture of a catalyst deactivated by thiophene and alumina yielded almost the same amount of C, and c6 cyclic products (Fig. 10). These results were used as evidence that the “paraffin cyclization reaction can proceed by a dual-function mechanism in which olefins are produced by the dehydrogenation function and then cyclized on the acid function” (118a, p. 388). We wish to emphasize the importance of the reverse of this conclusion: over the Pt/C both types of ring systems formed, without doubt resulting from the metal function. Sinfelt et al. (120) observed a twofold increase in the n-heptane aromatization rate when the platinum content of their alumina-supported catalyst increased from 0.10 to 0.60%. At the same time, the rate of methylcyclopentane ring expansion remained constant. This result also serves as evidence for metal-catalyzed aromatization over dual-function catalysts without the participation of any C, cyclic intermediate. The cyclization activity of platinum itself was independent of the nature of the support (109). Pure acidic cyclization prevailed with olefin feed (30, 109). Whereas the cyclization step itself has been rather neglected, much effort has been concentrated on other acid-catalyzed reactions. Alkane isomerization was regarded earlier to be a fast reaction (122), ensuring even equilibra--f

314

ZOLTAN

PAL

Yield mole yo

B

4.0

0.259

0.259

a259

PtlC

“2’3

PtlC plus

30

0.509 A12% ’5

2.0

1.o

’5 ‘6

L FIG. 10. Comparison of ring production from pure (A) and sulfur containing (B) n-heptane with and without mixing of catalyst components ( 1 1 8 ~ ) .

tion of alkane isomers prior to cyclization. Selman and Voorhies (112), however, found that the ratio of metal and acid-catalyzed methylcyclopentane ring opening (with opposite selectivity) may be more important in determining the composition of isomer alkanes. Special attention has been paid to acid-catalyzed ring expansion. Sterba and Haensel (1Z9) reported that the rate of benzene formation from methylcyclopentane increases with increasing fluorine content of the catalyst (up to 1.O% F with 0.3% Pt on alumina). At the same time, increasing platinum content also promoted this reaction (up to 0.075% Pt with 0.77% F on alumina). This indicates “the remarkable cooperative action of a dual function catalyst” (119, p. 11). With platinum and palladium supported on acidic alumina, cyclopentanes are important intermediates of aromatization (44, 123-124). For example, n-heptane gave about 2-3 times more aromatic product than 2,4-dimethylpentane, whereas the formation of C, cyclic products was about the same from both alkanes. Alkylcyclopentanes aromatized at a reasonable rate (1234.

METAL-CATALYZED CYCLIZATION REACTIONS OF HYDROCARBONS

3 15

Aromatization with C, cyclic intermediates can occur in the absence of acidic function, too. Kazansky et al. (5) estimated the role of C, cyclic intermediates in aromatization to be about 5% over platinum on carbon. Dautzenberg and Platteeuw found about 11% C, cyclic pathway with nonacidic platinum on alumina (23). 2,2,4-Trimethylpentane is forced to produce aromatics via C5 cyclization because of its structure; here the quaternary carbon atom facilitates ring enlargement (5, 23). Davis also estimated the contribution of the C5 cyclic mechanism to be about 5% in the aromatization of 1-[14C]-and 4-['4C]-n-heptane over nonacidic Pt-alumina (125): I4C in the methyl group (%)

l-[l 4C]-n-Heptane

4-[14C]-n-Heptane

Predicted with C6 closure only

Observed

50 0

40-44 3-6

With the addition of Sn as a second metal the composition was closer to that predicted for pure C, cyclization. Rhenium had the opposite effect (107). Suppression of the acidity of the alumina support (of platinum or palladium) by incorporation of sodium nitrate (126) decreases first the C, -+ C, ring enlargement activity of the catalyst (124, 127). Potassium ions (94-94b) or sec-butylamine (70) have a similar effect. With hydrocarbons higher than C, , there are more reaction possibilities. The aromatization of 1-[14C]-n-heptaneover nonacidic platinum on alumina permitted the following estimates of various ring closure pathways (128) : C6 Ring closure C, Ring closure C, Ring closure + ring opening + C6 ring closure C, Ring closure .+ ring enlargement to C6 C, Ring closure -+ 3-ethylpentane + C5 ring closure + ring opening + C6 ring closure

54% 18% 8% 12%

8%

This is the only evidence thus far on C,-ring closure over platinum. With gern-dimethyl hydrocarbons, isomerization and demethylation compete with each other. Isomerization predominates in the presence of acidic centers. Over platinum supported by acidic alumina 1,1 -dimethylcyclohexane gave 18% toluene and 60% o-xylene (percentages of the total aro-

316

ZOLTAN PAAL

matics). With a nonacidic alumina support, this ratio was 53 :32 (129). In addition to ring contraction (to isopropylcyclopentane), a c6 3 C7 ring expansion was also supposed as a means of explaining the presence of m-and p-xylenes as well as ethylbenzene. Experiments with methylcycloheptane confirmed this hypothesis. In addition to the two “classical” functions several other types of interactions are possible between the components of the catalyst. For example, platinum seemed to neutralize some acidic sites of alumina (129). This may be another reason why the acidic properties of platinum itself (108) are not very likely. The addition of a second, third, etc. metal makes the problem still more difficult. Yermakov and Kuznetsov (130) assume that this second metal may form an alloy with the catalyst, may “anchor” it to the surface as a low valent ion, or may fulfill both functions. Enhanced C6 cyclization activity has been reported over platinum containing molybdenum and rhenium additives, whereas tungsten additive was reported to promote C5 cyclization. Another example is the formation and activity of the “complex” platinum discussed in Section II,B,4. A very striking observation is the reduction of alumina over 500°C and the formation of Pt-A1 alloy. The formation of A1 species with less than + 3 charge has also been shown by Auger spectroscopy of Pt/y-Al,O, catalysts (62e). Enhanced selectivity of c 6 product formation (including benzene) from hexane was attributed to ) out, “even such an this alloy (131). As Dessing and Ponec ( 1 3 1 ~pointed inert carrier as SiO, can have a stabilizingeffect on some types of Pt particles (i.e., sites) or Pt compounds (oxides, silicides) which are not present on a pure massive metal” (p. 251). These problems point far beyond classical metal catalysts; as long as further careful studies do not clarify these phenomena, the most reasonable approach is to beware of oversimplifications, and to treat results obtained over supported catalysts with extreme caution.

B . CYCLIZATION

OVER

OXIDECATALYSTS

A very brief survey of ring closure processes over oxides (first, chromia) may reveal some common features with metal catalysis. This must have led in some cases to very similar ideas (see also Section 11,B). 1. Stepwise c6 dehydrocyclization was observed over potassia-chromiaalumina as well as potassia-molybdena-alumina catalysts (9, 10). Higher operating temperatures (45O0-5OO0C) of these catalysts facilitate the appearance of unsaturated intermediates in the gas phase. Radiotracer studies indicate a predominant C6 ring closure of 4C-labeled n-heptane over pure chromia (132, 132~). 2. C5 cyclization is not favored over chromia. Alkylcyclopentanes and

METAL-CATALYZED CYCLIZATION REACTIONS OF HYDROCARBONS

3 17

alkylpentanes caused a strong carbonization of the catalyst (37, 90, Wa, 1 3 2 ~ )As . opposed to metals, chromia is quite active in producing both smaller (C3, C,) and larger (C7, c,) rings. 3. The production of larger rings is in agreement with stepwise aromatization. Pines and Goetschel (37) put forward the possibility of an octadiene intermediate. They attributed the decreasing contribution of the larger rings in overall aromatization to the more facile desorption of octatrienes over deactivated catalysts. Later, both C, and c8 ring closure were attributed to a thermal cyclization process followed by catalytic isomerization steps (94).This may be the reason why the formation of larger rings is less probable over metals where the cyclization step itself is also regarded as a catalytic one (Section 11,B,2). 4. Skeletal ring contraction steps of primary C, and cs rings are more probable than bicyclic intermediates (132b). Aromatization of methylcyclopentane indicated no carbonium mechanism with a nonacidic catalyst. Instead, Pines and Chen (132b)proposed a mechanism similar to that defined later as “bond shift.” This is a methyl shift. Two additional isomerization pathways characteristic of chromia have also been demonstrated : vinyl shift (94) and isomerization via C3 and C4cyclic intermediates (90~). These were discussed in Section 111. 1,l -Dimethylcyclohexane and 4,4-dimethylcyclohexene gave mainly toluene over various chromia catalysts. Thus, both skeletal isomerization and demethylation activities of chromia have been verified. The presence of an acidic alumina support enhances isomerization : “dual function” effects are thus also possible. Dehydrocyclization over any chromia-based catalyst has not been realized on a commercial scale. It would seem that the disadvantages of the process (regeneration after a few hours’ run, highly endothermic reaction, requirement of pure n-hydrocarbons as the feed) outweigh its advantages (low pressure and a very high aromatic selectivity). The present trends of reforming (lower pressure and continuous operation) create, however, a technological background that may well facilitate the realization of the chromia-catalyzed process. V. Interpretation of Metal Activity in Catalytic Cyclization

The following factors seem to be important in cyclization (and also other) reactions of hydrocarbons : 1. The nature of the metal: Dehydrogenating properties are important for C6 dehydrocyclization. Platinum, palladium, iridium, and rhodium

318

ZOLTAN

PAAL

catalyze C, cyclization. Here surface geometry seems to play an outstanding role. 2. Added “astoichiometric” surface components’ (hydrogen and carbon, eventually other metals) : these influence activity and selectivity. AND CATALYTIC ACTIVITY OF METALSURFACES A. STRUCTURE

Most metals active in cyclization belong to Group VIITB and have either face-centered-cubic (fcc) or hexagonal close packed (hcp) crystal structure. It seems to be realistic to relate catalytic activity to the most stable [l 111 plane of fcc metals. Bond (135)describes the electron structure of the this plane. So-called t2g electron orbitals point toward those interstices where metal atoms in the subsequent overlayer would be accommodated. These orbitals have metallic character. So-called e, orbitals point toward the next nearest neighbor. These are localized and able to form real covalent bonds. The degree of hybridization of these orbitals is unknown. Knor (136)assumes that only eg orbitals would stick out of the plane, but they are almost completely hybridized. He assumes that the t Z gelectrons are parts of the electron gas of the metal. The e, and t 2 gsites are by no means equivalent. The [l 111 plane the most closely packed (six-coordinated) surface of the fcc structure and is very stable. The five-coordinated B, structures may have [110] or [311] configuration (137). The four-coordinated [loo] plane rearranges reversibly to a six-coordinated structure (138). Carbon-platinum interactions cause “faceting” of the [ 1001 plane, giving rise to the formation of [211] or [311] structures (139). Following Balandin’s classical concept on sextet adsorption of cyclohexane (140), a similar accommodation of triene species has been suggested on a [l 111plane (54). Figure 1l a illustrates the geometry of adsorption without defining its character. Cyclization and polymerization are equally facilitated by catalyst geometry. Much later low-energy electron diffraction (LEED) studies demonstrated that the most likely acetylene adsorption over a [l 111 platinum surface has a geometry that is very similar to the above suggestion (Fig. 1lb) (141). The LEED pattern of adsorbed ethylene indicated an analogous surface species (141a). We suggest that such an adsorption also is active in stepwise aromatization. Palazov’s recent infrared studies supplied two additional items of evidence in favor of this suggestion (75) : Eidus applied this name to components which do not participate in the stoichiometry equation of a catalytic process, but whose presence is, nevertheless, important (133). Such an effect of hydrogen has been reported to be rather general in organic catalysis (134). Metalcarbon ensembles may serve as sites for hydrogen transfer ( 1 3 4 ~ ) .

METAL-CATALYZED CYCLIZATION REACTIONS OF HYDROCARBONS

3 19

FIG. 11. (a) Schematic representation of hydrocarbons adsorbed on the [ l l l ] plane of platinum. Intersections of the lines of triangular net denote positions of the centers of platinum atoms. (1) Cyclohexane; (2) “all-cis” conformation of cis-1,3,5-hexatriene; (3) “transoid” conformations of cis- and trans- I ,3,5-hexatriene (54).(b)Adsorption configurations of acetylene and ethylene found most probable according to LEED studies (141).

(i) Ethylene adsorption over platinum resulted in a band at 2890 cm-’, which can be attributed to a 3C-H bond. (ii) The frequency of the band near to 2050 cm-’ characteristic of adsorbed CO became lower after ethylene coadsorption (Oc0 = 0.3). As indicated by benzene plus CO coadsorption, this means the presence of a rc bond in the surface species (142). The model obtained by combining (i) and (ii) corresponds to that shown in Fig. 1 lb. With butene and butadiene coadsorption, a similar shift of the CO band was observed. The increase of absorbance near to 3000 cm- points again to >C-H bonds. The several possible configurations of adsorbed hydrocarbons (cf. Fig. I la) result, here, in a broadening of the band ( 1 4 2 ~ ) . Aromatization according to Fig. 1la requires fewer surface sites than coke formation. A high amount of additives [such as Pb (24), Sn (74), and Re (143)] may dilute the catalyst surface to an extent where aromatization still might proceed over a platinum island, but surface polymerization is not possible anymore. C, cyclization requires stricter geometric conditions than aromatization. This is in favor of the “dual-site” mechanism of C, cyclic reactions (25).All metals catalyzing it have an fcc lattice, and their atomic diameter lies between 0.269 and 0.277 nm. These two criteria must be fulfilled simultaneously. With such a distance between the two sites, the screening of the C-C bond adjacent to the preferably adsorbed tertiary C atom becomes evident. Figure

’

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320

FIG. 12. A possible accommodation of 3-methylpentane suitable for ring closure assuming positions on the top of metal atoms as active sites (144).

12 (144) shows one possible way of adsorption; another variant has been published by Paal and Tetenyi (42). The dual site C, cyclization concept is related to the one suggested by Van Schaik et al. (89), viz. that C, cyclic isomerization (with closure and opening of the cyclopentane ring) requires two sites, whereas bond shift requires only one. They explained alloy activity by the diluting effect of the added metal on the catalytically active ensembles. This also is supported by recent studies with platinum-rhenium catalysts (143). With a strong drop in catalytic activity with increasing rhenium content, the following selectivity values were obtained (2% metal on silica, n-heptane pulses into 1 atm hydrogen at 400°C): ~~

Re in (Pt

+ Re) (%)

Selectivities for

0

40

80

100

Isomerization C , Cyclization Aromatization

0.16 0.24 0.3 1

0.16 0.26 0.34

0.085 0.048 0.27

0.037 0.016 0.07

As the rhenium content is increased from 40 to SO%, the ratio of selectivity for aromatization as compared with C, cyclization is increased from 1.3 to 5.65. When platinum is alloyed with iridium (on an a-alumina support), which is a C , cyclizing metal, the C , cyclization selectivity of alloys containing 30-70% iridium remains nearly the same (about 0.15) and falls between that of platinum (about 0.6) and that of iridium (0.025). The aromatization (and

METAL-CATALYZED CYCLIZATION REACTIONS OF HYDROCARBONS

32 1

isomerization) selectivity of alloys was almost as high as that of pure platinum on alumina (144a). Geometry may be partly responsible for the different behavior of Pt/Cu and Pt/Au alloys reported by de Jongste et al. (14) (Fig. 13). When small Cu atoms are added, C, cyclization may be possible. As Au atoms are larger than Pt the distances between active sites become too large for dual-site C , cyclization : the methylcyclopentane yield decreases. The cyclohexane ring is too big for the dual-site ring closure and opening mechanism even over platinum. With Pt/Au alloys, however, such a reaction is not excluded (Fig. 13). The higher spacing of chromia (145) may be the reason why it promotes C , , C7, and even C , cyclization (132-132b). An interesting synergism with Co-Rh catalysts has been reported recently . catalysts, which had a surface by Anderson and Mainwaring ( 1 4 5 ~ )Their enriched in cobalt (up to 0.98 mole .fraction), produced high amounts of methylcyclopentanefrom n-hexane (with no aromatization and some skeletal isomerization). Rapid ring opening of methylcyclopentane took place under conditions where no hydrogenolysis occurred. Thus, the “cobalt monolayer the properties of which have been modified by being present on a rhodiumrich matrix” (1454p. 204) behaved like a C,-cyclizing metal although cobalt does not belong to this group (42), and pure rhodium does not produce C, cyclic products either. One may incline to the view that the distances between cobalt atoms on a rhodium matrix are greater than those between pure cobalt; thus, “dual-site’’ C5cyclic reactions become possible. Stepped surfaces withstand cyclic oxidation-reduction treatments (146) like [ l l l ] and some other low-index planes. Steps have either [311] or [110] structures. They are claimed to be the only places where orbital hybridization does not take place (136).No wonder that such platinum (138) and iridium (147)surfaces have enhanced activity in C6 dehydrocyclization of n-heptane.

I

FIG. 13. Comparison of product composition obtained over diluted platinum alloys (14). (A) 5.2%Pt in Cu, T = 350°C; (B) 4.2%Pt in Au, T = 375°C.

322

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Blakeley and Somorjai (147a) reported that cyclohexane dehydrogenation was independent of the step density over stepped platinum surfaces, whereas ring opening to n-hexane increased proportionally to the step and kink density. At higher pressures, steps were active mainly in cyclohexene formation (148). It may be tempting to assume that a Balandin-type geometric conformity exists between the C6H,, ring and Pt [ l l l ] plane, whereas the introduction of [ 1001 steps (and/or kinks) disturbs this geometric harmony to an extent that ring opening and “edgewise” dehydrogenation to cyclohexene may occur. With disperse catalysts edges and kinks may be carbonized rapidly and dehydrogenation activity remains. Crystallite size effects indicate that steps (or the almost synonymous B, sites) might be responsible for the formation of cyclopentane species (149). Would single crystal studies confirm the role of these surface structures in C , cyclization, too? C5 Cyclizing metals are “soft” metals with their large atoms and high number of electrons. Montarnal and Martino (150) argue that this is an important factor favoring (obviously C, cyclic) isomerization rather than hydrogenolysis. Their broad d band also renders their electrons less available for multiple adsorption : that is, singly adsorbed intermediates that lead to C 5cyclization become favored. Deeper surface dissociation over these “soft” metals probably gives species such as those shown in Fig. 11 rather than sigma-bonded intermediates for hydrogenolysis. The hexagonal metals with similar atomic diameters are almost inactive in C, cyclization (42) (Re, 0.277; Os, 0.273; Ru, 0.267 nm). This can be attributed to one (or some) of the following reasons : a. C, cyclization occurs on a fcc [l 111 plane : the occurrence of the identical [OOOl] plane of the hcp structure is, however, much less probable (151). b. Planes different of the fcc [ l l l ] may be active and these do not exist over hcp metals. c. Steps may also be important, and these are different with the aba sequence of hcp layers from those brought about with the abca sequence of fcc crystals (136). d. The hcp metals are “harder” than C, cyclizing catalysts. In fact, they are rather active in hydrogenolysis. Correlation with the d-band width is only approximate: for example, 0 s has a broader d band than Rh and Pd (152).

Future careful experiments may well permit one to select the most probable reason for the behavior of hcp metals. B. ASTOICHIOMETRIC COMPONENTS AND SURFACE ACTIVITY Present-day techniques for surface studies have revealed that “clean” metal surfaces do not exist even under extreme conditions. It was for this

METAL-CATALYZED CYCLIZATION REACTIONS OF HYDROCARBONS

323

reason that Tetenyi et al. suggested that it is more correct to speak about “catalytic systems” than “metal catalysts” under reaction conditions (153). Based on other experimental facts, Somorjai expressed the same opinion (248). Hydrogen is the most important astoichiometric component. Even the effect of other added components can sometimes be interpreted in terms of governing the availability of surface hydrogen. This explains why adding a second (catalytically inactive) metal to platinum may have the same effect on the selectivity as surface hydrogen or nonmetallic additives (107)(see also Section 11,B,5). The presence of surface carbon is often governed by the concentration of hydrogen. Its effects may be indirect and are dealt with in recent reviews (8, 148). Four different approaches will be offered to interpret hydrogen effects in metal-catalyzed cyclization reactions. 1. Competitive Adsorption Approach

The very rapid hydrogen isotope exchange in any hydrocarbons adsorbed over the metals in question indicates a rapid dissociation of their C-H bonds. The degree of dissociation is governed by the amount of hydrogen present. On the basis of hydrogen sensitivities, various metal catalyzed processes can be arranged in the following order (62). From left to right, higher and higher hydrogen pressures are favorable (Scheme XII) : [Bond shift type isomerization requires some hydrogen, but is practically insensitive to hydrogen pressure changes (78).]

Optimum hydrogen pressure SCHEME XI1

A similar type of ordering has been observed for platinum (62),palladium, (91a), iridium, and rhodium (36).

Low hydrogen pressures are favorable for the first two reactions with deeply dissociated intermediates. Hydrogen determines here the direction of the overall reactions, that is, the ratio of aromatic and coke formation. Stoichiometric hydrogen is necessary for hydrogenolysis ; therefore, its optimum hydrogen pressure is higher. There must be a hydrogen pressure range when the lifetime of singly dis-

ZOLTAN PAAL

324

sociated radicals is long enough that C, cyclic reactions might proceed at a reasonable rate. The role of surface hydrogen concentration is supported by the fact that with increasing temperature (that is, when higher hydrogen pressure is necessary to maintain the same surface concentration) the positions of the maxima are shifted toward higher hydrogen pressures (25, 77). This is true for each process. The sections of the bell-shaped curves (e.g., Fig. 6 ) right to their maxima correspond to as high hydrogen pressure as is sufficient to gradually suppress even primary adsorption, which involves the dissociation of one single C-H bond. The concept of “reactive chemisorption” by Frennet et al. (154) also must be mentioned here. Instead of assuming hydrocarbon dissociation over “clean” metal sites, a hydrogen atom plus z adjacent “free” sites are supposed to be active in chemisorption. Isotope exchange of one hydrogen atom in methane has been treated in these terms throughout a very wide hydrogen pressure range (from lo-’ up to lo5 Torr). Its rate can be described by a bell-shaped curve as a function of hydrogen pressure: at lower pressures surface carbonaceous species hinders the process; at higher values, sorbed hydrogen hinders the process. Obviously, the underlying ideas are similar to those discussed above. The assumption of “reactive chemisorption” may be useful for the surface intermediate of C , cyclic reactions. It may well be possible that a competition occurs between a “reactive” and a “dissociative” chemisorption : the former giving C5 the latter C6 cyclic products. There is a thermodynamic relationship between these two surface species (see Section 11,A,2). Scheme XI11 summarizes all the above-mentioned facts about hydrogen effects and various surface intermediates (31). I I CsHu(ads)

/H

’ \

CeH@i(adS) +

\ \

(as k&+,l, %Hu-XRX

sH‘

%-kb

tH

-(O-X)H

2. Surface Heterogeneity Approach

In a further approximation, not only the amount but also the position of surface hydrogen should be considered. Thermodesorption studies showed

METAL-CATALYZED CYCLIZATION REACTIONS OF HYDROCARBONS

325

four (155) or possibly five (1552) types of adsorbed hydrogen. Absorbed hydrogen may also be present (15%). The different reactivity of various types of retained hydrogen was shown experimentally (155b). Menon and Froment ( 1 5 5 ~studied ) the activity of Pt-AI,O, and Pt-black pretreated in hydrogen at various temperatures. The overall activity had a very sharp minimum as a function of the pre-reduction temperature (550°C for supported, 500°C for unsupported platinum). This was due mainly to the almost complete ceasing of hydrogenolysis, whereas the amount of c6 products altered to a lesser extent. The selectivity of hydrogenolysis decreased almost monotonically as the temperature of pretreatment increased, along with the amount of aromatics within the c6 products. Saturated C, products showed an opposite change (Table X). These data support the validity of Scheme XII. Parallel hydrogen thermodesorption (TD) studies showed that pretreatment at 400°C resulted in the presence of low-temperature TD peak(s) of hydrogen (T,,, : 100-2OO0C), whereas high-temperature (TmaX x 400°C) hydrogen was observed with pretreatments between 500" and 600°C. Thus, the competitive adsorption approach of Section V,B,l should be applied for at least two types of hydrogen present under certain circumstances simultaneously. Low-temperature hydrogen would promote reactions with more deeply dissociated surface intermediates. High-temperature hydrogen suppresses these (mainly hydrogenolysis) in a way similar to the (reversible) deactivation caused by C1 or S. At the same time, it promotes C , cyclic reactions. The similarity between Pt-Al,O, and Pt-black excludes any support effect. The importance of the presence of various types of hydrogen was also underlined by TPR studies (62d). TABLE X Selectivity of n-Hexane Transformations as a Function of Catalyst Pretreatment Temperature"(155d) Distribution of C, products (%)

Selectivity (%) Pretreatment T ("C) 400 400b 450 500 550 600


84 82 61 21 31

cs 9 15 18 39 19 69

Saturated

--

98 75 65 82 100 100

Aromatic -2 25 35 18 -0 -0

Pulse system, I , , , , , ~ ~=~ 4OO0C, pulses: 2 pI of n-hexane catalyst: 0.4725 g Pt-A1,0, (0.6% Pt) calcined in air at 500°C and pretreated in hydrogen; carrier g a s : H,, 60 cm3 min-'. * Regenerated.

326

ZOLTAN PAAL

It is more difficult to identify these various hydrogen species. The first hydrogen-metal interaction was reported with steps (156) then, above 0 = 0.25, with flat plane sites. Here “stronger” (e,) sites should be saturated first. Ordered adsorption of hydrogen has been reported over partially covered Nit1 111 (157) and perhaps Pt( (157a) surfaces. Field ionization microscope (FIM) studies showed a triangular symmetry for hydrogen over platinum (158) corresponding to hydrogen adsorption in every second interstice (for example, over eg sites). The step structure of fcc metals gives also a similar symmetry (136). The reactivity of adsorbed hydrogen with oxygen was highest over the [331] sites of platinum, owing to a large extent to the fact that these are the main “trapping centers” for hydrogen where the dissociation occurs most readily ( 1 5 8 ~ )With . iridium, [123] sites (kinks on the edges of atomic layers) were most active ( 1 5 8 ~ ) . If hydrogen occupies all eg sites, the “dual-site’’ mechanism may operate over two adjacent t2g sites (42). The importance of active site periodicity and the screening of the adjacent C-C bond is valid in this case, too. This (assumedly eg adsorbed) hydrogen does not participate in C, cyclic reactions. There is some indication, however, that it might be mobilized for cyclobutane ring opening (97, 97u). One carbon atom in a “wrong” interstice may block the C, cyclization activity of several surrounding sites. Therefore, C, cyclic reactions are suppressed first during catalyst deactivation, while aromatization activity lasts much longer (159). This again supports the reactive adsorption mechanism (154). A different type of deactivation was reported as being due to “disordered” and “ordered” surface carbonaceous deposits (138, 148). With partially carbonized catalysts, a competition takes place between hydrogen and gaseous hydrocarbons for the “free” surface sites ; therefore, the hindering effect of higher hydrogen pressures becomes much more marked (159).

3. Surface Charge Approach There are conflicting views as far as the charge of surface hydrogen is concerned. Both H + and H- have been assumed (160). Surface potential measurements indicate the presence of two types of hydrogen (161). Later studies suggested partial negative charge at hydrogen species on [110] planes (162) and on steps (156);and a partial positive charge for hydrogen on [l 111 planes (156). The reasoning was based on the disturbance of the electron gas of the metal by hydrogen particles (Fig. 14). No full ionization of hydrogen was assumed here. The position of the hydrogen atom with respect to the phase boundary (surface or subsurface) may also be important (1574 163).

METAL-CATALYZED CYCLIZATION REACTIONS OF HYDROCARBONS

327

FIG.14. Schematic model of adsorption of hydrogen on a stepped platinum surface [Pt(997) or Pt(S) - 9(111) x (1 1 l)]. Shading denotes various partial charge of hydrogen atoms (256).

Our present knowledge of the possible charges carried by different adsorbed hydrocarbon species is very limited. Attempts to correlate the position of primary adsorption of the hydrocarbon with the partial charge of different carbon atoms (8,68a, 71)have not led thus far to unambiguous results. Future research-excluding any support activation by hydrogen-may being some significant breakthrough in this field. Various “astoichiometric” components (hydrogen, carbon, and others, for example, silicium and aluminum) present may interact with localized and nearly free electrons to differing extents. According to the localized free electron interplay model of metal catalysts developed by Knor (163, 164) the ratio of the two types of electrons may influence the catalytic properties considerably. For example, a subsurface proton attracts nearly free electrons and thus uncovers some localized orbitals. Carbon may interact first with localized electrons (164). This may be one of the reasons why their effects are of opposite character. The collective efforts of catalytic and surface chemists are necessary to bring some clarity to the multitude of problems arising here. 4. Surface Reconstruction Approach Hydrogen may promote surface rearrangement in a way that surfaceactive sites be created for one or another type of reaction. Hydrogen occupying most interstitial sites may promote rehybridization of surface orbitals (165) so that an adsorption shown in Fig. 12 might be possible. Even migration of surface atoms is not excluded. Maire et al. (166) reported LEED studies that indicated the reconstruction of surface step structure when hydrogen gas is added to the system. Finely dispersed platinum black (having a mean crystallite size of about 8-10 nm) sinters to a low-surface, large crystallite species under the effect of a few hours’ treatment in hydrogen at 200”-400”C (167). A difference of 50”-60°C in the temperature of the treatment may result in a difference in the crystallite size by a factor of 2. These changes are very rapid: literally a few minutes in 1 atm hydrogen is sufficient to cause a complete recrystallization of platinum black at temperatures of 1300”-1500°C below its melting point. The penetration of hydrogen into the crystal lattice occurs through discrete surface “gates” (168). Lang

328

ZOLTAN

PAAL

et al. (169)reported these to be identical with stepped surfaces. The resulting crystallites may have ordered and less ordered regions in varying ratios : the latter type has been claimed to be responsible for hydrogen occlusion in the case of powders of Group VIIIB metals (170). With the knowledge of the role of hydrogen in selectivity, the structure after recrystallization is of paramount importance. Thus, sintering of platinum black in hydrogen at 200°C (Pt-200) gives smaller and less developed crystallites, whereas a similar procedure at 360°C (Pt-360) results in larger crystallites of regular shape. Pt-200 favors C,-cyclic reactions, obviously owing to its higher hydrogen content. n-Hexane at 300°C in hydrogen stream gave the following selectivity percentages (171) :

Pt-200 Pt-360

< Cs

Isomer hexane

Methylcyclopentane

Benzene

60 70

26 12.5

12 5

2 12.5

Further studies are necessary to clarify how different treatments affect not only the size but also the structure of metal crystallites. C. CONCLUDING REMARKS

It is possible to trace back the two types of cyclization to two basic types of primary surface species whose structures have been interpreted in agreement with the atomistic picture of the catalytic surface. The nature of primary adsorption is governed by the amount of hydrogen. Uniform types of hydrogen effects with various unsupported and supported catalysts (e.g., 92) point to the existence of similar structures and, consequently, to similar catalytic activity. The temperature and pressure ranges favorable for these structures increase in the following order : single crystals, massive metals (films or blacks), and disperse supported catalysts-in agreement with common experience. On this basis, it is possible to develop a generalized concept for the cyclization activity of catalytic systems consisting of metals and hydrogen (and, possibly, also carbon). ACKNOWLEDGMENTS The author wishes to express his sincere gratitude to Professor P. Tetenyi for help and encouragement during several years of collaboration as well as during preparation of this manuscript. He is also indebted to Dr. S. M. Csicsery for valuable advice. Stimulating discussions with several colleagues from various countries are gratefully acknowledged.

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METAL-CATALYZED CYCLIZATION REACTIONS OF HYDROCARBONS

62d. 62e. 625 63. 64. 65. 66. 66a.

666. 66c. 67.

68. 68a. 69. 70. 71. 72. 73. 74. 75. 75a. 76. 77. 78. 79. 80.

81.

82. 83. 84. 85. 85a. 86. 87.

88. 89. 90. 90a.

90b. 91.

33 1

Rozanov, V. V., Gland, J., and Sklyarov, A. V., Kinet. Katal. 20, 1249 (1979). Sklyarov, A. V., Krylov, 0. V., and Keulks, J., Kinei. Kaial. 18, 1487 (1977). Tktenyi, P., and Babernics, L., J . Catal. 8,215 (1967). McHenry, K. W., Bertolacini, R. J., Brennan, H. J., Wilson, J. L., and Seelig, H. S., Actes Congr. Ini. Catal., 2nd, 1960, Vol. 11, p. 2292 (1961). Kluksdahl, H. E., and Houston, R. J., J. Phys. Chem. 65, 1469 (1961). Sivasankar, S., Ramaswamy, A. V., and Ratnasamy, P., J . Catal. 46,420 (1977). Volter, J., Plenary Lect., Symp. Supported Metal Cutalysts Hydrocarbon Conversions, 1978, Vol. 11, p. 153 (1979). Bursian, N. R., Kogan, S. B., and Davydova, Z . A,, Kinei. Kutal. 9,661 (1968). Bursian, N. R., Kogan, S. B., Davydova, Z. A,, and Yakovlev, A. A,, Kinet. Katal. 15, 1486 (1974). Bursian, N . R., Kogan, S. B., and Bolshakov, P. P., Kinet. Karal. 17, 1548 (1976). Rozengart, M. I., Polinin, V. L., Bryukhanov, V. G., and Kazansky, B. A., Dokl. Akad. Nauk SSSR 187,585 (1969). Levitsky, I. I., Minachev, Kh. M., and Udaltsova, E. A,, Izv. Akad. Nauk SSSR, Ser. Khim. p. 300 (1973). Levitsky, I. I., and Minachev, Kh. M., in “Mechanisms of Hydrocarbon Reactions. A Symposium” (F. Marta and D. Kallo, eds.) p. 81. Akademiai Kiado, Budapest, 1975. Davis, B. H., J . Catal. 42, 376 (1976). Davis, B. H., Westfall, G. A,, and Naylor, R. W., J . Catal. 42, 238 (1976). Davis, B. H., Westfall, G. A,, Watkins, J., and Pezzanite, J., J . Catal. 42,247 (1976). Bond, G. C., and Sermon, P., Gold Bull. 6, 102 (1973), cited by Davis et al. (71). Altham, J. A,, and Webb, G., J . Catal. 18, 133 (1968). Berndt, H., Mehner, H., Volter, J., and Meisel, W., Z . Anorg. Ally. Chem. 429,47 (1977). Palazov, A,, private communication. Muller, A. C., Engelhard, P. A., and Weisang, J. E., J . Catal. 56,65 (1979). Szebenyi, I., Ackermann, L., Szkchy, G., and Gobolos, S . , Acta Chim. Acad. Sci. Hung. 90,313 (1976); Szebenyi, I., and Szechy, G., ibid. 98, 115 (1978). Paal, Z . , Matusek, K., and Tetenyi, P., Acta Chim. Acad. Sci. Hung. 94, 119 (1977). Paal, Z., and Tetenyi, P., Kem. Kozl. 43, 165 (1975). Liberman, A. L., Vasina, T. V., and Kazansky, B. A,, Dokl. Akad. Nauk SSSR 111,430 (1957). Kazansky, B. A,, Liberman, A. L., and Lapshina, T. V., Dokl. Akad. Nauk SSSR 105, 727 (1955). Vlasov, V. G., Barkar, E. P., Fomichov, Yu. V., and Kazansky, B. A,, Dokl. Akad. Nauk SSSR 202,837 (1972). Garin, F., and Gault, F. G., J . Am. Chem. Soc. 97,4466 (1975). Bragin, 0. V., Tovmasyan, V. G., Isagulyants, G. V., Greish, A. A., Kovalenko, L. I., and Liberman, A. L., Izv. Akad. Nauk SSSR, Ser. Khim. p. 2041 (1977). Paal, Z . , and Tetenyi, P., Acta Chim. Acad Sci. Hung. 72,277 (1972). Paal, Z., D.Sc. Thesis, Hung. Acad. Sci., Budapest, 1977. Amir Ebrahimi, V., Garin, F., Weisang, F., and Gault, F. G., Nouu. J . Chim. 3,529 (1979). Anderson, J. R., and Avery, N . R., J . Catal. 5, 446 (1966). Muller, J. M., and Gault, F. G., Presented at the Fourth Int. Congr. Catal., Moscow, 1968. Symposium: “Kinetics and Mechanism of Complex Catalytic Reactions,” Paper 15. McKervey, M. A,, Rooney, J. J., and Samman, N. G., J . Caral. 30, 330 (1973). Van Schaik, J. R. H., Dessing, R. P., and Ponec, V., J . Catal. 38, 273 (1975). Csicsery, S. M., and Pines, H., Chem. Ind. (London) p. 1398 (1961). Pines, H., and Csicsery, S . M., J. Catal. 1, 313 (1962). Bartok, M., J . Chem. Soc., Chem. Commun. p. 139 (1979). Paal, Z . , and Tetenyi, P., J . Cutul. 29, 175 (1973).

332

ZOLTAN

PAL

91a. Paal, A., and Tetenyi, P., to be published. 92. Bragin, 0. V., Karpinski, Z., Matusek, K., Paal, Z., and TCtenyi, P., J . Catal. 56, 219 (1979). 93. Muller, J. M., and Gault, F. G., Proc. Int. Congr. Catal., Sth, 1972 Vol. I , p. 743 (1973). 94. Pines, H., Intra-Sci. Chem. Rep. 6, 24 (1972). 94a. Pines, H., and Goetschel, C. T., J . Am. Chem. Soc. 87,4207 (1965). 94b. Davis, B. H., J . Catal. 23, 340 (1971). 95. Eliel, E . L., Allinger, N. L., Angyal, S. J., and Morrison, G. A., “Conformation Analysis,” Wiley (Interscience), New York, 1966; Allinger, N. L., Tribble, M. T., Miller, M. A,, and Wertz, D. H., J. Am. Chem. Soc. 93, 1637 (1971). 96. Tetenyi, P., Guczi, L., Paal, Z., and Sarkany, A., Kem. Kozl. 47,363 (1977). 96a. Sarkany, A,, Matusek, K., and Tetenyi, P., J. Chem. Soc., Faraday Trans. 1 73, 1699 (1977). 97. Paal, Z . , and Dobrovolszky, M., React. Kinef. Catal. Lett. 1,435 (1974). 970. PaaI, Z., Dobrovolszky, M., and Tetenyi, P., React. Kinet. Catal. Lett. 2,97 (1975). 98. Liberman, A. L., Bragin, 0.V., Guryanova, G. K., and Kazansky, B. A., Izv. Akad. Nauk SSSR, Ser. Khim. p. 1737 (1963); Bragin, 0.V., Olferyeva, T. G., Preobrazhensky, A. V., Liberman, A. L., and Kazansky, B. A,, Dokl. Akad. Nauk SSSR 202,339 (1972). 99. Miki, Y., Yamadaya, S., and Oba, M., J. Catal. 49, 278 (1977). 100. Lukina, M. Yu., Olferyeva, T. G., Bragin, 0. V., Liberman, A. L., and Kazansky, B. A., Dokl. Akad. Nauk SSSR 193,106 (1970); Kazansky, B. A., Bragin, 0. V., HelkovskayaSergeeva, E. G., and Liberman, A. L., ibid. 214, 103 (1974). 101. Maire, G., Plouidy, G., Prudhomme, J. C., and Gault, F. G., J. Catal. 4, 556 (1965). 101a. Bragin, 0. V., Helkovskaya-Sergeeva, E. G., and Liberman, A. L., lzu. Akad. Nauk SSSR, Ser. Khim. p. 1037 (1979). 102. Bragin, 0.V., and Liberman,A. L., Proc. Int. Congr. Catal.,4th, 1968Vol.I, p. 351 (1971). 103. Vedenyapin, A. A,, Balenkova, E. S., Bragin, 0. V., and Kazansky, B. A., Dokl. Akad. Nauk SSSR 191, 1053 (1970). 104. Paal, Z., Dobrovolszky, M.,and Tetenyi, P., All-Union Conj. Catalytic React. Liquid Phase, 1974, Vol. I , p. 187 (1974). 104a. Balenkova, E. S., Khromov, S. I., Shokova, E. A,, Kucheryavaya, N. N., Sterin, Kh. E., and Kazansky, B. A., Neftekhimiya 2, 275 (1962). 105. Kazansky, B. A., Shokova, E. A., Khromov, S. I., Aleksanyan, V. T., and Sterin, Kh. E., Dokl. Akad. Nauk SSSR 133, 1090 (1960); Vedenyapin, A. A., Balenkova, E. S., Khromov, S. I., and Kazansky, B. A,, Neftekhimiya 9, 821 (1969). 105a. Khromov, S. I., Balenkova, E. S., Lishenok, 0. E., and Kazansky, B. A., Dokl. Akad. Nauk SSSR 135,627 (1960). 106. Mirzaeva, A. K., Elagina, N. V., Sterin, Kh. E., and Kazansky, B. A,, Neftekhimiya 2, 31 (1962). 1060. Elagina, N . V., Mirzaeva, A. K., Sterin, Kh. E., and Kazansky, B. A,, Neftekhimiya 3, 663 (1963). 106b. Elagina, N . V., Mirzaeva, A. K., Sterin, Kh. E., Bobrova, A. V., and Kazansky, B. A,, Neftekhimiya 4, 241 (1964). 107. Davis, B. H . , J. Catal. 46, 348 (1977). 108. Lester, G. R., J . Catal. 13, 187 (1969). 109. Callender, W. L., Brandenberger, S. G., and Meerbott, W. K., Proc. Int. Congr. Catal., Sth, 1972 Vol. 11, p. 1265 (1973). 110. Kazansky, B. A., and Liberman, A. L., WorldPet. Congr., Proc., 8th, 1971 Vol. IV, p. 395 (1971). I l l . Selman, D. M., and Voorhies, A,, Ind. Eng. Chem., Prod. Res. Deu. 14, 118 (1975). 112. Brandenberger, S. G., Callender, W. I., and Meerbott, W. K., J . Catal. 42, 282 (1976).

METAL-CATALYZED CYCLIZATION REACTIONS OF HYDROCARBONS

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112a. Christoffel, E . G., and Rohschlager, K.-H., Ind. Eng. Chem. Proc. Res. Dev. 17, 331 (1978). 113. Sinfelt, J . H., and Rohrer, J. C., J . Phys. Chem. 65, 978 (1961). 114. Weisz, P. B., Ado. Catal. 13, 137 (1962). 115. Janowski, F., and Wolf, F., Chem. Tech. (Leiprig) 29,273 (1977). 116. Ciapetta, F. G., and Hunter, J. B., Ind. Eng. Chem. 45, 159 (1953); Ciapetta, F. G., ibid. p. 162. 117. Weisz, P. B . , and Swegler, E. V., Science 126, 31 (1957); Hindin, S. G., Weller, S. W., and Mills, G . A., J. Phys. Chem. 62,244 (1958). 118. Smith, R. L., Naro, P. A., and Silvestri, A. J., J . Catal. 20,359 (1971). 118a. Silvestri, A. J., Naro, P. A., and Smith, R. L., J. Catal. 14,386 (1969). 119. Sterba, M. J., and Haensel, V., Ind. Eng. Chem., Prod. Res. Dev. 15, 2 (1976). 120. Sinfelt, J. H., Hurwitz, H., and Rohrer, J. C., J . Catal. 1,481 (1962). 121. Mills, G . A,, Heinemann, H., Milliken, T. H., and Oblad, A. G . ,Ind. Eng. Chem. 45, 134 (1953). 122. Fogelberg, L.-G., Gore, R., and Ranby, B., Acta Chem. Scand. 21, 2041, 2050 (1967). 123. Kazansky, B. A., Fomichov, Yu. V., and Gostunskaya, I. V., Izv. Akad. Nauk SSSR, Ser. Khim. p. 1112 (1968). 123a. Kazansky, B. A., Gostunskaya, I. V., Solonina, L. S., and Fadeev, V. S . , Zzv. Akad. Nauk SSSR, Ser. Khim. p. 683 (1970). 124. Vlasov, V. G . , Fomichov, Yu. V., and Kazansky, B. A., Izir. Akad. Nauk SSSR, Ser. Khim. p. 65 (1970); Neffekhimiya 10,821 (1970); Ilyin, V. F., and Usov, Yu.N., ibid. 13, 387, 514 (1973). 125. Davis, B. H., J . Catal. 29, 398 (1973). 126. Maatman, R. W., Ind. Eng. Chem. 51, 913 (1959). 127. Kazansky, B. A., Gostunskaya, I. V., and Fomichov, Yu. V., Dokl. Akad. Nauk SSSR 180,383 (1968). 128. Noguiera, L., and Pines, H., unpublished results, cited by Pines, H., Proc. Int. Congr. Catal., 5th, 1972 Vol. 11, p. 1275 (1973). 129. Pines, H., and Greenlee, T. W., J . Org. Chem. 26, 1052 (1961). 130. Yermakov, Yu. I . , and Kuznetsov, B. N., Kinet. Katal. 18, 1167 (1977). 131. Den Otter, G. J., and Dautzenberg, F. M., J . Catal. 53, 116 (1978). 131a. Dessing, R. P., and Ponec, V., React. Kinet. Catal. Left. 5,251 (1976). 132. Chen, C.-T., Haag, W. O., and Pines, H., Chem. Ind. (London)p. 1379 (1959); Pines, H., and Chen, C.-T., J . Org. Chem. 26, 1057 (1961). 132a. Pines, H., and Chen, C.-T., Acres Congr. Int. Catal., 2nd, 1960 Vol. I , p. 367 (1961). 132b. Pines, H . , and Chen, C.-T., J. Am. Chem. Soc. 82,3562 (1960). 133. Eidus, Ya. T., Kinef. Katal. 11,422 (1970); 15, 1212 (1974). 134. Bartok, M . , React. Kinet. Catal. Lett. 3, 115 (1975); Acta Phys. Chem. 21,79 (1975). 134a. Thomson, S . J . , and Webb, G., J . Chem. Soc., Chem. Commun. p. 526 (1976). 135. Bond, G. C., Discus. Faraday Soc. 41,200 (1966). 136. Knor, Z . , Adv. Catal. 22,51 (1972). 137. Van Hardeveld, R., and Hartog, F., Adv. Catal. 22, 75 (1972). 138. Lang, B., Joyner, R. W., and Somorjai, G. A., J . Catal. 27,405 (1972). 139. Lang, B., Legare, P., and Maire, G., Surf. Sci. 47,89 (1975); Lang, B., ibid. 53,317 (1975). 140. Balandin, A. A,, 2. Phys. Chem., Abt. B 2, 289 (1929); Ado. Catal. 19, 1 (1969). 141. Kesmodel, L. L., Baetzold, R. C., and Somorjai, G. A,, Surf. Sci. 66,299 (1977). 141a. Stair, P. C., and Somorjai, G. A., J . Chem. Phys. 66, 2036 (1977). 142. Palazov, A,, J . Cafal.30, 13 (1973). 1420. Shopov, D., Andrew, A,, and Palazov, A,, Izv. Otd. Khim. Nauki (Bulg. Akad. Nauk) 2, 321 (1969).

334

ZOLTAN PAAL

143. Tournayan, L., Bacaud, R., Charcosset, H., and Leclercq, G., J . Chem. Res. (M) p. 3582 (1978). 144. Paal, Z., Proc. Int. Congr. Catal., 6th, 1976 Vol. 11, p. 926 (1977). 144a. Charcosset, H., Frety, R., Leclercq, G., Moraweck, B., Tournayan, L., and Varloud, J., React. Kinet. Catal. Lett. 10, 301 (1979). 145. Dyne, S. R., Butt, J. B., and Hailer, G. L., J . Cafal.25, 378 (1972). 145a. Anderson, J. R., and Mainwaring, D. E., Znd. Eng. Chem., Prod. Res. Deo. 17,202 (1978). 146. Blakely, D. W., and Somorjai, G. A,, Surf. Sci. 65,419 (1977). 147. Nieuwenhuys, B. E., and Somorjai, G. A., J . Catal. 46,259 (1977). 147a. Blakely, D. W., and Somorjai, G. A,, J . Cufal.42, 181 (1976). 148. Somorjai, G. A,, Adu. Catal. 26, 168 (1977). 149. Dartigues, J.-M., Chambellan, A,, and Gault, F. G., J . Am. Chem. Soc. 98, 856 (1976). 150. Montarnal, R., and Martino, G., Rev. Znst. Fr. Pet. 32, 367 (1977). 151. Anderson, J. R., “Structure of Metallic Catalysts.” Academic Press, New York, 1975. 152. Baer, Y., Heden, P. F., Hedman, J., Klasson, M., Nordling, C., and Siegbahn, K., Phys. Ser. 1, 55 (1970). 153. TCtCnyi, P., Guczi, L., and Paal, Z., Acta Chim. Acad. Sci. Hung. 83, 37 (1974). 154. Frennet, A,, Lienard, G., Crucq, A,, and Degols, L., J . Catal. 53, 150 (1978). 155. Tsuchiya, S., Amenomiya, A., and Cvetanovic, R. J., J . Catal. 19, 245 (1970). 155a. Moger, D., Besenyei, G., and Nagy, F., Magy. Kem. Foly. 81,284 (1975). Z55b. Paal, Z., and Thomson, S. J., J . Catal. 30,96 (1973). 155c. Menon, P. G., and Froment, G. F., J . Cutal. 59, 138 (1979); Menon, P. G., private communication. 156. Christmann, K., and Ertl, G., Surf. Sci.60,365 (1976). 157. Behm, J., Christmann, K., and Ertl, G., Solid State Commun. 25,763 (1978); Christmann, K., and Ertl, G., private communication. 157a. Christmann, K., Ertl, G., and Pignet, T., Surf. Sci. 54, 365 (1976). 158. Gorodetskii, V. V., andsavchenko, V. I., Proc. Int. Congr. Cutal.,Sth, 1972Vol. I, p. 513 (1973). 158a. Gorodetskii, V. V., Sobyanin, V. A,, Bulgakov, N. N., and Knor, Z., Surf. Sci. 82, 120 (1979). 159. Paal, Z., Dobrovolszky, M., and Tethyi, P., J. Catal. 46, 65 (1977). 160. MacKay, K . M., “Hydrogen Compounds of the Metallic Elements.” Spon, London, 1966. 161. Dus, R., andTompkins, F. C., J . Chem. Soc., Faruday Trans. 171,930 (1975); Nieuwenhuys, B. E., Surf. Sci. 59,430 (1976). 162. Miiller, E. W . ,Surf. Sci. I, 462 (1967). 163. Knor, Z . , Surf. Defect Prop. Solids 6, 139 (1977). 164. Knor, Z., Kinet. Katal. 21, 17 (1980). 165. Hayward, D. O., in “Chemisorption and Reactions on Metallic Films” (J. R. Anderson, ed.), Vol. I , p. 224. Academic Press, New York, 1971. 166. Maire, G . , Bernhardt, P., Legare, P., and Lindauer, G., Proc. Int. Vac. Congr., 7th, 1977 Vol. I, p. 861 (1977). 167. Baird, T., Paal, Z., and Thomson, S . J., J . Chem. Soc., Faraday Trans. 1 69, 50, 1237 (1973). 168. Gibb, T. R. P., Prog. Znorg. Chem. 3, 315 (1962). 169. Lang, B., Joyner, R. W., and Somorjai, G. A,, Surf. Sci. 30,454 (1972). 170. Wells, P. B., J . Cotal. 52, 498 (1978). 171. Paal, Z., Barna, A., Barna, B. P., and Toth, L.. to be published.

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 Abbas, N., 32(109), 38(127), 52,53 Abd El-Salaam, K. M., 125(55), 132(55), 149 Ackermann, L., 291(76), 331 Adams, C . R., 188, 196 Adams, D. L., 17(62), 18(62), 51 Addink, C . , 137(109), 150 Affrossman, S., 81(69), 95 Ahuja, V . K., 173, 195 Albers, H., 39(135), 53 Aldag. A. W., 110, 111, 125(13), 132(13), 138, 148

Aoyama, Y., 207(68,69), 226 Arai, H., 129(77), 132(77), 143, 150 Arnett, R. L., 277(28), 289(28), 329 Artamov. A. A,, 187(143), 196 Asano, T., 230(26). 271 Atkinson, G. B., 127(63), 132(63), 149 Atkinson, S. J., 66(21, 22), 80(22), 81(22), 93 Au, C . T., 80(66), 81(66), 82(66), 87(66, 68), 88(SS), 94, 95 Auger, P., 7, 8, 10,50 Avdeyev, V. I., 187, 196 Avery, N. R., 297, 331 Avilova, I. M., 129(79), 132(79), 150

Alekseeva, V. I., 176(91), 195 Alexanyan, V. T., 278(38, 38a), 292(38, 38a), 295(38, 38a), 306(105), 330, 332 B Ali, D., 163(64), 165(64), 194 Babernics, L., 287(62f), 331 Al-Joboury, M. I., 58(4), 93 Bacaud, R., 319(143), 320(143), 333 Alkahazov, T. G., 129(82), 132(82), I50 Bachrach, R. Z . , 78(61), 94 Allinger, N . L., 303(95), 332 Bacon, D. W., 132(106), 150 Al-Rawi, H., 220(1), 224 Baer, Y., 322(152), 334 Altham, J. A., 290(73), 331 Baetzold, R. C . , 98(4), 148, 318(141), 319 Amenomiya, A,, 325(155), 334 (141), 333 Amir-Ebrahim, V., 296(85a), 331 Baird, M. J., 124(44), 132(44), 149 Amirnazmi, A,, 123(27). 132(27), 149 Baird, T., 327(167), 334 Andeev, A,, 188, 196 Baker, A. D., 3(14), 50 Anderson, J. R., 274(7), 275(7), 279(7), 297, Baker, J. M., 14(42), 51 298, 321, 322(151), 329, 331, 334 Baker, R. H., 176(90), 195 Anderson, S., 18(73), 52 Andrew, A,, 185, 186(138), 187(147). 196, Baker, R. W., 278(32), 329 Balandin, A. A,, 187, 196, 278(35), 318, 322, 3 19(142a), 333 330,333 Andreu, P., 163(50, 63), 164(50), 165(63), 170 Balenkova, E. S., 274(1 I), 303(1 I ) , 306(1 I , (63), 194 103, 104a, 105), 307(11), 309(105a), 329, Angyal, S. J., 303(95), 332 332 Anju, Y., 163(42), 164(42), 167, 170(66), 171 Balenkova, J. C., 176(91), 195 (42), 194 Balikova, M., 124(35), 132(35), 149 Aomura, K., 163(62), 165(62), 194 Ballivet, D.. 139(112). 150 Aonuma, T., 180(112), 181, 195 335

336

AUTHOR INDEX

Banthorpe, D. V., 167(67), 194 Barkar, E. P., 295(81), 331 Barna, A., 328 (171), 334 Barna, B. P., 328(171), 334 Barnard, E. A., 208(2), 224 Barron, Y., 275(15a), 292,294(15a), 296(15a), 329 Bart, C. J., 261(55), 262(55), 263(55), 265(55, 60), 266(60), 271, 272 Barteau, M. A,, 6(27), 28(102), 31(102), 39 (136), 48(144), 5 0 , 5 2 , 5 3 Barthomeuf, D., 139(112), 150 Bartok, M., 298(906), 318(134), 331,333 Bartsch, R., 132(97), 133(97), 145, 150 Basinski, S., 170(72), 171(72), 194 Basset, J., 125(46), 132(46), 149 Batts, B. L., 219,224 Batuev, M. I., 279(47), 300(47), 330 Bauer, E., 4(18), 15(59), 50,51 Bauer, R. S., 78(61), 94 Baiant, V., 163(41, 61), 164(41), 165(61), 168 (41), 170(71), 176(71), 182(120, 123), 184, 186(142), 193, 194, 195, 196 Behm, J., 326(157), 334 Belomestnykh, I. P., 187(144), 196 Belousov, V. M., 186,196 Bender, M. L., 199(4, 6,27,46), 201,202, 203, 205, 208(7), 214, 223, 224, 224, 225, 226, 227 Bennett, C . O., 123(26), 132(26, loo), 133 (loo), 149, 150 Bennett, G. B., 208(91), 226 Benovic, S. J . , 199(15), 224 Benson, S. W., 48(143), 53 Bensiger, J. B., 6(28), 7(30a), 9(30a), 10(31), 14(45, 46, 47), 17(45, 60, 61), 19(30a), 24 ( 9 9 , 25(96), 26(96, 98), 27(98), 30(61), 32(1l I), 35(30a, 9 9 , 38(95, 125), 43(95), 50, 51, 52, 53 Beranek, L., 124(35), 132(35), 149, 155(13), 160(37), 163(13), 166(13), 167, 168, 170 (71, 74), 171(74), 176(71), 182(118), 189 (74), 190(74), 193, 194, 195 Bergmark, T., 11(33), 14(33), 51 Bernard, J. R.,131(94), 132(94), 150 Berndt, H., 290(74), 319(74), 331 Bernhardt, P., 327(166), 334 Bertolacini, R. J., 288(63), 330 Bertolini, J. C . , 86, 95 Besenyei, G., 325(155a), 334

Best, D. A , , 102, 148 Betteridge, D., 3(14), 50 Beyer, H., 129(75), 132(75), 150 Bickley, R. I., 67(30), 93 Biloen, P., 125(50), 132(50), 139, 149 Blakely, D. W., 67(33), 93, 321(146), 322. 334 Blassini, O., 163(50), 164(50), 194 Blout, E. R., 208(39), 225 Bobianko, I. I., 188, 196 Bobrova, A. V., 309(106b), 310(106b), 311 (106b), 332 Bogdanova, 0. K., 187(144), 196 Bohn, G. K., 7(30), 50 Bolshakov, P. P., 288(66c), 331 Bonczek, F., 15(59), 51 Bond, G. C., 290(72), 318,331, 333 Bonzel, H. P., 72(49), 94 Bootsma, G. A., 39(135), 53 Bordass, W. T., 58, 93 Boreskov, G. K., 72(50), 94 Bork, A. C . , 184(129), 196 Boronin, A. I., 72(50), 94 Boudart, M., 32(110), 52, 118(15), 123(27), 127(69), 131(89, 90), 132(27, 69, 89, 90), 137,142, 14a,149,150. 152,192 Bowker, M., 11(38), 12(38). 23(92). 25(92), 28(102), 31(102, 105), 36(122), 37(122), (39(136), 48(141, 144), 52, 52,53 Bowman, R.G., 265,272 Bradley, D. C . , 46(140), 53 Bradshaw, A. M., 68(37), 94 Bragg, J., 21(86), 52 Bragin, 0. V., 276(26), 283(58), 292(26), 295, 296(26, 83), 298(92), 303(98), 304, 306 (102, 103), 328(92), 329, 330, 331, 332 Braithwaite, M. J . , 62(15), 75(15), 79(15), 90 (13,93 Brandenberger, S. G., 312(109, 112), 313(109), 332 Brass, H. J., 202,224 Braun, R. M., 277(28), 289(28), 329 Brennan, H. J., 288(63), 331 Breslow, R.,202, 203, 204, 205(9), 222, 224, 225 Breza, J., 80(66), 81(66), 82(66), 87(66), 94 Brihta, I., 184(130), 196 Brill, W. W., 229(5), 270 Broukers, R.,37(123), 53 Brown, C. A,, 173, 195 Brown, H. C . , 170, 194, 232(40), 271

331

AUTHOR INDEX

Brown, W. G., 233(43), 271 Brucker, C. F., 11(35), 12(41), 51 Bruice, T. C., 199(15),208(16), 224, 224 Brundle, C. R., 3(7), 13(43), 14(43), 17(43), 50, 51, 57(3, 3a), 58(3, 3a), 61(3a), 62(3), 66(21, 22, 26), 75(56), 77(59), 80(3, 3a, 22, 65), 81(22, 65), 91(94), 92(94), 93, 94, 95 Brunelle, J . P., 278(39), 330 Bryukhanov, V. G., 283(55), 289(67), 330,331 Bulanova, T. F., 278(38), 292(38), 295(38), 330 Bulgakov, N. N., 326(158a). 334 Bursian, N. R., 288, 331 Burwell, R. L., Jr., 131(88), 132(88), 150, 155 ( I 6), 193, 265,272 Butt, 3. B., 131(88), 132(88), 150,321(145),334 Butt, P. V., 123(24), 125(24), 132(24), 136, 137 (24), 148

C Caccamese, S . , 234(50), 271 Callender, W. L., 312(109, 112), 313(109), 332 Campadelli, F., 265(60), 266(60), 272 Campbell, P., 202, 224 Cant, N. W., 129(76), 132(76), 143, 150 Carley, A. F., 59, 61, 70(45), 71(46), 77(10, 59), 78(10), 79(10). 89(92), 93, 94, 95 Carra, S., 124(43), 142(43), 149 Carrioza, I., 124(33), 132(33), 138, 149, 163 (44), 164(44), 194 Cashion, J. K., 58, 65, 93 Castellan, A., 261(55), 262(55), 263(55), 265 ( 5 5 , 60), 266(60), 271,272 Castner, D . D., 4(22), 7(22). 50 Catry, J. P., 163(48), 164(48), 189(48), 190 (48), 194 Catterick, T., 18(71), 52 Cederbaum, L. S., 68(37), 94 Cerveny, L., 159(33), 173, 174(33), 193, 194, 195 Chakravarty, P. K., 209(17), 224 Chambellan, A., 322(149), 334 Charcosset, H., 319(143), 320(143), 321( 144a), 333,334 Chebotareva, N. P..129(71), 132(71), 150 Chen, C.-T., 316(132, 132a), 317, 321(132, 132a, 132b), 333 Chinomi, K., 86(86a), 95 Cho, I., 213(77), 226

Chou Chin-Shen, 170(71), 176(71), 194 Christmann, K., 326(156, 157, 157a), 327 (156), 334 Christoffel, E. G., 277(30), 312(112a), 313(30), 329 Chuang, T. J., 91, 92(94), 95 Chuvylkin, N. D., 139(113), 150 Ciapetta, F. G., 278(32), 312(116), 329, 333 Clark, A., 110, 111, 125(13), 132(13), 138, 148 Clarke, J. K. A,, 155, 193, 274(8), 275(8), 276 (8), 278(41), 279(8, 43, 46), 289(8), 297 (8), 304, 323(8), 327(8), 329, 330 Clarke, T. A,, 65(19), 85, 93, 95 Clowes, G. A., 201,226,227 Cohen, J. B., 131(88), 132(88), 150 Collins, D. M . , 68(38), 92(38), 94 Congdon, W. I.. 218(40), 225 Conner, W. C., Jr., 124(40), 132(40), 138(40), 149 Corson, B. B., 230(29), 271 Craig, R. S., 127(67), 132(67), I49 Cramer, F., 199(18), 200,203,224,225 Criado, J. M., 21(78), 52, 159(35), 161, 170, 172, 193 Crocker, A,, 21(85), 52 Crucq, A,, 324(154), 326(154), 334 Csicsery, S. M., 274(12, 13), 279,289(12), 298, 309(12), 317(90, 90a), 328, 329, 330, 331 Cutlip, M. B., 132(100), 133(100), 150 Cvetanovic, R. J., 132, 133(107), 150, 325 (155), 334 Czarnieki, M . F., 202,225 Cziiros, Z., 170(73), 171(73), 194

D Dalla Betta, R. A., 127(61), 132(61), 141, 149, 150 Dartigues, J.-M., 322(149), 334 Dashevskii, M . M . , 232(41), 233(41), 271 Datka, J., 131(93), 132(93), 150 Dautzenberg, D., 169,194 Dautzenberg, F. M., 125(50), 132(50), 139, 149,276(23), 284,288,315,316(131), 329, 333 Davidova, H., 176(92), 195 Davidson, D., 233(46), 271 Davis, B. H., 289, 290, 291(69, 70, 711, 302, 311(107), 315(70, 94b, 107, 125). 323 (107), 327(71), 331, 332, 333

338

AUTHOR INDEX

Davis, L. E., 216(23), 225 Davison, C. J., 3(1), 50 Davydova, 2. A , , 288(66a, 66b), 331 Deak, G., 170(73), 171(73), 194 Degols, L., 324(154), 326(154), 334 de Jongste, H. C., 274(14), 279(14), 321, 329 de Kreuk, J. F., 183, 195 Demuth, J. E., 4(20a, 23), 7(20a), 11(36), 12 (40), 14(42), 36(121), 50, 51, 53, 85, 86 (85),95

Deneuville, A , , 88(89), 95 Den Otter, G. J., 316(131), 333 Derouane, E. G., 191(160), 196 Dessing, R. P., 298(89), 316,320(89), 331,333 Dewit, A,, 37(123), 53 Dhar, M. M., 209(17, 59, 60), 224, 225 Diakova, M. K., 176(94), 177(94), 178(94), 179, 195 Dickinson, J. T., 28( I O I ) , 52 Dignam, M., 18(72), 52 Dimitrov, C.. 124(42), 132(42), 149 Dixon, J . K., 38(128), 39(128), 53, 163(59), 165(59), 194 Dobres, R. M., 278(32), 329 Dobrovolny, F. J., 230(22), 271 Dobrovolszky, M., 276(25, 36), 293(25), 294 (25, 36), 295(25), 298(25), 303(97, 97a), 304, 305(97a), 306(104), 319(25), 323 (36), 324(25), 326(97,97a, 159), 329, 330, 332,334 Doherty, J. B., 203(12), 224 Dolgov, B. N., 180(1lo), 195 Domcke, W., 68(37), 94 Dowden, D. A., 82,95 Drayer, D. E., 229(8), 270 Dubinsky, Y. G., 283(55), 330 Duc, T. M., 88(89), 95 Duchet, J. C., 185, 196 Duell, M., 21(89), 52 Duke, C. B., 4(20), 50 Dumsha, T. C., 222(86), 226 Dunn, I. J., 160(38), 161, 193 Dus, R., 326(161), 334 Dwyer, D. J., 127(64), 132(64), 149 Dyatlov, A. A., 122(16), 132(16), 148 Dyne, S. R., 321(145), 334

E Eastman, D. E., 3(6), 11(36), 14(42), 50. 51, 58, 65, 66(23), 81(23), 85, 93, 95

Echevin, B., 124(38), 132(38), 149 Edwards, J., 132(100), 133(100), 150 Egelhoff, W. F., 72(50), 94 Ehrlich, G., 3(9), 15(49), 50,51 Eidus, Y. T., 318(133), 333 Einstein, T. L., 17(68), 51 Eischens, R. E., 21(80), 52 Eischens, R. P.,67, 93 Elagina, N. V., 309(106, 106a, 106b), 310 (106, 106a, 106b). 311(106b), 332 Eley, D. D., 82, 86, 95, 104, 107, 148 Eliel, E. L., 303(95), 332 Emmett, P. H., 154(5), 184(5), 193 Endjsova, J., 173(88), 175, 195 Engasser, J . - M . , 222(24), 225 Engelhard, P. A., 290(75a), 331 Engelhardt, H. A,, 39(134), 53 Erickson, N. E., 14(44), 51, 59(9), 72(51), 93, 94

Ermakov, Y. I., 132(68), 149 Ertl. G.. 3(1l), 50, 66(26), 74, 92(54), 93, 94, 326(156, 157, 157a), 327(156), 334 Evans, E. L., 79(62), 94 Evans, S., 61(13), 77(59), 79(62), 93,94 Exner, O., 161(39), 166(39), 193 Eyring, H., 105(9), 109(9), 148

F Fabian, D. J., 81(69), 95 Fadeev, V. S., 279(44), 283(56,56a), 296(56a), 314(44, 123a), 330,333 Fadley, C. S., 3(8), 50 Fahrenfort, J., 21(79), 30(79), 52, 82, 95 Falconer, J. L., 15(53, 54, 58), 26(97, 99), 27 (99), 33(99), 35(118), 36(118), 51, 52, 53, 122(20), 132(20), 148 Farberov, M. I., 230(35), 271 FarkaS, J., 182(124), 195 Farnsworth, H. E., 4(19), 50 Fasman, G. D., 208(39), 225 Fedyk, J., 18(72), 52 Feldman, A,, 11(33), 14(33), 51 Feofanova, L. M., 278(33), 329 Ferraris, G., 123(29), 132(29), 149 Ferreira, J. M., 125(51, 52), 132(51, 52), 149 Fetting, F., 277(30), 313(30), 329 Figueras, F., 125(46), 132(46), 149 Fijzeman, U. J., 39(135), 53 Finkelsthein, A. V., 154(7), 184, 193 Finocchiaro, P., 234(50), 271

339

AUTHOR IN I )EX

Fisher, G. B., 12(39), 36(120), 49(145), 51, 53 Flodstrom, S. A,, 78, 94 Flohr, K., 201(25), 225 Fluit, J . M., 37(123), 53 Fogelberg, L.-G., 313(122), 333 Fomichov, Y. V., 295(81), 314(123, 124), 315 (124, 127), 331, 333 Ford, R. R.,67(29), 93 Fox, J. S., 124(41), 132(41), 149 Frackiewicz, A., 122(17), 132(17), 148 Franklin, J. L., 163(51), 164(51), 194 Franz, W., 276(24), 288(24), 290(24), 291(24), 319(24), 329 Fredrickson, P. W., 129(76), 132(76), 143, 150 Freeouf, J. L., 65(20), 93 Freidlin, L. C., 173, 174(85), 195 French, D., 199(26),225 Frennet, A,, 324, 326(154), 334 Frety, R.,321(144a), 334 Friesema, C., 109(12), 148 Froitzheim, H., 18(70), 23(70), 51 Froment, G. F., 325,334 Fuggle, J. C . , 66(25), 68(36), 72, 74,75,93,94 Fujita, K., 202(95), 203(97), 226 Fukii, Y., 163(60), 165(60), 194 Fukuda, K., 21(82), 31(82), 52 Fuller, M . J., 129(84), 132(84), 144, 150

G Gaetjens, E., 210,226 Gallezot, P., 131(93), 132(93), 150 Galwey, A. K., 100(6), 148 Garin, F., 296(85a), 298, 331 Garnett, J. K., 185, 196 Gasan-Zade, G. Z., 129(82), 132(82), 150 Gault, F. G., 275(15a, 17), 278(40, 41a), 292 (15a, 17), 294(15a), 295, 296(15a, 41a, 85a), 297, 298, 300, 301(17), 304(101), 305(87), 322(149), 329, 330,331,332, 334 Gay, I. D., 65(19), 80(64), 85(79), 93, 94, 95 Geneste, P., 182(117), 195 Georgiev, C. D., 163(56), 164(56), 194 Germain, J. E., 132(102), 133(102), 146(102), 150

Germer, L. H., 3(1), 50 Gibb, T. R.P., 327(168), 334 Gigola, C. E., 131(92), 132(92), I50 Gimzewski, J. K., 81, 95 Giordano, N., 261, 262, 263, 265(55, 60), 266 (60), 271, 272

GI:i
Gltliraky, R.C., 213(75), 226 G ~ ) l ~ ~ \ S., l o s291(76), , 331 Gocrschel, C. T., 278(37), 302(94a), 315(94a), 3 17,330, 332 Golivets, G. I . , 232(41), 233(41), 271 Golivets, I. D., 232(41), 233(41), 271 Golodets, G. I., 122(21), 123(21), 129(71, 79), 132(21, 71, 79), 136, 148, 150, 188, 196 Goiiier, R.,3(12), 50 Goiiiez, R.,125(51, 52), 132(51, 52), 149 G i i i i ~ oE. , E., 131(90), 132(90), 150 Giiiid, G. M., 163(57), 164(57), 194 Goic. R., 313(122), 333 Goiodetskii, V. V., 326(158, 158a), 334 Goiokhovatskii, Y. B., 188, 196 Go\iunskaya, I. V., 276(26), 279(44), 283(56, 56a, 57), 292(26), 295(26), 296(26, 56a), 314(44,123, 123a), 315(127),329,330,333 Go\ inour, C. G., 67(30), 93 GIiu1, M., 92(96), 95 Gr<,cnlee,T. W., 316(129), 333 Grcish, A. A,, 283(58), 296(83), 330, 331 Grcrloble, D. C., 132(104), 133(104), 147, 150 Gritlith, T. J., 122(23), 132(23), 148 GI-illiths, D. W., 199(27),225 GI-iinley,T. B., 17(66, 67), 51 Gring, J . L., 278(34), 330 GI-ohman, W. D., 36(119), 53, 86(85), 95 Giohse, A. V., 273(4), 329 Griihn, W. B., 205, 214, 225 Griinwald, E., 156(20), 161(20), 166(20), 193 Griinze, M., 72(47b), 80(47b), 94 Giiiii, L., 125(54), 132(54, IOI), 133(54, 101), 146, 149, 150, 303(96), 323(153), 332, 334 Giitlkov, B., 186, 196 Guillot, G., 203(12), 224 Giic)ymour, C. G., 17(63), 51 Ciipta, R. K., 18(71), 52 Gupta, S. K., 208(63), 226 G ~ i r dF. , R. N., 208(39), 225 Giiryanova, G. K., 303(98), 332 Giihtafsson, T., 65, 93

H H.i.ig, W. O . , 316(132), 321(132), 333

H.iber, J., 86(87), 88, 91, 95 H,itlman, J., 11(33), 14(33), 51

340

AUTHOR INDEX

Haensel, V., 312(119), 313(119), 314,333 Hagstrom, S. B., 78(61), 94 Hajek, M., 185, 196 Hall, W. K., 124(40), 132(40), 138(40), 139, 149, 150, 163(43), 164(43), 165, 187, 194, 265, 266(62), 267(66), 272 Haller, G. L., 321(145), 334 Hammett, L. P., 156, 157, 158, 161, 175, 180, 193 Hamrin, K., 11(33), 14(33), 51 Hanson, F. V., 127(69), 132(69), 142, 149 Haraszthy-Papp, M., 170(73), 171(73), 194 Haro, J., 125(51), 132(51), 149 Harris, L. A., 3(3), 7(3), 50 Hartog, F., 318(137), 333 Hartsuck, J. A,, 198(31),225 Hashimoto, H., 125(45), 132(45), 149 Hashimoto, K., 261(56), 264,265,272 Hattori, K., 203(35), 225 Hayward, D. O., 135,150, 327(165), 334 Heath, C. E., 189(159), 190(159), 196 Heathcock, C. H., 182(121), 195 Heden, P. F., 322(152), 334 Hedman, J., 322(152), 334 Heilmann, P., 6(24), SO Heinemann, H., 313(121), 333 Heinz, K., 6(24), SO Heise, K., 176(100), 177, 178(100), 179, 186, 189(100), 190(100), 195 Helkovskaya-Sergeeva, E. G., 275(100), 304 (100, lola), 332 Heller, M. J., 209, 225 Helms, C. R., 33(113, 115), 52 Hendler, R. N., 39(130), 53 Hendrickson, J. G., 230(31), 271 Henke, B. L., 59,93 Herley, P. J., 135, 150 Hermann, M., 176(100), 177, 178(100), 179, 186, 189(100), 190(100), 195 Hernandez, L., 154(4), 173(4), 184(4), 193 Hershfield, R., 205,225 Hettinger, W. P., 278(34), 330 Hettler, H., 199(18), 224 Hightower, J. W., 139, I50 Hill, T., 18(69), 27(69), 51 Hindin, S. G., 312(117), 333 Hine, J., 156(22), 159(29), 193,219,225 Hingerty, B., 200(90), 226 Hirai, K., 125(45), 132(45), 149, 163(60), 165 (60), 194

Hishida, T., 158(25), 186, 193 Ho, M., 21(81), 52 Hoang-Van, C., 131(94), 132(94), 150 Hochmuth, J. K., 123(26), 132(26), 149 Hodges, R. J., 185. 196 Hoekstra, P., 137(109), 150 Hoffman, N. E., 185, 196 Hollinger, G., 88, 95 Hopster, H., 72(49), 94 Horie, T., 233(49), 271 Horiuti, J., 100, 113, 148 Hovarth, C., 222(24), 225 Houalla, M., 266(66), 267(66), 272 Houston, R. J., 288(64), 331 Hsien-Cheng Yao, 154(5), 184(5), 193 Huang, C. P., 127(62), 132(62), 142, 149 Hudgins, R. R., 129(78), 132(78), 150 Hughes, E. D., 155(11), 193 Hunter, J. B., 312(116), 333 Hurwitz, H., 312(120), 313(120), 333 Hussey, A. S., 176(90), 195

I Ibach, H., 18(70), 23(70), 51, 72(49), 94 Ibach, S., 86, 95 Ikeda, S., 68, 86(86a), 88(74a), 94, 95 Il’chenko, N. I., 122(21), 123(21), 129(71, 79), 132(21, 71, 79), 136, 148, IS0 Ilyin, V. F., 283(55), 314(124), 315(124), 330, 333 Imanaka, T., 180(111), 195 Imelik, B., 131(93), 132(93), I50 Inamura, K., 125(45), 132(45), 149 Ingold, C. K., 155(10), 193 Inoue, Y., 86(86a), 95 Irshad, M., 188,196 Irwin, W. J., 182(125), 191(125), 195 Isa, S. A,, 66(26), 83(75), 93, 95 Isagulyants, G. V., 283(55, 57, 58), 296(83), 330,331 Ishida, Y., 188, 196 Ishii, J., 163(60), 165(60), 194 Ishino, K., 240(52), 260(52), 265(52), 271 Ito, M., 23(94), 52 Iwakura, Y., 203(35), 225 Iwamoto, I., 180(112), 181, 195 Iwamoto, K., 208(70), 226 Iwasawa, Y., 67(32), 93, 129(80), 132(80), 150

AUTHOR INDEX

J Jacobs, P. A., 129(75), 132(75), 150 Jacono, M. L., 265(62, 63), 266(62), 272 Jaeger, H., 31(103), 52 Jaeger, J., 21(86, 87), 52 Jamieson, D. M . , 129(70), 132(70), 142,149 Janowski, F., 312(115), 333 Jardine, I., 173(87), 174, 175(87), 176(87), 195 Jencks, W. P., 199(36), 225 Jenkins, G. I., 86, 95 Jepsen, D. W., 4(20a), 7(20a), 50 Jewur, S. S., 125(56), 132(56), 138, 149 Johansson, G., 11(33), 14(33), 51 Johnson, D. W., 70(43, 44), 72(44, 47a), 73 (53), 74(53), 75. 80(47a), 94 Johnson, S., 15(55), 33(112), 38(126), 51, 52, 53 Johnson, T. W., 224(37, 38), 225 Jover, B., 124(37), 132(37), 149 Joyner, R. W., 21(90), 23(90), 52, 61, 62(15), 64(16), 65(16, 17), 66(24, 26, 28), 67(34), 75(15, 16), 76(58), 77(60a), 78(60a), 79 (15, 60a), 83(74, 75), 87(60a), 88(74), 90 (15), 93, 94, 95, 318(138), 321(138), 326 (138), 327(169), 328(169), 333, 334 Jungers, J. C., 163(48), 164(48), 176(95), 177 (95), 178(95), 180(109), 184(132), 189 (48), 190(48), 194, 195,196

K Kabanov, V . A,, 21 1(42), 215.225 Kabe, T., 261(57), 263, 264(57), 265(57), 266 (57), 272 Kadowaki, K., 230(26), 271 Kaiser, E. T., 201, 207(58), 223(57), 225, 226 Kalman, J., 132(101), 133(101), 146, 150 Kalman, V., 180(114), 184(114), 195 Kamusher, G. D., 273(2), 329 Kano, K., 206(71), 226 Kargin, V. A,, 21 1(42), 225 Karlson, S., 11(33), 14(33), 51 Karpeyskaya, E. I., 278(35), 330 Karpinski, Z., 278(41), 298(92), 304(92), 328 (92), 330,332 Karzhev, V. I., 273(3), 329 Katchalski, E., 208, 225 Kato, A,, 163(42), 164(42), 167, 170(66), 171 (42), 194

341

Kawai, T., 230(36, 37, 38, 39), 232(39, 42), 233(42, 48), 234(42, 51), 240(52), 241 (53), 244(53), 259(54), 260(52), 265(52), 269(67), 271, 272 Kawakubo, H., 202(95), 226 Kawanami, K., 207(72), 226 Kazanskii, B. A,, 163(56), 164(56), 194 Kazansky, B. A,, 273(1), 274, 275(5, lo), 276 (22), 278(38, 38a), 279, 283(22, 56, 56a), 284(22), 289(67), 292(38, 38a), 295, 296, 300, 303(9, 10, 11, 98), 304(100), 305(5), 306(11, 103, 104a, 105), 307(11), 309, 310 (106, 106a, 106b), 311(106b), 312(110), 314(44, 123, 123a, 124). 315, 316(9, lo), 329,330,331,332 Kazansky, V. B., 139(113), 150 Kazusaka, A., 114, 129(83), 132(83), 144, 148,150 Keith, C. D., 278(34), 330 Kemball, C., 277(29), 329 Kenney, C. N., 123(24), 125(24), 132(24), 136, 137(24), 148 Kesmodel, L. L., 318(141), 319(141), 333 Keulks, G. W., 176(90), 195 Keulks, J., 287(62e), 288(62e), 316(62e), 331 Kharasch, M. S., 232(40), 271 Kharlamov, V. V., 124(40), 132(40), 138(40), 149 Khromov, S. I., 176(91),195,274(1l), 303(11), 306(1 I , 104a, 105), 307(1 l), 309(105a), 329,332 Khromova, G. I., 176(91), 195 Kibby, C. L., 163(43), 164(43), 165, 187, 194 Kida, M., 207(68, 69), 226 Kieboom, A. P. G., 154(8), 155(18), 159(34), 173(89), 175, 177, 180(115), 181, 183, 191 (162). 193,195,196 Kiefer, H. C., 218, 225 Kiewkicz, W., 170(72), 171(72), 194 Kilty, P. A., 76, 94 Kim, C. J., 132(105), 133(105), I50 King, D. A,, 17(63), 51, 67(30), 93 Kirk, R. S., 163(52), 164(52), 189(52), 190(52), I94 Kirkuchi, J., 207(68), 226 Kirsh, Y. E., 211,215,225 Kishi, K., 66(27), 68, 72(40, 47c), 73(52), 75, 77(60, 60a), 78(60, 60a), 79(60, 60a), 80 (47c, 52), 82(60), 86(86a), 87(60, 60a), 88 (74a), 93, 94, 95

342

AUTHOR INDEX

Kishida, S., 180(111), 195 Kisliuk, M. U., 287(62a), 330 Kitamura, T., 189(157), 190(157), 196 Kitaura, Y., 205, 225 Kladnig, W., 155(12), 163(12), 166(12), 170 (12), 193 Klan, 200(90), 226 Klapwijk, P., 180,195 Klasson, M., 322(152), 334 Klaus, I. S., 21 1(54), 225 Klissurski, D. G., 122(23), 129(70), 132(23, 70), 142, 148, 149 Klotz, I. M., 209, 216, 217, 218(40, 44), 219 (92, 93), 224(37, 38, 87), 225,226 Kluksdahl, H. E., 288(64), 331 Kneringer, G., 6(26), 50 Knor, Z., 318, 321(136), 332(136), 326(136, 158a, 163), 327,333,334 Knozinger, H., 168, 169, 170(75), 171(75), 194 KO, E. I., 6(27, 28), 25(96), 26(96), 32(108, 11l), 38(125), 5 0 , 5 2 , 53 Kobal, H., 129(73), 132(73), 142, 150 Kobal, I., 129(73), 132(73), 142, 150 Kobayashi, D., 230(26), 271 Kobayashi, I., 181, 195 Kobliansky, G. G., 172(80). 194 Kochloefl, K., 132(103), 133(103), 147, 150, 163(41,45), 164(41,45), 168, 170(71), 176 (71), 180(113), 181, 182(119, 120), 185 (134), 193, 194, 195, 196 Kogan, S . B., 288(66a, 66b, 66c), 331 Koguchi, K., 230(9, 16, 25), 271 Kohn, H., 222(13), 224 Koltun, W. L., 208(39), 225 Komiyama, M., 199(6,46), 203(5, 6). 224,225 Kondo, H., 207(94), 226 Konig, P., 125(48, 49), 132(48, 49), 137, 149 Koppel, I. A , , 159(31), 193 Kopple, K. D., 208(47), 225 Korchak, V. N., 132(98), 133(98), 145, 150 Kovalenko, L. I., 283(55, 57), 296(83), 330, 331 Kozlova, I. P., 230(35), 271 Kranich, W. L., 163(40), 164(40), 166(40), 193 Kraus, M., 154(2), 155(13), 158, 163(13, 41, 45, 46, 55, 61), 164(41, 45, 46, 5 3 , 165 (61, 651, 166(13), 167, 168, 169(69), 170 (2, 71, 74), 171(2, 74), 173(88), 175(88), 176(71, 92), 177, 180(23), 182(118, 119, 123), 183, 184, 185, 186, 187, 188, 189

(74,158), 190(74,158), 192, 193,194,195, 196 Krimond, T. Y., 279(49a), 330 Kripylo, P., 163(64), 165(64), 194 Kropa, E. L., 163(59), 165(59), 194 Krosnar. T., 186(142), 196 Krupay, B. W., 125(47), 129(85), 132(47, 85), 144, 149, I50 Krylov, 0. V., 287(62e), 288(62e), 316(62e), 331 Kubelka, V., 186, 196 Kucheryavaya, N. N., 306(104a), 332 Kuhn, W., 176(99), 178(99), 179(99), 195 Kuijers, F. J., 274(14), 279(14), 321(14), 329 Kunitake, T., 214,225 Kiippers, J., 3(11), 50, 66(26), 93 Kuriacose, J. C., 125(56), 132(56), 138, 149 Kuroda, Y., 226 Kurono, Y., 202,225 Kuzmina, Z. M., 154(7), 184, 193 Kuznetsov, B. N., 132(68), 149,316,333 Kuznetsov, V. L., 132(68), 149

L Ladenheim, H., 210(52, 53), 225 Lagally, M. G., 17(65), 51 Laidler, K., 105(9), 109(9), 148 Lake, R. D., 230(29), 271 Lamaty, G., 182(117), 195 Landler, J. J., 3(2), 4(2), 50 Landis, P. S., 170(78), 194 Lang, B., 318(138, 139), 321(138), 326(138), 327, 328(169), 333,334 Lange, B., 176(99), 178(99), 179(99), 195 Lapshina, T. V., 295(80), 331 Larson, L. A., 28(101), 52 Latham, D., 57(3a), 58(3a), 61(3a), 80(3a), 93 Lauks, I. R., 92(96), 95 Law, B., 65(19), 85(79). 93, 95 Lawson, A., 21(88), 39(133), 52,53 Leach, H. F., 124(42), 132(42), 149 Lebedev, S. V., 172, 194 Leclercq, G., 132(96), 133(96), 145, 150, 182, 195, 319(143), 320(143), 321(144a), 333, 334 Leclercq, L., 132(96), 133(96), 145, 150, 182, 195 Lee, J., 212(78), 226 Lee, S. B., 74(54), 92(54), 94

343

AUTHOR INDEX

Lee, Ben-Kuo, 163(47), 164(47), 194 Lee, Yun-Zing, 163(47), 164(47), 194 Leffler, J. E., 156(20), 161(20), 166(20), 193 Legark, P., 318(139), 327(166), 333, 334 Lehwald, S., 18(70), 23(70), 51 Leland, T. W., 129(72), 132(72), 150 Lengyel, A,, 180(114), 184(114), 195 Le Nhu Thanh, 186,196 Leonard, A. J., 125(53), 132(53), 149 Lester, G. R., 311, 316(108), 332 Letsinger, R. L., 210, 211, 225 Leutic, P., 82, 95 Levine, H. L., 223, 225 Levitsky, I. I., 289, 290(68), 327(68a), 331 Lewhald, S., 86, 95 Lewis, K. E., 283(59), 330 Liberman, A. D., 276(26), 292(26), 296(26), 329 Liberman, A. L., 274(5), 275(5, 19), 278(38, 38a), 279(47), 283(58), 292, 294, 295(26, 38,38a, 79,80), 296(83), 300,303(98), 304 (100, lola), 305(5), 306(102), 312(110), 315(5), 329, 330, 331, 332 Lieber, E., 154(3), 175(3), 184(3), 192 Lienard, G., 324(154), 326(154), 334 Lietz, G., 276(24), 288(24), 290(24), 291(24), 319(24), 329 Lin,C. J., 110, 111, 125(13), 132(13), 138, 148 Lindau, I., 68(38), 92(38), 94 Lindauer, G., 327(166), 334 Lindberg, B., 11(33), 14(33), 51 Lindgren, I., 11(33), 14(33), 51 Linero, M. A,, 163(63), 165(63), 170(63), 194 Ling, D. T., 33(114), 52 Linn, W. J., 132(99), 133(99), 146, 150 Linnett, J. W., 58, 93 Lipscomb, W. N., 198(31), 225 Lipsey, C., 203(12), 224 Lishenok, 0. E., 309(105a), 332 Lister, D. G., 48(142), 53 Litvin, J. F., 173, 174, 195 Liu Ta-Chuang, 163(47), 164(47), 194 Lloyd, D. R., 65, 93 Loebl, E. M., 210(52), 225 Loffler, D. G., 123(30, 31), 132(30,31), 149 Lombardo, E. A,, 124(40), 132(40), 138, 149, 265(62), 266(62, 66), 267(66), 272 Longfield, J. E., 38(128), 39(128), 53 Lopez, F. J., 163(50), 164(50), 194 Lowe, G., 233(47), 271

Loza, G. V., 274(5), 275(5), 305(5), 315(5), 329 Lozovoi, A. V., 176(94, 101, 102), 177(94), 178 (94, 101, 102), 179, 186, 195 Luetic, P., 184(130), 196 Lukina, M. Y., 304(100), 332 Lunsford, J. H., 124(44), 129(83), 132(44, 83), 144, 149,150 Lu Tai-Chung, 163(47), 164(47), 194 Luth, H., 36(119), 53, 86(85), 95

M Maatman, R. W., 98(1, 2, 3), 104(3), 109(3, 12), 137(1, 109), 138(3), 148, 150,315 (126), 333 McCarroll, J. J., 284(61), 330 McCarty, J., 10(32), 32(106, 107), 51, 52 MacDonald, J. N., 48(142), 53 MachBEek, H., 182(119), 195 McHenry, K. W., 288,331 MacKay, K. M., 326(160), 334 McKee, C. S., 79(63), 94 Mackensen, G., 203,224,225 McKervey, M. A,, 297,300,331 McKinney, R. W., 39(130), 53 McQuillin, F. J., 173(87), 174(87), 175(87), 176(87), 182(125), 191(125), 195 Madden, H. H., 4(16), 50 Madey, T. E., 12(39), 14(44), 15(50), 36(120), 51, 53, 59, 66(25), 72(51), 93, 94 Madix, R. J., 6(27, 28), 10(31), 11(38), 12(38), 14(45,46,47), 15(53, 55a, 56, 58), 16(56), 17(45, 60, 61), 18(74), 20(75), 21(75), 22 (75, 91), 23(92), 24(95), 25(92, 961, 26 (96, 97, 98, 99), 27(98, 99, IOO), 28(102), 30(61, 102a), 31(102, 102a, 105), 32 (106, 107, 108, 109, 111),33(99, 112, 117), 34(100), 35(95, 100, 118), 36(118), 37 (74, 122, 124), 38(95, 124, 125, 126, 127), 39(124, 136), 40(137), 41(137, 138), 42 (137), 43(56, 95), 44(139), 45(139), 46 (139),48(137, 141, 144), 50,51,52,53,85, 95

Madon, R. J., 124(40), 131(89), 132(40, 891, 138(40), 149, 150 Mahaffy, P., 137(109), 150 Mailhe, A,, 39(131), 53 Mainwaring, D. E., 321, 334 Maire, G., 275(15a), 292(15a), 294(15a), 296

344

AUTHOR INDEX

(15a), 304, 318(139), 327, 329, 332, 333, 334 Maki, H., 212,226 Makinen, M. W., 207(58), 225 Makoveev, P. S., 188, 196 Malinowski, S., 170(72), 171(72), 194 Mandre, G., 163(58), 165(58), 194 Manecke, G., 215,226 Mano, Y.,129(81), 132(81), 143, 150 Manor, P. C., 200(90), 226 Marcus, P. M., 4(20a), 7(20a), 50 Marczewski, W., 86(87), 95 Mars, P., 21(76), 30(76), 52 Martino, G., 322, 334 Martinotti, G., 261(55), 262(55), 263(55), 265 (55), 271 Marukjan, G. M., 187(143), 196 Marvel, C. S., 233(45), 271 Mashkina, A. V., 188, 196 Mason, R., 65, 67, 68(39), 80(64), 85(79), 86, 93, 94, 95 Massardier, J., 131(93), 132(93), 150 Massoth, F. E., 261(58), 265,266, 272 MathC, T., 180(114), 184(114), 195 Mathieu, M. V., 125(46), 132(46), 149 Mathur, K. B., 209(17, 59,60), 224,225 Matloob, M. H., 70(42,43,44), 72(44, 47), 80 (42, 47), 82(42), 83(75), 87, 94, 95 Matsukawa, F., 129(81), 132(81), 143, 150 Matsumoto, K., 208(70), 226 Matusek, K., 292(77), 293(77), 295(77), 298 (92), 303(96a), 304(92), 324(77), 328(92), 331,332 Maurel, R., 132(96), 133(96), 145, 150, 182, 195 Maurel, T., 173, 174, 175(82), 189(82), 190 (82), 194 May, D. R., 163(59), 165(59), 194 Mayer, I., 100, 148 Mears, D. E., 118(15), 148 Meerbott, W. K., 312(109, 112), 313(109), 332 Mehner, H., 290(74), 319(74), 331 Mehta, R. V., 209(59,60), 225 Meisel, W., 290(74), 319(74), 331 Melander, L., 155(14), 193 Menon, P. G., 325,334 Menzel, D., 14(48), 15(52), 39(134), 51, 53, 66 (25), 74, 75(56), 93, 94 Meubus, P., 123(28), 132(28), 149 Meyers, W. E., 220(61, 62), 225,226

Mihajlova, D., 187(147), 188, 196 Miki, Y., 261(57), 263(59), 264(57), 265(57), 266(57), 272, 303(99), 306(99), 332 Miller, M. A,, 303(95), 332 Milliken, T. H., 313(121), 333 Mills, G. A., 312(117), 313, 333 Minachev, K. M., 124(40), 132(40), 138(40), 149, 278(33), 289(68), 290(68), 327(68a), 329,331 Mironov, G. S., 230(35), 271 Mirzaeva, A. K., 309(106, 106a, 106b), 310 (106, 106a, 106b), 311(106b), 332 Mitchell, A. R., 208(63), 226 Miyahara, K., 100, 113, 114, 131(86, 87), 132 (86,87), 144, 148, 150 Miyamoto, A,, 108, 148 Mochida, I., 158(24, 26), 159(32), 163(26, 42, 53, 54), 164(26, 42, 53, 54), 165(54), 167, 170(66), 171(42), 177, 193, 194 Moffat, J. B., 122(18), 124(34), 132(18, 34), 148, 149 Moger, D., 325(155a), 334 Moldavsky, B. L., 273(2), 329 Montarnal, R. E., 278(39), 322, 330,334 Montaudo, G., 234(50), 271 Moraweck, B., 321(144a), 334 Morawetz, H., 210, 224,225,226 Morgan, A. E., 6(25), 50 Morikawa, A., 124(39), 132(39), 138, 149 Morimoto, M., 213(77), 226 Moro-oka, Y., 189(157), 190(157), 196 Morrell, J. C., 273(4), 329 Morrison, G. A., 303(95), 332 Morrison, J., 3(2), 4(2), 50 Mortikov, E. S., 276(22), 279(22, 49a), 283 (22), 284(22), 329, 330 Moyes, R. B., 81(67), 91(67), 95 Mrnzel, D., 68(35), 93 Mulik, I. Y., 186, 196 Muller, A. C., 290(75a), 331 Muller, E. W., 326(162), 334 Muller, J. M., 127(66), 132(66), 149, 275(15a, 17), 292(15a, 17L 294(15a), 296(15a), 297, 300, 301(17), 305(87), 329, 331, 332 Muller, K., 6(24), 50 Munuera, G., 21(78), 52, 124(33), 132(33), 138, 149, 163(44), 164(44), 194 Murakami, Y., 180(111), 195, 206, 207, 208, 226 Murphy, W. R., 129(72), 132(72), I50

345

AUTHOR INDEX

N Nagashima, S . , 21(82), 31(82), 52 Nagy, F.. 325(155a), 334 Najemnik, J., 176(103). 177, 178(103), 186, 189(103), 190(103), 191(103), 195 Nakamura, E., 230(9, 16, 25), 271 Nakamura, M., 131(95), 132(95), 150 Nakano, A,, 207(69), 208(70), 226 Nakayama, T., 230(16, 25), 271 Naltemeyer, M., 200(90), 226 Naro, P. A,, 312(118, 118a), 313(118, 118a), 314(118a), 333 Naylor, R. W., 289(70), 291(70), 315(70), 331 Netzer, F. P., 6(26), 50 Nicholson, D. E., 163(51), 164(51), 194 Nicks, L. J., 127(63), 132(63), 149 Nicolaidas, J., 132(100), 133(100), 150 Nieuwenhuys, B. E., 321(147), 326(161), 334 Nieuwstad, T. J., 180, 195 Nishide, H., 215(73), 226 Nitecki, D. E., 208(47), 225 Nitscke, F., 66(26), 93 Noguchi, J., 209(74, IOl), 226, 227 Noguiera, N., 315(128), 333 Noller, H., 155(12), 163(12, 49, 50, 63), 164 (49, 50), 165(63), 166(12), 170, 193, 194 Nomura, O., 181, 195 Nondek, L., 187, 188,196 Nord, F. F., 154(4), 173(4), 184(4), 193 Nordberg, R., 11(23), 14(33), 51 Nordling, C., 11(33), 14(33), 51,322(152), 334 Norton, P. R., 77(59), 94 Noto, Y., 21(82), 31(82), 52 Novaro, O., 125(52), 132(52), 149 Nowack, G . P., 176(90), 195 Nozaki, F., 129(81), 132(81), 143, 150 Nystron, R. F., 233(43), 271

0 Oba, M., 303(99), 306(99), 332 Oblad, A. G . ,313(121), 333 O’Cinneide, A,, 278(40), 330 Ogasawara, S . , 129(80), 132(80), 150 Ogino, Y.,108, 148 Ohba, M., 261(57), 263(59), 264(57), 265(57), 266(57), 272 Ohmacht, R., 124(37), 132(37), 149 Okahata, Y., 214(48, 49, 50), 225

Okamoto, H., 207(72,94), 226 Oldenburg, C. C . , 180, 195 Olferyeva, T. G., 303(98), 304(100), 332 Ollis, D. F., 32(110), 52 Onishi, T., 21(82), 31(82), 52 Onozuka, S . , 203(35), 225 Osmanov, M. O., 129(82), 132(82), 150 Ostermeier, K., 163(49), 164(49), I94 Ostrovskii, V. E., 122(16), 132(16), 148 Ostero-Schipper, P. H., 131(88), 132(88), 150 Otsuka, K., 124(39), 132(39), 138, 149 Oudar, J., 4(15), 50 Overberger, C. G., 212,213,214,226,233(45), 271 Overman, L. E., 204,224 Owen, N. L., 48(142), 53 Ozaki, A., 122(19), 127(19), 132(19), 148, 189 (157), 190(157), 196

P Paal, Z,, 275(20, 21), 276(25, 27, 27a), 277 (31), 278(31, 36), 279(42, 50, 51), 280 (50, 52, 53, 54), 281(52, 54), 282(20, 53), 283(20, 53), 284(21, 62), 285(21), 286 (21, 62c), 287(62c), 289(21), 292(27, 27a, 77), 293(25, 77), 294(25, 31, 36, 78), 295 (25, 77), 296(21, 51, 84, 8% 298(25, 27, 27a, 62, 78, 91, 91a, 92), 299(91a), 300 (27a, 62, 78,91,91a), 303(42,62,91,91a, 96, 97, 97a), 304, 305(91, 91a, 97a), 306 (53, 104), 307(85), 308(78), 309(78), 318 (54), 319(25), 320, 321(42), 322(42), 323 (36, 78, 91a, 153), 324(25, 31, 77), 325 (155b), 326(42, 97, 97a, 159), 327(167), 328(92, 171). 329, 330, 332, 332, 334 Pachta, J., 159(33), 174(33), 193 Padalia, B. D., 81(69), 95 Paez, M., 163(50), 164(50), 194 Pajonk, G., 132(102), 133(102), 146(102), 150 Palazov, A., 187(147), 196, 290(75), 318, 319 (142, 142a), 331, 333 Palczewska, W., 122(17), 132(17), 148 Palmberg, P. W., 7(30), 50 Park, R. L., 4(16), 50 Parry, D. E., 79(62), 94 Paton, R. M., 201(25), 225,226 Penn, D. R., 61,93 Pennekamp, E. F. H., 176(97), 177, 178(97), 195

346

AUTHOR INDEX

Pepe, F., 123(29), 132(29), 149 Perderau, M., 4(15), 50 Peria, W. T., 3(4), 7(4), 50 Perkampus, H. H., 179(104), 195 Peter, A., 279(43), 330 Petrakis, L., 265, 266, 272 Petro, J., 180(114), 184(114), 195 Petrov, L., 185, 186(138), 196 Pezzanite, J., 289(71), 290(71), 291(71), 327 (71), 331 Photaki, I., 209, 226 Pianetta, P., 68(38), 92(38), 94 Pielaszak, J., 77(59), 94 Pignet, T., 123(32), 132(32), 136, 149, 326 (1 57a), 334 Piken, A. G., 127(61), 132(61), 149 Pimental, G. C . , 277(28), 329 Pines, H., 170(76), 171(76), 172, 194,278(37), 279,298,301,302(94a), 315(94,94a, 128), 316(129, 132, 132a), 317, 321(132, 132a, 132b), 330,331,332,333 Pirug, G., 72(49), 73, 94 Pitzer, K. S . , 277(28), 289(28), 329 Plate, A. F., 273(1), 329 Platteeuw, J. C . , 276(23), 284, 288, 315, 329 Pliskin, W. A., 21(80), 52 Plouidy, G., 304(101), 332 Plummer, E. W., ll(34, 37), 51, 65(20), 86, 93,95 Poleski, M., 122(17), 132(17), 148 Polinin, V. L., 289(67), 331 Pollack, R. N., 222(86), 226 Ponec, V., 131(91), 132(91), 150, 274(14), 279 (14, 45), 298(89), 316, 320(89), 321(14), 329,330,331,333 Poppa, H., 15(59), 51 Poulter, S . R., 182(121), 195 Pour, V., 127(66), 132(66), 149 Prank, R. A., 224(87), 226 Prebrazhensky, A. V., 303(98), 332 Prettre, M., 125(46), 132(46), 149 Price, W. C . , 58, 93 Prihradny, L., 170(73), 171(73), 194 Primet, M., 131(93), 132(93), 150 Prinzler, H., 163(64), 165(64), 194 Pritchard, D. J., 132, 250 Pritchard, J., 18(71),52 Pritchard, R. G., 61(13), 93 Prudhomme, J. C . , 304(101), 332 Puddu, S . , 131(91), 132(91), 150

Puthenpurackal, T., 185,196 Pyatnitzkii, Y.I., 188, 196

Q Quinn, C. M., 57(2), 75(2), 93

R Rader, C. P., 176(98), 178(98), 179, 195 Raesenberg, J. R., 154(3), 175(3), 184(3), I92 Ragaini, V., 124(43), 132(43), 149 Ralkovh, A., 180(113), 181, 195 Ramaswamy, A. V., 125(53), 132(53), 149, 288(65), 295(65), 332 R h b y , B., 313(122), 333 Rase, H. F., 163(52), 164(52), 180, 189(52), 190(52), 194, 195 Rasias, S . , 71(46), 94 Rathousky, J., 186(142), 196 Ratnasamy, P., 125(53), 132(53), 149,288(65), 295(65), 331 Redhead, P. A., 15(57),51 Regner, A., 127(66), 132(66), 149 Renny, L. V . , 79(63), 94 Reynolds, P. W., 82,95 Rhodin, T. N., 4(23), 11(35), 12(41), 50, 51 Rice, D. W., 91(94), 92(94), 95 Richardson, J . T., 127(62), 132(62), 142, 149 Rideal, E. K., 56, 86, 95 Riggs, A. S., 124(34), 132(34), 149 Roberts, J. K., 17(64), 51 Roberts, M. W., 21(90), 23(90), 52, 57(2, 3, 3a), 58(3, 3a), 59(10), 61(3a), 62(3, 1 3 , 64(l6), 65(16, l7), 66(21,22,24,26,27,28), 67(30), 68(40), 70(42, 43, 44,45), 71(46), 72(40, 44, 47, 47a, 47c), 73(52, 53), 74 (53), 76(58), 77(10, 60,60a, 63), 78(10, 60, 60a), 79(10, 15, 60,60a, 63), 80(3, 3a, 22, 42, 47, 47a, 47c, 52, 65, 66), 81(22, 65, 66, 67, 68), 82(42, 60, 66), 83(74, 75), 87, 88(74, 88), 89(92, 93), 90(15), 91(67), 93, 94, 95 Roberts, R. M., 163(57), 164(57), 194 Robertson, A. J. B., 21(85, 89), 52 Robschlager, K.-H., 312(112a), 332 RoEnakovh, M., 180(108), 195 Roeski, R. W., 208(63), 226 Rohrer, J. C . , 275(18), 312(18, 113, 120), 313 (120), 329,333

AUTHOR INDEX

Roiter, V. A,, 188, 196 Rol, N. C., 76(57), 94 Rooney, J. J., 155, 193,274(8), 275(8, 15), 276 (8), 279(8), 298(8), 292(15), 297, 300, 304, 323(8), 327(8), 329,331 Ross, R. A,, 122(23), 125(47), 129(70,85), 132 (23, 47, 70, 85), 142, 144, 148, 149, 150 Rossini, F. D., 277(28), 289(28), 329 Rosynek, M. P., 124(41), 132(41), 149 Rousseau, J., 86, 95 Royer, G. P., 216, 217, 218(44), 220(61, 62), 222(88), 225,226 Rozanov, V. V., 286(62c), 287(62a, 62b, 62c, 62d), 288(62d), 325(62d), 330,331 Rozengart, M. I., 276(22), 279(22, 49a, 50), 280(50), 283, 284(22), 289(67), 329, 330, 331 Rubanik, N. Y., 186,196 Rubloff, G., 36(119), 53 Rubloff, G. W., 12(40), 36(121), 51,53, 86,95 Ruiz-Vizcaya, M. E., 125(52), 132(52), 149 Russell, S. H., 107, 148 RuiiEka, V., 159(33), 173, 174(33), 180, 184, 193, 194,195. 196

S Sabatier, P., 39(131), 53 Sachtler, W. M. H., 21(79), 30(79), 52,76(57), 82(73), 94, 95, 125(50), 132(50), 139, 149, 279(45), 330 Saenger, W., 200(21,90), 225,226 St. Pierre, T., 212(78, 79), 226 Saito, Y., 158(26), 163(26), 164(26), 193 Sakarellou-Daitsiotou, M., 209,226 Salamone, J. C., 212(80, 81), 213(77, 81), 226 Salje, E., 89, 95 Samman, N. G., 297,300,331 Sanders, J., 21(86), 52 Sano, A., 230(37, 38), 271 Santrochova, H., 184,196 Sarkany, A,, 125(54), 132(54), 133(54), 146, 149, 303(96, 96a), 332 Sasaki, M., 269(67), 272 Sastri, M. V. C., 124(36), 132(36), 149 Sato, S., 131(86, 87), 132(86, 87), 144, 150 Sato, Y.,230(26), 271 Saunders, J. H., 233(45), 271 Saunders, K. W., 163(59), 165(59), 194 Savchenko, V. I., 326(158), 334

347

Savereide, T. J., 210, 211,225 Scarpa, I. S., 218(40, 44), 219(93), 225 Schepelin, A. P., 72(50), 94 Schiavello, 123(29), 132(29), 149 Schmidt, L. D., 3(10), 15(10, 51), 50, 51, 123 (30, 31, 32), 132(30, 31, 32), 136, 149 Schmir, G. L., 208(16), 224 Schneider, J. A,, 208(91), 226 Schneider, P., 163(61), 165(61), 194 Scholten, J. J. F., 21(76), 30(76), 52 Schrieffer, J. R., 17(68), 51 Schuster, C., 172, 173(81), 194 Schwab, G. M., 82,95, 163(58), 165(58), 194 Scofield, J. H., 61, 93 Scott, L. G., 122(18), 132(18), 148 Sebastian, J. F., 201,226,227 SedlaEek,J., 154(9), 169(69), 187,193,194,196 Seelig, H. S., 288(63), 331 Segal, E., 131(89), 132(89), 150 Seiyama, T., 163(42), 164(42), 167, 170(66), 171(42), 194 Seljakh, I. V., 180(110), 195 Selman, D. M., 312(111), 313(111), 314,332 Senegacnik, M., 129(73), 132(73), 142, I50 Seniavin, S. A,, 176(101, 102), 178(101, 102), 186(101, 102), 195 Sermon, P.,290(72), 331 Seshadri, K. S., 265, 266,272 Setinek, K., 160(37), 170(74), 171(74), 189 (74), 190(74), 193, 194 Severyanova, M. G., 273(3), 329 Sexton, B. A,, 23(93), 49(145), 52, 53, 85, 95, 127(65), 132(65), 149 Shacklett, C. D., 230(30), 232(44), 233(44),271 Shafer, J., 210, 226 Sheehan, J. C., 208(91), 226 Shelef, M., 127(61), 132(61), 141, 149, I50 Shephard, F. E., 275(15), 292(15), 329 Shimokawa, N., 203(97), 226 Shirakata, H., 203(97), 226 Shokova, E. A., 306(104a, 105), 332 Shopov, D., 185, 186(138), 187(147), 196, 319 (142a), 333 Shorter, J., 159(30), 193 Shuikin, N. I., 278(33), 329 Sica, A. M., 131(92), 132(92), I50 Siegbahn, K., 11(33), 14(33), 51, 57, 93, 322 (152), 334 Siegel, B., 222(13), 224 Silva, A. E. M., 129(78), 132(78), 150

348

AUTHOR INDEX

Silveston, P. L., 129(78), 132(78), 150 Silvestri, A. J., 312(118, 118a), 313,314(118a), 333 Sim Do-Chen, 154(2), 170(2), 171(2), 192 Simonik, J., 170(76), 171(76), 172, 194 Simonikova, J., 180(113), 181, 195 Simons, E., 208(39), 225 Simonyi, M., 100, 148 Sinfelt, J. H., 275(18), 312(18, 113, 120), 313, 329,333 Singh-Bopari, S., 76(58), 94 Singleton, D. L., 133, 150 Siova, A. N., 273(3), 329 Sivasanker, S., 125(53), 132(53), 149, 288(65), 295(65), 331 Sklyarov, A. V., 286(62c), 287(62a, 62b, 62c, 62d, 62e), 288(62d, 62e), 316(62e), 325 (62d), 330,331 Sleight, A. W., 132(99), 133(99), 146, 150 Sloniewsky, A. R., 216(45), 225 Smart, R. St. C., 89(93), 95 Smejkal, J., 182(124), 195 Smith, G. B. L., 154(3), 175(3). 184(3), 192 Smith, H. A,, 160(36), 176(97, 98), 177, 178 (97, 98), 179, 193, 195, 230(30), 232(44), 233(44), 271 Smith, L. I., 230(22), 271 Smith, R. L., 312(118, 118a), 313(118, 118a), 314(118a), 333 Smith, T. W., 214,226 Smolik, J., 185, 196 Sobyanin, V. A,, 326(158a), 334 Solonina, L. S., 314(123a), 333 Somorjai, G. A,, 4(22), 6(25), 7(22), 50, 67, 86,93,95,98(4), 127(64, 65), 132(64, 65), 148, 149, 318(138, 141), 319(141), 321 (138, 146, 147), 322, 323(148), 326(148), 327(169), 328(169), 333, 334 Sosnovsky, H., 21(83, 84), 52 Sosnovsky, M. D., 31(104), 52 Spatz, H.-C., 200(21), 225 Spetnagel, W. J., 219(92), 226 Spicer, W. E., 3(5), 33(114, 116), 50, 52, 68 (38), 92, 94 Sporka, K., 180,195 Srihari, V., 122(22), 124(22), 132(22), 148 Stacey, K. A., 219(100), 220(1), 224,227 Stair, P. C., 318(141a), 333 Stamoudis, V., 202(51), 225 Stauffer, J. E., 163(40), 164(40), 166(40), 193

Stein, W. D., 208(2), 224 Steiner, H., 274(6), 283(59), 329, 330 Steinkilberg, M., 66(25), 93 Sterba, M. J., 312(119), 313(119), 314,333 Sterin, K. E., 278(38, 38a), 292(38, 38a), 295 (38, 38a). 306(104a, 105), 309(106, 106a, 106b), 310(106, 106a, 106b), 311(106b), 330,332 Stoch, J., 86(87), 88(90), 95 Stock, L. M., 170(79), 194 Storck, W., 215(73), 226 Storey, W., 67(30), 93 Strnad, P., 163(55), 164(55), 189(158), 190 (I%), 194,196 Suetaka, W., 23(94), 52 Sugier, A. A,, 278(39), 330 Sugioka, M., 163(62), 165(62), 194 Suh, J., 219,226 Sultanov, M. Y.,129(82), 132(82), I50 Sunamoto, J., 206(71), 207,226 Sun Cheng-E, 184(131), 196 Susko, A., 35(118), 36(118), 53 Svetaka, W., 21(81), 52 Swegler, E. V., 312(117), 333 Szabo, Z. G., 124(37), 132(37), 149 Szebenyi, I., 291,331 Szkchy, G., 291(76), 331 Szekely, G., 277(31), 278(31), 294(31), 324 (31), 329 Szepanska, S., 170(72), 171(72), 194

T Tabushi, I., 202(95), 226 Tabushi, K., 203,226 Tahara, T., 214(50), 225 Tzira, K., 207(94), 226 Takagi, Y.,181,195 Takahashi, T., 163(60), 165(60), 194 Takaoka, H., 163(60), 165(60), 194 Take, J., 158(26), 163(26), 164(26), 193 Takeda, A., 199(46), 225 Takeshita, T., 127(67), 132(67), 149 Tamaru, K., 21(82), 31(82), 52 Tamm, P. W., 3(10), 15(10), 50 Tanaka, K., 181, 195 Tanielian, C., 132(97), 133(97), 145, 150 Tapping, R. L., 77(59), 94 Tarama, K., 261(56), 264(56), 265(56), 272 Taya, K., 154(6), 184(6), 193

349

AUTHOR INDEX

Teichner, S. J., 124(38), 131(94), 132(38, 94, 102), 133(102), 146(102), 149, I50 Tellier, J., 173, 174, 175(82), 189(82), 190(82), 194 Teranishi, S., 180( 11I), 195 Tetenyi, P., 125(48, 49, 54), 132(48, 49, 54), 133(54), 137, 146, 149, 275(20, 21), 276 (25, 27a), 277(31), 278(31,36), 279(42,48, 511, 280(52, 53, 541, 281(52, 54), 282(20, 53), 283(20, 531, 284(21, 621, 285(21), 286 287(62f), 289(21), 292(27a, 77), 293 (25, 77), 294(25, 31, 36, 78), 295(25, 77), 296(21, 51, 84), 298(25, 27a, 62, 78, 91, 91a, 92), 299(91a), 300(27a, 62, 78, 91, 91a), 303(42, 62, 91, 91a, 96, 96a, 97a), 304(52, 92, 97a), 305(91, 91a, 97a). 306 (48, 53, 104), 308(78), 309(78), 318(54), 319(25), 320, 321(42), 322(42), 323, 324 (25, 31, 771, 326(42, 97a, 159), 328, 329, 330,331,332,334 Teter, J. W., 278(34), 330 Textor, M., 67(32), 80(64), 93, 94 Textor, R., 68(39), 86,94 Thomas, G. E., 72(50), 73,94 Thomas, J. M., 61, 77(59), 79(62), 93, 94 Thomas, M. D., 39(132), 53 Thomson, S. J., 39(133), 53, 284(61), 318 (134a), 325(155b), 327(167), 330,333,334 Thonon, C . , 184(132), 196 Tilyaev, S. K., 173, 174(85), 195 Toda, F., 203(35), 225 Todd, G., 15(59),51 Tolstopyatova, A. A., 278(35), 330 Tominaga, H., 129(77), 132(77), 143, 150 Tompkins, F. C., 135, 150, 326(161), 334 Torto, F. G., 233(47), 271 Toth, L., 328(171), 334 Tottrup, P. B., 123(25), 132(25), 136, 149 Tournayan, L., 319(143), 320(143), 321(144a), 333,334 Tovmasyan, V. G., 276(26), 292(26), 295(26), 296(26, 83), 329, 331 Toyoshima, I., 100, 113, 148 Tracy, J. V., 7(30), SO Trambouze, Y . , 139(112), 150 Trapnell, B. M. W., 86, 95 Treshchova, E. G . , 278(33), 329 Tret’yakov, I. I., 132(98), 133(98), 145, I50 Tribble, M. T., 303(95), 332 Tricker, M. J., 79(62), 94

Trillo, J. M., 21(78), 52 Trimm, D. L., 188, 196 Tsuchiya, J., 129(77), 132(77), 143, I50 Tsuchiya, S . , 325(155), 334 Tsuno, Y., 158, 193 Tsurugaya, M., 240(52), 259(54), 260(52), 265 (52), 271 Turner, D. W., 58, 93 Turnquest, B. W., 208(7), 224 Twigg, G. H., 275,279, 284(60), 329,330

U Uchijima, T., 158(25), 186, 188, 193, 196 Udaltsova, E. A., 289(68), 290(68), 327(68), 33 1 Uematsu, T., 125(45), 132(45), 149 Uemitsu, N., 188, 196 Ueno, A,, 123(26), 132(26), 149 Umbach, E., 75(56), 94 Uniger, L., 86(87), 88(90), 91, 95 Uno, K., 203(35), 225 Unterwald, F., 3(2), 4(2), SO Urabe, K., 122(19), 127(19), 132(19), I48 Usov, Y . N . , 283(55), 314(124), 315(124), 330, 333 Utaka, M., 199(46), 225 Uytterhoeven, J. B., 129(75), 132(75), 150

v Vaghi, A., 261(55), 262(55), 263(55), 265(55, 60), 266(60), 271, 272 Valles, E. M., 131(92), 132(92), I50 vanBekkum, H., 173(89), 175, 180, 181, 183, 191(161, 162), 195, 196 Vandamrne, L. J., 129(75), 132(75), 150 van de Putte, K. J. G., 180(115), 181, 195 VanderWal, N. J., 39(135), 53 Van Etten, R. L., 201,226,227 Van Hardeveld, R., 318(137), 333 Van Hove, M. A., 4(21), 50 van Mechelen, C . , 180(109), 195 Vannice, M. A., 118(15), 127(58. 59, 60), 132 (58,59, 60), 148,149 van Rantwijk, F., 154(8), 155(18), 191(161, 162), 193,196 Van Reyen, L. L., 82(73), 95 Van Santen, R. A., 279(45), 330 Van Schaik, J. R. H., 298,320,331

350

AUTHOR INDEX

van Vliet, A., 191(161), 196 Varadarajan, T. K., 124(36), 132(36), 149 Varloud, J., 321(144a), 334 Vasina, T. V., 274(5), 275(5), 295(79), 305(5), 315(5), 329, 331 Vedenyapin, A. A,, 306(103, 105), 332 Veerkamp, T. F., 129(72), 132(72), 150 Venuto, P. B., 170, 194 Vetrova, B. B., 230(35), 271 Vickerrnan, J . C., 127(57), 132(57), 141,149 Vidal, B., 182(117), 195 Vierrath, H., 277(30), 313(30), 329 Viswanathan, B., 124(36), 132(36), 149 Viswaneth, D. S., 122(22), 124(22), 132(22), 148 Vlasov, V. G., 295(81), 314(124), 315(124), 331,333 Vogel, B., 210(67a), 227 Volter, J., 176(99, loo), 177, 178(99, loo), 179, 186, 189(100), 190(100), 195, 276 (24), 288, 290(24, 66, 74), 291(24), 319 (24, 74), 329, 331 Voorhies, A., 312(111), 313(111), 314, 332 Vorburger, T. V., 11(34), 51, 86(80), 95 Vorchheimer, N., 212,226 Vorhoek, F. H., 21(77), 52

W Wachowski, L., 129(74), 132(74), 150 Wachs, I. E., 15(55a, 56), 16(56), 18(74), 22 (91), 37(74, 124), 38(124), 39(124), 40 (137), 41(137, 138), 42(137), 43(56), 44 (139), 45(139), 46(139), 48(137), 51, 52,53 Wachter, W. A,, 131(88), 132(88), 150 Waclawski, B. J., 11(34), 12(39), 36(120), 51, 53, 86(80), 95 Wadsworth, F. T., 230(31), 271 Walker, J. F., 39(129), 53 Walker, S. M., 17(66, 67), 51 Walkov, W., 276(24), 288(24), 290(24), 291 (24, 3 19(24), 329 Wallace, R. D., 39(130), 53 Wallace, W. E., 127(67), 132(67), 149 Walters, M. J., 79(62), 94 Walton, D. K., 21(77), 52 Wandelt, K., 66(26), 75(56), 93, 94 Wang, T. H., 71(46), 94 Wang Fu-An, 163(47), 164(47), 194

Wang Hsiu-Shan, 184(131), 196 Waquier, J. P., 176(95), 177(95), 178(95), 195 Warwick, M. E., 129(84), 132(84), 144, 150 Watanabe, S., 261(56), 264(57), 265(56), 272 Watkins, J., 289(71), 290(71), 291(71), 327 (71), 331 Watson, L. M., 81(69), 95 Weatherhead, R. H., 219, 220(1), 224,227 Webb, A. N., 67,93 Webb, G., 290(73), 318(134a), 331, 333 Weber, R. E., 3(4), 7(4), 50 Weedon, B. C. L., 233(47), 271 Weinberg, W. H., 72(50), 73, 94 Weisang, F., 278(41a), 296(41a, 85a), 330 Weisang, J. E., 290(75a), 331 Weiss, M., 74(54), 92(54), 94, 233(46), 271 Weissberger, A., 155(15), 193 Weisz, P. B., 312(114, 117), 333 Weller, S. W., 312(117), 333 Wells, B. R., 57(2), 75(2), 93 Wells, P. B., 328(170), 334 Wells, P. R., 156(21), 193 Wertz, D. H., 303(95), 332 Westfall, G. A,, 289(70, 71), 290(71), 291(70, 71), 315(70), 327(71), 331 Williams, A,, 219(100), 200(1), 224, 227 Wilson, J. L., 288(63), 331 Wise, H., 122(20), 131(95), 132(20, 95), 148, 150

Wojciechowski, B. W., 102, 148 Wolf, F . , 312(115), 333 Wood, E.A., 4(17), 50 Wood, P. R., 81(68), 95

Y Yakovlev, A. A,, 288(66b), 331 Yakubchik, A. O . , 172(80), 194 Yamadaya, M., 261(57), 263(59), 264(57), 265 (57), 266(57), 272 Yamadaya, S., 303(99), 306(99), 332 Yamamoto, H., 209(74, 101), 226,227 Yamamura, K., 207(58), 225 Yarnazaki, Y., 230(36,37, 38,39), 232(39,42), 233(42, 48), 234(42), 240(52), 241(53), 244(53), 259(54), 260(52), 265(52), 269 (67), 271,272 Yang Wen-Hsueh, 163(47), 164(47), 194 Yaroslavsky, C., 212(79), 226

35 1

AUTHOR INDEX

Yaroslavsky, S., 212(78, 79, 81), 213(81), 226 Yates, J. T., 12(39), 14(44), 36(120), 51,53, 59 (9), 72(51), 93, 94 Yates, J. T., Jr., 15(50), 51 Yates, K., 57(3a), 58(3a), 61(3a), 64(16), 75 (16), 80(3a), 93 Yermakov, Y.I., 316,333 Ying, D. H. S., 20(75), 21(75), 22(75), 27 (loo), 33(117), 34(100), 35(100), 52, 53 Yoneda, Y., 158, 159(32), 163(26, 53, 54), 164 (26, 53, 54), 165(54), 177, 186, 187, 193, 194, 196 Yoshida, K., 233(49), 271 Yoshida, T., 177(96), 180(112), 181, 189(96), 190(96), 195 Yu, K. Y., 33(113, 114, 116), 52, 68(38), 92 (38), 94 Yudkina, T. P., 278(33), 329

Yuhashi, S., 230(39), 232(39), 233(48,50), 271 Yukawa, Y., 158, 193 Ywasawa, Y., 80(64), 94

Z Zakharov, I. I., 187, 196 Zanderighi, L., 160(37), 193 Zdraiil, M., 176(103), 177, 178(103), 183, 186, 189(103, 156), 190(103, 156), 191, 195, 196 Zeif, A. P., 188, 196 Zhdan, P. A., 72(50), 94 Zhidomirov, G. M., 139(113), IS0 Zidan, F., 132(102), 133(102), 146,150 Zielinski, S., 129(74), 132(74), IS0 Zimmer, H., 286(62c), 287(62c), 330 Zwietering, P., 21(76), 30(76), 52

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Subject Index A Acetic acid, decomposition, 35-36 Acid- base catalysis, heterogeneous, 163172 Active species, Moo,-Al,O, catalyst, 258259,262-265 Adsorption alcohols, reactions, 36-37 approach, surface activity, 322-328 metal surfaces carbon monoxide, 65-68 nitric oxide, 68-73 nitrogen, 73-75 oxygen, 75-79 Adsorptivity, structure effect, 189-191 correlations, 189-190 AES, see Auger electron spectroscopy Alcohols dehydration, elimination of reactions, 165- 166 dehydrogenation, hydrocarbons, 186-188 reactions, 36-49 adsorption, 36-37 clean surfaces, 37-38 ethanol oxidation, 44-48 methanol oxidation, 38-44 oxidation on copper and silver, 38-39 oxidation reaction on silver, 48-49 Alkylbenzenes, hydrogenation, 176- 177 Alloys, surface segregation, 91-92 Amine acylation, nitrophenyle esters, reactions, 216-217 Amines, macrocycles, 205-206 Ammonia and hydrazine, chemisorption, 80 Analysis of reactions, rate determining steps, 121-148 one-reactant

inorganic, 134- 136 organic, 124-125, 137-140 two-reactants inorganic, 122- 123, 126- 129, 141- 144 organic, 144-147 symbols, 147-148 Argon pressure, silver signal intensity, 64-65 Aromatics formation, C , hydrocarbons, 306 Aromatization, temperature programmed studies, 286-288 Astoichiometric components, surface activity, 322-328 adsorption approach, 323-324 surface charge approach, 326-327 surface heterogeneity approach, 324-326 surface reconstruction approach, 327-328 Asymmetric diarylmethanes, hydrogenolytic behaviors, 229-270,247-252 catalytic hydrogenolysis, 243-258 kinetics and scheme, 252-258 MoO,-AI,O, catalyst, 259-269 relative reactivity, 255-257 schematic model, 254 Atomization aromatic selectivities, dehydrocyclization, 302 hydrocarbons, 289-292,300-302 isohexanes, 298-302 Auger electron spectra Fe (IOO), 7-9 HCOOH decomposition, 25-26 Auger electron spectroscopy, 7- 10 Auger structure, 10 schematic apparatus, 18- 19 spectra, 7-9,25-26 Auger structure, 10 353

354

SUBJECT INDEX

B Benzene nucleus, hydrogenation, 178- 179 temperature programmed desorption, 284, 286 Bifunctional catalysts, ring closure, cyclization, 3 1 1-3 16 C, cyclization, 3 1 1 C, dehydrocyclization, 3 11 ring enlargement, 31 1-316 Bimolecular surface reactions reactants adsorption, 111-1 12 with single reactant, 108-109 Bond shift, skeletal isomerization, 297-298 Branched synthetic polymer, polyethyleneimine. 215-220

C C-C bond, hydrogenolyses, 182-184 C-X bond, hydrogenolyses, 182- 184 C=C bond, hydrogenation, 172-176 C(ls) core level shift, 13-14 C, cyclics ethylcyclobutane, 304-305 isomerization, 298 methylcyclopentane, 304-305 C, cyclization bifunctional catalysts, 31 1 reactions, 292-296 dehydrogenative cyclization, 295-296 hydrogenative cyclization, 292-295 methylcyclopentane formation rate, 292-293 C, cyclization, direct, 288 C, dehydrocyclization atomization, hydrocarbons, 289-292 bifunctional catalysts, 31 1 cyclization direct, 288 steps, 283-286 mechanism, 279-283 temperature programmed reaction studies, 286-288 Carbonium ion type cyclization, 312 enlargement, 3 12 isomerization, 312 opening, 312

Carbon monoxide adsorption, polycrystalline iron, 13-14 chemisorption, 65-68 adsorption on metal surfaces, 65-68 dissociation and molecular adsorption, Pt, 67 emission spectra, 58-59 iron, 12 reactivity pattern, 67-68 surface structure correlations, 66-67 desorption, spectra, 15-17 Carboxylic acid, reactions, 21-36 acetic acid decomposition, 35-36 copper-nickel alloys, 33-35 DCOOH adsorption on Cu (1 lo), spectra, 23 formic acid decomposition, clean metals, 21-32 copper, 21-23 iron, 24-25 nickel, 25-28 ruthenium, 28 silver, 28 tungsten, 25 formic acid decomposition, metaladlayer surfaces, 32-35 formic anhydride adsorption, 27-28 HCOO decomposition on copper, spectra, 22 HCOOH adsorption in iron, spectra, 24-25 decomposition on W(100), spectra, 25-26 historical view, 21 kinetic parameters, decomposition, 28-29 surface carbides and oxides, 32-33 Catalysis cycloamyloses, 202-205 polyethyleneimine, 218-220 rates and equilibria, structure effects, 153-156 redox, see Heterogeneous redox catalysis structure effects, 153-156 surface, see Surface catalysis Catalysts active species, 262-265 bifunctional cyclization, 31 1-316 characterizations, energy slopes, 161-162 cyclization, 278-279

355

SUBJECT INDEX

hydrogenolysis, diarylmethanes, 239-241,262-265 active species of catalysts, 262-265 calcination temperatures, 259-260 catalyst pretreatment, effects, 259-269 hydrogen treatment, 261-262 MOO,-AI,O, catalyst, 258-269 MOO, contents, 260-261 water and ammonia effect, 264 platinum hydrogen adsorptivity, 290-291 n-heptane, atomization, 290-29 1 pretreatment temperature, 325 synthetic, enzyme-like, 197-224 Catalytic behaviors, metallic oxides, 240 cyclization, metal activity, 317-328 hydrogenolysis, diphenylmethane kinetics, 241-243 reaction mechanism, 267 hydrogenolysis, asymmetric diarylmethanes, 243-258 kinetics and scheme, 252-258 MoO,-Al,O, catalyst, 258-269 relative reactivity, 255-257 schematic model, 254 reactions hexatriene, 284-285 ring size effect, 176 Chemical composition, photoelectron spectroscopy, 62 Chemisorption carbon monoxide, 58-59 complex molecules, 80-88 ammonia and hydrazine, 80 formic acid, 82-85 hydrocarbons, 85-88 water, 80-82 diatomic molecules, 65-79 carbon monoxide, 65-79 nitric oxide, 68-73 nitrogen, 73-75 oxygen, 75-79 nickel, 58-59 Clean metals, formic acid decomposition, 21-32 Clean surfaces, alcohol reactions, 37-38 Complex molecules, chemisorption, 80-88 ammonia and hydrazine, 80 formic acid, 82-85

hydrocarbons, 85-88 water, 80-82 Copper and silver, oxidation, methanol and ethanol, 38-44 Copper-nickel alloys, formic acid, reactions, 33-35 Core level shift, C(ls), 13-14 Correlation data, structure effects, 159- 160 Correlations, adsorptivity, 189-190 Cycles, medium and large, 306-309 small, 303-305 Cyclic hydrocarbons, reactivities, 308-309 Cyclization dual function catalysts and oxides, 311-317 oxide catalysts, 316-317 ring closure, bifunctional catalysts, 311-316 hydrogenative versus dehydrogenative, 292-295 reactions C, cyclization, 292-296 C, dehydrocyclization, 279-292 catalysts, 278-279 forms of cyclization, 275-277 general problems, 275-296 hydrocarbons, metal-catalyzed, 273-328 thermodynamic considerations, , 277-218 skeletal rearrangements, 297-31 1 interconversion between ring systems, 303-3 1 I metal-catalyzed skeletal isomerization, 297-298 open chain hydrocarbons, 298-302 steps, C, dehydrocyclization, 283-286 Cycloamylosis, synthetic catalysis, 199-205 catalysis, 202-205 D-glucopyranose, 199 structure and properties, 199-202 Cycloheptaamylose cavity, 204 Cyclooctane transformations, 307

D Decomposition acetic acid, 35-36 single crystals, kinetic parameters, 28-29

356

SUBJECT INDEX

Dehydration, alcohol, elimination of reactions, 165-166 Dehydrocyclization C, mechanism, 279-283 n-hexane, 284,286 Dehydrogenation alcohols, 184-185 cyclization, 292-295 hydrocarbons and alcohols, 186- 188 Desorption, benzene, temperature programmed, 284, 286 o-glucopyranose, cycloamylosis structure, 199 Diarylmethanes asymmetric, 229-270 catalytic hydrogenolysis, 243-258 hydrogenolytic behaviors, 229-270, 247-252 kinetics and scheme, 252-258 MOO,-AI,O, catalyst, 258-269 preparation, 232-238 properties, 234-238 relative reactivity, 255-257 schematic model, 254 hydrogenolysis, catalysts, 239-241 metallic oxides, behaviors, 240 thermal hydrocracking behavior, 269 Diatomic molecules, chemisorption, 65-79 carbon monoxide, 65-68 nitric oxide, 68-73 nitrogen, 73-75 oxygen, 75-79 Diphenylmethane, catalytic hydrogenolysis kinetics, 241-243 reduction mechanism, 267 Dual function catalysts and oxides, 31 1-317

E EELS, see Electron energy loss spectroscopy Electron binding energy, X-ray photoelectron spectroscopy, 14 energy loss spectroscopy, applications, 72-92 spectrometer, schematic diagram, 63-64 spectroscopy applications, 62-92 experimental strategy, 62-65

Electronic and steric effects, substitutents, 153-155 Elimination of reactions, 163- 170 Entropy of activation, rate determining steps, 117-121 moment of inertia, 120-121 tables, 119 Enzyme-like synthetic catalysts, 197-224 Enzymes, semisynthetic, 223 Ethanol oxidation, copper and silver, 38-44 Ethylcyclobutane, C , cyclics, 304-305

F Flash deposition, see Temperature programmed deposition First-order unimolecular surface reaction, 105-108 Formic acid chemisorption, 82-85 lead exposition, spectra, 84-85 decomposition, 21-35 clean metals, 21-33 metal-adlayer surface reactions, 32-35 reactions, 32-35 Formic anhydride adsorption, surface dipole arrangement, 27-28 Forms of cyclization, elementary acts, 275-277

H Helium, cyclooctane transformation, 307 Heterogeneous catalysis acid-base catalysis, 163-172 elimination of reactions, 163- 170 substitution of reactions, 170-172 linear free energy relationships, 158- 159 redox catalysis, 172- 189 metal oxides and sulfides, 186-189 metals, reactions, 172-186 High-conversion data, site density, 114- 1 17 High pressure electron spectrometer, diagram, 63-64 Histidine peptides, hydrolysis, 208 Horiuti, Miyahara, Toyoshima approach, site density, 113-1 14 Hydrazine and ammonia, chemisorption, 80

357

SUBJECT INDEX

Hydrocarbons alcohols, dehydrogenation, 186- 188 aromatization, 289-292, 300-302 chemisorption, 85-88 metal-alkene surfaces, spectra, 85-86 open chain, skeletal rearrangements, 298-302 structure dependence, 292-295 sulfides, oxidation, 188-189 Hydrocracking behavior, thermal, 269 Hydrogenation alkylbenzenes, 176-177, 186 aromatic nucleus, 176-180 benzene nucleus, 178- 179 C=C bond, 172-176 cyclization, 295-296 dehydrogenative, 296 ketone, 180-181 unsaturated noncyclic compounds, 172-1 73 Hydrogenolysis chemical bond, 182-184 diarylmethanes catalyst, 239-241 metallic oxides, catalytic behaviors, 240 MoO,-AI,O, catalyst, 258-259 trimethyldiphenylmethane, 239 Hydrogenolytic behavior asymmetric diarylmethanes, 229-270, 247-252 catalyst, MOO,-AI,O, calcination treatment effect, 259-260 hydrogen treatment effect, 261-262 MOO, content effect, 260-261 reduction time effect, 265-266 water and ammonia effect, 264 phenylarylmethanes, 244-246 Hydro 1y sis acetoxybenzene sulfonate, 21 1 nitrophenyl, 220-221 nitrophenyl acetate, 208 nitrophenyltrifluoroacetanilide,222 Hydrogen adsorption platinum catalyst, 290-291 schematic model, 326-327 cyclooctane transformations, 307 treatment effect, MoO,-AI,O, catalyst, 261-262

I Immobilized catalysts, synthetic, 220-222 Interactions, active species and substrates, 265-269 Interconversion between ring systems, 303-31 1 medium and large cycles, 306-309 ring systems, 309-31 1 small cycles, 303-305

K Ketone, hydrogenation, 180-181 Kinetic isotope effects, catalysis rates, 155 Kinetic parameters, decomposition, single crystal surfaces, 28-29 Kinetics, catalytic hydrogenolysis asymmetric diarylmethanes, 252-258 diphenylmethane, 241-243 kinetic data, 242 schematic model, 254

L Lead deposition, spectra, 84-85 LEED, see Low energy electron diffraction LFER, see Linear free energy relationships Linear correlations, structure effects, 160-161 Linear free energy relationships heterogeneous catalysis, 158-159 slope, catalyst characterization, 161-162 substitution reactions, 170-171 Linear polymers, synthetic catalysts, 208-215 polypeptides, 208-2 10 vinyl polymers, 210-215 Low energy electron diffraction applications, 62, 72-92 patterns for surface carbides, 6-7 schematic apparatus, 18-19 structure and real space, 4-7

M Macrocycles amines, 205-206 paracyclophanes, 206-208 synthetic catalysts, 205-208

358

SUBJECT INDEX

Mechanism, dehydrocyclization, 279-283 Metal activity, catalytic cyclization, 317-328 astoichiometric components, surface activity, 322-328 metal surfaces, structure and catalytic activity, 318-322 adlayer surfaces, formic acid decomposition, 32-35 copper-nickel alloys, 33-35 surface carbides and oxides, 32-33 catalyzed, cyclization reactions, 273-328 skeletal isomerization, 297-298 oxides, chemisorption, 88-91 chemisorption, 88-91 sulfides, reactions, 186-189 reactions, 172-186 single crystal surfaces, reactions, 1-49 surfaces, adsorption carbon monoxide, 65-68 nitric oxide, 68-73 nitrogen, 73-75 oxygen, 75-79 surfaces, overlayer structure, 7-8 surfaces, structure for catalytic activity, 318-322 Methanol oxidation, copper and silver, 38-44 Methylcyclopentane C 5 cyclics, 304-305 C, cyclization, formation rate, 292-293 Methyl orange, polyethyleneimine binding, 216-2 I7 Mobile activated complex, site density, 108

N Nitric oxide, chemisorption, 68-73 Nitrogen oxide, chemisorption, 73-75

0 Open chain hydrocarbons, skeletal rearrangements, 298-302 aromatic selectivities, 302 atomization, 298-302 cyclization, 298-302 Organic substituents, probes, surface catalysis, 151-192 Oxidation

hydrocarbons and sulfides, 188- 189 reactions, alcohols, 38-49 Oxide catalysts, cyclization, 316-3 17 Oxidized silver, spectra, 64-65 Oxygen, chemisorption, 75-79 adsorption, 76-77 Overlayer structure, adsorbates on metal surfaces, 7-8

P Pauling charge, electron binding energy, 14 Paracyclophanes, macrocycles, 206-208 Phenylarylmethanes, hydrogenolytic behavior, 244-246 Photoelectron spectra, metal-alkene surfaces, 85-86 spectroscopy chemical composition, 62 metal oxide surfaces, 55-92 solid surfaces, 55-92 surface chemistry, 55-92 yield, chemisorbed layer, 59-62 factors affecting yield, 61 Platinum catalyst, hydrogen adsorptivity, 290-29 1 surfaces, dissociation and molecular adsorption, 67 Polyethyleneimine branched synthetic polymers, 215-220 binding of small molecules, 216-218 catalysis, 218-220 structure, 215-216 preparation, steps, 220-221 Polymers, linear, synthetic catalysts, 208-215

Q Quantitative treatment, structure effects, 155-162

R Reactions alcohols, 36-49 adsorption, 36-37 clean surfaces, 37-38 ethanol oxidation, 44-48 methanol oxidation. 38-44

SUBJECT INDEX

oxidation on copper and silver, 38-48 oxidation reaction, silver, 48-49 kinetics and mechanism, 1-49 metal oxides and sulfides, 186-189 alcohols, dehydrogenation, 186- 188 alkylbenzenes, hydrogenation, 186 hydrocarbons, 186-189 sulfides, oxidation, 188-189 metals, heterogeneous redox catalysis, 172-186 dehydrogenation, alcohols, 184- 185 hydrogenation, 172-178, 180-182 hydrogenolysis, 182-184 Real space, surface structure, 6-7 Redox catalysis, heterogeneous, 172- 189 Reduction mechanism, catalytic hydrogenolysis, 267 Reduction time effect, hydrogenolytic behavior, 265-266 Relative reactivities cyclic hydrocarbons, 308-309 diarylmethanes, 255-257 Ring closure, bifunctional catalysts, 311-316 Ring enlargement, bifunctional catalysts, 311-316 carbonium ion type, 3 12 Ring production comparison, 314 Ring size effect, catalytic reactions, 176 Ring systems, interconversion, 309-31 1

S Semisynthetic enzymes, 223 Silver and copper, oxidation, methanol and ethanol, 38-44 Site densities, rate determining steps, 107-1 I7 adsorption, 102-105 bimolecular surface reaction reactants adsorption, 111-112 single reactant, 108-109 calcination, 102-105, 114-1 17 entropy criteria, 97-148 first-order unimolecular surface reactions, 105-108 Horiuti, Miyahara, Toyoshima approach, 113-114 reactants migration, 109- 111

359

Skeletal isomerization, metal catalyzed, 297-298 Small molecules binding, polyethyleneimine, 216-218 Solid-crystalized reactions, site density, 97-148 Spirocyclic hydrocarbons, transformation, 309-310 Stereochemical effects, catalysis rates, 155-156 Structure and catalytic activity, metal surfaces, 318-322 Structure and properties, cycloamylosis, 199-202 Structure effects adsorptivity, 189-191 quantitative treatment, 156-162 catalyst characterization, 161-162 correlation data, 159-160 heterogeneous catalysis, 158-159 linear correlations, 160-161 types of correlations, 156-158 rates and equilibria, catalysis, 153-165 electronic and steric effects, 153-155 kinetic isotope effects, 155 stereochemical effects, 155-1 56 Structure, polyethyleneimine, 21 5-216 Substitution and addition reactions, 170171 Sulfides and hydrocarbons, oxidation, 188- 189 Surface activity, astoichiometric components, 322-328 Surface carbides and oxides, formic acid decomposition, 32-33 Surface catalysis, mechanism organic substituent, probes, 151-192 heterogeneous acid-base catalysis, 163-172 heterogeneous redox catalysis, 172-189 structure effects adsorptivity, 189-191 quantitative treatments, 156-162 rates and equilibria, 153-156 Surface charge approach, surface activity, 326-327 Surface chemistry, photoelectron spectroscopy, 55-92 Surface concentrations escape depth data, 61

360

SUBJECT INDEX

photoelectron intensity data, 59-62 ionization, 61 Surface dipoles arrangements, 27-28 Surface heterogeneity approach, 324-326 Surface reactions bimolecular reactants adsorption, 11-112 single reactant, 108-109 unimolecular, first-order, 105- 108 Surface reactivity, tools, 3-21 Auger electron spectroscopy, 7- 10 low energy electron diffraction, 4-7 temperature programmed desorption, 15-118 temperature programmed reaction spectroscopy, 10-14 ultraviolet photoelectron spectroscopy, 10-14 X-ray photoelectron spectroscopy, 10-14 Surface reconstruction approach, 327-328 Surface structure, binding energy, 66-67 Synthetic catalysts, enzyme-like, 197-224 cycloamylosis, 199-205 immobilized catalysts, 220-222 linear polymer, 208-215 macrocycles, 205-208 polyethyleneimine, branched polymers, 215-220 semisynthetic enzymes, 223 Synzymes, catalysts, 197-224

T Taft relationship slopes, 167- 168 Temperature programmed desorption benzene, 284,286 spectra, 15-16, 17 tool, 15-18 programmed reaction spectroscopy adsorption, spectra, 18- 19 decomposition, spectra, 20 schematic apparatus, 18-19 tool, 18-21 Thermal hydrocracking behavior, diarylmethanes, 269

Thermodynamic considerations, cyclization reactions, 277-278 TPD, see Temperature programmed desorption TPRS, see Temperature programmed reaction spectroscopy Transformation, spirocyclic hydrocarbons, 309-31 1 Transition state theory, rate determining steps, 99-102 Types of correlations, structure effects, 156- 158

U Ultraviolet photoelectron spectroscopy, 56-59 applications, solid surfaces, 58-92 schematic apparatus, 18-19 spectra, 11-12, 23 tool, 10-14 Unimolecular surface reactions, 105- 108 Unsaturated noncyclic compounds, hydrogenation, 172-173 UPS, see Ultraviolet photoelectron spectroscopy

V Vacuum generator spectrometer, 58 Vinyl polymers. 210-215

w Water and ammonia effect, hydrogenolytic behavior, 264 Water chemisorption, 80-82 spectra, 81

x X-ray spectroscopy, 56-59 applications, solid surfaces, 57-92 energies, HCOOH adsorption, 24-25 schematic apparatus, 18-19 spectra, 13-14 surface sensitivity, 57-58 tool, 10-14

Contents of Previous Volumes Volume 1

The Heterogeneity of Catalyst Surfaces for Chemisorption HUGHS. TAYLOR Alkylation of Isoparaffins 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 AND I. FANKUCHEN Volume 2

The Fundamental Principles of Catalytic Activity FREDERICK SEITZ The Mechanism of the Polymerization of Alkenes LOUISSCHMERLING AND V. N. IPATIEFF Early Studies of Multicomponent Catalysts ALWINMITTASCH Catalytic Phenomena Related to Photographic Development T. H. JAMB Catalysis and the Adsorption of Hydrogen on Metal Catalysts OTTOBEECK Hydrogen Fluoride Catalysis J. H. SIMONS Entropy of Adsorption CHARLES KEMBALL

About the Mechanism of Contact Catalysis GEORG-MARIA SCHWAB 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. HENRYRUSHTON AND K. A. KRIEGER The Poisoning of Metallic Catalysts E. B. MAXTED Catalytic Cracking of Pure Hydrocarbons VLADIMIRHAENSEL 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 AHLBORN WHEELER Nickel Sulfide Catalysts WILLIAM J. KIRKPATRICK

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 Molecular Structure R. P. BELL Theory of Physical Adsorption L. HILL TERRELL 361

362

CONTENTS OF PREVIOUS VOLUMES

The Role of Surface Heterogeneity in Adsorption GEORGE D. 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 JOSEPHWEISS The Specific Reactions of Iron in Some Hemoproteins PHILIPGEORGE

Volume 5 Latest Developments in Ammonia Synthesis ANDERS NIELSEN 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 J. G. TOLPIN,G. s. JOHN,AND E. FIELD The Elucidation of Reaction Mechanisms by the Method of Intermediates in QuasiStationary Concentrations J. A. CHRISTIANSEN Iron Nitrides as Fischer-Tropsch Catalysts ROBERT B. ANDERSON Hydrogenation of Organic Compounds with Synthesis Gas MILTONORCHIN The Uses of Raney Nickel EUGENE LIEBERAND FREDL. MORRITZ

Some General Aspects of Chemisorption and Catalysis TAKAOKWAN Nobel Metal-Synthetic Polymer Catalysts and Studies on the Mechanism of Their Action WILLIAM P. DUNWORTH AND F. F. NORD Interpretation of Measurement in Experimental Catalysis P. B. WEISZAND C. D. PRATER Commercial Isomerization B. L. EVERING Acidic and Basic Catalysis MARTINKILPATRICK Industrial Catalytic Cracking RODNEY V. SHANKLAND

Volume 7 The Electronic Factor in Heterogeneous Catalysis M. McD. BAKERAND G. I. JENKINS Chemisorption and Catalysis on Oxide Semiconductors G. PARRAVANO AND M. BOUDART The Compensation Effect in Heterogeneous Catalysis E. CREMER Field Emission Microscopy and Some Applications to Catalysis and Chemisorption ROBERTGOMER Adsorption on Metal Surfaces and Its Bearing on Catalysis JOSEPHA. BECKER The Application of the Theory of Semiconductors to Problems of Heterogeneous Catalysis K, HAUFFE Surface Barrier Effects in Adsorption, Illustrated by Zinc Oxide S. ROYMORRISON 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 AND J. R. COLEY V. I. KOMAREWSKY Polymerization of Olefins from Cracked Gases EDWINK. JONES Coal-Hydrogenation Vapor-Phase Catalysts E. E. DONATH The Kinetics of the Cracking of Cumene by Silica-Alumina Catalysts CHARLESD. PRATERAND RUDOLPHM. LAGO Volume 9 Proceedings of the International Congress on Catalysis, Philadelphia, Pennsylvania, 1956 Volume 10 The Infrared Spectra of Adsorbed Molecules R. P. EISCHENS AND W. A. PLISKIN The Influence of Crystal Face in Catalysis ALLANT. GWATHMEY AND ROBERTE. CUNNINGHAM The Nature of Active Centres 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. SCHUITAND L. L. VAN REIJEN Volume 11 The Kinetics of the Stereospecific Polymerization of &-Olefins G. NATTAAND I. PASQUON

363

Surface Potentials and Adsorption Process on Metals R. V. CULVERAND F. C. TOMPKINS Gas Reactions of Carbon RUSINKO,JR., P. L. WALKER,JR., FRANK AND L. G. AUSTIN The Catalytic Exchange of Hydrocarbons with Deuterium C. KEMBALL Immersional 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. O'REILLY Bare-Catalyzed Reactions of Hydrocarbons HERMAN PINES AND LUKE A. 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

.

364

CONTENTS OF PREVIOUS VOLUMES

The Structure and Analysis of Complex Reaction Systems JAMESWEI AND CHARLES D. PRATER Catalytic Effect in Isocyanate Reactions A. FARKAS AND G . A. MILLS

Volume 14 Quantum Conversion in Chloroplasts MELVIN CALVIN The Catalytic Decomposition of Formic Acid P. MARS, J. J. F. SCHOLLEN,AND P. ZWIETERING Application of Spectrophotometry to the Study of Catalytic Systems H. P. LEFTINAND M. C. HOBSON, JR. Hydrogenation of Pyridines and Quinolines MORRISFREIFELDER Modern Methods in Surface Kinetics: Flash, Desorption, Field Emission Microscopy, and Ultrahigh Vacuum Techniques GERTEHRLICH Catalytic Oxidation of ilydrocarbons 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. BONDAND P. B. WELLS Electronic Spectroscopy of Absorbed Gas Molecules A. TERENIN The Catalysis of Isotopic Exchange in Molecular Oxygen G. K. BORE~KOV

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 VIII Transition Metal Catalysts J. L. GARNETTAND W. A. SOLLICHBAUMGARTNER Stereochemistry and the Mechanism of Hydrogenation of Unsaturated Hydrocarbons SAMUEL SIEGEL Chemical Identification of Surface Groups H. P. BOEHM

Volume 17 On the Theory of Heterogeneous Catalysis JUROHORIUTIAND 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 AND Y. AMENOMIYA Catalytic Oxidation of Olefins HERVEY H. VOGEAND CHARLESR. ADAMS The Physical-Chemical Properties of Chromia-Alumina Catalysts CHARLES P. POOLE,JR. AND D. S. MACIVER Catalytic Activity and Acidic Property of Solid Metal Sulfates Kozo TANABE AND TSUNEICHI TAKESHITA Electrocatal ysis S. SRINIVASEN, H. WROBLOWA, AND 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 ELLISON H. TAYLOR Organic Catalysis over Crystalline Aluminosilicates P. B. VENUTO AND P. S. LANDIS On the Transition Metal-Catalyzed Reactions of Norbornadiene and the Concept of K 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, AND R. N. HAWARR Dynamic Methods for Characterization of Adsorptive Properties of Solid Catalysts L. POLINSKI AND L. NAPHTALI Enhanced Reactivity at Dislocations in Solids J. M. THOMAS

Volume 20

365

AND JEANEUGENE GERMAIN MICHELBLANCHARD 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 Mieellar Solutions H. MORAWETZ Catalytic Activities of Thermal Polyanhydroa-Amino Acids AND SIDNEY w . FOX DUANEL. ROHLFING

Volume 21 Kinetics of Adsorption and Desorption and the Elovich Equation AND F. C. TOMPKINS C. AHARONI Carbon Monoxide Adsorption on the Transition Metals R. R. FORD Discovery of Surface Phases by Low Energy Electron Diffraction (LEED) JOHNW. MAY Sorption, Diffusion, and Catalytic Reaction in Zeolites L. RIEKERT Adsorbed Atomic Species as Intermediates in Heterogeneous Catalysis CARLWAGNER

Chemisorptive and Catalytic Behavior of Chromia ROBERT L. BURWELL, JR., GARYL. HALLER, KATHLEEN C. TAYLOR, AND JOHNF. READ Volume 22 Correlation among Methods of Preparation Hydrogenation and Isomerization over Zinc of Solid Catalysts, Their Structures, and Oxide Catalytic Activity KIYOSHI MORIKAWA, TAKAYASU SHIRASAKI, R. J. KOKESAND A. L. DENT Chemisorption Complexes and Their Role in AND MASAHIDE OKADA Catalytic Reactions on Transition Metals Catalytic Research on Zeolites Z. KNOR J. TURKEVICH AND Y. ONO Influence of Metal Particle Size in NickelCatalysis by Supported Metals on-Aerosil Catalysts on Surface Site DisM. BOUDART tribution, Catalytic Activity, and SelecCarbon Monoxide Oxidation and Related tivity Reactions on a Highly Divided Nickel R. VANHARDEVELD AND F. HARTOG Oxide Adsorption and Catalysis on Evaporated P. C. GRAVELLE AND S. J . TEICHNER Alloy Films Acid-Catalyzed Isomerization of Bicyclic R. L. Moss AND L. WHALLEY Olefins

366

CONTENTS OF PREVIOUS VOLUMES

Heat-Flow Microcalorimetry and Its Applications to Heterogeneous Catalysis P. C. GRAVELLE Electron Spin Resonance in Catalysis JACKH. LUNSFORD

R. P. COONEY, G . CURTHOYS, AND NGUYEN THETAM Analysis of Thermal Desorption Data for Adsorption Studies MILOS SMUTEK, SLAVOJ CERNQ, AND FRANTISEK BUZEK

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 JAMESWEI The Metathesis of Unsaturated Hydrocarbons Catalyzed by Transition Metal Compounds J. C. MOLAND J. A. MOULIIN One-Component Catalysts for Polymerization of Olefins Yu. YERMAKOV AND V. ZAKHAROV The Economics of Catalytic Processes AND D . S. DAVIES J. DEWING Catalytic Reactivity of Hydrogen on Palladium and Nickel Hydride Phases W. PALCZEWSKA Laser Raman Spectroscopy and Its Application to the Study of Adsorbed Species

Volume 25 Application of Molecular Orbital Theory to Catalysis ROGERC. BAETZOLD The Stereochemistry of Hydrogenation of a$-Unsaturated Ketones ROBERTL. AUGUSTINE Asymmetric Homogeneous Hydrogenation J . D. MORRISON,W. F. MASLER,AND M. K. NEUBERG Stereochemical Approaches to Mechanisms of Hydrocarbon Reactions on Metal Catalysts AND J. J. ROONEY J. K. A. CLARKE Specific Poisoning and Characterization of Catalytically Active Oxide Surfaces HELMUTKNOZINGER Metal-Catalyzed Oxidations of Organic Compounds in the Liquid Phase: A Mechanistic Approach AND JAYK. KOCHI ROGERA. SHELDON Volume 26 Active Sites in Heterogeneous Catalysis G. A. SOMORJAI Surface Composition and Selectivity of Alloy Catalysts W. M. H . SACHTLER AND R. A. VAN SANTEN Mossbauer Spectroscopy Applications to Heterogeneous Catalysis JAMES A. DUMESIC AND HENRIK TOPS~E Compensation Effect in Heterogeneous Catalysis A. K. GALWEY Transition Metal-Catalyzed Reactions of Organic Halides with CO, Olefins, and Acetylenes R. FT HECK

CONTENTS OF PREVIOUS VOLUMES

Manual of Symbols and Terminology for Physicochemical Quantities and UnitsAppendix I1 Part 11: Heterogeneous Catalysis

Characterization of Molybdena Catalysts F. E. MASSOTH Poisoning of Automative Catalysts M. SHELEF, K. OTTO,AND N. C. OTTO

Volume 27

Electronics of Supported Catalysts GEORG-MARIA SCHWAB The Effect of a Magnetic Field on the Catalyzed Nondissocitive Parahydrogen Conversion Rate P. W. SELWOOD Hysteresis and Periodic Activity Behavior in Catalytic Chemical Reaction Systems VLADlMrR HLAVACEK AND JAROSLAV VOTRUBA Surface Acidity of Solid Catalysts H. A. BENESIAND B. H. C. WINQUIST Selective Oxidation of Propylene GEORGEW. KEULKS,L. DAVIDKRENZKE, AND THOMAS N. NOTERMANN u-n Rearrangements and Their Role in Catalysis BARRYGOREWITAND MINORUTSUTSUI

367

Volume 28

Elementary Steps in the Catalytic Oxidation of Carbon Monoxide on Platinum Metals T. ENGELAND G. ERTL The Binding and Activation of Carbon Monoxide, Carbon Dioxide, and Nitric Oxide and Their Homogeneously Catalyzed Reactions RICHARDEISENBERG AND DAN E. HENDRIKSEN

The Kinetics of Some Industrial Heterogeneous Catalytic Reactions M. I. TEMKIN Metal-Catalyzed Dehydrocyclization of Alkylaromatics SIGMUND M. CSICSERY Metalloenzyme Catalysis JOSEPHJ. VILLAFRANCA AND FRANKM. RAUSHEL

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