ADVANCES IN CATALYSIS VOLUME 29
Advisory Board
G. K. BORESKOV Novosibirsk, U . S . S . R .
M. BOUDART Stanford, Cal...
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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.
ACADEMIC PRESS,INC.
111 Fifth Avenue, New York, New York 10003
United Kingdom Edition published by ACADEMIC PRESS, INC. (LONDON) LTD. 24/28 Oval Road, London NW1 I D X
LIBRARY OF
CONGRESS CATALOG CARD
NUMBER:49-1755
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 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-
CRITERIA IN SOLID-CATALYZED REACTIONS
135
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
-
136
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,
CRITERIA IN SOLID-CATALYZED REACTIONS
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
138
RUSSELL W. MAATMAN
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
CRITERIA IN SOLID-CATALYZED REACTIONS
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
140
RUSSELL W. MAATMAN
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.
’.
CRITERIA IN SOLID-CATALYZED REACTIONS
141
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
142
RUSSELL W. MAATMAN
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
CRITERIA IN SOLID-CATALYZED REACTIONS
143
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.
144
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;
146
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
147
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.
150
RUSSELL W. MAATMAN
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
Copyright 0 1980 by Academic Press. Inc . All rights of reproduction in any form reserved . ISBN 0-12-007829-5
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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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153
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
154
M. KRAUS
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-
SUBSTITUENT EFFECTS AS PROBES
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-
156
M. KRAUS
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
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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
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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
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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.
REFERENCES 1 . Boudart, M., Chem. Eng. Prog. 57, No. 8,33 (1961). 2. Sim Do-Chen, and Kraus, M., Colleci. Czech. Chem. Commun. 32,2972 (1967). 3. Raesenberg, J. R., Lieber, E., and Smith, G. B. L., J . Am. Chem. Soc. 61,384 (1939).
SUBSTITUENT EFFECTS AS PROBES
193
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).
194
M. KRAUS
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
195
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.
196
M. KRAUS
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).
ENZYME-LIKE SYNTHETIC CATALYSTS
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.
225
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).
226
G . P. ROYER
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.
RFJERENCES
Mid-Century Co., U. S. Patent 2,833,816 (1958). Mid-Century Co., U. S. Patents 3,089,906 and 3,089,907 (1958). Furukawa Electric Co. Ltd., Ger. Offen. 2,112,009 (1971). Standard Oil Co., U. S. Patent 3,532,746 (1970). Brill, W. W., Ind. Eng. Chem. 52,837 (1960). Mitsubishi Yuka Co. Ltd., Japan Open 45-4,978 (1970). Teijin Kasei Co., Japan Open 46-14,332 (1971). 8. Drayer, D. E., Hydrocarbon Process. 50, 143 (1971). 1. 2. 3. 4. 5. 6. 7.
HYDROGENOLYTIC BEHAVIORS OF ASYMMETRIC DIARYLMETHANES
27 1
9. Nakamura, E., and Koguchi, K., J . Jpn. Pet. Inst. 12, 612 (1969). 10. Standard Oil Co., U. S. Patent 2,564,073 (1951). 11. Standard Oil Co., U. S. Patent 2,795,630 (1957).
12. Standard Oil Co., U. S. Patent 2,803,681 (1957). 13. California Research Corp., U. S. Patent 2,910,514 (1959). 14. Sinclair Research Inc., U . S. Patent 3,233,002 (1966). 15. Sinclair Research Inc., U. S. Patent 3,260,764 (1966). 16. Koguchi, K., Nakamura, E., and Nakayama, T., J . Jpn. Pet. Inst. 17, 1022 (1974). 17. Universal Oil Products Co., U. S. Patent 2,447,599 (1948). 18. Shell Development Co., U. S. Patent 2,756,261 (1956). 19. Mid-Century Corp., British Patent 794,693 (1958). 20. Mitsubishi Yuka Co. Ltd., Japan Open 41-20340 (1966). 21. Mitsubishi Yuka Co. Ltd., Japan Open 41-20341 (1966). 22. Smith, L. I., and Dobrovolny, F. J., J. Am. Chem. Soc. 48, 1413 (1926); Smith, L. I., Org. Synth. Collect. Vol. 2,248 (1943). 23. Esso Research and Engineering Co., U. S. Patent 3,031,513 (1962). 24. Chem. Abstr. 66, 104,753b (1967). 25. Koguchi, K., Nakamura, E., and Nakayama, T., J . Jpn. Pet. Inst. 17, 1028 (1974). 26. Kobayashi, D., Asano, T., Kadowaki, K., and Sato, Y., J . Jpn. Pet. Inst. 13, 775 (1970). 27. N.V.DE Bataafsche Petroleum Maatshappij, British Patent 766,498 (1957). 28. Shell Development Co., U. S. Patent 2,819,322 (1958). 29. Lake, R. D., and Corson, B. B., J . Org. Chem. 24, 1823 (1959). 30. Shacklett, C. D., and Smith, H. A,, J . Am. Chem. Soc. 73, 766 (1951). 31. Hendrickson, J. G . , and Wadsworth, F. T., Ind. Eng. Chem. 50,877 (1958). 32. American Oil Co., U. S. Patent 2,906,785 (1959). 33. British Hydrocarbon Chemicals Ltd., U. S. Patent 2,977,396 (1961). 34. N.V.DE Bataafsche Petroleum Maatschappij, Netherlands Patent 96,232 (1960). 35. Mironov, G . S . , Vetrova, B. B., Kozlova, I. P., and Farberov, M. I., Zh. Prikl. Khim. 39 (7), 1614 (1966). 36. Kawai, T., and Yamazaki, Y., J . J p k Pet. Inst. 16,484 (1973). 37. Kawai, T., Yamazaki, Y., and Sano, A,, J . Jpn. Pet. Inst. 16,490 (1973). 38. Kawai, T., Yamazaki, Y., and Sano, A,, J . Jpn. Pet. Inst. 16, 567 (1973). 39. Kawai, T., Yamazaki, Y., and Yuhashi, S . , J . Jpn. Pet. Inst. 17, 678 (1974). 40. Kharasch, M. S., and Brown, H. C., J. Am. Chem. SOC.61, 2142 (1939). 41. Golivets, G. I., Dashevskii, M. M., and Golivets, I. D., Zh. Prikl. Khim. 41, 148 (1968). 42. Kawai, T., and Yamazaki, Y., J . Jpn. Pet. Inst. 17,657 (1974). 43. Nystron, R. F., and Brown, W. G . , J . Am. Chem. Soc. 69,2548 (1947). 44. Shacklett, C. D., and Smith, H. A,, J . Am. Chem. SOC. 73,766 (1951). 45. Marvel, C. S., Saunders, J. H., and Overberger, C. G., J . Am. Chem. SOC.68, 1085 (1946). 46. Davidson, D., and Weiss, M., Org. Synth. Collect. Vol. 2, 590 (1955). 47. Lowe, G.,Torto, F. G . , and Weedon, B. C. L., J . Chem. Soc. p. 1855 (1958). 48. Kawai, T., Yamazaki, Y., and Yuhashi, S., Bull. Chem. Soc. Jpn. 47,2613 (1974). 49. Horie, T., and Yoshida, K., J . Jpn. Pet. Inst. 17,46 (1974). 50. Montaudo, G., Caccamese, S., and Finocchiaro, P., J . Am. Chem. Soc. 93,4202 (1971). 51. Kawai, T., Doctoral Thesis, Tokyo Metropolitan University, Tokyo, Japan. 52. Kawai, T., Yamazaki, Y., Tsurugaya, M., and Ishino, K., J . Jpn. Pet. Inst. 19, 33 (1976). 53. Kawai, T., and Yamazaki, Y., J . Jpn. Pet. Inst. 18,692 (1975). 54. Kawai, T., Yamazaki, Y., and Tsurugaya, M., Bull. Jpn. Pet. Inst. 18, No. 1, 20 (1976). 55. Giordano, N., Bart, J. C. J., Vaghi, A,, Castellan, A,, and Martinotti, G., J . Cuful.36,81 (1975).
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56. Hashimoto, K., Watanabe, S., and Tarama, K., Nippon Kugaku Kaishi p. 591 (1975).
57. Kabe, T., Yamadaya, M., Ohba, M., and Miki, Y., J . Chem. SOC.Jpn., Ind. Chem. Sect. 74, 1566 (1971). 58. Massoth, F. E., J . Cufal.30,204 (1973). 59. Kabe, T., Yamadaya, M., Miki, Y., and Ohba, T., Shokubai 17, No. 1,23P (1975). 60. Giordano, N., Castellan, A,, Bart, C. J., Vaghi, A,, and Campadelli, F., J . Cufal. 37, 204 (1975). 61. Seshadri, K. S., and Petrakis, L., J . Catul. 30,195 (1973). 62. Lombardo, E. A., Jacono, M. L., and Hall, W. K., J . Catul. 51,243 (1978). 63. Jacono, M. L., and Hall, W. K., J . Colloid Interface Sci. 58, 76 (1977). 64. Bowman, R. G., Ph.D. Dissertation, Northwestern University, Evanston, Illinois (1978). 65. Bowman, R. G., and Burwell, R. L., Private communication. 66. Lombardo, E. A., Houalla, M., and Hall, W. K., J . Cutul. 51, 256 (1978). 67. Kawai, T., Yamazaki, Y., and Sasaki, M., J . Jpn. Pet. Insf. 20,486 (1977).
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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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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ZOLTAN PAAL
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).
278
ZOLTAN PAL
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
PAAL
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
284
ZOLTAN
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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
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
’
ZOLTAN PAAL
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
ZOLTAN
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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